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+Project Gutenberg's Form and Function, by E. S. (Edward Stuart) Russell
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Form and Function
+       A Contribution to the History of Animal Morphology
+
+Author: E. S. (Edward Stuart) Russell
+
+Release Date: January 23, 2007 [EBook #20426]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK FORM AND FUNCTION ***
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+Produced by Suzanne Lybarger, Turgut Dincer and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
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+
+
+
+FORM AND FUNCTION
+
+A CONTRIBUTION TO THE
+HISTORY OF ANIMAL MORPHOLOGY
+
+By E.S. RUSSELL,
+M.A., B.Sc., F.Z.S.
+
+ILLUSTRATED
+
+LONDON
+
+JOHN MURRAY, ALBEMARLE STREET, W.
+
+1916
+
+_All rights reserved_
+
++---------------------------------------+
+| Transcriber's Note: Obvious printer   |
+| errors have been corrected, all other |
+| inconsistencies in spelling and       |
+| punctuation are as in the original.   |
++---------------------------------------+
+
+
+PREFACE
+
+
+This book is not intended to be a full or detailed history of animal
+morphology: a complete account is given neither of morphological
+discoveries nor of morphological theories. My aim has been rather to
+call attention to the existence of diverse typical attitudes to the
+problems of form, and to trace the interplay of the theories that have
+arisen out of them.
+
+The main currents of morphological thought are to my mind three--the
+functional or synthetic, the formal or transcendental, and the
+materialistic or disintegrative.
+
+The first is associated with the great names of Aristotle, Cuvier, and
+von Baer, and leads easily to the more open vitalism of Lamarck and
+Samuel Butler. The typical representative of the second attitude is E.
+Geoffroy St. Hilaire, and this habit of thought has greatly influenced
+the development of evolutionary morphology.
+
+The main battle-ground of these two opposing tendencies is the problem
+of the relation of function to form. Is function the mechanical result
+of form, or is form merely the manifestation of function or activity?
+What is the essence of life--organisation or activity?
+
+The materialistic attitude is not distinctively biological, but is
+common to practically all fields of thought. It dates back to the
+Greek atomists, and the triumph of mechanical science in the 19th
+century has induced many to accept materialism as the only possible
+scientific method. In biology it is more akin to the formal than to
+the functional attitude.
+
+In the course of this book I have not hidden my own sympathy with the
+functional attitude. It appears to me probable that more insight will
+be gained into the real nature of life and organisation by
+concentrating on the active response of the animal, as manifested both
+in behaviour and in morphogenesis, particularly in the post-embryonic
+stages, than by giving attention exclusively to the historical aspect
+of structure, as is the custom of "pure morphology." I believe we
+shall only make progress in this direction if we frankly adopt the
+simple everyday conception of living things--which many of us have had
+drilled out of us--that they are active, purposeful agents, not mere
+complicated aggregations of protein and other substances. Such an
+attitude is probably quite as sound philosophically as the opposing
+one, but I have not in this place attempted any justification of it. I
+have touched very lightly upon the controversy between vitalism and
+materialism which has been revived with the early years of the present
+century. It hardly lends itself as yet to historical treatment, and I
+could hardly hope to maintain with regard to it that objective
+attitude which should characterise the historian.
+
+The main result I hope to have achieved with this book is the
+demonstration, tentative and incomplete as it is, of the essential
+continuity of animal morphology from the days of Aristotle down to our
+own time. It is unfortunately true that modern biology, perhaps in
+consequence of the great advances it has made in certain directions,
+has to a considerable extent lost its historical consciousness, and if
+this book helps in any degree to counteract this tendency so far as
+animal morphology is concerned, it will have served its purpose.
+
+I owe a debt of gratitude to my friends Dr James F. Gemmill and Prof.
+J. Arthur Thomson for much kindly encouragement and helpful criticism.
+The credit for the illustrations is due to my wife, Mrs Jehanne A.
+Russell. One is from Nature; the others are drawn from the original
+figures.
+
+E.S.R.
+
+CHELSEA, 1916.
+
+
+
+
+CONTENTS
+
+
+CHAP.                                                    PAGE
+
+I. THE BEGINNINGS OF COMPARATIVE ANATOMY                    1
+
+II. COMPARATIVE ANATOMY BEFORE CUVIER                      17
+
+III. CUVIER                                                31
+
+IV. GOETHE                                                 45
+
+V. ETIENNE GEOFFROY ST HILAIRE                             52
+
+VI. THE FOLLOWERS OF ETIENNE GEOFFROY ST HILAIRE           79
+
+VII. THE GERMAN TRANSCENDENTALISTS                         89
+
+VIII. TRANSCENDENTAL ANATOMY IN ENGLAND--RICHARD OWEN     102
+
+IX. KARL ERNST VON BAER                                   113
+
+X. THE EMBRYOLOGICAL CRITERION                            133
+
+XI. THE CELL-THEORY                                       169
+
+XII. THE CLOSE OF THE PRE-EVOLUTIONARY PERIOD             190
+
+XIII. THE RELATION OF LAMARCK AND DARWIN TO MORPHOLOGY    213
+
+XIV. ERNST HAECKEL AND CARL GEGENBAUR                     246
+
+XV. EARLY THEORIES ON THE ORIGIN OF VERTEBRATES           268
+
+XVI. THE GERM-LAYERS AND EVOLUTION                        288
+
+XVII. THE ORGANISM AS AN HISTORICAL BEING                 302
+
+XVIII. THE BEGINNINGS OF CAUSAL MORPHOLOGY                314
+
+XIX. SAMUEL BUTLER AND THE MEMORY THEORIES OF HEREDITY    335
+
+XX. THE CLASSICAL TRADITION IN MODERN MORPHOLOGY          345
+
+INDEX                                                     365
+
+
+
+
+ILLUSTRATIONS
+
+
+FIG.                                                     PAGE
+
+1. HYOID ARCH OF THE CONGER. (ORIGINAL.)                   58
+
+2. "VERTEBRA" OF A PLEURONECTID. (GEOFFROY.)               61
+
+3. ABDOMINAL SEGMENT OF THE LOBSTER. (GEOFFROY.)           63
+
+4. IDEAL TYPICAL VERTEBRA. (OWEN.)                        102
+
+5. NATURAL TYPICAL VERTEBRA. (OWEN.)                      103
+
+6. THE ARCHETYPE OF THE VERTEBRATE SKELETON. (OWEN.)      105
+
+7. IDEAL TRANSVERSE SECTION OF A VERTEBRATE EMBRYO.
+     (VON BAER.)                                          119
+
+8. GILL-SLITS OF THE PIG EMBRYO. (RATHKE.)                134
+
+9. MECKEL'S CARTILAGE AND EAR-OSSICLES IN EMBRYO OF
+     PIG. (REICHERT.)                                     145
+
+10. CRANIAL VERTEBRÆ AND VISCERAL ARCHES IN EMBRYO
+      OF PIG. (REICHERT.)                                 148
+
+11. EMBRYONIC CRANIUM OF THE ADDER. (RATHKE.)             152
+
+12. TRANSVERSE SECTION OF CHICK EMBRYO. (REMAK.)          211
+
+13. DEVELOPMENT OF THE ASCIDIAN LARVA (KOWALEVSKY.)       272
+
+14. TRANSVERSE SECTION OF THE WORM _NAIS_. (SEMPER.)      280
+
+15. THE FIVE PRIMARY STAGES OF ONTOGENY. (HAECKEL.)       292
+
+
+FORM AND FUNCTION
+
+
+
+
+CHAPTER I
+
+THE BEGINNINGS OF COMPARATIVE ANATOMY
+
+
+The first name of which the history of anatomy keeps record is that of
+Alcmaeon, a contemporary of Pythagoras (6th century B.C.). His
+interests appear to have been rather physiological than anatomical. He
+traced the chief nerves of sense to the brain, which he considered to
+be the seat of the soul, and he made some good guesses at the
+mechanism of the organs of special sense. He showed that, contrary to
+the received opinion, the seminal fluid did not originate in the
+spinal cord. Two comparisons are recorded of his, one that puberty is
+the equivalent of the flowering time in plants, the other that milk is
+the equivalent of white of egg.[1] Both show his bias towards looking
+at the functional side of living things. The latter comparison
+reappears in Aristotle.
+
+A century later Diogenes of Apollonia gave a description of the venous
+system. He too placed the seat of sensation in the brain. He assumed a
+vital air in all living things, being in this influenced by Anaximenes
+whose primitive matter was infinite air. In following out this thought
+he tried to prove that both fishes and oysters have the power of
+breathing.[2]
+
+A more strictly morphological note is struck by a curious saying of
+Empedocles (4th century B.C.), that "hair and foliage and the thick
+plumage of birds are one."[3]
+
+In the collected writings of Hippocrates and his school, the _Corpus
+Hippocraticum_, of which no part is later than the end of the 5th
+century, there are recorded many anatomical facts. The author of the
+treatise "On the Muscles" knew, for instance, that the spinal marrow
+is different from ordinary marrow and has membranes continuous with
+those of the brain. Embryos of seven days (!) have all the parts of
+the body plainly visible. Work on comparative embryology is contained
+in the treatise "On the Development of the Child."[4]
+
+The author of the treatise "On the Joints," which Littré calls "the
+great surgical monument of antiquity," is to be credited with the
+first systematic attempt at comparative anatomy, for he compared the
+human skeleton with that of other Vertebrates.
+
+Aristotle (384-322 B.C.)[5] may fairly be said to be the founder of
+comparative anatomy, not because he was specially interested in
+problems of "pure morphology," but because he described the structure
+of many animals and classified them in a scientific way. We shall
+discuss here the morphological ideas which occur in his writings upon
+animals--in the _Historia Animialium_, the _De Partibus Animalium_,
+and the _De Generatione Animalium_.
+
+The _Historia Animalium_ is a most comprehensive work, in some ways
+the finest text-book of Zoology ever written. Certainly few modern
+text-books take such a broad and sane view of living creatures.
+Aristotle never forgets that form and structure are but one of the
+many properties of living things; he takes quite as much interest in
+their behaviour, their ecology, distribution, comparative physiology.
+He takes a special interest in the comparative physiology of
+reproduction. The _Historia Animalium_ contains a description of the
+form and structure of man and of as many animals as Aristotle was
+acquainted with--and he was acquainted with an astonishingly large
+number. The later _De Partibus Animalium_ is a treatise on the causes
+of the form and structure of animals. Owing to the importance which
+Aristotle ascribed to the final cause this work became really a
+treatise on the functions of the parts, a discussion of the problems
+of the relation of form to function, and the adaptedness of structure.
+
+Aristotle was quite well aware that each of the big groups of animals
+was built upon one plan of structure, which showed endless variations
+"in excess and defect" in the different members of the group. But he
+did not realise that this fact of community of plan constituted a
+problem in itself. His interest was turned towards the functional side
+of living things, form was for him a secondary result of function.
+
+Yet he was not unaware of facts of form for which he could not quite
+find a place in his theory of organic form, facts of form which were
+not, at first sight at least, facts of function. Thus he was aware of
+certain facts of "correlation," which could not be explained off-hand
+as due to correlation of the functions of the parts. He knew, for
+instance, that all animals without front teeth in the upper jaw have
+cotyledons, while most that have front teeth on both jaws and no horns
+have no cotyledons (_De Gen._, ii. 7).
+
+Speaking generally, however, we find in Aristotle no purely
+morphological concepts. What then does morphology owe to Aristotle? It
+owes to him, _first_, a great mass of facts about the structure of
+animals; _second_, the first scientific classification of animals;[6]
+_third_, a clear enunciation of the fact of community of plan within
+each of the big groups; _fourth_, an attempt to explain certain
+instances of the correlation of parts; _fifth_, a pregnant distinction
+between homogeneous and heterogeneous parts; _sixth_, a generalisation
+on the succession of forms in development; and _seventh_, the first
+enunciation of the idea of the _Échelle des êtres_.
+
+(1) What surprises the modern reader of the _Historia Animalium_
+perhaps more than anything else is the extent and variety of
+Aristotle's knowledge of animals. He describes more than 500 kinds.[7]
+Not only does he know the ordinary beasts, birds, and fishes with
+which everyone is acquainted, but he knows a great deal about
+cuttlefish, snails and oysters, about crabs, crawfish (_Palinurus_),
+lobsters, shrimps, and hermit crabs, about sea-urchins and starfish,
+sea-anemones and sponges, about ascidians (which seem to have puzzled
+him not a little!). He has noticed even fish-lice and intestinal
+worms, both flat and round. Of the smaller land animals, he knows a
+great many insects and their larvæ. The extent of his anatomical
+knowledge is equally surprising, and much of it is clearly the result
+of personal observation. No one can read his account of the internal
+anatomy of the chameleon (_Hist. Anim._, ii.), or his description of
+the structure of cuttlefish (_Hist. Anim._, iv), or that touch in the
+description of the hermit crab (_Hist. Anim._, iv.)--" Two large eyes
+... not ... turned on one side like those of crabs, but straight
+forward"--without being convinced that Aristotle is speaking of what
+he has seen. Naturally he could not make much of the anatomy of small
+insects and snails, and, to tell the truth, he does not seem to have
+cared greatly about the minutiæ of structure. He was too much of a
+Greek and an aristocrat to care about laborious detail.
+
+Not only did he lay a foundation for comparative anatomy, but he made
+a real start with comparative embryology. Medical men before him had
+known many facts about human development; Aristotle seems to have been
+the first to study in any detail the development of the chick. He
+describes this as it appears to the naked eye, the position of the
+embryo on the yolk, the palpitating spot at the third day, the
+formation of the body and of the large sightless eyes, the veins on
+the yolk, the embryonic membranes, of which he distinguished two.
+
+(2) Aristotle had various systems of classifying animals. They could
+be classified, he thought, according to their structure, their manner
+of reproduction, their manner of life, their mode of locomotion, their
+food, and so on. Thus you might, in addition to structural
+classifications, divide animals into gregarious, solitary and social,
+or land animals into troglodytes, surface-dwellers, and burrowers
+(_Hist. Anim._, i.).
+
+He knew that dichotomous classifications were of little use for
+animals (_De Partibus_, i. 3) and he explicitly and in so many words
+accepted the principle of all "natural" classification, that
+affinities must be judged by comparing not one but the sum total of
+characters. As everyone knows, he was the first to distinguish the big
+groups of animals, many of which were already distinguished roughly by
+the common usages of speech. Among his Sanguinea he did little more
+than define with greater exactitude the limits of the groups
+established by the popular classification. Among the "exsanguineous"
+animals, however, corresponding to our Invertebrates, he established a
+much more definite classification than the popular, which is apt to
+call them indiscriminately "shellfish," "insects," or "creeping
+things." He went beyond the superficialities of popular
+classification, too, in clearly separating Cetacea from fishes. He had
+some notion of species and genera in our sense. He distinguished many
+species of cuttlefish--_Octopus (Polypus)_ of which there were many
+kinds, _Eledone (Moschites)_ which he knew to have only one row of
+suckers while _Octopus_ has two, _Argonauta, Nautilus, Sepia_, and
+apparently _Loligo media_ (= his Teuthis) and _L. vulgaris_(or
+_forbesii_) which seems to be his Teuthos. He had a grasp of the
+principles which should be followed in judging of the natural
+affinities of species. For example, he knew that the cuckoo resembles
+a hawk. "But," he says, "the hawk has crooked talons, which the cuckoo
+has not, nor does it resemble the hawk in the form of its head, but in
+these respects is more like the pigeon than the hawk, which it
+resembles in nothing but its colour; the markings, however, upon the
+hawk are like lines, while the cuckoo is spotted" (_Hist. Anim._,
+Cresswell's trans., p. 147, London, 1862).
+
+The groups he distinguished were--man, viviparous quadrupeds,
+oviparous quadrupeds, birds, fishes, Cetacea, Cephalopoda,
+Malacostraca (= higher Crustacea), Insecta (= annulose animals),
+Testacea (= molluscs, echinoderms, ascidians). A class of Acalephæ,
+including sea-anemones and sponges, was grouped with the Testacea. The
+first five groups were classed together as sanguineous, the others as
+exsanguineous, from the presence or absence of red blood.
+
+Besides these classes "there are," he says, "many other creatures in
+the sea which it is not possible to arrange in any class from their
+scarcity" (Creswell, _loc. cit._, p. 90).
+
+(3) Aristotle's greatest service to morphology is his clear
+recognition of the unity of plan holding throughout each of the great
+groups.
+
+He recognises this most clearly in the case of man and the viviparous
+quadrupeds, with whose structure he was best acquainted. In the
+_Historia Animalium_ he takes man as a standard, and describes his
+external and internal parts in detail, then considers viviparous
+quadrupeds and compares them with man. "Whatever parts a man has
+before, a quadruped has beneath; those that are behind in man form the
+quadruped's back" (Cresswell, _loc. cit._, p. 26). Apes, monkeys, and
+Cynocephali combine the characteristics of man and quadrupeds. He
+notices that all viviparous quadrupeds have hair. Oviparous quadrupeds
+resemble the viviparous, but they lack some organs, such as ears with
+an external pinna, mammæ, hair. Oviparous bipeds, or birds, also "have
+many parts like the animals described above." He does not, however,
+seem to realise that a bird's wings are the equivalent of a mammal's
+arms or fore-legs. Fishes are much more divergent; they possess no
+neck, nor limbs, nor testicles (meaning a solid ovoid body such as the
+testis in mammals), nor mammæ. Instead of hair they have scales.
+
+Speaking generally, the Sanguinea differ from man and from one another
+in their parts, which may be present or absent, or exhibit differences
+in "excess and defect," or in form. Unity of plan extends to all the
+principal systems of organs. "All sanguineous animals have either a
+bony or a spinous column. The remainder of the bones exist in some
+animals; but not in others, for if they have the limbs they have the
+bones belonging to them" (Cresswell, _loc. cit._, p. 60). "Viviparous
+animals with blood and feet do not differ much in their bones, but
+rather by analogy, in hardness, softness, and size" (Cresswell, _loc.
+cit._, p. 59). The venous system, too, is built upon the same general
+plan throughout the Sanguinea. "In all sanguineous animals, the nature
+and origin of the principal veins are the same, but the multitude of
+smaller veins is not alike in all, for neither are the parts of the
+same nature, nor do all possess the same parts" (Cresswell, _loc.
+cit._, p. 56). It will be noticed in the first and last of these three
+quotations that Aristotle recognises the fact of correlation between
+systems of organs,--between limbs and bones, and between blood-vessels
+and the parts to which they go.
+
+Sanguineous animals all possess certain organs--heart, liver, spleen,
+kidneys, and so on. Other organs occur in most of the classes--the
+oesophagus and the lungs. "The position which these parts occupy is
+the same in all animals [sc. Sanguinea]" (Cresswell, _loc. cit._, p.
+39).
