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Title: Form and Function - A Contribution to the History of Animal Morphology
Author: E. S. (Edward Stuart) Russell
Language: English
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FORM AND FUNCTION

A CONTRIBUTION TO THE
HISTORY OF ANIMAL MORPHOLOGY

By E.S. RUSSELL,
M.A., B.Sc., F.Z.S.

ILLUSTRATED

LONDON

JOHN MURRAY, ALBEMARLE STREET, W.

1916

_All rights reserved_

+---------------------------------------+
| Transcriber's Note: Obvious printer   |
| errors have been corrected, all other |
| inconsistencies in spelling and       |
| punctuation are as in the original.   |
+---------------------------------------+


PREFACE


This book is not intended to be a full or detailed history of animal
morphology: a complete account is given neither of morphological
discoveries nor of morphological theories. My aim has been rather to
call attention to the existence of diverse typical attitudes to the
problems of form, and to trace the interplay of the theories that have
arisen out of them.

The main currents of morphological thought are to my mind three--the
functional or synthetic, the formal or transcendental, and the
materialistic or disintegrative.

The first is associated with the great names of Aristotle, Cuvier, and
von Baer, and leads easily to the more open vitalism of Lamarck and
Samuel Butler. The typical representative of the second attitude is E.
Geoffroy St. Hilaire, and this habit of thought has greatly influenced
the development of evolutionary morphology.

The main battle-ground of these two opposing tendencies is the problem
of the relation of function to form. Is function the mechanical result
of form, or is form merely the manifestation of function or activity?
What is the essence of life--organisation or activity?

The materialistic attitude is not distinctively biological, but is
common to practically all fields of thought. It dates back to the
Greek atomists, and the triumph of mechanical science in the 19th
century has induced many to accept materialism as the only possible
scientific method. In biology it is more akin to the formal than to
the functional attitude.

In the course of this book I have not hidden my own sympathy with the
functional attitude. It appears to me probable that more insight will
be gained into the real nature of life and organisation by
concentrating on the active response of the animal, as manifested both
in behaviour and in morphogenesis, particularly in the post-embryonic
stages, than by giving attention exclusively to the historical aspect
of structure, as is the custom of "pure morphology." I believe we
shall only make progress in this direction if we frankly adopt the
simple everyday conception of living things--which many of us have had
drilled out of us--that they are active, purposeful agents, not mere
complicated aggregations of protein and other substances. Such an
attitude is probably quite as sound philosophically as the opposing
one, but I have not in this place attempted any justification of it. I
have touched very lightly upon the controversy between vitalism and
materialism which has been revived with the early years of the present
century. It hardly lends itself as yet to historical treatment, and I
could hardly hope to maintain with regard to it that objective
attitude which should characterise the historian.

The main result I hope to have achieved with this book is the
demonstration, tentative and incomplete as it is, of the essential
continuity of animal morphology from the days of Aristotle down to our
own time. It is unfortunately true that modern biology, perhaps in
consequence of the great advances it has made in certain directions,
has to a considerable extent lost its historical consciousness, and if
this book helps in any degree to counteract this tendency so far as
animal morphology is concerned, it will have served its purpose.

I owe a debt of gratitude to my friends Dr James F. Gemmill and Prof.
J. Arthur Thomson for much kindly encouragement and helpful criticism.
The credit for the illustrations is due to my wife, Mrs Jehanne A.
Russell. One is from Nature; the others are drawn from the original
figures.

E.S.R.

CHELSEA, 1916.



CONTENTS


CHAP.                                                    PAGE

I. THE BEGINNINGS OF COMPARATIVE ANATOMY                    1

II. COMPARATIVE ANATOMY BEFORE CUVIER                      17

III. CUVIER                                                31

IV. GOETHE                                                 45

V. ETIENNE GEOFFROY ST HILAIRE                             52

VI. THE FOLLOWERS OF ETIENNE GEOFFROY ST HILAIRE           79

VII. THE GERMAN TRANSCENDENTALISTS                         89

VIII. TRANSCENDENTAL ANATOMY IN ENGLAND--RICHARD OWEN     102

IX. KARL ERNST VON BAER                                   113

X. THE EMBRYOLOGICAL CRITERION                            133

XI. THE CELL-THEORY                                       169

XII. THE CLOSE OF THE PRE-EVOLUTIONARY PERIOD             190

XIII. THE RELATION OF LAMARCK AND DARWIN TO MORPHOLOGY    213

XIV. ERNST HAECKEL AND CARL GEGENBAUR                     246

XV. EARLY THEORIES ON THE ORIGIN OF VERTEBRATES           268

XVI. THE GERM-LAYERS AND EVOLUTION                        288

XVII. THE ORGANISM AS AN HISTORICAL BEING                 302

XVIII. THE BEGINNINGS OF CAUSAL MORPHOLOGY                314

XIX. SAMUEL BUTLER AND THE MEMORY THEORIES OF HEREDITY    335

XX. THE CLASSICAL TRADITION IN MODERN MORPHOLOGY          345

INDEX                                                     365



ILLUSTRATIONS


FIG.                                                     PAGE

1. HYOID ARCH OF THE CONGER. (ORIGINAL.)                   58

2. "VERTEBRA" OF A PLEURONECTID. (GEOFFROY.)               61

3. ABDOMINAL SEGMENT OF THE LOBSTER. (GEOFFROY.)           63

4. IDEAL TYPICAL VERTEBRA. (OWEN.)                        102

5. NATURAL TYPICAL VERTEBRA. (OWEN.)                      103

6. THE ARCHETYPE OF THE VERTEBRATE SKELETON. (OWEN.)      105

7. IDEAL TRANSVERSE SECTION OF A VERTEBRATE EMBRYO.
     (VON BAER.)                                          119

8. GILL-SLITS OF THE PIG EMBRYO. (RATHKE.)                134

9. MECKEL'S CARTILAGE AND EAR-OSSICLES IN EMBRYO OF
     PIG. (REICHERT.)                                     145

10. CRANIAL VERTEBRÆ AND VISCERAL ARCHES IN EMBRYO
      OF PIG. (REICHERT.)                                 148

11. EMBRYONIC CRANIUM OF THE ADDER. (RATHKE.)             152

12. TRANSVERSE SECTION OF CHICK EMBRYO. (REMAK.)          211

13. DEVELOPMENT OF THE ASCIDIAN LARVA (KOWALEVSKY.)       272

14. TRANSVERSE SECTION OF THE WORM _NAIS_. (SEMPER.)      280

15. THE FIVE PRIMARY STAGES OF ONTOGENY. (HAECKEL.)       292


FORM AND FUNCTION



CHAPTER I

THE BEGINNINGS OF COMPARATIVE ANATOMY


The first name of which the history of anatomy keeps record is that of
Alcmaeon, a contemporary of Pythagoras (6th century B.C.). His
interests appear to have been rather physiological than anatomical. He
traced the chief nerves of sense to the brain, which he considered to
be the seat of the soul, and he made some good guesses at the
mechanism of the organs of special sense. He showed that, contrary to
the received opinion, the seminal fluid did not originate in the
spinal cord. Two comparisons are recorded of his, one that puberty is
the equivalent of the flowering time in plants, the other that milk is
the equivalent of white of egg.[1] Both show his bias towards looking
at the functional side of living things. The latter comparison
reappears in Aristotle.

A century later Diogenes of Apollonia gave a description of the venous
system. He too placed the seat of sensation in the brain. He assumed a
vital air in all living things, being in this influenced by Anaximenes
whose primitive matter was infinite air. In following out this thought
he tried to prove that both fishes and oysters have the power of
breathing.[2]

A more strictly morphological note is struck by a curious saying of
Empedocles (4th century B.C.), that "hair and foliage and the thick
plumage of birds are one."[3]

In the collected writings of Hippocrates and his school, the _Corpus
Hippocraticum_, of which no part is later than the end of the 5th
century, there are recorded many anatomical facts. The author of the
treatise "On the Muscles" knew, for instance, that the spinal marrow
is different from ordinary marrow and has membranes continuous with
those of the brain. Embryos of seven days (!) have all the parts of
the body plainly visible. Work on comparative embryology is contained
in the treatise "On the Development of the Child."[4]

The author of the treatise "On the Joints," which Littré calls "the
great surgical monument of antiquity," is to be credited with the
first systematic attempt at comparative anatomy, for he compared the
human skeleton with that of other Vertebrates.

Aristotle (384-322 B.C.)[5] may fairly be said to be the founder of
comparative anatomy, not because he was specially interested in
problems of "pure morphology," but because he described the structure
of many animals and classified them in a scientific way. We shall
discuss here the morphological ideas which occur in his writings upon
animals--in the _Historia Animialium_, the _De Partibus Animalium_,
and the _De Generatione Animalium_.

The _Historia Animalium_ is a most comprehensive work, in some ways
the finest text-book of Zoology ever written. Certainly few modern
text-books take such a broad and sane view of living creatures.
Aristotle never forgets that form and structure are but one of the
many properties of living things; he takes quite as much interest in
their behaviour, their ecology, distribution, comparative physiology.
He takes a special interest in the comparative physiology of
reproduction. The _Historia Animalium_ contains a description of the
form and structure of man and of as many animals as Aristotle was
acquainted with--and he was acquainted with an astonishingly large
number. The later _De Partibus Animalium_ is a treatise on the causes
of the form and structure of animals. Owing to the importance which
Aristotle ascribed to the final cause this work became really a
treatise on the functions of the parts, a discussion of the problems
of the relation of form to function, and the adaptedness of structure.

Aristotle was quite well aware that each of the big groups of animals
was built upon one plan of structure, which showed endless variations
"in excess and defect" in the different members of the group. But he
did not realise that this fact of community of plan constituted a
problem in itself. His interest was turned towards the functional side
of living things, form was for him a secondary result of function.

Yet he was not unaware of facts of form for which he could not quite
find a place in his theory of organic form, facts of form which were
not, at first sight at least, facts of function. Thus he was aware of
certain facts of "correlation," which could not be explained off-hand
as due to correlation of the functions of the parts. He knew, for
instance, that all animals without front teeth in the upper jaw have
cotyledons, while most that have front teeth on both jaws and no horns
have no cotyledons (_De Gen._, ii. 7).

Speaking generally, however, we find in Aristotle no purely
morphological concepts. What then does morphology owe to Aristotle? It
owes to him, _first_, a great mass of facts about the structure of
animals; _second_, the first scientific classification of animals;[6]
_third_, a clear enunciation of the fact of community of plan within
each of the big groups; _fourth_, an attempt to explain certain
instances of the correlation of parts; _fifth_, a pregnant distinction
between homogeneous and heterogeneous parts; _sixth_, a generalisation
on the succession of forms in development; and _seventh_, the first
enunciation of the idea of the _Échelle des êtres_.

(1) What surprises the modern reader of the _Historia Animalium_
perhaps more than anything else is the extent and variety of
Aristotle's knowledge of animals. He describes more than 500 kinds.[7]
Not only does he know the ordinary beasts, birds, and fishes with
which everyone is acquainted, but he knows a great deal about
cuttlefish, snails and oysters, about crabs, crawfish (_Palinurus_),
lobsters, shrimps, and hermit crabs, about sea-urchins and starfish,
sea-anemones and sponges, about ascidians (which seem to have puzzled
him not a little!). He has noticed even fish-lice and intestinal
worms, both flat and round. Of the smaller land animals, he knows a
great many insects and their larvæ. The extent of his anatomical
knowledge is equally surprising, and much of it is clearly the result
of personal observation. No one can read his account of the internal
anatomy of the chameleon (_Hist. Anim._, ii.), or his description of
the structure of cuttlefish (_Hist. Anim._, iv), or that touch in the
description of the hermit crab (_Hist. Anim._, iv.)--" Two large eyes
... not ... turned on one side like those of crabs, but straight
forward"--without being convinced that Aristotle is speaking of what
he has seen. Naturally he could not make much of the anatomy of small
insects and snails, and, to tell the truth, he does not seem to have
cared greatly about the minutiæ of structure. He was too much of a
Greek and an aristocrat to care about laborious detail.

Not only did he lay a foundation for comparative anatomy, but he made
a real start with comparative embryology. Medical men before him had
known many facts about human development; Aristotle seems to have been
the first to study in any detail the development of the chick. He
describes this as it appears to the naked eye, the position of the
embryo on the yolk, the palpitating spot at the third day, the
formation of the body and of the large sightless eyes, the veins on
the yolk, the embryonic membranes, of which he distinguished two.

(2) Aristotle had various systems of classifying animals. They could
be classified, he thought, according to their structure, their manner
of reproduction, their manner of life, their mode of locomotion, their
food, and so on. Thus you might, in addition to structural
classifications, divide animals into gregarious, solitary and social,
or land animals into troglodytes, surface-dwellers, and burrowers
(_Hist. Anim._, i.).

He knew that dichotomous classifications were of little use for
animals (_De Partibus_, i. 3) and he explicitly and in so many words
accepted the principle of all "natural" classification, that
affinities must be judged by comparing not one but the sum total of
characters. As everyone knows, he was the first to distinguish the big
groups of animals, many of which were already distinguished roughly by
the common usages of speech. Among his Sanguinea he did little more
than define with greater exactitude the limits of the groups
established by the popular classification. Among the "exsanguineous"
animals, however, corresponding to our Invertebrates, he established a
much more definite classification than the popular, which is apt to
call them indiscriminately "shellfish," "insects," or "creeping
things." He went beyond the superficialities of popular
classification, too, in clearly separating Cetacea from fishes. He had
some notion of species and genera in our sense. He distinguished many
species of cuttlefish--_Octopus (Polypus)_ of which there were many
kinds, _Eledone (Moschites)_ which he knew to have only one row of
suckers while _Octopus_ has two, _Argonauta, Nautilus, Sepia_, and
apparently _Loligo media_ (= his Teuthis) and _L. vulgaris_(or
_forbesii_) which seems to be his Teuthos. He had a grasp of the
principles which should be followed in judging of the natural
affinities of species. For example, he knew that the cuckoo resembles
a hawk. "But," he says, "the hawk has crooked talons, which the cuckoo
has not, nor does it resemble the hawk in the form of its head, but in
these respects is more like the pigeon than the hawk, which it
resembles in nothing but its colour; the markings, however, upon the
hawk are like lines, while the cuckoo is spotted" (_Hist. Anim._,
Cresswell's trans., p. 147, London, 1862).

The groups he distinguished were--man, viviparous quadrupeds,
oviparous quadrupeds, birds, fishes, Cetacea, Cephalopoda,
Malacostraca (= higher Crustacea), Insecta (= annulose animals),
Testacea (= molluscs, echinoderms, ascidians). A class of Acalephæ,
including sea-anemones and sponges, was grouped with the Testacea. The
first five groups were classed together as sanguineous, the others as
exsanguineous, from the presence or absence of red blood.

Besides these classes "there are," he says, "many other creatures in
the sea which it is not possible to arrange in any class from their
scarcity" (Creswell, _loc. cit._, p. 90).

(3) Aristotle's greatest service to morphology is his clear
recognition of the unity of plan holding throughout each of the great
groups.

He recognises this most clearly in the case of man and the viviparous
quadrupeds, with whose structure he was best acquainted. In the
_Historia Animalium_ he takes man as a standard, and describes his
external and internal parts in detail, then considers viviparous
quadrupeds and compares them with man. "Whatever parts a man has
before, a quadruped has beneath; those that are behind in man form the
quadruped's back" (Cresswell, _loc. cit._, p. 26). Apes, monkeys, and
Cynocephali combine the characteristics of man and quadrupeds. He
notices that all viviparous quadrupeds have hair. Oviparous quadrupeds
resemble the viviparous, but they lack some organs, such as ears with
an external pinna, mammæ, hair. Oviparous bipeds, or birds, also "have
many parts like the animals described above." He does not, however,
seem to realise that a bird's wings are the equivalent of a mammal's
arms or fore-legs. Fishes are much more divergent; they possess no
neck, nor limbs, nor testicles (meaning a solid ovoid body such as the
testis in mammals), nor mammæ. Instead of hair they have scales.

Speaking generally, the Sanguinea differ from man and from one another
in their parts, which may be present or absent, or exhibit differences
in "excess and defect," or in form. Unity of plan extends to all the
principal systems of organs. "All sanguineous animals have either a
bony or a spinous column. The remainder of the bones exist in some
animals; but not in others, for if they have the limbs they have the
bones belonging to them" (Cresswell, _loc. cit._, p. 60). "Viviparous
animals with blood and feet do not differ much in their bones, but
rather by analogy, in hardness, softness, and size" (Cresswell, _loc.
cit._, p. 59). The venous system, too, is built upon the same general
plan throughout the Sanguinea. "In all sanguineous animals, the nature
and origin of the principal veins are the same, but the multitude of
smaller veins is not alike in all, for neither are the parts of the
same nature, nor do all possess the same parts" (Cresswell, _loc.
cit._, p. 56). It will be noticed in the first and last of these three
quotations that Aristotle recognises the fact of correlation between
systems of organs,--between limbs and bones, and between blood-vessels
and the parts to which they go.

Sanguineous animals all possess certain organs--heart, liver, spleen,
kidneys, and so on. Other organs occur in most of the classes--the
oesophagus and the lungs. "The position which these parts occupy is
the same in all animals [sc. Sanguinea]" (Cresswell, _loc. cit._, p.
39).

Unity of plan is observable not only in the Sanguinea, but also within
each of the other large groups. Aristotle recognises that all his
cuttlefish are alike in structure. Among his Malacostraca he compares
point by point the external parts of the carabus (_Palinurus_), and
the astacus (_Homarus_), and he compares also the general internal
anatomy of the various "genera" he distinguishes. As regards Testacea,
he writes, "The nature of their internal structure is similar in all,
especially in the turbinated animals, for they differ in size and in
the relations of excess; the univalves and bivalves do not exhibit
many differences" (Cresswell, _loc. cit._, p. 83). There is an
interesting remark about "the creature called carcinium"
(hermit-crab), that it "resembles both the Malacostraca and the
Testacea, for this in its nature is similar to the animals that are
like carabi, and it is born naked" (Cresswell, _loc. cit._, p. 85). In
the last phrase we may perhaps read the first recognition of the
embryological criterion.

With the recognition of unity of plan within each group necessarily
goes the recognition of what later morphology calls the homology of
parts. The parts of a horse can be compared one by one with the parts
of another viviparous quadruped; in all the animals belonging to the
same class the parts are the same, only they differ in excess or
defect--these remarks are placed in the forefront of the _Historia
Animalium_. Generally speaking, parts which bear the same name are for
Aristotle homologous throughout the class. But he goes further and
notes the essential resemblance underlying the differences of certain
parts. He classes together nails and claws, the spines of the
hedgehog, and hair, as being homologous structures. He says that teeth
are allied to bones, whereas horns are more nearly allied to skin
(_Hist. Anim._, iii.). This is an astonishingly happy guess,
considering that all he had to go upon was the observation that in
black animals the horns are black but the teeth white. One cannot but
admire the way in which Aristotle fixes upon apparently trivial and
commonplace facts, and draws from them far-reaching consequences. He
often goes wrong, it is true, but he always errs in the grand manner.

While Aristotle certainly recognised the existence of homologies, and
even had a feeling for them, he did not clearly distinguish homology
from analogy. He comes pretty near the distinction in the following
passage. After explaining that in animals belonging to the same class
the parts are the same, differing only in excess or defect, he says,
"But some animals agree with each other in their parts neither in form
nor in excess and defect, but have only an analogous likeness, such as
a bone bears to a spine, a nail to a hoof, a hand to a crab's claw,
the scale of a fish to the feather of a bird, for that which is a
feather in the bird is a scale in the fish" (Cresswell, _loc. cit._,
p. 2). One of these comparisons is, however, a homology not an
analogy, and the last phrase throws a little doubt upon the whole
question, for it is not made clear whether it is position or function
that determines what are equivalent organs.

In the _De Partibus Animalium_ there occurs the following
passage:--"Groups that only differ in degree, and in the more or less
of an identical element that they possess, are aggregated under a
single class; groups whose attributes are not identical but analogous
are separated. For instance, bird differs from bird by gradation, or
by excess and defect; some birds have long feathers, others short
ones, but all are feathered. Bird and Fish are more remote and only
agree in having analogous organs; for what in the bird is feather, in
the fish is scale. Such analogies can scarcely, however, serve
universally as indications for the formation of groups, for almost all
animals present analogies in their corresponding parts."[8] It is thus
similarity in form and structure which determines the formation of the
main groups. Within each group the parts differ only in degree, in
largeness or smallness, softness and hardness, smoothness or
roughness, and the like (_loc. cit._, i., 4, 644^b). These passages
show that Aristotle had some conception of homology as distinct from
analogy. He did not, however, develop the idea. What Aristotle sought
in the variety of animal structure, and what he found, were not
homologies, but rather communities of function, parts with the same
attributes. His interest was all in _organs_, in functioning parts,
not in the mere spatial relationship of parts.

This comes out clearly in his treatise _On the Parts of Animals_,
which is subsequent to, and the complement of, his _History of
Animals_. The latter is a description of the variety of animal form,
the former is a treatise on the functions of the parts. He describes
the plan of the _De Partibus Animalium_ as follows:--"We have, then,
first to describe the common functions, common, that is, to the whole
animal kingdom, or to certain large groups, or to members of a
species. In other words, we have to describe the attributes common to
all animals, or to assemblages, like the class of Birds, of closely
allied groups differentiated by gradation, or to groups like Man not
differentiated into subordinate groups. In the first case the common
attributes may be called analogous, in the second generic, in the
third specific" (i, 5, 645^b, trans. Ogle). The alimentary canal is a
good example of a part which is "analogous" throughout the animal
kingdom, for "all animals possess in common those parts by which they
take in food, and into which they receive it" (Cresswell, _loc. cit._,
p. 6).

The _De Partibus Animalium_ becomes in form a comparative
organography, but the emphasis is always on function and community of
function. Thus he treats of bone, "fish-spine," and cartilage together
(_De Partibus_, ii., 9, 655^a), because they have the same function,
though he says elsewhere that they are only analogous structures (ii.,
8, 653^b). In the same connection he describes also the supporting
tissues of Invertebrates--the hard exoskeleton of Crustacea and
Insects, the shell of Testacea, the "bone" of _Sepia_ (ii., 8,
654^a). Aristotle took much more interest in analogies, in organs of
similar function, than in homologies. He did recognise the existence
of homologies, but rather _malgré lui_, because the facts forced it
upon him.

His only excursion into the realm of "transcendental anatomy" is his
comparison of a Cephalopod to a doubled-up Vertebrate whose legs have
become adherent to its head, whose alimentary canal has doubled upon
itself in such a way as to bring the anus near the mouth (_De
Partibus_, iv., 9, 684^b). It is clear, however, that Aristotle did
not seek to establish by this comparison any true homologies of parts,
but merely analogies, thus avoiding the error into which Meyranx and
Laurencet fell more than two thousand years later in their paper
communicated to the Académie des Sciences, which formed the
starting-point of the famous controversy between Cuvier and E.
Geoffroy St Hilaire (see Chap. V., below).

Moreover, Aristotle did not so much compare a Cephalopod with a
doubled-up Vertebrate as contrast Cephalopods (and also Testacea) with
all other animals. Other animals have their organs in a straight line;
Cephalopods and Testacea alone show this peculiar doubling up of the
body.

(4) Aristotle was much struck with certain facts of correlation, of
the interdependence of two organs which are not apparently in
functional dependence on one another. Such correlation may be positive
or negative; the presence of one organ may either entail the presence
of the other, or it may entail its absence. Aristotle has various ways
of explaining facts of correlation. He observed that no animal has
both tusks and horns, but this fact could easily be explained on the
principle that Nature never makes anything superfluous or in vain. If
an animal is protected by the possession of tusks it does not require
horns, and _vice versa_. The correlation of a multiple stomach with
deficient development of the teeth (as in Ruminants) is accounted for
by saying that the animal needs its complex stomach to make up for the
shortcomings of its teeth! (_De Partibus_, iii., 14, 674^b.) Other
examples of correlation were not susceptible of this explanation in
terms of final causes. He lays stress on the fact, in the main true,
of the inverse development of horns and front teeth in the upper jaw,
exemplified in Ruminants. He explains the fact in this way. Teeth and
horns are formed from earthy matter in the body and there is not
enough to form both teeth and horns, so "Nature by subtracting from
the teeth adds to the horns; the nutriment which in most animals goes
to the former being here spent on the augmentation of the latter" (_De
Partibus_, iii., 2, 664^a, trans. Ogle). A similar kind of explanation
is offered of the fact that Selachia have cartilage instead of bone,
"in these Selachia Nature has used all the earthy matter on the skin
[_i.e._, on the placoid scales]; and she is unable to allot to many
different parts one and the same superfluity of material" (_De
Partibus_, ii., 9, 655^a, trans. Ogle). Speaking generally, "Nature
invariably gives to one part what she subtracts from another" (_loc.
cit._, ii., 14, 658^a).

This thought reappears again in the 19th century in E. Geoffroy St
Hilaire's _loi de balancement_ and also in Goethe's writings on
morphology. For Aristotle it meant that Nature was limited by the
nature of her means, that finality was limited by necessity. Thus in
the larger animals there is an excess of earthy matter, as a necessary
result of the material nature of the animal; this excess is turned by
Nature to good account, but there is not enough to serve both for
teeth and for horns (_loc. cit._, iii., 2, 663^b).

But there are other instances of correlation which seem to have taxed
even Aristotle's ingenuity beyond its powers. Thus he knew that all
animals (meaning viviparous quadrupeds) with no front teeth in the
upper jaw have cotyledons on their foetal membranes, and that most
animals which have front teeth in both jaws and no horns have no
cotyledons (_De Generatione_, ii., 7). He offers no explanation of
this, but accepts it as a fact.

We may conveniently refer here to one or two other ideas of Aristotle
regarding the causes of form. He makes the profound remark that the
possible range of form of an organ is limited to some extent by its
existing differentiation. Thus he explains the absence of external
(projecting) ears in birds and reptiles by the fact that their skin is
hard and does not easily take on the form of an external ear (_De
Partibus_, ii, 12). The fact of the inverse correlation is certain;
the explanation is, though very vague, probably correct.

In one passage of the _De Partibus_ Aristotle clearly enunciates the
principle of the division of labour, afterwards emphasised by H.
Milne-Edwards. In some insects, he says, the proboscis combines the
functions of a tongue and a sting, in others the tongue and the sting
are quite separate. "Now it is better," he goes on, "that one and the
same instrument shall not be made to serve several dissimilar ends;
but that there shall be one organ to serve as a weapon, which can then
be very sharp, and a distinct one to serve as a tongue, which can then
be of spongy texture and fit to absorb nutriment. Whenever, therefore,
Nature is able to provide two separate instruments for two separate
uses, without the one hampering the other, she does so, instead of
acting like a coppersmith who for cheapness makes a spit and
lampholder in one" (iv., 6, 683^a).

(5) The first sentence of the _Historia Animalium_ formulates, with
that simplicity and directness which is so characteristic of
Aristotle, the distinction between homogeneous and heterogeneous
parts, in the mass the distinction between tissues and organs. "Some
parts of animals are simple, and these can be divided into like parts,
as flesh into pieces of flesh; others are compound, and cannot be
divided into like parts, as the hand cannot be divided into hands, nor
the face into faces. All the compound parts also are made up of simple
parts--the hand, for example, of flesh and sinew and bone" (Cresswell,
_loc. cit_., p. I).

In the _De Partibus Animalium_ he broadens the conception by adding
another form of composition. "Now there are," he says, "three degrees
of composition; and of these the first in order, as all will allow, is
composition out of what some call the elements, such as earth, air,
water, fire.... The second degree of composition is that by which the
homogeneous parts of animals, such as bone, flesh, and the like, are
constituted out of the primary substances. The third and last stage is
the composition which forms the heterogeneous parts, such as face,
hand, and the rest" (ii., 1, 646^a, trans. Ogle).

In the _Historia Animalium_ the homogeneous parts are divided into (1)
the soft and moist (or fluid), such as blood, serum, flesh, fat, suet,
marrow, semen, gall, milk, phlegm, fæces and urine, and (2) the hard
and dry (or solid), such as sinew, vein, hair, bone, cartilage, nail,
and horn. It would appear from this enumeration that Aristotle's
distinction of simple and complex parts does not altogether coincide
with our distinction of tissues and organs. We should not call vein a
tissue, nor do we include under this heading non-living secretions.
But in the _De Partibus Animalium_ Aristotle, while still holding to
the distinction set forth above, is alive to the fact that his simple
parts include several different sorts of substances. He distinguishes
among the homogeneous parts three sets. The first of these comprises
the tissues out of which the heterogeneous parts are constructed,
_e.g._, flesh and bone; the second set form the nutriment of the
parts, and are invariably fluid; while the third set are the residue
of the second and constitute the residual excretions of the body (ii.,
2, 647^b). He sees clearly the difficulty of calling vein or
blood-vessel a simple part, for while a bloodvessel and a part of it
are both blood-vessel, as we should say vascular tissue, yet a part of
a blood-vessel is not a bloodvessel. There is form superadded to
homogeneity of structure (ii., 2, 647^b). Similarly for the heart and
the other viscera. "The heart, like the other viscera, is one of the
homogeneous parts; for, if cut up, its pieces are homogeneous in
substance with each other. But it is at the same time heterogeneous in
virtue of its definite configuration" (ii., 1, 647^a, trans. Ogle).

Aristotle, therefore, came very near our conception of tissue. He was
of course not a histologist; he describes not the structure of
tissues, which he could not know, but rather their distribution within
the organism; his section on the homogeneous parts of Sanguinea
(_Historia Animalium_, iii., second half) is largely a comparative
topographical anatomy; in it, for instance, he describes the venous
and skeletal systems.

This distinction which Aristotle drew plays an important part in all
his writings on animals, particularly in his theory of development. It
was a distinction of immense value, and is full of meaning even at the
present day. No one has ever given a better definition of organ than
is implied in Aristotle's description of the heterogeneous parts--"The
capacity of action resides in the compound parts" (Cresswell, _loc.
cit._, p. 7). The heterogeneous parts were distinguished by the
faculty of doing something, they were the active or executive parts.
The homogeneous parts were distinguished mainly by physical characters
(_De Generatione_, i., 18), but certain of them had other than purely
physical properties, they were the organs of touch (_De Partibus_,
ii., 1, 647^a).

(6) In a passage in the _De Generatione_ (ii, 3) Aristotle says that
the embryo is an animal before it is a particular animal, that the
general characters appear before the special. This is a foreshadowing
of the essential point in von Baer's law (see Chap. IX. below).

He considers also that tissues arise before organs. The homogeneous
parts are anterior genetically to the heterogeneous parts and
posterior to the elementary material (_De Partibus_, ii., 1, 646^b).

(7) We meet in Aristotle an idea which later acquired considerable
vogue, that of the _Échelle des êtres_(or "scale of beings"), that
organisms, or even all objects organic or inorganic, can be arranged
in a single ascending series. The idea is a common one; its first
literary expression is found perhaps in primitive creation-myths, in
which inorganic things are created before organic, and plants before
animals. It may be recognised also in Anaximander's theory that land
animals arose from aquatic animals, more clearly still in Anaxagoras'
theory that life took its origin on this globe from vegetable germs
which fell to earth with the rain. Anaxagoras considered animals
higher in the scale than plants, for while the latter participated in
pleasure (when they grew) and pain (when they lost their leaves),
animals had in addition "Nous." In Empedocles' theory of evolution,
the vegetable world preceded the animal. Plato, in the _Timaeus_,
describes the whole organic world as being formed by degradation from
man, who is created first. Man sinks first into woman, then into brute
form, traversing all the stages from the higher to the lower animals,
and becoming finally a plant. This is a reversal of the more usual
notion, but the idea of gradation is equally present.

Aristotle seems not to have believed in any transformation of species,
but he saw that Nature passes gradually from inanimate to animate
things without a clear dividing line. "The race of plants succeeds
immediately that of inanimate objects" (Cresswell, _loc. cit._, p.
94). Within the organic realm the passage from plants to animals is
gradual. Some creatures, for example, the sea-anemones and sponges,
might belong to either class.

Aristotle recognised also a natural series among the groups of
animals, a series of increasing complexity of structure. He begins his
study of structure with man, who is the most intricate, and then takes
up in turn viviparous and oviparous quadrupeds, then birds, then
fishes. After the Sanguinea he considers the Exsanguinea, and of the
latter first the most highly organised, the Cephalopods, and last the
simplest, the lower members of his class of the Testacea. In treating
of generation (in _Hist. Animalium_, v.) he reverses this order. In
the _De Generatione_ (Book ii., I) there is given another serial
arrangement of animals, this time in relation to their manner of
reproduction. There is a gradation, he says, of the following kind:--

1. Internally viviparous Sanguinea  } producing a perfect
2. Externally viviparous Sanguinea  }       animal.
3. Oviparous Sanguinea--producing a perfect egg.
4. Animals producing an imperfect egg (one which
     increases in size after being laid).
5. Insects, producing a scolex (or grub).

In Aristotle's view the gradation of organic forms is the consequence,
not the cause, of the gradation observable in their activities. Plants
have no work to do beside nutrition, growth, and reproduction; they
possess only the nutritive soul. Animals possess in addition sensation
and the sensitive or perceptive soul--"their manner of life differs in
their having pleasure in sexual intercourse, in their mode of
parturition and rearing their young" (_Hist. Anim._, viii., trans.
Cresswell, p. 195). Man alone has the rational soul in addition to the
two lower kinds.

As it is put in the _De Partibus_ (ii., 10, 656^a, trans. Ogle),
"Plants, again, inasmuch as they are without locomotion, present no
great variety in their heterogeneous parts. For, where the functions
are but few, few also are the organs required to effect them....
Animals, however, that not only live but feel, present a greater
multiformity of parts, and this diversity is greater in some animals
than in others, being most varied in those to whose share has fallen
not mere life but life of high degree. Now such an animal is man."

With the great exception of Aristotle, the philosophers of Greece and
Rome made little contribution to morphological theory. Passing mention
may be made of the Atomists--Leucippus, Democritus, and their great
disciple Lucretius, who in his magnificent poem "De Natura Rerum" gave
impassioned expression to the materialistic conception of the
universe. But the full effect of materialism upon morphology does not
become apparent till the rise of physiology in the 17th and 18th
centuries, and reaches its culmination in the 19th century. The
evolutionary ideas of Lucretius exercised no immediate influence upon
the development of morphology.

    [1] E. Zeller, _Greek Philosophy_, Eng. trans., i., 522
    f.n., London 1881. Other particulars as to Alcmaeon in
    T. Gomperz, _Greek Thinkers_, Eng. trans., i., London,
    1901.

    [2] Zeller, _loc. cit._, i., p. 297.

    [3] Gomperz, _loc. cit._, i., p. 244.

    [4] R. Burckhardt, _Biologie u. Humanismus_, p. 85,
    Jena, 1907.

    [5] See the interesting account of Aristotle's
    biological work in Prof. D'Arcy W. Thompson's Herbert
    Spencer lecture (1913) and his translation of the
    _Historia Animalium_ in the Oxford series.

    [6] On Aristotle's forerunners, see R. Burckhardt, "Das
    koïsche Tiersystem, eine Vorstufe des zoologischen
    Systematik des Aristoteles." _Verh. Naturf. Ges. Basel_,
    xx., 1904.

    [7] T.E. Lones, _Aristotle's Researches in Natural
    Science_, pp. 82-3, London, 1912.

    [8] _De Partibus Animalium_, i., 4, 644^a trans. W.
    Ogle, Oxford, 1911.



CHAPTER II

COMPARATIVE ANATOMY BEFORE CUVIER


For two thousand years after Aristotle little advance was made upon
his comparative anatomy. Knowledge of the human body was increased not
long after his death by Herophilus and Erasistratus, but not even
Galen more than four centuries later made any essential additions to
Aristotle's anatomy.

During the Middle Ages, particularly after the introduction to Europe
in the 13th century of the Arab texts and commentaries, Aristotle
dominated men's thoughts of Nature. The commentary of Albertus Magnus,
based upon that of Avicenna, did much to impose Aristotle upon the
learned world. Albertus seems to have contented himself with following
closely in the footsteps of his master. There are noted, however, by
Bonnier certain improvements made by Albertus on Aristotle's view of
the seriation of living things. "He is the first," writes Bonnier, "to
take the correct view that fungi are lower plants allied to the most
lowly organised animals. From this point there start, for Albertus
Magnus, two series of living creatures, and he regards the plant
series as culminating in the trees which have well-developed
flowers."[9]

Aristotle's influence is predominant also in the work of Edward Wotton
(1492-1555), who in his book _De differentiis animalium_ adopted a
classification similar to that proposed by Aristotle. He too laid
stress upon the gradation shown from the lower to the higher forms.

In the 16th century, two groups of men helped to lay foundations for a
future science of comparative anatomy--the great Italian anatomists
Vesalius, Fallopius and Fabricius, and the first systematists (though
their "systems" were little more than catalogues) Rondeletius,
Aldrovandus and Gesner.

The anatomists, however, took little interest in problems of pure
morphology; the anatomy of the human body was for them simply the
necessary preliminary of the discovery of the functions of the
parts--they were quite as much physiologists as anatomists.

One of them, Fabricius, made observations on the development of the
chick (1615). Harvey, who was a pupil of Fabricius, likewise published
an account of the embryology of the chick.[10] In his philosophy and
habit of thought Harvey was a follower of Aristotle. It is worth
noting that in his _Exercitationes anatomicae de motu cordis_ (1628)
there is a passage which dimly foreshadows the law of recapitulation
in development which later had so much vogue.[11]

A stimulating contribution to comparative anatomy was made by
Belon,[12] who published in 1555 a _Histoire de la nature des Oyseaux_,
in which he showed opposite one another a skeleton of a bird and of a
mammal, giving the same names to homologous bones. The anatomy of
animals other than man was indeed not altogether neglected at this
time. Coiter (1535-1600) studied the anatomy of Vertebrates,
discovering among other things the fibrous structure of the brain.
Carlo Ruini of Bologna wrote in 1598 a book on the anatomy of the
horse.[13] Somewhat later Severino, professor at Naples, dissected many
animals and came to the conclusion that they were built upon the same
plan as man.[14] Willis, of Oxford and London, in his _Cerebri Anatome_
(1659) recognised the necessity for comparative study of the structure
of the brain. He found out that the brain of man is very like that of
other mammals, the brain of birds, on the contrary, like that of
fishes![15] He described the anatomy of the oyster and the crayfish. He
had, however, not much feeling for morphology.

The foundation of the Jardin des Plantes at Paris in 1626 and the
subsequent addition to it of a Museum of Natural History and a
menagerie gave a great impulse to the study of comparative anatomy by
supplying a rich material for dissection. Advantage was taken of these
facilities, particularly by Claude Perrault and Duverney.[16] In a
volume entitled _De la Mécanique des Animaux_, Perrault recognises
clearly the idea of unity of type, and even pushes it too far, seeking
to prove that in plants there exists an arterial system and veins
provided with valves.[17]

The beginning of the 17th century saw the invention of the microscope,
which was to have such an enormous influence upon the development of
biological studies. It did not come into scientific use until well on
in the middle of the century. Just before it came into use Francis
Glisson (1597-1677), an Englishman, gave in the introduction to his
treatise on the liver an account of the notions then current on the
structure of organic bodies. He classifies the parts as "similar" and
"organic," the former determined by their material, the latter by the
form which they assume. The similar parts are divided into the
sanguineous or rich in blood and the spermatic. Both sets are further
subdivided according to their physical characters,[18] the latter, for
instance, into the hard, soft, and tensile tissues. The classification
resembles greatly that propounded by Aristotle, though it is notably
inferior in the details of its working out.

For Aristotle, as for all anatomists before the days of the
microscope, the tissues were not much more than inorganic substances,
differing from one another in texture, in hardness, and other physical
properties. They possessed indeed properties, such as contractility,
which were not inorganic, but as far as their visible structure was
concerned there was little to raise them above the inorganic level.
The application of the microscope changed all that, for it revealed in
the tissues an organic structure as complex in its grade as the gross
and visible structure of the whole organism. Of the four men who first
made adequate use of the new aid, Malpighi, Hooke, Leeuenhoek, and
Swammerdam, the first-named contributed the most to make current the
new conceptions of organic structure. He studied in some detail the
development of the chick. He described the minute structure of the
lungs (1661), demonstrating for the first time, by his discovery of
the capillaries, the connection of the arteries with the veins. In his
work, _De viscerum structura_ (1666), he describes the histology of
the spleen, the kidney, the liver, and the cortex of the brain,
establishing among other things the fact that the liver was really a
conglomerate gland, and discovering the Malpighian bodies in the
kidney. This work was done on a broad comparative basis. "Since in the
higher, more perfect, red-blooded animals, the simplicity of their
structure is wont to be involved by many obscurities, it is necessary
that we should approach the subject by the observation of the lower,
imperfect animals."[19] So he wrote in the _De viscerum structura_, and
accordingly he studied the liver first in the snail, then in fishes,
reptiles, mammals, and finally man. In the introduction to his
_Anatome plantarum_ (1675), in which he laid the foundations of plant
histology, he vindicates the comparative method in the following
words:--"In the enthusiasm of youth I applied myself to Anatomy, and
although I was interested in particular problems, yet I dared to pry
into them in the higher animals. But since these matters enveloped in
peculiar mystery still lie in obscurity, they require the comparison
of simpler conditions, and so the investigation of insects[20] at once
attracted me; finally, since this also has its own difficulties I
applied my mind to the study of plants, intending after prolonged
occupation with this domain, to retrace my steps by way of the
vegetable kingdom, and get back to my former studies. But perhaps not
even this will be sufficient; since the simpler world of minerals and
the elements should have been taken first. In this case, however, the
undertaking becomes enormous and far beyond my powers."[21] There is
something fine in this life of broad outlines, devoted whole-heartedly
to an idea, to a plan of research, which required a lifetime to carry
out.

An important histological discovery dating from this time is that of
the finer structure of muscle, made by Stensen (or Steno) in 1664. He
described the structure of muscle-fibres, resolving them into their
constituent fibrils.

To the microscope we owe not only histology but the comparative
anatomy of the lower animals. Throughout the 17th and 18th centuries
the discovery of structure in the lower animals went on continuously,
as may be read in any history of Zoology.[22] We content ourselves here
with mentioning only some representative names.

In the 17th century Leeuenhoek, applying the microscope almost at
random, discovered fact after fact, his most famous, discovery being
that of the "spermatic animalcules."

Swammerdam studied the metamorphoses of insects and made wonderfully
minute dissections of all sorts of animals, snails and insects
particularly. He described also the development of the frog. It is
curious to see what a grip his conception of metamorphosis had upon
him when he homologises the stages of the frog's development with the
Egg, the Worm, and the Nymph of insects (_Book of Nature_, p. 104,
Eng. trans., 1785). He even speaks of the human embryo as being at a
certain stage a Man-Vermicle.

In the 18th century, Réaumur and Bonnet continued the minute study of
insects, laying more stress, however, on their habits and physiology
than upon their anatomy. Lyonnet made a most laborious investigation
of the anatomy of the willow-caterpillar (1762). John Hunter (1728-93)
dissected all kinds of animals, from holothurians to whales. His
interest was, however, that of the physiologist, and he was not
specially interested in problems of form. It is interesting to note a
formulation in somewhat confused language of the recapitulation
theory. The passage occurs in his description of the drawings he made
to illustrate the development of the chick. It is quoted in full by
Owen (J. Hunter, _Observations on certain Parts of the Animal
OEconomy_, with Notes by Richard Owen. London, 1837. Preface, p.
xxvi). We give here the last and clearest sentence--"If we were to
take a series of animals from the more imperfect to the perfect, we
should probably find an imperfect animal corresponding with some stage
of the most perfect."

The tendency of the time was not towards morphology, but rather to
general natural history and to systematics, the latter under the
powerful influence of Linnæus (1707-1778). The former tendency is
well represented by Réaumur (1683-1757) with his observations on
insects, the digestion of birds, the regeneration of the crayfish's
legs, and a hundred other matters. To this tendency belong also
Trembley's famous experiments on Hydra (1744), and Rösel von
Rosenhof's _Insektenbelustigungen_ (1746-1761).

Bonnet (1720-1793) deserves special mention here, since in his _Traité
d'Insectologie_ (1745), and more fully in his _Contemplation de la
Nature_ (1764), he gives the most complete expression to the idea of
the _Échelle des êtres_.

This idea seems to have taken complete possession of his imagination.
He extends it to the universe. Every world has its own scale of
beings, and all the scales when joined together form but one, which
then contains all the possible orders of perfection. At the end of the
Preface to his _Traité_ _d'Insectologie_ (OEuvres, i., 1779) he
gives a long table, headed "Idée d'une Échelle des êtres naturels,"
and rather resembling a ladder, on the rungs of which the following
names appear:--

MAN.
Orang-utan.
Ape.

QUADRUPEDS.
Flying squirrel.
Bat.
Ostrich.

BIRDS.
Aquatic birds.
Amphibious birds.
Flying Fish.

FISH.
Creeping fish.
Eels.
Water serpents.

SERPENTS.
Slugs.
Snails.

SHELL FISH.
Tube-worms.
Clothes-moths.

INSECTS.
Gall insects.
Taenia.
Polyps.
Sea Nettles.
Sensitive plant.

PLANTS.
Lichens.
Moulds.
Fungi, Agarics.
Truffles.
Corals, and Coralloids.
Lithophytes.
Asbestos.
Talcs, Gypsums.
Selenites, Slates.

STONES.
Figured stones.
Crystals.

SALTS.
Vitriols.

METALS.

HALF-METALS.

SULPHURS.
Bitumens.

EARTHS.
Pure earth.

WATER.

AIR.

FIRE.

More subtile matter.

The nature of the transitional forms which he inserts between his
principal classes show very clearly his entire lack of morphological
insight--the transitions are functional. The positions assigned to
clothes-moths and corals are very curious! The whole scheme, so
fantastic in its details, was largely influenced by Leibniz's
continuity philosophy, and is in no way an improvement on the older
and saner Aristotelian scheme.

Robinet, in the fifth volume of his book _De la nature_ (1761-6),
foreshadows the somewhat similar views of the German
transcendentalists. "All beings," he writes, "have been conceived and
formed on one single plan, of which they are the endlessly graduated
variations: this prototype is the human form, the metamorphoses of
which are to be considered as so many steps towards the most excellent
form of being."[23]

The idea of a gradation of beings appears also in Buffon (1707-1788),
but here it takes more definitely its true character as a functional
gradation.[24] "Since everything in Nature shades into everything
else," he says, "it is possible to establish a scale for judging of
the degrees of the intrinsic qualities of every animal."[25]

He is quite well aware that the groups of Invertebrates are different
in structural plan from the Vertebrates--"The animal kingdom includes
various animated beings, whose organisation is very different from our
own and from that of the animals whose body is similarly constructed
to ours."[26]

He limits himself to a consideration of the Vertebrates, deeming that
the economy of an oyster ought not to form part of his subject matter!
He has a clear perception of the unity of plan which reigns throughout
the vertebrate series.[27] What is new in Buffon is his interpretation
of the unity of plan. For the first time we find clearly expressed the
thought that unity of plan is to be explained by community of origin.

Buffon's utterances on this point are, as is well known, somewhat
vacillating. The famous passage, however, which occurs in his account
of the Ass shows pretty clearly that Buffon saw no theoretical
objection to the descent of all the varied species of animals from one
single form. Once admit, he argues, that within the bounds of a single
family one species may originate from the type species by
"degeneration," then one might reasonably suppose that from a single
being Nature could in time produce all the other organised beings.[28]
Elsewhere, _e.g._, in the discourse _De la Dégéneration des
Animaux_,[29] Buffon expresses himself with more caution. He finds that
it is possible to reduce the two hundred species of quadrupeds which
he has described to quite a small number of families "from which it is
not impossible that all the rest are derived."[30] Within each of the
families the species branch off from a parent or type species. This we
may note is a great advance on the linear arrangement implied in the
idea of an _Échelle des êtres_.[31]

It is a mistake to suppose that Buffon was par excellence a maker of
hypotheses. On the contrary he saw things very sanely and with a very
open mind. He expressly mentions the great difficulties which one
encounters in supposing that one species may arise from another by
"degeneration." How does it happen that two individuals "degenerate"
just in the right direction and to the right stage so as to be capable
of breeding together? How is it that one does not find intermediate
links between species? One is reminded of the objections, not
altogether without validity, which were made to the Darwinian theory
in its early days. I cannot agree with those who think that Buffon was
an out-and-out evolutionist, who concealed his opinions for fear of
the Church. No doubt he did trim his sails--the palpably insincere
"Mais non, il est certain, par la révélation, que tous les animaux ont
également participé à la grace de la création,"[32] following hard upon
the too bold hypothesis of the origin of all species from a single
one, is proof of it. But he was too sane and matter-of-fact a thinker
to go much beyond his facts, and his evolution doctrine remained
always tentative. One thing, however, he was sure of, that evolution
would give a rational foundation to the classification which, almost
in spite of himself, he recognised in Nature. If, and only if, the
species of one family originated from a single type species, could
families, be founded rationally, _avec raison_.

Buffon was, curiously enough, rather unwilling to recognise any
systematic unit higher than the species. Strictly; speaking there are
only individuals in Nature; but there are also groups of individuals
which resemble one another from generation to generation and are able
to breed together. These are species--Buffon adheres to the genetic
definition of species--and the species is a much more definite unit
than the genus, the order, the class, which are not divisions imposed
by us upon Nature. Species are definitely discontinuous,[33] and this
is the only discontinuity which Nature shows us. Buffon put his views
into practice in his _Histoire Naturelle_, where he describes species
after species, never uniting them into larger groups. We have seen,
however, how the facts forced upon him the conception of the "family."

Buffon was no morphologist. He left to Daubenton what one might call
the "dirty work" of his book, the dissection and minute description of
the animals treated.

But Buffon was a man of genius, and accordingly his ideas on
morphology are fresh and illuminating. Few naturalists have been so
free from the prejudices and traditions of their trade. He makes in
the _Discours sur la Nature des Animaux_[34] a distinction, which
Bichat and Cuvier later developed with much profit, between the
"animal" and the "vegetative" part of animals.[35] The vegetative or
organic functions go on continuously, even in sleep, and are performed
by the internal organs, of which the heart is the central one. The
active waking life of the animal, that part of its life which
distinguishes it from the plant, involves the external parts--the
sense-organs and the extremities. An animal is, as it were, made up of
a complex of organs performing the vegetative functions, assimilation,
growth, and reproduction, surrounded by an envelope formed by the
limbs, the sense-organs, the nerves and the brain, which is the centre
of this "envelope."[36] Animals may differ from one another enormously
in the external parts, particularly in the appendicular skeleton,
without showing any great difference in the plan and arrangement of
their internal organs. Quadrupeds, Cetacea, birds, amphibians and fish
are as unlike as possible in external form and in the shape of their
limbs; but they all resemble one another in their internal organs. Let
the internal organs change, however--the external parts will change
infinitely more, and you will get another animal, an animal of a
totally different nature. Thus an insect has a most singular internal
economy, and, in consequence, you find it is in every point different
from any vertebrate animal.

In this contrast, on the whole justified, between the importance of
variations in the "vegetative" and variations in the "animal" parts,
one may see without doing violence to Buffon's thought, an indication
of the difference between homology and analogy. It is usually in the
external parts, in the organs by which the animal adapts itself to its
environment, that one meets with the greatest number of analogical
resemblances. This contrast of vegetative and animal parts and their
relative importance for the discovery of affinities was at any rate a
considerable step towards an analysis of the concept of unity of plan.

To Xavier Bichat (1771-1802) belongs the credit of working out in
detail the distinction drawn by Aristotle and Buffon between the
animal and the vegetative functions. Bichat was not a comparative
anatomist; his interest lay in human anatomy, normal and pathological.
So his views are drawn chiefly from the consideration of human
structure.

He classifies functions into those relating to the individual and
those relating to the species. The functions pertaining to the
individual may be divided into those of the animal and those of the
organic life.[37] "I call _animal life_ that order of functions which
connects us with surrounding bodies; signifying thereby that this
order belongs only to animals" (p. lxxviii.). Its organs are the
afferent and efferent nerves, the brain, the sense-organs and the
voluntary muscles; the brain is its central organ. "Digestion,
circulation, respiration, exhalation, absorption, secretion,
nutrition, calorification, or production of animal heat, compose
organic life, whose principal and central organ is the heart" (p.
lxxix.).

The contrast of the animal and the organic life runs through all
Bichat's work; it receives classical expression in his _Recherches
Physiologiques sur la Vie et la Mort_ (1800). The plant and the animal
stand for two different modes of living. The plant lives within
itself, and has with the external world only relations of nutrition;
the animal adds to this organic life a life of active relation with
surrounding things (3rd ed., 1805, p. 2). "One might almost say that
the plant is the framework, the foundation of the animal, and that to
form the animal it sufficed to cover this foundation with a system of
organs fitted to establish relations with the world outside. It
follows that the functions of the animal form two quite distinct
classes. One class consists in a continual succession of assimilation
and excretion; through these functions the animal incessantly
transforms into its own substance the molecules of surrounding bodies,
later to reject these molecules when they have become heterogeneous to
it. Through this first class of functions the animal exists only
within itself; through the other class it exists outside; it is an
inhabitant of the world, and not, like the plant, of the place which
saw its birth. The animal feels and perceives its surroundings,
reflects its sensations, moves of its own will under their influence,
and, as a rule, can communicate by its voice its desires and its
fears, its pleasures or its pains. I call organic life the sum of the
functions of the former class, for all organised creatures, plants or
animals, possess them to a more or less marked degree, and organised
structure is the sole condition necessary to their exercise. The
combined functions of the second class form the 'animal' life, so
named because it is the exclusive attribute of the animal kingdom"
(pp. 2-3).

In both lives there is a double movement, in the animal life from the
periphery to the centre and from the centre to the periphery, in the
organic life also from the exterior to the interior and back again,
but here a movement of composition and decomposition. As the brain
mediates between sensation and motion, so the vascular system is the
go-between of the organs of assimilation and the organs of
dissimilation.

The most essential structural difference between the organs of animal
life and the organs of organic life is in man and the higher animals
at least, the symmetry of the one set and the irregularity of the
other--compare the symmetry of the nerves and muscles of the animal
life with the asymmetrical disposition of the visceral muscles and the
sympathetic nerves, which belong to the organic life.

Noteworthy differences exist between the two lives with respect to the
influence of habit. Everything in the animal life is under the
dominion of habit. Habit dulls sensation, habit strengthens the
judgment. In the organic life, on the contrary, habit exercises no
influence. The difference comes out clearly in the development of the
individual. The organs of the organic life attain their full
perfection independently of use; the organs of the animal life require
an education, and without education they do not reach perfection
(_Loc. cit._, p. 127).

Bichat was the founder of what was known for a time as General
Anatomy--the study of the constituent tissues of the body in health
and disease. His classification of tissues was macroscopical and
physiological; he relied upon texture and function in distinguishing
them rather than upon microscopical structure. The tissues he
distinguished are as follows:--[38]

1. The cellular membrane.
2. Nerves of animal life.
3. Nerves of organic life.
4. Arteries.
5. Veins.
6. Exhalants.
7. Absorbents and glands.
8. Bones.
9. Medulla.
10. Cartilage.
11. Fibrous tissue.
12. Fibro-cartilage.
13. Muscles of organic life.
14. Muscles of animal life.
15. Mucous membrane.
16. Serous membrane.
17. Synovial membrane.
18. The Glands.
19. The Dermis.
20. Epidermis.
21. Cutis.

The "cellular membrane" seems to mean undifferentiated connective
tissue; "exhalants" are imperceptible tubes arising from the
capillaries and secreting fat, serum, marrow, etc.; the "absorbents
and glands" are the lymphatics and the lymphatic glands.

In Bichat's eyes this resolution of the organism into tissues had a
deeper significance than any separation into organs, for to each
tissue must be attributed a _vie propre_, an individual and peculiar
life. "When we study a function we must consider the complicated organ
which performs it in a general way; but if we would be instructed in
the properties and life of that organ we must absolutely resolve it
into its constituent parts."[39] The tissues have, too, a great
importance for pathology, for diseases are often diseases of tissues
rather than of organs.[40]

    [9] _Le Monde végétal_, p. 41, Paris, 1907.

    [10] _Exercitationes de generatione animalium_,1651. For
    an account of Harvey's work on generation and
    development, see Em. Rádl's masterly _Geschichte der
    biologischen Theorien_, i., pp. 31-8, Leipzig, 1905.

    [11] The passage runs:--"Sic natura perfecta et divina
    nihil faciens frustra, nec quipiam animali cor addidit,
    ubi non erat opus, neque priusquam esset ejus usus,
    fecit; sed iisdem gradibus in formatione cujuscumque
    animalis, transiens per omnium animalium constitutiones
    (ut ita dicam) ovum, vermem, foetum, perfectionem in
    singulis acquirit."

    [12] See I. Geoffroy St Hilaire, _Essais de Zoologie
    générale_, p. 71, Paris, 1841.

    [13] M. Foster, _Lectures on the History of Physiology_,
    Cambridge, p. 53, 1901.

    [14] _Zootomia democritea_, Nuremberg, 1645;
    _Antiperipatias, seu de respiratione piscium_,
    Amsterdam, 1661.

    [15] Rádl, _loc. cit._, i., p. 50.

    [16] Perrault et Duverney, _Mémoires pour servir à
    l'histoire des Animaux_, Paris, 1699.

    [17] F. Houssay, _Nature et Sciences naturelles_, Paris,
    p. 76, n.d.

    [18] Foster, _loc. cit._, p. 85.

    [19] Trans, by Foster, _loc. cit._, p. 113.

    [20] He made a careful study of the silkworm.

    [21] "Etenim, ferventi aetatis calore, Anatomica
    aggressus, licet circa peculiaria fuerim solicitus, in
    _perfectioribus_ tamen haec rimari sum ausus. Verum, cum
    haec propriis tenebris obscura jaceant, simplicium
    analogismo egent; inde _insectorum_ indago illico
    arrisit; quae cum et ipsa suas habeat difficultates ad
    Plantarum perquisitionem animum _postremo_ adjeci, ut
    diu hoc lustrato mundo gressu retroacto Vegetantis
    Naturae gradu, ad prima studia iter mihi aperirem. Sed
    nec forte hoc ipsum sufficiet cum simplicior _Mineralium
    Elementorumque_ mundus praeire debeat. At in immensum
    excrescit opus, et meis viribus omnino impar," _Opera
    Omnia_, i., p. 1, London, 1686.

    [22] See particularly E. Rádl, _loc. cit._. I Teil. J. V.
    Carus, _Geschichte der Zoologie_, München, 1872.

    [23] For a good historical account of the gradation
    theories see Thienemann's paper in the _Zoologische
    Annalen_(Würzburg) iii., pp. 185-274, 1910, from which
    the quotation from Robinet is taken.

    [24] _Histoire naturelle_, i., p. 13; ii, p. 9; iv., p.
    101; and xiv., pp. 28-9, 1749 and later.

    [25] No translation can render the beauty of the
    original--"Comme tout se fait et que tout est par nuance
    dans la Nature ..." (iv., p. 101).

    [26] _Hist. nat._, iv., p. 5.

    [27] See particularly his comparison of the skeleton of
    the horse with that of man. _Hist. Nat._, iv., p. 381,
    also p. 13.

    [28] _Loc. cit._, p. 382.

    [29] Tome xiv., pp. 311-374.

    [30] Tome xiv., p. 358.

    [31] See also "Oiseaux," Tome i., pp. 394, 395. Pallas in
    1766 adopted for the whole animal kingdom this branching
    arrangement.

    [32] "But this cannot be, for it is certain by revelation
    that all animals have equally participated in the grace
    of creation."

    [33] iv., p. 385.

    [34] iv., pp. 3-110.

    [35] It has been revived in our own days by Bergson,
    _Matière et Mémoire_, p. 57.

    [36] iv., pp. 7-15.

    [37] _Anatomie Générale_, Paris, 1801, Eng. trans. 1824.

    [38] _Anatomie Générale_, Eng. trans., i., p. lii.

    [39] _Anatomie Générale_, Eng. trans., i., p. lviii.

    [40] _Loc cit._, i., sect. vii.



CHAPTER III

CUVIER


Cuvier was perhaps the greatest of comparative anatomists; his work
is, in the best sense of the word, classical.

Like all his predecessors, like Aristotle, like the Italian
anatomists, Cuvier studied structure and function together, even gave
function the primacy.

Some functions, he says,[41] are common to all organised bodies--origin
by generation, growth by nutrition, end by death. There are also
secondary functions. Of these the most important, in animals at least,
are the faculties of feeling and moving. These two faculties are
necessarily bound up together; if Nature has given animals sensation
she must also have given them the power of movement, the power to flee
from what is harmful and draw near to what is good. These two
faculties determine all the others. A creature that feels and moves
requires a stomach to carry food in. Food requires instruments to
divide it, liquids to digest it. Plants, which do not feel and do not
move, have no need of a stomach, but have roots instead. Thus the
"Animal Functions" of feeling and moving determine the character of
the organs of the second order, the organs of digestion. These in
their turn are prior to the organs of circulation, which are a means
to the end of distributing the nutrient fluid or blood to all parts of
the body. These organs of the third order are not only dependent on
those of the second order, but are also not even necessary, for many
animals are without them. Only animals with a circulatory system can
have definite breathing organs--lungs or gills. Plants, and animals
without a circulation, breathe by their whole surface.

There is accordingly a rational order of functions, and therefore of
the systems of organs which perform them. The most important are the
Animal Functions, with their great organ-system, the neuro-muscular
mechanism. Then come the digestive functions, and after them, and in a
sense accessory to them, the functions and organs of circulation and
respiration. The last three may be grouped as the Vital Functions.

The Animal Functions not only determine the character of the Vital
Functions, but influence also the primary faculty of generation, for
animals' power of movement has rendered their mode of fecundation more
simple, has therefore had an effect on their organs of generation.

This division into "Animal" and "Vital" functions recalls Buffon's and
Bichat's distinction of the "animal" and the "vegetative" lives.
Cuvier apparently took this idea from Buffon, for he says that a plant
is an animal that sleeps.[42] But the idea is as old as Aristotle, who
discusses the "sleep" of embryos and of plants in the last book of the
_De Generatione animalium_. The distinction between animal and
vegetative life is, of course, based for Aristotle in the difference
between the [Greek: psychê aisthêtikê] and the [Greek: psychê
threptikê]. Cuvier, like Aristotle, Buffon, and Bichat, makes the
heart the centre of the "vegetative" organs.

It is important to note that Cuvier puts function before structure,
and infers from function what the organ will be. "Plants," he writes,
"having few faculties, have a very simple organisation."[43] It is only
after having discussed and classified functions that Cuvier goes on to
examine organs.

First his views on the composition of the animal body. Aristotle
distinguished three degrees of composition--the "elements," the
homogeneous parts, and the heterogeneous parts or organs. Cuvier does
the same. Some small advance has been made in the two thousand years'
interval, due in the first place to the progress of chemistry, and in
the second to the invention of the microscope. To the first
circumstance Cuvier owes his knowledge that the inorganic substances
forming the first degree of composition are principally C, N, H, O,
and P, combined to form albumen, fibrine, and the like, which are in
their turn combined to form the solids and fluids of the body. To the
latter circumstance Cuvier owes the statement that the finest
fragments into which mechanical division can resolve the organism are
little flakes and filaments, which, joined up loosely together, form a
"cellulosity." The discovery of the true cellular nature of animal
tissues did not come till much later, till some years after Cuvier's
death in 1832. Knowledge of histological detail was, however,
considerable by the beginning of the 19th century. Cuvier knew, for
example, that each muscle fibre has its own nerve fibre. But he gives
no elaborate account of the homogeneous parts, no detailed histology.
On the other hand his treatment of the heterogeneous parts or organs
is detailed and masterly.[44]

The main systems of organs are, in order of importance, the nervous
and muscular, the digestive, the circulatory, and the respiratory.
Each organ or system of organs may have many forms. If any form of any
organ could exist in combination with any form of all the others there
would be an enormous number of combinations theoretically possible.
But these combinations do not all exist in Nature, for organs are not
merely assembled (_rapproché's_), but act upon one another, and act
all together for a common end. Accordingly only the combinations that
fulfil these conditions exist in Nature. Cuvier thus dismisses the
question of a science of possible organic forms and considers only the
forms or combinations actually existing. This question of the
possibility of a "theoretical" morphology of living things, after the
fashion of the morphology of crystals with their sixteen possible
types, was raised in later years by K. G. Carus, Bronn, and Haeckel.

Organisms, then, are harmonious combinations of organs, and the
harmony is primarily a harmony of functions. Every function depends
upon every other, and all are necessary. The harmony of organs and
their mutual dependence are the results of the interdependence of
function. This thought, the recognition of the functional unity of the
organism, is the fundamental one at the base of all Cuvier's work.
Before him men had recognised more or less clearly the harmony of
structure and function, and had based much of their work upon this
unanalysed assumption. Cuvier was the first naturalist to raise this
thought to the level of a principle peculiar to natural history. "It
is on this mutual dependence of the functions and the assistance which
they lend one to another that are founded the laws that determine the
relations of their organs; these laws are as inevitable as the laws of
metaphysics and mathematics, for it is evident that a proper harmony
between organs that act one upon another is a necessary condition of
the existence of the being to which they belong."[45]

This rational principle, peculiar to natural history, Cuvier calls the
principle of the conditions of existence, for the following
reason:--"Since nothing can exist that does not fulfil the conditions
which render its existence possible, the different parts of each being
must be co-ordinated in such a way as to render possible the existence
of the being as a whole, not only in itself, but also in its relations
with other beings, and the analysis of these conditions often leads to
general laws which are as certain as those which are derived from
calculation or from experiment."[46]

By "conditions of existence" he means something quite different from
what is now commonly understood. The idea of the external conditions
of existence, the environment, enters very little into his thought. He
is intent on the adaptations of function and organ within the living
creature--a point of view rather neglected nowadays, but essential for
the understanding of living things. The very condition of existence of
a living thing, and part of the essential definition of it, is that
its parts work together for the good of the whole.

The principle of the adaptedness of parts may be used as an
explanatory principle, enabling the naturalist to trace out in detail
the interdependence of functions and their organs. When you have
discovered how one organ is adapted to another and to the whole, you
have gone a certain way towards understanding it. That is using
teleology as a regulative principle, in Kant's sense of the word.
Cuvier was indeed a teleologist after the fashion of Kant, and there
can be no doubt that he was influenced, at least in the exposition of
his ideas, by Kant's _Kritik der Urtheilskraft_, which appeared ten
years before the publication of the _Leçons d'Anatomie Comparée_.
Teleology in Kant's sense is and will always be a necessary postulate
of biology. It does not supply an explanation of organic forms and
activities, but without it one cannot even begin to understand living
things. Adaptedness is the most general fact of life, and innumerable
lesser facts can be grouped as particular cases of it, can be, so far,
understood.

Cuvier's famous principle of correlation, the corner-stone of his
work, is simply the practical application to the facts of structure of
the principle of functional adaptedness. By the principle of
correlation, from one part of an animal, given sufficient knowledge of
the structure of its like, you can in a general way construct the
whole. "This must necessarily be so: for all the organs of an animal
form a single system, the parts of which hang together, and act and
re-act upon one another; and no modifications can appear in one part
without bringing about corresponding modifications in all the
rest."[47] The logical basis of the principle is sound. The functions
of the parts are all intimately bound up with one another, and one
function cannot vary without bringing in its train corresponding
modifications in the others. Structure and function are bound up
together; every modification of a function entails therefore the
modification of an organ. Hence from the shape of one organ you can
infer the shape of the other organs--if you have sufficiently
extensive empirical knowledge of functions, and of the relation of
structure to function in each kind of organ. Given an alimentary canal
capable of digesting only flesh, and possessing therefore a certain
form, you know that the other functions must be adapted to this
particular function of the alimentary canal. The animal must have keen
sight, fine smell, speed, agility, and strength in paws and jaws.
These particular functions must have correspondingly modified organs,
well-developed eyes and ears, claws and teeth. Further, you know from
experience that such and such definitely modified organs are
invariably found with the carnivorous habit, carnassial teeth, for
example, and reduced clavicles. From a "carnivorous" alimentary canal,
then, you can infer with certainty that the animal possessed
carnassial teeth and the other structural peculiarities of carnivorous
animals, _e.g._, the peculiar coronoid process of the mandible. From
the carnassial tooth you can infer the reduced clavicle, and so on.
"In a word, the form of the tooth implies the form of the condyle;
that of the shoulder blade that of the claws, just as the equation of
a curve implies all its properties."[48]

Similarly the great respiratory power of birds is correlated with
their great muscular strength, and renders necessary great digestive
powers. Hence the correlated structure of lungs, muscles and their
attachments, and alimentary canal, in birds.

Not only do systems of organs, by being adjusted to special
modifications of function, influence one another, but so also do parts
of the same organ. This is noticeably the case with the skeleton,
where hardly a facet can vary without the others varying
proportionately, so that from one bone you can up to a certain point
deduce all the rest.

We deduce the necessity, the constancy, of these co-existences of
organs from the observed reciprocal influence of their functions. That
being established, we can argue from observed constancy of relation
between two organs an action of one upon the other, and so be led to a
discovery of their functions. But even if we do not discover the
functional interdependencies of the parts, we can use the established
fact of the constant co-existence of two parts as proof of a
functional correlation between them.

Correlation is either a rational or an empirical principle, according
as we know or do not know the interdependence of function of which it
is the expression. Even when we apply the rational principle of
correlation it would be useless in our hands if we had not extensive
empirical knowledge; when we use an empirical rule of correlation we
depend entirely upon observation. "There are a great many cases,"
writes Cuvier,[49] "where our theoretical knowledge of the relations of
forms would not suffice, if it were not filled out by observation,"
that is to say, there are many cases of correlation not yet explicable
in terms of function. From a hoof you can deduce the main characters
of herbivores (with a certain amount of assistance from your empirical
knowledge of herbivores), but could you from a cloven hoof deduce that
the animal is a ruminant, unless you had observed the constancy of
relation, not directly explicable in terms of function, between cloven
hoofs and chewing the cud? Or could you deduce from the existence of
frontal horns that the animal ruminates? "Nevertheless, since these
relations are constant, they must necessarily have a sufficient cause;
but as we are ignorant of this cause, observation must supplement
theory; observation establishes empirical laws which become almost as
certain as the rational laws, when they are based upon a sufficient
number of observations.... But that there exist all the same hidden
reasons for all these relations is partly revealed by observation
itself, independently of general philosophy."[50] That is to say, even
correlations for which no explanation in terms of function can be
supplied are probably in reality functional correlations. This may, in
some cases, be inferred from the graded correspondence of two sets of
organs. For example, ungulates which do not ruminate, and have not a
cloven hoof, have a more perfect dentition and more bones in the foot
than the true cloven-hoofed ruminants. There is a correlation between
the state of development of the teeth and of the foot. This
correlation is a graded one, for camels, which have a more perfect
dentition than other ruminants, have also a bone more in their tarsus.
It seems probable, therefore, that there is some reason, that is, some
explanation in terms of function, for this case of correlation.

Nevertheless, the fact remains that many correlations are not
explicable in terms of function, and the substitution of correlation
as an empirical principle for correlation as a rational principle
marks for Cuvier a step away from his functional comparative anatomy
towards a pure morphology. It is significant that in later times the
term correlation has come to be applied more especially to the purely
empirical constancies of relation, and has lost most of its functional
significance. But the correlation of the parts of an organism is no
mere mathematical concept, to be expressed by a coefficient, but
something deeper and more vital.

Cuvier interpreted the functional dependence of the parts in terms of
what we now call the general metabolism. He had a clear vision of the
constant movement of molecules in the living tissue, combining and
recombining, of the organism taking in and intercalating molecules
from outside from the food and rejecting molecules in the excretions,
a ceaseless _tourbillon vital_. "This general movement, universal in
every part, is so unmistakably the very essence of life that parts
separated from a living body straightway die."[51] The organisation of
the body, the arrangement of its solids and liquids, is adapted to
further the _tourbillon vital_. "Each part contributes to this general
movement its own particular action and is affected by it in particular
ways, with the result that, in every being, life is a unity which
results from the mutual action and reaction of all its parts."[52]

Cuvier, however, did not resolve life into metabolism, nor reduce
vital happenings to the chemical level. The form of organised bodies
is more essential than the matter of which they are composed, for the
matter changes ceaselessly while the form remains unchanged. It is in
form that we must seek the differences between species, and not in the
combinations of matter, which are almost the same in all.[53] The
differences are to be sought at the level of the second and third
degrees of composition.

The existence of differences of form introduces a new problem, the
problem of diversity. There are only a few possible combinations of
the principal organs, but as you get down to less important parts the
possible scope of variation is greatly increased, and most of the
possible variations do exist. Nature seems prodigal of form, of form
which needs not to be useful in order to exist. "It needs only to be
possible, _i.e._, of such a character that it does not, destroy the
harmony of the whole."[54] We seize here the relation of the principle
of the adaptedness of parts to the problem of the variety of form. The
former is in a sense a regulative and conservative principle which
lays down limits beyond which variation may not stray. In itself it is
not a fountain of change; there must be another cause of change. This
thought is of great importance for theories of descent.

Cuvier has no theory to account for the variety of form: he contents
himself with a classification. There are two main ways of classifying
forms; you may classify according to single organs or according to the
totality of organs. By the first method you can have as many
classifications as you have organs, and the classifications will not
necessarily coincide. Thus you can divide animals according to their
organs of digestion into two classes, those in which the alimentary
canal is a sac with one opening (zoophytes) and those in which the
canal has two openings,[55] a curious forestalment, in the rough, of
the modern division of Metazoa into Coelentera and Coelomata.

It is only by taking single organs that you can arrange animals into
long series, and you will have as many series as you take organs. Only
in this way can you form any _Échelle des êtres_ or graded series; and
you can get even this kind of gradation only within each of the big
groups formed on a common plan of structure; you can never grade, for
example, from Invertebrates to Vertebrates through intermediate
forms[56] (which is perfectly true, in spite of Amphioxus and
Balanoglossus!).

In the _Règne Animal_ Cuvier restricts the application of the idea of
the _Échelle_ within even narrower limits, refusing to admit its
validity within the bounds of the vertebrate phylum, or even within
the vertebrate classes. This seems, however, to refer to a seriation
of whole organisms and not of organs, so that the possibility of a
seriation of organs within a class is not denied. Cuvier was, above
all, a positive spirit, and he looked askance at all speculation which
went beyond the facts. "The pretended scale of beings," he wrote, "is
only an erroneous application to the totality of creation of partial
observations, which have validity only when confined to the sphere
within which they were made."[57] This remark, which is after all only
just, perfectly expresses Cuvier's attitude to the transcendental
theories, and was probably a protest against the sweeping
generalisations of his colleague, Etienne Geoffroy St Hilaire.

A true classification should be based upon the comparison of all
organs, but all organs are not of equal value for classification, nor
are all the variations of each organ equally important. In estimating
the value of variations more stress should be laid on function than on
form, for only those variations are important which affect the mode of
functioning. These are the principles on which Cuvier bases the
classification of animals given in the _Leçons_, Article V., "Division
des animaux d'après l'ensemble de leur organisation." The scheme of
classification actually given in the _Leçons_ recalls curiously that
of Aristotle, for there is the same broad division into Vertebrates,
with red blood, and Invertebrates, almost all with white blood. Nine
classes altogether are distinguished--Mammals, Birds, Reptiles,
Fishes, Molluscs, Crustacea, Insects, Worms, Zoophytes (including
Echinoderms and Coelenterates).

A maturer theory and practice of classification is given in the _Règne
Animal_ of seventeen years later. Here the principle of the
subordination of characters (which seems to have been first explicitly
stated by the younger de Jussieu in his _Genera Plantarum_, 1789,[58])
is more clearly recognised. The properties or peculiarities of
structure which have the greatest number of relations of
incompatibility and coexistence, and therefore influence the whole in
the greatest degree, are the important or dominating characters, to
which the others must be subordinated in classification. These
dominant characters are also the most constant.[59] In deciding which
characters are the most important Cuvier makes use of his fundamental
classification of functions and organs into two main sets. "The heart
and the organs of circulation are a kind of centre for the vegetative
functions, as the brain and the spinal cord are for the animal
functions."[60] These two organ-systems vary in harmony, and their
characters must form the basis for the delimitation of the great
groups. Judged by this standard there are four principal types of
form,[61] of which all the others are but modifications. These four
types are Vertebrates, Molluscs, Articulates, and Radiates. The first
three have bilateral, the last has radial symmetry. Vertebrates and
Molluscs have blood-vessels, but Articulates show a functional
transition from the blood-vessel to the tracheal system. Radiates
approach the homogeneity of plants; they appear to lack a distinct
nervous system and sense organs, and the lowest of them show only a
homogeneous pulp which is mobile and sensitive. All four classes are
principally distinguished from one another by the broad structural
relations of their neuromuscular system, of the organs of the animal
functions. Vertebrates have a spinal cord and brain, an internal
skeleton built on a definite plan, with an axis and appendages; in
Molluscs the muscles are attached to the skin and the shell, and the
nervous system consists of separate masses; Articulates have a hard
external skeleton and jointed limbs, and their nervous system consists
of two long ventral cords; Radiates have ill-defined nervous and
muscular systems, and in their lowest forms possess the animal
functions without the animal organs.

This well-rounded classification of animal forms is in a sense the
crown of Cuvier's work, for the principle of the subordination of
characters, in the interpretation which he gives to it, is a direct
application of his principle of functional correlation. Each of the
great groups is built upon one plan. The idea of the unity of plan has
become for Cuvier a commonplace of his thought, and it is tacitly
recognised in all his anatomical work. But he never takes it as a
hard-and-fast principle which must at all costs be imposed upon the
facts.

Cuvier has become known as the greatest champion of the fixity of
species, but it is not often recognised that his attitude to this
problem is at least as scientific as that of the evolutionists of his
own and later times. No doubt he became dogmatic in his rejection of
evolution-theory, but he was on sure ground in maintaining that the
evolutionists of his day went beyond their facts. He considered that
certain forms (species) have reproduced themselves from the origin of
things without exceeding the limits of variation. His definition of a
species was, "the individuals descended from one another or from
common parents, together with those that resemble them as much as they
resemble one another."[62] "These forms are neither produced nor do
they change of themselves; life presupposes their existence, for it
cannot arise save in organisations ready prepared for it."[63]

He based his rejection of all theories of descent upon the absence of
definite evidence for evolution. If species have gradually changed, he
argued, one ought to find traces of these gradual modifications.[64]
Palæontology does not furnish such traces. Again, the limits of
variation, even under domestication, are narrow, and the most extreme
variation does not fundamentally alter the specific type. Thus the dog
has varied perhaps most of all, in size, in shape, in colour. "But
throughout all these variations the relations of the bones remain the
same, and the form of the teeth never changes to an appreciable
extent; at most there are some individuals in which an additional
false molar develops on one side or the other."[65] This second
objection is the objection of the morphologist. It would be an
interesting study to compare Cuvier's views on variation with those of
Darwin, who was essentially a systematist.

Cuvier's first objection was of course determined to some extent by
the imperfection of the palæontological knowledge of his time. But
even at the present day the objection has a certain force, for
although we have definite evidence of many serial transformations of
one species into another along a single line, for example, Neumayr's
_Paludina_ series, yet at any one geological level the species, the
lines of descent, are all distinct from one another.[66]

Cuvier recognised very clearly that there is a succession of forms in
time, and that on the whole the most primitive forms are the earliest
to appear. Mammals are later than reptiles, and fishes appear earlier
than either. As Depéret puts it, "Cuvier not only demonstrated the
presence in the sedimentary strata of a series of terrestrial faunas
superimposed and distinct, but he was the first to express, and that
very clearly, the idea of the gradual increase in complexity of these
faunas from the oldest to the most recent" (p. 10).

He did not believe that the fauna of one epoch was transformed into
the fauna of the next. He explained the disappearance of the one by
the hypothesis of sudden catastrophes, and the appearance of the next
by the hypothesis of immigration. He nowhere advanced the hypothesis
of successive new creations. "For the rest, when I maintain that the
stony layers contain the bones of several genera and the earthy layers
those of several species which no longer exist, I do not mean that a
new creation has been necessary to produce the existing species, I
merely say that they did not exist in the same localities and must
have come thither from elsewhere."[67] It was left to d'Orbigny to
teach the doctrine of successive creations, of which he distinguished
twenty-seven (_Cours élémentaire de palaeontologie stratigraphique_,
1849).

Cuvier, however, can hardly have believed that all species were
present at the beginning, since he does admit a progression of forms.
Probably he had no theory on the subject, for theories without facts
had little interest for him. At any rate it is a mistake to think that
Cuvier was a supporter of the theological doctrine of special
creation. His philosophy of Nature was mechanistic, and he dedicated
his _Recherches sur les Ossemens Fossiles_ to his friend Laplace. He
admitted the idea of evolution at least so far as to conceive of a
development of man from a savage to a civilised state.[68] He refused
to accept the extravagant evolutionary theory of Demaillet and the
somewhat confused theory of Lamarck (whom he joins with Demaillet),[69]
just as he rejected the transcendental theories of Geoffroy St
Hilaire, because they seemed to him not based upon facts.

    [41] _Leçons d'Anatomie Comparée_, tome i., pp. 10 _et
    scq._, 1800.

    [42] _Leçons d'Anatomie Comparée_, i., p. 18.

    [43] _Loc. cit._, i., p. 13.

    [44] _Leçons d'Anatomie Comparée_, tome i., Articles
    iii.-iv., 1800.

    [45] _Leçons d'Anatomie Comparée_, i., p. 47.

    [46] _Le Règne Animal_, i., p. 6, 1817.

    [47] _Histoire des Progrès des Sciences naturelles depuis
    1789_, i., p. 310, 1826.

    [48] _Recherches sur les Ossemens Fossiles_, i., p. 60,
    1812.

    [49] _Ossemens fossiles_, i., p. 60.

    [50] _Loc. cit._, i., p. 63.

    [51] _Leçons d'Anatomie Comparée_, i., p. 6.

    [52] _Le Règne Animal_, i., p. 16.

    [53] _Hist. Prog. Sci. Nat._, i., p. 187, 1826.

    [54] _Leçons_, i., p. 58.

    [55] _Loc. cit._, i., Article iii.

    [56] _Loc. cit._, i., p. 60.

    [57] _Règne Animal_, i., p. xx.

    [58] Cuvier, _Hist. Prog. Sci. Nat._, i., p. 288, 1826.

    [59] _Règne Animal_, i., p. 10.

    [60] _Règne Animal_, p. 55.

    [61] First propounded by Cuvier in 1812, _Ann. Mus.
    d'Hist. Nat._, xix.

    [62] _Règne Animal_, i., p. 19.

    [63] _Loc. cit._, p. 20.

    [64] _Recherches sur les Ossemens Fossiles_, i., p. 74,
    1812.

    [65] _Loc. cit._, p. 79.

    [66] See C. Depéret, _Les transformations du Monde
    animal_, Paris, 1907, and G. Steinmann, _Die
    geologischen Grundlagen der Abstammungslehre_, Leipzig,
    1908.

    [67] _Recherches_, i., p. 81.

    [68] _Règne Animal_, i., p. 91.

    [69] _Ossemens Fossiles_, i., p. 26.



CHAPTER IV

GOETHE


Science, in so far as it rises above the mere accumulation of facts,
is a product of the mind's creative activity. Scientific theories are
not so much formulæ extracted from experience as intuitions imposed
upon experience. So it was that Goethe, who was little more than a
dilettante,[70] seized upon the essential principles of a morphology
some years before that morphology was accepted by the workers.

Goethe is important in the history of morphological method because he
was the first to bring to clear consciousness and to express in
definite terms the idea on which comparative anatomy before him was
based, the idea of the unity of plan. We have seen that this idea was
familiar to Aristotle and that it was recognised implicitly by all who
after him studied structure comparatively. In Goethe's time the idea
had become ripe for expression. It was used as a guiding principle in
Goethe's youth particularly by Vicq d'Azyr and by Camper. The former
(1748-1794), who discovered[71] in the same year as Goethe (1784) the
intermaxillary bone in man, pointed out the homology in structure
between the fore limb and the hind limb, and interpreted certain
rudimentary bones, the intermaxillaries and rudimentary clavicles, in
the light of the theory that Vertebrates are built upon one single
plan of structure.

"Nature seems to operate always according to an original and general
plan, from which she departs with regret and whose traces we come
across everywhere" (Vicq d'Azyr, quoted by Flourens, _Mém. Acad.
Sei._, XXIII., p. xxxvi.).

Peter Camper (1722-1789), we are told by Goethe himself in his
_Ostéologie_, was convinced of the unity of plan holding throughout
Vertebrates; he compared in particular the brain of fishes with the
brain of man.

The idea of the unity of plan had not yet become limited and defined
as a strictly scientific theory; it was an idea common to philosophy,
to ordinary thought, and to anatomical science. We find it expressed
by Herder (who perhaps got it from Kant) in his _Ideen sur Philosophie
der Geschichte der Menschheit_ (1784), and it is possible that Goethe
became impressed with the importance of the idea through his
conversations with Herder. Be that as it may, it is certain that
Goethe sought for the intermaxillaries in man only because he was
firmly convinced that the skeleton in all the higher animals was built
upon one common plan and that accordingly bones such as the
intermaxillaries, found well developed in some animals, must also be
found in man. The idea was not drawn from the facts, but the facts
were interpreted and even sought for in the light of the idea. "I
eagerly worked upon a general osteological scheme, and had accordingly
to assume that all the separate parts of the structure, in detail as
in the whole, must be discoverable in all animals, because on this
supposition is built the already long begun science of comparative
anatomy."[72]

The principle comes to clear expression in his _Erster Entwurf einer
allgemeinen Einleitung in die vergleichende Anatomie_ (1795).[73] He
writes:--"On this account an attempt is here made to arrive at an
anatomical type, a general picture in which the forms of all animals
are contained in potentia, and by means of which we can describe each
animal in an invariable order."[74] His aim is to discover a general
scheme of the constant in organic parts, a scheme into which all
animals will fit equally well, and no animal better than the rest.
When we remember that the type to which anatomists before him had,
consciously or unconsciously, referred all other structure was man
himself, we see that in seeking after an abstract generalised type
Goethe was reaching out to a new conception. The fact that only the
structure of man and the higher animals was at all well-known in his
time led Goethe to think that his general Typus would hold for the
lower animals as well, though it was to be arrived at primarily from a
study of the higher animals. All he could assert of the entire animal
kingdom was that all animals agreed in having a head, a middle part,
and an end part, with their characteristic organs, and that
accordingly they might, in this respect at least, be reduced to one
common Typus. Goethe's knowledge of the lower animals was not
extensive.

Though Goethe did not work out a criterion of the homology of parts
with any great clearness, he had an inkling of the principle later
developed by E. Geoffroy St Hilaire, and called by him the "Principle
of Connections." According to this principle, the homology of a part
is determined by its position relative to other parts. Goethe
expresses it thus:--"On the other hand the most constant factor is the
position in which the bone is invariably found, and the function to
which it is adapted in the organic edifice."[75] But from this sentence
it is not clear that Goethe understood the principle as one of form
independent of function, for he seems to consider that the homology of
an organ is partly determined by the function which it performs for
the whole. He wavers between the purely formal or morphological
interpretation of the principle of connections and the functional. We
find him in the additions to the _Entwurf_ (1796), saying:--"We must
take into consideration not merely the spatial relations of the parts,
but also their living reciprocal influence, their dependence upon and
action on one another." [76] But in seeking for the intermaxillary bone
in man he was guided by its position relative to the maxillaries--it
must be the bone between the anterior ends of the maxillaries, a bone
whose limits are indicated in the adult only by surface grooves.

