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Title: The Evolution of Man, V.1.

Author: Ernst Haeckel

Release Date: September, 2004 [EBook #6430]
[Yes, we are more than one year ahead of schedule]
[This file was first posted on December 13, 2002]

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*** START OF THE PROJECT GUTENBERG EBOOK THE EVOLUTION OF MAN, V.1. ***




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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.

At the same time, I must admit that our knowledge of the evolution of
functions is very far from being as complete as our acquaintance with
the evolution of structures. One might say, in fact, that the whole
science of evolution has almost confined itself to the study of
structures; the evolution of FUNCTIONS hardly exists even in name.
That is the fault of the physiologists, who have as yet concerned
themselves very little about evolution. It is only in recent times
that physiologists like W. Engelmann, W. Preyer, M. Verworn, and a few
others, have attacked the evolution of functions.

It will be the task of some future physiologist to engage in the study
of the evolution of functions with the same zeal and success as has
been done for the evolution of structures in morphogeny (the science
of the genesis of forms). Let me illustrate the close connection of
the two by a couple of examples. The heart in the human embryo has at
first a very simple construction, such as we find in permanent form
among the ascidiae and other low organisms; with this is associated a
very simple system of circulation of the blood. Now, when we find that
with the full-grown heart there comes a totally different and much
more intricate circulation, our inquiry into the development of the
heart becomes at once, not only an anatomical, but also a
physiological, study. Thus it is clear that the ontogeny of the heart
can only be understood in the light of its phylogeny (or development
in the past), both as regards function and structure. The same holds
true of all the other organs and their functions. For instance, the
science of the evolution of the alimentary canal, the lungs, or the
sexual organs, gives us at the same time, through the exact
comparative investigation of structure-development, most important
information with regard to the evolution of the functions of these
organs.

This significant connection is very clearly seen in the evolution of
the nervous system. This system is in the economy of the human body
the medium of sensation, will, and even thought, the highest of the
psychic functions; in a word, of all the various functions which
constitute the proper object of psychology. Modern anatomy and
physiology have proved that these psychic functions are immediately
dependent on the fine structure and the composition of the central
nervous system, or the internal texture of the brain and spinal cord.
In these we find the elaborate cell-machinery, of which the psychic or
soul-life is the physiological function. It is so intricate that most
men still look upon the mind as something supernatural that cannot be
explained on mechanical principles.

But embryological research into the gradual appearance and the
formation of this important system of organs yields the most
astounding and significant results. The first sketch of a central
nervous system in the human embryo presents the same very simple type
as in the other vertebrates. A spinal tube is formed in the external
skin of the back, and from this first comes a simple spinal cord
without brain, such as we find to be the permanent psychic organ in
the lowest type of vertebrate, the amphioxus. Not until a later stage
is a brain formed at the anterior end of this cord, and then it is a
brain of the most rudimentary kind, such as we find permanently among
the lower fishes. This simple brain develops step by step,
successively assuming forms which correspond to those of the amphibia,
the reptiles, the duck-bills, and the lemurs. Only in the last stage
does it reach the highly organised form which distinguishes the apes
from the other vertebrates, and which attains its full development in
man.

Comparative physiology discovers a precisely similar growth. The
function of the brain, the psychic activity, rises step by step with
the advancing development of its structure.

Thus we are enabled, by this story of the evolution of the nervous
system, to understand at length THE NATURAL DEVELOPMENT OF THE HUMAN
MIND and its gradual unfolding. It is only with the aid of embryology
that we can grasp how these highest and most striking faculties of the
animal organism have been historically evolved. In other words, a
knowledge of the evolution of the spinal cord and brain in the human
embryo leads us directly to a comprehension of the historic
development (or phylogeny) of the human mind, that highest of all
faculties, which we regard as something so marvellous and supernatural
in the adult man. This is certainly one of the greatest and most
pregnant results of evolutionary science. Happily our embryological
knowledge of man's central nervous system is now so adequate, and
agrees so thoroughly with the complementary results of comparative
anatomy and physiology, that we are thus enabled to obtain a clear
insight into one of the highest problems of philosophy, the phylogeny
of the soul, or the ancestral history of the mind of man. Our chief
support in this comes from the embryological study of it, or the
ontogeny of the soul. This important section of psychology owes its
origin especially to W. Preyer, in his interesting works, such as The
Mind of the Child. The Biography of a Baby (1900), of Milicent
Washburn Shinn, also deserves mention. [See also Preyer's Mental
Development in the Child (translation), and Sully's Studies of
Childhood and Children's Ways.]

In this way we follow the only path along which we may hope to reach
the solution of this difficult problem.

Thirty-six years have now elapsed since, in my General Morphology, I
established phylogeny as an independent science and showed its
intimate causal connection with ontogeny; thirty years have passed
since I gave in my gastraea-theory the proof of the justice of this,
and completed it with the theory of germinal layers. When we look back
on this period we may ask, What has been accomplished during it by the
fundamental law of biogeny? If we are impartial, we must reply that it
has proved its fertility in hundreds of sound results, and that by its
aid we have acquired a vast fund of knowledge which we should never
have obtained without it.

There has been no dearth of attacks--often violent attacks--on my
conception of an intimate causal connection between ontogenesis and
phylogenesis; but no other satisfactory explanation of these important
phenomena has yet been offered to us. I say this especially with
regard to Wilhelm His's theory of a "mechanical evolution," which
questions the truth of phylogeny generally, and would explain the
complicated embryonic processes without going beyond by simple
physical changes--such as the bending and folding of leaves by
electricity, the origin of cavities through unequal strain of the
tissues, the formation of processes by uneven growth, and so on. But
the fact is that these embryological phenomena themselves demand
explanation in turn, and this can only be found, as a rule, in the
corresponding changes in the long ancestral series, or in the
physiological functions of heredity and adaptation.


CHAPTER 1.2. THE OLDER EMBRYOLOGY.

It is in many ways useful, on entering upon the study of any science,
to cast a glance at its historical development. The saying that
"everything is best understood in its growth" has a distinct
application to science. While we follow its gradual development we get
a clearer insight into its aims and objects. Moreover, we shall see
that the present condition of the science of human evolution, with all
its characteristics, can only be rightly understood when we examine
its historical growth. This task will, however, not detain us long.
The study of man's evolution is one of the latest branches of natural
science, whether you consider the embryological or the phylogenetic
section of it.

Apart from the few germs of our science which we find in classical
antiquity, and which we shall notice presently, we may say that it
takes its definite rise, as a science, in the year 1759, when one of
the greatest German scientists, Caspar Friedrich Wolff, published his
Theoria generationis. That was the foundation-stone of the science of
animal embryology. It was not until fifty years later, in 1809, that
Jean Lamarck published his Philosophie Zoologique--the first effort to
provide a base for the theory of evolution; and it was another
half-century before Darwin's work appeared (in 1859), which we may
regard as the first scientific attainment of this aim. But before we
go further into this solid establishment of evolution, we must cast a
brief glance at that famous philosopher and scientist of antiquity,
who stood alone in this, as in many other branches of science, for
more than 2000 years: the "father of Natural History," Aristotle.

