THE EVOLUTION OF MAN.
HAECKEL'S EVOLUTION OF MAN. PLATE I.
DEVELOPMENT OF THE FACE (THIRD STAGE).
EXPLANATION UK CHAP. XXI.
M. Man. B. Bat. S. Sheep. C. Cat.
THE
EVOLUTION OF MAN
A POPULAR EXPOSITION
OF THE
PRINCIPAL POINTS OF HUMAN ONTOGENY AND PHYLOGENY.
FROM THE GERMAN OF
ERNST HAECKEL,
PROFESSOR IN THE FNIVEBSITY OF JENA, AUTHOR OF "THE HISTORY OF C E E A T I O N." BTC.
IN TWO VOLUMES. VOL. I.
NEW YORK: D. APPLETON AND COMPANY,
72 FIFTH AVENUE. 1898.
Authorized Edition.
SAOTA BARBARA COLLEGE LIBRARY
v /
CONTENTS OF VOL. I.
List of Plates ... ... ... ... ... ... xiv
List of Woodcuts ... ... ... ... ... xv
List of Genetic Tables ... ... ... ... ... xviii
Preface to the First Edition ... ... ... xix
Preface to the Third Edition ... ... ... ... xxvii
Prometheus ... ... ... ... ... xxxvii
Faust ... ... ... ... ... xxxvii
CHAPTER I.
THE FUNDAMENTAL LAW OF THE EVOLUTION OF ORGANISMS.
General Significance of the History of the Evolution of Man. — Ignor- ance of it among the so-called Educated Classes. — The Two Branches of the History of Evolution. — Ontogeny, or the History of Germs (Embryos), and Phylogeny, or the History of Descent (or of the Tribes). — Causal Connection between the Two Series of Evolution. — The Evolution of the Tribe determines the Evolution of the Germ. — Ontogeny as an Epitome or Recapitulation of Phy- logeny. The Incompleteness of this Epitome. — The Fundamental Law of Bicgeny. — Heredity and Adaptation are the two Formativu Functions, or the two Mechanical Causes, of Evolution. — Absence of Purposive Causes. — Validity of Mechanical Causes only. — Sub- stitution of the Monistic or Unitary for the Dualistic or Binary Cosmology. — Radical Importance of the Facts of Embryology to Monistic Philosophy. — Palingenesis, or Derived History, and Keno- Kcuesis, or Vitiated History. — History of the Evolution of Forms and Functions. — Necessary Connection between Physiogeny and
VI CONTENTS.
Morphogeny.— The History of Evolution as yet almost entirely the Product of Morphology, and not of Physiology. — The History of the Evolution of the Central Nervous System (Brain and Spinal Marrow) is involved in that of the Psychic Activities, or the Miud 1
CHAPTER II.
THE EARLIER HISTORY OF ONTOGENY. CASPAR FEIEDEICH WOLFF.
The Evolution of Animals as known to Aristotle. — His Knowledge of the Ontogeny of the Lower Animals. — Stationary Condition of the cientific Study of Nature during the Christian Middle Ages — First Awakening of Ontogeny in the Beginning of the Seventeenth Century. — Fabricius ab Aqnapendente. — Harvey. — Marcello Mai- ighi.— Importance of the Incubated Chick. — The Theories of Pre- formation and Encasement (Evolution and Pre-delineation). — Theories of Male and Female Encasement. — Either the Sperm, animal or the Egg as the Pre-forrued Individual. — Animalculists : Leeuwenhoek, Hartsoeker, Spallanzani. — Ovnlists : Haller, Leib- nitz, Bonnet. — Victorv of the Theory of Evolution owing to the uthority of Haller and Leibnitz.— Caspar Friedrich Wolff.— His Fate and Works. — Tne Theoria Generationis. — Re-formation, or Epigenesis. — The History of the Evolution of the Intestinal Canal. — The Foundations of the Theory of Germ-layers (Four Layers, Leaves).— The Metamorphosis of Plants.— The Germs of the Cellular Theory.— Wolff's Monistic Philosophy ...
CHAPTER III.
MODERN ONTOGENY. KARL ERNST BAER.
Karl Ernst Baer, the Principal Disciple of Wolff.— The Wiirzhnrg School of Embryologists : Dollinger, Pander, Baer. — Pander's Theory of Germ-layers. — Its Full Development by Baer. — The Disc-shaped first parts into Two Germ-layers, each of which again divides into Two Strata. The Skin or Flesh-stratum arises from the Outer or Animal Germ- layer. The Tascular or Mucous Stratum arises from the Inner or Vegetative Germ-layer. The Significance of the Germ-layers. — The Modification of the Layers into Tubes. — Baer's Discovery of the Human Egg, the Germ-vesicle, and Chorda Dor-
CONTENTS. Vli
PAOB
salia. — The Four Types of Evolution in the Four Main Groups of the Animal Kingdom. — Baer's Law of the Type of Evolution and the Degree of Perfection. — Explanation of this Law by the Theory of Selection. — Baer's Successors : Rathke, Johannes Miiller, Bis- choff, Kolliker.— The Cell Theory : Schleiden, Schwann.— Its Appli- cation to Ontogeny : Robert Remak. — Retrogressions in Ontogeny : Beichert and His. — Extension of the Domain of Ontogeny : Darwin 48
CHAPTER IV.
THE EARLIER HISTORY OF PHTLOGENT. JEAN LAMARCK.
Phylogeny before Darwin. — Origin of Species. — Karl Linnaeus' Idea of Species, and Assent to Moses' Biblical History of Creation. — The Deluge. — Palaeontology. — George Cuvier's Theory of Catastrophes. — Repeated Terrestrial Revolutions, and New Creations. — Lyell's Theory of Continuity.— The Natural Causes of the Constant Modi- fication of the Earth. — Supernatural Origin of Organisms. — Immanuel Kant's Dualistic Philosophy of Nature. — Jean Lamarck. — Monistic Philosophy of Nature. — The Story of his Life. — His Philosophic Zoologique. — First Scientific Statement of the Doctrine of Descent. — Modification of Organs by Practice and Habit, in Conjunction with Heredity. — Application of the Theory to Man. — Descent of Man from the Ape. — Wolfgang Goethe. — His Studies in Natural Science. — His Morphology. — His Studies of the " Formation and Transformation of Organisms." — Goethe's Theory of the Tendency to Specific Differences (Heredity) and of Meta- morphosis (Adaptation) ... ... ... ... ... 70
CHAPTER V.
MODERN PHYLOGENY. CHARLES DAKWIN.
Kolation of Modern to Earlier Phylogeny. — Charles Darwin's Work on the Origin of Species. — Causes of its Remarkable Success. — The Theory of Selection : the Interrelation of Hereditary Transmission and Adaptation in the Struggle for Existence". — Darwin's Life and Voyage Round the World — His Grandfather, Erasmus Darwin. — Charles Darwin's Study of Domestic Animals and Plants. — Com.
vlii CONTENTS.
PAC.B
parison of Artificial with Natural Conditions of Breeding. — The Struggle for Existence. — Necessary Application of the Theory of Descent to Man. — Descent of Man from the Ape. — Thomas Hux- ley.— Karl Vogt.— Friedrich Kolle.— The Pedigrees in the Generelle Morphologie and the " History of Creation." — The Genealogical Alternative. — The Descent of Man from Apes deduced from the Theory of Descent. — The Theory of Descent as the Greatest Induc- tive Law of Biology. — Foundation of this Induction. — Palaeon- tology.— Comparative Anatomy. — The Theory of Rudimentary Organs. — Purposelessness, or Dysteleology. — Genealogy of the Natural System.— Chorology.— ffikology.— Ontogeny.— Refutation of the Dogma of Species. — The " Monograph on the Chalk Spongea ;" Analytic Evidence for the Theory of Descent ... 93
CHAPTER VI.
THE EGG-CELL AND THE AMOEBA.
The Egg of Man and of other Animals is a Simple Cell.— Import and Essential Principles of the Cell Theory. — Protoplasm (Cell-snb- stance), and the Nucleus (Cell-kernel), as the Two Essential Con- stitnent Parts of every Genuine Cell. — The Undifferentiated Egg- cell, compared with a highly Differentiated Mind-cell or Nerve-cell of the Brain. — The Cell as an Elementary Organism, or an Indi- vidual of the First Order.— The Phenomena of its Life.— The Special Constitution of the Egg-cell. — Yelk. — The Germ-vesicle. — The Germ-spot. — The Egg-membrane, or Chorion. — Application of the Fundamental Principle of Biogeny to the Egg.cell. — One-celled Organisms. — The Amoebae. — Organization and Vital Phenomena. — Their Movements. — Amoeboid Cells in Many-celled Organisms. — Movements of such Cells, and Absorption of Solid Matter. — Absor- bent Blood Corpuscles. — Comparison of Amoeba with Egg-cell. — Amoeboid Egg-cells of Sponges. — The Amoeba as the Common Ancestral Form of Many-celled Organisms ... ... ... 120
CHAPTER VIL
THE PROCESSES OF EVOLUTION AND IMPREGNATION.
Development of the Many-celled from the One-celled Organism. — The Cell-hermit and the Cell-state.— The Principles of the Formation of the State. — The Differentiation of the Individuals as the
CONTENTS.
PACK
Standard of Measurement for the Grade of the State. — Parallel between the Processes of Individual and of Race Development. — The Functions of Evolution. — Growth. — Inorganic and Organic Growth. — Simple and Complex Growth. — Nourishment and Change of Substance. — Adaptation and Modification. — Reproduction. — Asexual and Sexual Reproduction. — Heredity. — Division of Labour, or Differentiation. — Atavism, or Reversion. — Coalescence. — The Functions of Evolution as yet very little studied by Physiology, and hence the Evolutionary Process has often been misjudged. — The Evolution of Consciousness, and the Limits to the Knowledge of Nature. — Fitful and Gradual Evolution. — Fertilization. — Sexual Generation. — The Egg-cell and the Sperm-cell. — Theory of the Sperm-animals. — Sperm-cells a form of Whip-cell. — Union of the Male Sperm-cell with the Female Egg-cell.— The Product of this is the Parent-cell, or Cytula. — Nature of the Process of Fertilization. — Relation of the Kernel (Nucleus) to this Process. — Disappear- ance of the Germ-vesicle. — Monerala. — Reversion to the Monera- form.— The Cytula ... ... ... ... ... ...148
CHAPTER VIII.
EGG-CLEAVAGE AND THE FORMATION OF THE GERM-LAYERS
First Processes after the Fertilization of the Egg-cell is complete. — Original or Palingenetic Form of Egg-cleavage. — Significance of the Cleavage-process. — Mulberry-germ, or Morula. — Germ-vesicle, or Blastula Germ-membrane, or Blastoderm. — Inversion (In- vagination) of the Germ-vesicle. — Formation of the Gastrula. — Primitive Intestine and Primitive Mouth. — The Two Primary Germ-layers ; Exoderm and Entoderm.— Kenogenetic Form of Egg- cleavage. — Unequal Cleavage (segmentatio inequalis) and Hood- gastrula (Amphigastrula) of Amphibia and Mammalia. — Total and Partial Cleavage. — Holoblastic and Meroblastic Eggs. — Discoidal Cleavage (segmentatio discoidalis) and Disc-gastrula (Discogastrula) of Fishes, Reptiles, Birds. — Superficial Cleavage {segmentatio super- ficialis) and Vesicular Gastrula (Peri-Gastrula) of Articulates (Arthropoda),— Permanent Two-layered Body-form of Lower
Animals. — The Two-layered Primaeval Parent-form ; Gastreea.
Homology of the Two Primary Germ-layers in all Intestinal Animals (Metaz'a), — Significance of the Two Primary Germ- layers. — Origin and Significance of the Four Secondary Germ- layers.— The Exoderm or Skin-layer gives rise to the Skin-sensory
X CONTENTS.
PAGI
Layer and the Skin-fibrous Layer.— The Entodenn or Intestinal Layer gives rise to the Intestinal-fibrous Layer and the Intestinal- glandular Layer 184
CHAPTER IX.
THE VERTEBRATE NATURE OP MAN.
Relation of Comparative Anatomy to Classification. — The Family -rela- tionship of the Types of the Animal Kingdom. — Different Signi- ficance and Unequal Value of the Seven Animal Types. — The Gastrcea Theory, and the Phylogenetic Classification of the Animal Kingdom. — Descent of the Gastraea from the Protozoa. — Descent of Plant-animals and Worms from the Gastrasa. — Descent of the Four Higher Classes of Animals from Worms. — The Verte- brate Nature of Man. — Essential and Unessential Parts of the Vertebral Organism. — The Amphioxus, or Lancelet, and the Ideal Primitive Vertebrate in Longitudinal and Transverse Sections. — The Notochord.— The Dorsal Half and the Ventral Half.— The Spinal Canal.— The Fleshy Covering of the Body.— The Leather- skin (corium). — The Outer-skin (epidermis). — Body-cavity (cceloma). —The Intestinal Tube.— The Gill-openings.— The Lymph- vessels. — The Blood-vessels.— The Primitive Kidneys and Organs of He- production. — The Products of the Four Secondary Germ-layers ... 241
CHAPTER X.
THE CONSTRUCTION OF THE BODY FROM THE GERM- LAYERS.
The Original (Palingenetic) Development of the Vertebrate Body from the Gastrnla. — Relation of this Process to the Later (Kenogenetic) Germination, as it occurs in Mammals. — The most important act in the Formation of the Vertebrate. — The Primary Germ-layers, and also the Secondary Germ-layers, which arise by Fission of the Prima- ries, originally form Closed Tabes. — Contemporaneously with the Completion of the Yelk-sac, the Germ-layers flatten, and only later again assume a Tabular Form. — Origin of the Disc-shaped Mamma- lian Germ-area. — Light Germ-area (area pellucida) and Dark Germ- area (area opaca). — The Oval Germ-shield, which afterwards assumes the Shape of the Sole of a Shoe, appears in the Centre of the Light Germ-area (a. pellucida) .— The Primitive Streak
CONTENTS. XI
PAGE
separates the Germ-shield into a Eight and Left Half.— Below the Dorsal Furrow the Central Germ-layer parts into the Notochord and the Two Side-layers. — The Side-layers split horizontally into Two Layers : The Skin-fibrous Layer and the Intestinal-fibrous Layer. — The Primary Vertebral Cords separate from the Side- layers. — The Skin-sensory Layer separates into Three Parts : the Horny Layer, Spinal Canal, and Primitive Kidney. — Formation of the Coeloin and the First Arteries. — The Intestinal Canal proceeds from the Intestinal Furrow. — The Embryo separates from the Germ- vesicle. — Around it is formed the Amnion-fold, which coalesces over the back of the Embryo, so as to form a Closed Sac. — The Amnion. — The Amnion-water. — The Yelk-sac, or Navel-vesicle.— The Closing of the Intestinal and Ventral Walls occasions the Formation of the Navel.— The Dorsal and Ventral Walls ... 274
CHAPTER XL
GENERAL STRUCTURE AND ARTICULATION OF THE INDIVIDUAL. '
Sssential Agreement between the Chief Palingenetic Germ Processes in the case of Man and in that of other Vertebrates. — The Human Body,like that of all Higher Animals, develops from Two Primary and Four Secondary Germ-layers. — The Skin-sensory Layer forms the Horn-plate, the Medullary Tube, and the Primitive Kidneys. — The Middle Layer (Mesoderm) breaks up into the Central Notochord, the Two Primitive Vertebral Cords, and the Two Side-layers.— The latter split up into the Skin-fibrous Layer and the Intestinal- fibrous Layer. — The Intestinal-glandular Layer forms the Epi- thelium of the Intestinal Canal, and of all its Appendages. — Onto- genetic and Phylogenetio Fission of the Germ-layers. — Formation of the Intestinal Canal. — The Two-layered Globular Intestinal Germ-vesicle of Mammals represents the Primitive Intestine.— Head Intestinal Cavity, and Pelvic Intestinal Cavity. — Mouth Groove and Anal Groove. — Secondary Formation of Mouth and Anus. — Intestinal Navel and Skin-navel. — Movement of the Primitive Kidneys from the Outside to the Inside.— Separation of the Brain and Spinal Marrow. — Rudiments of the Brain-bladders. The Articulation or Metameric Structure of the Body. — The Primitive Vertebrae (Trunk- Segments, or Metamera). — The Con- struction and Origin of ihe Vertebral Column. — Vertebral Bodies and Vertebral Arches. — Skeleton-plate and Muscle-plate. — Forma-
CONTENTS.
MOi
tion of the Skull from the Head-plates. — Gill-openings and Gill- arches. — Sense-organs. — Limbs. — The Two Front Limbs and the Two Hind Limbs ... SL'S
CHAPTER XII.
THE GERM-MEMBRANES AND THE FIRST CIRCULATION OF THE BLOOD.
The Mammalian Organization of Man. — Man has the same Bodily Structure as all other Mammals, and his Embryo develops iu exactly the same way. — In its Later Stages the Human Embryo is not essentially different from those of the Higher Mammals, and in its Earlier Stages not even from those of all Higher Vertebrates. — The Law of the Ontogenetic Connection of Systematically Related Forms. — Application of this Law to Man. — Form and Size of the Human Embryo in the First Four Weeks. — The Human Embryo iu the First Month of its Development is formed exactly like that of any other Mammal. — In the Second Month the First Noticeable Differences .appear. — At first, the Human Embryo resembles those of all other Mammals ; later, it resembles only those of the Higher Mammals. — The Appendages and Membranes of the Human Embryo. — The Yelk-sac. — The Allantois and the Placenta. — The Amnion. — The Heart, the First Blood-vessels, and the First Blood, arise from the Intestinal-fibrous Layer. — The Heart separates itself from the Wall of the Anterior Intestine. — The First Circulation of the Blood in the Germ-area (a. germinativa) : Yelk- arteries and Yelk-veins. — Second Embryonic Circulation of the Blood, in the Allantois : Navel-arteries and Navel-veins. — Divisions of Human Germ-history C<G3
CHAPTER XIIL
THE STRUCTURE OF THE BODY OF THE AMPHIOXUS AND OF THE ASCIDIAN.
Causal Significance of the Fundamental Law of Biogeny. — Influence of Shortened and Vitiated Heredity. — Kenogenetic Modification of Palingenesis.— The Method of Phylogeny based on the Method of Geology. — Hypothetic Completion of the Connected Evolutionary Series by Apposition of the Actual Fragments. — Phylogenetio Hypotheses are Reliable and Justified.— Importance of the Amphi-
CONTEXTS. xiii
PACK
oxns and the Ascidian.— Natural History and Anatomy of the Ainphioxus. — External Structure of the .Body. — Skin-covering. — Outer-skin (Epidermis) and Leather-skin (Corium). — Notochord. — Medullary Tube. — Organs of Sense. — Intestine with an Anterior Respiratory Portion (Gill-intestine) and a Posterior Digestive Portion (Stomach-intestine). — Liver. — Pulsating Blood-vessels. — Dorsal Vessel over the Intestine (Gill-vein and Aorta). — Ventral Vessel -under the Intestine (Intestinal Vein and Gill-artery). — Movement of the Blood. — Lymph-vessels. — Ventral Canals and Side Canals — Body-cavity and Gill-cavity. — Gill-covering. — Kidneys. — Sexual Organs. — Testes and Ovaries. — Vertebrate Nature of Amphioxns. — Comparison of Amphioxus and Young Lamprey (Petromyzon) . — Comparison of Amphioxns and Ascidian. — Cellulose Tunic. — Gill-sac. — Intestine. — Nerve-centres. — Heart. — Sexual Organs ... ... ... ... ... ... 40fi
CHAPTER XIV.
GERM-HISTORY OF THE AMPHIOXUS AND OF THE ASCIDIAN.
Relationship of the Vertebrates and Invertebrates. — Fertilization of the Amphioxus. — The Egg undergoes Total Cleavage, and changes into a Spherical Germ-membrane Vesicle (Blastula). — From this the Intestinal Larva, or Gastrula, originates by Inversion. — The Gastrnla of the Amphioxus forms a Medullary Tube from a Dorsal Furrow, and between this and the Intestinal Tube, a Notochord : on both Sides the latter is a Series of Muscle-plates ; the Mate.mera. — Fate of the Four Secondary Germ-layers. — The Intestinal Canal divides into an Anterior Gill-intestine, and a Posterior Stomach, intestine.— Blood-vessels and an Intestinal-muscle Wall originate from the Intestinal-fibrous Layer. — A Pair of Skin-folds (Gill, roofs) grow out from the Side-wall of the Body, and, by Coales- cence, form the Ventral Side of the Large Gill-cavity. — The Ontogeny of the Ascidian is, at first, identical with that of the Amphioxns. — The same Gastrula is Developed, which forms a Notochord between the Medullary and Intestinal Tubes. — Retrogressive Development of the same. — The Tail with the Notochord is shed. — The Ascidian attaches itself firmly, and envelops itself in its Cellulose Tunic. — Appendicnlaria, a Tunicate which remains throughout Life in the Stage of the Larval Ascidian and retains the Tail-fin with the Chorda (Chordonia).— General Comparison and Significance of the Amphioxus and the Ascidian 43'j
LIST OF PLATES.
Plate 1. (Frontinpiece). Development of the face in Mammals (Man. Bat, Cat, Sheep) in three different stages
Explanation vol. ii. 34C
Plate II. (between p. 240 and p. 241). Total egg-cleavage. Gas- trulation of holoblastic eggs (primordial and unequal cleavage)
Explanation 240
Plate III. (between p. 240 and p. 241). Partial egg-cleavage. Gastrulation of meroblastic eggs (discoidal and superficial cleavage) ... ... ... ... Explanation 240
Plate IV. (between p. 320 and p. 321 ). Diagrammatic transverse section through various ontogenetic and phylogenetic stages in the development of the human body, showing the formation of this from the four secondary germ-layers ... Explanation 321
Plates V. (between p. 320 and p. 321). Diagrammatic longitu- dinal sections through various germ and tribal forms of Man, showing their formation from the four secondary germ-layers
Explanation 323
Plate VI. (between p. 362 and p. 363). Comparison of the embryos of a Fish, an Amphibian, a Reptile, and a Bird, in three different stages of evolution ... ... Explanation 362
Plate VII. (between p. 362 and p. 363). Comparison of the embryos of four different Mammals (Pig, Ox, Rabbit, and Man) in three different stages of evolution ... Explanation 362
Plate VIII. (between p. 404 and p. 405). Representation of two human embryos, the one of nine, the other of twelve weeks : the latter within the egg-membranes ... Explanation 405
Plate IX. (between p. 404 and p. 405). Representation of a human embryo of five months, natural size, within the egg- membranes ... ... ... ... Explanation 405
Plate X. (between p. 438 and p. 439). Germ-history of Ascidian and Amphioxus ... ... ... ... Explanation 436
Plate XI. (between p. 438 and p. 439). Structure of the body of Ascidian, Amphioxus, and larva of Petromyzon
Explanation 437
LIST OF WOODCUTS.