As a matter of fact Goethe's morphological views are neither very
clearly expressed nor very consistent. This comes out in his treatment
of the relation between structure and function. Sometimes he takes the
view that structure determines function. "The parts of the animal," he
writes, "their reciprocal forms, their relations, their particular
properties determine the life and habits of the creature."[77] We are
not to explain, he says, the tusks of the _Babirussa_ by their
possible use, but we must ask how it comes to have tusks. In the same
way we must not suppose that a bull has horns in order to gore, but we
must investigate the process by which it comes to have horns to gore
with. This is the rigorous morphological view. On the other hand he
admits elsewhere that function may influence form. Apparently he did
not work out his ideas on this point to logical clearness, and Rádl[78]
is probably correct in saying that the following quotation with its
double assertion represents most nearly Goethe's position:--

"Also bestimmt die Gestalt die Lebensweise des Thieres, Und die Weise
zu leben, sie wirkt auf alle Gestalten Mächtig zurück."[79]

His best piece of purely morphological work was his theory of the
metamorphosis of plants. Stripped of its vaguer elements, and of the
crude attempt to explain differences in the character of plant organs
by differences in the degree of "refinement" of the sap supplied to
them, the theory is that stem-leaves, sepals, petals, and stamens are
all identical members or appendages. These appendages differ from one
another only in shape and in degree of expansion, stem-leaves being
expanded, sepals contracted, petals expanded, and so on alternately.
It is equally correct to call a stamen a contracted petal, and a petal
an expanded stamen, for no one of the organs is the type of the
others, but all equally are varieties of a single abstract
plant-appendage.

What Goethe considered he had proved for the appendages of plants he
extended to all living things. Every living thing is a complex of
living independent beings, which "der Idee, der Anlage nach," are the
same, but in appearance may be the same or similar, different or
unlike.[80] Not only is there a primordial animal and a primordial
plant, schematic forms to which all separate species are referable,
but the parts of each are themselves units, which "der Idee nach," are
identical _inter se_. This fantasy can hardly be taken seriously as a
scientific theory; it seems, however, to have been what guided Goethe
in his "discovery" of the vertebral nature of the skull. Just as the
fore limb can be homologised with the hind limb, so, reasoning by
analogy, the skull should be capable of being homologised with the
vertebræ. To what ludicrous extremes this doctrine of the repetition
of parts within the organism was pushed we shall see when we consider
the theories of the German transcendentalists of the early nineteenth
century.

Though Goethe's morphological views were lacking in definiteness he
hit upon one or two ideas which proved useful. Thus he enunciated the
"law of balance" long before Etienne Geoffroy St Hilaire, the law
"that to no part can anything be added, without something being taken
away from another part, and _vice versa_."[81] He saw, too, what a help
to the interpretation of adult structure the study of the embryo would
be, for many bones which are fused in the adult are separate in the
embryo.[82] This also was a point to which the later transcendentalists
gave considerable attention.

So far we have spoken of Goethe as if he were merely the prophet of
formal morphology; we have pointed out how he brought to clear
expression the morphological principle implicit in the idea of unity
of type, and how he seized upon some important guiding ideas, such as
the principle of connections. But Goethe was not a formalist, and he
was very far from the static conception of life which is at the base
of pure morphology. His interest was not in _Gestalt_ or fixed form,
_Bildung_ or form change. He saw that _Gestalt_ was but a momentary
phase of _Bildung_, and could be considered apart and in itself only
by an abstraction fatal to all understanding of the living thing.
Mephistopheles scoffs at the scholars who would explain a living
creature by anatomising it:

 "Dann hat er die Theile in seiner Hand,
  Fehlt leider! nur das geistige Band."[83]

Goethe kept clear of this mistake; he knew that the artist comes
nearer to the truth than the analyst.

In the fragment entitled _Bildung und Umbildung organischer Naturen_
(1807), introductory to a reprint of his paper on the "Metamorphosis
of Plants," we get an exposition of his general views on living
things. He points out there how we try to understand things by
separating them into their parts. We can, it is true, resolve the
organism into its structural elements, but we cannot recompose it or
endow it with life by joining up the parts. Hence we require some
other means of understanding it. "In all ages even among scientific
men there can be discerned a yearning to apprehend the living form as
such, to grasp the connection of their external visible parts, to
interpret them as indications of the inner activity, and so, in a
certain measure, to master the whole conceptually." This science which
should discover the inner meaning of organic _Bildung_ is called
Morphology.[84] In Morphology we should not speak of _Gestalt_ or fixed
form, or if we do we should understand by it only a momentary phase of
_Bildung_. Form is of interest not in itself but only as the
manifestation of the inner activity of the living being. Over
development, he says elsewhere, there presides a formative force, a
_bildende Kraft_ or _Bildungstrieb_, which works out the idea of the
organism. Living things, in his view of them, strive to manifest an
idea. They are Nature's works of art--and so, incidentally, they
require an artist to interpret them.

This profound conception of the nature of life is applied not only to
the growing changing individual but also to the whole changing world
of organisms. They are all manifestations of a living shaping power
which moulds them. This shaping power, immanent in all life, is
conceived to work according to a general plan, and so we get an
explanation of the fact that living things seem simply varieties of
one common type.

"If we once recognise," says Goethe, "that the creative spirit brings
into being and shapes the evolution of the more perfect organic
creatures according to a general scheme, is it altogether impossible
to represent this original plan if not to the senses at least to the
mind...?"[85]

Such an interpretation of the unity of plan reaches perhaps beyond the
bounds of science.

    [70] _See_ Kohlbrugge, "Hist. krit. Studien über Goethe
    als Naturforscher," _Zool. Annalen._ v., 1913, pp.
    83-231.

    [71] Or re-discovered, according to Kohlbrugge.

    [72] Cotta ed., vol. ix., p. 448.

    [73] "First Draft of a General Introduction to
    Comparative Anatomy."

    [74] Cotta ed., ix., p. 463.

    [75] Cotta ed., p. 478.

    [76] _Loc. cit._, p. 491.

    [77] _Entwurf_, Cotta ed., ix., p. 465.

    [78] _Geschichte der biologischen Theorien_, i., p. 266.

    [79] "So the form determines the manner of life of the
    animal, and the manner of life in its turn reacts
    powerfully upon all forms."

    [80] _Bildung und Umbildung organischer Naturen_, 1807.

    [81] Cotta ed., ix., p. 466.

    [82] _Loc. cit._, pp. 474-5.

    [83] Then he has all the parts within his hand, excepting
    only, sad to say, the living bond.

    [84] Goethe was the inventor of the word.

    [85] Cotta ed., ix., p. 490.



CHAPTER V

ETIENNE GEOFFROY SAINT-HILAIRE


E. Geoffrey made an experiment, unsuccessful but instructive. He tried
to found a science of pure morphology; he failed: his failure showed,
once and for all, that a pure morphology of organic forms is
impracticable.

Already, in 1796, in one of his earliest memoirs,[86] Geoffroy was
guided by the idea that Nature has formed all living things upon one
plan. Organs which seem anomalous are merely modifications of the
normal; the trunk of an elephant is formed by the excessively
prolonged nostrils, the horn of a rhinoceros is simply a mass of
adhering hairs. In general, however varied their form, all organs are
simply variations of a common scheme; Nature employs no new organs.
Organs which are rudimentary, such as the clavicles in the ostrich and
the nictitating membrane in man, bear witness to the unity of plan. In
this Geoffroy goes no further than his predecessors. They too had
recognised homologies of organs; they too had interpreted rudimentary
organs as vestiges of an original plan.

In a series of papers published in 1807, Geoffroy took a further step,
and sought to establish homologies which were not obvious--homologies,
too, not so much of organs as of parts.

These memoirs (published in the _Annales du Muséum d'Histoire
naturelle_, vols. ix. and x., 1807) dealt with the homology between
the bones of the pectoral fin and girdle in fish and the bones of the
arm and shoulder-girdle in higher Vertebrates, with the homologies of
the bones of the sternum, and with the determination of the pieces of
the skull, particularly in the crocodile. All Geoffroy's morphological
doctrine is found in them, but for the full expression of his views we
must take his chief work, the _Philosophie anatomique_, particularly
the first volume (1818). This volume contains, beside the important
"Discours préliminaire" and "Introduction" which we shall presently
consider in detail, five memoirs, which deal with the various bones
connected with the respiratory organs in fishes (the bones of the
operculum, of the hyoid, of the branchial arches, of the pectoral
girdle), and seek to discover their homologies with corresponding
bones in air-breathing Vertebrates.

"Can the organisation of vertebrated animals be referred to one
uniform type?" This is the question with which the _Philosophie
anatomique_ opens, the question to which the whole book is an answer.
But is it not generally acknowledged by naturalists that Vertebrates
are built upon one uniform plan, that, for instance, the fore limb may
be modified for running, climbing, swimming, or flying, yet the
arrangement of the bones remain the same? How else could there be a
"natural method" of classification?[87]

But the homologies so drawn repose upon a vague and confused feeling for
likenesses; they are not based upon an explicit principle. What general
principle can be applied? "Now it is evident that the sole general
principle one can apply is given by the position, the relations, and the
dependencies of the parts, that is to say, by what I name and include
under the term of _connections_." For instance, the part known as the
hand in man and generally as the fore foot in other Vertebrates, is the
fourth part in order in the anterior member, and its homologue can
always be recognised by this fact of its connections (p. xxvi.). The
principle of connections serves as a guide in tracing an organ through
all its functional transformations, for "an organ can be deteriorated,
atrophied, annihilated, but not transposed" (p. xxx.).

It is this principle which enables one to follow out in detail the
further fundamental conception that in every Vertebrate there are found
the same "organic materials," or units of construction. This conception,
which Geoffroy calls the _Théorie des analogues_ (p. xxxii.), is clearly
one part of the old idea of the unity of type; it teaches the _unity of
composition_ of organic beings, while the _Principe des connexions_ adds
the _unity of plan_.

Both conceptions are logically implicit in the vague notion of unity of
type; Geoffroy disengaged them, and pushed each to its logical extreme.

Most of the ordinary homologies of structure in air-breathing
Vertebrates have already been seized, he continues, for they are more or
less obvious, and many intermediate states exist (p. xxxiv.). But
ordinary methods of comparison fail when the attempt is made to
homologise the structure of fishes with that of air-breathing
Vertebrates, for the homologies are anything but obvious and no
intermediate organs are found.

Most air-breathing Vertebrates have a larynx, a trachea, and bronchi,
which are absent in fish; and fish have many parts which seem to be
absent in higher Vertebrates. But apply the "Theory of Analogues"; it
teaches that there can be no organ peculiar to fish and not found in
other Vertebrates; apply the "Principle of Connections," it will show
which organs are homologous in the two types (p. xxxv.).

Comparative anatomists, with few exceptions, had hitherto taken man as
the type, and referred all structure to his; Geoffroy's principles led
him to give preference to no one animal in particular, but to seize upon
each part in the species in which it reaches the maximum of its
development (p. xxxvi.). He is thus led to refer all structures to a
generalised abstract type. In this abstract type each organ exists at
the maximum of its development, each organ shows all its potentialities
realised. In a way, therefore, this type, this abstraction, gives the
scheme of the possible transformations of each organ.

It is true Geoffroy does not refer to this "Archetype" in so many words,
but it must always have been vaguely present in his mind. He has this
idea in his head when he says in one of his later works, "There is,
philosophically speaking, only a single animal."[88] The "single animal"
is simply the generalised type.

Having laid down his two principles Geoffroy goes on to apply them to
the difficult case of the comparison of the skeleton of fish with the
skeleton of the higher Vertebrates. "My present task is to demonstrate
that there is no part of the bony framework of fishes that cannot find
its analogue in the other vertebrated animals."[89] It seems at first
sight that many bones are peculiar to fish, formed expressly for
performing the functions which fish do not share with higher animals.
These are the bones connected with respiration--the operculum, the
branchiostegal rays, the branchial arches, and others. That the peculiar
bones should be connected with the respiratory functions is only
natural, for the contrast between fish and higher Vertebrates is
essentially a contrast between water-breathing and air-breathing
animals. Considering first the general form of the skeleton in fish, we
are met at once with a difficulty; there is no obvious homologue in
fishes of the neck, the trunk, and the abdomen of higher animals. What
apparently corresponds to the trunk is in fishes crowded close up under
the head. But, after all, it is not of the essence of the vertebrate
type to have the trunk and the abdomen attached at definite and
invariable distances along the vertebral column--that is a notion
surviving from the anatomy which made man its type. The "trunk" differs
in position according to the class, in quadrupeds, birds, and fishes (p.
9). Now, says Geoffroy, allow me this one hypothesis, that the trunk
with its organs can, as it were, move bodily along the vertebral column,
so as to be found in one class near the front end of the vertebral
column, in another about the middle, and in a third near the end, then I
can show you in detail that the constituent parts of this trunk are
found in all classes to be invariably in the same positions relatively
to one another (p. 10). It is important to note this hypothesis of a
"metastasis" which Geoffroy makes, for it is the key to the
understanding of many of the far-fetched homologies which he tries to
establish. It is, of course, clear that this hypothesis is in formal
contradiction with his principal hypothesis of the invariability of
connections, and that he, so to speak, gets a hold on his fish to apply
his principle of connections only by admitting at the very outset an
exception to his primary principle. A further application of the
hypothesis of metastasis will be noticed below in connection with the
determination of the sternum of fishes. We note here an interpretation
of the first metastasis in terms of functional adaptation. "The constant
and violent action of the tail, if it does not go so far as actually to
displace and move forward the internal organs, at least fits in well
with an arrangement in which the organs are so disposed" (p. 99).

The first memoir deals with the homologies of the opercular bones.
Geoffroy considers that the external opening of the ear corresponds to
the external opening of the gill-chamber, which lies between the
operculum and the pectoral girdle. The ear communicates with the buccal
cavity by the Eustachian tube, so does the branchial chamber by means of
the gill-slits. The auditory chamber of higher Vertebrates is,
therefore, the homologue of the branchial chamber in fish; the opercular
bones in fish and the ossicles of the ear in other Vertebrates stand in
close relation to this chamber; therefore the opercular bones are the
homologues of the ossicles of the ear, the interoperculum corresponding
to the malleus, the suboperculum to the lenticular, the minute lower
part of the suboperculum to the incus, the operculum to the stapes, and
the pre-operculum to the tympanic ring. In making these particular
determinations Geoffroy professes to be led by his principle of
connections. The pre-operculum has, he says, the same connections with
neighbouring bones as the tympanic bone in other Vertebrates, and the
other pieces of the gill-cover are homologised with particular
ear-ossicles according to the order in which they stand to one another.
The second memoir in the book deals with the sternum, and affords a very
good example of Geoffroy's method of dealing with the facts of
structure. We shall omit here any detailed reference to the other three
memoirs, which deal with the hyoid, with the branchial arches and the
structures which correspond in air-breathing Vertebrates, and with the
bones of the shoulder-girdle.

In the memoir on the sternum Geoffroy's first care is to arrive at a
definition of what a sternum is. He defines it partly by its functions,
partly by its connections, as the system of bones which covers and
protects the thorax, and gives attachment to certain groups of muscles.

The most highly developed sternum (according to this definition) is the
plastron of the tortoise, whose structure it dominates (p. 103). It is
important, therefore, to determine of how many bones the plastron is
composed, since the full number of elementary parts of which an organ is
composed is best seen when the organ is at the maximum of its
development. There are nine bones in the plastron of the tortoise. "The
conclusion to be drawn from this is that every sternum, provided that it
is not inhibited in its development by some obstacle, is composed of
_nine elementary parts_" (p. 105). These nine bones are in Geoffroy's
nomenclature, the episternals, the hyosternals, the hyposternals, the
xiphisternals, which are all paired bones, and the entosternal, which is
unpaired. The arrangement of them is in the tortoise:--

Episternal---------------------------Episternal
    |\__                               __/|
    |   \__                         __/   |
    |      \__                   __/      |
    |         \__ Entosternal __/         |
    |       __/                 \__       |
    |    __/                       \__    |
    | __/                             \__ |
    |/                                   \|
Hyosternal                           Hyosternal
    |                                     |
    |                                     |
    |                                     |
    |                                     |
Hyposternal-------------------------Hyposternal
    |                                     |
    |                                     |
    |                                     |
    |                                     |
Xiphisternal------------------------Xiphisternal.

The articulations in the tortoise are indicated by the connecting
lines. Geoffroy tries to show that the sternum in other animals is
composed of these nine bones, or at least of a certain number of them,
always in the same invariable relative positions. Thus in birds the
sternum consists of five pieces, of a huge keeled entosternal, and of
two "annexes" on either side, which are the hyo-and hyposternals.
These are separate only in young birds. Occasionally, especially in
young birds, rudiments of episternals and xiphisternals also occur.
The minuteness of the episternals and the xiphisternals may be
attributed to the gigantic size of the entosternal, in accordance with
the _Loi de balancement_. In the other air-breathing Vertebrates the
nine sternal elements can according to Geoffroy be discovered without
great difficulty. But when we come to the determination of the sternum
in fishes, difficulties abound, which Geoffroy solves in the following
way. He points out that between the clavicles (_cleithra_) and the
hyoid bone (_basihyal_) in fishes there is a long median bone
(_urohyal_) which is attached in front by two strong tendons to the
horns of the hyoid and is free behind (see Fig. 1). Gouan (1720) had
seen in this bone the homologue of the sternum. Geoffroy adopts this
view, but considers that this bone alone cannot represent the whole
sternum. He finds the representatives of other bones of the sternum in
the large bones (_epihyal_ and _ceratohyal_, or the two pieces of the
_ceratohyal_) which are comprised in the hyoid arch. But he is
immediately met by the difficulty that this complex of bones is
situated in front of the pectoral girdle, whereas the sternum in
higher Vertebrates lies behind the pectoral girdle. He reflects,
however, that the gills of fish, situated in front of the clavicles,
are merely the lungs under another name. The gills have become shifted
forward by a metastasis similar to that which brought the whole
thoracic organs far forward in fish. This being so, their supporting
elements, the sternum and the ribs, must have moved with them, and are
hence to be found in front of the pectoral girdle.

[Illustration: FIG. 1.--Hyoid Arch of the Conger. (Original.)]

Geoffroy's next step is to point out that the only possible homologues
of sternal ribs are the branchiostegal rays, which arise from the large
bones of the hyoid arch. If these are sternal ribs, the bones to which
they are attached must be the hyo- and hyposternals or "annexes," the
bones from which in birds the ribs take their origin.

The unpaired sternal bone (_urohyal_) cannot be homologous with the
entosternal, for it has no connections with the annexes. He decides that
it must represent the episternals, for in some young birds there is a
two-headed episternal to which two strong tendons are attached, just in
the same way as the unpaired piece in fish is bound to the bones of the
hyoid by two tendons. "Thus it is not the sternum as a whole that has
shifted in front of the clavicles and covered with its side pieces the
gills placed there; it is a piece exclusively piscine, in the sense that
it is only in the class of fishes that it reaches the _maximum_ of its
development" (p. 83).

To sum up, the sternum in all four vertebrate classes is composed of the
same elements, arranged always in the same way. "One is ... led to the
conception of an ideal type of sternum for all Vertebrates, which then,
considered from a lower standpoint, resolves itself into several
secondary forms according as the whole or the majority of the
constituent materials are employed, or even as these elements come to
change their respective dimensions or proportions" (p. 134). As to the
elementary constituents, "they give proof of individuality, and
sometimes even, in certain abnormalities, of independence, and rise to
the level of primary organisatory materials" (p. 132). What holds good
for the sternum holds good for other organs--and accordingly the unity
of plan and composition can be demonstrated for all the organs of
Vertebrates.

Soon after the publication of the _Philosophie anatomique_ (1818)
Geoffroy went further in his search for unity, and maintained that the
structure of insects and Crustacea could be reduced to the vertebrate
type.

He proposed to replace Cuvier's classification of the animal kingdom
into the four large groups, Vertebrata, Mollusca, Articulata, and
Radiata by the following classification:--[90]

                Hauts-Vertébrés (Vertebrata, Cuv.).
     Vertébrés /
               \
                Dermo-Vertébrés (Articulata, Cuv.).


                Mollusques (Mollusca, Cuv.).
   Invertébrés /
               \
                Rayonnés (Radiata, Cuv.).

The idea upon which is based the comparison of Articulates with
Vertebrates is that each skeletal segment of Articulates is a vertebra.
In the Hauts-vertébrés the vertebræ are internal; in the
Dermo-vertébrés they are external. "_Every animal lives either outside
or inside its vertebral column_."[91] The essence of a vertebra is not
its form, nor its function, but its composition from four elementary
pieces which unite round a central space (_Isis, loc. cit._, p. 532).
Serres had shown that in the higher animals every vertebra is formed
from four centres of ossification, that the body of the vertebra is at
first tubular, and that afterwards it becomes filled up. In lobsters and
crabs each segment is composed of four elementary pieces, as may be seen
most easily in young ones. "Accordingly each segment corresponds to a
true vertebra in composition: there is the same number of 'materials,'
the same order in the course of ossification, the same kind of
articulation, the same annular arrangement, the same empty space in the
middle" (p. 534). The only difference is that in Articulates the central
space is very great and contains all the organs of the body, whereas in
the higher Vertebrates the body of the vertebra becomes completely
filled up. In the thoracic region of Crustacea it is not the whole
segment with part of the carapace which corresponds to a vertebra, but
merely the part round the ventral nerve-cord (endophragmal skeleton).

If the skeleton of the segment in Articulates corresponds to the body of
a vertebra and is here external, then the appendages of the Articulate
must correspond to ribs (p. 538). The full development of this thought
is found in a Memoir of 1822, "Sur la vertèbre."[92] He takes as the
typical vertebra that of a Pleuronectid, probably the turbot. His
original figure is reproduced (Fig. 2).

[Illustration: FIG. 2.--"Vertebra" of a Pleuronectid. (After Geoffroy.)]

He includes as part of the vertebra not only the neural (e', e'') and
hæmal (o', o'') arches, but also, above and below these, the radialia
(a'', u') and the fin-rays (a', u''). (Neither the radialia nor the
fin-rays are, by the way, in the same transverse plane as the body of
the vertebra). Every vertebra, he considers, contains these nine
pieces--the cycleal (or body), the two perials (e', e'') and the two
epials (a', a'') above, the two paraals (o', o'') and the two cataals (u',
u'') below. The epials and the cataals are in reality paired bones which
in fish mount one on top of the other to support the median fins. In the
cranial region--the skull is formed of modified vertebræ--the epials
and perials open out so as to form the walls and roof of the brain; in
the thoracic region the paraals and cataals reach their maximum of
development and perform the same service for the thoracic organs, the
paraals becoming vertebral, and the cataals sternal, ribs.

We have seen that in Arthropods the body of the vertebra (cycleal) forms
the open ring of the segment, which lies immediately under the skin, the
vertebral tube coinciding with the epidermal tube. The homologues of the
other eight pieces of the vertebra must accordingly be sought in the
external appendages. At first sight there seems here a contradiction of
the principle of connections, for the appendages in Arthropods are
lateral, whereas the paired bones of the vertebra are dorsal and
ventral. But there is in reality no contradiction, for "what our law of
connections absolutely requires is that all organs, whether internal or
external, should stand to one another in the same relations; but it is
all one whether the box (_coffre_) that encloses them lies with this or
that side on the ground. What similarities in the organisation of man
and the digitate mammals, and yet what differences between their
attitudes when standing! The same holds true as regards the normal
attitudes of the pleuronectids and the other fishes" (p. 107).

The exact way in which Geoffroy homologised the parts of the appendages
in Arthropods with the paired pieces of the typical vertebra is best
shown by the reproduction of his figure of an abdominal segment of the
lobster (Fig. 3), in which the parts homologous with those represented
in the figure of the typical vertebra (Fig. 2) are indicated by the same
letters. The ingenuity of the comparison is astonishing.

[Illustration: FIG. 3.--Abdominal Segment of the Lobster. (After
Geoffroy.)]

The comparison of the Arthropod with the Vertebrate is extended also to
the internal organs. The internal organs of the Arthropod are shown to
stand in the same order to one another as in the Vertebrate, only the
organs are inverted. Thus the nervous system is dorsal in the
Vertebrate, ventral in the Arthropod. Turn the Arthropod on its back and
the relative positions of the systems of organs are the same as in the
Vertebrate. The relation of the organs to the external tube is of course
different in Arthropods and Vertebrates, but this is no contradiction of
the principle of connections. "Such a tube, although it is the organs
essential to life that it contains, can yet behave in different ways
with regard to the mass of these organs: the principle of connections
demands only that all the organs maintain with one another fixed and
definite relations; but the principle would be in no way invalidated if
the whole mass had rotated inside the tube" (p. 112).

Geoffroy pushed the analogy between Arthropods and Vertebrates very far,
for he asserted that every piece in the skeleton of an insect was
homologous with some bone in Vertebrates, that it stood always in its
proper place, and remained faithful to at least one of its
connections.[93] It does not appear that he attempted to prove in detail
this very big assumption, but the beginnings of a detailed comparison
are found in the paper of 1820, _Sur l'organisation des insectes_. Six
segments are distinguished in an insect--the head, the three divisions
of the thorax, the abdomen, and the terminal segment of the abdomen (p.
455).

The skeleton of the insect's head is said to correspond to the bones of
the face, to the bones of the cerebrum and to the hyoid of higher
Vertebrates, the skeleton of the prothorax to the bones of the
cerebellum, of the palate, and the pieces of the larynx, the skeleton of
the mesothorax to the parietals, interparietals, and opercular bones,
and that of the metathorax to the skeleton of the thorax of Vertebrates.
The pieces of the abdomen and of the terminal segment correspond to the
bones of the abdomen and coccyx (p. 458). It does not need the
subsequent likening of the hind wings of insects to the air bladder of
fish, and of the stigmata to the pores of the lateral line, to convince
one finally of the fancifulness of the whole comparison.

In 1830 two young naturalists, Meyranx and Laurencet, presented to the
Académie des Sciences a memoir in which they likened a Cephalopod to a
Vertebrate bent back at the level of the umbilicus, saying that the
Vertebrate in this position had all its organs in the same order as in
the Cephalopod. Geoffroy took up this idea with enthusiasm, seeing in it
a further application of his master-idea of the unity of plan and
composition. By means of this comparison Mollusca definitely took their
place in the _Échelle des êtres_, after the Articulata, just as Geoffroy
had maintained in 1820, saying that crabs formed a link between the
other Crustacea and the molluscs.[94] The comparison brought him nearer
to the end he had in view, the reference of all animal structure to one
single type.

But in championing the memoir of Meyranx and Laurencet, Geoffroy found
himself in direct antagonism with Cuvier, who held that his four
"Embranchements" had each a separate and distinct plan of structure. In
a paper read to the Academy in February 1830,[95] Cuvier easily
demolished the crude comparison of the Cephalopod to the Vertebrate. He
gave diagrams of the internal organs of a Cephalopod and of a Vertebrate
bent back in the manner indicated by Meyranx and Laurencet, and he
showed in detail that the arrangement of the main organs was quite
different, that the likeness would have been much greater if the
Cephalopod had been likened to a Vertebrate doubled up the other way,[96]
but that even then the arrangement of the organs would not be the same.
The organs, too, of the Cephalopod are differently constructed. He sums
up his criticism by saying:--"I give true and summary expression to all
these facts when I say that Cephalopods have several organs in common
with Vertebrates, which fulfil in either case similar functions, but
that these organs are differently arranged with respect to one another,
and often constructed in a different way; that they are in Cephalopods
accompanied by several other organs which Vertebrates do not possess,
whilst the latter on their side have many organs which Cephalopods lack"
(p. 257). Geoffroy could not accept this commonsense view of the matter,
but made a fight for his transcendental theories. This was the beginning
of the famous controversy between Geoffroy and Cuvier which so excited
the interest of Goethe. It was a struggle between "comparative anatomy"
and "morphology," between the commonsense teleological view of structure
and the abstract, transcendental. Geoffroy brought forward all his
theories on the homology of the skeleton of fish with the skeleton of
higher Vertebrates, and tried to prove by them his great principle of
the unity of plan and composition; Cuvier took Geoffroy's homologies one
by one, and showed how very slight was their foundation. Cuvier was on
sure ground in insisting upon the observable diversities of structural
type, and his vast knowledge enabled him to score a decisive victory.[97]

The controversy was not, as we are sometimes told, a controversy between
a believer in evolution and an upholder of the fixity of species,
although it raised a question upon which evolution theory was to throw
some light.

In these Darwinian days Geoffroy has reaped a little posthumous glory as
an early believer in evolution. That he did believe in evolution to a
limited extent is certain; that his theory of evolution was, as it were,
a by-product of his life-work, is also certain. Geoffroy was primarily a
morphologist and a seeker after the unity hidden under the diversity of
organic form. His theory of evolution had as good as no influence upon
his morphology, for he did not to any extent interpret unity of plan as
being due to community of descent. His morphological, non-evolutionary
standpoint comes out quite clearly in several places in the _Philosophie
anatomique_. He does not derive the structure of the higher Vertebrates
from the simpler structure of the lower, but when he finds in fish a
part at the maximum of its development, he speaks of the same part,
rudimentary in the higher forms, as being, as it were, held in reserve
for use in the fish. Thus, speaking of the episternal in fish which
forms the central piece of its sternum, he says, "it is a bone that is
rudimentary in birds (one might almost add a bone that is held in
reserve in birds for this fate) which is destined to form in the centre
the principal keel of this new machine" (p. 84). Again, with reference
to the homology of the ossicles of the ear with the opercular bones in
fish, "employing other resources equally hidden and rudimentary, Nature
makes profitable use of the four tiny ossicles lodged in the auditory
passage, and, raising them in fish to the greatest possible dimensions,
forms from them these broad opercula...." (p. 85). Or you may take it
the other way about, and start from the organisation of fishes;
opercular bones are of no use to air-breathing animals, so they dwindle
away, and are pressed into the service of the ear, although they are of
little use in hearing (p. 46).

There is here no thought of evolution; in later years, however, his
researches upon fossil crocodilians led him to consider the possibility
that the living species were descended from the antediluvian. For the
factors of the transformation he refers to Lamarck's hypotheses.[98] In a
memoir of 1828,[99] dealing with the possible genetic relation of living
to fossil species, he still regards the question as more or less open.
Although fossil species are mostly different from living species are we
therefore to conclude, he asks, that they are not the ancestors of the
present day forms? "The contrary idea arises more naturally in the mind;
for otherwise the six-days' creation would have had to be repeated and
new beings produced by a fresh creation. Now this proposition, contrary
as it is to the most ancient historical traditions, is inadmissible" (p.
210). It is sufficiently clear from this quotation that Geoffroy was
thinking only of a transformation of the antediluvian species created by
God, and by no means of an evolution of all species from one primitive
type. In matters of religion Geoffroy was orthodox. He goes on to point
out how great a resemblance there is in essential structure between
fossil and living species. All find their place in one scheme of
classification; does it not seem that all are modifications "of one
single being, of that abstract being or common type, which it is always
possible to denote by the same name?" (p. 211). This type is abstract,
not actual, and it is certainly not conceived as an original ancestor of
all animals.

The fullest development of Geoffroy's views on evolution is found in his
memoir "Le degré d'influence du monde ambiant pour modifier les formes
animales."[100] Here the relation of his evolution-theory to his
morphology is pointed out. The principle of unity of plan and
composition cannot be the final goal of zoology; there must follow on it
a philosophical study of the _differences_ between organic forms. The
causes of these differences are to be found in the environment (pp.
66-7). Geoffroy seems here to be moving from a pure to a causal
morphology. It is probable, he continues, that living species have
descended by uninterrupted generation from the antediluvian species (p.
74), and that they have in the process become modified through external
influences.

Now of all functions respiration is the most important, and upon
respiration everything is regulated. "If it be admitted that the slow
progression of the centuries has brought in its train successive changes
in the proportion of the different elements of the atmosphere, it
follows as a rigorously necessary consequence that the organisation has
been proportionately influenced by them" (p. 76). The respiratory milieu
changes, the species change with it, or are eliminated (p. 79). We may
see, perhaps, in the stress which Geoffroy lays upon respiration and the
respiratory milieu a result of his constant obsession with the
comparison of fish with air-breathing Vertebrates.

In the first geological period, we read in another Memoir of the same
year,[101] when ammonites and _Gryphæa_ flourished, hot-blooded animals
with lungs could not exist. "A lung constructed like that of mammals and
birds would not have been adapted to the essence of the respiratory
element such as in my conception of it the system of the environing air
used to be"[102] (p. 58).

Geoffroy does not tell us exactly how the milieu is to act upon the
organism; the whole theory is little more than a sketch and a pointing
out of the way for future research--and in this prophetic enough. The
action of external agents was apparently considered as physical, and no
power of active adaptation was ascribed to the organism.

From a passage in the memoir "Sur la Vertèbre" we may perhaps infer that
he believed increasing complexity of structure to be due to a
realisation of potentialities, to the development of parts present in
the lower animals only in potency--"the organisation ... only awaits
favourable conditions to rise, by addition of parts, from the simplicity
of the first formations to the complication of the creatures at the head
of the scale" (p. 112). Evolution takes place as the environment allows,
and in a sense in opposition to the environment.

He believed in saltatory evolution, for he considered that the lower
oviparous Vertebrates could not be transformed into birds by slow
modification, but only by a sudden transformation of their lungs, which
would bring about the other characteristics of birds (p. 80). He
considered, too, that transformations could arise by means of monstrous
development (p. 86). In this connection the experiments which he made on
the hen's egg[103] in order to produce artificial monstrosities are
significant, though his purpose was rather to obtain proof of the
inadequacy of the preformation hypothesis.[104]

It seems probable enough that if Geoffroy had developed his views on
evolution he would finally have been led to interpret unity of plan in
terms of genetic relationship. But as it was he remained at his
morphological standpoint. He did not interpret rudimentary organs as
useless heritages of the past; he preferred to think that Nature had
prepared double means for the same function, one or other being
predominant according as the animal lived in the water or on the land.
"To the animal that lives exclusively in the air Nature has granted an
organisation suited to this mode of respiration, without however
suppressing the other corresponding means, that is to say, without
depriving it of a second system which is applicable only to the mode of
respiration by the intermediary of water, and _vice versa_."[105]

He seems, in one instance at least, to have hit upon the root-idea of
the biogenetic law, but he was far from appreciating its significance.
He recognised that an amphibian in its development passed through a
stage when it was in all essentials similar to a fish, and he saw in
this visible transformation a picture of the evolutionary
transformation. "An amphibian," he writes,[106] "is at first a fish under
the name of tadpole, and then a reptile [_sic_] under that of frog....
In this observed fact is realised what we have above represented as an
hypothesis, the transformation of one organic stage into the stage
immediately superior." But it is not clear that he considered the
development of the amphibian to be a _repetition_ of its ancestral
history.

He went, however, a certain length towards recognising the main
principle of a law which was a commonplace of German transcendental
thought, and was developed later by his disciple E. Serres, the law that
the higher animals repeat during their development the main features of
the adult organisation of animals lower in the scale. Thus he compared
fish as regards certain parts of their structure with the foetus of
mammals. He compared also Articulates with embryonic Vertebrates in
respect of their vertebræ, for in the higher Vertebrates the body of the
vertebra is tubular at an early stage of development, and in Articulates
the body of the vertebra remains tubular permanently (_supra_, p. 61).
As regards their vertebræ, "insects occupy a place in the series of the
ages and developments of the vertebrate animals, that is to say, they
realise one of the states of their embryo, as fishes do one of the
states of their foetal condition."[107]

This idea was destined to exercise a great influence upon the
development of morphology. A further development of the thought is that
certain abnormalities in the higher animals, resulting from arrest of
development, represent states of organisation which are permanent in the
lower animals.[108]

So far we have considered Geoffroy's theories in their application to
the facts. We go on to discuss the theories themselves, and the general
conception of living things which underlies them.

The principle of unity of plan and composition is the keynote of
Geoffroy's work. It states that the same materials of organisation are
to be found in all animals, and that these materials stand always in the
same general spatial relations to one another. The "materials of
organisation" are not necessarily organs in the physiological sense, and
indeed the principle of the unity of plan cannot be upheld if the unity
has reference to organs only. This became clear to Geoffroy, especially
in his later years. In 1835 he wrote, speaking of the principle of the
unity of plan, "I have, moreover, regenerated this principle, and
obtained for it universality of application, by showing that it is not
always the organs as a whole, but merely the materials composing each
organ, that can be reduced to unity."[109] Even in the _Philosophie
anatomique_ he deals rather with parts than with organs; he deals, for
instance, with the elementary parts of the sternum, not with the organ
"sternum" in its totality. The functions of the sternum vary, and the
primary protective function of the sternum may be assumed by quite other
parts, _e.g._, by the clavicles in fish, which protect the heart.[110]

True homologies can be established between materials of organisation but
not always between organs, which may be composed of different
"materials."

Almost as a corollary to this comes the further view that form is of
little importance in determining homologies. An organ is essentially an
instrument for doing a particular kind of work, and its form is
determined by its function. Organs which perform the same function are
usually similar in form though the elementary materials composing them
may be different. This is seen in many cases of convergence. Organs,
therefore, which perform the same function and are similar in external
form are not necessary homologous. Conversely, the same complex of
materials, say a fore limb, may take on the most varied shapes according
as the function of the organ changes--but homology remains though form
changes. Accordingly, form is one of the least important elements to be
considered in determining a homology. "Nature," he wrote in one of his
early papers, "tends to repeat the same organs in the same number and in
the same relations, and varies to infinity only their form. In
accordance with this principle I shall have to draw my conclusions, in
the determining the bones of the fish's skull, not from a consideration
of their form, but from a consideration of their connections."[111]

Again, after comparing a vertebra of the Aurochs with an abdominal
segment of the crab, he says, "I have insisted upon an identity which
has extended to the least important relation of all, that of form."[112]

Geoffroy's morphological units or materials of organisation were in the
case of the skeleton--with which his researches principally deal--the
single bones. But the interesting point is that he sought his
skeleton-units in the embryo, and considered each separate centre of
ossification as a separate bone. Coalescence of bones originally
separate is one of the most usual events in development, and it is an
occurrence which, more than any other, tends to obscure homologies.
Because of its coalescence with the maxillaries, the intermaxillary in
man was not discovered until Vicq d'Azyr and Goethe found it separate in
the embryo. Apparently quite independently of Goethe, Geoffroy hit upon
this plan of seeking in the embryo the primary elements or materials of
organisation. In an early paper on the skull of Vertebrates,[113] where he
is concerned with showing that each bone of the fish's skull has its
homologue in the skull of higher Vertebrates, he is faced with the
difficulty that the skull of the fish has more bones than the skull of
higher Vertebrates. "Having had the inspiration," he writes, "to reckon
as many bones as there are distinct centres of ossification, and having
made a consistent trial of this method, I have been able to appreciate
the correctness of the idea: fish, in their earliest stages, are in the
same conditions relatively to their development as the foetuses of
mammals, and hence bear out the theory" (p. 344). So, too, in dealing
with the homologies of the sternal elements (_supra_, p. 57) he treats
as separate bones the "annexes" of the sternum in birds, though these
are separate only in the young.

If the same materials of organisation are present in all animals, and if
they are arranged always in the same positions relatively to one
another, how does it come about that animal forms are so varied, what
explanation can be offered of the diversities of organic structure?
Geoffroy's main answer to this question is his _Loi de balancement_. The
law was enunciated by him already in 1807.[114] We take the following
quotation, which represents his thought most nearly, from the _Cours de
l'histoire naturelle des Mammifères_ (1829). "According to our manner of
regarding the organisation of mammals, there is only a single animal
modified by the inverse reciprocal variation of all or some of its
parts. Now, from the fact that there is only one single general animal,
it follows that for each section of its components or for each of its
organs there is available only a given quantity of formative materials.
Now suppose that the distribution of these materials has not been made
in such a way as to ensure an exact equilibrium between all the parts
concerned, one organ will get more than its share, another less. My law
of the compensation of organs is founded on these principles" (i.,
_Leçon_ 16, p. 12). "The atrophy of one organ turns to the profit of
another; and the reason why this cannot be otherwise is simple, it is
because there is not an unlimited supply of the substance required for
each special purpose."[115] The nutritive material available is limited
for each species; if one part gets more than its share the other parts
must get less--that is all the law means. As an example, take the
minuteness of the episternals and xiphisternals in birds, as contrasted
with the huge size of the entosternal. "The minuteness of the
episternals and xiphisternals might be imputed to this gigantic piece
diverting to its own profit the nutritive fluid, since the bigger it is
the smaller these are."[116]

One has constantly to remember in dealing with Geoffroy's theories that
he was not an evolutionist, but purely a morphologist. It is therefore,
perhaps, to ask too much to require of him an explanation of the causes
of diversity. The morphologist describes, classifies, generalises; he
does not seek for causes. But we must leave this question aside in order
to discuss how far Geoffroy's theory of the unity of plan and
composition fits the facts. As Geoffroy himself admitted on several
occasions, his theory was an _à priori_ one, a theory hit upon by hasty
induction, then erected into a principle and imposed upon the facts. No
more than Goethe did he extract his principle from a sufficient mass of
data.

Now he found his theory to be in its pure form unworkable; he found, for
example, that the skeleton of fishes could not be compared directly,
bone for bone, with the skeleton of higher Vertebrates; he had to admit
differences of position of whole sets of organs in the two groups, he
had to admit various _metastases_, before he could bring the skeleton of
fish into line. And these metastases are due to functional
requirements--for example, the forward position of sternum and thoracic
organs in fish is an adaptation to swimming.

So he does not so much demonstrate the unity of plan of whole organisms
as the unity of plan of particular corresponding parts of them. Thus he
does not prove or attempt to prove that Articulates are in all points
like Vertebrates, but simply that their skeleton is built upon the same
plan as that of Vertebrates. The rest of the organs, while still
comparable with the organs of Vertebrates, stand in different relations
to the skeleton. An Articulate therefore, on his own showing, is not,
_as a whole_, built upon the same general structural plan as a
Vertebrate.

Further, he does not always remain true to his principles, for he does
not establish homologies of parts entirely by their connections but
sometimes by their functions as well. Thus the sternum, or rather the
complex of sternal elements, is defined and discovered in particular
cases not by its connections only but also by its functions. The
framework of the gills is homologised part by part with the framework of
the lungs, not because the relations of the framework to the rest of the
skeleton are the same in fish and air-breathing Vertebrates, but simply
because gills are considered the equivalents of lungs--a comparison
which is purely physiological.

Even with these concessions to the functional view of living things,
Geoffroy was unable to make good his contention that all animals are
built upon the same plan. His arguments failed to carry conviction to
his contemporaries, and Cuvier in particular subjected them to
destructive, and indeed final, criticism.

The paper, already referred to, in which Cuvier disposed of the
transcendentalists' comparison of Cephalopods and Vertebrates is of
great significance, for it states in the clearest way the radical
opposition between the functional and the formal attitudes to living
things.

Cuvier points out that if by unity of composition is meant identity,
then the statement that all animals show the same composition is simply
not true--compare a polyp with a man!--on the other hand, if by unity is
meant simply resemblance or homology, the statement is true within
certain limits, but it has been employed as a principle since the days
of Aristotle, and the theory of unity of composition is original only in
so far as it is false. He admits, however, that Geoffroy has seized upon
many hidden homologies, especially by his valuable discovery of the
importance of foetal structure. In all this Cuvier is undoubtedly right.
Unity of plan and composition, as Geoffroy conceived it, simply does not
exist. Cuvier goes on to say that this principle of Geoffroy's, in the
greatly modified form in which it can be accepted, and has been accepted
from the dawn of zoology, is not the sole and unique principle of the
science. On the contrary, it is merely a subordinate principle,
subordinate to a higher and more fruitful principle, that, namely, of
the conditions of existence, of the adaptation (_convenance_) of the
parts, of the co-ordination of the parts for the rôle which the animal
is to play in Nature. "That is the true philosophical principle," he
says, "whence may be deduced the possibility of certain resemblances,
the impossibility of certain others; it is the rational principle from
which follows the principle of the unity of plan and composition, and in
which at the same time it finds those limits, which some would like to
disregard" (p. 248).

Geoffroy's position is the direct contrary. He holds that the principle
of the unity of plan and composition is the true base of natural
history,[117] and that this unity limits the possible transformations of
the organism. Thus, speaking of the influence of the respiratory medium,
he says, "All the same this influence of the external world, if it has
ever become a cause which disturbed organisation, must necessarily have
been confined within fairly narrow limits; animals must have opposed to
it certain conditions inherent to their nature, the existence of the
same materials composing them, and a manifest tendency to resemble one
another, and to reproduce invariably the same primordial type."[118] Unity
of plan and composition is, on this view, prior to adaptation and limits
adaptation. Cuvier's view, on the contrary, is that the necessity of
functional and ecological adaptation accounts for the repetition of the
same types of structure. There are, of all the possible combinations of
organs, only a few viable types--those whose structure is adapted to
their life. Therefore it is reasonable that these few types should be
repeated in innumerable exemplars. One must remember, in order to
appreciate Cuvier's view, that he was not obsessed, as we are, by the
idea of evolution.

Cuvier thought in terms of organs, not in terms of "materials of
organisation." He held that the resemblances between the organs of one
class of animals and the organs of another were due to the similarity of
their functions. "Let us conclude, then, that if there are resemblances
between the organs of fish and those of other classes, it is only in the
measure that there is a resemblance between their functions."[119] There
are only a few kinds of organs, each adapted for a particular function,
and these organs are necessarily repeated from class to class.--"As the
animal kingdom has received only a limited number of organs, it is
inevitable that some at least of these organs should be common to
several classes."[120]

Geoffroy thought in terms of "materials," of parts of indefinite
function, parts which might take on any function. He insists upon the
necessity of disregarding function when tracing out the unity of
composition. He considers, in direct opposition to Cuvier's
interpretation of structural resemblance as due to similarity of
function, that unity of composition is the primary fact, and similarity
of function subsidiary. In his reply in the _Mammifères_ (1829) to
Cuvier's criticisms in the _Histoire naturelle des Poissons_ (1828), he
insists on the necessity of excluding function from consideration in any
truly philosophical treatment of comparative anatomy (Discours prél., p.
25). Cuvier held that function determined structure, or at least that
the necessity of adaptation ruled the transformations of form. Geoffroy
considered that structure determined function, that changes of
structure, however they might arise, caused changes of function.
"Animals," he writes, "have no habits but those that result from the
structure of their organs; if the latter varies, there vary in the same
manner all their springs of action, all their faculties and all their
actions."[121]

Again, "a vegetarian régime is imposed upon the Quadrumana by their
possession of a somewhat ample stomach, and intestines of moderate
length."[122] The hand of the bat has become so modified as to constrain
the bat to live in the air.[123]

The best example of Geoffroy's insistence upon the priority of structure
to function, and so of his purely morphological attitude, is perhaps his
interpretation, already alluded to, of the appendages of Articulates.
The segments of the Articulate are, he says, the equivalents of the
bodies of the vertebræ of higher forms. Now "from the circumstance that
the vertebra is external, it results that the ribs must be so too; and,
as it is impossible that organs of such a size can remain passive and
absolutely functionless, these great arms, hanging there continually at
the disposition of the animal, are pressed into the service of
progression, and become its efficient instruments."[124] The ribs become
locomotory appendages.

We may compare the similar thought that the ear ossicles are simply
opercular bones reduced and turned to other uses.

Geoffroy could not but recognise the correlation of structure to
function, for this is a fact which imposes itself upon every observer.
He recognised also correlation between functions, as when he pointed out
the connection between increased respiration and enhanced muscular
activity in birds.[125] He interpreted structure at times in terms of
function, the short, strong clavicle of the mole as an adaptation to
digging, the keeled sternum of birds as an adaptation to flying, and so
on. But we may say that his whole tendency was to disregard function, to
look upon it as subsidiary. He protests against arguing from function
and habits to structure, as an "abuse of final causes."[126] He was not so
convinced as Cuvier was of the all-importance of functional correlation;
in this view he was probably confirmed by his work on teratology. It did
not surprise him that Insects, in which lungs, heart and circulation
have disappeared(!), should yet have a skeleton built upon the same plan
as the skeleton of Vertebrates, which possess these organs; the
correlation of organ-systems is not so close as to prevent this.[127] So
too, although the other organs of the insect are all inside the body of
the vertebræ, they are yet comparable with the organs of Vertebrates.[128]
The existence of rudimentary organs also seemed to him an argument
against too strict a correlation of parts.

The contrast between the teleological attitude, with its insistence upon
the priority of function to structure, and the morphological attitude,
with its conviction of the priority of structure to function, is one of
the most fundamental in biology.

Cuvier and Geoffroy are the greatest representatives of these opposing
views. Which of them is right? Is there nothing more in the unity and
diversity of organic forms than the results of functional adaptation, or
is Geoffroy right in insisting upon an element of unity which cannot be
explained in terms of adaptation? If there be an irreducible element of
unity, is there any truth in Geoffroy's suggestion that this unity
results from a power which is exercised in the world of atoms where are
elements of inalterable character?[129]

The problem as Geoffroy and Cuvier understood it was not an evolutionary
one. But the problem exists unchanged for the evolutionist, and
evolution-theory is essentially an attempt to solve it in the one
direction or the other. Theories such as Darwin's, which assume a random
variation which is not primarily a response to environmental changes,
answer the problem in Geoffroy's sense. Theories such as Lamarck's,
which postulate an active responsive self-adaptation of the organism,
are essentially a continuation and completing of Cuvier's thought.

    [86] "Mémoire sur les rapports naturels des makis,"
    _Magasin Encyclopèdique_, vii.

    [87] Discours préliminaire, pp. xv.-xxiv.

    [88] _Études progressives d'un Naturaliste_, p. 50,
    Paris, 1835.

    [89] _Philosophie Anatomique_., i., Introduction, p. 1.

    [90] "Sur une colonne vertébrale et ses côtes dans les
    insectes apiropodes," (_Acad. Sci._, Feb. 12, 1820).
    Printed in _Isis_, pp. 527-52, 1820 (2).

    [91] "Sur l'organisation des insectes," p. 458. _Isis_,
    pp. 452-62, 1820 (2).

    [92] _Mém. Mus. d'Hist. nat._, ix., pp. 89-119, Pls.
    v-vii.

    [93] _Sur l'organisation des insectes_, p. 459.

    [94] _Isis_, p. 549.

    [95] Published in _Ann. Sci. Nat._, xix., pp. 241-59,
    1830.

    [96] _Cf._ Aristotle (_supra_, p. 10).

    [97] For an account of the controversy reference may be
    made to I. Geoffroy St Hilaire, _Vie Travaux et Doctrine
    scientifique d'Etienne Geoffroy St Hilaire_, Paris,
    1847; also Semper, _Arb. zool. zoot. Instit. Würzburg_,
    iii., 1876-7, K. E. von Baer, _Lebensgeschichte Cuviers_,
    ed. L. Stieda, 1897, and J. Kohlbrugge, in _Zoolog.
    Annalen_, v., pp. 143-95. 1913.

    [98] "Recherches sur l'organisation des Gavials," _Mém.
    Mus. d'Hist. nat._, xii., 1825.

    [99] _Mém. Mus. d'Hist. nat._, xvii., pp. 209-29.

    [100] _Mém. Acad. Sci._, xii., pp. 63-92, 1833.

    [101] _Mém. Acad. Sci._, xii., pp. 43-61, 1833.

    [102] Geoffroy's French style is at times incredibly bad,
    and more or less literal translations of his sentences
    are apt to read queerly!

    [103] _Mém. Mus. d'Hist. nat._, xiii., p. 289, 1826.

    [104] _Mém. Mus. d'Hist. nat._, xviii., p. 221, 1828. His
    teratological work is important, and is chiefly
    contained in the second volume of the _Philosophie
    anatomique_.

    [105] _Phil. anat._, i., p. 449.

    [106] _Mém. Acad. Sci._, xii., p. 82, 1833.

    [107] _Mém. Mus. d'Hist. nat._, ix., p. 101, 1822.

    [108] _Cours de l'histoire naturelle des Mammifères_, i.,
    Leçon 3, p. 13, 1829.

    [109] _Études progressives d'un Naturaliste_, p. 59, f.n.,
    Paris, 1835.

    [110] _Phil. Anat._, i., p. 444.

    [111] _Ann. Mus. d'Hist. nat._, x., p. 344, 1807.

    [112] _Isis_, p. 534, 1820 (2).

    [113] _Ann. Mus. d'Hist. nat._, x., pp. 342-65, 1807.

    [114] _loc. cit._, x., p. 343.

    [115] _Phil. anat._, i., 450, f.n. _Cf._ Aristotle
    (_supra_, p. 11).

    [116] _Loc. cit._, p. 136.

    [117] _Mammifères_, i., Discours prél., p. 18.

    [118] _Phil. anat._, i., p. 208.

    [119] Cuvier and Valenciennes, _Hist. nat. Poissons_, i.,
    p. 550, 1828.

    [120] Cuvier and Valenciennes, _loc. cit._, p. 544.

    [121] _Mammifères_, i., _Leçon_ 4, p. 17.

    [122] _Loc. cit._, _Leçon_ 5, p. 8.

    [123] _Loc. cit._, _Leçon_ 13, p. 6.

    [124] _Isis_, p. 539, 1820 (2).

    [125] _Mammifères_, i., _Leçon_ 4, p. 6.

    [126] _Mammifères_, Discours prél., p. 7.

    [127] _Isis_, p. 460, 1820 (2).

    [128] _Mém. Mus. d'Hist. nat._, ix., p. 102, 1822.

    [129] _Mém. Acad. Sci._., xii., p. 76, 1833.



CHAPTER VI

THE FOLLOWERS OF ETIENNE GEOFFROY SAINT-HILAIRE


Geoffroy's theories were not generally accepted by his contemporaries,
but his methods had considerable influence, especially in France, where
many made essays in pure morphology.

His chief follower was Serres, who is mentioned indeed in the
_Philosophie anatomique_ as a fellow-worker. Serres was primarily a
medical anatomist; his interest lay in human anatomy and embryology,
normal and pathological.

His best early work was an _Anatomie comparée du cerveau_ (1824-26),
which met with a flattering reception from Cuvier.[130] He laid great
stress upon the development of the brain and spinal cord in the
different classes, and was quick to point out analogies not only between
adult but also between embryonic structures. He paid much attention to
cases of correlation, and noted a great many; he observed, for instance,
a constant relation between the development of the spinal cord and of
the corpora quadrigemina, and between the size of the corpora
quadrigemina and the volume of the optic nerves and eyes. In this the
influence of Cuvier is unmistakable.

Serres' early theoretical views are to be found in a series of papers in
the _Annales des Sciences naturelles_,[131] under the general title
_Recherches d'Anatomie transcendante, sur les Lois de l'Organogénie
appliquées à l'anatomie pathologique_, also published separately. We
follow these papers in our exposé of Serres' doctrine, reserving for a
future chapter (Chap. XII.) the consideration of his matured views of
thirty years later.

In the first of them he points out how neither position nor function has
proved altogether sufficient to establish homologies. In the early days
anatomists were guided by form; when form failed them, they traced an
organ in its changes throughout the series of animals by considering its
function. This method was satisfactory enough as regards the organs of
the nutritive life. But in the organs of the life of relation, in the
nervous system, the functions of the parts were difficult to discover,
and their form very changeful. Hence a new principle was required, and
Serres found it in the thought which he probably owed to the German
transcendentalists (see Chap. VII.), that the permanent structure of the
lower animals could be compared with phases in the development of the
higher, and particularly of man, or, as he put it, that comparative
anatomy was often only a fixed and permanent anthropogeny, and
anthropogeny a fugitive and transitory comparative anatomy (xi., p.
106).

"In rising towards the first formations," he writes, "transcendental
anatomy recognised that one and the same organ, however complicated its
definitive form might be, repeated in its transitory states the organic
simplicities of the lower classes. Thus the primitive heart of birds was
first of all a canal, then a pocket or single cavity, then finally the
complex organ of the class. Comparative anatomy was thus seen to be
repeated and reproduced by embryogeny" (xii., p. 85).

His explanation of the fact of repetition is that, "in animals belonging
to the lower classes the _formative force_, whatever it may be, has a
less energetic impulsion than in the higher animals, and hence the
organs pass through only a part of the transformations which those of
the higher forms undergo; and it is for this reason that they show
permanently the organic dispositions which are only transitory in the
embryo of man and the higher Vertebrates. Hence these double aortas,
these double venæ cavæ which one observes more or less constantly among
reptiles" (xxi., p. 48).

The number of stages in embryogeny is proportionate to the complexity of
the adult; the younger the embryo the simpler its organs--such is the
general formula of the relation between the embryo and the adult. But
here in Serres' doctrine of parallelism a complication enters. He
observed that embryonic organs did not always develop in a piece, by
simple growth, but often were formed by the union of separately formed
parts or layers. Thus the kidney in man is formed by the fusion of a
number of "little kidneys," and the spinal cord reaches its full
development by the laying down of successive layers within it. He was
greatly impressed with this fact, which, as a convinced believer in
epigenesis, he used with great effect against the preformistic theories.
"This method of isolated formation," he wrote, "is noticed in early
stages in the thyroid, the liver, the heart, the aorta, the intestinal
canal, the womb, the prostate, the clitoris, and the penis" (xi., p.
69). So, too, in the development of the skeleton, ossification proceeds
from separate centres, foramina are formed by the fusion of separate
bones round them. In his memoir, _Lois d'Osteogénie_ (1819), Serres
established several laws of ossification based upon this principle of
separate formation.[132]

How is the fact of multiple formation to be reconciled with the
principle of repetition, according to which organs are simplest in the
early embryo and in the lower animals? But observation shows that, as a
rule, the further down the scale you go the more divided organs
become--the more numerous the bones of the skull, for example. There is
thus a parallel between multiple formation of organs in the embryos of
the higher Vertebrates and their subdivided state in the lower. Take,
for example, the kidney. In the genus _Felis_, and in birds, each kidney
has two lobes, in the elephant four, in the otter ten, in the ox twelve
to fourteen. The human kidney in its development starts with about a
dozen lobes, and the number diminishes as the kidney grows. Thus the
permanent state of the kidney in the animals mentioned is reproduced by
the stages of its development in man (xii., p. 126).

So, too, at the second or third month the uterus of the human embryo is
bicornuate, and afterwards passes through stages comparable to the adult
and permanent uterus of rodents, ruminants, and carnivores. There is
indeed a time in the development of the human embryo when it resembles
in many of its organs the adult stage of various lower animals. It is
about this time that it possesses a tail.

We note that Serres' theory of parallelism applies, strictly speaking,
only to organs, not to organisms, although he, too, readily fell into
the error of supposing that the organisation of an embryo could be
compared as a whole with the adult organisation of an animal lower in
the scale. Thus he wrote in one of his later papers[133]--"As our
researches have made clear, an animal high in the organic scale only
reaches this rank by passing through all the intermediate states which
separate it from the animals placed below it. Man only becomes man after
traversing transitional organisatory states which assimilate him first
to fish, then to reptiles, then to birds and mammals." Serres was not
altogether free from the besetting sin of the transcendentalists--hasty
generalisation.

The law of parallelism applied not only to Vertebrates but also to
Invertebrates. In a short paper[134] of 1824 Serres attempted an
explanation of the nervous system of Invertebrates. Invertebrates, he
considered, lacked the cerebrospinal axis of Vertebrates, and their
nervous system was the homologue of the sympathetic system of
Vertebrates. The relation of the invertebrate to the vertebrate nervous
system being thus fixed, can the nervous system of Invertebrates be
reduced to one plan? It does not seem possible to establish a common
plan for the adult nervous systems. But apply the principle of
parallelism, which has proved so valuable within the limits of the
vertebrate series. Taking insects as the highest class, we find that
there are three stages in the development of their nervous system; in
the first the nervous system is composed of two separate strands, in the
second the strands unite round the oesophagus, in the third they unite
also behind. Now in _Bulla aperta_, stage (1) is permanent; in _Clio_,
_Doris_, _Aplysia_, _Tritonia_, _Sepia_, _Helix_, stage (2) is
permanent, and in _Unio_ stage (3). In fact, all the varieties of the
nervous system of molluscs fall into one or other of these three
classes. "It follows, then, that as regards their nervous system, the
Mollusca are more or less advanced larvæ of insects" (p. 380). The law
of parallelism is here applied to single organ-systems, but in later
years Serres applied it to whole organisations also, saying that the
lower Invertebrates were permanent embryos of the higher.

In the paper of 1834, already referred to, Serres pushed his
speculations further and attempted to establish the unity of type of all
animals, Vertebrates and Invertebrates alike--a favourite pastime of the
transcendentalists. It is incontestable, he admits, that adult
Invertebrates are quite different in structure from adult Vertebrates,
"but if one regards them as what I take them to be, namely, _permanent
embryos_, and if one compares their organisation with the embryogeny of
Vertebrates, one sees the differences disappear, and from their
analogies arise a crowd of unsuspected resemblances" (_loc. cit._, p.
247).

The last point of Serres' doctrine which calls for remark is his
interpretation of abnormalities as being often comparable to grades of
structure permanent in the lower animals. Thus the double aorta which
may occur as an abnormality in man is the normal and permanent state in
reptiles. This idea, of course, he got from Etienne Geoffroy St Hilaire.
It is further developed in his "_Théorie des formations et des
déformations organiques appliquée à l'anatomie comparée des
monstruosités_ (1832), and in his final large memoir of 1860 (see below,
p. 205).

In 1816 appeared a fine piece of work by J. C. Savigny on the homologies
of the appendages in Articulates. The standpoint was that of pure
morphology. "I am convinced," he wrote, "that when a more complete
examination has been made of the mouth of insects, properly so called,
that is to say, having six legs and two antennæ, it will be found that
whatever form it affects it is always essentially composed of the same
elements.... The organ remains the same, only the function is modified
or changed--such is Nature's constant plan."[135] In this the influence of
Geoffroy can be traced; but the work was very free from the
exaggerations of the transcendentalists, and many of Savigny's
homologies are accepted even to-day. The first memoir dealt with the
mouth-parts of insects; the second with the anterior appendages of
Articulates generally. Savigny shows that the mouth-parts of insects can
be reduced to the type shown in Orthoptera, where there are clearly two
mandibles, two maxillæ, and a lower lip formed by the fusion of two
second maxillæ. All other insects have these same mouth-parts, disposed
in the same order, however much their form may have been modified in
response to new functions. He goes on to compare the anterior set of
appendages in a long series of Articulates, in _Julus_, _Scolopendra_,
_Cancer_, _Gammarus_, _Cyamus_, _Nymphon_, _Phalangium_, _Apus_,
_Caligus_, _Limulus_, and a few others. For Crustacea he established the
homologies now accepted, of the mandibles with the mandibles of insects,
of the first and second pairs of maxillæ with the parts so named in
insects, and so on. He is quite clear that the maxillipedes of Crustacea
are the homologues of the feet of Hexapoda. "Their disposition must lead
one to think that the six anterior feet of _Julus_, that is to say, all
the feet of the Hexapoda, are here transformed into jaws" (_loc. cit._,
p. 48). In _Scolopendra_ also there is a similar transformation of two
pairs of legs into auxiliary jaws. In _Gammarus_, where there is only
the first pair of maxillipedes, the other two pairs have become
"retransformed" into feet. We find him supporting his comparison of the
three anterior pairs of legs in _Julus_ to the three pairs of legs in
insects by an argument drawn from embryology; for only the first three
pairs of feet are present in _Julus_ at birth (Degeer), "an observation,
which, together with their position, should cause them to be considered
as the representatives of the six thoracic feet of Hexapoda" (p. 44).

His comparison of the Arachnid appendages with those of insects and
Crustacea is very curious. As his starting-point he takes _Cyamus_,
which has antennæ (two pairs) and mouth parts (four pairs) as in many
Crustacea, and then seven pairs of legs; he compares with it _Nymphon_,
which has in all seven pairs of appendages. These appendages he
homologises with the seven pairs of legs of _Cyamus_, so that the first
appendage in _Nymphon_ corresponds to the seventh appendage of _Cyamus_.
This homology is extended to all Arachnids; their first two pairs of
appendages, however they may be modified as "false" mandibles and
"false" maxillæ, really correspond to the second and third maxillipedes
in Crustacea, and to the second and third pairs of feet in insects. It
is interesting to note that he treats _Limulus_ as an Arachnid, pointing
out that there is as much difference between _Apus_ and _Limulus_ as
between _Cancer_ and _Phalangium_. He describes the "gnathobases" in
_Phalangium_ and _Limulus_. We may note that he had just an inkling of
the modern doctrine that all the appendages of Articulates consist of a
basal joint bearing an inner and an outer terminal piece, for he
observes that the "cirri" of the maxillipedes of Crustacea give the
appendage the same bifid appearance as the appendages of the abdomen and
the thoracic legs of _Mysis_ (p. 50).

V. Audouin, in his memoir, _Recherches anatomiques sur le thorax des
animaux articulés_,[135] applied the principle of the unity of plan and
composition to the exoskeleton of insects, Crustaceans, and Arachnids.
His guiding ideas were, "(1) that the skeleton of articulated animals is
formed of a definite number of pieces, which are either distinct or
intimately fused with one another; (2) that in many cases, some pieces
diminish or altogether disappear, while others reach an excessive
development; (3) that the increase of one piece seems to exert on the
neighbouring pieces a kind of influence which explains all the
differences one finds between the individuals of each order, family and
genus" (Sep. copy, p. 16). Geoffroy had already stated, without proof,
that the parts of the Arthropod's skeleton, however they might change in
shape and size, remained faithful to the principle of connections, at
least at their points of insertion.[137] Audouin gave the detailed
demonstration of this by his accurate and minute determination of the
pieces of the arthropod skeleton. He recognised that the body of
Arthropods was made up of a series of similar rings, and that even the
compact head of insects consisted of fused segments. In each segment
Audouin distinguished a fixed number of hard chitinous parts, the dorsal
tergum, the ventral sternum, the lateral "flanc" of three pieces, all to
be recognised by their positions relative to one another. Many of the
names which he proposed are still in use; it was he who introduced the
terms prothorax, mesothorax, and metathorax, for the three segments of
the insect's thorax. He used Geoffroy's _Loi de balancement_ to explain
cases of correlative development, such as the relation between the size
of the front wings and the development of the mesothorax. In another
paper Audouin compared the three pieces of the dorsal skeleton of
Trilobites to the tergum and the upper part of the "flanc."[138] In a
third paper of about the same time he tried to establish the homologies
of the segments throughout the Articulate series--with less success than
Savigny.

Later on, in conjunction with Milne-Edwards, he demonstrated the unity
of composition of the nervous system in Crustacea, showing how the
concentrated system of the crab was formed by the same series of ganglia
as in the Macrura.

The entomologist Latreille also tackled the problem of the homologies of
the segments in the different classes of Arthropods (Cuvier, _loc.
cit._, p. cclxxii.). He thought he could find fifteen segments in all
Arthropods. He made the retrograde step of likening the head of insects
to a single segment. But some of his homologies showed morphological
insight, _e.g._, his comparison of the "first jaws" of Arachnids to
antennæ, because they were placed above the upper lip. It was he who
first pointed out the resemblance of the leaf-like gills of Ephemerid
larvæ to wings, and suggested that wings were "a sort of tracheal feet."

He made also a rather hazy and speculative contribution on Okenian lines
to the problem of the relation of Arthropods to Vertebrates, likening
the carapace of Crustacea to an enormously developed hyoid, the
appendages of the tail to the ventral and anal fins of fish. The
masticatory organs of Arthropods were jaws disjointed at their
symphysis; antennæ, nostrils turned outside in.

Dugès also made a comparison of Articulates with Vertebrates.[139] He did
not accept Geoffroy's vertebral theory of the Arthropod skeleton, though
he admitted that in Arthropods the dorsal surface was turned towards the
ground, basing this assumption on the position of the nervous system,
and also, curiously enough, on the inverted position of the embryo on
the lower surface of the yolk. He considered that the mandibles and
first maxillæ of Arthropods were the homologues of the upper and lower
jaws of Vertebrates, adducing as confirmatory evidence the fact that in
snakes the rami are separate. The labium was the equivalent of the
hyoid, the labial palps and maxillipedes the equivalent of the "hyoid"
elements which form the branchial arches.

But Dugès' main contribution to morphological method was his conception
of the living organism as a colony of lesser units, which were
themselves real "organisms." "By _organism_ the author means a complex
of organs which taken together suffice to constitute, ideally or
actually, a complete animal. An 'organism' is, as it were, an elementary
or simple animal; several organisms combined form a complex animal" (p.
255). Dugès hit upon this principle, which was first suggested to him by
A. Moquin-Tandon's work on the leech (1827), as a great aid in
demonstrating the unity of plan and composition throughout the animal
kingdom.[140] According to his view there are three main types of
animals--(1) Biserials, including bilaterally symmetrical animals,
composed of two parallel series of "organisms"; (2) Radiates, composed
of "organisms" arranged like the spokes of a wheel; and (3)
Raceme-animals, in which the separate "organisms" were disposed more or
less irregularly, in bunches (p. 257). The unitary "organism" is
supposed to be the same in all, only the arrangement differing. Dugès of
course admitted that the centralisation of the complete organism became
greater the higher it stood in the scale, and that this held good also
in individual development. The appendages of Articulates and Vertebrates
were thought of as the members of as many separate organisms. He went so
far as to suggest that the fingers of a man's hand were the free
extremities of as many thoracic members.

Dugès' conception of the organism has often been revived since in a
saner form, _e.g._, by E. Perrier, and it has a certain validity. It has
much affinity with the similar conceptions of Goethe and the German
transcendentalists.

    [130] _Mém. Acad. Sci._, iv., pp. cclxxxiv.-ccci., 1824.

    [131] _Ann. Sci. Nat._, xi., xii., 1827; xvi., 1829; xxi., 1830.

    [132] See Rádl, _loc. cit._, i., pp. 225-6.

    [133] _Ann. Sci. nat._ (2), ii., p. 248, 1834.

    [134] _Ann. Sci. nat._, iii., pp. 377-80, 1824.

    [135] _Mémoires sur les Animaux sans Vertèbres_, Part I.,
    p. 10, Paris, 1816.

    [136] _Ann. Sci. Nat._, (1), i., pp. 97-135, 416-432,
    1824.

    [137] _Isis_, p. 456, 1820 (2).

    [138] Cuvier, _Mém. Acad. Sci._, iv., p. cclxx., 1824.

    [139] _Acad. Sci._ 18th Oct. 1831. Extract in _Ann. Sci.
    Nat._, xxiv., pp. 254-60, 1831.

    [140] His views were more fully elaborated in his _Mémoire
    sur la conformité organique dans l'échelle animale_,
    Montpellier, 1832.



CHAPTER VII

THE GERMAN TRANSCENDENTALISTS


To complete our historical survey of the morphology of the early 19th
century we have now to turn back some way and consider the curious
development of morphological thought in Germany under the influence of
the _Philosophy of Nature_. We have already seen many of these notions
foreshadowed by Goethe, who had considerable affinity with the
transcendentalists, but the full development of transcendental habits of
thought comes a little later than the bulk of Goethe's scientific work,
and owes more to Kielmeyer and Oken than to Goethe himself.

A great wave of transcendentalism seems to have passed over biological
thought in the early 19th century, arising mainly in Germany, but
powerfully affecting, as we have seen, the thought of Geoffroy and his
followers. Many ideas were common to the French and German schools of
transcendental anatomy, the fundamental conception that there exists a
unique plan of structure, the idea of the scale of beings, the notion of
the parallelism between the development of the individual and the
evolution of the race. It is difficult to disentangle the part played by
each school and to determine which should have the credit for particular
theories and discoveries. The philosophy seems to have come chiefly from
Germany, the science from France. It must be borne in mind that German
comparative anatomy was largely derivative from French, that the Paris
Museum was the acknowledged anatomical centre, and that Cuvier was its
acknowledged head.

It is probably correct to say that the credit mainly belongs to the
German transcendental school for the law of the parallelism between the
stages of individual development and the stages of the scale of beings,
and the theory of the repetition or multiplication of parts within the
individual. The vertebral theory of the skull is a particular
application of the second of these generalisations.

The law of parallelism[141] seems to have been expressed first by
Kielmeyer (1793),[142] who gave to it a physiological form, saying that
the human embryo shows at first a purely vegetative life, then becomes
like the lower animals, which move but have no sensation, and finally
reaches the level of the animals that both feel and move.

The idea was next taught by Autenrieth in 1797.[143]

Oken (1779-1851) in his early tract _Die Zeugung_ (1805), and in his
_Lehrbuch der Naturphilosophie_ (1809-11) elaborated the thought, and
taught that every animal in its development passes through the classes
immediately below it. "During its development the animal passes through
all stages of the animal kingdom. The foetus is a representation of all
animal classes in time."[144] The Insect, for example, is at first Worm,
next Crab, then a perfect volant animal with limbs, a Fly (_ibid._, p.
542).

As Nature is "the representation of the individual activities of the
spirit," so the animal kingdom is the representation of the activities
or organs of man. The animal kingdom is therefore "a dismemberment of
the highest animal, _i.e._, of Man" (p. 494). Now "animals are gradually
perfected, entirely like the single animal body, by adding organ unto
organ"--the way of evolution is the way of development. Hence "animals
are only the persistent foetal stages or conditions of Man," who is the
microcosm, and contains within himself all the animal kingdom.

Oken was himself a careful student of embryology; von Baer[145] speaks of
his work (published in Oken and Kieser, _Beiträge zur vergleichenden
Zoologie, Anatomie und Physiologie_, 2 pts., 1806-7) as forming the
turning-point in our understanding of the mammalian ovum. He had
accordingly actually observed a resemblance in certain details of
structure between the human foetus and the lower animals; but the
peculiar form which the law took in his hands was a consequence of his
hazy philosophy. He saw the relation of teratological to foetal
structure, for he affirmed that "malformations are only persistent
foetal conditions" (p. 492).

The idea of comparing the embryo of higher animals with the adult of
lower was widely spread at this time among German zoologists. We find,
for example, in Tiedemann's brilliant little textbook[146] the statement
that "Every animal, before reaching its full development, passes through
the stage of organisation of one or more classes lower in the scale, or,
every animal begins its metamorphosis with the simplest organisation"
(p. 57).

Thus the higher animals begin life as a kind of fluid animal jelly which
resembles the substance of a polyp; the young mammal, like the lower
Vertebrates, has only a simple circulation, and, like them, lives in
water (the amniotic fluid); the frog is first like a worm, then develops
gills and becomes like a fish (p. 57). In his work on the anatomy of the
brain,[147] Tiedemann established the homology of the optic lobes in birds
by comparing them with foetal corpora quadrigemina in man (see Serres,
_Ann. Sci. nat._, xii., p. 112).

J. F. Meckel, in 1811, devoted a long essay to a detailed proof of the
parallelism between the embryonic states of the higher animals and the
permanent states of the lower animals. In a previous memoir in the same
collection[148] (i., 1, 1808) he had made some comparisons of this kind in
dealing with the development of the human foetus; in this memoir (ii.,
1, 1811) he brings together all the facts which seem to prove the
parallelism.

His collection of facts is a very heterogeneous one; he mingles
morphological with physiological analogies, and makes the most
far-fetched comparisons between organs belonging to animals of the most
diverse groups. He compares, for instance, the placenta with the gills
of fish, of molluscs and of worms, homologising the cotyledons with the
separate tufts of gills in _Tethys, Scyllæa_ and _Arenicola_(p. 26).
This is purely a physiological analogy. He compares the closed anus of
the early human embryo with the permanent absence of an anus in
Coelentera, and the embryo's lack of teeth with the absence of teeth in
many reptiles and fish, in birds, and in many Cetacea (p. 46).[149] These
are merely chance resemblances of no morphological importance. He
considers bladderworms as animals which have never escaped from their
amnion, and _Volvox_ as not having developed beyond the level of an egg
(p. 7). He lays much stress upon likeness of shape and of relative size,
comparing, for instance, the large multilobate liver of the human foetus
with the many-lobed liver of lower Vertebrates and of Invertebrates. In
general he shows himself, in his comparisons, lacking in morphological
insight.

His treatment of the vascular system affords perhaps the best example of
his method (pp. 8-25). The simplest form of heart is the simple tubular
organ in insects, and it is under this form that the heart first appears
in the developing chick. The bent form of the embryonic heart recalls
the heart of spiders; it lies at first free, as in the mollusc _Anomia_.
The heart consists at first of one chamber only, recalling the
one-chambered heart of Crustacea. A little later three chambers are
developed, the auricle, ventricle, and aortic bulb; at this stage there
is a resemblance to the heart of fish and amphibia. At the end of the
fourth day the auricle becomes divided into two, affording a parallel
with the adult heart of many reptiles.

In his large text-book of a somewhat later date, the _System der
vergleichenden Anatomie_ (i., 1821), he works out the idea again and
gives to it a much wider theoretic sweep, hinting that the development
of the individual is a repetition of the evolutionary history of the
race. Meckel was a timid believer in evolution. He thought it quite
possible that much of the variety of animal form was due to a process of
evolution caused by forces inherent in the organism. "The
transformations," he writes, "which have determined the most remarkable
changes in the number and development of the instruments of organisation
are incontestably much more the consequence of the tendency, inherent in
organic matter, which leads it insensibly to rise to higher states of
organisation, passing through a series of intermediate states."[150]

His final enunciation of the law of parallelism in this same volume
shows that he considered the development of the individual to be due to
the same forces that rule evolution. "The development of the individual
organism obeys the same laws as the development of the whole animal
series; that is to say, the higher animal, in its gradual evolution,
essentially passes through the permanent organic stages which lie below
it; a circumstance which allows us to assume a close analogy between the
differences which exist between the diverse stages of development, and
between each of the animal classes" (p. 514).

He was not, of course, able fully to prove his contention that the lower
animals are the embryos of the higher, and we gather from the following
passage that he could maintain it only in a somewhat modified form. "It
is certain," he writes, "that if a given organ shows in the embryo of a
higher animal a given form, identical with that shown throughout life by
an animal belonging to a lower class, the embryo, in respect of this
portion of its economy, belongs to the class in question" (p. 535). The
embryo of a Vertebrate might at a certain stage of development, be
called a mollusc, if for instance, it had the heart of a mollusc.

He admits, too, that the highest animal of all does not pass through in
his development the entire animal series. But the embryo of man always
and necessarily passes through many animal stages, at least as regards
its single organs and organ-systems, and this is enough in Meckel's eyes
to justify the law of parallelism (p. 535).

In his excellent discussion of teratology Meckel points out how the idea
of parallelism throws light upon certain abnormalities which are found
to be normal in other (lower) forms (p. 556).[151]

We may refer to one other statement of the law of parallelism--by K. G.
Carus in his _Lehrbuch der vergleichenden Anatomie_ (Leipzig, 1834). The
standpoint is again that of _Naturphilosophie_. It is a general law of
Nature, Carus thinks, that the higher formations include the lower; thus
the animal includes the vegetable, for it possesses the "vegetative" as
well as the "animal" organs. So it is, too, by a rational necessity that
the development of a perfect animal repeats the series of antecedent
formations.

As we have said, the main credit for the enunciation of the law of
parallelism belongs to the German transcendental school; but the law
owes much also to Serres, who, with Meckel, worked out its implications.
It might for convenience, and in order to distinguish it from the laws
later enunciated by von Baer and Haeckel, be called the law of
Meckel-Serres.

Under the "theory of the repetition or multiplication of parts within
the organism" may be included, first, generalisations on the serial
homology of parts, and second, more or less confused attempts to
demonstrate that the whole organisation is repeated in certain of the
parts. The recognition of serial homologies constituted a real advance
in morphology; the "philosophical" idea of the repetition of the whole
in the parts led to many absurdities. It led Oken to assert that in the
head the whole trunk is repeated, that the upper jaw corresponds to the
arms, the lower to the legs, that in each jaw the same bony divisions
exist as in the limbs, the teeth, for instance, corresponding to the
claws (_loc. cit._, p. 408). It led him to distinguish "two animals" in
every body--the cephalic and the sexual animal. Each of these has its
own organs; thus "in the perfect animal there are two intestinal systems
thoroughly distinct from each other, two intestines which belong to two
different animals, the sexual and cephalic animal, or the plant and the
animal" (p. 382). The intestine of the sexual animal is the large
intestine; the lungs of the sexual animal are the kidneys, its glottis
is the urethra, its mouth the anus. So, too, the mouth is the stomach of
the head. On another line of thought the sternum is a ventral vertebral
column. Limbs are connate ribs, the digits indicating the number of ribs
included (_cf._ Dugès, _supra_, p. 88).

J. F. Meckel[152] discusses "homologies" of this kind in the thorough and
pedestrian way so characteristic of him. Not only, he says, are the
right and left halves of the body comparable with one another, but also
the upper and the lower, the dividing line being drawn at the level of
the diaphragm. The lumbar complex corresponds to the skull, the anus to
the mouth, the urino-genital opening to the nasal opening; in general,
the urino-genital system corresponds to the respiratory, the kidneys to
the lungs, the ureters to bronchi, the testes and ovaries to the thymus
(he had observed the physiological relation between the development of
the thymus and the state of the genital organs), the prostate and the
uterus to the thyroid gland, and the penis and clitoris to the tongue.
The fore-limbs and girdle correspond in detail with the hind limbs and
the pelvis--a point already worked out by Vicq d'Azyr; the dorsal and
ventral halves of the body are likewise comparable in some respects, the
sternum, for example, answering in the arrangement of its bones, muscles
and arteries to the vertebral column. The skeleton of each member is in
some respects a repetition of the vertebral column.

His brother, D. A. Meckel,[153] worked out an elaborate comparison between
the alimentary canal and the genital organs, basing the legitimacy of
the comparison upon early embryological relations and upon the state of
things in Coelentera, where genital and digestive organs occupy the same
cavity. In his view the uterus corresponded to the stomach, the vagina
to the oesophagus, the fallopian tubes to the intestine, and so on.

The vertebral theory of the skull took its origin from the same habit of
thought. As part of the wider idea of the metameric repetition of parts
it had some scientific worth, but the theory was pushed too far, and the
facts were twisted to suit it. Among annulate animals the theory of
repetition found ample scope; Oken was able to compare with justice the
jaws of crabs and insects with their other limbs, as Savigny did later
in a more scientific way. Among Vertebrates the application of the
theory of serial repetition was not so obvious, except in the case of
the vertebræ. Goethe seems to have been the first to hit upon the idea
that the skull is composed of a number of vertebræ, serially homologous
with those of the vertebral column. He tells us that the idea flashed
into his mind when contemplating in the Jewish cemetery at Venice a
dried sheep's skull. The discovery was made in 1790, but not published
till 1820.[154]

The idea seems to have been taught by Kielmeyer, one of the earliest of
the "philosophers of nature," but it was not published by him.

In a book (_Cours d'Études médicales_), published in 1803, Burdin
assimilated the skull to the vertebral column.

Oken, in an inaugural dissertation (Programm) _Ueber die Bedeutung der
Schädelknochen_,[155] published in 1807, gave to the theory its necessary
development. Autenrieth, also in 1807,[156] distinguishing separate
ganglia in the brain, was not far from the hypothesis that each of these
ganglia must have its separate vertebra.

In 1808 Duméril read a paper to the Académie des Sciences in which he
compared the skull to a gigantic vertebra, basing his hypothesis on the
similarity existing between the crests and depressions on the hinder
part of the skull and those on the posterior surfaces of the vertebræ.

After Oken's work the vertebral theory was taken up generally by both
the German and the French anatomists. Spix published in 1815 a large
volume on the skull, entitled _Cephalogenesis_, distinguishing (as Oken
did at first) three cranial vertebræ. Bojanus in his _Anatome testudinis
europæae_ (1819), and in a series of papers in _Isis_ (1817-1819, and
1821) established the existence of a fourth cranial vertebra, and this
was accepted by Oken in the later editions of his _Lehrbuch_. Meckel and
Carus among the Germans, de Blainville and E. Geoffroy among the French,
contributed to the development of the theory. In England the theory was
championed particularly by Richard Owen.

It was one thing to assert in a moment of inspiration that the skull was
composed of modified vertebræ; it was quite another to demonstrate the
relation of the separate bones of the skull to the supposed vertebræ.
Upon this much uncertainty reigned; there was not even unanimity as to
the number of vertebræ to be distinguished. Goethe found six vertebræ in
the skull; Spix, and at first Oken, three only, Geoffroy seven; the
accepted orthodox number seems to have been four (Bojanus, Oken, Owen).

As an example of the method of treatment adopted we may take Oken's
matured account of the composition of the cranial vertebræ, as given in
the English translation of his _Lehrbuch_. "To a perfect vertebra," he
says, "belong at least five pieces, namely, the body, in front the two
ribs, behind the two arches or spinous processes" (p. 370). In the
cervical vertebræ the transverse processes represent the ribs. The skull
consists of four vertebræ, the occipital, the parietal, the frontal and
the nasal, or, named after the sense with which each is associated, the
auditory, the lingual, the ocular and the olfactory. The "bodies" of
these vertebræ are the body of the occipital (basioccipital), the two
bodies of the sphenoid (basi- and pre-sphenoid), and the vomer. The
transverse processes of each are the condyles of the occipitals
(exoccipitals), the alæ of the two sphenoids (alisphenoids and
orbitosphenoids) and the lateral surfaces of the vomer. The arches or
spinous processes are the occipital crest, the parietals, the frontals,
and the nasals.

The cranium is thus composed of four rings of bone, each composed of the
typical elements of a vertebra.

The arbitrary nature of the comparison is obvious enough. As Cuvier
pointed out in the posthumous edition of his _Leçons_, it is only the
occipital segment that shows any real analogy with a vertebra--an
analogy which Cuvier ascribed to similarity of function. He admitted a
faint resemblance of the parietal segment to a vertebra:--"The body of
the sphenoid does indeed look like a repetition of the basioccipital,
but having a different function it takes on another form, especially
above, by reason of its posterior clinoid apophyses."[157] He denied the
resemblance of the frontal and nasal "vertebræ" to true vertebræ,
pointing out that both parietals and frontals are bones specially
developed for the purpose of roofing over and protecting the cerebrum.

A very curious development was given to the vertebral theory by K. G.
Carus, who seems to have taken as his text a saying of Oken's, that the
whole skeleton is only a repeated vertebra.[158] His system is worthy of
some consideration, for he tries to work out a geometry of the
skeleton.[160]

His method of deduction is a good example of pure _Naturphilosophie_.
Life, he says, is the development of something determinate from
something indeterminate. A finite indeterminate thing, that is, a
liquid, must take a spherical form if it is to exist as an individual.
Hence the sphere is the prototype of every organic body. Development
takes place by antagonism, by polarity, typically by the division and
multiplication of the sphere. In the course of development the sphere
may change, by expansion into an egg-shaped body, or by contraction into
a crystalline form, the changes due to expansion being typical of living
things, those due to contraction being typical of dead. At the surface
of the primitive living sphere is developed the protective
_dermatoskeleton_, which naturally takes the shape of a hollow sphere;
round the digestive cavity which is formed in the living sphere is
developed the _splanchnoskeleton_; round the nervous system (which is,
as it were, the animal within the animal) is developed the
_neuroskeleton_. All skeletal formations belong to one or other of these
systems.