The extant scientific works of Aristotle deal with many different
sides of biological research; the most comprehensive of them is his
famous History of Animals. But not less interesting is the smaller
work, On the Generation of Animals (Peri zoon geneseos). This work
treats especially of embryonic development, and it is of great
interest as being the earliest of its kind and the only one that has
come down to us in any completeness from classical antiquity.

Aristotle studied embryological questions in various classes of
animals, and among the lower groups he learned many most remarkable
facts which we only rediscovered between 1830 and 1860. It is certain,
for instance, that he was acquainted with the very peculiar mode of
propagation of the cuttlefishes, or cephalopods, in which a yelk-sac
hangs out of the mouth of the foetus. He knew, also, that embryos come
from the eggs of the bee even when they have not been fertilised. This
"parthenogenesis" (or virgin-birth) of the bees has only been
established in our time by the distinguished zoologist of Munich,
Siebold. He discovered that male bees come from the unfertilised, and
female bees only from the fertilised, eggs. Aristotle further states
that some kinds of fishes (of the genus serranus) are hermaphrodites,
each individual having both male and female organs and being able to
fertilise itself; this, also, has been recently confirmed. He knew
that the embryo of many fishes of the shark family is attached to the
mother's body by a sort of placenta, or nutritive organ very rich in
blood; apart from these, such an arrangement is only found among the
higher mammals and man. This placenta of the shark was looked upon as
legendary for a long time, until Johannes Muller proved it to be a
fact in 1839. Thus a number of remarkable discoveries were found in
Aristotle's embryological work, proving a very good acquaintance of
the great scientist--possibly helped by his predecessors--with the
facts of ontogeny, and a great advance upon succeeding generations in
this respect.

In the case of most of these discoveries he did not merely describe
the fact, but added a number of observations on its significance. Some
of these theoretical remarks are of particular interest, because they
show a correct appreciation of the nature of the embryonic processes.
He conceives the development of the individual as a new formation, in
the course of which the various parts of the body take shape
successively. When the human or animal frame is developed in the
mother's body, or separately in an egg, the heart--which he regards as
the starting-point and centre of the organism--must appear first. Once
the heart is formed the other organs arise, the internal ones before
the external, the upper (those above the diaphragm) before the lower
(or those beneath the diaphragm). The brain is formed at an early
stage, and the eyes grow out of it. These observations are quite
correct. And, if we try to form some idea from these data of
Aristotle's general conception of the embryonic process, we find a dim
prevision of the theory which Wolff showed 2000 years afterwards to be
the correct view. It is significant, for instance, that Aristotle
denied the eternity of the individual in any respect. He said that the
species or genus, the group of similar individuals, might be eternal,
but the individual itself is temporary. It comes into being in the act
of procreation, and passes away at death.

During the 2000 years after Aristotle no progress whatever was made in
general zoology, or in embryology in particular. People were content
to read, copy, translate, and comment on Aristotle. Scarcely a single
independent effort at research was made in the whole of the period.
During the Middle Ages the spread of strong religious beliefs put
formidable obstacles in the way of independent scientific
investigation. There was no question of resuming the advance of
biology. Even when human anatomy began to stir itself once more in the
sixteenth century, and independent research was resumed into the
structure of the developed body, anatomists did not dare to extend
their inquiries to the unformed body, the embryo, and its development.
There were many reasons for the prevailing horror of such studies. It
is natural enough, when we remember that a Bull of Boniface VIII
excommunicated every man who ventured to dissect a human corpse. If
the dissection of a developed body were a crime to be thus punished,
how much more dreadful must it have seemed to deal with the embryonic
body still enclosed in the womb, which the Creator himself had
decently veiled from the curiosity of the scientist! The Christian
Church, then putting many thousands to death for unbelief, had a
shrewd presentiment of the menace that science contained against its
authority. It was powerful enough to see that its rival did not grow
too quickly.

It was not until the Reformation broke the power of the Church, and a
refreshing breath of the spirit dissolved the icy chains that bound
science, that anatomy and embryology, and all the other branches of
research, could begin to advance once more. However, embryology lagged
far behind anatomy. The first works on embryology appear at the
beginning of the sixteenth century. The Italian anatomist, Fabricius
ab Aquapendente, a professor at Padua, opened the advance. In his two
books (De formato foetu, 1600, and De formatione foetus, 1604) he
published the older illustrations and descriptions of the embryos of
man and other mammals, and of the hen. Similar imperfect illustrations
were given by Spigelius (De formato foetu, 1631), and by Needham
(1667) and his more famous compatriot, Harvey (1652), who discovered
the circulation of the blood in the animal body and formulated the
important principle, Omne vivum ex vivo (all life comes from
pre-existing life). The Dutch scientist, Swammerdam, published in his
Bible of Nature the earliest observations on the embryology of the
frog and the division of its egg-yelk. But the most important
embryological studies in the sixteenth century were those of the
famous Italian, Marcello Malpighi, of Bologna, who led the way both in
zoology and botany. His treatises, De formatione pulli and De ovo
incubato (1687), contain the first consistent description of the
development of the chick in the fertilised egg.

Here I ought to say a word about the important part played by the
chick in the growth of our science. The development of the chick, like
that of the young of all other birds, agrees in all its main features
with that of the other chief vertebrates, and even of man. The three
highest classes of vertebrates--mammals, birds, and reptiles (lizards,
serpents, tortoises, etc.)--have from the beginning of their embryonic
development so striking a resemblance in all the chief points of
structure, and especially in their first forms, that for a long time
it is impossible to distinguish between them. We have known now for
some time that we need only examine the embryo of a bird, which is the
easiest to get at, in order to learn the typical mode of development
of a mammal (and therefore of man). As soon as scientists began to
study the human embryo, or the mammal-embryo generally, in its earlier
stages about the middle and end of the seventeenth century, this
important fact was very quickly discovered. It is both theoretically
and practically of great value. As regards the THEORY of evolution, we
can draw the most weighty inferences from this similarity between the
embryos of widely different classes of animals. But for the practical
purposes of embryological research the discovery is invaluable,
because we can fill up the gaps in our imperfect knowledge of the
embryology of the mammals from the more thoroughly studied embryology
of the bird. Hens' eggs are easily to be had in any quantity, and the
development of the chick may be followed step by step in artificial
incubation. The development of the mammal is much more difficult to
follow, because here the embryo is not detached and enclosed in a
large egg, but the tiny ovum remains in the womb until the growth is
completed. Hence, it is very difficult to keep up sustained
observation of the various stages in any great extent, quite apart
from such extrinsic considerations as the cost, the technical
difficulties, and many other obstacles which we encounter when we
would make an extensive study of the fertilised mammal. The chicken
has, therefore, always been the chief object of study in this
connection. The excellent incubators we now have enable us to observe
it in any quantity and at any stage of development, and so follow the
whole course of its formation step by step.

