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FORM AND FUNCTION

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

./// /•;</// Is nsirvetl

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. Geoffrey 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 ipth 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

vi PREFACT.

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 prob-
ably 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
earl\- 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.
Gcmmill 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
figure's.

E. S. R.

Cmisr.A,

CONTENTS

CHAP. PAGE

I. THE BEGINNINGS OF COMPARATIVE ANATOMY . . i

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

FIO. 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 VERTEBRAE AND VISCERAL ARCHES IN EMBRYO

OF PIG. (REICHERT.) ..... 148

u. 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

IX

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

1 E. Zeller, Greek Philosophy, Eng. trans., i., 522 f.n., London iSSr.
Other particulars as to Alcmaeon in T. Gomperz, Greek Thinkers, Eng.
trans., i., London, 1901.

- Zeller, loc. cit., i., p. 297. 3 Gomperz, loc. cit., i., p. 244.

i' THE BEGINNINGS OF COMPARATIVE ANATOMY

In the collected writings of Hippocrates and his school,
the ( 'in-pus Hippocraticiun, 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."1

The author of the treatise " On the Joints," which Littre
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 r,.C.) 2 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 Ilistoria Anunalinni, the 7V Partibus Aniinalinin,
and the De General ionc Aniiiialiiun.

The Ilistoria Aniinalinin 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
vi<-w 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 Anhnaliiun 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 7V Ptirtibns
Aniinalimn is a treatise on the causes of the form and

1 R. Burckhnnlt, Hiolfl^ic u. Hinnanisiints, p. S5, Jena, 1907.

'-' Sec tin- interesting account uf Aristotle's hiolo^ii ,il work in I'rof.
D'Arcy \V. Thompson's Herbert Spencer lecture (1913) and his transla-
tion of the Historic .\niin<ilin»i in the Oxford series.

ARISTOTLE 3

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 (Dc 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;1 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 EcJielle dcs etres.

(i) What surprises the modern reader of the Historia
Animalinin perhaps more than anything else is the extent
and variety of Aristotle's knowledge of animals. He

1 On Aristotle's forerunners, see R. Burckhardt, "Das koi'sche
Tiersystem, eine Vorstufe des zoologischen Systematik des Aristoteles."
Verh. Naturf. Ges. Basel, xx., 1904.

4 THE BEGINNINGS OF COMPARATIVE ANATOMY

describes more than 500 kinds.1 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 (l\i/i/uints}, 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 larvae. 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. At/ini., ii.), or his
description of the structure of cuttlefish (Hist. Auini., iv.), or
that touch in the description of the hermit crab (Hist. Ani>u.,
iv.) — "Two large eyes . . . not . . . turned on one side like
those of crabs, but straight forward" -without being con-
vinced 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 docs not seem to
have cared greatly about the minutiae of structure. Me 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.

•&1

(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

1 T. E. Lones, Aristotle's Researches in A<itur<il Science, pp. 8^-3,
London,

ARISTOTLE'S CLASSIFICATION 5

might, in addition to structural classifications, divide animals
into gregarious, solitary and social, or land animals into
troglodytes, surface-dwellers, and burrowers (Hist. Auiin., 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 " classifica-
tion, 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 forbesit] 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. Anint., 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 Acalephae, including sea-anemones and

G THE HFXilNMNCiS OF COMPARATIVE ANATOMY

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 Aniuialimn 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, mammae, 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 mammas. 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" (Cress-
well, 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).

UNITY OF FLAN 7

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. at., 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 posi-
tion which these parts occupy is the same in all animals
[sc. Sanguinea] " (Cresswell, loc. ct't., 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
(Honiarus}, 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. tit., 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. «'/., 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 mor-
phology 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

B

THK BEGINNINGS OK COMrAKATIVK. ANATOMY

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 (//is/. Aniin., 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" (Cressvvell, he. 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 /)<• l\irtihns Aiiiinalimn 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,

HOMOLOGY AND ANALOGY 9

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."1 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 small-
ness, softness and hardness, smoothness or roughness, and
the like (loc. cit., i., 4, 644b). 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, 645b, 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 com-
parative 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, 65 5a),
because they have the same function, though he says

1 De Partibus Animalium, i., 4, 644% trans. W. Ogle, Oxford, 1911.

10 TIIK BEGINNINGSOF COMPARATIVE ANATOMY

elsewhere that they are only analogous structures (ii.,
81 653b). I'1 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*). Aristotle took much more interest in
analogies, in organs of similar function, than in homologies.
He did recognise the existence of homologies, but rather
nidlgre Ini, 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 Partibns, iv.,
9, 684b). 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 Academic 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 Cephal-
opod 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 docs not require horns, and vice
. The correlation of a multiple stomach with deficient

CORRELATION 11

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 Partibns,
iii., 14, 674b.) 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 Partibns,
iii., 2, 664a, trans. Ogle). A similar kind of explanation is
offered of the fact that Selachia have cartilage instead of
bone, "in these Selathia 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*, trans. Ogle).
Speaking generally, " Nature invariably gives to one part
what she subtracts from another " (loc. cit., ii., 14, 658a).

This thought reappears again in the ipth century in
E. Geoffroy St Hilaire's lot 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, 663b).

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

12 Till: BEGINNINGS OF COMPARATIVE ANATOMY

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 (A1
Prtrtibns, ii., 12). The fact of the inverse correlation is
certain ; the explanation is, though very vague, probably
correct.

In one passage of the De Partibns 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, 683a).

(5) The first sentence of the Historia Aniinaliiun
formulates, with that simplicity and directness which is so
characteristic of Aristotlc.thc 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 arc 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" (( 'rcssvvell, loc. cit., p. i).

In the DC /\rr/il>/is . Inimaltum he broadens the conception
by adding another form of composition. "Now there arc,"
he says, "three degrees of composition; and of these the
first in <>r<lrr, as all will allow, is composition out of what
some call the elements, such as earth, air, water, fire. . . .

DEGREES OF COMPOSITION 13

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., I,
646a, trans. Ogle).

In the Historia Aniinalium the homogeneous parts are
divided into (i) the soft and moist (or fluid), such as blood,
serum, flesh, fat, suet, marrow, semen, gall, milk, phlegm,
faeces 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 Aniinalium 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 homo-
geneous 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, 647b). He sees clearly the difficulty of
calling vein or blood-vessel a simple part, for while a blood-
vessel 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 blood-
vessel. There is form superadded to homogeneity of structure
(ii., 2, 647b). Similarly for the heart and the other viscera.
" The heart, like the other viscera, is one of the homo-
geneous 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., i,
647^ 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 Aniinalium, iii.,

14 THE BEGINNINGS OF COMPARATIVE ANATOMY

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., i, 647a).

(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 hetero-
geneous parts and posterior to the elementary material
(De Parti bus, ii., i, 646^.

(7) We meet in Aristotle an idea which later acquired
considerable vogue, that of the liclicllc dcs circs (or " scale of
beings"), that organisms, or even all objects organic or in-
organic, 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
arc 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

THE SCALE OF BEINGS 15

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 transforma-
tion 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. Aniinaliiiin, v.) he reverses
this order. In the De Generatione (Book ii., r) 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,

16 THE BEGINNINGS OF COMPARATIVE ANATOMY

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. Aiiiin., 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 Partibns (ii., 10, 656a, 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 im-
passioned 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
1 7th and iSth centuries, and reaches its culmination in the
1 9th century. The evolutionary ideas of Lucretius exer-
cised no immediate influence upon the development of
morphology.

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 intro-
duction to Europe in the I3th 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."1

Aristotle's influence is predominant also in the wrork
of Edward Wotton (1492- 1555), who in his book De differentiis
aniinalium 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 1 6th century, two groups of men helped to lay
foundations for a future science of comparative anatomy—

1 Le Monde vegetal, p. 41, Paris, 1907.

37

18 COMPARATIVE ANATOMY BEFORE CUVIER

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
fur them simply the necessary preliminary of the discovery of
the functions of the parts — the}' 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.1 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.2

A stimulating contribution to comparative anatomy was
made by Belon,3 who published in 1555 a Histoirc dc la
nature dcs Oyseau.v, 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.
Goiter (1535-1600) studied the anatomy of Vertebrates,
discovering among other things the fibrous structure of the
brain. Garlo Ruini of Bologna wrote in 1598 a book on the
anatomy of the horse.4 Somewhat later Severino, professor
at Naples, dissected many animals and came to the conclusion

1 R.vercitationes de generatione animalium, 1651 For an account of
Harvey's work on generation and development, see Em. Radl's masterly
Geschichte der biologischen Theoricn, i., pp. 31-8, Leipzig, 1905.

The passage runs : — " Sic natura perfecta et divina nihil faciens
frustra, nee quipiam animal! cor addidit, ubi non erat opus, neque
priusquam esset ejus usus, fecit ; sed iisdem gradibus in formationc
cujuscumque animalis, transiens per omnium animalium constitutiones
(ut ita diram) ovum, vermem, f<ctum, perfectionem in singulis
acquirit."

' Sec I. C.eofifroy St Hilairc, E ssai's de /.oologie generate, p. 71, Paris,
1841.

M. Foster, Lectures on the History of Physiology, Cambridge, p. 53,
1901.

INVENTION OF THE MICROSCOPE 19

that they were built upon the same plan as man.1 Willis, of
Oxford and London, in his Cerebri A natoine (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 ! 2 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, particu-
larly by Claude Perrault and Duverney.3 In a volume
entitled De la Mecaniqne dcs Aniinanx, 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.4

The beginning of the i/th century saw the invention of
the microscope, which was to have such an enormous in-
fluence 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,5 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.

1 Zootomia democri/ea, Nuremberg, 1645 '•> Antiperipatias, sen de
respiratione piscium, Amsterdam, i66r. •' Radl, loc. «'/., i., p. 50.

' Perrault et Duverney, Me moires pour servir a thistoire des Animaitx,
Paris, 1699.

1 F. Houssay, Nature et Sciences naturelles, Paris, p. 76, n.d.
5 Foster, loc. a'/., p. 85.

20 COMPARATIVE ANATOMY BKFOKK (TYIKK

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 visccruin stnictnra
(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."1
So he wrote in the De I'isccniin structure, and accordingly he
studied the liver first in the snail, then in fishes, reptiles,
mammals, and finally man. In the introduction to his
Aiuiloiiic plantarntn (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 par-
ticular problems, yet I dared to pry into them in the higher
animals. But since llu-M- matters enveloped in peculiar
mystery still lie in obscurity, the}' require the comparison
of simpler conditions, and so the investigation of insects'-'

1 Trans, by Foster, loc. cit., p. 1 13.

- He made a careful studv of the silkworm.

APPLICATION OF THE MICROSCOPE 21

at once attracted me; finally, since this also has its own
difficulties I applied my mind to the study of plants, intend-
ing 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."1 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 com-
parative anatomy of the lower animals. Throughout the I7th
and 1 8th centuries the discovery of structure in the lower
animals went on continuously, as may be read in any history of
Zoology.2 We content ourselves here with mentioning only
some representative names.

In the i /th 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

1 " Etenim, fervent! 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 mi hi aperirem. Sed nee forte hoc
ipsum sufficiet cum simplicior Mineralium Elementorumque mundus
praeire debeat. At in immensum excrescit opus, et meis viribus omnino
impar," Opera Omm'a, i., p. i, London, 1686.

2 See particularly E. Radl, loc. cit., \ Teil. J. V. Cams, Geschichte
der Zoologie, Miinchen, 1872.

22 COMPARATIVE ANATOMY HKIXWK Cl'YIKR

homologises the stages of the frog's development with the
Ei;g, the Worm, and the Nymph of insects (I>ook 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 1 8th century, Reaumur 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 recapitula-
tion 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 1 \irts of the Animal (Economy, 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 Linnaeus (1707-1778).
The former tendency is well represented by Reaumur
(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 Rosel von
Rosenhof's Insektenbelustigungen ( 1 746- 1 76 1 ).

1!' iniiet (1720-1793) deserves special mention here, since
in his Traitc d'lusectologic (1745), and more full}7 in his
Contemplation dc la Xatiire (1764), he gives the most com-
plete expression to the idea of the /:>//<•//<• des ctres.

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 Traitc

THE SCALE OF BEINGS 23

d* Insectologie (CEuvres, i., 1779) he gives a long table, headed
'•' Idee d'une Echelle des etres naturels," and rather resem-
bling a ladder, on the rungs of which the following names
appear : —

MAN. Tube-worms. STONES.

Orang-utan. Clothes-moths. Figured stones.

Ape. INSECTS. Crystals.

QUADRUPEDS. Gall insects. SALTS.

Flying squirrel. Taenia. Vitriols.

Bat. Polyps. METALS.

Ostrich. Sea Nettles. HALF-METALS.

BIRDS. Sensitive plant. SULPHURS.

Aquatic birds. PLANTS.

Amphibious birds. Lichens. Bitumens.

Flying Fish. EARTHS.

FISH. Moulds. Pure earth.

Creeping fish. Fungi, Agarics. WATER.

Eels. Truffles. AIR

Water serpents. Corals, and Coralloids.

SERPENTS. Lithophytes. FIRE.

Slugs. Asbestos.

Snails. Talcs, Gypsums. More subtile matter.

SHELL FISH. Selenites, Slates.

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." 1

1 For a good historical account of the gradation theories see
Thienemann's paper in the Zoologische Annalen (Wurzburg) iii., pp. 185-
274, 1910, from which the quotation from Robinet is taken.

C

24 COMPARATIVE ANATOMY HKl'ORK (T'YIKR

The idea of a gradation of beings appears also in Bullmi
(1707-1788), but here it takes more definitely its true
character as a functional gradation.1 "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."

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."

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.1
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 com-
munity 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 "degenera-
tion," then one might reasonably suppose that from a
single being Nature could in time produce all the other
organised beings.0 Elsewhere, <\<,r., in the discourse DC /<i
Degeneration dcs Aninuinx^ Buffon expresses himself with
iin ire caution. He finds that it is possible to reduce the two
hundred species of quadrupeds which he has described to

1 Histoire nafurelle, i., p. 13 ; ii , p. y ; iv., p. 101 ; and xiv., pp.
28-y, I74y and later.

' No translation can render the beauty of the original — "Coinnie
tout se fait et que tout est par nuance dans la Nature . . ." (iv., p. 101).

:; Hist, nat., iv., p. 5.

1 Sec particularly his comparison of the skeleton of the horse with
that of man. Hist. Nat., iv., p. 381, also p. 13.

0 Loc. «'/., p. 382.

0 Tome xiv., pp. 31 1 -374.

BUFFON 25

quite a small number of families " from which it is not
impossible that all the rest are derived."1 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 Echelle dcs
ctres?

It is a mistake to suppose that Buffbn 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 revelation, que tous les animaux out egalement participe
a la grace de la creation," 3 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 founda-
tion 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 recog-
nise any systematic unit higher than the species. Strictly
speaking there are only individuals in Nature ; but there

1 Tome xiv., p. 358.

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

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

20 ro.Mi'AKATiyi; ANATOMY HKIXWI; CUVIMK

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,1 and this is the only discontinuity which
Nature shows us. Buffon put his views into practice in his
///.vAv'/r *\<i//<;r//i\ where he describes species after species,
never uniting them into larger groups. \\'e have seen,
however, how the facts forced upon him the conception of
the " family."

Buffon was no morphologist. lie 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 dcs Aniinanx'- a distinction, which Bichat and
Cuvier later developed with much profit, between the
"animal" and the "vegetative" part of animals.3 The
vegetative or organic functions go on continuously, even
in sleep, and arc 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."1 Animals may differ from one another
enormously in the external parts, particularly in the appen-
dicular skeleton, without showing any great difference in
the plan and arrangement of their internal organs.

1 iv., p. 3X5. - iv., pp. 3- 1 10

:1 It has been revived in our oun clays by Bcrgson, M attire et
Miinoirc, p. 57.
4 iv., pp. 7-15-

BICHAT 27

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 ex-
ternal 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 work-
ing 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.1 " I call animal life that order
of functions which connects us with surrounding bodies ;
signifying thereby that this order belongs only to animals"
(p. Ixxviii.). 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, respira-
tion, exhalation, absorption, secretion, nutrition, calorification,
or production of animal heat, compose organic life, whose
principal and central organ is the heart " (p. Ixxix.).

The contrast of the animal and the organic life runs
1 Anatomic Generate, Paris, 1801, Eng. trans. 1824.

2S COMPARATIVE ANATOMY BKFORK CUVIER

through all Bichat's work ; it receives classical expression in
his Rcchcrc/tcs Physiologiques snr la \'ic ct In Mori (iSoo).
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., 1.^05, 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 excre-
tion ; through these functions the animal incessantly trans-
forms into its own substance the molecules of surround-
ing 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

ami 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

ins of animal life and the organs of organic life is, in

man and the higher animals at least, the symmetry of

GENERA L A NATOM Y

29

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 (Joe. tit., 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 tex-
ture and function in distinguishing them rather than upon
microscopical structure,
follows : — :

The tissues he distinguished are as

1. The cellular membrane.

2. Nerves of animal life.
Nerves of organic life.

3-
4-

5-
6.

Arteries.

Veins.

Exhalants.

7. Absorbents and glands.

8. Bones.

9. Medulla.

10. Cartilage.

1 1. Fibrous tissue.

IS-
1 6.

12. Fibro-cartilage.

13. Muscles of organic life.

14. Muscles of animal life.
Mucous membrane.
Serous membrane.

17. Synovial membrane.

1 8. 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

1 Anatomic Gcncrale, Eng. trans., i., p. Hi.

30 COMPARATIVE ANATOMY BEFORE CUVIER

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." l The tissues have, too,
a great importance for pathology, for diseases are often
diseases of tissues rather than of organs.2

1 Anatomic Generate, Eng. trans., i., p. Iviii.

2 Loc at., 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,1 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.

1 Lemons d' Anatomic Coinparre, tome i., pp. \ocf scq., 1800.

31

CUVIER

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 a-lso 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
1 In (Ton, for he says that a plant is an animal that sleeps.1
But the idea is as old as Aristotle, who discusses the
"sleep" of embryos and of plants in the last book of
the f>c Gcneratiouc anininlinin. The distinction between
animal and -vegetative life is, of course, based for Aristotle
in the difference bet\vecn the \^i>x>? aurOijTiKq and the
\f'"X'] OpeTTTiK^. 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." 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.
Ari-totle 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 circum-
stance Cuvier owes his knowledge that the inorganic
ms d? Anatomic Cf>mp;i>;:,\ i., p. iS. -' I.oc. <//., i., p. 13.

FUNCTIONAL HARMONY 33

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

o

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 iQth 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 hetero-
geneous parts or organs is detailed and masterly.1

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 (rapprockts), 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
1 Lecons (f Anatomic Comparce, tome i., Articles iii.-iv., 1800.

34 CUVIER

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 deter-
mine the relations of their organs ; these laws are as inevit-
able 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." y

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."

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 ma}- 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

1 Lemons d1 Anatomic Compares, i., p. 47.
'-' I.e Rt^nc Animal, i., p. 6, 1817.

TELEOLOGY AND CORRELATION 35

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 dcr Urtheilskraft, which appeared ten years
before the publication of the Lccons d' Anatomic Comparee.
Teleology in Kant's sense is and will always be a necessa-ry
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." * 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 bring-
ing in its train corresponding modifications in the others.
Structure and function are bound up together ; every modifi-
cation 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 par-
ticular functions must have correspondingly modified organs,

1 Histoire des Progrcs des Sciences nalurelles depuis 1789, i., p. 310,
1826.

36 rrWKH

well-developed eyes arm cars, 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."1

Similarly the great respiratory power of birds is corre-
lated 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-exis-
tences 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.

('orrelation 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. Fven 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 area great many cases," writes
1 Recherches sur les Ossancns Fossil es, i., p. £o, 1812.

CORRELATION: RATIONAL AND EMPIRICAL 37

Cuvier,1 "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 correla-
tion 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 con-
stancy 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 con-
stant, they must necessarily have a sufficient cause ; but
as we are ignorant of this cause, observation must supple-
ment 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." That is to say, even correlations
for which no explanation in terms of function can be sup-
plied are probably in reality functional correlations. This
may, in some cases, be inferred from the graded corre-
spondence 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
1 OssemensfossileS) i., p. 60. - Loc. tit., i., p. 63.

38 CUVIKll

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."1 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 par-
ticular 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."

Cuvier, however, did not resolve life into metabolism, nor
reduce vital happenings to the chemical level. The form of
onranised bodies is more essential than the matter of which

o

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 combina-
tions of matter, which are almost the same in all.3 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

1 Lemons it Anatomic Conif>iin:c, i., p. 6.

- Lf Rcgne Animal, i., p. 16.

3 Hist. Prog. Sci. Nat., i., p. 187, 1826.

CLASSIFICATION 39

such a character that it does not destroy the harmony of the
whole." l 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,'2 a curious
forestalment, in the rough, of the modern division of
Metazoa into Coelentera and Ccelomata.

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
EcJielle dcs ctres 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 forms3 (which is perfectly true, in spite of
Amphioxus and Balanoglossus !).

In the Rcgnc Animal Cuvier restricts the application of
the idea of the Ecliellc 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

1 Lemons, i., p. 58. '-' Loc. cif, i., Article iii.

3 Loc. cit., i., p. 60.

D

40 CUVIEU

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." l 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 Geoffrey 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 Lccons,
Article V., " Division des animaux d'apres 1'ensemble de leur
organisation." The scheme of classification actually given in
the Lccons 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 Ccelenterates).

A maturer theory and practice of classification is given
in the Rcgne 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 Plantaniui, I789,2) is more clearly recognised.
The properties or peculiarities of structure which have the
greatest number of relations of incompatibility and coexist-
ence, 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/5 In deciding which
characters arc 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

1 Ri'gne Animal, i., p. xx.

2 Cuvier, Hist. Prog. Set. Nat., i., p. 288, 1826.
' Ri'gne Animal, i., p. 10.

CLASSIFICATION 41

a kind of centre for the vegetative functions, as the brain
and the spinal cord are for the animal functions."1 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,'2 of which all the others are but modifica-
tions. These four types are Vertebrates, Molluscs, Articu-
lates, 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 ; Articu-
lates 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

1 Rcgnc A nimal, p. 55.

2 First propounded by Cuvier in 1812, Ann. Mus.d'Hist, Nat., xix.

42 CUVIEU

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
\vas on sure ground in maintaining that the evolutionists of
his day went beyond their facts. He considered that cer-
tain 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."1 "These forms are neither pro-
duced nor do they change of themselves; life presupposes
their existence, for it cannot arise save in organisations
ready prepared for it."

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.3 Palaeontology 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."4 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 palaeontological 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 Piilndina series,

Aiiiiitdl, i , p. 19.
. cit., p. 20.

1 Rcc here lies sitr les Osscinens rossilcs, i., p. 74, 1812.
1 Loc. cit., p. 79.

SUCCESSION OF FORMS 43

yet at any one geological level the species, the lines of
descent, are all distinct from one another.1

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 Deperet
puts it, " Cuvier not only demonstrated the presence in the
sedimentary strata of a series of terrestrial faunas super-
imposed 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." It was left to d'Orbigny to
teach the doctrine of successive creations, of which he
distinguished twenty-seven (Cours eleinentaire de palaeontologie
stratigraphique, 1 849).

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 Recherc/ics snr les Ossemeus 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

1 See C. Deperet, Les transformations dit Monde animal, Paris, 1907,
and G. Steinmann, Die geologischen Grundlagen der Abstammungslehre^
Leipzig, 1908.

2 Recherches, i., p. Si.

44 CUVIER

to a civilised state.1 He refused to accept the extravagant
evolutionary theory of Demaillet and the somewhat confused
theory of Lamarck (whom he joins with Demaillet),2 just as he
rejected the transcendental theories of Geoffrey St Hilaire,
because they seemed to him not based upon facts.

1 Rcgnc Animal, i., p. 91.
- Os semens Fossiles, i., p. 26.

CHAPTER IV

i

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 formulae extracted
from experience as intuitions imposed upon experience.
So it was that Goethe, who was little more than
a dilettante,1 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 conscious-
ness 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
discovered2 in the same year as Goethe (1784) the inter-
maxillary 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

1 See Kohlbrugge, "Hist. krit. Studien iiber Goethe als Natur-
forscher," Zool. Annalen. v., 1913, pp. 83-231.
- Or re-discovered, according to Kohlbrugge.

45

46 (JOKTIIE

whose traces we come across everywhere" (Vicq d'Azyr,
quoted by Flourens, Mem, A cad. Sci., xxni., p. xxxvi.).

Peter Camper (1722-1789), we are told by Goethe himself
in his Osteologie^ 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 com-
mon to philosophy, to ordinary thought, and to anatomical
science. We find it expressed by Herder (who perhaps got it
from Kant) in his Idcen zur Philosophie dcr Gcscliiclitc der
McuscJihcit (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 inter-
preted 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."1

The principle comes to clear expression in his J'.rstcr
r.nt-^'itrf cincr allgcuicinai I'.uilcitniig in die vcrglcicJictidc
. \nntomic (1795)." 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." 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
Ix-forc him had, consciously or unconsciously, referred all

1 Cotta cd., vol. i.\-., p. 448.

'• First I) raft of a (k-ncral Introduction to Comparative Anatomy."
:i Cotta ed., i\., p.

UNITY OF PLAN 47

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." l
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." 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
1 Cotta ed., p. 478. 2 Loc. ctt., p. 491.

48 GOETHE

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." T
\Ve are not to explain, he says, the tusks of the Babirnssa \
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 Radl2 is probably correct in saying
that the following quotation with its double assertion
represents most nearly Goethe's position : —

"Also bestimmt die Gestalt die Lebenswcise des Thieres,
Und die Wcise zu leben, sie wirkt auf alle Gestalten
Machtig zuriick.'):i

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 append-
ages of plants he extended to all living things. Every living
thing is a complex of living independent beings, which " dcr

1 Jtn/;curf, Cotta cd.,*ix., p. 465.

- Geschichte der biologischen Tltcoricn, i., p. 266.

•• Si i tin- form determines the manner of life of the animal, and the
manner of life in its turn reacts powerfully upon all forn

REPETITION OF PARTS 49

Idee, der Anlage nach," are the same, but in appearance may
be the same or similar, different or unlike.1 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 vertebrae. 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
Geoffrey St Hilaire, the law " that to no part can anything
be added, without something being taken away from another
part, and vice versa" 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.3 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, but in 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

1 Bildung und Umbildnng organischcr Naturen, 1807.

2 Cotta ed., ix., p. 466, :) Loc. ct/., pp. 474-5.

50 GOETHE

scoffs at the scholars who would explain a living creature by

anatomising it :

" Dann hat cr die Theile in seiner Hand,
Fehlt leider ! nur das geistige Band." '

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

In the fragment entitled Dilditng und Umbildung
org<itiisc1icr Xaturcn (1807), introductory to a reprint of his
paper on the " Metamorphosis of Plants," we get an exposi-
tion 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 indica-
tions 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 Bildnug is called
Morphology.2 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 Bilduiig. 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 bildcndc
Kraft or Bildnugstricb, 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 manifesta-
tions 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

1 Then he has all the parts within his hand, excepting only, sad to
say, tht living bond.

• Goethe was the inventor of the word.

INTERPRETATION OF UNITY OF PLAN 51

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. . . P"1

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

1 Cotta ed., ix., p. 490.

CHAPTER V

ETIENNE (.'.EUFFKOY SAIKT-IJILAIKli

K. GEOI-TKOY 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 1/96, in one of his earliest memoirs,1
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 — homologiVs, too, not so much of organs as
of parts.

These memoirs (published in the .\>/n<i/cs du Slnscnin
<nfis/<>in- natnrcllc, 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

1 " Mcmoirc sur les rapports nature-Is dcs makis,'; M>i£<isin Encyclo-
vii.

PRINCIPLE OF CONNECTIONS 53

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 anatoinique,
particularly the first volume (iSiS). This volume contains,
beside the important " Discours preliminaire" and " Introduc-
tion " 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
Philosophic atiatouiiqne 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 ? 1

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
1 Discours preliminaire, pp. xv.-xxiv.

54 KTIMNNK GEOF1 ROY SAINT-HILAIRE

Vertebrate there are found the same " organic materials," or
units of construction. This conception, which Geoffrey calls
the Tlicoric dcs 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 dcs
connc.vions adds the unity of plan.

Both conceptions are logically implicit in the vague
notion of unity of type ; Geoffrey 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 ;
Geoffrey'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 transforma-
tions 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

UNITY OF TYPE IN VERTEBRATES 55

speaking, only a single animal."1 The "single animal" is
simply the generalised type.

Having laid down his two principles Geoffrey 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." 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. Con-
sidering 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 Geoffrey, 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

1 Etudes progressives (Pun Naturaliste, p. 50, Paris, 1835.

2 Philosophic Anatomique., i., Introduction, p. I.

E

56 KTIKNNK GKOl-THOY SAINT-IIILAIKK

with his principal hypothesis of the invariability of con-
nections, 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 metas-
tasis 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. Geoffrey 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 Verte-
brates, 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
Geoffrey's method of dealing with the facts of structure. \\'e
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.