+
+Unity of plan is observable not only in the Sanguinea, but also within
+each of the other large groups. Aristotle recognises that all his
+cuttlefish are alike in structure. Among his Malacostraca he compares
+point by point the external parts of the carabus (_Palinurus_), and
+the astacus (_Homarus_), and he compares also the general internal
+anatomy of the various "genera" he distinguishes. As regards Testacea,
+he writes, "The nature of their internal structure is similar in all,
+especially in the turbinated animals, for they differ in size and in
+the relations of excess; the univalves and bivalves do not exhibit
+many differences" (Cresswell, _loc. cit._, p. 83). There is an
+interesting remark about "the creature called carcinium"
+(hermit-crab), that it "resembles both the Malacostraca and the
+Testacea, for this in its nature is similar to the animals that are
+like carabi, and it is born naked" (Cresswell, _loc. cit._, p. 85). In
+the last phrase we may perhaps read the first recognition of the
+embryological criterion.
+
+With the recognition of unity of plan within each group necessarily
+goes the recognition of what later morphology calls the homology of
+parts. The parts of a horse can be compared one by one with the parts
+of another viviparous quadruped; in all the animals belonging to the
+same class the parts are the same, only they differ in excess or
+defect--these remarks are placed in the forefront of the _Historia
+Animalium_. Generally speaking, parts which bear the same name are for
+Aristotle homologous throughout the class. But he goes further and
+notes the essential resemblance underlying the differences of certain
+parts. He classes together nails and claws, the spines of the
+hedgehog, and hair, as being homologous structures. He says that teeth
+are allied to bones, whereas horns are more nearly allied to skin
+(_Hist. Anim._, iii.). This is an astonishingly happy guess,
+considering that all he had to go upon was the observation that in
+black animals the horns are black but the teeth white. One cannot but
+admire the way in which Aristotle fixes upon apparently trivial and
+commonplace facts, and draws from them far-reaching consequences. He
+often goes wrong, it is true, but he always errs in the grand manner.
+
+While Aristotle certainly recognised the existence of homologies, and
+even had a feeling for them, he did not clearly distinguish homology
+from analogy. He comes pretty near the distinction in the following
+passage. After explaining that in animals belonging to the same class
+the parts are the same, differing only in excess or defect, he says,
+"But some animals agree with each other in their parts neither in form
+nor in excess and defect, but have only an analogous likeness, such as
+a bone bears to a spine, a nail to a hoof, a hand to a crab's claw,
+the scale of a fish to the feather of a bird, for that which is a
+feather in the bird is a scale in the fish" (Cresswell, _loc. cit._,
+p. 2). One of these comparisons is, however, a homology not an
+analogy, and the last phrase throws a little doubt upon the whole
+question, for it is not made clear whether it is position or function
+that determines what are equivalent organs.
+
+In the _De Partibus Animalium_ there occurs the following
+passage:--"Groups that only differ in degree, and in the more or less
+of an identical element that they possess, are aggregated under a
+single class; groups whose attributes are not identical but analogous
+are separated. For instance, bird differs from bird by gradation, or
+by excess and defect; some birds have long feathers, others short
+ones, but all are feathered. Bird and Fish are more remote and only
+agree in having analogous organs; for what in the bird is feather, in
+the fish is scale. Such analogies can scarcely, however, serve
+universally as indications for the formation of groups, for almost all
+animals present analogies in their corresponding parts."[8] It is thus
+similarity in form and structure which determines the formation of the
+main groups. Within each group the parts differ only in degree, in
+largeness or smallness, softness and hardness, smoothness or
+roughness, and the like (_loc. cit._, i., 4, 644^b). These passages
+show that Aristotle had some conception of homology as distinct from
+analogy. He did not, however, develop the idea. What Aristotle sought
+in the variety of animal structure, and what he found, were not
+homologies, but rather communities of function, parts with the same
+attributes. His interest was all in _organs_, in functioning parts,
+not in the mere spatial relationship of parts.
+
+This comes out clearly in his treatise _On the Parts of Animals_,
+which is subsequent to, and the complement of, his _History of
+Animals_. The latter is a description of the variety of animal form,
+the former is a treatise on the functions of the parts. He describes
+the plan of the _De Partibus Animalium_ as follows:--"We have, then,
+first to describe the common functions, common, that is, to the whole
+animal kingdom, or to certain large groups, or to members of a
+species. In other words, we have to describe the attributes common to
+all animals, or to assemblages, like the class of Birds, of closely
+allied groups differentiated by gradation, or to groups like Man not
+differentiated into subordinate groups. In the first case the common
+attributes may be called analogous, in the second generic, in the
+third specific" (i, 5, 645^b, trans. Ogle). The alimentary canal is a
+good example of a part which is "analogous" throughout the animal
+kingdom, for "all animals possess in common those parts by which they
+take in food, and into which they receive it" (Cresswell, _loc. cit._,
+p. 6).
+
+The _De Partibus Animalium_ becomes in form a comparative
+organography, but the emphasis is always on function and community of
+function. Thus he treats of bone, "fish-spine," and cartilage together
+(_De Partibus_, ii., 9, 655^a), because they have the same function,
+though he says elsewhere that they are only analogous structures (ii.,
+8, 653^b). In the same connection he describes also the supporting
+tissues of Invertebrates--the hard exoskeleton of Crustacea and
+Insects, the shell of Testacea, the "bone" of _Sepia_ (ii., 8,
+654^a). Aristotle took much more interest in analogies, in organs of
+similar function, than in homologies. He did recognise the existence
+of homologies, but rather _malgré lui_, because the facts forced it
+upon him.
+
+His only excursion into the realm of "transcendental anatomy" is his
+comparison of a Cephalopod to a doubled-up Vertebrate whose legs have
+become adherent to its head, whose alimentary canal has doubled upon
+itself in such a way as to bring the anus near the mouth (_De
+Partibus_, iv., 9, 684^b). It is clear, however, that Aristotle did
+not seek to establish by this comparison any true homologies of parts,
+but merely analogies, thus avoiding the error into which Meyranx and
+Laurencet fell more than two thousand years later in their paper
+communicated to the Académie des Sciences, which formed the
+starting-point of the famous controversy between Cuvier and E.
+Geoffroy St Hilaire (see Chap. V., below).
+
+Moreover, Aristotle did not so much compare a Cephalopod with a
+doubled-up Vertebrate as contrast Cephalopods (and also Testacea) with
+all other animals. Other animals have their organs in a straight line;
+Cephalopods and Testacea alone show this peculiar doubling up of the
+body.
+
+(4) Aristotle was much struck with certain facts of correlation, of
+the interdependence of two organs which are not apparently in
+functional dependence on one another. Such correlation may be positive
+or negative; the presence of one organ may either entail the presence
+of the other, or it may entail its absence. Aristotle has various ways
+of explaining facts of correlation. He observed that no animal has
+both tusks and horns, but this fact could easily be explained on the
+principle that Nature never makes anything superfluous or in vain. If
+an animal is protected by the possession of tusks it does not require
+horns, and _vice versa_. The correlation of a multiple stomach with
+deficient development of the teeth (as in Ruminants) is accounted for
+by saying that the animal needs its complex stomach to make up for the
+shortcomings of its teeth! (_De Partibus_, iii., 14, 674^b.) Other
+examples of correlation were not susceptible of this explanation in
+terms of final causes. He lays stress on the fact, in the main true,
+of the inverse development of horns and front teeth in the upper jaw,
+exemplified in Ruminants. He explains the fact in this way. Teeth and
+horns are formed from earthy matter in the body and there is not
+enough to form both teeth and horns, so "Nature by subtracting from
+the teeth adds to the horns; the nutriment which in most animals goes
+to the former being here spent on the augmentation of the latter" (_De
+Partibus_, iii., 2, 664^a, trans. Ogle). A similar kind of explanation
+is offered of the fact that Selachia have cartilage instead of bone,
+"in these Selachia Nature has used all the earthy matter on the skin
+[_i.e._, on the placoid scales]; and she is unable to allot to many
+different parts one and the same superfluity of material" (_De
+Partibus_, ii., 9, 655^a, trans. Ogle). Speaking generally, "Nature
+invariably gives to one part what she subtracts from another" (_loc.
+cit._, ii., 14, 658^a).
+
+This thought reappears again in the 19th century in E. Geoffroy St
+Hilaire's _loi de balancement_ and also in Goethe's writings on
+morphology. For Aristotle it meant that Nature was limited by the
+nature of her means, that finality was limited by necessity. Thus in
+the larger animals there is an excess of earthy matter, as a necessary
+result of the material nature of the animal; this excess is turned by
+Nature to good account, but there is not enough to serve both for
+teeth and for horns (_loc. cit._, iii., 2, 663^b).
+
+But there are other instances of correlation which seem to have taxed
+even Aristotle's ingenuity beyond its powers. Thus he knew that all
+animals (meaning viviparous quadrupeds) with no front teeth in the
+upper jaw have cotyledons on their foetal membranes, and that most
+animals which have front teeth in both jaws and no horns have no
+cotyledons (_De Generatione_, ii., 7). He offers no explanation of
+this, but accepts it as a fact.
+
+We may conveniently refer here to one or two other ideas of Aristotle
+regarding the causes of form. He makes the profound remark that the
+possible range of form of an organ is limited to some extent by its
+existing differentiation. Thus he explains the absence of external
+(projecting) ears in birds and reptiles by the fact that their skin is
+hard and does not easily take on the form of an external ear (_De
+Partibus_, ii, 12). The fact of the inverse correlation is certain;
+the explanation is, though very vague, probably correct.
+
+In one passage of the _De Partibus_ Aristotle clearly enunciates the
+principle of the division of labour, afterwards emphasised by H.
+Milne-Edwards. In some insects, he says, the proboscis combines the
+functions of a tongue and a sting, in others the tongue and the sting
+are quite separate. "Now it is better," he goes on, "that one and the
+same instrument shall not be made to serve several dissimilar ends;
+but that there shall be one organ to serve as a weapon, which can then
+be very sharp, and a distinct one to serve as a tongue, which can then
+be of spongy texture and fit to absorb nutriment. Whenever, therefore,
+Nature is able to provide two separate instruments for two separate
+uses, without the one hampering the other, she does so, instead of
+acting like a coppersmith who for cheapness makes a spit and
+lampholder in one" (iv., 6, 683^a).
+
+(5) The first sentence of the _Historia Animalium_ formulates, with
+that simplicity and directness which is so characteristic of
+Aristotle, the distinction between homogeneous and heterogeneous
+parts, in the mass the distinction between tissues and organs. "Some
+parts of animals are simple, and these can be divided into like parts,
+as flesh into pieces of flesh; others are compound, and cannot be
+divided into like parts, as the hand cannot be divided into hands, nor
+the face into faces. All the compound parts also are made up of simple
+parts--the hand, for example, of flesh and sinew and bone" (Cresswell,
+_loc. cit_., p. I).
+
+In the _De Partibus Animalium_ he broadens the conception by adding
+another form of composition. "Now there are," he says, "three degrees
+of composition; and of these the first in order, as all will allow, is
+composition out of what some call the elements, such as earth, air,
+water, fire.... The second degree of composition is that by which the
+homogeneous parts of animals, such as bone, flesh, and the like, are
+constituted out of the primary substances. The third and last stage is
+the composition which forms the heterogeneous parts, such as face,
+hand, and the rest" (ii., 1, 646^a, trans. Ogle).
+
+In the _Historia Animalium_ the homogeneous parts are divided into (1)
+the soft and moist (or fluid), such as blood, serum, flesh, fat, suet,
+marrow, semen, gall, milk, phlegm, fæces and urine, and (2) the hard
+and dry (or solid), such as sinew, vein, hair, bone, cartilage, nail,
+and horn. It would appear from this enumeration that Aristotle's
+distinction of simple and complex parts does not altogether coincide
+with our distinction of tissues and organs. We should not call vein a
+tissue, nor do we include under this heading non-living secretions.
+But in the _De Partibus Animalium_ Aristotle, while still holding to
+the distinction set forth above, is alive to the fact that his simple
+parts include several different sorts of substances. He distinguishes
+among the homogeneous parts three sets. The first of these comprises
+the tissues out of which the heterogeneous parts are constructed,
+_e.g._, flesh and bone; the second set form the nutriment of the
+parts, and are invariably fluid; while the third set are the residue
+of the second and constitute the residual excretions of the body (ii.,
+2, 647^b). He sees clearly the difficulty of calling vein or
+blood-vessel a simple part, for while a bloodvessel and a part of it
+are both blood-vessel, as we should say vascular tissue, yet a part of
+a blood-vessel is not a bloodvessel. There is form superadded to
+homogeneity of structure (ii., 2, 647^b). Similarly for the heart and
+the other viscera. "The heart, like the other viscera, is one of the
+homogeneous parts; for, if cut up, its pieces are homogeneous in
+substance with each other. But it is at the same time heterogeneous in
+virtue of its definite configuration" (ii., 1, 647^a, trans. Ogle).
+
+Aristotle, therefore, came very near our conception of tissue. He was
+of course not a histologist; he describes not the structure of
+tissues, which he could not know, but rather their distribution within
+the organism; his section on the homogeneous parts of Sanguinea
+(_Historia Animalium_, iii., second half) is largely a comparative
+topographical anatomy; in it, for instance, he describes the venous
+and skeletal systems.
+
+This distinction which Aristotle drew plays an important part in all
+his writings on animals, particularly in his theory of development. It
+was a distinction of immense value, and is full of meaning even at the
+present day. No one has ever given a better definition of organ than
+is implied in Aristotle's description of the heterogeneous parts--"The
+capacity of action resides in the compound parts" (Cresswell, _loc.
+cit._, p. 7). The heterogeneous parts were distinguished by the
+faculty of doing something, they were the active or executive parts.
+The homogeneous parts were distinguished mainly by physical characters
+(_De Generatione_, i., 18), but certain of them had other than purely
+physical properties, they were the organs of touch (_De Partibus_,
+ii., 1, 647^a).
+
+(6) In a passage in the _De Generatione_ (ii, 3) Aristotle says that
+the embryo is an animal before it is a particular animal, that the
+general characters appear before the special. This is a foreshadowing
+of the essential point in von Baer's law (see Chap. IX. below).
+
+He considers also that tissues arise before organs. The homogeneous
+parts are anterior genetically to the heterogeneous parts and
+posterior to the elementary material (_De Partibus_, ii., 1, 646^b).
+
+(7) We meet in Aristotle an idea which later acquired considerable
+vogue, that of the _Échelle des êtres_(or "scale of beings"), that
+organisms, or even all objects organic or inorganic, can be arranged
+in a single ascending series. The idea is a common one; its first
+literary expression is found perhaps in primitive creation-myths, in
+which inorganic things are created before organic, and plants before
+animals. It may be recognised also in Anaximander's theory that land
+animals arose from aquatic animals, more clearly still in Anaxagoras'
+theory that life took its origin on this globe from vegetable germs
+which fell to earth with the rain. Anaxagoras considered animals
+higher in the scale than plants, for while the latter participated in
+pleasure (when they grew) and pain (when they lost their leaves),
+animals had in addition "Nous." In Empedocles' theory of evolution,
+the vegetable world preceded the animal. Plato, in the _Timaeus_,
+describes the whole organic world as being formed by degradation from
+man, who is created first. Man sinks first into woman, then into brute
+form, traversing all the stages from the higher to the lower animals,
+and becoming finally a plant. This is a reversal of the more usual
+notion, but the idea of gradation is equally present.
+
+Aristotle seems not to have believed in any transformation of species,
+but he saw that Nature passes gradually from inanimate to animate
+things without a clear dividing line. "The race of plants succeeds
+immediately that of inanimate objects" (Cresswell, _loc. cit._, p.
+94). Within the organic realm the passage from plants to animals is
+gradual. Some creatures, for example, the sea-anemones and sponges,
+might belong to either class.
+
+Aristotle recognised also a natural series among the groups of
+animals, a series of increasing complexity of structure. He begins his
+study of structure with man, who is the most intricate, and then takes
+up in turn viviparous and oviparous quadrupeds, then birds, then
+fishes. After the Sanguinea he considers the Exsanguinea, and of the
+latter first the most highly organised, the Cephalopods, and last the
+simplest, the lower members of his class of the Testacea. In treating
+of generation (in _Hist. Animalium_, v.) he reverses this order. In
+the _De Generatione_ (Book ii., I) there is given another serial
+arrangement of animals, this time in relation to their manner of
+reproduction. There is a gradation, he says, of the following kind:--
+
+1. Internally viviparous Sanguinea  } producing a perfect
+2. Externally viviparous Sanguinea  }       animal.
+3. Oviparous Sanguinea--producing a perfect egg.
+4. Animals producing an imperfect egg (one which
+     increases in size after being laid).
+5. Insects, producing a scolex (or grub).
+
+In Aristotle's view the gradation of organic forms is the consequence,
+not the cause, of the gradation observable in their activities. Plants
+have no work to do beside nutrition, growth, and reproduction; they
+possess only the nutritive soul. Animals possess in addition sensation
+and the sensitive or perceptive soul--"their manner of life differs in
+their having pleasure in sexual intercourse, in their mode of
+parturition and rearing their young" (_Hist. Anim._, viii., trans.
+Cresswell, p. 195). Man alone has the rational soul in addition to the
+two lower kinds.
+
+As it is put in the _De Partibus_ (ii., 10, 656^a, trans. Ogle),
+"Plants, again, inasmuch as they are without locomotion, present no
+great variety in their heterogeneous parts. For, where the functions
+are but few, few also are the organs required to effect them....
+Animals, however, that not only live but feel, present a greater
+multiformity of parts, and this diversity is greater in some animals
+than in others, being most varied in those to whose share has fallen
+not mere life but life of high degree. Now such an animal is man."
+
+With the great exception of Aristotle, the philosophers of Greece and
+Rome made little contribution to morphological theory. Passing mention
+may be made of the Atomists--Leucippus, Democritus, and their great
+disciple Lucretius, who in his magnificent poem "De Natura Rerum" gave
+impassioned expression to the materialistic conception of the
+universe. But the full effect of materialism upon morphology does not
+become apparent till the rise of physiology in the 17th and 18th
+centuries, and reaches its culmination in the 19th century. The
+evolutionary ideas of Lucretius exercised no immediate influence upon
+the development of morphology.
+
+    [1] E. Zeller, _Greek Philosophy_, Eng. trans., i., 522
+    f.n., London 1881. Other particulars as to Alcmaeon in
+    T. Gomperz, _Greek Thinkers_, Eng. trans., i., London,
+    1901.
+
+    [2] Zeller, _loc. cit._, i., p. 297.
+
+    [3] Gomperz, _loc. cit._, i., p. 244.
+
+    [4] R. Burckhardt, _Biologie u. Humanismus_, p. 85,
+    Jena, 1907.
+
+    [5] See the interesting account of Aristotle's
+    biological work in Prof. D'Arcy W. Thompson's Herbert
+    Spencer lecture (1913) and his translation of the
+    _Historia Animalium_ in the Oxford series.
+
+    [6] On Aristotle's forerunners, see R. Burckhardt, "Das
+    koïsche Tiersystem, eine Vorstufe des zoologischen
+    Systematik des Aristoteles." _Verh. Naturf. Ges. Basel_,
+    xx., 1904.
+
+    [7] T.E. Lones, _Aristotle's Researches in Natural
+    Science_, pp. 82-3, London, 1912.
