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THE EVOLUTION OF MAN

A POPULAR SCIENTIFIC STUDY

BY

ERNST HAECKEL

VOLUME 1.

HUMAN EMBRYOLOGY OR ONTOGENY.

TRANSLATED FROM THE FIFTH (ENLARGED) EDITION BY JOSEPH MCCABE.

[ISSUED FOR THE RATIONALIST PRESS ASSOCIATION, LIMITED.]

WATTS & CO., 17, JOHNSONS COURT, FLEET STREET, LONDON, E.C. 1912.

CONTENTS OF VOLUME 1.

LIST OF ILLUSTRATIONS.

GLOSSARY.

TRANSLATOR'S PREFACE.

TABLE: CLASSIFICATION OF THE ANIMAL WORLD.

CHAPTER 1.1. THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION.

CHAPTER 1.2. THE OLDER EMBRYOLOGY.

CHAPTER 1.3. MODERN EMBRYOLOGY.

CHAPTER 1.4. THE OLDER PHYLOGENY.

CHAPTER 1.5. THE MODERN SCIENCE OF EVOLUTION.

CHAPTER 1.6. THE OVUM AND THE AMOEBA.

CHAPTER 1.7. CONCEPTION.

CHAPTER 1.8. THE GASTRAEA THEORY.

CHAPTER 1.9. THE GASTRULATION OF THE VERTEBRATE.

CHAPTER 1.10. THE COELOM THEORY.

CHAPTER 1.11. THE VERTEBRATE CHARACTER OF MAN.

CHAPTER 1.12. THE EMBRYONIC SHIELD AND GERMINATIVE AREA.

CHAPTER 1.13. DORSAL BODY AND VENTRAL BODY.

CHAPTER 1.14. THE ARTICULATION OF THE BODY.

CHAPTER 1.15. FOETAL MEMBRANES AND CIRCULATION.

LIST OF ILLUSTRATIONS.

PORTRAIT OF ERNST HAECKEL FROM THE PAINTING BY FRANZ VON LEUBACH, 1899 (REPRODUCED BY "JUGEND").

FIGURE 1.1. THE HUMAN OVUM.

FIGURE 1.2. STEM-CELL OF AN ECHINODERM.

FIGURE 1.3. THREE EPITHELIAL CELLS.

FIGURE 1.4. FIVE SPINY OR GROOVED CELLS.

FIGURE 1.5. TEN LIVER-CELLS.

FIGURE 1.6. NINE STAR-SHAPED BONE-CELLS.

FIGURE 1.7. ELEVEN STAR-SHAPED CELLS.

FIGURE 1.8. UNFERTILISED OVUM OF AN ECHINODERM.

FIGURE 1.9. A LARGE BRANCHING NERVE-CELL.

FIGURE 1.10. BLOOD-CELLS.

FIGURE 1.11. INDIRECT OR MITOTIC CELL-DIVISION.

FIGURE 1.12. MOBILE CELLS.

FIGURE 1.13. OVA OF VARIOUS ANIMALS.

FIGURE 1.14. THE HUMAN OVUM.

FIGURE 1.15. FERTILISED OVUM OF HEN.

FIGURE 1.16. A CREEPING AMOEBA.

FIGURE 1.17. DIVISION OF AN AMOEBA.

FIGURE 1.18. OVUM OF A SPONGE.

FIGURE 1.19. BLOOD-CELLS, OR PHAGOCYTES.

FIGURE 1.20. SPERMIA OR SPERMATOZOA.

FIGURE 1.21. SPERMATOZOA OF VARIOUS ANIMALS.

FIGURE 1.22. A SINGLE HUMAN SPERMATOZOON.

FIGURE 1.23. FERTILISATION OF THE OVUM.

FIGURE 1.24. IMPREGNATED ECHINODERM OVUM.

FIGURE 1.25. IMPREGNATION OF THE STAR-FISH OVUM.

FIGURES 1.26 AND 1.27. IMPREGNATION OF SEA-URCHIN OVUM.

FIGURE 1.28. STEM-CELL OF A RABBIT.

FIGURE 1.29. GASTRULATION OF A CORAL.

FIGURE 1.30. GASTRULA OF A GASTRAEAD.

FIGURE 1.31. GASTRULA OF A WORM.

FIGURE 1.32. GASTRULA OF AN ECHINODERM.

FIGURE 1.33. GASTRULA OF AN ARTHROPOD.

FIGURE 1.34. GASTRULA OF A MOLLUSC.

FIGURE 1.35. GASTRULA OF A VERTEBRATE.

FIGURE 1.36. GASTRULA OF A LOWER SPONGE.

FIGURE 1.37. CELLS FROM THE PRIMARY GERMINAL LAYERS.

FIGURE 1.38. GASTRULATION OF THE AMPHIOXUS.

FIGURE 1.39. GASTRULA OF THE AMPHIOXUS.

FIGURE 1.40. CLEAVAGE OF THE FROG'S OVUM.

FIGURES 1.41 TO 1.44. SECTIONS OF FERTILISED TOAD OVUM.

FIGURES 1.45 TO 1.48. GASTRULATION OF THE SALAMANDER.

FIGURE 1.49. SEGMENTATION OF THE LAMPREY.

FIGURE 1.50. GASTRULATION OF THE LAMPREY.

FIGURE 1.51. GASTRULATION OF CERATODUS.

FIGURE 1.52. OVUM OF A DEEP-SEA BONY FISH.

FIGURE 1.53. SEGMENTATION OF A BONY FISH.

FIGURE 1.54. DISCOID GASTRULA OF A BONY FISH.

FIGURES 1.55 AND 1.56. SECTIONS OF BLASTULA OF SHARK.

FIGURE 1.57. DISCOID SEGMENTATION OF BIRD'S OVUM.

FIGURES 1.58 TO 1.61. GASTRULATION OF THE BIRD.

FIGURE 1.62. GERMINAL DISK OF THE LIZARD.

FIGURES 1.63 AND 1.64. GASTRULATION OF THE OPOSSUM.

FIGURES 1.65 TO 1.67. GASTRULATION OF THE OPOSSUM.

FIGURES 1.68 TO 1.71. GASTRULATION OF THE RABBIT.

FIGURE 1.72. GASTRULA OF THE PLACENTAL MAMMAL.

FIGURE 1.73. GASTRULA OF THE RABBIT.

FIGURES 1.74 AND 1.75. DIAGRAM OF THE FOUR SECONDARY GERMINAL LAYERS.

FIGURES 1.76 AND 1.77. COELOMULA OF SAGITTA.

FIGURE 1.78. SECTION OF YOUNG SAGITTA.

FIGURES 1.79 AND 1.80. SECTION OF AMPHIOXUS-LARVAE.

FIGURES 1.81 AND 1.82. SECTION OF AMPHIOXUS-LARVAE.

FIGURES 1.83 AND 1.84. CHORDULA OF THE AMPHIOXUS.