Carus defines his aim to be the discovery of the inner law which
presides over the formation of the skeleton throughout the animal
kingdom; he desires to know "how such and such a formation is realised
in virtue of the eternal laws of reason" (iii., p. 93). Here we touch
the kernel of _Naturphilosophie_--the search for rational laws which are
active in Nature; the discontent with merely empirical laws.

The thesis which Carus sustains is that all forms of skeleton, whether
of dermatoskeleton, splanchnoskeleton, or neuroskeleton, can be deduced
from the hollow sphere, which is the primary form of any skeleton
whatsoever (p. 95). That means, put empirically, that every skeleton can
be represented schematically by a number of hollow spheres, suitably
modified in shape, and suitably arranged. The chief modification in
shape exhibited by bones is one which is intermediate between the
organic and the crystalline series of modifications of the sphere. The
organic modifications are bounded by curved lines, the crystalline by
straight; the intermediate partly by curved and partly by straight
lines. They are the dicone (the shape of a diabolo) and the cylinder.
These forms must necessarily be of importance for the skeleton, which is
intermediate between the organic and the inorganic. "The dicone embodies
the real significance of the bone," writes Carus. Each dicone and
cylinder composing the skeleton is called by Carus a vertebra.

We may expect then all skeletons to be composed of spheres, cylinders
and dicones in diverse arrangements. Nature being infinite, all the
possible types of arrangement of these elements must exist in the test
or skeleton of some animal, living, fossil, or to come (p. 127). One
conceives easily what the main types of skeleton must be. In some
animals, _e.g._, sea-urchins, the skeleton is a simple sphere; in
others, _e.g._, starfish, secondary rows of spheres radiate out from a
central sphere or ring; in annulate animals the skeleton consists of a
row of partially fused spheres.

In Vertebrates the arrangement is more complex. There are first the
protovertebral rings of the dermatoskeleton, these being principally the
ribs, limb-girdles, and jaws. Round the central nervous system are
developed the deutovertebral rings of the neuroskeleton (vertebræ in the
ordinary sense). The apophyses and bodies of the vertebræ, and the bones
of the members[160] are composed of columns of tritovertebræ, or vertebræ
of the third order. Thus the whole vertebrate skeleton is a particular
arrangement of vertebræ, which in their turn are modifications of the
primary hollow sphere.

The German transcendentalists were more or less contemporary with E.
Geoffroy, and no doubt influenced him, especially in his later years, as
they certainly did his follower Serres. Oken indeed wrote, in a note[161]
appended to Geoffroy's paper on the vertebral column of insects, that
"Mr Geoffroy [_sic_] is without a doubt the first to introduce in France
_Naturphilosophie_ into comparative anatomy, that is to say, that
philosophy one of whose doctrines it is to seek after the
_signification_ of organs in the scale of organised beings." This is,
however, an exaggeration, for Geoffroy was primarily a morphologist,
whereas the morphology of the German transcendentalists was only a
side-issue of their _Naturphilosophie_.

Geoffroy, on his part, exercised some influence on the
transcendentalists. He asserts[162] indeed that Spix got some of the ideas
published in the _Cephalogenesis_ (1815) from attending his course of
lectures in 1809. It is certainly the case that Spix published before
Geoffroy the view that the opercular bones are homologous with the
ear-ossicles, adopting, however, a different homology for the separate
bones.[163]

Some speculations seem to have been common to both schools--for
instance, the law of Meckel-Serres, the vertebral theory of the skull,
and the recognition of serial homology in the appendages of Arthropods
(Savigny, Oken). Latreille and Dugès, as well as Serres, clearly show in
their theoretical views the influence of Oken and the other
transcendentalists. Geoffroy's principle of connections and law of
compensation were recognised by some at least of the Germans.

But whatever his actual historical relations may have been with the
German school, Geoffroy was vastly their superior in the matter of pure
morphology. He alone brought to clear consciousness the principles on
which a pure morphology could be based: the Germans were transcendental
philosophers first, and morphologists after.

One understands from this how J. F. Meckel, who was in some ways the
leading comparative anatomist in Germany at this time, could be at once
a transcendentalist and an opponent of Geoffroy. Meckel had a curiously
eclectic mind. A disciple of Cuvier, having studied in 1804-6 the rich
collections at the Museum in Paris, the translator of Cuvier's _Leçons
d'anatomie comparée_, he earned for himself the title of the "German
Cuvier," partly through the publication of his comprehensive textbook
(_System der vergl. Anatomie_, 5 vols.), partly by his extensive and
many-sided research work, partly by his authoritative teaching. His
_System_ shows in almost every page of its theoretical part the
influence of Cuvier; and it is through having assimilated Cuvier's
teaching as to the importance of function that Meckel combats Geoffroy's
law of connections, at least in its rigorous form. He submits that the
connections of bones and muscles must change in relation to functional
requirements. He rejects Geoffroy's theory of the vertebrate nature of
Articulates. Generally throughout his work the functional point of view
is well to the fore.

Yet at heart Meckel was a transcendentalist of the German school. His
vagaries on the subject of "homologues" leave no doubt about that, and,
in spite of Cuvier, he believed, though not very firmly, in the
existence of one single type of structure.

A Cuverian by training, his lack of morphological sense threw him into
the ranks of the transcendentalists, to whom perhaps he belonged by
nature.

    [141] For a full account, see Kohlbrugge, _Zool. Annalen_,
    xxxviii., 1911.

    [142] _Rede über das Verhältnis der organischen Kräfte_,
    Stuttgart u. Tübingen, 1793 (1814). See Rádl, _loc.
    cit._, i., p. 261; ii., p. 57.

    [143] _Supplem. ad historiam embryonis_, Tübingen, 1797.

    [144] _Lehrbuch der Naturphilosophie_, Eng. trans., p.
    491, 1847.

    [145] _Ueber Entwickelungsgeschichte der Thiere_, i., p.
    xvii., 1828.

    [146] _Zoologie_, Landshut, i., 1808.

    [147] _Anatomie u. Bildungsgeschichte des Gehirns im Fötus
    des Menschen_, Nürnberg, 1816.

    [148] _Beyträge zur vergleichende Anatomie_, Leipzig, i.,
    1808-9, ii., 1811-2.

    [149] Cetacea were generally considered at this time to be
    mammals of low organisation.

    [150] From the French trans., which appeared under the
    title _Traité gén. d'Anat. comparée_, i., p. 449, 1828.

    [151] _Cf._ Geoffroy (_supra_, p. 70).

    [152] _Beyträge_, ii., 2, 1812. Also in his _System d.
    vergl. Anat._, i., 1821.

    [153] In J. F. Meckel's _Beyträge_, ii.

    [154] _Zur Morphologie_, i., 2, p. 250, 1820; and ii., 2,
    pp. 122-4, 1824.

    [155] See translation, giving the gist of this paper, in
    Huxley's _Lectures on the Elements of Comparative
    Anatomy_, pp. 282-6, London, 1864.

    [156] Reil's _Archiv. f. Physiol._, vii., 1807.

    [157] _Leçons d'anatomie comparée_, 3rd ed., Brussels
    reprint, i., p. 414, 1836.

    [158] In his Programm, _U. d. Bedeut. d. Schädelknochen_,
    1807.

    [159] _Traité élémentaire d'anatomie comparée_ (French
    trans.), vol. iii., Paris, 1835. First developed in his
    volume _Von den Ur-Theilen des Knochen und
    Schalen-Gerustes_, Leipzig, 1828.

    [160] Dutrochet in 1821 had tried to prove that the bones
    of the members belong to the type of the vertebra--the
    dicone.

    [161] _Isis_, pp. 552-9, 1820 (2).

    [162] _Mém. Mus. d'Hist. nat._, ix., 1822.

    [163] Cuvier and Valenciennes, _Hist. nat. Poissons_, i.,
    p. 311, f.n.



CHAPTER VIII

TRANSCENDENTAL ANATOMY IN ENGLAND--RICHARD OWEN


Richard Owen is the epigonos of transcendental morphology; in him its
guiding ideas find clear expression, and in his writings are no
half-truths struggling for utterance. But he was, though a staunch
transcendentalist, an eclectic of the older ideas current in his time;
for he picked out what was best in the older systems--Cuvier's
teleology, Geoffroy's principle of connections, Oken's idea of the
serial repetition of parts. In particular, he assimilated the teaching
of Cuvier, the great opponent of the transcendentalists, and reconciled
it in part with his own transcendentalism. His main theoretical views
are to be found in his volume _On the Archetype and Homologies of the
Vertebrate Skeleton_ (London, 1848). The master-idea of the book is that
the vertebrate skeleton consists of a series of comparable segments,
each of which Owen calls a vertebra. His definition of a vertebra is,
"one of those segments of the endo-skeleton which constitute the axis of
the body, and the protecting canals of the nervous and vascular trunks"
(p. 81). The parts of a typical vertebra are shown in Fig. 4, which is
copied from Owen's Fig. 14.

                |||
  zygapophysis  ||| -- neural spine
             \  |||
              *//^\\*
diapophysis   //   \\ -- neurapohysis
         \   //  o  \\
          ===== --- =====
              /     \
       ===== |CENTRUM| O ===== -- peiurapophysis
              \     /
          ===== --- =====
         /    \\   //
parapophysis  *\\v//*
             /  |||
  zygapophysis  ||| -- hæmal spine
                |||

FIG. 4.--Ideal Typical Vertebra. (After Owen.)

In Fig. 5 (page 103) is shown an actual vertebra, as Owen conceives it,
the "vertebra" being that of a bird.

[Illustration: FIG. 5.--Natural Typical Vertebra; Thorax of a Bird.
(After Owen.)]

A segment of sternum is included as the "hæmal spine" of the vertebra
(_hs_); the vertebral rib is the "pleurapophysis" (_pl_); the sternal
rib the "hæmapophysis" (_h_); the uncinate process of the vertebral rib
is known as the "diverging appendage" (_a_). The whole vertebrate
skeleton is composed of a series of vertebræ which show these typical
parts. We arrive thus at the conception of an "Archetype" of the
vertebrate skeleton, such as is represented in Fig. 6.

The archetype is only a scheme of what is usually constant in the
vertebrate skeleton, and both the number and the arrangement of the
bones in any real Vertebrate are subject to variation. "It has been
abundantly proved," Owen writes, towards the end of his volume, "that
the idea of a natural segment (vertebra) of the endoskeleton does not
necessarily involve the presence of a particular number of pieces, or
even a determinate and unchangeable arrangement of them. The great
object of my present labour has been to deduce ... the relative value
and constancy of the different vertebral elements, and to trace the kind
and extent of their variations within the limits of a plain and obvious
maintenance of a typical character" (p. 146).

It goes without saying that Owen considered the skull to be formed of
vertebræ--the vertebral theory of the skull was, in his system, a
deduction from the vertebral theory of the skeleton. He recognised four
cranial vertebræ; the arrangement of them, and the relation of their
constituent bones to the parts of the typical vertebra are shown in the
table appearing on page 106. So far as their first three elements are
concerned, these vertebræ are practically identical with the vertebræ
distinguished in the classical vertebral theory of the skull, as
enunciated by Oken. A divergence appears with the determination of the
other elements of the vertebræ. The upper and lower jaws are associated
with the nasal and frontal vertebræ respectively, not however as limbs
of the head, but as constituent elements of these vertebræ. In the same
way the hyoid apparatus is part and parcel of the parietal vertebra, and
the pectoral girdle and fore-limbs part of the occipital vertebra.

[Illustration: FIG. 6.--The Archetype of the Vertebrate Skeleton. (After
Owen.)]

Cranial Vertebræ.[164] (After Owen, 1848, p. 165.)

+---------------+---------------+----------------+---------------+-------------+
|  Vertebræ.    |   Occipital.  |  Parietal.     |    Frontal.   |    Nasal.   |
+===============+===============+================+===============+=============+
|Centra.        |Basioccipital. |Basisphenoid.   |Presphenoid.   |Vomer.       |
+---------------+---------------+----------------+---------------+-------------+
|Neurapophyses. |Exoccipital.   |Alisphenoid.    |Orbitosphenoid.|Prefrontal.  |
+---------------+---------------+----------------+---------------+-------------+
|Neural Spines. |Supraoccipital.|Parietal.       |Frontal.       |Nasal.       |
+---------------+---------------+----------------+---------------+-------------+
|Parapophyses.  |Paroccipital.  |Mastoid.        |Postfrontal.   |None.        |
+---------------+---------------+----------------+---------------+-------------+
|Pleurapophyses.|Scapular.      |Stylohyal.      |Tympanic.      |Palatal.     |
+---------------+---------------+----------------+---------------+-------------+
|Hæmapophyses.  |Coracoid.      |Ceratohyal.     |Articular.     |Maxillary.   |
+---------------+---------------+----------------+---------------+-------------+
|Hæmal Spines.  |Episternum.    |Basihyal.       |Dentary.       |Premaxillary.|
+---------------+---------------+----------------+---------------+-------------+
|  Diverging    |Fore-limb or   |Branchiostegals.|Operculum.     |Pterygoid and|
|  Appendage.   |  Fin.         |                |               |  Zygoma.    |
+---------------+---------------+----------------+---------------+-------------+

Owen's reasons for considering the pectoral girdle and the fore-limb
part of the occipital vertebra are as follows. In fish the pectoral
girdle is slung to the skull by means of the post-temporal bone
(supra-scapula, according to Owen) which abuts on the occipital arch. In
_Lepidosiren_, whose skeleton resembles the archetype in many ways, the
pectoral girdle is likewise attached to the occipital segment.

In most other Vertebrates the pectoral girdle has shifted backwards
along the vertebral column, by a "metastasis" (Geoffroy) similar to that
by which the pelvic fins in many fish have shifted up close to the
pectoral girdle. The scapula (with supra-scapula) is the pleurapophysis,
the coracoid the hæmapophysis, of the occipital vertebra. The clavicle
is homologised with the slender bone in fish now known as the
post-clavicle, which shows a connection with the first or atlas vertebra
of the vertebral column, forming, according to Owen, the hæmapophysis of
the atlas. Owen considers it no objection to this view that in other
Vertebrates the clavicle is anterior to the coracoid--"its anterior
position to the coracoid in the air-breathing Vertebrata is no valid
argument against the determination, since in these we have shown that
the true scapular arch is displaced backwards" (_On the Nature of
Limbs_, p. 63, London, 1849). In the pelvic girdle the ilium corresponds
to the scapula, the ischium to the coracoid, the pubis to the clavicle.
Hence the ilium is a pleurapophysis, the ischium and pubis are both
hæmapophyses. The fore-limb is the developed "appendage" of the
occipital vertebra, the hind-limb the developed "appendage" of the
pelvic vertebra. They are serially homologous with, for example, the
uncinate processes of the ribs in birds (see Figs. 5 and 6). The
fore-limb is a simple filament in _Lepidosiren_, and presents few joints
in _Proteus_ and _Amphiuma_; in other air-breathing Vertebrates it shows
a more complete development, the humerus, radius and ulna, and the bones
of the wrist and hand becoming differentiated out.

As the fore-limb is equivalent to a single bone of the archetype, it is
said to be, in its developed state, "teleologically compound" (p. 103).

Since in the archetype every vertebra has its appendage, more than two
pairs of locomotory limbs might have been developed. "Any given
appendage might have been the seat of such developments as convert that
of the pelvic arch into a locomotive limb; and the true insight into the
general homology of limbs leads us to recognise many potential pairs in
the typical endoskeleton. The possible and conceivable modifications of
the vertebrate archetype are far from having been exhausted in the forms
which have hitherto been recognised, from the primæval fishes of the
palæozoic ocean of this planet up to the present time" (p. 102). It is
not of the essence of the vertebrate type to be tetrapodal.

In determining homologies Owen remained true to Geoffroy's principle of
connections. Speaking of an attempt which had been made to determine
homologies by the mode of development, he writes, "There exists
doubtless a close general resemblance in the mode of development of
homologous parts; but this is subject to modification, like the forms,
proportions, functions, and very substance of such parts, without their
essential homological relationships being thereby obliterated. These
relationships are mainly, if not wholly, determined by the relative
position and connection of the parts, and may exist independently of
form, proportions, substance, function and similarity of development.
But the connections must be sought for at every period of development,
and the changes of relative position, if any, during growth, must be
compared with the connections which the part presents in the classes
where vegetative repetition is greatest and adaptive modification least"
(p. 6). It is interesting to note that in Owen's opinion comparative
anatomy explains embryology. Thus the scapula, which is the
pleurapophysis of the occipital vertebra, is vertical on its first
appearance in the embryo of tetrapoda, and lies close up to the head
(_On the Nature of Limbs_, p. 49)--the embryo shows a greater
resemblance to the archetype than the adult. "We perceive a return to
it, as it were, in the early phases of development of the highest
organised of the actually existing species, or we ought rather to say
that development starts from the old point; and thus, in regard to the
scapula, we can explain the constancy of its first appearance close to
the head, whether in the human embryo or in that of the swan, also its
vertical position to the axis of the spinal column, by its general
homology as the rib or 'pleurapophysis' of the occipital
vertebra" (_Limbs_, p. 56).

We owe to Owen the first clear distinction between "homologous" and
"analogous" organs; it was he who first proposed the terms "homologue"
and "analogue," which he defined as follows:--"_Analogue_. A part or
organ in one animal which has the same function as another part or organ
in a different animal." "_Homologue_. The same organ in different
animals under every variety of form and function."[165]

He introduced also useful distinctions between Special, General, and
Serial Homology. "The relations of homology," he writes, "are of three
kinds: the first is that above defined, viz., the correspondency of a
part or organ, determined by its relative position and connections, with
a part or organ in a different animal; the determination of which
homology indicates that such animals are constructed on a common type;
when, for example, the correspondence of the basilar process of the
human occipital bone with the distinct bone called 'basi-occipital' in a
fish or crocodile is shown, the _special homology_ of that process is
determined. A higher relation of homology is that in which a part or
series of parts stands to the fundamental or general type, and its
enunciation involves and implies a knowledge of the type on which a
natural group of animals, the Vertebrate, for example, is constructed.
Thus when the basilar process of the human occipital bone is determined
to be the 'centrum' or 'body' of the last cranial vertebra, its _general
homology_ is enunciated.

"If it be admitted that the general type of the vertebrate endoskeleton
is rightly represented by the idea of a series of essentially similar
segments succeeding each other longitudinally from one end of the body
to the other, such segments being for the most part composed of pieces
similar in number and arrangement, and though sometimes extremely
modified for special functions, yet never so as to wholly mask their
typical character--then any given part of one segment may be repeated in
the rest of the series, just as one bone may be reproduced in the
skeletons of different species, and this kind of repetition or
representative relation in the segments of the same skeleton I call
'serial homology'" (p. 7). As an example of serial homology we might
take the centra of the vertebræ--the vomer, the presphenoid, the
basisphenoid, the basioccipital and the series of centra in the spinal
column. Such serially repeated parts are called _homotypes_ (p. 8).

Not all the bones of the vertebrate skeleton are included in the
archetype as constituents of the vertebræ. Thus the branchial and
pharyngeal arches are accounted part of the splanchnoskeleton, as
belonging to the same category as the heart bone of some ruminants, and
the ossicles of the stomach in the lobster (p. 70). The ossicles of the
ear in mammals are "peculiar mammalian productions in relation to the
exalted functions of a special organ of sense" (p. 140, f.n.). This
recognition of a possible development of new organs to meet new
functions shows unmistakably the influence of Cuvier. Owen was indeed
well aware of the importance of the functional aspect of living things,
and he often adopted the teleological point of view. As a true
morphologist, however, he held that the principle of adaptation does not
suffice to explain the existence of special homologies. The ossification
of the bones of the skull from separate centres may be purposive in
Eutheria, in that it prevents injury to the skull at birth; but how
explain on teleological principles the similar ossification from
separate centres in marsupials, birds and reptiles? How explain above
all the fact that the centres are the same in number and relative
position in all these groups? Surely we must accept the idea of an
archetype "on which it has pleased the divine Architect to build up
certain of his diversified living works" (p. 73).

In his study of centres of ossification, Owen made in point of theory a
distinct advance on his predecessors. We saw that Geoffroy recognised
the importance of studying the ossification of the skeleton, and that
Cuvier accepted such embryological evidence as an aid in determining
homologies. Owen pointed out that it was necessary to distinguish
between centres of ossification which were teleological in import and
such as were purely indicative of homological relationships. Many bones,
single in the adult, arise from separate centres of ossification, but we
must distinguish between "those centres of ossification that have
homological relations, and those that have only teleological ones;
_i.e._, between the separate points of ossification of a human bone
which typify vertebral elements, often permanently distinct bones in the
lower animals; and the separate points which, without such
signification, facilitate the progress of osteogeny, and have for their
obvious final cause the well-being of the growing animal" (p. 105).
There is, for example, a teleological reason why in mammals and leaping
Amphibia (_e.g._, frogs), the long bones should ossify first at their
ends, for the brain is thus protected from concussion; in reptiles that
creep there is less danger of concussion, and the long bones ossify in
the middle (p. 105). But there is no teleological reason why the
coracoid process of the scapula should in all mammals develop from a
separate centre. The coracoid is however a real vertebral element
(hæmapophysis), and in monotremes, birds and reptiles it is in the adult
a large and separate bone. Its ossification from a separate centre in
mammals has therefore a homological significance. The scapula in mammals
is an example of what Owen calls a "homologically compound" bone. All
those bones which are formed by a coalescence of parts answering to
distinct elements of the typical vertebra are "homologically compound"
(p. 105). On the other hand, "All those bones which represent single
vertebral elements are 'teleologically compound' when developed from
more than one centre, whether such centres subsequently coalesce, or
remain distinct, or even become the subject of individual adaptive
modifications, with special joints, muscles, etc., for particular
offices" (p. 106). The limb-skeleton, corresponding as it does to a
single bone of the archetype, is the typical example of a teleologically
compound bone. Owen in his definition of teleological compoundness has
combined two kinds of adaptation--(1) temporary adaptation of bones to
the exigencies of development, birth and growth (_e.g._, development of
long bones from separate centres); (2) definitive adaptation of a
skeletal part to the functions which it has to perform (_e.g._,
teleological structure of limbs). Such adaptations are, so to speak,
grafted on the archetype.

Owen's general views on the nature of living things merit some
attention. Organic forms, according to Owen, result from the
antagonistic working of two principles, of which one brings about a
vegetative repetition of structure, while the other, a teleological
principle, shapes the living thing to its functions. The former
principle is illustrated in the archetype of the vertebrate skeleton, in
the segmentation of the Articulates, in the almost mathematical symmetry
of Echinoderms, and the actually crystalline spicules of sponges. It is
the same principle which causes repetition of the forms of crystals in
the inorganic world. "The repetition of similar segments in a vertebral
column, and of similar elements in a vertebral segment, is analogous to
the repetition of similar crystals as the result of polarising force in
the growth of an inorganic body" (p. 171). This "general polarising
force" it is which mainly produces the similarity of forms, the
repetition of parts, and generally the signs of the unity of
organisation. The adaptive or "special organising force" or [Greek:
idea], on the other hand, produces the diversity of organic beings. In
every species these two forces are at work, and the extent to which the
general polarising or "vegetative-repetition-force" is subdued by the
teleological is an index of the grade of the species.

This view is analogous to the Geoffroyan conception that the diversity
of form is limited by the unity of plan. Owen thus ranges himself with
Geoffroy against Cuvier, who considered that diversity of form is
limited only by the principle of the adaptation of parts.

    [164] Owen introduced most of the names of bones now
    current.

    [165] _Lectures on Invertebrate Animals_, pp. 374, 379,
    1843.



CHAPTER IX

KARL ERNST VON BAER


Von Baer was recognised as the founder of embryology even by his
contemporaries. His predecessors, Aristotle,[166] Fabricius,[167]
Harvey,[168] Malpighi,[169] Haller,[170] Wolff,[171] had made a
beginning with the study of development; von Baer, by the thoroughness
of his observation and the strength of his analysis, made embryology a
science.

It was to one of the German transcendentalists that von Baer owed the
impulse to study development. Ignatius Döllinger, Professor in Würzburg,
induced three of his pupils, Pander, d'Alton and von Baer, to devote
themselves to embryological research. The development of animals was at
this time little known, in spite of recent work by Meckel (1815 and
1817), Tiedemann (_Anatomie u. Bildungsgeschichte des Gehirns_, 1816),
by Oken (_loc. cit., supra_, p. 90), and some others.

Pander, with whom apparently Döllinger and d'Alton collaborated, was the
first to publish his results;[172] von Baer, who through absence from
Würzburg had for a time dropped his embryological studies, started to
work in 1819, after the publication of Pander's treatise, and produced
in 1828 the first volume of his master-work, _Ueber
Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion_
(Königsberg, 1828). The second volume followed in 1837, but dates really
from 1834, and was published in an incomplete form. This second volume
is intended as an introduction to embryology for the use of doctors and
science students. In it von Baer describes in full detail the
development of many vertebrate types--chick, tortoise, snake, lizard,
frog, fish, several mammals and man, basing his remarks largely upon his
personal observations, but taking account also of all contemporary work.
A separate account of the development of a fish (_Cyprinus blicca_)
appeared in 1835.[173]

We shall concentrate attention on the first volume. This volume contains
the first full and adequate account of the development of the chick,
followed by a masterly discussion of the laws of development in general.

When we consider that von Baer worked chiefly with a simple microscope
and dissecting needles, the minuteness and accuracy of his observations
are astonishing. He described the main facts respecting the development
of all the principal organs, and if, through lack of the proper means of
observation, he erred in detail, he made up for it by his masterly
understanding and profound analysis of the essential nature of
development. His account of the development of the chick is a model of
what a scientific memoir ought to be; the series of "Scholia" which
follow contain the deductions he made from the data, and, in so far as
they are direct generalisations from experience, they are valid for all
time.

The first Scholion is directed against the theory of preformation, and
succeeds in refuting it on the ground of simple observation. The theme
of the second Scholion is that the essential nature (_die Wesenheit_) of
the animal determines its differentiation, that no stage of development
is solely determined by the antecedent stage, but that throughout all
stages the _Wesenheit_ or idea of the definitive whole exercises
guidance. This guidance is shown most clearly in the regulatory
processes of the germ, whereby the large individual variations commonly
presented by the early embryo are compensated for or neutralised in the
course of further development. Baer in this shows himself a vitalist.

It is, however, the third and subsequent Scholia which must here
particularly occupy our attention, for it is in these that von Baer
comes to grips with morphological problems. Already in the second
Scholion he had definitely enunciated the law which runs as a theme
throughout the volume, the observational and the theoretical part alike,
the law that development is essentially a process of differentiation by
which the germ becomes ever more and more individualised. "The essential
result of development," he writes, "when we consider it as a whole, is
the increasing independence (_Selbständigkeit_) of the developing
animal" (p. 148). In the third Scholion he elaborates this thought and
shows that differentiation takes place in triple wise. The three
processes of differentiation are "primary differentiation" or
layer-formation, "histological differentiation" within the layers, and
the "morphological differentiation" of primitive organs.

The first of these differentiations in time is the formation of the
germ-layers, which takes place by a splitting or separation of the
blastoderm into a series of superimposed lamellæ. Baer's account of the
process in the chick is as follows:--

"First of all, the germ separates out into heterogeneous layers, which
with advancing development acquire ever greater individuality, but even
on their first appearance show rudiments of the structures which will
characterise them later. Thus in the germ of the bird, so soon as it
acquires consistency at the beginning of incubation, we can distinguish
an upper smooth continuous surface and a lower more granular surface.
The blastoderm separates thereupon into two distinct layers, of which
the lower develops into the plastic body-parts of the embryo, the upper
into the animal parts; the lower shows clearly a further division into
two closely connected subsidiary layers--the mucous layer and the
vessel-layer; the original upper layer also shows a division into two,
which form respectively the skin and the parts which I have called the
true ventral and dorsal plates--parts which contain in an
undifferentiated state the skeletal and muscular systems, the connective
tissues, and the nerves belonging to these. In order to have a
convenient term for future use, I have named this layer the
muscle-layer" (p. 153).

The process of delamination results then in the formation of four
layers, of which the upper two (composing the "animal" or "serous"
layer) will give origin to the animal (neuromuscular) part of the body,
the lower pair to the plastic or vegetative organs. The uppermost layer
will form the external covering of the embryo, and also the amniotic
folds; from it there differentiates out at a very early stage the
rudiment of the central nervous system, forming a more or less
independent layer. Below the outermost layer lies the layer from which
are formed the muscular and skeletal systems, and beneath this
"muscle-layer" comes the "vessel-layer," which gives origin to the main
blood-vessels. The innermost layer of the four will form the mucous
membrane of the alimentary canal and its dependencies; at the present
stage, however, it is, like the other layers, a flat plate.

From all these layers tubes are developed by the simple bending round of
their edges. The outermost layer becomes the investing skin-tube of the
embryo; the layer for the nervous system forms the tubular rudiment of
the brain and spinal cord; the mucous layer curls round to form the
alimentary tube; the muscle layer grows upwards and downwards to form
the fleshy and osseous tube of the body wall; even the vessel layer
forms a tube investing the alimentary canal, but a part of it goes to
form the medial "Gekröse," or mesenterial complex, which departs
considerably from the tubular form.

When these tubes or "fundamental organs" are formed the process of
primary differentiation is complete. The fundamental organs, however,
have at no time actually the form of tubes; they exist as tubes only
ideally, for morphological and histological differentiation go on
concurrently with the process of primary differentiation.

Through morphological differentiation the various parts of the
fundamental organs become specialised, through unequal growth, first
into the primitive organs and then into the functional organs of the
body. "Single sections of the tubes originally formed from the layers
develop individual forms, which later acquire special functions: these
functions are in the most general way subordinate elements of the
function of the whole tube, but yet differ from the functions of other
sections. Thus the nerve-tube differentiates into sense-organs, brain
and spinal cord, the alimentary tube into mouth cavity, oesophagus,
stomach, intestine, respiratory apparatus, liver, bladder, etc. This
specialisation in development is bound up with increased or diminished
growth" (p. 155). Rapid growth concentrated at one point brings about an
evagination; in this manner are formed the sense-organs from the
nerve-tube, the liver and lungs from the alimentary tube. Or increased
growth over a section of a tube causes it to swell out; in this wise the
brain develops from the nerve-tube, the stomach from the alimentary
tube. The segmentation which soon becomes so marked, particularly in the
muscle layer, is also due to a process of morphological differentiation.

At the same time that the organs of the body are being thus roughly
blocked out and moulded from the germ-layers the third process of
differentiation is actively going on. "In addition to the
differentiation of the layers, there follows later another
differentiation in the substance of the layers, whereby cartilage,
muscle and nerve separate out, a part also of the mass becoming fluid
and entering the bloodstream" (p. 154). Through histological
differentiation the texture of the layers and incipient organs becomes
individualised. In its earliest appearance the germ consists of an
almost homogeneous mass, containing clear or dark globules suspended in
its substance (ii., p. 92). This homogeneity gives place to
heterogeneity; the structureless mass becomes fibrous to form muscles,
hardens to form cartilage or bone, becomes liquid to form the blood,
differentiates in a hundred other ways--into absorbing and secreting
tissues, into nerves and ganglia, and so forth. It will be noticed that
the concept of histological differentiation is independent of the
cell-theory; it signifies that textural differentiation which leads to
the formation of tissues in Bichat's sense. The tissues and the
germ-layers stand in fairly close relation with one another, for while
certain tissues are formed chiefly but not exclusively in one layer,
others are formed only in one layer and never elsewhere. For example,
peripheral nerves are for the most part formed in the muscle layer,
though the bulk of the nervous tissue is formed in the walls of the
nerve tube; similarly blood and blood-vessels may arise from almost any
layer, though their chief seat of origin is the vessel-layer; on the
other hand, bone is formed only in the muscle-layer (i., p. 155, ii.,
pp. 92-3).

This relation of tissue to germ-layer was more fully discussed and
brought into greater prominence by Remak, from the standpoint of the
cell-theory, and it will occupy us in a later chapter (Chap. XII.).

The fourth Scholion elaborates the analysis of developmental processes
still further, and discusses in particular the scheme of development
which is shown by the Vertebrata. The characteristic structure of the
vertebrate body is brought about by a "double symmetrical" rolling
together of the germ-layers, whereby two main tubes are formed, one
above and one below the axis of the body, which is the chorda. The
dorsal tube is formed by the two animal layers, the ventral tube by all
the layers combined (see Fig. 7).

The process is indicated with sufficient clearness in the diagram. It
will be seen that the real foundation and framework of the arrangement
is the muscle-layer, with its two tubes, one surrounding the central
nervous system and forming the "dorsal plates," the other surrounding
the body cavity and forming the "ventral plates." In the dorsal plates,
which early show metameric segmentation, the investing skeleton of the
neural axis develops; in the ventral plates are formed the ribs, the
ventral arches of the vertebræ, the hyoid, the lower jaw and other
skeletal structures.

The alimentary or "mucous" tube and the part of the vessel layer which
invests it become so closely bound up with one another as to form a
single primitive organ--the alimentary canal. The muscles of the
alimentary canal are accordingly in all probability developed in the
investing part of the vessel layer. From the "Gekröse," or remaining
part of the vessel layer develop the Wolffian bodies (_Urnieren_,
Pronephros), the kidneys, the sex glands, and the series of
"blood-glands"--suprarenals, thyroid, thymus and spleen. Baer did not
attach any special morphological significance to the peritoneal lining
of the body cavity, as is done in more modern forms of the germ-layer
theory. The gill-slits were largely formed by outgrowths from the
alimentary canal.

_a._ Chorda.
_b._ Dorsal plates.
_c._ Ventral plates.
_d._ Spinal cord.
_e._ Vessel-layer.
_f._ Alimentary tube.
_g._ Pronephros.
_h._ Skin.
_i._ Amnion.
_k._ Serous membrane.
_l._ Yolk-sac.

In his germ-layer theory von Baer was influenced a good deal by
Pander, to whom the actual discovery of the process of layer-formation
is due. Pander, however, had distinguished only three germ-layers, an
upper "serous" layer, a lower "mucous" layer and a middle
"vessel-layer." He it was who introduced the terms "Keimhaut"
(blastoderm) and "Keimblatt" (germ-layer).

[Illustration: FIG. 7.--Ideal Transverse Section of a Vertebrate Embryo.
(After von Baer.)]

The honour of being the founder of the germ-layer theory is sometimes
attributed to C. F. Wolff, notably by Kölliker and O. Hertwig. Wolff, it
is true, in his memoir _De formatione intestinorum_ (1768-9) showed that
the alimentary canal was first formed as a flat plate which folded round
to form a tube, and in a somewhat vaguely worded passage he hinted that
a similar mode of origin might be found to hold good for the other
organ-systems. But it seems clear that Wolff had no definite conception
of the process of layer-formation as the first and necessary step in all
differentiation. This, at any rate, was von Baer's opinion, who assigns
to Pander the glory of the discovery of the germ-layers. "You," he
writes, "through your clearer recognition of the splitting of the
germ--a process which remained dark to Wolff--have shed a light upon all
forms of development" (p. xxi.).

We have now seen, following von Baer's exposition, how development is
essentially a process of differentiation, a progress from the general to
the special, from the homogeneous to the heterogeneous; we have analysed
the process into its three subordinate processes--primary, histological
and morphological differentiation. So far we have considered development
in general and the laws which govern it; we have now to consider the
varieties of development which the animal kingdom offers in such
profusion, in order to discover what relations exist between them. This
is the problem set in the fifth Scholion. Baer at once brings us face to
face with the solution of the problem attempted in the Meckel-Serres
law. It is a generally received opinion, he writes, that the higher
animals repeat in their development the adult stages of the lower, and
this is held to be the essential law governing the relation of the
variety of development to the variety of adult form. This opinion arose
when there was little real knowledge of embryology; it threw light
indeed upon certain cases of monstrous development, but it was pushed
altogether too far. It complicated itself with a belief in a historical
evolution;--"People gradually learnt to think of the different animal
forms as developed one from another--and seemed, in some circles at
least, determined to forget that this metamorphosis could only be
conceptual" (p. 200). At the same time the theory of parallelism led men
to rehabilitate the outworn conception of the scale of beings, to
maintain that animals form one single series of increasing complexity, a
scale which the higher members must mount step by step in their
development--from which it followed that evolution, whether conceived as
an ideal or as an historical process, could take place only along one
line, could be only progressive or regressive. Not all the supporters of
the theory of parallelism held these extreme views, but conclusions of
this kind were natural and logical enough.

Von Baer had soon found in the course of his embryological studies that
the facts did not at all fit in with the doctrine of parallelism; the
developing chick, for example, was at a very early stage demonstrably a
Vertebrate, and did not recapitulate in its early stages the
organisation of a polyp, a worm or a mollusc. He had published his
doubts in 1823, but his final confutation of the theory of parallelism
is found in this Scholion.

If it were true, he says, that the essential thing in the development of
an animal is this repetition of lower organisations, then certain
deductions could be drawn, which one would expect to find confirmed in
Nature. The first deduction would be that no structures should appear in
the embryo of the higher animals that are not found in the lower
animals. But this is not confirmed by the facts--no adult among the
lower animals, for instance, has a yolk-sac like that of the chick
embryo. Again, if the law of parallelism were true, the mammalian embryo
would have to repeat the organisation of, among other groups, insects
and birds. But the embryo _in utero_ is surrounded by fluid and cannot
possibly breathe free air, so it cannot possibly repeat the structure of
either insects or birds, which are pre-eminently air-organisms.
Generally speaking, indeed, we find in all the higher embryos special
structures which adapt them to the very special conditions of their
development, and these we never find as permanent structures in the
lower animals. The supporters of the theory of parallelism might,
however, admit the existence of such special embryonic organs without
greatly prejudicing their case, for these temporary organs stand to some
extent outside the scope of the theory.

But they would have to face a second and more important deduction from
their views, namely, that the higher animals should repeat at every
stage of their development the whole organisation of some lower animal,
and not merely agree with them in isolated details of structure. The
deduction is, however, not borne out by the facts. The embryo of a
mammal resembles in many points, at different stages of its development,
the adult state of a fish; it has gill-slits and complete aortic arches,
a two-chambered heart, and so on. But at no time does it combine all the
essential characters of a fish; nor has it ever the tail of a fish, nor
the fins, nor the shape. Any recapitulation there may be is a
recapitulation of single organs, there is never a repetition of the
complete organisation of a fish. This is indeed the fundamental
criticism of the theory of parallelism; and if it applies even within
the limits of the vertebrate phylum, so much the more does it apply to
comparisons between embryonic Vertebrates and adult Invertebrates.

There are also some lesser arguments which might be urged against the
theory of parallelism. If the theory were strictly true, no state which
is permanent in a higher animal could be passed through by an animal
lower in the scale. But birds, which are lower in the scale than
mammals, pass through a stage in which they resemble mammals in certain
respects much more than they do when adult, for in an embryonic
condition they agree with mammals in having no feathers, no air sacs, no
pneumatic sacs in the bones, no beak. Their brain also resembles that of
mammals more in an earlier stage than it does later. So, too, myriapods
and hydrachnids have at birth three pairs of feet, and resemble at this
stage adult insects, which form a higher class.

Again, were the analogy between the development of the individual and
the evolution of the _Échelle des êtres_ complete, organs and
organ-systems ought to develop in the individual in the order in which
they appear in the scale of beings. But this is not always the case. In
fish the hinder extremity develops only its terminal joint, while in the
embryos of higher animals the basal joint is the first to appear.

Another consequence one would expect to find realised, were the theory
of parallelism correct, is the late appearance in development of parts
which are confined to the higher animals. In the development of a
Vertebrate accordingly one would not expect the vertebræ to appear
before the embryo had passed through many Invertebrate stages. But
experience shows the direct contrary, for in the chick the rudiments of
the vertebral axis appear sooner than any other part.

The theory of parallelism or recapitulation then is not borne out by the
facts, and clearly cannot be the law which we are seeking. But what then
is the true relation between the variety of development and the variety
of adult structure? Before answering this question we must review the
varied forms of adult organisation and consider in what relations they
stand to one another. In particular we must enquire whether they belong
to one type or to many. One point is here cardinal--we must distinguish
between the _type_ of organisation and the _grade_ of differentiation.
By "type" von Baer means the structural plan of the organism. "I call
the _type_ the spatial relationship of the organic elements and organs"
(p. 208). Each type of organisation characterises one of the big groups
of animals; the lesser groups represent "grade" modifications of the
type. "The product of the degree of differentiation and the type gives
the several great groups of animals which are called classes" (p. 208).
_Ausbildung_ (differentiation) takes place in one or other of several
directions, in adaptation, for instance, to life in the water or to life
in the air.

There are, von Baer considers, four main types--(1) the peripheral or
radiate type, (2) the longitudinal type, (3) the massive or molluscan
type, (4) the vertebrate type. The radiate type is shown by discoid
infusoria, by medusæ, by starfish and their allies. The longitudinal
type characterises such genera as _Vibrio_, _Filaria_, _Gordius_, and
all the annulate animals. Mollusca, rotifers, polyzoa, and such
infusoria as are not included in types (1) and (2) belong to the massive
type, in which the body and its parts form rounded masses. The
longitudinal type is predominantly "animal," the massive type
predominantly "plastic" (vegetative). The vertebrate type has both the
"animal" and the "plastic" organs highly developed. In the symmetrical
arrangement of the animal parts it resembles the longitudinal type; its
plastic parts with their asymmetrical arrangement and rounded shape
belong to the massive type.

These types of von Baer inevitably recall the "Embranchements" of
Cuvier, with which they more or less coincide. It seems that von Baer
arrived at his types (from the study of adult structure) independently
of Cuvier, though the priority of publication rests with Cuvier.[174]

Now it is clear that the development of the individual, which is
essentially an _Ausbildung_, a differentiation, is directly comparable
with the grade-differentiation of forms within the type. And just as the
type rules all its varied modifications, so does the development of the
individual take place always within the bounds imposed by type. This is
von Baer's chief contribution to the theory of embryonic
relationships--the law that "the type of organisation determines the
manner of development" (p. xxii.). Development is not merely from the
general to the special--there are at least four distinct "general"
types, from which the special is developed. The type is fixed in the
very earliest stages of development--the embryo of a Vertebrate is from
the very beginning a Vertebrate (p. 220), and it shows at no time any
agreement in total organisation with any Invertebrate. The types are
independent of one another; differentiation and development follow a
different course in each of them. Not but what some analogies can be
found between the very earliest stages of embryos of different type.
Thus vertebrate and annulate embryos agree in certain points at the time
of the formation of the primitive streak. And in the earliest stage of
all, the egg-stage, there is probably agreement between all the types.
In eggs with yolk, whether vertebrate or annulate, there is always a
separation into an animal and a plastic layer. It seems, too, as if a
hollow sphere were a constant stage in the development of all animals
(pp. 224, 258). Apart from these analogies, development takes an
entirely independent course in each of the four main types, and no
embryo of one of the higher types repeats in its development the
peculiar organisation of any adult of the lower types.

If we consider now development within the type, which is the only
legitimate thing to do, we arrive at certain laws governing the relation
of embryos to one another. For instance, at a certain stage vertebrate
embryos are uncommonly alike. Von Baer had two in spirit which he was
unable to assign to their class among amniotes; they might have been
lizard, bird, or mammal, he could not say definitely which.[175] Generally
the farther back we go in the development of Vertebrates the more alike
we find the embryos. The type-characters are first to appear, then the
class characters, then the characters distinguishing the lesser
classificatory groups. "From a more general type the special gradually
emerges" (p. 221). The chick is first a Vertebrate, then a
land-vertebrate, then a bird, then a land-bird, then a gallinaceous
bird, and finally _Gallus domesticus_. Development within the type is a
progress from the general to the special, a real evolution. The more
divergent two adults are, the farther back we must go in their
development to find an agreement between their embryos. We can sum up
the case in the following laws:--

"(1) _That the general characters of the big group to which the embryo
belongs appear in development earlier than the special characters._ In
agreement with this is the fact that the vesicular form is the most
general form of all; for what is common in a greater degree to all
animals than the opposition of an internal and an external surface?

"(2) _The less general structural relations are formed after the more
general, and so on until the most special appear._

"(3) _The embryo of any given form, instead of passing through the state
of other definite forms, on the contrary separates itself from them._

"(4) _Fundamentally the embryo of a higher animal form never resembles
the adult of another animal form, but only its embryo_" (p. 224).

These laws relating to development within the limits of type are
destructive of even a limited application of the theory of parallelism,
for not even within the limits of the type is there a real scale which
the higher forms must mount; each embryo develops for itself, and
diverges sooner or later from the embryos of other species, the
divergence coming earlier the greater the difference between the adult
forms. It is only because the lower less-differentiated adult forms
happen to be little divergent from the generalised or embryonic type,
that they show a certain similarity with the embryos of the higher more
differentiated members of the group. Such similarity, however, is due to
no necessary law governing the development of the higher animals; it is,
on the contrary, merely a consequence of the organisation of these lower
animals (p. 224).

Von Baer goes on to show what are the distinguishing embryological
characters of the types and classes, working out a dichotomous schema of
development, which each embryo must follow, branching off early or late
to its terminal point, according to the lower or higher goal it has to
reach.

One important consequence for morphology results from von Baer's laws of
differentiation within the type. If the embryo develops from the general
to the special, then the state in which each organ or organ-system first
appears must represent the general or typical state of that organ within
the group. Embryology will therefore be of great assistance to
comparative anatomy, whose chief aim it is to discover the generalised
type, the common plan of structure, upon which the animals of each big
group are built. And the surest way to determine the true homologies of
parts will be to study their early development. "For since each organ
becomes what it is only through the manner of its development, its true
value can be recognised only from its method of formation. At present,
we form our judgments by an undefined intuition, instead of regarding
each organ merely as an isolated product of its fundamental organ, and
discerning from this standpoint the correspondences and dissimilarities
in the different types" (p. 233). Parts, therefore, which develop from
the same "fundamental organ," and in the last resort from the same
germ-layer, have a certain kinship, which may even reach the degree of
exact homology.

Now since the mode of development in each type is peculiar to that type,
organs of the same name in different types must not necessarily be
accounted homologous, even if they correspond exactly with one another
in their general _functional_ relations to the rest of the organs. Thus
the central nervous system of Arthropods must not be homologised with
the central nervous system of Vertebrates, for it develops in a
different manner. So, too, the brain of Arthropods or of Mollusca is not
strictly comparable with the brain of Vertebrates. Again, the air-tubes
or tracheæ of insects are, like the trachea and bronchi of many
Vertebrates, air-breathing organs. But the two organs are not
homologous, for the air-tubes of Vertebrates are developed from the
alimentary tube ("fundamental organ" of the alimentary system, developed
from the vegetative layer), while the air-tubes of insects arise either
by histological differentiation, or by invagination of the skin (p.
236). Organs can be homologous only within the limits of the big groups;
there can be no question of homology between members of different types.

The development of plants, like the development of animals, is
essentially a progress from the general to the special (p. 242).
Botanists have not been troubled by any recapitulation theory, and in
founding their big groups, Acotyledons, Monocotyledons, and
Dicotyledons, upon embryological characters, they were guided by true
principles, which ought indeed to be followed in zoology. If we knew the
development of all kinds of animals sufficiently well, then the best way
to classify them would be according to the characters they show in their
early development, for it is in early development that they show the
characters of the type in their most generalised form. As it is, we have
in our ignorance to establish the big groups by the study of adult
structure, but we find, on putting together all we know of comparative
embryology, that a classification of animals according to the mode of
their development gives, as is only natural, the same four groups as
does the study of adult structure. The four types of development are
thus:--

(1) The double-symmetrical, which is found in Vertebrates. It is called
the double-symmetrical, because in Vertebrates development takes place
from a central axis (notochord) in two directions, upwards and
downwards, in such a way that two tubes are formed, one above and one
below the axis. (2) The second type is the symmetrical, which is shown
by Annulates. A primitive streak is formed on the ventral surface of the
yolk; development proceeds symmetrically on both sides of the streak.
(3) Radiate development is probably typical of the radiate structural
type. (4) In the massive type, the development seems to be a spiral one.

Common to most modes is a separation of the germ into animal and plastic
layers, a separation which seems to be conditioned largely by the
presence of yolk. A classification based upon embryological characters
ought to be applied even to the lesser groups and would here prove
itself of service. Embryology, for instance, fully supports de
Blainville's separation of Batrachia from true reptiles,[176] for reptiles
develop an amnion and Batrachia do not.

We come now to the sixth and last Scholion. Development is a true
evolution of the special from the general, so runs von Baer's most
general law of all. This can be expressed in a slightly different way,
and the words which he chooses in the sixth Scholion to express this
final and most general result are these:--"The developmental history of
the individual is the history of the growing individuality in every
respect" (p. 263). The greatest modern treatise on embryology ends on a
splendid note. One creative thought rules all the forms of life. And
more--"It is this same thought that in cosmic space gathered the
scattered masses into spheres and bound them together in the solar
system, the same that from the weathered dust on the surface of the
metallic planets brought forth the forms of life. And this thought is
nought else but life itself, and the words and syllables in which life
expresses itself are the varied forms of the living" (p. 264).

Von Baer reminds one greatly of Cuvier. There is the same sheer
intellectual power, the same sanity of mind, the same synthetic grip.
Von Baer, like Cuvier, never forgot that he was working with living
things; he was saturated, like Cuvier, with the sense of their
functional adaptedness. In his paper on the external and internal
skeleton[177] he gives a masterly analysis of the functional modifications
of the limbs in Vertebrates, and the whole paper indeed, with its sober
attack on transcendentalism, is a vindication as much of the functional
point of view as of the importance of embryology.

Both Cuvier and von Baer, by the very sanity of their views, found
themselves in partial opposition to the theories current in their time.
Cuvier was the critic of Geoffroy and the transcendentalists, of Lamarck
and the believers in the _Échelle des êtres_, evolutionary or ideal. Von
Baer also, though influenced greatly by _Naturphilosophie_, turned
against the exaggerations of the transcendental school, and by his
unanswerable criticism of the theory of parallelism took away the ground
from those who too easily believed in an historical evolution.[178]

We have seen what were von Baer's criticisms of the theory of
parallelism. If we turn to the later writings of Cuvier we find the
essential criticism expressed in similar terms. Speaking of an attempt
which had been made to show that fish were molluscs developed to a
higher degree, he wrote in 1828,[179] "Let us draw the conclusion that
even if these animals can be spoken of as ennobled molluscs, as molluscs
raised to a higher power, or if they are embryos of reptiles, the
beginnings of reptiles, this can be true of them only in an abstract and
metaphysical sense, and that even this abstract statement would be very
far from giving an accurate idea of their organisation." From the fact
that the respiratory and circulatory organs of fish greatly resemble
those of tadpoles the conclusion has been drawn that fish are in a sense
embryos of Amphibia (p. 547). But this manner of viewing things is none
the less vicious, "for this reason ... that it considers only one or two
points and neglects all the others" (p. 548), and is directly contrary
to common sense. There is never a recapitulation of total organisations,
only at the most of single organs.

It will be remembered that Cuvier opposed and demolished the theory of
the _Échelle des êtres_, not only by showing that there were in Nature
four entirely different plans of animal structure, but also by
demonstrating that even the animals of each single _Embranchement_ could
not readily be arranged in one series, that a serial arrangement was
really valid only for their separate organs. Von Baer also held that
there are four distinct types of structure; he, too, combated the idea
of gradation within the limits of the type. In so far as species
represent successive stages in the development, the _Ausbildung_, of the
type, so far can the idea of a scale of beings be applied. But the
members of a type follow not one line of evolution but several diverging
lines, in direct adaptation to different environmental conditions, so
that a serial arrangement of them is not as a rule possible. It may be
possible to establish a serial arrangement of single organs from the
simplest to the most complex. But each organ or organ-system will
require a different serial arrangement, for the different systems vary
on different lines and an animal may be highly developed in respect of
one system and little developed in respect of all the others. Man, for
instance, is the highest animal only in respect of his nervous system.
The idea of the scale of beings has therefore only a very limited
application even within the limits of the type. Applied to the whole
animal kingdom it becomes merely absurd.

Another point of resemblance between Cuvier and von Baer was that
Cuvier, though essentially a student of adult structure, did recognise
the importance of embryology; following up some observations of
Dutrochet he studied the foetal membrane of mammals and tried to
establish their homologies.[180] And in his criticism of the vertebral
theory of the skull he advanced as an argument against the
basisphenoid being a vertebral centrum the fact (established
by Kerkring, 1670), that it develops from two centres.[181] Von Baer's
relation to transcendental anatomy was in some ways a close one, though
he was a trenchant critic of the extreme views of the school.[182] He took
from Oken the idea that a simple fundamental plan rules the organisation
of all Vertebrates; "That jaws and limbs are modifications of one
fundamental form is readily apparent, and, after Oken, the fact ought to
be accepted by the majority of those naturalists who do not refuse to
admit the existence of a general type from which the diversity of
structure is developed" (i., p. 192). He accepted the vertebral theory
of the skull in its main lines, and used his embryological knowledge to
support the idea that jaws correspond to limbs--the latter point as part
of the transcendental idea that the hind end of the body repeats the
organisation of the anterior part (i., p. 192). The particular form
which his theory of the relation of jaws to limbs took is shown in the
following passage:--"The maxillary bone has ... the significance of an
extremity and at the same time that of a rib or lower arch of a
vertebra, just as the pelvic bones unite in themselves the signification
of ribs and proximal members of the hinder extremity" (Meckel's
_Archiv_, p. 367, 1826).

He appreciated the morphological idea of the serial repetition of parts,
and gave it accurate formulation. The whole vertebrate body, he
considered, was composed of a longitudinal series of _morphological
elements_, each of which was made up a section from each of the
fundamental organs--a vertebra, a section of the nerve-cord, and so on
(_Entwickelungsgeschichte_, ii., p. 53). Groups of these morphological
elements formed _morphological divisions_, such as the vertebral
segments of the head with their highly developed neural arches, or the
segments of the neck with their undeveloped hæmal arches. The
morphological elements are clearly shown only in the animal parts, but
there are indications in the embryo of a segmentation also of the
vegetative parts,--the gill-slits, for instance, and the vascular
arches. The vegetative parts, however, develop on the whole
unsymmetrically (_cf._ Bichat). These elements which von Baer
distinguishes are morphological units, as he himself points out,
contrasting them with organs which are not usually units in a
morphological sense. "We call organ," he writes, "each part that has by
reason of its form or its function a certain distinctiveness, but this
concept is very indefinite, and possesses, from a morphological point of
view, little value. For this reason it seems necessary to introduce into
scientific morphology the concepts of morphological elements and
divisions" (ii., p. 84).

Von Baer exercised a very considerable influence upon the subsequent
trend of morphological theory. By his criticism of the Meckel-Serres
theory, he rid morphology for a time of an idea which was leading it
astray; by his substitution of the law that development is always from
the general to the special, he set morphologists looking for the
archetype in the embryo, not in the adult alone, and made them realise
that homologies could often best be sought in the earliest stages of
development; by formulating the germ-layer theory he supplied
morphologists with a new criterion of homology, based upon the special
relations of the parts (germ-layers) which are first differentiated in
all development. He made the study of development an essential part of
morphology.

    [166] _De generatione Animalium_.

    [167] _De formato foetu_, ? 1600; _De formatione
    foetus_, 1604.

    [168] _Exercitationes de generatione animalium_, 1651.

    [169] _De formatione pulli in ovo_, 1673; _De ovo
    incubato_, 1686.

    [170] _De formatione pulli in ovo_, 1757-8; _Sur la
    formation du coeur dans le poulet_, 1758.

    [171] _Theoria generatioinis_, 1759; _De formatione
    intestinorum_, 1768-9.

    [172] _Beiträge zur Entwickelung des Hühnchens im Ei._
    Würzburg, 1818. Also in Latin in shorter form, 1817.

    [173] _Untersuchungen ü. die Entwickelungsgeschichte der
    Fische_; Leipzig, 1835.

    [174] Cuvier, in 1812, _Ann. Mus. d'Hist. Nat._, xix.; von
    Baer in 1816, _Nova Acta Acad. Nat. Cur._ See
    _Entwickelungsgeschichte der Thiere_, i., p. vii., f.n.

    [175] Compare a parallel passage in Prévost et Dumas:--"At
    the very first sight one will be struck with the
    resemblance between the forms of the very early embryos
    of these two classes, a resemblance so extraordinary
    that one cannot refuse to admit the conclusions
    resulting from it. The resemblance is so striking that
    one can defy the most experienced observer to
    distinguish in any way the embryos of dog or rabbit ...
    from those of fowls or ducks of a corresponding
    age."--_Ann. Sci. nat._, iii., p. 132, 1824.

    [176] _De l'organisation des Animaux_, i., p. 140, 1822.

    [177] "Ueber das äussere und innere Skelet," Meckel's
    _Archiv für Anat. u. Physiol._, pp. 327-76, 1826. See,
    too, his _Entwickelungsgeschichte_, i., pp. 181, ff.

    [178] Von Baer wrote an appreciative biography of Cuvier,
    published posthumously in 1897, _Lebensgeschichte
    Cuviers_, ed. L. Stieda. French trans. in _Ann. Sci.
    Nat._ (_Zool._), ix., 1907.

    [179] Cuvier et Valenciennes, _Histoire naturelle des
    Poissons_, i., p. 550.

    [180] _Mém. Mus. d'Hist. Nat._, iii., pp. 98-119, 1817.

    [181] _Leçons d'Anatomie comparée_, 3rd ed., vol. i., p.
    414, Bruxelles, 1836.

    [182] In the aforementioned paper in Müller's _Archiv_ he
    criticises Carus vigorously and is sarcastic on
    Geoffroy.



CHAPTER X

THE EMBRYOLOGICAL CRITERION


Pander's work of 1817 was the forerunner of an embryological period in
which men's hopes and interest centred round the study of development.
"With bewilderment we saw ourselves transported to the strange soil of a
new world," wrote Pander, and many shared his hopeful enthusiasm. K. E.
von Baer's _Entwickelungsgeschichte_ was by far the greatest product of
this time, but it stands in a measure apart; we have in this chapter to
consider the lesser men who were Baer's contemporaries, friends,
followers or critics.

It was largely a German science, this new embryology, and its leaders
were all personally acquainted. Pander, von Baer and Rathke were on
friendly terms with one another; von Baer dedicated his master-work to
Pander; Rathke dedicated the second volume of his _Abhandlungen_ to von
Baer. Interest in the new science was, however, not confined to Germany.
In Italy, Rusconi commenced in 1817 his pioneer researches on the
development of the Amphibia with a _Descrizione anatomica degli organi
della circolazione delle larve delle Salamandre aquatiche_ (Pavia), in
which he traced the metamorphoses of the aortic arches. This was
followed in 1822 by his _Amours des Salamandres aquatiques_ (Milan), and
in 1826 by his memoir _Du développement de la grenouille_ (Milan). In
this last paper he described how the dark upper hemisphere of the frog's
egg grows down over the lower white hemisphere and leaves free only the
yolk plug; he observed the segmentation cavity and the archenteron, but
thought that the former became the alimentary canal; he observed and
interpreted rightly the formation of the medullary folds. The circular
blastopore in the frog in later years often went by the name of the anus
of Rusconi.

In France Dutrochet[183] investigated the foetal membranes in various
vertebrate classes; Prévost and Dumas studied the very earliest stages
of development in birds, mammals and amphibia (_Ann. Sci. nat._, ii.,
iii., 1824, xii., 1827).

A little later came Dugès' studies of the osteology and myology of
developing amphibia (1834),[184] and Coste's careful researches into the
early developmental history of mammals.[185]

[Illustration: FIG. 8.--Gill-slits of the Pig Embryo. (After Rathke.)]

It was in 1825 that Heinrich Rathke (1793-1860), published his famous
discovery of gill-slits in the embryo of a mammal,[186] a discovery which
aroused considerable interest, and greatly stimulated embryological
research. He describes how in a young embryo of a pig he saw four slits
in the region of the neck, going right through into the oesophagus. They
were separated by partitions which he called _Kiemenbogen_
(gill-arches), and immediately in front of the first gill-slit lay the
developing lower jaw. He compared these gill-slits with those of a
dogfish. We reproduce his drawing of the pig-embryo (_Isis_, Pl. IV.,
fig. 1).

Later in the same year Rathke discovered gill-slits in the chick,[187] in
this case finding only three. He described growing out from in front of
the first slit a structure which he compared to the operculum or
gill-cover of a fish.

These discoveries were confirmed and extended for the chick[188] by the
embryologist Huschke, a pupil of Oken. Like Rathke, he found only three
indubitable gill-slits, but he noticed that the body-wall in front of
the first gill-slit was really composed of two arches, which were on the
whole similar to the gill-arches. The hinder of these two seemed to him
to be a horn of the hyoid, the front one, which was bent at an angle, to
be the rudiment of the upper and lower jaws (p. 401). Between these two
arches he found an opening, just as between two gill-arches a gill-slit.
This opening led into the mouth-cavity, and according to Huschke it
became the external ear-passage. He discovered also three pairs of
aortic arches in close relation with the gill-arches, so close indeed,
that he did not hesitate to call them gill-arteries, and to recognise
their resemblance with the aortic arches of fish. He traced, in part at
least, the metamorphosis which these aortic arches undergo. This part of
his discovery he developed in fuller detail in a paper of 1828,[189] in
which he gave some excellent figures.

Shortly after Huschke's first paper, von Baer published his views and
observations on this subject in a short memoir in Meckel's _Archiv_.[190]
In this paper he confirmed Rathke's discovery, and described the slits
and arches in the dog and the chick. Both Rathke and he found gill-slits
in the human embryo about this time (p. 557). There were generally
present, he found, four gill-slits, and, as Rathke had suggested, the
first gill-arch became the lower jaw. Von Baer also confirmed Rathke's
discovery of the operculum, assigning it, however, to the second
gill-arch. He refused to accept Huschke's derivation of the auditory
meatus from the first gill-slit. Von Baer saw what had escaped Rathke
and Huschke, that there were, not three nor four, but as many as five
aortic arches.

In his view of the metamorphosis of the aortic arches in the chick the
first two pairs disappeared completely, the third pair gave rise to the
arteries of the head and the fore-limbs, the right side of the fourth
arch became the aorta, the left half of the fourth and the right half of
the fifth arch became the pulmonary arteries, while the left half of the
fifth arch disappeared. This schema, which for the last three arches was
the same as Huschke's, von Baer upheld for the chick even in the second
volume of his _Entwickelungsgeschichte_ (p. 116); he rectified it,
however, for mammals in the same volume (p. 212), deriving both
pulmonary arteries from the fifth arch, and the aorta from the fourth
left. He fully recognised the great analogy of the embryonic arrangement
of gill-arches and gill-arteries in Tetrapoda with their arrangement in
fish (i., pp. 53, 73).

Huschke, in a paper of 1832,[191] chiefly devoted to the development of
the eye, figured and described the developing upper and lower jaws, and
maintained against von Baer that the first slit turns into the auditory
meatus and the Eustachian tube.

These were the first papers of the embryological period. Before going on
to discuss the principles which guided embryological research during the
next ten or twenty years it is convenient to note what were the main
lines of work characterising the period.

The typical figure of the period is Rathke, who produced a great deal of
first-class embryological work. He was, even more than von Baer, a
comparative embryologist, and there were few groups of animals that he
did not study. His first large publication, the _Beiträge zur Geschichte
der Thierwelt_ (i.-iv., Halle, 1820-27), contained much anatomical work
in addition to the purely embryological; he commenced here his series of
papers on the development of the genital and urinary organs, continued
in the _Abhandlungen zur Bildungsund Entwickelungs-Geschichte des
Menschen und der Thiere_ (i., ii., Leipzig, 1832-3). A fellow-worker in
this line was Johannes Müller, whose _Bildungsgeschichte der Genitalien_
(Düsseldorf) appeared in 1830.

In a memoir on the development of the crayfish which appeared in
1829,[192] Rathke found in an Invertebrate confirmation of the germ-layer
theory propounded by Pander and von Baer. He was greatly struck by the
inverted position of the embryo with respect to the yolk. In following
out the development of the appendages he noticed how much alike were
jaws and legs in their earliest stage, and how this supported Savigny's
contention that the limbs of Arthropods belonged to one single type of
structure. In his paper (1832) on the development of the fresh-water
Isopod, _Asellus_,[193] Rathke returns to this point. Commenting on the
original similarity in development of antennæ, jaws and legs, he writes,
"Whatever the doubts one may have reserved as to the intimate relation
existing between the jaws and feet of articulate animals after the
researches of Savigny on this subject and mine on developing crayfish,
they must all fall to the ground when one examines with care the
development of the fresh-water Asellus" (p. 147 of French translation).

Further comparative work by Rathke is found in the two volumes of
_Abhandlungen_ and in a book, _Zur Morphologie, Reisebemerkungen aus
Taurien_ (1837), which contains embryological studies of many different
types, including a study of the uniform plan of arthropod limbs. Later
on Rathke devoted himself more to vertebrate embryology, producing among
other works his classical papers on the development of the adder (1839),
of the tortoise (1848), and of the crocodile (1866). He laid the
foundations of all subsequent knowledge of the development of the
blood-vascular system in a series of papers of various dates from 1838
to 1856. The diagrams in his paper on the aortic arches of reptiles
(1856) were for long copied in every text-book.

Rathke was a foremost worker in another important line of embryological
work, the study of the development of the skeleton and particularly of
the skull. We shall discuss the history of the embryological study of
the skull in some detail below; meantime, we note the two other
important lines of research which characterise this period. One is the
intensive study of the development of the human embryo, a study pursued
by, among others, Pockels, Seiler, Breschet, Velpeau, Bischoff, Weber,
Müller, and Wharton Jones.[194] The other important line--the early
development of the Mammalia--was worked chiefly by Valentin,[195]
Coste,[196] and, above all, by Bischoff, whose series of papers[197] was
justly recognised as classical.

What interests us chiefly in the work of this embryological period is,
of course, the relation of embryology to comparative anatomy and to pure
morphology. The embryologists were not slow to see that their work threw
much light upon questions of homology, and upon the problem of the unity
of plan. Von Baer, we have seen, recognised this clearly in 1828;
Rathke, in one of his most brilliant papers, the
_Anatomisch-philosophische Untersuchungen über den Kiemenapparat und das
Zungenbein_ (Riga and Dorpat, 1832), used the facts of development with
great effect to show the homology of the gill-arches and hyoid
throughout the vertebrate series; Johannes Müller made great use of
embryology in his classical _Vergleichende Anatomie der Myxinoiden_ (i.
Theil, 1836), and, according to his pupil Reichert, firmly held the
opinion that embryology was the final court of appeal in disputed points
of comparative anatomy;[198] Reichert himself in a book of 1838
(_Vergleichende Entwickelungsgeschichte des Kopfes der nackten
Amphibien_) discussed the two different methods of arriving at the
"Type"--the anatomical method of comparing adults, and the embryological
method of comparing embryogenies. Of the embryological method, he says,
"Its aim is to distinguish during the formation of the organism the
originally given, the essence of the type, and to classify and interpret
what is added or altered in the further course of development.
Embryologists watch the gradual building up of the organism from its
foundations, and distinguish the fundament, the primordial form, the
type, from the individual developments; they reach thus, following
Nature in a certain measure, the essential structure of the organism,
and demonstrate the laws that manifest themselves during embryogeny" (p.
vi.). The embryologists, influenced in this greatly by von Baer,
gradually felt their way to substituting for the "Archetype" of pure
morphology what one may perhaps best call the _embryological archetype_.
How the transition was made we can best see by following out the course
of discovery in one particular line. We choose for this purpose the
development of the skull, a subject which excited much interest at this
time and upon which much quite fundamental work was done, particularly
by Rathke and Reichert.

Following up his discovery of gill-slits and arches in the embryos of
birds and mammals, Rathke in two papers of 1832[199] and 1833[200] worked
out the detailed homologies of the gill-arches in the higher
Vertebrates. He describes how in the embryo of the Blenny there is a
short, thick arch between the first gill-slit and the mouth. A furrow
appears down the middle of the arch dividing it incompletely into two.
In the anterior halves a cartilaginous rod is developed which is
connected with the skull; these rods become on either side the lower jaw
and "quadrate." In the posterior halves two similar rods are formed
which develop into the hyoid. The hyoid is at first connected with the
skull, but afterwards frees itself and becomes slung to the "quadrate."
From the hinder edge of the hyoid arch grows out the membranous
operculum, in which develop later the opercular bones and branchiostegal
rays. The upper jaw is an independent outgrowth of the serous layer.

The serial homology of the lower jaw and quadrate with the hyoid and
with the true gill-arches was thus established in fish, and Rathke had
little difficulty in demonstrating a similar origin of lower jaw and
hyoid in the embryos of higher Vertebrates. He could even, as we have
noted before, find the homologue of the operculum in a flap which grows
out from the hyoid arch in the embryo of birds.

But Rathke could not altogether shake himself free from the
transcendental notion of the homology of jaws with ribs, and this led
him to draw a certain distinction between the first two and the
remaining gill-arches, by which the homology of the former with the ribs
was asserted and the homology of the latter denied. He thought he could
show that the skeletal structures (lower jaw, "quadrate," and hyoid) of
the first two arches were formed in the serous layer, just like true
ribs, and like them in close connection with the vertebral skeletal
axis. The other, "true," gill-arches appeared to him to be formed in the
mucous layer, in the lining of the alimentary canal. They had no direct
connection with the vertebral column, and seemed therefore to belong to
what Carus[201] had called the visceral or splanchno-skeleton. He did not,
however, let this distinction hinder him from asserting the substantial
homology of all the gill-arches _inter se_, the first two included.

Rathke's discoveries relative to the development of the jaws, the hyoid
and the operculum, enabled him to make short work of the homologies
proposed for them by the transcendentalists. He could prove from
embryology that the jaws were not the equivalent of limbs, as so many
Okenians believed. He could reject, with a mere reference to the facts
of development, Geoffroy's comparison of the hyoid and the
branchiostegal rays in fish with sternum and ribs. He could show the
emptiness of the attempts made by Carus, Treviranus, de Blainville and
Geoffroy, to establish by anatomical comparison the homologies of the
opercular bones, for he could show that these bones were peculiar to
fish, and were scarcely indicated, and that only temporarily, in the
development of other Vertebrates.[202] He did not, however, himself
realise the relation of the ear-ossicles to the gill-arches, though he
knew that Spix and Geoffroy were quite wrong in homologising them with
the opercular bones in fish. He described, it is true, the development
of the external meatus of the ear and the Eustachian tube from the slit
which appears between the first and the second arch, as Huschke had done
before him; he described, in confirmation of Meckel, the "Meckelian
process" of the hammer running down inside the lower jaw; but the
discovery of the true homologies of the ear-ossicles was not made until
a year or two later by Reichert.

In his further study of the development of _Blennius viviparus_, Rathke
observed some important facts about the development of the vertebral
column and skull. He found that the vertebral centra were first formed
as rings in the chorda-sheath, which give off neural and hæmal
processes. The vertebra later ossifies from four centres. The chorda
(notochord) is prolonged some little way into the head, and the base of
the cranium is formed by the expanded sheath, which reaches forward in
front of the end of the notochord. This cranial basis shows a division
into three segments, in which Rathke was inclined to see an indication
of three cranial vertebræ. (It turned out that this division into three
segments did not really exist, and Rathke later acknowledged that he had
made an error of observation.) The side walls of the skull grow out from
this base and form a fibrous capsule for the brain. The cranial section
of the chorda itself shows no sign of segmentation; but later on the
cranial portion of the chorda-sheath ossifies, like the vertebræ, from
several centres. The vomer, which, in the classical form of the
vertebral theory of the skull, was the centrum of the fourth, or
foremost, cranial vertebra, does not, according to Rathke, develop in
continuity with the cranial basis and the chorda sheath, but develops
separately in the facial region.

Von Baer, like Rathke at this time, was also to some extent a believer
in the vertebral theory of the skull. In his second volume (1834, pub.
1837) he holds that the development of the skull, as the sum of the
anterior vertebral arches, is in general the same as that of the other
neural arches, and is modified only by the great bulk of the brain
(_Entwickelungsgeschichte_, ii., p. 99). He had, however, some doubts as
to the entire correctness of the vertebral theory, doubts suggested by a
study of the developing skull. "In the course of the formation of the
head in the higher animals, something additional is introduced which
does not originally belong to the cranial vertebræ. At first we see the
vertebration in the hinder region of the skull very clearly. Afterwards
it becomes suddenly indistinct, as if some new formation overlaid it"
(i., p. 194).

Even more clearly is his doubt expressed in his paper on _Cyprinus_.
"Upon the formation of the vertebral column only this need be said, that
at this stage the notochord is very clearly seen, and the upper and
lower arches and spinous processes are visible right to the end of the
tail, but the separation into vertebræ ceases abruptly where the back
passes into the head. I do not hesitate to assert _that bony fish, too,
have at this stage an unsegmented cartilaginous cranium_ (as
cartilaginous fish have all their life), the prominences and hollows of
which constitute its only resemblance with the vertebral type" (1835, p.
19).

A convinced supporter of the vertebral theory was Johannes Müller, who,
in his classical memoir on the Myxinoids,[203] discussed at some length
the relation between the development of the vertebræ and the development
of the skull. His memoir is principally devoted to comparative anatomy,
but in treating of the skeleton he pays much attention to development.
He describes the formation of the vertebræ in elasmobranch embryos; for
the facts regarding other Vertebrates he relies largely on work by
Rathke (_Blennius_, 1833) and Dugès (1834). He recognises as the basis
of his comparisons the homology of the notochord in all vertebrate
embryos with the persistent notochord which forms the chief part or the
whole of the vertebral column in the Cyclostomes. The notochord
possesses an inner and an outer sheath and the outer sheath is
continuous with the _basis cranii_ (p. 92). It is in the outer sheath
that the vertebræ develop--from four separate pieces, in fish at least,
plus an additional element which helps to form the centrum. The skull of
Vertebrates consists, according to Müller, of three vertebræ, whose
centra are the basioccipital, the basisphenoid and the presphenoid.
Other bones besides those belonging to the vertebræ are present, but
this formation out of three vertebræ gives the essential schema for the
skull. Now the brain capsule, like the sheath of the spinal cord, is a
development from the outer sheath of the notochord. If the skull
consists of vertebræ we should expect the centra of the skull-vertebræ
to develop in the outer sheath at the sides of the cranial section of
the notochord as two separate halves, just as do the bodies of the
vertebræ; we should expect further the cartilaginous side-walls of the
cranium to develop in the membranous brain-sheath just as the neural
arches develop in the membranous sheath of the spinal column. In
Rathke's discovery (!) of a segmentation of the _basis cranii_ into
three parts, and of the isolated formation of the vomer, Müller sees a
confirmation of his view that the skull is composed of three and not
four vertebræ. But there is nothing in Rathke's observations to support
the idea that the centra of the cranial vertebræ are formed from
separate halves. Müller has to be content with a reference to the state
of things in _Ammocoetes_ (which, by the way, he did not know to be the
young of _Petromyzon_). In the simple skull of _Ammocoetes_ the base is
formed chiefly by two cartilaginous bars lying more or less parallel
with the longitudinal axis of the skull and embracing with their hinder
ends the cranial portion of the notochord.

These bars, declares Müller, are clearly the still separate halves of
the _pars basilaris cranii_, and represent the divided centra of the two
hinder cranial vertebræ. To complete the parallel between the
development of the skull and of the vertebræ, it would have been
necessary to show that the side walls of the cranium developed in a
similar manner from separate pieces. Müller could not prove this point
from the available embryological data, and indeed the facts which he did
use had to be twisted to suit his theory. A curious apparent
confirmation of his idea that the centra of the cranial vertebræ are
formed from separate halves was supplied in 1839 by Rathke's discovery
of the trabeculæ in the embryonic skull of the adder.

The next big step in the study of the development of the skull was
taken by a pupil of Müller, C. B. Reichert, who showed in his work
very distinct traces of his master's influence. Reichert's first and
most important contribution to the subject was his paper on the
metamorphosis of the gill, or, as he called them, the visceral arches
in Vertebrates,[204] particularly in the two higher classes. Reichert
describes the similar origin in embryo of bird and mammal (pig) of
three "visceral" arches. These arches stand in close relation to the
three cranial vertebræ which Reichert, like Müller, distinguishes. He
makes the retrograde step of admitting only three aortic arches, and
he is not inclined to consider the three visceral arches as equivalent
to the gill-arches of fish--in his opinion they have more analogy with
ribs, though differing somewhat from ribs in their later
modifications. The visceral arches are processes of the visceral
plates (von Baer), which grow downwards and meet in the middle line,
leaving between one another and the undivided body wall three visceral
slits opening into the pharynx. The first visceral process is
different in shape from the others, for it sends forward, parallel
with the head and at right angles to its downward portion, an upper
portion in which later the upper jaw is formed. The other two
processes are straight. From the hinder edge of the second visceral
arch there develops, as Rathke had seen, a fold which is comparable
with the operculum of fish. The first slit develops externally into
the ear-passage, internally into the Eustachian tube, and in the
middle a partition forms the tympanic ring and tympanum. Inside each
of the visceral processes on either side a cartilaginous rod develops.
In the first process this rod shows three segments, of which the first
lies inside that portion of the process which is parallel with the
head. This upper segment forms the foundation for the bones of the
upper jaw. The lowest segment of the cartilaginous rod becomes
Meckel's cartilage, and on the outer side of this the bones of the
lower jaw are formed. The middle segment becomes in mammals the incus
(one of the ear-ossicles), and in birds the quadrate. Meckel's
cartilage, which was discovered by Meckel[205] in fish, amphibians and
birds, is a long strip of cartilage which runs from the ear-ossicle
known as the hammer in mammals,[206] to the inside of the mandible.
Reichert shows how this relation comes about. The hammer, according to
his observations on the embryo of the pig, is simply the proximal end
of Meckel's cartilage, which later becomes separated off from the long
distal portion (see Fig. 9). The third ear-ossicle of mammals, the
stapes, comes not from the first arch but from the second. The
cartilaginous rod of the second arch segments like the first into
three pieces. Of these the uppermost disappears, the middle one, which
lies close up to the labyrinth of the ear, becomes the stapes, and the
lowest becomes the anterior horn of the hyoid. The stapes forms a
close connection with the hammer and the incus. In birds, where there
is a single ear-ossicle, the columella, the middle piece of arch I
forms, as we have seen, the quadrate, by means of which the lower jaw
is joined to the skull. The proximal end of Meckel's cartilage, which
in mammals forms the hammer, here gives the articular surface between
the lower jaw and the quadrate. The columella is formed from the
middle piece of the three into which the cartilage of the second arch
segments. It is, therefore, the homologue of the stapes in mammals.
The third arch takes a varying share, together with the second, in the
formation of the hyoid apparatus.

[Illustration: FIG. 9.--Meckel's Cartilage and Ear-ossicles in Embryo
of Pig. (After Reichert.)]

In this paper Reichert made a distinct advance on the previous workers
in the same field--Rathke, Huschke, von Baer, Martin St Ange, Dugès.
Huschke was indeed the first to suggest that both upper and lower jaws
were formed in the first gill-arch. But both von Baer and Rathke[207] held
that the upper jaw developed as a special process independent of the
lower jaw rudiment, and the actual proof that the upper jaw is a
derivative of the first visceral arch seems to have been first supplied
by Reichert. His brilliant work on the development of the ear-ossicles
founded what we may justly call the classical theory of their
homologies. His views were attacked and in some points rectified, but
the main homologies he established are even now accepted by many,
perhaps the majority of morphologists.

In a paper of 1838 on the comparative embryology of the skull in
Amphibia,[208] Reichert added to his results for mammals and birds an
account of the fate of the first and second visceral arches in Anura and
Urodela.

The first visceral arch, he found, gave in Amphibia practically the same
structures as in the higher Vertebrates. Its skeleton segmented, as in
mammals and birds, into three parts; the upper part gave rise to the
palatine and pterygoid in Anura, but seemed to disappear in Urodeles,
where the so-called palatine and pterygoid developed in the mucous
membrane of the mouth; the middle part gave, as in birds, the quadrate,
which formed a suspensorium for both arches; the lower part, as Meckel's
cartilage, formed a foundation for the bones of the lower jaw. Of arch
II., the lower part became the horn of the hyoid, the upper part had a
varying fate. In some Anura it formed the ossicle of the ear (homologue
of the columella of birds and the stapes of mammals), in others it
disappeared. In reptiles the upper segment of the second arch formed, as
in birds, the columella.

The account of the metamorphoses of the visceral arches in Amphibia
forms only a small part of Reichert's memoir of 1838, the chief object
of which was to discover the general "typus" of the vertebrate skull,
and to follow out its modifications in the different classes. Von Baer
had shown that the generalised type appeared most clearly in the early
embryo; Reichert therefore sought the archetype of the skull in the
developing embryo. He brought to his task the preconceived notion that
the skull could be reduced to an assemblage of vertebræ, but he saw that
comparative anatomy alone could not effect this reduction; he had
recourse, therefore, to embryology, hoping to find in the simplified
structure of the embryo clear indications of three primitive cranial
vertebræ (p. 121, 1837).

In the head he distinguished two tubes, the upper formed by the dorsal
plates, the lower by the ventral or visceral plates. Both of these tubes
were derived from the serous or animal layer (_cf._ von Baer, _supra_,
p. 118). The walls of the lower tube were formed by the visceral
processes, within which later the skeleton of the visceral arches
developed. The walls of the upper tube formed the bones and muscles of
the cranium proper. The facial part of the head was formed by elements
from both upper and lower tubes. The dorsal tube showed signs of a
division into three cranial vertebræ (_Urwirbeln_, primitive vertebræ).
In mammals and birds, as Reichert had shown in his 1837 paper, the three
cranial vertebræ were indicated by transverse furrows on the ventral
surface of the still membranous skull (see Fig. 10, p. 148).

Even in mammals and birds, however, the positions of the eye, the
ear-labyrinth, and the three visceral arches were the safest guides to
the delimitation of the cranial vertebræ (pp. 134-138, 1837). In
Amphibia generally there were no definite lines of separation on the
skull itself. "At this stage," he writes of the cartilaginous cranium of
the frog, "we find no trace of a veritable division into vertebræ in the
cartilaginous trough formed by the _basis cranii_ and the side parts. On
the contrary, it is quite continuous, as it is also in the higher
Vertebrates during the process of chondrification" (p. 44, 1838). The
vertebræ in the membranous or cartilaginous skull could be delimited in
Amphibia by the help of the eye and the ear-labyrinth, which lie more or
less between the first and second, and the second and third vertebræ,
but, above all, by the vesicles of the brain.

As in the higher Vertebrates, the visceral arches are associated with
the cranial vertebræ as their ventral extensions, being equivalent to
the visceral plates which form the ventral portion of the "primitive
vertebræ" or primitive segments of the trunk.

[Illustration: FIG. 10.--Cranial Vertebræ and Visceral Arches in Embryo
of Pig. Ventral Aspect. (After Reichert.)]

If the three cranial vertebræ are not very distinct in the early stages
of development when the skull is still membranous or cartilaginous, they
become clearly delimited when ossification sets in. Three rings of bone
forming three more or less complete vertebræ are the final result of
ossification. The composition of these rings is as follows:--

+-------------------------------------------------------------------+
|                |   Base.       |    Sides.       |  Top.          |
|----------------+---------------+-----------------+----------------|
|First vertebra  |Presphenoid    |Orbitosphenoids  |Frontals        |
|                |               |                 |                |
|Second vertebra |Basisphenoid   |Alisphenoids     |Parietals       |
|                |               |                 |                |
|Third vertebra  |Basioccipital  |Exoccipitals     |Supraoccipital  |
+-------------------------------------------------------------------+

The other bones of the skull are not included in the vertebræ, and this
is in large part due to the fact that the sense capsules are formed
separately from the cranium (p. 29, 1838). The ear-labyrinth, it is
true, fuses indissolubly with the cranium at a later period, but the
bones which develop in its capsule are not for all that integral parts
of the primitive cranial vertebræ. This point, it is interesting to
note, had already been made by Oken in his _Programm_ (1807). But many
of the bones developed in relation to the sense organs can find their
place in the generalised embryonic schema or archetype of the vertebrate
skull, for they are of very constant occurrence during early
development.

Having arrived at a generalised embryonic type for the vertebrate skull,
of which the fundamental elements are the three cranial vertebræ and
their arches, Reichert goes on to discuss the particular forms under
which the skull appears in adult Vertebrates. He accepts in general von
Baer's law that the characters of the large groups appear earlier in
embryogeny than the characters of the lesser classificatory divisions.
"When we observe new and not originally present rudiments in very early
embryonic stages, as, for instance, that for the lacrymals, the
probability is that they belong to the distinctive development of one of
the _larger_ vertebrate groups. From these are to be carefully
distinguished such rudiments as arise later during ossification, mostly
as _ossa intercalaria_, in order to give greater strength to the skull
in view of the greater development of the brain, etc.; the latter give
their individual character to the _smaller_ vertebrate groups, and
comprise such bones as the _vomer_, the _Wormian bones_, the lowermost
turbinal, etc." (p. 63, 1838).

He did not accept the Meckel-Serres law of parallelism. He recognised
the great similarity between the unsegmented cartilaginous cranium of
Elasmobranchs, and the primordial cranium of the embryos of the higher
Vertebrates, but he did not think that the cranium of Elasmobranchs was
simply an undeveloped or embryonic stage of the skulls of the higher
forms. Rather "do the _Holocephala_, _Plagiostomata_, and _Cyclostomata_
appear to us to be lower developmental stages individually
differentiated, so that the other fully differentiated Vertebrates
cannot easily be referred directly to their type" (p. 152, 1838). The
skull of these lower fishes is itself a specialised one; it is an
individualised modification of a simple type of skull. And this holds
good in general of the skulls of the lower Vertebrates--they are
individualised exemplars of a simple general type, not merely unmodified
embryonic stages of the greatly differentiated skulls of the higher
Vertebrates (p. 250, 1838). Differentiation within the vertebrate phylum
is therefore not uniserial, but takes place in several directions.
Reichert describes two sorts of modifications of the typical
skull--class modifications and functional modifications. The causes of
the modifications which characterise classificatory groups are unknown;
the second class of modifications occur in response to adaptational
requirements.

Reichert's two papers are of considerable importance, and Müller's
remark in his review[209] of them is on the whole justified. "These
praiseworthy investigations supply from the realm of embryology new and
welcome foundations for comparative anatomy" (p. clxxxvii.).

The development of the skull was, however, more thoroughly worked out by
Rathke, and with less theoretical bias, in his classical paper on the
adder.[210] This memoir of Rathke's is an exhaustive one and deals with
the development of all the principal organ-systems, but particularly of
the skeletal and vascular. He confirmed in its essentials Reichert's
account of the metamorphoses of the first two visceral arches,
describing how the rudiment of the skeleton of the first arch appears as
a forked process of the cranial basis, the upper prong developing into
the palatine and pterygoid, the lower forming Meckel's cartilage, while
the quadrate develops from the angle of the fork. The actual bone of the
upper jaw (maxillary) develops outside and separate from the
palato-pterygoid bar. The cartilaginous rod supporting the second
visceral arch divides into three pieces on each side, of which the lower
two form the hyoid, the uppermost the columella. Like Reichert he held
the visceral arches to be parts of the visceral plates, containing,
however, elements from all three germ-layers--the serous, mucous, and
vessel layers.