+
+    [8] _De Partibus Animalium_, i., 4, 644^a trans. W.
+    Ogle, Oxford, 1911.
+
+
+
+
+CHAPTER II
+
+COMPARATIVE ANATOMY BEFORE CUVIER
+
+
+For two thousand years after Aristotle little advance was made upon
+his comparative anatomy. Knowledge of the human body was increased not
+long after his death by Herophilus and Erasistratus, but not even
+Galen more than four centuries later made any essential additions to
+Aristotle's anatomy.
+
+During the Middle Ages, particularly after the introduction to Europe
+in the 13th century of the Arab texts and commentaries, Aristotle
+dominated men's thoughts of Nature. The commentary of Albertus Magnus,
+based upon that of Avicenna, did much to impose Aristotle upon the
+learned world. Albertus seems to have contented himself with following
+closely in the footsteps of his master. There are noted, however, by
+Bonnier certain improvements made by Albertus on Aristotle's view of
+the seriation of living things. "He is the first," writes Bonnier, "to
+take the correct view that fungi are lower plants allied to the most
+lowly organised animals. From this point there start, for Albertus
+Magnus, two series of living creatures, and he regards the plant
+series as culminating in the trees which have well-developed
+flowers."[9]
+
+Aristotle's influence is predominant also in the work of Edward Wotton
+(1492-1555), who in his book _De differentiis animalium_ adopted a
+classification similar to that proposed by Aristotle. He too laid
+stress upon the gradation shown from the lower to the higher forms.
+
+In the 16th century, two groups of men helped to lay foundations for a
+future science of comparative anatomy--the great Italian anatomists
+Vesalius, Fallopius and Fabricius, and the first systematists (though
+their "systems" were little more than catalogues) Rondeletius,
+Aldrovandus and Gesner.
+
+The anatomists, however, took little interest in problems of pure
+morphology; the anatomy of the human body was for them simply the
+necessary preliminary of the discovery of the functions of the
+parts--they were quite as much physiologists as anatomists.
+
+One of them, Fabricius, made observations on the development of the
+chick (1615). Harvey, who was a pupil of Fabricius, likewise published
+an account of the embryology of the chick.[10] In his philosophy and
+habit of thought Harvey was a follower of Aristotle. It is worth
+noting that in his _Exercitationes anatomicae de motu cordis_ (1628)
+there is a passage which dimly foreshadows the law of recapitulation
+in development which later had so much vogue.[11]
+
+A stimulating contribution to comparative anatomy was made by
+Belon,[12] who published in 1555 a _Histoire de la nature des Oyseaux_,
+in which he showed opposite one another a skeleton of a bird and of a
+mammal, giving the same names to homologous bones. The anatomy of
+animals other than man was indeed not altogether neglected at this
+time. Coiter (1535-1600) studied the anatomy of Vertebrates,
+discovering among other things the fibrous structure of the brain.
+Carlo Ruini of Bologna wrote in 1598 a book on the anatomy of the
+horse.[13] Somewhat later Severino, professor at Naples, dissected many
+animals and came to the conclusion that they were built upon the same
+plan as man.[14] Willis, of Oxford and London, in his _Cerebri Anatome_
+(1659) recognised the necessity for comparative study of the structure
+of the brain. He found out that the brain of man is very like that of
+other mammals, the brain of birds, on the contrary, like that of
+fishes![15] He described the anatomy of the oyster and the crayfish. He
+had, however, not much feeling for morphology.
+
+The foundation of the Jardin des Plantes at Paris in 1626 and the
+subsequent addition to it of a Museum of Natural History and a
+menagerie gave a great impulse to the study of comparative anatomy by
+supplying a rich material for dissection. Advantage was taken of these
+facilities, particularly by Claude Perrault and Duverney.[16] In a
+volume entitled _De la Mécanique des Animaux_, Perrault recognises
+clearly the idea of unity of type, and even pushes it too far, seeking
+to prove that in plants there exists an arterial system and veins
+provided with valves.[17]
+
+The beginning of the 17th century saw the invention of the microscope,
+which was to have such an enormous influence upon the development of
+biological studies. It did not come into scientific use until well on
+in the middle of the century. Just before it came into use Francis
+Glisson (1597-1677), an Englishman, gave in the introduction to his
+treatise on the liver an account of the notions then current on the
+structure of organic bodies. He classifies the parts as "similar" and
+"organic," the former determined by their material, the latter by the
+form which they assume. The similar parts are divided into the
+sanguineous or rich in blood and the spermatic. Both sets are further
+subdivided according to their physical characters,[18] the latter, for
+instance, into the hard, soft, and tensile tissues. The classification
+resembles greatly that propounded by Aristotle, though it is notably
+inferior in the details of its working out.
+
+For Aristotle, as for all anatomists before the days of the
+microscope, the tissues were not much more than inorganic substances,
+differing from one another in texture, in hardness, and other physical
+properties. They possessed indeed properties, such as contractility,
+which were not inorganic, but as far as their visible structure was
+concerned there was little to raise them above the inorganic level.
+The application of the microscope changed all that, for it revealed in
+the tissues an organic structure as complex in its grade as the gross
+and visible structure of the whole organism. Of the four men who first
+made adequate use of the new aid, Malpighi, Hooke, Leeuenhoek, and
+Swammerdam, the first-named contributed the most to make current the
+new conceptions of organic structure. He studied in some detail the
+development of the chick. He described the minute structure of the
+lungs (1661), demonstrating for the first time, by his discovery of
+the capillaries, the connection of the arteries with the veins. In his
+work, _De viscerum structura_ (1666), he describes the histology of
+the spleen, the kidney, the liver, and the cortex of the brain,
+establishing among other things the fact that the liver was really a
+conglomerate gland, and discovering the Malpighian bodies in the
+kidney. This work was done on a broad comparative basis. "Since in the
+higher, more perfect, red-blooded animals, the simplicity of their
+structure is wont to be involved by many obscurities, it is necessary
+that we should approach the subject by the observation of the lower,
+imperfect animals."[19] So he wrote in the _De viscerum structura_, and
+accordingly he studied the liver first in the snail, then in fishes,
+reptiles, mammals, and finally man. In the introduction to his
+_Anatome plantarum_ (1675), in which he laid the foundations of plant
+histology, he vindicates the comparative method in the following
+words:--"In the enthusiasm of youth I applied myself to Anatomy, and
+although I was interested in particular problems, yet I dared to pry
+into them in the higher animals. But since these matters enveloped in
+peculiar mystery still lie in obscurity, they require the comparison
+of simpler conditions, and so the investigation of insects[20] at once
+attracted me; finally, since this also has its own difficulties I
+applied my mind to the study of plants, intending after prolonged
+occupation with this domain, to retrace my steps by way of the
+vegetable kingdom, and get back to my former studies. But perhaps not
+even this will be sufficient; since the simpler world of minerals and
+the elements should have been taken first. In this case, however, the
+undertaking becomes enormous and far beyond my powers."[21] There is
+something fine in this life of broad outlines, devoted whole-heartedly
+to an idea, to a plan of research, which required a lifetime to carry
+out.
+
+An important histological discovery dating from this time is that of
+the finer structure of muscle, made by Stensen (or Steno) in 1664. He
+described the structure of muscle-fibres, resolving them into their
+constituent fibrils.
+
+To the microscope we owe not only histology but the comparative
+anatomy of the lower animals. Throughout the 17th and 18th centuries
+the discovery of structure in the lower animals went on continuously,
+as may be read in any history of Zoology.[22] We content ourselves here
+with mentioning only some representative names.
+
+In the 17th century Leeuenhoek, applying the microscope almost at
+random, discovered fact after fact, his most famous, discovery being
+that of the "spermatic animalcules."
+
+Swammerdam studied the metamorphoses of insects and made wonderfully
+minute dissections of all sorts of animals, snails and insects
+particularly. He described also the development of the frog. It is
+curious to see what a grip his conception of metamorphosis had upon
+him when he homologises the stages of the frog's development with the
+Egg, the Worm, and the Nymph of insects (_Book of Nature_, p. 104,
+Eng. trans., 1785). He even speaks of the human embryo as being at a
+certain stage a Man-Vermicle.
+
+In the 18th century, Réaumur and Bonnet continued the minute study of
+insects, laying more stress, however, on their habits and physiology
+than upon their anatomy. Lyonnet made a most laborious investigation
+of the anatomy of the willow-caterpillar (1762). John Hunter (1728-93)
+dissected all kinds of animals, from holothurians to whales. His
+interest was, however, that of the physiologist, and he was not
+specially interested in problems of form. It is interesting to note a
+formulation in somewhat confused language of the recapitulation
+theory. The passage occurs in his description of the drawings he made
+to illustrate the development of the chick. It is quoted in full by
+Owen (J. Hunter, _Observations on certain Parts of the Animal
+OEconomy_, with Notes by Richard Owen. London, 1837. Preface, p.
+xxvi). We give here the last and clearest sentence--"If we were to
+take a series of animals from the more imperfect to the perfect, we
+should probably find an imperfect animal corresponding with some stage
+of the most perfect."
+
+The tendency of the time was not towards morphology, but rather to
+general natural history and to systematics, the latter under the
+powerful influence of Linnæus (1707-1778). The former tendency is
+well represented by Réaumur (1683-1757) with his observations on
+insects, the digestion of birds, the regeneration of the crayfish's
+legs, and a hundred other matters. To this tendency belong also
+Trembley's famous experiments on Hydra (1744), and Rösel von
+Rosenhof's _Insektenbelustigungen_ (1746-1761).
+
+Bonnet (1720-1793) deserves special mention here, since in his _Traité
+d'Insectologie_ (1745), and more fully in his _Contemplation de la
+Nature_ (1764), he gives the most complete expression to the idea of
+the _Échelle des êtres_.
+
+This idea seems to have taken complete possession of his imagination.
+He extends it to the universe. Every world has its own scale of
+beings, and all the scales when joined together form but one, which
+then contains all the possible orders of perfection. At the end of the
+Preface to his _Traité_ _d'Insectologie_ (OEuvres, i., 1779) he
+gives a long table, headed "Idée d'une Échelle des êtres naturels,"
+and rather resembling a ladder, on the rungs of which the following
+names appear:--
+
+MAN.
+Orang-utan.
+Ape.
+
+QUADRUPEDS.
+Flying squirrel.
+Bat.
+Ostrich.
+
+BIRDS.
+Aquatic birds.
+Amphibious birds.
+Flying Fish.
+
+FISH.
+Creeping fish.
+Eels.
+Water serpents.
+
+SERPENTS.
+Slugs.
+Snails.
+
+SHELL FISH.
+Tube-worms.
+Clothes-moths.
+
+INSECTS.
+Gall insects.
+Taenia.
+Polyps.
+Sea Nettles.
+Sensitive plant.
+
+PLANTS.
+Lichens.
+Moulds.
+Fungi, Agarics.
+Truffles.
+Corals, and Coralloids.
+Lithophytes.
+Asbestos.
+Talcs, Gypsums.
+Selenites, Slates.
+
+STONES.
+Figured stones.
+Crystals.
+
+SALTS.
+Vitriols.
+
+METALS.
+
+HALF-METALS.
+
+SULPHURS.
+Bitumens.
+
+EARTHS.
+Pure earth.
+
+WATER.
+
+AIR.
+
+FIRE.
+
+More subtile matter.
+
+The nature of the transitional forms which he inserts between his
+principal classes show very clearly his entire lack of morphological
+insight--the transitions are functional. The positions assigned to
+clothes-moths and corals are very curious! The whole scheme, so
+fantastic in its details, was largely influenced by Leibniz's
+continuity philosophy, and is in no way an improvement on the older
+and saner Aristotelian scheme.
+
+Robinet, in the fifth volume of his book _De la nature_ (1761-6),
+foreshadows the somewhat similar views of the German
+transcendentalists. "All beings," he writes, "have been conceived and
+formed on one single plan, of which they are the endlessly graduated
+variations: this prototype is the human form, the metamorphoses of
+which are to be considered as so many steps towards the most excellent
+form of being."[23]
+
+The idea of a gradation of beings appears also in Buffon (1707-1788),
+but here it takes more definitely its true character as a functional
+gradation.[24] "Since everything in Nature shades into everything
+else," he says, "it is possible to establish a scale for judging of
+the degrees of the intrinsic qualities of every animal."[25]
+
+He is quite well aware that the groups of Invertebrates are different
+in structural plan from the Vertebrates--"The animal kingdom includes
+various animated beings, whose organisation is very different from our
+own and from that of the animals whose body is similarly constructed
+to ours."[26]
+
+He limits himself to a consideration of the Vertebrates, deeming that
+the economy of an oyster ought not to form part of his subject matter!
+He has a clear perception of the unity of plan which reigns throughout
+the vertebrate series.[27] What is new in Buffon is his interpretation
+of the unity of plan. For the first time we find clearly expressed the
+thought that unity of plan is to be explained by community of origin.
+
+Buffon's utterances on this point are, as is well known, somewhat
+vacillating. The famous passage, however, which occurs in his account
+of the Ass shows pretty clearly that Buffon saw no theoretical
+objection to the descent of all the varied species of animals from one
+single form. Once admit, he argues, that within the bounds of a single
+family one species may originate from the type species by
+"degeneration," then one might reasonably suppose that from a single
+being Nature could in time produce all the other organised beings.[28]
+Elsewhere, _e.g._, in the discourse _De la Dégéneration des
+Animaux_,[29] Buffon expresses himself with more caution. He finds that
+it is possible to reduce the two hundred species of quadrupeds which
+he has described to quite a small number of families "from which it is
+not impossible that all the rest are derived."[30] Within each of the
+families the species branch off from a parent or type species. This we
+may note is a great advance on the linear arrangement implied in the
+idea of an _Échelle des êtres_.[31]
+
+It is a mistake to suppose that Buffon was par excellence a maker of
+hypotheses. On the contrary he saw things very sanely and with a very
+open mind. He expressly mentions the great difficulties which one
+encounters in supposing that one species may arise from another by
+"degeneration." How does it happen that two individuals "degenerate"
+just in the right direction and to the right stage so as to be capable
+of breeding together? How is it that one does not find intermediate
+links between species? One is reminded of the objections, not
+altogether without validity, which were made to the Darwinian theory
+in its early days. I cannot agree with those who think that Buffon was
+an out-and-out evolutionist, who concealed his opinions for fear of
+the Church. No doubt he did trim his sails--the palpably insincere
+"Mais non, il est certain, par la révélation, que tous les animaux ont
+également participé à la grace de la création,"[32] following hard upon
+the too bold hypothesis of the origin of all species from a single
+one, is proof of it. But he was too sane and matter-of-fact a thinker
+to go much beyond his facts, and his evolution doctrine remained
+always tentative. One thing, however, he was sure of, that evolution
+would give a rational foundation to the classification which, almost
+in spite of himself, he recognised in Nature. If, and only if, the
+species of one family originated from a single type species, could
+families, be founded rationally, _avec raison_.
+
+Buffon was, curiously enough, rather unwilling to recognise any
+systematic unit higher than the species. Strictly; speaking there are
+only individuals in Nature; but there are also groups of individuals
+which resemble one another from generation to generation and are able
+to breed together. These are species--Buffon adheres to the genetic
+definition of species--and the species is a much more definite unit
+than the genus, the order, the class, which are not divisions imposed
+by us upon Nature. Species are definitely discontinuous,[33] and this
+is the only discontinuity which Nature shows us. Buffon put his views
+into practice in his _Histoire Naturelle_, where he describes species
+after species, never uniting them into larger groups. We have seen,
+however, how the facts forced upon him the conception of the "family."
+
+Buffon was no morphologist. He left to Daubenton what one might call
+the "dirty work" of his book, the dissection and minute description of
+the animals treated.
+
+But Buffon was a man of genius, and accordingly his ideas on
+morphology are fresh and illuminating. Few naturalists have been so
+free from the prejudices and traditions of their trade. He makes in
+the _Discours sur la Nature des Animaux_[34] a distinction, which
+Bichat and Cuvier later developed with much profit, between the
+"animal" and the "vegetative" part of animals.[35] The vegetative or
+organic functions go on continuously, even in sleep, and are performed
+by the internal organs, of which the heart is the central one. The
+active waking life of the animal, that part of its life which
+distinguishes it from the plant, involves the external parts--the
+sense-organs and the extremities. An animal is, as it were, made up of
+a complex of organs performing the vegetative functions, assimilation,
+growth, and reproduction, surrounded by an envelope formed by the
+limbs, the sense-organs, the nerves and the brain, which is the centre
+of this "envelope."[36] Animals may differ from one another enormously
+in the external parts, particularly in the appendicular skeleton,
+without showing any great difference in the plan and arrangement of
+their internal organs. Quadrupeds, Cetacea, birds, amphibians and fish
+are as unlike as possible in external form and in the shape of their
+limbs; but they all resemble one another in their internal organs. Let
+the internal organs change, however--the external parts will change
+infinitely more, and you will get another animal, an animal of a
+totally different nature. Thus an insect has a most singular internal
+economy, and, in consequence, you find it is in every point different
+from any vertebrate animal.
+
+In this contrast, on the whole justified, between the importance of
+variations in the "vegetative" and variations in the "animal" parts,
+one may see without doing violence to Buffon's thought, an indication
+of the difference between homology and analogy. It is usually in the
+external parts, in the organs by which the animal adapts itself to its
+environment, that one meets with the greatest number of analogical
+resemblances. This contrast of vegetative and animal parts and their
+relative importance for the discovery of affinities was at any rate a
+considerable step towards an analysis of the concept of unity of plan.
+
+To Xavier Bichat (1771-1802) belongs the credit of working out in
+detail the distinction drawn by Aristotle and Buffon between the
+animal and the vegetative functions. Bichat was not a comparative
+anatomist; his interest lay in human anatomy, normal and pathological.
+So his views are drawn chiefly from the consideration of human
+structure.
+
+He classifies functions into those relating to the individual and
+those relating to the species. The functions pertaining to the
+individual may be divided into those of the animal and those of the
+organic life.[37] "I call _animal life_ that order of functions which
+connects us with surrounding bodies; signifying thereby that this
+order belongs only to animals" (p. lxxviii.). Its organs are the
+afferent and efferent nerves, the brain, the sense-organs and the
+voluntary muscles; the brain is its central organ. "Digestion,
+circulation, respiration, exhalation, absorption, secretion,
+nutrition, calorification, or production of animal heat, compose
+organic life, whose principal and central organ is the heart" (p.
+lxxix.).