THE ARCHETYPAL STERNUM

57

In the memoir on the sternum Geoffrey'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 Geoffrey's
nomenclature, the episternals, the hyosternals, the hypo-
sternals, the xiphisternals, which are all paired bones, and
the entosternal, which is unpaired. The arrangement of
them is in the tortoise :—

Episternal

Episternal

Hyosternal

Hyosternal

Hyposternal

Hyposternal

Xiphisternal

Xiphisternal.

The articulations in the tortoise are indicated by the
connecting lines. Geoffrey 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

58

KTIKNNi: (.KOITKOY SAINT-HILAIRE

young birds, rudiments of episternals and xiphisternals also
occur. The minuteness of the episternals and the xiphi-

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sternals may be attributed to the gigantic size of the ento-
stcrnal, in accordance with the I.oi <tc balancement,
In the other air-breathing Vertebrates the nine sternal
elements can according to Geoffrey be discovered without

THE STERNUM IN FISHES 59

great difficulty. But when we come to the determina-
tion of the sternum in fishes, difficulties abound, which
Geoffrey solves in the following way. He points out that
between the clavicles (cleithrd) and the hyoid bone (basihyaT)
in fishes there is a long median bone (urohyat) which is
attached in front by two strong tendons to the horns of the
hyoid and is free behind (see Fig. i). 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 repre-
sentatives 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 immedi-
ately 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 metas-
tasis similar to that which brought the whole thoracic

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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.

Geoffrey'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 homo-
logous 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

GO KTIKNM: (;I:OITROY SAINT-IIILAIKK

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 ahvays 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
auatouiiauc (1818) Geoffrey 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 classifica-
tion : — ]

... , ( Hauts-Vertebres (Vcrtebrata, Cuv.).

Vertebres , ^ . . ' '

tDermo-Vcrtebres (Articulata, Cuv.).

., , I Mollusques (Mollusca, Cuv.).
Invertebres * , '

i Rayon nes (Radiata, Cuv.).

The idea upon which is based the comparison of Articu-
lates with Vertebrates is that each skeletal segment of
Articulates is a vertebra. In the Hauts-vcrtebres the
vertcbiM- are internal; in the Dcrmo-vcrtebres they are
external. " /:rv/T animal /ires cither outside or inside its
vertebral column"- The essence of a vertebra is not its
form, nor its function, but its composition from four

i "Snr unr . oloiinc vrert^bralc <-t ses c6tes dans Ics insectes
apiropodi . Acad. Set'., Kcl>. 12, iSso). Printed in his, pp. 5-7-52>
i

" Sur I'organisation des insi - tes," |». 45s- /J/X I'l1- 452'6-» lS2° (2)-

THE VERTEBRAE OF ARTICULATES

61

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 com-
posed of four elementary pieces, as may be
seen most easily in young ones. " Accord-
ingly 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 Articu-
lates 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 (endo-
phragmal skeleton).

If the skeleton of the segment in Articu-
lates 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
vertebre." x He takes as the typical vertebra
that of a Pleuronectid, probably the turbot.
His original figure is reproduced (Fig. 2).

He includes as part of the vertebra not
only the neural (e', e") and haemal (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,
1 Mem. Mus. d'Hist. nat., ix., pp. 89-119, Pis. v-vii.

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02 ETIENNE GEOFFROV SAINT-HILAIRE

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 vertebrae — 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, \vhether internal or
external, should stand to one another in the same relations ;
but it is all one whether the box (coffrc) 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 "

The exact way in which Geoffrey 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.

THE ARTHROPOD AND THE VERTEBRATE 63

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 Arthro-
pods and Vertebrates, but this
is no contradiction of the prin-
ciple 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 re-
lations ; but the principle
would be in no way invali-
dated if the whole mass had
rotated inside the tube" (p.
112).

Geoffrey pushed the anal-
ogy 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

FlG. 3. — Abdominal Segment of the
Lobster. (After Geoffrey.)

always in its proper place, and remained faithful to at
least one of its connections.1 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 F organisation dcs insectcs. Six segments
are distinguished in an insect— the head, the three divisions
1 Sur P organisation des insec/es, p. 459.

64 KTIRNNK GEOFFROY SAINT-HILAIRE

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 mesothcrax to the parietals,
interparietals, and opercular bones, and that of the meta-
thorax 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 Academic 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. Geoffrey 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 l-.cJicllc
dcs cfrcs, after the Articulata, just as Gcoffroy had maintained
in 1820, saying that crabs formed a link between the other
Crustacea and the molluscs.1 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, Geoffrey 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,- Cuvier easily
demolished the crude comparison of the Cephalopod to the
Vertebrate, lie gave diagrams of the internal organs of a
Ccphalopod and of a Vertebrate bent back in the manner
indicated by Meyranx and Laurencet, and he showed in

1 /si's, p. 549.

: I'ulilislic.l in Ann. Sci. AW/., xix., |>p. 241-59, 1830.

THE CEPHALOrOD AND THE VERTEBRATE 65

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,1 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). Geoffrey 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 Geoffrey 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. Geoffrov brought forward all his theories

* o

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
Geoffrey'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.2

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.

1 Cf. Aristotle (supra, p. 10).

' For an account of the controversy reference may be made to
I. Geoffroy St Hilaire, Vie Travaux •<?/ Doctrine scientifique (PEticnne
Geoffrey Si Hilaire, Paris, 1847 ; also Semper, Arb. zool. zoot. Instit.
Wiirzburg, iii., 1876-7, K. E. von Baer, Lebensgeschichte Cuiners,
ed. L. Stieda, 1897, and J. Kohlbrugge, in Zoolog. Annalen, v., pp.

H3-95)

66 ETIENNE GEOFFROY SAINT-HILAIRE

In these Darwinian days Geoffrey 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. Geoffrey 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
Philosopliie anatoiiiiqne. 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 car 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.1 In a memoir of i8jS,-;

1 <: Kcchcn In ^ ,ur 1'or-anisation dcs C.aviaK,' .1 /<///. .1///\-. tf } 'list.
., xii., 1825.
M> 'in. Mus. if Hist, nut., xvii., pp. 209-29.

VIEWS ON EVOLUTION 67

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 Geoffrey 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 Geoffrey's views on evolution
is found in his memoir " Le degre d'influence du monde
ambiant pour modifier les formes animales."1 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
1 Mem. Acad. Sa'., xii., pp. 63-92, 1833.

68 KTIKNM; CJKOFFKOY SAINT-HILAIRE

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,1 when ammonites and GrypJuca 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"- (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 Vertcbre " 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.

Me 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,

1 M,'in. Ac<ui. Si'i., xii., ]>p. 43-61, 1833.

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

BIOGENETIC LAW 69

that transformations could arise by means of monstrous
development (p. 86). In this connection the experiments
which he made on the hen's egg1 in order to produce
artificial monstrosities are significant, though his purpose was
rather to obtain proof of the inadequacy of the preformation
hypothesis.2

It seems probable enough that if Geoffrey 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 corre-
sponding 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."3

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 transfor-
mation. " An amphibian," he writes,4 " 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

1 Mem, Mus. d'Hisf. nat., xiii., p. 289, 1826.

2 Mem. Mus. (PHist. naf., xviii., p. 221, 1828. His teratological
work is important, and is chiefly contained in the second volume of the
Philosophic anatomique.

3 Phil, anaf., i., p. 449.

. Acad. Sci.t xii., p. 82, 1833.

70 ETIKNNK GEOFFROY SAINT-HILAIRE

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 vertebrae, 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 vertebrae, " 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."1

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.2

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 unit}7 of plan, " I have, more-
over, regenerated this principle, and obtained for it univer-
sality of application, by showing that it is not always the
organs as a whole, but merely the materials composing each

1 Mt'in. Mns. d' Hist. ;/<//., ix., ]>. 101, 1822.

-' dntrs dc Phistoirc natitrcllc dcs Altuninifcrcs^ i., Le^on 3, p. 13,
1829.

CRITERIA OF HOMOLOGY 71

organ, that can be reduced to unity." 1 Even in the PJiilo-
sopJiie 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 pro-
tective function of the sternum may be assumed by
quite other parts, e.g., by the clavicles in fish, which protect
the heart.2

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 considera-
tion of their connections."3

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." 4

1 Etudes progressives d'un Naturalistc, p. 59, f.n., Paris, 1835.

2 Phil. Anat., i., p. 444.

3 Ann. Mus. d'Hist. na/., x., p. 344, 1807.

4 Isis, P- 534, 1820 (2).

F

72 ETIENNE GEOlvFROY SAINT-HILAIRE

Geoffrey's morphological units or materials of organisa-
tion 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, Geoffrey 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,1 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 fetuses of mammals,
and hence bear out the theory " (p. 344). So, too, in dealing
with the homologies of the sternal elements (snprn, 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? Geoffrey's
main answer to this question is his Loi dc balancement. The
law was enunciated by him already in 1807.- \Ye take the
following quotation, which represents his thought most
nearly, from the Conrs dc riiistoirc natiircUc </,'s Mauuni-
fcres (1829). "According to our manner of regarding the

1 Ann. Mus. </ ' I fist, nut., x., pp. 342-65, 1807.
'-' he. cif., x , p. 343.

LAW OF COMPENSATION 73

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., Lecon 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."1 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." -

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 a 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 un-
workable ; he found, for example, that the skeleton of fishes

1 Phil, anat., i., 450, f.n. Cf. Aristotle (supra, p. i i).

2 Loc. cit., p. 136.

74 ETIENNE GEOITROV SAINT-HILAIUE

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 wctastases, 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 Verte-
brates, 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 con-
temporaries, 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

GEOFFROY AND CUVIER 75

1

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
Geoffrey 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 Geoffrey conceived it, simply does not exist.
Cuvier goes on to say that this principle of Geoffrey'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 role 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).

Geoffrey'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,1 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

1 MammifcreS) i., Discours prel., p. 18.

76 ETIENNE GEOFFROY SAINT-HILAIRE

manifest tendency to resemble one another, and to reproduce
invariably the same primordial type." l 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. There-
fore 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."
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."3

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 Mammiftres (1829) to
Cuvier's criticisms in the Histoirc ihitnrclle dcs l\nssons
(1828), he insists on the necessity of excluding function from
consideration in any truly philosophical treatment of com-
parative anatomy (Discours prel., p. 25). Cuvier held that
function determined structure, or at least that the necessity

1 /'////. a nn/., ']., p. 208.

- Cuvier and Valenciennes, Hist. ti<tf. rrissons, i., ]). 550, 1828.

3 Cuvier and Valenciennes, he. cit., p. 544.

STRUCTURE AND FUNCTION 77

of adaptation ruled the transformations of form. Geoffrey
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."1

Again, "a vegetarian regime is imposed upon the Ouad-
rumana by their possession of a somewhat ample stomach,
and intestines of moderate length." 2 The hand of the bat
has become so modified as to constrain the bat to live in
the air.3

The best example of Geoffrey's insistence upon the
priority of structure to function, and so of his purely morpho-
logical 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
vertebrae 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."4 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.5
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

1 MammifereS) i., Le<;on 4, p. 17.

2 Loc. cit., Le$on 5, p. 8. 3 Loc. cit,, Lecon 13, p. 6.

4 Jsi's, p. 539, 1820 (2). 5 MammifereS) i., Legon 4, p. 6.

78 ETIENNE GEOFFROY SAINT-IIILAIRE

of final causes." l 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.'2 So too, although
the other organs of the insect are all inside the body of the
vertebrae, they are yet comparable with the organs of Verte-
brates.3 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
Geoffrey's suggestion that this unity results from a power
which is exercised in the world of atoms where are elements
of inalterable character ? 4

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 Geoffrey's sense. Theories such as
Lamarck's, which postulate an active responsive self-adapta-
tion of the organism, arc essentially a continuation and
completing of Cuvier's thought.

1 Mam»iir,-rcs,, Discours prcl., p. 7. - Jsr's, p. 460, 1820(2).

M ,".-. .Ui/s. it' I fiat. n<tt., ix., p. 102, 1822.
1 Mcin. Acnit. .SV/., xii., p. 76, 1833.

CHAPTER VI

THE FOLLOWERS OF ETIENNE GEOFFROY SAINT-IHLAIRE

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 Philosophic 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 Anatomic compare? du cerveau
(1824-26), which met with a flattering reception from Cuvier.1
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? under the
general title Reclierches d1 Anatomic transcendante, sur les Lois
de £ Organogenic applique es d I' anatomic pathologique, also
published separately. We follow these papers in our expose
of Serres' doctrine, reserving for a future chapter (Chap. XII.)
the consideration of his matured views of thirty years later.

1 Mem. Acad. Sd., iv., pp. cclxxxiv.-ccci., 1824.

- Ann, Set. Nat., xi., xii., 1827 ; xvi., 1829 ; xxi., 1830.

79

80 THE FOLLOWERS OF GEOFFKOY

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 trans-
cendentalists (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, "trans-
cendental 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 vena:: cav;e 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'

SERRES 81

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'Osteogenie (1819), Serres
established several laws of ossification based upon this
principle of separate formation.1

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
Felts, 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
1 See Radl, loc. cit., i.. pp. 225-6.

THE FOLLOWERS OF GEOFFROY

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 1 — " 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 paper2 of 1824 Serres
attempted an explanation of the nervous system of Inverte-
brates. Invertebrates, he considered, lacked the cerebro-
spinal 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 llnlla apcrta, stage (i) is
permanent ; in Clwt Doris, Aplysia, Tritotiia, Sepia, Jldi.v,
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 larvae of insects " (p. 380). The law of parallelism

1 Ann. Sci. not. (2), ii., p. 248, 1834.
" Ann. Sci. //<//., iii., pp. 377-80, 1824.

SAVIGNY 83

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 incontest-
able, 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 (Joe. cit.,
p. 247).

The last point of Serres' doctrine which calls for remark
is his interpretation of abnormalities as being often com-
parable 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
' T/i^orie des formations et des deformations organiques
applique e a I 'anatomic comparee des monstruosites (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 con-
vinced," he wrote, " that when a more complete examina-
tion has been made of the mouth of insects, properly so
called, that is to say, having six legs and two antennae, 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."1 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

1 Mcmoires sur les Aniuianx sans Vertcbres, Part I., p. 10, Paris, 1816.

84: Till: FOLLOWERS OF GKOFFROY

second with the anterior appendages of Articulates gener-
ally. Savigny shows that the mouth-parts of insects can
be reduced to the type shown in Orthoptera, where there
are clearly two mandibles, two maxillae, and a lower lip
formed by the fusion of two second maxillae. 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
fnlns, Scolopcndra, Caticcr, Gam-morns > Cyamus, XyinpJwn,
Phalangium^ Apns> Caligits, Limit Ins, 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 maxillae 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 Jnlus, that is to say, all the feet of the
Hexapoda, are here transformed into jaws " (loc. cit., p. 4,S).
In Scolopatdm also there is a similar transformation of two
pairs of legs into auxiliary jaws. In Gaiiniianis, where there
is only the first pair of maxillipedes, the other two pairs
have become " retransformed " into feet. \Ve find him
supporting his comparison of the three anterior pairs of legs
\\-\ Jnlns 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 Jnlns 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).

1 1 is comparison of the Arachnid appendages with those
of insects and Crustacea is very curious. As his starting-
point he takes Cydi/ms, which has antenn;e (two pairs) and
mouth parts (four pairs) as in many Crustacea, and then
seven pairs of legs ; he compares with it Xyniplion, which has
in all seven pairs of appendages. These appendages he
homologises with the seven pairs of legs of Cyainns, so that
the first appendage in Xymplioii corresponds to the seventh
appendage of Cy^tnns. This homology is extended to
all Arachnids ; their first two pairs of appendages, however

AUDOUIN 85

they may be modified as " false " mandibles and " false "
maxillae, really correspond to the second and third maxilli-
pedes in Crustacea, and to the second and third pairs of feet in
insects. It is interesting to note that he treats Liuinlus as an
Arachnid, pointing out that there is as much difference between
Apns and Liinulus as between Cancer and PJialanginin. He
describes the " gnathobases " in Phalangiuin 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
My sis (p. 50).

V. Audouin, in his memoir, Recherches anatoniiqncs snr Ic
thorax des aninianx articules} applied the principle of the
unity of plan and composition to the exoskeleton of insects,
Crustaceans, and Arachnids. His guiding ideas were, "(i)
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. 1 6). 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.2 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

1 Ami. Sci. Nat. (i), i., pp. 97-135, 416-432, 1824.

2 Jsis, p. 456, 1820 (2).

86 Till: FOLLOWERS OF (iKOlTUOY

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 Geoffrey's
Lot de balanccinait 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." l 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

o

Savigny.

Later on, in conjunction with Milne-Edwards, he demon-
strated 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
antennas, because they were placed above the upper lip. It
was he who first pointed out the resemblance of the leaf-like
gills of Ephemerid larva; to wings, and suggested that wings
were " a sort of tracheal feet."

He made also a rather hazy and speculative contribution
on Okcnian 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 ; antenna-,
nostrils turned outside in.

Duges also made a comparison of Articulates with
Vertebrates.- He did not accept Geoffrey's \\-rtebral theory

1 Cuvicr, M,:in. Ac<ui. Set'., iv., p. cclxx., 1824.

- Ac<id. Sci. 1 8th Oct. 1831. Extract in Ann. Sci. Nat., xxiv., pp.
254-60, 1831.

DUGES 87

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 maxillae 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 equiva-
lent of the " hyoid " elements which form the branchial
arches.

But Duges' 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). Duges 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.1
According to his view there are three main types of animals —
(i) 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. Duges
of course admitted that the centralisation of the com-
plete organism became greater the higher it stood in
the scale, and that this held good also in individual
development. The appendages of Articulates and Verte-
brates were thought of as the members of as many
separate organisms. He went so far as to suggest that the

1 His views were more fully elaborated in his Mcmoire sur la
conformite organique dans I'cchelle animale, Montpellier, 1832.

G

88 TIIK FOLLOWERS OF GKOFFHOV

fingers of a man's hand were the free extremities of as many
thoracic members.

Dugcs' 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.

CHAPTER VII

THE GERMAN TRANSCENDENTALISTS

To complete our historical survey of the morphology of the
early ipth 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 trans-
cendental 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 igth century, arising
mainly in Germany, but powerfully affecting, as we have seen,
the thought of Geoffrey 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

89

90 THE GERMAN TRANSCENDENTALISTS

of the parallelism between the stages of individual develop-
ment 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 1 seems to have been expressed
first by Kielmeyer (i793),'2 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 I797-3

Oken (1779-1851) in his early tract Die Zcugiing (1805),
and in his LehrbncJi dcr Naturphilosophie (1809-1 1) 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."4 The Insect, for example, is at first
Worm, next Crab, then a perfect volant animal with limbs,
a Fix- (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
festal 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
Huer:' speaks of his work (published in Oken and Kieser,
AV//;v>j,v .::/(/- vergleichenden Zoo/o^ic, Anatomic and Pliysi-

1 For a full account, sec Kohlbrugge, Zool. Anim/en, xxxviii., 191 1.
- Rcdc iiber das Verhaltnis der organischen Kriifie, Stuttgart u.
Tubingen, 1793 (1814). See Kadi, he. cit., i., p. 261 ; ii., p. 57.
; Supplcm. nd historical emfayonis, Tubingen, 1797.
1 Lehrbuch der Naturphilosophie^ En-, trans., p. 491, 1847.
•' Ueber Enhvickelungsgeschichte dcr Thiere, i., p. xvii., 1828.

LAW OF PARALLELISM 91

ologic" 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 con-
sequence of his hazy philosophy. He saw the relation of
teratological to foetal structure, for he affirmed that " mal-
formations 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 x 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,2
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 collection3
(i., i, 1808) he had made some comparisons of this kind in
dealing with the development of the human foetus ; in this
memoir (ii., i, 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

1 Zoologie, Landshut, i., 1808.

2 Anatomic u. Bildungsgeschichtc des Gehirns im Fotus des Menschen,
Niirnberg, 1816.

3 Beytrcige zur vergleichende Anatomic, Leipzig, i., 1808-9, ii., 1811-2.

92 THE GERMAN TRANSCENDENTALISTS

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 Tct/iys, ScylUca 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).1 These are merely chance
resemblances of no morphological importance. He considers
bladderworms as animals which have never escaped from
their amnion, and Volvo.v 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 f/i-r I'cr^lcicJicuden Anatomic (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

1 Cetacea were generally considered at this lime to be mammals of
low organisation.

LAW OF PARALLELISM 93

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."1

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 evolu-
tion. " 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 through-
out 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

1 From the French trans., which appeared under the title Traite gen.
d'Anat. compare'e, i.^ p. 449, 1828,

94 THE GERMAN TRANSCENDENTALISTS

abnormalities which are found to be normal in other (lower)
forms (p. 556).1

We may refer to one other statement of the law of
parallelism — by K. G. Carus in his Lclirbnch dcrverglcicliaidcn
Anatomic (Leipzig, 1834). The standpoint is again that
of NaturpJiilosopJiic. 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, generalisa-
tions 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 (Joe.
ci/., 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

1 Cf. Geoffrey (sit/>r<i, p. 70).

LAW OF REPETITION 95

sternum is a ventral vertebral column. Limbs are connate
ribs, the digits indicating the number of ribs included (cf.
Duges, supra.) p. 88).

J. F. Meckel 1 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 corre-
sponds 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 develop-
ment 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 arrange-
ment 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,2 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
Ccelentera, 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

1 Beytriige, ii., 2, 1812. Also in his System d. vergl. Anat.^ i., 1821.

2 In J. F. Meckel's Beytrdge, ii.

96 THE GERMAN TRANSCENDENTALISTS

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 vertebrae.
Goethe seems to have been the first to hit upon the idea that
the skull is composed of a number of vertebras, 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 I.82O.1

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' Etudes medicales], published in 1803,
Burdin assimilated the skull to the vertebral column.

Oken, in an inaugural dissertation (Programrn) Ueber die
Bedeutung der ScJiadclknochen* published in 1807, gave to the
theory its necessary development. Autenrieth, also in i8o7,3
distinguishing separate ganglia in the brain, was not far from
the hypothesis that each of these ganglia must have its
separate vertebra.

In 1808 Dumeril read a paper to the Academic des
Sciences in which he compared the skull to a gigantic
VQrtebra, 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 vertebrae.

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 Okcn did at first) three
cranial vertebrae. Bojanus in his Anatome testudinis europceae
(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
Lchrlnich. Meckel and Carus among the Germans, do
Blainville and E. Geoffrey among the French, contributed to

1 Zur Morphologic, i., 2, p. 250, 1820 ; and ii., 2, pp. 122-4, '824.
- See translation, giving the gist of this paper, in Huxley's Lectures on
the Elements of Comparative Anatomy, pp. 282-6, London, 1864.
3 Reil's Archii'.f. rhysiol., vii , 1807.

VERTEBRAL THEORY OF THE SKULL 97

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 vertebrae ; it was
quite another to demonstrate the relation of the separate
bones of the skull to the supposed vertebrae. Upon this
much uncertainty reigned ; there was not even unanimity as
to the number of vertebrae to be distinguished. Goethe
found six vertebrae 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 vertebrae, as given in the English translation of his
Lehrbnch. " 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 vertebrae the transverse processes represent the ribs.
The skull consists of four vertebras, 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 vertebrae 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 alae 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
Lecons, 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

98 THE GERMAN TRANSCENDENTALISM

clinoicl apophyscs."1 He denied the resemblance of the
frontal and nasal " vertebnu " to true vertebrae, 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.2 His system is worthy of some considera-
tion, for he tries to work out a geometry of the skeleton/5

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 nenroskclctoii. All skeletal forma-
tions belong to one or other of these systems.

Carus defines his aim to be the discovery of the inner la\v
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 Xatnr-
pJiilosopliic — the search for rational laws which are active in
Nature, the discontent with merely empirical laws.

1 Lemons tf anatomic compare, 3rd cd., Brussels reprint, i., p. 414, 1836.

-' In his I'ro^ramm, U. ii. Hcdcut. </. Schddelknochen^ 1807.

:; Trait:' ,'1,'incntairc d'unaioinie coin/xurc (French trans.), vol. iii.,
Paris, 1835. First developed in his volume Von den Ur-Thcilen dcs
Knochenund Schalen-GerusteS) Lcip/iy, 1828.

K. G. CARUS 99

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.t 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 (vertebrae in the ordinary sense).
The apophyses and bodies of the vertebrae, and the bones
of the members1 are composed of columns of tritovertebrae,
or vertebrae of the third order. Thus the whole vertebrate
skeleton is a particular arrangement of vertebrae, which

1 Dutrochet in 1821 had tried to prove that the bones of the
members belong to the type of the vertebra — the dicone.

100 TIIK GERMAN TRANSCENDENTALISTS

in their turn are modifications of the primary hollow
sphere.

The German transcendentalists were more or less
contemporary with E. Geoffrey, and no doubt influenced him,
especially in his later years, as they certainly did his
follower Serres. Oken indeed wrote, in a note 1 appended
to Geoffrey's paper on the vertebral column of insects, that
" Mr Geoffrey [sic] is without a doubt the first to introduce in
France NaturphilosopJiic 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 Geoffrey was
primarily a morphologist, whereas the morphology of the
German transcendentalists was only a side-issue of their
NaturpJiilosopliie.

Geoffrey, on his part, exercised some influence on the
transcendentalists. He asserts2 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 Geoffrey the view that the
opercular bones are homologous with the ear-ossicles,
adopting, however, a different homology for the separate
bones.3

Some speculations seem to have been common to both
schools — for instance, the law of Meckcl-Serres, the vertebral
theory of the skull, and the recognition of serial homology
in the appendages of Arthropods (Savigny, Oken). Latreille
and Dugcs, as well as Serres, clearly show in their
theoretical views the influence of Oken and the other
transcendentalists. Geoffrey'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, Geoffrey 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 transcen-
dental philosophers first, and morphologists after.

1 Jsis, pp. 552-9, 1820 (2). - Mem. Mus. it Hist, na/., i.x., 1822

:1 Cuvier and Valenciennes, Hist. nat. Poissons, i., p. 311, f.n.

J. F. MECKEL 101

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 Geoffrey. 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 Leqons danatomie comparee, he earned for himself
the title of the " German Cuvier," partly through the publica-
tion of his comprehensive textbook (System der vergl.
Anatomic, 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 Geoffrey'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 require-
ments. 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 " homo-
logues " 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.

CHAPTER VIII

TRAXSCKXDK.XTAL AXATn.MY IX KXGI.AXD-
RICHARD OWKN

RICHARD OWEN is the epigonos of transcendental mor-
phology ; 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 transcendcntalist, an eclectic of

zygsupophysis .

dia.pophysis .

pa.rsbpophysis

., neuraJ spine

_ . . - neur&pophysis

---^ - - - p/eurapoph/sis

haemapophysis

-p ophysis

- - hasmaJ spine
FlG. 4. — Idcul Typical Vcrlclmi. (After Owen.)

the older ideas current in his time ; for he picked out wh.it
was best in the older systems — Cuvier's teleology, Geoffrey's
principle of connections, Okcn'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

THE VERTEBRATE SKELETON

103

in part with his own transcendentalism. His main theoretical
views are to be found in his volume On tJie 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

d-.

hs

FlG. 5. — Natural Typical Vertebra ; Thorax of a Bird.
(After Owen.)

which Owen calls a vertebra. His definition of a vertebra
is, " one of those segments of the endo-skeleton which con-
stitute the axis of the body, and the protecting canals of
the nervous and vascular trunks" (p. Si). The parts of a
typical vertebra are shown in Fig. 4, which is copied from
Owen's Fig. 14.

H

104 TRANSCENDENTAL ANATOMY IN KN(iLANl)

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

A segment of sternum is included as the "haemal spine"
of the vertebra (//.v) ; the vertebral rib is the " pleura-
pophysis " (/>/) ; the sternal rib the " haemapophysis " (//) ;
the uncinate process of the vertebral rib is known as the
"diverging appendage" (a). The whole vertebrate skeleton
is composed of a series of vertebrae 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 arrange-
ment 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 vertebrae — the vertebral theory of the skull was,
in his system, a deduction from the vertebral theory of the
skeleton. He recognised four cranial vertebra- ; the arrange-
ment 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 vertebra: are practically identical with
the vertebra- 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 vertebra-.
The upper and lower jaws are associated with the nasal and
frontal vertebra' respectively, not however as limbs of the
head, but as constituent elements of these vertebra-. In the
>ame way the hyoid apparatus is part and parcel of the
paru-tal vertebra, and the pectoral girdle and fore-limbs part
of the occipital vertebra.

£j Neural spine.
\%& Neurapophysis.
[ 1 Diapophysis.
| Centrum.
[OJ] Parapophysis.
§$g Pleurapophysis
§ Ha-majiophysis.
^^ n;«mal spine.
H Appendage.

FlG. 6.— The Archetype of the
Vertebrate Skeleton. (After
Owen.)