By the end of the seventeenth century Malpighi had advanced as far as
it was possible to do with the imperfect microscope of his time in the
embryological study of the chick. Further progress was arrested until
the instrument and the technical methods should be improved. The
vertebrate embryos are so small and delicate in their earlier stages
that you cannot go very far into the study of them without a good
microscope and other technical aid. But this substantial improvement
of the microscope and the other apparatus did not take place until the
beginning of the nineteenth century.

Embryology made scarcely any advance in the first half of the
eighteenth century, when the systematic natural history of plants and
animals received so great an impulse through the publication of
Linne's famous Systema Naturae. Not until 1759 did the genius arise
who was to give it an entirely new character, Caspar Friedrich Wolff.
Until then embryology had been occupied almost exclusively in
unfortunate and misleading efforts to build up theories on the
imperfect empirical material then available.

The theory which then prevailed, and remained in favour throughout
nearly the whole of the eighteenth century, was commonly called at
that time "the evolution theory"; it is better to describe it as "the
preformation theory."* (* This theory is usually known as the
"evolution theory" in Germany, in contradistinction to the "epigenesis
theory." But as it is the latter that is called the "evolution theory"
in England, France, and Italy, and "evolution" and "epigenesis" are
taken to be synonymous, it seems better to call the first the
"pre-formation theory.") Its chief point is this: There is no new
formation of structures in the embryonic development of any organism,
animal or plant, or even of man; there is only a growth, or unfolding,
of parts which have been constructed or pre-formed from all eternity,
though on a very small scale and closely packed together. Hence, every
living germ contains all the organs and parts of the body, in the form
and arrangement they will present later, already within it, and thus
the whole embryological process is merely an evolution in the literal
sense of the word, or an unfolding, of parts that were pre-formed and
folded up in it. So, for instance, we find in the hen's egg not merely
a simple cell, that divides and subdivides and forms germinal layers,
and at last, after all kinds of variation and cleavage and
reconstruction, brings forth the body of the chick; but there is in
every egg from the first a complete chicken, with all its parts made
and neatly packed. These parts are so small or so transparent that the
microscope cannot detect them. In the hatching, these parts merely
grow larger, and spread out in the normal way.

When this theory is consistently developed it becomes a "scatulation
theory."* (* "Packing theory" would be the literal translation.
Scatula is the Latin for a case or box.--Translator.) According to its
teaching, there was made in the beginning one couple or one individual
of each species of animal or plant; but this one individual contained
the germs of all the other individuals of the same species who should
ever come to life. As the age of the earth was generally believed at
that time to be fixed by the Bible at 5000 or 6000 years, it seemed
possible to calculate how many individuals of each species had lived
in the period, and so had been packed inside the first being that was
created. The theory was consistently extended to man, and it was
affirmed that our common parent Eve had had stored in her ovary the
germs of all the children of men.

The theory at first took the form of a belief that it was the FEMALES
who were thus encased in the first being. One couple of each species
was created, but the female contained in her ovary all the future
individuals of the species, of either sex. However, this had to be
altered when the Dutch microscopist, Leeuwenhoek, discovered the male
spermatozoa in 1690, and showed that an immense number of these
extremely fine and mobile thread-like beings exist in the male sperm
(this will be explained in Chapter 2.7). This astonishing discovery
was further advanced when it was proved that these living bodies,
swimming about in the seminal fluid, were real animalcules, and, in
fact, were the pre-formed germs of the future generation. When the
male and female procreative elements came together at conception,
these thread-like spermatozoa ("seed-animals") were supposed to
penetrate into the fertile body of the ovum and begin to develop
there, as the plant seed does in the fruitful earth. Hence, every
spermatozoon was regarded as a homunculus, a tiny complete man; all
the parts were believed to be pre-formed in it, and merely grew larger
when it reached its proper medium in the female ovum. This theory,
also, was consistently developed in the sense that in each of these
thread-like bodies the whole of its posterity was supposed to be
present in the minutest form. Adam's sexual glands were thought to
have contained the germs of the whole of humanity.

This "theory of male scatulation" found itself at once in keen
opposition to the prevailing "female" theory. The two rival theories
at once opened a very lively campaign, and the physiologists of the
eighteenth century were divided into two great camps--the
Animalculists and the Ovulists--which fought vigorously. The
animalculists held that the spermatozoa were the true germs, and
appealed to the lively movements and the structure of these bodies.
The opposing party of the Ovulists, who clung to the older "evolution
theory," affirmed that the ovum is the real germ, and that the
spermatozoa merely stimulate it at conception to begin its growth; all
the future generations are stored in the ovum. This view was held by
the great majority of the biologists of the eighteenth century, in
spite of the fact that Wolff proved it in 1759 to be without
foundation. It owed its prestige chiefly to the circumstance that the
most weighty authorities in the biology and philosophy of the day
decided in favour of it, especially Haller, Bonnet, and Leibnitz.

Albrecht Haller, professor at Gottingen, who is often called the
father of physiology, was a man of wide and varied learning, but he
does not occupy a very high position in regard to insight into natural
phenomena. He made a vigorous defence of the "evolutionary theory" in
his famous work, Elementa physiologiae, affirming: "There is no such
thing as formation (nulla est epigenesis). No part of the animal frame
is made before another; all were made together." He thus denied that
there was any evolution in the proper sense of the word, and even went
so far as to say that the beard existed in the new-born child and the
antlers in the hornless fawn; all the parts were there in advance, and
were merely hidden from the eye of man for the time being. Haller even
calculated the number of human beings that God must have created on
the sixth day and stored away in Eve's ovary. He put the number at
200,000 millions, assuming the age of the world to be 6000 years, the
average age of a human being to be thirty years, and the population of
the world at that time to be 1000 millions. And the famous Haller
maintained all this nonsense, in spite of its ridiculous consequences,
even after Wolff had discovered the real course of embryonic
development and established it by direct observation!

Among the philosophers of the time the distinguished Leibnitz was the
chief defender of the "preformation theory," and by his authority and
literary prestige won many adherents to it. Supported by his system of
monads, according to which body and soul are united in inseparable
association and by their union form the individual, or the "monad,"
Leibnitz consistently extended the "scatulation theory" to the soul,
and held that this was no more evolved than the body. He says, for
instance, in his Theodicee: "I mean that these souls, which one day
are to be the souls of men, are present in the seed, like those of
other species; in such wise that they existed in our ancestors as far
back as Adam, or from the beginning of the world, in the forms of
organised bodies."