+
+The contrast of the animal and the organic life runs through all
+Bichat's work; it receives classical expression in his _Recherches
+Physiologiques sur la Vie et la Mort_ (1800). The plant and the animal
+stand for two different modes of living. The plant lives within
+itself, and has with the external world only relations of nutrition;
+the animal adds to this organic life a life of active relation with
+surrounding things (3rd ed., 1805, p. 2). "One might almost say that
+the plant is the framework, the foundation of the animal, and that to
+form the animal it sufficed to cover this foundation with a system of
+organs fitted to establish relations with the world outside. It
+follows that the functions of the animal form two quite distinct
+classes. One class consists in a continual succession of assimilation
+and excretion; through these functions the animal incessantly
+transforms into its own substance the molecules of surrounding bodies,
+later to reject these molecules when they have become heterogeneous to
+it. Through this first class of functions the animal exists only
+within itself; through the other class it exists outside; it is an
+inhabitant of the world, and not, like the plant, of the place which
+saw its birth. The animal feels and perceives its surroundings,
+reflects its sensations, moves of its own will under their influence,
+and, as a rule, can communicate by its voice its desires and its
+fears, its pleasures or its pains. I call organic life the sum of the
+functions of the former class, for all organised creatures, plants or
+animals, possess them to a more or less marked degree, and organised
+structure is the sole condition necessary to their exercise. The
+combined functions of the second class form the 'animal' life, so
+named because it is the exclusive attribute of the animal kingdom"
+(pp. 2-3).
+
+In both lives there is a double movement, in the animal life from the
+periphery to the centre and from the centre to the periphery, in the
+organic life also from the exterior to the interior and back again,
+but here a movement of composition and decomposition. As the brain
+mediates between sensation and motion, so the vascular system is the
+go-between of the organs of assimilation and the organs of
+dissimilation.
+
+The most essential structural difference between the organs of animal
+life and the organs of organic life is in man and the higher animals
+at least, the symmetry of the one set and the irregularity of the
+other--compare the symmetry of the nerves and muscles of the animal
+life with the asymmetrical disposition of the visceral muscles and the
+sympathetic nerves, which belong to the organic life.
+
+Noteworthy differences exist between the two lives with respect to the
+influence of habit. Everything in the animal life is under the
+dominion of habit. Habit dulls sensation, habit strengthens the
+judgment. In the organic life, on the contrary, habit exercises no
+influence. The difference comes out clearly in the development of the
+individual. The organs of the organic life attain their full
+perfection independently of use; the organs of the animal life require
+an education, and without education they do not reach perfection
+(_Loc. cit._, p. 127).
+
+Bichat was the founder of what was known for a time as General
+Anatomy--the study of the constituent tissues of the body in health
+and disease. His classification of tissues was macroscopical and
+physiological; he relied upon texture and function in distinguishing
+them rather than upon microscopical structure. The tissues he
+distinguished are as follows:--[38]
+
+1. The cellular membrane.
+2. Nerves of animal life.
+3. Nerves of organic life.
+4. Arteries.
+5. Veins.
+6. Exhalants.
+7. Absorbents and glands.
+8. Bones.
+9. Medulla.
+10. Cartilage.
+11. Fibrous tissue.
+12. Fibro-cartilage.
+13. Muscles of organic life.
+14. Muscles of animal life.
+15. Mucous membrane.
+16. Serous membrane.
+17. Synovial membrane.
+18. The Glands.
+19. The Dermis.
+20. Epidermis.
+21. Cutis.
+
+The "cellular membrane" seems to mean undifferentiated connective
+tissue; "exhalants" are imperceptible tubes arising from the
+capillaries and secreting fat, serum, marrow, etc.; the "absorbents
+and glands" are the lymphatics and the lymphatic glands.
+
+In Bichat's eyes this resolution of the organism into tissues had a
+deeper significance than any separation into organs, for to each
+tissue must be attributed a _vie propre_, an individual and peculiar
+life. "When we study a function we must consider the complicated organ
+which performs it in a general way; but if we would be instructed in
+the properties and life of that organ we must absolutely resolve it
+into its constituent parts."[39] The tissues have, too, a great
+importance for pathology, for diseases are often diseases of tissues
+rather than of organs.[40]
+
+    [9] _Le Monde végétal_, p. 41, Paris, 1907.
+
+    [10] _Exercitationes de generatione animalium_,1651. For
+    an account of Harvey's work on generation and
+    development, see Em. Rádl's masterly _Geschichte der
+    biologischen Theorien_, i., pp. 31-8, Leipzig, 1905.
+
+    [11] The passage runs:--"Sic natura perfecta et divina
+    nihil faciens frustra, nec quipiam animali cor addidit,
+    ubi non erat opus, neque priusquam esset ejus usus,
+    fecit; sed iisdem gradibus in formatione cujuscumque
+    animalis, transiens per omnium animalium constitutiones
+    (ut ita dicam) ovum, vermem, foetum, perfectionem in
+    singulis acquirit."
+
+    [12] See I. Geoffroy St Hilaire, _Essais de Zoologie
+    générale_, p. 71, Paris, 1841.
+
+    [13] M. Foster, _Lectures on the History of Physiology_,
+    Cambridge, p. 53, 1901.
+
+    [14] _Zootomia democritea_, Nuremberg, 1645;
+    _Antiperipatias, seu de respiratione piscium_,
+    Amsterdam, 1661.
+
+    [15] Rádl, _loc. cit._, i., p. 50.
+
+    [16] Perrault et Duverney, _Mémoires pour servir à
+    l'histoire des Animaux_, Paris, 1699.
+
+    [17] F. Houssay, _Nature et Sciences naturelles_, Paris,
+    p. 76, n.d.
+
+    [18] Foster, _loc. cit._, p. 85.
+
+    [19] Trans, by Foster, _loc. cit._, p. 113.
+
+    [20] He made a careful study of the silkworm.
+
+    [21] "Etenim, ferventi aetatis calore, Anatomica
+    aggressus, licet circa peculiaria fuerim solicitus, in
+    _perfectioribus_ tamen haec rimari sum ausus. Verum, cum
+    haec propriis tenebris obscura jaceant, simplicium
+    analogismo egent; inde _insectorum_ indago illico
+    arrisit; quae cum et ipsa suas habeat difficultates ad
+    Plantarum perquisitionem animum _postremo_ adjeci, ut
+    diu hoc lustrato mundo gressu retroacto Vegetantis
+    Naturae gradu, ad prima studia iter mihi aperirem. Sed
+    nec forte hoc ipsum sufficiet cum simplicior _Mineralium
+    Elementorumque_ mundus praeire debeat. At in immensum
+    excrescit opus, et meis viribus omnino impar," _Opera
+    Omnia_, i., p. 1, London, 1686.
+
+    [22] See particularly E. Rádl, _loc. cit._. I Teil. J. V.
+    Carus, _Geschichte der Zoologie_, München, 1872.
+
+    [23] For a good historical account of the gradation
+    theories see Thienemann's paper in the _Zoologische
+    Annalen_(Würzburg) iii., pp. 185-274, 1910, from which
+    the quotation from Robinet is taken.
+
+    [24] _Histoire naturelle_, i., p. 13; ii, p. 9; iv., p.
+    101; and xiv., pp. 28-9, 1749 and later.
+
+    [25] No translation can render the beauty of the
+    original--"Comme tout se fait et que tout est par nuance
+    dans la Nature ..." (iv., p. 101).
+
+    [26] _Hist. nat._, iv., p. 5.
+
+    [27] See particularly his comparison of the skeleton of
+    the horse with that of man. _Hist. Nat._, iv., p. 381,
+    also p. 13.
+
+    [28] _Loc. cit._, p. 382.
+
+    [29] Tome xiv., pp. 311-374.
+
+    [30] Tome xiv., p. 358.
+
+    [31] See also "Oiseaux," Tome i., pp. 394, 395. Pallas in
+    1766 adopted for the whole animal kingdom this branching
+    arrangement.
+
+    [32] "But this cannot be, for it is certain by revelation
+    that all animals have equally participated in the grace
+    of creation."
+
+    [33] iv., p. 385.
+
+    [34] iv., pp. 3-110.
+
+    [35] It has been revived in our own days by Bergson,
+    _Matière et Mémoire_, p. 57.
+
+    [36] iv., pp. 7-15.
+
+    [37] _Anatomie Générale_, Paris, 1801, Eng. trans. 1824.
+
+    [38] _Anatomie Générale_, Eng. trans., i., p. lii.
+
+    [39] _Anatomie Générale_, Eng. trans., i., p. lviii.
+
+    [40] _Loc cit._, i., sect. vii.
+
+
+
+
+CHAPTER III
+
+CUVIER
+
+
+Cuvier was perhaps the greatest of comparative anatomists; his work
+is, in the best sense of the word, classical.
+
+Like all his predecessors, like Aristotle, like the Italian
+anatomists, Cuvier studied structure and function together, even gave
+function the primacy.
+
+Some functions, he says,[41] are common to all organised bodies--origin
+by generation, growth by nutrition, end by death. There are also
+secondary functions. Of these the most important, in animals at least,
+are the faculties of feeling and moving. These two faculties are
+necessarily bound up together; if Nature has given animals sensation
+she must also have given them the power of movement, the power to flee
+from what is harmful and draw near to what is good. These two
+faculties determine all the others. A creature that feels and moves
+requires a stomach to carry food in. Food requires instruments to
+divide it, liquids to digest it. Plants, which do not feel and do not
+move, have no need of a stomach, but have roots instead. Thus the
+"Animal Functions" of feeling and moving determine the character of
+the organs of the second order, the organs of digestion. These in
+their turn are prior to the organs of circulation, which are a means
+to the end of distributing the nutrient fluid or blood to all parts of
+the body. These organs of the third order are not only dependent on
+those of the second order, but are also not even necessary, for many
+animals are without them. Only animals with a circulatory system can
+have definite breathing organs--lungs or gills. Plants, and animals
+without a circulation, breathe by their whole surface.
+
+There is accordingly a rational order of functions, and therefore of
+the systems of organs which perform them. The most important are the
+Animal Functions, with their great organ-system, the neuro-muscular
+mechanism. Then come the digestive functions, and after them, and in a
+sense accessory to them, the functions and organs of circulation and
+respiration. The last three may be grouped as the Vital Functions.
+
+The Animal Functions not only determine the character of the Vital
+Functions, but influence also the primary faculty of generation, for
+animals' power of movement has rendered their mode of fecundation more
+simple, has therefore had an effect on their organs of generation.
+
+This division into "Animal" and "Vital" functions recalls Buffon's and
+Bichat's distinction of the "animal" and the "vegetative" lives.
+Cuvier apparently took this idea from Buffon, for he says that a plant
+is an animal that sleeps.[42] But the idea is as old as Aristotle, who
+discusses the "sleep" of embryos and of plants in the last book of the
+_De Generatione animalium_. The distinction between animal and
+vegetative life is, of course, based for Aristotle in the difference
+between the [Greek: psychê aisthêtikê] and the [Greek: psychê
+threptikê]. Cuvier, like Aristotle, Buffon, and Bichat, makes the
+heart the centre of the "vegetative" organs.
+
+It is important to note that Cuvier puts function before structure,
+and infers from function what the organ will be. "Plants," he writes,
+"having few faculties, have a very simple organisation."[43] It is only
+after having discussed and classified functions that Cuvier goes on to
+examine organs.
+
+First his views on the composition of the animal body. Aristotle
+distinguished three degrees of composition--the "elements," the
+homogeneous parts, and the heterogeneous parts or organs. Cuvier does
+the same. Some small advance has been made in the two thousand years'
+interval, due in the first place to the progress of chemistry, and in
+the second to the invention of the microscope. To the first
+circumstance Cuvier owes his knowledge that the inorganic substances
+forming the first degree of composition are principally C, N, H, O,
+and P, combined to form albumen, fibrine, and the like, which are in
+their turn combined to form the solids and fluids of the body. To the
+latter circumstance Cuvier owes the statement that the finest
+fragments into which mechanical division can resolve the organism are
+little flakes and filaments, which, joined up loosely together, form a
+"cellulosity." The discovery of the true cellular nature of animal
+tissues did not come till much later, till some years after Cuvier's
+death in 1832. Knowledge of histological detail was, however,
+considerable by the beginning of the 19th century. Cuvier knew, for
+example, that each muscle fibre has its own nerve fibre. But he gives
+no elaborate account of the homogeneous parts, no detailed histology.
+On the other hand his treatment of the heterogeneous parts or organs
+is detailed and masterly.[44]
+
+The main systems of organs are, in order of importance, the nervous
+and muscular, the digestive, the circulatory, and the respiratory.
+Each organ or system of organs may have many forms. If any form of any
+organ could exist in combination with any form of all the others there
+would be an enormous number of combinations theoretically possible.
+But these combinations do not all exist in Nature, for organs are not
+merely assembled (_rapproché's_), but act upon one another, and act
+all together for a common end. Accordingly only the combinations that
+fulfil these conditions exist in Nature. Cuvier thus dismisses the
+question of a science of possible organic forms and considers only the
+forms or combinations actually existing. This question of the
+possibility of a "theoretical" morphology of living things, after the
+fashion of the morphology of crystals with their sixteen possible
+types, was raised in later years by K. G. Carus, Bronn, and Haeckel.
+
+Organisms, then, are harmonious combinations of organs, and the
+harmony is primarily a harmony of functions. Every function depends
+upon every other, and all are necessary. The harmony of organs and
+their mutual dependence are the results of the interdependence of
+function. This thought, the recognition of the functional unity of the
+organism, is the fundamental one at the base of all Cuvier's work.
+Before him men had recognised more or less clearly the harmony of
+structure and function, and had based much of their work upon this
+unanalysed assumption. Cuvier was the first naturalist to raise this
+thought to the level of a principle peculiar to natural history. "It
+is on this mutual dependence of the functions and the assistance which
+they lend one to another that are founded the laws that determine the
+relations of their organs; these laws are as inevitable as the laws of
+metaphysics and mathematics, for it is evident that a proper harmony
+between organs that act one upon another is a necessary condition of
+the existence of the being to which they belong."[45]
+
+This rational principle, peculiar to natural history, Cuvier calls the
+principle of the conditions of existence, for the following
+reason:--"Since nothing can exist that does not fulfil the conditions
+which render its existence possible, the different parts of each being
+must be co-ordinated in such a way as to render possible the existence
+of the being as a whole, not only in itself, but also in its relations
+with other beings, and the analysis of these conditions often leads to
+general laws which are as certain as those which are derived from
+calculation or from experiment."[46]
+
+By "conditions of existence" he means something quite different from
+what is now commonly understood. The idea of the external conditions
+of existence, the environment, enters very little into his thought. He
+is intent on the adaptations of function and organ within the living
+creature--a point of view rather neglected nowadays, but essential for
+the understanding of living things. The very condition of existence of
+a living thing, and part of the essential definition of it, is that
+its parts work together for the good of the whole.
+
+The principle of the adaptedness of parts may be used as an
+explanatory principle, enabling the naturalist to trace out in detail
+the interdependence of functions and their organs. When you have
+discovered how one organ is adapted to another and to the whole, you
+have gone a certain way towards understanding it. That is using
+teleology as a regulative principle, in Kant's sense of the word.
+Cuvier was indeed a teleologist after the fashion of Kant, and there
+can be no doubt that he was influenced, at least in the exposition of
+his ideas, by Kant's _Kritik der Urtheilskraft_, which appeared ten
+years before the publication of the _Leçons d'Anatomie Comparée_.
+Teleology in Kant's sense is and will always be a necessary postulate
+of biology. It does not supply an explanation of organic forms and
+activities, but without it one cannot even begin to understand living
+things. Adaptedness is the most general fact of life, and innumerable
+lesser facts can be grouped as particular cases of it, can be, so far,
+understood.
+
+Cuvier's famous principle of correlation, the corner-stone of his
+work, is simply the practical application to the facts of structure of
+the principle of functional adaptedness. By the principle of
+correlation, from one part of an animal, given sufficient knowledge of
+the structure of its like, you can in a general way construct the
+whole. "This must necessarily be so: for all the organs of an animal
+form a single system, the parts of which hang together, and act and
+re-act upon one another; and no modifications can appear in one part
+without bringing about corresponding modifications in all the
+rest."[47] The logical basis of the principle is sound. The functions
+of the parts are all intimately bound up with one another, and one
+function cannot vary without bringing in its train corresponding
+modifications in the others. Structure and function are bound up
+together; every modification of a function entails therefore the
+modification of an organ. Hence from the shape of one organ you can
+infer the shape of the other organs--if you have sufficiently
+extensive empirical knowledge of functions, and of the relation of
+structure to function in each kind of organ. Given an alimentary canal
+capable of digesting only flesh, and possessing therefore a certain
+form, you know that the other functions must be adapted to this
+particular function of the alimentary canal. The animal must have keen
+sight, fine smell, speed, agility, and strength in paws and jaws.
+These particular functions must have correspondingly modified organs,
+well-developed eyes and ears, claws and teeth. Further, you know from
+experience that such and such definitely modified organs are
+invariably found with the carnivorous habit, carnassial teeth, for
+example, and reduced clavicles. From a "carnivorous" alimentary canal,
+then, you can infer with certainty that the animal possessed
+carnassial teeth and the other structural peculiarities of carnivorous
+animals, _e.g._, the peculiar coronoid process of the mandible. From
+the carnassial tooth you can infer the reduced clavicle, and so on.
+"In a word, the form of the tooth implies the form of the condyle;
+that of the shoulder blade that of the claws, just as the equation of
+a curve implies all its properties."[48]
+
+Similarly the great respiratory power of birds is correlated with
+their great muscular strength, and renders necessary great digestive
+powers. Hence the correlated structure of lungs, muscles and their
+attachments, and alimentary canal, in birds.
+
+Not only do systems of organs, by being adjusted to special
+modifications of function, influence one another, but so also do parts
+of the same organ. This is noticeably the case with the skeleton,
+where hardly a facet can vary without the others varying
+proportionately, so that from one bone you can up to a certain point
+deduce all the rest.
+
+We deduce the necessity, the constancy, of these co-existences of
+organs from the observed reciprocal influence of their functions. That
+being established, we can argue from observed constancy of relation
+between two organs an action of one upon the other, and so be led to a
+discovery of their functions. But even if we do not discover the
+functional interdependencies of the parts, we can use the established
+fact of the constant co-existence of two parts as proof of a
+functional correlation between them.
+
+Correlation is either a rational or an empirical principle, according
+as we know or do not know the interdependence of function of which it
+is the expression. Even when we apply the rational principle of
+correlation it would be useless in our hands if we had not extensive
+empirical knowledge; when we use an empirical rule of correlation we
+depend entirely upon observation. "There are a great many cases,"
+writes Cuvier,[49] "where our theoretical knowledge of the relations of
+forms would not suffice, if it were not filled out by observation,"
+that is to say, there are many cases of correlation not yet explicable
+in terms of function. From a hoof you can deduce the main characters
+of herbivores (with a certain amount of assistance from your empirical
+knowledge of herbivores), but could you from a cloven hoof deduce that
+the animal is a ruminant, unless you had observed the constancy of
+relation, not directly explicable in terms of function, between cloven
+hoofs and chewing the cud? Or could you deduce from the existence of
+frontal horns that the animal ruminates? "Nevertheless, since these
+relations are constant, they must necessarily have a sufficient cause;
+but as we are ignorant of this cause, observation must supplement
+theory; observation establishes empirical laws which become almost as
+certain as the rational laws, when they are based upon a sufficient
+number of observations.... But that there exist all the same hidden
+reasons for all these relations is partly revealed by observation
+itself, independently of general philosophy."[50] That is to say, even
+correlations for which no explanation in terms of function can be
+supplied are probably in reality functional correlations. This may, in
+some cases, be inferred from the graded correspondence of two sets of
+organs. For example, ungulates which do not ruminate, and have not a
+cloven hoof, have a more perfect dentition and more bones in the foot
+than the true cloven-hoofed ruminants. There is a correlation between
+the state of development of the teeth and of the foot. This
+correlation is a graded one, for camels, which have a more perfect
+dentition than other ruminants, have also a bone more in their tarsus.