FTOJBB
1. Human egg-cell
2. Human liver-cell .
3. Epithelium cell from tongue 124
4. Thorny cells of epidermis
5. Human bone-cells
6. Enamel cells of tooth
7. A mind-cell
8. Blood-cells in process of
division . . • *
9. Active lymph-cells .
10. Primitive eggs of various
animals ....
11. Mammalian egg-cell .
12. Egg-cell of Hen
13. An Amoeba . . .
14. Egg-cell of a Chalk-sponge
15. Blood-cells absorbing mat-
tor ....
16. Blood-cells dividing .
17. Sperm-cells (seed-cells) .
18. Fertilization of mammalian
egg ....
19. Monerula of Mammal
20. Moneron dividing . . 12. Cytula of Mammal . .
|
PAGE 122 |
FIGURE 22. G |
|
124 |
23. G |
|
124 |
24. G |
|
125 |
25. G |
|
126 |
26. G |
|
126 |
27. G |
|
128 |
28. G |
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29. G |
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|
131 |
30. C |
|
132 |
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31. C |
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|
134 |
32-35 |
|
136 |
86. It |
|
139 |
37. C |
|
142 |
38. B |
|
144 |
39. E |
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40. E |
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|
145 |
41. G |
|
159 |
42. E |
|
173 |
43. G |
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44. E |
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|
175 |
45. E |
|
179 |
46. ft |
|
180 |
47. B |
|
181 |
48. I |
PAOE
Germination of a Coral . 190
Gastrulaof Gastrophysema 193
Gastrula of Sagitta . . 193
Gastrula of Uraster . . 193
Gastrula of Nanplius . 193
Gastrula of Limnseus . 193
Gastrula of Amphioxus . 193
Gastrula of Olynthus . 195 Cells of primary germ- layers . . . .198
Cleavage of Frog's egg . 203
35. Gastrulationof the Toad 206
Monerula of Eabbit . . 210
Cytula of Babbit . . 210
Babbit-egg with two cells 210
Babbit-egg with four cells 212
Babbit-egg with eight cells 212
Gastrula of Babbit . . 213
Egg of an Osseous Fish . 217
Gastrnla of an Osseous Fish 219
Egg-cell of Hen . . 223
Egg-cleavage of Bird . 225
Mulberry-germ of Chick . 228
Bladder-germ of Chick . 228
Invaginated germ of Chick 228
FIGURE
49. Gastrula of Chick
50, 51. Four secondary germ
layers .
52-56. Diagrammatic longitu- dinal and transverse sec- tions through the ideal Primitive Vertebrate
57, 58. „
59. „
60. „ 61.
62-C9. Diagrammatic trans- verse sections through the mostimportant germ- forms of the ideal Primi- tive Vertebrate
70. Diagrammatic transverse
sections through various mammalian germs (ex- plaining the separation of the intestine from the yelk-sac).
71. Gastrula of Mammal .
72. Intestinal germ-vesicle of
Mammal
73. Transverse section through
the intestinal germ- vesicle of Mammal
74. Exoderm-cells of the above 7 5. Entcderm-cells of the above 76. Transverse section through
77-81. Intestinal germ-vesicle
of Babbit
82, 83. Germ-area of Rabbit 84. So.
|
LIST |
OF WOODCUTS. |
||
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PACK FIGURE |
TAOK |
||
|
QOQ |
86. Sole-shaped germ-shield |
||
|
jerm- |
of Dog. |
298 |
|
|
. |
236 |
87. Sole-shaped germ-shield |
|
|
gitu- |
of Chick |
298 |
|
|
sec- |
88. Transverse section |
||
|
ideal |
through germ-shield |
300 |
|
|
e |
256 |
89. |
301 |
|
259 |
90. |
302 |
|
|
263 |
91. |
304 |
|
|
264 |
92. |
306 |
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|
267 |
93. |
309 |
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rang. |
94. Development of egg-mem- |
||
|
ongh |
branes .... |
312 |
|
|
erm- |
95. Transverse section |
||
|
rimi- |
through germ of Chick . |
317 |
|
|
. |
276 |
96, 97. „ |
318 |
|
?erse |
98. |
319 |
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rious |
99. |
331 |
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|
(ex- |
100. Separation of the intes- |
||
|
ition |
tine from the yelk-sac |
||
|
i the |
(diagrammatic) |
333 |
|
|
283 |
101. Longitudinal section |
||
|
288 |
through embryo Chick . |
336 |
|
|
e of |
102. Longitudinal section |
||
|
289 |
through head of an |
||
|
)ugh |
embryo Chick |
337 |
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|
L. |
103-105. Lyre-shaped Chick |
||
|
289 |
embryo |
342 |
|
|
bove |
290 |
106, 107. Germ-disc or germ- |
|
|
bove |
290 |
area of Babbit |
344 |
|
mgh |
108, 109. „ |
345 |
|
|
. |
293 |
110, 111. Skeleton of Man . |
351 |
|
side |
112. Transverse section |
||
|
. |
294 |
through germ of Chick . |
352 |
|
it . |
296 |
113. Human neck-vertebra |
854 |
|
297 |
114. Human chest-vertebra . |
354 |
|
|
298 |
115. Human lumbar-vertebra . |
354 |
LIST OF WOODCUTS.
XVII
116, 117. Head of embryo
Chick . . . .356
118. Head of embryo Dog . 356
119. Rudiments of the limbs . 357
120. „ „ 359
121. Lyre-shaped germ of Dog 367
122. Human germs from the
second to the fifteenth week . . . .368 123, 124. Anatomy of human germs (four and five weeks) . . . .370
125. Head of Nose -ape . . 374
126. Head of Julia Pastrana . 374 127-131. Human eggs and
germs from second to sixteenth weeks . . 376 132,133. „ „ 377
134. „ „ 378
135. Chick germ with allantois 380
136. Dog germ with allantois . 381
137. „ „ 382
138. Pregnant human uterus
with egg - membranes
and navel-cord • . 384
FIOtTllB PAOB
139. Development of egg-mem-
branes . . . . 385
140. Development of amnion . 387
141. „ 388
142. „ „ 389 143,144. Development of heart 392 145,146. „ „ 393
147. „ „ 395
148. First circulation of the
blood .... '396
149. „ „ 397
150. „ „ 398
151. Amphioxus lanceolatns . 420
152. Transverse section
through Amphioxus . 424
153. AnAscidian . . .431
154. Another Ascidian . . 434
155. Gastrula of Amphioxus . 444
156. Gastrnla of Sponge . 445
157. Transverse section
through Amphioxu s larva 447
158-160. „ „ 452
161. Transverse section
through Vertebrate . 457
162. Appendicularia . . 459
LIST OF GENETIC TABLES.
TABLE PAGE
I. Systematic Survey of the main branches of Biogeny ... 24 II. Systematic Survey of the constituent parts of the one- celled germ-form before and after fertilization ... 183
III. Systematic Survey of the most important differences in
the egg-cleavage and gastrulation of animals . . . 241
IV. Systematic Survey of the five first germinal stages of
animals, with reference to the four main forms of egg-cleavage ... ... ... ... ... 242
V. Systematic Survey of some of the most important obser- vations in the rhythm of egg-cleavage ... ... 243
VL Systematic Survey of some of the most important organs of the ideal Primitive Vertebrates, and their de- velopment from the germ-layers ... ... ... 273
VIL Systematic Survey of the development of the human
organ-systems from the germ-layers ... ... 327
VIII. Systematic Survey of the most important section of
human germ-history ... ... ... ... 402
IX. Systematic Survey of the most important homologies between the embryo of Man, and the embryo of the Ascidian and the Amphioxus in a corresponding stage of development, on the one hand, and the developed Man on the other ... ... ... 465
X. Systematic Survey of the relationship in form of the Ascidian and Amphioxus on the one hand, of the Fish and Man on the other, in a fully developed condition 466
XI Ontogenetic cell pedigree of the Amphioaus ... ... 407
PREFACE TO THE FIRST EDITION.
THESE chapters on Anthropogeny are the first attempt to render the facts of human germ-history accessible to a wider circle of educated people, and to explain these facts by human tribal history. I have not overlooked the great difficulty and danger involved in thus entering for the first time on ground which is so especially full of risks. No other branch of natural science yet remains so ex- clusively confined to its own technical students ; no other branch has been so wilfully obscured and mystified, by priestly influence, as has the germ-history of Man. If, even now, we say that each human individual develops from an egg, the only answer, even of most so-called edu- cated men, will be an incredulous smile ; if we show them the series of embryonic forms developed from this human egg, their doubt will, as a rule, change into disgust. Few educated men have any suspicion of the fact, that these human embryos conceal a greater wealth of important truths, and form a more abundant source of knowledge than is afforded by the whole mass of most other sciences and of all so-called "revelations."
XX PREFACE TO THE FIRST EDITION.
Nor is this surprising, when we see what a little way the knowledge of human evolution has spread even among the very students of Nature. Even in most works devoted to the Natural History, Anatomy, Physiology, Ethnology, and Psychology of Man, it is evident at a glance that their authors, if not ignorant, have at least a very superficial knowledge of human germ-history, and that tribal history lies far beyond them. The name of Darwin is, indeed, in every mouth. But few persons have really assimilated the theory of descent, as reformed by him ; few have made it part of themselves. To show how far even biologists of repute are from thoroughly understanding the history of evolution, no more remarkable recent instance can be found than the well-known address, on "The Limits of Natural Knowledge," delivered by the celebrated physio- logist, Du Bois Eeymond, in 1873, before the naturalisto assembled at Leipzig. This eloquent address, the source of such triumph to the opponents of the theory of evolu- tion, the cause of such pain to all friends of intellectual advance, is essentially a great denial of the history of evolution. No thoughtful naturalist will disagree with the Berlin physiologist when, in the first half of his address, he explains the limits of natural knowledge, as they are at present set to man by his vertebrate nature. But it is equally certain that every monistic naturalist will protest against the second half of the address, in which, not only is another limit, assumed to be different (but in reality identical), indicated for human knowledge, but the con- clusion is finally drawn, that man will never pass over these limits : " We shall never know that ! Ignorabimus! "
As the unanimous thanks of the Ecclesia militans have
PREFACE TO THE FIRST EDITION. XXi
been gained by the author of this " Ignorabimus," the most deserving student of the electricity of nerves and muscles, we must here most emphatically protest in the name of advancing natural knowledge and of all science capable of development. Had our one-celled Amoeba-ancestors of the Laurentian Period been told that their descendants would afterwards, in the Cambrian Period, produce a many- celled Worm-like organism possessed of skin and intestine, muscles and nerves, kidneys and blood-vessels, they would certainly not have believed ; nor, again, would these Worms have believed, had they been told that their descendants would develop into skull-less Vertebrates, such as the Amphioxus ; nor would these Skull-less Animals have credited that their posterity would ever become Skulled Animals (Craniota). Our Silurian Primitive-fish ancestors would have been equally hard to convince that their off- spring of the Devonian Period would acquire amphibian form, and yet later, in the Triassic Period, would appear as Mammals ; the latter, again, would have deemed it im- possible that, in Tertiary times, a very late descendant of theirs would acquire human form, and would gather the splendid fruits of the tree of knowledge. All these would have answered : " We shall never change, nor shall we ever understand the history of our evolution! Nunquam mutabimur ! Semper ignordbimus ! "
With this Ignorabimus the Berlin school of Biology tries to stop science in its advance along the paths of evolution. This seemingly humble but really audacious "Ignorabimus" is the " Ignoratis" of the infallible Vatican and of the " black international " which it leads ; that mischievous host, against which the modern civilized
XX11 PREFACE TO THE FIRST EDITION.
state has now at last begun in earnest the "struggle for culture." In this spiritual warfare, which now moves all thinking humanity, and which prepares the way for a future existence more worthy of man, spiritual freedom and truth, reason and culture, evolution and progress stand on the one side, marshalled under the bright banner of science ; on the other side, marshalled under the black flag of hierarchy, stand spiritual servitude and falsehood, want of reason and barbarism, superstition and retrogres- sion. The trumpet of this gigantic spiritual warfare marks the dawn of a new day and the end of the long darkness of the Middle Ages. For modern civilization, in spite of the progress of culture, lies bound in the fetters of the hierarchy of the Middle Ages ; and social and civil life is ruled, not by the science of truth, but by the faith of the church. We need but mention the mighty influence which irrational dogmas still exercise on the elementary education of our youth; we need but mention that the state yet permits the existence of cloisters and of celibacy, the most immoral and baneful ordinances of the " only- saving " church ; we need but mention that the civilized state yet divides the most important parts of the civil year in accordance with church festivals ; that in many countries it allows public order to be disturbed by church processions, and so on. We do indeed now enjoy the unusual pleasure of seeing "most Christian bishops" and Jesuits exiled and imprisoned for their disobedience to the laws of the state. But this same state, till very recently, harboured and cherished these most dangerous enemies of reason.
In this mighty " war of culture," affecting as it does
PREFACE TO THE FIRST EDITION.
the whole history of the World, and in which we may well deem it an honour to take part, no better ally than Anthro- pogeny can, it seems to me, be brought to the assistance of struggling truth. The history of evolution is the heavy artillery in the struggle for truth. Whole ranks of dualistic sophisms fall before the monistic philosophy, as before the chain shot of artillery, and the proud structure of the Roman hierarchy, that mighty stronghold of infallible dogmatism, falls like a house of cards. Whole libraries of church wisdom and false philosophy melt away as soon as they are seen in the light afforded by the history of evolution. The church militant itself furnishes the most striking evidences of this, for it never ceases to give the lie to the plain facts of human germ-history, condemning them as "diabolical inventions of materialism." In so doing it gives the most brilliant witness that it recognizes as unavoidable the conclusions which we have drawn from these facts as to tribal history, as to the true causes of these facts.
In order to render these little known facts of germ- history and their causal explanation by tribal history accessible to as wide a circle of educated readers as pos- sible, I have followed the same course as that which I adopted, six years ago, in my "Natural History of Creation," of which the " Anthropogeny " forms a second, more detailed part. In the summer of 1873 I had the academical lectures, on the outlines of the history of human evolution, which I have delivered during the last twelve years in Jena before a mixed audience of students of all faculties, taken down in shorthand by two of that nudience, Messrs. Kiessling and Schlawe. The task I
XXiV PREFACE TO THE FIRST EDITION.
undertook in publishing these was indeed much harder than that incurred in the " Natural History of Creation; " for while the latter passed lightly through the widest circle of biological phenomena, and touched only on the most interesting points, I was obliged, in the " History of the Evolution of Man," to exhibit a much more limited series of phenomena in their proper connection, of which, indeed, each individual one is interesting in its proper place, although they are of very various degrees of interest. Moreover, the comprehension of form -phenomena, with which human germ- history deals, is among the most difficult of morphological tasks ; the academical lectures on the history of human evolution are rightly considered even by medical men, who are previously acquainted with the anatomical features of the human body, as the most difficult to understand. I saw, therefore, that, if I desired to make the road into this dark region, entirely closed as yet to most men, really accessible to the educated laity, I must, on the one hand, limit myself as far as possible in my selection from the abundance of empiric matter, and yet, on the other hand, that I must be careful not to pass entirely over any essential part of this matter.
Although, therefore, I have throughout taken pains to present the scientific problem of Anthropogeny in as popular a form as possible, I do not imagine that I have completely accomplished this very difficult task. I shall, however, have gained my object if I succeed in affording educated persons an approximate conception of the most essential outlines of human germ-history, and in con- vincing them that the sole explanation and comprehension of the matter is afforded by the corresponding tribal
PREFACE TO THE FIRST EDITION. XXV
history. Perhaps, at the same time, I may hope to con- vince some of those specialists, who deal indeed daily with the facts of germ-history, but who neither know nor wish to know anything about the true causes of these, which lie hid in tribal history. As this is quite the first attempt to present the Ontogeny and Phylogeny of man in their whole causal connection, I fear that, at best, the point at which I aim lies far beyond the point gained. But by this each thinking man will, it is to be hoped, be convinced that only by recognizing this connection does the history of human evolution become a science. On- togeny can only be really understood through Phylogeny. The history of the tribe lays bare the true causes of the history of the germ.
ERNST HEINKICH HAECKEL.
Jena, July 13, 1874.
PREFACE TO THE THIRD EDITION.
WHEN, two years ago, I published the first edition of the "History of the Evolution of Man," and this was followed, a few months later, by an unaltered second edition, I was fully conscious of the hazard involved in so doing, and was prepared to meet with numerous attacks. These were not slow to come; and if I were now obliged to answer all my opponents, this third edition might easily be doubled in size. I think, however, that I may satisfy myself with but a few remarks.
The great majority of my opponents are determined enemies of the Doctrine of Descent, who altogether deny a natural evolution of organic nature, and who can only explain both the origin of man and that of animal and plant species with the help ot miracles, by super- natural creative acts. These adherents of the Creation Theory I need not answer ; for Anthropogeny, as the special application of the Theory of Descent to Man, naturally starts from the recognition of this latter theory : ten years ago, in my Generelle Morphologic, and again in the " Natural History of Creation," I explained my own conception of this in sufficient detail.
XXviii PREFACE TO THE THIRD EDITION.
I cannot, however, refrain from defending my stand- point against those naturalists, who, taking their position indeed on the Theory of Descent and on Darwinism, yet combat my individual conception of this, and, especially, regard my application of the theory to Anthropogeny as erroneous. Many of these naturalists, who were formerly determined opponents of the Theory of Descent, have recently passed over to Darwin's camp, merely in order not to stand entirely inactive at the barren standpoint offered by negation. Against two of these false Darwinists, Wilhelm His and Alexander Goette, I have defended myself in a special work on " The Aims and Methods of the Modern History of Evolution" (" Ziele und Wege derHeuti- gen Entwickelungsgeschichte." Jena, 1875). To that work I now refer. On the other hand, I have been forcibly attacked by naturalists who are really esteemed as well- known and convinced adherents of the Theory of Evolu- tion. Of these, Karl Vogt and Albert Kolliker require a few words of answer.
Vogt, whose many services in furthering Zoology I have always most readily acknowledged, ranked second to Huxley among those naturalists who, but a few years after the appearance of Darwin's "Origin of Species," attempted to apply the theory contained in that work to Man and represented this as necessary. He afterwards, however, made no further progress in the same direction. While, as I am convinced, the mass of facts already accumulated in Comparative Anatomy, Ontogeny, Palaeontology, and Sys- tematic Zoology, is amply sufficient to afford the most general points on which to base the hypothetic human pedigree, Karl Vogt now holds opposed views, and entirely
PREFACE TO THE THIRD EDITION.
rejects the ancestral series as I have arranged it. He says : " We have been able to prove the assertion that Men and Apes must have originated from a common line ; — more than this we have never asserted, and further back than this it is absolutely impossible to prove anything or even to show with any degree of probability more than that, at farthest, the higher Mammals may perhaps have de- veloped from Pouched Animals (Marsupialia)." Against this view of Vogt's, I assert, that with the same logical " certainty or probability " the common descent of all Mammals from lower Vertebrates, primarily from Am- phibia, less immediately from Fishes, may be " proved." With the same " certainty or probability " — I assert again — the descent of all these Skulled Animals (Craniota) from Skull-less forms (Acrania, allies of Amphioxus),the descent of these latter from Chorda Animals (Chordonia, forms allied to Ascidia), and the descent of these Chorda Animals from low Worms, "may be proved." With the same " certainty or probability " — I say finally — " we have been able to prove the assertion," that these Worms must, in their turn, have originated from a Gastrsea (resembling the gastrula), and these Gastrseads from a one-celled organism (resembling the undifferentiated Amoeba). Proofs, as I believe, of these assertions are given in Chapters XIII.-XXV. of this edition.
The whole of this hypothetic pedigree Karl Vogt entirely rejects, without, however, substituting another. He espe- cially denies our relationship with the Selachii and the Amphioxus, with the Ascidia and the Gastraea, although the especially great phylogenetic significance of these instruc- tive animal-forms is almost unanimously recognized by the
XXX PREFACE TO THE THIRD EDITION.
first authorities in our science. Whilst Vogt completely opposes himself to these important views, which from day to day become more firmly established, he refers to Karl Semper, a "gifted" naturalist, who shares these views of Vogt's, and who derives Vertebrates from Einged Worms (Annelida). I regret that I can make no use of this reference; nor do I find reason to answer Semper's polemic on "Haeckelism in Zoology" (" Haeckelismus in der Zoologie." Hamburg, 1876) ; for, apart from his de- fective education and his insufficient acquaintance with the whole subject of Zoology, this "gifted" zoologist is so much at variance with logic, as also with truth, that refutation seems superfluous. (Cf. vol. i. p. 91 and p. 426.) An example is sufficient to show this : In order to indicate the scientific value of "Haeckelism," and in order "to show that this tendency must continually diverge more and more widely from the really scientific study of nature," Semper brings forward the fact that, " according to Haeckel's own statement, Darwinism should be the religion of every naturalist." This last statement, which I consider absurd, is not mine, but that of my determined opponent,. Professor Rutimeyer, and I quoted the sentence in the preface to the third edition of the " Natural History of Creation " merely to show the singular ground occupied by its author.
The wide cleft which separates my standpoint of the history of evolution and of natural science, as a whole, from that of Vogt and Semper cannot be better indicated than by our mutual position towards philosophy. Karl Vogt, like his friend Karl Semper, was a sworn contemner of all philosophy. The former seizes every opportunity to
PKEFACE TO THE THIRD EDITION. XXXI
mock at philosophic tendencies and researches ; and the latter knows no more severe charge to bring against me than that I seek to unite empiricism and philosophy, experience and idea, " observation and reflection." I am certainly firmly convinced that a really scientific study of nature can no more dispense with philosophic reflection, than can healthy philosophy ignore the results of natural scientific experience. "An exact empiricism," without those philosophic thoughts which combine and explain the raw material of facts, merely results in the accumulation of a lifeless store of knowledge ; on the other hand, " speculative philosophy " which knows nothing of the firm basis afforded by natural scientific observation, can only produce transient cloud-pictures. The most intimate com- bination and blending of empiricism and philosophy can alone enable us to construct a permanent and sure scientific structure. I still hold as decidedly as ever the much- abused views which I expressed, ten years ago, about this matter in my Generelle Morphologic, and the fundamental ideas which I have here reproduced.