The first gill-slit, or, as Rathke here prefers to call it, pharyngeal
slit, closes completely in snakes and in Urodeles. It forms the
Eustachian tube in all other Tetrapoda. As regards the vertebræ, Rathke
describes them as being formed in the sheath of the chorda from paired
rudiments, each of which sends two branches upwards, and two branches
downwards. The two inner pairs of processes coalesce round the chorda,
and later form the centrum; the upper outer pair meet above the spinal
column; the lower outer pair form ribs. The odontoid process of the axis
vertebra is the centrum of the atlas (p. 120). The formation of
vertebral rudiments begins close behind the ear-labyrinth, but in front
of this the chorda-sheath gives origin to a flat membranous plate which
afterwards becomes cartilaginous. This plate reaches forward below the
third cerebral vesicle as far as the infundibulum. The notochord ends in
this plate, which is the _basis cranii_, just at the level of the
ear-labyrinth. In no Vertebrate does the notochord extend farther
forward (p. 122). The _basis cranii_ gives off three trabeculæ. The
middle one is small and sticks up behind the infundibulum; it is absent
in fish and Amphibia, and soon disappears during the development of the
higher forms. The lateral trabeculæ are long bars which curve round the
infundibulum and reach nearly to the front end of the head. Together
they are lyre-shaped. The cranial basis and the trabeculæ are formed,
like the vertebræ, in the sheath of the notochord, and the only
differences between the two in the early stage of their development are
that the formative mass for the cranial basis is much greater in amount
than that for the vertebræ, and that the cranial basis by means of its
processes, the trabeculæ, reaches well in front of the terminal portion
of the notochord (p. 36). The capsule for the ear-labyrinth develops
quite independently of the cranial basis and the notochord. It resembles
on its first appearance, in form, position, composition, and
connections, the ear-capsule of Cyclostomes, and so do the ear-capsules
of all embryonic Vertebrates (p. 39). It manifests clearly the embryonic
archetype, ... "there exists one single and original plan of formation,
as we may suppose, upon which is built the labyrinth of Vertebrates in
general" (p. 40). When ossification sets in, the ear-capsule forms three
bones, of which two fuse with the supraoccipital and exoccipitals.

[Illustration: FIG. 11.--Embryonic Cranium of the Adder. Ventral Aspect.
(After Rathke.)]

During the formation of the ear-capsule the cranial basis develops from
a plate to a trench, for in its hinder section the side parts grow up to
form the side walls of the brain, in exactly the same way as the
processes of the vertebral rudiments grow up to enclose the spinal
column (pp. 122, 192). The foundations of the skull are now complete,
and ossification gradually sets in. The basioccipital is formed
in the posterior part of the _basis cranii_, and the exoccipitals in the
side walls of the trench in continuity with the fundament of the
basioccipital (see Fig. 11). The supraoccipital is formed in cartilage
above the exoccipitals. The basisphenoid develops, like the
basioccipital, in the flat _basis cranii_, but towards its anterior
edge, between the large foramen (_h_) and the pituitary space (_i_). It
is formed from two centres, each of which is originally a ring round the
carotid foramen. The presphenoid develops in isolation between the
lateral trabeculæ, just behind the point where they fuse. The side parts
of the basisphenoid and presphenoid (forming the alisphenoids and the
orbitosphenoids respectively) develop in cartilage separately from the
cranial basis, not like the exoccipitals in continuity with it. The
hinder parts of the trabeculæ become enclosed by two processes of the
basisphenoid; their front parts remain in a vestigial and cartilaginous
state alongside the presphenoid. The frontals and parietals show a
peculiar mode of origin in the adder, differing from their origin in
other Vertebrates. The frontals develop in continuity with the
orbitosphenoids, the parietals in continuity with the alisphenoids, and
so have much resemblance with the vertebral neural arches which surround
the spinal column (p. 195).

Through Rathke's work the real embryonic archetype of the vertebrate
skull was for the first time disclosed. Rathke discussed this archetype
and its relation to the vertebral theory of the skull in another paper
of the same year (1839), but before going on to this paper, we shall
quote from the paper on the adder the following passage, remarkable for
the clear way in which the idea of the embryological archetype is
expressed. "Whatever differences may appear in the development of
Vertebrates, there yet exists for the different classes and orders a
universally valid idea (plan, schema, or type) ruling the first
formation of their separate parts. This idea must first be worked out,
though possibly with modifications, before more special ideas can find
play. The result of the latter process, however, is that what was formed
by the first idea is not so much hidden as partially or wholly
destroyed" (p. 135).

Rathke's general paper on the development of the skull in Vertebrates[211]
treats the matter on a broader comparative basis than his paper on the
adder, and takes into account all the vertebrate classes, in so far as
their development was then known. He here makes the interesting
suggestion, later entirely confirmed, that the _basis cranii_ or basilar
plate is first laid down as two strips, one on each side of the
chorda--the structures now known as parachordals (pp. 6, 27). For this
supposition, he thinks, speaks the structure of the skull in
_Ammocoetes_, which in this respect is the simplest of all Vertebrates
(pp. 6, 22). In _Ammocoetes_, as Johannes Müller had shown, the
foundation of the skull is formed by two long cartilaginous bars,
between the hinder portions of which the notochord ends. In these Rathke
was inclined to see the homologues of his trabeculæ, and of the
parachordals which he was ready to assume from his embryological
observations.

Müller was, of course, very ready to accept Rathke's opinions on this
subject, for he considered that they supported his own theory of the
vertebral nature of the skull. After describing in his _Handbuch der
Physiologie_ the cartilaginous bands in _Ammocoetes_ and their highly
differentiated homologues in the Myxinoids, he writes in the later
editions, "Hence we see that in the cranium, as in the spinal column,
there are at first developed at the sides of the chorda dorsalis two
symmetrical elements, which subsequently coalesce, and may wholly
enclose the chorda. Rathke has recently observed, in the embryos of
serpents and other animals, before the formation of the proper cranial
vertebræ, two symmetrical bands of cartilage, similar to those which I
discovered as a persistent structure in _Ammocoetes_.... At a later
period the _basis cranii_ of vertebrate animals contains three parts
analogous to the bodies of vertebræ, the most anterior of which, in the
majority of animals, is generally small, and its development frequently
abortive, whilst in man and mammiferous animals the three are very
distinct. These parts are developed by the formation of three distinct
points of ossification, one behind the other, in the basilar
cartilage."[212]

Rathke was very cautious about accepting the vertebral theory of the
skull; he saw that the facts of development were not altogether
favourable to the theory, and he gave his adherence with many
reservations and saving clauses. His general attitude may be summed up
as follows.[213]

The chorda sheath is the common matrix of the vertebræ and of a large
part of the skull. The basilar plate and the trabeculæ, which are
developed from the chorda sheath, give origin to three bones, which
might possibly be considered equivalent to vertebral centra--the
basioccipital, the basisphenoid, and the _Riechbein_ (ethmoid). The
_Riechbein_ develops from the fused ends of the trabeculæ. The
presphenoid might also be considered as a vertebral body, but it
develops independently of the basilar plate and trabeculæ.

Now of these bones, the basioccipital is in every way equivalent to a
vertebral centrum, for it develops in the basilar plate round the
notochord. With the exoccipitals, which arise just like neural arches,
it forms a true vertebra. The supraoccipital is an accessory bone
developed in relation to bigger brains. The basisphenoid appears in the
basilar plate, but in front of the notochord, nor does it arise in
exactly the same way as the centrum of a vertebra. The basisphenoid with
the alisphenoids, which develop independently in the side walls of the
brain, may, however, still be considered as forming a vertebra, though
the resemblance is not so great as in the case of the occipital ring.
The presphenoid, being long and pointed, is very unlike a vertebral
body. The orbitosphenoids develop separately from it. The ethmoid also
differs from a vertebra, for it surrounds not the whole nervous axis as
the two hinder "vertebræ" do, but only two prolongations of it, the
olfactory lobes. In its development and final form it shows no
particular resemblance to a vertebra. Its body, the _pars
perpendicularis_ (mesethmoid) shows no similarity with a vertebral
centrum. Completing the three hinder cranial "vertebræ" and roofing in
the brain are the supraoccipital, the parietals and the frontals. The
premaxillaries, vomer, and nasals do not belong to the cranial scheme;
they are covering bones connected with the ethmoid. So, too, the
ear-capsule is not part of the cranial vertebræ, but is rather to be
compared to the intercalary bones in the vertebral column of certain
fish. Summing up as regards the cranial vertebræ Rathke writes, "We find
that the four different groups of bones, consisting of the basioccipital
with its intercalary (the supraoccipital), the basisphenoid with its
intercalaries (parietals), the presphenoid with its intercalaries
(frontals), and the ethmoid with its outgrowths (turbinals and
cribriform plate), taking them in order from behind forwards, show an
increasing divergence from the plan according to which vertebræ as
commonly understood develop, so that the basioccipital shows the
greatest resemblance to a vertebra, the ethmoid the least" (p. 30).

In a posthumous volume published in 1861 the same opinion is put
forward. "In the head, too," he writes, "some vertebræ can be
recognised, although in a more or less modified form. Yet at most only
four cranial vertebræ can be assumed, and these differ from ordinary
well-developed vertebræ in their manner of formation the more the
farther forward they lie."[214]

Rathke was an able and careful critic of the vertebral theory of the
skull, but he accepted it in the main. Actual attack on the theory upon
embryological grounds was begun by C. Vogt, in his work on the
development of _Coregonus_,[215] and in his paper on the development of
_Alytes_.[216] He described for _Coregonus_ an origin of the skull in the
main similar to that established by Rathke for the adder. There was a
"nuchal plate" in which the front end of the notochord was imbedded; the
notochord ended at the level of the labyrinth; there were two lateral
bands, comparable to Rathke's lateral trabeculæ; a "facial plate" was
also formed, which seems on the whole equivalent to the plate formed by
the fused anterior ends of the trabeculæ. A little later the cranium
formed a complete cartilaginous box surrounding the brain, very similar
to the adult cranium of a shark.

In his criticism of the vertebral theory of the skull, Vogt started by
defining the vertebra as a ring formed round the chorda. Now since only
the occipital segment of the skull is formed actually round the
notochord, the parts of the skull lying in front of this cannot
themselves be vertebræ, though they may be considered as prolongations
of the occipital or nuchal vertebra. "We must regard the nuchal plate as
a true vertebra, modified, it is true, in its formation and development
by its particular functions. Now, since the notochord ends with the
nuchal plate we can no longer regard as vertebræ the parts of the skull
that lie beyond, such as the lateral processes of the cranium and the
facial plate, for they have no relation with the notochord" (p. 123).

To support this view he adduced the fact that the vertebral divisions
(primitive vertebræ) visible in the trunk do not extend into the head.
He used precisely the same arguments in his paper on _Alytes_ to destroy
the vertebral theory of the skull. We quote the following passage
translated by Huxley (1864, p. 295) from this paper. "It has therefore
become my distinct persuasion that the occipital vertebra is indeed a
true vertebra, but that everything which lies before it is not fashioned
upon the vertebrate type at all, and that efforts to interpret it in
such a way are vain; that, therefore, if we except that vertebra
(occipital) which ends the spinal column anteriorly, there are no
cranial vertebræ at all."

L. Agassiz, himself a pupil of Döllinger, in the general part (1844) of
his _Recherches sur les Poissons fossiles_ (Neuchâtel, 1833-43), repeats
in the main his pupil Vogt's criticism of the vertebral theory (vol. i.,
pp. 125-9).

These arguments of Vogt and Agassiz were not considered by Müller to
dispose of the theory,[217] which maintained a firm hold even upon
embryologists. It was still upheld by Reichert, and Kölliker in 1849
showed himself convinced of its general validity.

A useful step in the analysis of the concept "vertebra" was taken by
Remak,[218] who showed what a complex affair the formation of vertebræ
really is, involving as it does a complete resegmentation
(_Neugliederung_) of the vertebral column, whereby the original
vertebral bodies were replaced by the secondary definitive bodies (p.
143). Remak showed, as he thought, that the protovertebral segmentation
of the dorsal muscle-plates did not extend into the head, and he denied
Reichert's assertion (1837) that the cranial basis in mammals showed
transverse grooves delimiting three cranial vertebræ (p. 36). The
gill-slits, he considered, could not possibly be regarded as marking the
limits of head vertebræ.

In 1858 appeared Huxley's well-known Croonian Lecture, _On the Theory of
the Vertebrate Skull_,[219] in which he stated with great clearness and
force the case for the embryological method of determining homologies,
and criticised with vigour the vertebral theory of the skull. By this
time the two rival methods in morphology had become clearly
differentiated, and Huxley was able to contrast them, or at least to
show how necessary the new embryological method was as a corrective and
a supplement to the older anatomical, or, as he calls it, "gradation"
method. Applied to the "Theory of the Skull," the gradation method
consists in comparing the parts of the skull and vertebral column in
adult animals with respect to their form and connections. "Using the
other method, the investigator traces back skull and vertebral column to
their earliest embryonic states and determines the identity of parts by
their developmental relations" (p. 541). This second method is the final
and ultimate. "The study of the gradations of structure presented by a
series of living beings may have the utmost value in suggesting
homologies, but the study of development alone can finally demonstrate
them" (p. 541). As an example of the utility and, indeed, the necessity
of applying the embryological method Huxley takes the case of the
quadrate bone in birds. This bone had been generally regarded by
anatomists as the equivalent of the tympanic of mammals, on account of
its connection with the tympanum; but Reichert showed (1837) that the
same segment of the first visceral arch developed into the incus in
mammals, and into the quadrate in birds, and that therefore the quadrate
was homologous with the incus. Similarly, on developmental grounds, the
malleus or hammer of mammals is the homologue of the articular of birds,
since both are developed from a portion of Meckel's cartilage identical
in form and connections in the two groups. The homologies of the bones
connected with the jaws in bony fishes had long been a subject of
contention among comparative anatomists; Huxley shows from his personal
observations how the development of the visceral arches throws light
upon these difficulties. The mandibular arch in the developing fish is
abruptly angled, as in the embryo of Tetrapoda; the upper prong of it
ossifies into the palatine and pterygoid; at the angle is formed the
quadrate (jugal, Cuvier), and to the quadrate is articulated the lower
jaw, which ossifies round the lower prong or Meckel's cartilage. The
scheme of development of the jaws is accordingly similar in fish to what
it is in other Vertebrates, and this similarity of development enables
Huxley to recognise what are the true homologues of the quadrate, the
palatine and the pterygoid in adult bony fish, and to prove that the
symplectic and the metapterygoid (tympanal, Cuvier) are bones peculiar
to fish. In developing Amphibia Huxley found a suspensorium of hyoid and
mandibular arches similar to the hyomandibular of fish.

Tackling his main problem of the unity of plan of the vertebrate skull,
Huxley shows, by a careful discussion of the anatomical relationships of
the chief bones in typical examples of all vertebrate classes, that
there is on the whole unity of plan as regards the osseous skull. This
unity of composition can be established, on the gradation method, by
considering the connections of the bones of the skull with one another,
their relations to the parts of the brain and to the foramina of the
principal cranial nerves. The assistance of the embryological method is,
however, necessary in determining many points with regard to the bones
developed in relation to the visceral arches. But there is a further
step to be taken. "Admitting ... that a general unity of plan pervades
the organisation of the ossified skull, the important fact remains that
many vertebrated animals--all those fishes, in fact, which are known as
_Elasmobranchii_, _Marsipobranchii_, _Pharyngobranchii_ and _Dipnoi_
have no bony skull at all, at least in the sense in which the words have
hitherto been used" (p. 571). The membranous or cartilaginous skull of
these fishes shows a general resemblance in its main features to the
ossified skull of other Vertebrates; the relations of the ear to the
vagus and trigeminal nerves are, for instance, the same in both; the
main regions of the cartilaginous skull can be homologised with definite
bones or groups of bones in the bony skull; but discrepancies occur. It
is again to development that we must turn to discover the true
relationship of the cartilaginous to the ossified skull. "The study of
the development of the ossified vertebrate skull ... satisfactorily
proves that the adult crania of the lower _Vertebrata_ are but special
developments[220] of conditions through which the embryonic crania of
the highest members of the sub-kingdom pass" (p. 573). It is with the
embryonic cranium of higher Vertebrates that the adult skull of the
lower fishes must be compared, and the comparison will show a
substantial though not a complete agreement between them. Thus, speaking
of the development of the frog's skull, Huxley writes:--"If, bearing in
mind the changes which are undergone by the palatosuspensorial
apparatus, ... we now compare the stages of development of the frog's
skull with the persistent conditions of the skull in the _Amphioxus_,
the lamprey, and the shark, we shall discover the model and type of the
latter in the former. The skull of the _Amphioxus_ presents a
modification of that plan which is exhibited by the frog's skull when
its walls are still membranous and the notochord is not yet embedded in
cartilage. The skull of the lamprey is readily reducible to the same
plan of structure as that which is exhibited by the tadpole when its
gills are still external and its blood colourless. And finally, the
skull of the shark is at once intelligible when we have studied the
cranium in further advanced larvæ, or its cartilaginous basis in the
adult frog" (p. 577). Development, therefore, proves what comparative
anatomy could only foreshadow--the unity of plan of all vertebrate
skulls, ossified and unossified alike. "We have thus attained to a
theory or general expression of the laws of structure of the skull. All
vertebrate skulls are originally alike; in all (save _Amphioxus_?) the
base of the primitive cranium undergoes the mesocephalic flexure, behind
which the notochord terminates, while immediately in front of it the
pituitary body is developed;[221] in all, the cartilaginous cranium has
primarily the same structure--a basal plate enveloping the end of the
notochord and sending forth three processes, of which one is short and
median, while the other two, the lateral trabeculæ, pass on each side of
the space on which the pituitary body rests, and unite in front of it;
in all, the mandibular arch is primarily attached behind the level of
the pituitary space, and the auditory capsules are enveloped by a
cartilaginous mass, continuous with the basal plate between them. The
amount of further development to which the primary skull may attain
varies, and no distinct ossifications at all may take place in it; but
when such ossification does occur, the same bones are developed in
similar relations to the primitive cartilaginous skull" (p. 578).

In a word, there is a general plan or primordial type which is
manifested in the higher forms most clearly in their earliest
development--an embryological archetype therefore.

Huxley now goes on to consider the relation of this general plan or type
of the skull to the structure and development of the vertebral column.
Does the skull in its development show any signs of a composition out of
several vertebræ? The vertebral column develops as a segmented structure
round the notochord; the skull develops first as an unsegmented plate
extending far beyond the notochord. The processes of this basilar plate,
the trabeculæ, are quite unlike anything in the vertebral column. It is
true that when the process of ossification begins, separate bones are
differentiated in the basilar plate one in front of the other, giving an
appearance of segmentation. The hindmost of these bones, the
basioccipital, ossifies round the notochord, quite like a vertebral
centrum, and its side parts which form the occipital arch develop in a
"remotely similar" way to the neural arches of the vertebræ. The next
bone, however, the basisphenoid, develops in front of the notochord, and
shows very little analogy with a vertebral body. The analogy is even
more far-fetched when applied to the axial bones in front of the
basisphenoid. The cranium might indeed be divided upon ossification into
a series of segments bearing a more or less remote analogy with
vertebræ. "In the process of ossification there is a certain analogy
between the spinal column and the cranium, but that analogy becomes
weaker and weaker as we proceed towards the anterior end of the skull"
(p. 585). The best way to state the facts is to say that both skull and
vertebral column start in their development from the same point, but
immediately begin to diverge. The clear indications of segmentation
which fully ossified adult skulls undoubtedly show are, therefore,
secondary, and the vertebral theory of the skull, which was originally
based upon the appearance of such fully ossified crania, is on the whole
negatived by embryology.

We have now to turn back a few years in order to follow up another line
of discovery which had an important bearing upon the theory of the
vertebrate skull--the working out of the distinction between membrane
and cartilage bones.

As early as 1731, R. Nesbitt,[222] in two lectures delivered to the Royal
College of Surgeons, demonstrated that in the human foetus some bones
were formed not in cartilage but directly in fibrous tissue, and this
observation was confirmed by other human anatomists, particularly by
Sharpey at a considerably later date. In 1822 Arendt[223] focussed
attention upon the remarkable structure of the skull of the Pike, with
its cartilaginous brain-box studded all over with bony plaques, an
arrangement which had already attracted the interest of Cuvier and
Meckel. K. E. von Baer[224] in 1826 discussed at some length the relation
between the bony and the cartilaginous skull in fishes, with particular
reference to the sturgeon, coming to the following just conclusion:--"If
we consider the fibrous skeleton of _Ammocoetes_ as the first foundation
of the skeleton of Vertebrates, we can form a series among the
cartilaginous fishes, according as a cartilaginous skeleton penetrates
more and more into this fibrous foundation. In the same way the process
of ossification supplants the cartilaginous skeleton. So long as the
ossifications lie in the skin, as in the sturgeon, they form corneous
bones (_Hornknochen_), but when they lie under the skin, they form true
bones, _e.g._, the bones of the skull in the pike" (p. 374).

Embryologists soon become aware that a similar distinction between a
primitive cartilaginous foundation and a secondary overlying
ossification of the skull showed itself in the development of all
Vertebrates. Dugès, in his _Recherches sur l'ostéologie et la myologie
des Batraciens_ (1834), distinguished between such bones as are formed
by direct ossification of the cartilaginous groundwork of the skull, and
such as are developed in the periosteal fibrous tissue.

Reichert in 1838[225] noted that several of the skull bones in Amphibia
are formed without the intermediary of cartilage, such as the nasals,
the maxillaries and the lacrymals. So, too, the frontals and parietals
of Teleosts developed independently of the cartilaginous skull, and
belonged to the skeletal system of the skin, not to the true vertebral
axial skeleton (pp. 215-6). Even more interesting was his discovery,
afterwards confirmed by Hertwig,[226] that in the newt several bones
connected with the palate were formed in the mucous membrane of the
mouth by the fusion of a number of little conical teeth (p. 97). Certain
of these bones he considered to be the substitutes, not the equivalents,
of the palatine and pterygoid of other Vertebrates, which are formed
from the upper part of the first visceral arch, a part missing in the
newt (p. 100). Owing to the difference of development he would not
homologise these bones in the newt with the palatine and pterygoid of
other Vertebrates. He recognised also that the bone now known as the
parasphenoid was developed in the frog in the mucous membrane of the
mouth, and had originally no connection with the cranial basis (p. 34).
Rathke in 1839 also allowed the distinction between cartilage and
membrane bone, but laid no stress upon it (_Entw. d. Natter._, p. 197).

Jacobson in 1842[227] introduced the useful term, "primordial cranium,"
for the primitive cartilaginous foundation of the skull, and drew a
sharp distinction between cartilage bones and membrane bones.

In his _Recherches sur les Poissons fossiles_,[228] L. Agassiz used Vogt's
work on the development of _Coregonus_ to establish a classification of
the bones of the skull in fish, a classification which had the merit of
drawing a sharp distinction between the cartilaginous groundwork and
the "protective plates" of the fish's skull. He recognised that the
protective plates developed in a different way from the other bones of
the skull. "We must distinguish," he writes, "two kinds of ossification;
one which tends to transform the primitive parts of the embryonic
cranium directly into bone, and another which leads to the deposition of
protective plates round this core, which develop not only upon the upper
surface, as has hitherto been supposed, but also on the lateral walls
and on the lower surface of the cranium" (p. 112). In the skull of all
fish there are three elements--(1) the cartilaginous base, including the
nuchal plate, the trabeculæ and the facial plate, together with the
auditory capsules; (2) the cartilaginous cerebral envelope; (3) the bony
protective plates (absent in Elasmobranchs). The bones developed in
relation to these cranial elements can be classified as follows:--(1)
the basioccipital, exoccipitals (paroccipitals?), supraoccipital and
"petrous" (_rocher_), developed from the nuchal plate; the ali- and
orbito-sphenoids developed from the trabeculæ; the "cranial ethmoid"[229]
developed from the facial plate; (2) the parietals, frontals and nasals
formed from the "superior" protective plate; the "anterior" and
"posterior" frontals and the temporal, from the "lateral" plates; the
body of the sphenoid and the vomer from the "inferior" plates. The other
element, the cartilaginous brain-box, does not ossify, and tends to
become absorbed (p. 124).

In 1849 Kölliker published a paper[230] dealing with the morphological
significance of the distinction between membrane and cartilage bones,
and in 1850[231] he defended his views against the criticisms of
Reichert[232] in a further note entitled _Die Theorie des
Primordialschädels festgehalten_. It is convenient to consider these
papers together. Kölliker held that there was (1) a histological and (2)
a morphological difference between the two categories of bones. The
histological development of the two kinds was different, but this
difference was not sufficient to establish a morphological distinction
between them, a distinction in their anatomical _Bedeutung_. The true
morphological distinction between them was their development in
different skeleton-forming layers. Membrane bones were developed in
fibrous tissue lying between the skin and the deep layer which formed
the primordial cranium, and it was this formation in a separate layer
that gave them a different morphological significance from the bones
formed directly in the deep layer. Kölliker's distinction, therefore,
was between the bones formed in the primordial cartilaginous cranium on
the one hand, and the superficial ossifications in fibrous tissue on the
other hand. The cartilaginous cranium in Kölliker's opinion was formed
upon the vertebral type, and the membrane bones were accessory. This, at
least, was his opinion in 1849. In 1850, after Stannius had shown that
membrane bones occurred as integral parts of the vertebræ in certain
fish, he modified his view of the membrane bones, and admitted them, at
least in some cases, as constituents of the cranial vertebræ.

On this morphological distinction of membrane and cartilage bones future
comparative osteology was to be based:--

"My sole aim is to state again the principle upon which comparative
osteology is to be based and extended, and this is that first place
should be assigned to anatomical considerations, and among these to the
manner of origin of the whole bone in relation to the skeleton-forming
layers" (1850, p. 290).

The homologies established by this new principle might run counter to
the homologies indicated by the study of adult structure. "Thus, for
instance, although the lower jaw in position, function, form and shape,
appears to be the same bone throughout, yet it must be admitted that it
shows a difference in the different classes. In Mammals and Man it is an
entirely secondary bone (an extremity according to Reichert), in Birds,
Amphibia and Fishes only partially so, for its articular belongs to
Meckel's cartilage and is accordingly analogous to a rib; indeed, in the
Plagiostomes, etc., the whole lower jaw along with the articular is a
persistent Meckel's cartilage" (p. 290, 1850).

So, too, the supraoccipital in man cannot be fully homologised with the
supraoccipital of many mammals, for its upper half arises at first in
isolation as a secondary bone (p. 290).

Reichert objected to the distinction drawn by Kölliker, and denied that
there was either a histological or a morphological difference between
membrane and cartilage bones. It was shown a few years later by H.
Müller[233] that there was in truth no essential difference in
histological development between the two categories of bone, that the
cartilage cells were replaced by bone cells identical with those taking
part in the formation of membrane bones. The morphological distinction
continued however to be recognised, particularly by the embryologists.
Rathke in his volume of 1861[234] classified the bones of the skull
according to their origin from the primordial cranium or from the
overlying fibrous layer, distinguishing as membrane bones, the
parietals, frontals, nasals, lachrymals, maxillaries and premaxillaries,
jugals, tympanic, parts of the "temporal," vomer, part of the
supraoccipitals in some mammals, and the mandible (with the exception of
the articular in such as have a quadrate bone). Huxley was also inclined
in 1864[235] to recognise the distinction, but he writes with some
reserve:--"Is there a clear line of demarcation between membrane bones
and cartilage bones? Are certain bones always developed primarily from
cartilage, while certain others as constantly originate in membrane? And
further, if a membrane bone is found in the position ordinarily occupied
by a cartilage bone, is it to be regarded merely as the analogue and not
as the homologue of the latter?" (p. 296).

We may note here that many comparative anatomists of the period were
quite ready to decide Huxley's last question in a sense favourable to
the older, purely anatomical, view of homology. Owen, for instance, held
that difference of development did not disturb homologies established by
form and connections. "Parts are homologous," he writes, "in the sense
in which the term is used in this work, which are not always similarly
developed: thus the 'pars occipitalis stricte dicta,' etc., of
Soemmering is the special homologue of the supraoccipital bone of the
cod, although it is developed out of pre-existing cartilage in the fish
and out of aponeurotic membrane in the human subject."[236] Similarly he
pointed to the diversities of development of the vertebral centrum in
the different vertebrate classes as proof that development could not
always be relied upon in deciding homologies (p. 89). But he could not
deny that the archetype was better shown in the embryo than in the adult
(_supra_, p. 108).

J. V. Carus[237] likewise stood firm for the older method of determining
homologies by comparison of adult structure. "We can regard as
homologous," he writes, "only those parts which in the fully formed
animal possess a like position and show the same topographical relations
to the neighbouring parts" (p. 389). Parts homologous in this sense
might develop in different ways, but no great importance was to be
attached to such a circumstance. Membrane and cartilage bones developed
in practically the same way, from the same skeleton-forming layer, and
no morphological significance attached to their distinction (pp. 227,
457). Embryology was of considerable value in helping to determine
homologies, but the evidence that it supplied was contributory, not
conclusive. Perhaps the greatest service which the study of development
rendered was to disentangle, by a comparison of the earliest embryos,
the generalised type (p. 389).

We have now traced, by our historical study of the theory of the skull,
the gradual evolution of the tendency to find in development the surest
guide to determining homologies. We have seen how the embryological
"type" came to be substituted, in whole or in part, for the anatomical
"type" derived from the study of adult structure. But we have had to do
only with a modification, not with a transformation, of the criterion of
homology recognised by the anatomists. Homology is still determined by
position, by connections, in the embryo as in the adult. "Similarity of
development" has become the criterion of homology in the eyes of the
embryologist, but "similarity of development" means, not identity of
histological differentiation, but similarity of connections throughout
the course of development. For the purposes of morphology, development
has to be considered as an orderly sequence of successive forms, not in
its real nature as a process essentially continuous. Morphology has to
replace the living continuity by a kinematographic succession of stages.
Since it is the earliest of these stages that manifest the simplest and
most generalised structural relations of the parts, it is in the earlier
stages that homologies can be most easily determined. But these
homologies are still determined solely by the relative positions and
connections of the parts, just as homologies are determined in the last
of all the stages of development, the adult state. And since the
generalised type is shown most clearly in the earliest stages and tends
to become obscured by later differentiation, homologies observed in
embryonic life are to be upheld even if the relations in adult life seem
to indicate different interpretations.

    [183] See review by Cuvier, _Mém. Mus. Hist, nat._, iii.,
    pp. 82-97, 1817.

    [184] _Mém. Savans étrangers_, vi. Extract in _Ann. Sci.
    nat._ (2) i. (_Zool._), pp. 366-72, 1834.

    [185] _Recherches sur la génération des Mammifères_, 1834.
    _Embryogénie comparée_, 1837.

    [186] "Kiemen bey Säugthieren," _Isis_, pp. 747-9, 1825.

    [187] "Kiemen bey Vögeln," _Isis_, pp. 1100-1, 1825.

    [188] "Ueber die Kiemenbogen und Kiemengefässe beym
    bebrüteten Hühnchen," _Isis_, xx., pp. 401-3, 1827.
    (Read in Sept. 1826 to the _Versammlung der deutschen
    Naturforscher und Aerzte_, then recently founded by
    Oken).

    [189] _Isis_, pp. 160-4, Pl. II., 1828.

    [190] "Ueber die Kiemen und Kiemengefässe in den Embryonen
    der Wirbelthiere," Meckel's _Archiv_ for 1827, pp.
    556-68. Also in _Ann. Sci. nat._, xv., pp. 266-80,
    280-4, 1828.

    [191] Meckel's _Archiv_, vi., pp. 1-47, 1832.

    [192] _Untersuchungen über die Bildung und Entwickelung
    der Fluss-Krebses_, Leipzig, folio, 1829. Preliminary
    notice in _Isis_, pp. 1093-1100, 1825.

    [193] "Untersuchungen über die Bildung und Entwickelung
    der Wasser-Assel.," _Abh. z. Bild. u. Entwick.-Gesch._,
    i., pp. 1-20, 1832. Translated in _Ann. Sci. nat._ (2),
    ii., (_Zool._), pp. 139-57, 1834.

    [194] Kölliker, _Entwickelungsgeschichte_, 2nd ed., p. 17,
    Leipzig, 1879.

    [195] _Handbuch der Entwickelungsgeschichte des Menschen
    und ... der Säugethiere und Vögel_, Berlin, 1835.

    [196] _Embryogénie comparée_, 1837; _Histoire générale du
    développement des corps organisés_, 1847-49.

    [197] _Entwickelungsgeschichte des Kaninchen-Eies_,
    Braunschweig, 1842; _Entwickelungsgeschichte des
    Hunde-Eies_, Braunschweig, 1845;
    _Entwickelungsgeschichte des Meerschweinchens_, Giessen,
    1852; _Entwickelungsgeschichte des Rehes_, Giessen,
    1854.

    [198] "It is the rôle of embryology, as my great teacher
    says, to form the court of appeal for comparative
    anatomy, and it is from embryology particularly, which
    has in the last decades provided such signal instances
    of the unravelling of obscure problems, that we have to
    expect a definite clearing up of the problems relating
    to the development of the head."--Müller's _Archiv_, p.
    121, 1837.

    [199] _Anat.-phil. Unters. ü. d. Kiemenapparat u. d. Zungenbein_, Riga
    and Dorpat, 1832.

    [200] "Bildungs- und Entwickelungs-geschichte des Blennius viviparus,"
    _Abhandl. z. Bild. u. Entwick.-Gesch. des Menschen u. der Thiere_,
    ii., pp. 1-68, Leipzig, 1833.

    [201] _Von den Ur-Theilen des Knochen und
    Schalen-Gerustes_, Leipzig, 1828.

    [202] _Kiemenapparat_, pp. 107-118.

    [203] _Vergleichende Anatomie der Myxinoiden_. Part I.
    (Osteology and Myology). (_Abh. königl. Akad. Wiss.
    Berlin_, for 1834, pp. 65-340, 9 pls., 1836.) Also
    separately.

    [204] "Ueber die Visceralbogen der Wirbelthiere in
    Allgemeinen und deren Metamorphosen bei den Vögeln und
    Säugethiere," Müller's _Archiv_, pp. 120-222, 1837.

    [205] _Handbuch d. menschl. Anatomie_, iv., p. 47.

    [206] This was shown by Serres (_Ann. Sci. nat._, xi., p.
    54 f.n., 1827), who found in a human embryo a long
    cartilaginous piece extending from the ear-ossicles to
    the inside of the lower jaw, and suggested that it was
    the foundation of the permanent mandible.

    [207] _Abhandl._, i., p. 102, 1832; ii., p. 25, 1833. (_Blennius_
    paper).

    [208] _Vergleichende Entwickelungsgeschichte des Kopfes der nackten
    Amphibien_, Königsberg, quarto, 276 pp., 1838.

    [209] Müller's _Archiv_ for 1838.

    [210] _Entwickelungsgeschichte der Natter_, Königsberg,
    1839.

    [211] _Bemerkungen über die Entwickelung des Schädels der
    Wirbelthiere_, Königsberg, 1839.

    [212] _Handbuch der Physiologie des Menschen_, Koblenz,
    1835; Eng. trans. by W. Baly, ii., p. 1615, 1838.

    [213] For a full statement of Rathke's conclusions, see
    the translation given by Huxley in _Lectures on the
    Elements of Comparative Anatomy_, London, 1864.

    [214] _Entwickelungsgeschichte der Wirbelthiere_, p. 142,
    1861.

    [215] _Embryologie des Salmones_. A separate volume of L.
    Agassiz's _Histoire naturelle des Poissons d'Eau douce
    de l'Europe centrale_, Neuchâtel, 1842.

    [216] _Untersuchungen über die Entwickelungsgeschichte der
    Gebürtshelferkröte_, Solothurn, 1842.

    [217] Müller's _Archiv_ for 1843, p. ccxlviii.

    [218] _Untersuchtingen über die Entwickelung der
    Wirbelthiere_, Berlin, 1850-55.

    [219] Delivered 17th June 1858. Reprinted in _The
    Scientific Memoirs of T. H. Huxley_, edited by M. Foster
    and E. Ray Lankester, vol. i., pp. 538-606 (1898).

    [220] _Cf._ Reichert, _supra_, p. 149.

    [221] The origin of the pituitary body from the roof of
    the mouth was first described by Rathke (1839).

    [222] _Human Osteogeny explained in two Lectures_, London,
    1736.

    [223] _De capitis ossei Esocis lucii structura singulari.
    Dissert. inaug._ Regiomonti, 1822.

    [224] "Ueber das äussere und innere Skelet," Meckel's
    _Archiv_, pp. 327-76, 1826.

    [225] _Vergl. Entwick. d. Kopfes d. nackten Amphibien_ (p.
    186).

    [226] _Arch. f. mikr. Anat._, xi., Suppl., 1874.

    [227] "Om Primordial-Craniet," _Förhandlingar Skand.
    Naturf. Möle_, Stockholm, 1842.

    [228] Vol. I., General part, pub. 1844.

    [229] _Entosphenoid_, Owen.

    [230] _Zweiter Bericht zootom. Anstalt zu Würzburg_, 1849.

    [231] _Zeits. f. wiss. Zool._, ii., pp. 281-91.

    [232] Müller's _Archiv_ for 1849, pp. 443-515.

    [233] _Zeits. f. wiss Zool._, ix., 1858.

    [234] _Entw. d. Wirbelthiere_, pp. 139-40, 1861.

    [235] _Lectures on the Elements of Comparative Anatomy_.

    [236] _On the Archetype of the Vertebrate Skeleton_, p. 5,
    1848.

    [237] _System der thierischen Morphologie_, Leipzig, 1853.



CHAPTER XI

THE CELL-THEORY.


With the founding of the cell-theory by Schwann in 1839 an important
step was taken in the analysis of the degrees of composition of the
animal body. Aristotle had distinguished three--the unorganised
material, itself compounded of the four primitive elements, earth and
water, air and fire, the homogeneous parts or tissues and the
heterogeneous parts or organs, and this conception was retained with
little change even to the days of Cuvier and von Baer. Those of the old
anatomists who speculated on the relations of organic elements to one
another were dominated by Aristotle's simple and profound
classification, and proposed schemes which differed from his only in
detail. Bichat enlarged and deepened the concept of tissue, but the
degree of composition below this was for him, as for all anatomists of
his time, a fibrous or pulpy "cellulosity," living, indeed, but showing
no uniform and elemental structure. It was Schwann's merit to interpose
between the tissue and the mere unorganised material a new element of
structure, the cell. And, as it happened, a few years before Schwann
published his cell-theory, Dujardin hinted at another degree of
composition which was later to take its place between the cell and the
chemical elements--sarcode or protoplasm.

As is well known, the concept of the cell arose first in botany. Robert
Hooke discovered cells in cork and pith in 1667, and his discovery was
followed up by Grew and Malpighi in 1671, and by Leeuenhoek in 1695. But
they did not conceive the cell as a living, independent, structural
unit. They were interested in the physiology of the plant as a whole,
how it lived and nourished itself, and they studied cells and
sieve-tubes, wood fibres and tracheæ with a view rather to finding out
their functions and their significance for the life of the plant than to
discovering the minutiæ of their structure. The same attitude was taken
up by the few botanists who in the 18th century paid any heed to the
microscopical anatomy of plants. For C. F. Wolff,[238] the formation of
cells was a result of the secretion of drops of sap in the fundamental
substance of the plant, this substance remaining as cell-walls when
cell-formation was completed--no idea here of cells as units of
structure.

In the early 19th century, interest in plant anatomy revived somewhat,
and much work was done by Treviranus, Mirbel, Moldenhawer, Meyen and von
Mohl.[239] As a result of their work the fact was established that the
tissues of plants are composed of elements which can, with few
exceptions, be reduced to one simple fundamental form--the spherical
closed cell. Thus the vessels of plants are formed by coalescence of
cells, fibres by the elongation of cells and the thickening and
toughening of their walls. At this time, interest was concentrated on
the cell-wall, to the almost total neglect of the cell-contents; the
"matured framework" of plant cells, to use Sach's convenient phrase, was
the chief, almost the sole, object of study. And it was natural enough
that the mere architecture of the plant should monopolise interest, that
the composition of the tissues out of the cells, and the fitting
together of the tissues to form the plant should awaken and hold the
curiosity of the investigator; even the modifications of the cell-walls
themselves, their rings and spiral thickenings and pits, offered a
fascinating field of enquiry.

The idea that the cell-contents might show a characteristic and
individual structure had hardly dawned upon botanists when Schleiden
published his famous paper, _Beiträge zur Phytogenesis_.[240] Schleiden's
theme in this paper is the origin and development of the plant cell, a
subject then very obscure, in spite of pioneer work by Mirbel. A few
years before, Robert Brown had called attention to the presence in the
epidermal cells of orchids and other plants of a characteristic spot
which he called the areola or nucleus.[241] Schleiden saw the importance
of this discovery, confirmed the constant presence of the nucleus in
young cells, and held it to be an elementary organ of the cell. He named
it the cytoblast because, in his opinion, it formed the cell. It was
embedded in a peculiar gummy substance, the cytoblastem, which formed a
lining to the cellulose cell-wall. Within the nucleus there was often a
small dark spot or sphere--the nucleolus. The nucleus, Schleiden
thought, originated as a minute granule in the cytoblastem which
gradually increased in size, becoming first a nucleolus (_Kernchen_),
and then, by further condensation of matter round it, a nucleus. Several
nuclei might be formed in this way in a single cell. New cells took
their origin directly from a full-grown nucleus, in a peculiar way which
Schleiden describes as follows:--"As soon as the cytoblasts have reached
their full size a delicate transparent vesicle arises on their surface;
this is the young cell, which at first takes the shape of a very flat
segment of a sphere, of which the plane surface is formed by the
cytoblast, the convex side by the young cell itself, which lies upon the
cytoblast like a watch-glass on a watch" (p. 145). The young cells
increase in size and fill up the cavity of the old cell, which is in
time resorbed. Cell-development always takes place within existing
cells, and either one or many new cells may be formed within the
mother-cell. Schleiden's views on cell-formation were drawn from some
rather imperfect observations on the embryo-sac and pollen-tube, but he
extended his theory to cell-formation in general. Though wrong in almost
all respects the theory had at least the merit of fixing attention upon
the really important constituents of the cell, the nucleus and the
cell-plasma. To Schleiden, too, we owe the conception of the cell as a
more or less independent living unity, whose life is not entirely
identified with the life of the plant as a whole. "Each cell," he
writes, "carries on a double life; one a quite independent and
self-contained life, the other a dependent life in so far as the cell
has become an integral part of the plant" (p. 138).

So long as the definition of the plant cell embraced little more than
the hardened cell-wall it was little wonder that "cells" in this sense
were not recognised in animal tissues, except in a few exceptional
cases--as in the notochord by Johannes Müller.[242] Careful observation of
animal tissues discovered in some cases the existence of discontinuous
units of structure, but these were not, as a rule, recognised before
1838 as analogous to plant cells. Von Baer, for example, observed that
the young chick embryo was composed partly of an albuminous mass and
partly of _Kügelchen_ or little globules suspended in it
(_Entwickelungsgeschichte_, i., pp. 19, 144). Since such _Kügelchen_
disposed in a row formed the notochord (i., p. 145) it seems probable
that his _Kügelchen_ were really cells. Similarly A. de Quatrefages[243]
in 1834 saw and figured segmentation spheres in the developing egg of
_Limnæa_, but he called them globules and did not recognise their
analogy with the cells of plants. According to M'Kendrick,[244] Fontana,
so far back as 1781,[245] described cells with nuclei in various tissues,
and used acids and alkalis to bring out their structure more clearly.
But it was not till 1836-7-8 that a fairly widespread occurrence of
cells in animal tissues was recognised. The pioneer in this seems to
have been Purkinje, who described cells in the choroidal plexus in
1836,[246] and compared gland cells with the cells of plants in 1837.[247]
Henle in 1837[248] and 1838[249] described various kinds of epithelial
tissue, distinguishing them according to the kind of cell composing
them; he also discovered the mode of growth of stratified epithelium.
Valentin[250] appears to have seen cells in cartilage and epithelium even
before Henle, and to have observed cells in the blastoderm of the chick.
In his report on the progress of anatomy during 1838 Johannes Müller was
able to refer to quite a number of papers dealing with the occurrence of
cells in animal tissues. In addition to those already noted, he mentions
work by Breschet and Gluge on the cells of the umbilical cord, by
Dumortier on the cells in the liver of molluscs, by Remak and by
Purkinje on nerve cells, by Donné on the cells of the conjuctiva, cornea
and lens. He reports, too, that Turpin had compared the epithelial cells
of the vagina with the cell-tissue of plants. Müller himself had not
only recognised the cellular nature of the notochord, but had observed
the cells of the vitreous humour, fat cells and pigment cells, and even
the nuclei of cartilage cells. From Schwann (1839) we learn that C. H.
Schults had followed back the corpuscles of the blood to their original
state of nucleated cells, and that Werneck had recognised cells in the
embryonic lens. A preliminary notice of Schwann's own work appeared in
1838 (Froriep's _Notizen_, No. 91, 1838), the full memoir in 1839, under
the title _Mikroskopische Untersuchungen über die Uebereinstimmung in
der Struktur und dem Wachstume der Tiere und Pflanzen_.[251]

Theodor Schwann was a pupil of Johannes Müller, and we know that Müller
took much interest in the new histology. It is probably to his influence
that we owe Schwann's brilliant work on the cell, which appeared just
after Schwann left Berlin for Löwen. Schwann was himself, as his later
work showed, more a physiologist than a morphologist; he did quite
fundamental work on enzymes, discovering and isolating the pepsin of the
gastric juice; he proved that yeast was not an inorganic precipitate but
a mass of living cells; he carried out experiments directed to show that
spontaneous generation does not occur. We shall see in his treatment of
the cell-theory clear indications of his physiological turn of mind.
Schwann was only twenty-nine when his master-work appeared, and the book
is clearly the work of a young man. It has the clear structure, the
logical finish, which the energy of youth imparts to its chosen work. So
the work of Rathke's prime, the _Anatomische-philosophische
Untersuchungen_ of 1832 shows more vigour and a more reasoned structure
than his later papers. Schwann's book is indeed a model of construction
and cumulative argument, and even for this reason alone justly deserves
to rank as a classic.

The first section of his book is devoted to a detailed study of the
structure and development of cartilage cells and of the cells of the
notochord, and to a comparison of these with plant cells. He accepts
Schleiden's account of the origin and development of nuclei and cells as
a standard of comparison; and he seeks to show that nucleus and
nucleolus, cell-wall and cell-contents, show the same relations and
behave in the same manner in these two types of animal cells as in the
plant-cells studied by Schleiden. The types of cell which he chose for
this comparison are the most plant-like of all animal cells, and he was
even able to point to a thickening of the cell-wall in certain cartilage
cells, analogous to the thickening which plays so important a part in
the outward modification of plant-cells. The analogy indeed in structure
and development between chorda and cartilage cells and the cells of
plants seemed to him complete. The substance of the notochord consisted
of polyhedral cells having attached to their wall an oval disc similar
in all respects to the nucleus of the plant-cell, and like it containing
one or more nucleoli. Inside the mother-cell were to be found young
developing cells of spherical shape, lacking however a nucleus.
Cartilage was even more like plant tissue. It was composed of cells,
each with its cell membrane. The cells lay close to one another,
separated only by their thickened cell-wall and the intercellular
matrix, showing thus even the general appearance of the cellular tissue
of plants. They contained a nucleus with one or two nucleoli, and the
nucleus was often resorbed, as in plants, when the cell reached its full
development. Other nuclei were in many cases present in the cell, round
which young cells could be seen to develop, in exactly the same manner
as in plants. These nuclei had accordingly the same significance as the
nuclei of plants, and deserved the same name of cytoblasts or
cell-generators. The true nucleus of the cartilage cell was probably in
the same way the original generator of the mother-cell.

Having proved the identity in structure and function of the cells of
these selected tissues with the cells of plants, as conceived by
Schleiden, Schwann had still to show that the generality of animal
tissues consisted either in their adult or in their embryonic state of
similar cells. This demonstration occupies the second and longest
section of his book.

His method is throughout genetic; he seeks to show, not so much that all
animal tissues are actually in their finished state composed of cells
and modifications of cells, as that all tissues, even the most complex,
are developed from cells analogous in structure and growth with the
cells of plants.

All animals develop from an ovum; it was his first task to discover
whether the ovum was or was not a cell. It happened that, some years
before Schwann wrote, a good deal of work had been done on the minute
structure of the ovum, particularly by Purkinje and von Baer. Purkinje
in 1825[252] discovered and described in the unfertilised egg of the fowl
a small vesicle containing granular matter, which he named the
_Keimbläschen_ or germinal vesicle. It disappeared in the fertilised
egg. As early as 1791 Poli had seen the germinal vesicle in the eggs of
molluscs, but the first adequate account was given by Purkinje. In
1827[253] von Baer discovered the true ova of mammals and cleared up a
point which had been a stumbling block ever since the days of von Graaf,
who had described as the ova the follicles now bearing his name.[254] Even
von Graaf had noticed that the early uterine eggs were smaller than the
supposed ovarian eggs; Prévost and Dumas[255] had observed the presence in
the Graafian follicle of a minute spherical body, which, however, they
hesitated to call the ovum; it was left to von Baer to elucidate the
structure of the follicle and to prove that this small sphere was indeed
the mammalian ovum. His discovery was confirmed by Sharpey and by Allen
Thomson. Von Baer found the germinal vesicle in the eggs of frogs,
snakes, molluscs, and worms, but not in the mammalian ovum; he
considered the whole mammalian ovum to be the equivalent of the germinal
vesicle of birds--a comparison rightly questioned by Purkinje (1834). In
1834 Coste[256] discovered in the ovum of the rabbit a vesicle which he
considered to be the germinal vesicle of Purkinje; he observed that it
disappeared after fertilisation. Independently of Coste, and very little
time after him, Wharton Jones[257] found the germinal vesicle in the
mammalian ovum. Valentin in 1835,[258] Wagner in 1836,[259] and Krause in
1837,[260] added considerably to the existing knowledge of the structure
of the ovum. Wagner in his _Prodromus_ called attention to the
widespread occurrence, within the germinal vesicle of a darker speck
which he called the _Keimfleck_ or germinal spot, known sometimes as
Wagner's spot. He recognised the _Keimfleck_ in the ova of many classes
of animals from mammals to polyps. Frequently more than one _Keimfleck_
occurred.

Schwann had therefore a good deal of exact knowledge to go upon in
discussing the significance of the ovum for the cell-theory. There were
two possible interpretations. Either the ovum was a cell and the
germinal vesicle its nucleus, or else the germinal vesicle was itself a
cell within the larger cell of the ovum and the germinal spot was its
nucleus. Schwann had some difficulty in deciding which of these views to
adopt, but he finally inclined to the view that the ovum is a cell and
the germinal vesicle its nucleus, basing his opinion largely upon
observations by Wagner which tended to prove that the germinal vesicle
was formed first and the ovum subsequently formed round it. But the ovum
was not, in Schwann's view, a simple cell, for within it were contained
yolk-granules, one set apparently containing a nucleus, the others not.
Even the second set, those composing the yellow yolk, were considered by
Schwann to deserve the name of cells, because, although a nucleus could
not be observed in them, they had a definite membrane, distinct from
their contents--a conception of the cell obviously dating from the
earliest botanical notions of cells as little sacs. The yolk cells were
not mere dead food material but living units which took part in the
subsequent development of the egg. The relation between the unfertilised
egg and the blastoderm which arises from it is not made altogether clear
by Schwann. According to his account the cells of the blastoderm are
formed actually in the ovum. Round the nucleus of the egg appears a
_Niederschlag_ or precipitate which is the rudiment of the blastoderm
(p. 68). When the egg leaves the ovary the nucleus disappears, leaving
behind it this rudiment of the blastoderm, which rapidly grows and
increases in size. The blastoderm of the chick before incubation is
found to be composed of spherical anucleate bodies which Schwann
considers to be cells, because they almost certainly develop into the
cells of the incubated blastoderm, which are clearly recognisable as
such after eight hours' incubation. The serous and mucous layers can be
distinguished after sixteen hours' incubation, and it is found that the
cells of the serous layer contain definite nuclei, though such seem to
be absent in the cells of the mucous layer. Between the two layers other
cells are formed belonging to the vessel layer, which is, however, in
Schwann's opinion not a very definitely individualised layer.

Schwann's next step is a detailed demonstration of the origin of each
tissue from simple cells such as those composing the incubated
blastoderm.

"The foregoing investigation has taught us that the whole ovum shows
nothing but a continual formation and differentiation of cells, from the
moment of its appearance up to the time when, through the development of
the serous and mucous layers of the blastoderm, the foundation is given
for all the tissues subsequently appearing: we have found this common
parent of all tissues itself to consist of cells; our next task must be
to demonstrate not only in this general way that tissues originate from
cells, but also that the special formative mass of each tissue is
composed of cells, and that all tissues are either constituted by simple
cells or by one or other of the manifold kinds of modified cells" (p.
71). Five classes of tissue can be distinguished, according to the
extent and manner of the modifications which the cells composing them
have undergone. There are first of all independent and isolated cells,
such as the corpuscles of the blood and lymph, not forming a coherent
tissue in the ordinary sense. Next there are the assemblages of cells
lying in contiguity with one another, but not in any way fused; examples
of this class are the epidermal tissues and the lens of the eye. In the
third class come tissues the cells of which have fused by their walls,
but whose cell-cavities are not in continuity, such as osseous tissue
and cartilage. In the tissues of the fourth class, comprising the most
highly specialised of all, not only are the cell-walls continuous but
also the cell-cavities; to this class belong muscle, nerve and capillary
vessels. A fifth class, of rather a special nature, includes the fibrous
tissues of all kinds. This is the first classification of tissues upon a
cellular basis, and it marks the foundation of a new histology which
took the place of the "general anatomy" of Bichat. The exhaustive
account which Schwann gives of the structure and development of the
tissues in this section of his book constitutes the first systematic
treatise on histology in the modern sense, and it is still worth
reading, in spite of many errors in detail.

Schwann found it easy to demonstrate the cellular nature of the tissues
of his first three classes. With the other two classes he had more
difficulty. Fibres of all kinds, he considered, arose by an elongation
of cells, which afterwards split longitudinally into long strips,
forming as the case might be white or elastic fibrous tissue.
Muscle-fibres and nerve-fibres were formed in a totally different way,
by coalescence of cells; each separate muscle-fibre and nerve-fibre was
thus a compound cell. Capillaries, Schwann held, were formed by cells
hollowed out like drain-pipes, and set end to end--a mistaken view soon
corrected by Vogt (_Embryologie des Salmones_, p. 206, 1842).

In this detail part of his book Schwann accumulates material for a
general theory of the cell which he develops in the third and last
section. Taking up the physiological or dynamical standpoint, he points
out that one process is common to all growth and development of tissues
both in animals and plants, namely, the formation of cells, a process
which he conceives to take place in the following manner. There is,
first of all, a structureless substance, the cytoblastem, the matrix in
which all cells originate. The cytoblastem may be either inside the
cells, or, more usually, in the spaces between them. It is not a
substance of definite chemical and physical properties, for the matrix
of cartilage and the plasma of the blood alike come within the
definition. It has largely the significance of food material for the
developing cells. In plants, according to Schleiden, cells are never
formed in the intercellular substance--the cytoblastem is within the
cells; but extracellular cell formation seems to be the general rule in
animals. An intracellular formation of cells occurs only in the ovum, in
cartilage cells and chorda cells and in a few others, and even there it
is not the exclusive method of formation; a formation of cells within
cells never occurs in muscles and nerves, nor in fibrous tissue (p.
204). In the cytoblastem granules appear, which gradually increase in
size and take on the characteristic shape of nuclei; round each of these
a young cell is formed. Sometimes the young cells appear to have no
nuclei, as in the intracellular brood of chorda cells, but, as a rule, a
nucleus is clearly visible. The nucleus is indeed the most
characteristic constituent of the cell. "The most important and most
constant criterion of the existence of a cell is the presence or absence
of the nucleus," writes Schwann near the beginning of his book (p. 43).

As a general rule the nucleolus is formed first, and round it by a sort
of condensation or concretion the nucleus, which is frequently hollow,
and round this again, by a somewhat similar process, the cell. "The
whole process of the formation of a cell consists in the precipitation
round a small previously formed corpuscle (the nucleolus) of first one
layer (the nucleus) and then later round this a second layer (the cell
substance)" (p. 213). The outermost layer of the cell usually thickens
to form the membrane, but this membrane formation does not always occur,
and the membrane is not present in all cells. The nucleus is formed in
exactly the same manner as the cell, and it might with much truth itself
be called a cell--a cell of the first order, while ordinary nucleated
cells might be designated cells of the second order (p. 212). In
anucleate cells there is probably only a single process of layer
formation round an infinitely small nucleolus. In almost all nucleate
cells the nucleus is resorbed when the cell reaches its full
development, and it is larger and more important the younger the cell
is.

The cell was for Schwann not a morphological concept at all, but a
physiological; the cell was a dynamical, not a statical unit.
Cell-formation was the process at the back of all production of life,
and cells were the centres of all vital activity. Each cell was itself
an organism, and its life and activities were to some extent independent
of the lives and activities of all the other cells. The multicellular
organism was a colony of unicellular organisms, and its life was a sum
of the lives of its constituent elements. This "theory of the organism,"
which holds so important a place in biology even at the present day, is
developed by Schwann in the concluding pages of his book.

He begins by contrasting the teleological with the materialistic
conception of living things. In the teleological view, a special force
works in the living organism, guiding and directing its activities
towards a purposeful end. According to the materialistic view there are
no other forces at work in the living organism than those which act in
the inorganic realm, or at least there are none but forces at one with
these in their blindness and necessity. True, the purposiveness of
living processes cannot be denied; but its ground lies, according to
this view, not in a vital force which guides and rules the individual
life, but in the original creation and collocation of matter according
to a rational plan. The purposiveness of life is part of the
purposiveness of the universe; just as the stars circle for ever in
harmoniously adjusted paths, so do the processes of life work together
towards a common end. Both are the inevitable result of the original
distribution of matter in the primitive chaos, a distribution fixed by a
rational and foreknowing Being (p. 222).

Which of the two conceptions is to be adopted in biology? Teleological
explanations have long been banished from the physical sciences, and in
biology they are only a last resort when physical explanations have
proved incomplete (p. 223). And if the ground of the purposiveness of
living Nature is the same as the ground of the purposiveness of the
universe, is it not reasonable to suppose that explanations which have
proved satisfactory for inorganic things will in time with sufficient
knowledge prove adequate also for organic things?

The teleological conception, again, leads to difficulties particularly
when it is applied to the facts of reproduction. If we suppose that a
vital force unifies and coordinates the organism and is its very
essence, we must also suppose that this force is divisible and that a
part of it--separated in reproduction--can bring about the same results
as the whole. If on the contrary the forces having play in the organism
are the mere result of the particular combination of the matter
composing it, the reconstruction of a particular combination of
molecules in the ovum is all that is necessary to set development
a-going along exactly the course taken by the ovum of the parent.
Another argument against the teleological view is derived from the facts
of the cell-theory. The cell-theory tells us that the molecules of the
living body are not immediately built up in manifold combinations to
form the organism, but are formed first into unit-constructions or
cells, and that these units of composition are invariably formed in all
development, of plants and animals alike, however diverse the goal of
development may be. If there were a vital principle would we not expect
to find that, scorning this roundabout way of reaching its goal, it went
straight to the mark, taking a different and distinctive course for each
individual development, building up the organism direct without the
intermediary of cells? But since there is a universal principle of
development, namely, the formation of cells, does it not seem that the
cells must be the true organisms, that the whole "individual" organism
must be an aggregate of cells, and that the concept of individuality
applied to the organism is accordingly a logical fiction? And it is just
upon this notion of the individuality of the organism that the
teleological concept is based. The teleological view can perhaps not be
completely refuted until the adequacy of materialistic explanations has
been finally shown; but it is certain that the most promising method for
research is the materialistic (p. 226).

"We start out then from the assumption that the basis of the organism is
not a force acting according to a definite plan; on the contrary, the
organism arises through the action of blind and necessary laws, of
forces which are as much implicit in matter as those of the inorganic
world. Since the chemical elements in organic Nature differ in no way
from those of inorganic Nature, the ground or cause of organic phenomena
can consist only in a different mode of combination of matter, either in
a peculiar mode of combination of the elementary atoms to form atoms of
the second order, or in the particular arrangement of these compound
molecules to form the separate morphological units of the organism or
the whole organism itself" (p. 226). Accepting then the materialistic
conception of the organism, we have to consider this further problem.
Does the ground of organic processes lie in the whole organism or in its
elementary parts? Translated into terms of metabolism--note the
physiological point of view--the question runs, are metabolic processes
the result of the molecular construction of the organism as a whole, or
does the centre of metabolic activity lie in the cell? Is it the cell
rather than the organism that is the immediate agent of assimilatory
processes? In the first alternative the cause of the growth of the
constituent parts lies in the totality of the organism; in the other
alternative:--"Growth is not the result of a force having its ground in
the organism as a whole, but each of the elementary parts possesses a
force of its own, a life of its own, if you will; that is to say, in
each elementary part the molecules are so combined as to set free a
force whereby the cell is enabled to attract new molecules and so to
grow, and the whole organism exists only through the reciprocal action
of the single elementary parts.... In this eventuality it is the
elementary parts that form the active element in nutrition, and the
totality of the organism can be indeed a condition, but on this view it
cannot be a cause" (p. 227).

To help in the decision of this question, appeal must be made to the
facts established as to the cellular nature of the organism and of its
reproductive elements. We know that every organism is composed of cells,
which are formed and grow according to the same laws wherever they are
found, whose formation therefore is everywhere due to the same forces.
If we find that certain of these cells--all of which we know to be
essentially identical one with another--have the power when separated
from the others of growing and developing into new organisms, we can
infer that not only such cells but also all other cells have this
assimilatory power. The ova of animals, the spores of plants, the
isolated cells of lower organisms in general, all show the power of
separate assimilation and development. "We must therefore, in general,
ascribe to the cell an individual life, that is to say, the combination
of the molecules in the single cell does suffice to produce the force
whereby the cell is enabled to draw to itself new molecules. The ground
of nutrition and growth lies not in the organism as a whole, but in the
separate elementary parts, the cells. The fact that it is not every cell
that can continue to grow when separated from the organism is not in
itself an objection to this theory, any more than it is an objection to
the individual life of a bee that it cannot continue to exist apart from
the swarm. The activation of the forces existing within the cell depends
on conditions which the cell encounters only in connection with the
whole" (pp. 228-9).

Schwann's next step is to discover what are the essential forces active
in the cell, and here he enters the realm of hypothesis. He finds they
can be reduced to two--an attractive force and a metabolic force. The
attractive force is seen in the process of cell-formation, where first
of all the nucleolus is formed by a concentration and precipitation of
substances found free in the cytoblastem, and in the same way the
nucleus and later the cell are laid down as concentric precipitates from
the cytoblastem. Cell-formation also involves the second or metabolic
force, by means of which the cell alters the chemical composition of the
medium surrounding it so as to prepare it for assimilation. Schwann's
attractive force brings about the actual taking up of the prepared
substance; his metabolic force is the cause of the digestion of food
substances, and is nearly identical with enzyme action. With what
inorganic process, he now asks (p. 239), can the process of
cell-formation be most nearly compared, and the answer obviously is,
with the process of crystallisation. Cells are, it is true, quite
different in shape and consistency from crystals, and they grow by
intussusception, not by apposition--their plastic or attractive forces
seem therefore to be different. A still more important difference is
that the metabolic force is peculiar to the cell. Yet there are
important analogies between crystals and cells. They agree in the
important respect that they both grow in solutions at the cost of the
dissolved substance, according to definite laws, and develop a definite
and characteristic shape. It might even be maintained, Schwann thinks,
that the attractive force of crystals is really identical with that of
cells, and that the difference in result is due merely to the difference
between the substance of the cell and the substance of the crystal. He
points out how organic bodies are remarkable for their powers of
imbibition, and he seeks to show that the cell is the form under which a
body capable of imbibition must necessarily crystallise, and that the
organism is an aggregate of such imbibition-crystals. The analogy
between crystallisation and cell-formation he works out in the following
manner:--"The substance of which cells are composed possesses the power
of chemically transforming the substance with which it is in immediate
contact, in somewhat the same way as the well-known preparation of
platinum changes alcohol into acetic acid. Each part of the cell
possesses this property. If now the cytoblastem is altered by an already
formed cell in such a way that a substance is formed that cannot become
part of the cell, it crystallises out first as the nucleolus of a new
cell. This in its turn alters the composition of the cytoblastem. A part
of the transfomed substance may remain in solution in the cytoblastem or
may crystallise out as the beginning of a new cell; another part, the
cell-substance, crystallises round the nucleolus. The cell-substance is
either soluble in the cytoblastem and crystallises out only when the
latter is saturated with it, or it is insoluble and crystallises as soon
as it is formed, according to the aforementioned laws of the
crystallisation of imbibition-bodies; it forms thus one or more layers
round the nucleolus, etc. If one imagines cell-formation to take place
in this way, one is led to think of the plastic force of the cell as
identical with the force by means of which a crystal grows" (pp.
249-50).

Two difficulties have to be faced by this theory--(1) the origin of the
metabolic power of the cells, (2) the reason why the cells arrange
themselves so as to form an organism of complex and definite structure.
Schwann tries to explain the origin of the "metabolic" action, the
analogy of which with the contact-action of colloidal platinum he
recognises, by attributing it to the peculiar structural arrangements of
molecules. In attempting to account for the harmonious structure of the
organism he points to the analogy of ordinary crystals, which often form
complex and regular tree-like arrangements; plants in particular
resemble these regularly shaped crystal-aggregates.

The whole ingenious theory is offered merely as an hypothesis and a
guide to research. It is interesting as one of the most carefully
thought-out attempts ever made to give a thorough-going materialistic
account of the origin and development of organic form, and it arose
directly out of the cell-theory.

Schleiden and Schwann started out from an erroneous theory of the origin
and development of cells, which impaired to some extent the value of
their results. It was not long, however, before their theory of the
origin of cells by "crystallisation" from an intra- or extra-cellular
cytoblastem was challenged and overthrown, and the generalisation that
cells originate by division from pre-existing cells put in its place.

This was established for plant cells by Meyen, Unger, von Mohl, Naegeli
and Hofmeister in or about the forties.[261] Criticism of the
Schwann-Schleiden theory from the zoological side was suggested by the
study of the segmentation of the ovum--the developmental process in
which the multiplication of cells is most easily observed. The
segmentation of the ovum was well known to Schwann, for the process had
been described in the frog by Prévost and Dumas in 1824,[262] in the frog
and newt by Rusconi,[263] and an elaborate study of the process in the
frog had been made by von Baer.[264] Schwann indeed suspected that there
must be some connection between the segmentation of the ovum and the
formation of cells, but he did not realise that the cellular blastoderm
of the chick was formed by the division or segmentation of the egg-cell.

Segmentation was soon found to be of widespread occurrence. Von Siebold
in 1837 described the process in Entozoa,[265] and in the same year Lovén
saw segmentation in _Campanularia_,[266] and Sars in the starfish and in
Nudibranchs.[267]

In 1838 Bischoff[268] observed segmentation in the mammalian ovum, and the
whole course of segmentation in the ovum of the rabbit from the 2-celled
to the morula stage was carefully described and figured by Barry[269] in
1839. C. Vogt[270] in 1842 described segmentation in _Coregonus_ and
_Alytes_. The discovery of segmentation in the ovum of birds was not
made until 1847, by Bergmann,[271] confirmed independently by Coste[272]
in 1850. By 1848 segmentation had been noted in _Hydra_ and various
hydroids, in acalephs, in starfish, polyzoa, nematodes, rotifers,
leeches, oligochætes, polychætes, in most groups of molluscs and
arthropods, and in all the vertebrate classes.[273]

The process was at first held to be merely one of yolk-division, or
_Dotterfurchung_, and its details were by most interpreted in the light
of the Schleiden-Schwann theory of cell-formation.

The first steps towards a truer conception of the process seem to have
been taken by Bergmann, who in 1841[274] called attention to the presence
of nuclei in the segmentation-spheres of the frog's egg, and by Bagge in
the same year, who observed that division of the nuclei preceded the
multiplication of the segmentation spheres.[275] He considered the nuclei
to be anucleate cells, and the same view was taken by Kölliker in
1843.[276] Next year, however, in his classical paper on Cephalopod
development[277] Kölliker came to the opinion that they were really
nuclei. He showed that segmentation was brought about by cell-division,
that between "total" and "partial" segmentation there was a difference
of degree and not of kind, and that the cells of the body were formed by
division of the segmentation spheres. He held, however, that the nuclei
multiplied endogenously and not by division. The division of nuclei was
observed by Coste in 1846.[278] Leydig in 1848[279] took the necessary step
in advance and maintained that the nuclei as well as the cells increased
always by division. He was supported by Remak, who in a paper of
1852,[280] and more fully in his monumental _Untersuchungen über die
Entwickelung der Wirbelthiere_ (Berlin, 1850-55), proved that in the
frog's egg at least segmentation was a simple process of cell-division,
initiated always by division of the nucleus.[281]

One point Remak left undecided--the fate of the _Keimbläschen_ or
egg-nucleus. It was generally held, even so late as the 'fifties, that
the egg-nucleus disappeared just before segmentation began--Bischoff
clung to this belief even in 1877.[282] Though Barry had held in 1839 that
the egg-nucleus does not disappear in segmentation, J. Müller seems to
have been the first actually to prove that it forms by division the
nuclei of the first two segmentation spheres. He furnished the
demonstration in the egg of _Entoconcha mirabilis_,[283] and his paper was
known to Remak, who could not, however, observe a similar division of
the egg-nucleus in the frog. Müller's discovery was confirmed for
_Oceania armata_ by Gegenbaur,[284] and for _Notommata sieboldii_ by
Leydig.[285]

In 1854 Virchow,[286] previously a supporter of Schwann, crystallised the
new views in the famous phrase--_Omnis cellula e cellula_--and gave wide
publicity to them in his classical lectures on Cellular Pathology,
delivered in 1858.[287] The new doctrine of cell-formation was also taught
by Leydig[7] in his text-book of histology, published in 1857.

The Schleiden-Schwann theory of the origin of cells by generation in a
cytoblastem was now definitely overthrown.

The importance of the protoplasmic content of the cell was brought into
prominence through the work of Dujardin,[289] Purkinje,[290] Cohen[291] and
Max Schultze.[292] The last-named in 1861 proposed a definition of the
cell which might be accepted at the present day. "A cell," he wrote, "is
a little blob of protoplasm containing a nucleus" (p. 11).

    [238] _Theoria generationis_, Halae, 1759.

    [239] See J. v. Sachs, _Geschichte der Botanik_, book ii.,
    Eng. Trans., 2nd impr., 1906.

    [240] Müller's _Archiv_, pp. 137-76, 1838.

    [241] _Trans. Linnean Soc._, xvi., p. 710, 1833.

    [242] _Myxinoiden_, i. Theil., p. 89, 1835.

    [243] _Ann. Sci. nat._ (2) (_Zool._) ii., pp. 107-18, pl.
    11, 1834.

    [244] _Proc. Phil. Soc. Glasgow_, xix., pp. 71-125,
    1887-8.

    [245] _Traité sur le venin de la vipère_, 1781.

    [246] Müller's _Archiv_, 1836.

    [247] J. Müller, _Jahresbericht ü. d. Fortschritte der
    anat.-physiol. Wissenschaften im Jahre_ 1838. Müller's
    _Archiv_, 1838.

    [248] _Symbolæ ad anatomiam villorum imprimis eorum
    epithelii_, Berlin, 1837.

    [249] _U. d. Ausbreitung des Epitheliums im menschlichen
    Körper_. Müller's _Archiv_, 1838.

    [250] See Schwann's _Bemerkungen_ at the end of his
    _Mikroskopische Untersuchungen_.

    [251] Republished in Ostwald's _Klassiker der exakten
    Wissenschaften_, No. 176, Leipzig, 1910. References in
    the text are to the original pagination.

    [252] _Symbolæ ad ovi avium historiam_.

    [253] _De ovi mammalium et hominis genesi_.

    [254] _De mulierum organis_, 1672.

    [255] _Ann. Sci. nat._, iii., p. 135, 1842.

    [256] _Recherches sur la génération des Mammifères_.
    Report by Academy Committee. _Ann. Sci. nat._ (2)
    (_Zool._) ii., pp. 1-18, 1834; also _Embryogénie
    comparée_, 1837.

    [257] _Lond. and Edin. Phil. Mag._ (3) vii., 1835; _Phil.
    Trans._ 1837.

    [258] _Handbuch der Enfwickelungsgeschichte_, 1835, and
    Müller's _Archiv_, 1836.

    [259] _Prodromus historiæ generationis hominis atque
    animalium_, Lipsiæ, 1836.

    [260] Müller's _Archiv_, 1837.

    [261] Sachs, _History of Botany_, Book ii.

    [262] _Ann. Sci. nat._, i., pp. 110-14, 1824. Swammerdam
    is said to have observed the 2-celled stage in the egg
    of the frog (_Bibl. Nat._, 1752), and Rösel v. Rosenhof
    the same stage in the tree-frog (_Hist. nat. ranarum
    nostratium_, 1758).

    [263] _Développement de la grenouille commune_, Milan,
    1826. _Biblioteca italiana_, lxxix., 1836, and Müller's
    _Archiv_, 1836. Agassiz is said by Vogt (1842) to have
    seen segmentation in the Perch as early as 1831.

    [264] Müller's _Archiv_, 1836.

    [265] In Burdach, _Die Physiologie als
    Erfahrungswissenschaft_, 2nd Ed., vol. ii.

    [266] Wiegmann's _Archiv_, 1837.

    [267] _Bericht Versamml. deutsch. Naturf. in Prag_, 1837.

    [268] _Bericht Versamm. deutsch. Naturf. in Freiburg_,
    1838. Later in his _Entw. d. Wirbelth_., and in his
    papers on the development of the rabbit.

    [269] _Phil. Trans._, 1839. See particularly Pl. vi.,
    figs. 105-12.

    [270] _Embryologie des Salmones_ 1842.

    [271] Müller's _Archiv_, 1847.

    [272] _C.R. Acad. Sci._, xxx., p. 638.

    [273] See review by Leydig in _Isis_, 1848, pp. 161-193.

    [274] Müller's _Archiv_, pp. 89-102, 1841.

    [275] _De evolution Stronzyli auric. el Ascaridis acum._,
    Erlangen, 1841.

    [276] Müller's _Archiv_, pp. 66-141, 1843.

    [277] _Entwickelungsgeschichte der Cephalopoden_, Zurich,
    1844.

    [278] _Froriep's Notizen_, No. 800, 1846.

    [279] _Isis_, 1848.

    [280] Müller's _Archiv_, p. 47, 1852, also 1854 and 1858.

    [281] See particularly Plate IX., figs. 3-7.

    [282] _Hist.-krit. Bemerkungen zu den neuesten
    Mittheilungen ü. d. erste Entwickelung d.
    Säugethiereier_, München, 1877.

    [283] _Monatsber. Akad. Wiss. Berlin_, 1851.

    [284] _Zur Lehre von Generationswechsel u. d. Fortpflanzen
    d. Medusen u. Polypen_.

    [285] _U. d. Bau u. d. system. Stellung d. Räderthiere_,
    1854.

    [286] _Arch f. path. Anat. Phys._, vii., pp. 1-39, 1854.
    Also in his _Beiträge z. spec. Path. u. Therapie_.

    [287] _Die Cellularpathologie_, Berlin, 1858.

    [288] _Lehrbuch der Histologie_, 1857.

    [289] _Ann, Sci. nat._ (2) iii., pp. 108-9 and pp. 312-4,
    1835. Also iv, pp. 343-77.

    [290] 1839 or 1840.

    [2913] _Nova Acta Acad. Leop._, xxii., 1850. Trans. in 1853
    for Ray Society.

    [292] _Arch. f. Anat. u. Physiol._, pp. 1-27, 1861.



CHAPTER XII

THE CLOSE OF THE PRE-EVOLUTIONARY PERIOD


The influence of the cell-theory on morphology was not altogether happy.
The cell-theory was from the first physiological; cells were looked upon
as centres of force rather than elements of form, and the explanation of
all the activities of the organism was sought in the action of these
separate dynamic centres. There resulted a certain loss of feeling for
the problems of form. The organism was seen no longer as a cunningly
constructed complex of organs, tissues and cells; it had become a mere
cell-aggregate; the higher elements of form were disregarded and
ignored.

We have seen this physiological attitude expressed with the utmost
clearness by the founder of the cell-theory himself; we shall see the
same attitude taken up by most of his successors. Thus Vogt, who was
later to become one of the protagonists of materialism in Germany,
developed in his memoir on the embryology of _Coregonus_[293] the theory
of the independent or individual life of the cell. "Each cell," he
wrote, "represents in some measure a separate organism, and while their
development necessarily conforms to the general plan and the particular
tendencies of the parent organism, they nevertheless each follow their
own particular tendency and do not lose their independence until, by
reason of the metamorphoses which they undergo, they lose their cellular
nature" (p. 275).

And again, "... we are obliged to admit the existence in the cell of an
independent life, which makes its development self-sufficient.... Each
cell consequently represents a little independent organism, which
assimilates foreign substances, builds them up, and rejects those that
are useless; from this point of view the embryo can be compared up to a
certain point with a zoophyte stock, of which each polyp, while living
its own independent life, is yet incorporated in the common corm, which
impresses its distinctive character upon every polyp" (p. 293).

Classical expression was given to the "colonial theory" of the organism
by Virchow in his lectures on "Cellular Pathology."[294] For Virchow the
organism resolves itself into an assemblage of living centres, the
cells; the organism has no real existence as a unity, for there is no
one single centre from which its activities are ruled. Even the nervous
system, which appears to act as a co-ordinating centre, is itself an
aggregate of discrete cells. "A tree is a body of definite and orderly
composition, the ultimate elements of which, in every part of it, in
leaf and root, in stem and flower, are cellular elements--so also are
animal forms. _Every animal is a sum of vital units_, each of which
possesses the full characteristics of life. The character and the unity
of life cannot be found in one definite point of a higher organisation,
for example in the brain of man, but only in the definite, constantly
recurring disposition shown individually by each single element. It
follows that the composition of the major organism, the so-called
individual, must be likened to a kind of social arrangement or society,
in which a number of separate existences are dependent upon one another,
in such a way, however, that each element possesses its own particular
activity, and, although receiving the stimulus to activity from the
other elements, carries out its own task by its own powers" (2nd ed.,
pp. 12-13).

Analysis, decomposition, or disintegration of the organism is here
pushed to its extreme point, and the problem of recomposition, synthesis
and co-ordination shirked or forgotten.

The harmful influence of the cell-theory upon morphology did not pass
unnoticed by the broader-minded zoologists of the day. Virchow's earlier
paper[295] on the application of the cell-theory to physiology and
pathology called forth a vigorous protest from Reichert,[296] who
discussed in a very instructive way the contrast between the older
"systematic" and the newer "atomistic" attitude to living Nature.

Is it really true, he asks, that the cell is the dominant element in all
organisation; is the cell comparable in importance to the atom of the
chemists; or is it not rather the servant of a higher regulatory power?
Johannes Müller, who was Reichert's master, had in his _Physiology_[297]
argued splendidly for the existence of a creative force which guides and
rules development, and brings to pass that unity and harmony of
composition which distinguish living things from inorganic products.
Reichert sought in vain in the writings of the biological "atomists" for
any smallest recognition of these broader characteristics of living
things upon which Müller had rightly laid stress. For the atomists the
cell was the only element of form; they ignored the combination of cells
to form tissues, of tissues to form organs, of organs to form an
organism. For the morphologists the cell was one element among many, and
the lowest of all.

The difference of attitude is clearly shown if we consider from the two
points of view a complicated organ-system such as the central nervous
system. The atomist sees in this a mere aggregate of cells or at the
most of groups of cells. "The morphologist," on the other hand, "sees in
the central nervous system a _proximate_ element in the composition of
the body--a primitive organ. From this point of view he apprehends and
judges its morphological relations with, in the first place, the other
co-ordinated primitive organs in the system as a whole; in all this the
cells remain in the background, and have nothing to do directly with the
determination of these morphological relations" (p. 6). Within the
nervous system there are separate organs which stand to one another in a
definite morphological and functional relationship. These organs are, it
is true, composed of cells; but between the form and connections of
these organs and the cells which compose them there is no direct and
necessary relation (p. 6). It is true that the cell is the ultimate
element of organic form, and that all development takes place by
multiplication and form-change of cells. Yet is the cell in all this not
independent of the unity of the developing embryo, and what the cells
produce, they produce, so to speak, not of their own free will, nor by
chance, but under the guiding influence of the unity of the whole, and
in a certain measure as its agents (p. 7). The atomists will not admit
the truth of this; they see in development nothing more than a process
of the form-change and multiplication of cells. The full meaning of
development escapes them, for they take no cognisance of the increasing
complexity of the embryo, of the separating-out of tissues, of the
moulding of organs, of the harmonious adaptation and adjustment of the
parts to form a working whole.

In general, the fault of the atomists is that they do not respect the
limits which Nature herself has prescribed to the process of logical
analysis and disintegration of the organism; they do not recognise the
existence of natural and rational units or unities; they forget the one
great principle of rational analysis, "that, by universally valid,
inductive, logical method, natural objects must in all cases be accepted
and dealt with in the combination and concatenation in which they are
given" (p. 10).

The atomists at least recognised one natural organic element, the cell;
the materialistic physiologists of the time resolved even this unity
into an aggregate of inorganic compounds, and regarded the organism
itself as nothing but a vastly complicated physico-chemical mechanism.
From this point of view morphology had no right of existence, and we
find Ludwig, one of the foremost of the materialistic school,
maintaining that morphology was of no scientific importance, that it was
nothing more than an artistic game, interesting enough, but completely
superseded and robbed of all value by the advance of materialistic
physiology.[298]

Naturally enough, morphologists did not accept this rather contemptuous
estimate of their science, but held firmly to the morphological
attitude. So Leuckart in his reply to Ludwig, so Rathke in a letter to
Leuckart published in that reply, so Reichert in his _Bericht_, so J. V.
Carus in his _System der thierischen Morphologie_,[299] upheld the
validity, the independence, of morphological methods. Leuckart and
Rathke called attention to the absolute impossibility of explaining by
materialistic physiology the unity of plan underlying the diversity of
animal form. J. V. Carus, who was convinced of the validity of
physiological methods within their proper sphere, drew a sharp
distinction between systematics and morphology on the one hand, and
physiology on the other. Physiology had nothing to do with the problems
of form at all; its business was to study the physical and chemical
processes which lay at the base of all vital activities. Morphology, on
its part, had to accept form as something given, and to study the
abstract relations of forms to one another. "On this point," he writes,
"stress is to be laid, that morphology has to do with animal form as
something _given_ by Nature, that though it follows out the changes
taking place during the development of an animal and tries to explain
them, it does not enquire after the conditions whose necessary and
physical consequence this form actually is" (p. 24). He expressed indeed
a pious hope (p. 25) that physiology might one day be so far advanced
that it could attempt with some hope of success to discover the
physico-chemical determinism of form, but this remained with him merely
a pious hope. Reichert, in his _Bericht_, applied to the rather wild
theorisings of the physiologist Ludwig the same clear commonsense
criticism that he bestowed on the other "atomists."

It would take too long to describe the great development that
materialistic physiology took at this time, and to show how the
separation of morphology from physiology, which originally took place
away back in the 17th century, had by this time become almost absolute.
The years towards the end of the first half of the century marked indeed
the beginning of the classical period as well of physiology as of
dogmatic materialism. Moleschott and Buchner popularised materialism in
Germany in the 'fifties, while Ludwig, du Bois Reymond and von Helmholtz
began to apply the methods of physics to physiology. In France, Claude
Bernard was at the height of his activity, rivalled by workers almost as
great. The doctrine of the conservation of energy was established about
this same time.

Between the cell-theory on the one side, and physiology on the other, it
was a wonder that morphology kept alive at all. The only thing that
preserved it was the return to the sound Cuvierian tradition which had
been made by many zoologists in the 'thirties and 'forties. It is a
significant fact that this return to the functional attitude coincided
in the main with the rise of marine zoology, and that the man who most
typically preserved the Cuvierian attitude, H. Milne-Edwards, was also
one of the first and most consistent of marine biologists. Milne-Edwards
describes in his interesting _Rapport sur les Progrès récents des
Sciences zoologiques en France_ (Paris) 1867, how "About the year 1826,
two young naturalists, formed in the schools of Cuvier, Geoffroy and
Majendie, considered that zoology, after having been purely descriptive
or systematic and then anatomical, ought to take on a more physiological
character; they considered that it was not enough to observe living
objects in the repose of death, and that it was desirable to get to
understand the organism in action, especially when the structure of
these animals was so different from that of man that the notions
acquired as to the special physiology of man could not properly be
applied to them" (p. 17). The two young naturalists were H.
Milne-Edwards and V. Audouin. In pursuance of these excellent ideas they
set to work to study the animals of the seashore, producing in 1832-4
two volumes of _Recherches pour servir à l'histoire naturelle du
littoral de la France_. After Audouin's early death A. de Quatrefages
was associated with Milne-Edwards in this pioneer work, and their
valiant struggles with insufficient equipment and lack of all laboratory
accommodation, and the rich harvest they reaped, may be read of in
Quatrefage's fascinating account of their journeyings.[300] Note that
though they called themselves physiologists they meant by physiology
something very different from the mere physical and chemical study of
living things. They were interested, as Cuvier was, primarily in the
problems of form; they sought to penetrate the relation between form and
function; their chief aim was, therefore, the study not of physiology[301]
in the restricted sense, but physiological morphology. As a matter of
fact they produced more taxanomic and anatomical work than work on
physiological morphology, but this was only natural, since such a wealth
of new forms was disclosed to their gaze. Milne-Edwards' masterly
_Histoire Naturelle des Crustacés_[302] and A. de Quatrefage's _Histoire
Naturelle des Annelés marins et d'eau douce_[303] were typical products of
their activity.

In the North, men like Sars and Lovén were starting to work on the
littoral fauna of the fjords; in Britain, Edward Forbes was opening up
new worlds by the use of the dredge; Johannes Müller was using the
tow-net to gather material for his masterly papers on the metamorphoses
of Echinoderms.[304] Work on the taxonomy and anatomy of marine animals
was in general in full swing by the 'fifties and 'sixties.

This return to Nature and to the sea had a very beneficial effect upon
morphology, bringing it out from the laboratory to the open air and the
seashore. It saved morphology from formalism and aridity, and in
particular from a certain narrowness of outlook born of too close
attention paid to the details of microscopical anatomy. It brought
morphologists face to face again with the wonderful diversity of organic
forms, with the unity of plan underlying that diversity, with the
admirable adjustment of organ to function and of both to the life of the
whole.

Milne-Edwards' theoretical views, as expounded in his _Introduction à la
zoologie générale_ (1851), well reflect this Cuvierian attitude.[305] He
acknowledges himself the debt he owes to Cuvier; "the further I advance
in the study of the sciences which he cultivated with so sure a hand,"
he writes in 1867, "the more I venerate him."