\J

106 TRANSCENDENTAL ANATOMY IN ENGLAND

Cranial Vertebra.1 (After Owen, 1848, p. 165.)

Vertebra;.

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.

Haemnpophyses.

Coracoid.

Ceratohyal.

Articular.

Maxillary.

Haemal Spines.

Episternum.

Basihyal.

Dentary.

Premaxillary.

Diverging Ap-
pendage.

Fore -limb or
Fin.

Branchiostegals.

Operculum.

Pterygoid and
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 Lcpidosircn, 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
h;umapophysis, 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 h;emapophysis of the atlas. Owen considers
it no objection to this view that in other Vertebrates the

1 Owen introduced most of the names of bones now current.

THE ARCHETYPE 107

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 back-
wards " (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 haemapophyses. 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 Amphimna ; 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, " Ideologically
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 conceiv-
able modifications of the vertebrate archetype are far from
having been exhausted in the forms which have hitherto
been recognised, from the primaeval fishes of the palaeozoic
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
Geoffrey'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 homo-
logous parts ; but this is subject to modification, like the
forms, proportions, functions, and very substance of such

108 TllANSCKNDKNTAL ANATOMY IN ENGLAND

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 develop-
ment, 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 tJie -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." " ! foniologue. The same organ in different
animals under every variety of form and function."1

Me introduced also useful distinctions between Special,
( u neral,und 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
1 I. <\ lures on Inrcrtclnate Animals, pp. 374, 379, 1843.

HOMOLOGY 109

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 honiology 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 longi-
tudinally 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 vertebras — 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 vertebrae. 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

110 TRANSCENDENTAL ANATOMY IN' ENGLAND

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 distfnct 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 deter-
mining 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 (<\{,r., frogs), the long bones should ossify first at their
(.•nds, 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 clement (haemapophysis),
and in monotremcs, birds and reptiles it is in the adult a
large and separate bom-. Its ossification from a separate

HOMOLOGY AND TELEOLOGY 111

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 com-
pound" (p. 105). On the other hand, "All those bones
which represent single vertebral elements are ' Ideologically
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 par-
ticular offices" (p. 106). The limb-skeleton, corresponding
as it does to a single bone of the archetype, is the typical
example of a ideologically compound bone. Owen in his
definition of teleological compoundness has combined two
kinds of adaptation — (i) temporary adaptation of bones to
the exigencies of development, birth and growth (e.g., develop-
ment 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 adap-
tations 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 arche-
type 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
ic>e'a, on the other hand, produces the diversity of organic

112 TRANSCENDENTAL ANATOMY IN ENGLAND

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.

CHAPTER IX

KARL ERNST VON BAER

VON BAER was recognised as the founder of embryology
even by his contemporaries. His predecessors, Aristotle,1
Fabricius,2 Harvey,3 Malpighi,4 Haller,5 Wolff,0 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
Dollinger, Professor in Wiirzburg, 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 {Anatoinie 21. Bildungsgeschichte
des GcJiirns, 1816), by Oken (Joe. cit., supra, p. 90), and some
others.

Pander, with whom apparently Dollinger and d'Alton
collaborated, was the first to publish his results;7 von Baer,
who through absence from Wiirzburg 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, UeberEntwickelungsgeschichte

I De generatione Aniinalium.

' De formato fa'/u, ? 1600 ; De forinatione fattus, 1604.

3 Exercitationes de generatione animaliuui, 1651.

4 De forinatione pulli in ovo, 1673 '•> De ovo incubato, 1686.

5 De fonnatione pulli in ovo, 1757-8 ; Sur la formation du cccur dans
le poulet, 1758.

II Theoria genera tionis, 1759 ; De fo rmatione intestinorinn, 1768-9.

7 Beitrcige zur Entivickclung des Hiihnchens im Ei. Wiirzburg, 1818.
Also in Latin in shorter form, 1817.

ns

114 KARL ERNST VON BAER

der Thicrc. Beobachtnng iuid Reflexion (Konigsberg, 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 observa-
tions, but taking account also of all contemporary work. A
separate account of the development of a fish {Cyprinus blicca)
appeared in I835.1

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 ]]7esenheit} of the animal
determines its differentiation, that no stage of development
is solely determined by the antecedent stage, but that
throughout all stages the M'csoiheit 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

' Untersuchungen it. die I'.nt-«.'ickelun%sgcsclri<.htc tier FiscJic ; Leipzig
1835.

FORMATION OF GERM LAYERS 115

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
{Selbstdndigkeif) 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
lamellae. 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

116 KARL KKNST VON BAKU

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 delarnination 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
" Gekrose," or mesenterial complex, which departs consider-
ably from the tubular form.

When these tubes or " fundamental organs " are formed
the process of primary differentiation is compKir The
fundamental organs, however, have at no time actually the
form of tubes; they exist as tubes only ideally, for morpho-
logical 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

DIFFERENTIATION 1 1 7

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 appa-
ratus, 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 segmen-
tation 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 blood-
stream" (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 bjlood,
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

118 KARL ERNST VON BAER

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 develop-
mental processes still further, and discusses in particular
the scheme of development which is shown by the Verte-
brata. 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 invest-
ing skeleton of the neural axis develops; in the ventral plates
are formed the ribs, the ventral arches of the vertebras, 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 " Gekrose," or remaining part
of the vessel layer develop the Wolffian bodies (Urnicrui,

GERM-LAYER THEORY

119

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.

In his germ-layer theory von Baer was influenced a good
deal by Pander, to whom the actual discovery of the process

FlG. 7. — Ideal Transverse Section of a Vertebiate Embryo.
(After von Baer.)

a. Chorda.

b. Dorsal plates.

c. Ventral plates.

d. Spiual cord.

e. Vessel-layer.
/. Alimentary tube.
g. Pronephros.
h. Skin.

i. Amnion.

k. Serous membrane.

I. Yolk-sac.

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 " Keim-
blatt " (germ-layer).

The honour of being the founder of the germ-layer theory
is sometimes attributed to C. F. Wolff, notably by Kolliker
and O. Hertwig. Wolff, it is true, in his memoir Deformatione
intestinorum (1768-9) showed that the alimentary canal was

120 KAUI. KRNST VON BAKU

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

CRITICISM OF THEORY OF PARALLELISM 121

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 embryo-
logical 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 organisa-
tions, 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 organisa-
tion 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 con-
ditions 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

122 KAUL KKNST VON BAKU

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, uor the shape. Any recapitulation there ma}' 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 Eclicllc tics it res
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 <>f parallelism correct, is the late appearance
in development of parts which arc confined to the higher
animals. In the development of a Vertebrate accordingly

DOCTRINE OF TYPES 123

one would not expect the vertebrae 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 organisa-
tion 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). Ansbildiuig (differen-
tiation) 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 — (i) 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 medusae, by
starfish and their allies. The longitudinal type characterises
such genera as Vibrio, Filaria, Gordins, and all the annulate
animals. Mollusca, rotifers, polyzoa, and such infusoria as
are not included in types (i) 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

124 KARL ERNST VON BAER

plastic parts with their asymmetrical arrangement and
rounded shape belong to the massive type.

These types of von Baer inevitably recall the " Embranche-
ments" 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.1

Now it is clear that the development of the individual,
which is essentially an Aitsbildniig, 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.). Develop-
ment is not merely from the general to the special — there arc
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 In vertebrate. 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.

1 Cuvier, in 1812, Ann. MHS. if' /fist. Nat., xix. ; von llacr in 1816,
Nova Acta Acdd. l\>it. ('/ft: Sec Entwickelungsgcsckichte dcr Thiere,
i., p. vii., f.n.

LAWS OF DEVELOPMENT 125

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 un-
commonly 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.1 Generally the farther back we go in the develop-
ment 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 domestic/is.
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 :—

"(i) That tJie general characters of the Ing 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 t/ie 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

1 Compare a parallel passage in Prevost 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 extra-
ordinary 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 clucks of a corresponding age." — Ann. Set. nat.,
iii., p. 132, 1824.

126 KARL ERNST VON BAER

ncrcr rescmb/cs tlic adult of anotJicr 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 homologics of parts will be
to study their early development. " For since each organ
becomes what it is only through the manner of its develop-
ment, 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

THE EMBRYOLOGICAL CRITERION 127

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 tracheae
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

128 KARL ERNST VON BAER

groups as does the study of adult structure. The four types
of development are thus :—

( i ) 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 symmetri-
cally 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,1 for
reptiles develop an amnion and Batrachia do not.

We come now to the sixth and last Scholion. Develop-
ment 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 arc the
varied forms of the living" (p. 264).

Von Baer reminds one greatly of Cuvier. There is

1 Ih' F organisation i/t-x Aniin<ni.\\ i., p. 1^0, 18^2.

VON BAER AND CUVIER 129

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 l 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
Echelle des ctrcs^ evolutionary or ideal. Von Baer also,
though influenced greatly by NaturpJiilosophie, 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.'2

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 i823,3 " 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

1 " Ueber das iiussere und innere Skelet," Meckel's Archiv fiir Anat.
it. Physiol., pp. 327-76, 1826. See, too, his Entwickelungsgeschichte, i.,
pp. 181, ff.

'• Von Baer wrote an appreciative biography of Cuvier, published
posthumously in 1897, Lebensgeschichte Cuviers, ed. L. Stieda. French
trans, in Ann. Set. Nat. (Zool.\ ix., 1907.

3 Cuvier et Valenciennes, Histoire naturelle des Poissons, i., p. 550.

130 KARL ERNST VON BAER

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 lichcllc des ctrcs, 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. Jn 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 environ-
mental 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 <>f adult
structure, did recognise the importance of embryology;
following up some observations of Dutrochet he studied the
f-i-tal membrane of mammals and trird to establish their
homologies.1 And in his criticism of the vertebral theory of
the skull he advanced as an argument against the basi-

/.r. d'Hist. t\<it., iii., pp. 9^-119, 1817.

RELATION TO TRANSCENDENTALISTS 131

sphenoid being a vertebral centrum the fact (established by
Kerkring, 1670), that it develops from two centres.1

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.2 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 Arc/iiv, 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
(EutwickelungsgeschicJite, ii., p. 53). Groups of these morpho-
logical elements formed morpJiological divisions, such as the
vertebral segments of the head with their highly developed
neural arches, or the segments of the neck with their un-
developed haemal arches. The morphological elements are
clearly shown only in the animal parts, but there are indica-
tions in the embryo of a segmentation also of the vegetative
parts,— the gill-slits, for instance, and the vascular arches.

1 Lemons d? Anatomic comparee, 3rd ed., vol. i., p. 4^4, Bruxelles, 1836.

2 In the aforementioned paper in Miiller's Archiv he criticises
Carus vigorously and is sarcastic on Geoffroy.

132 KARL KHNST VON BAKU

The vegetative parts, however, develop on the whole unsym-
metrically (cf. Bichat). These elements which von Baer dis-
tinguishes 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 develop-
ment. He made the study of development an essential
part of morphology.

CHAPTER X

THE EMBRYOLOGICAL CRITERION

PANDER'S work of 1817 was the forerunner of an embryo-
logical 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 Abhandlnngen 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 Descrisione anatomica dcgli organi della
circolazione delle larvc dellc Salamandre aqnaticJie (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 Dn developpe-
ment 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

133

134 THi; K.MIMYOIXXJICAL CRITERION

the alimentary canal ; he observed and interpreted rightly
the formation of the medullary folds. The circular blasto-
pore in the frog in later years often went by the name of the
anus of Rusconi.

In France Dutrochet l investigated the ftetal membranes
in various vertebrate classes ; Prevost and Dumas studied
the very earliest stages of development in birds, mammals
and amphibia (Ann. Sci. uat., ii., iii., 1824, xii., 1827).

A little later came Duges' studies of the osteology and
myology of developing amphibia (1834),- and Coste's careful

researches into the early develop-
mental history of mammals.3

It was in 1825 that Heinrich
Rathke (1793-1860), published his
famous discovery of gill-slits in the
embryo of a mammal,4 a discovery
which aroused considerable interest,
and greatly stimulated embryological
research. He describes how in a
young embryo of a pig he saw four

-Gi":f s « U;e. *« slits in the region of the neck, going
Embryo. (After Rathke.)

right through into the oesophagus.

They were separated by partitions which he called Kictnen-
bogen (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 (Jsis, PI. IV., fig. i).

Later in the same year Rathke discovered gill-slits in the
chick/" 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

1 See review by Cuvier, Mem. Afus. Hist. H<I/., iii., pp. 82-97,
1817.

- Mem. SaiHins ctr<in^crs, vi. Kxtract in Ann. Sci. rial. (2) i. (Zool.),
pp. 366-72, 1834.

:t Recherches sur la generation ties Manuni feres, 1834. Embryogcnie
comparee, 1837.

1 " Kiemen bey Saugthieren," Isis, pp. 747-9, 1825.

' " Kieineii bey Voyeln," /sis, pp. 1100-1, 1825.

HUSCHKE. VON BAER 135

chick l 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 meta-
morphosis which these aortic arches undergo. This part
of his discovery he developed in fuller detail in a paper of
1828,- 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 MeckeFs Arc/iiv? 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.

1 " Ueber die Kiemenbogen und Kiemengefasse beym bebriiteten
Hiihnchen," Isz's, xx., pp. 401-3, 1827. (Read in Sept. 1826 to the
Versammlung der deutschen Naturforscher und Aerzte, then recently
founded by Oken).

2 /si's, pp. 160-4, PL II., 1828.

3 " Ueber die Kiemen und Kiemengefasse 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.

K

136 TIIK EMBRYOLOGICAL CRITERION

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 dis-
appeared. 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. 1 16) ; he recti-
fied 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 I832,1 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 Eus-
tachian 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 Rathkc, 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 ]>citriigc :jnr Gi-sc/iic/ite dcr Thicncclt
(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 lUldinigs-
nnd Entwickelungs-Geschichte dcs Mcnschcn mid dcr TJiicrc
(i., ii., Leipzig, 1832-3). A fellow-worker in this line was
Johannes Miiller, whose Bildungsgeschichte dcr (icn it alien
(Diisseldorf) appeared in iS3o.

In a memoir on the development of the crayfish which
1 Meckel's Arc/u'v, vi., pp. 1-47, 1832.

RATHKE 137

appeared in I829,1 Rathke found in an Invertebrate confirma-
tion 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, Ascllus?
Rathke returns to this point. Commenting on the original
similarity in development of antennae, 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 Hlor-
phologie, Reiscbemerkungen aus Taurien (1837), which con-
tains 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

1 Untersuchungen iiber die Bildiing und Entwickclung der Fluss-
Krebses, Leipzig, folio, 1829. Preliminary notice in his, pp. 1093-1100,
1825.

" " Untersuchungen iiber die Bildung und Entwickelung der Wasser-
Assel.," Abh. s-. Bild. u. Entwick.-Gesch., i., pp. 1-20, 1832. Translated in
Ann. Sci. nat. (2), ii., (ZooL), pp. 139-57, 1834.

138 THE EMBRYOLOGICAL CRITERION

history of the cmbryological 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, Miiller, and Wharton Jones.1 The
other important line - - the early development of the
Mammalia — was worked chiefly by Valentin,- Coste,15 and,
above all, by Bischoff, whose series of papers4 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 iibcr den Kiancu-
appai-at und das Zungcnbciu (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 Miiller made great use of
embryology in his classical }'crglciclicndc Anatomic do-
My.vinoiden (i. Theil, 1836), and, according to his pupil
Rcichert, firmly held the opinion that embryology was
the final court of appeal in disputed points of comparative
anatomy ;5 Reichert himself in a book of 1838 ( Vcrglciclicndc

1 Kolliker, Entwickelungsgeschichte^ 2nd ed., p. 17, Leip/ig, 1879.

- I/andbuch dcr Entwickelungsgeschiclite dcs Menschcn nnd . . . dcr
Sdugethiere und Vi>£cl, Berlin, 1835.

3 Embryo^c'nic compan'c, 1837 ; Histoirc generate du cUveloppement
dcs corps organist's, 1847-49.

1 Entwickelungsgeschichte des Kaninchen-EieS) Braunschweig, 1842 ;
Entwickelungsgeschichte dcs IIunde-Eics, Braunschweig, 1^45 ; Ent-
wickelungsgeschichte dcs Meersckweinchens^ (iiessen, 1852; Entwickc-
lungsgeschichte des Itches^ (I lessen, 1854.

" "It is the role 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 \ve have to expect
a definite clearing up of the problems relating to the development
of the head." — Muller's Archiv, p. 121, 1837.

THE EMBRYOLOGICAL ARCHETYPE 139

Entwickelungsgeschichte dcs 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 dis-
tinguish 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 x and 1833 '2 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,

1 Anat.-phil. Unters. si. d. Kicmenappamt n. d. Zttngenbein, Riga and
Dorpat, 1832.

14 " Bildungs- and Entwickelungs-geschichte des Blennius viviparus,"
Abhandl' z. Bild. n. Entivick.-Gesch. dcs Mcnschen u. der Thiere, ii.,
pp. 1-68, Leipzig, 1833.

140 THE EMBRYOLOGICAL CRITERION

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
Carus1 had called the visceral or splanchno-skeleton. He
did not, however, let this distinction hinder him from assert-
ing 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. I Ie could prove from embryology that
the jaws were not the equivalent of limbs, as so many
Okcnians believed. He could reject, with a mere reference
to the facts of development, Geoffrey's comparison of the
hyoid and the branchiostegal rays in fish with sternum and
ribs. lie could show the emptiness of the attempts made

1 I'tvi i/,->i Ur-Thcilen tics Knoclicn- und Schalcn-Gcrustcs, Leipzig,
1828.

DEVELOPMENT OF SKULL: RATHKE 141

by Carus, Treviranus, de Blainville and Geoffrey, 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.1 He did not,
however, himself realise the relation of the ear-ossicles to the
gill-arches, though he knew that Spix and Geoffrey 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 vivi-
parus, 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 haemal 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 vertebrae. (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 vertebrae, 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.

1 Kiemenapparat) pp. 107-118.

142 THE EMBRYOLOGICAL CRITERION

Von Baer, like Rathkc 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 develop-
ment 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
(Entwickelungsgeschichtet 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 vertebrae. At first we see the vertebra-
tion in the hinder region of the skull very clearly. After-
wards it becomes suddenly indistinct, as if some new forma-
tion overlaid it" (i., p. 194).

Even more clearly is his doubt expressed in his paper on
Cyprinns. " 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 vertebrae ceases abruptly where the back
passes into the head. I do not hesitate to assert tJiat bony
fish, too, have at this stage an unscgiuoitcd cartilaginous
cranium (as cartilaginous fish have all their life), the pro-
minences and hollows of which constitute its only resem-
blance with the vertebral type" (1835, p. 19).

A convinced supporter of the vertebral theory was
Johannes M tiller, who, in his classical memoir on the
Myxinoids,1 discussed at some length the relation between the
development of the vertebra and the development of the
skull. Mis 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 vertebra: in elasmobranch embryos ; for the facts
regarding other Vertebrates he relies largely on work by
Rathkc (r>lcnirins, 1833) and Duges (1834). He recognises
as the basis of his comparisons the homology of the notochord

ihcndc An<i/<>mit- dcr M^y.vinoidcn. I'art I. (Osteology and
Myology). (Abh. kthiigl. Akatl. U'iss. Berlin, for 1834, pp. 65-340, 9
pis., 1836.) Also separate! v.

DEVELOPMENT OF SKULL: MULLER 143

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 vertebra; 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
Miiller, of three vertebrae, whose centra are the basioccipital,
the basisphenoid and the presphenoid. Other bones
besides those belonging to the vertebrae are present, but this
formation out of three vertebrae 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 vertebrae we should
expect the centra of the skull-vertebrae 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 vertebrae ; 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, Miiller sees
a confirmation of his view that the skull is composed of three
and not four vertebrae. But there is nothing in Rathke's
observations to support the idea that the centra of the cranial
vertebrae are formed from separate halves. Miiller has to be
content with a reference to the state of things in Aunnocoetes
(which, by the way, he did not know to be the young of
Petromyzon). In the simple skull of Aunnocoetes 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 Miiller, are clearly the still separate
halves of the pars basilaris cranii, and represent the divided
centra of the two hinder cranial vertebrae. To complete the
parallel between the development of the skull and of the
vertebrae, it would have been necessary to show that the side
walls of the cranium developed in a similar manner from

144 THE EMBRYOLOGICAL CRITERION

separate pieces. M tiller 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 vertebrae are formed from separate halves was
supplied in 1839 by Rathke's discovery of the trabeculas 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 Miiller, 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,1
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 vertebrae which Reichert, like Miiller,
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 car-passage, internally into the
Kustachian tube, and in the middle a partition forms the
tympanic ring and tympanum. Inside each of the visceral
processes on cither side a cartilaginous rod develops. In

1 "I'eber die Visccralbo^en clcr \Yirbelthicre in All^cmeinen und
dercn Mctamorphoscn bci den V<i-<-ln und Siiugetliiere," Miiller's
Arclih1, |>|). 120-2-22, 1837.

DEVELOPMENT OF SKULL: REICHERT 145

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 l in fish, amphibians and
birds, is a long strip of cartilage which runs from the ear-
ossicle known as the hammer in mammals,2 to the inside of
the mandible. Reichert shows how this relation comes

FlG. 9. — Meckel's Cartilage and Ear-ossicles in Embryo
of Pig. (After Reichert.)

a. Mandible. h. Hammer. k. Incus.

g. Meckel's cartilage. i. Handle of Hammer. n. Stapes.

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

1 Handbuch d. menschl. Anatomic, iv., p. 47.

2 This was shown by Serres (Ann. Set. 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.

140 THE EMBRYOLOGICAL CRITERION

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.

In this paper Reichert made a distinct advance on the
previous workers in the same field — Rathke, Huschke, von
Baer, Martin St Ange, Duges. 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 Rathke1 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. II is
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,'2 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 practi-
cally 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 ptervgoid developed in the mucous
membrane of the mouth; the middle part gave, as in birds,

1 Ahhandl., i., p. 102, 1832 ; ii., p. 25, 1^33. (R/cnnius pnpcr).

r,-f^l,-ic/icni/t- Entwickelungsgeschickte iicx Kopfrs tier mickten
A)iip/iil>i<'!i} Konigsberg, quarto, 276 pp., 1838.

ARCHETYPE OF SKULL 147

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 modifica-
tions 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 precon-
ceived notion that the skull could be reduced to an
assemblage of vertebrae, 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 vertebras (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, snpra, p. nS). 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 vertebrae
(Urwirbeln, primitive vertebrae). In mammals and birds,
as Reichert had shown in his 1837 paper, the three cranial
vertebras 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 vertebrae

148

THE EMBRYOLOGICAL CRITERION

(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 vertebrae in the
cartilaginous trough formed by the twsis 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 vertebrae
in the membranous or carti-
laginous skull could be de-
limited 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 vertebra;, but, above all,
by the vesicles of the brain.

As in the higher Verte-
brates, the visceral arches are
associated with the cranial ver-
tebrae as their ventral exten-
sions, being equivalent to the
visceral plates which form the

ventral portion of the "primitive vertebnu " or primitive
segments of the trunk.

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

FIG. 10. — Cnuiial Vertebrae and
Visceral Arches in Embryo of
Pij*. Ventral Aspect. (After
Reicheit.)

JJase.

Sides.

Top,

First vertebra

Presphenoid

Orbitosphenoids

Frontala

Second vertebra .

Basisphenoid

Alisphenoids

Parietals

Third vertebra

Basioccipital

Exoci ipit.ils

Supraoccipital

ARCHETYPE OF SKULL 149

The other bones of the skull are not included in the

*

vertebrae, 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 vertebrae. This point, it is interesting to
note, had already been made by Oken in his Prograinm (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 vertebrae 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 Wonnian
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 HolocepJiala, Plagiostomata,
and Cydostomata appear to us to be lower developmental
stages individually differentiated, so that the other fully
differentiated Vertebrates cannot easily be referred directly

150 THE EMBRYOLOG1CAL CRITERION

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 modifica-
tions 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 t\vo papers are of considerable importance,
and Miiller's remark in his review1 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.'2 This memoir of
Rathke's is an exhaustive one and d'eals with the develop-
ment 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 ptery-
goid.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.

1 Miiller's Archiv for 1838.

- Entwickelungsgeschichte der Natter ) Konigsberg, 1839.

ARCHETYPE OF SKULL

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 vertebra;, 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

o

the ear-labyrinth, but in front of this the chorda-sheath
gives origin to a flat membranous plate which after-
wards 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 trabeculae. 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 trabeculae 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 trabeculae are formed, like the vertebrae,
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 vertebrae, and that the cranial
basis by means of its processes, the trabeculae, 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 con-
nections, 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"

L

152

TIIK EMBRYOLOGICAL CRITERION

(p. 40). When ossification sets in, the ear-capsule forms
three bones, of which two fuse with the supraoccipital and
exoccipitals.

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

FIG. ii. — Embryonic Cranium of the Adder. Ventral

(After Rathke.)

a. Basioccipiul.
'-. Kxoreipital.
c. Ear capsule.

('. Bssisphenoid.

c. Alispheiioid.
/. OrbitosphenoiJ.

Ii. Foramen.
. I'ituilar.v space.

in the posterior part of the basis cran'ii, and the exoccipitals
in the side walls of the trench in continuity with the
fundament of the basioccipital (see Fig. 11). The supra-
occipital is formed in cartilage above the exoccipitals. The
basisphenoid develops, like the basioccipital, in the flat basis
era nii, but towards its anterior edge, between the large
foramen (//) and the pituitary space (/). 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 trabeculae, just behind the point where

ARCHETYPE OF SKULL 153

they fuse. The side parts of the basisphenoid and pre-
sphenoid (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 trabeculae 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 hkiden as partially or
wholly destroyed" (p. 135).

Rathke's general paper on the development of the skull
in Vertebrates l 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

1 Bemerkungen iibcr die Entivickelung des Schadels der Wirbclthicre^
Konigsberg, 1839.

Till: EMBRYOLOGICAL CRITERION

skull in Annnocoetes, which in this respect is the simplest of
all Vertebrates (pp. 6, 22). In Aininococtcs, as Johannes
Mtiller 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 trabecuku, and of the para-
chordals which he was ready to assume from his embryo-
logical observations.

Miiller 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 HatidbncJi der PJiysiologic the cartila-
ginous 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 vertebras,
two symmetrical bands of cartilage, similar to those which I
discovered as a persistent structure in Ainniococtcs. . . .
At a later period the basis cranii of vertebrate animals
contains three parts analogous to the bodies of vertebrae,
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." l

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. -

1 Ilandbuch der Physiologic dcs Afcnsc/tcn, Koblenz, 1835 ; Eng.
trans, by W. lialy, ii., p. 1615, 1838.

; For a full staiemcnt of Rathke ;s conclusions," see the translation
given by Huxley in Lectures on the Elements of Comparative Anatomy,
London, 1864.

CRITICISM OF VERTEBRAL THEORY 155

The chorda sheath is the common matrix of the vertebrae
and of a large part of the skull. The basilar plate and the
trabeculae, which are developed from the chorda sheath, give
origin to three bones, which might possibly be considered
equivalent to vertebral centra — the basioccipital, the basi-
sphenoid, and the Riechbcin (ethmoid). The Rieclibcin
develops from the fused ends of the trabeculae. The pre-
sphenoid might also be considered as a vertebral body, but it
develops independently of the basilar plate and trabeculae.

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 independ-
ently 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 pre-
sphenoid, 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 " vertebrae " do, but
only two prolongations of it, the olfactory lobes. In its
development and final form it shows no particular resem-
blance to a vertebra. Its body, the pars perpendicularis
(mesethmoid) shows no similarity with a vertebral centrum.
Completing the three hinder cranial " vertebras " 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 vertebrae, but is rather to be com-
pared to the intercalary bones in the vertebral column of
certain fish. Summing up as regards the cranial vertebrae
Rathke writes, "We find that the four different groups of
bones, consisting of the basioccipital with its intercalary
(the supraoccipital), the basisphenoid with its intercalates

156 THE EMBRYOLOGICAL CRITERION

(parietals), the presphenoid with its intercalaries (frontals),
and the ethmoid with its outgrowths (turbinals and cribri-
form plate), taking them in order from behind forwards,
show an increasing divergence from the plan according to
which vertebra; 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
vertebra; can be recognised, although in a more or less
modified form. Yet at most only four cranial vertebra.- can
be assumed, and these differ from ordinary well-developed
vertebra: in their manner of formation the more the farther
forward they lie." l

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? and in his paper on the development of Alytcs"*
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 trabeculre ; a " facial plate " was also
formed, which seems on the whole equivalent to the plate
formed by the fused anterior ends of the trabeculaj. 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

1 Enttvickelungsgeschickte der \\'irlh-ltlricre,\>. 142, 1861.

-' /tnil>n't>lni;i<- des Sdlinoncs. A separate volume of L. Ayassiz's
Histoire nuturellc <lcs I'nissons d' Ran douce dc r Europe <-cn/r<i/c,
Neuchatel, 184?.

' Untersuckungen fiber die Entwickelungsgeschichte der Gchiirtshel-
ferkri>(t, Solotluirn, 1842.