+It seems probable, therefore, that there is some reason, that is, some
+explanation in terms of function, for this case of correlation.
+
+Nevertheless, the fact remains that many correlations are not
+explicable in terms of function, and the substitution of correlation
+as an empirical principle for correlation as a rational principle
+marks for Cuvier a step away from his functional comparative anatomy
+towards a pure morphology. It is significant that in later times the
+term correlation has come to be applied more especially to the purely
+empirical constancies of relation, and has lost most of its functional
+significance. But the correlation of the parts of an organism is no
+mere mathematical concept, to be expressed by a coefficient, but
+something deeper and more vital.
+
+Cuvier interpreted the functional dependence of the parts in terms of
+what we now call the general metabolism. He had a clear vision of the
+constant movement of molecules in the living tissue, combining and
+recombining, of the organism taking in and intercalating molecules
+from outside from the food and rejecting molecules in the excretions,
+a ceaseless _tourbillon vital_. "This general movement, universal in
+every part, is so unmistakably the very essence of life that parts
+separated from a living body straightway die."[51] The organisation of
+the body, the arrangement of its solids and liquids, is adapted to
+further the _tourbillon vital_. "Each part contributes to this general
+movement its own particular action and is affected by it in particular
+ways, with the result that, in every being, life is a unity which
+results from the mutual action and reaction of all its parts."[52]
+
+Cuvier, however, did not resolve life into metabolism, nor reduce
+vital happenings to the chemical level. The form of organised bodies
+is more essential than the matter of which they are composed, for the
+matter changes ceaselessly while the form remains unchanged. It is in
+form that we must seek the differences between species, and not in the
+combinations of matter, which are almost the same in all.[53] The
+differences are to be sought at the level of the second and third
+degrees of composition.
+
+The existence of differences of form introduces a new problem, the
+problem of diversity. There are only a few possible combinations of
+the principal organs, but as you get down to less important parts the
+possible scope of variation is greatly increased, and most of the
+possible variations do exist. Nature seems prodigal of form, of form
+which needs not to be useful in order to exist. "It needs only to be
+possible, _i.e._, of such a character that it does not, destroy the
+harmony of the whole."[54] We seize here the relation of the principle
+of the adaptedness of parts to the problem of the variety of form. The
+former is in a sense a regulative and conservative principle which
+lays down limits beyond which variation may not stray. In itself it is
+not a fountain of change; there must be another cause of change. This
+thought is of great importance for theories of descent.
+
+Cuvier has no theory to account for the variety of form: he contents
+himself with a classification. There are two main ways of classifying
+forms; you may classify according to single organs or according to the
+totality of organs. By the first method you can have as many
+classifications as you have organs, and the classifications will not
+necessarily coincide. Thus you can divide animals according to their
+organs of digestion into two classes, those in which the alimentary
+canal is a sac with one opening (zoophytes) and those in which the
+canal has two openings,[55] a curious forestalment, in the rough, of
+the modern division of Metazoa into Coelentera and Coelomata.
+
+It is only by taking single organs that you can arrange animals into
+long series, and you will have as many series as you take organs. Only
+in this way can you form any _Échelle des êtres_ or graded series; and
+you can get even this kind of gradation only within each of the big
+groups formed on a common plan of structure; you can never grade, for
+example, from Invertebrates to Vertebrates through intermediate
+forms[56] (which is perfectly true, in spite of Amphioxus and
+Balanoglossus!).
+
+In the _Règne Animal_ Cuvier restricts the application of the idea of
+the _Échelle_ within even narrower limits, refusing to admit its
+validity within the bounds of the vertebrate phylum, or even within
+the vertebrate classes. This seems, however, to refer to a seriation
+of whole organisms and not of organs, so that the possibility of a
+seriation of organs within a class is not denied. Cuvier was, above
+all, a positive spirit, and he looked askance at all speculation which
+went beyond the facts. "The pretended scale of beings," he wrote, "is
+only an erroneous application to the totality of creation of partial
+observations, which have validity only when confined to the sphere
+within which they were made."[57] This remark, which is after all only
+just, perfectly expresses Cuvier's attitude to the transcendental
+theories, and was probably a protest against the sweeping
+generalisations of his colleague, Etienne Geoffroy St Hilaire.
+
+A true classification should be based upon the comparison of all
+organs, but all organs are not of equal value for classification, nor
+are all the variations of each organ equally important. In estimating
+the value of variations more stress should be laid on function than on
+form, for only those variations are important which affect the mode of
+functioning. These are the principles on which Cuvier bases the
+classification of animals given in the _Leçons_, Article V., "Division
+des animaux d'après l'ensemble de leur organisation." The scheme of
+classification actually given in the _Leçons_ recalls curiously that
+of Aristotle, for there is the same broad division into Vertebrates,
+with red blood, and Invertebrates, almost all with white blood. Nine
+classes altogether are distinguished--Mammals, Birds, Reptiles,
+Fishes, Molluscs, Crustacea, Insects, Worms, Zoophytes (including
+Echinoderms and Coelenterates).
+
+A maturer theory and practice of classification is given in the _Règne
+Animal_ of seventeen years later. Here the principle of the
+subordination of characters (which seems to have been first explicitly
+stated by the younger de Jussieu in his _Genera Plantarum_, 1789,[58])
+is more clearly recognised. The properties or peculiarities of
+structure which have the greatest number of relations of
+incompatibility and coexistence, and therefore influence the whole in
+the greatest degree, are the important or dominating characters, to
+which the others must be subordinated in classification. These
+dominant characters are also the most constant.[59] In deciding which
+characters are the most important Cuvier makes use of his fundamental
+classification of functions and organs into two main sets. "The heart
+and the organs of circulation are a kind of centre for the vegetative
+functions, as the brain and the spinal cord are for the animal
+functions."[60] These two organ-systems vary in harmony, and their
+characters must form the basis for the delimitation of the great
+groups. Judged by this standard there are four principal types of
+form,[61] of which all the others are but modifications. These four
+types are Vertebrates, Molluscs, Articulates, and Radiates. The first
+three have bilateral, the last has radial symmetry. Vertebrates and
+Molluscs have blood-vessels, but Articulates show a functional
+transition from the blood-vessel to the tracheal system. Radiates
+approach the homogeneity of plants; they appear to lack a distinct
+nervous system and sense organs, and the lowest of them show only a
+homogeneous pulp which is mobile and sensitive. All four classes are
+principally distinguished from one another by the broad structural
+relations of their neuromuscular system, of the organs of the animal
+functions. Vertebrates have a spinal cord and brain, an internal
+skeleton built on a definite plan, with an axis and appendages; in
+Molluscs the muscles are attached to the skin and the shell, and the
+nervous system consists of separate masses; Articulates have a hard
+external skeleton and jointed limbs, and their nervous system consists
+of two long ventral cords; Radiates have ill-defined nervous and
+muscular systems, and in their lowest forms possess the animal
+functions without the animal organs.
+
+This well-rounded classification of animal forms is in a sense the
+crown of Cuvier's work, for the principle of the subordination of
+characters, in the interpretation which he gives to it, is a direct
+application of his principle of functional correlation. Each of the
+great groups is built upon one plan. The idea of the unity of plan has
+become for Cuvier a commonplace of his thought, and it is tacitly
+recognised in all his anatomical work. But he never takes it as a
+hard-and-fast principle which must at all costs be imposed upon the
+facts.
+
+Cuvier has become known as the greatest champion of the fixity of
+species, but it is not often recognised that his attitude to this
+problem is at least as scientific as that of the evolutionists of his
+own and later times. No doubt he became dogmatic in his rejection of
+evolution-theory, but he was on sure ground in maintaining that the
+evolutionists of his day went beyond their facts. He considered that
+certain forms (species) have reproduced themselves from the origin of
+things without exceeding the limits of variation. His definition of a
+species was, "the individuals descended from one another or from
+common parents, together with those that resemble them as much as they
+resemble one another."[62] "These forms are neither produced nor do
+they change of themselves; life presupposes their existence, for it
+cannot arise save in organisations ready prepared for it."[63]
+
+He based his rejection of all theories of descent upon the absence of
+definite evidence for evolution. If species have gradually changed, he
+argued, one ought to find traces of these gradual modifications.[64]
+Palæontology does not furnish such traces. Again, the limits of
+variation, even under domestication, are narrow, and the most extreme
+variation does not fundamentally alter the specific type. Thus the dog
+has varied perhaps most of all, in size, in shape, in colour. "But
+throughout all these variations the relations of the bones remain the
+same, and the form of the teeth never changes to an appreciable
+extent; at most there are some individuals in which an additional
+false molar develops on one side or the other."[65] This second
+objection is the objection of the morphologist. It would be an
+interesting study to compare Cuvier's views on variation with those of
+Darwin, who was essentially a systematist.
+
+Cuvier's first objection was of course determined to some extent by
+the imperfection of the palæontological knowledge of his time. But
+even at the present day the objection has a certain force, for
+although we have definite evidence of many serial transformations of
+one species into another along a single line, for example, Neumayr's
+_Paludina_ series, yet at any one geological level the species, the
+lines of descent, are all distinct from one another.[66]
+
+Cuvier recognised very clearly that there is a succession of forms in
+time, and that on the whole the most primitive forms are the earliest
+to appear. Mammals are later than reptiles, and fishes appear earlier
+than either. As Depéret puts it, "Cuvier not only demonstrated the
+presence in the sedimentary strata of a series of terrestrial faunas
+superimposed and distinct, but he was the first to express, and that
+very clearly, the idea of the gradual increase in complexity of these
+faunas from the oldest to the most recent" (p. 10).
+
+He did not believe that the fauna of one epoch was transformed into
+the fauna of the next. He explained the disappearance of the one by
+the hypothesis of sudden catastrophes, and the appearance of the next
+by the hypothesis of immigration. He nowhere advanced the hypothesis
+of successive new creations. "For the rest, when I maintain that the
+stony layers contain the bones of several genera and the earthy layers
+those of several species which no longer exist, I do not mean that a
+new creation has been necessary to produce the existing species, I
+merely say that they did not exist in the same localities and must
+have come thither from elsewhere."[67] It was left to d'Orbigny to
+teach the doctrine of successive creations, of which he distinguished
+twenty-seven (_Cours élémentaire de palaeontologie stratigraphique_,
+1849).
+
+Cuvier, however, can hardly have believed that all species were
+present at the beginning, since he does admit a progression of forms.
+Probably he had no theory on the subject, for theories without facts
+had little interest for him. At any rate it is a mistake to think that
+Cuvier was a supporter of the theological doctrine of special
+creation. His philosophy of Nature was mechanistic, and he dedicated
+his _Recherches sur les Ossemens Fossiles_ to his friend Laplace. He
+admitted the idea of evolution at least so far as to conceive of a
+development of man from a savage to a civilised state.[68] He refused
+to accept the extravagant evolutionary theory of Demaillet and the
+somewhat confused theory of Lamarck (whom he joins with Demaillet),[69]
+just as he rejected the transcendental theories of Geoffroy St
+Hilaire, because they seemed to him not based upon facts.
+
+    [41] _Leçons d'Anatomie Comparée_, tome i., pp. 10 _et
+    scq._, 1800.
+
+    [42] _Leçons d'Anatomie Comparée_, i., p. 18.
+
+    [43] _Loc. cit._, i., p. 13.
+
+    [44] _Leçons d'Anatomie Comparée_, tome i., Articles
+    iii.-iv., 1800.
+
+    [45] _Leçons d'Anatomie Comparée_, i., p. 47.
+
+    [46] _Le Règne Animal_, i., p. 6, 1817.
+
+    [47] _Histoire des Progrès des Sciences naturelles depuis
+    1789_, i., p. 310, 1826.
+
+    [48] _Recherches sur les Ossemens Fossiles_, i., p. 60,
+    1812.
+
+    [49] _Ossemens fossiles_, i., p. 60.
+
+    [50] _Loc. cit._, i., p. 63.
+
+    [51] _Leçons d'Anatomie Comparée_, i., p. 6.
+
+    [52] _Le Règne Animal_, i., p. 16.
+
+    [53] _Hist. Prog. Sci. Nat._, i., p. 187, 1826.
+
+    [54] _Leçons_, i., p. 58.
+
+    [55] _Loc. cit._, i., Article iii.
+
+    [56] _Loc. cit._, i., p. 60.
+
+    [57] _Règne Animal_, i., p. xx.
+
+    [58] Cuvier, _Hist. Prog. Sci. Nat._, i., p. 288, 1826.
+
+    [59] _Règne Animal_, i., p. 10.
+
+    [60] _Règne Animal_, p. 55.
+
+    [61] First propounded by Cuvier in 1812, _Ann. Mus.
+    d'Hist. Nat._, xix.
+
+    [62] _Règne Animal_, i., p. 19.
+
+    [63] _Loc. cit._, p. 20.
+
+    [64] _Recherches sur les Ossemens Fossiles_, i., p. 74,
+    1812.
+
+    [65] _Loc. cit._, p. 79.
+
+    [66] See C. Depéret, _Les transformations du Monde
+    animal_, Paris, 1907, and G. Steinmann, _Die
+    geologischen Grundlagen der Abstammungslehre_, Leipzig,
+    1908.
+
+    [67] _Recherches_, i., p. 81.
+
+    [68] _Règne Animal_, i., p. 91.
+
+    [69] _Ossemens Fossiles_, i., p. 26.
+
+
+
+
+CHAPTER IV
+
+GOETHE
+
+
+Science, in so far as it rises above the mere accumulation of facts,
+is a product of the mind's creative activity. Scientific theories are
+not so much formulæ extracted from experience as intuitions imposed
+upon experience. So it was that Goethe, who was little more than a
+dilettante,[70] seized upon the essential principles of a morphology
+some years before that morphology was accepted by the workers.
+
+Goethe is important in the history of morphological method because he
+was the first to bring to clear consciousness and to express in
+definite terms the idea on which comparative anatomy before him was
+based, the idea of the unity of plan. We have seen that this idea was
+familiar to Aristotle and that it was recognised implicitly by all who
+after him studied structure comparatively. In Goethe's time the idea
+had become ripe for expression. It was used as a guiding principle in
+Goethe's youth particularly by Vicq d'Azyr and by Camper. The former
+(1748-1794), who discovered[71] in the same year as Goethe (1784) the
+intermaxillary bone in man, pointed out the homology in structure
+between the fore limb and the hind limb, and interpreted certain
+rudimentary bones, the intermaxillaries and rudimentary clavicles, in
+the light of the theory that Vertebrates are built upon one single
+plan of structure.
+
+"Nature seems to operate always according to an original and general
+plan, from which she departs with regret and whose traces we come
+across everywhere" (Vicq d'Azyr, quoted by Flourens, _Mém. Acad.
+Sei._, XXIII., p. xxxvi.).
+
+Peter Camper (1722-1789), we are told by Goethe himself in his
+_Ostéologie_, was convinced of the unity of plan holding throughout
+Vertebrates; he compared in particular the brain of fishes with the
+brain of man.
+
+The idea of the unity of plan had not yet become limited and defined
+as a strictly scientific theory; it was an idea common to philosophy,
+to ordinary thought, and to anatomical science. We find it expressed
+by Herder (who perhaps got it from Kant) in his _Ideen sur Philosophie
+der Geschichte der Menschheit_ (1784), and it is possible that Goethe
+became impressed with the importance of the idea through his
+conversations with Herder. Be that as it may, it is certain that
+Goethe sought for the intermaxillaries in man only because he was
+firmly convinced that the skeleton in all the higher animals was built
+upon one common plan and that accordingly bones such as the
+intermaxillaries, found well developed in some animals, must also be
+found in man. The idea was not drawn from the facts, but the facts
+were interpreted and even sought for in the light of the idea. "I
+eagerly worked upon a general osteological scheme, and had accordingly
+to assume that all the separate parts of the structure, in detail as
+in the whole, must be discoverable in all animals, because on this
+supposition is built the already long begun science of comparative
+anatomy."[72]
+
+The principle comes to clear expression in his _Erster Entwurf einer
+allgemeinen Einleitung in die vergleichende Anatomie_ (1795).[73] He
+writes:--"On this account an attempt is here made to arrive at an
+anatomical type, a general picture in which the forms of all animals
+are contained in potentia, and by means of which we can describe each
+animal in an invariable order."[74] His aim is to discover a general
+scheme of the constant in organic parts, a scheme into which all
+animals will fit equally well, and no animal better than the rest.
+When we remember that the type to which anatomists before him had,
+consciously or unconsciously, referred all other structure was man
+himself, we see that in seeking after an abstract generalised type
+Goethe was reaching out to a new conception. The fact that only the
+structure of man and the higher animals was at all well-known in his
+time led Goethe to think that his general Typus would hold for the
+lower animals as well, though it was to be arrived at primarily from a
+study of the higher animals. All he could assert of the entire animal
+kingdom was that all animals agreed in having a head, a middle part,
+and an end part, with their characteristic organs, and that
+accordingly they might, in this respect at least, be reduced to one
+common Typus. Goethe's knowledge of the lower animals was not
+extensive.
+
+Though Goethe did not work out a criterion of the homology of parts
+with any great clearness, he had an inkling of the principle later
+developed by E. Geoffroy St Hilaire, and called by him the "Principle
+of Connections." According to this principle, the homology of a part
+is determined by its position relative to other parts. Goethe
+expresses it thus:--"On the other hand the most constant factor is the
+position in which the bone is invariably found, and the function to
+which it is adapted in the organic edifice."[75] But from this sentence
+it is not clear that Goethe understood the principle as one of form
+independent of function, for he seems to consider that the homology of
+an organ is partly determined by the function which it performs for
+the whole. He wavers between the purely formal or morphological
+interpretation of the principle of connections and the functional. We
+find him in the additions to the _Entwurf_ (1796), saying:--"We must
+take into consideration not merely the spatial relations of the parts,
+but also their living reciprocal influence, their dependence upon and
+action on one another." [76] But in seeking for the intermaxillary bone
+in man he was guided by its position relative to the maxillaries--it
+must be the bone between the anterior ends of the maxillaries, a bone
+whose limits are indicated in the adult only by surface grooves.