FIGURES 1.85 AND 1.86. CHORDULA OF THE AMPHIBIA.

FIGURES 1.87 AND 1.88. SECTION OF COELOMULA-EMBRYOS OF VERTEBRATES.

FIGURES 1.89 AND 1.90. SECTION OF COELOMULA-EMBRYO OF TRITON.

FIGURE 1.91. DORSAL PART OF THREE TRITON-EMBRYOS.

FIGURE 1.92. CHORDULA-EMBRYO OF A BIRD.

FIGURE 1.93. VERTEBRATE-EMBRYO OF A BIRD.

FIGURES 1.94 AND 1.95. SECTION OF THE PRIMITIVE STREAK OF A CHICK.

FIGURE 1.96. SECTION OF THE PRIMITIVE GROOVE OF A RABBIT.

FIGURE 1.97. SECTION OF PRIMITIVE MOUTH OF A HUMAN EMBRYO.

FIGURES 1.98 TO 1.102. THE IDEAL PRIMITIVE VERTEBRATE.

FIGURE 1.103. REDUNDANT MAMMARY GLANDS.

FIGURE 1.104. A GREEK GYNECOMAST.

FIGURE 1.105. SEVERANCE OF THE DISCOID MAMMAL EMBRYO.

FIGURES 1.106 AND 1.107. THE VISCERAL EMBRYONIC VESICLE.

FIGURE 1.108. FOUR ENTODERMIC CELLS.

FIGURE 1.109. TWO ENTODERMIC CELLS.

FIGURES 1.110 TO 1.114. OVUM OF A RABBIT.

FIGURES 1.115 TO 1.118. EMBRYONIC VESICLE OF A RABBIT.

FIGURE 1.119. SECTION OF THE GASTRULA OF FOUR VERTEBRATES.

FIGURES 1.120 TO 1.123. EMBRYONIC SHIELD OF A RABBIT.

FIGURE 1.124. COELOMULA OF THE AMPHIOXUS.

FIGURE 1.125. CHORDULA OF A FROG.

FIGURE 1.126. SECTION OF FROG-EMBRYO.

FIGURES 1.127 AND 1.128. DORSAL SHIELD OF A CHICK.

FIGURE 1.129. SECTION OF HIND END OF A CHICK.

FIGURE 1.130. GERMINAL AREA OF THE RABBIT.

FIGURE 1.131. EMBRYO OF THE OPOSSUM.

FIGURE 1.132. EMBRYONIC SHIELD OF THE RABBIT.

FIGURE 1.133. HUMAN EMBRYO AT THE SANDAL-STAGE.

FIGURE 1.134. EMBRYONIC SHIELD OF RABBIT.

FIGURE 1.135. EMBRYONIC SHIELD OF OPOSSUM.

FIGURE 1.136. EMBRYONIC DISK OF A CHICK.

FIGURE 1.137. EMBRYONIC DISK OF A HIGHER VERTEBRATE.

FIGURES 1.138 TO 1.142. SECTIONS OF MATURING MAMMAL EMBRYO.

FIGURES 1.143 TO 1.146. SECTIONS OF EMBRYONIC CHICKS.

FIGURE 1.147. SECTION OF EMBRYONIC CHICK.

FIGURE 1.148. SECTION OF FORE-HALF OF CHICK-EMBRYO.

FIGURES 1.149 AND 1.150. SECTIONS OF HUMAN EMBRYOS.

FIGURE 1.151. SECTION OF A SHARK-EMBRYO.

FIGURE 1.152. SECTION OF A DUCK-EMBRYO.

FIGURES 1.153 TO 1.155. SOLE-SHAPED EMBRYONIC DISK OF CHICK.

FIGURES 1.156 AND 1.157. EMBRYO OF THE AMPHIOXUS.

FIGURES 1.158 TO 1.160. EMBRYO OF THE AMPHIOXUS.

FIGURES 1.161 AND 1.162. SECTIONS OF SHARK-EMBRYOS.

FIGURE 1.163. SECTION OF A TRITON-EMBRYO.

FIGURES 1.164 TO 1.166. VERTEBRAE.

FIGURE 1.167. HEAD OF A SHARK-EMBRYO.

FIGURES 1.168 AND 1.169. HEAD OF A CHICK-EMBRYO.

FIGURE 1.170. HEAD OF A DOG-EMBRYO.

FIGURE 1.171. HUMAN EMBRYO OF THE FOURTH WEEK.

FIGURE 1.172. SECTION OF SHOULDER OF CHICK-EMBRYO.

FIGURE 1.173. SECTION OF PELVIC REGION OF CHICK-EMBRYO.

FIGURE 1.174. DEVELOPMENT OF THE LIZARD'S LEGS.

FIGURE 1.175. HUMAN-EMBRYO FIVE WEEKS OLD.

FIGURES 1.176 TO 1.178. EMBRYOS OF THE BAT.

FIGURE 1.179. HUMAN EMBRYOS.

FIGURE 1.180. HUMAN EMBRYO OF THE FOURTH WEEK.

FIGURE 1.181. HUMAN EMBRYO OF THE FIFTH WEEK.

FIGURE 1.182. SECTION OF TAIL OF HUMAN EMBRYO.

FIGURES 1.183 AND 1.184. HUMAN EMBRYO DISSECTED.

FIGURE 1.185. MISS JULIA PASTRANA.

FIGURES 1.186 TO 1.190. HUMAN EMBRYOS.

FIGURE 1.191. HUMAN EMBRYOS OF SIXTEEN TO EIGHTEEN DAYS.

FIGURES 1.192 AND 1.193. HUMAN EMBRYO OF FOURTH WEEK.

FIGURE 1.194. HUMAN EMBRYO WITH ITS MEMBRANES.

FIGURE 1.195. DIAGRAM OF THE EMBRYONIC ORGANS.

FIGURE 1.196. SECTION OF THE PREGNANT WOMB.

FIGURE 1.197. EMBRYO OF SIAMANG-GIBBON.

FIGURE 1.198. SECTION OF PREGNANT WOMB.

FIGURES 1.199 AND 1.200. HUMAN FOETUS AND PLACENTA.

FIGURE 1.201. VITELLINE VESSELS IN GERMINATIVE AREA.

FIGURE 1.202. BOAT-SHAPED EMBRYO OF THE DOG.

FIGURE 1.203. LAR OR WHITE-HANDED GIBBON.

FIGURE 1.204. YOUNG ORANG.

FIGURE 1.205. WILD ORANG.

FIGURE 1.206. BALD-HEADED CHIMPANZEE.

FIGURE 1.207. FOETAL MEMBRANES AND CIRCULATION.

FIGURE 1.208. FEMALE GORILLA.

FIGURE 1.209. MALE GIANT-GORILLA.

GLOSSARY.

ACRANIA: animals without skull (cranium).

ANTHROPOGENY: the evolution (genesis) of man (anthropos).

ANTHROPOLOGY: the science of man.