Moreover, he must be very one-sided or short-sighted who does not recognize the natural approximation, which is now becoming more close in all branches of human knowledge, between experimental and reflective study. The enormous enlargement of the field of empiric knowledge which has been brought about by the progress of the last half-century, has resulted in a corresponding specialization of separate researches, and consequently in an isolation of diverging aims which cannot possibly continue to satisfy. All thoughtful observers feel, more acutely in consequence of this, that they must raise themselves from the wearisome
XXxil PREFACE TO THE THIRD EDITION.
task of accumulating dry details to wider views, and thus to gain sympathy with allied aims. On the other side, the sterility of such pure speculative philosophy as ignores all those enormous advances in empiric knowledge, has so forced its way into the consciousness of all sound thinkers, that they earnestly desire to fall back on the firm basis afforded by experimental science.
The ever-increasing flood of writings on natural philo- sophy, and essays on the relation of philosophy to natural science, plainly indicates the happy growth of this scientific unitary tendency. Nothing is more favourable to this, nothing better advances the combination of the various scientific lines, than the new theory of evolution. The extraordinary importance ascribed to this theory, rests especially on the fact that it supplies a philosophic central point, and just for this very reason it has in so short a time gained the active interest of all thoughtful minds. It raises us from a knowledge of facts to a knowledge of causes, and thus affords a deeper satisfaction to the demand for causality innate in human reason than a mere experimental science could ever supply. When, therefore, Karl Vogt and many other naturalists entirely reject philo- sophy, and will not allow that it has any point of union with what is called " exact " natural science — they volun- tarily renounce all the higher aims of investigation. (Cf. vol. ii. p. 387.)
Albert Kolliker occupies a similarly one-sided stand- point. This author, in the second edition of his " History of the Evolution of Man and the Higher Animals" ("Ent- wickelungsgeschichte des Menschen und der Hoheren Thiere," 1876), in especially attacking the fundamenlal law
PREFACE TO THE THIRD EDITION.
of Biogeny, has impugned the very foundation on which Anthropogeny rests. Most of his objections are, it appears to me, refuted by the explanations which I have given in this third edition as to the very important relations of Palingenesis and Kenogenesis. (Compare especially Chapters L, VIII., and X.) Kolliker will not recognize the Gastrsea Theory because he has been unable to discover a gastrula in Mammals and Birds. But his experiences are opposed to the most recent researches of Van Beneden and Rauber, of whom the former in the case of the Eabbit, the latter in the case of the Chick, describes a kenogenetic gastrula-form, which, in accordance with the Gastrsea theory, may easily be referred to the palingenetic gastrula of the Amphioxus. Kolliker says finally : "As the last and most important argument, I bring forward the fact that Phylogeny as read by Darwin and Haeckel does not, it appears to me, represent the truth." This " most im- portant argument " is a simple petitio principii. The sen- tence might as well be, " phylogeny is not true because it does not represent the truth."
How very different in other respects Kolliker's concep- tion of the history of evolution is from mine is most clearly indicated in the " General Observations " (§ 29) at the end of his book. The learned Wiirzburg anatomist there explains with reference to germ -history, his " essential agreement in fundamental conceptions " with the un- learned Leipzig anatomist Wilhelm His. I have explained the nature of these " mechanical fundamental conceptions " in Chapter XXIV. of this book (vol. ii. p. 352), and in greater detail in my work on "The Aims and Methods of the Modern History of Evolution " (" Ziele und Wege der
3
PREFACE TO THE THIRD EDITION.
Heutigen Entwickelungsgeschichte "). The celebrated theories of His, of which I have spoken as the " envelope theory," "gum-pouch theory," "waste-rag theory," etc., are the brilliant results of that " gifted " author's efforts and mathematical calculations. And yet many have allowed themselves to be dazzled by the " exact " appear- ance of his mathematical formula The history of the evolution of organisms, equally with the history of human civilization, can never be the subject of " exact " investi- gation. The history of evolution is in its very nature an historic natural science, as is geology. To regard and treat these and other historic natural sciences as " exact " leads to the greatest errors. This is as true of germ- history (Ontogeny) as of tribal history (Phylogeny) ; foi between the two there is the most intimate causal connection.
Many naturalists have especially blamed the diagram- matic figures given in the Anthropogeny. Certain tech- nical embryologists have brought most severe accusations against me on this account, and have advised me to substi- tute a larger number of elaborated figures, as accurate as possible. I, however, consider that diagrams are much more instructive than such figures, especially in popular scientific works. For each simple diagrammatic figure gives only those essential form-features which it is intended to explain, and omits all those unessential details which in finished, exact figures, generally rather disturb and confuse than instruct and explain. The more complex are the form-features, the more do simple diagrams help to make them intelligible. For this reason, the few diagrammatic figures, simple and rough as they were, with which Baer
PREFACE TO THE THIRD EDITION. XXXV
half a century ago accompanied his well-known " History of the Evolution of Animals," have been more serviceable in rendering the matter intelligible than all the numerous and very careful figures, elaborated with the aid of camera lucida, which now adorn the splendid and costly atlases of His, Goette, and others. If it is said that my diagrammatic figures are "inaccurate," and a charge of " falsifying science " is brought against me, this is equally true of all the very numerous diagrams which are daily used in teaching. All diagrammatic figures are "inaccurate."
The important advances in many different directions made daring the last two years, both by germ-history and tribal history, especially the reconstruction of the germ- layer theory and the development of the Gastraea theory, have compelled me essentially to modify the second and third sections of the Anthropogeny. Chapters VIII., IX., XVI., and XIX. especially appear in a new form ; but even in Sections I. and IX. I have been compelled to modify much and to improve many parts. At the same time I have exerted myself to the utmost, by improving the formal exposition, to render the extremely dry and unacceptable matter more interesting. This is, of course, an unusally hard task, and I am well aware how far even this third edition, in spite of all my efforts, is from affording a really popularly intelligible explanation of the Ontogeny and Phylogeny of Man. Because the defective natural scien- tific instruction in our schools, even in the present day, leaves educated men quite or nearly ignorant of the struc- ture and arrangement of their bodies, the anatomical and physiological foundation is usually wanting, on which alone a true knowledge of human germ-history, and consequently
PREFACE TO TIIE THIRD EDITION.
of human tribal history, can be based. And yet, as Baer says, "no investigation is more worthy of a free and thoughtful man than the study of himself." (Cf. vol. i. p. 244.) Hoping, as I do, that I may have aided to some extent to bring about this true self-knowledge, I shall have gained my purpose if my labours arouse an active interest in wider circles in the historic evolution of our animal organism, and if they advance the knowledge of this most significant process.
ERNST HEINRICH HAECKEL.
Jena, October 6, 1876.
PROMETHEUS.
ENVEIL thine heaven, Zeus, with vaporous cloud.
And practise, like a boy beheading thistles,
On oaks and mountain summits ;
Yet must thou let my earth alone to stand,
And these my dwellings, which thou didst not bniUlj
And these my flocks, for whose bright glow
Thou enviest me.
I know not aught more wretched
Beneath the sun than you, ye Gods !
Who nourish piteously,
With tax of sacrifice and reek of prayer ; your glory
Would starve, if children were not yet, and suppliants,
So full of hope — and fools.
When I was young, and knew not whence nor whither,
I used to turn my dazzled eyes to the sun,
As if above me were
An ear to listen to my crying,
A heart, like mine, to pity those oppress'd.
Who aided me against the Titans' arrogance ?
Who rescued me from death, from slavery ?
'Tis thou alone hast wrought it all, thou holy, glowing heart.
Thou didst glow young and fresh, though cheated ; thanks for
saving That slumbering one above.
Why should I honour thee ?
Hast thou e'er lighten 'd the woes of the laden ones ?
Hast thou e'er dried the tears of the sorrowful?
[t was not thou who welded me to manhood,
But Time the almighty, Fate the everlasting,
My Lords and thine.
ii FAUST.
Dost fondly fancy I shall hate my life, And hie me to the waste, because not all My blossom-dreams bear fruit ?
Here sit I forming manhood in ray image,
A race resembling me,
To sorrow, and to weep,
To taste, to hold, to enjoy,
And not take heed of thoe,
^a i | GOETHE.
FAUST.
Earth's narrow circle is well known to mo ; What is above the eye can never see. Fool, who peers thither with his vision dim, And feigns a crowd of beings like to him !
Let him look round him, standing without fear ; This world speaks plain for who has ears to hear : He need not stray within the vast to be, But clasp what he can feel and see.
So let him wander all his earthly day ;
Though ghosts should walk, still let him go his way:
In every progress woe and joy betide,
Though eveiy moment be unsatisfied.
Y"es, in this thought, I fix unswerving ;
Wisdom gives thus her judgment form ; Those are of Freedom, Life, deserving,
Who daily take them both by storm.
Goran
THE EVOLUTION OF MAN.
CHAPTER I.
THE FUNDAMENTAL LAW OF THE EVOLUTION OF ORGANISMS.
General Significance of the History of the Evolution of Man. — Ignorance of it among the so-called Educated Classes. — The Two Branches of the History of Evolution. — Ontogeny, or the History of Germs (Embryos) , and Phylogeny, or the History of Descent (or of the Tribes). — Causal Connection between the Two Series of Evolution. — The Evolution of the Tribe determines the Evolution of the Germ. — Ontogeny as an Epitome or Recapitulation of Phylogeny. The Incompleteness of this Epitome. — The Fundamental Law of Biogeny. — Heredity and Adapta- tion are the two Formative Functions, or the two Mechanical Causes, of Evolution. — Absence of Purposive Causes. — Validity of Mechanical Causes only. — Substitution of the Monistic or Unitary for the Dualistic, or Binary Cosmology. — Eadical Importance of the Facts of Embryology to Monistic Philosophy. — Palingenesis, or Derived History, and Keno- genesis, or Vitiated History. — History of the Evolution of Forms and Functions. — Necessary Connection between Physiogeny and Morpho- geny. — The History of Evolution as yet almost entirely the Product of Morphology, and not of Physiology. — The History of the Evolution of the Central Nervous System (Brain and Spinal Marrow) is involved in that of the Psychic Activities, or the Mind.
" The History of the Evolution of Organisms consists of two kindred and closely connected parts : Ontogeny, which is the history of the evolution of individual organisms, and Phylogeny, which is the historyof the evolution of organic tribes. Ontogeny is a brief and rapid recapitulation of Phylogeny, dependent on the physiological functions of Heredity (reproduo-
2 THE EVOLUTION OF MAN.
tion) and Adaptation (nutrition). The individual organism reproduces fn the rapid and short course of its own evolution the most important of the changes in form through which its ancestors, according to laws of Heredity and Adaptation, have passed in the slow and long course of their palaeonto- logioal evolution." — HAECKEL'S Qenerelle Morphologic (1866).
THE natural phenomena of the evolutionary history of man claim an entirely peculiar place in the wide range of the scientific study of nature. There is surely no subject of scientific investigation touching man more closely, or in the knowledge of which he is more deeply concerned, than the human organism itself; and of all the various branches of the science of man, or anthropology, the history of his natural evolution should excite his highest interest. For it affords a key for the solution of the greatest of those problems at which human science is striving. The greatest problems with which human science is occupied — the inquiry into the true nature of man, or, as it is called, the question of " Man's Place in Nature," which deals with the past and primitive history, the present condition, and future of Man — are all most directly and intimately linked to this branch of scientific research, which is called The History of the Evolution of. Man, or briefly, "Anthropogeny."1 It is, however a most astonishing but incontestable fact, that the history of the evolution of man as yet constitutes no part of general education. Indeed, our so-called " edu- cated classes" are to this day in total ignorance of the most important circumstances and the most remarkable phenomena which Anthropogeny has brought to light.
In corroboration of this most astounding fact, I will only mention that most "educated people" do not even know that each human individual is developed from an egg, and that this egg is a simple cell, like that of any
GENERAL IGNORANCE OF THE HISTORY OF EVOLUTION. 3
animal or plant. They are also ignorant of the fact that, in the development of this egg, an organism is first formed which is entirely different from the fully developed human body, to which it bears no trace of resemblance. The majority of "educated people" have never seen such a human germ, or embryo, in the early stages of development,2 nor are they aware that it is not at all different from those of other animals. They do not know that, at a certain period, this embryo has essentially the anatomical structure of a Lancelot, later of a Fish, and in subsequent stages those of Amphibian and Mammal forms ; and that in the further evolution of these mammal forms those first appear which stand lowest in the series, namely, forms allied to the Beaked Animals (Ornithorhynchus) ; then those allied to Pouched Animals (Marsupialia), which are followed by forms most resembling Apes ; till at last the peculiar human form is produced as the final result. These significant facts are so little known that, when incidentally mentioned, they are commonly doubted, or are even regarded as unfounded inventions. Every one knows that the butterfly proceeds from a pupa, the pupa from a caterpillar, to which it bears no resemblance, and again the caterpillar from the egg of the butterfly. But few, except those of the medical profession, are aware that man, in the course of his individual evolution, passes through a series of transformations no less astonishing and remarkable than the well-known metamorphoses of the butterfly. The mere tracing of this wonderful series of forms, through which the human embryo passes in the course of its development, is, of course, of great general interest. But our understanding will be satisfied in a far higher degree, if we refer these remarkable facts to their final causes, and recognize
4 THE EVOLUTION OF MAN.
that these natural phenomena are of the utmost importance to the entire range of human knowledge. They are of special importance to the " History of Creation," and, in connection with this, to philosophy in general, — as we shal] presently see. Further, as the general results of all human striving after knowledge are summed up in philosophy, it follows that every branch of scientific research comes more or less in contact with, and is influenced by, the History of the Evolution of Man.
In undertaking to describe the most important character- istics of these significant phenomena, and to trace them back to their final causes, I shall assign a much greater scope and aim to the History of the Evolution of Man than is usual. The lectures given on this subject in German universities during the past fifty years have been exclusively designed for medical students. It is true that the physician is most deeply interested in becoming acquainted with the development of the bodily organization of man, with which he deals, practically, from day to day, in his profession. I shall not here attempt to give a special account of the course of the evolution of the individual, such as has usually been given in embryological lectures, because few of my readers have studied human anatomy, or are acquainted with the physical structure of the developed man. Hence, I shall have to confine myself in many points to general outlines, neglecting many of the remarkable details, which would have to be discussed in treating of the evolution of special human organs, but which from their complicated nature, and because they are not easy to describe, can only be completely understood by the aid of an intimate ac- quaintance with human anatomy. I shall strive, however
OBJECT OF THE HISTORY OF EVOLUTION. 5
to present this branch of the science in as popular a form as possible. A satisfactory general idea of the course of the evolution of the human embryo can, indeed, be given without going very deeply into anatomical details. As numerous successful attempts have recently been made to awaken the interest of larger classes of educated persons in other branches of Science, I also may hope to succeed in this department, though it is in many respects especially beset with difficulties.
The History of the Evolution of Man, as it has been usually treated in lectures for medical students at the universities, has only concerned itself with Embryology,3 so-called, or more correctly with Ontogeny,4 in other words, with the history of the evolution of individual human organisms. This, however, is only the first part of the task before us, only the first half of the History of the Evolution of Man in the wider sense which will here be attributed .0 the term. The second part, equal in importance and interest, is Phylogeny,5 which is the history of the evolution of the descent of man, that is, of the evolution of the various animal forms through which, in the course of count- less ages, mankind has gradually passed into its present form. All my readers know of the very important scientific movement which Charles Darwin caused fifteen years ago, by his book on the Origin of Species. The most important direct consequence of this work, which marks a fresh epoch, has been to cause new inquiries to be made into the origin of the human race, which have proved the natural evolution of man through lower animal forms. The Science which treats of the development of the human race from the animal kingdom is called Phylogeny, or the tribal
6 THE EVOLUTION OF MAN.
history of man. The most important source from which the science derives its material, is Ontogeny, or the history of germs, in other words, of the evolution of the individual. Palaeontology, or the science of petrifactions, and, in a yet greater degree, Comparative Anatomy, also afiord most im- portant aid to Phylogeny.
These two divisions of our science, Ontogeny, or the history of the germ, Phylogeny, or the history of the tribe, are most intimately connected, and the one cannot be understood without the other. The close intertwining of both branches, the increased proportions which germ- history and tribal history lend to each other, alone raise Biogeny 6 (or the history of organic evolution, in the widest sense) to the rank of a philosophic natural science. The connection between the two is not external and superficial, but deeply internal and causal Our knowledge of this connection has been but very recently obtained ; it is most clearly and accurately expressed in the comprehensive state- ment which I call "the fundamental law of organic evolution" or more briefly, " the first principle of Biogeny." 7
This fundamental law, to which we shall recur again and again, and on the recognition of which depends the thorough understanding of the history of evolution, is briefly expressed in the proposition : that the History of the Gerin is an epitome of the History of the Descent ; or, in other words : that Ontogeny is a recapitulation of Phylogeny ; or, somewhat more explicitly : that the series of forms through which the Individual Organism passes during its progress from the egg cell to its fully developed state, is a brief, compressed reproduction of the long series of forms through which the animal ancestors of that organism (or the ancestral forms
THE INFLUENCE OF PHYLOGENY ON ONTOGENY. 7
of its species) have passed from the earliest periods of so- called organic creation down to the present time.
The causal nature of the relation which connects the History of the Germ (Embryology, or Ontogeny) with that of the tribe (Phylogeny) is dependent on the phenomena of Heredity and Adaptation. When these are properly understood, and their fundamental importance in deter- mining the forms of organisms recognized, we may go a step further, and say: Phylogenesis is the mechanical cause of Ontogenesis. The Evolution of the Tribe, which is dependent on the laws of Heredity and Adaptation, effects all the events which take place in the course of the Evolution of the Germ or Embryo.
The chain of different animal forms which, according to the Theory of Descent, constitutes the series of ancestors, or chain of forefathers of every higher organism, and hence also of man, always forms a connected whole. This un- broken succession of forms may be represented by the letters of the Alphabet A, B, C, D, E, etc., down to Z, in their alphabetical order. In apparent contradiction to this, the history of the individual evolution, or the Ontogeny of most organisms show us only a fragment of this series of forms, so that the interrupted chain of embryonic forms would be represented by something like : A, B, F, H, I, K, L, etc. ; or, in other cases, thus : B, D, H, L, M, N, etc. Several evolu- tionary forms have, therefore, usually dropped out of t'he originally unbroken chain of forms. In many cases also (retaining the figure of the repeated alphabet) one or more letters, representing ancestral forms, are replaced in the corresponding places among the embryonic forms by equi- valent letters of another alphabet. Thus, for example, in
8 THE EVOLUTION OF MAN.
place of the Latin B or D, a Greek B or A is often found Here, therefore, the text of the biogenetic first principle is vitiated, while in the former case it was epitomized This gives more importance to the fact that, notwithstanding this, the sequence remains the same, so that we are enabled to recognize its original order.
Indeed, there is always a complete parallelism between the t wo series of evolution. This is, however, vitiated by the fact that in most cases many forms which formerly existed and actually lived in the phylogenetic series are now wanting, and have been lost from the ontogenetic series of evolution. If the parallelism between the two series were perfect, and if this great fundamental law of the causal connection between Ontogeny and Phylogeny, in the strict sense of the word, had full and unconditional sway, we should only have to ascertain, with the aid of microscope and scalpel, the series of forms through which the fertilized human egg passes before it attains its complete development. Such an examination would at once give us a complete picture of the remarkable series of forms through which the animal ancestors of the human race have passed, from the beginning of organic creation to the first appearance of man. But this repro- duction of the Phylogeny in the Ontogeny is complete only in rare instances, and seldom corresponds to the entire series of the letters of the alphabet. In fact, in most cases the epitome is very incomplete, and greatly altered and per- verted by causes which we shall investigate hereafter. Hence we are seldom able to determine directly, by means of its Ontogeny, the different forms through which the ancestry of each organism has passed; on the contrary, we commonly find, — and not less so in the Phylogeny of man, — a number
CONNECTION BETWEEN PHYLOGENY AND ONTOGENY. 9
of gaps. We are, however, able to bridge over the greater part of these gaps satisfactorily by the help of Compa- rative Anatomy, though not to fill them up directly by ontogenetic research. It is therefore all the more im- portant that we are acquainted with a considerable number of lower animal forms which still find place in the history of the individual evolution of man. In such cases, from the nature of the transient individual form, we may quite safely infer the nature of the ancestral animal form.
For example, from the fact that the human egg is a simple cell, we may at once infer that there has been at a very remote time a unicellular ancestor of the human race resembling an Amoeba. Again, from the fact that the human embryo originally consists merely of two simple germ-layers, we may at once safely infer that a very ancient ancestral form is represented by the two-layered Gastrsea. A later embryonic form of the human being points with equal certainty to a primitive worm-like ancestral form which is related to the sea-squirts or Ascidians of the present day. But the low animal forms which constitute the ancestral line between the unicellular amceba and the gastrsea, and further between the gastrsea and the ascidian form, can only be approximately conjectured with the aid of Comparative Anatomy and Ontogeny. On account of a shortened process of Heredity, various ontogenetic intermediate forms, which must have existed phylogenetically, or in the ancestral lineage, have in the course of historic evolution gradually dropped out from these gaps. But notwithstanding these numerous and sometimes very considerable gaps, there is, on the whole, complete agreement between the two series of evolution. Indeed, it will be one of my principal objects to
IO THE EVOLUTION OF MAN.
prove the deep harmony, and original parallelism, be- tween the two series. By adducing numerous facts, I hope to convince my readers that from the actually existing series of embryonic forms which can be shown at any time, we are able to draw the most important conclusions as to the genealogical tree of the human species. We shall thus be able to form a general picture of the series of animal forms which succeeded each other as the direct ancestors of man, in the long course of the history of the organic world.