CRITICISM OF VERTEBRAL THEORY 157

lying in front of this cannot themselves be vertebrae, 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 vertebrae 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 vertebras) 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 vertebrae at all."

L. Agassiz, himself a pupil of Dollinger, in the general
part (1844) of his Recherches sur les Poissons fossilcs
(Neuchatel, 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 Muller to dispose of the theory,1 which maintained a firm
hold even upon embryologists. It was still upheld by
Reichert, and Kolliker in 1849 showed himself convinced of
its general validity.

A useful step in the analysis of the concept " vertebra "
was taken by Remak,2 who showed what a complex affair the
formation of vertebrae really is, involving as it does a complete
resegmentation (Neuglie'derung] 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

1 Miiller's Archiv for 1843, p. ccxlviii.

! Unfersuchungcn iibcr die Entivickelung dcr Wirbelthierc, Beilin,
1850-55.

158 THE EMBRYOLOG1CAL CRITERION

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 vertebrae
(p. 36). The gill-slits, he considered, could not possibly be
regarded as marking the limits of head vertebrae.

In 1858 appeared Huxley's well-known Croonian Lecture,
0/t tJic TJicory of the Vertebrate Skull} 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 embryo-
logical 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 arc developed from a portion

1 Delivered i/tli June 1858. Reprinted in The Scientific Memoirs of
T. //. 7/n.r/cy, edited l>y M. Foster and K. Ray Lankcster, vol. i., pp.

5;vs-r,,,r, (1898).

DEVELOPMENT OF SKULL: HUXLEY 159

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 conten-
tion 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 MeckeFs carti-
lage. The scheme of development of the jaws is accord-
ingly 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 offish.

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 deter-
mining 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 ElasinobrancJiii, 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

160 THE EMBHYOLOGICAL CRITERION

features to the ossified skull of other Vertebrates; the

relations of the ear to the vagus and trigcminal 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 developments1 of conditions

through which the embryonic crania of the highest members

of the sub-kingdom pass" (p. 5/3). 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

Ainpliioxns, the lamprey, and the shark, we shall discover the

model and type of the latter in the former. The skull of the

Ajiip/iio.vus presents a modification of that plan which is

exhibited by the frog's skull when its walls arc 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 \vhen 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 larvae, 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 arc originally

alike; in all (save .li/ip/i io.v/is ?} the base of the primitive

cranium undergoes the mcsoccphalic flexure, behind which

the notochord terminates, while immediately in front of it

1 Cf. Rcichert, st//>m, p. 149.

CRITICISM OF VERTEBRAL THEORY 161

the pituitary body is developed ; l 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 trabeculae, 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. 573).

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
vertebrae? 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 trabeculae, 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 vertebrae. 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

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

162 THE KMBRYOLOGICAL CRITERION

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 vertebra?. " 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,1 in two lectures delivered
to the Royal College of Surgeons, demonstrated that in the
human fiutus 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- 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:! 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

1 Human Ostcogcny e.\-/>l<iin<-tt in two Lectures, London, 1736.

l>, ,v////.v ossci Rsocis lucii structura sin«ul<iri. nisscrt. inaug.
Rejjiomonti, 1X22.

:) "Uebcr das ausscrc und inncrc Skclct," Meckcl's /Itr/iiv, pp. 327-

MEMBRANE AND CARTILAGE BONES 163

series among the cartilaginous fishes, according as a cartila-
ginous skeleton penetrates more and more into this fibrous
foundation. In the same way the process of ossification sup-
plants 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 distinc-
tion between a primitive cartilaginous foundation and a
secondary overlying ossification of the skull showed itself in
the development of all Vertebrates. Duges, in his RecJiercJies
sur I* osteologie et la myologie dcs Batraciens ( 1 834), 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 I8381 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,2 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 (Entiv. d. Natter., p. 197).

1 Vergl. Entwick. d. Kopfes d. nackten AmpJiibicn (p. 186).
- Arch.f. mikr. Anat., xi., Suppl., 1874.

1G4 THE EMBRYOLOGICAL CRITERION

Jacobson in 1842 1 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 RccJierches snr les Poissons fossiles* 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 — (i) the
cartilaginous base, including the nuchal plate, the trabeculae
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 : —
(i) the basioccipital, exoccipitals (paroccipitals ? ), supra-
occipital and " petrous " (rocher), developed from the nuchal
plate ; the ali- and orbito-sphenoids developed from the
trabecula: ; the "cranial ethmoid "3 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 vomcr from the "inferior"
plates. The other element, the cartilaginous brain-box, docs
not ossify, and tends to become absorbed (p. 124).

In 1849 Kollikcr published a paper1 dealing with the

1 " Om Primordial-Cranict," ForhancUingar Skand. Naturf. Jlfi>/e,
Stockholm, 1842.

- Vol. I., General part, pub. 1844.

Entosphenoid, Owen.
1 Ziucitcr Bericht zootom. Anstalt zu \\'itrzburg,

MEMBRANE AND CARTILAGE BONES 165

morphological significance of the distinction between
membrane and cartilage bones, and in IS5O1 he defended
his views against the criticisms of Reichert 2 in a further
note entitled Die Theorie des Primordialschadels festgeJialten.
It is convenient to consider these papers together. Kolliker
held that there was (i) a histological and (2] a morphological
difference between the two categories of bones. The histo-
logical 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. Kolliker'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
Kolliker'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
vertebrae in certain fish, he modified his view of the mem-
brane bones, and admitted them, at least in some cases, as
constituents of the cranial vertebrae.

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 considera-
tions, 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

1 Zeits.f. iviss. Zool., ii., pp. 281-91.
- Muller's Archiv for 1849, PP- 443-5I5-

100 TIIK EMBRYOLOGICAL CRITERION

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 Meckcl'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 homo-
logised 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 Kolliker,
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. M tiller1 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 '2 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 prcmaxillaries, 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 :!
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,

I /.cits. f. MSS Zi>o/., ix., 1858.

-' Kntiv. d. \\'irl>clthic>\\ pp. 139-40, 1861.

II Lectures on the Elements of Comparative Anatomy.

CRITERIA OF HOMOLOGY 167

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."1 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 '2 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).

1 On the Archetype of the Vertebrate Skeleton, p. 5, 1848.
* System der thierischen Morphologic, Leipzig, 1853.

M

168 THE EMBRYOLOGICAL CRITERION

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.

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 com-
pounded of -the four primitive elements, earth and water, air
and fire, the homogeneous parts or tissues and the hetero-
geneous 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 struc-
ture. 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

169

170 THE CELL-THEORY

as a whole, how it lived and nourished itself, and they
studied cells and sieve-tubes, wood fibres and tracheae with
a view rather to finding out their functions and their
significance for the life of the plant than to discovering
the minutiae of their structure. The same attitude was
taken up by the few botanists who in the iSth century
paid any heed to the microscopical anatomy of plants. For
C. F. Wolff,1 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 ipth century, interest in plant anatomy
revived somewhat, and much work was done by Treviranus,
Mirbel, Moldenhawer, Meyen and von Mohl.'2 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, In'ifni^'c r-itr
Phytogenesis? Schleiden's theme in this paper is the origin

1 Theoria generationiS) Halae, 1759.

- See J. v. Sachs, Geschichtc dcr Botanik, book ii., Eng. Trans., 2nd
iinpr., 1906.

3 Miiller's Archiv, pp. 137-76, 1838.

SCHLEIDEN 171

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 character-
istic spot which he called the areola or nucleus.1 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 (Kcriic/iett), 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 observa-
tions 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
1 Trans. Linnean Soc., xvi., p. 710, 1833.

172 THE CELL-THEORY

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 Muller.1 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 Kugelchen or little
globules suspended in it (Entwickelungsgeschichte^ i., pp. 19,
144). Since such KiigclcJicn disposed in a row formed the
notochord (i., p. 145) it seems probable that his Kilgelchen
were really cells. Similarly A. de Ouatrefages'2 in 1834
saw and figured segmentation spheres in the developing
egg of Liuimca, but he called them globules and did not
recognise their analogy with the cells of plants. According
to M'Kendrick,;J Fontana, so far back as i/Si,4 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 i836,5 and compared gland cells with the cells
of plants in 1837.'' Henle in 1837 ~ and 1838 s 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.

1 Myxinoiden, i. Theil., p. 89, 1835.
- Ann. Sci. nat. (2) (Zool.} ii., pp. 107-18, pi. u, 1834.
3 Proc. Phil. Soc. G/astfw, xix., pp. 71-125, 1887-8.
1 Traitc sur le venin de la vipire, 1781.
1 Miiller's Archiv, 1836.

'; J. Muller, Jithrcsbcricht ii. d. Fortschritte deranat.-physiol. Wisscn-
schaftcn i»i jahre 1838. Miiller's Arc/iiv, 1838.

~ Symbolce ad anatomiam villorum imprimis eorum cpitJiclii, Berlin,

1837-

< U. d. Ausbreitung dcs Epitheliums im menschlichen Kvrper,
Miiller's Arc/ii7', 1838.

SCHWANN 173

Valentin l 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 Miiller 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
Donne 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 olants. Miiller himself had

O A

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 Notisen, No. 91, 1838), the
full memoir in 1839, under the title Mikroskopische Unter-
sucJiungcn fiber die Uebereinstimmung in der Struktur und
dem WacJistume der Tiere und Pflanzen?

Theodor Schwann was a pupil of Johannes Miiller, and
we know that Miiller took much interest in the new his-
tology. It is probably to his influence that we owe Schwann's
brilliant work on the cell, which appeared just after Schwann
left Berlin for Lowen. 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 physio-

1 See Schwann's Bemerkungcn at the end of his Mikroskopische
Untersuch ungen.

2 Republished in Ostwald's Klassiker der exakten Wissenschaften,
No. 176, Leipzig, 1910. References in the text are to the original
pagination.

174 THE CELL-THEORY

logical 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 Anatotnische-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 resorbecl, 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

STRUCTURE OF OVUM 175

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 1 discovered and described in the unfertilised egg of the
fowl a small vesicle containing granular matter, which
he named the Keimblaschen or germinal vesicle. ' It dis-
appeared 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 2 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.3 Even von Graaf had noticed that the
early uterine eggs were smaller than the supposed ovarian
eggs; Prevost and Dumas4 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

1 Symbolae ad ovi avium historiam.

'2 De ovi mammalium et ho minis genesi.

3 De mulieruin organis, 1672. '* Ann. Set. nat., iii., p. 135, 1842.

176 THE CELL-THEORY

that this small sphere was indeed the mammalian ovum.
Ilis discovery was confirmed by Sharpey and by Allen
Thomson. Von Baer found the germinal vesicle in the eggs

o o o

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 l 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 Jones2 found the germinal vesicle in the
mammalian ovum. Valentin in i835,3 Wagner in i836,4
and Krause in 1837;"' 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 Keiinflcck or germinal spot, known sometimes
as Wagner's spot. He recognised the Kcimflcck in the ova
of many classes of animals from mammals to polyps.
Frequently more than one Kcimjlcck 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 difficult)' 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

1 Rccherclies sur !<i generation lies Mamniifires. Report by Academy
Committee. Ann. >V/. nat. (2) (Zool.~) ii., pp. i-lS, 1834 ; also Einbryo-
X<'nic I'l'iiifiart'e, 1837.

- I.oinL and J'lilin. Phil. M<i£. (3) vii., 1835 ; Phil. Trans. 1837.

:i Handbuch der Entwickelungsgeschichtet 1835, and M tiller's Archiv,
1836.

' rroiiroinns histoHa generati&nis hominis ntijuc aniinaliuin, Lipsiae,
1836.

•'• Mullcr's Archil', 1837.

FORMATION OF BLASTODERM 177

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

o

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

178 THE CELL-THEORY

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 difficult}-. 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

CELL-FORMATION 179

set end to end — a mistaken view soon corrected by Vogt
(Euibryologic des Salmoncs, 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 extra-
cellular 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,

o

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

180 THE CELL-THEORY

(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 pur-
posiveness 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

MATERIALISM AND TELEOLOGY 181

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 reproduc-
tion. If we suppose that a vital force unifies and co-
ordinates 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

182 THE CELL-THEORY

organisms, that the whole "individual" organism must be an
aggregate °f 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 meta-
bolic 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 assimi-
latory 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

INDIVIDUAL LIFE OF THE CELL 183

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 xan 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

N

184 THE CELL-THEORY

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 intussuscep-
tion, 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 trans-

CELLS AS IMBIBITION-CRYSTALS 185

formed 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 — (i) 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 tp 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

186 THE CELL-THEORY

forties.1 Criticism of the Schwann-Schleiden theory from
the zoological side was suggested by the study of the seg-
mentation 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 Provost and
Dumas in i82/j.,'2 in the frog and newt by Rusconi/' and an
elaborate study of the process in the frog had been made by
von Baer.4 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,5 and in the same year Loven saw segmen-
tation in CainfxTtnulana^ and Sars in the starfish and in
Nudibranchs. 7

In 1838 Bischoff8 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 '•' in 1839.
C. Vogt10 in 1842 described segmentation in Coirgoniis
and Alytes. The discovery of segmentation in the ovum of

1 Sachs, History of Botany, Book ii.

- Ann. Sci. nat., i., pp. 110-14, 1824. Swatnmerdam is said to have
observed the 2-cellcd stage in the egg of the frog (Ribl. Nat., 1752), and
Rosel v. Rosenhof the same stage in the tree-frog (f/ist. tint, rananim
nostratium, 1758).

*D£ueloppementdelagrenouille commune, Milan, 1826. Biblioteca
italiana, Ixxix., 1836, and Muller's Archi-v, 1836. Agassiz is said by Vogt
(1842) to have seen segmentation in the Perch as early as 1831.

1 Muller's Archiv, 1836.

•"' In Burdach, Die Physiologic ah Erfahrungswissenschaft, 2nd Ed.,
vol. ii.

'• Wiegmann's Archiv, 1837.

' Bcricht Vcrsamml. dcutsch. Naturf. in 1'rag, 1837.

H Bcricht Versamm. dcutsch. Naturf. in Freiburg, 1838. Later in
his Entw. d. WirbcltJi., and in his papers on the development of the
rabbit.

'•' Phil. Trans., 1839. See particularly PI. vi., figs. 105-12.
'" Embryologie des Salmones 1842.

SEGMENTATION 187

birds was not made until 1847, by Bergmann,1- confirmed
independently by Coste'2 in 1850. By 1848 segmentation
had been noted in Hydra and various hydroids, in acalephs,
in starfish, polyzoa, nematodes, rotifers, leeches, oligochsetes,
polychaetes, in most groups of molluscs and arthropods, and
in all the vertebrate classes.3

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 i84i4
called attention to the presence of nuclei in the segmenta-
tion-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.5 Reconsidered
the nuclei to be anucleate cells, and the same view was
taken by Kolliker in 1843.''' Next year, however, in his
classical paper on Cephalopod development 7 Kolliker 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 i846.s Leydig in 1848° 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 i852,10 and more fully in his monumental

1 Muller's Archiv, 1847.

- C.R. Acad. Sci., xxx., p. 638.

3 See review by Leydig in Isis, 1848, pp. i6[-ig3.

4 Muller's Archiv, pp. 89-102, 1841.

•' De evolutions Stron^yli auric, ct Ascaridis acum., Erlangen,
1841.

0 Muller's Archiv, pp. 66-141, 1843.

7 Entwickelungsgeschichte der Ccphalopoden, Zurich, 1844.

3 Froriep's Notizen, No. 800, 1846.

9 his, 1848.

1(1 Muller's Archiv, p. 47, 1852, also 1854 and 1858.

188 THE CELL-THEORY

Untersuchungen iibcr die Kiit-^'ickclnng tier WirbeltJiiere
(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.1

One point Remak left undecided — the fate of the
Keimblaschen 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 iS//.2 Though Barry had held in 1839 that the
egg-nucleus does not disappear in segmentation, J. Muller
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
finfoconc/Kr mirabilis? and his paper was known to Remak,
who could not, however, observe a similar division of the
egg-nucleus in the frog. Muller's discovery was confirmed
for Oceania annata by Gegenbaur,4 and for Notommata
sicboldii by Ley dig.5

In 1854 Virchow,6 previously a supporter of Schwann,
crystallised the new views in the famous phrase — Oinnis ccllnla
e cellula — and gave wide publicity to them in his classical
lectures on Cellular Pathology, delivered in i858.7 The new
doctrine of cell-formation was also taught by Leydig8 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,9

1 See particularly Plate IX., figs. 3-7.

- Ilist.-krit. BancrkutigeH zu den ncitestcn Mitthcilun^en ii. d. erste
Entwickelung d. Sdugethiereier^ Mtinchen, 1877.

:! Monatsber. Akad. Wiss. Berlin, 1851.

1 Znr Lehrc von Gcnerationswcchsel u. d. Fortpflanxen d. Mcduscn u.
Polypen.

' U. d. Ban 11. d. system. Stellit/n; d Rndcrtltierc, 1854.

" Arch f. path. Anat. rhys., vii., ])p. 1-39, 1854. Also in his Keitnige
s. sp,-<\ f'tit/i. it. Tlicrapic.

~ J)ic Cclliilarf>,ttlwlo£it\ I'.crlin, 1858.

" 1.1-hrlnich dcr I listolo^ic, 1857.

:i Ann. Sci. n<it. (2) iii., pp. 108-9 and pp. 312-4, 1835. Also iv,

PP 343-77-

DEFINITION OF THE CELL 189

Purkinje,1 Cohen2 and Max Schultze.3 The last-named in
1 86 1 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. 1 1).

1 1839 or 1840.

2 Nova Ada Acad. Lcop., xxii., 1850. Trans, in 1853 for Ray
Society.

3 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 him-
self; 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 Coregonus1 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 develop-
ment self-sufficient. . . . Each cell consequently represents
a little independent organism, which assimilates foreign sub-
stances, builds them up, and rejects those that are useless ;

1 Rinbryologic (/,-s Sn///io>ies, 1842.
100

CELLS AS VITAL UNITS 191

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."1 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 for-
gotten.

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 2 on the application of the

1 Die Celhilarpathologie in ihrer Begriindung aiif physiologische tend
pathologische Gcwebelchre, Berlin, 2nd ed. 1859 ; Eng. trans., by Chance,
1860.

- Arch. path. Anat. Phys., vii., pp. 1-39 (1854).

192 CLOSE OF THE PRE-E VOLUTION A RY PERIOD

cell-theory to physiology and pathology called forth a vigorous
protest from Reichert,1 who discussed in a very instructive
\vay 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
M tiller, who was Reichert's master, had in his Physiology-
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 M tiller 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, com-
posed of cells; but between the form and connections of

1 Ilcricht iibcr (Ue I'orlschritlc dcr mikroskopischen An itoinic i»i

1854. Miiller's Archiv, 1855. Sec also
-' l/ndb. if. rhysiol., i., 1835.

DISINTEGRATION 193

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 develop-
ment 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 con-
catenation 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 i
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.1

Naturally enough, morphologists did not accept this
rather contemptuous estimate of their science, but held

1 See Leuckart's reply to Lud wig's criticism, in Zcit. f. wiss. ZooL. ii.,
p. 271, 1850.

194 CLOSE OF THE PRE-EVOLUTIONARY PERIOD

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 dcr thierischcn Morphologic? 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 />V;vV///,
applied to the rather wild theorisings of the physiologist
Ludvvig 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 i/th 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

1 Leipzig, 1853.

MARINE ZOOLOGY 195

Raymond 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 Progres recents des Sciences zoologiques en
France" (Paris) 1867, how " About the year 1826, two young
naturalists, formed in the schools of Cuvier, Geoffrey 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 a Ihistoire 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.1 Note that though they called themselves

1 Souvenirs (fun Naturaliste, 2 vols., Paris, 1854. Eng. Trans, as
Rambles of a Naturalist on the Coasts of France, Spain, and Italy, 2
vols.j 1857.

196 CLOSE OF THE PRE-EVOLUTIONARY PERIOD

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 x in the restricted sense, but physio-
logical morphology. As a matter of fact they produced more
ta>«0momic 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 Natnrelle ties Cntstaces'*1 and A. de
Ouatrefage's Histoire Naturelle des Anneles marins ct (feau
douce'A were typical products of their activity.

In the North, men like Sars and Loven 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 M tiller was using the tow-net to gather material for
his masterly papers on the metamorphoses of Echinoderms.4
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 a la zoologie gene rale (1851), well reflect this
Cuvierian attitude/' He acknowledges himself the debt he

1 Milne-Edwards later published a classical textbook on comparative
anatomy and physiology — Lemons sur la Physiologic et f Anatomic

rers, 14 vols., Paris, 1857-80.
''• Paris, 1834-40. Three volumes of the Suites <i Bujfon.
' Paris, 1865. Two volumes of the Suites >"i Huff on.
1 U. <{. Metamorphose dcr ^/>hiun-fi it. Secigcl., Berlin, 1848. U. d.
Metamorphose der Holothurien it. Asterien., Berlin, 1851.

1 As I have been unable to obtain a copy of the Introduction^

MILNE-EDWARDS 197

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 stand-
point, 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 instru-
ments" (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 <jf 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 organisa-
tion. This can only be attained by a division of labour l

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.

1 This principle was first developed by Milne-Edwards in 1827,
in the Dictionnaire classique ctHist. 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 special-
isation of the appendages and the ganglionic chain.

198 CLOSE OF THE PRE-EVOLUTIONARY PERIOD

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 undiffcrentiated part to special functions, or
she may even convert to other uses an organ already
specialised (p. 464). So, for example, the function of respira-
tion 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

MILNE-EDWARDS 199

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 ex-
traordinary parallelism between the families of marsupials
and the orders of placentals,1 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

'•Studied by Isidore Geoffrey St Hilaire in his paper Classification
parallclique des Maniiniferes, C. R. Acad. Set., xx., 1845. Remarked
upon by Cuvier, Rcgne animal. , i., p. 171, 1817, also by de Blainville.

O

200 CLOSE OF THE PRE-EVOLUTIONARY PERIOD

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 par-
ticular 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.1 So, too, Johannes Miiller, 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.2

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.3

Not many years after the publication of Milne-Edwards'
Introduction a la soologie gtntrale (1851) there appeared a
book by II. G. Bronn in which was offered a very similar
analysis of organic diversity. The curious thing was that

1 Cuvier et Valenciennes, Hist. nat. Jcs Foissons, i., p. 550, 1828.
-' Myxinoidcn, Th. I. Abh. k. Akad. Wiss. Berlin for 1834, pp.
loo, no, 179, etc.

1 Vcrgl. Enlw. Kopf. nackt. Amphibicn, p. 101, 1838.

H. G. BllONN 201

Bronn approached the problem from quite a different stand-
point, from the standpoint, indeed, of Naturphilosopkie. Of
this the title of the book is itself sufficient proof-
Morpkologische Studien fiber die Gestaltungs-gesetze der
Naturkorper iiberhaupt mid der organischen insbesondere
(Leipzig and Heidelberg, I858).1 The linking up of
organic with inorganic form is characteristic ; there is much
talk, too, in the book of Urstoffe and Urkrdfte, but underlying
the Naturphilosophie we can trace the same Cuvierian'treat-
ment 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 2 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 Amor-
phozoa, Actinozoa, and Hemisphenozoa — are essentially
adaptations to particular modes of locomotion. Animals
either are fixed, or they move in all directions without refer-
ence 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 Ccelentera, which are fixed, and the Echino-
derms, which have a central point and move indifferently
along any radial axis ; their form differs from the strobi-
loid 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

1 I have not seen the companion volume on palaeontological pro-
gression, Unters. ii. d. Entwickelungsgesetse der organischen Welt
wtihrend der Bildungszeit unserer Erdobcrfliiche, Stuttgart, 1858.

2 "Strobiloid" because of its spiral development. The theory of
the spiral growth of plants played an important part in botanical
morphology about this time.

202 CLOSE OF THE PRE-EYOEUTIONARY PERIOD

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 development1 (Entivickclmigsgesetze}, 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 differ-
entiation is by far the most important, and most of the
others arc 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. Hronn himself recognised the great
similarity of this law of progressive differentiation to Milne-
Edwards' principle of the division of labour; he seems, how-
ever, to have arrived at it independently.

1 Cf. Meckel's Principle of progressive Evolution, su/»v, p. 93.

H. G. BRONN 203

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, medusae, etc.) " (p. 141).

It was characteristic of the 'forties and 'fifties that trans-
cendental 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 Kolliker, and we find J. V. Carus in I8531
taking it as almost conclusively proved.2

We may mention, too, as showing clear marks of the
influence of transcendental ideas, L. Agassiz's work on the
principles of classification.3 And Serres, who was Geoffrey's

1 System der thierischen Morphologic, pp. 33, 457. Also C. Bruch,
Die Wirbellheorie des Schcidels, am Skelette des Lachses gepriift,
Frankfort-on-Main, 1862.

2 In France the vertebral theory was advocated by Lavocat in his
Notivelle Ostcologie comparee de la tete des animau.v doinestiques, Toulouse,
1864. It seems also that Lacaze-Duthiers held fast to it even in 1872—
Arch. zool. exp. gen., i., p. 51, 1872.

3 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.

204 CLOSE OF THE PRE-EVOLUTIONARY PERIOD

chief disciple, recanted not a whit of his doctrine of recapitu-
lation, but re-affirmed and expanded it from time to time,
and particularly in a lengthy memoir published in i860.1
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,2 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. Mt'»n>irs, vol. i., p. 571).

It is instructive to find that between Serres and Milne-
Edwards there existed the same antagonism as between von

1 " Principes d'Embryogcnie, de Zoogcnie ct de Terntogcnie," Mem.
Acad. Sci., xxv., pp. 1-943, pis. xxv., 1860.

2 "On the Morphology of the Cephalous Mollusca," Phil. Trans.,
1853, Sci. Memoirs, i., pp. 152-92.

SERRES AND MILNE-EDWARDS 205

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,1 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 i844,2 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, Etienne Serres, who in a number of
papers published in the 'thirties and 'forties,3 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,

1 " Observations sur les changements de forme que les divers Crus-
taces eprouvent," Ann. Set. nat. (i) xxx., p. 360, 18.33.

2 " Considerations sur quelques principes relatifs a la classification
naturelle des animaux," Ann. Set. nat. (3) i., p. 65, 1844.

3 Supra, pp. 79-83. Also Precis cPanatomie transcendante, principes
(f organogenic, Paris, 1842.

206 CLOSE OF THE PRE-EVOLUTIONARY PERIOD

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,1 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. S33).2

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 develop-
ment of Vertebrates and Invertebrates." A memoir was
presented the next year by Lereboullet 3 which met with
the approval cf the Academy in so far as its statements of
fact were concerned, but seemed to them to require amplifica-

1 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).

; It is wo! tli while recording that Serrcs 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 cmbryologists.

: " Kmbnolngic comparee du Brochet, de la Perche, et de 1'Ecrc-
visse," Ann. Set. nat. (4), i., p. 237, 1854 ; ii., p. 39, 1854. M<»i.
Savons etrangers^ xvii.

LEREBOULLET 207

tion 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 monographs1
on the embryology of the trout, the lizard and the pond-
snail LyvifHza,a.nd rounded offhis work with a full discussion2
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 forma-
tion. 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 there-

1 Ann. Set. nat. (4) xvi., p. 113, 1861 ; xvii., p. 88, 1862 ; xviii., p. 5,
1862 ; xix., p. 5, 1863.

2 xx., p. 5, 1863.

208 CLOSE OF THE PRE-EVOLUTIONARY PERIOD

fore 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 prim- '
ordial distinction " (p. 58).

In other directions also von Baer's work was confirmed
and extended by later observers — those parts of it particu-
larly 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 up-
holder of the doctrine, and made considerable use of it in
his writings.1 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 ex-
pressive phrase.2

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.3 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 Medusa; 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 physio-
logical 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 genera-
tion " (p. 24). Von Baer had already hinted at this homology

1 Particularly in his lUcnnius (1833) a"d Natter ( \ 839).

•' In the " preliminary notice "of his Crayfish paper — /si's, pp 1093-
1 100, 1825.

' "On the Anatomy and the Affinities of the Family of the Medusre,"
/'////. Trans., 1849 ; Set. Memoirs, i., pp. 9-32.

REMAK 209

in the second volume of his Entwickelungsgeschichte (1837),
where he says with reference to the separation of the blasto-
derm 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 Allman1 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 inter-
preting the formation of the germ-layers. Microscopical
technique also had been greatly improved since i828.2

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,3 two cell-layers, of which the undermost

1 Phil, Trans., cxliii., p. 368, 1853.

' The principle of achromatism was discovered (by Fraunhofer) and
achromatic microscopes introduced in the early part of the igth century.
The use of chemical reagents, such as acetic acid, and various hardening
fluids, came into fashion not long after. J. Muller 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;.