ARCHI-: (in compounds) the first or typical—as, archi-cytula, archi-gastrula, etc.

BIOGENY: the science of the genesis of life (bios).

BLAST-: (in compounds) pertaining to the early embryo (blastos = a
bud); hence:—
Blastoderm: skin (derma) or enclosing layer of the embryo.
Blastosphere: the embryo in the hollow sphere stage.
Blastula: same as preceding.
Epiblast: the outer layer of the embryo (ectoderm).
Hypoblast: the inner layer of the embryo (endoderm).

BRANCHIAL: pertaining to the gills (branchia).

CARYO-: (in compounds) pertaining to the nucleus (caryon); hence:—
Caryokineses: the movement of the nucleus.
Caryolysis: dissolution of the nucleus.
Caryoplasm: the matter of the nucleus.

CENTROLECITHAL: see under LECITH-.

CHORDARIA and CHORDONIA: animals with a dorsal chord or back-bone.

COELOM or COELOMA: the body-cavity in the embryo; hence:—
Coelenterata: animals without a body-cavity.
Coelomaria: animals with a body-cavity.
Coelomation: formation of the body-cavity.

CYTO-: (in compounds) pertaining to the cell (cytos); hence:—
Cytoblast: the nucleus of the cell.
Cytodes: cell-like bodies, imperfect cells.
Cytoplasm: the matter of the body of the cell.
Cytosoma: the body (soma) of the cell.

CRYPTORCHISM: abnormal retention of the testicles in the body.

DEUTOPLASM: see PLASM.

DUALISM: the belief in the existence of two entirely distinct principles (such as matter and spirit).

DYSTELEOLOGY: the science of those features in organisms which refute the "design-argument."

ECTODERM: the outer (ekto) layer of the embryo.

ENTODERM: the inner (ento) layer of the embryo.

EPIDERM: the outer layer of the skin.

EPIGENESIS: the theory of gradual development of organs in the embryo.

EPIPHYSIS: the third or central eye in the early vertebrates.

EPISOMA: see SOMA.

EPITHELIA: tissues covering the surface of parts of the body (such as the mouth, etc.)

GONADS: the sexual glands.

GONOCHORISM: separation of the male and female sexes.

GONOTOMES: sections of the sexual glands.

GYNECOMAST: a male with the breasts (masta) of a woman (gyne).

HEPATIC: pertaining to the liver (hepar).

HOLOBLASTIC: embryos in which the animal and vegetal cells divide equally (holon = whole).

HYPERMASTISM: the possession of more than the normal breasts (masta).

HYPOBRANCHIAL: underneath (hypo) the gills.

HYPOPHYSIS: sensitive-offshoot from the brain in the vertebrate.

HYPOSOMA: see SOMA.

LECITH-: pertaining to the yelk (lecithus); hence:—
Centrolecithal: eggs with the yelk in the centre.
Lecithoma: the yelk-sac.
Telolecithal: eggs with the yelk at one end.

MEROBLASTIC: cleaving in part (meron) only.

META-: (in compounds) the "after" or secondary stage; hence:—
Metagaster: the secondary or permanent gut (gaster).
Metaplasm: secondary or differentiated plasm.
Metastoma: the secondary or permanent mouth (stoma).
Metazoa: the higher or later animals, made up of many cells.
Metovum: the mature or advanced ovum.

METAMERA: the segments into which the embryo breaks up.

METAMERISM: the segmentation of the embryo.

MONERA: the most primitive of the unicellular organisms.

MONISM: belief in the fundamental unity of all things.

MORPHOLOGY: the science of organic forms (generally equivalent to anatomy).

MYOTOMES: segments into which the muscles break up.

NEPHRA: the kidneys; hence:—
Nephridia: the rudimentary kidney-organs.
Nephrotomes: the segments of the developing kidneys.

ONTOGENY: the science of the development of the individual (generally equivalent to embryology).

PERIGENESIS: the genesis of the movements in the vital particles.

PHAGOCYTES: cells that absorb food (phagein = to eat).

PHYLOGENY: the science of the evolution of species (phyla).

PLANOCYTES: cells that move about (planein).

PLASM: the colloid or jelly-like matter of which organisms are
composed; hence:—
Caryoplasm: the matter of the nucleus (caryon).
Cytoplasm: the matter of the body of the cell.
Deutoplasm: secondary or differentiated plasm.
Metaplasm: secondary or differentiated plasm.
Protoplasm: primitive or undifferentiated plasm.

PLASSON: the simplest form of plasm.

PLASTIDULES: small particles of plasm.

POLYSPERMISM: the penetration of more than one sperm-cell into the ovum.

PRO- or PROT: (in compounds) the earlier form (opposed to META); hence:—
Prochorion: the first form of the chorion.
Progaster: the first or primitive stomach.
Pronephridia: the earlier form of the kidneys.
Prorenal: the earlier form of the kidneys.
Prostoma: the first or primitive mouth.
Protists: the earliest or unicellular organisms.
Provertebrae: the earliest phase of the vertebrae.
Protophyta: the primitive or unicellular plants.
Protoplasm: undifferentiated plasm.
Protozoa: the primitive or unicellular animals.

RENAL: pertaining to the kidneys (renes).

SCATULATION: packing or boxing-up (scatula = a box).

SCLEROTOMES: segments into which the primitive skeleton falls.

SOMA: the body; hence:—
Cytosoma: the body of the cell (cytos).
Episoma: the upper or back-half of the embryonic body.
Somites: segments of the embryonic body.
Hyposoma: the under or belly-half of the embryonic body.

TELEOLOGY: the belief in design and purpose (telos) in nature.

TELOLECITHAL: see LECITH-.

UMBILICAL: pertaining to the navel (umbilicus).

VITELLINE: pertaining to the yelk (vitellus).

***

PREFACE.

[BY JOSEPH MCCABE.]

The work which we now place within the reach of every reader of the English tongue is one of the finest productions of its distinguished author. The first edition appeared in 1874. At that time the conviction of man's natural evolution was even less advanced in Germany than in England, and the work raised a storm of controversy. Theologians—forgetting the commonest facts of our individual development—spoke with the most profound disdain of the theory that a Luther or a Goethe could be the outcome of development from a tiny speck of protoplasm. The work, one of the most distinguished of them said, was "a fleck of shame on the escutcheon of Germany." To-day its conclusion is accepted by influential clerics, such as the Dean of Westminster, and by almost every biologist and anthropologist of distinction in Europe. Evolution is not a laboriously reached conclusion, but a guiding truth, in biological literature to-day.