+
+As a matter of fact Goethe's morphological views are neither very
+clearly expressed nor very consistent. This comes out in his treatment
+of the relation between structure and function. Sometimes he takes the
+view that structure determines function. "The parts of the animal," he
+writes, "their reciprocal forms, their relations, their particular
+properties determine the life and habits of the creature."[77] We are
+not to explain, he says, the tusks of the _Babirussa_ by their
+possible use, but we must ask how it comes to have tusks. In the same
+way we must not suppose that a bull has horns in order to gore, but we
+must investigate the process by which it comes to have horns to gore
+with. This is the rigorous morphological view. On the other hand he
+admits elsewhere that function may influence form. Apparently he did
+not work out his ideas on this point to logical clearness, and Rádl[78]
+is probably correct in saying that the following quotation with its
+double assertion represents most nearly Goethe's position:--
+
+"Also bestimmt die Gestalt die Lebensweise des Thieres, Und die Weise
+zu leben, sie wirkt auf alle Gestalten Mächtig zurück."[79]
+
+His best piece of purely morphological work was his theory of the
+metamorphosis of plants. Stripped of its vaguer elements, and of the
+crude attempt to explain differences in the character of plant organs
+by differences in the degree of "refinement" of the sap supplied to
+them, the theory is that stem-leaves, sepals, petals, and stamens are
+all identical members or appendages. These appendages differ from one
+another only in shape and in degree of expansion, stem-leaves being
+expanded, sepals contracted, petals expanded, and so on alternately.
+It is equally correct to call a stamen a contracted petal, and a petal
+an expanded stamen, for no one of the organs is the type of the
+others, but all equally are varieties of a single abstract
+plant-appendage.
+
+What Goethe considered he had proved for the appendages of plants he
+extended to all living things. Every living thing is a complex of
+living independent beings, which "der Idee, der Anlage nach," are the
+same, but in appearance may be the same or similar, different or
+unlike.[80] Not only is there a primordial animal and a primordial
+plant, schematic forms to which all separate species are referable,
+but the parts of each are themselves units, which "der Idee nach," are
+identical _inter se_. This fantasy can hardly be taken seriously as a
+scientific theory; it seems, however, to have been what guided Goethe
+in his "discovery" of the vertebral nature of the skull. Just as the
+fore limb can be homologised with the hind limb, so, reasoning by
+analogy, the skull should be capable of being homologised with the
+vertebræ. To what ludicrous extremes this doctrine of the repetition
+of parts within the organism was pushed we shall see when we consider
+the theories of the German transcendentalists of the early nineteenth
+century.
+
+Though Goethe's morphological views were lacking in definiteness he
+hit upon one or two ideas which proved useful. Thus he enunciated the
+"law of balance" long before Etienne Geoffroy St Hilaire, the law
+"that to no part can anything be added, without something being taken
+away from another part, and _vice versa_."[81] He saw, too, what a help
+to the interpretation of adult structure the study of the embryo would
+be, for many bones which are fused in the adult are separate in the
+embryo.[82] This also was a point to which the later transcendentalists
+gave considerable attention.
+
+So far we have spoken of Goethe as if he were merely the prophet of
+formal morphology; we have pointed out how he brought to clear
+expression the morphological principle implicit in the idea of unity
+of type, and how he seized upon some important guiding ideas, such as
+the principle of connections. But Goethe was not a formalist, and he
+was very far from the static conception of life which is at the base
+of pure morphology. His interest was not in _Gestalt_ or fixed form,
+_Bildung_ or form change. He saw that _Gestalt_ was but a momentary
+phase of _Bildung_, and could be considered apart and in itself only
+by an abstraction fatal to all understanding of the living thing.
+Mephistopheles scoffs at the scholars who would explain a living
+creature by anatomising it:
+
+ "Dann hat er die Theile in seiner Hand,
+  Fehlt leider! nur das geistige Band."[83]
+
+Goethe kept clear of this mistake; he knew that the artist comes
+nearer to the truth than the analyst.
+
+In the fragment entitled _Bildung und Umbildung organischer Naturen_
+(1807), introductory to a reprint of his paper on the "Metamorphosis
+of Plants," we get an exposition of his general views on living
+things. He points out there how we try to understand things by
+separating them into their parts. We can, it is true, resolve the
+organism into its structural elements, but we cannot recompose it or
+endow it with life by joining up the parts. Hence we require some
+other means of understanding it. "In all ages even among scientific
+men there can be discerned a yearning to apprehend the living form as
+such, to grasp the connection of their external visible parts, to
+interpret them as indications of the inner activity, and so, in a
+certain measure, to master the whole conceptually." This science which
+should discover the inner meaning of organic _Bildung_ is called
+Morphology.[84] In Morphology we should not speak of _Gestalt_ or fixed
+form, or if we do we should understand by it only a momentary phase of
+_Bildung_. Form is of interest not in itself but only as the
+manifestation of the inner activity of the living being. Over
+development, he says elsewhere, there presides a formative force, a
+_bildende Kraft_ or _Bildungstrieb_, which works out the idea of the
+organism. Living things, in his view of them, strive to manifest an
+idea. They are Nature's works of art--and so, incidentally, they
+require an artist to interpret them.
+
+This profound conception of the nature of life is applied not only to
+the growing changing individual but also to the whole changing world
+of organisms. They are all manifestations of a living shaping power
+which moulds them. This shaping power, immanent in all life, is
+conceived to work according to a general plan, and so we get an
+explanation of the fact that living things seem simply varieties of
+one common type.
+
+"If we once recognise," says Goethe, "that the creative spirit brings
+into being and shapes the evolution of the more perfect organic
+creatures according to a general scheme, is it altogether impossible
+to represent this original plan if not to the senses at least to the
+mind...?"[85]
+
+Such an interpretation of the unity of plan reaches perhaps beyond the
+bounds of science.
+
+    [70] _See_ Kohlbrugge, "Hist. krit. Studien über Goethe
+    als Naturforscher," _Zool. Annalen._ v., 1913, pp.
+    83-231.
+
+    [71] Or re-discovered, according to Kohlbrugge.
+
+    [72] Cotta ed., vol. ix., p. 448.
+
+    [73] "First Draft of a General Introduction to
+    Comparative Anatomy."
+
+    [74] Cotta ed., ix., p. 463.
+
+    [75] Cotta ed., p. 478.
+
+    [76] _Loc. cit._, p. 491.
+
+    [77] _Entwurf_, Cotta ed., ix., p. 465.
+
+    [78] _Geschichte der biologischen Theorien_, i., p. 266.
+
+    [79] "So the form determines the manner of life of the
+    animal, and the manner of life in its turn reacts
+    powerfully upon all forms."
+
+    [80] _Bildung und Umbildung organischer Naturen_, 1807.
+
+    [81] Cotta ed., ix., p. 466.
+
+    [82] _Loc. cit._, pp. 474-5.
+
+    [83] Then he has all the parts within his hand, excepting
+    only, sad to say, the living bond.
+
+    [84] Goethe was the inventor of the word.
+
+    [85] Cotta ed., ix., p. 490.
+
+
+
+
+CHAPTER V
+
+ETIENNE GEOFFROY SAINT-HILAIRE
+
+
+E. Geoffrey made an experiment, unsuccessful but instructive. He tried
+to found a science of pure morphology; he failed: his failure showed,
+once and for all, that a pure morphology of organic forms is
+impracticable.
+
+Already, in 1796, in one of his earliest memoirs,[86] Geoffroy was
+guided by the idea that Nature has formed all living things upon one
+plan. Organs which seem anomalous are merely modifications of the
+normal; the trunk of an elephant is formed by the excessively
+prolonged nostrils, the horn of a rhinoceros is simply a mass of
+adhering hairs. In general, however varied their form, all organs are
+simply variations of a common scheme; Nature employs no new organs.
+Organs which are rudimentary, such as the clavicles in the ostrich and
+the nictitating membrane in man, bear witness to the unity of plan. In
+this Geoffroy goes no further than his predecessors. They too had
+recognised homologies of organs; they too had interpreted rudimentary
+organs as vestiges of an original plan.
+
+In a series of papers published in 1807, Geoffroy took a further step,
+and sought to establish homologies which were not obvious--homologies,
+too, not so much of organs as of parts.
+
+These memoirs (published in the _Annales du Muséum d'Histoire
+naturelle_, vols. ix. and x., 1807) dealt with the homology between
+the bones of the pectoral fin and girdle in fish and the bones of the
+arm and shoulder-girdle in higher Vertebrates, with the homologies of
+the bones of the sternum, and with the determination of the pieces of
+the skull, particularly in the crocodile. All Geoffroy's morphological
+doctrine is found in them, but for the full expression of his views we
+must take his chief work, the _Philosophie anatomique_, particularly
+the first volume (1818). This volume contains, beside the important
+"Discours préliminaire" and "Introduction" which we shall presently
+consider in detail, five memoirs, which deal with the various bones
+connected with the respiratory organs in fishes (the bones of the
+operculum, of the hyoid, of the branchial arches, of the pectoral
+girdle), and seek to discover their homologies with corresponding
+bones in air-breathing Vertebrates.
+
+"Can the organisation of vertebrated animals be referred to one
+uniform type?" This is the question with which the _Philosophie
+anatomique_ opens, the question to which the whole book is an answer.
+But is it not generally acknowledged by naturalists that Vertebrates
+are built upon one uniform plan, that, for instance, the fore limb may
+be modified for running, climbing, swimming, or flying, yet the
+arrangement of the bones remain the same? How else could there be a
+"natural method" of classification?[87]
+
+But the homologies so drawn repose upon a vague and confused feeling for
+likenesses; they are not based upon an explicit principle. What general
+principle can be applied? "Now it is evident that the sole general
+principle one can apply is given by the position, the relations, and the
+dependencies of the parts, that is to say, by what I name and include
+under the term of _connections_." For instance, the part known as the
+hand in man and generally as the fore foot in other Vertebrates, is the
+fourth part in order in the anterior member, and its homologue can
+always be recognised by this fact of its connections (p. xxvi.). The
+principle of connections serves as a guide in tracing an organ through
+all its functional transformations, for "an organ can be deteriorated,
+atrophied, annihilated, but not transposed" (p. xxx.).
+
+It is this principle which enables one to follow out in detail the
+further fundamental conception that in every Vertebrate there are found
+the same "organic materials," or units of construction. This conception,
+which Geoffroy calls the _Théorie des analogues_ (p. xxxii.), is clearly
+one part of the old idea of the unity of type; it teaches the _unity of
+composition_ of organic beings, while the _Principe des connexions_ adds
+the _unity of plan_.
+
+Both conceptions are logically implicit in the vague notion of unity of
+type; Geoffroy disengaged them, and pushed each to its logical extreme.
+
+Most of the ordinary homologies of structure in air-breathing
+Vertebrates have already been seized, he continues, for they are more or
+less obvious, and many intermediate states exist (p. xxxiv.). But
+ordinary methods of comparison fail when the attempt is made to
+homologise the structure of fishes with that of air-breathing
+Vertebrates, for the homologies are anything but obvious and no
+intermediate organs are found.
+
+Most air-breathing Vertebrates have a larynx, a trachea, and bronchi,
+which are absent in fish; and fish have many parts which seem to be
+absent in higher Vertebrates. But apply the "Theory of Analogues"; it
+teaches that there can be no organ peculiar to fish and not found in
+other Vertebrates; apply the "Principle of Connections," it will show
+which organs are homologous in the two types (p. xxxv.).
+
+Comparative anatomists, with few exceptions, had hitherto taken man as
+the type, and referred all structure to his; Geoffroy's principles led
+him to give preference to no one animal in particular, but to seize upon
+each part in the species in which it reaches the maximum of its
+development (p. xxxvi.). He is thus led to refer all structures to a
+generalised abstract type. In this abstract type each organ exists at
+the maximum of its development, each organ shows all its potentialities
+realised. In a way, therefore, this type, this abstraction, gives the
+scheme of the possible transformations of each organ.
+
+It is true Geoffroy does not refer to this "Archetype" in so many words,
+but it must always have been vaguely present in his mind. He has this
+idea in his head when he says in one of his later works, "There is,
+philosophically speaking, only a single animal."[88] The "single animal"
+is simply the generalised type.
+
+Having laid down his two principles Geoffroy goes on to apply them to
+the difficult case of the comparison of the skeleton of fish with the
+skeleton of the higher Vertebrates. "My present task is to demonstrate
+that there is no part of the bony framework of fishes that cannot find
+its analogue in the other vertebrated animals."[89] It seems at first
+sight that many bones are peculiar to fish, formed expressly for
+performing the functions which fish do not share with higher animals.
+These are the bones connected with respiration--the operculum, the
+branchiostegal rays, the branchial arches, and others. That the peculiar
+bones should be connected with the respiratory functions is only
+natural, for the contrast between fish and higher Vertebrates is
+essentially a contrast between water-breathing and air-breathing
+animals. Considering first the general form of the skeleton in fish, we
+are met at once with a difficulty; there is no obvious homologue in
+fishes of the neck, the trunk, and the abdomen of higher animals. What
+apparently corresponds to the trunk is in fishes crowded close up under
+the head. But, after all, it is not of the essence of the vertebrate
+type to have the trunk and the abdomen attached at definite and
+invariable distances along the vertebral column--that is a notion
+surviving from the anatomy which made man its type. The "trunk" differs
+in position according to the class, in quadrupeds, birds, and fishes (p.
+9). Now, says Geoffroy, allow me this one hypothesis, that the trunk
+with its organs can, as it were, move bodily along the vertebral column,
+so as to be found in one class near the front end of the vertebral
+column, in another about the middle, and in a third near the end, then I
+can show you in detail that the constituent parts of this trunk are
+found in all classes to be invariably in the same positions relatively
+to one another (p. 10). It is important to note this hypothesis of a
+"metastasis" which Geoffroy makes, for it is the key to the
+understanding of many of the far-fetched homologies which he tries to
+establish. It is, of course, clear that this hypothesis is in formal
+contradiction with his principal hypothesis of the invariability of
+connections, and that he, so to speak, gets a hold on his fish to apply
+his principle of connections only by admitting at the very outset an
+exception to his primary principle. A further application of the
+hypothesis of metastasis will be noticed below in connection with the
+determination of the sternum of fishes. We note here an interpretation
+of the first metastasis in terms of functional adaptation. "The constant
+and violent action of the tail, if it does not go so far as actually to
+displace and move forward the internal organs, at least fits in well
+with an arrangement in which the organs are so disposed" (p. 99).
+
+The first memoir deals with the homologies of the opercular bones.
+Geoffroy considers that the external opening of the ear corresponds to
+the external opening of the gill-chamber, which lies between the
+operculum and the pectoral girdle. The ear communicates with the buccal
+cavity by the Eustachian tube, so does the branchial chamber by means of
+the gill-slits. The auditory chamber of higher Vertebrates is,
+therefore, the homologue of the branchial chamber in fish; the opercular
+bones in fish and the ossicles of the ear in other Vertebrates stand in
+close relation to this chamber; therefore the opercular bones are the
+homologues of the ossicles of the ear, the interoperculum corresponding
+to the malleus, the suboperculum to the lenticular, the minute lower
+part of the suboperculum to the incus, the operculum to the stapes, and
+the pre-operculum to the tympanic ring. In making these particular
+determinations Geoffroy professes to be led by his principle of
+connections. The pre-operculum has, he says, the same connections with
+neighbouring bones as the tympanic bone in other Vertebrates, and the
+other pieces of the gill-cover are homologised with particular
+ear-ossicles according to the order in which they stand to one another.
+The second memoir in the book deals with the sternum, and affords a very
+good example of Geoffroy's method of dealing with the facts of
+structure. We shall omit here any detailed reference to the other three
+memoirs, which deal with the hyoid, with the branchial arches and the
+structures which correspond in air-breathing Vertebrates, and with the
+bones of the shoulder-girdle.
+
+In the memoir on the sternum Geoffroy's first care is to arrive at a
+definition of what a sternum is. He defines it partly by its functions,
+partly by its connections, as the system of bones which covers and
+protects the thorax, and gives attachment to certain groups of muscles.
+
+The most highly developed sternum (according to this definition) is the
+plastron of the tortoise, whose structure it dominates (p. 103). It is
+important, therefore, to determine of how many bones the plastron is
+composed, since the full number of elementary parts of which an organ is
+composed is best seen when the organ is at the maximum of its
+development. There are nine bones in the plastron of the tortoise. "The
+conclusion to be drawn from this is that every sternum, provided that it
+is not inhibited in its development by some obstacle, is composed of
+_nine elementary parts_" (p. 105). These nine bones are in Geoffroy's
+nomenclature, the episternals, the hyosternals, the hyposternals, the
+xiphisternals, which are all paired bones, and the entosternal, which is
+unpaired. The arrangement of them is in the tortoise:--
+
+Episternal---------------------------Episternal
+    |\__                               __/|
+    |   \__                         __/   |
+    |      \__                   __/      |
+    |         \__ Entosternal __/         |
+    |       __/                 \__       |
+    |    __/                       \__    |
+    | __/                             \__ |
+    |/                                   \|
+Hyosternal                           Hyosternal
+    |                                     |
+    |                                     |
+    |                                     |
+    |                                     |
+Hyposternal-------------------------Hyposternal
+    |                                     |
+    |                                     |
+    |                                     |
+    |                                     |
+Xiphisternal------------------------Xiphisternal.
+
+The articulations in the tortoise are indicated by the connecting
+lines. Geoffroy tries to show that the sternum in other animals is
+composed of these nine bones, or at least of a certain number of them,
+always in the same invariable relative positions. Thus in birds the
+sternum consists of five pieces, of a huge keeled entosternal, and of
+two "annexes" on either side, which are the hyo-and hyposternals.
+These are separate only in young birds. Occasionally, especially in
+young birds, rudiments of episternals and xiphisternals also occur.
+The minuteness of the episternals and the xiphisternals may be
+attributed to the gigantic size of the entosternal, in accordance with
+the _Loi de balancement_. In the other air-breathing Vertebrates the
+nine sternal elements can according to Geoffroy be discovered without
+great difficulty. But when we come to the determination of the sternum
+in fishes, difficulties abound, which Geoffroy solves in the following
+way. He points out that between the clavicles (_cleithra_) and the
+hyoid bone (_basihyal_) in fishes there is a long median bone
+(_urohyal_) which is attached in front by two strong tendons to the
+horns of the hyoid and is free behind (see Fig. 1). Gouan (1720) had
+seen in this bone the homologue of the sternum. Geoffroy adopts this
+view, but considers that this bone alone cannot represent the whole
+sternum. He finds the representatives of other bones of the sternum in
+the large bones (_epihyal_ and _ceratohyal_, or the two pieces of the
+_ceratohyal_) which are comprised in the hyoid arch. But he is
+immediately met by the difficulty that this complex of bones is
+situated in front of the pectoral girdle, whereas the sternum in
+higher Vertebrates lies behind the pectoral girdle. He reflects,
+however, that the gills of fish, situated in front of the clavicles,
+are merely the lungs under another name. The gills have become shifted
+forward by a metastasis similar to that which brought the whole
+thoracic organs far forward in fish. This being so, their supporting
+elements, the sternum and the ribs, must have moved with them, and are
+hence to be found in front of the pectoral girdle.
+
+[Illustration: FIG. 1.--Hyoid Arch of the Conger. (Original.)]
+
+Geoffroy's next step is to point out that the only possible homologues
+of sternal ribs are the branchiostegal rays, which arise from the large
+bones of the hyoid arch. If these are sternal ribs, the bones to which
+they are attached must be the hyo- and hyposternals or "annexes," the
+bones from which in birds the ribs take their origin.