In this phylogenetic significance of ontogenetic phe- nomena, it is of course most important to distinguish clearly and exactly between the original, palingenetic processes of evolution, and the later kenogenetic processes of the same. The term Palingenetic process8 (or reproduction of the history of the germ) is applied to all such phenomena in the history of evolution as are exactly reproduced, in consequence of conservative heredity, in each succeeding generation, and which, therefore, enables us directly to infer the corre- sponding processes in the tribal history of the developed ancestors. The term Kenogenetic process9 (or vitiation of the history of the germ) is applied to all such processes in the germ-history as are not to be explained by heredity from primaeval parent-forms, but which have been acquired at a later time in consequence of the adaptation of the germ, or embryo form, to special conditions of evolution These kenogenetic processes are recent additions, which do not allow of direct inference as to the corresponding pro- cesses in the tribal history of the ancestral line, but which i-ather falsify and conceal the latter.
This critical distinction between the primary palinge- netic, and the secondary kenogenetic processes is of course
PALINGENESIS AND KENOGEXESIS. II
of the greatest importance to scientific Phylogeny, which, from the available empiric material supplied by Ontogeny, by Comparative Anatomy, and by Palaeontology, seeks to infer the long extinct historical processes of tribal evolution. It is of the same importance to the student of evolution as is the critical distinction between corrupt and genuine passages in the text of an old writer to the philologist ; the separation of the original text from interpolations and corrupt readings. This distinction between Palingenesis or inherited evolution, and Kenogenesis or vitiated evolution, has not, however, yet been sufficiently appreciated by naturalists. But I believe that it is the first condition requisite, if the history of evolution is to be really understood, and I think that two separate main divisions, based on this distinction, must be made in germ-history; Palingenesis or inherited history, and Kenogenesis or vitiated history.
Let us illustrate this highly important distinction by a few examples taken from the evolution of man. In Man, as in all other higher Vertebrates, the following incidents of germ- history must be regarded as palingenetic processes : the formation of the two primary germ-layers, the appearance of a simple notochord (Chorda) between the spinal tube and the intestinal tube, the transitory formation of gill-arches and gill-openings, of primitive kidneys, of the primitive brain bladder, the hermaphrodite rudiment of the sexual organSj etc All these, and many other important phenomena have evidently been accurately handed down, by constant heredity, from the primaaval ancestors of Mammals, and must, there- fore, be referred directly to corresponding palseontological evolutionary incidents in the history of the tribe. On the other hand, this is not the case with the following germinal
12 THE EVOLUTION OF MAN.
incidents, which must be explained as kenogenetie pro- cesses ; the formation of the yelk- sac, of the allantois and placenta, of the amnion and chorion, and, generally, of the different egg-membranes and the corresponding systems of blood-vessels; also the transitory separation of the primitive vertebrate plates and the side-plates, the secondary closing of the stomach wall and the intestinal wall, the formation of the navel, etc. All these, and many other phenomena are evidently not referable to corresponding conditions of an earlier, independent, and fully developed parent form, but must be explained as solely due to adaptation to the peculiar conditions of egg-life or embryo-life (within the egg-membranes). With reference to this fact we may now define our "first principle of Biogeny" more exactly as follows : " The evolution of the germ (Ontogeny) is a com- pressed and shortened reproduction of the evolution of the tribe (Phylogeny) ; and, moreover, this reproduction is more complete, in proportion as, in consequence of constant heredity, the original inherited evolution (Palingenesis) is more closely retained; on the other hand, the repetition is more incomplete, in proportion as the later vitiated evolution (Kenogenesis) is introduced by changing adapta- tion."10
The kenogenetic vitiations of the original, palingenetic incidents of evolution depend in great measure on a gradually occurring displacement of the phenomena, which is effected in the course of many thousands of years by adaption to the changed conditions of embryonic existence. This displace- ment may effect either the place or the time of the phenomena. If the former, it is called Heterotopy ; if the latter, Heterochrony.
HETEROTOPY AND HETEROCHRONY. 13
" Displacement in position, " or " Heterotopy," especially affects the cells or elementary parts which compose the organs; but it also affects the organs themselves. For example, the sexual organs of the human embryo, as well as those of many higher animals, appear to originate from the middle germ-layer. But the comparative Ontogeny of the lower animals shows, on the other hand, that these organs did not originally arise from this layer, but from one of the primary germ-layers ; the male sexual organs from the outer germ-layer, the female from the inner. Gradually, however, the germ-cells have altered their original site, and have made their way, at an early period, from their original position into the middle germ-layer, so that they now appear actually to originate in the latter. An analogous heterotopism affects the primitive kidneys in the higher Vertebrates. Even the appearance of the mesoderm itself is very greatly affected by a displacement in position, which is connected with the transition of embryo cells from one germ-layer into another.
The kenogenetic " displacements in time," or " Hetero- chronisms," are equally significant. They are seen in the fact that in the germ-history (Ontogeny) the sequence in which the organs appears differs from that which, judging from the tribal history (Phylogeny), would be expected. By hcterotopy the sequence in position is vitiated; similarly, by heterochrony the sequence in time is vitiated. This vitiation may effect either an acceleration or a retardation in the appearance of the organs. We must regard the following incidents in the germ-history of man as examples of ontogenetic acceleration : the early appearance of the heart, the gill-openings, the brain, the eyes, the chorda,
14 THE EVOLUTION OF MAN.
etc. It is evident that these organs appear earlier in relation to others than was originally the case in the history of the tribe. The reverse is true of the retarded completion of the intestinal canal, the body-cavity, and the sexual organs. It is evident that in these cases there is an ontogeuetic postponement or retardation.
It is only by critically appreciating these kenogenetic incidents in relation to the palingenetic, and by constantly allowing for the changes in inherited evolution effected by vitiated evolution, that it is possible to recognize the fundamental significance of the first principle of Biogeny, which in this way attains its true value as the most im- portant explanatory principle of the history of evolution. When it is thus critically appreciated, this first principle also proves to be the " red thread " on which we can string every one of the phenomena in this wonderful domain ; this is the thread of Ariadne, with the aid of which alone we are able to find an intelligible course through this com- plicated labyrinth of forms. Even at an earlier period, when the history of the evolution of the human and the animal individual first became somewhat more accurately known — which is hardly half a century ago ! — people were greatly surprised at the wonderful similarity existing in the onto- genetic forms, or the stages of the individual evolution, of very different animals. They noticed also the remarkable resemblance between these and certain developed animal forms of allied lower groups. Even the older natural philo- sophers recognized the fact that in a certain way these lower animals permanently represent in the system of the animal kingdom forms which appear transiently in the evolution of individuals of higher groups. But formerly
HEREDITY AND ADAPTATION. 15
it was impossible to understand and interpret aright this remarkable resemblance. Darwin's greatest merit is that he has now enabled us to understand this circumstance. This gifted naturalist was the first to place the pheno- mena of Heredity on the one hand, and of Adaptation on the other, in their true light, and to show the fundamental significance of their constant interaction in the production of organic forms. He was the first to point out the im- portant part played by the continual Struggle for Existence in which all organisms take part, and how under its in- fluence, through Natural Selection, new species of organisms have arisen, and still arise, entirely by the interaction of Heredity and Adaptation. Darwin thus enabled us properly to understand the immensely important relation existing between the two divisions of the History of Evolution : Ontogeny, and Phylogeny.
If the phenomena of Heredity and Adaptation are left unnoticed, if these two formative physiological functions of the organism are not taken into account, then it is entirely impossible thoroughly to understand the History of Evolution; so that before the time of Darwin we had no clear idea of the real nature and caus-es of the development of germs. It was utterly impossible to explain the strange series of forms through which a human being passes in its embryonic evolution ; it was impossible to comprehend the reason of the curious series of various animal-like forms which appear in the Ontogeny of man. Previously it was even generally believed that the whole human being, with all its parts foreshadowed, existed even in the egg, and that his evolution was only an unfolding of the form, a simple process of growth. But this is not at all the case. On the contrary,
1 6 THE EVOLUTION OF MAN.
the entire process of the evolution of the individual presents to the eye a connected series of diverse animal forms ; and these various animal forms exhibit very diverse conditions of external and internal structure. The reason why every human individual must pass through this series of forms in the course of his embryonic evolution, was first explained to us by the Theory of Descent of Lamarck and Darwin. From this theory we first learn the efficient causes (causce effiaientes) of individual evolution ; by the aid of this theory we first perceive that such mechanical causes alone suffice to effect the evolution of the individual organism, and that the co-operation of designing, or teleological causes (causce finales), which were formerly universally assumed, is unnecessary. Of course, these final causes still play an important part in the prevailing school-philosophy ; but in our new natural philosophy we are enabled to replace them entirely by the efficient causes.
I allude to this matter at this early stage, in order to call attention to one of the most important advances made in any branch of human knowledge during the past ten years. The history of philosophy shows that in the cosmology of our day, as in that of antiquity, final causes are almost universally deemed to be the real ultimate causes of the phenomena of organic life, and especially those of the life of man. The prevailing Doctrine of Design, or Teleology, assumes that the phenomena of organic life, and in particular those of evolution, are explicable only by purposive causes, and that, on the contrary, they in no way admit of a mechanical explanation, that is, one entirely based on natural science. The most difficult problems in this respect which have been before us, and which seemed capable of
MONISM AND DUALISM. \J
solution only by means of Teleology, are, however, precisely those which have been mechanically solved in the Theory of Descent. The reconstruction of the history of the evolu- tion of man, which this theory has effected, has actually removed the greatest difficulties. We shall see in the course of our inquiries how, through Darwin's reform of the Doctrine of Evolution, the most wonderful problems, hitherto deemed unapproachable, of the organization of man and animals have admitted of a natural solution, of a mechanical explanation, by non-purposive causes. It has enabled us to substitute everywhere unconscious causes acting from necessity, for conscious purposive causes.11
If the recent progress in the Doctrine of Evolution had accomplished only this, every thoughtful person must have admitted that even in this an immense advance had been made in knowledge. In consequence of it, the tendency called unitary or monistic, in contradistinction to the dual- istic, or binary, which has heretofore prevailed in speculative philosophy, must ultimately prevail throughout philosophy.12 This is the point at which the history of the evolution of man at once penetrates deeply into the very foundations of philosophy. For this reason alone it is very much to be desired, in fact is indispensable, that any one who aspires to philosophic culture should learn the most important facts in this field of research.
The significance of the facts of Ontogeny is so great and so evident that the dualistic teleological philosophy, finding them extremely inconvenient, has of late endeavoured to meet them by simple denial Such, for instance, has been the case with the fact that every human being develops from an egg, and that this egg is a simple cell, like the 3gg-
1 8 THE EVOLUTION OF MAN.
cell of all other animals. When in my "History of Creation" I had discussed this fundamental fact, and had directed attention to its immense significance, several theological periodicals pronounced it a malicious invention of my own, The evident fact that at a certain stage of their evolution the embryos of Man and of the Dog are entirely in- distinguishable from one another was also denied.
The fact is that an examination of the human embryo in the third or fourth week of its evolution shows it to be altogether different from the fully developed Man, and that it exactly corresponds to the undeveloped embryo-form presented by the Ape, the Dog, the Rabbit, and other Mammals, at the same stage of their Ontogeny. At this stage it is a bean-shaped body of very simple structure, with a tail behind, and two pairs of paddles, resembling the fins of a fish, and totally dissimilar to the limbs of man and other mammals, at the sides. Nearly the whole of the front half of the body consists of a shapeless head without a face, on the sides of which are seen gill-fissures and gill-arches as in Fishes. (Of. Plate VII. at the end of Chapter XI.) In this stage of evolution the human embryo differs in no essential way from the embryo of an Ape, Dog, Horse, Ox, etc., at a corresponding age. Even such facts as these., which can be easily and promptly demonstrated at any time by placing side by side the corresponding embryos of Man, a Dog, a Horse, etc., have been spoken of by theologians and teleological philosophers as inventions of materialism ; and even naturalists, who were presumably acquainted with them, have tried to deny them. No stronger proof, surely, of the immense radical importance of these embryological facts, in favour of the monistic philosophy can be given than
INTERRELATION OF FORMS AND FUNCTIONS. 19
these efforts on the part of the dualistic school to meet them by simple denial or utter silence. They are indeed extremely distasteful to that school, and are totally irreconcilable with their teleological cosmology. We must therefore take especial care to place them in their true light. We are entirely of the opinion of Huxley, who, in his able " Evidence as to Man's Place in Nature," says that these facts, " though ignored by many of the professed instructors of the public mind, are easy of demonstration, and are universally agreed to by men of science; while their significance is so great, that whoso has deeply pondered over them will, I think, find little to startle him in the other revelations of Biology."
Although our chief inquiry is primarily directed to the history of the evolution of the bodily form of Man and of his organs, and to their external and internal structural relations, I must here at once observe that the history of the evolution of the functions is inseparably connected with this. Everywhere in Anthropology, just as in Zoology, of which the former is but a part, and throughout the whole field of Biology, these two branches of research are thus inseparably connected. The peculiar form of the organism and its organs, both internal and external, is always closely related to the peculiar manifestations of life, of the organism and its organs, or, in other words, to the physiological func- tions performed by these. This intimate relation between form and function is also shown in the evolution of the organ- ism and its various parts. The history of the evolution ot forms, which primarily occupies us, is at the same time the history of the evolution of functions ; and this is equally true of the human and of all other organisms.
20 THE EVOLUTION OF MAN.
But I must here add at once, that our knowledge of th« evolution of functions is as yet far from being so advanced as our knowledge of the evolution of forms. Indeed, properly speaking, the entire history of evolution, or Biogeny, includ- ing both Ontogeny and Phylogeny, has as yet been almost exclusively a history of the evolution of forms, while the Biogeny of functions hardly exists even in name. The fault lies solely with Physiology, which has as yet scarcely given a thought to the history of evolution, which it has left entirely to the care of Morphology.
The two chief divisions of biological research — Mor- phology and Physiology — have long travelled apart, taking different patha This is perfectly natural, for the aims, as well as the methods, of the two divisions are different. Morphology, the science of forms, aims at a scientific under- standing of organic structures, of their internal and external proportions of form. Physiology, the science of functions, on the other hand, aims at a knowledge of the functions of organs, or, in other words, of the manifestations of life.13 Physiology, however, has, especially during the last twenty years, been far more one-sided in its progress than Mor- phology. Not only has it entirely neglected to apply the comparative method, by which Morphology has gained its greatest results, but it has altogether disregarded the History of Evolution. Hence it has come to pass that, within the past few decades, Morphology has advanced far beyond Physiology, although the latter is pleased to look haughtily down upon the former.' It is Morphology which has gained the greatest results in the fields of Comparative Anatomy and Biogeny, and almost everything stated in these pages as to the History of the Evolution of Man. is due to the
DEFECTIVE STATE OF PHYSIOLOGY. 21
exertions of morphologists, and not of physiologists. Indeed the direction at present taken by Physiology is so one- sided that it has even neglected the recognition of the most important functions of Evolution, namely, Heredity and Adaptation, and has left this entirely physiological task to morphologists. We owe to morphologists, and not to physi- ologists, nearly all that we yet know of Heredity and Adaptation. The latter still works as little at the functions of evolution as at the evolution of the functions.
It will, therefore, be the task of a future Physiogeny to grasp the history of the evolution of the functions with the same earnestness, and with the same success, with which Morphogeny has long ago undertaken the study of the history of the evolution of forms. A few instances will show how closely the two are connected. The heart of the human embryo has at first a very simple structure, such as appears permanently only in Asc'dians and other inferior Worms, and connected with it is a circulation of the blood oi the most simple kind. When, on the other hand, we see that with the fully developed form of the human heart there is connected a function of the circulation of the blood totally different from the former one, and far more complicated, the study of the evolution of the heart necessarily enlarges from a task which was originally morphological to one which is physiological also. It is the same in the case oi all other organs and their activities.
Thus, for instance, a careful comparative study of the history of the evolution of the form of the intestinal canal, the lungs, and the organs of generation, affords us also most important information as to the evolution of the respective functions of these organs.
22 THE EVOLUTION OF MAN.
This important relation is most clearly seen in th« history of the evolution of the nervous system. In the economy of the human body, this system performs the func- tions of sensation, of voluntary movement, volition, and finally the highest psychical functions, namely, those of thought ; in a word, every one of the various activities which constitute the special subject of Psychology, or the science of the mind. Modern Anatomy and Physiology have demon- strated that these functions of the mind, or psychic activities, are immediately dependent upon the more delicate structure of the central nervous system, upon the internal conditions of the form of the brain and the spinal marrow. Here are placed the extremely complex mechanism of cells, whose physiological function constitutes the mind-life of Man. It is so complex that to most people its function appears to be something supernatural, and incapable of mechanical explanation. But the history of the evolution of the in- dividual furnishes us with the most surprising and signi- ficant information as to the gradual origin and progressive formation of this most important system of organs. For the first rudiment of the central nervous system in the human embryo makes its appearance in the same most simple form in which Ascidians and other inferior Worms retain it throughout life. A perfectly simple spinal marrow, without brain, such as throughout its existence represents the organ of the mind of the Amphioxus, the lowest of Vertebrates, first develops from this rudiment. It is only at a later period that a brain develops from the anterior extremity of this spinal cord, and this brain is of the simplest form, similar to the permanent form of this organ in the lower Fishes. Step by step this simple brain develops still further, passing through forms corresponding to those of
ONTOGENY AND PHYLOGENY. 23
the Amphibia, Beaked Animals (Ornithostoma), Pouched Animals, or Marsupials, and Semi-apes (Prosimice), until the highly organized form is reached which distinguishes the Apes from all other Vertebrates, and which finally attains its highest development in the human brain. But step by step with this progressive evolution of the form of the brain, the evolution of its peculiar function, the psychical activities, moves on hand in hand, and it is therefore the history of the evolution of the central nervous system which for the first time enables us to understand the origin of life of the human mind from natural causes, and the gradual historic development of the psychic activities of man. It i1* impossible without the aid of Ontogeny to perceive how these highest and most brilliant functions of the animal organism have been historically developed. In a word, the history of the evolution of the spinal marrow and the brain of the human embryo at the same time directly leads us to understand the Phylogeny of the human mind, that most sublime activity of life which in the developed human being we are accustomed to regard as something wonderful and supernatural.
There is no doubt that this special result of the study of the history of evolution is among the greatest and most important. Happily, our knowledge of the Ontogeny of the central nervous system of Man is so satisfactory, and agrees so perfectly with the supplementary results of Comparative Anatomy and Physiology, that it affords us a perfectly clear insight into one of the highest problems of philosophy, namely, the Phylogeny of the psyche, the mind, or the history of the ancestral lineage of Man's psychic activities, and leads us into the only path by which we shall ever be able to solve this the highest of all problems.
THE EVOLUTION OF MAN.
TABLE I.
LIST of the principal branches of BIOOENY, or the HISTORY OF ORGAN i. EVOLUTION, with reference to the four chief stages of Organic In dividuality— Cell, Organ, Person, and Race.14
First branch of Biogeny, or of the history of the evolution of organisms: GERM-HISTORY, or On- togeny (history of the development of the embryo of the in- dividual organism).
. Germ-history of
Forms. (Morphogeny.)
'1. Germ-history of the cells (and cytods) and of the tissues composed of the cells llistogeny.
2. Germ-history of the organs, and of the systems and apparatus composed of the organs. Organogeny.
3. Germ-history of the persons (called " the history of the evolution of bodily form "). ISlastogeny.
4. Genn-hisfory of races (or of social aggregates composed of persons: fa- milies, communities, states, etc. Cor- viogeny.
iThe germ-history of the functions or the history of the development of vital activities in the individual, has not yet been accurately and scientifically in- vestigated.
n.
Second branch of Biogeny, or of the history of the evolution of organisms: TRIBAL HISTORY, or Phylogeny (history of the palseontological evo- lution of organic
3. Tribal history of Forms.
1. Tribal history of the cells (hardly at- tempted as yet). Histophyly.
2. Tribal history of organs (an unrecog- nized main object of comparative ana- tomy). Organophyly.
3. Tribal history of persons (an unrecog- nized main object of the natural system of classification). Blastophyly.
4. Tribal history of races (or of social aggregates composed of persons : fa- milies, communities, states, etc. Cor-
. mophyly.
0 tribal history of the functions, or ttii listory of the palaeontological develop, nent of vital activities, has, in the case f most organisms, not yet been ex- mined. In the case of man, a large art of the history of culture fcUls unrleJ hi* head.
CHAPTEE II. THE EARLIER HISTORY OF ONTOGENY.
CASPAR FRIEDRICH WOLFF.
The Evolution of Animals as known to Aristotle. — His Knowledge of the Ontogeny of the Lower Animals. — Stationary Condition of the Scien- tific Study of Nature during the Christian Middle Ages. — First Awaken- ing of Ontogeny in the Beginning of the Seventeenth Century. — Fa- bricius ab Aquapendente. — Harvey. — Marcello Malpighi. — Importance of the Incubated Chick. — The Theories of Pre-formation and Encase- ment (Evolution and Pre -delineation). — Theories of Male and Female Encasement. — Either the Sperm-animal or the Egg as the Pre-formed Individual. — Animalculists : Leeuwenhoek, Hartsoeker, Spallanzani. — Ovnlists : Haller, Leibnitz, Bonnet. — Victory of the Theory of Evolution owing to the Authority of Haller and Leibnitz. — Caspar Friedrich Wolff. — His Fate and Works. — The Theoria Generationis. — Ke-formation, or Epigenesis.— The History of the Evolution of the Intestinal Canal.— The Foundations of the Theory of Germ-layers (Four Layers, or Leaves). —The Metamorphosis of Plants.— The Germs of the Cellular Theory. —Wolff's Monistic Philosophy.
"He who wishes to explain Generation must take for his theme the organic body and its constituent parts, and philosophize about them ; he must show how these parts originated, and how they came to be in that rela- tion in which they stand to each other. But he who learns to know a thing not only directly from its phenomena, but also its reasons and causes j and who, therefore, not by the phenomena merely, but by these also, is compelled to say : ' The thing must be so, and it cannot be otherwise ; it is necessarily of such a character ; it must have such qualities ; and it is impossible for it to possess others' — understands the thing not only historically but truly philosophically, and he has a philosophic knowledge of it. Our own
2(5 THE EVOLUTION OF MAN.
Theory of Generation is to be such a philosophic comprehension of an organic body, very different from one merely historical."— CASPAK FEIEDEICH WOLPJ (1764).