3 Untersuchungen iiber die Entwickclung der Wirbelthiere, folio, pp.
xxxvii + 195, 12 plates, Berlin, 1850-1855.

210 CLOSE OF THE PRE-EVOLUTIONARY PERIOD

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
Hornblatf], 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. Ail 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 con-
nective 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 pre-
eminently the layer that forms muscular, nervous, vascular
and connective tissue. In view of its functional significance,
it may be called the molory 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 (Sciten-
plattcn}. The former exhibit almost as soon as they are

REMAK

211

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 Darmfaserplatteti], and the
mother-tissue of the generative organs (the Mittelplatteri}.

FlG. 12. — Transverse Section of Chick Embryo.
(After Remak.)

h. Epidermis.

m. Spinal cord.

mw. Dorsal plate.

ug. Pronephric duct.

pa. Aortic root.

hp.\

and r" Hautplatte."
itm. J

mp. " Mittelplatte."
d/. " Darmfaserplatte."

x. Edge of amniotic fold.
ph. Pleuro-peritoneal
cavity.

d. Epithelium of alimen-
tary canal.

From the Hautplatten there develops, without the dorsal
plates seeming to take any part in the process, the rudiment
of the extremities " (p. 79).

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 histo-

212 CLOSE OF THE PRE-EVOLUTIONARY PERIOD

logical 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.

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,1 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,2 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

1 Radl, loc. «'/., i., p. 71. 2 Kritik der Urtheilskraft^ 1790.

213

214 LAMARCK AND DARWIN

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."1

So, too, Buffon's evolutionism was suggested by his
study of the structural affinities of animals, and Erasmus
Darwin in his Zoononria (1794) brought forward as one of
the strongest proofs of evolution, " the essential unity of
plan in all warm-blooded animals."'2

But, as a matter of historical fact, no morphologist, not
even Geoffrey, 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 iSth and ipth centuries arose in connection with the
transcendental notion of the Eclicllc tics circs, 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 i8th century, and received complete and even
exaggerated expression from Bonnet and Robinct. Buffon
also was influenced by it. Towards the beginning of the
iQth century the idea was taken up eager!}' by the trans-
cendental school and by them given, in their theories of the

1 Eng. Trans, by J. II. Hcrnard, p. 337, London, 1892.
•' II. F. Osborn, From the Greeks to Daru-in, p. 145, New York and
London, 1894.

TRANSCENDENTALISM AND EVOLUTION 215

"one animal," a more morphological turn. Their recapitula-
tion 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.1 Many of them wavered between the
conception of evolution as an ideal process, as a Vorstel-
hmgsart, 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 evolu-
tion to be a real process.

We have already in previous chapters '2 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.3 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 Eclielle ties ctres had for him much less a

1 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.

2 Chapters vii. and ix.

3 On early evolution-theories see, in addition to Osborn and Rndl,
J. Arthur Thomson, The Science of Life, 1899, and the opening essay in
Darwin and Modern Science, Cambridge, 1909.

P

216 LAMARCK AND DARWIN

morphological orientation than it had even for the transcen-
dentalists, 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 naturellc dcs Aniinanx sans
Vertcbrcs (1816-22). His speculative work on biology is
contained in three publications, the small book entitled
Considerations snr r organisation dcs corps vivants (1802), the
larger work of 1809, the Philosophie zoologique, and the
introductory matter to his Animanx sans Vertcbres (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 PJiilosopJiic zcologiquc 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 ethers regard as
forming a separate species."1

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

1 Phil, zool,, ed. Ch. Martins, vol. i., p. 75, 1873.

LAMARCK: SCALE OF BEING 217

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 Echelle des etres.

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. 6. Arachnids. 11. Fishes.

2. Polyps. 7. Crustacea. 12. Reptiles.

3. Radiates. 8. Annelids. 13. Birds.

4. Worms. 9. Cirripedes. 14. Mammals.

5. Insects. 10. Molluscs.

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 Eclielle is in no way a morphological one,
and was not intended to be such. It is a scale of increasing

o

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

218 LAMARCK AND DARWIN

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 organi-
sation 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, proceed-
ing 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 Histoirc natit relic
dcs Aiiiinan.v sans Vcrtebrcs^ 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: — " i. No kind or particle of matter

1 Quotations in the text arc from the 2nd Edit. (Ueshayes and Milne-
Edwards), i., Paris, 1835.

LAMARCK'S ZOOLOGICAL AXIOMS 219

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 maybe,
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 material-
ism, 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 interienr. He has some
difficulty in defining exactly what he means by it: — "I

220 LAMARCK AND DARWIN

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 arc endowed with intelligence it nevertheless
acts in such a case entirely without premeditation and
before any mental operation has brought its -icill 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 trans-
forming external into internal excitations.

To this second group of animals, possessing the sentiment
intcricur, 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
interienr 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 (<VW//.v)
experienced by the sentiment interienr. The sentiment
interieitr is therefore the cause not only of instinctive action
but also of all morphogenetic processes. Will and intellig-
ence (which are confined to a relative!)' 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 l-.elielle des
ctrcs. What he wrote in the riiilosopliie zoologitjue 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
I'xuinet (p. no), but refuses to accept his view of an ILeliellc
extcnding down into the inorganic. Like Bonnet, however,

LAMARCK'S FOUR LAWS 221

j

and like the German transcendentalists, Lamarck makes
man the goal of evolution (p. 116). He makes it quite clear
that his Echelle 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
(An/in, s. Vert., p. 134). To the latter cause are due the
lacuna, 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 Laiv. — 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 Laiu. — The degree of development of organs and
their force of action are always proportionate to the use made
of these organs.

" Fourth Laiv. — All that has been acquired, imprinted or
changed jn the organisation of the individual during the
course of its life is preserved by generation and transmitted

222 LAMARCK AND DARWIN

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.1

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 inte'ricnr.

1 For instance, Lucretius : —

" Is titii nunc animus <|iuli sit mrpon- ft limit-
constiteril pcn^mi rationcm rc<Mrn- ilictis.
Prinripi" i-- e aio persubtilem atquc mimitis
pi-ii|ii:ini corporibus fartum constare."

— Hi- /\,-i //>n \iiitir<i, iii., w. 177-80.

LAMARCK: SENTIMENT INTERIEUR 223

In the large group of apathetic or insensitive animals,
which do not possess this faculty, needs cannot be experi-
enced ; 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 per-
forming 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 interieur 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

224 LAMARCK AND DARWIN

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

LAMARCK'S VITALISM 225

state in which we now see them, become easily determin-
able" (p. 1 68).

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.1

" 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 material-
istic philosophy ?

We have seen that irritability, the sentiment interieur,
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
PJiilosopliie zoologiqne and the introduction to the Animaux
sans Vertcbres 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 interieur acts by directing the movements of
the subtle fluids of the body (which are themselves modifica-
tions 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 interieur 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 senti-
ment interieur 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

1 Contrast Treviranus — •" In every living being there exists a capa-
bility 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.

226 LAMARCK AND DARWIN

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 psycho-
logical 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 assimila-
tion of organ-formation to instinctive behaviour, his constant
insistence on the importance of bcsoin and Juibititdc.

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.1 He took no interest in the morpho-
logical 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 cairi'awiicc tics parties, the marvellous
co-ordination of parts to form a whole; he had little concep-
tion 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

1 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. Sec S. I'.utler, Evolution, Olii «ml New, London,
1879, for an excellent account of Erasmus Darwin.

LAMARCK A SYSTEMATIST 227

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 environ-
ments. 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 sim-

228 LAMARCK AND DARWIN

plicity 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 . — Scries of Non-articulated 1 1. — Series of Articulated

Animals. Animals.

Infusoria.

I
Polyps.

'^ rt

'« &

c .5 '

1) C

Ascidians. Radiates. Worms.

It

in C

C 'P

o> ~,

C/J <

Epizoa.
Acephala.

Annelids. Insects.

Molluscs.

Crustacea.
Cirripedes.

Arachnids.

Fishes.

^'B I Reptiles.

Birds.
Mammals.

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 Cuvicr paid him at
least the compliment of criticising his theory,1 not distinguish-
ing 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

1 As did also Lyell in his Principles of Geology, 1830.

VON BAKU ON EVOLUTION 229

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 iQth century evolution was
in the air. There were few of von Baer's contemporaries
who had not read Lamarck ; l Erasmus Darwin's Zoonoinia
ran through three editions, and was translated into German,
French and Italian ;2 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 evolu-
tion 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? 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 palaeontology, 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 him-
self 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 develop-
ment, 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

1 K. E. von Baer, Reden, i., p. 37, Petrograd, 1864.

2 Radl, loc. cit., i., p. 296. 3 Reprinted in his Reden^ i,, 1864.

230 LAMARCK AM) DARWIN

attitude to Darwinism as he did to the evolution theories in
vogue in his youth.

Although in the twenty or thirty years before the publica-
tion 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 Geoffrey
St Ililaire, the son of the great Etienne Geoffrey St
Hilaire, seems to have held that species might be formed
from varieties. The law which L. Agassiz thought he could
establish,1 of the parallelism between pala^ontological
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.2

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 environ-
ment. 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. lie
was not satisfied with the theories of evolution that had been
proposed by his grandfather, by Lamarck, and by K.
Geoffrey St Hilaire — he did not indeed understand these
theories any too well, lie resolved to work out the problem
in h-is own way, for his own satisfaction. He tells us all this
very clearly in his autobiography. " During the voyage

1 See Huxley's criticism of it in a Royal Institution lecture of 1851,
republished in AV/. .!/<•;//., i., pp. 300 4. On its relation to Hacckd's
biogenetic law, see below, p. 255.

- System dcr thierischcn Morphologic, p. 5, 1853.

DARWIN AS FIELD NATURALIST 231

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." x

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

1 Life and Letters of Charles Darwin, ed. F. Darwin, i., p. 82,
3rd ed., 1887.

Q

232 LAMARCK AND DARWIN

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 convenancc des parties and the inter-
action 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 prob-
lems 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 morphologi-
cal 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 morpho-
logical facts and its relation to the current morphology of his
time.

The sketch of his theory, written in I842,1 shows a very
significant division into two parts — the first dealing with the
positive facts of variability and the theory of natural selection,

1 T/ie Foundations of the Origin of Species^ a Sketch written in 1842.
Kd. F. Darwin, Cambridge, 1909.

DARWIN ON MORPHOLOGY 233

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
fcetal 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 fcetus — 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. 5I).1

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

1 Cf. a parallel passage in the Origin^ 1st ed., pp. 485-6.

234 LAMARCK AND DARWIN

of Species (ist 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 indica-
tions 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 con-
sider 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. Geoffrey St
Ililaire, and Owen, referring indeed specifically to Geoffrey's
law of connections. " What can be more curious," he asks,

DARWIN : UNITY OF TYPE 235

"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 por-
poise, and the wing of the bat, should all be constructed on
the same pattern, and should include similar bones, in the
same relative positions? Geoffrey St Hilaire has strongly
insisted on the high importance of relative position or con-
nection 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

o o

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 " meta-
morphosis," 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, " meta-

236 LAMARCK AND DARWIN

morphosis " 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 recapitu-
lation, 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 develop-
ment. 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.1

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,

1 In the ist ed. (p. 439), Darwin makes the curious mistake
of attributing this story to Agassiz.

DARWIN: EMBRYOLOGICAL ARCHETYPE 237

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 mani-
festly tend to resemble each other much more closely than
do the adults, just as we have seen in the case of pigeons"

(PP- 446-7)-

Since the embryo shows the generalised type, the struc-
ture 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. " Em-
bryology 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

1 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,

238 LAMARCK AND DARWIN

naturally turns to great advantage in his argument for
evolution. Throughout the whole chapter Darwin's pre-
occupation with the problems of classification is clearly
manifest.

On the question as to whether descent was mono-
phyletic 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,1 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,'2 and followed
with interest the progress of evolutionary morphology, incor-
porating 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-
Kdwards, 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 " corre-

1 See his Letters, passim.

'• Writing to Huxley on the subject of the hitter'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. Scward, 1903, i., p. 73.

DARWIN AND CUVIER 2 -.9

lation " 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 : x 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.

1 Italics mine.

240 LAMARCK AND DARWIN

Darwin's conception of correlation was singularly in-
complete. 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. 1/7). 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,1 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 develop-
ment, 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

1 Das Problem des Lebens. Biologischc Studicn. Bad Sacha, 1906.
See also E. Radl, JUol. Ccntralblatt, xxi., 1901.

DARWIN ON CORRELATION 241

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 Radl, 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.
W'ith 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. " Darvvinismus " 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

242 LAMARCK AND DARWIN

of development, that there must have been manifested in
evolution some definite and orderly tendency towards
perfection.1

No one expressed this objection with greater force than
did von Baer, in a series of masterly essays2 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 (Zi'eF) — " 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 evolntio 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 Kollikcr,
who shortly after the publication of the Origin pro-

1 See the excellent treatment of the difference between the " realism "
of Darwin and the "rationalism" of his critics, in Kadi, ii., particularly
pp. 109, 135. The most elaborate criticism of Darwinism from the
older standpoint was that given by A. Wigand in Der Danoinismus
nnd <iic Naturforschung New tons und Citviers, 3 vols., Braunschweig,
1X72.

' In vol. ii. of his Rcden, St Petersburg (Petrograd), 1876—
Uebcr den Ziveck in den Vbrgdngen der Natur ; Uebcr Ziektrebigkeit in
den organischen l\<<rpcrn insbesondere j and Uebcr Da nviri's LeJirc,

KOLLIKEirS CRITICISM OF DARWIN 243

mulgated in a critical note on Darwinism a sketch of his
theory of " heterogeneous generation." l

Kolliker's attitude is typical of that taken up by many
of the morphologists of the day.2 He accepts evolution
completely, but rejects Darwinism because it recognises
no Eiitivickelungsgesetz, 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 Kolliker laid more stress upon the Entwickelnngsgesetz
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 mani-
fests 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
Kolliker regarded his principle of evolution as mechanical.

1 " Ueber die Darwinische Schopfungstheorie," Zeitx. f. wtss. Zool.t
xiv., pp. 74-86, 1864. Elaborated in Anat. u. syst. Beschreibung d.
Alcyonarien, 1872.

2 Cf. for instance Nageli's theory of a perfecting principle, first
developed in his Entstchung u. Begrijf der natiirhistorischer Ari,
MUnchen, 1865.

244 LAMARCK AND DARWIN

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 IS681 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 PJiilosopJiic ties Unbewussten
(1868) and in his valuable essay on Wahrheit tffnd Irrtum im
Dartvinismus (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'2 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

1 Anatomy of Vertebrates, iii., 1868.

- Rapport sur les I'rogrcs rccents des Sciences zoologiques en France.
Paris, 1867.

MILNE-EDWARDS ON "CREATION11 245

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
palaeontology 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 interven-
tion 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.

CHAPTER XIV

KRXST IIAECKEL AND CARL (JKGKNUAUR

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 com-
parison of adult structure ; and second, by the comparative
study of developing embryos. It \vas 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

240

MORPHOLOGY AND EVOLUTION 247

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 MorpJwlogy of Organisms} 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-

1 Gene re lie Morphologie der Organisuien. Allgevieine Grundstige der
organischen Formenivissenschaft, nicchanisch begriindet durch die von
C/t. Darwin reformierte Descendenzthcorie. Berlin, 1866. Reprinted in
part as Prinzipien der generellen Morphologie der Organismen. Berlin,
1906.

R

248 ERNST HAKCKEL AM) CAUL GEGENBAUR

Darwinian thought. It was a medley of dogmatic materialism,
idealistic morphology, and evolution theory ; its sources
were, approximately, Biichner, 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 Natur philosophic.

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 reproduc-
tion, 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 Dar-
winism.

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 i 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 ('uvicr called the principle of
the "conditions of existence." The concept of correlation
had simply no meaning for Haeckel. The analysis and

HAECKEL: IDEALISTIC MORPHOLOGY 249

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 ho\v Bronn tried in a modest
way to found a stereometrical morphology, but had the grace
not to push his stereometry a routrauce, recognising very
wisely that the greater part of organic form is functionally
determined. Haeckel took over this idea1 and pushed it to
wild extremes, founding a new science of " Promorphology " of
which he was the greatest — and only — exponent.2

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 signi-
ficance; 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 : — (i) Plastids (Cytodes and cells) ; (2) Organs
(including cell-fusions, tissues, organs, organ-systems) ; (3)
Antimeres (homotypic parts, i.e., halves or rays) ; (4) Meta-
meres (homodynamic parts, i.e., segments); (5) Persons
(individuals in the ordinary sense) ; (6) Corms (colonial
animals).

The thought is essentially transcendental, and recalls

1 He mentions as his predecessors in this field, Bronn, J. Muller,
Burmeister, and G. Jager.

2 In Grundriss einer Allgeineinen Naturgeschichte der Radiolarien,
Berlin, 1887, and Kunstforinen der Natitr, Suppl. Heft, Leipzig.

250 ERNST MAECKEL AND CARL GEGENBAUR

the " theory of the repetition of parts," of which so much
use was made by the German transcendentalists, such as
Goethe,1 Oken, Meckel and K. G. Carus, as well as by
Duges.

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. \Ye
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 elaborat-
ing, 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 con-
nected 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

1 Ilacckel had an intense admiration for. Goethe's morphological
work. It is a curious coincidence that the work of Goethe, Oken
and Hacckel was closely associated with the town of Jena.

HAECKEL: GENEALOGICAL TREES 251

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 MorpJwlogy. The genealogical trees depicted in
the second volume (plates i.-viii.) are nothing more than
graphic representations of the ordinary systematic relation-
ships 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 (Formenbuschel) "
(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.1 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
1 But he himself would not admit this ! See Gen. Morph., ii., p. ii.

252 ERNST HAECKEL AND CARL GEGENBAUR

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 Miiller 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 book1 in which he showed that individual development

clue to ancestral history.

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. ill). 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 develop-
ment of the species will be mirrored in its developmental
i_history " (p. 112).

"TOf course the recapitulation of ancestral history will be
neither literal nor extended. " The historical record preserved
in developmental history is gradually effaced as the develop-
ment strikes into a constantly straighter course from the
egg to the perfect animal, and it is frequently sopJiisticatcd
by the struggle for existence which the free-living larv.x-
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 c<>n-

1 h'iir Darwin, 1864. KM;.;, trans, by D.illas &S, Facts and Arguments
for Darwin^ London, i86y.

HAECKEL: BIOGENETIC LAW 253

ceived 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 Mala-
costraca, and that accordingly the nauplius and the zoaea
larvae 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. Miiller.

In much the same form as it was propounded by Miiller
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 GrundgesetrJ] — 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 palaeontological evolution
according to the laws of heredity and adaptation.

254 ERNST HAECKEL AND CARL GEGENBAUR

" 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 bion1 during its individual develop-
ment adapts itself to new conditions: accordingly the
repetition is the more accurate the greater the resem-
blance 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. Miiller.

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

1 The bion is the physiological, as the morphon is the morphological,

individual.

THE THREE-FOLD PARALLELISM 255

parallelism Haeckel merely resuscitated in an evolutionary
form a doctrine widely discussed in the 'forties and 'fifties,1
and championed particularly by L. Agassiz,2 a doctrine
which must be regarded as a development or expansion of
the Meckel-Serres law.3 It is the view that a parallelism
exists between the natural system, embryonic develop-
ment, and palaeontological 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 palaeozoic 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 arrange-
ment 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

1 See Vogt, Embryologie des Salmones, p. 259, 1842, and supra, p. 230.

2 An Essay on Classification, London, 1859.

3 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 dis-
appear during the development of the individual animal'' (p. 73, i8o8j.

256 ERNST HAECKEL AND CARE GEGENBAUR

real species at the present day ; they might or might not
be discoverable as fossils. That they had real existence

j

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 palaeontology.

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 hypo-
thetical, 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 con-
crete 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 /:>//<•//<• idea : l — " 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 transforma-
tions 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

1 The Ilitlory of Creation, vol. i., p. 310, 1876. Translation of the
Natitrlichc ScMpfttngSgeschii htc, 1868.

HAECKEL: BIOGENETIC LAW 257

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.'1

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
AKjIaeckel, phytogeny is_the cause of ontogeny. This thought,
"I that the or^amiTsrrris^efore 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 organisa-
tion, 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. Miiller, that the true course of recapitulation was
frequently modified by larval and fcetal adaptations. As
time went on, he was forced to hedge more and more on this
point, and finally in his Anthropogenic (1874) and his second
1 Cf. a parallel passage from Serres, supra, p. 82.

258 ERNST HAECKEL AND CARL GEGENBAUR

paper on the Gastr.ta theory (I875),1 he had to worR 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 fo)-in,
and are transmitted from this by Jiercdity, obviously possess
primary importance for the understanding of the casual-
physiological relations ; on the other hand, those develop-
mental processes which appear subsequently through
adaptation to the needs of embryonic or larval life, and
accordingly can not be regarded as repeating the organi-
sation 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 ceno-
genetic. Considered from this critical standpoint, the \vhole
of ontogeny falls into two main parts: — First, palingenesis, or
'epitomised history' {Auszugsgeschichte}^ and second, ceno-
gencsis, or ' counterfeit history ' (Falschungsgeschichte). 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."

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 noto-
chord 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 aorta; 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

1 Jenai\chc /.citschrift, ix., pp. 402-508, 1875.

2 Loc. cit.. ix., p. 409.

HAECKEL: CENOGENESIS 259

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 accelera-
tion 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,

260 KKNST IIAKCKKL AM) CAUL (il-XJKMJArU

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 morpholo-
gists 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 con-
temporaries.

His friend, Carl Gegenbaur, who gained a great and
well-deserved reputation by his masterly studies on verte-
brate morphology,1 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 Grundzngc der vergleichenden Anatomic, the second
edition of which, published in 1870, soon came to be
regarded as the classical text-book of evolutionary morphol-
ogy, Gegenbaur enunciated very much the same general
principles as Haeckel, and referred to the Gcncrcllc Morphol-
ogic as the chief and fundamental work on animal mor-
phology. 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. Accord-
ing to Haeckel,- Gegenbaur was greatly influenced by
J. M tiller, 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 arc
considered by Gegenbaur to be entirely distinct sciences,
with different subject-matter and different methods. " The
task of physiology is the investigation of the function's of the
animal body or of its parts, the referring back of these
functions to elementary processes and their explanation by

1 Untcrsuchiin^cn zur vergl. Anatomic d. //';>/>< 7////V;v, Leipzig, i.,
1864 ; ii., 1865 ; and iii., 1872.

- " U. d. Uiologie in Jena wiihrend des 19 Jahrlumderts," Jcnaische
Zt'itschriff, xxxix., pp. 713-26, 1905.

. GEGENBAUR: METHOD OF MORPHOLOGY 261

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 — compara-
tive 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

262 ERNST HAECKEL AND CARL GEGENBAUR

fact is of great importance, for those changes \vhich 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 \vhich 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
Meckcl-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, palaeontological 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"1 (p. 6).

It is worth noting that in Gegenbaur's opinion compara-

1 Grundriss der vergl. Anatomic, 1874, 2nd ed., 1878. Trans, by
F. Jeffrey Hell, revised by E. Ray Lankester, as Elements of Comparative
Anatomy, London, 1878.

GEGENBAUR ON ADAPTATION 263

tive 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 (Gmndsiige, pp. 7-8).

While Gegenbaur was at one with all " pure " morpholo-
gists, 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 inter-
preted the homological resemblances of animals as being
due to heredity, their differences as due to adaptation,1 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 v/hich 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, pre-
served 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 con-

1 " 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 "
(Grundziige, p. 19).

S

264 ERNST IIAECKEL AND CAUL GEGENBAUR

servativc principle. . . . Adaptation is commenced by a
change in the function of organs, so that the physiological
relatio)is 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 physio-
logical" (Elements, pp. 8-9). Gegenbaur recognised also that
morphological differentiation depended largely on the
physiological division of labour (Grundziigc, 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."1 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" (Grundziige} 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 cor-
relation a knowledge of functions, and of the functional
relations of the organism to its environment, is clearly indis-
pensable.

1 History cf Creation, i., pp. 241-2.

GEGENBAUR: GENEALOGICAL STANDPOINT 265

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 . . . corre-
sponds to the hypothetical genetic relationship. In the
more or the less clear homology, we have the expres-
sion of the more or less intimate degree of relationship.
Blood - relationship becomes dubious exactly in propor-
tion 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 " (Grundsiige,
p. 75). The real ancestral forms cannot, as a rule, be dis-
covered 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

266 ERNST HAECKKL AND CARL GEGENBAUR

with the ancestral form, which remains essentially hypo-
thetical" (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 physio-
logically eqivalent, 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 resem-
blances of organs within the organism, and includes four
kinds of resemblance, homotypy, homodynamv, homonomy
and homonymy. Right and left organs are homotypic,
metameric organs are homodynamic ; homonomy is the rela-
tion 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 (Grundzitgc, p. 80).

Special homology, on the other hand, relates to re-
semblances 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"
(fc/t-iin-Hfs, 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 separ-

LANKESTER ON HOMOGENY 267

ated 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 pro-
posed in 1870 a genetic definition of homology.1 He pro-
posed, 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 liomogenous (p. 36). All
other resemblances were to be called homoplastic. " Homo-
plasy 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 morpholo-
gists 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.

1 " 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

KAKIA" THEORIES ON THE ORIGIN OF VERTEBRATES

HAKCKKL and Gegenbaur set the fashion for ptiylogenetic
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 micro-
scopical anatomy with its endless variety of stains and
reagents made it possible for the tyro to confirm in a day
what von Baer and Miiller had taken weeks of painful
endeavour to discover.1 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

1 The stages in the development of microscopical technique are well
summarised by R. lUirckhardt, (.Icsihichtc tier /.oolo^ie, p. 121, Leipx.ig
1907.

208

A. KOWALEVSKY 269

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 Gastraea 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 Gastraea and Ccelom
theories.

In the first period we might again distinguish two main
tendencies, according as speculations were based mainly
upon anatomical or mainly upon embryological considera-
tions, 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

270 THEORIES ON THE ORIGIN OF VERTEBRATES

Russian on the development of Amphioxus, published in
1865. This subject was followed up in two papers which
appeared in 1867* and 1877.- In his papers on Amphioxus
Kowalevsky made out the main features in the develop-
ment 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 sur-
mised that it was formed from the cavities of the " primitive
vertebrae," 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,3 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 communi-
cated memoirs of even greater interest,4 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-

1 " Kntwickclungsgeschichte dcs Amphioxus lanceolatus," Mem. Acad.
Sci. St Petersbourg (Petrograd) (v-ii.), xi., No. 4, 1867, 17 pp., 3 pis.

- " Weitere Studien vi. die Entwickelungsgeschichte des Amphioxus
l.inccolatus," Arch, fiir mikr. Anat., xiii., pp. 181-204, 1877.

3 Particularly by Hatschek (1881) and Uoveri (1892).

4 " Entwickelungsgeschichte der einfachen Ascidien," Mem. Acad.
Sci. St Petersbourg (Petrograd), (vii.), x., No. 15, 1866, 19 pp., 3 pis.
"Weitere Studien u. die Entwicklung der einfachen Ascidien/' Arch. f.
mikr. Anal., vii., pp. 101-130, 1871.

THE ASCIDIAN THEORY 271

known tailed larvae, 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 ecto-
dermal 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.

Kowalevsky's results were accepted by Haeckel, Gegen-
baur, Darwin,1 and many others as conclusive evidence of
the origin of Vertebrates from a form resembling the ascidian
tadpole; they were extended and amplified by Kupffer2 in
1870, later by van Beneden and Julin3 and numerous other
workers ; they were adversely criticised by Metschnikoff 4
and von Baer,5 as well as by H. de Lacaze-Duthiers and
A. Giard.6 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 for-

1 Descent of Man, i., p. 205, 1871.

2 Arch. f. mikr. Ana?., vi., 1870, and viii., 1872.

3 Archives de Biologie, 1884, 1885, and 1887.

4 Bull. Acad. Set. St Petersbourg (Petrograd) xiii., 1869, and Zeits.
f. iviss. Zool., xxii., 1872.

5 Mem. Acad. Set. St Petersboiirg (Petrograd) (7), xix., 1873.

6 Giard, Arch. zool. cxper. gen., i., 1872, and Lacaze-Duthiers, ibid.)
iii., 1874.

FlG. 13.— Development of the Ascidian Larva. (After Kowalevsky.)

GIARD'S CRITICISMS 273

gotten 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 estab-
lished 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.1 The fall of the Ascidians
from their larval high estate provided the text for many
a Darwinian sermon.

1 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 Herouard, Traite de Zoologie concrete. Tome viii., Paris, 1898.

274 THEORIES ON THE ORIGIN OF VERTEBRATES

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 Ursprungder
WirbcltJiiere nnd das Princip des Functiotiswechsel (Leipzig).
He followed this up by a long series of studies on vertebrate
anatomy and embryology,1 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 cnira ccrcbclli in the A'.ow rlioinboiJca,
and that in Vertebrates this mouth has been replaced
functionally by a new ventrally placed mouth, formed by the

1 "Studien zur Urgeschichte des \Virbelthierkorpers," MitthciL
Zool. Staf. ,V,v//"/, 1882-1907.