+
+The unpaired sternal bone (_urohyal_) cannot be homologous with the
+entosternal, for it has no connections with the annexes. He decides that
+it must represent the episternals, for in some young birds there is a
+two-headed episternal to which two strong tendons are attached, just in
+the same way as the unpaired piece in fish is bound to the bones of the
+hyoid by two tendons. "Thus it is not the sternum as a whole that has
+shifted in front of the clavicles and covered with its side pieces the
+gills placed there; it is a piece exclusively piscine, in the sense that
+it is only in the class of fishes that it reaches the _maximum_ of its
+development" (p. 83).
+
+To sum up, the sternum in all four vertebrate classes is composed of the
+same elements, arranged always in the same way. "One is ... led to the
+conception of an ideal type of sternum for all Vertebrates, which then,
+considered from a lower standpoint, resolves itself into several
+secondary forms according as the whole or the majority of the
+constituent materials are employed, or even as these elements come to
+change their respective dimensions or proportions" (p. 134). As to the
+elementary constituents, "they give proof of individuality, and
+sometimes even, in certain abnormalities, of independence, and rise to
+the level of primary organisatory materials" (p. 132). What holds good
+for the sternum holds good for other organs--and accordingly the unity
+of plan and composition can be demonstrated for all the organs of
+Vertebrates.
+
+Soon after the publication of the _Philosophie anatomique_ (1818)
+Geoffroy went further in his search for unity, and maintained that the
+structure of insects and Crustacea could be reduced to the vertebrate
+type.
+
+He proposed to replace Cuvier's classification of the animal kingdom
+into the four large groups, Vertebrata, Mollusca, Articulata, and
+Radiata by the following classification:--[90]
+
+                Hauts-Vertébrés (Vertebrata, Cuv.).
+     Vertébrés /
+               \
+                Dermo-Vertébrés (Articulata, Cuv.).
+
+
+                Mollusques (Mollusca, Cuv.).
+   Invertébrés /
+               \
+                Rayonnés (Radiata, Cuv.).
+
+The idea upon which is based the comparison of Articulates with
+Vertebrates is that each skeletal segment of Articulates is a vertebra.
+In the Hauts-vertébrés the vertebræ are internal; in the
+Dermo-vertébrés they are external. "_Every animal lives either outside
+or inside its vertebral column_."[91] The essence of a vertebra is not
+its form, nor its function, but its composition from four elementary
+pieces which unite round a central space (_Isis, loc. cit._, p. 532).
+Serres had shown that in the higher animals every vertebra is formed
+from four centres of ossification, that the body of the vertebra is at
+first tubular, and that afterwards it becomes filled up. In lobsters and
+crabs each segment is composed of four elementary pieces, as may be seen
+most easily in young ones. "Accordingly each segment corresponds to a
+true vertebra in composition: there is the same number of 'materials,'
+the same order in the course of ossification, the same kind of
+articulation, the same annular arrangement, the same empty space in the
+middle" (p. 534). The only difference is that in Articulates the central
+space is very great and contains all the organs of the body, whereas in
+the higher Vertebrates the body of the vertebra becomes completely
+filled up. In the thoracic region of Crustacea it is not the whole
+segment with part of the carapace which corresponds to a vertebra, but
+merely the part round the ventral nerve-cord (endophragmal skeleton).
+
+If the skeleton of the segment in Articulates corresponds to the body of
+a vertebra and is here external, then the appendages of the Articulate
+must correspond to ribs (p. 538). The full development of this thought
+is found in a Memoir of 1822, "Sur la vertèbre."[92] He takes as the
+typical vertebra that of a Pleuronectid, probably the turbot. His
+original figure is reproduced (Fig. 2).
+
+[Illustration: FIG. 2.--"Vertebra" of a Pleuronectid. (After Geoffroy.)]
+
+He includes as part of the vertebra not only the neural (e', e'') and
+hæmal (o', o'') arches, but also, above and below these, the radialia
+(a'', u') and the fin-rays (a', u''). (Neither the radialia nor the
+fin-rays are, by the way, in the same transverse plane as the body of
+the vertebra). Every vertebra, he considers, contains these nine
+pieces--the cycleal (or body), the two perials (e', e'') and the two
+epials (a', a'') above, the two paraals (o', o'') and the two cataals (u',
+u'') below. The epials and the cataals are in reality paired bones which
+in fish mount one on top of the other to support the median fins. In the
+cranial region--the skull is formed of modified vertebræ--the epials
+and perials open out so as to form the walls and roof of the brain; in
+the thoracic region the paraals and cataals reach their maximum of
+development and perform the same service for the thoracic organs, the
+paraals becoming vertebral, and the cataals sternal, ribs.
+
+We have seen that in Arthropods the body of the vertebra (cycleal) forms
+the open ring of the segment, which lies immediately under the skin, the
+vertebral tube coinciding with the epidermal tube. The homologues of the
+other eight pieces of the vertebra must accordingly be sought in the
+external appendages. At first sight there seems here a contradiction of
+the principle of connections, for the appendages in Arthropods are
+lateral, whereas the paired bones of the vertebra are dorsal and
+ventral. But there is in reality no contradiction, for "what our law of
+connections absolutely requires is that all organs, whether internal or
+external, should stand to one another in the same relations; but it is
+all one whether the box (_coffre_) that encloses them lies with this or
+that side on the ground. What similarities in the organisation of man
+and the digitate mammals, and yet what differences between their
+attitudes when standing! The same holds true as regards the normal
+attitudes of the pleuronectids and the other fishes" (p. 107).
+
+The exact way in which Geoffroy homologised the parts of the appendages
+in Arthropods with the paired pieces of the typical vertebra is best
+shown by the reproduction of his figure of an abdominal segment of the
+lobster (Fig. 3), in which the parts homologous with those represented
+in the figure of the typical vertebra (Fig. 2) are indicated by the same
+letters. The ingenuity of the comparison is astonishing.
+
+[Illustration: FIG. 3.--Abdominal Segment of the Lobster. (After
+Geoffroy.)]
+
+The comparison of the Arthropod with the Vertebrate is extended also to
+the internal organs. The internal organs of the Arthropod are shown to
+stand in the same order to one another as in the Vertebrate, only the
+organs are inverted. Thus the nervous system is dorsal in the
+Vertebrate, ventral in the Arthropod. Turn the Arthropod on its back and
+the relative positions of the systems of organs are the same as in the
+Vertebrate. The relation of the organs to the external tube is of course
+different in Arthropods and Vertebrates, but this is no contradiction of
+the principle of connections. "Such a tube, although it is the organs
+essential to life that it contains, can yet behave in different ways
+with regard to the mass of these organs: the principle of connections
+demands only that all the organs maintain with one another fixed and
+definite relations; but the principle would be in no way invalidated if
+the whole mass had rotated inside the tube" (p. 112).
+
+Geoffroy pushed the analogy between Arthropods and Vertebrates very far,
+for he asserted that every piece in the skeleton of an insect was
+homologous with some bone in Vertebrates, that it stood always in its
+proper place, and remained faithful to at least one of its
+connections.[93] It does not appear that he attempted to prove in detail
+this very big assumption, but the beginnings of a detailed comparison
+are found in the paper of 1820, _Sur l'organisation des insectes_. Six
+segments are distinguished in an insect--the head, the three divisions
+of the thorax, the abdomen, and the terminal segment of the abdomen (p.
+455).
+
+The skeleton of the insect's head is said to correspond to the bones of
+the face, to the bones of the cerebrum and to the hyoid of higher
+Vertebrates, the skeleton of the prothorax to the bones of the
+cerebellum, of the palate, and the pieces of the larynx, the skeleton of
+the mesothorax to the parietals, interparietals, and opercular bones,
+and that of the metathorax to the skeleton of the thorax of Vertebrates.
+The pieces of the abdomen and of the terminal segment correspond to the
+bones of the abdomen and coccyx (p. 458). It does not need the
+subsequent likening of the hind wings of insects to the air bladder of
+fish, and of the stigmata to the pores of the lateral line, to convince
+one finally of the fancifulness of the whole comparison.
+
+In 1830 two young naturalists, Meyranx and Laurencet, presented to the
+Académie des Sciences a memoir in which they likened a Cephalopod to a
+Vertebrate bent back at the level of the umbilicus, saying that the
+Vertebrate in this position had all its organs in the same order as in
+the Cephalopod. Geoffroy took up this idea with enthusiasm, seeing in it
+a further application of his master-idea of the unity of plan and
+composition. By means of this comparison Mollusca definitely took their
+place in the _Échelle des êtres_, after the Articulata, just as Geoffroy
+had maintained in 1820, saying that crabs formed a link between the
+other Crustacea and the molluscs.[94] The comparison brought him nearer
+to the end he had in view, the reference of all animal structure to one
+single type.
+
+But in championing the memoir of Meyranx and Laurencet, Geoffroy found
+himself in direct antagonism with Cuvier, who held that his four
+"Embranchements" had each a separate and distinct plan of structure. In
+a paper read to the Academy in February 1830,[95] Cuvier easily
+demolished the crude comparison of the Cephalopod to the Vertebrate. He
+gave diagrams of the internal organs of a Cephalopod and of a Vertebrate
+bent back in the manner indicated by Meyranx and Laurencet, and he
+showed in detail that the arrangement of the main organs was quite
+different, that the likeness would have been much greater if the
+Cephalopod had been likened to a Vertebrate doubled up the other way,[96]
+but that even then the arrangement of the organs would not be the same.
+The organs, too, of the Cephalopod are differently constructed. He sums
+up his criticism by saying:--"I give true and summary expression to all
+these facts when I say that Cephalopods have several organs in common
+with Vertebrates, which fulfil in either case similar functions, but
+that these organs are differently arranged with respect to one another,
+and often constructed in a different way; that they are in Cephalopods
+accompanied by several other organs which Vertebrates do not possess,
+whilst the latter on their side have many organs which Cephalopods lack"
+(p. 257). Geoffroy could not accept this commonsense view of the matter,
+but made a fight for his transcendental theories. This was the beginning
+of the famous controversy between Geoffroy and Cuvier which so excited
+the interest of Goethe. It was a struggle between "comparative anatomy"
+and "morphology," between the commonsense teleological view of structure
+and the abstract, transcendental. Geoffroy brought forward all his
+theories on the homology of the skeleton of fish with the skeleton of
+higher Vertebrates, and tried to prove by them his great principle of
+the unity of plan and composition; Cuvier took Geoffroy's homologies one
+by one, and showed how very slight was their foundation. Cuvier was on
+sure ground in insisting upon the observable diversities of structural
+type, and his vast knowledge enabled him to score a decisive victory.[97]
+
+The controversy was not, as we are sometimes told, a controversy between
+a believer in evolution and an upholder of the fixity of species,
+although it raised a question upon which evolution theory was to throw
+some light.
+
+In these Darwinian days Geoffroy has reaped a little posthumous glory as
+an early believer in evolution. That he did believe in evolution to a
+limited extent is certain; that his theory of evolution was, as it were,
+a by-product of his life-work, is also certain. Geoffroy was primarily a
+morphologist and a seeker after the unity hidden under the diversity of
+organic form. His theory of evolution had as good as no influence upon
+his morphology, for he did not to any extent interpret unity of plan as
+being due to community of descent. His morphological, non-evolutionary
+standpoint comes out quite clearly in several places in the _Philosophie
+anatomique_. He does not derive the structure of the higher Vertebrates
+from the simpler structure of the lower, but when he finds in fish a
+part at the maximum of its development, he speaks of the same part,
+rudimentary in the higher forms, as being, as it were, held in reserve
+for use in the fish. Thus, speaking of the episternal in fish which
+forms the central piece of its sternum, he says, "it is a bone that is
+rudimentary in birds (one might almost add a bone that is held in
+reserve in birds for this fate) which is destined to form in the centre
+the principal keel of this new machine" (p. 84). Again, with reference
+to the homology of the ossicles of the ear with the opercular bones in
+fish, "employing other resources equally hidden and rudimentary, Nature
+makes profitable use of the four tiny ossicles lodged in the auditory
+passage, and, raising them in fish to the greatest possible dimensions,
+forms from them these broad opercula...." (p. 85). Or you may take it
+the other way about, and start from the organisation of fishes;
+opercular bones are of no use to air-breathing animals, so they dwindle
+away, and are pressed into the service of the ear, although they are of
+little use in hearing (p. 46).
+
+There is here no thought of evolution; in later years, however, his
+researches upon fossil crocodilians led him to consider the possibility
+that the living species were descended from the antediluvian. For the
+factors of the transformation he refers to Lamarck's hypotheses.[98] In a
+memoir of 1828,[99] dealing with the possible genetic relation of living
+to fossil species, he still regards the question as more or less open.
+Although fossil species are mostly different from living species are we
+therefore to conclude, he asks, that they are not the ancestors of the
+present day forms? "The contrary idea arises more naturally in the mind;
+for otherwise the six-days' creation would have had to be repeated and
+new beings produced by a fresh creation. Now this proposition, contrary
+as it is to the most ancient historical traditions, is inadmissible" (p.
+210). It is sufficiently clear from this quotation that Geoffroy was
+thinking only of a transformation of the antediluvian species created by
+God, and by no means of an evolution of all species from one primitive
+type. In matters of religion Geoffroy was orthodox. He goes on to point
+out how great a resemblance there is in essential structure between
+fossil and living species. All find their place in one scheme of
+classification; does it not seem that all are modifications "of one
+single being, of that abstract being or common type, which it is always
+possible to denote by the same name?" (p. 211). This type is abstract,
+not actual, and it is certainly not conceived as an original ancestor of
+all animals.
+
+The fullest development of Geoffroy's views on evolution is found in his
+memoir "Le degré d'influence du monde ambiant pour modifier les formes
+animales."[100] Here the relation of his evolution-theory to his
+morphology is pointed out. The principle of unity of plan and
+composition cannot be the final goal of zoology; there must follow on it
+a philosophical study of the _differences_ between organic forms. The
+causes of these differences are to be found in the environment (pp.
+66-7). Geoffroy seems here to be moving from a pure to a causal
+morphology. It is probable, he continues, that living species have
+descended by uninterrupted generation from the antediluvian species (p.
+74), and that they have in the process become modified through external
+influences.
+
+Now of all functions respiration is the most important, and upon
+respiration everything is regulated. "If it be admitted that the slow
+progression of the centuries has brought in its train successive changes
+in the proportion of the different elements of the atmosphere, it
+follows as a rigorously necessary consequence that the organisation has
+been proportionately influenced by them" (p. 76). The respiratory milieu
+changes, the species change with it, or are eliminated (p. 79). We may
+see, perhaps, in the stress which Geoffroy lays upon respiration and the
+respiratory milieu a result of his constant obsession with the
+comparison of fish with air-breathing Vertebrates.
+
+In the first geological period, we read in another Memoir of the same
+year,[101] when ammonites and _Gryphæa_ flourished, hot-blooded animals
+with lungs could not exist. "A lung constructed like that of mammals and
+birds would not have been adapted to the essence of the respiratory
+element such as in my conception of it the system of the environing air
+used to be"[102] (p. 58).
+
+Geoffroy does not tell us exactly how the milieu is to act upon the
+organism; the whole theory is little more than a sketch and a pointing
+out of the way for future research--and in this prophetic enough. The
+action of external agents was apparently considered as physical, and no
+power of active adaptation was ascribed to the organism.
+
+From a passage in the memoir "Sur la Vertèbre" we may perhaps infer that
+he believed increasing complexity of structure to be due to a
+realisation of potentialities, to the development of parts present in
+the lower animals only in potency--"the organisation ... only awaits
+favourable conditions to rise, by addition of parts, from the simplicity
+of the first formations to the complication of the creatures at the head
+of the scale" (p. 112). Evolution takes place as the environment allows,
+and in a sense in opposition to the environment.
+
+He believed in saltatory evolution, for he considered that the lower
+oviparous Vertebrates could not be transformed into birds by slow
+modification, but only by a sudden transformation of their lungs, which
+would bring about the other characteristics of birds (p. 80). He
+considered, too, that transformations could arise by means of monstrous
+development (p. 86). In this connection the experiments which he made on
+the hen's egg[103] in order to produce artificial monstrosities are
+significant, though his purpose was rather to obtain proof of the
+inadequacy of the preformation hypothesis.[104]
+
+It seems probable enough that if Geoffroy had developed his views on
+evolution he would finally have been led to interpret unity of plan in
+terms of genetic relationship. But as it was he remained at his
+morphological standpoint. He did not interpret rudimentary organs as
+useless heritages of the past; he preferred to think that Nature had
+prepared double means for the same function, one or other being
+predominant according as the animal lived in the water or on the land.
+"To the animal that lives exclusively in the air Nature has granted an
+organisation suited to this mode of respiration, without however
+suppressing the other corresponding means, that is to say, without
+depriving it of a second system which is applicable only to the mode of
+respiration by the intermediary of water, and _vice versa_."[105]
+
+He seems, in one instance at least, to have hit upon the root-idea of
+the biogenetic law, but he was far from appreciating its significance.
+He recognised that an amphibian in its development passed through a
+stage when it was in all essentials similar to a fish, and he saw in
+this visible transformation a picture of the evolutionary
+transformation. "An amphibian," he writes,[106] "is at first a fish under
+the name of tadpole, and then a reptile [_sic_] under that of frog....
+In this observed fact is realised what we have above represented as an
+hypothesis, the transformation of one organic stage into the stage
+immediately superior." But it is not clear that he considered the
+development of the amphibian to be a _repetition_ of its ancestral
+history.
+
+He went, however, a certain length towards recognising the main
+principle of a law which was a commonplace of German transcendental
+thought, and was developed later by his disciple E. Serres, the law that
+the higher animals repeat during their development the main features of
+the adult organisation of animals lower in the scale. Thus he compared
+fish as regards certain parts of their structure with the foetus of
+mammals. He compared also Articulates with embryonic Vertebrates in
+respect of their vertebræ, for in the higher Vertebrates the body of the
+vertebra is tubular at an early stage of development, and in Articulates
+the body of the vertebra remains tubular permanently (_supra_, p. 61).
+As regards their vertebræ, "insects occupy a place in the series of the
+ages and developments of the vertebrate animals, that is to say, they
+realise one of the states of their embryo, as fishes do one of the
+states of their foetal condition."[107]
+
+This idea was destined to exercise a great influence upon the
+development of morphology. A further development of the thought is that
+certain abnormalities in the higher animals, resulting from arrest of
+development, represent states of organisation which are permanent in the
+lower animals.[108]
+
+So far we have considered Geoffroy's theories in their application to
+the facts. We go on to discuss the theories themselves, and the general
+conception of living things which underlies them.