There was ample evidence to substantiate the conclusion even in the first edition of the book. But fresh facts have come to light in each decade, always enforcing the general truth of man's evolution, and at times making clearer the line of development. Professor Haeckel embodied these in successive editions of his work. In the fifth edition, of which this is a translation, reference will be found to the very latest facts bearing on the evolution of man, such as the discovery of the remarkable effect of mixing human blood with that of the anthropoid ape. Moreover, the ample series of illustrations has been considerably improved and enlarged; there is no scientific work published, at a price remotely approaching that of the present edition, with so abundant and excellent a supply of illustrations. When it was issued in Germany, a few years ago, a distinguished biologist wrote in the Frankfurter Zeitung that it would secure immortality for its author, the most notable critic of the idea of immortality. And the Daily Telegraph reviewer described the English version as a "handsome edition of Haeckel's monumental work," and "an issue worthy of the subject and the author."

The influence of such a work, one of the most constructive that Haeckel has ever written, should extend to more than the few hundred readers who are able to purchase the expensive volumes of the original issue. Few pages in the story of science are more arresting and generally instructive than this great picture of "mankind in the making." The horizon of the mind is healthily expanded as we follow the search-light of science down the vast avenues of past time, and gaze on the uncouth forms that enter into, or illustrate, the line of our ancestry. And if the imagination recoils from the strange and remote figures that are lit up by our search-light, and hesitates to accept them as ancestral forms, science draws aside another veil and reveals another picture to us. It shows us that each of us passes, in our embryonic development, through a series of forms hardly less uncouth and unfamiliar. Nay, it traces a parallel between the two series of forms. It shows us man beginning his existence, in the ovary of the female infant, as a minute and simple speck of jelly-like plasm. It shows us (from analogy) the fertilised ovum breaking into a cluster of cohering cells, and folding and curving, until the limb-less, head-less, long-tailed foetus looks like a worm-shaped body. It then points out how gill-slits and corresponding blood-vessels appear, as in a lowly fish, and the fin-like extremities bud out and grow into limbs, and so on; until, after a very clear ape-stage, the definite human form emerges from the series of transformations.

It is with this embryological evidence for our evolution that the present volume is concerned. There are illustrations in the work that will make the point clear at a glance. Possibly TOO clear; for the simplicity of the idea and the eagerness to apply it at every point have carried many, who borrow hastily from Haeckel, out of their scientific depth. Haeckel has never shared their errors, nor encouraged their superficiality. He insists from the outset that a complete parallel could not possibly be expected. Embryonic life itself is subject to evolution. Though there is a general and substantial law—as most of our English and American authorities admit—that the embryonic series of forms recalls the ancestral series of forms, the parallel is blurred throughout and often distorted. It is not the obvious resemblance of the embryos of different animals, and their general similarity to our extinct ancestors in this or that organ, on which we must rest our case. A careful study must be made of the various stages through which all embryos pass, and an effort made to prove their real identity and therefore genealogical relation.

This is a task of great subtlety and delicacy. Many scientists have worked at it together with Professor Haeckel—I need only name our own Professor Balfour and Professor Ray Lankester—and the scheme is fairly complete. But the general reader must not expect that even so clear a writer as Haeckel can describe these intricate processes without demanding his very careful attention. Most of the chapters in the present volume (and the second volume will be less difficult) are easily intelligible to all; but there are points at which the line of argument is necessarily subtle and complex. In the hope that most readers will be induced to master even these more difficult chapters, I will give an outline of the characteristic argument of the work. Haeckel's distinctive services in regard to man's evolution have been:

1. The construction of a complete ancestral tree, though, of course, some of the stages in it are purely conjectural, and not final.

2. The tracing of the remarkable reproduction of ancestral forms in the embryonic development of the individual. Naturally, he has not worked alone in either department.

The second volume of this work will embody the first of these two achievements; the present one is mainly concerned with the latter. It will be useful for the reader to have a synopsis of the argument and an explanation of some of the chief terms invented or employed by the author.

The main theme of the work is that, in the course of their embryonic development, all animals, including man, pass roughly and rapidly through a series of forms which represents the succession of their ancestors in the past. After a severe and extensive study of embryonic phenomena, Haeckel has drawn up a "law" (in the ordinary scientific sense) to this effect, and has called it "the biogenetic law," or the chief law relating to the evolution (genesis) of life (bios). This law is widely and increasingly accepted by embryologists and zoologists. It is enough to quote a recent declaration of the great American zoologist, President D. Starr Jordan: "It is, of course, true that the life-history of the individual is an epitome of the life-history of the race"; while a distinguished German zoologist (Sarasin) has described it as being of the same use to the biologist as spectrum analysis is to the astronomer.

But the reproduction of ancestral forms in the course of the embryonic development is by no means always clear, or even always present. Many of the embryonic phases do not recall ancestral stages at all. They may have done so originally, but we must remember that the embryonic life itself has been subject to adaptive changes for millions of years. All this is clearly explained by Professor Haeckel. For the moment, I would impress on the reader the vital importance of fixing the distinction from the start. He must thoroughly familiarise himself with the meaning of five terms.

BIOGENY is the development of life in general (both in the individual and the species), or the sciences describing it.

ONTOGENY is the development (embryonic and post-embryonic) of the individual (on), or the science describing it.

PHYLOGENY is the development of the race or stem (phulon), or the science describing it.

Roughly, ontogeny may be taken to mean embryology, and phylogeny what we generally call evolution.

Further, the embryonic phenomena sometimes reproduce ancestral forms, and they are then called PALINGENETIC (from palin = again): sometimes they do not recall ancestral forms, but are later modifications due to adaptation, and they are then called CENOGENETIC (from kenos = new or foreign).

These terms are now widely used, but the reader of Haeckel must understand them thoroughly.

The first five chapters are an easy account of the history of embryology and evolution. The sixth and seventh give an equally clear account of the sexual elements and the process of conception. But some of the succeeding chapters must deal with embryonic processes so unfamiliar, and pursue them through so wide a range of animals in a brief space, that, in spite of the 200 illustrations, they will offer difficulty to many a reader. As our aim is to secure, not a superficial acquiescence in conclusions, but a fair comprehension of the truths of science, we have retained these chapters. However, I will give a brief and clear outline of the argument, so that the reader with little leisure may realise their value.

When the animal ovum (egg-cell) has been fertilised, it divides and subdivides until we have a cluster of cohering cells, externally not unlike a raspberry or mulberry. This is the morula (= mulberry) stage. The cluster becomes hollow, or filled with fluid in the centre, all the cells rising to the surface. This is the blastula (hollow ball) stage. One half of the cluster then bends or folds in upon the other, as one might do with a thin indiarubber ball, and we get a vase-shaped body with hollow interior (the first stomach, or "primitive gut"), an open mouth (the first or "primitive mouth"), and a wall composed of two layers of cells (two "germinal layers"). This is the gastrula (stomach) stage, and the process of its formation is called gastrulation. A glance at the illustration (Figure 1.29) will make this perfectly clear.