IN approaching each science it is, in several respects, pro- fitable to glance at the course of its evolution The well- known principle that " whatever has come into being can only be known from the process by which it came into being" is applicable to science. By tracing its gradual development, we shall most clearly perceive its tasks and aims. We shall also find that the present condition of the History of the Evolution of Man, with all its peculiar cir- cumstances, can only be properly understood by taking into consideration the history of the evolution of the science itself. The examination will not detain us long ; for the History of the Evolution of Man is one of the very youngest of the Natural Sciences. This is equally true of its two divisions : the History of the Germ, or Ontogeny, and the History of the Tribe, or Phylogeny.
Passing over such most ancient germs of the science as are found in classical antiquity, and which we shall have to discuss presently, the true History of the Evolution of Man, as a science, really begins in the year 1759, when Caspar Friedrich Wolff, one of the most eminent of German naturalists, published his Theoria Generations. This was the first foundation-stone for a true history of animal germs. In 1809, exactly fifty years later, Jean Lamarck published the Philosophic Zoologique, the first attempt at a History of Descent ; and in 1859, another half century later, appeared Darwin's work, which must be regarded as the first to give a scientific basis to that attempt. But, before carefully examining this as the real foundation of the
ARISTOTLE ON DEVELOPMENT. 2J
History of the Evolution of Man, we must rapidly glance at the great philosopher and naturalist of antiquity, who, in this as well as in all other branches of research in Natural Science, stands quite alone for a period of more than two thousand years. This was Aristotle, "the Father of Natural History."
Among the extant writings of Aristotle on Natural History, treating of various aspects of biological research, and the most important of which is the History of Animals, there occurs also a smaller work, specially confined to the History of Evolution. It is entitled Peri Zoon Geneseos ("On the Generation and Development of Animals").15 This work is of great interest, if merely because it is the most ancient, and the only one of its kind, which has reached us from classical antiquity in a fairly complete condition. It is important also because, like others of Aristotle's writings on subjects of Natural History, it entirely controlled the science for two thousand years. The philosopher was a careful observer and an ingenious thinker ; yet, while his importance as philosopher has never been doubted, his merits as an observant naturalist have only lately been duly appreciated. Those students of Nature who have lately more accurately examined his writings on Natural History, have been astonished at the mass of interesting statements, and the remarkable observa- tions which abound in them. With regard to the History of Evolution, it is specially noticeable that Aristotle traced it in the most diverse classes of animals, and that he was acquainted, especially in connection with the lower animals, with several of the most remarkable facts which we have re-discovered only towards the middle of the present century. K
28 THE EVOLITIOX OF MAN.
It is certain, for example, that he was thoroughly acquainted with the entirely peculiar method of propagation and development of the Cuttle-fishes, or Cephalapods, the embryo of which has a bag of yelk protruding from the mouth. He knew, also, that embryos of Bees can be developed from the egg even when it has not been fertilized. The so-called parthenogenesis, or virginal generation, of Bees has been proved in our days only lately by the meritorious zoologist, Siebold, of Munich, who also showed that male Bees develop from unimpregnated, and female bees only from impregnated eggs.16 Aristotle further relates that some Fishes (of the species Serranus) are hermaphrodites, inasmuch as each individual has male and female organs, and impregnates itself. This fact, also, has only lately been established. He also knew that the embryos of several species of Sharks are connected with the mother's womb by a sort of placenta — an organ of nourishment, full of blood, which otherwise occurs only in Man and the higher Mammals. This placenta of the Shark was for a long time considered mythical, until, in 1839, Johannes Muller, of Berlin, proved it to be a fact. We might quote many other remarkable observations from Aristotle's account of Evolution, which would prove the accuracy of this great naturalist's acquaintance with onto- genetic investigations, and the great degree in which he was in advance of subsequent times in this respect.
In most of his observations he was not satisfied with merely stating the facts, but he added reflections on their significance. Some of these theoretical thoughts are of special interest, because they indicate a right fundamental perception of the nature of the processes of evolution. He
ARISTOTLE AS A NATURALIST. 29
conceives the evolution of the individual to be a new formation, in which the several parts of the body develop one after the other. According to him, when the human or animal individual develops, either within the mother's body or out of it in the egg, the heart is formed first, and is the beginning and the centre of the body. After the heart has been formed, the other organs appear ; of these the interior precede the exterior, and the upper, or those above the diaphragm, precede the lower, or those below it. The brain is formed at a very early stage, and out of it grow the eyes. This assertion is, indeed, quite accurate. On trying to obtain from these statements of Aristotle an idea of his conception of the processes of evolution, we find that they indicate a faint presentiment of that theory of evolution which is now called Epigenesis, and which Wolff, some two thousand years later, first proved. It is especially remark- able that Aristotle altogether denied the eternity of the individual. He admitted that the kind or species, formed from individuals of the same kind, might possibly be eternal ; but asserted that the individual itself was tran- sient, that it came into being anew in the act of genera- tion, and perished at death.
During the two thousand years after Aristotle no essential progress in Zoology in general, or in the History of Evolution in particular, is to be recorded. People were content to expound Aristotle's zoological writings, to copy them, to deface them greatly by additions, and to translate them into other languages. There was hardly any independent research during this long period. During the Middle Ages of Christianity, when insurmountable obstacles were laid in the way of independent researches in
JO THE EVOLUTION OF MAN.
natural science by the development and diffusion of influential conceptions of faith, a re-commencement of biological researches was especially out of the question. Even when, in the sixteenth century, human Anatomy again began to be studied, and independent investigations of the structure of the body of the developed human being were again first made, anatomists dared not extend their investigations into the condition of the yet undeveloped human body, into the formation and development of the embryo.
The prevailing fear of such researches was due to several causes. This seems but natural when we remember that by the bull of Pope Boniface VIII. greater excom- munication was pronounced against all who dared to dis- member a human corpse. While anatomical investiga- tion of the developed human body was a crime which drew down the curse of the Church, it is evident that the examination of the body of the child, hidden in the mother's womb, and which the Creator himself seemed, by its concealed position, to have intentionally withdrawn from the curious gaze of naturalists, would have appeared much more criminal and impious. The omnipotence of the Christian Church, which at that time caused many thousands to be executed and burned for heresy, and which even then with correct instinct foresaw danger threatened to itself from the deadly enemy which was then growing up in Natural Science, took care that the latter should not make too rapid strides.
It was only when the Reformation broke the all- embracing power of the Only-Saving Church, and a new and fresh intellectual impulse began to release enslaved
EARLIER STUDENTS OF EVOLUTION. 31
science from the iron chains of dogmatism, that human Anatomy and the History of the Evolution of Man could move again more freely, with the re-opening of research in other natural sciences. But Ontogeny remained far behind Anatomy, and it was only in the beginning of the seven- teenth century that the first ontogenetic publications appeared. The first to begin was the Italian anatomist, Fabricius ab Aquapendente, Professor at Padua, who pub- lished two works — De Formato Foetu (1600), and De Formations Fo3tus (1604), — which contain the oldest figures and descriptions of the embryo of Man and other Mammals, and also of the Chick. Similar imperfect representations were given soon after by Spigelius — De Formato Fcetu (1631) — by the Englishman, Needham (1667), and his celebrated countryman, Harvey (1652). The latter discovered the circulation of the blood in the animal body, and made the important assertion : Omne vivum ex ovo ("Everything living comes from an egg"). The Dutch naturalist, Swammerdam, in his " Bible of Nature," pub- lished the results of the first investigations into the embryology of the Frog, and the so-called segmentation of its yelk. The most important ontogenetic researches of the seventeenth century, however, were those of the Italian, Marcello Malpighi of Bologna, who gave a fresh impetus both to Zoology and to Botany. His two dissertations, De Formatione Pulli, and De Ovo Incubato (1687), contain the first connected description of the history of the development of the chick in the incubated egg.
Here I must make some remarks on the great importance of the Chick in relation to our science. The history of the formation of a Chick, as well as of all birds, accurately
j2 THE EVOLUTION OF MAN.
corresponds in its essential characteristics with that ^of all other higher Vertebrates; and, therefore, also of Man. The three higher classes" of Vertebrates, Mammals, Birds, and Reptiles (Lizards, Snakes, Turtles, etc.), are from the beginning of their individual development so surprisingly similar in all essential features of their bodily structure, especially in the earlier stages, that for a long while it is impossible to distinguish them. (Of. Plates VI. and VII.) It has long been known that the accurate study of the evolution of the embryo of the Bird, which is most readily obtained as the subject of research, is all that is necessary in order to learn the essentially similar mode of evolution of Mammals, therefore also of Man. Even as early as the middle and the end of the seventeenth century, when human embryos, as well as those of all other Mammals, began to be examined in their earlier stages, this most important fact was soon recognized. It is of the greatest importance, both for theoretical and for practical purposes. Conclusions of the highest importance to the theory of evolution may be drawn from the similarity of structure of the embryos of widely differing animals. This simi- larity is invaluable in practical ontogenetic research, because the ontogeny of Birds, which is accurately known, most completely supplements and explains the embryology of Mammals, which has been but imperfectly studied. Hen's eggs can be obtained at all times and in any quan- tity, and by hatching them artificially the evolution of the embryo may be traced step by step. On the other hand, it is much more difficult to study the evolution of Mammals, because the embryo of these does not develop in a large egg that has been laid, or, in other words, in an
IMPORTANCE OF THE CHICK. 33
independent and isolated body, but in a small egg, which, until maturity, remains enclosed and concealed in the body of the mother. For this reason it is very difficult to pro- cure all the stages of development in any large number, for the purpose of making connected investigations, not to mention external reasons, such as the great cost, the technical difficulties, and the many other obstacles, which lie in the way of any extended series of researches into fecundated mammals. For this reason, from that time to the present day, the Chick during the process of incubation has been the subject oftenest and most closely investigated. The perfection of hatching-machines has made it yet easier to obtain embryo-chicks in any required stage of evolution and in any quantity, in order to examine the whole process of formation step by step.
About the end of the seventeenth century the history of the evolution of the incubated Chick had already been advanced as far, and its more essential, external, and less delicate conditions were as well known, owing to the labours of Malpighi, as investigations with the imperfect microscopes of that time rendered possible. Of course, the perfection of the microscope and of technical methods of research was a necessary condition for more accurate em- bryological research. For vertebrate embryos in their earlier stages are so small and delicate, that it is impossible to examine them without a good microscope, and without applying peculiar technical methods. But these means were not applied, and the microscope was not essentially perfected till the beginning of our century.
Throughout the whole of the- first half of the eighteenth century, during which time the systematic Natural History
34 THE EVOLUTION OF MAN.
of animals and plants received so great an impulse from Linnaeus' famous Systema Naturae, the History of Evolution made scarcely any progress. It was in the year 1759 that Caspar Friedrich Wolff made his appearance, and his genius gave an entirely new direction to this science. Until then Embryology was almost exclusively occupied in unsuccessful attempts to construct various theories of evolution from the scanty material already acquired.
The theory which at that time gained almost universal acceptance, and which continued to be generally received during the entire eighteenth century, is in Germany com- monly called the Theory of Unfolding (Auswickelung), or Evolution, but is better spoken of as the Theory of Pre- formation.17 Its main idea is the following : no really new formation takes place during the evolution of each indi- vidual organism, animal or plant, including therefore Man : there is only a growth and an unfolding of parts, all of which have, from eternity, been present, pre-formed, and complete, though only very minute, and wrapped together. Every organic germ, therefore, contains all the parts and organs of the body pre-formed and represented in their subsequent form, position, and connection, and the entire course of the evolution of the individual, the entire onto- genetic process, is nothing but an evolution in the most exact meaning of the word; namely, an unwrapping of wrapped-up parts already formed. Hence, for example, in a hen's egg we do not find a simple cell which undergoes division, and the generation of cells of which form layers of germs, and by various changes, separations, and new for- mations, ultimately bring into being the body of the Bird ; but every hen's egg contains from the beginning a complete
THEORIES OF PRE -FORMATION AND ENCASEMENT. 35
Chick, with all its parts pre-formed and wrapped together, and during the development of the incubated egg these parts are merely drawn out and grow.
As soon as this theory was carried out logically, it necessarily led to the Theory of Encasement. According to this, every species of animal or plant was originally creatt3d only as a pair or as a single individual ; but this one indi- vidual already contained, encased within itself, the germs of all the other individuals of its species which have ever lived or will live. As at that time the age of the earth was calculated, according to the Biblical history of creation, at five or six thousand years, people thought they could approximately calculate the number of germs of every species of organism which had lived during that period, and consequently the number which had existed encased in the first " created " individual of the species. The theory was logically extended to mankind, and it was accordingly maintained that our first common mother Eve held in her ovary the germs of all the children of men, one encased in the other.
This Theory of Encasement was then developed so that the female individuals, were considered to be the created beings which were encased one in another. It was believed that only a single pair of each species was originally created ; but the ovary of the female individual contained, encased within it, alJ the germs of all the individuals of the kind, of both sexes, which were ever to develop. But the Theory of Pre-formation took quite another shape when, in 1690, Leeuwenhoek, the Dutch microscopist, discovered the human spermatozoids, or seminal threads, and proved that a large number of extremely delicate and actively
36 THE EVOLUTION OF MAN.
moving threads exist in the sperm or seminal fluid of the male. (Cf. Fig. 17 in Chap. VII.) This astonishing discovery was at once interpreted to the effect that these minute living bodies, briskly swimming about in the seminal fluid, were genuine animals, the pre-formed germs of future generations. When at the time of fecundation the two generative substances, male and female, came in contact with each other, these thread-like seminal animalcules were to penetrate into the fruitful soil of the ovary and there to attain their development like vegetable seeds in the fruitful soil of the earth. According to this theory every single seminal animalcule of Man is a complete human being; all the separate parts of the body would be entirely pre-formed in it, and subject only to a mere unwrapping and enlargement as soon as they reached the favourable matrix of the female egg. This theory also was logically carried out to the effect that in every single thread-like body were contained all the subsequent generations of its descendents, one encased in the other, each in the most extreme degree of fineness, and of the minutest size. The seminal gland of Adam, therefore, contained the germs of all the children of men who have ever peopled our planet, who inhabit it at present, or will occupy it in the future " until the end of the world."
Of course, this Doctrine of Encasement in the Male was utterly opposed to the Doctrine of Encasement in the Female, which had previously prevailed. The only ground common, to both was the false idea that the germs of innumerable generations, previously formed and encased one in another, existed in every organism ; a conception on which was also founded the curious Prolepsis Theory of Linnseus.
The two opposite theories of encasement soon began a
ANIMALCULISTS V. OVULISTS. 37
vigorous contest, "which resulted in the division of the physiologists of the eighteenth century into two large bodies of combatants, entirely opposed and contending vehemently. These were the Animalculists, and the Ovn- lists. The dispute between these two parties appears laughable to us now, for the theory of the one is just as unfounded as that of the other. As Alfred Kirchhoff says, in an excellent biographical sketch of Wolff, " this dispute was as little capable of settlement, as the inquiry whether the angels lived in the East or in the West of the heavenly rogions." 18
The Animalculists, or the Believers in Sperm, looked upon the moving seminal threads as the real animal germs, and they found support on the one hand in the lively movement, and on the other in the form of these seminal animalcules. For in the case of man, as well as of a large majority of other animals, they appear to have a somewhat oblong, egg-like, or pear-like head, a thin intermediate segment, and a very thin tail, narrowing to a hair-like form (Fig. 17). In reality, the whole formation is but a simple whip-shaped cell. The head is the cellular nucleus, surrounded by cell-matter, which is protracted into the thinner portions in the middle, and to the hair-like, move- able iail ; the latter is the whip, or thread-like appendage of other whip-shaped cells. The Animalculists, however, con- Bidered the head to be a real animal head, and the rest of the body to be a complete animal body. Leeuwenhoek, Haitsoeker, and Spallanzani were the chief defenders of this theory of Pre-delineation.
The opposite party, the Ovulists (Ovists), or Believers in ^ggs, who adhered to the older Theory (if Evolution,
3 8 THE EVOLUTION OF MAN.
maintained that the egg was the real animal germ, and that the seminal animalcules, at the time of fecundation, only gave the impulse which caused the unfolding of the eo-g in which all generations were encased one in the other. This opinion prevailed with the majority of biologists during the whole of the last century, though Wolff, in 1759, demonstrated its utter want of foundation. Its acceptance was specially due to the fact that the most celebrated biological and philosophical authorities of that time had pronounced in its favour, — among them princi- pally Haller, Bonnet, and Leibnitz.
Albrecht Haller, Professor at Gottingen, who has often been called " the Father of Physiology," was a very learned and comprehensively educated man, but, as an interpreter of the more profound natural phenomena, occupied no very high position. He has best described himself in the celebrated and often-cited saying, that " Into the inner side of Nature no created mind ever penetrates; happy he to whom she shows only her outer husk ! " The best answer to this " husk " view of nature was given by Goethe, in hi* splendid poem which ends with the lines :
" Nor hnsk nor kernel Nature brings —
For all one only type of things ; . Yet prove thyself, and seek to know
If husk or kernel thou dost show."
Attempts have, however, been recently made to justi fy Haller's " husk " view. Wilhelm His has made himself the special defender of this strange conception. Haller, in his well-known work,Elementa Physiologies, adopted the Theory of Evolution (Theory of Pre-formation) in a most decided manner, in these words : " There is no coming into being J
HALLER AND LEIBNITZ. 39
(Nulla est epigenesis). No part of the animal body was made previous to another, and all were created simultaneously (Nulla in corpore animali pars ante aliam facto, est, et omnes simul creates existunfy." In reality, therefore, he denied any actual evolution in the natural sense, and in this went so far as to maintain even the existence of a beard in the new-born boy, and the existence of the horns in the hornless fawn ; all the parts were already present in a complete state, but hidden for a while from the human eye. Haller even calculated the number of human beings which God, on the sixth day of His work of creation, at once created and encased in the ovary of Eve, the Mother of all. He estimated them at two hundred thousand millions, by assuming the creation of the world to have been six thousand years ago, the average human life thirty years, and the number of human beings alive at the same time one thousand million. And the celebrated Haller advocated all this rampant nonsense, and the inferences drawn from it, most successfully, even after Wolff had dis- covered the true Epigenesis, and proved it by investigation. Leibnitz was the most important of the philosophers who adopted the Theory of Evolution (Pre-formation), and by his great authority, as well as by his talented exposition, gained numerous followers for it. Based upon his Theory of Monads, according to which soul and body are in an eternally inseparable union, and in their bi-unity constitute Ihe individual (the Monad), Leibnitz quite logically applied the Theory of Encasement to the soul also, and denied all real development for it, equally with the body. In his Theodicce, for instance, he says : " I think that souls, which will some day be human souls, as in the case of those of
^O THE EVOLUTION OF MAN.
other species, pre-existed in the semen ; that they existed in the ancestors as far back as Adam, therefore since the beginning of things, always in the form of organized bodies."
The Theory of Encasement seemed to receive its most important experimental support in the researches of Bonnet one of its most zealous adherents. He observed, for the first time, in Plant-lice, the so-called " virginal generation," or parthenogenesis, which is an interesting form of propaga- tion lately proved by Siebold and others, in many other articulated animals, such as various Crabs and Insects.16 The females of these and other lower animals of certain groups propagate for several generations without having been impregnated by a male. Such eggs, which for their evolution do not require to be impregnated, are called " false eggs," Pseudova, or Spores. Bonnet, in 1745, for the first time observed that a female Plant-louse, which he had completely shut off, as in a nunnery, and shielded from all contact with males, after shedding its skin four times, gave birth on the eleventh day to a living female, and within the next twenty days produced as many as ninety-four other females ; and that soon all of these, without having come in contact with a male, multiplied again in the same virgin manner. Thereupon, of course, it seemed that a tangible proof of the truth of the Theory of Encasement, according to the interpretation of Ovulists, had been Abundantly furnished, and it naturally became almost uni- versally accepted in this sense.
The case stood thus, when suddenly, in the year 1759, 'Jaspar Friedrich Wolff, then a young man, appeared, and with his new Theory of Epigenesis gave the death-blow to the entire Theory of Pre-formation. Wolff was born at
CASPAR FRIEDRIUH WOLFF. 41
Berlin, in 1733. He was the son of a tailor, and studied natural science and medicine at first in Berlin, at the Medico-surgical College, under the celebrated anatomist Meckel, and subsequently in Halle. Here, in the twenty- sixth year of his age, he passed his examination for his doctor's degree ; and on the 28th of November, 1759, in his dissertation as doctor, he defended the new doctrine of true evolution, the Theoria Generationis, founded on Epigenesis. This dissertation, in spite of its small limits and difficult language, ranks among the most important essays ever written in the whole range of biological literature. It is equally distinguished by its abundance of new and most careful researches, and by its far-reaching and very sug- gestive ideas given in connection with the observations, which latter he developed into a brilliant Theory of Evolu- tion entirely true to nature. Yet this remarkable publica- tion had at first no results whatever. Although the study of Natural Science was then flourishing in consequence of the impetus given by Linnaeus; although botanists and zoologists soon numbered, not dozens, but hundreds; yet hardly anybody took any interest in Wolffs Theory of Generation. And the few who had read it, foremost among whom was Haller, considered it totally false.
Although Wolff proved the truth of Epigenesis by means of the most accurate research, and refuted the un- founded hypotheses of the Theory of Pre-formation, yet the "exact" physiologist Haller continued to be the most zealous adherent of the latter, and rejected the correct doctrine of Wolff with his dictatorial decree : Nulla est epigenesis ! It is not surprising that the entire body of physiological scholars of the second half of the eighteenth
4.2 THE EVOLUTION OF MAN.
century submitted to the dictum of this physiological pope, and opposed Epigenesis as a dangerous innovation. More than half a century elapsed before Wolff's labours met with their deserved acknowledgment. Only after Meckel, in the year 1812, had translated into German another most im- portant publication of Wolffs, " On the Formation of the Intestinal Canal " (published 1764), and had drawn atten- tion to its extraordinary significance, people began to re- occupy themselves with this almost forgotten author, who, of all the naturalists of the preceding century, had made the deepest progress into the knowledge of the living organism.
Thus, as so often happens in the history of human know- ledge, new-born truth succumbed to all-powerful error, upheld by the weight of authority. The knowledge of Epi- genesis, clear as the sun, was not able to pierce through the thick fog of the Dogma of Pre-formation, and its ingenious discoverer was vanquished in the fight for the truth by the overwhelming power of the enemy.