The theory seemed to receive considerable support from the
observations of one of its most zealous supporters, Bonnet. In 1745 he
discovered, in the plant-louse, a case of parthenogenesis, or
virgin-birth, an interesting form of reproduction that has lately been
found by Siebold and others among various classes of the articulata,
especially crustacea and insects. Among these and other animals of
certain lower species the female may reproduce for several generations
without having been fertilised by the male. These ova that do not need
fertilisation are called "false ova," pseudova or spores. Bonnet saw
that a female plant-louse, which he had kept in cloistral isolation,
and rigidly removed from contact with males, had on the eleventh day
(after forming a new skin for the fourth time) a living daughter, and
during the next twenty days ninety-four other daughters; and that all
of them went on to reproduce in the same way without any contact with
males. It seemed as if this furnished an irrefutable proof of the
truth of the scatulation theory, as it was held by the Ovulists; it is
not surprising to find that the theory then secured general
acceptance.

This was the condition of things when suddenly, in 1759, Caspar
Friedrich Wolff appeared, and dealt a fatal blow at the whole
preformation theory with his new theory of epigenesis. Wolff, the son
of a Berlin tailor, was born in 1733, and went through his scientific
and medical studies, first at Berlin under the famous anatomist
Meckel, and afterwards at Halle. Here he secured his doctorate in his
twenty-sixth year, and in his academic dissertation (November 28th,
1759), the Theoria generationis, expounded the new theory of a real
development on a basis of epigenesis. This treatise is, in spite of
its smallness and its obscure phraseology, one of the most valuable in
the whole range of biological literature. It is equally distinguished
for the mass of new and careful observations it contains, and the
far-reaching and pregnant ideas which the author everywhere extracts
from his observations and builds into a luminous and accurate theory
of generation. Nevertheless, it met with no success at the time.
Although scientific studies were then assiduously cultivated owing to
the impulse given by Linne--although botanists and zoologists were no
longer counted by dozens, but by hundreds, hardly any notice was taken
of Wolff's theory. Even when he established the truth of epigenesis by
the most rigorous observations, and demolished the airy structure of
the preformation theory, the "exact" scientist Haller proved one of
the most strenuous supporters of the old theory, and rejected Wolff's
correct view with a dictatorial "There is no such thing as evolution."
He even went on to say that religion was menaced by the new theory! It
is not surprising that the whole of the physiologists of the second
half of the eighteenth century submitted to the ruling of this
physiological pontiff, and attacked the theory of epigenesis as a
dangerous innovation. It was not until more than fifty years
afterwards that Wolff's work was appreciated. Only when Meckel
translated into German in 1812 another valuable work of Wolff's on The
Formation of the Alimentary Canal (written in 1768), and called
attention to its great importance, did people begin to think of him
once more; yet this obscure writer had evinced a profounder insight
into the nature of the living organism than any other scientist of the
eighteenth century.

Wolff's idea led to an appreciable advance over the whole field of
biology. There is such a vast number of new and important observations
and pregnant thoughts in his writings that we have only gradually
learned to appreciate them rightly in the course of the nineteenth
century. He opened up the true path for research in many directions.
In the first place, his theory of epigenesis gave us our first real
insight into the nature of embryonic development. He showed
convincingly that the development of every organism consists of a
series of NEW FORMATIONS, and that there is no trace whatever of the
complete form either in the ovum or the spermatozoon. On the contrary,
these are quite simple bodies, with a very different purport. The
embryo which is developed from them is also quite different, in its
internal arrangement and outer configuration, from the complete
organism. There is no trace whatever of preformation or in-folding of
organs. To-day we can scarcely call epigenesis a THEORY, because we
are convinced it is a fact, and can demonstrate it at any moment with
the aid of the microscope.

Wolff furnished the conclusive empirical proof of his theory in his
classic dissertation on The Formation of the Alimentary Canal (1768).
In its complete state the alimentary canal of the hen is a long and
complex tube, with which the lungs, liver, salivary glands, and many
other small glands, are connected. Wolff showed that in the early
stages of the embryonic chick there is no trace whatever of this
complicated tube with all its dependencies, but instead of it only a
flat, leaf-shaped body; that, in fact, the whole embryo has at first
the appearance of a flat, oval-shaped leaf. When we remember how
difficult the exact observation of so fine and delicate a structure as
the early leaf-shaped body of the chick must have been with the poor
microscopes then in use, we must admire the rare faculty for
observation which enabled Wolff to make the most important discoveries
in this most difficult part of embryology. By this laborious research
he reached the correct opinion that the embryonic body of all the
higher animals, such as the birds, is for some time merely a flat,
thin, leaf-shaped disk--consisting at first of one layer, but
afterwards of several. The lowest of these layers is the alimentary
canal, and Wolff followed its development from its commencement to its
completion. He showed how this leaf-shaped structure first turns into
a groove, then the margins of this groove fold together and form a
closed canal, and at length the two external openings of the tube (the
mouth and anus) appear.

Moreover, the important fact that the other systems of organs are
developed in the same way, from tubes formed out of simple layers, did
not escape Wolff. The nerveless system, muscular system, and vascular
(blood-vessel) system, with all the organs appertaining thereto, are,
like the alimentary system, developed out of simple leaf-shaped
structures. Hence, Wolff came to the view by 1768 which Pander
developed in the Theory of Germinal Layers fifty years afterwards. His
principles are not literally correct; but he comes as near to the
truth in them as was possible at that time, and could be expected of
him.

Our admiration of this gifted genius increases when we find that he
was also the precursor of Goethe in regard to the metamorphosis of
plants and of the famous cellular theory. Wolff had, as Huxley showed,
a clear presentiment of this cardinal theory, since he recognised
small microscopic globules as the elementary parts out of which the
germinal layers arose.

Finally, I must invite special attention to the MECHANICAL character
of the profound philosophic reflections which Wolff always added to
his remarkable observations. He was a great monistic philosopher, in
the best meaning of the word. It is unfortunate that his philosophic
discoveries were ignored as completely as his observations for more
than half a century. We must be all the more careful to emphasise the
fact of their clear monistic tendency.


CHAPTER 1.3. MODERN EMBRYOLOGY.