So much for the embryonic process in itself. The application to evolution has been a long and laborious task. Briefly, it was necessary to show that ALL the multicellular animals passed through these three stages, so that our biogenetic law would enable us to recognise them as reminiscences of ancestral forms. This is the work of Chapters 1.8 and 1.9. The difficulty can be realised in this way: As we reach the higher animals the ovum has to take up a large quantity of yelk, on which it may feed in developing. Think of the bird's "egg." The effect of this was to flatten the germ (the morula and blastula) from the first, and so give, at first sight, a totally different complexion to what it has in the lowest animals. When we pass the reptile and bird stage, the large yelk almost disappears (the germ now being supplied with blood by the mother), but the germ has been permanently altered in shape, and there are now a number of new embryonic processes (membranes, blood-vessel connections, etc.). Thus it was no light task to trace the identity of this process of gastrulation in all the animals. It has been done, however; and with this introduction the reader will be able to follow the proof. The conclusion is important. If all animals pass through the curious gastrula stage, it must be because they all had a common ancestor of that nature. To this conjectural ancestor (it lived before the period of fossilisation begins) Haeckel gives the name of the Gastraea, and in the second volume we shall see a number of living animals of this type ("gastraeads").

The line of argument is the same in the next chapter. After laborious and careful research (though this stage is not generally admitted in the same sense as the previous one), a fourth common stage was discovered, and given the name of the Coelomula. The blastula had one layer of cells, the blastoderm (derma = skin): the gastrula two layers, the ectoderm ("outer skin") and entoderm ("inner skin"). Now a third layer (mesoderm = middle skin) is formed, by the growth inwards of two pouches or folds of the skin. The pouches blend together, and form a single cavity (the body cavity, or coelom), and its two walls are two fresh "germinal layers." Again, the identity of the process has to be proved in all the higher classes of animals, and when this is done we have another ancestral stage, the Coelomaea.

The remaining task is to build up the complex frame of the higher animals—always showing the identity of the process (on which the evolutionary argument depends) in enormously different conditions of embryonic life—out of the four "germinal layers." Chapter 1.9 prepares us for the work by giving us a very clear account of the essential structure of the back-boned (vertebrate) animal, and the probable common ancestor of all the vertebrates (a small fish of the lancelet type). Chapters 1.11 to 1.14 then carry out the construction step by step. The work is now simpler, in the sense that we leave all the invertebrate animals out of account; but there are so many organs to be fashioned out of the four simple layers that the reader must proceed carefully. In the second volume each of these organs will be dealt with separately, and the parallel will be worked out between its embryonic and its phylogenetic (evolutionary) development. The general reader may wait for this for a full understanding. But in the meantime the wonderful story of the construction of all our organs in the course of a few weeks (the human frame is perfectly formed, though less than two inches in length, by the twelfth week) from so simple a material is full of interest. It would be useless to attempt to summarise the process. The four chapters are themselves but a summary of it, and the eighty fine illustrations of the process will make it sufficiently clear. The last chapter carries the story on to the point where man at last parts company with the anthropoid ape, and gives a full account of the membranes or wrappers that enfold him in the womb, and the connection with the mother.

In conclusion, I would urge the reader to consult, at his free library perhaps, the complete edition of this work, when he has read the present abbreviated edition. Much of the text has had to be condensed in order to bring out the work at our popular price, and the beautiful plates of the complete edition have had to be omitted. The reader will find it an immense assistance if he can consult the library edition.

JOSEPH MCCABE.

Cricklewood, March, 1906.

***

HAECKEL'S CLASSIFICATION OF THE ANIMAL WORLD.

UNICELLULAR ANIMALS (PROTOZOA).

1. Unnucleated.

Bacteria.
Protamoebae.

Monera.

2. Nucleated.

2A. Rhizopoda.

Amoebina.
Radiolaria.

2B. Infusoria.

Flagellata.
Ciliata.

3. Cell-Colonies.

Catallacta.
Blastaeada.

MULTICELLULAR ANIMALS (METAZOA).

1. COELENTERIA, COELENTERATA, OR ZOOPHYTES. Animals without body-cavity, blood or anus.

1A. Gastraeads.

Gastremaria.
Cyemaria.

1B. Sponges.

Protospongiae.
Metaspongiae.

1C. Cnidaria (Stinging Animals).

Hydrozoa.
Polyps.
Medusae.

1D. Platodes (Flat-Worms).

Platodaria.
Turbellaria.
Trematoda.
Cestoda.

2. COELOMARIA OR BILATERALS. Animals with body-cavity and anus, and generally blood.

2A. Vermalia (Worm-Like).

Rotatoria.
Strongylaria.
Prosopygia.
Frontonia.

2B. Molluscs.

Cochlides.
Conchades.
Teuthodes.

2C. Articulates.

Annelida.
Crustacea.
Tracheata.

2D. Echinoderms.

Monorchonia.
Pentorchonia.

2E. Tunicates.

Copelata.
Ascidiae.
Thalidiae.

2F. Vertebrates.

2F.1. Acrania-Lancelet (Without Skull).

2F.2. Craniota (With Skull).

2F.2A. Cyclostomes. ("Round-Mouthed").

2F.2B. Fishes.

Selachii.
Ganoids.
Teleosts.
Dipneusts.

2F.2C. Amphibia.

2F.2D. Reptiles.

2F.2E. Birds.

2F.2F. Mammal.

Monotremes.

Marsupials.

Placentals:—
Rodents.
Edentates.
Ungulates.
Cetacea.
Sirenia.
Insectivora.
Cheiroptera.
Carnassia.
Primates.

(This classification is given for the purpose of explaining Haeckel's use of terms in this volume. The general reader should bear in mind that it differs very considerably from more recent schemes of classification. He should compare the scheme framed by Professor E. Ray Lankester.)

***

THE EVOLUTION OF MAN.

CHAPTER 1.1. THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION.

The field of natural phenomena into which I would introduce my readers in the following chapters has a quite peculiar place in the broad realm of scientific inquiry. There is no object of investigation that touches man more closely, and the knowledge of which should be more acceptable to him, than his own frame. But among all the various branches of the natural history of mankind, or anthropology, the story of his development by natural means must excite the most lively interest. It gives us the key of the great world-riddles at which the human mind has been working for thousands of years. The problem of the nature of man, or the question of man's place in nature, and the cognate inquiries as to the past, the earliest history, the present situation, and the future of humanity—all these most important questions are directly and intimately connected with that branch of study which we call the science of the evolution of man, or, in one word, "Anthropogeny" (the genesis of man). Yet it is an astonishing fact that the science of the evolution of man does not even yet form part of the scheme of general education. In fact, educated people even in our day are for the most part quite ignorant of the important truths and remarkable phenomena which anthropogeny teaches us.