Milne-Edwards frankly takes up the teleological standpoint, and
interprets organic forms on the assumption that they are purposive and
rationally constructed. "To arrive at an understanding of the harmony of
the organic creation," he writes, "it seemed to me that it would be well
to accept the hypothesis that Nature has gone about her work as we would
do ourselves according to the light of our own intelligence, if it were
given us to produce a similar result. Comparing and studying living
things as if they were machines created by the industry of man, I have
tried to grasp the manner in which they might have been invented, and
the principles whose application would have led to the production of
such an assemblage of diversified instruments" (p. 435). The problem is
to discover the laws which rule the diversity of organic forms. The
first and most obvious of these laws is the "law of economy," or the law
of unity of type. Nature, as Cuvier pointed out, has not had recourse to
all the possible forms and combinations of organs; she appears to work
with a limited number of types and to get the greatest possible
diversity out of these by varying the proportions of the constitutive
materials of structure. Within the limits of each type Nature has
brought about diversity by raising her creatures to different degrees of
perfection. This is the second law of organic form, and it is this law
that Milne-Edwards chiefly elaborates. Degrees of perfection mean for
him, as for Aristotle, primarily degrees of perfection of function, but
since structure is necessarily in close relation with function,
perfection of function brings in its train increased perfection of
organisation. This can only be attained by a division of labour[306] among
the organs and by their consequent differentiation. An animal is like a
workshop where some complicated product is manufactured, and the organs
are like the workmen. Each workman has his own special piece of work to
do, at which he becomes thoroughly expert; and the finished product is
manufactured more rapidly and efficiently by the co-operation of workers
each skilled in one department than it would be if each workman had to
produce the whole. Applied to the organism this principle of the
division of labour means the differentiating out of the separate
functions, their localisation in different parts of the organism, and
their co-ordination to produce a combined result.

This differentiation of functions implies a corresponding
differentiation of organs, but it is functional differentiation which
always takes the lead. "Where division of labour has not been introduced
into the organism there must exist a great simplicity of structure. But
just as uniformity in the functions of the different parts of the body
implies a uniformity in their mode of constitution, so diversity in
function must be accompanied by particularities in structure; and, in
consequence also, the number of dissimilar parts must be augmented and
the complication of the machine increased" (p. 463). Since function
comes before form there is not always a special organ for every
function. "It is a grave error to believe that a particular function can
be performed only by one and the same organ. Nature can arrive at the
desired result by various ways, and when we look down through the animal
kingdom from the highest to the lowest forms we see that the function
does not disappear even when the special instrument provided for the
purpose in the higher types ceases to exist" (p 470).

Nature, holding fast to the law of economy, does not even always create
a new organ for a new function; she may simply adapt an undifferentiated
part to special functions, or she may even convert to other uses an
organ already specialised (p. 464). So, for example, the function of
respiration is in the lowest animals diffused indifferently over the
whole surface of the body, and only as organisation advances is it
localised in special organs, such as gills. Now suppose that Nature
wishes to adapt a fish, which breathes by gills, to life in the air; she
does not create an organ specially for this purpose, but utilises the
moist gill-chamber (_e.g._, in _Anabas scandens_), modifying it in
certain ways so that the fish can take advantage of the oxygen it
contains. But this gill-chamber lung is at best a makeshift, and when
she comes to the more definitely terrestrial Amphibia Nature gives up
the attempt to use the gill-chamber as a lung, and creates a new organ,
the true vertebrate lung, specially adapted for breathing air (p. 475).

But whatever means Nature adopts, her aim is always the same--to
specialise, to differentiate, to produce diversity from uniformity.

Differentiation not only raises the level of organisation; it usually
also takes the direction of adaptation to particular habits of life, and
this is perhaps the most fruitful cause of diversity. Everywhere we find
animals specialised in adaptation to their environment--to life in air
or water, or on land--and many of their most striking differences are
due to this cause. But adaptation may also act in reducing diversity,
for there necessarily occur many instances of parallel adaptation or
convergence. So we get the extraordinary parallelism between the
families of marsupials and the orders of placentals,[307] the remarkable
similarity between the respiratory organs of land-crabs and
air-breathing fish--to mention only two out of an immense range of
analogous facts.

The last cause of diversity that Milne-Edwards adduces is what he calls
a "borrowing" of peculiarities of structure from another systematic
group. Thus, "among reptiles, the tortoises seem to have borrowed from
birds some of their characteristic features of organisation; and among
the sauroid fishes the piscine type seems to have been influenced by the
type from which reptiles are derived" (p. 479). So many riddles that, a
little later on, stimulated the ingenuity of the evolutionists!

Such, then, were the factors which Milne-Edwards considered adequate to
explain the rich variety of animal forms. We cannot do better than quote
his own summary of his doctrine:--"To sum up, then, the great
differences introduced by Nature into the constitution of animals seem
to depend essentially upon the existence of a certain number of general
plans or distinct types, upon the perfecting in various degrees either
of the whole or of parts of each of these structural plans, upon the
adaptation of each type to varied conditions of existence, and upon the
secondary imitation of foreign types by certain derivatives of each
particular type" (p. 480).

We have laid stress on the fact that Milne-Edwards put function before
form, for this is the mark of the true Cuvierian. With it goes the
belief that Nature forms new parts to meet new requirements, that she is
not limited, as Geoffroy thought, to a definite number of "materials of
organisation," but can produce others at need. Cuvier held, for example,
that many of the muscles and even the bones of fish were peculiar to
them, and without homologues in the other Vertebrates, having been
created by Nature for special ends.[308] So, too, Johannes Müller, who in
many ways and not least in his sane vitalism was a follower of the
Cuvierian tradition, recognised that many of the complicated cartilages
in the skull of Cyclostomes were specially formed for the important
function of sucking, and had no equivalent in other fish.[309]

So, too, the embryologists after Cuvier often came across instances of
the special formation of parts to meet temporary needs. Thus Reichert
interpreted the "palatine" and "pterygoid," which are formed in the
mouth of the newt larva by a fusion of conical teeth, as special
adaptations to enable the little larva to lead a carnivorous life.[310]

Not many years after the publication of Milne-Edwards' _Introduction à
la zoologie générale_ (1851) there appeared a book by H. G. Bronn in
which was offered a very similar analysis of organic diversity. The
curious thing was that Bronn approached the problem from quite a
different standpoint, from the standpoint, indeed, of
_Naturphilosophie_. Of this the title of the book is itself sufficient
proof--_Morphologische Studien über die Gestaltungs-gesetze der
Naturkörper überhaupt und der organischen insbesondere_ (Leipzig and
Heidelberg, 1858).[311] The linking up of organic with inorganic form is
characteristic; there is much talk, too, in the book of _Urstoffe_ and
_Urkräfte_, but underlying the _Naturphilosophie_ we can trace the same
Cuvierian treatment of form, and see crystallise out laws of progressive
development that bear no small analogy with the laws established by
Milne-Edwards.

According to Bronn, the ideal fundamental form of the plant is an ovoid
or strobiloid[312] body, for a plant reaches out in two directions in
search of food--towards the sun and towards the earth. Animals differ
from plants in being endowed with sensation and mobility (_cf._
Aristotle and Cuvier), and it is this characteristic that gives them
their distinctive form. The main types of animal form--the Amorphozoa,
Actinozoa, and Hemisphenozoa--are essentially adaptations to particular
modes of locomotion. Animals either are fixed, or they move in all
directions without reference to any definite axis, or they move in one
main direction.

The Amorphozoa or shapeless animals include many of the Protozoa and
sponges; they have no typical form, and most of them are sessile. The
Actinozoa include such animals as the Coelentera, which are fixed, and
the Echinoderms, which have a central point and move indifferently along
any radial axis; their form differs from the strobiloid mainly in having
radiate rather than spiral symmetry. The Hemisphenozoa, or bilaterally
symmetrical animals, include all those that habitually move forward;
they have a front end and a hind end, a dorsal surface and a ventral,
and the mouth, sense-organs and "brain" are concentrated in the front
end to form a head--all in direct adaptation to this forward movement;
they make up the vast majority of animals.

The fundamental forms of living things are, however, merely so many
themes on which a multitude of further variations are woven, through the
action of the laws which rule the detail of organic diversities. These
further laws may be set down under four main heads. Under the first
comes the law of the existence of certain fundamentally distinct
structural types, which are distinguished from one another by their
ground-form, by the number of organ-systems, and by the number of
homotypic organs they possess, but principally by the relative position
of the organs to one another (principle of connections). The form and
connections of the nervous system are of particular importance in
distinguishing the types (_cf._ Cuvier). The second factor in the
diversity of organic form is the action of certain laws of progressive
development[313] (_Entwickelungsgesetze_), which bear the same relation to
the development of the animal kingdom as the laws of individual
development bear to the development of the embryo, for organs appear in
the different animal series in much the same order and manner as they
develop in the individual. These laws are (1) progressive
differentiation of functions and organs; (2) numerical reduction of
serially repeated parts; (3) concentration of functions and their organs
in particular parts of the body; (4) centralisation of organ-systems and
parts of such, so that they come to depend upon one central organ; (5)
internalisation of the "noblest" organs, unless these are necessarily
external, and (6) increase in size of the whole or of parts. Of these
the law of differentiation is by far the most important, and most of the
others are in a sense merely special cases of this fundamental law. To
this law of differentiation is due the increase in complexity or
perfection of organisation which is shown by all the animal series.
Bronn himself recognised the great similarity of this law of progressive
differentiation to Milne-Edwards' principle of the division of labour;
he seems, however, to have arrived at it independently.

Bronn's third factor in the production of variety of form is adaptation
to environment, or better, functional response to environment. Bronn
gives an excellent account of adaptational modifications and calls
attention, just as Milne-Edwards did, to the numerous analogies of
structure which adaptation brings about. He works out the interesting
view that there is some connection between classificatory groups and
adaptational forms, especially such as are connected with the function
of locomotion:--"Based upon a common characteristic method of locomotion
are whole or nearly whole sub-phyla (Hexapoda), classes (mammals and
reptiles, birds, fishes, gastropods, pteropods, brachiopods, Bryozoa,
Rotifera, jelly-fish, polypes, sponges), sub-classes (mobile and
immobile lamellibranchs, echinoderms, walking and swimming Crustacea,
parasitic and free-living worms, and so on), often, however, only orders
and quite small groups (snakes, eels, bats, sepias, medusæ, etc.)" (p.
141).

It was characteristic of the 'forties and 'fifties that transcendental
anatomy, along with Nature-philosophy, went rather out of fashion, its
false simplicities and premature generalisations being overwhelmed by
the flood of new discoveries. A few stalwarts indeed upheld
transcendental views. We have already discussed the morphological system
built up by Richard Owen in the late 'forties, a system transcendental
in its main lines. We have seen the vertebral theory of the skull still
maintained in the 'fifties by such men as Reichert and Kölliker, and we
find J. V. Carus in 1853[314] taking it as almost conclusively proved.[315]

We may mention, too, as showing clear marks of the influence of
transcendental ideas, L. Agassiz's work on the principles of
classification.[316] And Serres, who was Geoffroy's chief disciple,
recanted not a whit of his doctrine of recapitulation, but re-affirmed
and expanded it from time to time, and particularly in a lengthy memoir
published in 1860.[317] But in general we may say that pure morphology in
the Geoffroyan or Okenian sense was becoming gradually discredited. A
curious indication of this is seen in the fact that not only the idea
but the very word "Archetype" came to be regarded with suspicion. Thus
even J. V. Carus, who had much affinity with the transcendentalists,
wrote of the vertebrate archetype (which he took over almost bodily from
Owen)--"It may here be observed that this schema may be used as a
methodological help, but it is not to be placed in the foreground"
(_loc. cit._, p. 395). Huxley, who was definitely a follower of von
Baer, was much more outspoken with regard to ideal types. In an
important memoir on the general anatomy of the Gastropoda and
Cephalopoda,[318] he set himself the task of reducing all their complex
forms to one type. In summing up, he writes:--"From all that has been
stated, I think that it is now possible to form a notion of the
archetype of the Cephalous Mollusca, and I beg it to be understood that
in using this term, I make no reference to any real or imaginary 'ideas'
upon which animal forms are modelled. All that I mean is the conception
of a form embodying the most general propositions that can be affirmed
respecting the Cephalous Mollusca, standing in the same relation to them
as the diagram to a geometrical theorem, and like it, at once imaginary
and true" (i., p. 176). Again, in his Croonian lecture on the theory of
the vertebrate skull, he remarks that a general diagram of the skull
could easily be given. "There is no harm," he continues, "in calling
such a convenient diagram the 'Archetype' of the skull, but I prefer to
avoid a word whose connotation is so fundamentally opposed to the spirit
of modern science" (_Sci. Memoirs_, vol. i., p. 571).

It is instructive to find that between Serres and Milne-Edwards there
existed the same antagonism as between von Baer and the German
transcendentalists. Milne-Edwards was a constant critic of the law of
parallelism which Serres continued to uphold with little modification
for over thirty years, just as von Baer was a critic of that form of the
doctrine which was current in the early part of the century. As early as
1833, Milne-Edwards, through his studies of crustacean development,[319]
had come to the conclusion, independently of von Baer, that development
always proceeded from the general to the special; that class characters
appeared before family characters, generic characters before specific.
In an interesting paper published in 1844,[320] he discussed the relation
of this law of development to the problems of classification, and
arrived at results almost identical with those set forth by von Baer in
his Fifth Scholion.

Like von Baer he rejected completely the theory of parallelism and the
doctrine of the scale of beings; like von Baer he held that the type of
organisation--of which there are several--is manifested in the very
earliest stages and becomes increasingly specialised throughout the
course of further development; like von Baer, too, he sketched a
classification based upon embryological characters.

These views were further developed in his volume of 1851, and also in
his _Rapport_ of 1867.

They brought him into conflict with his confrere in the Academy of
Sciences, Étienne Serres, who in a number of papers published in the
'thirties and 'forties,[321] and particularly in his comprehensive memoir
of 1860, still maintained the theory of parallelism and the doctrine of
the absolute unity of type. His memoir of 1860 shows how completely
Serres was under the domination of transcendental ideas. Much of it
indeed goes back to Oken. "The animal kingdom," he writes, "may be
considered in its entirety as a single ideal and complex being" (p.
141). His views have become a little more complicated since his first
exposition of them in 1827, and he has been forced to modify in some
respects the rigour of his doctrine. But he still holds fast to the main
thesis of transcendentalism--the absolute unity of plan of all animals,
vertebrate and invertebrate alike,[322] the gradual perfecting of
organisation from monad to man, the repetition in the embryogeny of the
higher animals of the "zoogeny" of the lower.

He recognised, however, that the idea of a simple scale of beings is
only an abstraction, and that the true repetition is of organs rather
than of organisms. He was willing even to admit, at least in the later
pages of his memoir, that there might be not one animal series but
several parallel series, as had been suggested by Isidore Geoffroy St
Hilaire (p. 749). In general, his views are now less dogmatic than they
were in his earlier writings, but they are not for all that changed in
any essential. For, in summing up his main results, he writes, "The
whole animal kingdom can in some measure be regarded ideally as a single
animal, which, in the course of formation and metamorphosis in its
diverse manifestations, here and there arrests its own development, and
thus determines at each point of interruption, by the very state it has
reached, the distinctive characters of the phyla, the classes, families,
genera, and species" (p. 833).[323]

To settle the dispute pending between two of its most illustrious
members, the Academy proposed in 1853, as the subject of one of its
prizes, "the positive determination of the resemblances and differences
in the comparative development of Vertebrates and Invertebrates." A
memoir was presented the next year by Lereboullet[324] which met with the
approval of the Academy in so far as its statements of fact were
concerned, but seemed to them to require amplification in its
theoretical part. But even in this memoir Lereboullet was able to show
that the balance of evidence was greatly in favour of Milne-Edwards'
views, and his general conclusions in 1854 were that "in the presence of
such fundamental differences, one is obliged to give up the idea of one
single plan in the formation of animals; while, on the contrary, the
existence of diverse plans or types is clearly demonstrated by all the
facts" (p. 79). To fulfil the Academy's requirements, Lereboullet
continued his work, and in 1861-63 he published a series of elaborate
monographs[325] on the embryology of the trout, the lizard and the
pond-snail _Lymnæa_, and rounded off his work with a full discussion[326]
of the theoretical questions involved. In this considered and
authoritative judgment he completely disposed of Serres' theories of the
unity of plan and the unity of genetic formation. Except in the very
earliest stages of oogenesis there is no real similarity between the
development of a Zoophyte, a Mollusc, an Articulate and a Vertebrate,
but each is stamped from the beginning with the characteristics of its
type. The lower animals are not, and cannot possibly be the permanent
embryos of the higher animals. "The results which I have obtained," he
writes, "are diametrically opposed to the theory of the zoological
series constituted by stages of increasing perfection, a theory which
tries to demonstrate in the embryonic phases of the higher animals a
repetition of the forms which characterise the lower animals, and which
has led to the assertion that the latter are permanent embryos of the
former. The embryo of a Vertebrate shows the vertebrate type from the
very beginning, and retains this type throughout the whole course of its
development; it never is, and never can be, either a Mollusc or an
Articulate" (xx., p. 54).

"We are led to establish ... as the general result of our researches,
the existence of several types, and, consequently, of different plans,
in the development of animals. These different types are manifested from
the very beginning of embryonic life; the characters distinguishing them
are therefore primordial, and we can say with M. Milne-Edwards that
_everything goes to prove that the distinction established by Nature
between animals belonging to different phyla is a primordial
distinction_" (p. 58).

In other directions also von Baer's work was confirmed and extended by
later observers--those parts of it particularly that had reference to
the germ-layer theory, and to the concept of histological
differentiation. His germ-layer theory was accepted in its main lines by
Rathke, Bischoff and Lereboullet, and applied by them to the multitude
of new facts they discovered. Rathke, in particular, was a firm upholder
of the doctrine, and made considerable use of it in his writings.[327]
Even before the publication of von Baer's book he had interpreted in
terms of the germ-layer theory sketched by his friend Pander the
splitting of the blastoderm which occurs in the early development of
_Astacus_, whereby there are formed a serous and a mucous layer, one
inside the other--like the coats of an onion, to use his own expressive
phrase.[328]

An ingenious application of the Pander-Baer theory was made by Huxley,
who compared the outer and inner cell-layers which form the groundwork
of the Coelentera with the serous and mucous layers of the vertebrate
germ.[329] He laid stress, it is true, rather on the physiological than on
the morphological resemblance. "A complete identity of structure," he
writes, "connects the 'foundation membranes' of the Medusæ with the
corresponding organs in the rest of the series; and it is curious to
remark, that throughout, the outer and inner membranes appear to bear
the same physiological relation to one another as do the serous and
mucous layers of the germ; the outer becoming developed into the
muscular system, and giving rise to the organs of offence and defence;
the inner, on the other hand, appearing to be more closely subservient
to the purposes of nutrition and generation" (p. 24). Von Baer had
already hinted at this homology in the second volume of his
_Entwickelungsgeschichte_ (1837), where he says with reference to the
separation of the blastoderm of the chick into two layers. "Yet
originally there are not two distinct or even separable layers, it is
rather the two surfaces of the germ which show this differentiation,
just as polyps show the same contrast of an external surface and an
internal digestive surface. In between the two layers there is in our
germ as in the polyp an indifferent mass" (p. 67). The terms ectoderm
and entoderm were introduced by Allman[330] in 1853 for the two
cell-layers in the Hydrozoa.

Remak is the second great name in the history of the germ-layer theory.
He had the great advantage over von Baer of being able to make use of
the cell-theory in interpreting the formation of the germ-layers.
Microscopical technique also had been greatly improved since 1828.[331]

Remak's greatest service was that he put the germ-layer theory in direct
relation with the cell-theory by demonstrating the cellular continuity
from egg-cell to tissue, and by showing that each germ-layer possessed
distinctive histological characteristics. Hardly less important was his
clear marking-off of the "middle layer" as a separate and distinct layer
of the germ. He it was who introduced the modern conception of the
mesoderm, and cleared up the confusion in which Pander and von Baer had
left the organs formed between the serous and the mucous layer. Remak's
middle layer was a different thing from Pander's ill-defined
"vessel-layer"; it included and unified from a new point of view the
"vessel" and "muscle" layers of von Baer.

There are in the unincubated blastoderm of the chick, according to
Remak,[332] two cell-layers, of which the undermost subsequently splits
into two. Three layers are thus formed--the upper, middle and lower. The
upper layer differentiates into a medullary plate and an epidermic plate
(Remak's _Hornblatt_), and gives origin to the medullary tube with all
its evaginations, and to the skin with all its derivatives and pockets.
It forms such diverse structures as the brain, the spinal cord, the eye,
the ear, the mouth, hairs, feathers, nails, sweat-glands, lacrymal
glands, and so forth. All these parts are connected directly or
indirectly with sensation, and the upper germ-layer may accordingly be
called the _sensory_ layer. The lower layer gives rise to the epithelium
and the proper tissue of the alimentary canal and its derivatives, as
the liver, lungs, pancreas, kidneys, thyroid, thymus, etc. These parts
are all concerned in the processes of assimilation and dissimilation,
and the lower layer may accordingly be called the _trophic_ layer. Now
between the upper or sensory layer and the lower or trophic layer there
exists, in spite of their very different functions, a close histological
likeness, for both are essentially epithelial layers. The resemblance is
particularly strong if we compare the lower layer with the _Hornblatt_
of the upper layer--both consist of epithelial tissue, and of its
derivative, glandular tissue, and form neither vessels nor nerves. The
middle layer, on the contrary, forms nerves and muscles, vessels and
connective tissue, and little or no epithelium. It does not form all the
blood-vessels without exception (and so cannot be called the
vessel-layer), for the blood-vessels of the central nervous system are
in all probability formed from the upper layer. So, too, it does not
form all the nerves and muscles--the optic and auditory nerves and the
nerves and muscles of the iris probably arise in the upper layer. But,
in spite of these exceptions, its general histological character is so
well defined that it may be contrasted with the other two as
preeminently the layer that forms muscular, nervous, vascular and
connective tissue. In view of its functional significance, it may be
called the _motory_ layer, or better, since it forms also the sexual
glands, the _motor-germinative_ layer. The middle layer, early in its
history, shows a division into dorsal plates (_Urwirbelplatten_) and
ventral plates (_Seitenplatten_). The former exhibit almost as soon as
they are formed the characteristic proto-vertebral segmentation, the
latter split to form the pleuro-peritoneal or body-cavity. Remak
describes the latter process as follows:--"In the region of the trunk,
where a greater independence of the fate of the alimentary canal and its
annexes becomes necessary for the voluntary executive organs, the
ventral plates undergo a process of splitting, leading to the formation
of the sensitive part of the integument (the _Hautplatten_), the
muscular part of the alimentary tube (the _Darmfaserplatten_), and the
mother-tissue of the generative organs (the _Mittelplatten_). From the
_Hautplatten_ there develops, without the dorsal plates seeming to take
any part in the process, the rudiment of the extremities" (p. 79).

[Illustration: FIG. 12.--Transverse Section of Chick Embryo. (After
Remak.)]

His _Darmfaserplatten_ form the nervous and muscular tissue of the
alimentary canal and its dependencies, and also the heart; the
_Hautplatten_ form the general body-wall (exclusive of the skin) and the
appendages. In the embryo they line the amniotic cavity. The skeleton
and peripheral nerves originate wholly within the middle layer.

Remak's conception of the relations of the three germ-layers to one
another and to the body-cavity is well illustrated in Fig. 12.

In his germ-layer theory Remak's standpoint is histological rather than
morphological. The distinction which he draws between the sensory and
trophic layers on the one hand, and the motor-germinative layer on the
other, is entirely a histological one. The greater part of his book,
indeed, is devoted to a study of the histogenesis of the different
organs of the body; he is bent chiefly upon unravelling the part which
each germ-layer takes in the formation of each tissue and organ.

His generalisation that two of the germ-layers give rise exclusively or
almost exclusively to one kind of tissue excited great interest at the
time, and gave the direction to histogenetic research for quite a number
of years, though in the end it turned out to be insufficiently founded.

Though Remak's germ-layer theory had thus principally a histological
orientation, it laid down the main lines of the modern morphological
treatment of the germ-layers.

    [293] _Embryologie des Salmones_, 1842.

    [294] _Die Cellularpathologie in ihrer Begründung auf
    physiologische und pathologische Gewebelehre_, Berlin,
    2nd ed. 1859; Eng. trans., by Chance, 1860.

    [295] _Arch. path. Anat. Phys_., vii., pp. 1-39 (1854).

    [296] _Bericht über die Fortschritte der mikroskopischen
    Anatomie im jahre 1854._ Müller's _Archiv_, 1855. See
    also 1856.

    [297] _Hndb. d. Physiol._, i., 1835.

    [298] See Leuckart's reply to Ludwig's criticism, in
    _Zeit. f. wiss. Zool._, ii., p. 271, 1850.

    [299] Leipzig, 1853.

    [300] _Souvenirs d'un Naturaliste_, 2 vols., Paris, 1854.
    Eng. Trans. as _Rambles of a Naturalist on the Coasts of
    France, Spain, and Italy_, 2 vols., 1857.

    [301] Milne-Edwards later published a classical textbook
    on comparative anatomy and physiology--_Leçons sur la
    Physiologie et l'Anatomie comparées_, 14 vols., Paris,
    1857-80.

    [302] Paris, 1834-40. Three volumes of the _Suites à
    Buffon_.

    [303] Paris, 1865. Two volumes of the _Suites à Buffon_.

    [304] _U. d. Metamorphose der Ophiuren u. Seeigel._,
    Berlin, 1848. _U. d. Metamorphose der Holothurien u.
    Asterien._, Berlin, 1851.

    [305] As I have been unable to obtain a copy of the
    _Introduction_, the passages which follow are taken from
    the _Rapport_ of 1867, where Milne-Edwards gives a
    complete exposition of his doctrine, sometimes in the
    words of the original.

    [306] This principle was first developed by Milne-Edwards
    in 1827, in the _Dictionnaire classique d'Hist.
    naturelle_. It was probably suggested to him by his
    studies on the Crustacea, among which the principle is
    so beautifully exemplified in the concentration and
    specialisation of the appendages and the ganglionic
    chain.

    [307] Studied by Isidore Geoffroy St Hilaire in his paper
    _Classification parallélique des Mammifères, C. R. Acad.
    Sci._, xx., 1845. Remarked upon by Cuvier, _Règne
    animal_., i., p. 171, 1817, also by de Blainville.

    [308] Cuvier et Valenciennes, _Hist. nat. des Poissons_,
    i., p. 550, 1828.

    [309] _Myxinoiden_, Th. I. _Abh. k. Akad. Wiss. Berlin_
    for 1834, pp. 100, 110, 179, etc.

    [310] _Vergl. Entw. Kopf. nackt. Amphibien_, p. 101, 1838.

    [311] I have not seen the companion volume on
    palæontological progression, _Unters. ü. d.
    Entwickelungsgesetze der organischen Welt während der
    Bildungszeit unserer Erdoberfläche_, Stuttgart, 1858.

    [312] "Strobiloid" because of its spiral development. The
    theory of the spiral growth of plants played an
    important part in botanical morphology about this time.

    [313] _Cf._ Meckel's Principle of progressive Evolution,
    _supra_, p. 93.

    [314] _System der thierischen Morphologie_, pp. 33, 457.
    Also C. Bruch, _Die Wirbeltheorie des Schädels, am
    Skelette des Lachses geprüft_, Frankfort-on-Main, 1862.

    [315] In France the vertebral theory was advocated by
    Lavocat in his _Nouvelle Ostéologie comparée de la tête
    des animaux domestiques_, Toulouse, 1864. It seems also
    that Lacaze-Duthiers held fast to it even in
    1872--_Arch. zool. exp. gén._, i., p. 51, 1872.

    [316] _An Essay on Classification_, Boston, 1857, London,
    1859. He considered the classificatory categories to be
    the categories of the Creator's thought, and hence
    natural, and in no sense mere conventions.

    [317] "Principes d'Embryogénie, de Zoogénie et de
    Teratogénie," _Mém. Acad. Sci._, xxv., pp. 1-943, pls.
    xxv., 1860.

    [318] "On the Morphology of the Cephalous Mollusca,"
    _Phil. Trans._, 1853, _Sci. Memoirs_, i., pp. 152-92.

    [319] "Observations sur les changements de forme que les
    divers Crustacés éprouvent," _Ann. Sci. nat._ (1) xxx.,
    p. 360, 1833.

    [320] "Considérations sur quelques principes relatifs à la
    classification naturelle des animaux," _Ann. Sci. nat._
    (3) i., p. 65, 1844.

    [321] _Supra_, pp. 79-83. Also _Précis d'anatomie
    transcendante, principes d'organogénie_, Paris, 1842.

    [322] The inversion of the organs shown by Vertebrates as
    compared with Invertebrates is due to the reversed
    position of the embryo relatively to the yolk! (pp.
    821-6).

    [323] It is worth while recording that Serres enunciated a
    "law of symmetry" according to which the embryo is
    formed by the union of its two symmetrical halves--a law
    which recalls the "concrescence theory" of His and some
    modern embryologists.

    [324] "Embryologie comparée du Brochet, de la Perche, et
    de l'Ecrévisse," _Ann. Sci. nat._ (4), i., p. 237, 1854;
    ii., p. 39, 1854. _Mém. Savans etrangers_, xvii.

    [325] _Ann. Sci. nat._ (4) xvi., p. 113, 1861; xvii., p.
    88, 1862; xviii., p. 5, 1862; xix., p. 5, 1863.

    [326] xx., p. 5, 1863.

    [327] Particularly in his _Blennius_ (1833) and _Natter_
    (1839).

    [328] In the "preliminary notice" of his Crayfish
    paper--_Isis_, pp 1093-1100, 1825.

    [329] "On the Anatomy and the Affinities of the Family of
    the Medusæ," _Phil. Trans._, 1849; _Sci. Memoirs_, i.,
    pp. 9-32.

    [330] _Phil. Trans._, cxliii., p. 368, 1853.

    [331] The principle of achromatism was discovered (by
    Fraunhofer) and achromatic microscopes introduced in the
    early part of the 19th century. The use of chemical
    reagents, such as acetic acid, and various hardening
    fluids, came into fashion not long after. J. Müller
    seems to have been one of the first to realise their
    importance. Remak himself invented one or two fixing and
    hardening mixtures (pp. 87, 127, 1855), which enabled
    him to cut excellent hand sections. Section-cutting
    machines were not invented till later (V. Hensen, 1866,
    His, 1870).

    [332] _Untersuchungen über die Entwickelung der
    Wirbelthiere_, folio, pp. xxxvii + 195, 12 plates,
    Berlin, 1850-1855.



CHAPTER XIII

THE RELATION OF LAMARCK AND DARWIN TO MORPHOLOGY.


It is a remarkable fact that morphology took but a very little part in
the formation of evolution-theory. When one remembers what powerful
arguments for evolution can be drawn from such facts as the unity of
plan and composition and the law of parallelism, one is astonished to
find that it was not the morphologists at all who founded the theory of
evolution.

It is true that the noticeable resemblances of animals to one another,
the possibility of arranging them in a system, the vague perception of
an all-pervading plan of structure, did suggest to many minds the
thought that systematic affinities might be due to blood-relationship.
Thus Leibniz considered that the cat tribe might possibly be descended
from a common ancestor,[333] and another great philosopher, Immanuel Kant,
was led by his perception of the unity of type to suggest as possible
the derivation of the whole organic realm from one parent form, or even
ultimately from inorganic matter. In the course of his masterly
discussion of mechanism and teleology,[334] he writes, "The agreement of
so many genera of animals in a certain common schema, which appears to
be fundamental not only in the structure of their bones, but also in the
disposition of their remaining parts--so that with an admirable
simplicity of original outline, a great variety of species has been
produced by the shortening of one member and the lengthening of another,
the involution of this part and the evolution of that--allows a ray of
hope, however faint, to penetrate into our minds, that here something
may be accomplished by the aid of the principle of the mechanism of
Nature (without which there can be no natural science in general). This
analogy of forms, which with all their differences seem to have been
produced according to a common original type, strengthens our suspicions
of an actual relationship between them in their production from a common
parent, through the gradual approximation of one animal-genus to
another--from those in which the principle of purposes seems to be best
authenticated, _i.e._, from man down to the polype, and again from this
down to mosses and lichens, and finally to the lowest stage of Nature
noticeable by us, viz., to crude matter."[335]

So, too, Buffon's evolutionism was suggested by his study of the
structural affinities of animals, and Erasmus Darwin in his _Zoonomia_
(1794) brought forward as one of the strongest proofs of evolution, "the
essential unity of plan in all warm-blooded animals."[336]

But, as a matter of historical fact, no morphologist, not even Geoffroy,
deduced from the facts of his science any comprehensive theory of
evolution. The pre-Darwinian morphologists were comparatively little
influenced by the evolution-theories current in their day, and it was in
the anatomist Cuvier and the embryologist von Baer that the early
evolutionists found their most uncompromising opponents.

Speaking generally, and excepting for the moment the theory of Lamarck,
we may say that the evolution-theories of the 18th and 19th centuries
arose in connection with the transcendental notion of the _Échelle des
êtres_, or scale of perfection. This notion, which plays so great a part
in the philosophy of Leibniz, was very generally accepted about the
middle of the 18th century, and received complete and even exaggerated
expression from Bonnet and Robinet. Buffon also was influenced by it.
Towards the beginning of the 19th century the idea was taken up eagerly
by the transcendental school and by them given, in their theories of the
"one animal," a more morphological turn. Their recapitulation theory was
part and parcel of the same general idea.

One understands how easily the notion of evolution could arise in minds
filled with the thought of the ideal progression of the whole organic
kingdom towards its crown and microcosm, man. Their theory of
recapitulation led them to conceive evolution as the developmental
history of the one great organism.[337] Many of them wavered between the
conception of evolution as an ideal process, as a _Vorstellungsart_, and
the conception of it as an historical process. Bonnet, Oken, and the
majority of the transcendentalists seem to have chosen the former
alternative; Robinet, Treviranus, Tiedemann, Meckel, and a few others
held evolution to be a real process.

We have already in previous chapters[338] briefly noticed the relation of
one or two of the transcendental evolution-theories to morphology, and
there is little more to be said about them here. They had as good as no
influence upon morphological theory, nor indeed upon biology in
general.[339] It is different with the theory of Lamarck, which, although
it had little influence upon biological thought during and for long
after the lifetime of its author, is still at the present day a living
and developing doctrine.

Lamarck's affinity with the transcendentalists was in many ways a close
one, but he differed essentially in being before all a systematist. Nor
is the direct influence of the German transcendentalists traceable in
his work--his spiritual ancestors are the men of his own race, the
materialists Condillac and Cabanis, and Buffon, whose friend he was. The
idea of a gradation of all animals from the lowest to the highest was
always present in Lamarck's mind, and links him up, perhaps through
Buffon, with the school of Bonnet. The idea of the _Échelle des êtres_
had for him much less a morphological orientation than it had even for
the transcendentalists, for he was lacking almost completely in the
sense for morphology. Lamarck's scientific, as distinguished from his
speculative work, was exclusively systematic, and it was systematics of
a very high order. He introduced many reforms into the general
classification of animals. He was the first clearly to separate
Crustacea (1799), and a little later (1800) Arachnids, from insects. He
reduced to a certain orderliness the neglected tribes of the
Invertebrates, and wrote what was for long the standard work on their
systematics--the _Histoire naturelle des Animaux sans Vertèbres_
(1816-22). His speculative work on biology is contained in three
publications, the small book entitled _Considérations sur l'organisation
des corps vivants_ (1802), the larger work of 1809, the _Philosophie
zoologique_, and the introductory matter to his _Animaux sans Vertèbres_
(vol. i., 1816).

It is no easy matter to give in short compass an account of Lamarck's
biological philosophy. He is an obscure writer, and often
self-contradictory.

In the first part of the _Philosophie zoologique_ Lamarck is largely
pre-occupied with the problem of whether species are really distinct, or
do not rather grade insensibly into one another. As a systematist of
vast experience Lamarck knew how difficult it is in practice to
distinguish species from varieties. "The more," he writes, "we collect
the productions of Nature, the richer our collections become, the more
do we see almost all the gaps filled up and the lines of separation
effaced. We find ourselves reduced to an arbitrary determination, which
sometimes leads us to seize upon the slightest differences of varieties,
and form from them the distinctive character of what we call a species,
and at other times leads us to consider as a variety of a certain
species individuals a little bit different, which others regard as
forming a separate species."[340]

For Lamarck, as for Darwin later, the chief problem was not the
evolution and differentiation of types of structure, but the mode of
origin of species.

Lamarck is at great pains to show how arbitrary are our determinations
of species, and how artificial the classificatory groups which we
distinguish in Nature. Strictly speaking, there are in Nature only
individuals, "... this is certain, that among her products Nature has in
reality formed neither classes, nor orders, nor families, nor genera,
nor constant species, but only individuals which succeed one another and
resemble those that produced them. Now, these individuals belong to
infinitely diversified races, which shade into one another under all the
forms and in all the degrees of organisation, and each of which
maintains itself without change, so long as no cause of change acts upon
it" (p. 41).

But there is a natural order in the animal kingdom, a progression from
the simpler to the more complex organisations, a natural _Échelle des
êtres_.

This order is shown by the relation to one another of the large
classificatory groups, for they can be arranged in series from the
simplest to the most complex, somewhat as follows:--

1. Infusoria.
2. Polyps.
3. Radiates.
4. Worms.
5. Insects.
6. Arachnids.
7. Crustacea.
8. Annelids.
9. Cirripedes.
10. Molluscs.
11. Fishes.
12. Reptiles.
13. Birds.
14. Mammals.

But the order of Nature is essentially continuous, and the limits of
even the best defined of these classes are in reality artificial--"if
the order of Nature were perfectly known in a kingdom, the classes which
we should be forced to establish in it would always constitute entirely
artificial sections" (p. 45).

In the same way the lesser classificatory groups represent smaller
sections of the one unique order of Nature. Note that Lamarck's
_Échelle_ is in no way a morphological one, and was not intended to be
such. It is a scale of increasing physiological differentiation, and the
stages of it are marked by the acquirement of this or that new organ
(_cf._ Oken). "Observation of their state convinces one that in order to
produce them successively Nature has proceeded gradually from the
simpler to the more complex. Now Nature, having had in mind the
realisation of a plan of organisation which would permit of the greatest
perfecting (that of the Vertebrates), a plan very different from those
which she has been obliged to form as a preliminary to reaching it, one
understands that, among the multitude of animals, one must necessarily
come across not a single system of organisation which has become
progressively perfected, but diverse very distinct systems, each of
which has come into existence at the moment when each primary organ
first put in its appearance" (p. 171).

For Lamarck this order of Nature was not merely ideal--Nature had
actually formed the classes successively, proceeding from the simpler to
the more complex; she had brought about this evolution by transforming
the primitive species of animals, raising them to higher degrees of
organisation, and modifying them in relation to the environment in which
they found themselves.

Lamarck's theory of evolution is worked out in great detail in his
_Philosophie zoologique_, but the exposition is diffuse and
disconnected; it is better in giving an account of it to follow the more
concise, mature and general exposition which he gives in the
Introduction to his _Histoire naturelle des Animaux sans Vertèbres_.[341]
Near the beginning of the Introduction Lamarck gives us in a few short
"Fundamental Principles" the main lines of his general philosophy. He is
a confirmed materialist. Every fact and phenomenon is essentially
physical and owes its existence or production entirely to material
bodies or to relations between them. All change and all movement is in
the last resort due to mechanical causes. Every fact or phenomenon
observed in a living body is at once a physical fact or phenomenon and a
product of organisation (p. 19). Life, thought and sensation are not
properties of matter, but result from particular material combinations.

His thorough-going materialism is most clearly shown in its relation to
living things in the first three of the "Zoological Principles and
Axioms," which are developed further on in the book.

These are as follows:--"1. No kind or particle of matter can have in
itself the power of moving, living, feeling, thinking, nor of having
ideas; and if, outside of man, we observe bodies endowed with all or one
of these faculties, we ought to consider these faculties as physical
phenomena which Nature has been able to produce, not by employing some
particular kind of matter which itself possesses one or other of these
faculties, but by the order and state of things which she has
constituted in each organisation and in each particular system of
organs.

"2. Every animal faculty, of whatever nature it may be, is an organic
phenomenon, and results from a system of organs or an organ-apparatus
which gives rise to it and upon which it is necessarily dependent.

"3. The more highly a faculty is developed the more complex is the
system of organs which produces it, and the higher the general
organisation; the more difficult also does it become to grasp its
mechanism. But the faculty is none the less a phenomenon of
organisation, and for that reason purely physical" (p. 104).

According to these "axioms" function is a direct and mechanical effect
of structure.

The curious thing is that in spite of his avowed materialism, Lamarck's
conception of life and evolution is profoundly psychological, and from
the conflict of his materialism and his vitalism (of which he was
himself hardly conscious), arise most of the obscurities and the
irreductible self-contradiction of his theory.

Lamarck divided animals (psychologically!) into three great
groups--apathetic or insensitive animals, animals endowed with
sensation, and intelligent animals. The first group, which comprise all
the lower Invertebrates, are distinguished from other animals by the
fact that their actions are directly and mechanically due to the
excitations of the environment; they have no principle of reaction to
external influences, but passively prolong into action the excitations
they receive from without. They are _irritable_ merely. The second group
are distinguished from the first by their possessing, in addition to
irritability, a power which Lamarck calls the _sentiment intérieur_. He
has some difficulty in defining exactly what he means by it:--"I have no
term to express this internal power possessed not only by intelligent
animals but also by those that are endowed merely with the faculty of
sensation; it is a power which, when set in action by the feeling of a
need, causes the individual to act at once, _i.e._, in the very moment
of the sensation it experiences; and if the individual is of those that
are endowed with intelligence it nevertheless acts in such a case
entirely without premeditation and before any mental operation has
brought its _will_ into play" (p. 24).

It is the power we call instinct in animals (p. 25), and it implies
neither consciousness nor will. It acts by transforming external into
internal excitations.

To this second group of animals, possessing the _sentiment intérieur_,
belong the higher Invertebrates, notably insects and molluscs. Only
animals possessed of a more or less centralised nervous system can
manifest this _sentiment_, or principle of (unconscious) reaction to
external stimuli.

The higher animals, or the four Vertebrate classes, form the group of
"intelligent animals." In virtue of their more complex organisation they
possess in addition to the _sentiment intérieur_ the faculties of
intelligence and will.

Now, broadly put, Lamarck's theory of evolution is that new organs are
formed in direct reaction to needs (_besoins_) experienced by the
_sentiment intérieur_. The _sentiment intérieur_ is therefore the cause
not only of instinctive action but also of all morphogenetic processes.
Will and intelligence (which are confined to a relatively small number
of animals) have little or nothing to do directly with evolution.

To understand the working-out of Lamarck's evolution-theory we must
revert to his conception of the _Échelle des êtres_. What he wrote in
the _Philosophie zoologique_ is here repeated in the work of 1816 with
little modification.

There is a real progression from the simpler to the more complex
organisations; Nature has gradually complicated her creatures by giving
them new organs and therefore new faculties.

It is interesting to note that Lamarck expressly refers to Bonnet (p.
110), but refuses to accept his view of an _Échelle_ extending down into
the inorganic. Like Bonnet, however, and like the German
transcendentalists, Lamarck makes man the goal of evolution (p. 116). He
makes it quite clear that his _Échelle_ is a functional one, for he
links Vertebrates to molluscs even while expressly admitting that they
are not connected by any structural intermediates (p. 123). He does not
fall into the error of the transcendentalists and assume that
Vertebrates and Invertebrates alike are formed upon one common plan of
structure.

The progression of organisation shown by the animal kingdom has not been
altogether regular and uninterrupted:--"The progression in complexity of
organisation shows here and there, in the general animal series,
anomalies induced by the influence of environment and by the influence
of the habits contracted" (_Phil. zool._, i., p. 145).

There are thus really two causes at work to produce the variety of
organisation as it appears to us, one which tends to produce a regular
increase in complexity, and one which disturbs and diversifies this
regular advance.

The first cause Lamarck calls the vital power (_pouvoir de la vie_); the
other may be called the influence of circumstance (_Anim. s. Vert._, p.
134). To the latter cause are due the lacunæ, the blind alleys, and the
complications which the otherwise simple scale of perfection shows.

To explain both these aspects of evolution Lamarck propounded in his
volume of 1816 four laws, which read as follows:--

"_First Law_.--Life, by its own forces, tends continually to increase
the volume of every body possessing it, and to extend the dimensions of
its parts, up to a limit which it brings about itself.

"_Second Law_.--The production of a new organ in an animal body results
from the arisal and continuance of a new need, and from the new movement
which this need brings into being and sustains.

"_Third Law_.--The degree of development of organs and their force of
action are always proportionate to the use made of these organs.

"_Fourth Law_.--All that has been acquired, imprinted or changed in the
organisation of the individual during the course of its life is
preserved by generation and transmitted to the new individuals that
descend from the individual so modified" (pp. 151-2).

It is mainly but not entirely by reason of the first of these laws that
organisation tends to progress, and mainly by reason of the second and
third that difference of environment brings about diversity of
organisation. In virtue of the fourth law the acquirements of the
individual become the property of the race.

Lamarck's exposition of his first law, that life tends by its own powers
to enlarge and extend its bodily instrument, is vague and difficult to
understand. He has already explained some pages back how the first
organisms arose by spontaneous generation in the form of minute
gelatinous utricles (_cf._ Oken). He conceives that it is in the
movements of the fluids proper to the organism that the power resides to
enlarge and extend the body. Nutrition alone is not sufficient to bring
about extension; a special force is required, acting from within
outwards (p. 153). In the most primitive organisms the movements of the
vital fluids are weak and slow, but in the course of evolution they
gradually accelerate, and, becoming more rapid, trace out canals in the
delicate tissue which contains them, and finally form organs.

Subtle fluids play a great part in Lamarck's biology: they take the
place of the soul or entelechy which the vitalists would postulate to
explain organic happenings. Lamarck seems in this to follow certain of
the old materialists, who conceived the soul to be formed of a matter
more subtle than the ordinary.[342]

In his second law Lamarck's essentially vitalistic attitude comes out
very clearly, for it states that a psychological moment enters into all
new production of form, that the ultimate cause of the development of
new form is the need felt by the organism. This need is of course not a
conscious one, it is a need perceived by the _sentiment intérieur_.

In the large group of apathetic or insensitive animals, which do not
possess this faculty, needs cannot be experienced; accordingly new
organs are here formed directly and mechanically, by the movements of
the vital fluids set in action by excitations from without--the
evolution, like the behaviour, of these animals is due to the direct and
physical action of the environment. "But this is not the case with the
more highly organised animals which possess _feeling_. They experience
needs, and each need felt, acting upon their 'inner feeling,'
immediately directs the fluids and the forces to the part of the body
where action can satisfy the need. Now, if there exists at this point an
organ capable of performing the required action, it is quickly
stimulated to act; and if the organ does not exist and the need is
pressing and sustained, bit by bit the organ is produced and developed
in proportion to the continuity and the energy of its use" (p. 155).

In intelligent animals the _sentiment intérieur_ may be moved by thought
or will.

As an example of the way in which the law works Lamarck takes the
hypothetical case of a gastropod mollusc, which as it creeps along
experiences dimly the need to feel the objects in front of it. It makes
an effort (unconscious, be it noted) to touch these objects with the
anterior portions of its head, and sends forward continually to these
parts a great volume of nervous and other fluids. From these efforts and
the repeated afflux of fluids there must result a development of the
nerves supplying these parts. And as, along with the nervous fluids,
nutritive juices constantly flow to the parts, there must result the
formation of two or four tentacles in the places to which these fluids
are directed. A curious mixture of mechanistic "explanations" and
vitalistic hypothesis!

In his third law, that use and disuse are powerful to modify organs,
Lamarck is upon more solid ground, and can point to many instances of
the visible effect of these factors of change. It is of course rather
closely bound up with his second law and may even be regarded as an
extension of it.

The law has reference to one of the most powerful means employed by
Nature to diversify species, a means which comes into play whenever the
environment changes. The cause of the great diversity shown by animal
species is indeed ultimately to be sought in the environment. As the
imperfect and earliest forms developed they spread over the earth and
invaded the utmost corners of it:--"One can imagine what an enormous
variety of habitats, stations, climates, available foods, environing
media, etc., animals and plants have had to endure, as the existing
species were forced to change their place of abode. And although these
changes have taken place with extreme slowness ... their reality,
necessitated by various causes, has none the less induced the species
affected by them slowly to change their manner of life and their
habitual actions. Through the effects of the second and third of the
laws cited above, these induced activity-changes must have brought into
being new organs, and must have been able to develop them further if
more frequent use was made of them; they must in the same way have been
capable of bringing about the degeneration and finally the complete
disappearance of existing organs which had become useless" (p. 161).

On the other hand, if the environment does not change, species remain
constant.

It is to be noted that change in environment is rather the occasion than
the cause of modification; the environment induces the organism to
change its habitual way of life; it sets up new needs, to satisfy which
the organism must modify its structure. It is the organism that takes
the active part in all this, the action of the environment is indirect.

Of Lamarck's fourth law, which asserts the transmission of acquired
characters, little need here be said in the way of exposition. Upon the
truth of it depends of course Lamarck's whole theory. He himself never
dreamed that anyone would ever dispute it.

Lamarck sums up as follows:--"By the four laws which I have just
enunciated all the facts of organisation seem to me to be easily
explained; the progression in the complexity of organisation of animals,
and in their faculties, seems to me easy to conceive; so, too, the means
which Nature has employed to diversify animals, and bring them to the
state in which we now see them, become easily determinable" (p. 168).

It is never made quite clear, we may note in passing, how far his second
and third laws tend to bring about an increase in complexity, in
addition to diversifying animals.[343]

"The function creates the organ," this would seem to be the kernel of
Lamarck's doctrine. But how does he reconcile this essentially
vitalistic conception with his strictly materialistic philosophy?

We have seen that irritability, the _sentiment intérieur_, and
intelligence itself, are the effects of organisation. We are told
farther on that both the _sentiment_ and intelligence are caused by
nervous fluids. A great part of both the _Philosophie zoologique_ and
the introduction to the _Animaux sans Vertèbres_ is given up to the
exposition of a materialistic psychology of animals and man, based
entirely upon this hypothesis of nervous fluids. Thus habits are due to
the fluids hollowing out definite paths for themselves.

The _sentiment intérieur_ acts by directing the movements of the subtle
fluids of the body (which are themselves modifications of the nervous
fluids) upon the parts where a new organ is needed. But if it is itself
only a result of the movement of nervous fluids? Again, how can a need
be "felt" by a nervous fluid? This is an entirely psychological notion
and cannot be applied to a purely material system. Whence arises the
power of the _sentiment intérieur_ to canalise the energies of the
organism, so to direct and co-ordinate them that they build up purposive
structures, or effect purposive actions (as in all instinctive
behaviour)? Either the _sentiment intérieur_ is a psychological faculty,
or it is nothing.

There is no doubt that, as expressed by Lamarck, the conception conceals
a radical confusion of thought. It is not possible to be a
thorough-going materialist, and at the same time to believe that new
organs are formed in direct response to needs felt by the organism.
Lamarck could never resolve this antinomy, and his speculations were
thrown into confusion by it. To this cause is due the frequent obscurity
of his writings.

Should we be right in laying stress upon the psychological side of
Lamarck's theory, and disregarding the materialistic dress in which,
perhaps under the influence of the materialism current in his youth, he
clothed his essentially vitalistic thought? Everything goes to prove
it--his constant preoccupation with psychological questions, his tacit
assimilation of organ-formation to instinctive behaviour, his constant
insistence on the importance of _besoin_ and _habitude_.

Let us not forget the profundity of his main idea, that, exception made
for the lower forms, the animal is essentially active, that it always
_reacts_ to the external world, is never passively acted upon. Let us
not forget that he pointed out the essentially psychological moment
implied in all processes of individual adaptation. With keen insight he
realised that conscious intelligence counts for little in evolution, and
focussed attention upon the unconscious but obscurely psychical
processes of instinct and morphogenesis.

Not without reason have the later schools of evolutionary thought, who
developed the psychological and vitalistic side of his doctrine, called
themselves Neo-Lamarckians.

We shall say then that Lamarck, in spite of his materialism, was the
founder of the "psychological" theory of evolution.

Lamarck stood curiously aloof and apart from the scientific thought of
his day.[344] He took no interest in the morphological problems that
filled the minds of Cuvier and Geoffroy; he had indeed no feeling at all
for morphology. He did not realise, like Cuvier, the _convenance des
parties_, the marvellous co-ordination of parts to form a whole; he had
little conception of what is really implied in the word "organism." He
was not, like Geoffroy, imbued with a lively sense of the unity of plan
and composition, and of the significance of vestigial organs as
witnesses to that unity. He seems not to have known of the
recapitulation theory, of which he might have made such good use as
powerful evidence for evolution. Even with the German
transcendentalists, with whom in the looseness of his generalisations he
shows some affinity, he seems not to have been specially acquainted.

He was interested more in the problems suggested to him by his daily
work in the museum. He wanted to know why species graded so annoyingly
into one another; he wanted to examine critically his haunting suspicion
that species were really not distinct, and that classification was
purely conventional. The question, too, of the adaptation of species to
their environment, the problem of ecological adaptation, in distinction
to that of functional adaptation which interested Cuvier so greatly,
came vividly before him as he worked through the vast collections of the
museum. He was the first systematist to occupy himself in a
philosophical manner with the problems of general biology. He introduced
new problems and a new way of looking at old. With Lamarck the problem
of species and the problem of ecological adaptation enter into general
biology.

The one point in which he does definitely carry on the thought of his
predecessors is his conception of the animal kingdom as forming a scale
of (functional) perfection. He did not go to the same extreme as Bonnet;
he did not even consider that the animal series was a continuation of
the vegetable series; in his opinion they formed two diverging scales.
He recognised, too, that among animals there was no simple and regular
gradation from the lowest to the highest, but that the orderly
progression was disturbed and diverted by the necessity of adaptation to
different environments. It is interesting to note that in developing
this idea he arrived at a roughly accurate distinction between
homologous and analogous structures. More importance, he thought, was to
be attributed in classifying animals to characters which appeared due to
the "plan of Nature" than to such as were produced by an external
modifying cause (p. 299). But he did not formulate the distinction in
any strictly morphological way.

As his ideas developed he laid less stress upon the simplicity and
continuity of the scale; in his supplementary remarks to the
Introduction of 1816 he admits that the series is really very much
branched, and even that there may be two distinct series among animals
instead of one. His last schema of the course of evolution shows no
little analogy with the genealogical trees of Darwinian speculation. It
is headed "The presumed _Order_ of the formation of Animals, showing two
separate partly-branching series," and it reads as follows:--

   I.--_Series of Non-articulated_    II.--_Series of Articulated_
                 _Animals_.                      _Animals_.
"
I    ¦--         Infusoria.
n    ¦               ¦
s A  ¦             Polyps.
e n  ¦               ¦
n i  ¦        ----------------
s m  ¦        ¦              ¦
i a  ¦     Ascidians.     Radiates.                   Worms.
t l  ¦        ¦                                         ¦
i s  ¦        ¦                                  --------------
v .  ¦        ¦                                  ¦            ¦
e    ¦        ¦                                  ¦         Epizoa.
"    ¦--      ¦                                  ¦            ¦
              ¦                                  ¦            ¦
"    ¦--      ¦                                  ¦            ¦
S A  ¦     Acephala.                          Annelids.    Insects.
e n  ¦        ¦                                               ¦
n i  ¦        ¦                                               ¦
s m  ¦     Molluscs.                                  -------------
i a  ¦                                                ¦           ¦
t l  ¦                                                ¦       Arachnids.
i s  ¦                                            Crustacea.
v .  ¦                                                ¦
e    ¦                                                ¦
"    ¦--                                          Cirripedes.

I
n    ¦--
t A  ¦
e n  ¦                          Fishes.
l i  ¦                          Reptiles.
l m  ¦                          Birds.
i a  ¦                          Mammals.
g l  ¦
e s  ¦--
n .
t

It is interesting to note that Vertebrates are placed between the two
series, and are now not linked on directly to any Invertebrate group.

Lamarck's theory had little success. There is evidence, however, that
both Meckel and Geoffroy owed a good many of their evolutionary ideas to
Lamarck, and Cuvier paid him at least the compliment of criticising his
theory,[345] not distinguishing it, however, very clearly from the
evolutionary theories of the transcendentalists. But, speaking
generally, Lamarck's theory of evolution exercised very little influence
upon his contemporaries. This was probably due partly to the obscurity
and confusion of his thought, partly to his lack of sympathy with the
biological thought of his day, which was preponderatingly morphological.

It was not that men's minds were not ripe for evolution, for in the
early decades of the 19th century evolution was in the air. There were
few of von Baer's contemporaries who had not read Lamarck;[346] Erasmus
Darwin's _Zoonomia_ ran through three editions, and was translated into
German, French and Italian;[247] German philosophy was full of the idea of
evolution.

There was no unreadiness to accept the derivation of present-day species
from a primordial form--if only some solid evidence for such derivation
were forthcoming. Cuvier and von Baer, as we have seen, combated the
current evolution theories on the ground that the evidence was
insufficient, but von Baer at least had no rooted objection to
evolution. In an essay of 1834, entitled _The Most General Law of Nature
in all Development_,[348] von Baer expressed belief in a limited amount of
evolution. In this paper he did not admit that all animals have
developed from one parent form, and he refused to believe that man has
descended from an ape; but, basing his supposition upon the facts of
variability and upon the evidence of palæontology, he went so far as to
maintain that many species have evolved from parent stocks. In the
absence of conclusive proofs he did not commit himself to a belief in
any extended or comprehensive process of evolution.

Imbued as he was with the idea of development von Baer saw in evolution
a process essentially of the same nature as the development of the
individual. Evolution, like development, was due to a _Bildungskraft_ or
formative force. The ultimate law of all becoming was that "the history
of Nature is nothing but the history of the ever-advancing victory of
spirit over matter" (p. 71). In a later essay (1835) in the same volume
he says that all natural science is nothing but a long commentary on the
single phrase _Es werde!_. (p. 86).

As we shall see, von Baer adopted in later years the same attitude to
Darwinism as he did to the evolution theories in vogue in his youth.

Although in the twenty or thirty years before the publication of the
_Origin of Species_ (1859) no evolution theory of any importance was
published, and although the great majority of biologists believed in the
constancy of species, there were not wanting some who, like von Baer,
had an open mind on the subject, or even believed in the occurrence of
evolutionary processes of small scope. Isidore Geoffroy St Hilaire, the
son of the great Etienne Geoffroy St Hilaire, seems to have held that
species might be formed from varieties. The law which L. Agassiz thought
he could establish,[349] of the parallelism between palæontological
succession, systematic rank, and embryological development, tended to
help the progress of evolutionary ideas. J. V. Carus, who afterwards
became a supporter of Darwin, seems already, in 1853, to have inferred
from Agassiz's law the probability of evolution.[350]

But no evolution theory was taken very seriously before 1859, when the
_Origin of Species_ was published.

Like Lamarck, Charles Darwin was, neither by inclination nor by
training, a morphologist. In his youth he was a collector, a sportsman
and a field geologist. His voyage round the world on the _Beagle_
aroused in him keen interest in the problem of species--their variety,
their variation according to place and time, their adaptedness to
environment. The conviction gradually took possession of his mind that
the puzzling facts of geographical range and geological succession which
he observed wherever he went were explicable only on the hypothesis that
species change. He was not satisfied with the theories of evolution that
had been proposed by his grandfather, by Lamarck, and by E. Geoffroy St
Hilaire--he did not indeed understand these theories any too well. He
resolved to work out the problem in his own way, for his own
satisfaction. He tells us all this very clearly in his autobiography.
"During the voyage of the _Beagle_ I had been deeply impressed by
discovering in the Pampean formation great fossil animals covered with
armour like that on the existing armadillos; secondly, by the manner in
which closely allied animals replace one another in proceeding
southwards over the continent; and thirdly, by the South American
character of most of the productions of the Galapagos archipelago, and
more especially by the manner in which they differ slightly on each
island of the group; some of the islands appearing to be very ancient in
a geological sense.

"It was evident that such facts as these, as well as many others, could
only be explained on the supposition that species gradually become
modified; and the subject haunted me. But it was equally evident that
neither the action of the surrounding conditions, nor the will of the
organisms (especially in the case of plants) could account for the
innumerable cases in which organisms of every kind are beautifully
adapted to their habits of life--for instance, a woodpecker or a
tree-frog to climb trees, or a seed for dispersal by hooks or plumes. I
had always been much struck by such adaptations, and until these could
be explained it seemed to me almost useless to endeavour to prove by
indirect evidence that species have been modified."[351]

All Darwin's varied subsequent work revolved round these, for him,
essential problems--How do species change, and how do they become
adapted to their environment? He never ceased to be essentially a field
naturalist, and his theory of natural selection would have been an empty
and abstract thing if his vast knowledge and understanding of the "web
of life" had not given it colour and form. He never lost touch with the
living thing in its living, breathing reality--even plants he rightly
regarded as active things, full of tricks and contrivances for making
their way in the world. No one ever realised more vividly than he the
delicacy and complexity of the adaptations to environment which are the
necessary condition of success in the struggle for existence. Almost his
greatest service to biology was that he made biologists realise as they
never did before the vast importance of environment. He took biology
into the open air, away from the museum and the dissecting-room.

Naturally this attitude was not without its drawbacks. It led him to
take only a lukewarm interest in the problems of morphology. It is true
he used the facts of morphology with great effect as powerful arguments
for evolution, but it was not from such facts that he deduced his theory
to account for evolution. It is questionable indeed whether the theory
of natural selection is properly applicable to the problems of form. It
was invented to account for the evolution of specific differences and of
ecological adaptations; it was not primarily intended as an explanation
of the more wonderful and more mysterious facts of the _convenance des
parties_ and the interaction of structure and function. Perhaps Darwin
did not realise this inner aspect of adaptation quite so vividly as he
did the more superficial adaptation of organisms to their environment.
It was, perhaps, his lack of morphological training and experience that
led him to disregard the problems of form, or at least to realise very
insufficiently their difficulty.

It is in any case very significant that only a small part of his _Origin
of Species_ is devoted to the discussion of morphological
questions--only one chapter out of the fourteen contained in the first
edition.

Though the theory of natural selection took little account of the
problems of form, Darwin's masterly vindication of the theory of
evolution was of immense service to morphology, and Darwin himself was
the first to point out what a great light evolution threw upon all
morphological problems. In a few pages of the _Origin_ he laid the
foundations of evolutionary morphology.

We have here to consider his interpretation of morphological facts and
its relation to the current morphology of his time.

The sketch of his theory, written in 1842,[352] shows a very significant
division into two parts--the first dealing with the positive facts of
variability and the theory of natural selection, the second with the
general evidence for evolution. It is in the second part that the
paragraphs on morphological matters occur. In paragraph 7, on affinities
and classification, Darwin points out that on the theory of evolution
homological relationship would be real relationship, and the natural
system would really be genealogical. In the next paragraph he notes that
evolution would account for the unity of type in the great classes, for
the metamorphosis of organs, and for the close resemblance which early
embryos show to one another. It is of special interest to note that he
definitely rejects the Meckel-Serres theory of recapitulation. "It is
not true," he writes, "that one passes through the form of a lower
group, though no doubt fish more nearly related to foetal state" (p.
42). The greater divergence which adults show seems to him to be due to
the fact that selection acts more on the later than on the embryonic
stages. He realises very clearly how illuminative the theory of
evolution is when applied to the puzzling facts of embryonic
development. "The less differences of foetus--this has obvious meaning
on this view: otherwise how strange that a horse, a man, a bat should at
one time of life have arteries, running in a manner which is only
intelligibly useful in a fish! The natural system being on theory
genealogical, we can at once see why foetus, retaining traces of the
ancestral form, is of the highest value in classification" (p. 45).

Abortive organs, too, gain significance on the evolutionary hypothesis.
"The affinity of different groups, the unity of types of structure, the
representative forms through which foetus passes, the metamorphosis of
organs, the abortion of others, cease to be metaphorical expressions and
become intelligible facts" (p. 50).

In general, organisms can be understood only if we take into account the
cardinal fact that they are historical beings. "We must look at every
complicated mechanism and instinct as the summary of a long history of
useful contrivances much like a work of art" (p. 51).[353]

Already in 1842 Darwin had seized upon the main principles of
evolutionary morphology: the indications then given are elaborated in
the thirteenth chapter of the _Origin of Species_ (1st ed., 1859). A
good part of this chapter is given up to a discussion of the principles
of classification, only a few pages dealing with morphology proper. But,
as Darwin rightly saw, the two things are inseparable.

We note first that there is no hint of the "scale of beings"--Darwin
conceives the genealogical tree as many branched. Animals can be classed
in "groups under groups," and cannot be arranged in one single series.

He discusses first what kind of characters have the greatest
classificatory value. Certain empirical rules have been recognised, more
or less consciously, by systematists--that analogical characters are
less valuable than homological, that characters of great physiological
importance are not always valuable for classificatory purposes, that
rudimentary organs are often very useful, and so on. He finds that as a
general rule "the less any part of the organisation is concerned with
special habits, the more important it becomes for classification" (p.
414), and adduces in support Owen's remark that the generative organs
afford very clear indications of affinities, since they are unlikely to
be modified by special habits. These rules of classification can be
explained "on the view that the natural system is founded on descent
with modification; that the characters which naturalists consider as
showing true affinity ... are those which have been inherited from a
common parent, and, in so far, all true classification is genealogical;
that community of descent is the hidden bond which naturalists have been
unconsciously seeking, and not some unknown plan of creation, or the
enunciation of general propositions, and the mere putting together and
separating objects more or less alike" (p. 420).

In general, then, homological characters are more valuable for
classificatory purposes because they have a longer pedigree than
analogical characters, which represent recent acquirements of the race.

Coming to morphology proper, Darwin takes up the question of the unity
of type, and the homology of parts, for which the unity of type is but a
general expression.

He treats this on the same lines as E. Geoffroy St Hilaire, and Owen,
referring indeed specifically to Geoffroy's law of connections. "What
can be more curious," he asks, "than that the hand of a man, formed for
grasping, that of a mole for digging, the leg of a horse, the paddle of
the porpoise, and the wing of the bat, should all be constructed on the
same pattern, and should include similar bones, in the same relative
positions? Geoffroy St Hilaire has strongly insisted on the high
importance of relative position or connection in homologous parts; they
may differ to almost any extent in form and size, and yet remain
connected together in the same invariable order" (p. 434).

The unity of plan cannot be explained on teleological grounds, as Owen
has admitted in his _Nature of Limbs_, nor is it explicable on the
hypothesis of special creation (p. 435). It can be understood only on
the theory that animals are descended from one another and retain for
innumerable generations the essential organisation of their ancestors.
"The explanation is to a large extent simple on the theory of the
selection of successive slight modifications--each modification being
profitable in some way to the modified form, but often affecting by
correlation other parts of the organisation. In changes of this nature,
there will be little or no tendency to alter the original pattern or to
transpose the parts.... If we suppose that the ancient progenitor, the
archetype as it may be called, of all animals, had its limbs constructed
on the existing general pattern, for whatever purpose they served, we
can at once perceive the plain significance of the homologous
construction of the limbs throughout the whole class" (p. 435).

We may note three important points in this passage--first, the
identification of the archetype with the common progenitor; second, the
view that progressive evolution is essentially adaptive, and dominated
by natural selection; and third, the _petitio principii_ involved in the
assumption that adaptive modification brings inevitably in its train the
necessary correlative changes.

In his section on morphology Darwin shows clearly the influence of Owen,
and through him of the transcendental anatomists. He refers to the
transcendental idea of "metamorphosis," as exemplified in the vertebral
theory of the skull and the theory of the plant appendage, and shows
how, on the hypothesis of descent with modification, "metamorphosis" may
now be interpreted literally, and no longer figuratively merely (p.
439).

Very great interest attaches to Darwin's treatment of development, for
post-Darwinian morphology was based to a very large extent on the
presumed relation between the development of the individual and the
evolution of the race. Just as he kept clear of the notion of the scale
of beings, so he avoided the snare of the Meckel-Serres theory of
recapitulation, according to which the embryo of the highest animal,
man, during its development climbs the ladder upon the rungs of which
the whole animal series is distributed, in its gradual progression from
simplicity to complexity. The law of development which he adopts is that
of von Baer, which states that development is essentially
differentiation, and that as a result embryos belonging to the same
group resemble one another the more the less advanced they are in
development. There can be little doubt that he was indebted to von Baer
for the idea, and in the later editions of the _Origin_ he acknowledges
this by quoting the well-known passage in which von Baer tells how he
had two embryos in spirit which he was unable to refer definitely to
their proper class among Vertebrates.[354]

Not only are embryos more alike than adults, because less
differentiated, but it is in points not directly connected with the
conditions of existence, not strictly adaptive, that their resemblance
is strongest (p. 440)--think, for instance, of the arrangement of aortic
arches common to all vertebrate embryos. Larval forms are to some extent
exceptions to this rule, for they are often specially adapted to their
particular mode of life, and convergence of structure may accordingly
result. All these facts require an explanation. "How, then, can we
explain these several facts in embryology--namely, the very general, but
not universal, difference in structure between the embryo and the
adult--of parts in the same individual embryo, which ultimately become
very unlike and serve for different purposes, being at this early period
of growth alike--of embryos of different species within the same class,
generally but not universally, resembling each other--of the structure
of the embryo not being closely related to its conditions of existence,
except when the embryo becomes at any period of life active and has to
provide for itself--of the embryo apparently having sometimes a higher
organisation than the mature animal, into which it is developed" (pp.
442-3). Obviously all these facts are formally explained by the doctrine
of descent. But Darwin goes further, he tries to show exactly how it is
that the embryos resemble one another more than the adults. He thinks
that the phenomenon results from two principles--first, that
modifications usually supervene late in the life of the individual; and
second, that such modifications tend to be inherited by the offspring at
a corresponding, not early, age (p. 444).

Thus, applying these principles to a hypothetical case of the origin of
new species of birds from a common stock, he writes:--"... from the many
slight successive steps of variation having supervened at a rather late
age and having been inherited at a corresponding age, the young of the
new species of our supposed genus will manifestly tend to resemble each
other much more closely than do the adults, just as we have seen in the
case of pigeons"[355] (pp. 446-7).

Since the embryo shows the generalised type, the structure of the embryo
is useful for classificatory purposes. "For the embryo is the animal in
its less modified state; and in so far it reveals the structure of its
progenitor" (p. 449)--the embryological archetype reveals the ancestral
form. "Embryology rises greatly in interest, when we thus look at the
embryo as a picture, more or less complete, of the parent form of each
great class of animals" (p. 450)--a prophetic remark, in view of the
enormous subsequent development of phylogenetic speculation.

We may sum up by saying that Darwin interpreted von Baer's law
phylogenetically.

The rest of the chapter is devoted to a discussion of abortive and
vestigial organs, whose existence Darwin naturally turns to great
advantage in his argument for evolution. Throughout the whole chapter
Darwin's preoccupation with the problems of classification is clearly
manifest.

On the question as to whether descent was monophyletic or polyphyletic
Darwin expressed no dogmatic opinion. "I believe that animals have
descended from at most only four or five progenitors, and plants from an
equal or lesser number.... I should infer from analogy that probably all
the organic beings which have ever lived on this earth have descended
from one primordial form, into which life was first breathed" (p. 484).

Darwin rightly laid much stress upon the morphological evidence for
evolution,[356] which he considered to be weighty. It probably contributed
greatly to the success of his theory. Though he himself did little or no
work in pure morphology, he was alive to the importance of such work,[357]
and followed with interest the progress of evolutionary morphology,
incorporating some of its results in later editions of the _Origin_, and
in his _Descent of Man_ (1871).

In his morphology Darwin was hardly up to date. He does not seem to have
known at first hand the splendid work of the German morphologists, such
as Rathke and Reichert; he pays no attention to the cell-theory, nor to
the germ-layer theory. His sources are, in the main, Geoffroy St
Hilaire, Owen, von Baer, Agassiz, Milne-Edwards, and Huxley.

Perhaps his greatest omission was that he did not give any adequate
treatment of the problem of functional adaptation and the correlation of
parts. It is not too much to say that Darwin not only disregarded these
problems almost entirely, but by his insistence upon ecological
adaptation and upon certain superficial aspects of correlation,
succeeded in giving to the words "adaptation" and "correlation" a new
signification, whereby they lost to a large extent their true and
original functional meaning.

It is true that Darwin himself, as well as his successors, believed that
natural selection was all-powerful to account for the evolution of the
most complicated organs, but it may be questioned whether he realised
all the conditions of the problem of which he thus easily disposed. He
says, rightly, in an important passage, that "It is generally
acknowledged that all organic beings have been formed on two great
laws--Unity of Type, and the Conditions of Existence. By unity of type
is meant that fundamental agreement in structure which we see in organic
beings of the same class, and which is quite independent of their habits
of life. On my theory, unity of type is explained by unity of descent.
The expression of conditions of existence, so often insisted upon by the
illustrious Cuvier, is fully embraced by the principle of natural
selection. For natural selection acts by either now adapting _the
varying parts of each being to its organic and inorganic conditions of
life_:[358] or by having adapted them during past periods of time: the
adaptations being aided in many cases by the increased use or disuse of
parts, being affected by the direct action of the external conditions of
life, and subjected in all cases to the several laws of growth and
variation. Hence, in fact, the law of the Conditions of Existence is the
higher law; as it includes, through the inheritance of former variations
and adaptations, that of Unity of Type" (_Origin_, 6th ed., Pop.
Impression, pp. 260-1). It is clear that Darwin took the phrase
"Conditions of Existence" to mean the environmental conditions, and the
law of the Conditions of Existence to mean the law of adaptation to
environment. But that is not what Cuvier meant by the phrase: he
understood by it the principle of the co-ordination of the parts to form
the whole, the essential condition for the existence of any organism
whatsoever (see above, Chap. III., p. 34).

Of this thought there is in Darwin little trace, and that is why he did
not sufficiently appreciate the weight of the argument brought against
his theory that it did not account for the correlation of variations.

Darwin's conception of correlation was singularly incomplete. As
examples of correlation he advanced such trivial cases as the relation
between albinism, deafness and blue eyes in cats, or between the
tortoise-shell colour and the female sex. He used the word only in
connection with what he called "correlated variation," meaning by this
expression "that the whole organisation is so tied together during its
growth and development, that when slight variations in any one part
occur, and are accumulated through natural selection, other parts become
modified" (6th ed., p. 177). He took it for granted that the "correlated
variations" would be adapted to the original variation which was acted
upon by natural selection, and he saw no difficulty in the gradual
evolution of a complicated organ like the eye if only the steps were
small enough. "It has been objected," he writes, "that in order to
modify the eye and still preserve it as a perfect instrument, many
changes would have to be effected simultaneously, which, it is assumed,
could not be done through natural selection; but as I have attempted to
show in my work on the variation of domestic animals, it is not
necessary to suppose that the modifications were all simultaneous, if
they were extremely slight and gradual" (6th ed., p. 226).

In post-Darwinian speculation the difficulty of explaining correlated
variation by natural selection alone became more acutely realised, and
it was chiefly this difficulty that led Weismann to formulate his
hypothesis of germinal selection as a necessary supplement to the
general selection theory.

The change in the conception of correlation which Darwin's influence
brought about has been very clearly stated by E. von Hartmann,[359] from
whom the following is taken:--"While the correlation of parts in the
organism was before Darwin regarded exclusively from the standpoint of
morphological systematics, Darwin tried to look at it from the
standpoint of physiological and genealogical development, and in so
doing he put the standpoint of morphological systematics in the shade.
But the more we are now beginning to realise that systematic
relationship does not necessarily imply genetic affinity the more must
the correlation of parts come back into favour as a systematic
principle. While Darwin only, as it were, against his will, relied on
the law of correlation as a last resort when all other help failed, this
law must be regarded, from the standpoint of the orderly inner
determination of all organic form-change, as having the rank of the
highest principle of all, a principle which rules parallel, divergent
and convergent evolution" (pp. 47-8).

Further on, following Rádl, he characterises Darwin's attitude to the
law of correlation in these terms:--"Darwin's interest is entirely
focussed on the variation, the function, the causes of form-production,
in short, upon evolution. Accordingly he regards correlation essentially
as correlative variation in the sense of a _departure_ from the given
type. With morphological correlation in _different_ types Darwin
troubles himself not at all, nor with correlation in the normal
development of a type" (p. 49).

Cuvier's conception of the _convenance des parties_, essential to all
biology, remained on the whole foreign to Darwin's thought, and to the
thought of his successors.

It was indeed one of their boasts that they had finally eliminated all
teleology from Nature. The great and immediate success which Darwinism
had among the younger generation of biologists and among scientific men
in general was due in large part to the fact that it fitted in well with
the prevailing materialism of the day, and gave solid ground for the
hope that in time a complete mechanistic explanation of life would be
forthcoming. "Darwinismus" became the battle-cry of the militant spirits
of that time.

It was precisely this element in Darwinism that was repugnant to most of
Darwin's opponents, in whose ranks were found the majority of the
morphologists of the old school. They found it impossible to believe
that evolution could have come about by fortuitous variation and
fortuitous selection; they objected to Darwin that he had enunciated no
real _Entwickelungsgesetz_, or law governing evolution. They were not
unwilling to believe that evolution was a real process, though many drew
the line at the derivation of man from apes, but they felt that if
evolution had really taken place, it must have been under the guidance
of some principle of development, that there must have been manifested
in evolution some definite and orderly tendency towards perfection.[360]

No one expressed this objection with greater force than did von Baer, in
a series of masterly essays[361] which the Darwinians, through sheer
inability to grasp his point of view, dismissed as the maunderings of
old age. In these essays von Baer pointed out the necessity for the
teleological point of view, at least as complementary to the
mechanistic. His general position is that of the "statical"
teleology--to use Driesch's term--of Kant and Cuvier. His attitude to
Darwinism is determined by his teleology. He admits, just as in 1834, a
limited amount of evolution; he criticises the evolution theory of
Darwin on the same lines exactly as forty or fifty years previously he
had criticised the recapitulation and evolution-theories of the
transcendentalists--principally on the ground that their deductions far
outrun the positive facts at their disposal. He rejects the theory of
natural selection entirely, on the ground that evolution, like
development, must have an end or purpose (_Ziel_)--"A becoming without a
purpose is in general unthinkable" (p. 231); he points out, too, the
difficulty of explaining the correlation of parts upon the Darwinian
hypothesis. His own conception of the evolutionary process is that it is
essentially _zielstrebig_ or guided by final causes, that it is a true
_evolutio_ or differentiation, just as individual development is an
orderly progress from the general to the special. He believed in
saltatory evolution, in polyphyletic descent, and in the greater
plasticity of the organism in earlier times.

The idea of saltatory evolution he took from Kölliker, who shortly after
the publication of the _Origin_ promulgated in a critical note on
Darwinism a sketch of his theory of "heterogeneous generation."[362]

Kölliker's attitude is typical of that taken up by many of the
morphologists of the day.[363] He accepts evolution completely, but
rejects Darwinism because it recognises no _Entwickelungsgesetz_, or
principle of evolution. For the Darwinian theory of evolution through
the selection of small fortuitous variations he would substitute the
theory of evolution through sudden, large variations, brought about by
the influence of a general law of evolution. This is his theory of
heterogeneous generation. "The fundamental idea of this hypothesis is
that under the influence of a general law of evolution creatures produce
from their germs others which differ from them" (p. 181). It is to be
noticed that Kölliker laid more stress upon the _Entwickelungsgesetz_
than upon the saltatory nature of variation, for he says a few pages
further on--"the notion at the base of my theory is that a great
evolutionary plan underlies the development of the whole organised
world, and urges on the simpler forms towards ever higher stages of
complexity" (p. 184). Saltatory evolution was not the essential point of
the theory:--"Another difference between the Darwinian hypothesis and
mine is that I postulate many saltatory changes, but I will not and
indeed cannot lay the chief stress upon this point, for I have not
intended to maintain that the general law of evolution which I hold to
be the cause of the creation of organisms, and which alone manifests
itself in the activity of generation, cannot also so act that from one
form others quite gradually arise" (p. 185). He put forward the
hypothesis of saltatory variation because it seemed to him to lighten
many of the difficulties of Darwinism--the lack of transition forms, the
enormous time required for evolution, and so on. It should be noted that
Kölliker regarded his principle of evolution as mechanical.

It would take too long to show in detail how a belief in innate laws of
evolution was held by the majority of Darwin's critics. A few further
examples must suffice.

Richard Owen, who in 1868[364] admitted the possibility of evolution, held
that "a purposive route of development and change, of correlation and
interdependence, manifesting intelligent Will, is as determinable in the
succession of races as in the development and organisation of the
individual. Generations do not vary accidentally, in any and every
direction; but in pre-ordained, definite, and correlated courses" (p.
808).

He conceived change to have taken place by abrupt variation, independent
of environment and habit, by "departures from parental type, probably
sudden and seemingly monstrous, but adapting the progeny inheriting such
modifications to higher purposes" (p. 797). He believed spontaneous
generation to be a phenomenon constantly taking place, and constantly
giving the possibility of new lines of evolution.

E. von Hartmann in his _Philosophie des Unbewussten_ (1868) and in his
valuable essay on _Wahrheit und Irrtum im Darwinismus_ (1874) criticised
Darwinism in a most suggestive manner from the vitalistic standpoint. He
drew attention to the importance of active adaptation, the necessity for
assuming definite and correlated variability, and to the evidence for
the existence of an immanent, purposive, but unconscious principle of
evolution, active as well in phylogenetic as in individual development.

In France H. Milne-Edwards[365] stated the problem thus:--"In the present
state of science, ought we to attribute to modifications dependent on
the action of known external agents the differences in the organic types
manifested by the animals distributed over the surface of the globe
either at the present day, or in past geological ages? Or must the
origin of types transmissible by heredity be attributed to causes of
another order, to forces whose effects are not apparent in the present
state of things, to a creative power independent of the general
properties of organisable matter such as we know them to-day?" (p. 426)

He concluded that the action of environment, direct or indirect, was
insufficient to account for the diversity of organic forms, and rejected
Darwin's theory completely. He thought it likely that the successive
faunas which palæontology discloses have originated from one another by
descent. But he thought that the process by which they evolved should
rightly be called "creation." The word was of course not to be taken in
a crude sense. When the zoologist speaks of the "creation" of a new
species, "he in no way means that the latter has arisen from the dust,
rather than from a pre-existing animal whose mode of organisation was
different; he merely means that the known properties of matter, whether
inert or organic, are insufficient to bring about such a result, and
that the intervention of a hidden cause, of a power of some higher
order, seems to him necessary" (p. 429).

The criticism of Darwinism exercised by the older currents of thought
remained on the whole without influence. It was under the direct
inspiration of the Darwinian theory that morphology developed during the
next quarter of a century.

    [333] Rádl, _loc. cit._, i., p. 71.

    [334] _Kritik der Urtheilskraft_, 1790.

    [335] Eng. Trans. by J. H. Bernard, p. 337, London, 1892.

    [336] H. F. Osborn, _From the Greeks to Darwin_, p. 145,
    New York and London, 1894.

    [337] See Meckel, _supra_, p. 93; _cf._ Tiedemann,
    _Zoologie_, p. 65, 1808. "Even as each individual
    organism transforms itself, so the whole animal kingdom
    is to be thought of as an organism in course of
    metamorphosis." Also p. 73 of the same book.

    [338] Chapters vii. and ix.

    [339] On early evolution-theories see, in addition to
    Osborn and Rádl, J. Arthur Thomson, _The Science of
    Life_, 1899, and the opening essay in _Darwin and Modern
    Science_, Cambridge, 1909.

    [340] _Phil. zool._, ed. Ch. Martins, vol. i., p. 75,
    1873.

    [341] Quotations in the text are from the 2nd Edit.
    (Deshayes and Milne-Edwards), i., Paris, 1835.

    [342] For instance, Lucretius:--

    "Is tibi nunc animus quali sit corpore et unde
    constiterit pergam rationem reddere dictis. Principio
    esse aio persubtilem atque minutis perquam corporibus
    factum constare."

    --_De Rerum Natura_, iii., vv. 177-80.

    [343] Contrast Treviranus--"In every living being there
    exists a capability of an endless variety of
    form-assumption; each possesses the power to adapt its
    organisation to the changes of the outer world, and it
    is this power, put into action by the change of the
    universe, that has raised the simple zoophytes of the
    primitive world to continually higher stages of
    organisation, and has introduced a countless variety of
    species into animate Nature." Quoted by Haeckel in
    _History of Creation_, i., p. 93, 1876.

    [344] There is no evidence that he was influenced by
    Erasmus Darwin, who forestalled his evolution theory, and
    was indeed more aware of its vitalistic implications. See
    S. Butler, _Evolution, Old and New_, London, 1879, for an
    excellent account of Erasmus Darwin.

    [345] As did also Lyell in his _Principles of Geology_,
    1830.

    [346] K. E. von Baer, _Reden_, i., p. 37, Petrograd, 1864.

    [347] Rádl, _loc. cit._, i., p. 296.

    [348] Reprinted in his _Reden_, i., 1864.

    [349] See Huxley's criticism of it in a Royal Institution
    lecture of 1851, republished in _Sci. Mem._, i., pp.
    300-4. On its relation to Haeckel's biogenetic law, see
    below, p. 255.

    [350] _System der thierischen Morphologie_, p. 5, 1853.

    [351] _Life and Letters of Charles Darwin_, ed. F. Darwin,
    i., p. 82, 3rd ed., 1887.

    [352] _The Foundations of the Origin of Species, a Sketch
    written in 1842_. Ed. F. Darwin, Cambridge, 1909.

    [353] _Cf._ a parallel passage in the _Origin_, 1st ed.,
    pp. 485-6.

    [354] In the 1st ed. (p. 439), Darwin makes the curious
    mistake of attributing this story to Agassiz.

    [355] In which nestlings of the different varieties are
    much more alike than adults. Darwin attached much
    importance to this idea, see _Life and Letters_, i., p.
    88, and ii., p. 338.

    [356] See his _Letters, passim_.

    [357] Writing to Huxley on the subject of the latter's
    work on the morphology of the Mollusca (1853), he
    says:--"The discovery of the type or 'idea' (in your
    sense, for I detest the word as used by Owen, Agassiz &
    Co.) of each great class, I cannot doubt, is one of the
    very highest ends of Natural History."--_More Letters_,
    ed. F. Darwin and A. C. Seward, 1903, i., p. 73.

    [358] Italics mine.

    [359] _Das Problem des Lebens. Biologische Studien_. Bad
    Sacha, 1906. See also E. Rádl, _Biol. Centralblatt_,
    xxi., 1901.

    [360] See the excellent treatment of the difference
    between the "realism" of Darwin and the "rationalism" of
    his critics, in Rádl, ii., particularly pp. 109, 135.
    The most elaborate criticism of Darwinism from the older
    standpoint was that given by A. Wigand in _Der
    Darwinismus und die Naturforschung Newtons und Cuviers_,
    3 vols., Braunschweig, 1872.

    [361] In vol. ii. of his _Reden_, St Petersburg
    (Petrograd), 1876--_Ueber den Zweck in den Vorgängen der
    Natur; Ueber Zielstrebigkeit in den organischen Körpern
    insbesondere_; and _Ueber Darwin's Lehre_.

    [362] "Ueber die Darwinische Schöpfungstheorie," _Zeits.
    f. wiss. Zool._, xiv., pp. 74-86, 1864. Elaborated in
    _Anat. u. syst. Beschreibung d. Alcyonarien_, 1872.

    [363] _Cf._ for instance Nägeli's theory of a perfecting
    principle, first developed in his _Entstehung u. Begriff
    der naturhistorischer Art_, München, 1865.

    [364] _Anatomy of Vertebrates_, iii., 1868.

    [365] _Rapport sur les Progrès récents des Sciences
    zoologiques en France_. Paris, 1867.