The result was that all progress in the History of Evo- lution was for a while arrested. This is all the more to be regretted because Wolff was finally compelled, by untoward circumstances, to quit his German Fatherland. From the first without means, he had only been able to finish his clas- sical work in the face of great difficulties, and was then com- pelled to earn his bread as a practising physician. During the Seven Years' War he was busy in the Silesian hospital?, and gave excellent lectures on Anatomy in the field hospital of Breslau, attracting the attention of Cothenius, the eminent Director of Hospitals. When peace had been con- cluded, this distinguished patron tried to procure a chaii in Berlin for Wolff, but failed on account of the narrow-
HISTORY OF WOLFF. 43
mindedness of the professors of the Berlin Medico-surgical College, who were averse to all scientific progress. This most learned faculty persecuted the Theory of Epigenesis as one of the most dangerous heresies ; just as is the case now with the Theory of Descent. Although Cothenius, and other patrons in Berlin, took a warm interest in Wolff, it was impossible even to procure permission for him to give public lectures on Physiology in Berlin. The conse- quence was, that Wolff was obliged to accept a summons with which the Empress Catherine of Russia honoured him in 17G6. He went to St. Petersburg, where he remained for twenty-seven years, devoting himself in undisturbed quiet to his deep researches, and enriching the publications of the St. Petersburg Academy with the productions of his brilliant talents. He died there in 1794.19
The progress which Wolff made in the entire science of Biology was so great that the naturalists of that time could not grasp it. The mass of important new researches, and of fruitful and great ideas accumulated in his publications, is so enormous that their full value has only been gradually appreciated, and their bearing properly understood during the present century. Wolff opened up the right path into the most various branches of biological investigations. Firstly, and above all, by the Theory of Epigenesis, he first made the real nature of organic evolution intelligible. lie proved satisfactorily that the evolution of every organ- ism consists of a series of new formations, and that no trace of the form of the developed organism exists either in the egg or in the semen of the male. These are simple bodies of an entirely different significance. The germ, or embryo which develops from the egg, shows in the various
44 THE EVOLUTION OF MAN.
phases of its evolution an internal structure and an external form totally different from those of the developed organism. In none of these phases do we find any pre-formed parts ; nowhere any encasement. In these days we can scarcely continue to call this Theory of Epigenesis a theory, for NQ have been thoroughly convinced of its correctness in fact, and we are able to demonstrate it in any moment under the microscope. Nor, during the last decade, has any doubt of the truth of Epigenesis been expressed.
Wolff supplied detailed proof of his Theory of Epigenesis in his scholarly treatise "On the Formation of the Intestinal Canal (1768)." In its complete condition the intestinal canal of the Chick is a very complex, long tube, to which the lungs, the liver, the salivary, and many smaller glands are attached. Wolff showed that there is no trace of this complex tube, with all its various parts, in the embryo Chick during the first period of incubation, but that in its place there is a flat, leaf-shaped body j and that the whole embryo-body in the earliest period is also of a flat, oblong, leaf-like form. Considering the difficulty of accurately ex- amining conditions so extremely minute and delicate as the first leaf-shaped beginnings of the body of the bird with the indifferent microscopes of the last century, we cannot but admire the rare talent for observation possessed by Wolff, who actually proved the most important facts known in this the darkest portion of Embryology. From this very difficult investigation he even drew the correct conclusion that the entire embryonic body of all higher animals, as well as of birds, is for a while a flat, thin, leaf-shaped plate, which at first appears simple, but subsequently as if composed of several layers. The lowest of all these
WOLFF ON GERM-LAYERS. 45
layers, or leaves, is the intestinal canal, the development of which Wolff examined thoroughly, from its beginning to its completion. He showed that the leaf-like rudiment first forms a groove, the edges of which curve towards each other, thus growing into a closed tube, and that, finally, at the ends of this tube the two openings, mouth and anus, arise.
Nor did Wolff overlook the fact that the other organic systems of the body originate, in an entirely similar way from leaf-shaped rudiments, which afterwards assume the form of tubes. Like the intestinal canal, the nerve, muscle, and vascular systems, with all the various organs belonging to the last, develop from a simple layer-like or leaf-like rudiment. Thus in 1768 Wolff learned the very significant fact, which, half a century later, was first formulated by Pander, in the fundamental "germ-layer theory." The sentence in which Wolff expressed the main idea of this theory is so remarkable, that I quote it. "This very wonderful analogy between parts which in Nature are so widely separated, an analogy which is not imaginary, but is founded on the most reliable observations, is in the highest degree worthy of the attention of physiologists ; for it will be granted that it has a deep significance and that it is most intimately connected with the generation, and with the nature of animals. The different systems which compose the whole animal seem to be successively formed, at different times but on one plan; and these systems are therefore like one another, even though in their nature they are distinct. The system which is first pro- duced, which first assumes a peculiar definite form, is the nerve-system. When this is completed the flesh-mass, which properly speaking constitutes the embryo, is formed
4.6 THE EVOLUTION OF MAN.
on the same plan. A third, the vascular system, now appears, which is certainly sufficiently similar to the two earlier structures to allow of its form being easily recognized as that which has been described as approximately common to all the systems. The fourth system, the intestinal canal, now follows ; this, again, is formed on the same plan, and, when completed and closed, resembles the three earlier systems." In this most important discovery Wolff laid the first foundations of the fundamental " germ-layer theory " which was not completely developed till long afterwards, by Pander (1817) and by Baer (1828). It is true that Wolff's propositions are verbally incorrect, but in them he reached the truth as nearly as was then possible, and as was to be expected. We shall presently see how nearly he approached to the real state of the case.
Wolff owes much of his comprehensive conception of nature to the fact, that he was as good a botanist as a zoologist. He studied the history of the development of plants also, and in the field of botany first founded the theory which Goethe afterwards developed in his brilliant treatise on the " Metamorphosis of Plants." Wolff was the first to show that all the various parts of plants may be traced back to the leaf as their common rudiment, or ' fundamental organ." Flower and fruit, with all their parts, consist only of modified leaves. This discovery must have seemed all the more surprising to Wolff, from the fact that he had discovered a simple leaf-like rudiment to be the first form of the embryonic body of animals, as it is of planta
We therefore find in Wolff distinct traces of those theories of which, at a much later period, other gifted
WOLFF AS A MONISTIC PHILOSOPHER. 47
naturalists were to construct the foundation of the know- ledge of the morphology of the animal and vegetable body. But our admiration for this eminent genius is still greater when we discover that he also first indicated the famous cellular theory. Indeed, Wolff had, as Huxley first pointed out, an evident presentiment of this fundamental theory, for he considered minute microscopical vesicles to be the real elementary parts constituting the germ-layers.
Finally, particular attention must be directed to the monistic character of the profound philosophical reflections which Wolff published in connection with all his admirable investigations. Wolff was a great monistic Natural Phi- losopher, in the best and most correct sense of the word. It is true that his philosophical researches, like his ex- perimental ones, were ignored for more than half a century, and have not even yet met with the recognition which they deserve ; but we therefore emphasize yet more strongly the fact that their tendency was strictly in that line of philosophy which we call monistic, and which alone Ciui be considered correct
CHAPTER III. MODERN ONTOGENY.
KA.RL EIINST BAEB.
Karl Ernst Baer, the Principal Disciple of Wolff.— The Wiirzbnrg School of Embryologists : Dollinger, Pander, Baer. — Pander's Theory of Germ- layers. — Its Full Development by Baer. — The Disc-shaped first parts into two Germ-layers, each of which again divides into Two Strata. The Skin or Flesh-stratum arises from the Outer or Animal Germ-layer. The Vascular or Mucous Stratum arises from the Inner or Vegetative Germ-layer. The Significance of the Germ-layers. — The Modification of the Layers into Tubes. — Baer's Discovery of the Human Egg, the Germ-vesicle, and Chorda Dorsalis. — The Four Types of Evolution in the Four Main Groups of the Animal Kingdom. — Baer's Law of the Type of Evolution and the Degree of Perfection. — Explanation of this Law by the Theory of Selection. — Baer's Successors : Rathke, Johannes Miiller, Bischoff, Kolliker.— The Cell Theory : Schleiden, Schwann.— Its Appli- cation to Ontogeny : Robert Remak. — Retrogressions in Ontogeny : Eeichert and His. — Extension of the Domain of Ontogeny : Darwin.
" The History of Evolution is the real source of light in the investigation of organic bodies. It is applicable at every step, and all our ideas of th« correlation of organic bodies will be swayed by our knowledge of the history of evolution. To carry the proof of it into all branches of research would be an almost endless task." — KARL ERNST BAER (1828).
IF we wish to separate our historic survey of the course of the development of the Science of Human Ontogeny into parts, it is most convenient to make three. The first of these occupied the last chapter, and includes the whole preparatory period of embryologies! researches; it extends
THREE PERIODS IN THE HISTORY OF ONTOGENY. 49
from Aristotle to Caspar Friedricli Wolff, to the year 1759, when the Theoria Generationis appeared and laid the foundation for future work. The second, to which we now turn our attention, comprises exactly a century ; that is, to the year 1859, in which appeared Darwin's work on " The Origin of Species," which reformed the whole basis of the science of Biology, and especially of Ontogeny. The beginning of the third division is as recent as the time of Darwin.
As Wolff's labours remained entirely unnoticed during half a century — till the year 1812 — we are not quite accurate in assigning the exact duration of a century to the second division. During fifty-three years not one book appeared which followed in the lines laid down by Wolff, and carried on his Theory of Evolution, His opinions, which were perfectly correct and founded directly on actual observations, were only occasionally mentioned, and then only to be rejected as erroneous. His opponents, followers of the prevalent and mistaken theory of Pre -formation, did not even deign to refute him. This was owing, as I have said before, to the extraordinary authority possessed by Albrecht Haller, Wolff's distinguished opponent, and the circumstance furnishes one of the most remarkable examples of the influence which a great authority may, as such, long exert against the clear recognition of facts. The neglect of Wolff's labours was so universal that in the beginning of this century two naturalists, Oken (1806) and Kieser (1810), undertook independent investigations into the development of the intestinal canal in the Chick, and obtained a correct insight into Ontogeny, without being aware of the existence of Wolffs important work in the
£0 THE EVOLUTION OF MAN.
same field, and trod in his very footsteps unconsciously. That they really did not know his works is proved by the fact that they did not advance as far as Wolff had done. Tn the year 1812 when Meckel translated Wolff's book on the Evolution of the Intestinal Canal into German, and called attention to its great importance, the eyes of anato- mists and physiologists were for the first time suddenly opened, and a great number of Biologists soon after under- took new embryological investigations, following out and corroborating Wolff's theory step by step.
This revival of Ontogeny, and the first confirmation and further development of the only true theory of Epigenesis, started from the university of Wiirzburg. The distinguished biologist, Db'llinger, was then lecturing there. He was the father of the famous theologian of Munich, who has done such good service in our day by his opposition to the new dogma of papal infallibility. Dollinger was both a thought- ful natural philosopher, and an accurate biological observer. He felt the greatest interest in the History of Evolution, and was much occupied with it. Yet he himself was unable to produce any very important work in this department, from want of means. But in the year 1816, a young doctor of medicine, who had just graduated, and whom we shall soon learn to know as the most important follower of Wolff, came to Wiirzburg. This was Karl Ernst Baer. His con- versations with Dollinger on the History of Evolution resulted in a renewal of the investigations. Dollinger ex- pressed a wish that, under his direction, some young naturalist should undertake a series of independent re- searches into the evolution of the Chick during the hatching of the egg. But neither he nor Baer possessed the con>
DOLLINGER, BAER, AND PANDER. 51
siderable pecuniary means then necessary to provide a hatching-apparatus, such as would afford uninterrupted observations of the process, or to pay a skilled artist t\> depict in a reliable form the successive stages of develop- ment. They, therefore, confided the execution of the plar. to Christian Pander, a wealthy, early friend of Baer's, by whom he had been induced to come to Wurzburg. Dalton, a skilful artist, was engaged to prepare the necessary copper- plates.
Thus was formed, as Baer says, " that combination, ever memorable in the history of science, in which a veteran, grown gray in physiological researches (Dollinger), a youth glowing with zeal for science (Pander), and an artist without a peer (Dalton), united their powers to lay a firm foundation for the History of the Evolution of the Animal Organism." In a short time the history of the evolution of the Chick, in which Baer took, though Indirectly, a most active part, was so far advanced that Pander, in his dissertation 20 for the degree of doctor, published in 1817, was able to give the first complete sketch of the history of the evolution of the Chick on the basis of Wolffs theory. He was able to define clearly Wolff's Theory of Germ-leaves, and to prove from observation the evolution of the complex system of organs from simple leaf-shaped primitive organs, as anti- cipated by Wolff. According to Pander, the leaf-shaped germinal appendage of the hen's egg separates before the twelfth hour of incubation into two distinct layers — an outer serous layer, and an inner mucous layer. Between the two, a third, vascular layer, subsequently developa
Baer, who was one of those most active in inducing Pander to make his investigations, and who retained the
52 THE EVOLUTION OF MAN.
liveliest interest in them after his departure from Wurzburg, began his own much more comprehensive researches in ] 819, and nine years later published, as the fruit of these i esearches, a work on " The History of the Evolution of Animals," which even now is generally and rightly con- sidered the most important and valuable contribution to embryological literature. This book, a true model of careful, experimental investigation, combined with ingenious philo- sophical speculation, appeared in two parts ; the first in the year 1828, the second in 1837.21 It is the firm founda- tion on which the whole history of the evolution of the individual rests to this day, and so far surpasses its pre- decessors, including Pander's outline, that, next to the labours of Wolff, it must be regarded as the most important basis of modern Ontogeny. As Baer, who died at Dorpat in November, 1876, was one of the greatest naturalists of our century, and has exerted a most important influence on other branches of Biology also, it may be of interest to give some account of the life of this extraordinary man.
Karl Ernst Baer was born in 1792, in Esthonia, on the little estate of Piep, which his father owned. He studied at Dorpat from 1810 to 1814, and then went to Wurzburg, where Dollinger not only initiated him into Comparative Anatomy and Ontogeny, but also exercised over him, by his own interest in philosophical studies, a highly stimu- lating influence. From Wiirzburg Baer went to Berlin, and then, accepting a call from the physiologist Burdach, to Konigsberg. There he delivered lectures on Zoology and Evolution, with some interruptions, until 1834, and com- pleted his most important works. In 1834 he went to St Petersburg as a member of the Academy of that place
BAERS WORK. 53
There, however, he forsook almost entirely his former field of labour, and occupied himself with researches of a totally different nature, in various branches of Natural Science, especially in Geography, Geology, Ethnography, and Anthro- pology. His works on the History of the Evolution of Animals are far the most important; nearly all of these were completed while he was in Kbnigsberg, though some of them were not published until later. Their merits, like those of Wolffs writings, are many-sided, and extend over the whole domain of Ontogeny in very various directions.
Baer especially perfected the fundamental Theory of Germ-layers, as a whole as well as in detail, so clearly and completely, that his idea of it yet forms the safest basis of our knowledge of Ontogeny. He showed that in Man and the other Mammals, as in the Chick — in short, as in all Ver- tebrates— first two, and then four germ-layers are formed, always in the same manner, and that the modification of these into tubes gives rise to the first fundamental organs of the body. According to Baer, the first rudiment of the body of a Vertebrate, as it appears on the globular yelk of the fertilized egg, is an oblong disc, which firtst separates into two leaves or layers. From the upper or animal layer evolve all the organs which produce the phenomena of animal life : the functions of sensation, of motion, and the covering of the body. From the lower or vegetative layer proceed all the organs which bring about the growth of the body : the vital functions of nutrition, digestion, blood- making, breathing, secretion, reproduction, and the like. Each of these two original germ-layers separates again into two thinner layers, or lamellae, one lying above the other. First, the animal layer separates into two, which Baer calls
54 THE EVOLUTION OF MAN.
the skin, or dermal layer, and the flesh, or muscular layei. From the uppermost of these two lamellae, the skin-layer, are formed the outer skin, the covering of the body, and the central nervous system, the spinal cord, the brain, and the organs of sensation. From the lower, or flesh-layer, the muscles, or fleshy parts, the internal or bony skeleton, — in short, the organs of motion, arise. Secondly, the lower, or vegetative germ-layer, parts in the same way into two lamellae, which Eaer distinguishes as the vascular and the mucous layer. From the outer of the two, the vascular layer, proceed the heart and the blood-vessels, the spleen, and the other so-called blood-vessel glands, the kidneys, and the sexual glands. Finally, from the lowest, and fourth or mucous layer, arises the inner alimentary membrane of the intestinal canal, with all its appendages, liver, lungs, salivary glands. Baer traced the transformation of these four secondary germ-layers into tube-shaped fundamental organs as ingeniously as he had successfully determined their import and their formation in pairs by the segmen- tation of the two primary germ-layers. He was the first to solve the difficult problem as to the process by which the entirely different body of the vertebrate develops from this flat, leaf-shaped, four-layered original germ ; the process was the transformation of the layers into tubes.
In accordance with certain laws of growth, the flat layers bend, and become arched ; the edges grow towards each other so that the distance between them is continually decreased ; finally they unite at the point of contact. By this process the flat intestinal layer changes into a hollow intestinal tube ; the flat spinal layer becomes a hol]o\v spinal tube, the skin-layer becomes a skin-tube, etc.
DISCOVERY OF THE HUMAN EGG. 55
Among the many and great services which Baer ren- dered in detail to Ontogeny, especially to that of Vertebrates, his discovery of the human egg must be especially men- tioned here. Most, even of the earlier naturalists, had assumed that man proceeds, like other animals, from an egg. The Theory of Evolution (pre-formation) had, more- over, assumed that all past, present, and future generations of the human race existed encased in the ova of Eve, the common mother. Yet the ova of Man and other Mammals were not actually known till the year 1827. For the egg is exceedingly small, a spherical vesicle or bladder of only one-tenth of a line in diameter, which can be seen with the naked eye only under very favourable circumstances. This spherical vesicle, when in the ovary of the mother, is en- closed in a number of peculiar spherical vesicles of much larger size, called Graafian follicles, after their discoverer Graaf, and these were formerly universally regarded as the actual eggs. It was not until the year 1827 — not fifty years ago — that Baer proved that these Graafian follicles are not the actual eggs, which are much smaller, and only imbedded in the Graafian follicles. (Of. end of Chapter XXV.)
Baer was also the first to observe the so-called germinal vesicle of Mammals, that is, the little spherical bladder which is first developed from the impregnated egg, and the (Lin wall of which consists of a single layer of uniform polygonal cells. (See Chapter VIII.) Another discovery of Baer's, of great importance in understanding the types of the lineage of the Vertebrates, and the characteristic organization of this group of animals in which Man is included, was that of the Chorda Dorsalis. This is a long, thin, cylindrical cartilaginous cord, which in all Vertebrates
56 THE EVOLUTION OF MAN.
passes lengthwise through the whole body of the embryo. It is developed at a very early stage, and is the first form?,- tion of the spine, the firm axis of Vertebrates. In the Lancelet (Amphioxus), the lowest of all Vertebrates, the entire inner skeleton is limited to this Chorda throughout life. But in Man and all the higher Vertebrates, first the spine, and later the skull, are developed round this cord.
Important as these and many other discoveries of Baer's were in the Ontogeny of Vertebrates, yet the great im- portance of his researches rested especially on the fact that he was the first to apply the comparative method to the study of the evolution. It was, of course, the Ontogeny of Vertebrates, and principally of Birds and Fishes, that Baer first and especially investigated. Yet he by no means limited himself to these ; for he included various Inverte- brates in his investigations. The most general result of these comparative embryological researches was that Baer assumed four totally different courses of evolution for the four principal groups of the animal kingdom. These four chief groups, or types, which at that time had just begun to be distinguished, in consequence of George Cuvier's researches in Comparative Anatomy, are : (1) Vertebrates (Vertebrata) ; (2) Articulated animals (Arthropoda) ; (3) Soft-bodied animals (Mollusca) ; and (4) the lower animals, which at that time were all erroneously grouped under the term Radiata. Cuvier, in the year 1816, demonstrated for the first time that these four groups of the animal kingdom show very essential and typical distinctions in their whole inner structure, and in the arrangement and position of the organic systems; that, on the other hand, the internal structure of all animals of one type, for example, of all Ver-
FOUR TYFES OF DEVELOPMENT. 57
fcebrates, is essentially similar, notwithstanding the great variety of outward forms. Baer, however, independently and almost simultaneously, furnished proof that the four groups develop from the egg by entirely different processes, and further, that the order of the series of embryonic forms in the course of evolution is from the very beginning identical in all animals of the same type, but, on the other hand, different- in those of different types. Up to that time, in making a classification of the animal kingdom, an endeavour had always been made to arrange all animals, from the lowest to the highest, from the infusoria to man, in a single connected series of forms; and the false idea had always been maintained, that there was a single unbroken gradation of development from the lowest animal to the highest. Cuvier and Baer proved that this conception is totally erroneous, — and that, on the contrary, there are four wholly distinct types of animals, which must be distin- guished not only as to their anatomical structure, but also as to their embryonic evolution.
As a result of this discovery, Baer succeeded in estab- lishing a very important law, which we shall name in his honour Baer's Law, and which he expresses as follows : " The evolution of an individual of a certain animal form is determined by two conditions : firstly, by a continuous perfection of the animal body by means of an increasing histological and morphological differentiation, or an increas- ing number and diversity of tissues and organic forms; secondly, and at the same time, by the continual transition from a more general form of the type to one more specific." The degree of perfection of the animal body depends on the greater or less amount of heterogeneity there is in its
58 THE EVOLUTION OF MAN.
elementary parts, and in the segments of its composite organs, — in a word, in the degree of histological and mor- phological differentiation. The type, on the other hand, is the order of the arrangement of the organic elements and of the organs. The type is quite distinct from the degree of perfection; the same type may exist in several degrees of perfection ; and, conversely, the same grade of perfection may be reached in several types. This explains the phe- nomenon that the most perfect animals of any type, — for example, the highest Arthropods and Molluscs, — are much more perfectly organized, or more highly differentiated, than the most imperfect animals of other types, — for ex- ample, than the lowest Vertebrates and Star-animals.