As an illustration of this curious state of things, it may be pointed out that most of what are considered to be "educated" people do not know that every human being is developed from an egg, or ovum, and that this egg is one simple cell, like any other plant or animal egg. They are equally ignorant that in the course of the development of this tiny, round egg-cell there is first formed a body that is totally different from the human frame, and has not the remotest resemblance to it. Most of them have never seen such a human embryo in the earlier period of its development, and do not know that it is quite indistinguishable from other animal embryos. At first the embryo is no more than a round cluster of cells, then it becomes a simple hollow sphere, the wall of which is composed of a layer of cells. Later it approaches very closely, at one period, to the anatomic structure of the lancelet, afterwards to that of a fish, and again to the typical build of the amphibia and mammals. As it continues to develop, a form appears which is like those we find at the lowest stage of mammal-life (such as the duck-bills), then a form that resembles the marsupials, and only at a late stage a form that has a resemblance to the ape; until at last the definite human form emerges and closes the series of transformations. These suggestive facts are, as I said, still almost unknown to the general public—so completely unknown that, if one casually mentions them, they are called in question or denied outright as fairy-tales. Everybody knows that the butterfly emerges from the pupa, and the pupa from a quite different thing called a larva, and the larva from the butterfly's egg. But few besides medical men are aware that MAN, in the course of his individual formation, passes through a series of transformations which are not less surprising and wonderful than the familiar metamorphoses of the butterfly.

The mere description of these remarkable changes through which man passes during his embryonic life should arouse considerable interest. But the mind will experience a far keener satisfaction when we trace these curious facts to their causes, and when we learn to behold in them natural phenomena which are of the highest importance throughout the whole field of human knowledge. They throw light first of all on the "natural history of creation," then on psychology, or "the science of the soul," and through this on the whole of philosophy. And as the general results of every branch of inquiry are summed up in philosophy, all the sciences come in turn to be touched and influenced more or less by the study of the evolution of man.

But when I say that I propose to present here the most important features of these phenomena and trace them to their causes, I take the term, and I interpret my task, in a very much wider sense than is usual. The lectures which have been delivered on this subject in the universities during the last half-century are almost exclusively adapted to medical men. Certainly, the medical man has the greatest interest in studying the origin of the human body, with which he is daily occupied. But I must not give here this special description of the embryonic processes such as it has hitherto been given, as most of my readers have not studied anatomy, and are not likely to be entrusted with the care of the adult organism. I must content myself with giving some parts of the subject only in general outline, and must not enter upon all the marvellous, but very intricate and not easily described, details that are found in the story of the development of the human frame. To understand these fully a knowledge of anatomy is needed. I will endeavour to be as plain as possible in dealing with this branch of science. Indeed, a sufficient general idea of the course of the embryonic development of man can be obtained without going too closely into the anatomic details. I trust we may be able to arouse the same interest in this delicate field of inquiry as has been excited already in other branches of science; though we shall meet more obstacles here than elsewhere.

The story of the evolution of man, as it has hitherto been expounded to medical students, has usually been confined to embryology—more correctly, ontogeny—or the science of the development of the individual human organism. But this is really only the first part of our task, the first half of the story of the evolution of man in that wider sense in which we understand it here. We must add as the second half—as another and not less important and interesting branch of the science of the evolution of the human stem—phylogeny: this may be described as the science of the evolution of the various animal forms from which the human organism has been developed in the course of countless ages. Everybody now knows of the great scientific activity that was occasioned by the publication of Darwin's Origin of Species in 1859. The chief direct consequence of this publication was to provoke a fresh inquiry into the origin of the human race, and this has proved beyond question our gradual evolution from the lower species. We give the name of "Phylogeny" to the science which describes this ascent of man from the lower ranks of the animal world. The chief source that it draws upon for facts is "Ontogeny," or embryology, the science of the development of the individual organism. Moreover, it derives a good deal of support from paleontology, or the science of fossil remains, and even more from comparative anatomy, or morphology.

These two branches of our science—on the one side ontogeny or embryology, and on the other phylogeny, or the science of race-evolution—are most vitally connected. The one cannot be understood without the other. It is only when the two branches fully co-operate and supplement each other that "Biogeny" (or the science of the genesis of life in the widest sense) attains to the rank of a philosophic science. The connection between them is not external and superficial, but profound, intrinsic, and causal. This is a discovery made by recent research, and it is most clearly and correctly expressed in the comprehensive law which I have called "the fundamental law of organic evolution," or "the fundamental law of biogeny." This general law, to which we shall find ourselves constantly recurring, and on the recognition of which depends one's whole insight into the story of evolution, may be briefly expressed in the phrase: "The history of the foetus is a recapitulation of the history of the race"; or, in other words, "Ontogeny is a recapitulation of phylogeny." It may be more fully stated as follows: The series of forms through which the individual organism passes during its development from the ovum to the complete bodily structure is a brief, condensed repetition of the long series of forms which the animal ancestors of the said organism, or the ancestral forms of the species, have passed through from the earliest period of organic life down to the present day.

The causal character of the relation which connects embryology with stem-history is due to the action of heredity and adaptation. When we have rightly understood these, and recognised their great importance in the formation of organisms, we can go a step further and say: Phylogenesis is the mechanical cause of ontogenesis.* (* The term "genesis," which occurs throughout, means, of course, "birth" or origin. From this we get: Biogeny = the origin of life (bios); Anthropogeny = the origin of man (anthropos); Ontogeny = the origin of the individual (on); Phylogeny = the origin of the species (phulon); and so on. In each case the term may refer to the process itself, or to the science describing the process.—Translator.) In other words, the development of the stem, or race, is, in accordance with the laws of heredity and adaptation, the cause of all the changes which appear in a condensed form in the evolution of the foetus.

The chain of manifold animal forms which represent the ancestry of each higher organism, or even of man, according to the theory of descent, always form a connected whole. We may designate this uninterrupted series of forms with the letters of the alphabet: A, B, C, D, E, etc., to Z. In apparent contradiction to what I have said, the story of the development of the individual, or the ontogeny of most organisms, only offers to the observer a part of these forms; so that the defective series of embryonic forms would run: A, B, D, F, H, K, M, etc.; or, in other cases, B, D, H, L, M, N, etc. Here, then, as a rule, several of the evolutionary forms of the original series have fallen out. Moreover, we often find—to continue with our illustration from the alphabet—one or other of the original letters of the ancestral series represented by corresponding letters from a different alphabet. Thus, instead of the Roman B and D, we often have the Greek Beta and Delta. In this case the text of the biogenetic law has been corrupted, just as it had been abbreviated in the preceding case. But, in spite of all this, the series of ancestral forms remains the same, and we are in a position to discover its original complexion.

In reality, there is always a certain parallel between the two evolutionary series. But it is obscured from the fact that in the embryonic succession much is wanting that certainly existed in the earlier ancestral succession. If the parallel of the two series were complete, and if this great fundamental law affirming the causal connection between ontogeny and phylogeny in the proper sense of the word were directly demonstrable, we should only have to determine, by means of the microscope and the dissecting knife, the series of forms through which the fertilised ovum passes in its development; we should then have before us a complete picture of the remarkable series of forms which our animal ancestors have successively assumed from the dawn of organic life down to the appearance of man. But such a repetition of the ancestral history by the individual in its embryonic life is very rarely complete. We do not often find our full alphabet. In most cases the correspondence is very imperfect, being greatly distorted and falsified by causes which we will consider later. We are thus, for the most part, unable to determine in detail, from the study of its embryology, all the different shapes which an organism's ancestors have assumed; we usually—and especially in the case of the human foetus—encounter many gaps. It is true that we can fill up most of these gaps satisfactorily with the help of comparative anatomy, but we cannot do so from direct embryological observation. Hence it is important that we find a large number of lower animal forms to be still represented in the course of man's embryonic development. In these cases we may draw our conclusions with the utmost security as to the nature of the ancestral form from the features of the form which the embryo momentarily assumes.