CHAPTER XIV

ERNST HAECKEL AND CARL GEGENBAUR


At the time when Darwin's work appeared there already existed, as we
have seen, a fully formed morphology with set and definite principles.
The aim of this pre-evolutionary morphology had been to discover and
work out in detail the unity of plan underlying the diversity of forms,
to disentangle the constant in animal form and distinguish from it the
accessory and adaptive. The main principle upon which this work was
based was the principle of connections, so clearly stated by Geoffroy.
The principle of connections served as a guide in the search for the
archetype, and this search was prosecuted in two directions--first, by
the comparison of adult structure; and second, by the comparative study
of developing embryos. It was found that the archetype was shown most
clearly by the early embryo, and this embryological archetype came to be
preferred before the archetype of comparative anatomy. It became
apparent also that the parts first formed (germ-layers) were of primary
importance for the establishing of homologies.

While practically all morphologists were agreed as to the main
principles of their science, they yet showed, as regards their general
attitude to the problems of form, a fairly definite division into two
groups, of which one laid stress upon the intimate relation existing
between form and function, while the other disregarded function
completely, and sought to build up a "pure" or abstract morphology. In
opposition to both groups, in opposition really to morphology
altogether, a movement had gained strength which tended towards the
analysis and disintegration of the organism. This movement took its
origin in the current materialism of the day, and found expression
particularly in the cell-theory and in materialistic physiology.

The separation between morphology as the science of form and physiology
as the science of the physics and chemistry of the living body had by
Darwin's day become well-nigh absolute.

The morphology of the 'fifties lent itself readily to evolutionary
interpretation. Darwin found it easy to give a formal solution of all
the main problems which pre-evolutionary morphology had set--he was able
to interpret the natural system of classification as being in reality
genealogical, systematic relationship as being really
blood-relationship; he was able to interpret homology and analogy in
terms of heredity and adaptation; he was able to explain the unity of
plan by descent from a common ancestor, and for the concept of
"archetype" to substitute that of "ancestral form."

The current morphology, Darwin found, could be taken over, lock, stock
and barrel, to the evolutionary camp.

In what follows we shall see that the coming of evolution made
surprisingly little difference to morphology, that the same methods were
consciously or unconsciously followed, the same mental attitudes taken
up, after as before the publication of the _Origin of Species_.

Darwin himself was not a professional morphologist; the conversion of
morphology to evolutionary ideas was carried out principally by his
followers, Ernst Haeckel and Carl Gegenbaur in Germany, Huxley,
Lankester, and F. M. Balfour in England.

It was in 1866 that Haeckel's chief work appeared, a _General Morphology
of Organisms_,[366] which was intended by its author to bring all
morphology under the sway and domination of evolution.

It was a curious production, this first book of Haeckel's, and
representative not so much of Darwinian as of pre-Darwinian thought. It
was a medley of dogmatic materialism, idealistic morphology, and
evolution theory; its sources were, approximately, Büchner, Theodor
Schwann, Virchow, H. G. Bronn, and, of course, Charles Darwin.

It was scarcely modern even on its first appearance, and many regarded
it, not without reason, as a belated offshoot of _Naturphilosophie_.

Its materialism is of the most intransigent character. The form and
activities of living things are held to be merely the mechanical result
of the physical and chemical composition of their bodies. The simplest
living things, the Monera, are nothing more than homogeneous masses of
protein substance. "They live, but without organs of life; all the
phenomena of their life, nutrition and reproduction, movement and
irritability, appear here as merely the immediate outcome of formless
organic matter, itself an albumen compound" (p. 63, 1906).

Teleology, the Achilles' heel of Kant's (otherwise sound!) philosophy,
is to be regarded as a totally refuted and antiquated doctrine,
definitely put out of court by Darwinism.

Haeckel works out his materialistic philosophy of living things very
much after the fashion of Schwann. There is the same talk of cells as
organic crystals, of crystal trees, of the analogy between assimilation
by the cell and the growth of crystals in a mother liquid. Heredity and
adaptation are shown equally as well by crystals as by organisms; for
heredity, or the internal _Bildungstrieb_ (!), is the mechanical effect
of the material structure of the crystal or the germ, and adaptation, or
the external _Bildungstrieb_, is a name for the modifications induced by
the environment. Adaptation so defined comes to be synonymous with the
fortuitous variation which plays so great a part in Darwin's theory of
natural selection.

It goes without saying that Haeckel allowed to the organism no other nor
higher individuality than belongs to the crystal, and took no account at
all of that harmonious interaction of the organs which Cuvier called the
principle of the "conditions of existence." The concept of correlation
had simply no meaning for Haeckel. The analysis and disintegration of
the organism was pushed by him to its logical extreme, and in this also
he was a child of his time.

A no less important influence clearly visible in the _General
Morphology_ is the idealistic morphology of men like K. G. Carus and H. G.
Bronn. In previous chapters we have seen how K. G. Carus attempted to
work out a geometry of the organism, and how Bronn tried in a modest way
to found a stereometrical morphology, but had the grace not to push his
stereometry _à l'outrance_, recognising very wisely that the greater
part of organic form is functionally determined. Haeckel took over this
idea[367] and pushed it to wild extremes, founding a new science of
"Promorphology" of which he was the greatest--and only--exponent.[368]

This "science" dealt with axes and planes, poles and angles, in a
veritable orgy of barbarous technical terms. It was intended to be a
"crystallography of the organic," and to lay the foundations of a
mechanistic morphology, or morphography at least.

How it was to be linked up with the physics and chemistry of living
matter on the one hand and with the ordinary morphology of real animals
on the other, was never made quite clear.

The science of Promorphology has no historical significance; it is
interesting only because it illustrates Haeckel's close affinity with
the idealistic morphologists.

Another abortive science of Haeckel's, the science of Tectology, was
equally a heritage from idealistic morphology. Tectology is the science
of the composition of organisms from individuals of different orders.
There were six orders of individuals:--(1) Plastids (Cytodes and cells);
(2) Organs (including cell-fusions, tissues, organs, organ-systems); (3)
Antimeres (homotypic parts, _i.e._, halves or rays); (4) Metameres
(homodynamic parts, _i.e._, segments); (5) Persons (individuals in the
ordinary sense); (6) Corms (colonial animals).

The thought is essentially transcendental, and recalls the "theory of
the repetition of parts," of which so much use was made by the German
transcendentalists, such as Goethe,[369] Oken, Meckel and K. G. Carus, as
well as by Dugès.

The third, and naturally the most important, ingredient in the _General
Morphology_ was the doctrine of evolution, in the form given to it by
Darwin. We have here no concern with Haeckel's evolutionary philosophy,
with the way in which he combined his evolutionism and his materialism
to form a queer Monism of his own. We are interested only in the way he
applied evolution to morphology, what modifications he introduced into
the principles of the science, and in general in what way he interpreted
the facts and theories of morphology in the light of the new knowledge.

We find that he repeats very much what Darwin said, giving, of course,
more detail to the exposition, and elaborating, particularly in his
recapitulation theory or "biogenetic law," certain doctrines not
explicitly stated by Darwin.

Like Darwin he held that the natural system is in reality genealogical.
"There exists," he writes, "one single connected natural system of
organisms, and this single natural system is the expression of real
relations which actually exist between all organisms, alike those now in
being on the earth and those that have existed there in some past time.
The real relations which unite all living and extinct organisms in one
or other of the principal groups of the natural system, are
genealogical: their relationship in form is blood-relationship; the
natural system is accordingly the genealogical tree of organisms, or
their genealogema.... All organisms are in the last resort descendants
of autogenous Monera, evolved as a consequence of the divergence of
characters through natural selection. The different subordinate groups
of the natural system, the categories of the class, order, family,
genus, etc., are larger or smaller branches of the genealogical tree,
and the degree of their divergence indicates the degree of genealogical
affinity of the related organisms with one another and with the common
ancestral form" (ii., p. 420).

The degree of systematic relationship is thus the degree of genealogical
affinity. It follows that the natural system of classification may be
converted straightway into a genealogical tree, and this is actually
what Haeckel does in the _General Morphology_. The genealogical trees
depicted in the second volume (plates i.-viii.) are nothing more than
graphic representations of the ordinary systematic relationships of
organisms, with a few hypothetical ancestral groups or forms thrown in
to give the whole a genealogical turn.

If the genealogical tree is truly represented by the natural system, it
would seem that for each genus a single ancestral form must be
postulated, for each group of genera a single more primitive form, and
so in general for each of the higher classificatory categories, right up
to the phylum. Species of one genus must be descended from a generic
ancestral form, genera of one family from a single family _Urform_, and
so on for the higher categories.

This consequence was explicitly recognised by Haeckel. "Genera and
families," he writes, "as the next highest systematic grades, are
extinct species which have resolved themselves into a divergent bunch of
forms (_Formenbüschel_)" (ii., p. 420).

The archetype of the genus, family, order, class and phylum was thus
conceived to have had at some past time a real existence.

The natural system of classification is based upon a proper appreciation
of the distinction between homological and analogical characters.
Haeckel, following Darwin, naturally interprets the former as due to
inheritance, the latter as due to adaptation, using these words, we may
note, in their accepted meaning and not in the abstract empty sense he
had previously attributed to them.[370] Similarly the "type of
organisation," in von Baer's sense, was due to heredity, the "grade of
differentiation" to adaptation.

So far Haeckel merely emphasised what Darwin had already said in the
_Origin of Species_. But by his statement of the "biogenetic law," and
particularly by the clever use he made of it, Haeckel went a step beyond
Darwin, and exercised perhaps a more direct influence upon evolutionary
morphology than Darwin himself.

Haeckel was not the original discoverer of the law of recapitulation. It
happened that a few years before the publication of Haeckel's _General
Morphology_, a German doctor, Fritz Müller by name, stationed in Brazil,
had been working on the development of Crustacea under the direct
inspiration of Darwin's theory, and had published in 1864 a book[371] in
which he showed that individual development gave a clue to ancestral
history.

He conceived that progressive evolution might take place in two
different ways. "Descendants ... reach a new goal, either by deviating
sooner or later whilst still on the way towards the form of their
parents, or by passing along this course without deviation, but then
instead of standing still advancing still farther" (Eng. trans., p.
111). In the former case the developmental history of descendants agrees
with that of the ancestors only up to a certain point and then diverges.
"In the second case the entire development of the progenitors is also
passed through by the descendants, and, therefore, so far as the
production of a species depends upon this second mode of progress, the
historical development of the species will be mirrored in its
developmental history" (p. 112).

Of course the recapitulation of ancestral history will be neither
literal nor extended. "The historical record preserved in developmental
history is gradually _effaced_ as the development strikes into a
constantly straighter course from the egg to the perfect animal, and it
is frequently _sophisticated_ by the struggle for existence which the
free-living larvæ have to undergo" (p. 114).

It follows that "the primitive history of a species will be preserved in
its developmental history the more perfectly the longer the series of
young stages through which it passes by uniform steps; and the more
truly, the less the mode of life of the young departs from that of the
adults, and the less the peculiarities of the individual young states
can be conceived as transferred back from later ones in previous periods
of life, or as independently acquired" (p. 121).

Applying these principles to Crustacea, he concluded that the shrimp
_Peneus_ with its long direct development gave the best and truest
picture of the ancestral history of the Malacostraca, and that
accordingly the nauplius and the zoaea larvæ represented important
ancestral stages. He conceived it possible so to link up the various
larval forms of Crustacea as to weave a picture of the primeval history
of the class, and he made a plucky attempt to work out the phylogeny of
the various groups.

The thought that development repeats evolution was already implicit in
the first edition of the _Origin_, but the credit for the first clear
and detailed exposition of it belongs to F. Müller.

In much the same form as it was propounded by Müller it was adopted by
Haeckel, and made the corner-stone of his evolutionary embryology.
Haeckel gave it more precise and more technical formulation, but added
nothing essentially new to the idea.

It is convenient to use his term for it--the biogenetic law
(_Biogenetische Grundgesetz_)--to distinguish it from the laws of
Meckel-Serres and von Baer, with which it is so often confused.

Haeckel's statement of it may best be summarised in his own words,
"Ontogeny, or the development of the organic individual, being the
series of form-changes which each individual organism traverses during
the whole time of its individual existence, is immediately conditioned
by phylogeny, or the development of the organic stock (phylon) to which
it belongs.

"Ontogeny is the short and rapid recapitulation of phylogeny,
conditioned by the physiological functions of heredity (reproduction)
and adaptation (nutrition). The organic individual (as a morphological
individual of the first to the sixth order) repeats during the rapid and
short course of its individual development the most important of the
form-changes which its ancestors traversed during the long and slow
course of their palæontological evolution according to the laws of
heredity and adaptation.

"The complete and accurate repetition of phyletic by biontic development
is obliterated and abbreviated by secondary contraction, as ontogeny
strikes out for itself an ever straighter course; accordingly, the
repetition is the more complete the longer the series of young stages
successively passed through.

"The complete and, accurate repetition of phyletic by biontic
development is falsified and altered by secondary adaptation, in that
the bion[372] during its individual development adapts itself to new
conditions: accordingly the repetition is the more accurate the greater
the resemblance between the conditions of existence under which
respectively the bion and its ancestors developed" (ii., p. 300).

The last two propositions, it will be observed, are taken over almost
verbally from F. Müller.

Now we have seen that the natural system of classification gives a true
picture of the genealogical relationships of organisms, that the smaller
and larger classificatory groups correspond to greater or lesser
branches of the genealogical tree. If ontogeny is a recapitulation of
phylogeny, we must expect to find the embryo repeating the organisation
first of the ancestor of the phylum, then of the ancestor of the class,
the order, the family and the genus to which it belongs. There must be a
threefold parallelism between the natural system, ontogeny and phylogeny
(ii., pp. 421-2).

It will be observed that there is here implied an analogy between the
biogenetic law and the law of von Baer, for both assert that development
proceeds from the general to the special, that the farther back in
development you go the more generalised do you find the structure of the
embryo; both assert, too, that differentiation of structure takes place
not in one progressive or regressive line, but in several diverging
directions.

But the analogy between the biogenetic law and the Meckel-Serres law is
even more obvious, and the resemblance between the two is much more
fundamental. It is a significant fact that in his theory of the
threefold parallelism Haeckel merely resuscitated in an evolutionary
form a doctrine widely discussed in the 'forties and 'fifties,[373] and
championed particularly by L. Agassiz,[374] a doctrine which must be
regarded as a development or expansion of the Meckel-Serres law.[375] It
is the view that a parallelism exists between the natural system,
embryonic development, and palæontological succession. Actually, as
Agassiz stated it, the doctrine applied neither to types, nor as a
general rule to classes, but merely to orders. It was well exemplified,
he thought, in Crinoids:--"The successive stages of the embryonic growth
of Crinoids typify, as it were, the principal forms of Crinoids which
characterise the successive geological formations. First, it recalls the
Cistoids of the palæozoic rocks, which are represented in its simple
spheroidal head; next the few-plated Platycrinoids of the Carboniferous
period; next the Pentacrinoids of the Lias and Oolite with their whorls
of cirrhi; and finally, when freed from its stem, it stands as the
highest Crinoid, as the prominent type of the family in the present
period" (p. 171).

The Meckel-Serres law, it will be remembered, expressed the idea that
the higher animals repeat in their ontogeny the adult organisation of
animals lower in the scale. Since Haeckel recognised clearly that a
linear arrangement of the animal kingdom was a mere perversion of
reality, and that a branching arrangement of groups more truly
represented the real relations of animals to one another, he could not
of course entertain the Meckel-Serres theory in its original form. But
he accepted the main tenet of it when he asserted that each stage of
ontogeny had its counterpart in an adult ancestral form. Such ancestral
forms might or might not be in existence as real species at the present
day; they might or might not be discoverable as fossils. That they had
real existence either now or at some past epoch Haeckel never doubted.
In his construction of phylogenetic trees he was so confident in the
truth of his biogenetic law that he largely disregarded and consistently
minimised the importance of the evidence from palæontology.

The biogenetic law differed from the Meckel-Serres law chiefly in the
circumstance that many of the adult lower forms whose organisation was
supposed to be repeated in the development of the higher animals were
purely hypothetical, being deduced directly from a study of ontogeny and
systematic relationships. The hypothetical ancestral forms which the
theory thus postulated naturally took their place in the natural system,
for they were merely the concrete projections or archetypes of the
classificatory groups.

The transcendentalists, of course, conceived evolution, whether real or
ideal, as a uniserial process, whereas Haeckel conceived it as
multiserial and divergent. It is here that the superficial agreement of
the biogenetic law with the law of von Baer comes in.

We might almost sum up the relation of the biogenetic law to the laws of
von Baer and Meckel-Serres by saying that it was the Meckel-Serres law
applied to the divergent differentiation upheld by von Baer instead of
to the uniserial progression believed in by the transcendentalists.

How near in practice Haeckel's law came to the recapitulation theory of
the transcendentalists may be seen in passages like the following, with
its partial recognition of the _Échelle_ idea:[276]--"As so high and
complicated an organism as that of man ... rises upwards from a simple
cellular state, and as it progresses in its differentiating and
perfecting, it passes through the same series of transformations which
its animal progenitors have passed through, during immense spaces of
time, inconceivable ages ago.... Certain very early and low stages in
the development of man, and other vertebrate animals in general,
correspond completely in many points of structure with conditions which
last for life in the lower fishes. The next phase which follows on this
presents us with a change of the fish-like being into a kind of
amphibious animal. At a later period the mammal, with its special
characteristics, develops out of the amphibian, and we can clearly see,
in the successive stages of its later development, a series of steps of
progressive transformation which evidently correspond with the
differences of different mammalian orders and families."[377]

The biogenetic law went beyond both the Meckel-Serres law and the law of
von Baer in that it recognised that the ancestral history of the species
accounts in part for the course which the development of the individual
takes, that in a certain sense, though not in the crude way supposed by
Haeckel, phylogeny is the cause of ontogeny. This thought, that the
organism is before all an historical being, is of course implied in the
evolution idea, is indeed the essential core of it. Take away this
element from the biogenetic law--not a difficult matter--and it becomes
merely a law of idealistic morphology, applicable to evolution
considered as an ideal process, as the progressive development in the
Divine thought of archetypal models.

As a book, the _General Morphology_ suffers a good deal from the arid,
schematic, almost scholastic manner of exposition adopted. Haeckel's
Prussian mania for organisation, for absolute distinctions, for
iron-bound formalism, is here given full scope. A treatment less
adequate to the variety, fluidity and changeableness of living things
could hardly be imagined.

His doctrine, though it remains essentially unchanged, receives in his
later works a less formal and more concrete expression, and, in
particular, his views on the biogenetic law undergo some small
modification.

Even in the _General Morphology_ Haeckel had recognised that ontogeny is
neither a complete nor an entirely accurate recapitulation of phylogeny;
he had admitted, following F. Müller, that the true course of
recapitulation was frequently modified by larval and foetal adaptations.
As time went on, he was forced to hedge more and more on this point, and
finally in his _Anthropogenie_ (1874) and his second paper on the
Gastræa theory (1875),[378] he had to work out a distinction between
palingenetic and cenogenetic characters, of which much use was made by
subsequent writers.

The distinction may be given in Haeckel's own words:--"Those ontogenetic
processes," he writes, "which are to be referred immediately, in
accordance with the biogenetic law, to an earlier completely developed
_independent ancestral form_, and are transmitted from this by
_heredity_, obviously possess _primary_ importance for the understanding
of the casual-physiological relations; on the other hand, those
developmental processes which appear subsequently through _adaptation_
to the needs of embryonic or larval life, and accordingly can _not_ be
regarded as repeating the organisation of an earlier independent
ancestral form, can clearly have for the understanding of the ancestral
history only a quite subordinate and _secondary_ importance.

"The first I have named _palingenetic_, the second _cenogenetic_.
Considered from this critical standpoint, the whole of ontogeny falls
into two main parts:--First, _palingenesis_, or 'epitomised history'
(_Auszugsgeschichte_), and second, _cenogenesis_, or 'counterfeit
history' (_Fälschungsgeschichte_). The first is the true ontogenetic
epitome or short recapitulation of past evolutionary history; the second
is the exact contrary, a new foreign ingredient, a falsifying or
concealing of the epitome of phylogeny."[379]

As examples of palingenetic processes in the development of Amniotes,
for instance, may be quoted the separation of two primary germ-layers,
the formation of a simple notochord between medullary tube and
alimentary canal, the appearance of a simple cartilaginous cranium, of
the gill-arches and their vessels, of the primitive kidneys, the
primitive tubular heart, the paired aortæ and the cardinal veins, the
hermaphroditic rudiment of the gonads, and so on. Cenogenetic processes,
on the other hand, include such phenomena as the formation of yolk and
the embryonic membranes, the temporary allantoic circulation, the navel,
the curved and contracted shape of the embryo, and the like.

The most important phenomena to be included under the general heading of
cenogenesis are, first, the occurrence of food-yolk, and second, those
anomalies of development which are classed by Haeckel as heterochronies
and heterotopies.

It is to the influence of the different amounts of yolk present in the
egg that are due the great differences in the segmentation and
gastrulation processes, which almost mask their true significance.

Heterochronic processes are such as arise through the dislocation of the
proper phylogenetic order of succession: heterotopic processes in the
same way are caused by a wandering of cells from one germ-layer to
another. The two classes of phenomena are disturbances either of the
proper spatial or of the proper temporal relation of the parts during
development.

Heterochrony shows itself, as a rule, either as an acceleration or as a
retardation of developmental events, as compared with their relative
time of occurrence during phylogeny. Thus the notochord, the brain, the
eyes, the heart, appear earlier in the ontogenetic than in the
phylogenetic series, while, on the other hand, the septum of the
auricles appears in the development of the higher Vertebrates before the
ventricular septum, which is undoubtedly a reversal of the phylogenetic
order.

Cases of heterotopy, or of organs being developed in a position or a
germ-layer other than that in which they originally arose in phylogeny,
are not so easy to find. According to Haeckel, the origin of the
generative products in the mesoderm is a heterotopic phenomenon, for he
considers that they must have originated phylogenetically in one of the
two primary layers, ectoderm or endoderm.

It is worthy of note that the help of comparative anatomy is admittedly
required in deciding what processes are palingenetic and what
cenogenetic (p. 412).

Haeckel's morphological notions, and particularly his biogenetic law,
excited a good deal of adverse criticism from men like His, Claus,
Salensky, Semper and Goette. Nor was his principal work, the _General
Morphology_, received with much favour. Nevertheless, since he did
express, though in a crude, dogmatic and extreme manner, the main
hypotheses upon which evolutionary morphology is founded, his historical
importance is considerable. He cannot perhaps be regarded as typical of
the morphologists of his time--he was too trenchantly materialistic, too
much the populariser of a crude and commonplace philosophy of Nature. In
point of concrete achievement in the field of pure research he fell
notably behind many of his contemporaries.

His friend, Carl Gegenbaur, who gained a great and well-deserved
reputation by his masterly studies on vertebrate morphology,[380] was a
sounder man, and probably exercised a wider and certainly a more
wholesome influence upon the younger generation of professional
morphologists than the more brilliant Haeckel. It is true that in his
famous _Grundzüge der vergleichenden Anatomie_, the second edition of
which, published in 1870, soon came to be regarded as the classical
text-book of evolutionary morphology, Gegenbaur enunciated very much the
same general principles as Haeckel, and referred to the _Generelle
Morphologie_ as the chief and fundamental work on animal morphology. But
in Gegenbaur's pages the Haeckelian doctrines are modified and subdued
by the strong commonsense and thorough appreciation of the older
classical or Cuvierian morphology that characterise Gegenbaur's work.
According to Haeckel,[381] Gegenbaur was greatly influenced by J. Müller,
who, as we know, laid as much stress on function as on form.

The "General Part" of Gegenbaur's text-book is in many ways a
significant document and deserves close attention.

We note first of all that physiology and morphology are considered by
Gegenbaur to be entirely distinct sciences, with different
subject-matter and different methods. "The task of physiology is the
investigation of the functions of the animal body or of its parts, the
referring back of these functions to elementary processes and their
explanation by general laws. The investigation of the material
substratum of these functions, of the form of the body and its parts,
and the explanation of this form, constitute the task of Morphology"
(2nd ed., p. 3).

Morphology falls naturally into two divisions--comparative anatomy and
embryology. The method of comparative anatomy is _comparison_ (p. 6),
and in employing this method account is to be taken of "the spatial
relations of the parts to one another, their number, extent, structure,
and texture." Through comparison one is enabled to arrange organs in
continuous series, and it comes out very clearly during this proceeding
"that the physiological value of an organ is by no means constant
throughout the different form-states of the organ, that an organ,
through the mere modification of its anatomical relations, can subserve
very different functions. Exclusive regard for their physiological
functions would place morphologically related organs in different
categories. From this it follows that in comparative anatomy we should
never in the first place consider the function of an organ. The
physiological value comes only in the second place into consideration,
when we have to reconstruct the relations to the organism as a whole of
the modification which an organ has undergone as compared with another
state of it. In this way comparative anatomy shows us how to arrange
organs in series; within these series we meet with variations which
sometimes are insignificant and sometimes greater in extent; they affect
the extent, number, shape, and texture of the parts of an organ, and can
even, though only in a slight degree, lead to alterations of position"
(p. 6).

Geoffroy St Hilaire would have subscribed to every word of this
vindication of his "principle of connections."

Between comparative anatomy and embryology there exists a close
connection, for the one throws light on the other. "While in some cases
the same organ shows only slight modifications in its development from
its early beginnings to its perfect state, in other cases the organ is
subjected to manifold modifications before it reaches its definitive
form; we see parts appear in it which later disappear, we observe
alterations in it in all its anatomical relations, alterations which may
even affect its texture. This fact is of great importance, for those
changes which an organ undergoes during its individual development lead
through states which the organ in other cases permanently shows, or at
the least the first appearance of the organ is the equivalent of a
permanent state in another organism. If then the fully developed organ
is in any special case so greatly modified that its proper relation to
some organ-series is obscured, this relation may be cleared up by a
knowledge of the organ's development. The earlier state indicated in
this way enables one to find with ease the proper place for the organ
and so insert it into an already known series. The relations which we
observe in an organ-seriation are then the equivalent of processes which
in certain cases take place in a similar manner during the individual
development of an organ. Embryology enters therefore into the closest
connection with comparative anatomy.... It teaches us to know organs in
their earliest states, and connects them up with the permanent states of
others, whereby they fill up the gaps which we meet with in the various
series formed by the fully developed organs of the body" (pp. 6-7).

This recognition of the parallelism between comparative anatomy and
embryology is, of course, the kernel of the Meckel-Serres law. For
Gegenbaur it had a very definite evolutionary meaning--he subscribed to
the evolutionary form of it, the biogenetic law. How near his conception
of the relation between ontogeny and phylogeny came to the old
Meckel-Serres law may be gauged from the following passage, taken from a
later work:--"Ontogeny thus represents, to a certain degree,
palæontological development abbreviated or epitomised. The stages which
are passed through by higher organisms in their ontogeny correspond to
stages which are maintained in others as the definitive organisation.
These embryonic stages may accordingly be explained by comparing them
with the mature stages of lower organisms, since we regard them as forms
inherited from ancestors belonging to such lower stages"[382] (p. 6).

It is worth noting that in Gegenbaur's opinion comparative anatomy was
prior in importance to embryology, that embryology could hardly exist as
an independent science, since it must seek the interpretation of its
facts always in the facts of comparative anatomy (_Grundzüge_, pp. 7-8).

While Gegenbaur was at one with all "pure" morphologists, whether
evolutionary or pre-evolutionary, in minimising as far as possible the
importance of function in the study of form, he was too cautious and
sober a thinker not to recognise the immense part which function really
plays. Thus he classified organs, according to their function, into
those that established relations with the external world and those that
had to do with nutrition and reproduction, very much as Bichat had done
before him.

Like Darwin, Haeckel and most evolutionists, he interpreted the
homological resemblances of animals as being due to heredity, their
differences as due to adaptation,[383] but he did not adopt Haeckel's
crude and shallow definition of these terms. For Gegenbaur heredity was
a convenient expression for the fact of transmission, and was not
explained offhand as the mere mechanical result of a certain material
structure handed down from germ to germ. Adaptation he defined in a way
which took the fullest account of function, and was as far as possible
removed from Haeckel's definition of it as the direct mechanical effect
of the environment upon the organism. "The organism is altered," writes
Gegenbaur, "according to the conditions which influence it. The
consequent _Adaptations_ are to be regarded as gradual, but steadily
progressive, changes in the organisation, which are striven after during
the individual life of the organism, preserved by transmission in a
series of generations, and further developed by means of natural
selection. What has been gained by the ancestor becomes the heritage of
the descendant. Adaptation and Transmission are thus alternately
effective, the former representing the modifying, the latter the
conservative principle.... Adaptation is commenced by a change in the
function of organs, so that the _physiological relations_ of organs play
the most important part in it. Since adaptation is merely the material
expression of this change of function, the modification of the function
as much as its expression is to be regarded as a gradual process. In
Adaptation, the closest connection between the function and the
structure of an organ is thus indicated. Physiological functions govern,
in a certain sense, structure; and so far what is morphological is
subordinated to what is physiological" (_Elements_, pp. 8-9). Gegenbaur
recognised also that morphological differentiation depended largely on
the physiological division of labour (_Grundzüge_, p. 49).

It is clear that Gegenbaur realised vividly the importance of function,
and in this respect, as in others, he is far beyond Haeckel. The same
thing comes out markedly in his treatment of correlation. Haeckel had no
slightest feeling for the true meaning of correlation. For him, as for
Darwin, it reduced itself to a law of correlative variation, according
to which "actual adaptation not only changes those parts of the organism
which are directly affected by its influence, but other parts also, not
directly affected by it."[384] Such "correlative adaptation" was due to
nutrition being a "connected, centralised activity."

Gegenbaur, on the contrary, had a firm grasp of the Cuvierian
conception, and expressed it in unmistakable terms. "As indeed follows
from the conception of life as the harmonious expression of a sum of
phenomena rigorously determining one another, no activity of an organ
can in reality be thought of as existing for itself. Each kind of
function (_Verrichtung_) presupposes a series of other functions, and
accordingly every organ must possess close relations with, and be
dependent on, all the others" (_Grundzüge_, p. 71). The organism must be
regarded as an individual whole which is as much conditioned by its
parts as one part is conditioned by the others. For an understanding of
correlation a knowledge of functions, and of the functional relations of
the organism to its environment, is clearly indispensable.

Gegenbaur's morphological system was out-and-out evolutionary. "The most
important part of the business of comparative anatomy," in Gegenbaur's
eyes, "is to find indications of genetic connection in the organisation
of the animal body" (_Elements_, p. 67).

The most important clue to discovering this genetic connection is of
course that given by homology; it is indeed the main principle of
evolutionary morphology that what is common in organisation is due to
common descent, what is divergent is due to adaptation. "Homology ...
corresponds to the hypothetical genetic relationship. In the more or the
less clear homology, we have the expression of the more or less intimate
degree of relationship. Blood-relationship becomes dubious exactly in
proportion as the proof of homologies is uncertain" (_Elements_, p. 63).

It is worth noting that while Gegenbaur agrees with Haeckel generally
that morphological relationships are really genealogical, that, for
instance, each phylum has its ancestral form, he enters a caution
against too hastily assuming the existence of a genetic relation between
two forms on the basis of the comparison of one or two organs. "In
treating comparative anatomy from the genealogical standpoint required
by the evolution-theory," he writes, "we have to take into consideration
the fact that the connections can almost never be discovered in the real
genealogically related objects, for we have almost always to do with the
divergent members of an evolutionary series. We derive, for instance,
the circulatory system of insects from that of Crustacea ... but there
exists neither a form that leads directly from Crustacea to insects nor
any organisatory state (_Organisationszustand_), which as such shows the
transition. Even when one point of organisation can be denoted as
transitional, numerous other points prevent us from regarding the whole
organism strictly in the same light" (_Grundzüge_, p. 75). The real
ancestral forms cannot, as a rule, be discovered among living species,
nor often as extinct. "When we arrange allied forms in series by means
of comparison, and seek to derive the more complex from the simpler, we
recognise in the lower and simpler forms only similarities with the
ancestral form, which remains essentially hypothetical" (p. 75).

The facts of development, Gegenbaur goes on to say, help us out greatly
in our search for ancestral forms, for the early stages in the ontogeny
of a highly organised animal give us some idea of the organisation of
its original ancestor. Characters common to the early ontogeny of all
the members of a large group are particularly important in this respect
(_cf._ von Baer's law).

Gegenbaur distinguishes homologous or morphologically equivalent
structures from such as are analogous or physiologically equivalent,
just as did Owen and the older anatomists. Like von Baer he recognises
homologies, as a rule, only within the type.

He contributed, however, to the common stock a useful analysis of the
concept of homology, and established certain classes and degrees of it.
He distinguished first between general and special homology, in quite a
different sense from Owen.

General homology, in Gegenbaur's sense, relates to resemblances of
organs within the organism, and includes four kinds of resemblance,
homotypy, homodynamy, homonomy and homonymy. Right and left organs are
homotypic, metameric organs are homodynamic; homonomy is the relation
exemplified by fin-rays or fingers, which are arranged with reference to
a transverse axis of the body; homonymy is a sort of metamerism in
secondary parts (not the main axis) of the body, and is shown by the
various divisions of the appendages (_Grundzüge_, p. 80).

Special homology, on the other hand, relates to resemblances between
organs in different animals. The interesting thing is that Gegenbaur
defines it genetically. Special homology is the name we give "to the
relations which obtain between two organs which have had a common
origin, and which have also a common embryonic history" (_Elements_, p.
64). This is his definition; but, in practice, Gegenbaur establishes
homologies by comparison just as the older anatomists did, and infers
common descent from homology, not homology from common descent.

"Special homology," he continues, "must be again separated into
sub-divisions, according as the organs dealt with are essentially
unchanged in their morphological characters, or are altered by the
addition or removal of parts" (p. 65). In the former case the homology
is said to be "complete," in the latter "incomplete." Thus the bones of
the upper arm are completely homologous throughout all vertebrate
classes from Amphibia upwards, while the heart of a fish is incompletely
homologous with the heart of a mammal.

Independently of Gegenbaur, Sir E. Ray Lankester proposed in 1870 a
genetic definition of homology.[385] He proposed, indeed, to do away with
the term homology altogether, on the ground that it included many
resemblances which were obviously not due to common descent--as, for
instance, the resemblance of metameres. So, too, organs which were
homologous in the ordinary sense, as the heart of birds and mammals,
might have arisen separately in evolution. He proposed, therefore, that
"structures which are genetically related, in so far as they have a
single representative in a common ancestor," should be called
_homogenous_(p. 36). All other resemblances were to be called
_homoplastic_. "Homoplasy includes all cases of close resemblance of
form which are not traceable to homogeny, all details of agreement not
homogenous, in structures which are broadly homogenous, as well as in
structures having no genetic affinity" (p. 41). Serial homology, for
instance, was a case of homoplasy.

The term "analogy" was to be retained for cases of functional
resemblance, whether homogenetic or not.

The attempt was an interesting one, but most morphologists wisely
adhered to the old concept of homology, in spite of Lankester's
declaration that this belonged to an older "Platonic" philosophy, and
ought to be superseded by a term more consonant with the new philosophy
of evolution.

    [366] _Generelle Morphologie der Organismen. Allgemeine
    Grundzüge der organischen Formenwissenschaft, mechanisch
    begründet durch die von Ch. Darwin reformierte
    Descendenztheorie_. Berlin, 1866. Reprinted in part as
    _Prinzipien der generellen Morphologie der Organismen_.
    Berlin, 1906.

    [367] He mentions as his predecessors in this field,
    Bronn, J. Müller, Burmeister, and G. Jäger.

    [368] In _Grundriss einer Allgemeinen Naturgeschichte der
    Radiolarien_, Berlin, 1887, and _Kunstformen der Natur_,
    Suppl. Heft, Leipzig.

    [369] Haeckel had an intense admiration for Goethe's
    morphological work. It is a curious coincidence that the
    work of Goethe, Oken and Haeckel was closely associated
    with the town of Jena.

    [370] But he himself would not admit this! See _Gen.
    Morph._, ii., p. 11.

    [371] _Für Darwin_, 1864. Eng. trans, by Dallas as _Facts
    and Arguments for Darwin_, London, 1869.

    [372] The bion is the physiological, as the morphon is the
    morphological, individual.

    [373] See Vogt, _Embryologie des Salmones_, p. 259, 1842,
    and _supra_, p. 230.

    [374] _An Essay on Classification_, London, 1859.

    [375] It was hinted at by Tiedemann. "It is clear that,
    proceeding from the earlier to the more recent strata, a
    gradation in fossil forms can be established from the
    simplest organised animals, the polyps, up to the most
    complex, the mammals, and that accordingly the animal
    kingdom as a whole has its developmental periods just
    like the single individual organism. The species and
    genera which have become extinct during the evolutionary
    process may be compared with the organs which disappear
    during the development of the individual animal" (p. 73,
    1808).

    [376] _The History of Creation_, vol. i., p. 310, 1876.
    Translation of the _Natürliche
    Schöpfungsgeschichte_, 1868.

    [377] _Cf._ a parallel passage from Serres, _supra_, p.
    82.

    [378] _Jenaische Zeitschrift_, ix., pp. 402-508, 1875.

    [379] _Loc. cit._, ix., p. 409.

    [380] _Untersuchungen zur vergl. Anatomie d.
    Wirbelthiere_, Leipzig, i., 1864; ii., 1865; and iii.,
    1872.

    [381] "U. d. Biologie in Jena während des 19
    Jahrhunderts," _Jenaische Zeitschrift_, xxxix., pp.
    713-26, 1905.

    [382] _Grundriss der vergl. Anatomie_, 1874, 2nd ed.,
    1878. Trans. by F. Jeffrey Bell, revised by E. Ray
    Lankester, as _Elements of Comparative Anatomy_, London,
    1878.

    [383] "This theory (evolution) shows that what was
    formerly called 'structural plan' or 'type' is the sum
    of the dispositions (_Einrichtungen_) of the animal
    organisation which are perpetuated by heredity, while it
    explains the modifications of these dispositions as
    adaptive states. Heredity and adaptation are thus the
    two important factors through which both the unity and
    the variety of organisation can be understood"
    (_Grundzüge_, p. 19).

    [384] _History of Creation_, i., pp. 241-2.

    [385] "On the use of the term Homology in Modern Zoology,
    and the distinction between Homogenetic and Homoplastic
    agreements," _Ann. Mag. Nat. Hist._ (4), vi., pp. 35-43,
    1870.



CHAPTER XV

EARLY THEORIES ON THE ORIGIN OF VERTEBRATES


Haeckel and Gegenbaur set the fashion for phylogenetic speculation, and
up to the middle 'eighties, when the voice of the sceptics began to make
itself heard, the chief concern of the younger morphologists was the
construction of genealogical trees. The period from about 1865 to 1885
might well be called the second speculative or transcendental period of
morphology, differing only from the first period of transcendentalism by
the greater bulk of its positive achievement. It must be remembered that
the later workers (at least towards the end of this period) had immense
advantages over their predecessors in the matter of equipment and
technique; they possessed well-fitted laboratories in the university
towns and by the sea; they had at their command perfected microscopes
and microtomes; while the whole new technique of microscopical anatomy
with its endless variety of stains and reagents made it possible for the
tyro to confirm in a day what von Baer and Müller had taken weeks of
painful endeavour to discover.[386] But the democratisation of morphology
which followed upon the facilitation of its means of research left an
evil heritage of detailed and unintelligent work to counterbalance the
very great and real advances which technical improvements alone rendered
possible.

This period of rapid development, which set in soon after the coming of
evolution and multiplied the concrete facts of morphology an
hundredfold, may for our present purpose be conveniently divided into
two somewhat overlapping periods, of which the second may be said to
begin with the enunciation by Haeckel of his Gastræa theory. Within the
first period fall the evolutionary speculations associated with the
names of Kowalevsky, Dohrn, Semper, and others; the characteristic of
the second period is the preponderating influence exercised upon
phylogenetic speculations by the germ-layer doctrine in its two main
evolutionary developments, the Gastræa and Coelom theories.

In the first period we might again distinguish two main tendencies,
according as speculations were based mainly upon anatomical or mainly
upon embryological considerations, and it so happens that these two
tendencies are very well illustrated by the various theories as to the
origin of Vertebrates which began to appear towards the 'seventies. We
shall accordingly, in this chapter, consider very briefly the history of
the earlier views on the phylogeny of the vertebrate stock.

In the early days, before the other claimants to the dignity of
ancestral form to the Vertebrates--_Balanoglossus_, Nemertines and the
rest--had put in an appearance, there were two main views on the
subject, one upheld by Haeckel, Kowalevsky and others, to the effect
that the proximate ancestor of Vertebrates was a form somewhat
resembling the ascidian tadpole, the other supported principally by
Dohrn and Semper that Vertebrates and Arthropods traced their descent to
a common segmented annelid or pro-annelid ancestor. The former view is
historically prior, and arose directly out of the brilliant
embryological investigations of A. Kowalevsky, who proved himself to be
a worthy successor of the great comparative embryologist Rathke. His
work was indeed a true continuation of Rathke's. It was not directly
inspired by evolution, though it supplied much useful confirmation of
the theory--you may read Kowalevsky's earlier memoirs and not realise
that they were written several years after the publication of the
_Origin of Species_.

His first paper of evolutionary importance was a note in Russian on the
development of Amphioxus, published in 1865. This subject was followed
up in two papers which appeared in 1867[387] and 1877.[388] In his
papers on Amphioxus Kowalevsky made out the main features in the
development of this primitive form, and showed that the chief organs
were formed in essentially the same way as in Vertebrates; he described
the formation of the archenteron by invagination, the appearance of the
medullary folds, which coalesced to form the neural canal, the formation
of the notochord and of the gill-slits. At first he made the mistake of
supposing that the body-cavity arose from the segmentation-cavity, but
in his later paper he rightly surmised that it was formed from the
cavities of the "primitive vertebræ," or mesodermal segments. The origin
of the notochord from the endoderm was also not made out by Kowalevsky
in his paper of 1867.

Although many important details remained to be discovered by later
investigators,[389] Kowalevsky's work at once made the development of
Amphioxus the key to vertebrate embryology, the typical ontogeny with
which all others could be compared.

Meanwhile, in 1866 and 1871, Kowalevsky had communicated memoirs of even
greater interest,[390] in which he showed that the simple Ascidians
developed in an extraordinarily similar way to Amphioxus and hence to
Vertebrates in general. His proof that Ascidians also develop on the
vertebrate type aroused great interest at the time, and was naturally
acclaimed by the evolutionists as a striking piece of evidence in favour
of their doctrine. The systematic position of the Ascidians was at that
time quite uncertain; they were grouped, as a rule, with the Mollusca,
and certainly no one suspected that their well-known tailed larvæ, first
seen by Savigny, showed any but the most superficial analogy with the
tadpoles of Amphibia. Kowalevsky's papers put a different complexion on
the matter. In the first of them he showed how the nervous system of the
simple Ascidian developed from ectodermal folds just as it did in
Amphioxus and Vertebrates, how gill-slits were formed in the walls of
the pharynx, and how there existed in the ascidian larva a structure
which in position and mode of development was the strict homologue of
the vertebrate notochord. In his second paper he entered into much more
detail, and published some excellent figures, often reproduced since
(see Fig. 13), but the proof of the affinity between Vertebrates and
Ascidians was in all essentials complete in his paper of 1866.

[Illustration: FIG. 13.--Development of the Ascidian Larva. (After
Kowalevsky.)]

Kowalevsky's results were accepted by Haeckel, Gegenbaur, Darwin,[391]
and many others as conclusive evidence of the origin of Vertebrates
from a form resembling the ascidian tadpole; they were extended and
amplified by Kupffer[392] in 1870, later by van Beneden and Julin[393]
and numerous other workers; they were adversely criticised by
Metschnikoff[394] and von Baer,[395] as well as by H. de
Lacaze-Duthiers and A. Giard.[396] Lacaze-Duthiers and von Baer both
held fast to the old view that Ascidians were directly comparable with
Lamellibranch molluscs; they denied the homology of the ascidian
nervous system with that of Vertebrates, von Baer being at great pains
to show that the ascidian nerve-centre was really ventral in position.
He pointed out also that the "notochord" was confined to the tail of
the ascidian larva. Giard's attitude was by no means so
uncompromising, and the criticisms he passed on the Kowalevsky theory
are both subtle and instructive. He admits that there exists a real
homology between, for instance, the notochord of Vertebrates and that
of Ascidians. "But," he adds, "it is too often forgotten that homology
does not necessarily mean an immediate common origin or close
relationship. There exist, doubtless, homologies of great atavistic
importance--I consider as such, for example, the formation of the
cavity of Rusconi [the archenteron] in Ascidians and lower
Vertebrates. But there are also adaptive and purely analogical
homologies, such as the interdigital palmation of aquatic birds,
amphibians and mammals. These are not purely analogous organs, for
they can be superposed one on another, which is not the case with
simply analogous structures (the bat's wing, for example, cannot be
superposed on the bird's wing); they are homologous formations,
resulting from the adaptation of the same fundamental organs to
identical functions. Such is, in my opinion, the nature of the
homology existing between the tail of the ascidian tadpole and that of
Amphioxus or of young amphibians. The ascidian larva, having no cilia
and being necessarily motile, requires for the insertion of its
muscles or contractile organs ... a central flexible axis, a true
chorda dorsalis analogous to that of Vertebrates" (pp. 278-9). This
point of view is strengthened by the fact that in _Molgula_, studied
by Lacaze-Duthiers, the embryo is practically stationary, and forms no
notochord, nor ever develops sense-organs in the cerebral vesicle.

Giard's general conclusion is that "the true homology with Vertebrates
ceases after the formation of the cavity of Rusconi and the medullary
groove: the homologies established by Kowalevsky for the notochord and
the relations of the digestive tube and nervous systems are not
atavistic, but adaptive, homologies" (p. 282). There is accordingly no
close genetic relationship between Ascidians and Vertebrates.

Giard's criticisms did not avail to check the vogue of the new theory,
which soon became an accepted article of faith in most morphological
circles.[397] The fall of the Ascidians from their larval high estate
provided the text for many a Darwinian sermon.

Some years after the genetic relationship of Ascidians and Vertebrates
had been established, a rival theory of the origin of Vertebrates made
its appearance--a theory which was practically a rehabilitation in a
somewhat altered form of the old Geoffroyan conception that Vertebrates
are Arthropods walking on their backs. This was the so-called Annelid
theory of Dohrn and Semper. Both Dohrn and Semper started out from the
fact that Annelids and Vertebrates are alike segmented animals, and it
was an essential part of their theory that this resemblance was due to
descent from a common segmented ancestor. Both laid great stress on the
fact that the main organs in Vertebrates are arranged in the same way as
in an Annelid lying on its back, the nervous system being uppermost, the
alimentary system coming next, and below this the vascular.

Dohrn's earlier views are contained in the fascinating little book
published in 1875, which bears the title _Der Ursprung der Wirbelthiere
und das Princip des Functionswechsel_ (Leipzig). He followed this up by
a long series of studies on vertebrate anatomy and embryology,[398] in
which he modified his views in certain details. We shall confine our
attention to the first sketch of his theory.

If the Vertebrate is conceived to have evolved from a primitive Annelid
which took to creeping or swimming ventral surface uppermost, a
difficulty at once arises with regard to the relative positions of the
"brain" and the mouth. In Vertebrates the brain, like the rest of the
nervous system, is dorsal to the mouth and the alimentary canal; in an
inverted Annelid, however, the brain is ventral to the mouth and is
connected with the dorsal nerve cord by commissures passing round the
oesophagus. It would seem, therefore, that the primitive Vertebrate must
have acquired either a new brain or a new mouth. Dohrn took the latter
view. He supposed that the original mouth of the primitive ancestor lay
between the _crura cerebelli_ in the _fossa rhomboidea_, and that in
Vertebrates this mouth has been replaced functionally by a new ventrally
placed mouth, formed by the medial coalescence of a pair of
gill-slits.[399] Probably the two mouths at one period co-existed, and the
older one was ousted by the growing functional importance of the newer
mouth.

The gill-slits were considered by Dohrn to be derived from the segmental
organs of Annelids, which were present originally in every segment of
the primitive ancestor. The gills were at first external, like the gills
of many Chætopods at the present day. For their support cartilaginous
gill-arches naturally arose in the body-wall, and the superficial
musculature became attached to these bars. "There existed in all the
segments of the Annelid-ancestors of Vertebrates gills with
cartilaginous skeleton and gill-arches in the body wall. Each gill had
its veins and arteries, each had its branch of the ventral nerve-cord,
and between each successive pair of gills a segmental organ opened to
the exterior" (p. 14, 1875). The paired fins and limbs of the Vertebrate
arose by the functional transformation of two pairs of these gills. The
anterior gills became the definitive internal gills of the Vertebrate,
for they gradually shifted into the mouths of the anterior segmental
organs, which had already acquired an opening into the pharynx and had
been transformed into true gill-slits. The posterior gills degenerated
and disappeared, but their arches remained as ribs. Gill-arches and ribs
were accordingly homologous structures and formed a _parietal_ skeleton.
The vertebrate anus, like the mouth, was probably secondary and formed
from a pair of gill-slits, the post-anal gut of vertebrate embryos
hinting that the original anus was terminal as in Annelids. The unpaired
fins of fish were originally paired and possibly arose from the
coalescence of rows of parapodia. Dohrn assumed also that the primitive
Annelid ancestor must have possessed a notochord to give support in
swimming.

If Vertebrates arose from primitive Annelid ancestors, how account for
Amphioxus and the Ascidians, which seem to be the most primitive living
Vertebrates and yet show no particular annelidan affinities? Dohrn tries
to answer this awkward question by showing that these forms are not
primitive but degenerate. He points out first that Cyclostomes are
degenerate fish, half specialised and half degraded in adaptation to a
parasitic mode of life. He thinks that if an _Ammocoetes_ were to become
sexually mature and degenerate still further, forms would result which
would resemble Amphioxus, and ultimately, if the process of degeneration
went far enough, larval Ascidians. Amphioxus therefore might well be
considered an extremely simplified and degenerate Cyclostome, and the
ascidian larva the last term of this degeneration-series. Both Amphioxus
and the Ascidians would accordingly be descended from fish, instead of
fish being evolved from them.

Dohrn conceived that the transformation of the Annelid into the
Vertebrate took place mainly by reason of an important transforming
principle, which he calls the principle of function-change. Each organ,
Dohrn thinks, has besides its principal function a number of subsidiary
functions which only await an opportunity to become active. "The
transformation of an organ takes place by reason of the succession of
the functions which one and the same organ possesses. Each function is a
resultant of several components, of which one is the principal or
primary function, while the others are the subsidiary or secondary
functions. The weakening of the principal function and the strengthening
of a subsidiary function alters the total function; the subsidiary
function gradually becomes the chief function, the total function
becomes quite different, and the consequence of the whole process is the
transformation of the organ" (p. 60). Examples of function-change are
not difficult to find. Thus the stomach in most Vertebrates performs
both a chemical and a mechanical function, but in some forms a part of
it specialises in the mechanical side of the work and becomes a gizzard,
while the remaining part confines its energies to the secretion of the
gastric juice. So, too, it is through function-change that certain of
the ambulatory appendages of Arthropods have become transformed into
jaws--their function as graspers of food has gradually prevailed over
their main function as walking limbs. In the evolution of Vertebrates
from Annelids the principle came into action in many connections--in the
formation of a new mouth from gill-slits, in the transformation of gills
into fins and limbs, of segmental organs into gill-slits, and so on.
Dohrn tells us that the principle of function-change was suggested to
him by Mivart's _Genesis of Species_ (1870), and he points out how it
enables a partial reply to be made to the dangerous objection raised
against the theory of natural selection that the first beginnings of new
organs are necessarily useless in the struggle for existence.

We may note in passing that a somewhat similar idea was later applied by
Kleinenberg to the explanation of some of the ancestral features of
development. He pointed out in his classical memoir on the embryology of
the Annelid _Lopadorhynchus_[400] that many embryonic organs seem to be
formed for the sole purpose of providing the necessary stimulus for the
development of the definitive organs. Thus the notochord is the
necessary forerunner of the vertebral column, cartilage the precursor of
bone. "From this point of view," he writes, "many rudimentary organs
appear in a different light. Their obstinate reappearance throughout
long phylogenetic series would be hard to understand were they really no
more than reminiscences of bygone and forgotten stages. Their
significance in the processes of individual development may in truth be
far greater than is generally recognised. When in the course of the
phylogeny they have played their part as intermediary organs
(_Vermittelungsorgane_) they assume the same function in the ontogeny.
Through the stimulus or by the aid of these organs, now become
rudimentary, the permanent parts of the embryo appear and are guided in
their development; when these have attained a certain degree of
independence, the intermediary organ, having played its part, may be
placed upon the retired list."[401]

Dohrn was well aware of the functional, or as he calls it, the
physiological, orientation of his principle, and he rightly regarded
this as one of its chief merits. He held that morphology became too
abstract and one-sided if it disregarded physiology completely; he saw
clearly that the evolution of function was quite as important a problem
as the evolution of form, and that neither could be solved in isolation
from the other. "The concept of function-change is purely
physiological;" he writes, "it contains the elements out of which
perhaps a history of the evolution of function may gradually arise, and
for this very reason it will be of great utility in morphology, for the
evolutionary history of structure is only the concrete projection of the
content and course of the evolution of function, and cannot be
comprehended apart from it" (p. 70).[402]

It is very instructive in this connection to note that Dohrn was not,
like so many of his contemporaries, a dogmatic materialist, but upheld
the commonsense view that vital phenomena must, in the first instance at
least, be accepted as they are. "It is for the time being irrelevant,"
he writes, "to squabble over the question as to whether life is a result
of physico-chemical processes or an original property (_Urqualität_) of
all being.... Let us take it as given" (p. 75).

Semper's speculations on the genetic affinity of Articulates and
Vertebrates are contained in two papers[403] which appeared about the same
time as Dohrn's. He openly acknowledges that his work is essentially a
continuation of Geoffroy's transcendental speculations, and gives in his
second paper a good historical account of the views of his great
predecessor. It is a significant fact that evolutionary morphologists
very generally held that Geoffroy was right in maintaining against
Cuvier[404] the unity of plan of the whole animal kingdom, for they saw in
this a strong argument for the monophyletic descent of all animals from
one common ancestral form.

In his first paper Semper does little more than break ground; he insists
on the fact that both Annelids and Vertebrates are segmented animals,
and he points out how close is the analogy between the nephridia or
"segmental organs" of the former and the excretory (mesonephric) tubules
of the latter, upon which he published in the same volume an extensive
memoir. At this time he considered _Balanoglossus_--by reason of its
gill-slits (its notochord he did not know)--to be the nearest living
representative of the ancestral form of Vertebrates and Annelida.

His second paper is a more exhaustive piece of work and deals with every
aspect of the problem, both from an anatomical and from an embryological
standpoint. It is consciously and admittedly an attempt to apply
Geoffroy's principle of the unity of plan and composition to the three
great metameric groups, the Annelida, Arthropoda, and Vertebrata. Semper
follows Geoffroy's lead very closely in maintaining that it is not the
position of the organs relative to the ground that must be taken into
account in establishing their homologies, but solely their spatial
relations one to another. He holds that dorsum and venter are terms of
purely physiological import, and he proposes to substitute for them the
terms neural and cardial (better, hæmal) surfaces, either of which may
be either dorsal or ventral in position.

Having established this primary principle, Semper has little difficulty
in showing that the main organs of the body lie to one another in the
same relative positions in Annelida, Arthropoda, and Vertebrata; and
this, together with the metameric segmentation common to them all,
constitutes his first great argument in favour of their genetic
relationship. But he has still to show that Annelids possess at least
the rudiments of certain organs which seem to be peculiar to
Vertebrates, as the gill-slits, the notochord, and a nervous system
developed from the ectoderm of the "dorsal" surface. He takes particular
cognisance also of the old distinction drawn by von Baer, that
Vertebrates show a "double-symmetrical" mode of development (_evolutio
bigemina_), the dorsal muscle-plates forming a tube above the notochord,
the ventral plates a tube below the notochord, whereas Articulates do
not possess this axis, and form only one tube, namely, that round the
"vegetative" organs (_evolutio gemina_). Semper is at pains to prove
that _evolutio bigemina_ is characteristic also of Annelidan
development.

[Illustration: FIG. 14.--Transverse Section (Inverted) of the Worm
_Nais_. (After Semper.)]

He gets his facts from an elaborate study of the process of budding in
the _Naidæ_, making the somewhat risky assumption that regeneration
takes essentially the same course as embryonic development.

He succeeds in showing--to his own satisfaction at least--that in the
formation of new segments in _Nais_ and _Chætogaster_ a strand of cells
appears between the alimentary canal and the nerve-cord, and that from
this axial strand the hæmal muscle-plates grow out dorsally round the
alimentary canal and the neural muscle-plates ventrally round the
nerve-cord (see Fig. 14).

This strand of cells, he concludes, must clearly be the notochord, and
the type of development is obviously the double-symmetrical met with in
Vertebrates.

The nervous system Semper found to develop in the buds of _Nais_ and
_Chætogaster_ by an ectodermal thickening, just as in some Vertebrates.
The cerebral ganglion was formed by the ends of the nerve-cord growing
up round the oesophagus and fusing with the paired "sense-plates" which
develop from the ectoderm of the head. The cerebral ganglion is
accordingly only secondarily hæmal in position, and there is no need
therefore to seek in Vertebrates for the homologue of the oesophageal
commissures of Annelids, as, for instance, Schneider did.

Since the mouth opens on the neural surface in Annelids and on the hæmal
surface in Vertebrates, Semper considers that they cannot be equivalent
structures, and he finds the homologue of the Vertebrate mouth in a
little pit on the hæmal surface of the head in the leech _Clepsine_ (also
in the true mouth of Turbellaria and the proboscis-opening in
Nemertines). The primitive Annelid mouth, however, does not appear in
the embryogeny of Vertebrates, for the great development of the brain
crowds it out of existence.

The homologues of the gill-slits Semper finds in two little canals in
the head of _Chætogaster_, which open from the pharynx to the exterior.
In Sabellids he describes an elaborate system of gill-canals, with a
supporting cartilaginous framework which forms a real _Kiemenkorb_ or
gill-basket, comparable with that of Amphioxus.

Gill-slits, notochord, relation of nervous system, mesonephric tubules,
are thus common to Annelids and Vertebrates--what further proof could
one desire of the close relationship of these groups? Yet Semper enters
into refinements of comparison, seeing, for instance, in the lateral
portions of the ventral ganglia (Fig. 14, _sp. g._) the homologues of
the spinal ganglia of Vertebrates, and comparing the lateral line of
sense organs in Annelids with the lateral line in Anamnia.

He will not admit that Amphioxus and the Ascidians show a closer
resemblance to Vertebrates than his beloved Annelids. Amphioxus, he
thinks, is not a Vertebrate, and Ascidians, though sharing with Annelids
the possession of a notochord, gill-slits, and a "dorsal" nervous
system, yet are further removed from Vertebrates than the latter by
reason of their lacking that essential characteristic of Vertebrates,
metameric segmentation.

Not content with establishing the unity of plan of Annelids, Arthropods,
and Vertebrates, Semper tries to link on the Annelids, as the most
primitive group of the three, to the unsegmented worms, and particularly
to the Turbellaria. His speculations on this matter may be summed up
somewhat as follows:--The common ancestor of all segmented animals is a
segmented worm-like form, not quite like any existing type, resembling
the Turbellaria in having two nerve strands on the dorsal side and no
oesophageal ring, potentially able to develop either the Vertebrate or
the Annelid mouth, and so to give origin both to the Articulate and to
the Vertebrate series. The common ancestor alike of unsegmented worms
and of all segmented types is probably the trochosphere larva, which in
the Vertebrates is represented by the simple _Keimblase_ or blastula.

The Annelid theory of Dohrn and Semper was perhaps not so widely
accepted as the rival Ascidian theory, but it counted not a few
adherents and gave a certain stimulus to comparative morphology. F. M.
Balfour, who pointed out about the same time as Semper the analogy
between the nephridia of Annelids and the mesonephric tubules of
Vertebrates,[405] while not accepting the actual theories of Dohrn and
Semper, took up a distinctly favourable attitude to the general idea
that Annelids and Vertebrates were descended from a common segmented
ancestor. Discussing this question in his classical work on the
development of Elasmobranch fishes,[406] Balfour came to the conclusion
"that we must look for the ancestors of the Chordata, not in allies of
the present Chætopoda, but in a stock of segmented forms descended from
the same unsegmented types as the Chætopoda, but in which two lateral
nerve-cords, like those of Nemertines, coalesced dorsally instead of
ventrally to form a median nervous cord. This group of forms, if my
suggestion as to their existence is well founded, appears now to have
perished."[407]

He held that while there was much to be said for the interchange of
dorsal and ventral surfaces postulated by Dohrn and Semper, the
difficulties involved in the supposition were too great; he preferred,
therefore, to assume that the present Vertebrate mouth was primitive,
and not a secondary formation.

His views as to the phylogeny of the Chordata and the genetic relation
of the various classes to one another are exhibited in the following
schema,[408] names of hypothetical groups being printed in capitals, names
of degenerate groups in italics:--


 Mammalia.                   Sauropsida.
    |                            |
    |____________________________|
                    |
               Proto-Amniota.         Amphibia.
                    |                     |
                    |_____________________|
                             |
                  Proto-Pentadactyloidei.
                             |
      Teleostei.             |
          |                  |
       Ganoidei.             |____________Dipnoi
          |                  |
          |__________________|
                             |
                       Proto-Ganoidei.
                             |
                             |____________Holocephali.
                             |
                             |____________Elasmobranchii.
                             |
                    Proto-Gnathostomata.
                             |
         ____________________|
         |                   |
   _Cyclostomata_.           |
                             |
                             |
                     Proto-Vertebrata.
                             |
                             |
                             |
                             |
         ____________________|______________________
         |                                         |
  _Cephalochorda_.    Protochordata.          _Urochorda_.


The hypothetical ancestral forms (Protochordata) possessed a notochord,
a ventral suctorial mouth and numerous gill-slits, and were presumably
descended from the common ancestor of Annelids and Vertebrates.
Amphioxus and the Ascidians found their place in this schema as
degenerate offshoots of the ancestral Protochordates, while the
Cyclostomes were in the same way the degenerate modern representatives
of the ancestral Protovertebrates.

Balfour's suggestion, that the nervous system in Annelids and
Vertebrates might have arisen by the dorsal or ventral coalescence of
the lateral nerve cords found in their common ancestor, bore fruit in
the speculations of Hubrecht,[409] on the relation of Nemertines to
Vertebrates.

The Annelid theory was firmly supported by Eisig, who in his elaborate
monograph on the _Capitellidæ_[410] maintained against Fürbringer the
genetic identity of the Annelidan nephridia with the kidney tubules of
Vertebrates. The independent discovery by E. Meyer[411] and J. T.
Cunningham,[412] of an internal segmental duct in _Lanice_, into which
several nephridia opened, seemed to strengthen this view.

Following Ehlers,[413] Eisig found the homologue of the notochord in the
accessory intestine of the _Capitellidæ_ and _Eunicidæ_, which he
supposed might easily be transformed, according to the principle of
function-change, from a respiratory to a supporting organ. He finally
disposed of the alternative notion that the notochord was represented in
Annelids by the "giant-fibres" or neurochordal strands which lie close
above the nerve-cord, a view held by Kowalevsky,[414] and for a time by
Semper. These strands were shown by Eisig, and by Spengel, to be the
neurilemmar sheaths of thick nerve fibres which had in many cases
degenerated. The view that the content of the neurochordal tubes was
nervous in nature was first promulgated by Leydig in 1864.

Much difference of opinion reigned as to the true homologies of the
brain and mouth of Annelids and Vertebrates. Beard[415] and others got
over the difficulty of the hæmal position of the cerebral ganglion in
Annelids by supposing that it degenerated and disappeared altogether in
the Annelidan ancestor of Vertebrates, and that accordingly it had no
homologue in the Vertebrate nervous system. Beard put forward also the
ingenious theory that the hypophysis represents the old Annelidan mouth.

Van Beneden and Julin[416] assumed that in the ancestors of Vertebrates
the oesophagus shifted forward between the still unconnected lobes of
the brain to open on the hæmal surface.

The fundamental assumption of the Annelid theory, that dorsal and
ventral surfaces are morphologically interchangeable, seemed rather bold
to many zoologists, and Gegenbaur[417] voiced a common opinion when he
rejected as unscientific the comparison of the ventral nerve cord of
Articulates with the dorsal nervous system of Vertebrates.

The _Balanoglossus_ theory of Vertebrate descent also belongs, at least
in its first form, to the earlier group of evolutionary speculations.
The gill-slits of _Balanoglossus_ were discovered by Kowalevsky as early
as 1866.[418] _Tornaria_ was discovered by J. Müller in 1850, but by him
considered an Asterid larva; its true nature as the larva of
_Balanoglossus_ was made out by Metschnikoff in 1870, who also remarked
upon its extraordinary likeness to the larvæ of Echinoderms.[419] That it
had some relationship with Vertebrates was recognised by Semper,
Gegenbaur and others, but the full working-out of its Vertebrate
affinities is due to Bateson.[420]

Bateson broke completely with the Dohrn-Semper view that the metamerism
of Articulates and Vertebrates must be put down to inheritance from a
common ancestor. He held that metamerism was merely a special
manifestation of the general property of repetition, common to all
living things (_cf._ Owen's "vegetative force"), and that accordingly
"however far back a segmented ancestor of a segmented descendant may
possibly be found, yet ultimately the form has still to be sought for in
which these repetitions had their origin" (p. 549). The meaning of the
phenomenon was obscure, but he was convinced that the explanation was
not to be found in ancestry. "This much alone is clear," he wrote, "that
the meaning of cases of complex repetition will not be found in the
search for an ancestral form, which, itself presenting this same
character, may be twisted into a representation of its supposed
descendant. Such forms there may be, but in finding them the real
problem is not even resolved a single stage; for from whence was their
repetition derived? The answer to this question can only come in a
fuller understanding of the laws of growth and of variation, which are
as yet merely terms" (pp. 548-9). It was in following up this line of
thought that Bateson produced his monumental _Materials for the Study of
Variation_ (1894).

He found a strong positive argument for his theory that Vertebrates are
descended from unsegmented forms in the fact that the notochord arises
as an unsegmented structure. With the notochord he homologised the
supporting rod in the proboscis of _Balanoglossus_, which like the
notochord arises from the dorsal wall of the archenteron, and has a
vacuolated structure. The gill-slits of _Balanoglossus_, with their
close resemblance in detail to those of Amphioxus, Bateson also used as
an argument in favour of the phylogenetic relationship of the
Enteropneusta and Vertebrata, together with the formation from the
ectoderm of a dorsal nerve tube.

Bateson's views attracted considerable attention, and were thought by
many to lighten appreciably the obscurity in which the origin of
Vertebrates was wrapped. Thus Lankester wrote in his article on
Vertebrates[421] in the _Encyclopedia Britannica_:--"It seems that in
_Balanoglossus_ we at last find a form which, though no doubt
specialised for its burrowing sand-life, and possibly to some extent
degenerate, yet has not to any large extent fallen from an ancestral
eminence. The ciliated epidermis, the long worm-like form, and the
complete absence of segmentation of the body-muscles lead us to forms
like the Nemertines. The great proboscis of _Balanoglossus_ may well be
compared to the invaginable organ similarly placed in the Nemertines.
The collar is the first commencement of a structure destined to assume
great importance in _Cephalochorda_ and _Craniata_, and perhaps
protective of a single gill-slit in _Balanoglossus_ before the number of
those apertures had been extended. Borrowing, as we may, the nephridia
from the Nemertines, and the lateral in addition to the dorsal nerve, we
find that _Balanoglossus_ gives the most hopeful hypothetical solution
of the pedigree of Vertebrates."

Much doubt was cast upon the Chordate affinities of the Enteropneusta by
Spengel in his monograph of the group,[422] but when the development of
the coelom came to be more thoroughly worked out in _Balanoglossus_ and
Amphioxus, the striking resemblance in this respect between the two
forms gave additional support to the Batesonian view.[423]

    [386] The stages in the development of microscopical
    technique are well summarised by R. Burckhardt,
    _Geschichte der Zoologie_, p. 121, Leipzig 1907.

    [387] "Entwickelungsgeschichte des Amphioxus lanceolatus,"
    _Mém. Acad. Sci. St Pétersbourg_ (Petrograd) (vii.),
    xi., No. 4, 1867, 17 pp., 3 pls.

    [388] "Weitere Studien ü. die Entwickelungsgeschichte des
    Amphioxus lanceolatus," _Arch. für mikr. Anat._, xiii.,
    pp. 181-204, 1877.

    [389] Particularly by Hatschek (1881) and Boveri (1892).

    [390] "Entwickelungsgeschichte der einfachen Ascidien,"
    _Mém. Acad. Sci. St Pétersbourg_ (Petrograd), (vii.),
    x., No. 15, 1866, 19 pp., 3 pls. "Weitere Studien ü. die
    Entwicklung der einfachen Ascidien," _Arch. f. mikr.
    Anat._, vii., pp. 101-130, 1871.

    [391] _Descent of Man_, i., p. 205, 1871.

    [392] _Arch. f. mikr. Anat._, vi., 1870, and viii., 1872.

    [393] _Archives de Biologie_, 1884, 1885, and 1887.

    [394] _Bull. Acad. Sci. St Pétersbourg_ (Petrograd) xiii.,
    1869, and _Zeits. f. wiss. Zool._, xxii., 1872.

    [395] _Mém. Acad. Sci. St Pétersbourg_(Petrograd)(7),
    xix., 1873.

    [396] Giard, _Arch. zool. expér. gén._, i., 1872, and
    Lacaze-Duthiers, _ibid._, iii., 1874.

    [397] For the later history of the Amphioxus-Ascidian
    theory the reader may be referred to A. Willey's
    well-known work, _Amphioxus and the Ancestry of the
    Vertebrates_, New York and London, 1894, and to Delage
    et Hérouard, _Traité de Zoologie concrète_, Tome viii.,
    Paris, 1898.

    [398] "Studien zur Urgeschichte des Wirbelthierkörpers,"
    _Mittheil. Zool. Stat. Neapel_, 1882-1907.

    [399] Leydig (_Vom Baue des thierischen Körpers_,
    Tübingen, 1864), who, in a measure, forestalled Dohrn
    and Semper by comparing Vertebrates with reversed
    Arthropods, specially insects, supposed the old mouth to
    pass between the _crura cerebri_.

    [400] _Zeits. f. wiss. Zool._, xliv., 1886.

    [401] Quoted by E. B. Wilson, _Wood's Holl Biological
    Lectures for 1894_, p. 121.

    [402] _Cf._ Metschnikoff, _Quart. Journ. Microsc. Sci._,
    xxiv., pp. 89-111, 1884.

    [403] "Die Stammesverwandschaft der Wirbelthiere und
    Wirbellosen," _Arb. zool.-zoot. Instit. Würzburg_, ii.,
    pp. 25-76, 1875; "Die Verwandschaftsbeziehungen der
    gegliederten Thiere," _Ibid._, iii., pp. 115-404,
    1876-7.

    [404] Abuse of Cuvier also dates from the early days of
    evolution, see Rádl, ii., pp. 12-17.

    [405] "On the origin and history of the urino-genital
    organs of Vertebrates," _Journ. Anat. Phys._, x., 1876.
    The conclusions of Balfour and Semper were adversely
    criticised by M. Fürbringer (_Morph. Jahrb._, iv.,
    1878), and were negatived by later research.

    [406] _A Monograph on the Development of Elasmobranch
    Fishes_, London, 1878.

    [407] _A Treatise on Comparative Embryology_, vol. ii., p.
    311, London, 1881.

    [408] _Loc. cit._, vol. ii., p. 327.

    [409] "On the Ancestral Form of the Chordata," _Q.J.M.S._,
    xxiii., 1883. "The Relation of the Nemertea to the
    Vertebrata," _ibid._, xxvii., 1887. Hubrecht gives the
    credit for the first indication of the relationship of
    Nemertines and Vertebrates to Harting (_Leerboek van de
    Grondbeginselen der Dierkunde_, 1874).

    [410] "Monographie der Capitelliden des Golfes von
    Neapel," _Fauna u. Flora des Golfes von Neapel_, Monog.
    xvi., Berlin, 1887.

    [411] _Mitt. Zool. Stat. Neapel_, vii., 1887.

    [412] _Nature_, xxxvi., p. 162, 1887.

    [413] "Nebendarm und Chorda dorsalis," _Nachr. Ges. Wiss.
    Göttingen_, p. 390, 1885.

    [414] "Embryologische Studien an Würmern u. Arthropoden,"
    _Mém. Acad. Sci. St Pétersbourg_ (Petrograd), (7), xvi.,
    1870. And in _Arch. f. mikr. Anat._, vii., p. 122, 1871.

    [415] "The Old Mouth and the New," _Anat. Anz._, iii.,
    1888. _Nature_, xxxix., 1889.

    [416] "Recherches sur la Morphologie des Tuniciers,"
    _Arch. de Biol._, vi., 1887.

    [417] "Die Stellung u. Bedeutung der Morphologie," _Morph.
    Jahrb._, i., pp. 1-19, 1876.

    [418] "Anatomie des Balanoglossus," _Mém. Acad. Sci. St
    Pétersbourg_ (Petrograd), (7), x., 1866.

    [419] _Zeit. f. wiss. Zool._, xx., 1870. For a recent view
    of the relation of the Enteropneusta to the Echinoderma,
    see J. F. Gemmill, _Phil. Trans._ B., ccv., pp. 213-94,
    1914.

    [420] In a series of papers published in 1884-6, the
    speculative results being discussed in his memoir on
    "The Ancestry of the Chordata," _Q.J.M.S._ (n.s.), xxvi.,
    pp. 535-71, 1886.

    [421] Reprinted in _Zoological Articles_, London, 1891.

    [422] "Die Enteropneusten des Golfes von Neapel," _Fauna
    und Flora des Golfes von Neapel_, Monog. xviii., Berlin,
    1893.

    [423] See Macbride, "A Review of Prof. Spengel's Monograph
    on Balanoglossus," _Q.J.M.S._, xxxvi., 1894, and "The
    Early Development of Amphioxus," _Q.J.M.S._, xl., 1898.



CHAPTER XVI

THE GERM-LAYERS AND EVOLUTION


In his papers of 1866 and 1867 Kowalevsky had remarked upon the
widespread occurrence of a certain type or fundamental plan of early
embryonic development, characterised by the formation, through
invagination, of a two-layered sac, whose cavity became the alimentary
canal. This developmental archetype was manifested in, for instance,
_Sagitta_,[424] _Rana_,[425] _Lymnæa_,[426] _Astacus_,[427]
_Phoronis_,[428] _Asterias_,[429] _Ascidia_,[428] the _Ctenophora_,[428]
and _Amphioxus_.[428] He noticed also that the invagination-opening
often became the definitive anus. Further instances of this mode of
development were later observed by Metschnikoff[430] and by
Kowalevsky[431] himself, but it was left to Haeckel to generalise these
observations and build up from them his famous Gastræa theory. This was
first enunciated in his monograph of the calcareous sponges,[432] and
worked out in detail in a series of papers published in 1874-76.[433]

Haeckel maintained that the "gastrula" stage occurred in the development
of all Metazoa, and that it was typically formed, by invagination, from
a hollow sphere of cells or "blastula." This typical formation might be
masked by cenogenetic modifications caused chiefly by the presence of
yolk. The gastrula stage was the palingenetic repetition of the
ancestral form of all Metazoa, the Gastræa.

From the Gastræa theory there followed at once two consequences, (1)
that ectoderm and endoderm, invagination-cavity (_Urdarm_) and
gastrula-mouth (_Urmund_ or _Protostoma_), were, with all their
derivatives, homologous, because homogenous, throughout the Metazoa, and
(2) that the descent of the Metazoa had been monophyletic, since all
were derived from the ancestral Gastræa. Huxley's suggestion (_supra_,
p. 208) that the outer and inner layers in Coelentera were homologous
with the ectoderm and endoderm of the germ was thus fully confirmed and
greatly extended.

The great importance of the Gastræa theory lay in the fact that it
linked up, by means of the biogenetic law, the germ-layer theory with
the doctrine of evolution. It supplied an evolutionary interpretation of
the earliest and most important of embryogenetic events, the process of
layer-formation. Upon the Gastræa theory or its implications were
founded most of the phylogenetic speculations which subsequently
appeared.

Upon the Gastræa theory Haeckel based a system of phylogenetic
classification which was intended to replace Cuvier's and von Baer's
doctrine of Types. This took the form of a monophyletic ancestral tree.
Its main outlines are given on p. 290 in graphic form, combined and
modified from the table on p. 53 of the 1874 paper and the genealogical
tree given in the _Kalkschwämme_.[434]

_Monophyletic Genealogical Tree of the Animal Kingdom, based upon the
Gastræa Theory and the Homology of the Germ Layers_.

_______________________________________________________________________
|      |                                                       |    . |
|      |                                                       |    m |
|      |                               _Vertebrata_.           |    o |
|    . |                                    |                  |    l |
|    m |               _Arthropoda_.        |                  |    e |
|    r |                     |              |                  |    o |
|    e |                     |              |                  |    c |
|    d |_Echinoderma_.       |              |  _Mollusca_.     |      |
|    d |      |              |              |       |          |    a |
|    n |      |              |   Sagitta.   \______ | ______/  |  |   |
|    e |      |              |      |              \|/         | .  d |
|      |      |              |      |               |          | a  n |
|    y |      |              |      | Nematoda.   Himatega.    | i  a |
|    b |      |              |      |     |            |       | r    |
|      |      |              |      |     |            |       | a  d |
|      |      |              |      |     |            |       | t  o |
|      |      \______________|______|_  __|____________|_____/ | a  o |
|      |                              \/                       | m  l |
|      |                                                       | æ  b |
|      |                            _Coelomati_                | H    |
|      |                    (worms with body-cavity}.          |  | h |
|      |                             \      /                  |    t |
|      |                              \    /                   |    i |
|      |                               \  /                    |    W |
|      |________________________________\/_____________________|______|
| .    |                                |                      |      |
| )  d |                                |                      |    . |
| s  e |    _Zoophyta_                  |      Plathelminthes. |    m |
| l  n |   (Coe;enterata).              |               |      |    o |
| a  i |                                |               |      |    l |
| m  l |                Acalephæ.       \______________ |_____/|    e |
| i    |                    |                          \/      |    o |
| n  , |  Spongiæ.          |                  _Acoelomi_      |    c |
| a  t |     |              |                (Worms without    |      |
|    u | Archispongia.   Archydra.            body cavity).    |    o |
| t  g |     |              |                         |        |    n |
| u    |     |              |                         |        |      |
| G  e |     \______  ______/                         |        |  | d |
| (  u |            \/                                |        | a  n |
|    r |         Protascus.                      Prothelmis.   | i  a |
|    t |             |                                |        | r    |
|      |             |                                |        | a  d |
|    A |       Gastræa radialis            Gastræs bilateralis | æ  o |
|  |   |          (sedens).                       (repens).    | n  o |
| a  . |             |                                |        | A  l |
| o  s |             |                                |        |  | b |
| z  r |             \_______________  _______________/        |      |
| a  e |                             \/                        |    o |
| t  y |                         _Gastræa_                     |    N |
| e  a |                   (Ontogeny : Gastrula).              |      |
| M  l |                             |                         |      |
|  |   |                             |                         |      |
|    m |                             |                         |      |
|    r |                             |                         |      |
|    e |                             |                         |      |
|    g |                             |                         |      |
|      |                             |                         |      |
|    y |                             |                         |      |
|    r |                             |                         |      |
|    a |                             |                         |      |
|    m |                             |                         |      |
|    i |                             |                         |      |
|    r |                             |                         |      |
|    P |                             |                         |      |
|      |                             |                         |      |
|    o |                             |                         |      |
|    w |                             |                         |      |
|    T |                             |                         |      |
|______|                    _________|_________________________|______|
|      |                    |                                         |
|      |          __________|                                         |
|      |          |                                                   |
|    . |          |                                                   |
|    t |      Planaeada               Acinetæ.             Ciliata.   |
|    u | (Ontogeny : Planula).            |                    |      |
|    g |          |                       \_________  _________/      |
| >    |          |                                 \/                |
| i  o |          |                             Infusoria.            |
| /  n |          |                                  |                |
| <    |          |                                  |                |
| a  , |      Synamoebæ            Gregarinæ         |                |
| o  s | (Ontogeny : Morula).          |             |                |
| z  r |          |                    |             |                |
| o  e |          |                    \_____  ______/                |
| t  y |          |                          \/                       |
| o  a |          |                        Amoebina.                  |
| r  l |          |                           |                       |
| P    |          \____________  _____________/                       |
| >  m |                       \/                                     |
| i  r |                    _Amoebæ_                    ?   ?   ?     |
| <  e |              (Ontogeny : Ovulum).              |   |   |     |
|    g |                        |                       |   |   |     |
|      |                        |                       |   |   |     |
|    o |                    _Monera_                     Monera.      |
|    N |              (Ontogeny : Monerula).                          |
|      |                                                              |
|______|______________________________________________________________|


The scheme is in many respects an interesting and important one. The
great contrast between the Protozoa, or animals with neither gut nor
germ-layers, and the Metazoa, which possess both structures, is for the
first time clearly brought out. The derivation of all the Metazoa from a
single ancestral form, the Gastræa, leads to the conclusion that the
types are not distinct from one another as Cuvier and von Baer supposed,
but agree in the one essential point, in the possession of an
_archenteron_ (Lankester, 1875), and an ectoderm and endoderm which are
homologous throughout all the Metazoan phyla. Finally, in the separation
of the sponges, Coelenterata and Acoelomi as animals lacking a body
cavity or coelom[435] from the four higher phyla, which are essentially
Coelomati, there is contained the germ of a conception which later
became of importance.

Somewhat similar views as to the importance of the germ-layer theory for
the phylogenetic classification of animals were published by Sir E. Ray
Lankester in 1873.[436] He distinguished three grades of animals--the
Homoblastica, Diploblastica, and Triploblastica. The first included the
Protozoa, the second the Coelenterata, the third the other five phyla,
distinguished by the possession of a third layer, the mesoderm, and a
"blood-lymph" cavity enclosed therein. He used the germ-layer theory to
prove the essential unity of type of all the Triploblastica.

The Gastræa theory gave point and substance to the biogenetic law, and
enabled Haeckel to state much more concretely the parallelism existing
between ontogeny and phylogeny. He was able to assert that five
primordial stages, each representing a primitive ancestral form,
recurred with regularity in the very earliest development of all
Metazoa.[437] These were the monerula, cytula, morula, blastula, and
gastrula (see Fig. 15). The monerula was the fertilised ovum after the
disappearance of the germinal vesicle;[438] it was the equivalent of
the primordial anucleate Monera which are the ancestors of all
animals. The ovum after the nucleus had been re-formed became the
cytula, which was the ontogenetic counterpart of the amoeba. The
morula, a compact mulberry-like congeries of segmentation-cells,
corresponded to the synamoeba, or earliest association of
undifferentiated amoeboid cells to form the first multicellular
organism. The blastula, or hollow sphere of segmentation cells,
usually ciliated, was reminiscent of the planæa, an ancestral
free-swimming form whose nearest living relation is the spherical
_Magosphæra_. The gastrula, finally, is the two-layered sac formed
from the blastula, typically by invagination of its wall. It repeats
the organisation of the gastræa, which is the common ancestor of all
Metazoa, and finds its nearest living counterpart in the simple
"sponges" _Haliphysema_ and _Gastrophysema_.[439] The ancestral line
of all the higher animals begins with the five hypothetical forms of
the moneron, amoeba, synamoeba, planæa, and gastræa.

[Illustration: FIG. 15.--The Five Primary Stages of Ontogeny. (After
Haeckel.) 1. Monerula. 2. Cytula. 3. Morula. 4. Blastula. 5. Gastrula.]

We may take the following account[440] of the phylogeny of the human
species, from the gastræa stage onwards, as typical of Haeckel's
speculations on the evolution of the higher forms. The progenitors of
man are, after the Gastræada:--


1. Turbellaria.
*2. Scolecida. (Worms with a coelom, probably represented
        at the present day by _Balanoglossus_.)
*3. Himatega. (Evolved from Scolecida by formation of
        dorsal nerve-tube and chorda, and resembling tailed
        larvæ of Ascidians.)
4. Acrania. (With metameric segmentation. Including
        Amphioxus.)
5. Monorrhina. (Cyclostomes.)
6. Selachia.
7. Dipneusta.
8. Sozobranchia. (Amphibia with permanent gills.)
9. Sozura. (Tailed Amphibia.)
*10. Protamnia.
*11. Promammalia.
12. Marsupialia.
13. Prosimiæ.
14. Menocerca. (Tailed apes.)
15. Anthropoides.
16. Pithecanthropi.
17. Homines.

It will be noticed that except for the hypothetical forms (marked with
an asterisk), which are themselves generalised classificatory groups,
the ancestral forms belong to long-recognised classes. The whole course
of the evolution follows well-worn systematic lines. This is typical of
Haeckel's phylogenetic speculations.

A more abstractly morphological scheme of the evolution of Vertebrates
is given in the _Systematic Phylogeny_ of 1895.[441] The ontogenetic and
ancestral stages are arranged in parallel columns thus:--

Cytula.                            Cytæa (Protozoa).
Morula.                            Moræa (Coenobium of Protozoa).
Blastula.                          Blastæa (_Volvocina_, etc.).
Depula (invaginated blastula).     Depæa.
Gastrula.                          Gastræa (cf. _Olynthus_, _Hydra_, and
                                      primitive Coelentera).
Coelomula (with one pair           Coelomæa (cf. _Sagitta_, _Ascidia_,
  of coelom-pockets).                 and primitive Helminthes).
Chordula (with medullary           Chordæa (_cf._ Ascidian larva and
  tube and chorda).                   larva of Amphioxus).
Spondula (with segmented           Prospondylus (Primitive Vertebrate).
  mesoderm).

This scheme differs from the earlier one chiefly in taking into account
certain advances, notably as regards the cytology of the fertilised ovum
and the true nature of the coelom, which had been made in the interval
of some twenty years.

Haeckel's Gastræa theory, though it exercised a great influence upon the
subsequent trend of phylogenetic speculation, was by no means
universally accepted _telle quelle_. Opinions differed considerably as
to the primitive mode of origin of the two-layered sac which was very
generally admitted to be of constant occurrence in early embryogeny. Ray
Lankester, in his paper of 1873, and more fully in 1877,[442] propounded a
"Planula" theory, according to which the ancestral form of the Metazoa
was a two-layered closed sac formed typically by delamination, less
often by invagination. He denied that the invagination opening (which he
named the blastopore) represented the primitive mouth,[443] holding that
this was typically formed by an "inruptive" process at the anterior end
of the planula, which led to the formation of a "stomodæum." A similar
process at the posterior end gave rise to the anus and the "proctodæum."

The question as to whether delamination or invagination was to be
considered the more primitive process was discussed in detail by
Balfour,[444] without, however, any very definite conclusion being
reached. He held that both processes could be proved in certain cases to
be purely secondary or adaptive, and that accordingly there was nothing
to show that either of them reproduced the original mode of transition
from the Protozoa to the ancestral two-layered Metazoa (p. 342). He by
no means rejected the theory that the Gastræa, "however evolved, was a
primitive form of the Metazoa," but, having regard to the great
variations shown in the relation of the blastopore to mouth and anus
(pp. 340-1), he was inclined to think that if the gastrula had any
ancestral characters at all, these could only be of the most general
kind. Balfour's attitude perhaps best represents the general consensus
of opinion with regard to the Gastræa theory.