We may distinguish three chief periods in the growth of our science of
human embryology. The first has been considered in the preceding
chapter; it embraces the whole of the preparatory period of research,
and extends from Aristotle to Caspar Friedrich Wolff, or to the year
1759, in which the epoch-making Theoria generationis was published.
The second period, with which we have now to deal, lasts about a
century--that is to say, until the appearance of Darwin's Origin of
Species, which brought about a change in the very foundations of
biology, and, in particular, of embryology. The third period begins
with Darwin. When we say that the second period lasted a full century,
we must remember that Wolff's work had remained almost unnoticed
during half the time--namely, until the year 1812. During the whole of
these fifty-three years not a single book that appeared followed up
the path that Wolff had opened, or extended his theory of embryonic
development. We merely find his views--perfectly correct views, based
on extensive observations of fact--mentioned here and there as
erroneous; their opponents, who adhered to the dominant theory of
preformation, did not even deign to reply to them. This unjust
treatment was chiefly due to the extraordinary authority of Albrecht
von Haller; it is one of the most astonishing instances of a great
authority, as such, preventing for a long time the recognition of
established facts.

The general ignorance of Wolff's work was so great that at the
beginning of the nineteenth century two scientists of Jena, Oken
(1806) and Kieser (1810), began independent research into the
development of the alimentary canal of the chick, and hit upon the
right clue to the embryonic puzzle, without knowing a word about
Wolff's important treatise on the same subject. They were treading in
his very footsteps without suspecting it. This can be easily proved
from the fact that they did not travel as far as Wolff. It was not
until Meckel translated into German Wolff's book on the alimentary
system, and pointed out its great importance, that the eyes of
anatomists and physiologists were suddenly opened. At once a number of
biologists instituted fresh embryological inquiries, and began to
confirm Wolff's theory of epigenesis.

This resuscitation of embryology and development of the
epigenesis-theory was chiefly connected with the university of
Wurtzburg. One of the professors there at that time was Dollinger, an
eminent biologist, and father of the famous Catholic historian who
later distinguished himself by his opposition to the new dogma of
papal infallibility. Dollinger was both a profound thinker and an
accurate observer. He took the keenest interest in embryology, and
worked at it a good deal. However, he is not himself responsible for
any important result in this field. In 1816 a young medical doctor,
whom we may at once designate as Wolff's chief successor, Karl Ernst
von Baer, came to Wurtzburg. Baer's conversations with Dollinger on
embryology led to a fresh series of most extensive investigations.
Dollinger had expressed a wish that some young scientist should begin
again under his guidance an independent inquiry into the development
of the chick during the hatching of the egg. As neither he nor Baer
had money enough to pay for an incubator and the proper control of the
experiments, and for a competent artist to illustrate the various
stages observed, the lead of the enterprise was given to Christian
Pander, a wealthy friend of Baer's who had been induced by Baer to
come to Wurtzburg. An able engraver, Dalton, was engaged to do the
copper-plates. In a short time the embryology of the chick, in which
Baer was taking the greatest indirect interest, was so far advanced
that Pander was able to sketch the main features of it on the ground
of Wolff's theory in the dissertation he published in 1817. He clearly
enunciated the theory of germinal layers which Wolff had anticipated,
and established the truth of Wolff's idea of a development of the
complicated systems of organs out of simple leaf-shaped primitive
structures. According to Pander, the leaf-shaped object in the hen's
egg divides, before the incubation has proceeded twelve hours, into
two different layers, an external serous layer and an internal mucous
layer; between the two there develops later a third layer, the
vascular (blood-vessel) layer.* (* The technical terms which are bound
to creep into this chapter will be fully understood later
on.--Translator.)

Karl Ernst von Baer, who had set afoot Pander's investigation, and had
shown the liveliest interest in it after Pander's departure from
Wurtzburg, began his own much more comprehensive research in 1819. He
published the mature result nine years afterwards in his famous work,
Animal Embryology: Observation and Reflection (not translated). This
classic work still remains a model of careful observation united to
profound philosophic speculation. The first part appeared in 1828, the
second in 1837. The book proved to be the foundation on which the
whole science of embryology has built down to our own day. It so far
surpassed its predecessors, and Pander in particular, that it has
become, after Wolff's work, the chief base of modern embryology.

Baer was one of the greatest scientists of the nineteenth century, and
exercised considerable influence on other branches of biology as well.
He built up the theory of germinal layers, as a whole and in detail,
so clearly and solidly that it has been the starting-point of
embryological research ever since. He taught that in all the
vertebrates first two and then four of these germinal layers are
formed; and that the earliest rudimentary organs of the body arise by
the conversion of these layers into tubes. He described the first
appearance of the vertebrate embryo, as it may be seen in the globular
yelk of the fertilised egg, as an oval disk which first divides into
two layers. From the upper or animal layer are developed all the
organs which accomplish the phenomena of animal life--the functions of
sensation and motion, and the covering of the body. From the lower or
vegetative layer come the organs which effect the vegetative life of
the organism--nutrition, digestion, blood-formation, respiration,
secretion, reproduction, etc.

Each of these original layers divides, according to Baer, into two
thinner and superimposed layers or plates. He calls the two plates of
the animal layer, the skin-stratum and muscle-stratum. From the upper
of these plates, the skin-stratum, the external skin, or outer
covering of the body, the central nervous system, and the
sense-organs, are formed. From the lower, or muscle-stratum, the
muscles, or fleshy parts and the bony skeleton--in a word, the motor
organs--are evolved. In the same way, Baer said, the lower or
vegetative layer splits into two plates, which he calls the
vascular-stratum and the mucous-stratum. From the outer of the two
(the vascular) the heart, blood-vessels, spleen, and the other
vascular glands, the kidneys, and sexual glands, are formed. From the
fourth or mucous layer, in fine, we get the internal and digestive
lining of the alimentary canal and all its dependencies, the liver,
lungs, salivary glands, etc. Baer had, in the main, correctly judged
the significance of these four secondary embryonic layers, and he
followed the conversion of them into the tube-shaped primitive organs
with great perspicacity. He first solved the difficult problem of the
transformation of this four-fold, flat, leaf-shaped, embryonic disk
into the complete vertebrate body, through the conversion of the
layers or plates into tubes. The flat leaves bend themselves in
obedience to certain laws of growth; the borders of the curling plates
approach nearer and nearer; until at last they come into actual
contact. Thus out of the flat gut-plate is formed a hollow gut-tube,
out of the flat spinal plate a hollow nerve-tube, from the skin-plate
a skin-tube, and so on.

Among the many great services which Baer rendered to embryology,
especially vertebrate embryology, we must not forget his discovery of
the human ovum. Earlier scientists had, as a rule, of course, assumed
that man developed out of an egg, like the other animals. In fact, the
preformation theory held that the germs of the whole of humanity were
stored already in Eve's ova. But the real ovum escaped detection until
the year 1827. This ovum is extremely small, being a tiny round
vesicle about the 1/120 of an inch in diameter; it can be seen under
very favourable circumstances with the naked eye as a tiny particle,
but is otherwise quite invisible. This particle is formed in the ovary
inside a much larger globule, which takes the name of the Graafian
follicle, from its discoverer, Graaf, and had previously been regarded
as the true ovum. However, in 1827 Baer proved that it was not the
real ovum, which is much smaller, and is contained within the
follicle. (Compare the end of Chapter 2.29.)