DOHRN 275

medial coalescence of a pair of gill-slits.1 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 Chaetopods
at the present day. For their support cartilaginous gill-arches
naturally arose in the body-wall, and the superficial muscu-
lature 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 disap-
peared, 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

1 Leydig (Vom Bauc des thierischen Kbrpers, Tiibingen, 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.

L'TG THEORIES ON THE ORIGIN OE VERTEBRATES

be the most primitive living Vertebrates and yet show no
particular anneliclan affinities ? Dohrn tries to answer this
awkward question by showing that these forms are not
primitive but degenerate. He points out first that Cyclo-
stomes 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 de-
generate still further, forms would result which would
resemble Amphioxus, and ultimatel)', if the process of de-
generation 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 trans-
formation of an organ takes place by reason of the succes-
sion 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 Arthro-
pods have become transformed into jaws — their function as
graspers of food has gradually prevailed over their main

PRINCIPLE OF FUNCTION-CHANGE 277

function as walking limbs. In the evolution of Vertebrates

o

from Annelids the principle came into action in many con-
nections— 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^ 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."

Dohrn was well aware of the functional, or as he calls

1 Zcits.f.iviss. Zool., xliv., 1886.

• Quoted by E. B. Wilson, Wood's Holl Biological Lectures for 1894,

p. 121.

278 THEORIES ON THE ORIGIN OF VERTEBRATES

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 com-
prehended apart from it " (p. /o).1

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
( Urqualitdf] of all being. . . . Let us take it as given "

(P- 75)-

Semper's speculations on the genetic affinity of Articu-
lates and Vertebrates are contained in two papers- 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 Geoffrey was right in
maintaining against Cuvier3 the unity of plan of the whole

1 Cf. Metsclmikoff, Quart. Journ. Mtcrosc. Set., xxiv., pp. 89-111,
1884.

2 "Die Stammesverwandschaft der Wirbelthiere und Wirbelloscn,"
Arb. zool.-zoot. Instit. \Viirzburg, ii., pp. 25-76, 1875 ; "Die Verwand-
schaftsbexiehungen dcr gegliederten Thicre," Ibid., iii., pp. 115-404,

1876-7.

3 Abuse of Cuvier also dates from the early days of evolution, see
Racll, ii., pp. 12-17.

SEMPER 279

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 (rnesonephric)
tubules of the latter, upon which he published in the same
volume an extensive memoir. At this time he considered
Balanogtossus — 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 Geoffrey's
principle of the unity of plan and composition to the three
great metameric groups, the Annelida, Arthropoda, and
Vertebrata. Semper follows Geoffrey'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, haemal) 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 bigeuiina], the

T

280 THEORIES ON THE ORIGIN OE VERTEBRATES

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 (cvolntio gcinina}.
Semper is at pains to prove that evolutio bigeinina is charac-
teristic also of Annelidan development.

n

FlG. 14. — Transverse Section (Inverted) of the Worm .\',tis. (After Semper.)

a.c. Alimentary canal.

n.c. Nervi; conl.

sp.tj. Sniiuil ganglion.
n. Notochord.

d.p. Neural inii^rlr. plate.
..'. 1 1 ai'inal muscle-plate.

He gets his facts from an elaborate study of the process
of budding in the Nnirfu-, 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 Xais and

Chictogaslcr a strand of cells appears between the alimentary

canal and the nerve-cord, and that from this axial strand the

THE ANNELID THEORY 281

haemal 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 CJitztogaster 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 haemal 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 haemal 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
haemal 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 Chcetogaster, 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, meso-
nephric tubules, are thus common to Annelids and Vertebrates
— what further proof could one desire of the close relation-
ship 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

282 THEORIES ON THE ORIGIN OF VERTEBRATES

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
cesophageal 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 Verte-
brates is represented by the simple Keiinblase 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 analog)- between
the nephridia of Annelids and the mesonephric tubules of
Vertebrates,1 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,- Balfour came to the conclusion " that
we must look for the ancestors of the Chordata, not in allies

1 "On the origin ;md history of the urino-genital organs of
Vertebrates," Journ. Anat. P/iys., x., 1876. The conclusions of Balfour and
Semper were adversely criticised by M. Fiirbrmger (MorpJi. Jahrb., iv.,
1878), and were negatived by later research.

- A Monograph on the Development of Elasmobranch Fishes, London,
1878.

F. M. BALFOUR 283

of the present Chaetopoda, but in a stock of segmented forms
descended from the same unsegmented types as the
Chcttopoda, 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 sugges-
tion as to their existence is well founded, appears now to
have perished." x

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 supposi-
tion 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,2 names of hypothetical
groups being printed in capitals, names of degenerate groups
in italics :—

Mammalia. Sauropsida.

PROTO-AMNIOTA. Amphibia.

I _ I

PROTO-PENTADACTYLOIDEI.

Teleostei.

Ganoidei.
I

Dipnoi.

PROTO-GANOIDEI.

— Holocephali.
— E lasmobranchii.

PROTO-GNATHOSTOMATA.

Cyclostomata.

PROTO-VERTEBRATA.

r i

Cephalochorda. PROTOCHORDATA. Urockorda.

1 A Treatise on Comparative Embryology, vol. ii , p. 311, London,
1 88 1. 2 Loc. tit., vol. ii., p. 327.

284 THEORIES ON THE ORIGIN OF VERTEBRATES

The hypothetical ancestral forms (Protochordata) pos-
sessed 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 Cyclo-
stomes 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,1 on the
relation of Nemertines to Vertebrates.

The Annelid theory was firmly supported by Eisig, who
in his elaborate monograph on the Capitcllidie" maintained
against Fiirbringer the genetic identity of the Annelidan
nephridia with the kidney tubules of Vertebrates. The
independent discovery by E. Meyer :? and J. T. Cunningham,4
of an internal segmental duct in Lanicc, into which several
nephridia opened, seemed to strengthen this view.

Following Ehlers/' Eisig found the homologue of the
notochord in the accessory intestine of the CapitellidcB and
Eunicidtc, which he supposed might easily be transformed,
according to the principle of function -change, from a
respiratory to a supporting organ. lie 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,(i and for a time by Semper. These strands were

1 "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 (Lccrbock van dc Grondbegimelen
dcr Dicrkundc, 1874).

2 " Monographic der Capitelliden des Golfes von Neapcl," Fauna
11. I-'lora dcs Golfes von A'w/V/, Monog. xvi., Berlin, 1887.

3 Mitt. tool. Stat. Neapcl, vii., 1887. ' Nature, xxxvi., p. 162, 1887.
" Ncbendarm und Chorda dorsalis," Nachr. Gcx. ll'iss. Gottingcn,

p. 390, 1885.

11 " Embryologische Studicn an Wiirmern u. Arthropoden,:: J /<•>//.
Acad. Sri. \/ r.'tersbourg (I'etrograd), (7), xvi., 1870. And in Arch f.
inikr. Anat., vii., p. 122, 1871.

VARIANTS OF ANNELID THEORY 285

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 homo-
logies of the brain and mouth of Annelids and Vertebrates.

o

Beard l and others got over the difficulty of the haemal
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 hypo-
physis represents the old Annelidan mouth. •

Van Beneden and Julin2 assumed that in the ancestors of
Vertebrates the oesophagus shifted forward between the still
unconnected lobes of the brain to open on the haemal surface.

The fundamental assumption of the Annelid theory, that
dorsal and ventral surfaces are morphologically interchange-
able, seemed rather bold to many zoologists, and Gegenbaur3
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 i866.4 Tornana
was discovered by J. M tiller 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 larvae of Echinoderms.5

1 "The Old Mouth and the New," Anat. A/is., iii., 1888. Nature,
xxxix., 1889.

2 " Recherches sur la Morphologie des Tuniciers," Arch, dc Biol.t
vi., 1887.

3 " Die Stellung u. Bedeutung der Morphologic,'' Morph. Jahrb., i.,
pp. 1-19, 1876.

4 "Anatomic des Balanoglossus,'' Mem. Acad. Sci. St Pctersbourg
(Petrograd), (7), x., 1866.

5 Zcit.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.

286 THEORIES ON THE ORIGIN OF VERTEBRATES

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.1

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 pro-
duced his monumental Materials for the Study of Variation
(1894).

He found a strong positive argument for his theory that
Vertebfates 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 archcnteron, and has a
vacuolated structure. The gill-slits of Halanoglossns, with
their close resemblance in detail to those of Amphioxus,
Bateson also used as an argument in favour .of the phylo-
gcnetic relationship of the Entcropneusta and Vertebrata,

1 In a scries 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,

THE BALANOGLOSSUS THEORY 287

together with the formation from the ectoderm of a dorsal

o

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 l 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,2
but when the development of the ccelom 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.3

1 Reprinted in Zoological Articles, London, 1891.

2 " Die Enteropneusten des Golfes von Neapel," Fauna und Flora
des Golfes von Neapel, Monog. xviii., Berlin, 1893.

3 See Macbride, "A Review of Prof. Spengel's Monograph on
Balanoglossus," OJ.M.S., xxxvi., 1894, and "The Early Development of
Amphioxus," Q.J.M.S., xl., 1898.

CHAPTER XVI

TIIK (1EKM-LAYKRS AND EVOLUTION

IN his papers of 1866 and 1867 Kovvalevsky had remarked
upon the widespread occurrence of a certain type or funda-
mental plan of early embryonic development, characterised
by the formation, through invagination, of a two-layered sac,
whose cavity became the alimentary canal. This develop-
mental archetype was manifested in, for instance, Sagitfn?-
Rrtiin? Lyimuca? AstacHs? P/wronis? Asterias? Ascidia?
the Ctenophora? and Amphioxus? He noticed also that
the invagination-opening often became the definitive anus.
Further instances of this mode of development were later
observed by Metschnikoff" and by Kowalevsky s himself, but
it was left to Haeckel to generalise these observations and
build up from them his famous Gastraea theory. This was
first enunciated in his monograph of the calcareous sponges,9
and worked out in detail in a series of papers published in
iS/4-76.10

1 Gegenbaur, Zcits.f. ii.'iss. ZooL, v., 1853.

-' Rcmak, loc. <://., p. 183, pi. xii.

! Lereboullet, Ann. Sci. nat. (4) xviii., pp. 1 18-9, 1862.

I Lereboullet, in Remak, p. 183 f.n.

'• Kowalevsky, Mt'-m. Acad. Set. St Pctcrsbourq (Petrograd), (7), x.
and xi., 1866 and 1867.

0 A. Agassi/, Contrib. Nat. Hist. United States, v., 1864.

" A/t'm. Acad. Set. St Pctersbourg (Petrograd), (7), xiv., 1869.

s " Embryolog. Studien an Wiirmern u. Arthropoden," JA'w. Ai'<ni.
Sci. St Pctersbourg (Petrograd), (7), xvi., 1870.

II Die K'nl/csc/i'n.'iiiiime, 3 vols., Berlin, 1872. General chapters
translated in Ann. Afiitf. Nat. Hist. (4), xi., pp. 241-62, 421-30, 1873.

"' "Die Gastraea-Theorie, die phylogenetische Classification des
Thierrcii hs und die Homologie der Keimblatter." Jcnaischc /.eilschrift,
viii., pp. 1-55, 1874. " Die Gastrula und die Eifurchung der Thiere,"

288

GASTR^A THEORY 289

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 Gastraea.

From the Gastrsea theory there followed at once two
consequences, (i) that ectoderm and endoderm, invagination-
cavity ( Urdarni] and gastrula-mouth (Unnundor Protostoina),
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 Gastraea. Huxley's suggestion
(supra, p. 208) that the outer and inner layers in Ccelentera
were homologous with the ectoderm and endoderm of the

o

germ was thus fully confirmed and greatly extended.

The great importance of the Gastraea 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 Gastraea theory or its implica-
tions were founded most of the phylogenetic speculations
which subsequently appeared.

Upon the Gastraea 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 Kalkschw dunned

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,

ibid., ix., pp. 402-508, 1875. "Die Physemarien, Gastneaden der
Gegemvart," and " Nachtrage zur Gastrasa-Theorie,'; ibid., x., pp. 55-98,
1876. Republished in Biologiiche Studien, 2nd part, Studicn zur
Gasirtza-Thcoric, 270 pp., 14 pis., Jena, 1877.
1 See Ann. Mag. Nat. Hist. (4), xi., p. 253.

290 THE GERM-LAYERS AND EVOLUTION

Monophyletic Genealogical Tree of tJie Animal Kingdom, based
upon the (jastrica TJieory and the H ontology of the Germ
Layers.

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MONOPHYLETISM 291

which possess both structures, is for the first time clearly
brought out. The derivation of all the Metazoa from a single
ancestral form, the Gastraea, 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 arcJienteron (Lankester, 1875), and an
ectoderm and endoderm which are homologous throughout
all the Metazoan phyla. Finally, in the separation of the
sponges, Ccelenterata and Accelomi as animals lacking a body
cavity or coelom x from the four higher phyla, which are
essentially Ccelomati, 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 iS/3.2
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 Gastraea 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.3 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 ; 4
it was the equivalent of the primordial anucleate Monera

1 Term first introduced in Die Kalkschwamme, p. 468, 1872.

2 "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.

3 First distinguished in Die Kalkschwiimme, i., p. 465.

4 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).

L

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1. Muiicrulu. 2. Cytula. 8. Murula. 4. Blastula. [>. Uastrula.

THE PRIMARY STAGES OF PHYLOGENY 293

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 synamceba, or earliest association of
undifferentiated amoeboid cells to form the first multi-
cellular organism. The blastula, or hollow sphere of seg-
mentation cells, usually ciliated, was reminiscent of the
plansea, an ancestral free-swimming form whose nearest
living relation is the spherical MagospJicera. The gastrula,
finally, is the two-layered sac formed from the blastula,
typically by invagination of its wall. It repeats the organisa-
tion of the gastraea, which is the common ancestor of all

o '

Metazoa, and finds its nearest living counterpart in the
simple " sponges " Haliphysema and Gastrophyscina)- The
ancestral line of all the higher animals begins with the
five hypothetical forms of the moneron, amceba, synamceba,
plansea, and gastraea.

We may take the following account2 of the phylogeny
of the human species, from the gastraea stage onwards,
as typical of Haeckel's speculations on the evolution of
the higher forms. The progenitors of man are, after the
Gastraeada :—

i. Turbellaria.
*2. Scolecida. (Worms with a ccelom, probably represented

at the present day by Balanoglossns.}
*3. Himatega. (Evolved from Scolecida by formation of

dorsal nerve-tube and chorda, and resembling tailed

larvae of Ascidians.)

4. Acrania. (With metameric segmentation. Including

Amphioxus.)

5. Monorrhina. (Cyclostomes.)

6. Selachia.

7. Dipneusta.

8. Sozobranchia. (Amphibia with permanent gills.)

1 Studicn z. Gastraa-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.

2 History of Creation, Eng. Trans., ii., pp. 278 ff.

294 THE GERM-LAYERS AND EVOLUTION

9. Sozura. (Tailed Amphibia.)
*io. Protamnia.
*ii. Promammalia.

12. Marsupialia.

13. Prosimue.

14. Menocerca. (Tailed apes.)

15. Anthropoides.

1 6. 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 evolu-
tion of Vertebrates is given in the Systematic Phylogeny Ql
1 895.* The ontogenetic and ancestral stages are arranged in
parallel columns thus :—

Cytula. Cytrca (Protozoa).

Morula. Monea (Coenobium of Protozoa).

Blastuhi. l!lubta;a (Volvocina, etc.).

Depula (invaginated bias- Deprea.

tula).

Gastrula. Gastrrea (cf. Olynthiis, Hydra, and primi-
tive Coclentera).

Cculomula (with one pair Cceh>m;ea (cf. Sagitta^ Ascidia, and

of adom-pockets). primitive Helminthes).

Chordula (with medullary Chorda^a (cf. Ascidian larva and larva of

tube and chorda). Amphioxus).

Spondula (with segmented Prospondylus (Primitive Vertebrate).

mesodcrm).

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 cculom,
which had been made in the interval of some twenty years.

Hieckel's Gastnua theory, though it exercised a great
influence upon the subsequent trend of phylogenetic
speculation, was by no means universally accepted tcllc ijncllc.
Opinions differed considerably as to the primitive mode of

i.- I'hylogcnie, iii., p. 41, Dcrlin,

INVAGINATION OR DELAMINATION 295

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 iS//,1
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,2
holding that this was typically formed by an "inruptive"
process at the anterior end of the planula. which led to the
formation of a " stomodaeum." A similar process at the
posterior end gave rise to the anus and the "proctodaeum."

The question as to whether delamination or invagination
was to be considered the more primitive process was
discussed in detail by Balfour,3 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 Gastraea, "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 Gastrsea theory.

From the same origins as the Gastraea 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
Gastraea theory — A. Agassiz, Metschnikoff and Kowalevsky.

1 "Notes on the Embryology and Classification of the Animal
Kingdom," Q.J.M.S, (n.s.), xvii., pp. 399-454, 1877

2 It was "part of the non-historic mechanism of growth" (loc. cif., p.
418).

3 Treatise on Comparative Embryology, ii., chap, xiii., 1881. For
a modern 'discussion of this problem, see Hubrecht, Q./.Af.S., xlix.,
1906.

U

29G THE GERM-LAYEKS AND EVOLUTION

But it was not Haeckel himself who enunciated the ccelom
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;1 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 observa-
tions 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,2 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
archentcron of the embryo, whilst a second outgrowth gave
rise to their ambulacral system; and in 1869 Metschnikoff
(A ft in. dc l}Acad. i 'nipt 'n ;alc dcs Sciences dc St J\:tcrsl>onrg,
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.

1 See Balfour, he. cit., Chapter xiii.

- A Treatise on Zoology, Pt. ii., 1900. Introduction by Sir E. Ray
Lankester.

ORIGIN OF C(KLOM 297

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 ( VerwandschaftsverJialtnisse 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.
Set., 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 enteroccel,
formed by pouching of the archenteron, as in Echinoderms,
Sagitta and Brachiopoda ; and the epiccel. . . . Immediately
after this I put forward the theory of the uniformity of
origin of the coelom as an enterocoel (Quart. Journ. Micr. Sri.,
April, 1875). • • • My theory of the ccelom as an enteroccel 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 ccelom " (pp. 16-18).

The enteroccel ic theory was taken up by O. and R.
Hertwig as an essential part of their Ccelomtkeorze.^ In

1 Studien zur Bliitterthcorie, Jena, 1879-80. "Die Coelomtheorie,
Versuch einer Erklarung des mittleren Keimblattes,'; Jenaische Zeitschrift,
xv., pp. 1-150, 1882.

298 THE GERM-LAYERS AND EVOLUTION

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 follow-
ing 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 Gastraea ; they form the limits
of the organism towards the exterior and towards the arch-
enteron. 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
enteroccel, which is to be regarded as a nipped-offdiverticulum
of the archenteron. Just as the two-layered animals can be
derived from the Gastraea, so can the four-layered animals
be derived from a Cctlom 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 (Secretgewebf) 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 I laeckel to be descended from the Gastnca,
so all Cu-'lomati were held by the Hertwigs to be derived from
an original caelomate Crforni. In both cases an embryo-
logical archetype becomes a hypothetical ancestral form.

The CcL'lom theory was considerably modified, extended

SEDGWICK 299

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.1 We shall
here focus attention upon one interesting line of speculation
followed out in this country particularly by Sedgvvick — 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 Kowa-
levsky 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 Gastrasa theory, for one of his main con-
tentions is that the blastopore or Urmund is homologous
throughout at least the three metameric phyla. In follow-
ing up Balfour's observations on the development of
Peripatus? 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 enteroccelic pouches of the
Ccelomati. He was led to enunciate the following theses: — 3
(i) that the mouth and anus of Vermes, Mollusca, Artho-
poda, 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 Medusae ; (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

1 For an historical account of this work, see Lankester, loc, cit., pp.
21-37.

2 Proc. Roy. Soc., 1883, and Q./.M.S., xxiii., 1883.

3 " Origin of Metameric Segmentation," Q.J.M.S., xxiv., pp. 43-82
1884.

300 THE GERM-LAYERS AND EVOLUTION

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 tracheae 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 Scyphomedusae. 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 Ccelenterate-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 Triplo-
blastica are found in a rudimentary condition in the Coelen-
terata; and that all the Triploblastica referred to must be
traced back to a diploblastic ancestor common to them
and the Ccelentcrata " (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.1

The characteristic relation of the central nervous system
to the blastopore in Annelida and Vertebrates had already
been pointed out by Kowalevsky,2 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.

1 See further the same author's article "Embryology" in the Ency.

/.Vv7., vol. xi., Mil) r<L ( '.iinKrid-v, \')lo.

- Arch.f. mikt: An>if., xiii., pp. 181-204, 1877.

LANG. HUBRECHT 301

In 1882, before the publication of Sedgwick's papers,
A. Lang1 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 enteroccelic pouches of
higher animals. This view, however, he soon gave up.2
Sedgwick's views found a supporter in A. A. W. Hubrecht,3
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 stomodaeum, and the ccelom from the
peripheral parts of the gastric cavity (p. 169, 1909).

1 " Der Bau von Gunda segmentata,'' Mitth. Zool. Stat. Neap., iii.,
pp. 187-250, 1882.

2 "Die Polycladen," Fauna u. Flora des Golfes von Neapel, Monog.
v., Leipzig, 1884, and " Beitrage zu einer Trophocceltheorie," Jen. Zeits.,
xxxviii., pp. 1-373, 1904 (which see for a modern account of theories of
metamerism).

3 "'Die Abstammung der Anneliden u. Chordaten,"/<?«- 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./.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
Cuvicrian 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 intro-
duced, 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 transcendcntalists. The principle
of connections still remained the guiding thread of morpho-
logical work ; the search for archetypes, whether anatomical

802

EVOLUTIONISTS AND TRANSCENDENTALISTS 303

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 classifica-
tion 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.
Geoffrey 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 trans-
cendentalists 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 Gastraea.

304 THE ORGANISM AS AN HISTORICAL BEING

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 transcen-
dental— 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

FORMAL AND FUNCTIONAL STANDPOINTS 305

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 simplifi-
cation 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.

Geoffrey 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.

306 THE ORGANISM AS AN HISTORICAL BEING

" 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 orien-
tation 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 Dens c.v niacJiina, ' New forma-
tion,' which is neither better nor worse than Gcucmtio
cijnivoca" (p. 2l).

Of a similar nature was the objection to convergence.1

Why, we may ask, were morphologists so unwilling to

1 The importance of convergence came to be realised after the
\o.Mie of phylogenetic speculation had passed — see Fricdmann, Die
Konvergenz der Organismeri) l'-crlin, 1904, and A. Willey, Convergence in
Evolution, London, 1911. Also L. Vialleton, Elements dc morphologic
des I'ertcbrcs, Paris, 1912.

THE ORGANISM AS ACTIVE 307

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 inventions1 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

o . o

main principle of pure morphology itself, that the unity of
plan imposes an iron limit upon adaptation, and in the

1 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.)

308 THE ORGANISM AS AN HISTORICAL BKING

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 some-
thing 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 morpho-
logical point of view when he wrote, with reference, to the
ancestral significance of the blastopore : — " If there is any-
thing 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

HISTORICAL INTERPRETATION OF FORM 309

could no longer be explained idealistically as the manifesta-
tion 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 interpre-
tation 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 require-
ments, 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 phylo-
genetic interpretation of the contrast between " membrane "
and "cartilage" bones.

In his Grundzuge of 1870, Gegenbaur made the suggestion
that the investing or membrane bones were derived phylo-
genetically from integumentary ossifications, and this was
worked out in detail a few years later by O. Hertwig.1

Many years before, several observers — J. Miiller, William-
son, and Steenstrup — had been struck with the resemblance
existing between the placoid scales and the teeth of Elasmo-
branch fishes. Hertwig followed up this clue, and came to
the conclusion not only that placoid scales and teeth were

1 Arch. f. mikr. Anat., xi. (suppl.), 1874; Morph. Jahrb., ii., 1876,
v. 1879, an<3 vii., 1882.

310 THE ORGANISM AS AN HISTORICAL BEING

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 (Deckk/toc/icn} 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 Kolliker 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 x consistently upheld the phylogenetic deriva-
tion 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. -

1 Vergldch. Anat, </. Wirbelthiere^ i., pp. 200-1, 1898.

-' For a full historical account of work on membrane and cartilage
bones (as well as on the theory of the skull) sec E. Gaupp, " Altere und
neuere Arbeiten liber den \Virbclthicrschadel," Ergeb. Anat. Enhv., x.,
1901, and " Die Entwickclung des Kopfskelettes," in liertwiy's Handbuch
vergl. cxper. Entivickclttngslehre d. Wirbeltkiere? iii., 2, pp. 573-874,
1905.

HISTORICAL INTERPRETATION OF FORM 311

A further instance of the historical interpretation of
animal structure, taken from quite a different field, is afforded
by the speculations of Dollo l on the ancestral history of the
Marsupials. In a brilliant paper of iSSo2 Huxley made the
suggestion that the ancestors of Marsupials were arboreal
forms. " I think it probable," lie 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

1 "Les Ancetres des Marsupiaux etaient-ils arboricoles ? " Trav.
Stat. zool. Wimereux, vii., pp. 188-203, pis. xi.-xii., 1899. See also
Bensley, Trans. Linn. Soc. (2) ix., pp. 83-214, 1903.

- Proc. Zool. Soc., pp. 649-62, 1880. Set. Mem., iv., pp. 457-72.

X

312 THE OIKiAMS.M AS AN HISTORICAL HKINCJ

achieved. Homologies were considered more interesting
than analogies, vestigial organs more interesting than fcutal
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.

Hut 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 legitim-
ately 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 recapitula-
tion 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 1 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.

6 From the point of view of the pure morphologist the
ipitulation theory is an instrument of researcli enabling
1 J. F. Gcinmill, Phil. Trans. Z>, ccv., p. 255, 1914.

TRANSFORMATION OF MORPHOLOGY 313

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, par-
ticularly by Roux and Samuel Butler, working with the
fruitful Lamarckian conception of the transforming power of
function.

CHAPTER XVIII

TJIK I;KI;INI\'IN<;S OK CAUSAL MOKIMIOLOOV

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. clu 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

314

EARLY WORKERS 315

that form-production was due to a complex of physico-
chemical causes, which they hoped some day to unravel;1
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 VV. 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.2

Though Roux was the only begetter of the science of
EntwicklungsmecJianik, he was, of course, not the first to
investigate experimentally the formative processes of animal
life. Study of regeneration dates back to Trembley (174°-
44), Reaumur (1742), Bonnet (1745), and Spallanzani (1768-
82),3 and in the years preceding Roux's activity good work
was done by Philipeaux. A beginning had been made with
experimental teratology by E. Geoffrey St Hilaire and
others, and the work of C. Dareste4 remains classical. Back
in the iSth century, some of John Hunter's experiments had
a bearing upon the problems of form ; his work on transplan-
tation was followed up in the ipth century by Flourens,
P. Bert, Oilier and many others. In founding in 1872 the
Archives de Zoologie experinientale et generate H. de Lacaze-

1 See Carus's remark, referred to on p. 194, above.
' Roux, Die Entwicklungsmechanik, p. 26, Leipzig, 1905.
:! T. H. Morgan, Regeneration, p. i, New York and London, 1901.
4 Recherches sur la production artificielle des Monstruositcs, Paris,
1877, and many later papers.

316 THE BEGINNINGS OF CAUSAL MORPHOLOGY

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 Enti^ick-
lungsmechanik was His's attempt in iS/41 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.":

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 paper3 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 iS/9.4

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 iSSi by his well-known

1 Unsere /\'<>rf>i-rf<»-//i nnd <i 'is f>/iyxi«!(>^isi-/ie Problem Hirer
Entxtclnuiy^ Leipzig, 1874.

: J. \V. Jenkinson, /',".i/w/w<y/A// /•1in/>ryt>l<w, p. 3, Oxford, 1909.

" I'rlx-r die VerzweigUllgen der I'lut-cf.^sc drs Mcnschen, "//•/;.
/.cit., xii., 1X78.

1 "Ucbcr die IJedeututi^ dcr Alilciikim^ dcs Arterienstammes bci
der As-.-di-.d)c," J<n. /.<•{(., xui., 1X79.

ROUX 317

book, Der Kampf der TJici/c iui Orgamsums, 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 mor-
phology, was naturally not so fully appreciated.

During the next few years Roux continued his studies
on functional adaptation,1 and at the same time made a new
departure by inaugurating, almost contemporaneously with
the physiologist Pfltiger, the study of experimental embry-
ology. Isolated observations had previously been made upon
the development of single blastomeres or parts of blastula?,
by Haeckel and Chun for instance,2 but Roux3 and Pfliiger4
were the first to investigate the subject systematically,
choosing for their work the egg of the frog/' Roux con-
tinued for many years to follow up this line of work.0

In 1890 he drew up a programme and manifesto7 of
Entwicklungsmechanik as " an anatomical science of the

1 " Beitriige zur Morphologic der funktionellen Anpassung. I.
Struktur eines hochdifferenzierten bindgevvebigen Organes (der Schwanz-
flosse des Delphin)," Arch. Ana/. Physio! . (Anat. Abt.} for 1883. II.
"Ueber die Selbstregulation der ' morphologischen ' Lange der
Skeletmuskeln des Menschen," Jen. Zeit., xvi., 1883. III. " Beschrei-
bting . . . einer Kniegelenkeknochenankylose," Arch. Anat. Physiol.
(Anat. Abt.} for 1885.