Baer's Law has been of the greatest importance in advancing our knowledge of animal organization ; though it was not until a later period that Darwin enabled us to perceive and value its real significance. Here we may at once remark that it can only be really understood by means of the Theory of Descent, by the recognition of the very important part played by Heredity and Adaptation in the construction of organic form. As I have shown in my Generclle Morphologie (vol. ii. p. 10), the type of evolution is the mechanical result of Heredity; the degree of perfection is the mechanical result of Adaptation. Heredity and Adaptation are the mechanical factors in the production of organic forms, which were first brought to bear on Ontogeny by Darwin's Theory of Selection, and which have enabled us for the first time to understand Baer's Law.
Baer's labours marked the beginning of a new epoch, and aroused an extraordinary interest in embryological
FRESH IMPETUS GIVEN TO ONTOGENY. 59
research throughout a very wide circle. We find, therefore, that a large number of investigators occupied the newly found field of research, and, with praiseworthy industry, made a great number of distinct new facts in a short time. The majority of these new embryologists are industrious specialists, who have been very useful in collecting fresh materials, but who have, as a rule, done but little to ad- vance the general problem of the History of Germs. I can, therefore, limit myself to the mention of a few names. Of special importance are the investigations of Heinrich Rathke, of Konigsberg (died 1861), who did much to advance the History of the Evolution of Invertebrates (Crabs, In- sects, Molluscs), as well as of Vertebrates (Fishes, Turtles, Snakes, Crocodiles). In the subject of the Embryology of Mammals, the widest conclusions are due to the careful experiments of Wilhelm Bischoff, of Munich. His History of the Evolution of the Rabbit (1840), of the Dog (1842), of the Guinea-Pig (1852), and of the Roe-Deer (1854), are as yet the most important basis of study in this department. Among the numerous works on the History of the Evolution of Invertebrates, those of the well-known zoologist, Johannes Miiller, of Berlin, on Star-animals (Echinoderma), are espe- cially noteworthy ; also those of Albert Kolliker, of Wiirz- burg, on Cuttle-fishes (Cephalopoda); of Siebold and Huxley, on Worms and Plant-animals ; of Fritz Miiller (Desterro), on the Crustacea ; of Weismann, on Insects, etc. The number of labourers in this field has of late greatly increased, although not very much of special importance has been accomplished. It is evident, from the majority of recent publications on Ontogeny, that their authors are not familiar enough with Comparative Anatomy. The most important of the latest
6O THE EVOLUTION OF MAN.
ontogenetic works are those of Kowalevsky, E. Ray Lan- kester, and Eduard van Beneden, to which we shall presently again refer.22
A more decided advance in general knowledge than waa effected by all these separate investigations, dates from the year 1838, when the proof of the Cellular Theory suddenly opened a new field of research in the History of Evolution. The distinguished botanist, Schleiden, of Jena, having proved by means of the microscope that -every vegetable body is composed of innumerable elementary parts, the so- called cells, Theodor Sch wann, of Berlin, a pupil of Johannes Miiller, applied this discovery directly to the animal body.23 He showed, that not only in plants, but also in the bodies of the most dissimilar animals, these same cells are dis- tinguishable, under the microscope, in all the tissues, and that they form the actual building material of organisms. .All the numerous tissues of the animal body, such as the entirely dissimilar tissues of the nerves, muscles, bones, outer skin, mucous skin, and of other similar parts, are originally composed of cells; and the same is true of all the various tissues of the vegetable body. These cells, which we shall hereafter consider more closely, are inde- pendent living beings, the citizens of the state, which con- stitute the entire multi-cellular organism. The knowledge of this most important fact was, of course, of direct service to the History of Evolution also, in that it raised many new questions, chiefly the.se : What relation have the cells to the germ-layers ? Are the germ -layers already com- posed of cells, and how are they related to the cells of the tissues which afterwards appear ? What place does tlie egg hold in the Cell Theory ? Is it itself a cell, or is it
THE CELL THEORY. 6l
composed of cells ? These were the important questions which the Cell Theory at once raised in the study of Em- bryology.
Several naturalists attempted in different ways to furnish the right answers, but the excellent " Investigations into the Evolution of Vertebrates," by Robert Remak, of Berlin (1851), became conclusive. By somewhat remoulding the Cellular Theory of Schleiden and Schwann, this gifted naturalist was able to overcome the great obstacles which this theory, in its first form, had placed in the way of Embryology. It is true that the anatomist, Karl Boguslaus Reichert, of Berlin, had previously attempted to explain the origin of the tissues. But this attempt was necessarily a total failure, owing to the fact that the extraordinarily confused mind of the author was equally destitute of every correct idea of the History of Evolution, of the Cellular Theory as a whole, and of a sound view of the structure and development of tissues in particular. The inaccuracy of Reichert's observations, and the falsity of the conclusions drawn from them, is shown by every accurate test applied to his so-called discoveries. By way of illustration, it may be said that he declared the whole of the upper germ-layer, from which the most important parts of the body — brain, spinal cord, outer skin, and the like — proceed, to be merely a transient " enveloping-skin " of the embryo, and that it had nothing to do with the formation of the body ; that many of the first formations of the separate organs did not proceed from the primary germ-layers, but came one by one from the yelk of the egg, and joined the layers after- ward. Reichert's preposterous embryological labours suc- ceeded in gaining a passing attention, only because they
62 THE EVOLUTION OF MAN.
were put forward with unusual presumption, and professed to disprove Baer's Theory of Germ-layers. They are written in so clumsy and confused a style, that no one could quite understand them ; but for this very reason they won the admiration of many readers, who supposed that a nucleus of profound wisdom was hidden somewhere be- hind these obscure oracular and mysterious sayings.
Remak was the first to throw full light on the great conftbdon which Reichert had caused, by explaining, in the simplest possible manner, the evolution of the tissues. Ac- cording to him, the egg of animals is always a simple cell, and the germ-layers, which proceed from the egg, are also composed only of cells, and those cells, which alone constitute the egg, are produced in a very simple manner by the continuous and repeated segmentation or dividing up of the original simple egg-cell. This cell divides, or parts, first into two, and then into four; from these four arise eight, then sixteen, and then thirty-two, and so on. Hence, in the individual evolution of every animal, as well as of every plant, from the one simple cell, constituting the egg, is formed, by repeated segmentation, an aggregate of cells, as Kolliker had already maintained in 1844. The cells of such a mass spread themselves out flatly, and so form into layers, so that every one of these layers is originally composed of but one kind of cell. The cells of the layers differentiate themselves, or assume various forms ; and then there is a further differentiation, or, in other words, a division of labour of the cells within the layers themselves, and this latter differentiation produces all the various tissues of the body.
These are the very simple principles of Histogeny, 01
REMAK. 63
the Science of the Evolution of Tissues, as first elaborated by Remak and by Kblliker in this comprehensive sense. By thus proving more definitely the part which the germ- layers take in the formation of the various tissues and systems of organs, and applying the Theory of Epigenesis to the cells and the tissues formed from them, Remak raised the Germ-layer Theory, at least as regards Vertebrates, to that degree of perfection in which we shall find it hereafter when we examine it in detail. According to him, the two germ-layers, of which the so-called germinal disc, the first simple leaf-shaped formation of the body of a Vertebrate, is composed, are soon increased by another layer, produced by the lower layer separating into two. These three have, entirely distinct relations to the various tissues. First, from the upper layer proceed those cells which compose the outer skin (epidermis) of the body, together with the parts belonging and necessary to it (hair, nails, and the like) — that is, the external covering which envelops the whole body ; and, remarkable as it is, it produces also the cells which constitute the central nervous system, — the brain and spinal marrow. Secondly, from the lower germ-layer spring the cells which form the intestinal epithelium, — that is the whole inner coating of the intestinal canal and its append- ages (liver, lungs, salivary glands, and the like) ; in other words, the tissues which take up the food of the animal body and attend to its digestion. Finally, from the middle layer, lying between these two, arise all the other tissues of the body of the Vertebrate; flesh and blood, bones and liga- ments, and the like. Remak also proved that the middle layer, which he calls the " motor-germinative " layer, again separates, secondarily, into two layers. In this way we get
64 THE EVOLUTION OF MAN.
the same four layers which Baer had previously assumed. The upper part of the middle layer after its cleavage (Baer's "Flesh-stratum"), Remak calls the skin-lamella (Hautplatte, or better, Hautfaserplatte); it forms the outer wall of the body (the true skin, cutis vena, the muscles, bones, and the like). The lower part (Baer's "Vascular stratum"), he calls the intestinal-fibrous lamella (Darm- faserplatte) ; it forms the outer covering of the intestinal canal, and of the heart, the blood-vessels, and so on.
Based on the firm foundation which Remak thus supplied to the History of the Evolution of the Tissues, or the science called Histogeny, numerous investigations of special points which have considerably extended our information have been made. Of course many attempts have been made to give much narrower limitations to Remak's doctrines, or to remodel them altogether. Reichert, of Berlin, and Wil- helm His, of Leipsic, have specially busied themselves to establish, in comprehensive works, an entirely new view of the evolution of the body of Vertebrates, according to which the rudiments of the body of the Vertebrate does not consist solely of the two primary germ-layers. But these works, owing to their total lack of the necessary knowledge of Comparative Anatomy, and clear knowledge of Ontogeny, and to the fact that they do not even glance at Phylogeny, could exert but a very transient influence. Only the total want of critical ability and comprehension of the real problems of the History of Evolution can explain the fact that many people for a time regarded the strange fancies of Reichert and His as a great gain.
His, in 1868, in a large book, on " The Eaily Evolution of the Chick in the Egg," detailed his entirely erroneous
HIS AND GOETTK. 6$
views in a very learned form, and under the banner of a new and very exact mathematical and physical method, he has recently expressed the same views in a general form in his book on " Our Body and the Physiological Problem of its Origin" (Leipsic, 1875). As His, in order to increase the circulation of the latter book, has allowed it to be publicly advertised as " important to readers of Haeckel's Anthropogenic," I shall only remark that my treatise on " The Aims and Methods of the History of Evolution " (Jena, 1875) frees me from the necessity of further answer. To the most important points in his false theories I shall refer again. (See Chapter XXIV.)
Quite recently, however, His and Reichert's books on Ontogeny, which had previously ranked as the most per- verted and unfortunate of the larger works on this science, have been far eclipsed, in that respect, by a ponderous work by Alexander Goette, of Strasburg, on the " History of the Evolution of Bombinator igneus, as the Basis of a Com- parative Morphology of Vertebrates " (Leipsic, 1875). This monograph is the biggest existing contribution to the literature of Ontogeny — a thick volume of 964 pages, ac- companied by a very beautiful folio atlas of 22 plates. These splendid plates, containing as many as 382 accurate and very carefully executed drawings, representing the history of the development of the Bombinator, are the result of years of incessant labour, and excite a most favourable interest in the huge work. Unfortunately, however, the reader who is induced by this splendid picture-book to expect a corresponding degree of excellence in the voluminous text, will be sadly disappointed. Not only is the whole account most obscure, confused, and
66 THE EVOLUTION OF MAN.
contradictory, but, further, the entire treatment shows that, by his whole scientific education, the author is incapable of the heavy task. I should not pronounce this harsh judgment, but that Goette natters himself that, as the reformer of the science, he is about to place it on an entirely new basis ; and but that, consequently, he treats the great leaders of the science — Baer, Remak, Gegenbaur, and others — in the most insolent manner, as narrow-minded labourers who, " by reason of their lack of knowledge of the history of evolution, have missed their aim." The following samples seem to show the mode in which the new science is constituted by Goette : " Perfect life renders evolution impossible. The capacity of evolution in the mature egg excludes real life. Egg-cleavage is not a living process of evolution. The egg neither as a whole nor as to its parts, neither in its origin nor in its complete state, is a cell. The cells of the various tissues are not organisms, are not organic individuals. The individuality of an organism is only a peculiar expression of the end of its evolution," and so on.
In these and many other statements Goette abruptly upsets the whole science, as at present constituted. The Cell Theory and the Protoplasmic Theory are rejected as worthless ; even Comparative Anatomy is, according to this writer, of no scientific value ; Phylogeny is no science, and so on. I have explained the most incredible of Goette's assertions and his most unexampled errors in my work on " The Aims and Methods of the History of Evolution " (Leipsic, 1875) ; in which book I have also criticized the views held by His and Agassiz. Errors of this sort are no longer possible in other sciences. Their occurrence in the History of Evolution is explained partly by the great
HUXLEY AND KOWALEVSKY. 67
difficulty of the very complex task which lies before this science, and partly by the insufficient general preparation possessed by most of the more recent students.
All valuable modern investigations into Animal Onto- genesis have only tended to confirm and add to the Theory of Germ-layers as established by Baer and Remak. As the most important advance made in this direction, it is deserv- ing of mention, that the same two primary germ-layers, from which the body of Vertebrates, including Man, develops, have recently been shown to exist in all inver- tebrate animals also, with the single exception of the lowest group, that of the Primaeval animals (Protozoa.} The dis- tinguished English naturalist, Huxley, in the year 1849. had already shown that this is also true of Plant-animals (Meduscs). He drew attention to the fact that the two cell-layers, from which the body of this Plant-animal develops, correspond, morphologically as well as physio- logically, to the two primary germ -layers of Vertebrates. The upper germ-layer, from which the outer skin and the flesh proceed, he named Ectoderm, or Outer layer ; the lower, which forms the organs of digestion and reproduc- tion, he called the Entoderm, or Inner layer. But during the past ten years, the two germ-layers have been found to exist among many other Invertebrates, The indefatigable Russian zoologist, Kowalevsky, has found them among widely differing groups of Invertebrates, in Worms, Star-animals (Echinoderma), Soft-bodied animals (Mollusca), Articulates (Arthropoda), and the like.
In my Monograph on the Calcareous Sponges, which appeared in 1872, 1 have shown that this same pair of primary germ-layers forms the basis of the body of the Sponges, and
68 THE EVOLUTION OF MAN.
that they are to be regarded as occupying the same relative place, or as being homologous, throughout all the various classes of animals, from the Sponges to Man. This homology of the two primary germ-layers, which is of extraordinary significance, extends throughout the animal kingdom, with only a few exceptions in the lowest class, the Primaeval animals (Protozoa). These animals are of an exceedingly low organization, and do not advance to the stage of form- ing germ-layers, and consequently never form real tissues. The whole body merely consists, either of a single cell, as in Amrebae and Infusoria, or of a loose mass of but slightly differentiated cells, or, as in Monera, it does not even attain a form as high as that of a cell. But from the egg-cell of all other animals two primitive germ-layers first proceed, the outer, animal layer (Ectoderm or Exoderm), and the inner, or vegetative layer (Entoderm), and from these the various tissues and organs arise. This is equally true of Sponges, of the other Plant-animals, and of Worms ; it is as true of Soft-bodied animals (Molluscd), Star-animals (Echin- oderma), and Articulates (Arthropoda), as of Vertebrates. All these animals may be comprised under the head of Intestinal Animals (Metazoa), in distinction from the Primaeval Animals (Protozoa), which have no intestine.
It is perhaps more correct not to place the Protozoa among the true animals at all, but to class them in the neutral kingdom of the Protista, those humblest primaeval beings which are neither true animals nor true plants. According to this view the Metazoa can alone be considered as true animals, and the origin from two primary germ- layers may be held to form the primary character of tho animal kingdom.
DARWIN. 69
In the lowest forms of Metazoa, the body consists throughout life of these two primary germ-layers. But in all higher Intestinal Animals, each of these forms by cleavage two other layers, so that the body is thenceforward composed of four secondary germ-layers. In my " Gastraea Theory " (1873), I have tried to show the general homology of these four layers in all Metazoa, and I have pointed out the important bearing of this fact on the natural system of the animal kingdom.24
But though the most important facts in the individual evolution of the human and animal body had been suffi- ciently established by these advances in Animal Ontogeny, yet the most difficult task remained, — namely, the discovery of 'the causes by which the evolution of organisms and the production of their forms is effected. The real mechanical causes of individual evolution were first explained in 1859, in Darwin's work, in which the facts of Heredity and Adaptation were for the first time scientifically discussed, and their bearing on Ontogeny correctly interpreted. Only by the Theory of Descent, and by the aid of the laws of Heredity and Adaptation, are we really able to understand the facts of individual evolution, and to explain them by efficient causes. This is the point in which the Darwinian Theory is so important to the History of the Evolution of Man and to the immediate connection of the first part of our science, Germ-history, or Ontogeny, with the second part, Tribal-history, or Phylogeny.
CHAPTER IV. THE EARLIER HISTORY OF PHYLOGENY.
y
JEAN LAMARCK.
Phylogeny before Darwin. — Origin of Species. — Karl Linnaeus* Idea of Species, and Assent to Moses' Biblical History of Creation.— The Deluge.— Palaeontology.— George Clavier's Theory of Catastrophes. — Eepeated Terrestrial Revolutions, and New Creations.— Lyell's Theory of Continuity.— The Natural Causes of the Constant Modification of the Earth. — Supernatural Origin of Organisms. — Immanuel Kane's Dualistic Philosophy of Nature. — Jean Lamarck. — Monistic Philosophy of Nature. — The Story of his Life.— His Philosophie Zoologique.— First Scientific Statement of the Doctrine of Descent. — Modification of Organs by Practice and Habit, in Conjunction with Heredity. — Applica- tion of the Theory to Man.— Descent of Man from the Ape.— Wolfgang Goethe.— His Studies in Natural Science.— His Morphology. — His Studies of the "Formation and Transformation of Organisms." — Goethe's Theory of the Tendency to Specific Differences (Heredity and of Metamorphosis (Adaptation).
*' It wonld be an easy task to show that the characteristics in the organi. ration of man, on account of which the human species and races are grouped as a distinct family, are all results of former changes of occu- pation, and of acquired habits, which have come to be distinctive of indi- viduals of his kind. When, compelled by circumstances, the most highly developed apes accustomed themselves to walking erect, they gained the ascendant over the other animala. The absolute advantage they
NEW ERA BEGUN BY DARWIN. 7 1
tmjoyed, and the new requirements imposed on them, made them change their mode of life, which resulted in the gradual modification of their organization, and in their acquiring many new qualities, and among them the wonderful power of speech." — JEAN LAMAECK (1809).
THOSE researches into the history of the individual evolution of man and animals, the history of which we surveyed in the last two chapters, had until recently hardly any other object than that of practically determining the changes of form undergone by the organism in the course of its growth. But until within the past fifteen years, no one dared to seek for the causes of these phenomena. During the entire century, from the year 1759, the date of the publication of Wolffs Theoria Generations, until the year 1859, when Darwin published his " Origin of Species," the causes of the evolution of the germ remained entirely hidden. During the whole century nobody thought of seriously ex- amining the real causes of the changes of form which take place in the evolution of the animal organism. Indeed, the task was looked upon as so difficult that it entirely surpassed the powers of human comprehension. It was reserved for Charles Darwin to declare all these causes. We may therefore point to this gifted naturalist, who, in other respects, has effected a complete revolution throughout the whole range of Biology, as the founder of a new era in the field of Ontogeny also. It is true that Darwin himself has not really entered very deeply into embryological investigations, and even in his well-known chief work on the phenomena of individual evolution has but casually touched upon these, yet, by his reform of the Theory of Descent, and by constructing what he has named the Theory of Selection, he has placed in our hands the
72 THE EVOLUTION OF MAN.
means of tracing the causes of the Evolution of Forms. It seems to me that it is in this respect that this great naturalist has had such an extraordinary effect on the entire subject of the History of Evolution.
In glancing, as we must now do, at the last period, but just begun, of ontogenetic research, we enter at the same time into the second division of the History of Evolu- tion, namely, the History of the Descent, or Tribe (Phylogeny). In the first chapter I drew attention to the exceedingly important and intimate causal connec- tion which exists between these two main branches of the History of Evolution, — that of the individual, and that of his ancestors. We stated this connection in the funda- mental Law of Biogeny : the brief Ontogeny, or the Evolution of the Individual, is a swift and contracted reproduction, a compressed recapitulation, of the Phylogeny, or the Evolution of the Species. This proposition in reality comprises everything essentially relating to the causes of evolution, and we shall try everywhere, in the course of these chapters, to establish it, and to uphold its truth, by adducing actual facts in proof. The meaning of this fundamental Law of Biogeny, in relation to this causal significance, is perhaps yet better expressed as follows : " The evolution of the species, or tribes (phyla), contains, in the functions of heredity and adaptation, the determin- ing cause of the evolution of individual organisms ; " or, quite briefly : '" Phylogeny is the mechanical cause of Ontogeny."
It is owing to Darwin that we are now able to trace the causes of individual evolution, which were previously deemed quite unapproachable, and to understand their real
LINN^US' "SYSTEMA NATURE." 73
nature ; we therefore give his name to the new era of the History of Evolution. But before considering the grand discovery by means of which Darwin enabled us to under- f land the causes of evolution, we must glance rapidly at the efforts made by earlier naturalists in the same direction. The historical survey of these endeavours will be much shorter even than that of the labours in the field of On- togeny. There are really but few names to be mentioned. At the head stands the great French naturalist, Jean Lamarck, who, in 1809, for the first time gave a scientific value to the so-called Theory of Descent. But even before this, the most important German philosopher, Kant, and the greatest German poet, Goethe, had both entertained the idea. During the previous half-century, however, their statements on this matter remained almost unnoticed. It was only in the commencement of our century that "Natural Philosophy " took up the question. Previously no one even dared to inquire seriously into the Origin of Species, which, properly speaking, is the culminating point of the History of Descent, or Phylogeny.