Baer was also the first to observe what is known as the segmentation
sphere of the vertebrate; that is to say, the round vesicle which
first develops out of the impregnated ovum, and the thin wall of which
is made up of a single layer of regular, polygonal (many-cornered)
cells (see the illustration in Chapter 1.12). Another discovery of his
that was of great importance in constructing the vertebrate stem and
the characteristic organisation of this extensive group (to which man
belongs) was the detection of the axial rod, or the chorda dorsalis.
There is a long, round, cylindrical rod of cartilage which runs down
the longer axis of the vertebrate embryo; it appears at an early
stage, and is the first sketch of the spinal column, the solid
skeletal axis of the vertebrate. In the lowest of the vertebrates, the
amphioxus, the internal skeleton consists only of this cord throughout
life. But even in the case of man and all the higher vertebrates it is
round this cord that the spinal column and the brain are afterwards
formed.

However, important as these and many other discoveries of Baer's were
in vertebrate embryology, his researches were even more influential,
from the circumstance that he was the first to employ the comparative
method in studying the development of the animal frame. Baer occupied
himself chiefly with the embryology of vertebrates (especially the
birds and fishes). But he by no means confined his attention to these,
gradually taking the various groups of the invertebrates into his
sphere of study. As the general result of his comparative
embryological research, Baer distinguished four different modes of
development and four corresponding groups in the animal world. These
chief groups or types are: 1, the vertebrata; 2, the articulata; 3,
the mollusca; and 4, all the lower groups which were then wrongly
comprehended under the general name of the radiata. Georges Cuvier had
been the first to formulate this distinction, in 1812. He showed that
these groups present specific differences in their whole internal
structure, and the connection and disposal of their systems of organs;
and that, on the other hand, all the animals of the same type--say,
the vertebrates--essentially agreed in their inner structure, in spite
of the greatest superficial differences. But Baer proved that these
four groups are also quite differently developed from the ovum; and
that the series of embryonic forms is the same throughout for animals
of the same type, but different in the case of other animals. Up to
that time the chief aim in the classification of the animal kingdom
was to arrange all the animals from lowest to highest, from the
infusorium to man, in one long and continuous series. The erroneous
idea prevailed nearly everywhere that there was one uninterrupted
chain of evolution from the lowest animal to the highest. Cuvier and
Baer proved that this view was false, and that we must distinguish
four totally different types of animals, on the ground of anatomic
structure and embryonic development.

Baer's epoch-making works aroused an extraordinary and widespread
interest in embryological research. Immediately afterwards we find a
great number of observers at work in the newly opened field, enlarging
it in a very short time with great energy by their various discoveries
in detail. Next to Baer's comes the admirable work of Heinrich Rathke,
of Konigsberg (died 1860); he made an extensive study of the
embryology, not only of the invertebrates (crustaceans, insects,
molluscs), but also, and particularly, of the vertebrates (fishes,
tortoises, serpents, crocodiles, etc.). We owe the first comprehensive
studies of mammal embryology to the careful research of Wilhelm
Bischoff, of Munich; his embryology of the rabbit (1840), the dog
(1842), the guinea-pig (1852), and the doe (1854), still form
classical studies. About the same time a great impetus was given to
the embryology of the invertebrates. The way was opened through this
obscure province by the studies of the famous Berlin zoologist,
Johannes Muller, on the echinoderms. He was followed by Albert
Kolliker, of Wurtzburg, writing on the cuttlefish (or the
cephalopods), Siebold and Huxley on worms and zoophytes, Fritz Muller
(Desterro) on the crustacea, Weismann on insects, and so on. The
number of workers in this field has greatly increased of late, and a
quantity of new and astonishing discoveries have been made. One
notices, in several of these recent works on embryology, that their
authors are too little acquainted with comparative anatomy and
classification. Palaeontology is, unfortunately, altogether neglected
by many of these new workers, although this interesting science
furnishes most important facts for phylogeny, and thus often proves of
very great service in ontogeny.

A very important advance was made in our science in 1839, when the
cellular theory was established, and a new field of inquiry bearing on
embryology was suddenly opened. When the famous botanist, M.
Schleiden, of Jena, showed in 1838, with the aid of the microscope,
that every plant was made up of innumerable elementary parts, which we
call cells, a pupil of Johannes Muller at Berlin, Theodor Schwann,
applied the discovery at once to the animal organism. He showed that
in the animal body as well, when we examine its tissues in the
microscope, we find these cells everywhere to be the elementary units.
All the different tissues of the organism, especially the very
dissimilar tissues of the nerves, muscles, bones, external skin,
mucous lining, etc., are originally formed out of cells; and this is
also true of all the tissues of the plant. These cells are separate
living beings; they are the citizens of the State which the entire
multicellular organism seems to be. This important discovery was bound
to be of service to embryology, as it raised a number of new
questions. What is the relation of the cells to the germinal layers?
Are the germinal layers composed of cells, and what is their relation
to the cells of the tissues that form later? How does the ovum stand
in the cellular theory? Is the ovum itself a cell, or is it composed
of cells? These important questions were now imposed on the
embryologist by the cellular theory.

The most notable effort to answer these questions--which were attacked
on all sides by different students--is contained in the famous work,
Inquiries into the Development of the Vertebrates (not translated) of
Robert Remak, of Berlin (1851). This gifted scientist succeeded in
mastering, by a complete reform of the science, the great difficulties
which the cellular theory had at first put in the way of embryology. A
Berlin anatomist, Carl Boguslaus Reichert, had already attempted to
explain the origin of the tissues. But this attempt was bound to
miscarry, since its not very clear-headed author lacked a sound
acquaintance with embryology and the cell theory, and even with the
structure and development of the tissue in particular. Remak at length
brought order into the dreadful confusion that Reichert had caused; he
gave a perfectly simple explanation of the origin of the tissues. In
his opinion the animal ovum is always a simple cell: the germinal
layers which develop out of it are always composed of cells; and these
cells that constitute the germinal layers arise simply from the
continuous and repeated cleaving (segmentation) of the original
solitary cell. It first divides into two and then into four cells; out
of these four cells are born eight, then sixteen, thirty-two, and so
on. Thus, in the embryonic development of every animal and plant there
is formed first of all out of the simple egg cell, by a repeated
subdivision, a cluster of cells, as Kolliker had already stated in
connection with the cephalopods in 1844. The cells of this group
spread themselves out flat and form leaves or plates; each of these
leaves is formed exclusively out of cells. The cells of different
layers assume different shapes, increase, and differentiate; and in
the end there is a further cleavage (differentiation) and division of
work of the cells within the layers, and from these all the different
tissues of the body proceed.