'• In 1869 and 1877 respectively (Rons, p. 53, 1905).

3 Ueber die Zeit. der Bcstimmung der Hauptrichtungen des Frosch-
embryo, Leipzig, 1883.

4 " Ueber den Einfluss der Schvverkraft auf die Teilung der Zellen,"
Pfliiger's Archiv, xxxi., 1883. Also subsequent papers in same journal.

•' 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.

fl In a series of " Beitnige 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. Enlw.-niecJi., from
1895 onwards.

' Die Entwicklungsmechanik der Organismen, cine anatomische
IVisscnschaft der Zuknnft, Wien, 1890.

318 THE BEGINNINGS OF CAUSAL MORPHOLOGY

future," and in 1895 he founded the famous Arc/iiv fi'tr
Entwicklungsmechanik^ publishing in the same year the two
large volumes of his collected papers,- of which the first
volume dealt with functional adaptation, the second with
experimental embryology.

1 1 is subsequent work includes several important general
papers ;:! besides a number of special memoirs dealing with
the factors of development, and with his original subject,
functional adaptation.4

In our sketch of his views we shall have occasion to refer
particularly to his publications of iSSi, 1895 (the Einleitung\
1902, 1905, and 1910.

Although Roux's biological philosophy is out-and-out
mechanistic, he yet recognises the difficulty, even the impossi-
bility, 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 " meta-
physics " 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 ;'iii incdid adopted by Roux is the analysis of
development, not directly into simple physico-chemical
processes, but into more complex organic processes dependent

1 The first volume contains the important Einleitung or general
Introduction.

2 Gesammelte Al>Jiandl linden iibcr Entwicklungsmechanik der Or^an-
isinai, 2 vols., Leipzig, 1895.

;! " Fiir unscr I'rogramm und seine Verwirklichung," A.E.M., v.,
])|). i-.So and 219 342, 1897. " Ueber die Selbstregulation der Lebc-
wesen," A.E.M.,\\\\., pp. 010-5, '9O2- "Die Entwicklungsmechanik,
ein ncuer /.urig der biologischcn Wissenschaft," Heft I. of the I'orlrii^c
7/ Aufstitzc iibtr Enfrwicklungsmechanik der Organistnen^ Leipzig, 1905.
< )ppel and Roux, u Ueber die gestaltlirhc Anpassung der Hltitgcfiisse,"
Heft x., of the Vortra^c n. Aufsiitzc, Leipzig, 1910.

1 " I'ebcr d. funkt. Anpassung des Muskclmagcns der Cans," A.E.Af.,
xxi., pp. 401 -91), 1906.

ROUX: "COMPLEX COMPONENTS11 319

upon the fundamental properties of living matter. The aim
of Entwicblungsmechanik is defined by Roux to be the
reduction of developmental events to the fewest and simplest
Wirkungsweisen, or causal processes.1 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 Entwick-
lungsmechanik 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

The exact quantitative formulation of a Wirkungsivcise constitutes
a law. The word itself is perhaps most conveniently rendered as
" causal process."

320 TIIK BEGINNINGS OF CAUSAL MORPHOLOGY

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 enumera-
tion 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 Kainpf dcr Tlicilc
( 1 88 1 ), 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 develop-
ment, 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

ROUX: THE TWO PERIODS OF DEVELOPMENT 32 1

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 /car' e£ox>'iv, 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 differentia-
tion 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 accom-
panied 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

322 THE HEGINNINGS OF CAUSAL MORPHOLOGY

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 inJierited 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, /.<'.,
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 ma}' 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 undrrstanding of the historical element in organic
form and activity. It supplies a practical criterion for the
distinguishing of "heritage" characters from acquired

ROUX : HEREDITY AND FUNCTION 323

characters, of palingenetic from cenogenetic-- a criterion
which descriptive morphology was unable to find.1 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 organisa-
tion. Roux himself was to a certain extent a preformationist,
for the development of his first-period characters is con-
ditioned 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 Anlagoi 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 develop-
ment, 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
Kainpf 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

1 M. Fiirbringer, perhaps under the influence of Roux, emphasised
the importance, from a morphological point of view, of studying post-
embryonic (functional) development, Unters. z. Morpk. u. Syst. der
Vbgel, ii., Amsterdam, p. 925, 1888.

o24 THE HK<;iNNIN(;S OF CAUSAL MOKPIIOUHi Y

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.1
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 inde-
pendence 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 auto-
matic 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 fibrilhu 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, tin-
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 six.e of lumen, elasticity of wall, direction
of branching, to conduct the blood to all parts of the body

1 See, for the development of this idea, Oppel, in Roux-Oppel, 1910. •

UOUX: FUNCTIONAL ADAPTATION 325

with the least possible waste of the propelling power through
frictional resistance. So, too, the spongy substance of the
long bones is arranged in lamella; 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 non-
functional 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."

320 Till: Hl-X;iNNIN<;s OF CAITSAL MORPHOLOGY

" From such temporary changes are sharply to be dis-
tinguished 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 them-
selves 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 adaptntiotis. . . . 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.1 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 tissurs we may
adduce the development and regulation of the blood-

1 Cf. the controversy between Herbert Spencer and Weismann on
the subject of " coadaptation " in the Contemporary Review for 1893
and 1894. See also YVeismann's paper in Darwin <uul Modern Science,
Cambridge, 1909.

ROUX: FUNCTIONAL ADAPTATION 327

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 : —

"(i) 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 hsemodynamically
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,
z>., 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,

Y

328 TIIK HK(,l.\MN(,S OF CAl'SAI. MORPHOLOGY

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 ] 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 muscu-
lature.

"(9) The faculty of being stimulated by increased blood-
pressure to produce the same structural changes as men-
tioned in par. S, 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
1 That is, the length they take up when .separated from the body.

ROUX : FORM AND FUNCTION 329

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
processor mode of assimilation. The faculty of "morpho-
logical 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

330 THE BEGINNINGS OF CAUSAL MORPHOLOGY

achieved by Roux's fello\v-\vorkers in the field of causal
morphology. As D. Barfurth points out,1 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 Pfliiger on the sexual difference in frogs (1881), by
Pfliiger 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.
Pfliiger and \V. Roux : Pfliiger publishing his researches
on 'the influence of gravity on cell -division,' Roux his
experimental investigations on 'the time of the determina-
tion 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
Pfliiger's gravity-experiments with animal eggs, in which
took part Pfliiger, 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-experi-
ments on the frog's egg (in the Sf///<\\: (icsc/l. /. ratci'l.
K/i/fnr, 1 5th Feb. 1884), also the important researches of
M. Nussbaum and Grubcr (followed up later by Yerworn,
Hofer and Balbiani) on Protozoa, and other experimental
work" (pp. xi.-xii.).

In 1 888 appeared a famous paper by W. Roux,- in which
he described how he had succeeded in killing by means of
a hot needle one of the two first bias torn eres of the frog's
egg, and how a half-embryo had developed from the
uninjured cell. Some years before :; he had enunciated, at
about the same time as \\Vismann, the viVw that development

1 "\Vilhelm Roux xinn 60. Cicburtsta-c,'' Arch. f. /i>/Av.-J/<v//., xxx.
/•', .v/.\r ///•//"/ //cr 1'rof. AVv/.r, 1't. i, 1910.

Vin limv's Aichir, cxiv., I 888. First announced in Sept. 1887.
' Ucbcr die Bedeutung der Kernteilungsfiguren^ Leipzig,

EXPERIMENTAL EMBRYOLOGY 331

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,1 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,2 which strongly supported the
Roux-Weismann theory. Considerable astonishment was
therefore caused by Driesch's announcement in 1891 3 that
he had obtained complete larvae from single blastomeres of
the sea-urchin's egg isolated at the two-celled stage. He
followed this up in the next year3 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 Amphioxusf and
Zoja on medusae.5 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, 6 and on Nereis by E. B.
Wilson,7 with analogous results.

In 1895 O. Schultze8 showed that if the frog's egg is held
between two plates and inverted at the two-celled stage

1 Bresl. iirtz. Zeitschr., 1885.

2 Journ, de VAnat. et de la Physiologic, xxiii., 1887.

3 Zeits.f. wzss. ZooL, liii., 1891 and 1892.

4 Journ. Morph., viii., 1893.

5 Arch. f. Ent.-Mech., i., 1895 5 ii-> l896-

6 Arch. f. mikr. Ana/., xliii., 1893.

7 Arch.f. Ent.-Mech., iii., 1896.
s Arch.f. Ent.-Mech., i., 1895.

332 TIIK BEGINNINGS OF CAUSAL MORPHOLOGY

there are formed two embryos instead of one. In the same
year T. H. Morgan l 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'2 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 i896,8 and they made
the further interesting discovery that the same defective
larvae 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,4 who removed the anucleate "yolk-lobe" from the
egg of the mollusc I/rdnassa at the two-celled stage, and
obtained larva;: which lacked a mesoblast. This result was
brilliantly confirmed and extended some years later by
I-",. 1). Wilson/' working on the egg of Dcntalinui. He found
that if the similar anucleate "polar lobe" of this form is
removed at the two-celled stage, deficient larvre arc formed,
in which the post - trochal region and the apical organ
are absent. lie 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 seg-
mentation-cells into which these cytoplasmic substances
arc thus segregated show a marked specificity of develop-

1 Ana/. Anz., x., i :•' : • Arch. /'. /•>//.- J/<v//., iv.

.Irch.f. /i>;/. -.I/", r//., ii., i.S«A ' Arch /. Knt.-.Wcch., iii., iSc;6.
iyV;-. /.i>i>/., i.. P

EXPERIMENTAL MORPHOLOGY 333

ment, 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. Corrobo-
rative 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 x
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.2
The formative power of function was studied in Germany by
Roux and his pupils, Fuld, O. Levy, Schepelmann and
others, particularly by E. Babak. In France, F. Houssay
inaugurated3 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.4 The principles of his
dynamical morphology were first laid down in his book
La Forme ct la Vie ( 1 900).

The famous experiments of Loeb, Delage and others on

1 Uftsere Kbr-perform, p. 19, Leipzig, 1874.

2 Biolog. Centrlbl., xiv., 1894, xv., 1895. Formative Reize in der
thicrischen Ontogenese, Leipzig, 1901.

3 " La Morphologic dynamique," No. i. of the Collection tie Mor-
pJiologie dynamiqitc, Paris, 1911.

4 " Forme, Puissance et Stabilite des Poissons," No. iv. of the
Collection, Paris, 1912,

334 TIIK BEGINNINGS OF CATSAL MORPHOLOGY

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.

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 auto-
matic 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 role
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 ipth 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

335

.l.'.G SAMUEL Bl'TLER AND MEMORY THEORIES

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 *) : — "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 amuse-
ment, 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
cculd 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 Involution, 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 L'ncotiscions Memory (1880) and Luck
or Cunning (1887).

Butler's main thesis is that living things arc 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

• nature itself is that same "sense of need" upon which

I In quotations arc taken from the l<;io reprint, London, Fifield. fc

BUTLER'S LAMARCKISM 337

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

338 SAMUEL BUTLER AND MEMORY THEORIES

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 iira 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 suc-
cessor is bo)ia fide but a part of the life of his progenitor,
imbued with all his memories, profiting by all his experi-
ences— which are, in fact, his own — and only uncon-
scious 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 con-
tinues— " I. That we are most conscious of, atid hare 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

BUTLER ON HABIT AND MEMORY 339

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

340 SAMl'EL WTTLEU AM) MEMORY THEORIES

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, \ve 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 develop-
ment 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 preserve 1
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 <>f memory we art attributing to it, would baffle that
memory, or which have rarely or never been included in the
category of its recollections, // acts precise/}1 .-/.v </ crcnhirc nets

BUTLERS TELEOLOGY 341

when its recollection is disturbed, or when it is required to do
soviet Ji ing iv/iich it has never done before''1 (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 concep-
tions 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 specu-
lations 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."1 Butler was not

1 Ueber das Gedachtnis als erne allgcmeine Funk lion der organisiertcn
Materic, Wien, 1870.

342 SAMTKL IH'TLKK AM) MKMOKY Til K< MIES

aware of Hering's work when he published his Life <nul
Habit, but in Unconscious Memory (1880) he gave full credit
to Ilering 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 conscious-
ness 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."1
Hering's views were supported by Haeckel.-

In 1893 an American, H. F. Orr,:: tried to work out a
theory of development and heredity based upon the funda-
mental idea " that the property which is the basis of bodily
development in organisms is the same property which we
recognise a§ the basis of psychic activity and psychic develop-
ment." He tried also to explain the recapitulation of
phylog<*f[y"by ontogeny as due to habit.

The neo-Lamarckian school of American palaeontologists
were also in sympathy with the memory idea, and this was
expressed most clearly perhaps by Copr. '

In 1904 appeared the work on this subject which has
attracted the most attention — R. Semon's Hie Mneme:'

. trans, in K. Hering, Memory * p. 9, Chicago and London, 1913.
-' Die Perigenesis der PlastiduU) I ma, 1875.
'•' A Theory of Development and Heredity^ New York, 1893.

The /V-///.-.//T Factors of Organic Ki'oliitit>n, Chicago, 1896.
•"• nic Mncmc ti/s erhaltendes I'rinzip ii/i ll'echscl ties organischen
Geschchens, Leipzig, 1904 ; 2nd ed., 1908.

SEMON. WARD. KIGNANO 343

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 stand-
point Semon expressly combats. As Ward1 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 interpre-
tation 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 transmissi-
bilite des caracfcres acquis;- 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

1 Heredity and Memory, Cambridge, 1913.

' Paris, 1906. Also in Italian and German. Eng. trans, by B. C. H.
Harvey, Chicago, 1911.

Z

344 SAMl'EI. W'TLKK AND MEMORY THEORIES

hypothesis set up is that every functional stimulus is trans-
formed 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 develop-
ment 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 con-
ditions 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 l and Sir Francis Darwin. -

Sec /'/-(>/>/ a/is of Life and Reproduction, London, 1913.
- 1' residential Address to the British Association, 1908.

CHAPTER XX

0

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, France, 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. Miiller. It may well be that the
intransigent materialism of the ipth 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.

345

:;4ii THE CLASSICAL TRADITION

Two movements seem significant in the morphology of
the last decade or so of the iQth century — first, the experi-
mental 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 Bcneden and others on the behaviour of
the nuclei in fertilisation and maturation.1 This line of
work gained added importance in connection with contem-
porary 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- and Whitman,3 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 pur-
posive 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 1^95.' The experiment was
later repeated and confirmed by Fischel and other workers.

1 Sec V.. B. Wilson's masterly book, The Cell in Development umf
Inheritance, New York and London, lyoo. - Q.J.Af.S., xxvi., 1886.

1 ll'ood's I loll I lio lexical Lectures for 1893.
4 Arch.f. Enf.-AIech., i., pp. 380-90, 1895.

ACTIVE RESPONSE OF ORGANISM 347

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.1 And,
as is well known, Driesch derived one of his most powerful
arguments in favour of vitalism from the extraordinary
regenerative processes shown by Tnbularia 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.2

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.3 A. Giard in particular called attention to
a remarkable group of facts which went to show that
embryos or larva; of the same or closely allied species might
develop in most dissimilar ways according to the conditions
in which they found themselves.4 His classical case of

1 Beitriige zur Kritik der Darwinschen Lehre, Leipzig, 1898.

2 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, Alanchester Lit. Phil. Sot:., 1906, and Vertebrate
Embryology, Oxford, 1913; A. Sedgwick, article "Embryology" in
Ency. Brtt., p. 318, vol. xi., nth Ed. (1910).

3 For a detailed treatment of this important point see the
remarkable volume of E. Schulz (Petrograd), Prinzipien der rationellen
•vergleichenden Embryologie. Leipzig, 1910.

4 " La Pcecilogonie," £;///. Set. France et Belgique, xxxix., pp. 153-87,
1905.

348 THE CLASSICAL TRADITION

" poecilogeny " was that of the shrimp J^i/innoiietcs i

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
clement 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 bio-
genetic 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,1 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 OppePs work of iSgi.2 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

1 Un problhne dc Devolution. I.u loi Ino^'ni'litjue fondamnitiilc.
Paris and Montprllirr, 1908.

: VergUichvng des Entovickelungsgrades lierOrganc
Entwickelungsxeiten l>ei IVh-hdtiercn, Jena,

OPPEL. KEIBEL 349

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."1

Oppel's methods were employed by Keibel 2 in his
investigations on the development of the pig, which formed
the model for the well-known series of Normentafdn 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

1 Quoted by Keibel, Ergebn. Anat. Entwick., vii., p. 741.

2 "Studien zur Entwickelungsgeschichte des Schweines," Schwalbe's
Morphol. Arbeiten, iii., 1893, and v., 1895.

Normentafeln zur Entwickelungsgeschichte des Sc/i?vetncs,]e-na., 1897.

" Das biogenetische Grundgesetz und die Cenogenese," Ergebn.
Anat. Entw., vii., pp. 722-92, 1897.

" U. d. Entwickelungsgrad der Organe," Handb. vergl. exper.
Enhvick. der Wirbelthiere, iii., 3, pp. 131-48, 1906.

350 THE CLASSICAL TRADITION

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, iSQ7).

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,1 Mehnert,- and Fischel.:!
Fischel found that variability was greatest among the
younger embryos, and became progressively less in later
stages. Like von Baer (s/t/>nr, 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 '
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. lie finds that in all cases
a pentadactylate rudiment is formed, even in those forms in

1 " Hcitra'tfe zur Embryologie der Wiederkaucr," Arch. Anat. Fnfw.,
1889.

"Die Individ. Variation d. Wirbeltiercmbryo," Morf>h. Arbeit., v.,
1895.

"U. Yariabilitat u. YVaclistum d. embryonalen Korpers," Morph.
J<ihrb., xxiv., i.

"(i.-tstrulation u. Keimblatterbildun- dcr Emyx /nftin\i fuuri, ;/,'
Morfih. Arbeit., i., i8«;i. " Kain.^cnrsr," .l/,v///. Arbeit., vii., pp. 1-156,
i . and .ilsn M-p.u.itely. Hioniefhiinik, crsclilosscn ,mx <l,-»i 1'rin //,•
. fena, r

MEHNERT 351

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.1 The acceleration or re-
tardation 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 trans-
mission 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

1 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" (Miiller's Archiv, p. 135, 1837). See also his
Entivick. Kopf. nackt. Amphib., p. 198, 1838. So, too, Ralhke notes how
the elongated shape of the snake appears even in very early embryonic
stages (Entivick. Natter., p. in, 1839).

352 THE CLASSICAL TRADITION

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 urgano-
genesis is exclusively dependent on phylogeny. But we
must not expect to find that all the stages in the develop-
ment of the separate organs, which coexisted in any member
of the phylogenetic series, appear at the. same tnne 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 differenti-
ated 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 arc at a phyletically
carl}' stage of development, while others are at a phyletically
older stage." 1

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. -

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. Agassi/. :; had remarked

1 Quoted by Keibel (p. 790, 1897) from the niomechanik.

/<*/<• /.die und die Geivcbe, Jena, 1898, and the subsequent editions
of this text-book, published under the title of Allgemcinc liioloj^ie.
/)ie Entwickelung der Hioh^ie i»i ncunzchntcn fahrhuniiert, Jena, 1900,
2nd cd., 1908. " Ueber die Stellung der vergl. Entwickelun»slehre ?.ur
vcr-l. Anatomic, zur Systematik und Dcscendcn/theorie," Ilandh.
>\ /><•>-. Entviickelvngslehre der \\'h-l>cliierc. iii., 3, pp. 149-80, Jena,
i'»o6, b). Also in 1't. I. of Vol. I. (1906, a).

-•/// Essay on Classification, I <>nd<m, 1859.

HIS. SEDGWICK

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,1 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 dis-
tinguished 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 character-
istics" (201).

This specificity of embryos was affirmed with even greater
confidence by Sedgwick in a paper critical of von Baer's law.2
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 develop-
ment ; 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 distinguish-
able from its allies from the very earliest stages all through
the deve-lopment, although these embryonic differences do
not necessarily implicate the same organs as do the adult
differences" (p. 39).

1 Unsere Korperform, Leipzig, 1874.
- Q.J.M.S., xxxvi., pp. 35-52, 1894.

\ '

r.4 THE CLASSICAL TRADITION

\

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 phylo-
genetic 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 soon. "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 In-
vertebrates of a segmentation-process, a morula - stage, a
blastula and agastrula. If now these developmental processes
do not depend on chance, but, on the contrary, are rooted in
the nature of the animal cell itself, wi- have no reason for
inferring from the recurrence of a similar segmentation-
process, inorula, blastula, and gastrula in all classes of the
animal kingdom the- common de-scent of all animals from one

O. HERTWIG 355

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 onto-
f geny 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
I multicellular organism only by the method of cell-division.
C Hence, in all Metazoa, ontogeny must start with a segmen-
/ tation-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
palaeontology), 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 explana-
tion 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

356 Till: CLASSICAL TRADITION

of morphology, but, on the contrary, morphology which has
to decide as to the possibility of descent."1

Hertwig, in a word, reverts to the pre-evolutionary

/conception of homology. " We see in homology," he writes,

7 "only the expression of regularities (Gesetzmdssigkeiten) 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

L. present an open one, requiring for its solution investigations

/ specially directed towards its elucidation" (p. 1/9, 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, 1'roamphibia and Proreptilia " (p. 155, 1906, b).
He rejects on the same general grounds the evolutionary
dogma of monophylctic or almost monophyletic descent,
and admits with Kolliker, von Baer, Wigand, Naegcli 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, Kolliker, Wigand, E. von
Hartmann and others. It is true the philosophical stand-
point 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 tic& petitio principii involved in the assumption that the

1 Quoted by Hertwig. See also K. Gocbel, " Die Grundprobleme
der heutigen Pflanxcnmorphologie," Rial. Centrbl., xxv., pp. 65-83,

PHYLOGENY AND PALEONTOLOGY 357

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 trans-
cendentalists) ; 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 palaeontology, and both insist upon the unique
importance of the palaeontological evidence.1

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
palaeontologists,2 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
palaeontology, for the evidence acquired through the study
of fossil remains was distinctly unfavourable to the pretty
schemes they evolved.

As Neumayr, Zittel, Deperet, Steinmann and others have
pointed out, the palaeontological 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 Archczopteryx, 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

1 This is also emphasised by Fleischmann in his critical study of
evolutionary morphology entitled Die Descendenztheorie, Leipzig, 1901.

• 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 palaeontologists by profession, such as Cope.

358 THK CLASSICAL TRADITION

been found as fossils. The great classificatory groups are
almost as distinct in early fossiliferous strata as they are at
the present day. As Deperet says in his admirable book,1 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 Archajopteryx, 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 bifurca-
tion 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. Wood-
ward,- after discussing the phylogcny 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 lacuna,'
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 paheontological 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
Hedged, as it were. No form has as yet been discovered of
which it could be said that it in any way approached the

1 Lcs Transformations du Monde animal^ 1'aris, 1907.
- " Malacology versus Palaeoconchology," I1 roc. Malacological Soc.,
viii., pp. 66-83, 1908.

PHYLOGENY AND PALAEONTOLOGY 359

hypothecated prorhipidoglossate mollusc, still less one
linking all the classes" (p. 79).

Pointing in the same direction as the absence of transi-
tional 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 repre-
sentatives 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 palaeontology 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
palaeontology. 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.1 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 palaeontologists
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 repre-
sented groups. Classical examples were the researches of
Hilgendorf- on the evolution of Planorbis uiultifonnis in the
lacustrine deposits of Steinheim, those of Waagen 3 on the

1 Particularly by E. Perrier, " La Tachygenese," Ann. Sci. nat.
(Zoo/.) (8), xvi., 1903.

2 Monatsber. k. Akad. IViss.^ Berlin, pp. 474-504, 1866.

3 Geognost. u. Palceont. Beitriige^ ii., Heft 2, pp. 181-256, 1869.

2 A

oGO T1IK CLASSICAL TRADITION

phylogeny of Ammonites subradiatus^ and the work of
Neumayr and Paul1 on l\iliidina ( / 7r//w</).

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, Dcperet
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 Dino-
therian stock is represented at the beginning of the Miocene
by the relatively small form D. cni'ieri ; this changes
progressively throughout Miocene times into D. leterinst
D. gigcintenm, and D.gigantissimum, Among the Mastodons
two quite distinct phyletic series can be distinguished, the
first commencing with PalcRomastodon beadnelli of the
Oligocene, and evolving between the Miocene and Pliocene
into Mastodon (irvernensis, after traversing the forms J/.
angustidens and J/. longirostris, the second starting with the
M. t n rice ti sis of the Lower Miocene and evolving through
M. borsoni into the M. auicricanns of the Quaternary. The
phyletic series of the true elephants in Europe are relatively
short, and go back only to the Quaternary, l:.lcplias antiqnns
giving origin to the Indian elephant, E. pn'scns 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 exempli-
fied in the case of the Proboscidea, and many other examples
could be quoted. Almost all rich genera arc polyphylctic in
the sense that their component species evolve along separate
and parallel lines of descent.- "Such great genera as the
genus Hoplites among the Ammonites, the genus Ceritliinin
among the Gastropoda, the genus J \-eten or the genus

1 Al>ii<intt. k.k. (lc«l. RficAsanstaft, vii., \Yien, 1875.

The case for polyphyk'tism is very strongly put l>y < 1. Stcinmunn
in his book, Die geologischen Grundlagen dcr Abstaminungslchre,
Leipzig, 1908.

PHYLETIC SKRIKS 361

Trigonia among the Lamellibranchs, each comprise perhaps
more than twenty independent phyletic series" (Deperet,
p. 200).

Variation along the phyletic lines is gradual l 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 palaeontology 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 " (Deperet, p. 249).

It is one of the paradoxes of biological history that the
palaeontologists 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 palaeon-
tologists in particular — Cope, Hyatt, Ryder, Ball, 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 palaeontologists, L, Riitimeyer (1825-
1895) and V. O. Kowalevsky (1842-83), who seem to have
carried on the splendid tradition of Cuvier. Speaking of

1 The steps in this chronological variation were termed by Waagen
" mutations."

302 TIIL CLASSICAL TRADITION

Kowalevsky's classical memoir, Vcrsnch cincr natiirliclicn
Classification dcr fossilcn Hiiftliicrc, Osborn 1 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 environ-
ment, 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 palaeontologists, including Osborn himself.

Osborn's law of " adaptive radiation," which links on to
Darwin's law of divergence,- constitutes a brilliant vindication
of the functional point of view. " According to this law each
isolated region, if large and sufficiently varied in its topog-
raphy, 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, theivforo 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

1 The A^c of Mammals in Europe, Asia, and North America, Nc\v
York, K;IO.

- Origin of Species, Oth cd., Chap. IV.

OSBORN: ADAPTIVE RADIATION

363

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.

FOSSORIAI,.

ARBOREAL.

Short-limbed, plantigrade,
pentadactyl, unguiculate
Stem.

AMBULATORY

or
TERRESTRIAL.

NATATORIAL.

Amphibious.

CUKSORIAL
Digitigrnde.

Aquatic.

TEETH.
OMNIVOROUS.

CARNIVOROUS

(Fish.
] Flesh.
I Carrion.

HERBIVOROUS

Unguligrade.

(Grass.

Herb.
. - Shrub.

Fruit.
[Root.

MVRMECOPHAGOUS.

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

364 THE CLASSICAL TRADITION

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 ant-
eaters, 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 dog-
matic 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, passion-
ate beings like ourselves, and we shall seek in our mor-
phology 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.

INDEX

ACTINOZOAN THEORY of Verte-
brate Descent, 299-300
Adaptation as Conservative Prin-
ciple—

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 Classi-
fication—
Bronn, 203

Adaptation of Parts. See "Corre-
lation, Functional," and
" Conditions of Existence "
Adaptive Radiation (Osborn), 362-4
Agassiz, A., 288 f.n., 295

On Ccelom, 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
Alcmjiean, i

365

Aldrovandus, 18

Allman, 209

Analogy. See also Homology.

Aristotle, 8-10

Owen, 108

Haeckel, 251

Gegenbaur, 266

Lankester, 267
Anaxagoras, 14
Anaximander, 14
Anaximenes, i

Animal and Vegetative Lives-
Aristotle, 1 6, 32

Buffon, 26-7

Bergson, 26 f.n.