From the same origins as the Gastræa theory arose the theory of the
coelom. The term dates back to Haeckel in 1872, and the observations
which first led up to the theory were made by the men who supplied the
foundations of the Gastræa theory--A. Agassiz, Metschnikoff and
Kowalevsky. But it was not Haeckel himself who enunciated the coelom
theory.

It will be remembered that Remak introduced in 1855 the conception of
the mesoderm as an independent layer derived from the endoderm. The
pleuro-peritoneal or body-cavity was formed as a split in the "ventral
plates" of the mesoderm. Haeckel's "coelom" corresponded to the
"pleuro-peritoneal cavity" of Remak, but his view of the origin of the
mesoderm brought him much closer to von Baer's conception of the origin
of _two_ secondary layers from ectoderm and endoderm respectively than
to Remak's conception of the mesoderm as a single independent layer.

Much uncertainty reigned at the time as to the exact manner of origin of
the mesoderm;[445] some held that it developed from the ectoderm, others
that it originated in the endoderm, while still others, and among them
Haeckel, considered that part of it came from the ectoderm and part from
the endoderm (pp. 23-4, 1874).

The solution of the problem came from those observations on the
development of the lower forms to which we have just alluded.

The early history of these discoveries and of the theory which grew out
of them has been well summarised by Lankester,[446] and may conveniently
be given in his own words:--

"As far back as 1864 Alexander Agassiz ("Embryology of the Star-fish,"
in _Contributions to the Natural History of the United States_, vol. v.,
1864) showed in his account of the development of Echinoderma that the
great body-cavity of those animals developed as a pouch-like outgrowth
of the archenteron of the embryo, whilst a second outgrowth gave rise to
their ambulacral system; and in 1869 Metschnikoff (_Mém. de l'Acad.
impériale des Sciences de St Pétersbourg_, series vii., vol. xiv.,
1869), confirmed the observations of Agassiz, and showed that in
Tornaria (the larva of Balanoglossus) a similar formation of
body-cavities by pouch-like outgrowths of the archenteron took place.
Metschnikoff has further the credit of having, in 1874 (_Zeitsch. wiss.
Zoologie_, vol. xxiv., p. 15, 1874), revived Leuckart's theory of the
relationship of the coelenteric apparatus of the Enterocoela to the
digestive canal and body-cavities of the higher animals. Leuckart had in
1848 maintained that the alimentary canal and the body-cavity of higher
animals were united in one system of cavities in the Enterocoela
(_Verwandschaftsverhältnisse der wirbellosen Thiere_, Brunswick, 1848).
Metschnikoff insisted upon such a correspondence when comparing the
Echinoderm larva, with its still continuous enteron and coelom, to a
Ctenophor, with its permanently continuous system of cavities and
canals. Kowalevsky, in 1871, showed that the body-cavity of Sagitta was
formed by a division of the archenteron into three parallel cavities,
and in 1874 demonstrated the same fact for the Brachiopoda. In 1875
(_Quart. Journ. Micr. Sci._, vol. xv., p. 52) Huxley proposed to
distinguish three kinds of body-cavity: the schizocoel, formed by the
splitting of the mesoblast, as in the chick's blastoderm; the
enterocoel, formed by pouching of the archenteron, as in Echinoderms,
Sagitta and Brachiopoda; and the epicoel.... Immediately after this I
put forward the theory of the uniformity of origin of the coelom as an
enterocoel (_Quart. Journ. Micr. Sci._, April, 1875).... My theory of
the coelom as an enterocoel was accepted by Balfour and was greatly
strengthened by his observations on the derivation of both notochord and
mesoblastic somites from archenteron in the Elasmobranchs, and by the
publication in 1877 by Kowalevsky of his second paper on the development
of Amphioxus--in which the actual condition which I had supposed to
exist in the Vertebrata was shown to occur, namely, the formation of the
mesoblast as paired pouches in which a narrow lumen exists, but is
practically obliterated on the nipping-off of the pouch from the
archenteron, after which process it opens out again as coelom" (pp.
16-18).

The enterocoelic theory was taken up by O. and R. Hertwig as an
essential part of their _Coelomtheorie_.[447] In a lengthy series of
monographs these workers made a comparative study of the mode of
formation of the middle layer, and arrived at a coherent theory of its
origin. They distinguished in the middle layer two quite distinct
elements, the mesoblast proper, formed by the evagination of the walls
of the archenteron, and the mesenchyme, formed by free cells budded off
from the germ-layers. The following passage gives a good idea of their
views and of the phylogenetic implications involved:--"Ectoblast and
entoblast are the two primary germ-layers which arise from the
invagination of the blastula; they are always the first to be laid down,
and they can be directly referred back to a simple ancestral form, the
Gastræa; they form the limits of the organism towards the exterior and
towards the archenteron. The parietal and visceral mesoblast, or the two
middle layers, are always of later origin, and arise through evagination
or plaiting of the entoblast, the remainder of which can now be
distinguished as secondary entoblast from the primary. They form the
walls of a new cavity, the enterocoel, which is to be regarded as a
nipped-off diverticulum of the archenteron. Just as the two-layered
animals can be derived from the Gastræa, so can the four-layered animals
be derived from a Coelom form. Embryonic cells, which become singly
detached from their epitheliar connections we consider to be something
quite different from the germ-layers, and accordingly we call them by
the special name of mesenchyme germs or primary cells of the mesenchyme.
They may develop both in two-layered and in four-layered animals. Their
function is to form between the epithelial limiting layers a secreted
tissue (_Secretgewebe_) or connective tissue with scattered cells, which
cells can undergo, like the epithelial elements, the most varied
modifications.... This secreted tissue in its simple or in its
differentiated state, with all its derivatives, we call the mesenchyme"
(p. 122).

The important point for us is that, just as all Metazoa were considered
by Haeckel to be descended from the Gastræa, so all Coelomati were held
by the Hertwigs to be derived from an original coelomate _Urform_. In
both cases an embryological archetype becomes a hypothetical ancestral
form.

The Coelom theory was considerably modified, extended and developed by
later workers, particularly as regards the relations to the coelom of
the genital organs and ducts and the nephridia, but no special
methodological interest attaches to these further developments.[448] We
shall here focus attention upon one interesting line of speculation
followed out in this country particularly by Sedgwick--the theory of the
Actinozoan ancestry of segmented animals. Its relation to the Coelom
theory lies in the fact that Sedgwick regarded the segmentation of the
body as moulded upon the segmentation of the mesoblast, which in its
turn, as Kowalevsky and Hatschek had shown, was a consequence of its
mode of origin as a series of pouches of the archenteron. In other
respects Sedgwick's speculations link on more closely to the Gastræa
theory, for one of his main contentions is that the blastopore or
_Urmund_ is homologous throughout at least the three metameric phyla. In
following up Balfour's observations on the development of
_Peripatus_,[449] Sedgwick was struck with the close resemblance existing
between the elongated slit-like blastopore of this form (giving rise to
both mouth and anus), with its border of nervous tissue, and the
slit-like mouth of the Actinozoan (functioning both as mouth and anus),
round which, as the Hertwigs had shown, there lies a special
concentration of nerve cells and nerve fibres. He found another point of
resemblance in the gastric pouches of the Actinozoa, which he
homologised directly with the enterocoelic pouches of the Coelomati. He
was led to enunciate the following theses:--[450] (1) that the mouth and
anus of Vermes, Mollusca, Arthopoda, and probably Vertebrata, is derived
from the elongated mouth of an ancestor resembling the Actinozoa; (2)
that somites are derived from a series of archenteric pouches, like
those of Actinozoa and Medusæ; (3) that excretory organs (nephridia,
segmental organs) are derived from parts of these pouches which in the
ancestral form, as in many polyps, were connected by a circular or
longitudinal canal, and opened to the exterior by pores. This
longitudinal canal was lost in Invertebrates, but persisted in
Vertebrates as the pronephric duct, while the pores remained in
Invertebrates and disappeared in Vertebrates; (4) that the tracheæ of
Arthropods, as well as the canal of the central nervous system in
Vertebrates, are to be traced back to certain ectodermal pits in the
diploblastic ancestor comparable to the sub-genital pits of the
Scyphomedusæ. These ectodermal pits were all originally respiratory
organs. "The essence of all these propositions," he writes, "lies in the
fact that the segmented animals are traced back not to a triploblastic
unsegmented ancestor, but to a two-layered Coelenterate-like animal with
a pouched gut, the pouching having arisen as a result of the necessity
for an increase in the extent of the vegetative surfaces in a rapidly
enlarging animal (for circulation and respiration)" (p. 47). "I have
attempted to show," he writes further on, "that the majority of the
Triploblastica ... are built upon a common plan, and that that plan is
revealed by a careful examination of the anatomy of Coelenterata; that
all the most important organ-systems of these Triploblastica are found
in a rudimentary condition in the Coelenterata; and that all the
Triploblastica referred to must be traced back to a diploblastic
ancestor common to them and the Coelenterata" (p. 68). The main
assumption was that the neural or blastoporal surface must be homologous
throughout the Metazoa, though it was dorsal in the Chordata, ventral in
the Annelida and Arthropoda. He derived the central nervous system of
the Chordata from the circumoral ring of the common ancestor by means of
the hypothesis that both the pre-blastoporal and the post-blastoporal
parts of it disappeared.[451]

The characteristic relation of the central nervous system to the
blastopore in Annelida and Vertebrates had already been pointed out by
Kowalevsky,[452] who had also sketched a theory of the common descent of
these two phyla from an ancestral form in which the nervous system
encircled the blastopore.

In 1882, before the publication of Sedgwick's papers, A. Lang[453] had put
forward the somewhat similar view that the stomach-diverticula of the
Turbellaria, which he had found to be segmentally arranged in certain
Triclads, were the morphological equivalents of the enterocoelic pouches
of higher animals. This view, however, he soon gave up.[454] Sedgwick's
views found a supporter in A. A. W. Hubrecht,[455] who utilised them in
connection both with his speculations on the relation of Nemertines to
Vertebrates, and with his exhaustive work on the early development of
the Mammalia. He postulated as the far-back ancestor of Vertebrates, "an
actinia-like, vermiform being, elongated in the direction of the
mouth-slit" (p. 410, 1906), and derived the central nervous system from
the circum-oral ring of this primitive form, the notochord from its
stomodæum, and the coelom from the peripheral parts of the gastric
cavity (p. 169, 1909).

    [424] Gegenbaur, _Zeits. f. wiss. Zool._, v., 1853.

    [425] Remak, _loc. cit._, p. 183, pl. xii.

    [426] Lereboullet, _Ann. Sci. nat._ (4) xviii., pp. 118-9,
    1862.

    [527] Lereboullet, in Remak, p. 183 f.n.

    [428] Kowalevsky, _Mém. Acad. Sci. St
    Pétersbourg_ (Petrograd), (7), x. and xi., 1866 and 1867.

    [429] A. Agassiz, _Contrib. Nat. Hist. United States_, v.,
    1864.

    [430] _Mém. Acad. Sci. St Pétersbourg_ (Petrograd), (7),
    xiv., 1869.

    [431] "Embryolog. Studien an Würmern u. Arthropoden,"
    _Mém. Acad. Sci. St Pétersbourg_ (Petrograd), (7), xvi.,
    1870.

    [432] _Die Kalkschwämme_, 3 vols., Berlin, 1872. General
    chapters translated in _Ann. Mag. Nat. Hist._ (4), xi.,
    pp. 241-62, 421-30, 1873.

    [433] "Die Gastræa-Theorie, die phylogenetische
    Classification des Thierreichs und die Homologie der
    Keimblätter." _Jenaische Zeitschrift_, viii., pp. 1-55,
    1874. "Die Gastrula und die Eifurchung der Thiere,"
    _ibid._, ix., pp. 402-508, 1875. "Die Physemarien,
    Gastræaden der Gegenwart," and "Nachträge zur
    Gastræa-Theorie," _ibid._, x., pp. 55-98, 1876.
    Republished in _Biologische Studien_, 2nd part, _Studien
    zur Gastræa-Theorie_, 270 pp., 14 pls., Jena, 1877.

    [434] See _Ann. Mag. Nat. Hist._ (4), xi., p. 253.

    [435] Term first introduced in _Die Kalkschwämme_, p. 468,
    1872.

    [436] "On the Primitive Cell-layers of the Embryo as the
    Basis of Genealogical Classification of Animals, and on
    the Origin of Vascular and Lymph Systems," _Ann. Mag.
    Nat. Hist._ (4), xi., pp. 321-38, 1873.

    [437] First distinguished in _Die Kalkschwämme_, i., p.
    465.

    [438] Even in the 'seventies it was still believed by many
    that the egg-nucleus disappeared on fertilisation. The
    true nature of the process was not fully made out till
    1875, when O. Hertwig observed the fusion of egg- and
    sperm-nuclei in _Toxopneustes (Morph. Jahrb._, i.,
    1876).

    [439] _Studien z. Gastræa-Theorie_, p. 214, 1877. These
    forms were known even in 1870 (Carter, _Ann. Mag. Nat.
    Hist._ (4), vi., pp. 346-7), to be Foraminifera. The
    figures of supposed collar-cells, etc., do credit to
    Haeckel's imagination.

    [440] _History of Creation_, Eng. Trans., ii., pp. 278 ff.

    [441] _Systematische Phylogenie_, iii., p. 41, Berlin,
    1895.

    [442] "Notes on the Embryology and Classification of the
    Animal Kingdom," _Q.J.M.S._ (n.s.), xvii., pp. 399-454,
    1877.

    [443] It was "part of the non-historic mechanism of
    growth" (_loc. cit._, p. 418).

    [444] _Treatise on Comparative Embryology_, ii., chap.
    xiii., 1881. For a modern discussion of this problem,
    see Hubrecht, _Q.J.M.S._, xlix., 1906.

    [445] See Balfour, _loc. cit._, Chapter xiii.

    [446] _A Treatise on Zoology_, Pt. ii., 1900. Introduction
    by Sir E. Ray Lankester.

    [447] _Studien zur Blättertheorie_, Jena, 1879-80. "Die
    Coelomtheorie, Versuch einer Erklärung des mittleren
    Keimblattes," _Jenaische Zeitschrift_, xv., pp. 1-150,
    1882.

    [448] For an historical account of this work, see
    Lankester, _loc. cit._, pp. 21-37.

    [449] _Proc. Roy. Soc._, 1883, and _Q.J.M.S._, xxiii.,
    1883.

    [450] "Origin of Metameric Segmentation," _Q.J.M.S._,
    xxiv., pp. 43-82 1884.

    [451] See further the same author's article "Embryology"
    in the _Ency. Brit._, vol. xi., 11th ed., Cambridge,
    1910.

    [452] _Arch. f. mikr. Anat._, xiii., pp. 181-204, 1877.

    [453] "Der Bau von Gunda segmentata," _Mitth. Zool. Stat.
    Neap._, iii., pp. 187-250, 1882.

    [454] "Die Polycladen," _Fauna u. Flora des Golfes von
    Neapel_, Monog. v., Leipzig, 1884, and "Beiträge zu
    einer Trophocoeltheorie," _Jen. Zeits._, xxxviii., pp.
    1-373, 1904 (which see for a modern account of theories
    of metamerism).

    [455] "Die Abstammung der Anneliden u. Chordaten," _ Jen.
    Zeits._, xxxix., pp. 151-76, 1905. "The Gastrulation of
    the Vertebrates," _Q.J.M.S._, xlix., pp. 403-19, 1906.
    "Early Ontogenetic Phenomena in Mammals," _Q.J.M.S._,
    liii., pp. 1-181, 1909.



CHAPTER XVII

THE ORGANISM AS AN HISTORICAL BEING


"Of late the attempt to arrange genealogical trees involving
hypothetical groups has come to be the subject of some ridicule, perhaps
deserved. But since this is what modern morphological criticism in great
measure aims at doing, it cannot be altogether profitless to follow this
method to its logical conclusions. That the results of such criticism
must be highly speculative, and often liable to grave error, is
evident."

The quotation is from Bateson's paper of 1886, and it is symptomatic of
the change which was soon to come over morphological thought. New
interests, new lines of work, began to usurp the place which pure
morphology had held so long.

This is accordingly a convenient stage at which to take stock of what
has gone before, to consider the relation of evolutionary morphology to
the transcendental and the Cuvierian schools of thought which preceded
it, and to make clear what new element evolution-theory added to
morphology.

The close analogy between evolutionary and transcendental morphology has
already been remarked upon and illustrated in the last three chapters.
We have seen that the coming of evolution made comparatively little
difference to pure morphology, that no new criteria of homology were
introduced, and that so far as pure morphology was concerned, evolution
might still have been conceived as an ideal process precisely as it was
by the transcendentalists. The principle of connections still remained
the guiding thread of morphological work; the search for archetypes,
whether anatomical or embryological, still continued in the same way as
before, and it was a point of subordinate importance that, under the
influence of the evolution-theory, these were considered to represent
real ancestral forms rather than purely abstract figments of the
intelligence. The law of Meckel-Serres was revived in an altered shape
as the law of the recapitulation of phylogeny by ontogeny; the natural
system of classification was passively inherited, and, by a _petitio
principii_, taken to represent the true course of evolution. It is true
that the attempt was made to substitute for the concept of homology the
purely genetic concept of homogeny, but no inkling was given of any
possible method of recognising homogeny other than the well-worn methods
generally employed in the search after homologies.

There was a close spiritual affinity between the speculative
evolutionists and the transcendentalists. Both showed the same
subconscious craving for simplicist conceptions--the transcendentalists
clung fast to the notion of the absolute unity of type, of the ideal
existence of the "one animal," and the evolutionists did precisely the
same thing when they blindly and instinctively accepted the doctrine of
the monophyletic descent of all animals from one primeval form. Geoffroy
persisted in regarding Arthropods as being built on the same plan as
Vertebrates: Dohrn and Semper did nothing different when they derived
both groups from an ancestor combining the main characters of both. The
determination to link together all the main phyla of the animal kingdom
and to force them all into a single mould was common to evolutionary and
pre-evolutionary transcendentalists alike.

From the fact that all Metazoa develop from an ovum which is a simple
cell, the evolutionists inferred that all must have arisen from one
primordial cell. From the fact that the next step in development is the
segmentation of the ovum, they argued that the ancestral Metazoa came
into being through the division of the primal Protozoon with aggregation
of the division-products. From the fact that a gastrula stage is very
commonly formed when segmentation has been completed, they assumed that
all germ-layered animals were descended from an ancestral Gastræa.

They quite ignored the possibility that a different explanation of the
facts might be given; they seized upon the simplest and most obvious
solution because it satisfied their overwhelming desire for
simplification. But is the simplest explanation always the
truest--especially when dealing with living things? One may be permitted
to doubt it. It is easy to account for the structural resemblance of the
members of a classificatory group, by the assumption that they are all
descended from a common ancestral form; it is easy to postulate any
number of hypothetical generalised types; but in the absence of positive
evidence, such simplicist explanations must always remain doubtful. The
evolutionists, however, had no such scruples.

Phylogenetic method differed in no way from transcendental--except
perhaps that it had learnt from von Baer and from Darwin to give more
weight to embryology. The criticisms passed by Cuvier and von Baer upon
the transcendentalists and their recapitulation theory might with equal
justice be applied to the phylogenetic speculations which were based on
the biogenetic law. There was the same tendency to fix upon isolated
points of resemblance and disregard the rest of the organisation. Thus,
on the ground of a presumed analogy of certain structures to the
vertebrate notochord, several invertebrate groups, as the Enteropneusta,
the Rhabdopleura, the Nemertea, were supposed to be, if not ancestral,
at least offshoots from the direct line of vertebrate descent. And if
other points of resemblance could in some of these cases be discovered,
yet no successful attempt was made to show that the total organisation
of any of these forms corresponded with that of the Vertebrate type.
With the possible exception of the Ascidian theory, all the numerous
theories of vertebrate descent suffered from this irremediable defect,
and none carried complete conviction.

In spite of the efforts of the evolutionists, as of those of the
transcendentalists, the phyla or "types" remained distinct, or at best
connected by the most general of bonds.

The close affinity of transcendentalists and evolutionists is shown very
clearly in their common contrast in habits of thought with the Cuvierian
school. It is the cardinal principle of pure morphology that function
must be excluded from consideration. This is a necessary and unavoidable
simplification which must be carried out if there is to be a science of
pure form at all. But this limitation of outlook, if carried over from
morphology to general biology becomes harmful, since it wilfully ignores
one whole side of life--and that the most important. The functional
point of view is clearly indispensable for any general understanding of
living things, and this is where the Cuvierian school has the advantage
over the transcendental--its principles are applicable to biology in
general.

Geoffroy and Cuvier in pre-evolutionary times well typified the contrast
between the formal and the functional standpoints. For Geoffroy form
determined function, while for Cuvier function determined form. Geoffroy
held that Nature formed nothing new, but adapted existing "materials of
organisation" to meet new needs. Cuvier, on the other hand, was always
ready to admit Nature's power to form entirely new organs in response to
new functional requirements.

The evolutionists followed Geoffroy rather than Cuvier. They laid great
store by homological resemblances, and dismissed analogies of structure
as of little interest. They were singularly unwilling to admit the
existence of convergence or of parallel evolution, and they held very
firmly the distinctively Geoffroyan view that Nature is so limited by
the unity of composition that she can and does form no new organs.

By no one has this underlying principle of evolutionary morphology been
more explicitly recognised than by Hubrecht, who in his paper of 1887,
after summarising the points of resemblance between Nemertines and
Vertebrates which led him to assume a genetic connection between them,
writes as follows:--"At the base of all the speculations contained in
this chapter lies the conviction, so strongly insisted upon by Darwin,
that new combinations or organs do not appear by the action of natural
selection unless others have preceded, from which they are gradually
derived by a slow change and differentiation.

"That a notochord should develop out of the archenteric wall because a
supporting axis would be beneficial to the animal may be a teleological
assumption, but it is at the same time an evolutional heresy. It would
never be fruitful to try to connect the different variations offered,
_e.g._, by the nervous system throughout the animal kingdom, if similar
assumptions were admitted, for there would be then quite as much to say
for a repeated and independent origin of central nervous systems out of
indifferent epiblast just as required in each special case. These would
be steps that might bring us back a good way towards the doctrine of
independent creations. The remembrance of Darwin's, Huxley's, and
Gegenbaur's classical foundations, and of Balfour's and Weismann's
brilliant superstructures, ought to warn us away from these dangerous
regions" (p. 644).

This same prejudice lies at the root of the idea of _Functionswechsel_,
in spite of the general functional orientation of that idea.

Dohrn's constant assumption is that Nature makes shift with old organs
wherever possible, instead of forming new ones. He derives gill-slits
from segmental organs, fins and limbs from gills, ribs from gill-arches,
and so on, instead of admitting that these organs might quite as well
have arisen independently. He objects on principle to the origin of
organs _de novo_. Thus, rebutting the suggestion that certain organs
which are not found in the lower Vertebrates might have arisen as new
formations, he writes:--"Against this supposition the whole weight of
all those objections can be directed that are to be brought in general
against the method of explanation which consists in appealing without
imperative necessity to the _Deus ex machina_, 'New formation,' which is
neither better nor worse than _Generatio equivoca_" (p. 21).

Of a similar nature was the objection to convergence.[456]

Why, we may ask, were morphologists so unwilling to admit the creative
power of life? Dohrn, for instance, was fully aware of the great
transforming influence exerted by function upon form--his theory of
_Functionswechsel_ regards as the most powerful agent of change the
activity of the animal, its effort to make the best use of its organs,
to apply them at need in new ways to meet new demands. Why then did he
not go a step further and admit that the animal could by its own
subconscious efforts form entirely new organs? Why did most
morphologists join with him in belittling the organism's power of
self-transformation?

The reasons seem to have been several. There is first the fundamental
reason, that the idea of an active creative organism is repugnant to the
intelligence, and that we try by all means in our power to substitute
for this some other conception. In so doing we instinctively fasten upon
the relatively less living side of organisms--their routine habits and
reflexes, their routine structure--and ignore the essential activity
which they manifest both in behaviour and in form-change.

We tend also to lay the causes of form-change, of evolution, as far as
possible outside the living organism. With Darwin we seek the
transforming factors in the environment rather than within the organism
itself. We fight shy of the Lamarckian conception that the living thing
obscurely works out its own salvation by blind and instinctive effort.
We like to think of organisms as machines, as passive inventions[457]
gradually perfected from generation to generation by some external
agency, by environment or by natural selection, or what you will. All
this makes us chary of believing that Nature is prodigal of new organs.

Other causes of the unwillingness of morphologists to admit the new
formation of organs are to be sought in the main principle of pure
morphology itself, that the unity of plan imposes an iron limit upon
adaptation, and in the powerful influence exercised at the time by
materialistic habits of thought. Teleology had become a bugbear to the
vast majority of biologists, and all real understanding of the Cuvierian
attitude seems, in most cases, to have been lost, although, curiously
enough, teleological conceptions were often unconsciously introduced in
the course of discussions on the "utility" of organs in the struggle for
existence.

Evolutionary morphology, being for the most part a form of pure or
non-functional morphology, agreed then in all essential respects with
pre-evolutionary or transcendental morphology.

But it contained the germ of a new conception which threw a new light
upon the whole science of morphology. This was the conception of the
organism as an historical being.

We have seen this thought expressed with the utmost clearness by Darwin
himself (_supra_, p. 233). In his eyes the structure and activities of
the living thing were a heritage from a remote past, the organism was a
living record of the achievements of its whole ancestral line. What a
light this conception threw upon all biology! "When we no longer look at
an organic being as a savage looks at a ship as something wholly beyond
his comprehension; when we regard every production of Nature as one
which has had a long history; when we contemplate every complex
structure and instinct as the summing-up of many contrivances, each
useful to the possessor, in the same way as any great mechanical
invention is the summing-up of the labour, the experience, the reason,
and even the blunders of numerous workmen; when we thus view each
organic being, how far more interesting--I speak from experience--does
the study of natural history become!" (_Origin_, 6th ed., pp. 665-6).

Sedgwick expressed the same thing from the morphological point of view
when he wrote, with reference to the ancestral significance of the
blastopore:--"If there is anything in the theory of evolution, every
change in the embryo must have had a counterpart in the history of the
race, and it is our business as morphologists to find it out" (p. 49,
1884).

By the evolution-theory the problems of form were linked indissolubly
with the problem of heredity. Unity of plan could no longer be explained
idealistically as the manifestation of Divine archetypal ideas; it had a
real historical basis, and was due to inheritance from a common
ancestor. The evolution-theory gave meaning and intelligibility to the
transcendental conception of the unity of plan; in particular it
supplied a simple and satisfying explanation of those puzzling vestigial
organs, whose existence was such a stumbling-block to the teleologists.
It enabled the biogenetic law to be substituted for the laws of
Meckel-Serres and von Baer, as being in some measure a combination and
interpretation of both.

Where the concept of evolution proved itself particularly useful was in
the interpretation of structures which were not immediately conditioned
by adaptation to present requirements, such as, for instance, the
arrangement of gill-slits and aortic arches in the foetus of land
Vertebrates. Such "heritage characters" could only be explained on the
hypothesis that they had once had functional or adaptational meaning.
Why, for instance, should the blastopore so often appear as a long slit,
closing by concrescence, unless this had been the original method of its
formation in remote Coelenterate ancestors?

The point hardly requires elaboration, since it has become an integral
part of all our thinking on biological problems. It may be as well,
however, for the sake of continuity, to give one or two examples of the
historical interpretation of animal structures. The first may
conveniently be the phylogenetic interpretation of the contrast between
"membrane" and "cartilage" bones.

In his _Grundzüge_ of 1870, Gegenbaur made the suggestion that the
investing or membrane bones were derived phylogenetically from
integumentary ossifications, and this was worked out in detail a few
years later by O. Hertwig.[458]

Many years before, several observers--J. Müller, Williamson, and
Steenstrup--had been struck with the resemblance existing between the
placoid scales and the teeth of Elasmobranch fishes. Hertwig followed up
this clue, and came to the conclusion not only that placoid scales and
teeth were strictly homologous, but also that all membrane bones were
derived phylogenetically from ossifications present in the skin or in
the mucous membrane of the mouth, just as cartilage bones were derived
from the cartilaginous skeletons of the primitive Vertebrates. In some
cases this manner of derivation could even be observed in ontogeny, as
Reichert had seen in the Newt, where certain bones in the roof of the
mouth are actually formed by the concrescence of little teeth, (_supra_,
p. 163). Hertwig considered that the following bones were originally
formed by coalescence of teeth--parasphenoid, vomer, palatine,
pterygoid, the tooth-bearing part of the pre-maxillary, the maxillary,
the dentary and certain bones of the hyo-mandibular skeleton of
Teleosts. All the investing bones (_Deckknochen_) of the skull were of
common origin, and could be traced back to integumentary skeletal
plates, which in the ancestral fish formed a dense carapace.

These conclusions were accepted by Kölliker himself, who wrote in his
_Entwickelungsgeschichte_ (1879)--"The distinction between the primary
or primordial, and the investing or secondary bones is from the
morphological standpoint sharp and definite. The former are
ossifications of the (cartilaginous) primordial skeleton, the latter are
formed outside this skeleton, and are probably all ossifications of the
skin or the mucous membrane" (p. 464).

Gegenbaur[459] consistently upheld the phylogenetic derivation of
investing bones from dermal ossifications, and even went further and
derived substitutionary bones as well from the integument, thus
establishing a direct comparison between the skeletal formations of
Vertebrates and Invertebrates. Investing bones were actual integumentary
ossifications which had gradually sunk beneath the skin to become part
of the internal skeleton; substitutionary bones were produced by cells
(osteoblasts) which were ultimately derived from the integument.[460]

A further instance of the historical interpretation of animal structure,
taken from quite a different field, is afforded by the speculations of
Dollo[461] on the ancestral history of the Marsupials. In a brilliant
paper of 1880[462] Huxley made the suggestion that the ancestors of
Marsupials were arboreal forms. "I think it probable," he wrote, "from
the character of the pes, that the primitive forms, whence the existing
Marsupialia have been derived, were arboreal animals; and it is not
difficult, I conceive, to see that, with such habits, it may have been
highly advantageous to an animal to get rid of its young from the
interior of its body at as early a period of development as possible,
and to supply it with nourishment during the later periods through the
lacteal glands, rather than through an imperfect form of placenta" (p.
655). Dollo followed up this suggestion, which had in the meantime been
strengthened by Hill's discovery of a true allantoic placenta in
_Perameles_, by demonstrating in the foot of present-day Marsupials
certain features which could only be interpreted as inherited from a
time when the ancestors of Marsupials were tree-living animals. These
were the occurrence of an opposable big toe (when this was present at
all), the great development of the fourth toe, the reduction and partial
syndactylism of the second and third toes, and in some cases the
regression of the nails. These characters were shown to be typical of
arboreal Vertebrates, and their occurrence in forms not arboreal
indicated that these were descended from tree-living ancestors. Traces
of an arboreal ancestry could be demonstrated even in the marsupial mole
_Notoryctes_.

These are only two examples out of hundreds that might be given. Present
day structure was interpreted in the light of past history; the common
element in organic form was seen to be due to common descent; the
existence of vestigial and non-functional organs was no longer a riddle.

There was even a tendency to concentrate attention upon the historical
side of structure, upon what the animal passively inherited rather than
upon what it personally achieved. Homologies were considered more
interesting than analogies, vestigial organs more interesting than
foetal and larval adaptations. Convergence was anathema. The dead-weight
of the past was appreciated at its full and more than its full value;
and the essential vital activity of the living thing, so clearly shown
in development and regeneration, was ignored or forgotten.

But evolutionary morphology for all practical purposes was a development
of pure or idealistic morphology, and was powerless to bring to fruit
the new conception with which evolution-theory had enriched it. The
reason is not far to seek. Pure morphology is essentially a science of
comparison which seeks to disentangle the unity hidden beneath the
diversity of organic form. It is not immediately concerned with the
causes of organic diversity--that is rather the task of the sciences of
the individual, heredity and development. To take an example--the
recapitulation theory may legitimately be used as a law of pure
morphology, as stating the abstract relation of ontogeny to phylogeny,
and the probable line of descent of any organism may be deduced from it,
as a mere matter of the ideal derivation of one form from another; but
an explanation of the reason for the recapitulation of ancestral history
during development can clearly not be given by pure morphology unaided.
From the fact that the common starfish shows in the course of its
development distinct traces of a stalk[463] it is possible to infer,
taking other evidence also into consideration, that the ancestors of the
starfish were at one stage of their existence stalked and sessile
organisms. But this leaves unanswered the question as to how and why the
starfish does still repeat after so many millions of years part of the
organisation of one of its remote ancestors. Why is this feature
retained, and by what means has it been conserved through countless
generations? It is clear that the answer can be given only by a science
of the causes of the production and retention of form, by a causal
morphology, based upon a study of heredity and development.

From the point of view of the pure morphologist the recapitulation
theory is an instrument of research enabling him to reconstruct probable
lines of descent; from the standpoint of the student of development and
heredity the fact of recapitulation is a difficult problem whose
solution would perhaps give the key to a true understanding of the real
nature of heredity.

To make full use of the conception of the organism as an historical
being it is necessary then to understand the causal nexus between
ontogeny and phylogeny.

We shall see in the next chapter that the transformation of morphology
from a comparative to a causal science did take place towards the end of
the century, and that some progress was made towards an understanding of
the relation between individual development and ancestral history,
particularly by Roux and Samuel Butler, working with the fruitful
Lamarckian conception of the transforming power of function.

    [456] The importance of convergence came to be realised
    after the vogue of phylogenetic speculation had
    passed--see Friedmann, _Die Konvergenz der Organismen_,
    Berlin, 1904, and A. Willey, _Convergence in Evolution_,
    London, 1911. Also L. Vialleton, _Elements de
    morphologie des Vertébrés_, Paris, 1912.

    [457] From this point of view there is a very profound
    analogy between artificial and natural selection. Upon
    the theory of natural selection organisms are lifeless
    constructs which are mechanically perfected by external
    agency, just as machines are improved by a process of
    conscious selection of the most successful among a
    number of competing models. (_Cf._ passage quoted below,
    on p. 308.)

    [458] _Arch. f. mikr. Anat._, xi. (suppl.), 1874; _Morph.
    Jahrb._, ii., 1876, v. 1879, and vii., 1882.

    [459] _Vergleich. Anat. d. Wirbelthiere_, i., pp. 200-1,
    1898.

    [460] For a full historical account of work on membrane
    and cartilage bones (as well as on the theory of the
    skull) see E. Gaupp, "Altere und neuere Arbeiten über
    den Wirbelthierschädel," _Ergeb. Anat. Entw._, x., 1901,
    and "Die Entwickelung des Kopfskelettes," in Hertwig's
    "_Handbuch vergl. exper. Entwickelungslehre d.
    Wirbelthiere_," iii., 2, pp. 573-874, 1905.

    [461] "Les Ancêtres des Marsupiaux étaient-ils
    arboricoles?" _Trav. Stat. zool. Wimereux_, vii., pp.
    188-203, pls. xi.-xii., 1899. See also Bensley, _Trans.
    Linn. Soc._ (2) ix., pp. 83-214, 1903.

    [462] _Proc. Zool. Soc._, pp. 649-62, 1880. _Sci. Mem._,
    iv., pp. 457-72.

    [463] J. F. Gemmill, _Phil. Trans. B_, ccv., p. 255, 1914.



CHAPTER XVIII

THE BEGINNINGS OF CAUSAL MORPHOLOGY


Until well into the 'eighties animal morphology remained a purely
descriptive science, content to state and summarise the relations
between the coexistent and successive form-states of the same and of
different animals. No serious attempt had been made to discover the
causes which led to the production of form in the individual and in the
race.

It is true that evolution-theory had offered a simple solution of the
great problem of the unity in diversity of animal forms, but this
solution was formal merely, and went little beyond that abstract
deduction of more complex from simpler forms, which had been the main
operation of pre-evolutionary morphology. Little was known of the actual
causes of ontogeny, and nothing at all of the causes of phylogeny; it
was, for instance, mere rhetoric on Haeckel's part to proclaim that
phylogeny was the mechanical cause of ontogeny.

Animal physiology, on its side, had developed in complete isolation from
morphology into a science of the functioning of the adult and finished
animal, considered as a more or less stable physico-chemical mechanism.
Since the days of Ludwig, Claude Bernard and E. du Bois Reymond, the
physiologists' chief care had been to analyse vital activities into
their component physical and chemical processes, and to trace out the
interchange of matter and energy between the organism and its
environment. Physiologists had left untouched, perhaps wisely, the much
more difficult problem of the causes of the development of form. For all
practical purposes they took the animal-machine as given, and did not
trouble about its mode of origin. They held indeed that form-production
was due to a complex of physico-chemical causes, which they hoped some
day to unravel;[464] but this future physiology of development remained
quite embryonic.

Physiology then had not really come into contact with the problems of
form, and it could give the morphologist no direct help when he turned
to investigate the causes of form-production. It had, however, a
determining influence upon the methods of those who first broke ground
in this No Man's Land between morphology proper and physiology. But it
is significant that it was a morphologist and not a physiologist that
did the first spade-work.

The pioneer in this field, both as investigator and as thinker, was W.
Roux, who sketched in the 'eighties the main outlines of a new science
of causal morphology, to which he gave the name of
_Entwicklungsmechanik_. The choice of name was deliberate, and the word
implied, first, that the new science was essentially an investigation of
the development of form, not of the mode of action of a formed
mechanism, and second, that the methods to be adopted were
mechanistic.[465]

Though Roux was the only begetter of the science of
_Entwicklungsmechanik_, he was, of course, not the first to investigate
experimentally the formative processes of animal life. Study of
regeneration dates back to Trembley (1740-44), Réaumur (1742), Bonnet
(1745), and Spallanzani (1768-82),[466] and in the years preceding Roux's
activity good work was done by Philipeaux. A beginning had been made
with experimental teratology by E. Geoffroy St Hilaire and others, and
the work of C. Dareste[467] remains classical. Back in the 18th century,
some of John Hunter's experiments had a bearing upon the problems of
form; his work on transplantation was followed up in the 19th century by
Flourens, P. Bert, Ollier and many others. In founding in 1872 the
_Archives de Zoologie expérimentale et générale_ H. de Lacaze-Duthiers
put forward in his introduction a powerful plea for the use of the
experimental method in zoology.

In some ways more directly connected with _Entwicklungsmechanik_ was
His's attempt in 1874[468] to explain on mechanical principles the
formation of certain of the embryonic organs by the bendings and
foldings of tubes or plates of cells. "His compared the various layers
of the chick embryo to elastic plates and tubes; out of these he
suggested that some of the principal organs might be moulded by mere
local inequalities of growth--the ventricles of the brain, for instance,
the alimentary canal, the heart--and he further succeeded in imitating
the formation of these organs by folding, pinching, and cutting
india-rubber tubes and plates in various ways."[469]

But Roux was undoubtedly the first to make a systematic survey of the
problems to be solved and to work out an organised method of attack. His
earliest work deals with the important problem of functional
adaptation--its importance to the organism, and its possible mechanistic
explanation. The first paper[470] was a study of the branching and
distribution of the arteries in the human body (1878), and a second
paper on the same subject followed in 1879.[471]

In these papers Roux showed how the development of the blood-vascular
system was largely determined by direct adaptation to functional
requirements, and he inferred the existence in the vascular tissues of
certain vital properties, in virtue of which the functional adaptation
of the blood-vessels came about. Thus the intima or inner lining must
possess the faculty of so reacting to the friction set up by the
blood-current as to oppose the least possible resistance to its flow;
the muscular coats must react to increased pressure by growing thicker,
and so on.

These papers were followed in 1881 by his well-known book, _Der Kampf
der Theile im Organismus_, which contained the working-out of his
mechanistic explanation of functional adaptation, and most of the
elements of his general "causal-analytical" theory of form production.
The significance of the book was popularly considered at the time to lie
in its supposed application of the selection idea to the explanation of
the internal adaptedness of animal structure--in the theory of "cellular
selection," and the book owed its success to its fitting in so well with
the prevalent Darwinism of the day. But its real importance, as a big
step towards causal morphology, was naturally not so fully appreciated.

During the next few years Roux continued his studies on functional
adaptation,[472] and at the same time made a new departure by
inaugurating, almost contemporaneously with the physiologist Pflüger,
the study of experimental embryology. Isolated observations had
previously been made upon the development of single blastomeres or parts
of blastulæ, by Haeckel and Chun for instance,[473] but Roux[474] and
Pflüger[475] were the first to investigate the subject systematically,
choosing for their work the egg of the frog.[476] Roux continued for many
years to follow up this line of work.[477]

In 1890 he drew up a programme and manifesto[478] of
_Entwicklungsmechanik_ as "an anatomical science of the future," and in
1895 he founded the famous _Archiv für Entwicklungsmechanik_,[479]
publishing in the same year the two large volumes of his collected
papers,[480] of which the first volume dealt with functional adaptation,
the second with experimental embryology.

His subsequent work includes several important general papers;[481]
besides a number of special memoirs dealing with the factors of
development, and with his original subject, functional adaptation.[482]

In our sketch of his views we shall have occasion to refer particularly
to his publications of 1881, 1895 (the _Einleitung_), 1902, 1905, and
1910.

Although Roux's biological philosophy is out-and-out mechanistic, he yet
recognises the difficulty, even the impossibility, of straightway
reducing development to the physico-chemical level. He tries to steer a
course midway between the simplicist conceptions of the materialists and
the "metaphysics" of the neo-vitalist school, which the experimental
study of development and regeneration soon brought into being. In 1895
he writes:--"The too simple mechanistic conception on the one hand, and
the metaphysical conception on the other represent the Scylla and
Charybdis, between which to sail is indeed difficult, and so far by few
satisfactorily accomplished; it cannot be denied that with the increase
of knowledge the seduction of the second has lately notably increased"
(p. 23).

The _via media_ adopted by Roux is the analysis of development, not
directly into simple physico-chemical processes, but into more complex
organic processes dependent upon the fundamental properties of living
matter. The aim of _Entwicklungsmechanik_ is defined by Roux to be the
reduction of developmental events to the fewest and simplest
_Wirkungsweisen_, or causal processes.[483] Two classes of causal
processes may be distinguished, as "complex components" and "simple
components" of development. The latter are directly explicable by the
laws of physics and chemistry; the former, while in essence
physico-chemical, are yet so very complicated that they cannot at
present be reduced to physico-chemical terms. The ultimate aim of
_Entwicklungsmechanik_ is to reduce development to its "simple
components," but its main task at the present day and for many years to
come is the analysis of development into its "complex components."

These complex components must be accepted as having much of the validity
of physical and chemical laws. They are mysterious in the sense that
they cannot yet be explained mechanistically, but they are constant in
their action, and under the same conditions produce always the same
effect--hence they may be made the subject of strictly scientific study.
They represent biological generalisations, in their way of equal
validity with the generalisations of physics and chemistry.

The principal "complex components" which Roux recognises are somewhat as
follows:--First come the elementary cell-functions of assimilation and
dissimilation, growth, reproduction and heredity, movement and
self-division (as a special co-ordination of cell-movements). Then at a
somewhat higher level, self-differentiation, and the trophic reaction to
functional stimuli. Components of even greater complexity may also be
distinguished, as, for instance, the biogenetic law. The various
tropisms exhibited in development may be regarded as "directive" complex
components. There must be added, not as being itself a component, but
rather as a mode or peculiar property of all functioning, the
omnipresent faculty of self-regulation.

It will be noticed that Roux's "complex components" are simply the
general properties or functions of organised matter.

Expressing Roux's thought in another way, we might say that life can
only be defined functionally, _i.e._, by an enumeration of the "complex
components" or elementary functions which all living beings manifest,
even down to the very simplest. "Living beings," writes Roux, "can at
present be defined with any approach to completeness only functionally,
that is to say, through characterisation of their activities, for we
have an adequate acquaintance with their functions in a general way,
though our knowledge of particulars is by no means complete" (p. 105,
1905). Defined in the most general and abstract way, living things are
material objects which persist in spite of their metabolism, and, by
reason of their power of self-regulation, in spite also of the changes
of the environment. This is the "functional minimum-definition of life"
(pp. 106-7, 1905).

We may now go on to consider the relation of function to form throughout
the course of development. Roux distinguishes in all development two
periods, in the first of which the organ is formed prior to and
independent of its function, while in the second the differentiation and
growth of the organ are dependent on its functioning. Latterly (1906 and
1910) Roux has distinguished three periods, counting as the second the
transition period when form is partly self-determined, partly determined
by functioning. As this conception of Roux's is of the greatest
importance we shall follow it out in some detail.

The idea was first elaborated in the _Kampf der Theile_ (1881), where he
wrote:--"There must be distinguished in the life of all the parts two
periods, an embryonic in the broad sense, during which the parts
develop, differentiate and grow of themselves, and a period of completer
development, during which growth, and in many cases also the balance of
assimilation over dissimilation, can come about only under the influence
of stimuli" (p. 180). There is thus a period of self-differentiation in
which the organs are roughly formed in anticipation of functioning, and
a period of functional development in which the organs are perfected
through functioning and only through functioning. The two periods cannot
be sharply separated from one another, nor does the transition from the
one to the other occur at the same time in the different tissues and
organs.

The conception is more fully expressed in 1905 as follows:--"This
separation (of development into two periods) is intended only as a first
beginning. The first period I called the embryonic period [Greek: kat'
exochên] or the period of organ-rudiments. It includes the 'directly
inherited' structures, _i.e._, the structures which are directly
predetermined in the structure of the germ-plasm, as, for instance, the
first differentiation of the germ, segmentation, the formation of the
germ-layers and the organ-rudiments, as well as the next stage of
'further differentiation,' and of _independent_ growth and maintenance,
that is, of growth and maintenance which take place without the
functioning of the organs.

"This is accordingly the period of direct fashioning through the
activity of the formative mechanism implicit in the germ-plasm, also the
period of the self-conservation of the formed parts without active
functioning.

"The second period is the period of 'functional form-development.' It
includes the further differentiation and the maintenance in their
typical form of the organs laid down in the first period; and this is
brought about by the exercise of the specific functions of the organs.
This period adds the finishing touches to the finer functional
differentiation of the organs, and so brings to pass the 'finer
functional harmony' of all organs with the whole. The formative activity
displayed during this period depends upon the circumstance that the
functional stimulus, or rather the exercise by the organs of their
specific functions, is accompanied by a subsidiary formative activity,
which acts partly by producing new form and partly by maintaining that
which is already formed.... Between the two periods lies presumably a
transition period, an intermediary stage of varying duration in the
different organs, in which both classes of causes are concerned in the
further building-up of the already formed, those of the first period in
gradually decreasing measure, those of the second in an increasing
degree" (pp. 94-6, 1905).

In the first period the organ forms or determines the function, in the
second period the function forms the organ, or at least completes its
differentiation. It is characteristic that in the first period
functionally adapted structure appears in the complete absence of the
functional stimulus.

The explanation of the difference between the two periods is to be found
in the different evolutionary history of the characters formed during
each. First-period characters are _inherited_ characters, and taken
together constitute the historical basis of the organism's form and
activity; second-period characters are those of later acquirement which
have not yet become incorporated in the racial heritage.

Inherited characters appear in development in the absence of the
stimulus that originally called them forth; acquired characters are
those that have not yet freed themselves from this dependence upon the
functional stimulus. First-period characters were originally, like
second-period characters, entirely dependent for their development upon
the functional stimuli in response to which they arose, and only
gradually in the course of generations did they gain that independence
of the functional stimulus which stamps them as true inherited
characters. Speaking of the formative stimuli which are active in
second-period development, Roux writes:--"These stimuli can also produce
new structure, which if it is constantly formed throughout many
generations finally becomes hereditary, _i.e._, develops in the
descendants in the absence of the stimuli, becomes in our sense
embryonic" (p. 180, 1881). Again, "form-characteristics which were
originally acquired in post-embryonic life through functional adaptation
may be developed in the embryo without the functional stimulus, and may
in later development become more or less completely differentiated, and
retain this differentiation without functional activity or with a
minimum of it. But in the continued absence of functional activity they
become atrophied ... and in the end disappear" (p. 201, 1881).

This conception of the nature of hereditary transmission is an important
one, and constitutes the first big step towards a real understanding of
the historical element in organic form and activity. It supplies a
practical criterion for the distinguishing of "heritage" characters from
acquired characters, of palingenetic from cenogenetic--a criterion which
descriptive morphology was unable to find.[484] The introduction of a
functional moment into the concept of heredity was a methodological
advance of the first importance, for it linked up in an understandable
way the problems of embryology, and indirectly of all morphology, with
the problem of hereditary transmission, and gave form and substance to
the conception of the organism as an historical being.

It is this element in Roux's theories that puts them so far in advance
of those of Weismann. Weismann did not really tackle the big problem of
the relation of form to function, and he left no place in his mechanical
system of preformation for functional or second-period development; he
conceived all development to be in Roux's sense embryonic, and due to
the automatic unpacking of a complex germinal organisation. Roux himself
was to a certain extent a preformationist, for the development of his
first-period characters is conditioned by the inherited organisation of
the germ-plasm, and is purely automatic. It was indeed his experiments
on the frog's egg (1888) that supplied some of the strongest evidence in
favour of the mosaic theory of development. The number of _Anlagen_
which he postulates in the germ is however small, and the germ-plasm in
his conception of it has a relatively simple structure (p. 103, 1905).

The transmission of acquired characters forms, of course, an integral
part of Roux's conception of heredity and development, for without this
transmission second-stage characters could not be transformed into
first-stage characters. He discusses this difficult question at some
length in the _Kampf der Theile_, coming to the conclusion that such
transmission takes place in small degree and gradually, and that many
generations are required before a new character can become hereditary.
He thinks that acquired characters are probably transmitted at the
chemical level. It is conceivable that acquired form-changes are
dependent on chemical changes, or are correlative with such, and that,
since the germ-cells stand in close metabolic relations with the soma,
these chemical changes may soak through to the germ-cells and so modify
them that a predisposition will appear in the descendants towards
similar form-changes.[485] From this point of view the problem of
transmission might be merged in the broader problem of the production of
form through chemical processes--the central problem of all development.

Inherited characters develop by an automatic process of
self-differentiation, and the separate parts of the embryo show during
this first period a surprising functional independence of one another.
But this state of things changes progressively as the second period is
reached, until finally all form-production and maintenance and all
correlation depend upon functioning. It is in the first period of
automatic development through internal "determining" factors that the
"developmental" functions in the strict sense, _e.g._ automatic growth,
division and self-differentiation, are most clearly shown. In the second
or "functional" period the formative influence of function upon
structure comes into play, and development becomes largely a matter of
"functional adaptation" to functional requirements.

All structure, according to Roux, is either functional or
non-functional. The former includes all structure that is adapted to
subserve some function. "Such 'functional structures' are, for example,
the composition of striated muscle fibres out of fibrillæ and these out
of muscle-prisms, or again the length and thickness of the muscles, the
static structure of the bones, the composition of the stomach and the
blood-vessels out of longitudinal and circular fibres, the external
shape of the vertebral centra and of the cuneiform bones of the foot"
(p. 73, 1910). Indeed, as Cuvier had already pointed out, practically
every organ in the body shows a functional structure which is accurately
and minutely adjusted to the function it is intended to perform. Thus,
to take some further examples, the arteries are admirably adapted as
regards size of lumen, elasticity of wall, direction of branching, to
conduct the blood to all parts of the body with the least possible waste
of the propelling power through frictional resistance. So, too, the
spongy substance of the long bones is arranged in lamellæ which take the
direction of the principal stresses and strains which fall upon the
bones in action.

Functional structure may be formed either in the first or in the second
period of development, may be either inherited or acquired, but it
reaches its full differentiation only in the second period, _i.e._,
under the influence of functioning. Practically speaking, functional
structure is directly dependent for its full development and for its
continued conservation upon the exercise of the particular function
which it serves. In the second period, but not in the first, increased
use leads to hypertrophy of the functional structure, disuse to atrophy.

From functional structure is to be distinguished nonfunctional
structure, which has no relation to the bodily functions--is neither
adapted to perform any of these, nor has arisen as a by-product of
functional activity. "To this category belong, for example, among
typical structures, the triangular form of the cross-section of the
tibia, the dolicocephalic or brachycephalic shape of the skull, most of
the external characters distinguishing genera and species, many of the
external features of the embryo which change in the course of
development, besides most of the abnormal forms shown by monstrosities,
tumours, etc." (p. 74, 1910). Non-functional structure is not affected
by functional adaptation, and may accordingly be left out of
consideration here.

Now the influence of functioning upon the form and structure of an organ
is twofold. There is first the immediate change brought about by the
very act of functioning--for example, the shortening and thickening of
skeletal muscles when they act. This is a purely temporary change, for
the organ at once returns to its normal quiescent state as soon as it
ceases to function. Such temporary functional change, brought about in
the moment of functioning, is usually dependent for its initiation upon
some neuro-muscular mechanism, though it may be elicited also by a
chemical stimulus. It is thus always a phenomenon of "behaviour." "From
such temporary changes are sharply to be distinguished all permanent
alterations which first appear in perceptible fashion through
oft-repeated or long-continued, enhanced functional activity. These
produce a new and lasting internal equilibrium of the organ, consisting
in an insertion of new molecules or a rearrangement of old. For this
reason they outlast the periods of functional form-change, or, if as in
the case of the muscles they themselves alter during functional
activity, they regain their state when the organ ceases to function" (p.
72, 1910). "Oft-repeated exercise or heightened exercise of the specific
functions, or repeated action of the functional stimuli which determine
them, produces, as we have said before, true form-changes as a
by-product. These are of two kinds. In so far as these form-changes
facilitate the repetition of the specific functions, I have called them
_functional adaptations_.... Such as do not improve the functioning of
the organ are indeed by-products of functioning, but without adaptive
character; they do not belong to the class of functional adaptations at
all" (p. 75, 1910).

We may now enquire in what way functional adaptations can arise as
by-products of functioning.

It is clear that natural selection in the sense of individual or
"personal" selection cannot adequately explain the origin of functional
structure and the functional harmony of structure, for thousands of
cells would have to vary together in a purposive way before any real
advantage could be gained in the struggle for existence, and it is in
the highest degree unlikely that this should come about by chance
variation.[486] The development of purposive internal structure is only to
be explained by the properties of the tissues concerned.

In illustration and proof of the statement that functional adaptation is
due to the properties of the tissues we may adduce the development and
regulation of the blood-vascular system, which has been thoroughly
studied from this point of view by Roux and Oppel (1910).

It appears that only the very first rudiments of the vascular system are
laid down in the short first period of automatic non-functional
development. All the subsequent growth and differentiation of the
blood-vessels falls into the second period, and is due wholly or in
great part to direct functional adaptation to the requirements of the
tissues. Thus from the rudiments formed in the first period there sprout
out the definitive vessels in direct adaptation to the food-consumption
of the tissues they are to supply. The size, direction and intimate
structure of these vessels are accurately adjusted to the part they play
in the economy of the whole, and this adjustment is brought about in
virtue of the peculiar properties or reaction-capabilities of the
different tissues of which the blood-vessels are composed.

The properties which Roux finds himself compelled to postulate in the
vascular tissues, after a thorough-going analysis of the different kinds
of functional adaptation shown by the blood-vessels, are summarised by
him as follows:--

"(1) The faculty--depending on a direct sensibility possessed by the
endothelium and perhaps also by the other layers of the intima--of
yielding to the impact of the blood, so far as the external relations of
the vessel permit. In this way the wall adapts itself to the
hæmodynamically conditioned 'natural' shape of the blood-stream, and
reaches this shape as nearly as possible." Through this faculty of the
lining tissue of the blood-vessels, the size of the lumen and the
direction of branching are so regulated as to oppose the least possible
resistance to the flow of the blood.

"(2) The faculty possessed by the endothelium of the capillaries of each
organ of adapting itself qualitatively to the particular metabolism of
the organ." This adaptedness of the capillaries is, however, more
usually an inherited state, _i.e._, brought about in the first period of
development.

"(3) The faculty possessed by the capillary walls of being stimulated to
sprout out and branch by increased functioning, _i.e._, by increased
diffusion, and their power to exhibit a chemically conditioned
cytotropism, which causes the sprouts to find one another and unite. A
similar process can be directly observed in isolated segmentation-cells,
which tend to unite in consequence of a power of mutual attraction.

"(4) The faculty of developing normal arterial walls in response to
strong intermittent pressure, and normal venous walls in response to
continuous lesser pressure." It has been shown, for instance, by Fischer
and Schmieden that in dogs a section of vein transplanted into an artery
takes on an arterial structure, at least as regards the circular
musculature, which doubles in thickness.

"(5) The power to regulate the normal[487] length of the arteries and
veins, in adaptation to the growth of the surrounding tissues, in such a
way that the stretching action of the blood-stream brings the vessel to
its proper functional length.

"(6) The power to form, in response to slight increases in longitudinal
tension, new structural parts which take their place alongside the
existing longitudinal fibres.

"(7) The power to regulate the width of the circular musculature
according to the degree of food-consumption by the tissues, in response
to nerve impulses initiated in these tissues.

"(8) The power possessed by the circular musculature of responding to
such continuous functional widening, by the formation of new structural
parts in the circular musculature, and so of widening the vessel
permanently or by this new formation of muscular fibres thickening the
circular musculature.

"(9) The faculty of being stimulated by increased blood-pressure to
produce the same structural changes as mentioned in par. 8, though here
the response is otherwise conditioned" (pp. 126-7, 1910).

It is by virtue of the tissue-properties detailed above that the complex
functional adaptations of the blood-vessels come about.

The development of the vascular system is no mere automatic and
mechanical production of form, apart from and independent of
functioning; it implies a living and co-ordinated activity of the
tissues and organs concerned, a power of active response to foreseen and
unforeseen contingencies. Form is then not something fixed and
congealed--it is the ever-changing manifestation of functional activity.
"Since most of the structure and form of the blood-vessels arises in
direct adaptation to function, the vessels of adult men and animals are
no fixed structures, which, once formed, retain their form and
structural build unchanged throughout life; on the contrary, they
require even for their continued existence the stimulus of functional
activity.... The fully formed blood-vessels are no static structures,
such as they appear to be according to the teaching of normal histology,
and such as they have long been taken to be. Observation and description
of normal development never shows us anything but the visible side of
organic happenings, the _products_ of activity, and leaves us ignorant
of the real processes of form-development and form-conservation, and of
their causes" (p. 125, 1910).

The real thing in organisation is not form but activity. It is in this
return to the Cuvierian or functional attitude to the problems of form
that we hold Roux's greatest service to biology to consist. The
attitude, however, seems to smack of vitalism, and Roux, as we have
seen, is no vitalist. He holds that the marvellous and apparently
purposive tissue-qualities which underlie all processes of functional
adaptation have arisen "naturally," in the course of evolution, by the
action of natural selection upon the various properties, useful and
useless, which appeared fortuitously in the primary living organisms. He
is, moreover, deeply imbued with the materialistic philosophy of his
youth, and it is indeed one of the chief characteristics of his system
that he states the fundamental properties or qualities of life in terms
of metabolism. A vital quality is for Roux a special process or mode of
assimilation. The faculty of "morphological assimilation" whereby form
is imposed upon formless chemical processes is the ultimate term of
Roux's analysis--"the most general, most essential, and most
characteristic formative activity of life" (p. 631, 1902).

We have now to consider very briefly the early results achieved by
Roux's fellow-workers in the field of causal morphology. As D. Barfurth
points out,[488] the years 1880-90 saw a general awakening of interest in
experimental morphology, and it is hard to say whether Roux's work was
cause or consequence. "There fall into this period," writes Barfurth,
"the experimental investigations by Born and Pflüger on the sexual
difference in frogs (1881), by Pflüger on the parthenogenetic
segmentation of Amphibian ova, on crossing among the Amphibia, and on
other important subjects (1882). In the following year (1883) appeared
two papers of fundamental importance, by E. Pflüger and W. Roux: Pflüger
publishing his researches on 'the influence of gravity on
cell-division,' Roux his experimental investigations on 'the time of the
determination of the chief planes in the frog-embryo.'... In the same
year appeared A. Rauber's experimental studies 'on the influence of
temperature, atmospheric pressure, and various substances on the
development of animal ova,' which have brought many similar works in
their train. The following year (1884) saw a lively controversy on
Pflüger's gravity-experiments with animal eggs, in which took part
Pflüger, Born, Roux, O. Hertwig and others, and in this year appeared
work by Roux dealing with the experimental study of development, and in
particular giving the results of the first definitely localised
pricking-experiments on the frog's egg (in the _Schles. Gesell. f.
vaterl. Kultur_, 15th Feb. 1884), also the important researches of M.
Nussbaum and Gruber (followed up later by Verworn, Hofer and Balbiani)
on Protozoa, and other experimental work" (pp. xi.-xii.).

In 1888 appeared a famous paper by W. Roux,[489] in which he described how
he had succeeded in killing by means of a hot needle one of the two
first blastomeres of the frog's egg, and how a half-embryo had developed
from the uninjured cell. Some years before[490] he had enunciated, at
about the same time as Weismann, the view that development was brought
about by a qualitative division of the germ-plasm contained in the
nucleus, and that the complicated process of karyokinetic or mitotic
division of the nucleus was essentially adapted to this end. He
conceived that development proceeded by a mosaic-like distribution of
potencies to the segmentation-cells, that, for instance, the first
segmentation furrow separated off the material and potencies for the
right half of the embryo from those for the left half. He had tried to
show experimentally that the first furrow in the frog's egg coincided
with the sagittal plane of the embryo,[491] and his later success in
obtaining a half-embryo from one of the first two blastomeres seemed to
establish the "mosaic theory" conclusively.

Roux's needle-experiment aroused much interest, especially as Weismann's
theory of heredity was then being keenly discussed. Chabry had published
in 1887 some interesting results on the Ascidian egg,[492] which strongly
supported the Roux-Weismann theory. Considerable astonishment was
therefore caused by Driesch's announcement in 1891[493] that he had
obtained complete larvæ from single blastomeres of the sea-urchin's egg
isolated at the two-celled stage. He followed this up in the next
year[493] by showing that whole embryos could be produced from one or more
blastomeres isolated at the four-cell stage. Similar or even more
striking results were obtained by E. B. Wilson on _Amphioxus_,[494] and
Zoja on medusæ.[495] Driesch succeeded also in disturbing the normal
course and order of segmentation by compressing the eggs of the
sea-urchin between glass plates, and yet obtained normal embryos.
Similar pressure-experiments were carried out on the frog by O.
Hertwig,[496] and on _Nereis_ by E. B. Wilson,[497] with analogous results.

In 1895 O. Schultze[498] showed that if the frog's egg is held between two
plates and inverted at the two-celled stage there are formed two embryos
instead of one. In the same year T. H. Morgan[499] repeated Roux's
fundamental experiment of destroying one of the two blastomeres, but
inverted the egg immediately after the operation--a whole embryo of half
size resulted. A year or two later Herlitzka[500] found that if the first
two blastomeres of the newt's egg were separated by constriction, two
normal embryos of rather more than half normal size were formed.

The main result of the first few years' work on the development of
isolated blastomeres was to show that the mosaic theory was not strictly
true, and that the hypothesis of a qualitative division of the nucleus
was on the whole negatived by the facts.

Evidence soon accumulated that the cytoplasm of the egg stood for much
in the differentiation of the embryo. A number of years previously Chun
had made the discovery that single blastomeres of the Ctenophore egg,
isolated at the two-celled stage, gave half-embryos. This was in the
main confirmed by Driesch and Morgan in 1896,[501] and they made the
further interesting discovery that the same defective larvæ could be
obtained by removing from the unsegmented egg a large amount of
cytoplasm. Conclusive proof of the importance of the cytoplasm was
obtained soon after by Crampton,[502] who removed the anucleate
"yolk-lobe" from the egg of the mollusc _Ilyanassa_ at the two-celled
stage, and obtained larvæ which lacked a mesoblast. This result was
brilliantly confirmed and extended some years later by E. B. Wilson,[503]
working on the egg of _Dentalium_. He found that if the similar
anucleate "polar lobe" of this form is removed at the two-celled stage,
deficient larvæ are formed, in which the post-trochal region and the
apical organ are absent. He further showed that in the unsegmented but
mature egg prelocalised cytoplasmic regions can be distinguished, which
later become separated from one another through the segmentation of the
egg. The segmentation-cells into which these cytoplasmic substances are
thus segregated show a marked specificity of development, giving rise,
even when isolated, to definite organs of the embryo. Wilson concluded
that the cytoplasm of the egg contains a number of specific
organ-forming stuffs, which have a definite topographical arrangement in
the egg. Development is thus due in part to a qualitative division not
of the nucleus but of the cytoplasm. Corroborative evidence of the
existence of cytoplasmic organ-forming stuffs has been supplied for
several other species, _e.g._, _Patella_ (Wilson), _Cynthia_ (Conklin),
_Cerebratulus_ (Zeleny), and _Echinus_ (Boveri).

It is interesting to recall that so long ago as 1874 W. His[504] put
forward the theory that there exist in the blastoderm and even in the
egg prelocalised areas, which contain the formative material for each
organ of the embryo, and from which the embryo is developed by a simple
process of unequal growth.

The experimental study of form was prosecuted in many other directions
besides that of experimental embryology. The study of regeneration and
of regulatory processes attracted many workers, among whom may be
mentioned T. H. Morgan, C. M. Child, and H. Driesch. In an interesting
series of papers C. Herbst applied the principles of the physiology of
stimulus to the interpretation of development.[505] The formative power of
function was studied in Germany by Roux and his pupils, Fuld, O. Levy,
Schepelmann and others, particularly by E. Babák. In France, F. Houssay
inaugurated[506] an important series of memoirs by himself and his pupils
on "dynamical morphology," the most important memoir being his own
valuable discussion of the functional significance of form in fishes.[507]
The principles of his dynamical morphology were first laid down in his
book _La Forme et la Vie_ (1900).

The famous experiments of Loeb, Delage and others on artificial
parthenogenesis may also be mentioned, though their connection with
morphology is somewhat remote.

The period was characterised also by the lively discussion of first
principles, in which Driesch took a leading part. Materialistic methods
of interpretation were upheld by perhaps the majority of biologists, but
vitalism found powerful support.

    [464] See Carus's remark, referred to on p. 194, above.

    [465] Roux, _Die Entwicklungsmechanik_, p. 26, Leipzig,
    1905.

    [466] T. H. Morgan, _Regeneration_, p. 1, New York and
    London, 1901.

    [467] _Recherches sur la production artificielle des
    Monstruosités_, Paris, 1877, and many later papers.

    [468] _Unsere Körperform und das physiologische Problem
    ihrer Entstehung_, Leipzig, 1874.

    [469] J. W. Jenkinson, _Experimental Embryology_, p. 3,
    Oxford, 1909.

    [470] "Ueber die Verzweigungen der Blutgefässe des
    Menschen," _Jen. Zeit_., xii., 1878.

    [471] "Ueber die Bedeutung der Ablenkung des
    Arterienstammes bei der Astabgabe," _Jen. Zeit_., xiii.,
    1879.

    [472] "Beiträge zur Morphologie der funktionellen
    Anpassung. I. Struktur eines hochdifferenzierten
    bindgewebigen Organes (der Schwanzflosse des Delphin),"
    _Arch. Anat. Physiol._ (_Anat. Abt._) for 1883. II.
    "Ueber die Selbstregulation der 'morphologischen' Länge
    der Skeletmuskeln des Menschen," _Jen. Zeit._, xvi.,
    1883. III. "Beschreibung ... einer
    Kniegelenkeknochenankylose," _Arch. Anat. Physiol._
    (_Anat. Abt._) for 1885.

    [473] In 1869 and 1877 respectively (Roux, p. 53, 1905).

    [474] _Ueber die Zeit. der Bestimmung der Hauptrichtungen
    des Froschembryo_, Leipzig, 1883.

    [475] "Ueber den Einfluss der Schwerkraft auf die Teilung
    der Zellen," Pflüger's _Archiv_, xxxi., 1883. Also
    subsequent papers in same journal.

    [476] For an account of the classical experiments on the
    frog's egg, see T. H. Morgan, _The Development of the
    Frog's Egg_, New York, 1897.

    [477] In a series of "Beiträge zur Entwicklungsmechanik
    des Embryo," published in various journals from 1884 to
    1891, all dealing with the frog's egg. Also in many
    papers in the _Archiv f. Entw. mech._, from 1895
    onwards.

    [478] _Die Entwicklungsmechanik der Organismen, eine
    anatomische Wissenschaft der Zukunft_, Wien, 1890.

    [479] The first volume contains the important _Einleitung_
    or general Introduction.

    [480] _Gesammelte Abhandlungen über Entwicklungsmechanik
    der Organismen_, 2 vols., Leipzig, 1895.

    [481] "Für unser Programm und seine Verwirklichung,"
    _A.E.M._, v., pp. 1-80 and 219-342, 1897. "Ueber die
    Selbstregulation der Lebewesen," _A.E.M._, xiii., pp.
    610-5, 1902. "Die Entwicklungsmechanik, ein neuer Zweig
    der biologischen Wissenschaft," Heft I. of the _Vorträge
    u. Aufsätze über Entwicklungsmechanik der Organismen_,
    Leipzig, 1905. Oppel and Roux, "Ueber die gestaltliche
    Anpassung der Blutgefässe," Heft x., of the _Vorträge u.
    Aufsätze_, Leipzig, 1910.

    [482] "Ueber d. funkt. Anpassung des Muskelmagens der
    Gans," _A.E.M._, xxi., pp. 461-99, 1906.

    [483] The exact quantitative formulation of a
    _Wirkungsweise_ constitutes a law. The word itself is
    perhaps most conveniently rendered as "causal process."

    [484] M. Fürbringer, perhaps under the influence of Roux,
    emphasised the importance, from a morphological point of
    view, of studying post-embryonic (functional)
    development, _Unters. z. Morph. u. Syst. der Vögel_,
    ii., Amsterdam, p. 925, 1888.

    [485] See, for the development of this idea, Oppel, in
    Roux-Oppel, 1910.

    [486] _Cf._ the controversy between Herbert Spencer and
    Weismann on the subject of "coadaptation" in the
    _Contemporary Review_ for 1893 and 1894. See also
    Weismann's paper in _Darwin and Modern Science_,
    Cambridge, 1909.

    [487] That is, the length they take up when separated from
    the body.

    [488] "Wilhelm Roux zum 60. Geburtstage," _Arch. f.
    Entw.-Mech._, xxx. _Festschrift für Prof. Roux_, Pt. i,
    1910.

    [489] Virchow's _Archiv_, cxiv., 1888. First announced in
    Sept. 1887.

    [490] _Ueber die Bedeutung der Kernteilungsfiguren_,
    Leipzig, 1883.

    [491] _Bresl. ärtz. Zeitschr._, 1885.

    [492] _Journ. de l'Anat. et de la Physiologie_, xxiii.,
    1887.

    [493] _Zeits. f. wiss. Zool._, liii., 1891 and 1892.

    [494] _Journ. Morph._, viii., 1893.

    [495] _Arch. f. Ent.-Mech._, i., 1895; ii., 1896.

    [496] _Arch. f. mikr. Anat._, xliii., 1893.

    [497] _Arch. f. Ent.-Mech._, iii., 1896.

    [498] _Arch. f. Ent.-Mech._, i., 1895.

    [499] _Anat. Anz._, x., 1895.

    [500] _Arch. f. Ent.-Mech._, iv. 1897.

    [501] _Arch. f. Ent.-Mech._, ii., 1896.

    [502] _Arch. f. Ent.-Mech._, iii., 1896.

    [503] _Journ. exper. Zool._, i., 1904.

    [504] _Unsere Körperform_, p. 19, Leipzig, 1874.

    [505] _Biolog. Centrlbl._, xiv., 1894, xv., 1895.
    _Formative Reize in der thierischen Ontogenese_,
    Leipzig, 1901.

    [506] "La Morphologie dynamique," No. i. of the
    _Collection de Morphologie dynamique_, Paris, 1911.

    [507] "Forme, Puissance et Stabilité des Poissons," No.
    iv. of the _Collection_, Paris, 1912.



CHAPTER XIX

SAMUEL BUTLER AND THE MEMORY THEORIES OF HEREDITY


We have laid stress upon the distinction established by Roux between the
two stages of development--the automatic and the functional--because of
the light which it seems to throw upon the phylogenetic relation of form
to function. We have pointed out, too, the paramount rôle that function
plays in Roux's theories of development and heredity, and we have
brought out the close kinship existing between his theory and that of
Lamarck. For Roux, as for Lamarck, the function creates the organ, and
it is only after long generations that the organ appears before the
function.

It so happened that just about the time when Roux's papers were
beginning to appear a brilliant attempt was made by Samuel Butler to
revive and complete the Lamarckian doctrine.

A man of singular freshness and openness of mind, combining in an
extraordinary degree extreme intellectual subtlety with a childlike
simplicity of outlook, Butler was one of the most fascinating figures of
the 19th century. He was not a professional biologist, and much of his
biological work is, for that reason, imperfect. But he brought to bear
upon the central problems of biology an unbiassed and powerful
intelligence, and his attitude to these problems, just because it is
that of a cultivated layman, is singularly illuminating.

He was not well acquainted with biological literature; he seems to have
hit upon the main ideas of his theory of life and habit in complete
independence of Lamarck, and only later to have become aware that
Lamarck had in a measure forestalled him. He puts this very beautifully
in the following passage from his chief biological work _Life and Habit_
(1877[508]):--"I admit that when I began to write upon my subject I did
not seriously believe in it. I saw, as it were, a pebble upon the
ground, with a sheen that pleased me; taking it up, I turned it over and
over for my amusement, and found it always grow brighter and brighter
the more I examined it. At length I became fascinated, and gave loose
rein to self-illusion. The aspect of the world changed; the trifle which
I had picked up idly had proved to be a talisman of inestimable value,
and had opened a door through which I caught glimpses of a strange and
interesting transformation. Then came one who told me that the stone was
not mine, but that it had been dropped by Lamarck, to whom it belonged
rightfully, but who had lost it; whereon I said I cared not who was the
owner, if only I might use it and enjoy it. Now, therefore, having
polished it with what art and care one who is no jeweller could bestow
upon it, I return it, as best I may, to its possessor" (p. 306). In one
of his later works, however, Butler made up for his first neglect of his
predecessors by giving what is undeniably the best account in English
literature of the work of Buffon, Lamarck, and Erasmus Darwin--in his
_Evolution, Old and New_ (1879). Many of his facts he took from Charles
Darwin, whose theory of natural selection he bitterly opposed, in the
two books just mentioned and in _Unconscious Memory_ (1880) and _Luck or
Cunning_ (1887).

Butler's main thesis is that living things are active, intelligent
agents, personally continuous with all their ancestors, possessing an
intense but unconscious memory of all that their ancestors did and
suffered, and moving through habit from the spontaneity of striving to
the automatism of remembrance.

The primary cause of all variation in structure is the active response
of the organism to needs experienced by it, and the indispensable link
between the outer world and the creature itself is that same "sense of
need" upon which Lamarck insisted. "According to Lamarck, genera and
species have been evolved, in the main, by exactly the same process as
that by which human inventions and civilisations are now progressing;
and this involves that intelligence, ingenuity, heroism, and all the
elements of romance, should have had the main share in the development
of every herb and living creature around us" (_Life and Habit_, p. 253).
Variations are indubitably the raw material of evolution--"The question
is as to the origin and character of these variations. We say they
mainly originate in a creature through a sense of its needs, and vary
through the varying surroundings which will cause those needs to vary,
and through the opening-up of new desires in many creatures, as the
consequence of the gratification of old ones; they depend greatly on
differences of individual capacity and temperament; they are
communicated, and in the course of time transmitted, as what we call
hereditary habits or structures, though these are only, in truth,
intense and epitomised memories of how certain creatures liked to deal
with protoplasm" (p. 267).

Butler's theory then is essentially a bold and enlightened Lamarckism,
completed and rounded off by the conception that heredity too is a
psychological process, of the same nature as memory.

In seeking to establish a close analogy between memory and heredity
Butler starts out from the fact of common experience, that actions which
on their first performance require the conscious exercise of will and
intelligence, and are then carried out with difficulty and hesitation,
gradually through long-continued practice come to be performed easily
and automatically, without the conscious exercise of intelligence or
will.

He tries to show that this is a general law--that knowledge and will
become intense and perfect only when through long-continued exercise
they become automatic and unconscious--and he applies this conception to
the elucidation of development.

Developmental processes, especially the early ones (of Roux's first
stage) are automatic and unconscious, and yet imply the possession by
the embryo of a wonderfully perfect knowledge of the processes to be
gone through, and an assured power of will and judgment. Is it
conceivable, says Butler, that the embryo can do all these things
without knowing how to do them, and without having done them before?
"Shall we say ... that a baby of a day old sucks (which involves the
whole principle of the pump, and hence a profound practical knowledge of
the laws of pneumatics and hydrostatics), digests, oxygenises its blood
(millions of years before Sir Humphrey Davy discovered oxygen), sees and
hears--all most difficult and complicated operations, involving a
knowledge of the facts concerning optics and acoustics, compared with
which the discoveries of Newton sink into utter insignificance? Shall we
say that a baby can do all these things at once, doing them so well and
so regularly, without being even able to direct its attention to them,
and without mistake, and at the same time not know how to do them, and
never have done them before?" (p. 54). Assuredly not.

The only possible explanation is that the embryo's ancestors have done
these things so often, throughout so many millions of generations, that
the embryo's knowledge of how to do them has become unconscious and
automatic by reason of this age-long practice. This implies that there
is in a very real sense actual personal continuity between the embryo
and all its ancestors, so that their experiences are his, their memory
also his. "We must suppose the continuity of life and sameness between
living beings, whether plants or animals, to be far closer than we have
hitherto believed; so that the experience of one person is not enjoyed
by his successor, so much as that the successor is _bona fide_ but a
part of the life of his progenitor, imbued with all his memories,
profiting by all his experiences--which are, in fact, his own--and only
unconscious of the extent of his own memories and experiences owing to
their vastness and already infinite repetitions" (p. 50). It is very
suggestive in this connection, he continues--"I. That we are _most
conscious of, and have most control over_, such habits as speech, the
upright position, the arts and sciences, which are acquisitions peculiar
to the human race, always acquired after birth, and not common to
ourselves and any ancestor who had not become entirely human.

"II. That we are _less conscious of, and have less control over_, eating
and drinking, swallowing, breathing, seeing and hearing, which were
acquisitions of our prehuman ancestry, and for which we had provided
ourselves with all the necessary apparatus before we saw light, but
which are, geologically speaking, recent, or comparatively recent.

"III. That we are _most unconscious of, and have least control over_,
our digestion and circulation, which belonged even to our invertebrate
ancestry, and which are habits, geologically speaking, of extreme
antiquity.... Does it not seem as though the older and more confirmed
the habit, the more unquestioning the act of volition, till, in the case
of the oldest habits, the practice of succeeding existences has so
formulated the procedure, that, on being once committed to such and such
a line beyond a certain point, the subsequent course is so clear as to
be open to no further doubt, to admit of no alternative, till the very
power of questioning is gone, and even the consciousness of volition"
(pp. 51-2).

The hypothesis then, that heredity and development are due to
unconscious memory, finds much to support it--"the self-development of
each new life in succeeding generations--the various stages through
which it passes (as it would appear, at first sight, without rhyme or
reason), the manner in which it prepares structures of the most
surpassing intricacy and delicacy, for which it has no use at the time
when it prepares them, and the many elaborate instincts which it
exhibits immediately on, and indeed before, birth--all point in the
direction of habit and memory, as the only causes which could produce
them" (p. 125). The hypothesis explains, for instance, the fact of
recapitulation:--"Why should the embryo of any animal go through so many
stages--embryological allusions to forefathers of a widely different
type? And why, again, should the germs of the same kind of creature
always go through the same stages? If the germ of any animal now living
is, in its simplest state, but part of the personal identity of one of
the original germs of all life whatsoever, and hence, if any now living
organism must be considered without quibble as being itself millions of
years old, and as imbued with an intense though unconscious memory of
all that it has done sufficiently often to have made a permanent
impression; if this be so, we can answer the above questions perfectly
well. The creature goes through so many intermediate stages between its
earliest state as life at all, and its latest development, for the
simplest of all reasons, namely, because this is the road by which it
has always hitherto travelled to its present differentiation; this is
the road it knows, and into every turn and up or down of which it has
been guided by the force of circumstances and the balance of
considerations" (pp. 125-6).

The hypothesis explains also the way in which the orderly succession of
stages in embryogeny is brought about, for we can readily understand
that the embryo will not remember any stage until it has passed through
the stage immediately preceding it. "Each step of normal development
will lead the impregnated ovum up to, and remind it of, its next
ordinary course of action, in the same way as we, when we recite a
well-known passage, are led up to each successive sentence by the
sentence which has immediately preceded it.... Though the ovum
immediately after impregnation is instinct with all the memories of both
parents, not one of these memories can normally become active till both
the ovum itself and its surroundings are sufficiently like what they
respectively were, when the occurrence now to be remembered last took
place. The memory will then immediately return, and the creature will do
as it did on the last occasion that it was in like case as now. This
ensures that similarity of order shall be preserved in all the stages of
development in successive generations" (pp. 297-8).

Abnormal conditions of development will cause the embryo to pause and
hesitate, as if at a loss what to do, having no ancestral experience to
guide it. Abnormalities of development represent the embryo's attempt to
make the best of an unexpected situation. Or, as Butler puts it, "When
... events are happening to it which, if it has the kind of memory we
are attributing to it, would baffle that memory, or which have rarely or
never been included in the category of its recollections, _it acts
precisely as a creature acts_ _when its recollection is disturbed, or
when it is required to do something which it has never done before_" (p.
132). "It is certainly noteworthy that the embryo is never at a loss,
unless something happens to it which has not usually happened to its
forefathers, and which in the nature of things it cannot remember" (p.
132).

Butler's teleological conception of organic evolution was of course
completely antagonistic to the naturalistic conceptions current in his
time. In one of his later books he repeats Paley's arguments in favour
of design, and to the question, "Where, then, is your designer of beasts
and birds, of fishes, and of plants?" he replies: "Our answer is simple
enough; it is that we can and do point to a living tangible person with
flesh, blood, eyes, nose, ears, organs, senses, dimensions, who did of
his own cunning, after infinite proof of every kind of hazard and
experiment, scheme out and fashion each organ of the human body. This is
the person whom we claim as the designer and artificer of that body, and
he is the one of all others the best fitted for the task by his
antecedents, and his practical knowledge of the requirements of the
case--for he is man himself. Not man, the individual of any given
generation, but man in the entirety of his existence from the dawn of
life onwards to the present moment" (_Evolution, Old and New_, p. 30,
1879).

Butler's theory of life and habit remained only a sketch, and he was
perhaps not fully aware of its philosophical implications. Since
Butler's time, a new complexion has been put upon biological philosophy
by the profound speculations of Bergson.

But it is not impossible that the future development of biological
thought will follow some such lines as those which he tentatively laid
down.

Butler was not the first to suggest that there is a close connection
between heredity and memory--it is a thought likely to occur to any
unprejudiced thinker. The first enunciation of it which attracted
general attention was that contained in Hering's famous lecture "On
Memory as a general Function of organised Matter."[509] Butler was not
aware of Hering's work when he published his _Life and Habit_, but in
_Unconscious Memory_ (1880) he gave full credit to Hering as the first
discoverer, and supplied an admirable translation of Hering's lecture.
As far as the assimilation of heredity to memory is concerned Hering and
Butler have much in common, but Hering did not share Butler's Lamarckian
and vitalistic views, preferring to hold fast, for the practical
purposes of physiology at all events, to the general accepted theory of
the parallelism between psychical and physical processes. He was
inclined to regard memory in the ordinary sense as a function of the
brain, and memory in general as a function of all organised matter.
Speaking of the psychical life, he says, "Thus the cause which produces
the unity of all single phenomena of consciousness must be looked for in
unconscious life. As we know nothing of this except what we learn from
our investigations of matter, and since in a purely empirical
consideration, matter and the unconscious must be regarded as identical,
the physiologist may justly define memory in a wider sense to be a
faculty of the brain, the results of which to a great extent belong to
both consciousness and unconsciousness."[510] Hering's views were
supported by Haeckel.[511]

In 1893 an American, H. F. Orr,[512] tried to work out a theory of
development and heredity based upon the fundamental idea "that the
property which is the basis of bodily development in organisms is the
same property which we recognise as the basis of psychic activity and
psychic development." He tried also to explain the recapitulation of
phylogeny by ontogeny as due to habit.

The neo-Lamarckian school of American palæontologists were also in
sympathy with the memory idea, and this was expressed most clearly
perhaps by Cope.[513]

In 1904 appeared the work on this subject which has attracted the most
attention--R. Semon's _Die Mneme_.[514] This was an elaborate treatment of
the question from the materialistic point of view, the main assumption
of Semon's theory being that the action of a stimulus upon the organism
leaves a more or less permanent material trace or "engramm," of such a
nature as to modify the subsequent action of the organism.

Applied to the explanation of heredity and development, Semon's theory
comes to very much the same as Weismann's, with engramms substituted for
determinants, but it has the great advantage of allowing for the
transmission of acquired characters. The application of the concept of
stimulus is valuable and suggestive, but it seems to us that the memory
theory of heredity can be properly utilised only by adopting a frankly
Lamarckian and vitalistic standpoint, and this standpoint Semon
expressly combats. As Ward[515] points out in his illuminating lecture on
heredity and memory--"Records or memoranda alone are not memory, for
they presuppose it. _They_ may consist of physical traces, but memory,
even when called 'unconscious,' suggests mind; for, as we have seen, the
automatic character implied by this term 'unconscious' presupposes
foregone experience.... The mnemic theory then, if it is to be worth
anything, seems to me clearly to require not merely physical records or
'engrams,' but living experience or tradition. The mnemic theory will
work for those who can accept a monadistic or pampsychist interpretation
of the beings that make up the world, who believe with Spinoza and
Leibniz that 'all individual things are animated albeit in divers
degree'" (pp. 55-6).

Perhaps the best and most ingenious treatment of memory and heredity
from a physical standpoint is that offered by E. Rignano in his book,
_Sur la transmissibilité des caractères acquis_.[516] Rignano seeks to
construct a physico-chemical "model" which will explain both heredity
and memory.

His system, which is based more firmly upon the facts of experimental
embryology than Semon's, postulates the existence of "specific nervous
accumulators." The essential hypothesis set up is that every functional
stimulus is transformed into specific vital energy, and deposits in the
nucleus of the cell a specific substance which is capable of
discharging, in an inverse direction, the nervous current which has
formed it, as soon as the dynamical equilibrium of the organism is
restored to the state in which it was when the original stimulus acted
upon it. These specific nuclear substances, different for each cell, are
accumulated also in the nuclei of the germinal substance, constituting
what Rignano calls the central zone of development. That is to say, each
functional adaptation changes slightly the dynamical equilibrium of the
organism, and this change in the system of distribution of the nervous
currents leads to the deposit in the central zone of development of a
new specific substance. In the development of the next individual this
new specific element enters into activity, and reproduces the nervous
current which has formed it, as soon as the organism reaches the same
conditions of dynamical equilibrium as those obtaining when the stimulus
acted on the parent.