To give a few examples, we can infer from the fact that the human ovum is a simple cell that the first ancestor of our species was a tiny unicellular being, something like the amoeba. In the same way, we know, from the fact that the human foetus consists, at the first, of two simple cell-layers (the gastrula), that the gastraea, a form with two such layers, was certainly in the line of our ancestry. A later human embryonic form (the chordula) points just as clearly to a worm-like ancestor (the prochordonia), the nearest living relation of which is found among the actual ascidiae. To this succeeds a most important embryonic stage (acrania), in which our headless foetus presents, in the main, the structure of the lancelet. But we can only indirectly and approximately, with the aid of comparative anatomy and ontogeny, conjecture what lower forms enter into the chain of our ancestry between the gastraea and the chordula, and between this and the lancelet. In the course of the historical development many intermediate structures have gradually fallen out, which must certainly have been represented in our ancestry. But, in spite of these many, and sometimes very appreciable, gaps, there is no contradiction between the two successions. In fact, it is the chief purpose of this work to prove the real harmony and the original parallelism of the two. I hope to show, on a substantial basis of facts, that we can draw most important conclusions as to our genealogical tree from the actual and easily-demonstrable series of embryonic changes. We shall then be in a position to form a general idea of the wealth of animal forms which have figured in the direct line of our ancestry in the lengthy history of organic life.

In this evolutionary appreciation of the facts of embryology we must, of course, take particular care to distinguish sharply and clearly between the primitive, palingenetic (or ancestral) evolutionary processes and those due to cenogenesis.* (* Palingenesis = new birth, or re-incarnation (palin = again, genesis or genea = development); hence its application to the phenomena which are recapitulated by heredity from earlier ancestral forms. Cenogenesis = foreign or negligible development (kenos and genea); hence, those phenomena which come later in the story of life to disturb the inherited structure, by a fresh adaptation to environment.—Translator.) By palingenetic processes, or embryonic recapitulations, we understand all those phenomena in the development of the individual which are transmitted from one generation to another by heredity, and which, on that account, allow us to draw direct inferences as to corresponding structures in the development of the species. On the other hand, we give the name of cenogenetic processes, or embryonic variations, to all those phenomena in the foetal development that cannot be traced to inheritance from earlier species, but are due to the adaptation of the foetus, or the infant-form, to certain conditions of its embryonic development. These cenogenetic phenomena are foreign or later additions; they allow us to draw no direct inference whatever as to corresponding processes in our ancestral history, but rather hinder us from doing so.

This careful discrimination between the primary or palingenetic processes and the secondary or cenogenetic is of great importance for the purposes of the scientific history of a species, which has to draw conclusions from the available facts of embryology, comparative anatomy, and paleontology, as to the processes in the formation of the species in the remote past. It is of the same importance to the student of evolution as the careful distinction between genuine and spurious texts in the works of an ancient writer, or the purging of the real text from interpolations and alterations, is for the student of philology. It is true that this distinction has not yet been fully appreciated by many scientists. For my part, I regard it as the first condition for forming any just idea of the evolutionary process, and I believe that we must, in accordance with it, divide embryology into two sections—palingenesis, or the science of recapitulated forms; and cenogenesis, or the science of supervening structures.

To give at once a few examples from the science of man's origin in illustration of this important distinction, I may instance the following processes in the embryology of man, and of all the higher vertebrates, as palingenetic: the formation of the two primary germinal layers and of the primitive gut, the undivided structure of the dorsal nerve-tube, the appearance of a simple axial rod between the medullary tube and the gut, the temporary formation of the gill-clefts and arches, the primitive kidneys, and so on.* (* All these, and the following structures, will be fully described in later chapters.—Translator.) All these, and many other important structures, have clearly been transmitted by a steady heredity from the early ancestors of the mammal, and are, therefore, direct indications of the presence of similar structures in the history of the stem. On the other hand, this is certainly not the case with the following embryonic forms, which we must describe as cenogenetic processes: the formation of the yelk-sac, the allantois, the placenta, the amnion, the serolemma, and the chorion—or, generally speaking, the various foetal membranes and the corresponding changes in the blood vessels. Further instances are: the dual structure of the heart cavity, the temporary division of the plates of the primitive vertebrae and lateral plates, the secondary closing of the ventral and intestinal walls, the formation of the navel, and so on. All these and many other phenomena are certainly not traceable to similar structures in any earlier and completely-developed ancestral form, but have arisen simply by adaptation to the peculiar conditions of embryonic life (within the foetal membranes). In view of these facts, we may now give the following more precise expression to our chief law of biogeny: The evolution of the foetus (or ontogenesis) is a condensed and abbreviated recapitulation of the evolution of the stem (or phylogenesis); and this recapitulation is the more complete in proportion as the original development (or palingenesis) is preserved by a constant heredity; on the other hand, it becomes less complete in proportion as a varying adaptation to new conditions increases the disturbing factors in the development (or cenogenesis).

The cenogenetic alterations or distortions of the original palingenetic course of development take the form, as a rule, of a gradual displacement of the phenomena, which is slowly effected by adaptation to the changed conditions of embryonic existence during the course of thousands of years. This displacement may take place as regards either the position or the time of a phenomenon.

The great importance and strict regularity of the time-variations in embryology have been carefully studied recently by Ernest Mehnert, in his Biomechanik (Jena, 1898). He contends that our biogenetic law has not been impaired by the attacks of its opponents, and goes on to say: "Scarcely any piece of knowledge has contributed so much to the advance of embryology as this; its formulation is one of the most signal services to general biology. It was not until this law passed into the flesh and blood of investigators, and they had accustomed themselves to see a reminiscence of ancestral history in embryonic structures, that we witnessed the great progress which embryological research has made in the last two decades." The best proof of the correctness of this opinion is that now the most fruitful work is done in all branches of embryology with the aid of this biogenetic law, and that it enables students to attain every year thousands of brilliant results that they would never have reached without it.

It is only when one appreciates the cenogenetic processes in relation to the palingenetic, and when one takes careful account of the changes which the latter may suffer from the former, that the radical importance of the biogenetic law is recognised, and it is felt to be the most illuminating principle in the science of evolution. In this task of discrimination it is the silver thread in relation to which we can arrange all the phenomena of this realm of marvels—the "Ariadne thread," which alone enables us to find our way through this labyrinth of forms. Hence the brothers Sarasin, the zoologists, could say with perfect justice, in their study of the evolution of the Ichthyophis, that "the great biogenetic law is just as important for the zoologist in tracing long-extinct processes as spectrum analyses is for the astronomer."