Cuvier, 26, 32

Bichat, 27-9

Oken, 94

K. G. Cams, 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. Geoffrey, 54, 67

Owen, 104-7, 1 10

]. V. Carus, Huxley, 204

C. Darwin, 238 f.n.
Archetype, Anatomical, as An-
cestral—

C. Darwin, 235, 247

Haeckel, 251

Gegenbaur, 265

Sedgwick, 300

Criticism of this idea—
O. Hertwig, 355-7

3G6

INDKX

Archetype, Embryological, 168,
246, 302-3

Von liacr, 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 Anima/iuiH, i, 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, 1 1

Division of Labour, 12

Degrees of Composition — homo-
geneous and heterogeneous
parts, 12-14, 169

Law of Development (Von Bacr),

M

Scale of l'>cings, 14-16
Functional attitude, 15-16, 197
Animal and Vegetative Lives,

1 6, 32
Ascidian Theory of Vertebrate

I >esccnt, 269-73, 304
Atomists, 16

Atomists, " I'.inlo-ii al," 102-4
Audouin, V.—

Unity of j)lan in Arthropods,

85-6

Law of Compensation, So
Marine /.oolo-y,
Autcnrielh, </>.
Avicenna, 17

l'.Ai:\K, E., 333

liaer, K. E. von, 113-32, 133, 251,

304, 345, 356

Founder of Embryology, 113
Entwickelungsgeschichte der

Thierc, 1 14
Regulation of Development, 1 14,

35°
Development as Differentiation,

115, 128
Germ-Layer Theory, 115-6, 118-

1 19, 208-9, 296
Morphological Differentiation,

116-7
Histological Differentiation,

117-8

Tissues and Germ- Layers, nS
Double symmetrical Develop-
ment, 1 1 8, 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 liaer and Cuvier, 128-30
Functional attitude, 129
Relation to Transcendentalists,

129, 131

Criticism of Scale of lieings, 130
Vertebral Theory of Skull, 131,

142

Serial Homology, 131-2
("•ill-slits, Gill-arches and Aortic

arches, 135-6, 146
Membrane and Cartilage Rones,

162-3

Degrees of Composition, 172
( )va of Mammals, 175-6
Segmentation of Ovum, 186
< 'ritirism of Evolution Theory,

229, 242

Influence on D.u ,\in, 236, 2^
( i itii IMU of I >ar\\ inism, 242

INDEX

367

Baer, K. E. von — contd.

Teleology and Correlation, 242
On Ascidians, 271

Bonnet, R., 350

Bonnier, G., on Albertus Magnus,
17

Baer's Law. See " Development, Born, G., 330

Von Baer's Law "
Bagge, 187

Boveri, T., 270 f.n., 333
Braem, 347 f.n.

Balanoglossus Theory of Vertebrate Braun, A., 355

Descent, 285-7
Balbiani, 330
Balfour, F. M., 247, 299

Annelid Theory, 282-4

Gastrulation and Gastraea
Theory, 295

Mesoderm, 296 f.n.

Coelom, 297
Barfurth, D., 330
Barry, M., 186, 188
Bateson, W.—

Metamerism, Vegetative Repeti-
tion, 286

Balanoglossus Theory, 286-7

On Phylogenetic Speculation, 302
Beard, J., 285
Belon, 1 8
Beneden van, and Julin, 27 r, 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, nS, 132, 169,
178, 263

Breschet, 138, 173
Bronn, H. G., 200-3,

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.
Biichner, 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

Animal and Vegetative Lives, Burmeister, 249 f.n.

27-9

" General Anatomy," 29-30

Vie propre of Tissues, 30
Biogenetic Law. See " Develop-
ment, Haeckel's Law"
Bischoff, 138

Segmentation, 186, iSS
Blainville, de, 96, 128, 141, 199 f.n.
Bojanus, 96, 97
Bonnet, C. —

Scale of Beings, 22-3, 220, 227

Evolution, 215

Regeneration, 315

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 Develop-
ment, 337-9

Consciousness and Habit, 337-9
Recapitulation Theory, 339-40
Teleology, 341

CABANIS, 215
Camper, P., 45, 46
Carter, 293 f.n.

368

INDEX

Carus, J. V.—

Criticism of Embryological Cri-
terion, 167
Morphology and Physiology,

'94

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, iSS

C. F. Wolff, 170

Schleiden, 170-2

Criticism of Schwann-Schleidcn

Theory, 185-8
Virchow, Leydig, 188
Cell-Theory and Germ-Layer

Theory —
Remak, 209-12

Cell-Theory as Disintegrate—
Schwann, 180-5, 24^
Vogt, 190-1
Virchow, 191
Ilaeckel, 248
Criticism of this idea—
Reichert, 192-3, 194
J. V. Carus, 194
Seclgwick, Whitman, 346
Cell-Theory, Influence on Mor-
phology, 190
Cenogenesis, 258-9, 323
Chabry, 331
Child, C. M., 333
Chun, C, 317, 332
Classification of Animals-
Aristotle, 4-6
Konddetius, Aldrovandus, Ges-

ncr, 1 8
I .innaeus, 22
r.uffon, 25-6
Cuvicr, 39-41
E. Gcoffroy, f»>

Classification of Animals — conid.

L. Agassiz, 203 f.n.

Lamarck, 216-7, 227> 228
Classification and Ecological

Adaptation (Bronn), 203
Classification as Genealogical—

IHiffon, 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
Clans, 259

Co-adaptation, 326 f.n.
Crclom—

Remak, 2 1 1

A. Kowalevsky, 270, 295, 297

Haeckel, 29 r, 295, 296

Lankester, 291, 297
Cidom, Theory of, 295-301
Cohen, 189
Goiter, 18
Colucci, 346
Compensation, Law of—

Aristotle, 1 1

Goethe, 49

E. Geoffroy, 72-3

Audouin, 86.

German Transcendentalists, loo
Condillac, 215

Conditions of Existence, Principle
of-

Cuvier, 34, 75-6, 239

Gegenbaur, 263-4

Roux, 324, 326

Spencer, Weismann, 326 f.n.

I >isregard for—
Lamarck, 226
C. Darwin, 232, 2^8-41
Ilaeckel, 248, 264
Conklin, 333
Connections, Principle of—

Goethe, 47

E. Gcoffroy, 53-4, 62-3, 71, 74, 261

Audouin, 85

INDEX

369

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 Evolu-
tionary Morphologists, 305, 3 1 2

Hubrecht, 305-6

Cope, E. D., 342, 357 f.n., 36 r, 362
Correlation, Functional —
Aristotle, 10-12
Cuvier, 35-8, 239, 241
E. Geoffroy, 77
Von Hartmann, 240-1
Radl, 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 Exist-
ence, 34, 75-6, 239

Cuvier — contd.

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,

4i;4, 129, 304
Variation, Limits of, 42
Palasontological Succession, 43
Polemic with Geoffroy, 64-5, 74-8
Criticism of Vertebral Theory

of Skull, 97-8

Influence on J. F. Meckel, lor
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,

3°4, 307, 336, 362
Systematist and Field Natura-
list, 230, 231

Palaeontological Succession, 231
Ecological Adaptation, 231-2,

235> 239

Species Problem, 231

Functional Adaptation, Disre-
gard for, 232, 238-41

Classification as genealogical,
_ 233, 234, 247

Unity of Plan due to Com-
munity of Descent, 233,
234-5> 239, 247

Embryological Archetype as
ancestral, 233, 236-7

370

INDl'.X

1 )ar\vin, Charles— contil.

Rejects Mcckel-Serres Law, 233,

236
Interpretation of Vestigial

Organs, 233, 237
Organism as Historical Being,

-i3, 308

Rejects Scale of Beings, 234
Homology, 234-5, 247
Principle of Connections, 234-5
Anatomical Archetype as an-
cestral, 235, 247
\'<>n Baer's Law interpreted

phylogenetically, 236-7
Modifications inherited at cor-
responding age, 237
Monophyletism and Polyphyle-

tism, 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, iSS
Von Baer, 172

Effect of Invention of Micro-
scope, 20

Relation to Cell-Theory, 169
Delage, 333

Delate and Ilerouard, 273 f.n.
Dclpino, 345
I )cmaillet, 44
Democritus, 16
Dcpent, C., 357
On < "nvier, 43
Al)scnce of intermediary forms

in Paleontology, 3,S
Phyletic scries and I'oly-

phyletism, 360-1
Development, Von Baer's Law
Aristotle, 14

Development, Von Baer's Law —

COflfd.

Von Baer, 124-6
Prevost and Dumas, 125 f.n.
Rcichert, 149-50, 351 f.n.
Milne-Edwards, 205-8
Lereboullct, 206-8
Criticised by—

Agassiz, 352-3

His, 353

Sedgwick, 353

O. Hertwig, 354
Phylogenetic Interpretation of —

I )arwin, 236-7

Gegenbaur, 266
Relation to Haeckel's Law, 254,

256, 257
Development, Biogcnetic Law

(Haeckel)-

Haeckel, 251, 253-9, 291-4
F. Miillcr, 252-3, 254. 257
Gegenbaur, 262
Roux, 319
Butler, 339-40
Orr, 342
Criticism of—

Viallcton, 348

Oppel, 348-9

Keibel, 349-5°

Mehncrt, 350-2

O. Hertwig, 352, 354-5

"is, 353

Relation to Laws of Meckel-
Scrrcs and Von Baer, 254,

-*><>, 257, 303, 3°9

Relation to Heredity and De-
velopment, 312-3

Influence of Causal Morphology,
347-8

1'ahront o logical Evidence for,359
1 )e\ elopment, Mcckcl-Scrres

Law

llarvi-y, iS

Hunter, 22

!•'.. ( 'leoffroy, 69-70, 72

S.-rres, 80-3, 94, 203-4, 205-6

Kielmcyer, Auteni ieth, ( )ken, 90

INDEX

371

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 Paral-
lelism—

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, i
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
Dollinger, I., 113, 157
Dollo, 311
Donne, 173
D'Orbigny, 43
Driesch, H., 242, 331, 332, 333, 334,

345, 346-7

Duges, A., 86-8, 100, 134, 142, 146
Unity of Plan, 87
Polyzoic conception of Organism,
87-8

Duges, A. — contd.

Membrane and Cartilage Bones,

163

Dujardin, 169, iSS
Dumas. See Prevost and Dumas
Dumeril, 96
Dumortier, 173
Dutrochet, 99 f.n., 130, 134
Duverney, 19

EAR-OSSICLES, Homology of—

E. Geoffrey, 56

Spix, 100

Rathke, 141, 150

Reichert, 144-7
Echclle des ctres. See " Scale

of Beings."
Ehlers, 284
Eisig, H., 284, 285
Embryology, Comparative, Early

Workers-
Aristotle, 4, 1 13

Fabricius, Harvey, 18, 113

Malpighi, 20, 113

Oken and Kieser, 90, 113

Haller, C. F. Wolff, J. F. Meckel,

Tiedemann, 1 13
Embryology, Experimental, 317,

3'8, 330-3
Embryological Archetype. See

"Archetype, Embryological"
Embryological Criterion of
Homology, 133-168, 347

Goethe, 49

E. Geoffrey, 72, 110

Cuvier, 75, 1 10, 130

Owen, 1 10-1

Von Baer, 126-8, 132, 138

Rathke, 138, 140-1

J. Miiller, 138

Reichert, 138-9, 144-7, 163

Vogt, 156-7

Huxley, 158-9, 166

Kolliker, 165-6

Criticised by-
Owen, J. V. Carus, 167
Empedocles, I, 15

372

Engraimn (Scinon), 343
Entwicklungsgesctz. See " Evolu-

lion, Intrinsic Laws of
Entovicklungsmeckanik, 315
Erasistratus, 17
Evolution Theory-
Lucretius, 16
r.uffon, 24-5, 214
Cuvier's criticism, 41-4, 129, 304
E. Geoffrey, 66-9, 73, 228
}. F. Mcckel, 92-3, 215, 228
Leibniz, 213
Kant, 213-4

Erasmus Darwin, 214, 229
C. lionnct, Oken, Robinet, Tre-

viranus, 215

Tiedemann, 215, 255 f.n.
Lamarck, 215-29
Von Uaer, 229, 242
I. Geoffroy St Hilaire, J. V.

Carus, 230

Charles Darwin, 230-41
Von Hartmann, 240-1, 244, 356
Kolliker, 243
Owen, 244

Milne-Edwards, 244-5
I laeckel, 250-9
Gegenbaur, 265
The Organism as an Historical

J icing, 308-13
C. Darwin, 233, 308
Haeckel, 252, 257
Sedgwick, 308-
Roux, 313, 327-4
Hutler, 313, 336-41
Evolution-Theory, Influence on

Morphology, 302-13
1. \cilution, Intrinsic Laws of,

241

J. F. Merki-1, 93
I'.ronn, 202

Von Jiaer, 229, 242, 356
Kullikcr, Nai-Ljeci, 243, 356
Owen, 244

Von Hartmann, 244, 356
Milne-Edwards, 244-5
O. Ik-rtwiy, 354-5, 35^7

Evolution, Intrinsic Laws of contd.
\Vi-and, 356
Dcpcret, 361

FAHRICIUS, 1 8, 113
Fallopius, 1 8

I-'ischcl, 346, 350
Fischer, 328
Flcischmann, 357 f.n.
Flourens, 46, 315
Fontana, 172
Forbes, E., 196
Formal Attitude, 240, 305

C.oethe, 49

E. Geoffroy, 62-3, 71, 75-8, 305

Haeckel, 249, 257, 260

Gegenbaur, 261, 203

Semper, 279

Adopted by Evolutionary Mor-
phologists, 302-8, 311-2, 314

Hubrecht, 305-6

Dohrn, 306
France, R., 345
Friedmann, 306 f.n.

F"ld> 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. Meckcl, 101
Owen, 109, no, MI
Von 13acr, 129

Milne- Edwards, n;5, 197-200
J. Muller, Reichert, 200
lironn, 201-3
Lamarck, 222-6, 307, 335
< '.r-i-nbaur, 260, 2(13-4
Dohrn, 277-8, 307
Roux, 320-9, 335
1 loussay, 333
liutler, ', v 4 i
i -. \\olff, 346
Dricsch, 346-7
Giard, 347

INDEX

373

Functional Attitude — could.

E. Schulz, 347 f.n.

Keibel, 349-5°

Mehnert, 350-1

American Palaeontologists, 361,
362

Rutimeyer, 361

V. O. Kowalevsky, 361-2

Osborn, 362-4
Function-Change, Principle of—

Dohrn, 276-8, 306, 307

Eisig, 284

Fiirbringer, M., 282 f.n., 284, 323
f.n.

GALEN, 17

Gastraea Theory, 269, 288-95, 298>

299-301, 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, iSS
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, 3io

Gemmill, J. F., 285 f.n., 312 f.n.
Geoffrey, 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

Geoffroy, Etienne, St Hilaire—

contd.
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, no
Law of Compensation, 72-3
Criticism of his Principles, 74
Relation to German Trans-

cendentalists, 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. Muller, Burmeister,

G. Jager, 249
Germinal Vesicle (Egg-nucleus),

175-7, 1 88, 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

374

INDEX

( .run - Layers ami < '.astriea
Theory

Haeckel, 289-95

Lankester, lialfour, 295
Germ-Layer Theory, Influence of

Causal Morphology on, 347
(icsncr, 1 8
(Hard, A.—

On Ascidian Theory, 271-3

Adaptive Homology, 273

Pcecilogcny, 347-8
Glisson, F., 19
Gluge, 173
Goebel, K., 356 f.n.
Goethe, 45-5 1, 65, 89, 250

Unity of Plan, 45-7, 51

Homology, 47

Principle of Connections, 47

Formal and Functional Alti-
tudes, 48-50

Teleology, 48

Metamorphosis of Plants, 48

Repetition of parts, 48-9

Vertebral Theory of Skull, 49,

<A 97
Law of Compensation, 49

Embryological Criterion, 49
Organisms as Nature's Works
of Art, 50

Goette, 259

( , r.iaf, von. 175

( '.rew, N., 169

( iruber, 330

II AKCKKI., Ernst, 247-60, 271, 314,

34?, 353. 357
His sources, 24^-50

Materialism, 248, 250

On Teleology, Heredity and
Adaptation, 248, 263

Correlation, Disregard for, 248,
204

Geometry of the Organism (Pro-
morphology), 249

Kepetition of Parts (Tcctology),
249-50

Haeckel, Ern.st coniti.

Classification as Genealogn.d

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 IJcings, 255, 256-7
Organism as an Historical

Being, 257
1'russianism, 257
Palingenesis, 258
Cenogenesis, 258-9
Heterotopy, Heterochrony, 259
Gastraea Theory, 269, 288-95
Phylogenetic Classification, 289-

94
Criticism of Theory of Types,

Monophyletism, 289, 291
Gastnea Theory and Biogenetic

Law, 291-4

Primary stages of Ontogeny
and Phylogeny, 291-3

Ciulom, 291, 295, 296

Experimental Embryology, 317
1 1 alter, 1 13
llarting, 284 f.n.
i lartmann, E. von—

On Darwin's conception of cor-
relation, 240-1

Evolution, 244, 356
Hartog, M., 344
Harvey, 18, 113
llatschek, 270 f.n., 299
Ilelmholt/, H. von, 195
Henle, 172
Hcnsen, V , 209 f.n.
Hcrbsl, C., 333
1 1 eider, 46

i Icrcdity and Mfinory, 330 44
llering, E , 341-2
" Heritage ;! Characters, 309, 322
I Irrlitzka, 332
1 lero|)hilus, 17
llcitwig, ()., 163, 330, 331, 346

i in C. K. Wolff, 119

INDEX

375

Hertwig, O. — contd.
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
Homologynot necessarily Homo-

geny, 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 Ccclentera,

299

Heterochrony, 259, 348, 349-52
Heterogeneous Generation (K61-

liker), 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-form-
ing 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"

Homology — contd.

Aristotle, 7-10

Belon, 1 8

Buffon, 27

Goethe, 47

E. Geoffroy, 53, 71

Serres, So

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,
1 66

Embryological Archetype, 1 59-61

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 Paral-
lelism, 230 f.n.

Coelom, 297

Ancestry of Marsupials, 31 1
Hyatt, A., 359, 361

INSTINCT and Morphogenesis,

Analogy of, vi., 307, 312
Lamarck, 220, 226

2 B

376

INDKX

jACOIiSON, 164

Jager, G., 249 f.n.

Jardin ties Plantcs, Paris, 19

Jcnkinson, J. W., 347 f.n.

On His, 316

Jones, Wliarton, 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 t'.n.
Kolliker, A.—

On C. F. Wolff, 119

Vertebral Theory of Skull, 157

Membrane and Cartilage Bones,
164-6, 310

Embryolugiral 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
Coclom, 270, 295, 297
( iastrula, 288
Knwalcvsky, V. O., 361-2
Krausc, 176
KupfiVr, 271

l,\r\/ic Duriiii.ks. II. de, 203 f.n.,

315-6

On Ascidians, 271, 273
l.amarck, 44, 66, 78, 215-29
Relation to BulTon, 215
Scale of I'.rni^. -IS--1"'1, 220-1,

2:7-8
As Evolutionary, 218, 220

Lamarck — condl.

Classification, 216-7, 2:7, 228
Species Problem, 216, 227
Materialism, 218-9, -22-j> 225-6
Psychological Vitalism, 219,

220-6, 307, 335
Sentiment intcricur, 219-20,

222-3, 225
Ecological Adaptation, 221, 222,

223, 224, 227
Laws of Evolution, 221-5
Transmission of Acquired Char-
acters, 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, Uomo-
plasy, and Analogy, 267

Balanoglossus Theory of Verte-
brate Descent, 287

Germ- Layer Theory and Phylo-
genetic Classification, 291

Planula Theory, 295

On Ccelom Theory, 296-7,

299 f.n.

I.atreille, 86, 100
Laurencet, 64
Lavocat, 203 f.n.
Leeuenhock, 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
1-^7, O., 333

l.cydig, 1X7, iS8, 275 f.n., 285
Linnaeus, 22
Loeb, J., 333, 347

INDEX

377

Loi de Balancement. See " Com-
pensation, Law of"
Loven, 186, 196
Lucretius, 16

On the Soul, 222 f.n.
Ludwig, 193, 194, 3M
Lyell, Sir C., 228 f.n.
Lyonnet, 22

MACBRIDE, E. W., 287 f.n.
M'Kendrick, J.-

On Fontana, 172
Mackenzie, \^., 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,

3 '4-5, 347
Lamarck, 218-9, 222-3, 225-6

The Darwinians, 241, 308

Haeckel, 248, 250

Roux, 315, 317, 3!S-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. .$>£" De-
velopment, 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

Ccelom, 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, Con-
vergence, 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,

•"* ^C\ "2

jj>u J
Muller, F., Biogenetic Law, 252-3,

254, 257
Muller, H., 166

378

INDKX

iMuller, J., 136, 209 f.n., 260, 285,

3°9< 345

Embryological Criterion, 138
Vertebral Theory of Skull, 142-4,

154, 157

On Reichert, 150
Cell Theory, 172-3
Division of Egg-nucleus, iSS
Vitalism, 192
Marine Zoology, 196
Functional Attitude, 200
Mutations (Waagen), 361 f.n.

NAEOELI, 185, 243 f.n., 356
Naturphilosophic. See "Phil-
osophy of Nature ''
Nesbitt, R., 162
Neumayr, 357, 360
Nussbaum, M., 330

OKEN, L., 89, 113, i3r, 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 Geoffrey, 100
Influence on Serres, 205
Evolution, 215

Oilier, 315

Oprjel, A., 318 f.n., 324 f.n., 327,

34S-9

Orr, H. F., 342

Osborn, H. F., 214 f.n., 361
i MI V '. ( ). Kowalevsky, 362
Functional Attitude, 362-4
Law of Adaptive Radiation,

362-4
( ) wen, R., 97, 102-12, 204

Eclecticism, 102

Vertebral Theory of Skeleton,
103-7

Archetype of Vertebrate-
Skeleton, 104-7, I I0

Vertebral Theory of Skull, 104-6

Metastasis, 106

Principle of Connections, 107-8

Owen, R.— contd.

Anatomy and Embryology, 108
Homology and Analogy, 108
Classes of Homology, 108-9, 266
Functional Attitude, 109, no,

1 1 1
Embryological Criterion, no,

167
Homological and Teleological

Compoundedness, 110-1
Vegetative Repetition of Parts,

in, 286
Unity of Plan as Conservative

Principle, 1 12
Influence on Darwin, 234, 235,

238
Evolution, 244

PACKARD, 361

Palaeontological Record, 357-61
Absence of connecting forms,

357-9

liiogenetic Law, 359

Phyletic Series, 359-61
Palaeontological 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 " De-
velopment, Meckel-Serres
Law "

Three-fold. Sec " Development,

Meckel-Serres Law"
Paris Museum <>t" .Natural History,

19,89, 10 1
Paul, 360
Pauly, A., 345
Perrault, C., 19
Perrier, E., 88, 359 f.n.
Pflu-er, E., 317, 330
Philipeaux, 31 5

INDEX

379

"Philosophy of Nature," 89, 94,

98, 203, 248
Phyletic Series, 359-61
Physiology, Separation from Mor-
phology, 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

Kolliker, Wigand, Naegeli, 356

Deperet, 360-1

Steinmann, 360 f.n.
Polyzoic Conception of Organism—

Duges, 87

Perrier, 88
Prevost and Dumas, 125 f.n., 134,

175, 1 86

Promorphology (Haeckel), 249
Protoplasm, 169, 188-9
Purkinje, 172, 173, i75> i?6, 189

QUATREFAGES, A. de, 172, 195-6

RADL, 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

Rathke, H. — contd.

Embryological Archetype, 151,

153
Membrane and Cartilage Bones,

163, 166

Rauber, A., 330
Reaumur, 22, 315

Recapitulation Theory. See " De-
velopment, Biogenetic Law"
Regeneration, 315, 318, 333, 346
Regulatory Processes in Develop-
ment, 114, 319, 333, 346-7, 350
Reichert, C. B., Embryological
Criterion, 138-9, 144-7, '63
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 Vertebras, 157
Cell Theory, 173, 187-8, 209
M icroscopical Technique, 209 f.n.
Germ-Layer Theory, 209-12, 296
Cells, Tissues and Germ-Layers,

209-12

Mesoderm, 209-11
Ccelom, 2 1 1,^296

Repetition of Parts within the
Organism, Theory of. See also
" Vertebral Theory of Skull "
Goethe, 48-9
Duges, 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, Rosel von, 22

380

INDEX

Roux, W., 313, 315-29, 344, 35'
Entwicklnngsmechanik) 3 1 5,

317-8
Materialistic Attitude, 315, 317,

3 '8-9, 329
Functional Adaptation, 316-7,

318, 320-9, 333
Experimental Embryology, 317,

Simple and Complex Com-

ponents, 318-20
Functional Definition of Life,

320

Functional Attitude, 320-9, 335
The Two Periods of Develop-

ment, 320-4, 325, 327, 335
Mosaic Theory of Development,

323, 33°-i

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 Ac-

tivity, 329
Ruini, C., 18
Rusconi, 133-4, 1 86
Riitimeyer, L., 361
Ryder, 361

SACHS, J. von, 170
St Ange, M., 146
S.ilensky, 259
Saltatory Variation—

I-.. C.eoffroy, 78

Von Baer, 242

Kolliker, 243

Owen, 244
Sarrodc, 169
Sars, M., 186, K/,
Savigny, J. C., 83-5, loo, 137, 271
S« ,ili- nf Brings, 89, 206, 214-5

Aristotle, 14-6

Anaximander, Anaxagoras, 14
s, I'lato, 15

Scale of Beings — contd.
Albertus Magnus, 17
C. Bonnet, 22-3
Robinct, 23
ButTon, 24
E. Geoffrey, 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
Lereboullct, 207
Darwin, 234
Haeckel, 255
Relation to Evolution-Theory,

214-5

Schepclmann, 333
Schlciden, 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, I So, 182
Development of Cells, 174-5,

179-80

Cellular Nature of Ovum, 175-71
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
Srdgwick, A., 347 f.n.

A' tino/oan Theory of Vertebrate

Descent, 299-300
Metamerism, 299
Embryological Archetype, 300
Organism as Historical Being,

308

Cell-Theory, 346
\'dii Baer's Law, 353

INDEX

381

Segmentation of Ovum, 186-8

Seller, 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 interieur • (Lamarck),

219-20, 222-3, 225
Serial Homology. See "Meta-
merism "
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

382

INDKX

Types, Theory of (Cuvicr and Von

Baer) — contd.
Criticised by—

E. Geoffrey, 60

I laeckel, 289, 291

Lankester, 291
Type-Theory and Evolution, 304

UNC-ER, 185

Unity of Composition, Principle of,
Geoffrey, 54, 70-2, 75-6, 200,

305
Unity of Plan, 88, 241, 278-9, 303,

3 1 2. 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
1 1 order, 46
Kant, 46, 213-4
E. Gcoffroy, 52-65, 70 ff.
Serres, 83, 205, 206
Savigny, 83
Audouin, 85-6
Latreille, 86
Duges, 86-7
J. F. Meckel, 101
Milne-Edwards, 197
Semper, 279

Hacckcl, 289, 291

Lankester, 291
Unity of 1'lan as due to Community

of Descent-
Darwin, 233, 234-5, 239, 247

I [aei l.d, 250-1

Gegenbaur, 263 f.n., 265

( 'riticisin of this idea

(). Il.-rtwi-, 355-7
Unity of 1'lan as Conservative
Principle—

E. < ieoffroy, 75, 78

( )w<-n, I I 2

lTnity of Plan as Conservative

Principle — contd.
Gegenbaur, 263-4
Evolutionary MorphologistS, 307

VALENTIN, 138, 173, 176
Variation, Limits of, Cuvier, 42
Vegetative Repetition of Parts-
Owen, in, 286
P.ateson, 286
Velpeau, 138

Vertebral Theory of Skull, 49, 96-9,
104-6, 131, 141-4, .147-9, '54-7,
161-2, 165, 203, 235, 310 f.n.
Vertebrate Descent, 269-87, 299-

3OI> 3°4
Verworn, M., 330

Vesalius, 18

Vestigial Organs, 233, 237, 309, 31 2

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, 34-
Ward, 343

Delpino, France, Pauly, A.
Wagner, Mackenzie, 345

Vogt, C.-

Criticism of Vertebral Theory,

1 56-7

Capillaries, 179
Segmentation, 186
Materialistic Attitude, 190-1
Threefold Parallelism, 255 f.n.

\YAAC.KN, 359, 3nl f-n-

\Va-ner, A., 345

Wagner, R., 176

Ward, J., 343

Weber, 13*

Weismann, A., 240, 323, 326 f.n.,

33°- >• 343
Werncck, 173
Whitman, C. O., 34''

\Vi-.md. A., 242 f.n., 350

INDEX 383

Willey, A., 273 f.n., 306 f.n. Wolff, G., 346-7

Williamson, 309 Woodward, B. B., 358

Willis, 19 Wotton, E., 17
Wilson, E. B., 331, 332-3, 346 f.n.,

347 f-n.

Wolff, C. F., 113 ZELENY, 333

Germ-layer Theory, 119-20 Zittel, K. von, 357, 358

Cells, 170 Zoja, 331

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