Development can thus be regarded as consisting of a number of stages, at
each of which new specific elements enter automatically into play and
lead the embryo from that stage to the stage succeeding. The germinal
substance on this theory of Rignano's is to be regarded as being
composed of a large number of specific elements, originally formed as a
result of each new functional adaptation, but now forming part of the
hereditary equipment.

The theory represents an advance upon the more static conceptions of
Semon. It owes much to Roux's influence.

In this country, the mnemic theories have been championed particularly
by M. Hartog[517] and Sir Francis Darwin.[518]

    [508] The quotations are taken from the 1910 reprint,
    London, Fifield.

    [509] _Ueber das Gedächtnis als eine allgemeine Funktion
    der organisierten Materie_, Wien, 1870.

    [510] Eng. trans, in E. Hering, _Memory_, p. 9, Chicago
    and London, 1913.

    [511] _Die Perigenesis der Plastidule_, Jena, 1875.

    [512] _A Theory of Development and Heredity_, New York,
    1893.

    [513] _The Primary Factors of Organic Evolution_, Chicago,
    1896.

    [514] _Die Mneme als erhaltendes Prinzip im Wechsel des
    organischen Geschehens_, Leipzig, 1904; 2nd ed., 1908.

    [515] _Heredity and Memory_, Cambridge, 1913.

    [516] Paris, 1906. Also in Italian and German. Eng. trans.
    by B. C. ,H. Harvey, Chicago, 1911.

    [517] See _Problems of Life and Reproduction_, London,
    1913.

    [518] _Presidential Address to the British Association_,
    1908.



CHAPTER XX

THE CLASSICAL TRADITION IN MODERN MORPHOLOGY


To write a history of contemporary movements from a purely objective
standpoint is well recognised to be an impossible task. It is difficult
for those in the stream to see where the current is carrying them: the
tendencies of the present will only become clear some twenty years in
the future.

I propose, therefore, in this concluding chapter to deal only with
certain characteristics of modern work on the problems of form which
seem to me to be derived directly from the older classical tradition of
Cuvier and von Baer.

The present time is essentially one of transition. Complete uncertainty
reigns as to the main principles of biology. Many of us think that the
materialistic and simplicist method has proved a complete failure, and
that the time has come to strike out on entirely different lines. Just
in what direction the new biology will grow out is hard to see at
present, so many divergent beginnings have been made--the materialistic
vitalism of Driesch, the profound intuitionalism of Bergson, the
psychological biology of Delpino, Francé, Pauly, A. Wagner and W.
Mackenzie. But if any of these are destined to give the future direction
to biology, they will in a measure only be bringing biology back to its
pre-materialistic tradition, the tradition of Aristotle, Cuvier, von
Baer and J. Müller. It may well be that the intransigent materialism of
the 19th century is merely an episode, an aberration rather, in the
history of biology--an aberration brought about by the over-rapid
development of a materialistic and luxurious civilisation, in which
man's material means have outrun his mental and moral growth.

Two movements seem significant in the morphology of the last decade or
so of the 19th century--first, the experimental study of form, and
second, the criticism of the concepts or prejudices of evolutionary
morphology.

The period was characterised also by the great interest taken in
cytology, following upon the pioneer work of Hertwig, van Beneden and
others on the behaviour of the nuclei in fertilisation and
maturation.[519] This line of work gained added importance in connection
with contemporary research and speculation on the nature of hereditary
transmission, and it has in quite recent years received an additional
stimulus from the re-discovery of Mendelian inheritance. Its importance,
however, seems to lie rather in its possible relation to the problems of
heredity than in any meaning it may have for the problems of form. More
significant is the revolt against the cell-theory started by Sedgwick[520]
and Whitman,[521] on the ground that the organism is something more than
an aggregation of discrete, self-centred cells.

The experimental work on the causes of the production and restoration of
form infused new life into morphology. It opened men's eyes to the fact
that the developing organism is very much a living, active, responsive
thing, quite capable of relinquishing at need the beaten track of normal
development which its ancestors have followed for countless generations,
in order to meet emergencies with an immediate and purposive reaction.
It was cases of this kind, cases of active regulation in development and
regeneration, that led men like G. Wolff and H. Driesch to cast off the
bonds of dogmatic Darwinism and declare boldly for vitalism and
teleology.

There was the famous case of the regeneration of the lens in Amphibia
from the edge of the iris--an entirely novel mode of origin, not
occurring in ontogeny. The fact seems to have been discovered first by
Colucci in 1891, and independently by G. Wolff in 1895.[522] The
experiment was later repeated and confirmed by Fischel and other
workers. Wolff drew from this and other facts the conclusion that the
organism possesses a faculty of "primary purposiveness" which cannot
have arisen through natural selection.[523] And, as is well known, Driesch
derived one of his most powerful arguments in favour of vitalism from
the extraordinary regenerative processes shown by _Tubularia_ and
_Clavellina_ in the course of which the organism actually demolishes and
rebuilds a part or the whole of its structure. But under the influence
of physiologists like Loeb many workers held fast to materialistic
methods and conceptions.

The great variety of regulative response of which the organism showed
itself capable made it very difficult for the morphologist to uphold the
generalisations which he had drawn from the facts of normal undisturbed
development. The germ-layer theory was found inadequate to the new
facts, and many reverted to the older criterion of homology based on
destiny rather than origin. The trend of opinion was to reject the
ontogenetic criterion of homology, and to refuse any morphological or
phylogenetic value to the germ-layers.[524]

The biogenetic law came more and more into disfavour, as the developing
organism more and more showed itself to be capable of throwing off the
dead-weight of the past, and working out its own salvation upon original
and individual lines.[525] A. Giard in particular called attention to a
remarkable group of facts which went to show that embryos or larvæ of
the same or closely allied species might develop in most dissimilar ways
according to the conditions in which they found themselves.[526] His
classical case of "poecilogeny" was that of the shrimp _Palæmonetes
varians_, the fresh-water form of which develops in an entirely
different way from the salt-water form.

Experimental workers indeed were inclined to rule the law out of
account, to disregard completely the historical element in development,
and this was perhaps the chief weakness of the neo-vitalist systems
which took their origin in this experimental work.

From the side also of descriptive morphology the biogenetic law
underwent a critical revision. It was studied as a fact of embryology
and without phylogenetic bias by men like Oppel, Keibel, Mehnert, O.
Hertwig and Vialleton,[527] and they arrived at a critical estimate of it
very similar to that of von Baer.

Theoretical objections to the biogenetic law had been raised from time
to time by many embryologists, but the positive testing of it by the
comparison of embryos in respect of the degree of development of their
different organs starts with Oppel's work of 1891.[528] He studied a large
number of embryos of different species at different stages of their
development, and determined the relative time of appearance of the
principal organs and their relative size. His results are summarised in
tabular form and have reference to all the more important organs. He was
led to ascribe a certain validity to the biogenetic law, but he drew
particular attention to the very considerable anomalies in the time of
appearance which are shown by many organs, anomalies which had been
classed by Haeckel under the name of heterochronies.

Oppel's main conclusions were as follows:--"There are found in the
developmental stages of different Vertebrates 'similar ontogenetic
series,' that is to say, Vertebrates show at definite stages
similarities with one another in the degree of development of the
different organs. Early stages resemble one another, so also do later
stages; equivalent stages of closely allied species resemble one
another, and older stages of lower animals resemble younger stages of
higher animals; young stages are more alike than old stages.... The
differences which these similar series show (for which reason they
cannot be regarded as identical) may be designated as temporal
disturbances in the degree of development of the separate organs or
organ-systems. Some organs show very considerable temporal dislocations,
others a moderate amount, others again an inconsiderable amount. Among
the developmental stages of various higher animals can be found some
which correspond to the ancestral forms and also to the lower types
which resemble these ancestral forms. On the basis of the tabulated data
here given there can be distinguished with certainty in the ontogeny of
Amniotes a pro-fish stage, a fish-stage, a land-animal stage, a
pro-amniote stage, and following on these a fully developed reptile,
bird or mammal stage."[529]

Oppel's methods were employed by Keibel[530] in his investigations on the
development of the pig, which formed the model for the well-known series
of _Normentafeln_ of the ontogeny of Vertebrates which were issued in
later years under Keibel's editorship. Keibel was more critical of the
biogenetic law than Oppel, and he held that the ancestral stages
distinguished by Oppel could not be satisfactorily established. He
suggested an interesting explanation of heterochrony in development,
according to which the premature or retarded appearance of organs in
ontogeny stands in close relation with the time of their entering upon
functional activity. Thus in many mammals the mesodermal part of the
allantois often appears long before the endodermal part, though this is
phylogenetically older. This Keibel ascribes to the fact that the
endodermal part is almost functionless. "One can directly affirm," he
writes, "that the time of appearance of an organ depends in an eminent
degree upon the time when it has to enter upon functional activity. This
moment is naturally dependent upon the external conditions. Among the
highest Vertebrates, the mammals, the traces of phylogeny shown in
ontogeny are to a great extent obliterated through the adaptation of
ontogeny to the external conditions, and through the modifications which
the germs of more highly organised animals necessarily exhibit from the
very beginning as compared with germs which do not reach such a high
level of development" (p. 754, 1897).

Study of individual variation in the time of appearance of the organs in
embryos of the same species was prosecuted with interesting results by
Bonnet,[531] Mehnert,[532] and Fischel.[533] Fischel found that variability
was greatest among the younger embryos, and became progressively less in
later stages. Like von Baer (_supra_, p. 114) he inferred that
regulatory processes were at work during development which brought
divergent organs back to the normal and enabled them to play their part
as correlated members of a functional whole.

Important theoretical views were developed by Mehnert[534] in a series of
publications appearing from 1891 to 1898. Like Keibel, Mehnert
emphasised the importance of function in determining the late or early
appearance of organs, but he conceived the influence of function to be
exerted not only in ontogeny, but also throughout the whole course of
phylogeny, by reason of the transmission to descendants of the effects
of functioning in the individual life.

In his paper of 1897 Mehnert details the results of an extensive
examination of the development of the extremities throughout the Amniote
series. He finds that in all cases a pentadactylate rudiment is formed,
even in those forms in which only a few of the elements of the hand or
foot come to full development. But whereas in forms with a normally
developed hand, _e.g._ the tortoise and man, all the digits develop and
differentiate at about the same rate, in forms which have in the adult
reduced digits, _e.g._ the ostrich and the pig, these vestigial digits
undergo a very slow and incomplete differentiation, while the others
develop rapidly and completely. He draws a general distinction between
organs that are phylogenetically progressive and such as are
phylogenetically regressive, and seeks to prove that progressive organs
show an ontogenetic acceleration and regressive organs a retardation.[535]
The acceleration or retardation affects not only the mass-growth of the
organs, but also their histological differentiation.

Now between progression and functioning and between regression and
functional atrophy there is obviously a close connection. Loss of
function is well known to be one of the chief causes of the degeneration
of organs in the individual life, and on the other hand, as Roux has
pointed out, all post-embryonic development is ruled and guided by
functioning. It is thus in the long run functioning that brings about
phylogenetic progression, absence of functional activity that causes
phylogenetic regression. This comes about through the transmission of
acquired functional characters, a transmission which Mehnert conceives
to be extraordinarily accurate and complete.

In general Mehnert adopts the functional standpoint of Cuvier, von Baer,
and Roux. His considered judgment as to the phylogenetic value of the
biogenetic law closely resembles that formed by von Baer, for he admits
recapitulation only as regards the single organs, not as regards the
organism as a whole. He has, however, much more sympathy with the law
than either Keibel or Oppel, though he agrees that it cannot be used for
the construction of ancestral trees. But he ascribes to it as a fact of
development considerable importance. The following passage gives a good
summary of his view as to the scope and validity of the law. "The
biogenetic law has not been shaken by the attacks of its opponents. The
assertion is still true that individual organogenesis is exclusively
dependent on phylogeny. But we must not expect to find that all the
stages in the development of the separate organs, which coexisted in any
member of the phylogenetic series, appear _at the same time_ in the
individual ontogeny of the descendants, because each organ possesses its
own specific rate of development. In this way it comes about naturally
that organs which become differentiated rapidly, as, for example, the
medullary tube, as a rule dominate earlier periods of ontogeny than do
the organs of locomotion. For the same reason the cerebral hemispheres
of man are almost as large in youth as in maturity. The picture which an
embryo gives is not a repetition in detail of one and the same
phylogenetic stage; it consists rather of an assemblage of organs, some
of which are at a phyletically early stage of development, while others
are at a phyletically older stage."[536]

A different line of attack was that adopted by O. Hertwig in a series of
papers, which contain also what is perhaps the best critical estimate of
the present position and value of descriptive morphology.[537]

It had not escaped the notice of many previous observers that quite
early embryos not infrequently show specific characters even before the
characters proper to their class, order and genus are developed--in
direct contradiction of the law of von Baer. Thus L. Agassiz[538] had
remarked in 1859 that specific characteristics were often developed
precociously. "The Snapping Turtle, for instance, exhibits its small
crosslike sternum, its long tail, its ferocious habits, even before it
leaves the egg, before it breathes through lungs, before its derm is
ossified to form a bony shield, etc.; nay, it snaps with its gaping jaws
at anything brought near, when it is still surrounded by its amnion and
allantois, and its yolk still exceeds in bulk its whole body" (p. 269).

Wilhelm His,[539] in the course of an acute and damaging criticism of the
biogenetic law as enunciated by Haeckel, showed clearly that by careful
examination the very earliest embryos of a whole series of Vertebrates
could be distinguished with certainty from one another. "An identity in
external form of different animal embryos, despite the common
affirmation to the contrary, does not exist. Even at early stages in
their development embryos possess the characters of their class and
order, nay, we can hardly doubt, of their species and sex, and even
their individual characteristics" (201).

This specificity of embryos was affirmed with even greater confidence by
Sedgwick in a paper critical of von Baer's law.[540] He wrote:--"If v.
Baer's law has any meaning at all, surely it must imply that animals so
closely allied as the fowl and duck would be indistinguishable in the
early stages of development; and that in two species so closely similar
that I was long in doubt whether they were distinct species, viz.,
_Peripatus capensis_ and _Balfouri_, it would be useless to look for
embryonic differences; yet I can distinguish a fowl and a duck embryo on
the second day by the inspection of a single transverse section through
the trunk, and it was the embryonic differences between the Peripatuses
which led me to establish without hesitation the two separate
species.... I need only say ... that a species is distinct and
distinguishable from its allies from the very earliest stages all
through the development, although these embryonic differences do not
necessarily implicate the same organs as do the adult differences" (p.
39).

Hertwig interprets this fact of the specific distinctness of closely
allied embryos in the light of the preformistic conception of heredity.
According to this view the whole adult organisation is represented in
the structure of the germ-plasm contained in the fertilised ovum, from
which it follows that the ova of two different species, and also their
embryos at every stage of development, must be as distinct from one
another as are the adults themselves, even though the differences may
not be so obvious. If this be the case there can be no real
recapitulation in ontogeny of the phylogeny of the race, for the
egg-cell represents not the first term in phylogeny, but the last. The
egg-cell _is_ the organism in an undeveloped state; it has a vastly more
complicated structure than was possessed by the primordial cell from
which its race has sprung, and it can in no way be considered the
equivalent of this ancestral cell.

Hertwig puts this vividly when he says that "the hen's egg is no more
the equivalent of the first link in the phylogenetic chain than is the
hen itself" (p. 160, 1906, b).

If ontogeny is not a recapitulation of phylogeny, how is it that the
early embryonic stages are so alike, even in animals of widely different
organisation? Hertwig's answer to this is very interesting. He takes the
view that many of the processes characterising early embryonic
development are the means necessarily adopted for attaining certain
ends. Such are the processes of segmentation, the formation of a
blastula, of cell-layers, of medullary folds where the nervous system is
a closed tube, the formation of the notochord as a necessary condition
of the development of the vertebral column, and so on. "Looked at from
this standpoint it cannot surprise us that in all animal phyla the
earliest embryonic processes take place in similar fashion, so that we
observe the occurrence both in Vertebrates and Invertebrates of a
segmentation-process, a morula-stage, a blastula and a gastrula. If now
these developmental processes do not depend on chance, but, on the
contrary, are rooted in the nature of the animal cell itself, we have no
reason for inferring from the recurrence of a similar
segmentation-process, morula, blastula, and gastrula in all classes of
the animal kingdom the common descent of all animals from one
blastula-like or gastrula-like ancestral form. We recognise rather in
the successive early stages of animal development only the manifestation
of special laws, by which the shaping of animal forms (as distinct from
plant forms) is brought about" (p. 178, 1906, b).

"The principal reason why certain stages recur in ontogeny with such
constancy and always in essentially the same manner is that they provide
under all circumstances the necessary pre-conditions through which alone
the later and higher stages of ontogeny can be realised. The unicellular
organism can by its very nature transform itself into a multicellular
organism only by the method of cell-division. Hence, in all Metazoa,
ontogeny must start with a segmentation-process, and a similar statement
could be made with regard to all the later stages" (p. 57, 1906, a).

Similarities in early development are therefore no evidence of common
descent, and in the same way the resemblances of adult animals, subsumed
under the concepts of homology and the unity of plan, are not
necessarily due to community of descent, but may also be brought about
by the similarity or identity of the laws which govern the evolution of
these animals. In the absence, therefore, of positive evidence as to the
actual lines of descent (to be obtained only from palæontology),
homological resemblance cannot be taken as proof of blood relationship,
for homology is a wider concept than homogeny. The only valid definition
of homology is that adopted in pre-evolutionary days, when those organs
were considered homologous "which agree up to a certain point in
structure and composition, in position, arrangement, and relation to the
neighbouring organs, and accordingly possess identical functions and
uses in the organism" (p. 151, 1906, b).

The concept of homology has thus a value quite independent of any
evolutionary interpretation which may be superadded to it. "Homology is
a mental concept obtained by comparison, which under all circumstances
retains its validity, whether the homology finds its explanation in
common descent or in the common laws that rule organic development" (p.
151, 1906, b). As A. Braun long ago pointed out, "It is not descent
which decides in matters of morphology, but, on the contrary, morphology
which has to decide as to the possibility of descent."[541]

Hertwig, in a word, reverts to the pre-evolutionary conception of
homology. "We see in homology," he writes, "only the expression of
regularities (_Gesetzmässigkeiten_) in the organisation of the animals
showing it, and we regard the question, how far this homology can be
explained by common descent and how far by other principles, as for the
present an open one, requiring for its solution investigations specially
directed towards its elucidation" (p. 179, 1906, b).

Holding, as he does, that no definite conclusions can be drawn from the
facts of comparative anatomy and embryology as to the probable lines of
descent of the animal kingdom, Hertwig accords very little value to
phylogenetic speculation. It is, he admits, quite probable that the
archetype of a class represents in a general sort of way the ancestral
form, but this does not, in his opinion, justify us in assuming that
such generalised types ever existed and gave origin to the present-day
forms. "It is not legitimate to picture to ourselves the ancestral forms
of the more highly organised animals in the guise of the lower animals
of the present day--and that is just what we do when we speak of
Proselachia, Proamphibia and Proreptilia" (p. 155, 1906, b).

He rejects on the same general grounds the evolutionary dogma of
monophyletic or almost monophyletic descent, and admits with Kölliker,
von Baer, Wigand, Naegeli and others that evolution may quite well have
started many times and from many different primordial cells.

There is indeed a great similarity between the views developed by O.
Hertwig and those held by the older critics of Darwinism--von Baer,
Kölliker, Wigand, E. von Hartmann and others. It is true the
philosophical standpoint is on the whole different, for while many of
that older generation were vitalists Hertwig belongs to the mechanistic
school.

But both Hertwig and the older school agree in pointing out the _petitio
principii_ involved in the assumption that the archetype represents the
ancestral form; both reject the simplicist conception of a monophyletic
evolution (which may be likened to the "one animal" idea of the
transcendentalists); both admit the possibility that evolution has taken
place along many separate and parallel lines, and explain the
correspondences shown by these separate lines by the similarity of the
intrinsic laws of evolution; finally, both emphasise the fact that we
know nothing of the actual course of evolution save the few indications
that are furnished by palæontology, and both insist upon the unique
importance of the palæontological evidence.[542]

It was a curious but very typical characteristic of evolutionary
morphology that its devotees paid very little attention to the positive
evidence accumulated by the palæontologists,[543] but shut themselves up
in their tower of ivory and went on with their work of constructing
ideal genealogies. It was perhaps fortunate for their peace of mind that
they knew little of the advances made by palæontology, for the evidence
acquired through the study of fossil remains was distinctly unfavourable
to the pretty schemes they evolved.

As Neumayr, Zittel, Depéret, Steinmann and others have pointed out, the
palæontological record gives remarkably little support to the ideal
genealogies worked out by morphologists. There is, for instance, a
striking absence of transition forms between the great classificatory
groups. A few types are known which go a little way towards bridging
over the gaps--the famous _Archæopteryx_, for example--but these do not
always represent the actual phylogenetic links. There is an almost
complete absence of the archetypal ancestral forms which are postulated
by evolutionary morphology. Amphibia do not demonstrably evolve from an
archetypal Proamphibian, nor do mammals derive from a single generalised
Promammalian type. Few of the hypothetical ancestral types imagined by
Haeckel have ever been found as fossils. The great classificatory groups
are almost as distinct in early fossiliferous strata as they are at the
present day. As Depéret says in his admirable book,[544] in the course of
a presentation of the matured views of the great Karl von Zittel, "We
cannot forget that there exist a vast number of organisms which are not
connected by any intermediate links, and that the relations between the
great divisions of the animal and vegetable kingdoms are much less close
than the theory [of evolution] demands. Even the Archæopteryx, the
discovery of which made so much stir and appeared to establish a genetic
relation between classes so distinct as Birds and Reptiles, fills up the
gap only imperfectly, and does not indicate the point of bifurcation of
these two classes. Intermediate links are lacking between Amphibia and
Reptiles. Mammals, too, occupy an isolated position, and no zoologist
can deny that they are clearly demarcated from other Vertebrates;
indeed, no fossil mammal is certainly known which comes nearer to the
lower Vertebrates than does Ornithorhynchus at the present day" (p.
115).

To take a parallel from the Invertebrata, B. B. Woodward,[545] after
discussing the phylogeny of the Mollusca as worked out by the
morphologists and comparing it with the probable actual course of the
evolution of the group, as evidenced by fossil shells, sums up as
follows:--"The lacunæ in our knowledge of the interrelationships of the
members of the various families and orders of Mollusca are slight
however, compared with the blank caused by the total absence from
palæontological history of any hint of passage forms between the classes
themselves, or between the Mollusca and their nearest allies. Nor is
this hiatus confined to the Molluscan phylum; it is the same for all
branches of the animal kingdom. There is circumstantial evidence that
transitional forms must have existed, but of actual proof none whatever.
All the classes of Mollusca appear fully fledged, as it were. No form
has as yet been discovered of which it could be said that it in any way
approached the hypothecated prorhipidoglossate mollusc, still less one
linking all the classes" (p. 79).

Pointing in the same direction as the absence of transitional forms is
the undeniable fact that all the great groups of animals appear with all
their typical characters at a very early geological epoch. Thus, in the
Silurian age a very rich fauna has already developed, and
representatives are found of all the main Invertebrate groups--sponges,
corals, hydroid colonies, five types of Echinoderms, Bryozoa,
Brachiopods, Worms, many types of Mollusca and Arthropoda. Of
Vertebrates, at least two types of fish are present--Ganoids and
Elasmobranchs. In the very earliest fossiliferous rocks of all, the
Precambrian formation, there are remains of Molluscs, Trilobites and
Gigantostraca, similar to those which flourished in Cambrian and
Silurian times.

The contributions of palæontology to the solution of the problems of
descent posed by morphology are, however, not all of this negative
character. The law of recapitulation is in some well-controlled cases
triumphantly vindicated by palæontology. Thus Hyatt and others found
that in Ammonites the first formed coils of the shell often reproduce
the characters belonging to types known to be ancestral, and what is
more they have demonstrated the actual occurrence of the phenomenon
known as acceleration or tachygenesis, often postulated by speculative
morphologists.[546] This is the tendency universally shown by embryos to
reproduce the characters of their ancestors at earlier and earlier
stages in their development.

The most valuable contribution made by palæontologists to morphology and
to the theory of evolution arose out of the careful and methodical study
of the actual succession of fossil forms as exemplified in limited but
richly represented groups. Classical examples were the researches of
Hilgendorf[547] on the evolution of _Planorbis multiformis_ in the
lacustrine deposits of Steinheim, those of Waagen[548] on the phylogeny of
_Ammonites subradiatus_, and the work of Neumayr and Paul[549] on
_Paludina_ (_Vivipara_).

These investigations demonstrated that it was possible to follow out
step by step in superjacent strata the actual evolution of fossil
species and to establish the actual "phyletic series."

To take an example from among the Vertebrates, Depéret has shown (_loc.
cit._, pp. 184-9), that the European Proboscidea, belonging to the three
different types of the Elephants, Mastodons and Dinotheria, have evolved
since the Oligocene epoch along five distinct but continuous lines. The
Dinotherian stock is represented at the beginning of the Miocene by the
relatively small form _D. cuvieri_; this changes progressively
throughout Miocene times into _D. laevius_, _D. giganteum_, and _D.
gigantissimum_. Among the Mastodons two quite distinct phyletic series
can be distinguished, the first commencing with _Palæomastodon
beadnelli_ of the Oligocene, and evolving between the Miocene and
Pliocene into _Mastodon arvernensis_, after traversing the forms _M.
angustidens_ and _M. longirostris_, the second starting with the _M.
turicensis_ of the Lower Miocene and evolving through _M. borsoni_ into
the _M. americanus_ of the Quaternary. The phyletic series of the true
elephants in Europe are relatively short, and go back only to the
Quaternary, _Elephas antiquus_ giving origin to the Indian elephant, _E.
priscus_ to the African.

The careful study of phyletic series brought to light the significant
fact that these lines of filiation tend to run for long stretches of
time parallel to, and distinct from one another, without connecting
forms. This is clearly exemplified in the case of the Proboscidea, and
many other examples could be quoted. Almost all rich genera are
polyphyletic in the sense that their component species evolve along
separate and parallel lines of descent.[550] "Such great genera as the
genus _Hoplites_ among the Ammonites, the genus _Cerithium_ among the
Gastropoda, the genus _Pecten_ or the genus _Trigonia_ among the
Lamellibranchs, each comprise perhaps more than twenty independent
phyletic series" (Depéret, p. 200).

Variation along the phyletic lines is gradual[551] and determinate, and
appears to obey definite laws. The earliest members of a phyletic series
are usually small in size and undifferentiated in structure, while the
later members show a progressive increase in size and complexity. Rapid
extinction often supervenes soon after the line has reached the maximum
of its differentiation.

The general picture which palæontology gives us of the evolution of the
animal kingdom is accordingly that of an immense number of phyletic
lines which evolve parallel to one another, and without coalescing,
throughout longer or shorter periods of geological times. "Each of these
lines culminates sooner or later in mutations of great size and highly
specialised characters, which become extinct and leave no descendants.
When one line disappears by extinction it hands the torch, so to speak,
to another line which has hitherto evolved more slowly, and this line in
its turn traverses the phases of maturity and old age which lead it
inevitably to its doom. The species and genera of the present day belong
to lines that have not reached the senile phase; but it may be surmised
that some of them, _e.g._ elephants, whales, and ostriches, are
approaching this final phase of their existence" (Depéret, p. 249).

It is one of the paradoxes of biological history that the
palæontologists have always laid more stress upon the functional side of
living things than the morphologists, and have, as a consequence, shown
much more sympathy for the Lamarckian theory of evolution. The American
palæontologists in particular--Cope, Hyatt, Ryder, Dall, Packard,
Osborn--have worked out a complete neo-Lamarckian theory based upon the
fossil record.

The functional point of view was well to the fore in the works of those
great palæontologists, L. Rütimeyer (1825-1895) and V. O. Kowalevsky
(1842-83), who seem to have carried on the splendid tradition of Cuvier.
Speaking of Kowalevsky's classical memoir, _Versuch einer natürlichen
Classification der fossilen Hufthiere_, Osborn[552] writes:--"This work is
a model union of the detailed study of form and function with theory and
the working hypothesis. It regards the fossil not as a petrified
skeleton, but as having belonged to a moving and feeding animal; every
joint and facet has a meaning, each cusp a certain significance. Rising
to the philosophy of the matter, it brings the mechanical perfection and
adaptiveness of different types into relation with environment, with
changes of herbage, with the introduction of grass. In this survey of
competition it speculates upon the causes of the rise, spread, and
extinction of each animal group. In other words, the fossil quadrupeds
are treated _biologically_--so far as is possible in the obscurity of
the past" (p. 8). The same high praise might with justice be accorded to
the work of Cope on the functional evolution of the various types of
limb-skeleton in Vertebrates, and on the evolution of the teeth as well
as to the work of other American palæontologists, including Osborn
himself.

Osborn's law of "adaptive radiation," which links on to Darwin's law of
divergence,[553] constitutes a brilliant vindication of the functional
point of view. "According to this law each isolated region, if large and
sufficiently varied in its topography, soil, climate, and vegetation,
will give rise to a diversified mammalian fauna. From primitive central
types branches will spring off in all directions, with teeth and
prehensile organs modified to take advantage of every possible
opportunity of securing food, and in adaptation of the body, limbs and
feet to habitats of every kind, as shown in the diagram [on p. 363]. The
larger the region and the more diverse the conditions, the greater the
variety of mammals which will result.

"The most primitive mammals were probably small insectivorous or
omnivorous forms, therefore with simple, short-crowned teeth, of
slow-moving, ambulatory, terrestrial, or arboreal habit, and with short
feet provided with claws. In seeking food and avoiding enemies in
different habitats the limbs and feet radiate in four diverse
directions; they either become _fossorial_ or adapted to digging habits,
_natatorial_ or adapted to _amphibious_ and finally to _aquatic_
habits, _cursorial_ or adapted to swift-moving, terrestrial progression,
_arboreal_ or adapted to tree life. Tree life leads, as its final stage,
into

                        LIMBS AND FEET.
                                                        Volant.
                                                          /
            Fossorial.                               Arboreal.
                \                                       /
        Short-limbed, plantigrade,      }       Ambulatory
          pentadactyl, unguiculate      }           or
          Stem.                         }       Terrestrial.
                /                                       \
          Natatorial.                                 Cursorial
         Amphibious.                                 Digitigrade.
             /                                             \
         Aquatic                                       Unguligrade.


                                  TEETH.
                                Omnivorous.
                                                              { Grass.
                        { Fish.     |                         { Herb.
Carnivorous             { Flesh.    |   Herbivorous           { Shrub.
            \           { Carrion.  |        /                { Fruit.
             \                      |       /                 { Root.
              \                     |      /
               \                    |     /   Myrmecophagous.
                \                   |    /        / Dentition reduced.
                 \                  |   /        /
                  \                 |  /        /
                   \                | /        /
                    \               |/        /
                        Stem: Insectivorous.


the parachute types of the flying squirrels and phalangers, or into the
true flying types of the bats.... Similarly in the case of the teeth,
insectivorous and omnivorous types appear to be more central and ancient
than either the exclusively carnivorous or herbivorous types. Thus the
extremes of carnivorous adaptation, as in the case of the cats, of
omnivorous adaptation, as in the case of the bears, of herbivorous
adaptation, as in the case of the horses, or myrmecophagous adaptation,
as in the case of the anteaters, are all secondary" (_loc. cit._, pp.
23-4).

We have now reached the end of our historical survey of the problems of
form. What the future course of morphology will be no one can say. But
one may hazard the opinion that the present century will see a return to
a simpler and more humble attitude towards the great and unsolved
problems of animal form. Dogmatic materialism and dogmatic theories of
evolution have in the past tended to blind us to the complexity and
mysteriousness of vital phenomena. We need to look at living things with
new eyes and a truer sympathy. We shall then see them as active, living,
passionate beings like ourselves, and we shall seek in our morphology to
interpret as far as may be their form in terms of their activity.

This is what Aristotle tried to do, and a succession of master-minds
after him. We shall do well to get all the help from them we can.

    [519] See E. B. Wilson's masterly book, _The Cell in
    Development and Inheritance_, New York and London, 1900.

    [520] _Q.J.M.S._, xxvi. 1886.

    [521] _Wood's Holl Biological Lectures_ for 1893.

    [522] _Arch. f. Ent.-Mech._, i., pp. 380-90, 1895.

    [523] _Beiträge zur Kritik der Darwinschen Lehre_,
    Leipzig, 1898.

    [524] See E. B. Wilson, "The Embryological Criterion of
    Homology," _Wood's Holl Biological Lectures_, Boston,
    pp. 101-24, 1895; Braem, _Biol. Centrblt._, xv., 1895;
    T. H. Morgan, _Arch. f. Ent.-Mech._, xviii.; J. W.
    Jenkinson, _Mem. Manchester Lit. Phil. Soc._, 1906, and
    _Vertebrate Embryology_, Oxford, 1913; A. Sedgwick,
    article "Embryology" in _Ency. Brit._, p. 318, vol. xi.,
    11th Ed. (1910).

    [525] For a detailed treatment of this important point see
    the remarkable volume of E. Schulz (Petrograd),
    _Prinzipien der rationellen vergleichenden Embryologie_,
    Leipzig, 1910.

    [526] "La Poecilogonie," _Bull. Sci. France et Belgique_,
    xxxix., pp. 153-87, 1905.

    [527] _Un problème de l'évolution. La loi biogénétique
    fondamentale_, Paris and Montpellier, 1908.

    [528] _Vergleichung des Entwickelungsgrades der Organe zu
    verschiedenen Entwickelungszeiten bei Wirbeltieren_,
    Jena, 1891.

    [529] Quoted by Keibel, _Ergebn. Anat. Entwick._, vii., p.
    741.

    [530] "Studien zur Entwickelungsgeschichte des Schweines,"
    Schwalbe's _Morphol. Arbeiten_, iii., 1893, and v.,
    1895.

      _Normentafeln zur Entwickelungsgeschichte des
      Schweines_, Jena, 1897.

      "Das biogenetische Grundgesetz und die Cenogenese,"
      _Ergebn. Anat. Entw._, vii., pp. 722-92, 1897.

      "U. d. Entwickelungsgrad der Organe," _Handb. vergl.
      exper. Entwick. der Wirbelthiere_, iii., 3, pp. 131-48,
      1906.

    [531] "Beiträge zur Embryologie der Wiederkäuer," _Arch.
    Anat. Entw._, 1889.

    [532] "Die individ. Variation d. Wirbeltierembryo,"
    _Morph. Arbeit._, v., 1895.

    [533] "U. Variabilität u. Wachstum d. embryonalen
    Körpers," _Morph. Jahrb._, xxiv., 1896.

    [534] "Gastrulation u. Keimblätterbildung der _Emys
    lutaria taurica_," _Morph. Arbeit._, i., 1891.
    "Kainogenese," _Morph. Arbeit._, vii., pp. 1-156, 1897,
    and also separately. _Biomechanik, erschlossen aus dem
    Prinzipe der Organogenese_, Jena, 1898.

    [535] This law was foreshadowed by Reichert in 1837, when he
    wrote:--"We notice in our investigation of embryos of different
    animal forms that it is those organs, those systems, which in the
    fully developed individual are peculiarly perfect, that in their
    earliest rudiments and also throughout the whole course of their
    development appear with the most striking distinctness" (Müller's
    _Archiv_, p. 135, 1837). See also his _Entwick. Kopf. nackt.
    Amphib._, p. 198, 1838. So, too, Rathke notes how the elongated
    shape of the snake appears even in very early embryonic stages
    (_Entwick. Natter._, p. 111, 1839).

    [536] Quoted by Keibel (p. 790, 1897) from the
    _Biomechanik_.

    [537] _Die Zelle und die Gewebe_, Jena, 1898, and the
    subsequent editions of this text-book, published under
    the title of _Allgemeine Biologie. Die Entwickelung der
    Biologie im neunzehnten Jahrhundert_, Jena, 1900, 2nd
    ed., 1908. "Ueber die Stellung der vergl.
    Entwickelungslehre zur vergl. Anatomie, zur Systematik
    und Descendenztheorie," _Handb. vergl. exper.
    Entwickelungslehre der Wirbeltiere_, iii., 3, pp.
    149-80, Jena, 1906. (1906, b). Also in Pt. I. of Vol. I.
    (1906, a).

    [538] _An Essay on Classification_, London, 1859.

    [539] _Unsere Körperform_, Leipzig, 1874.

    [540] _Q.J.M.S._, xxxvi., pp. 35-52, 1894.

    [541] Quoted by Hertwig. See also K. Goebel, "Die
    Grundprobleme der heutigen Pflanzenmorphologie," _Biol.
    Centrbl._, xxv., pp. 65-83, 1905.

    [542] This is also emphasised by Fleischmann in his critical study of
    evolutionary morphology entitled _Die Descendenztheorie_, Leipzig,
    1901.

    [543] The same remark applies to the bulk of speculation as to the
    factors of evolution, with the exception of the contributions made
    to evolution theory by the palæontologists by profession, such as
    Cope.

    [544] _Les Transformations du Monde animal_, Paris, 1907.

    [545] "Malacology _versus_ Palæoconchology," _Proc.
    Malacological Soc._, viii., pp. 66-83, 1908.

    [546] Particularly by E. Perrier, "La Tachygenèse," _Ann.
    Sci. nat._ (_Zool._) (8), xvi., 1903.

    [547] _Monatsber. k. Akad. Wiss._, Berlin, pp. 474-504,
    1866.

    [548] _Geognost. u. Palæont. Beiträge_, ii., Heft 2, pp.
    181-256, 1869.


    [549] _Abhand. k.k. Geol. Reichsanstalt_, vii., Wien,
    1875.

    [550] The case for polyphyletism is very strongly put by
    G. Steinmann in his book, _Die geologischen Grundlagen
    der Abstammungslehre_, Leipzig, 1908.

    [551] The steps in this chronological variation were
    termed by Waagen "mutations."

    [552] _The Age of Mammals in Europe, Asia, and North
    America_, New York, 1910.

    [553] _Origin of Species_, 6th ed., Chap. IV.



INDEX


ACTINOZOAN THEORY of Vertebrate Descent, 299-300

Adaptation as Conservative Principle--
  Cuvier, 39, 76

Adaptation, Ecological--
  Von Baer, 123
  H. Milne-Edwards, 199
  Lamarck, 221, 222, 223, 224, 227
  Treviranus, 225 f.n.
  C. Darwin, 231-2, 235, 239
  Haeckel, 248, 263
  Gegenbaur, 263
  V. O. Kowalevsky, 362
  Osborn, 362-4

Adaptation, Ecological, and Classification--
  Bronn, 203

Adaptation of Parts. _See_ "Correlation, Functional," and "Conditions of
  Existence"

Adaptive Radiation (Osborn), 362-4

Agassiz, A., 288 f.n., 295
  On Coelom, 296

Agassiz, L.--
  Criticism of Vertebral Theory of Skull, 157
  Membrane and Cartilage Bones, 164
  Transcendentalism, 203
  Classification, 203 f.n.
  Three-fold Parallelism, 230, 255
  Influence on Darwin, 238
  Specific Distinctness of Embryos, 353

Albertus Magnus, 17

Alcmæon, 1

Aldrovandus, 18

Allman, 209

Analogy. _See also_ Homology.
  Aristotle, 8-10
  Owen, 108
  Haeckel, 251
  Gegenbaur, 266
  Lankester, 267

Anaxagoras, 14

Anaximander, 14

Anaximenes, 1

Animal and Vegetative Lives--
  Aristotle, 16, 32
  Buffon, 26-7
  Bergson, 26 f.n.
  Cuvier, 26, 32
  Bichat, 27-9
  Oken, 94
  K. G. Carus, 94
  Von Baer, 116, 123, 131
  Remak (Sensory and trophic layers), 210
  Gegenbaur, 263

Annelid Theory of Vertebrate Descent, 274-85, 301

Archetype, Anatomical, 246, 302-3
  E. Geoffroy, 54, 67
  Owen, 104-7, 110
  J. V. Carus, Huxley, 204
  C. Darwin, 238 f.n.

Archetype, Anatomical, as Ancestral--
  C. Darwin, 235, 247
  Haeckel, 251
  Gegenbaur, 265
  Sedgwick, 300
  Criticism of this idea--
    O. Hertwig, 355-7

Archetype, Embryological, 168, 246, 302-3
  Von Baer, 126, 132
  Reichert, 139, 147, 149
  Rathke, 151, 153
  Huxley, 159-61

Archetype, Embryological, as Ancestral--
  C. Darwin, 233, 236-7
  Haeckel, 254, 289-91
  Gegenbaur, 266
  O. and R. Hertwig, 298
  Sedgwick, 300
  A. Kowalevsky, 300

Arendt, 162

Aristotle, 2-16, 17, 345, 364
  _Historia Animalium_, 2
  _De Partibus Animalium_, 2, 9
  Knowledge of Animals, 3, 4
  Comparative Embryology, 4
  Classification of Animals, 4-6
  Unity of Plan, 6-7, 10
  Homology and Analogy, 7-10
  Teleology and Correlation, 10-12
  Law of Compensation, 11
  Division of Labour, 12
  Degrees of Composition--homogeneous and heterogeneous parts, 12-14, 169
  Law of Development (Von Baer), 14
  Scale of Beings, 14-16
  Functional attitude, 15-16, 197
  Animal and Vegetative Lives, 16, 32

Ascidian Theory of Vertebrate Descent, 269-73, 304

Atomists, 16

Atomists, "Biological," 192-4

Audouin, V.--
  Unity of plan in Arthropods, 85-6
  Law of Compensation, 86
  Marine Zoology, 195

Autenrieth, 90, 96

Avicenna, 17


BABÁK, E., 333

Baer, K. E. von, 113-32, 133, 251, 304, 345, 356
  Founder of Embryology, 113
  _Entwickelungsgeschichte der Thiere_, 114
  Regulation of Development, 114, 350
  Development as Differentiation, 115, 128
  Germ-Layer Theory, 115-6, 118-119, 208-9, 296
  Morphological Differentiation, 116-7
  Histological Differentiation, 117-8
  Tissues and Germ-Layers, 118
  Double symmetrical Development, 118, 279
  Criticism of Meckel-Serres Law, 120-3, 304
  Theory of Types, 123-4, 289, 291
  Law of Development, 124-6
  Embryological Criterion, 126-8, 132, 138
  Embryological Archetype, 126, 132
  Types of Development, 127-8
  Von Baer and Cuvier, 128-30
  Functional attitude, 129
  Relation to Transcendentalists, 129, 131
  Criticism of Scale of Beings, 130
  Vertebral Theory of Skull, 131, 142
  Serial Homology, 131-2
  Gill-slits, Gill-arches and Aortic arches, 135-6, 146
  Membrane and Cartilage Bones, 162-3
  Degrees of Composition, 172
  Ova of Mammals, 175-6
  Segmentation of Ovum, 186
  Criticism of Evolution Theory, 229, 242
  Influence on Darwin, 236, 238
  Criticism of Darwinism, 242
  Teleology and Correlation, 242
  On Ascidians, 271

Baer's Law. _See_ "Development, Von Baer's Law"

Bagge, 187

_Balanoglossus_ Theory of Vertebrate Descent, 285-7

Balbiani, 330

Balfour, F. M., 247, 299
  Annelid Theory, 282-4
  Gastrulation and Gastræa Theory, 295
  Mesoderm, 296 f.n.
  Coelom, 297

Barfurth, D., 330

Barry, M., 186, 188

Bateson, W.--
  Metamerism, Vegetative Repetition, 286
  _Balanoglossus_ Theory, 286-7
  On Phylogenetic Speculation, 302

Beard, J., 285

Belon, 18

Beneden van, and Julin, 271, 285, 346

Bensley, A. B., 311 f.n.

Bergmann, 187

Bergson, H., 26 f.n., 341, 345

Bernard, Claude, 195, 314

Bert, P., 315

Bichat, X., 27-30, 118, 132, 169, 178, 263
  Animal and Vegetative Lives, 27-9
  "General Anatomy," 29-30
  _Vie propre_ of Tissues, 30

Biogenetic Law. _See_"Development, Haeckel's Law"

Bischoff, 138
  Segmentation, 186, 188

Blainville, de, 96, 128, 141, 199 f.n.

Bojanus, 96, 97

Bonnet, C.--
  Scale of Beings, 22-3, 220, 227
  Evolution, 215
  Regeneration, 315

Bonnet, R., 350

Bonnier, G., on Albertus Magnus, 17

Born, G., 330

Boveri, T., 270 f.n., 333

Braem, 347 f.n.

Braun, A., 355

Breschet, 138, 173

Bronn, H. G., 200-3, 248
  _Naturphilosophie_, 201
  Functional attitude, 201-3
  Geometry of Organism, 201, 249
  Theory of Types, 202
  Principle of Connections, 202
  Intrinsic Laws of Evolution, 202
  Division of Labour, 202
  Ecological  Adaptation and Classification, 203

Brown, R., 171

Bruch, C., 203 f.n.

Büchner, 194, 248

Buffon, 24-7, 336
  Scale of Beings, 24, 215
  Unity of Plan, 24
  Evolution, 24-5, 214
  Classification, 25-6
  Animal and Vegetative Lives, 26-7
  Homology and Analogy, 27

Burckhardt, R., 3 f.n., 268 f.n.

Burdin, 96

Burmeister, 249 f.n.

Butler, S., 226 f.n., 313, 335-42
  Relation to Lamarck, 335-7
  Psychological Vitalism, 336-41
  Heredity and Memory, 337-41
  The Two Stages of Development, 337-9
  Consciousness and Habit, 337-9
  Recapitulation Theory, 339-40
  Teleology, 341


CABANIS, 215

Camper, P., 45, 46

Carter, 293 f.n.

Carus, J. V.--
  Criticism of Embryological Criterion, 167
  Morphology and  Physiology, 194
  Vertebral Theory of Skull, 203
  On Archetype, 204
  Evolution, 230

Carus, K.G.--
  Law of Parallelism, 94, 249
  Vertebral Theory, 96
  Geometry of Skeleton, 98-100
  Splanchnoskeleton, 98, 140

Causal Morphology, 312-3, 315-34

Cell-Theory--
  Schwann, 169, 173-86, 188
  C. F. Wolff, 170
  Schleiden, 170-2
  Criticism of Schwann-Schleiden Theory, 185-8
    Virchow, Leydig, 188

Cell-Theory and Germ-Layer Theory--
  Remak, 209-12

Cell-Theory as Disintegrative--
  Schwann, 180-5, 248
  Vogt, 190-1
  Virchow, 191
  Haeckel, 248
  Criticism of this idea--
    Reichert, 192-3, 194
    J. V. Carus, 194
    Sedgwick, Whitman, 346

Cell-Theory, Influence on Morphology, 190

Cenogenesis, 258-9, 323

Chabry, 331

Child, C. M., 333

Chun, C, 317, 332

Classification of Animals--
  Aristotle, 4-6
  Rondeletius, Aldrovandus, Gesner, 18
  Linnæus, 22
  Buffon, 25-6
  Cuvier, 39-41
  E. Geoffroy, 60
  L. Agassiz, 203 f.n.
  Lamarck, 216-7, 227, 228

Classification and Ecological Adaptation (Bronn), 203

Classification as Genealogical--
  Buffon, 24-5
  Lamarck, 218, 228
  C. Darwin, 233, 234, 247
  Haeckel, 250-1, 254
  Criticism of this idea, 303, 304,
    O. Hertwig, 356

Classification, Phylogenetic--
  Haeckel's, 289-94

Claus, 259

Co-adaptation, 326 f.n.

Coelom--
  Remak, 211
  A. Kowalevsky, 270, 295, 297
  Haeckel, 291, 295, 296
  Lankester, 291, 297

Coelom, Theory of, 295-301

Cohen, 189

Coiter, 18

Colucci, 346

Compensation, Law of--
  Aristotle, 11
  Goethe, 49
  E. Geoffroy, 72-3
  Audouin, 86.
  German Transcendentalists, 100

Condillac, 215

Conditions of Existence, Principle of--
  Cuvier, 34, 75-6, 239
  Gegenbaur, 263-4
  Roux, 324, 326
  Spencer, Weismann, 326 f.n.
  Disregard for--
    Lamarck, 226
    C. Darwin, 232, 238-41
    Haeckel, 248, 264

Conklin, 333

Connections, Principle of--
  Goethe, 47
  E. Geoffroy, 53-4, 62-3, 71, 74, 261
  Audouin, 85

Connections, Principle of--_contd._
  German Transcendentalists, 100
  J. F. Meckel, 101
  Owen, 107-8
  Bronn, 202
  C. Darwin, 234-5
  Gegenbaur, 261
  Semper, 279
  In Embryology, 168
  Main Principle of Morphology, 246, 302

Convergence--
  Milne-Edwards, 199
  I. Geoffroy St Hilaire, 199 f.n., 206
  C. Darwin, 236
  Friedmann, Willey, Vialleton, 306 f.n.

Convergence, Rejected by Evolutionary Morphologists, 305, 312
  Hubrecht, 305-6

Cope, E. D., 342, 357 f.n., 361, 362

Correlation, Functional--
  Aristotle, 10-12
  Cuvier, 35-8, 239, 241
  E. Geoffroy, 77
  Von Hartmann, 240-1
  Rádl, 240 f.n., 241
  Von Baer, 242
  Gegenbaur, 264
  Disregarded by--
    C. Darwin, 235, 238-41
    Haeckel, 248, 264

Coste, 134, 138, 176, 187

Crampton, 332

Cunningham, J. T., 284

Cuvier, 26, 31-44, 89, 196, 197, 199 f.n., 278, 345, 361
  Functional attitude, 31-6, 65, 75-8, 200, 305
  Animal and Vegetative Lives, 32
  Degrees of Composition, 32-3
  Teleology, 33-5
  Functional Adaptedness, 33-5, 324
  Principle of Conditions of Existence, 34, 75-6, 239
  Correlation, 35-8, 239, 241
  Metabolism, 38
  Adaptation as Conservative Principle, 39, 76
  Classification, 39-41
  Principle of Subordination of Characters, 40
  Criticism of Scale of Beings, 39-40, 130
  Type Theory, 41, 124, 289, 291
  Criticism of Evolution-Theory, 41-4, 129, 304
  Variation, Limits of, 42
  Palæontological Succession, 43
  Polemic with Geoffroy, 64-5, 74-8
  Criticism of Vertebral Theory of Skull, 97-8
  Influence on J. F. Meckel, 101
  Criticism of Meckel-Serres Law, 129-30, 304
  As Embryologist, 130
  Criticism of Lamarck, 228

Cytology, 346

Cytoplasm of Egg, Organ-forming Stuffs, 332-3


DALL, 361

D'Alton, 113

Dareste, C., 315

Darwin, Charles, 78, 230-41, 271, 304, 307, 336, 362
  Systematist and Field Naturalist, 230, 231
  Palæontological Succession, 231
  Ecological Adaptation, 231-2, 235, 239
  Species Problem, 231
  Functional Adaptation, Disregard for, 232, 238-41
  Classification as genealogical, 233, 234, 247
  Unity of Plan due to Community of Descent, 233, 234-5, 239, 247
  Embryological Archetype as ancestral, 233, 236-7
  Rejects Meckel-Serres Law, 233, 236
  Interpretation  of  Vestigial Organs, 233, 237
  Organism as Historical Being, 233, 308
  Rejects Scale of Beings, 234
  Homology, 234-5, 247
  Principle of Connections, 234-5
  Anatomical Archetype as ancestral, 235, 247
  Von Baer's Law interpreted phylogenetically, 236-7
  Modifications inherited at corresponding age, 237
  Monophyletism and Polyphyletism, 238
  Causes of Success, 238, 241

Darwin, Erasmus, 214, 226 f.n., 229, 336

Darwin, Sir Francis, 344

Daubenton, 26

Degrees of Composition--
  Aristotle, 12-14, 169
  Glisson, 19
  Malpighi, 20
  Bichat, 29-30
  Cuvier, 32-3,
  Dujardin, 169, 188
  Von Baer, 172
  Effect of Invention of Microscope, 20
  Relation to Cell-Theory, 169

Delage, 333

Delage and Hérouard, 273 f.n.

Delpino, 345

Demaillet, 44

Democritus, 16

Depéret, C, 357
  On Cuvier, 43
  Absence of intermediary forms in Palæontology, 358
  Phyletic series and Polyphyletism, 360-1

Development, Von Baer's Law--
  Aristotle, 14
  Von Baer, 124-6
  Prévost and Dumas, 125 f.n.
  Reichert, 149-50, 351 f.n.
  Milne-Edwards, 205-8
  Lereboullet, 206-8
  Criticised by--
    Agassiz, 352-3
    His, 353
    Sedgwick, 353
    O. Hertwig, 354
  Phylogenetic Interpretation of--
    Darwin, 236-7
    Gegenbaur, 266
  Relation to Haeckel's Law, 254, 256, 257

Development, Biogenetic Law (Haeckel)--
  Haeckel, 251, 253-9, 291-4
  F. Müller, 252-3, 254, 257
  Gegenbaur, 262
  Roux, 319
  Butler, 339-40
  Orr, 342
  Criticism of--
    Vialleton, 348
    Oppel, 348-9
    Keibel, 349-50
    Mehnert, 350-2
    O. Hertwig, 352, 354-5
    His, 353
  Relation to Laws of Meckel-Serres and Von Baer, 254, 256, 257, 303, 309
  Relation to Heredity and Development, 312-3
  Influence of Causal Morphology, 347-8
  Palæontological Evidence for, 359

Development, Meckel-Serres Law--
  Harvey, 18
  Hunter, 22
  E. Geoffroy, 69-70, 72
  Serres, 80-3, 94, 203-4, 205-6
  Kielmeyer, Autenrieth, Oken, 90

Development, Meckel-Serres Law-_contd._
  Tiedemann, 91
  J. F. Meckel, 91-3
  K. G. Carus, 94
  Criticism of--
    Von Baer, 120-3, 304
    Cuvier, 129-30, 304
    Milne-Edwards, 205
    Lereboullet, 206-8
    C. Darwin, 233, 236
  Analogy with Biogenetic Law, 254-7, 262, 303, 304, 309

Development, Meckel-Serres Law, Theory of Three-fold Parallelism--
  L. Agassiz, 230, 255
  Tiedemann, Vogt, 255 f.n.
  Haeckel, 254-5

Development, The two periods of--
  Roux, 320-4, 325, 327, 335
  Butler, 337-9

Diogenes of Apollonia, 1

Disintegration. _See_ "Cell-Theory," and "Materialistic Attitude"

Division of Labour, Principle of--
  Aristotle, 12
  Milne-Edwards, 197-8
  Bronn, 202
  Gegenbaur, 264

Dohrn, A., 269, 274-8
  Annelid Theory of Vertebrate Descent, 274-7, 303
  Principle of Function-Change, 276-8, 307
  Functional Attitude, 277-8, 307
  Formal Attitude, 306

Döllinger, I., 113, 157

Dollo, 311

Donné, 173

D'Orbigny, 43

Driesch, H., 242, 331, 332, 333, 334, 345, 346-7

Dugès, A., 86-8, 100, 134, 142, 146
  Unity of Plan, 87
  Polyzoic conception of Organism, 87-8
  Membrane and Cartilage Bones, 163

Dujardin, 169, 188

Dumas. _See_ Prévost and Dumas

Duméril, 96

Dumortier, 173

Dutrochet, 99 f.n., 130, 134

Duverney, 19


EAR-OSSICLES, Homology of--
  E. Geoffroy, 56
  Spix, 100
  Rathke, 141, 150
  Reichert, 144-7

_Échelle des êtres. See_ "Scale of Beings."

Ehlers, 284

Eisig, H., 284, 285

Embryology, Comparative, Early Workers--
  Aristotle, 4, 113
  Fabricius, Harvey, 18, 113
  Malpighi, 20, 113
  Oken and Kieser, 90, 113
  Haller, C. F. Wolff, J. F. Meckel, Tiedemann, 113

Embryology, Experimental, 317, 318, 330-3

Embryological Archetype. _See_ "Archetype, Embryological"

Embryological Criterion of Homology, 133-168, 347
  Goethe, 49
  E. Geoffroy, 72, 110
  Cuvier, 75, 110, 130
  Owen, 110-1
  Von Baer, 126-8, 132, 138
  Rathke, 138, 140-1
  J. Müller, 138
  Reichert, 138-9, 144-7, 163
  Vogt, 156-7
  Huxley, 158-9, 166
  Kölliker, 165-6
  Criticised by--
    Owen, J. V. Carus, 167

Empedocles, 1, 15

Engramm (Semon), 343

_Entwicklungsgesetz._ _See_ "Evolution, Intrinsic Laws of"

_Entwicklungsmechanik_, 315

Erasistratus, 17

Evolution Theory--
  Lucretius, 16
  Buffon, 24-5, 214
  Cuvier's criticism, 41-4, 129, 304
  E. Geoffroy, 66-9, 73, 228
  J. F. Meckel, 92-3, 215, 228
  Leibniz, 213
  Kant, 213-4
  Erasmus Darwin, 214, 229
  C. Bonnet, Oken, Robinet, Treviranus, 215
  Tiedemann, 215, 255 f.n.
  Lamarck, 215-29
  Von Baer, 229, 242
  I. Geoffroy St Hilaire, J. V. Carus, 230
  Charles Darwin, 230-41
  Von Hartmann, 240-1, 244, 356
  Kölliker, 243
  Owen, 244
  Milne-Edwards, 244-5
  Haeckel, 250-9
  Gegenbaur, 265
  The Organism as an Historical Being, 308-13
    C. Darwin, 233, 308
    Haeckel, 252, 257
    Sedgwick, 308
    Roux, 313, 322-4
    Butler, 313, 336-41

Evolution-Theory, Influence on Morphology, 302-13

Evolution, Intrinsic Laws of, 241
  J. F. Meckel, 93
  Bronn, 202
  Von Baer, 229, 242, 356
  Kölliker, Naegeei, 243, 356
  Owen, 244
  Von Hartmann, 244, 356
  Milne-Edwards, 244-5
  O. Hertwig, 354-5, 356-7
  Wigand, 356
  Depéret, 361


FABRICIUS, 18, 113

Fallopius, 18

Fischel, 346, 350

Fischer, 328

Fleischmann, 357 f.n.

Flourens, 46, 315

Fontana, 172

Forbes, E., 196

Formal Attitude, 246, 305
  Goethe, 49
  E. Geoffroy, 62-3, 71, 75-8, 305
  Haeckel, 249, 257, 260
  Gegenbaur, 261, 263
  Semper, 279
  Adopted by Evolutionary Morphologists, 302-8, 311-2, 314
  Hubrecht, 305-6
  Dohrn, 306

Francé, R., 345

Friedmann, 306 f.n.

Fuld, 333

Functional Adaptation, 316-7, 318, 320-9, 333, 344, 351

Functional Attitude--
  Aristotle, 15-6, 197
  Bichat, 27-9
  Cuvier, 31-6, 65, 75-8, 200, 305
  Goethe, 49-50
  J. F. Meckel, 101
  Owen, 109, 110, 111
  Von Baer, 129
  Milne-Edwards, 195, 197-200
  J. Müller, Reichert, 200
  Bronn, 201-3
  Lamarck, 222-6, 307, 335
  Gegenbaur, 260, 263-4
  Dohrn, 277-8, 307
  Roux, 320-9, 335
  Houssay, 333
  Butler, 336-41
  G. Wolff, 346
  Driesch, 346-7
  Giard, 347
  E. Schulz, 347 f.n.
  Keibel, 349-50
  Mehnert, 350-1
  American Palæontologists, 361, 362
  Rütimeyer, 361
  V. O. Kowalevsky, 361-2
  Osborn, 362-4

Function-Change, Principle of--
  Dohrn, 276-8, 306, 307
  Eisig, 284

Fürbringer, M., 282 f.n., 284, 323 f.n.


GALEN, 17

Gastræa Theory, 269, 288-95, 298, 299-3O1, 303

Gastrula, Discovery of, 288

Gaupp, E., 310 f.n.

Gegenbaur, C, 247, 260-7, 271, 285, 286, 288 f.n.
  Division of Egg-nucleus, 188
  Functional Attitude, 260, 263-4
  Formal Attitude, 261, 263
  Principle of Connections, 261
  Embryology and Comparative Anatomy, 261-2, 263
  Biogenetic and Meckel-Serres Laws, 262
  Homology, 261, 263, 265, 266-7
  Adaptation and Correlation, 263-4
  Archetype as ancestral, 263 f.n, 265
  On Phylogenetic Speculation, 265-6
  Embryological Archetype, 266
  Membrane and Cartilage Bones, 309, 310

Gemmill, J. F., 285 f.n., 312 f.n.

Geoffroy, Etienne, St Hilaire, 40, 52-78, 141
  Unity of Plan, 52-65, 70 ff., as conservative, 75, 78
  Principle of Connections, 53-4, 62-3, 71, 74, 261
  Unity of Composition, 54, 70-1, 75-6, 200, 305
  Archetype, 54, 67
  Metastasis, 55-6, 59, 74
  Opercular Bones, 56
  Unity of Composition of Sternum, 57-60
  Classification, 60
  Vertebrates and Articulates, 60-4, 274, 278-9, 303
  Formal Attitude, 62-3, 65, 71, 75-8, 305
  Cephalopods and Vertebrates, 64-5
  Scale of Beings, 64
  Polemic with Cuvier, 64-5, 74-8
  Evolution, 66-9, 73, 228
  Biogenetic Law, 69
  Teratology, 69, 315
  Meckel-Serres Law, 70, 72
  Criteria of Homology, 71, 72, 110
  Law of Compensation, 72-3
  Criticism of his Principles, 74
  Relation to German Transcendentalists, 89, 100-1
  Vertebral Theory of Skull, 96, 97
  Influence on Darwin, 234-5, 238

Geoffroy, Isidore, St Hilaire, 65 f.n., 199 f.n., 230

Geometry of the Organism, 33
  K. G. Carus, 98-100, 249
  Bronn, 201, 249
  Haeckel, J. Müller, Burmeister, G. Jäger, 249

Germinal Vesicle (Egg-nucleus), 175-7, 188, 291 f.n.

Germ-Layer Theory--
  Von Baer, 115-6, 118-9, 208-9, 296
  Pander, 119-20, 209
  C. F. Wolff, 119-20
  Rathke, 136, 208
  Lereboullet, Bischoff, 208
  Huxley, 208, 289
  Remak, 209-12, 296

Germ-Layers and Gastræa Theory--
  Haeckel, 289-95
  Lankester, Balfour, 295

Germ-Layer Theory, Influence of Causal Morphology on, 347

Gesner, 18

Giard, A.--
  On Ascidian Theory, 271-3
  Adaptive Homology, 273
  Poecilogeny, 347-8

Glisson, F., 19

Gluge, 173

Goebel, K., 356 f.n.

Goethe, 45-51, 65, 89, 250
  Unity of Plan, 45-7, 51
  Homology, 47
  Principle of Connections, 47
  Formal and Functional Attitudes, 48-50
  Teleology, 48
  Metamorphosis of Plants, 48
  Repetition of parts, 48-9
  Vertebral Theory of Skull, 49, 96, 97
  Law of Compensation, 49
  Embryological Criterion, 49
  Organisms as Nature's Works of Art, 50

Goette, 259

Graaf, von, 175

Grew, N., 169

Gruber, 330

HAECKEL, Ernst, 247-60, 271, 314, 342, 353, 357
  His sources, 248-50
  Materialism, 248, 250
  On Teleology, Heredity and Adaptation, 248, 263
  Correlation, Disregard for, 248, 264
  Geometry of the Organism (Promorphology), 249
  Repetition of Parts (Tectology), 249-50
  Classification as Genealogical, 250-1, 254
  Archetype as ancestral, 251
  Homology and Analogy, 251
  Biogenetic  Law, 251, 253-9, 291-4
  Three-fold parallelism, 254-5
  Scale of Beings, 255, 256-7
  Organism as an Historical Being, 257
  Prussianism, 257
  Palingenesis, 258
  Cenogenesis, 258-9
  Heterotopy, Heterochrony, 259
  Gastræa Theory, 269, 288-95
  Phylogenetic Classification, 289-94
  Criticism of Theory of Types, Monophyletism, 289, 291
  Gastræa Theory and Biogenetic Law, 291-4
  Primary stages of Ontogeny and Phylogeny, 291-3
  Coelom, 291, 295, 296
  Experimental Embryology, 317

Haller, 113

Harting, 284 f.n.

Hartmann, E. von--
  On Darwin's conception of correlation, 240-1
  Evolution, 244, 356

Hartog, M., 344

Harvey, 18, 113

Hatschek, 270 f.n., 299

Helmholtz, H. von, 195

Henle, 172

Hensen, V., 209 f.n.

Herbst, C., 333

Herder, 46

Heredity and Memory, 336-44

Hering, E., 341-2

"Heritage" Characters, 309, 322

Herlitzka, 332

Herophilus, 17

Hertwig, O., 163, 330, 331, 346
  On C. F. Wolff, 119
  Fertilisation, 291 f.n.
  Membrane and Cartilage Bones, 309-10
  Biogenetic Law, 352, 354-5
  Von Baer's Law, 354
  Intrinsic Laws of Evolution, 354-5, 356-7
  Homology not necessarily Homogeny, 355-7
  Unity of Plan not necessarily due to Community of Descent, 355-7
  On Phylogenetic Speculation, 356

Hertwig, O. and R.--
  Coelom Theory, 297-8
  Nervous System of Coelentera, 299

Heterochrony, 259, 348, 349-52

Heterogeneous Generation (Kölliker), 243

Heterotopy, 259

Hilgendorf, 359

Hill, 311

Hippocratic Treatises, 2

His, W., 206 f.n., 209 f.n.
  Causal Morphology, 316
  Cytoplasm of Egg, Organ-forming Stuffs, 333
  Specific Distinctness of Embryos, 353

Histological Differentiation (von Baer), 117-8

Histology. _See also_ "Cell-Theory"
  Malpighi, 20
  Stensen, 21
  Bichat, 29-30, 169, 178
  Von Baer, 117-8
  Schwann, 178
  Remak, 209-12

Hofer, B., 330

Hofmeister, 185

Homogeny, 267, 303, 355

Homology, 168, 303, 355-7. _See also_ "Connections, Principle of," and
   "Embryological Criterion"
  Aristotle, 7-10
  Belon, 18
  Buffon, 27
  Goethe, 47
  E. Geoffroy, 53, 71
  Serres, 80
  Owen, 107-9
  Lamarck, 227
  C. Darwin, 234-5, 247
  Haeckel, 251
  Gegenbaur, 261, 263, 265, 266-7
  Giard, 273
  Semper, 279
  O. Hertwig, 355-7
  Braun, 355

Homology, Genetic Definition of--
  Gegenbaur, 266
  Lankester, 267
  O. Hertwig's criticism, 355-7

Homoplasy, 267

Hooke, R., 20, 169

Houssay, F., 19 f.n., 333

Hubrecht, A. A. W., 284, 295 f.n., 301, 305-6

Hunter, J., 22, 315

Huschke, 134-5, 136, 141, 146

Huxley, T. H., 157, 238, 247
  On Rathke, 154 f.n.
  Embryological Criterion, 158-9, 166
  Embryological Archetype, 159-61
  Criticism of Vertebral Theory of Skull, 161-2
  Membrane and Cartilage Bones, 166-7
  On Archetype, 204
  Germ-Layer Theory, 208, 289
  Criticism of Three-fold Parallelism, 230 f.n.
  Coelom, 297
  Ancestry of Marsupials, 311

Hyatt, A., 359, 361


INSTINCT and Morphogenesis, Analogy of, vi., 307, 312
  Lamarck, 220, 226

JACOBSON , 164

Jäger, G., 249 f.n.

_Jardin des Plantes_, Paris, 19

Jenkinson, J. W., 347 f.n.
  On His, 316

Jones, Wharton, 138, 176

Julin, C., 271, 285

Jussieu, de, 40


KANT, I.--
  Teleology, 35, 213, 242
  Unity of Plan, 46, 213-4
  Evolution, 213-4

Keibel, F., 348, 349-50

Kerkring, 131

Kielmeyer, 89, 90, 96

Kieser, 90

Kleinenberg, N., 277

Kohlbrugge, J., 44 f.n., 65 f.n.

Kölliker, A.--
  On C. F. Wolff, 119
  Vertebral Theory of Skull, 157
  Membrane and Cartilage Bones, 164-6, 310
  Embryological Criterion, 165-6
  Cell-division, 187
  Intrinsic Laws of Evolution, 243, 356
  Saltatory Variation, 243

Kowalevsky, A., 269-71, 284, 285, 299, 300
  Development of Amphioxus, 270
    Ascidians, 270-1
  Coelom, 270, 295, 297
  Gastrula, 288

Kowalevsky, V. O., 361-2

Krause, 176

Kupffer, 271


LACAZE-DUTHIERS, H. de, 203 f.n., 315-6
  On Ascidians, 271, 273

Lamarck, 44, 66, 78, 215-29
  Relation to Buffon, 215
  Scale of Beings, 215-8, 220-1, 227-8
  As Evolutionary, 218, 220
  Classification, 216-7, 227, 228
  Species Problem, 216, 227
  Materialism, 218-9, 222-3, 225-6
  Psychological Vitalism, 219, 220-6, 307, 335
  _Sentiment intérieur_, 219-20, 222-3, 225
  Ecological Adaptation, 221, 222, 223, 224, 227
  Laws of Evolution, 221-5
  Transmission of Acquired Characters, 221-2, 224
  Subtle Fluids, 222
  Use and Disuse, 223-4
  Independence of Current Thought, 226-7
  Homology and Analogy, 227
  Reception of his Theory, 228-9
  Lamarck and Butler, 335-7

Lang, A., 301

Lankester, Sir E. Ray, 247
  Homology, Homogeny, Homoplasy, and Analogy, 267
  _Balanoglossus_ Theory of Vertebrate Descent, 287
  Germ-Layer Theory and Phylogenetic Classification, 291
  Planula Theory, 295
  On Coelom Theory, 296-7, 299 f.n.

Latreille, 86, 100

Laurencet, 64

Lavocat, 203 f.n.

Leeuenhoek, 20, 21, 169

Leibniz, 23, 213, 343

Lereboullet--
  Von Baer's Law, 206-8
  Germ-layer Theory, 208
  Gastrula, 288 f.n.

Leucippus, 16

Leuckart, 193 f.n., 194, 297

Levy, O., 333

Leydig, 187, 188, 275 f.n., 285

Linnæus, 22

Loeb, J., 333, 347

_Loi de Balancement_. _See_ "Compensation, Law of"

Lovén, 186, 196

Lucretius, 16
  On the Soul, 222 f.n.

Ludwig, 193, 194, 314

Lyell, Sir C., 228 f.n.

Lyonnet, 22


MACBRIDE, E. W., 287 f.n.

M'Kendrick, J.--
  On Fontana, 172

Mackenzie, W., 345

Malpighi, M., 20-1, 113, 169

Marine Zoology, Rise of, 195-6

Materialistic Attitude, 246-7, 345, 364
  Schwann, 180-5
  Vogt, 190-1
  Virchow, 191
  Ludwig, 193
  Materialistic Physiology, 193-4, 314-5, 347
  Lamarck, 218-9, 222-3, 225-6
  The Darwinians, 241, 308
  Haeckel, 248, 250
  Roux, 315, 317, 318-9, 329
  Semon, 343
  Rignano, 344
  Loeb, 347
  Criticism of this attitude--
    Reichert, 192-3

Meckel, D. A., 95

Meckel, J. F., 113
  Meckel-Serres Law, 91-3
  Evolution, 92-3, 215, 228
  Teratology, 93-4
  Repetition of Parts, 95
  Vertebral Theory of Skull, 96
  Eclecticism, 101

Meckel's Cartilage, 141, 145

Meckel-Serres Law. _See_ "Development, Meckel-Serres Law"

Mehnert, E., 348, 350-2

Membrane and Cartilage Bones, 162-7, 309-10

Memory and Heredity, 336-44

Mendelism, 346

Mesenchyme, 298

Mesoderm, 209-11, 296, 297, 298

Metabolism--
  Cuvier, 38
  Schwann, 182-5
  Roux, 324, 329

Metamerism, 94, 95, 100, 109, 131-2, 266-7, 274-5, 279, 282, 286, 299, 301

Metamorphosis of Plants, 48, 235

Metastasis, Principle of--
  E. Geoffroy, 55-6, 59, 74
  Owen, 106

Metschnikoff, E., 278 f.n., 285, 288
  Criticism of Ascidian Theory, 271
  Coelom, 295, 296, 297

Meyen, 170, 185

Meyer, E., 284

Meyranx, 64

Microscope, Invention of, 19

Milne-Edwards, H., 12, 86, 238
  Marine Zoology, 195
  Functional Attitude, 195, 197-200
  Unity of Plan, 197
  Division of Labour, 197-8
  Ecological Adaptation, Convergence, 199
  Von Baer's Law, Polemic with Serres, 204-8
  Evolution, 244-5

Mirbel, 170, 171

Mivart, St G., 277

Mohl, von, 170, 185

Moldenhawer, 170

Moleschott, 194

Moquin-Tandon, A., 87

Morgan, T. H., 317 f.n., 332, 333, 347 f.n.

Mosaic Theory of Development, 330-3

Müller, F., Biogenetic Law, 252-3, 254, 257

Müller, H., 166

Müller, J., 136, 209 f.n., 260, 285, 309, 345
  Embryological Criterion, 138
  Vertebral Theory of Skull, 142-4, 154, 157
  On Reichert, 150
  Cell Theory, 172-3
  Division of Egg-nucleus, 188
  Vitalism, 192
  Marine Zoology, 196
  Functional Attitude, 200

Mutations (Waagen), 361 f.n.


NAEGELI, 185, 243 f.n., 356

_Naturphilosophie._ _See_ "Philosophy of Nature"

Nesbitt, R., 162

Neumayr, 357, 360

Nussbaum, M., 330


OKEN, L., 89, 113, 131, 134, 149
  Meckel-Serres Law, 90-1
  Teratology, 91
  Repetition of Parts, 94-5
  Serial Homology, 95-6, 100
  Vertebral Theory, 96, 97, 98
  On Geoffroy, 100
  Influence on Serres, 205
  Evolution, 215

Ollier, 315

Oppel, A., 318 f.n., 324 f.n., 327, 348-9

Orr, H. F., 342

Osborn, H. F., 214 f.n., 361
  On V. O. Kowalevsky, 362
  Functional Attitude, 362-4
  Law of Adaptive Radiation, 362-4

Owen, R., 97, 102-12, 204
  Eclecticism, 102
  Vertebral Theory of Skeleton, 103-7
  Archetype of Vertebrate Skeleton, 104-7, 110
  Vertebral Theory of Skull, 104-6
  Metastasis, 106
  Principle of Connections, 107-8
  Anatomy and Embryology, 108
  Homology and Analogy, 108
  Classes of Homology, 108-9, 266
  Functional Attitude, 109, 110, 111
  Embryological Criterion, 110, 167
  Homological and Teleological Compoundedness, 110-1
  Vegetative Repetition of Parts, 111, 286
  Unity of Plan as Conservative Principle, 112
  Influence on Darwin, 234, 235, 238
  Evolution, 244


PACKARD, 361

Palæontological Record, 357-61
  Absence of connecting forms, 357-9
  Biogenetic Law, 359
  Phyletic Series, 359-61

Palæontological Succession--
  Cuvier, 43
  E. Geoffroy, 67
  L. Agassiz, 230, 255
  C. Darwin, 231
  Milne-Edwards, 245
  Tiedemann, 255 f.n.

Paley, W., 341

Palingenesis (Haeckel), 258, 323

Pander, 113, 119-20, 133, 208, 209

Parallelism, Theory of. _See_ "Development, Meckel-Serres Law"
  Three-fold. _See_ "Development, Meckel-Serres Law"

Paris Museum of Natural History, 19, 89, 101

Paul, 360

Pauly, A., 345

Perrault, C., 19

Perrier, E., 88, 359 f.n.

Pflüger, E., 317, 330

Philipeaux, 315

"Philosophy of Nature," 89, 94, 98, 203, 248

Phyletic Series, 359-61

Physiology, Separation from Morphology, 194, 247, 260, 314

Physiology of Development, 315

Planula Theory (Lankester), 295

Plato, 15

Pockels, 138

Poecilogeny (Giard), 347-8

Poli, 175

Polyphyletism--
  Darwin, 238
  Von Baer, 242, 356
  Kölliker, Wigand, Naegeli, 356
  Depéret, 360-1
  Steinmann, 360 f.n.

Polyzoic Conception of Organism--
  Dugès, 87
  Perrier, 88

Prévost and Dumas, 125 f.n., 134, 175, 186

Promorphology (Haeckel), 249

Protoplasm, 169, 188-9

Purkinje, 172, 173, 175, 176, 189


QUATREFAGES, A. de, 172, 195-6


RÁDL, E., on Goethe, 48
  Correlation, 240 f.n., 241
  On Darwin's Critics, 242 f.n.
  On Cuvier's Critics, 278 f.n.

Rathke, H., 133, 136-7, 174, 194, 269, 351 f.n.
  Discovery of Gill-slits in Pig and Chick, 134
  Discovery of Gill-slits in Man, 135
  Germ-Layer Theory, 136, 208
  Embryological Criterion, 138, 140-1
  Homologies of Gill-arches, 139-41, 146, 150
  Development of Skull, 141, 150-4
  Vertebral Theory of Skull, 141, 154-6
  Embryological Archetype, 151, 153
  Membrane and Cartilage Bones, 163, 166

Rauber, A., 330

Réaumur, 22, 315

Recapitulation Theory. _See_ "Development, Biogenetic Law"

Regeneration, 315, 318, 333, 346

Regulatory Processes in Development, 114, 319, 333, 346-7, 350

Reichert, C. B., Embryological Criterion, 138-9, 144-7, 163
  Archetype, 139, 147, 149
  Homologies of Gill-arches and Ear-ossicles, 144-7
  Vertebral Theory of Skull, 147-9, 157
  Von Baer's Law, 149-50, 351 f.n.
  Membrane and Cartilage Bones, 163, 165, 166, 310
  Criticism of "Biological Atomists," 192-3, 194
  Functional Attitude, 193, 200

Remak, R., 118, 288 f.n.
  On Vertebræ, 157
  Cell Theory, 173, 187-8, 209
  Microscopical Technique, 209 f.n.
  Germ-Layer Theory, 209-12, 296
  Cells, Tissues and Germ-Layers, 209-12
  Mesoderm, 209-11
  Coelom, 211, 296

Repetition of Parts within the Organism, Theory of. _See also_
   "Vertebral Theory of Skull"
  Goethe, 48-9
  Dugès, 87-8
  Oken, 94-5
  J. F. Meckel, D. A. Meckel, 95
  Haeckel (Tectology), 249-50

Reymond, E. du Bois, 194, 314

Rignano, E., 343-4

Robinet, 23, 215

Rondeletius, 18

Rosenhof, Rösel von, 22

Roux, W., 313, 315-29, 344, 351
  _Entwicklungsmechanik_, 315, 317-8
  Materialistic Attitude, 315, 317, 318-9, 329
  Functional Adaptation, 316-7, 318, 320-9, 333
  Experimental Embryology, 317, 318, 330-1
  Simple and Complex Components, 318-20
  Functional Definition of Life, 320
  Functional Attitude, 320-9, 335
  The Two Periods of Development, 320-4, 325, 327, 335
  Mosaic Theory of Development, 323, 330-1
  Metabolism, 324, 329
  Structure, Functional and Non-functional, 324-6
  Functional Unity of Organism, 326
  Functional Adaptation of Blood-vessels, 326-9
  Form as manifestation of Activity, 329

Ruini, C., 18

Rusconi, 133-4, 186

Rütimeyer, L., 361

Ryder, 361


SACHS, J. von, 170

St Ange, M., 146

Salensky, 259

Saltatory Variation--
  E. Geoffroy, 78
  Von Baer, 242
  Kölliker, 243
  Owen, 244

Sarcode, 169

Sars, M., 186, 196

Savigny, J. C., 83-5, 100, 137, 271

Scale of Beings, 89, 206, 214-5
  Aristotle, 14-6
  Anaximander, Anaxagoras, 14
  Empedocles, Plato, 15
  Albertus Magnus, 17
  C. Bonnet, 22-3
  Robinet, 23
  Buffon, 24
  E. Geoffroy, 64
  Lamarck, 215-8, 220-1, 227-8
    As Evolutionary, 218, 220
  Haeckel, 256-7
  Criticism of this idea--
    Cuvier, 39-40, 130
    Von Baer, 130
    Milne-Edwards, 205
    Lereboullet, 207
    Darwin, 234
    Haeckel, 255
    Relation to Evolution-Theory, 214-5

Schepelmann, 333

Schleiden, 170-2

Schmieden, 328

Schults, C. H., 173

Schultze, Max, 189

Schultze, O., 331

Schulz, E., 347 f.n.

Schwann, Theodor, 169, 173-86, 248
  Physiological Standpoint, 173, 179, 180, 182
  Development of Cells, 174-5, 179-80
  Cellular Nature of Ovum, 175-7
  Development of Tissues from Cells, 177-8
  Histology, 178
  Materialism and Teleology, 180-3, 185
  Cell-metabolism, 182-5
  Cells as organic Crystals, 184-5

Sedgwick, A., 347 f.n.
  Actinozoan Theory of Vertebrate Descent, 299-300
  Metamerism, 299
  Embryological Archetype, 300
  Organism as Historical Being, 308
  Cell-Theory, 346
  Von Baer's Law, 353

Segmentation of Ovum, 186-8

Seiler, 138

Selection, Natural and Artificial, 307 f.n.

Self-Differentiation (Roux), 319, 320-1, 322, 323, 324, 327

Self-Regulation (Roux), 319

Semon, R., 342-3

Semper, C., 259, 269, 278-82, 284, 286
  Annelid Theory, 274, 278-82
  Metamerism, 274, 279, 282
  Follower of Geoffroy, 278
  Unity of Plan and Composition, 279, 303
  Principle of Connections, 279
  Formal Attitude, 279

_Sentiment intérieur_ (Lamarck), 219-20, 222-3, 225

Serial Homology. _See_ "Metamerism"

Serres, E., 79-83, 91, 100, 205-6, 257 f.n.
  Criteria of Homology, 80
  Law of parallelism, 80-3, 94, 203-4, 205-6
  Law of Multiple Formation, 80-1
  Unity of Plan, 83, 205, 206
  Teratology, 83
  Meckel's Cartilage, 145 f.n.
  Transcendentalism, 205-6
  Concrescence Theory, 206 f.n.

Severino, 18

Sharpey, 162, 176

Siebold, von, 186

Skull, Development of, 139-62.
  _See also_ "Vertebral Theory"

Spallanzani, 315

Species-Problem--
  Cuvier, 42
  Lamarck, 216, 227
  Darwin, 231

Spencer, H., 326 f.n.

Spengel, 285, 287

Spinoza, 343

Spix, 96, 97, 100, 141

Stannius, 165

Steenstrup, 309

Steinmann, G., 357, 360 f.n.

Stensen (Steno), 21

Swammerdam, 20, 21-2


TACHYGENESIS, 359

Technique, Microscopical, 209 f.n., 268

Tectology (Haeckel), 249

Teleology--
  Aristotle, 10
  Cuvier, 33-5
  Kant, 35, 213, 242
  Von Baer, 242
  Owen, Von Hartmann, 244
  Butler, 341
  G. Wolff, Driesch, 346
  Criticism of--
    Goethe, 48
    Schwann, 180-2
    The Darwinians, 241
    Haeckel, 248
    Evolutionary Morphologists, 308

Teratology, 69, 83, 91, 93, 315

Thienemann, 23 f.n.

Thompson, D'Arcy W., 2 f.n.

Thomson, A., 176

Thomson, J. Arthur, 215 f.n.

Tiedemann, 91, 113, 215, 255 f.n.

Tissues and Germ-Layers, 118, 209-12

Transcendental Anatomy, Relation to Evolutionary Morphology, 302-8, 312

Transcendentalism, French and German Schools, 89, 100

Trembley, 22, 315

Treviranus, 141, 170, 215, 225 f.n.

Turpin, 173

Types, Theory of (Cuvier and Von Baer)--
  Cuvier, 41, 124, 289, 291
  Von Baer, 123-4, 289, 291
  Bronn, 202
  Lereboullet, 207

Types, Theory of (Cuvier and Von Baer)--_contd._
  Criticised by--
    E. Geoffroy, 60
    Haeckel, 289, 291
    Lankester, 291

Type-Theory and Evolution, 304


UNGER, 185

Unity of Composition, Principle of, Geoffroy, 54, 70-2, 75-6, 200, 305

Unity of Plan, 88, 241, 278-9, 303, 312. _See also_ "Archetype"
  Aristotle, 6-7, 10
  Belon, Severino, 18
  Perrault, 19
  Robinet, 23
  Buffon, 24
  Cuvier, 41
  Goethe, 45-7, 51
  Vicq D'Azyr, 45
  Camper, 45, 46
  Herder, 46
  Kant, 46, 213-4
  E. Geoffroy, 52-65, 70 ff.
  Serres, 83, 205, 206
  Savigny, 83
  Audouin, 85-6
  Latreille, 86
  Dugès, 86-7
  J. F. Meckel, 101
  Milne-Edwards, 197
  Semper, 279
  Haeckel, 289, 291
  Lankester, 291

Unity of Plan as due to Community of Descent--
  Darwin, 233, 234-5, 239, 247
  Haeckel, 250-1
  Gegenbaur, 263 f.n., 265
  Criticism of this idea--
    O. Hertwig, 355-7

Unity of Plan as Conservative Principle--
  E. Geoffroy, 75, 78
  Owen, 112
  Gegenbaur, 263-4
  Evolutionary Morphologists, 307


VALENTIN, 138, 173, 176

Variation, Limits of, Cuvier, 42

Vegetative Repetition of Parts--
  Owen, 111, 286
  Bateson, 286

Velpeau, 138

Vertebral Theory of Skull, 49, 96-9, 104-6, 131, 141-4, 147-9, 154-7,
  161-2, 165, 203, 235, 310 f.n.

Vertebrate Descent, 269-87, 299-301, 304

Verworn, M., 330

Vesalius, 18

Vestigial Organs, 233, 237, 309, 312

Vialleton, L., 306 f.n., 348

Vicq d'Azyr, 45, 95

Virchow, R., 188, 191

Vitalism, Psychological--
  Lamarck, 219, 220-6, 307, 335
  Butler, 336-41
  Orr, Cope, 342
  Ward, 343
  Delpino, Francé, Pauly, A. Wagner, Mackenzie, 345

Vogt, C.--
  Criticism of Vertebral Theory, 156-7
  Capillaries, 179
  Segmentation, 186
  Materialistic Attitude, 190-1
  Threefold Parallelism, 255 f.n.


WAAGEN, 359, 361 f.n.

Wagner, A., 345

Wagner, R., 176

Ward, J., 343

Weber, 138

Weismann, A., 240, 323, 326 f.n., 330-1, 343

Werneck, 173

Whitman, C. O., 346

Wigand, A., 242 f.n., 356

Willey, A., 273 f.n., 306 f.n.

Williamson, 309

Willis, 19

Wilson, E. B., 331, 332-3, 346 f.n., 347 f.n.

Wolff, C. F., 113
  Germ-layer Theory, 119-20
  Cells, 170

Wolff, G., 346-7

Woodward, B. B., 358

Wotton, E., 17


ZELENY, 333

Zittel, K. von, 357, 358

Zoja, 331

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