The entire Phylogeny of Man, and also of other animals, is most intimately connected with the question as to the nature of species, and with the problem, how the distinct kinds of animals, which in systems are called species, really originated. The idea of species occupies the foreground. This idea was first presented by Linnaeus, who, in 1735, in iis Sy sterna Naturce, attempted the first accurate dis- crimination and nomenclature of animal and vegetable species, and made a systematic list of the species then known. Since that time species has retained its place in descriptive Natural History, in systematic Zoology and
74 THE EVOLUTION OF MAN.
Botany, as the most important collective term, although incessant strife has been waged as to the particular meaning of the term. Linnaeus himself gave no clear, scientific defi- nition of the real nature of organic kind, or species. On the contrary, he took as a basis the mythological views of this subject, which the prevailing religious " faith," grounded on the Mosaic History of Creation, had introduced, and which are even now very generally maintained. He even adhered directly to the Mosaic History of Creation, and assumed that, as it is written in Genesis " male and female created he them," there had originally been but one pair of each animal and vegetable kind, or species. He supposed that all the individuals of a kind were descendants of the original pair created on the sixth day of Creation. Lin- naeus held that only a single individual was created of those organisms which are hermaphrodite, that is, which unite in their bodies both sexual organs, for these already possessed in themselves the qualifications for propagating their own species. In further developing these mytho- logical ideas, Linnaeus adhered to the Mosaic account and utilized the so-called " Deluge," and the myth of the ark of Noah connected with it, to explain the choiology of organisms, the doctrine, that is, of the geographical and topographical distribution of animal and vegetable species. In harmony with Moses he assumed that all plants, animals, and human beings had been destroyed by the Deluge, with the exception of a single pair, which was saved in the ark to perpetuate the species, and which was put on land on Mount Ararat after the waters had subsided. Mount Ararat seemed to him specially adapted for this disembark- ation, because it is in a warm climate and rises to a height
CUVIER'S SYSTEM. ^5
of more than sixteen thousand feet, so that in its several zones of elevation it possessed all the climates necessary for the preservation of the various species of animals. The animals used to a cold climate could climb to the highest parts of the mountain ; those accustomed to a warm climate could descend to the foot ; and those from temperate zones could occupy the intermediate portions. From this moun- tain the animal and vegetable species could spread anew over the face of the earth.'25
A scientific development of the History of Creation was impossible in the time of Linnaeus, because, among other reasons, the science of petrifactions, or Palaeontology, one of its principal bases, did not as yet exist. This science of petrifactions, or of the remains of extinct animals and plants, is most intimately connected with the whole History of Creation. Without reference to it, it is impos- sible to answer the question as to the manner in which the animals and plants now in existence came into being. But the knowledge of these petrifactions arose in much later times, and the real founder of Palaeontology, as a science, was the eminent zoologist, George Cuvier, who followed Linnaeus in constructing a System of Animals, and who, in the beginning of this century, brought about a com- plete reform of Systematic Zoology. The influence of this celebrated naturalist, who displayed an especially great power with extraordinary results during the first thirty years of this century, was so great that he opened new paths in almost every branch of Zoology, but especially in Classification, Comparative Anatomy, and Palaeontology. It is, therefore, important to glance at his views of the nature of species. In this respect he followed Linnaeus and
76 THE EVOLUTION OF MAN.
the Mosaic account of Creation, though it was very difficult for him to do so, on account of the knowledge which he had of fossil animal forms. He was the first to show clearly that a number of totally different series of inhabitants had lived on our globe. He also showed that we must dis- tinguish at least ten or fifteen different main periods in the history of the earth, each of which exhibits a series of animals and plants of its own, peculiar to itself.
Of course, Cuvier was at once confronted with the ques- tion, whence these various series of inhabitants had come, and whether they had any connection with each other. He answered this question negatively, and maintained that these several " creations " were totally independent of each other; hence, that the supernatural act of creation by which, according to the received account of creation, the animal and vegetable species came into being, was repeated several times. Consequently, a series of quite distinct periods of creation must have followed one another, and in connection with them there must have occurred several vast alterations of thewhole surface of the earth, — revolutions and cataclysms similar to the mythical Flood. These catastrophes and upheavals were favourite subjects with Cuvier ; especially as at that time the science of geology was also beginning to move greatly, and made rapid progress towards a know- ledge of the structure and origin of the earth. Others, especially the geologist Werner and his school, were occupied in carefully examining the various layers of the crust of the earth, and systematically investigating the fossils found in these. The result of their researches also was the recog- nition of several periods of creation. The inorganic crust of the earth, the stratified surface, bore evidence of having
THEORY OF CATASTROPHES. 77
been just as different at every period as were the animals and plants then inhabiting it. Combining this view with the results of his own palaeontological and zoological researches, and striving to understand clearly the whole course of the evolution of Creation, Cuvier arrived at the hypothesis usually called the Theory of Cataclysms or Catastrophes, or the Doctrine of Violent Upheavals. According to it several revolutions occurred on our earth at certain times, suddenly destroying every living inhabitant ; and at the end of each of these catastrophes an entirely new creation of organisms took place. But as the latter cannot be conceived as having been effected wholly by natural means, we must suppose, in explanation, that the Creator supernaturally interfered in the natural course of things. This Doctrine of Revolutions, treated by Cuvier in a separate work, which has been translated into several modern languages, was soon generally accepted, and for half a century continued to prevail among biologists ; there are even yet a few prominent naturalists who advocate it.
It is true that more than forty years ago Cuvier's Doctrine of Catastrophes was altogether renounced by geologists ; and first of all by the English geologist, Charles Lyell, the most important authority in this branch of natural science. As early as the year 1830, in his famous "Principles of Geology," he proved that that doctrine is utterly false so far as the crust of the earth itself is con- cerned ; and he showed that in order to explain the structure and evolution of mountains, there is no need of having re- course to supernatural causes or universal catastrophes. On the contrary, the ordinary causes which even now unceasingly effect the transformation and reconstruction of the earth, are
78 THE EVOLUTION OF MAN.
amply sufficient to explain these phenomena. These causes are : atmospheric influences ; water in its various forms — such as snow and ice, fog and rain, the running stream and the surging sea; and finally, the volcanic phenomena contributed by the hot liquid mass in the interior of the earth. The most convincing proof was furnished by Lyell, that these natural causes are quite sufficient to explain all the phenomena of the structure and development of the crust of the earth. The geological teaching of Cuvier as to the revolutions and new creations was, therefore, soon totally abandoned, but in Biology the doctrine prevailed unopposed for thirty years longer. Zoologists and botanists, as far as they at all permitted themselves to think on the origin of organisms, adhered to Cuvier's false doctrine of repeated new creations and re-formations of the earth. This is cer- tainly one of the most curious examples of two closely related sciences long pursuing utterly divergent courses. One — Biology — remains far behind in the dualistic path, and even denies the possibility of solving "questions of creation " by the study of natural phenomena. The other — Geology — moves far ahead in the monistic path, and solves those very questions by the discovery of the actual causes.
As an instance how utterly biologists refrained from in- quiries into the origin of organisms, and the creation of the animal and vegetable species, during this period from 1830 to 1859, I mention, from my own experience, the fact that during all the whole course of my studies at the university, I never heard a single word on these most important and fundamental questions of biology. During this time, from 1852 to 1857, 1 had the good fortune to listen to the most distinguished teachers in. all branches of the science' of
CONSERVATISM OF BIOLOGY. 79
organic nature; but not one of them ever spoke of this fundamental point, or even once alluded to the question of the origin of species. Not a word was ever spoken in reference to the earlier attempts toward understanding the origin of the animal and vegetable species; it was never thought worth while to allude to Lamarck's valuable Philosophie Zoologique, in which that attempt had been made in the year 1809. The enormous opposition which Darwin met with when he first took up this question again may, therefore, be understood. His attempt seemed at first to be unsubstantial and unsupported by previous labours. Even in 1859 the entire problem of creation, the whole question of the origin of organisms, was considered by biologists as supernatural and transcendental. Even in speculative philosophy, in which this question should necessarily be approached from various sides, no one dared to take it seriously in hand.
The dualistic position taken by Immanuel Kant, and the extraordinary importance attached, during the whole of this century, to this most influential of modern philosophers, probably offer the best explanation of the last-mentioned fact. For while this great genius, equally excellent as n naturalist and a philosopher, in the field of inorganic nature aided essentially in constructing a "Natural History of Creation," he for the most part adopted the supernatural view of the origin of organisms. On the one hand, Kant, in his " Universal History of Nature and Theory of the Heavens," made a most successful and important "at- tempt to treat the constitution and the mechanical origin of the entire universe according to Newtonian principles," or, in other words, to treat it mechanically, to conceive
80 THE EVOLUTION OF MAN.
it monistically : and this attempt of his to explain the origin of the entire world by means of naturally working causes (causce ejfidentes), forms to this day the basis of all our natural cosmogony. But, on the other hand, Kant maintained that the " principle of the mechanism of nature here applied, without which, after all, there could be no science of nature," was wholly inadequate to explain the phenomena of organic nature, and especially the origin of organisms; that it was necessary to assume supernatural causes effecting a design (causce finales) for the origin of these natural bodies constructed with design. Indeed, he even went so far as to assert that "it is quite certain we cannot become adequately acquainted with organized beings, and their inner possibilities, by purely mechanical principles of nature, much less are we able to explain them ; and that this is so much the case that we may boldly assert that it is not rational for man even to enter upon such speculations, or to expect that a Newton will ever arise who, by natural laws not ordered by design, can render the production of a blade of grass intelligible ; in fact, we are compelled utterly to deny that it is possible for man ever to reach such knowledge." In these words Kant most definitely declared the dualistic and teleological standpoint which he adopted in the science of organic nature.
Kant sometimes, however, departed from this stand- point, especially in some very remarkable passages which I have discussed at some length in the fifth chapter of my " History of Creation," in which he has expressed himself in quite the opposite, or monistic sense. With reference tc these passages, as I there showed, he might even be declared
KANT. 8l
an adherent of the Theory of Descent. Several very sig- nificant expressions, to which Fritz Schultze, in his interest ing work on " Kant and Darwin,26 has lately again called attention, actually enable us to recognize Kant27 as the earliest prophet of Darwinism. He expresses with perfect c3 aarness the great idea of an all-embracing, uniform evolu- tion ; he assumes " a variation from the primitive type of the tribe as the result of natural wandering." He even declares that man originally moved on four feet, and that it was only gradually that the human race raised their heads proudly over those of their old comrades, the beasts. But all these evidently monistic utterances are but stray rays of light ; as a rule Kant adhered in Biology to those obscure dualistic notions according to which the powers which operate in organic nature are entirely different from those which prevail in the inorganic world. This dualistic, or two-sided conception of nature is still dominant in school-philosophy ; most philosophers still consider these two domains of natural phenomena as entirely different. On one side is the field of inorganic nature, the so-called "inanimate" world, where only mechanical laws (causes efficientes) are supposed to operate, of necessity and without purpose. On the other side is the field of " animated " organic nature, all the phenomena of which in their profoundest essence and first origin can be made intelligible only by assuming pre-ordained pur- poses, or so-called (causce finales) causes fulfilling a design.
Although the question of the origin of animal and •vegetable species, and the allied question as to the creation of man, remained until the year 1859 under the sway of these false dualistic prejudices, and were very generally
82 TIIE EVOLUTION OF MAN.
declared to be a subject beyond the reach of scientific knowledge, yet even in the beginning of our century there were independent eminent minds, who, undeterred by the prevailing doctrines, took these questions quite seriously in hand. The so-called earlier school of Natural Philosophy, which has so often been abused, deserves the highest praise in this respect. It was represented in France by Jean Lamarck, Buffon, Geoffroy St. Hilaire, and Ducrotay Blain- ville ; in Germany, by Wolfgang Goethe, Eeinhold Trevi- ranus, Schelling, and Lorenz Oken.
The gifted naturalist and philosopher who must here be mentioned first, is Jean Lamarck. He was born at Bazentin, in Picardy, August 1, 1744, and was the son of a clergyman who destined him for the Church. He, how- ever, first joined the army, and as a boy of sixteen dis- tinguished himself by his bravery in the battle of Lippstadt in Westphalia, which resulted unfavourably for the French. He was then stationed for several years in a. garrison in the south of France. Here he became acquainted with the interesting flora on the Mediterranean coast, which soon won him over to the study of botany. He resigned his commission, and published, as early as the year 1778, his valuable Flore Frangaise. For years he could gain no scientific position. It was only in his fiftieth year, in 1794, that he obtained a poor professorship of zoology at the museum of the Jardin de Plantes in Paris. His position caused him to enter more deeply into the study of zoology, towards the classification of which his labours were as valuable and important as those which he had dedicated to systematic botany. In 1302 he published his Considera- tions swr les corps vivants, which contains the first germs oj
HISTORY OF LAMARCK. 83
his Theory of Descent. In 1809 appeared the important Philosophie Zoologique, the principal work in which he elaborated this theory. In 1815 he gave to the world his comprehensive treatise on the Natural History of Inver- tebrates (Histoire naturelle des animaux sans vertebres\ in the Introduction to which the same theory is again developed. About this time Lamarck entirely lost his eye- sight. Grudging fate never favoured him. While his principal opponent, Cuvier, was lucky enough to gain an influential position and the highest rank of scientific fame in Paris, Lamarck, who far surpassed Cuvier m clear and high-minded conception of nature, was obliged to struggle in lonely seclusion for the very necessaries of life, and could obtain no recognition. In 1829 his laborious life closed in the midst of the most needy circumstances.28
Lamarck's Philosophie Zoologique was the first scientific outline of a real history of the evolution of Species, a natural history of the creation of plants, animals, and men. The effect produced by this remarkable and im- portant book was, like that of Wolff's, none : neither was understood. No naturalist felt called upon to interest him- self seriously in this book, and to forward the development of the rudiments of the most valuable progress in Biology which it laid down, The most eminent botanists and zoologists threw the book entirely aside, and did not con- sider it worth refuting. Cuvier, who taught and laboured in Paris as a contemporary of Lamarck, in his account of the progress made in Natural Science, in which the most insignificant observations were mentioned, did not deem it worth while to devote a syllable to this the greatest advance. In short, Lamarck's Zoological Philosophy shared the fate
84 THE EVOLUTION OF MAN.
of Wolffs Theory of Evolution, and was ignored for half a century. Even Oken and Goethe, the German natural philosophers, who were simultaneously employed in similar speculations, do not appear to have been aware of Lamarck's work. Had they known it, it would have been a great help to them, and they would have worked out the Theory of Evolution to a point beyond that which was otherwise possible to them.
To enable my readers to judge of the great value of the Philosophic Zoologique, I shall here briefly mention some of the most important of Lamarck's ideas. According to him. there is no essential difference between animate and inani- mate nature; all nature is a single world of connected phenomena, and the same causes which form and trans- form inanimate natural bodies are alone those which are at work in animate nature. Hence, we must apply the same methods of investigation and explanation to both. Life is only a physical phenomenon. The conditions of internal and external form of all orgaoisms — plants and animals, with man at their head — are to be explained, like those of minerals and other inanimate natural bodies, only by natural causes (causce efficientes), without the addition of purposive causes (causce finales). The same is true of the origin of the various species. Without contradicting nature, v^e can neither assume for them one original act of crea- tion, nor repeated new creations as implied in Cuvier's Doctrine of Catastrophes, — but only a natural, uninterrupted, and necessary evolution. The entire course of the evolu- tion of the earth and its inhabitants is continuous and connected. All the various species of animals and plants vvluch we now see around us, or which ever existed, have
LAMARCK'S THEORIES. 85
developed in a natural manner from previously existing, different species ; all are descendants of a single ancestral form, or at least of a few common forms. The most ancient ancestral forms must have been very simple organisms of the lowest grade, and must have originated from inorganic matter by means of spontaneous generation. Adaptation through practice and habit, to the changing external condi- tions of life, has ever been the cause of changes in the nature of organic species, and Heredity caused the transmission of these modifications to their descendants.
These are the principal outlines of the theory of Lamarck, now called the Theory of Descent or Transmuta- tion, and to which, fifty years later, attention was again called by Darwin, who firmly supported it with new proofs. Lamarck, therefore, is the real founder of this Theory of Descent or Transmutation, and it is a mistake to attribute its origin to Darwin. Lamarck was the first to formulate the scientific theory of the natural origin of all organisms, including man, and at the same time to draw the two ulti- mate inferences from this theory: firstly, the doctrine of the origin of the most ancient organisms through spon- taneous generation; and secondly, the descent of Man from the Mammal most closely resembling Man — the Ape.
Lamarck attempted to explain the latter process, a most important one, and of special interest to us here, by the same efficient causes to which he had also referred the natural origin of animal and vegetable species. He con sidered that, on the one hand, practice and habit (Adapta- tion), and, on the other, Heredity, are the most important of these causes. The chief modifications of the organs of animals and plants result, according to him, from the
86 THE EVOLUTION OF MAN.
functions or actions of the organs themselves, from the exercise or absence of exercise, the use or disuse of these organs. To mention examples, the Woodpecker and the Humming-bird owe their peculiarly long tongue to their habit of using these organs to take their food out oi narrow and deep crevices; the Frog acquired a web between its toes from the motions of swimming; the Giraffe gained its long neck by stretching it up to the branches of trees. Habits, the use and disuse of organs, are certainly of the greatest importance as efficient causes of organic form ; but they are insufficient to explain the modification of species. As a second and equally important cause, Lamarck fully perceived that Heredity must necessarily co-operate with Adaptation. He maintained that the variations of organs arising from habit or use are in themselves at first but insignificant in each separate individual ; but that by the accumulation of the effects produced in each individual, transmitted from generation to generation in an ever increas- ing number, they become very significant. This was quite a correct fundamental idea ; but Lamarck did not reach the principle which Darwin subsequently introduced as the most important factor in the Theory of Transmutation, namely, the principle of Natural Selection in the Struggle for Existence. Lamarck failed to discover this most im- portant causal relation, and this, together with the low condition of all biological sciences at that time, prevented him from more firmly establishing his theory of the common descent of animals and man.
Lamarck also attempted to explain the evolution of Man from the Ape, as principally due to the progress made by the Ape in its habits of life, the further development and
LAMARCK ON THE "APE QUESTION." 8?
increased use of its organs, and to the fact that it trans- mitted the improvements thus acquired to its descend- ants. Lamarck considered the most important of these ad- vantageous variations to be the erect gait of Man, the differ- ing form of the hands and feet, the growth of language, and the correlative higher development of the brain. He assumed that the Apes most closely akin to Man, those which became the ancestors of mankind, made the first step toward becoming human when they gave up the habit of climbing and living on trees, and accustomed themselves to an upright gait. This resulted in the carriage peculiar to Man and in the reconstruction of the spinal column and pelvis, as well as in the specialization of the two pairs of limbs — the fore pair developing into hands for the purpose of grasping and touching, while the hind pair were used only for walking, and thus developed into true feet. In con- sequence of the totally changed mode of life and of the correlation and interrelation of the various parts of the body and their functions, important changes occurred also in other organs and their functions. The change of food, for example, caused a change in the jaws and teeth, and, consequently, in the entire formation of the face. The tail, no longer used, gradually disappeared. As these Apes lived together in societies and acquired regulated family relations, such as are still found among the higher classes of Apes, the social habits, or so-called " social instincts," were especially developed. The Ape's language of mere soimds grew into the word-language of Man, and abstract ideas were accu- mulated from concrete impressions. The brain gradually developed in correlation with the larynx ; the organ of the mind in interrelation with that of speech. These important
88 THE EVOLUTION OF MAN.
ideas of Lamarck contain the first and oldest gerras of a real history of the human tribe.
Toward the end of the preceding and the beginning of this century, the great poet Goethe, whose genius was of the highest order, busied himself, independently of Lamarck, with the problem of creation, and his thoughts on this subject are of special interest. It is well known that Goethe's ready recognition of all the beauties of Nature, and his deep insight into her workings, early attracted him to natural scientific studies of the most various kinds. Throughout his life these formed the favourite occupation of his leisure hours. The theory of colours especially resulted in his well-known and compre- hensive work on this subject ; but the most valuable and important of Goethe's natural scientific studies are those in connection with organic bodies, with " Life, that splendid, priceless thing." In Morphology, the doctrine of forms, he made most unusually deep researches. Aided by Com- parative Anatomy, he obtained most brilliant results in this, and went far in advance of his time. His cranial theory, his discovery of the temporal jawbone in man, and his doctrine of the metamorphosis of plants, must be espe- cially mentioned here.29 These morphological studies led Goethe to make those researches into the formation and transformation of organisms which we must rank, after those of Lamarck, among the oldest and profoundest rudiments of phylogenetical science. He came so near the Theory of Descent that he must be classed with Lamarck among the founders of it. It is true that Goethe has nowhere given a connected scientific exposition of his theory of evolution ; but his brilliant miscellaneous writings, "Zw Morphologie*
GOETHE AS A NATURALIST. 89
abound in most excellent ideas. Some of them may indeed be called the rudiments of the Theory of Descent. In proof of this it is sufficient to adduce some of his most remarkable propositions. He says : " This, then, is what we have gained, fearlessly to assert that the more perfect natural organisms, such as Fishes, Amphibia, Birds, Mammals, and Man at the head of the last, have been formed after one primordial type, the very permanent parts of which only vary a little one way or another, and which in the course of reproduction is still being remoulded and perfected" (1796). This " primordial type " of Vertebrates, after which Man also has been shaped, answers to what we call " the common ancestral form of the vertebrate tribe," and from which all the various species of Vertebrates have arisen by constant "development, variation, and reproduction." In another passage Goethe says (1807) : " Plants and animals, regarded in their most imperfect condition, are hardly dis- tinguishable. This much, however, we may say, that from a condition in which plant is hardly to be distinguished from animal, creatures have appeared, gradually perfecting themselves in two opposite directions, — the plant is finally glorified into the tree, enduring and motionless, the animal into the human being, of the highest mobility and free- dom."
That Goethe, in these and other utterances, did not apeak merely figuratively, that he grasped the internal relation and connection of organic forms in a genealogical senoe, is yet more evident in remarkable separate passages in which he declares himself as to the causes of the external multiplicity of species, on the one hand, and of the internal unity of their structure on the other. He assumed that
90 THE EVOLUTION OF MAN.
every organism is the product of the co-operation of two contrary constructive forces, or formative tendencies. One, the internal formative tendency, "the centripetal force," is that of the type, or "the tendency toward specification," which constantly aims at maintaining uniform the organic forms of the species in the series of generations. This is Heredity. The other, the external formative tendency, " the centrifugal force," is variation, or " the tendency toward metamorphosis," which acts, through the continual changes made in the external conditions of their existence, so as continually to vary the species. This is Adapta tion.
In this significant conception, Goethe very nearly con- ceived the two great mechanical factors, Heredity and Adap- tation, whioh are, we assert, the most important efficient causes of the formation of species. For example, he says, that " at the foundation of all organization there is an original intrinsic kinship " (which is Heredity) ; " the variety of forms, however, is due to the conditions of relation necessarily held to the external world, on account of which we may properly assume, for the purpose of explaining the present forms, which are both varied and unvaried, that there was diversity, originally and simultaneously, and that a progressive transformation is continually going on" (which is Adaptation).
In order rightly to appreciate Goethe's morphological views it is, however, necessary to grasp the connection between the whole peculiar course of his monistic study of nature and his pantheistic conception of the world. Most significant in this respect is the lively and warm interest with which he followed the efforts which the French
GOETHE VIRTUALLY