Even at an earlier period, when a correct acquaintance with the evolution of the human and animal frame was only just being obtained—and that is scarcely eighty years ago!—the greatest astonishment was felt at the remarkable similarity observed between the embryonic forms, or stages of foetal development, in very different animals; attention was called even then to their close resemblance to certain fully-developed animal forms belonging to some of the lower groups. The older scientists (Oken, Treviranus, and others) knew perfectly well that these lower forms in a sense illustrated and fixed, in the hierarchy of the animal world, a temporary stage in the evolution of higher forms. The famous anatomist Meckel spoke in 1821 of a "similarity between the development of the embryo and the series of animals." Baer raised the question in 1828 how far, within the vertebrate type, the embryonic forms of the higher animals assume the permanent shapes of members of lower groups. But it was impossible fully to understand and appreciate this remarkable resemblance at that time. We owe our capacity to do this to the theory of descent; it is this that puts in their true light the action of heredity on the one hand and adaptation on the other. It explains to us the vital importance of their constant reciprocal action in the production of organic forms. Darwin was the first to teach us the great part that was played in this by the ceaseless struggle for existence between living things, and to show how, under the influence of this (by natural selection), new species were produced and maintained solely by the interaction of heredity and adaptation. It was thus Darwinism that first opened our eyes to a true comprehension of the supremely important relations between the two parts of the science of organic evolution—Ontogeny and Phylogeny.

Heredity and adaptation are, in fact, the two constructive physiological functions of living things; unless we understand these properly we can make no headway in the study of evolution. Hence, until the time of Darwin no one had a clear idea of the real nature and causes of embryonic development. It was impossible to explain the curious series of forms through which the human embryo passed; it was quite unintelligible why this strange succession of animal-like forms appeared in the series at all. It had previously been generally assumed that the man was found complete in all his parts in the ovum, and that the development consisted only in an unfolding of the various parts, a simple process of growth. This is by no means the case. On the contrary, the whole process of the development of the individual presents to the observer a connected succession of different animal-forms; and these forms display a great variety of external and internal structure. But WHY each individual human being should pass through this series of forms in the course of his embryonic development it was quite impossible to say until Lamarck and Darwin established the theory of descent. Through this theory we have at last detected the real causes, the efficient causes, of the individual development; we have learned that these mechanical causes suffice of themselves to effect the formation of the organism, and that there is no need of the final causes which were formerly assumed. It is true that in the academic philosophies of our time these final causes still figure very prominently; in the new philosophy of nature we can entirely replace them by efficient causes. We shall see, in the course of our inquiry, how the most wonderful and hitherto insoluble enigmas in the human and animal frame have proved amenable to a mechanical explanation, by causes acting without prevision, through Darwin's reform of the science of evolution. We have everywhere been able to substitute unconscious causes, acting from necessity, for conscious, purposive causes.* (* The monistic or mechanical philosophy of nature holds that only unconscious, necessary, efficient causes are at work in the whole field of nature, in organic life as well as in inorganic changes. On the other hand, the dualist or vitalist philosophy of nature affirms that unconscious forces are only at work in the inorganic world, and that we find conscious, purposive, or final causes in organic nature.)

If the new science of evolution had done no more than this, every thoughtful man would have to admit that it had accomplished an immense advance in knowledge. It means that in the whole of philosophy that tendency which we call monistic, in opposition to the dualistic, which has hitherto prevailed, must be accepted.* (* Monism is neither purely materialistic nor purely spiritualistic, but a reconciliation of these two principles, since it regards the whole of nature as one, and sees only efficient causes at work in it. Dualism, on the contrary, holds that nature and spirit, matter and force, the world and God, inorganic and organic nature, are separate and independent existences. Cf. The Riddle of the Universe chapter 12.) At this point the science of human evolution has a direct and profound bearing on the foundations of philosophy. Modern anthropology has, by its astounding discoveries during the second half of the nineteenth century, compelled us to take a completely monistic view of life. Our bodily structure and its life, our embryonic development and our evolution as a species, teach us that the same laws of nature rule in the life of man as in the rest of the universe. For this reason, if for no others, it is desirable, nay, indispensable, that every man who wishes to form a serious and philosophic view of life, and, above all, the expert philosopher, should acquaint himself with the chief facts of this branch of science.

The facts of embryology have so great and obvious a significance in this connection that even in recent years dualist and teleological philosophers have tried to rid themselves of them by simply denying them. This was done, for instance, as regards the fact that man is developed from an egg, and that this egg or ovum is a simple cell, as in the case of other animals. When I had explained this pregnant fact and its significance in my History of Creation, it was described in many of the theological journals as a dishonest invention of my own. The fact that the embryos of man and the dog are, at a certain stage of their development, almost indistinguishable was also denied. When we examine the human embryo in the third or fourth week of its development, we find it to be quite different in shape and structure from the full-grown human being, but almost identical with that of the ape, the dog, the rabbit, and other mammals, at the same stage of ontogeny. We find a bean-shaped body of very simple construction, with a tail below and a pair of fins at the sides, something like those of a fish, but very different from the limbs of man and the mammals. Nearly the whole front half of the body is taken up by a shapeless head without face, at the sides of which we find gill-clefts and arches as in the fish. At this stage of its development the human embryo does not differ in any essential detail from that of the ape, dog, horse, ox, etc., at a corresponding period. This important fact can easily be verified at any moment by a comparison of the embryos of man, the dog, rabbit, etc. Nevertheless, the theologians and dualist philosophers pronounced it to be a materialistic invention; even scientists, to whom the facts should be known, have sought to deny them.

There could not be a clearer proof of the profound importance of these embryological facts in favour of the monistic philosophy than is afforded by these efforts of its opponents to get rid of them by silence or denial. The truth is that these facts are most inconvenient for them, and are quite irreconcilable with their views. We must be all the more pressing on our side to put them in their proper light. I fully agree with Huxley when he says, in his "Man's Place in Nature": "Though these facts are ignored by several well-known popular leaders, they are easy to prove, and are accepted by all scientific men; on the other hand, their importance is so great that those who have once mastered them will, in my opinion, find few other biological discoveries to astonish them."

We shall make it our chief task to study the evolution of man's bodily frame and its various organs in their external form and internal structures. But I may observe at once that this is accompanied step by step with a study of the evolution of their functions. These two branches of inquiry are inseparably united in the whole of anthropology, just as in zoology (of which the former is only a section) or general biology. Everywhere the peculiar form of the organism and its structures, internal and external, is directly related to the special physiological functions which the organism or organ has to execute. This intimate connection of structure and function, or of the instrument and the work done by it, is seen in the science of evolution and all its parts. Hence the story of the evolution of structures, which is our immediate concern, is also the history of the development of functions; and this holds good of the human organism as of any other.