These are the simple foundations of histogeny, or the science that
treats of the development of the tissues (hista), as it was
established by Remak and Kolliker. Remak, in determining more closely
the part which the different germinal layers play in the formation of
the various tissues and organs, and in applying the theory of
evolution to the cells and the tissues they compose, raised the theory
of germinal layers, at least as far as it regards the vertebrates, to
a high degree of perfection.

Remak showed that three layers are formed out of the two germinal
layers which compose the first simple leaf-shaped structure of the
vertebrate body (or the "germinal disk"), as the lower layer splits
into two plates. These three layers have a very definite relation to
the various tissues. First of all, the cells which form the outer skin
of the body (the epidermis), with its various dependencies (hairs,
nails, etc.)--that is to say, the entire outer envelope of the
body--are developed out of the outer or upper layer; but there are
also developed in a curious way out of the same layer the cells which
form the central nervous system, the brain and the spinal cord. In the
second place, the inner or lower germinal layer gives rise only to the
cells which form the epithelium (the whole inner lining) of the
alimentary canal and all that depends on it (the lungs, liver,
pancreas, etc.), or the tissues that receive and prepare the
nourishment of the body. Finally, the middle layer gives rise to all
the other tissues of the body, the muscles, blood, bones, cartilage,
etc. Remak further proved that this middle layer, which he calls "the
motor-germinative layer," proceeds to subdivide into two secondary
layers. Thus we find once more the four layers which Baer had
indicated. Remak calls the outer secondary leaf of the middle layer
(Baer's "muscular layer") the "skin layer" (it would be better to say,
skin-fibre layer); it forms the outer wall of the body (the true skin,
the muscles, etc.). To the inner secondary leaf (Baer's "vascular
layer") he gave the name of the "alimentary-fibre layer"; this forms
the outer envelope of the alimentary canal, with the mesentery, the
heart, the blood-vessels, etc.

On this firm foundation provided by Remak for histogeny, or the
science of the formation of the tissues, our knowledge has been
gradually built up and enlarged in detail. There have been several
attempts to restrict and even destroy Remak's principles. The two
anatomists, Reichert (of Berlin) and Wilhelm His (of Leipzic),
especially, have endeavoured in their works to introduce a new
conception of the embryonic development of the vertebrate, according
to which the two primary germinal layers would not be the sole sources
of formation. But these efforts were so seriously marred by ignorance
of comparative anatomy, an imperfect acquaintance with ontogenesis,
and a complete neglect of phylogenesis, that they could not have more
than a passing success. We can only explain how these curious attacks
of Reichert and His came to be regarded for a time as advances by the
general lack of discrimination and of grasp of the true object of
embryology.

Wilhelm His published, in 1868, his extensive Researches into the
Earliest Form of the Vertebrate Body,* (* None of His's works have
been translated into English.) one of the curiosities of embryological
literature. The author imagines that he can build a "mechanical theory
of embryonic development" by merely giving an exact description of the
embryology of the chick, without any regard to comparative anatomy and
phylogeny, and thus falls into an error that is almost without
parallel in the history of biological literature. As the final result
of his laborious investigations, His tells us "that a comparatively
simple law of growth is the one essential thing in the first
development. Every formation, whether it consist in cleavage of
layers, or folding, or complete division, is a consequence of this
fundamental law." Unfortunately, he does not explain what this "law of
growth" is; just as other opponents of the theory of selection, who
would put in its place a great "law of evolution," omit to tell us
anything about the nature of this. Nevertheless, it is quite clear
from His's works that he imagines constructive Nature to be a sort of
skilful tailor. The ingenious operator succeeds in bringing into
existence, by "evolution," all the various forms of living things by
cutting up in different ways the germinal layers, bending and folding,
tugging and splitting, and so on.

His's embryological theories excited a good deal of interest at the
time of publication, and have evoked a fair amount of literature in
the last few decades. He professed to explain the most complicated
parts of organic construction (such as the development of the brain)
in the simplest way on mechanical principles, and to derive them
immediately from simple physical processes (such as unequal
distribution of strain in an elastic plate). It is quite true that a
mechanical or monistic explanation (or a reduction of natural
processes) is the ideal of modern science, and this ideal would be
realised if we could succeed in expressing these formative processes
in mathematical formulae. His has, therefore, inserted plenty of
numbers and measurements in his embryological works, and given them an
air of "exact" scholarship by putting in a quantity of mathematical
tables. Unfortunately, they are of no value, and do not help us in the
least in forming an "exact" acquaintance with the embryonic phenomena.
Indeed, they wander from the true path altogether by neglecting the
phylogenetic method; this, he thinks, is "a mere by-path," and is "not
necessary at all for the explanation of the facts of embryology,"
which are the direct consequence of physiological principles. What His
takes to be a simple physical process--for instance, the folding of
the germinal layers (in the formation of the medullary tube,
alimentary tube, etc.)--is, as a matter of fact, the direct result of
the growth of the various cells which form those organic structures;
but these growth-motions have themselves been transmitted by heredity
from parents and ancestors, and are only the hereditary repetition of
countless phylogenetic changes which have taken place for thousands of
years in the race-history of the said ancestors. Each of these
historical changes was, of course, originally due to adaptation; it
was, in other words, physiological, and reducible to mechanical
causes. But we have, naturally, no means of observing them now. It is
only by the hypotheses of the science of evolution that we can form an
approximate idea of the organic links in this historic chain.

All the best recent research in animal embryology has led to the
confirmation and development of Baer and Remak's theory of the
germinal layers. One of the most important advances in this direction
of late was the discovery that the two primary layers out of which is
built the body of all vertebrates (including man) are also present in
all the invertebrates, with the sole exception of the lowest group,
the unicellular protozoa. Huxley had detected them in the medusa in
1849. He showed that the two layers of cells from which the body of
this zoophyte is developed correspond, both morphologically and
physiologically, to the two original germinal layers of the
vertebrate. The outer layer, from which come the external skin and the
muscles, was then called by Allman (1853) the "ectoderm" (outer layer,
or skin); the inner layer, which forms the alimentary and reproductory
organs, was called the "entoderm" (= inner layer). In 1867 and the
following years the discovery of the germinal layers was extended to
other groups of the invertebrates. In particular, the indefatigable
Russian zoologist, Kowalevsky, found them in all the most diverse
sections of the invertebrates--the worms, tunicates, echinoderms,
molluscs, articulates, etc.

In my monograph on the sponges (1872) I proved that these two primary
germinal layers are also found in that group, and that they may be
traced from it right up to man, through all the various classes, in
identical form. This "homology of the two primary germinal layers"
extends through the whole of the metazoa, or tissue-forming animals;
that is to say, through the whole animal kingdom, with the one
exception of its lowest section, the unicellular beings, or protozoa.
These lowly organised animals do not form germinal layers, and
therefore do not succeed in forming true tissue. Their whole body
consists of a single cell (as is the case with the amoebae and
infusoria), or of a loose aggregation of only slightly differentiated
cells, though it may not even reach the full structure of a single
cell (as with the monera). But in all other animals the ovum first
grows into two primary layers, the outer or animal layer (the
ectoderm, epiblast, or ectoblast), and the inner or vegetal layer (the
entoderm, hypoblast, or endoblast); and from these the tissues and
organs are formed. The first and oldest organ of all these metazoa is
the primitive gut (or progaster) and its opening, the primitive mouth
(prostoma). The typical embryonic form of the metazoa, as it is
presented for a time by this simple structure of the two-layered body,
is called the gastrula; it is to be conceived as the hereditary
reproduction of some primitive common ancestor of the metazoa, which
we call the gastraea. This applies to the sponges and other zoophyta,
and to the worms, the mollusca, echinoderma, articulata, and
vertebrata. All these animals may be comprised under the general
heading of "gut animals," or metazoa, in contradistinction to the
gutless protozoa.

I have pointed out in my Study of the Gastraea Theory [not translated]
(1873) the important consequences of this conception in the morphology
and classification of the animal world. I also divided the realm of
metazoa into two great groups, the lower and higher metazoa. In the
first are comprised the coelenterata (also called zoophytes, or
plant-animals). In the lower forms of this group the body consists
throughout life merely of the primary germinal layers, with the cells
sometimes more and sometimes less differentiated. But with the higher
forms of the coelentarata (the corals, higher medusae, ctenophorae,
and platodes) a middle layer, or mesoderm, often of considerable size,
is developed between the other two layers; but blood and an internal
cavity are still lacking.

To the second great group of the metazoa I gave the name of the
coelomaria, or bilaterata (or the bilateral higher forms). They all
have a cavity within the body (coeloma), and most of them have blood
and blood-vessels. In this are comprised the six higher stems of the
animal kingdom, the annulata and their descendants, the mollusca,
echinoderma, articulata, tunicata, and vertebrata. In all these
bilateral organisms the two-sided body is formed out of four secondary
germinal layers, of which the inner two construct the wall of the
alimentary canal, and the outer two the wall of the body. Between the
two pairs of layers lies the cavity (coeloma).

Although I laid special stress on the great morphological importance
of this cavity in my Study of the Gastraea Theory, and endeavoured to
prove the significance of the four secondary germinal layers in the
organisation of the coelomaria, I was unable to deal satisfactorily
with the difficult question of the mode of their origin. This was done
eight years afterwards by the brothers Oscar and Richard Hertwig in
their careful and extensive comparative studies. In their masterly
Coelum Theory: An Attempt to Explain the Middle Germinal Layer [not
translated] (1881) they showed that in most of the metazoa, especially
in all the vertebrates, the body-cavity arises in the same way, by the
outgrowth of two sacs from the inner layer. These two coelom-pouches
proceed from the rudimentary mouth of the gastrula, between the two
primary layers. The inner plate of the two-layered coelom-pouch (the
visceral layer) joins itself to the entoderm; the outer plate
(parietal layer) unites with the ectoderm. Thus are formed the
double-layered gut-wall within and the double-layered body-wall
without; and between the two is formed the cavity of the coelom, by
the blending of the right and left coelom-sacs. We shall see this more
fully in Chapter 1.10.

The many new points of view and fresh ideas suggested by my gastraea
theory and Hertwig's coelom theory led to the publication of a number
of writings on the theory of germinal layers. Most of them set out to
oppose it at first, but in the end the majority supported it. Of late
years both theories are accepted in their essential features by nearly
every competent man of science, and light and order have been
introduced into this once dark and contradictory field of research. A
further cause of congratulation for this solution of the great
embryological controversy is that it brought with it a recognition of
the need for phylogenetic study and explanation.

Interest and practice in embryological research have been remarkably
stimulated during the past thirty years by this appreciation of
phylogenetic methods. Hundreds of assiduous and able observers are now
engaged in the development of comparative embryology and its
establishment on a basis of evolution, whereas they numbered only a
few dozen not many decades ago. It would take too long to enumerate
even the most important of the countless valuable works which have
enriched embryological literature since that time. References to them
will be found in the latest manuals of embryology of Kolliker,
Balfour, Hertwig, Kollman, Korschelt, and Heider.

Kolliker's Entwickelungsgeschichte des Menschen und der hoherer
Thiere, the first edition of which appeared forty-two years ago, had
the rare merit at that time of gathering into presentable form the
scattered attainments of the science, and expounding them in some sort
of unity on the basis of the cellular theory and the theory of
germinal layers. Unfortunately, the distinguished Wurtzburg anatomist,
to whom comparative anatomy, histology, and ontogeny owe so much, is
opposed to the theory of descent generally and to Darwinism in
particular. All the other manuals I have mentioned take a decided
stand on evolution. Francis Balfour has carefully collected and
presented with discrimination, in his Manual of Comparative Embryology
(1880), the very scattered and extensive literature of the subject; he
has also widened the basis of the gastraea theory by a comparative
description of the rise of the organs from the germinal layers in all
the chief groups of the animal kingdom, and has given a most thorough
empirical support to the principles I have formulated. A comparison of
his work with the excellent Text-book of the Embryology of the
Vertebrates (1890) [translation 1895] of Korschelt and Heider shows
what astonishing progress has been made in the science in the course
of ten years. I would especially recommend the manuals of Julius
Kollmann and Oscar Hertwig to those readers who are stimulated to
further study by these chapters on human embryology. Kollmann's work
is commendable for its clear treatment of the subject and very fine
original illustrations; its author adheres firmly to the biogenetic
law, and uses it throughout with considerable profit. That is not the
case in Oscar Hertwig's recent Text-book of the Embryology of Man and
the Mammals [translations 1892 and 1899] (seventh edition 1902). This
able anatomist has of late often been quoted as an opponent of the
biogenetic law, although he himself had demonstrated its great value
thirty years ago. His recent vacillation is partly due to the timidity
which our "exact" scientists have with regard to hypotheses; though it
is impossible to make any headway in the explanation of facts without
them. However, the purely descriptive part of embryology in Hertwig's
Text-book is very thorough and reliable.

A new branch of embryological research has been studied very
assiduously in the last decade of the nineteenth century--namely,
"experimental embryology." The great importance which has been
attached to the application of physical experiments to the living
organism for the last hundred years, and the valuable results that it
has given to physiology in the study of the vital phenomena, have led
to its extension to embryology. I was the first to make experiments of
this kind during a stay of four months on the