+
+The principle of unity of plan and composition is the keynote of
+Geoffroy's work. It states that the same materials of organisation are
+to be found in all animals, and that these materials stand always in the
+same general spatial relations to one another. The "materials of
+organisation" are not necessarily organs in the physiological sense, and
+indeed the principle of the unity of plan cannot be upheld if the unity
+has reference to organs only. This became clear to Geoffroy, especially
+in his later years. In 1835 he wrote, speaking of the principle of the
+unity of plan, "I have, moreover, regenerated this principle, and
+obtained for it universality of application, by showing that it is not
+always the organs as a whole, but merely the materials composing each
+organ, that can be reduced to unity."[109] Even in the _Philosophie
+anatomique_ he deals rather with parts than with organs; he deals, for
+instance, with the elementary parts of the sternum, not with the organ
+"sternum" in its totality. The functions of the sternum vary, and the
+primary protective function of the sternum may be assumed by quite other
+parts, _e.g._, by the clavicles in fish, which protect the heart.[110]
+
+True homologies can be established between materials of organisation but
+not always between organs, which may be composed of different
+"materials."
+
+Almost as a corollary to this comes the further view that form is of
+little importance in determining homologies. An organ is essentially an
+instrument for doing a particular kind of work, and its form is
+determined by its function. Organs which perform the same function are
+usually similar in form though the elementary materials composing them
+may be different. This is seen in many cases of convergence. Organs,
+therefore, which perform the same function and are similar in external
+form are not necessary homologous. Conversely, the same complex of
+materials, say a fore limb, may take on the most varied shapes according
+as the function of the organ changes--but homology remains though form
+changes. Accordingly, form is one of the least important elements to be
+considered in determining a homology. "Nature," he wrote in one of his
+early papers, "tends to repeat the same organs in the same number and in
+the same relations, and varies to infinity only their form. In
+accordance with this principle I shall have to draw my conclusions, in
+the determining the bones of the fish's skull, not from a consideration
+of their form, but from a consideration of their connections."[111]
+
+Again, after comparing a vertebra of the Aurochs with an abdominal
+segment of the crab, he says, "I have insisted upon an identity which
+has extended to the least important relation of all, that of form."[112]
+
+Geoffroy's morphological units or materials of organisation were in the
+case of the skeleton--with which his researches principally deal--the
+single bones. But the interesting point is that he sought his
+skeleton-units in the embryo, and considered each separate centre of
+ossification as a separate bone. Coalescence of bones originally
+separate is one of the most usual events in development, and it is an
+occurrence which, more than any other, tends to obscure homologies.
+Because of its coalescence with the maxillaries, the intermaxillary in
+man was not discovered until Vicq d'Azyr and Goethe found it separate in
+the embryo. Apparently quite independently of Goethe, Geoffroy hit upon
+this plan of seeking in the embryo the primary elements or materials of
+organisation. In an early paper on the skull of Vertebrates,[113] where he
+is concerned with showing that each bone of the fish's skull has its
+homologue in the skull of higher Vertebrates, he is faced with the
+difficulty that the skull of the fish has more bones than the skull of
+higher Vertebrates. "Having had the inspiration," he writes, "to reckon
+as many bones as there are distinct centres of ossification, and having
+made a consistent trial of this method, I have been able to appreciate
+the correctness of the idea: fish, in their earliest stages, are in the
+same conditions relatively to their development as the foetuses of
+mammals, and hence bear out the theory" (p. 344). So, too, in dealing
+with the homologies of the sternal elements (_supra_, p. 57) he treats
+as separate bones the "annexes" of the sternum in birds, though these
+are separate only in the young.
+
+If the same materials of organisation are present in all animals, and if
+they are arranged always in the same positions relatively to one
+another, how does it come about that animal forms are so varied, what
+explanation can be offered of the diversities of organic structure?
+Geoffroy's main answer to this question is his _Loi de balancement_. The
+law was enunciated by him already in 1807.[114] We take the following
+quotation, which represents his thought most nearly, from the _Cours de
+l'histoire naturelle des Mammifères_ (1829). "According to our manner of
+regarding the organisation of mammals, there is only a single animal
+modified by the inverse reciprocal variation of all or some of its
+parts. Now, from the fact that there is only one single general animal,
+it follows that for each section of its components or for each of its
+organs there is available only a given quantity of formative materials.
+Now suppose that the distribution of these materials has not been made
+in such a way as to ensure an exact equilibrium between all the parts
+concerned, one organ will get more than its share, another less. My law
+of the compensation of organs is founded on these principles" (i.,
+_Leçon_ 16, p. 12). "The atrophy of one organ turns to the profit of
+another; and the reason why this cannot be otherwise is simple, it is
+because there is not an unlimited supply of the substance required for
+each special purpose."[115] The nutritive material available is limited
+for each species; if one part gets more than its share the other parts
+must get less--that is all the law means. As an example, take the
+minuteness of the episternals and xiphisternals in birds, as contrasted
+with the huge size of the entosternal. "The minuteness of the
+episternals and xiphisternals might be imputed to this gigantic piece
+diverting to its own profit the nutritive fluid, since the bigger it is
+the smaller these are."[116]
+
+One has constantly to remember in dealing with Geoffroy's theories that
+he was not an evolutionist, but purely a morphologist. It is therefore,
+perhaps, to ask too much to require of him an explanation of the causes
+of diversity. The morphologist describes, classifies, generalises; he
+does not seek for causes. But we must leave this question aside in order
+to discuss how far Geoffroy's theory of the unity of plan and
+composition fits the facts. As Geoffroy himself admitted on several
+occasions, his theory was an _à priori_ one, a theory hit upon by hasty
+induction, then erected into a principle and imposed upon the facts. No
+more than Goethe did he extract his principle from a sufficient mass of
+data.
+
+Now he found his theory to be in its pure form unworkable; he found, for
+example, that the skeleton of fishes could not be compared directly,
+bone for bone, with the skeleton of higher Vertebrates; he had to admit
+differences of position of whole sets of organs in the two groups, he
+had to admit various _metastases_, before he could bring the skeleton of
+fish into line. And these metastases are due to functional
+requirements--for example, the forward position of sternum and thoracic
+organs in fish is an adaptation to swimming.
+
+So he does not so much demonstrate the unity of plan of whole organisms
+as the unity of plan of particular corresponding parts of them. Thus he
+does not prove or attempt to prove that Articulates are in all points
+like Vertebrates, but simply that their skeleton is built upon the same
+plan as that of Vertebrates. The rest of the organs, while still
+comparable with the organs of Vertebrates, stand in different relations
+to the skeleton. An Articulate therefore, on his own showing, is not,
+_as a whole_, built upon the same general structural plan as a
+Vertebrate.
+
+Further, he does not always remain true to his principles, for he does
+not establish homologies of parts entirely by their connections but
+sometimes by their functions as well. Thus the sternum, or rather the
+complex of sternal elements, is defined and discovered in particular
+cases not by its connections only but also by its functions. The
+framework of the gills is homologised part by part with the framework of
+the lungs, not because the relations of the framework to the rest of the
+skeleton are the same in fish and air-breathing Vertebrates, but simply
+because gills are considered the equivalents of lungs--a comparison
+which is purely physiological.
+
+Even with these concessions to the functional view of living things,
+Geoffroy was unable to make good his contention that all animals are
+built upon the same plan. His arguments failed to carry conviction to
+his contemporaries, and Cuvier in particular subjected them to
+destructive, and indeed final, criticism.
+
+The paper, already referred to, in which Cuvier disposed of the
+transcendentalists' comparison of Cephalopods and Vertebrates is of
+great significance, for it states in the clearest way the radical
+opposition between the functional and the formal attitudes to living
+things.
+
+Cuvier points out that if by unity of composition is meant identity,
+then the statement that all animals show the same composition is simply
+not true--compare a polyp with a man!--on the other hand, if by unity is
+meant simply resemblance or homology, the statement is true within
+certain limits, but it has been employed as a principle since the days
+of Aristotle, and the theory of unity of composition is original only in
+so far as it is false. He admits, however, that Geoffroy has seized upon
+many hidden homologies, especially by his valuable discovery of the
+importance of foetal structure. In all this Cuvier is undoubtedly right.
+Unity of plan and composition, as Geoffroy conceived it, simply does not
+exist. Cuvier goes on to say that this principle of Geoffroy's, in the
+greatly modified form in which it can be accepted, and has been accepted
+from the dawn of zoology, is not the sole and unique principle of the
+science. On the contrary, it is merely a subordinate principle,
+subordinate to a higher and more fruitful principle, that, namely, of
+the conditions of existence, of the adaptation (_convenance_) of the
+parts, of the co-ordination of the parts for the rôle which the animal
+is to play in Nature. "That is the true philosophical principle," he
+says, "whence may be deduced the possibility of certain resemblances,
+the impossibility of certain others; it is the rational principle from
+which follows the principle of the unity of plan and composition, and in
+which at the same time it finds those limits, which some would like to
+disregard" (p. 248).
+
+Geoffroy's position is the direct contrary. He holds that the principle
+of the unity of plan and composition is the true base of natural
+history,[117] and that this unity limits the possible transformations of
+the organism. Thus, speaking of the influence of the respiratory medium,
+he says, "All the same this influence of the external world, if it has
+ever become a cause which disturbed organisation, must necessarily have
+been confined within fairly narrow limits; animals must have opposed to
+it certain conditions inherent to their nature, the existence of the
+same materials composing them, and a manifest tendency to resemble one
+another, and to reproduce invariably the same primordial type."[118] Unity
+of plan and composition is, on this view, prior to adaptation and limits
+adaptation. Cuvier's view, on the contrary, is that the necessity of
+functional and ecological adaptation accounts for the repetition of the
+same types of structure. There are, of all the possible combinations of
+organs, only a few viable types--those whose structure is adapted to
+their life. Therefore it is reasonable that these few types should be
+repeated in innumerable exemplars. One must remember, in order to
+appreciate Cuvier's view, that he was not obsessed, as we are, by the
+idea of evolution.
+
+Cuvier thought in terms of organs, not in terms of "materials of
+organisation." He held that the resemblances between the organs of one
+class of animals and the organs of another were due to the similarity of
+their functions. "Let us conclude, then, that if there are resemblances
+between the organs of fish and those of other classes, it is only in the
+measure that there is a resemblance between their functions."[119] There
+are only a few kinds of organs, each adapted for a particular function,
+and these organs are necessarily repeated from class to class.--"As the
+animal kingdom has received only a limited number of organs, it is
+inevitable that some at least of these organs should be common to
+several classes."[120]
+
+Geoffroy thought in terms of "materials," of parts of indefinite
+function, parts which might take on any function. He insists upon the
+necessity of disregarding function when tracing out the unity of
+composition. He considers, in direct opposition to Cuvier's
+interpretation of structural resemblance as due to similarity of
+function, that unity of composition is the primary fact, and similarity
+of function subsidiary. In his reply in the _Mammifères_ (1829) to
+Cuvier's criticisms in the _Histoire naturelle des Poissons_ (1828), he
+insists on the necessity of excluding function from consideration in any
+truly philosophical treatment of comparative anatomy (Discours prél., p.
+25). Cuvier held that function determined structure, or at least that
+the necessity of adaptation ruled the transformations of form. Geoffroy
+considered that structure determined function, that changes of
+structure, however they might arise, caused changes of function.
+"Animals," he writes, "have no habits but those that result from the
+structure of their organs; if the latter varies, there vary in the same
+manner all their springs of action, all their faculties and all their
+actions."[121]
+
+Again, "a vegetarian régime is imposed upon the Quadrumana by their
+possession of a somewhat ample stomach, and intestines of moderate
+length."[122] The hand of the bat has become so modified as to constrain
+the bat to live in the air.[123]
+
+The best example of Geoffroy's insistence upon the priority of structure
+to function, and so of his purely morphological attitude, is perhaps his
+interpretation, already alluded to, of the appendages of Articulates.
+The segments of the Articulate are, he says, the equivalents of the
+bodies of the vertebræ of higher forms. Now "from the circumstance that
+the vertebra is external, it results that the ribs must be so too; and,
+as it is impossible that organs of such a size can remain passive and
+absolutely functionless, these great arms, hanging there continually at
+the disposition of the animal, are pressed into the service of
+progression, and become its efficient instruments."[124] The ribs become
+locomotory appendages.
+
+We may compare the similar thought that the ear ossicles are simply
+opercular bones reduced and turned to other uses.
+
+Geoffroy could not but recognise the correlation of structure to
+function, for this is a fact which imposes itself upon every observer.
+He recognised also correlation between functions, as when he pointed out
+the connection between increased respiration and enhanced muscular
+activity in birds.[125] He interpreted structure at times in terms of
+function, the short, strong clavicle of the mole as an adaptation to
+digging, the keeled sternum of birds as an adaptation to flying, and so
+on. But we may say that his whole tendency was to disregard function, to
+look upon it as subsidiary. He protests against arguing from function
+and habits to structure, as an "abuse of final causes."[126] He was not so
+convinced as Cuvier was of the all-importance of functional correlation;
+in this view he was probably confirmed by his work on teratology. It did
+not surprise him that Insects, in which lungs, heart and circulation
+have disappeared(!), should yet have a skeleton built upon the same plan
+as the skeleton of Vertebrates, which possess these organs; the
+correlation of organ-systems is not so close as to prevent this.[127] So
+too, although the other organs of the insect are all inside the body of
+the vertebræ, they are yet comparable with the organs of Vertebrates.[128]
+The existence of rudimentary organs also seemed to him an argument
+against too strict a correlation of parts.
+
+The contrast between the teleological attitude, with its insistence upon
+the priority of function to structure, and the morphological attitude,
+with its conviction of the priority of structure to function, is one of
+the most fundamental in biology.
+
+Cuvier and Geoffroy are the greatest representatives of these opposing
+views. Which of them is right? Is there nothing more in the unity and
+diversity of organic forms than the results of functional adaptation, or
+is Geoffroy right in insisting upon an element of unity which cannot be
+explained in terms of adaptation? If there be an irreducible element of
+unity, is there any truth in Geoffroy's suggestion that this unity
+results from a power which is exercised in the world of atoms where are
+elements of inalterable character?[129]
+
+The problem as Geoffroy and Cuvier understood it was not an evolutionary
+one. But the problem exists unchanged for the evolutionist, and
+evolution-theory is essentially an attempt to solve it in the one
+direction or the other. Theories such as Darwin's, which assume a random
+variation which is not primarily a response to environmental changes,
+answer the problem in Geoffroy's sense. Theories such as Lamarck's,
+which postulate an active responsive self-adaptation of the organism,
+are essentially a continuation and completing of Cuvier's thought.
+
+    [86] "Mémoire sur les rapports naturels des makis,"
+    _Magasin Encyclopèdique_, vii.
+
+    [87] Discours préliminaire, pp. xv.-xxiv.
+
+    [88] _Études progressives d'un Naturaliste_, p. 50,
+    Paris, 1835.
+
+    [89] _Philosophie Anatomique_., i., Introduction, p. 1.
+
+    [90] "Sur une colonne vertébrale et ses côtes dans les
+    insectes apiropodes," (_Acad. Sci._, Feb. 12, 1820).
+    Printed in _Isis_, pp. 527-52, 1820 (2).
+
+    [91] "Sur l'organisation des insectes," p. 458. _Isis_,
+    pp. 452-62, 1820 (2).
+
+    [92] _Mém. Mus. d'Hist. nat._, ix., pp. 89-119, Pls.
+    v-vii.
+
+    [93] _Sur l'organisation des insectes_, p. 459.
+
+    [94] _Isis_, p. 549.
+
+    [95] Published in _Ann. Sci. Nat._, xix., pp. 241-59,
+    1830.
+
+    [96] _Cf._ Aristotle (_supra_, p. 10).
+
+    [97] For an account of the controversy reference may be
+    made to I. Geoffroy St Hilaire, _Vie Travaux et Doctrine
+    scientifique d'Etienne Geoffroy St Hilaire_, Paris,
+    1847; also Semper, _Arb. zool. zoot. Instit. Würzburg_,
+    iii., 1876-7, K. E. von Baer, _Lebensgeschichte Cuviers_,
+    ed. L. Stieda, 1897, and J. Kohlbrugge, in _Zoolog.
+    Annalen_, v., pp. 143-95. 1913.
+
+    [98] "Recherches sur l'organisation des Gavials," _Mém.
+    Mus. d'Hist. nat._, xii., 1825.
+
+    [99] _Mém. Mus. d'Hist. nat._, xvii., pp. 209-29.
+
+    [100] _Mém. Acad. Sci._, xii., pp. 63-92, 1833.
+
+    [101] _Mém. Acad. Sci._, xii., pp. 43-61, 1833.
+
+    [102] Geoffroy's French style is at times incredibly bad,
+    and more or less literal translations of his sentences
+    are apt to read queerly!
+
+    [103] _Mém. Mus. d'Hist. nat._, xiii., p. 289, 1826.
+
+    [104] _Mém. Mus. d'Hist. nat._, xviii., p. 221, 1828. His
+    teratological work is important, and is chiefly
+    contained in the second volume of the _Philosophie
+    anatomique_.
+
+    [105] _Phil. anat._, i., p. 449.
+
+    [106] _Mém. Acad. Sci._, xii., p. 82, 1833.
+
+    [107] _Mém. Mus. d'Hist. nat._, ix., p. 101, 1822.
+
+    [108] _Cours de l'histoire naturelle des Mammifères_, i.,
+    Leçon 3, p. 13, 1829.
+
+    [109] _Études progressives d'un Naturaliste_, p. 59, f.n.,
+    Paris, 1835.
+
+    [110] _Phil. Anat._, i., p. 444.
+
+    [111] _Ann. Mus. d'Hist. nat._, x., p. 344, 1807.
+
+    [112] _Isis_, p. 534, 1820 (2).
+
+    [113] _Ann. Mus. d'Hist. nat._, x., pp. 342-65, 1807.
+
+    [114] _loc. cit._, x., p. 343.
+
+    [115] _Phil. anat._, i., 450, f.n. _Cf._ Aristotle
+    (_supra_, p. 11).
+
+    [116] _Loc. cit._, p. 136.
+
+    [117] _Mammifères_, i., Discours prél., p. 18.
+
+    [118] _Phil. anat._, i., p. 208.
+
+    [119] Cuvier and Valenciennes, _Hist. nat. Poissons_, i.,
+    p. 550, 1828.
+
+    [120] Cuvier and Valenciennes, _loc. cit._, p. 544.
+
+    [121] _Mammifères_, i., _Leçon_ 4, p. 17.
+
+    [122] _Loc. cit._, _Leçon_ 5, p. 8.
+
+    [123] _Loc. cit._, _Leçon_ 13, p. 6.
+
+    [124] _Isis_, p. 539, 1820 (2).
+
+    [125] _Mammifères_, i., _Leçon_ 4, p. 6.
+
+    [126] _Mammifères_, Discours prél., p. 7.
+
+    [127] _Isis_, p. 460, 1820 (2).
+
+    [128] _Mém. Mus. d'Hist. nat._, ix., p. 102, 1822.
+
+    [129] _Mém. Acad. Sci._., xii., p. 76, 1833.
+
+
+
+
+CHAPTER VI
+
+THE FOLLOWERS OF ETIENNE GEOFFROY SAINT-HILAIRE
+
+
+Geoffroy's theories were not generally accepted by his contemporaries,
+but his methods had considerable influence, especially in France, where
+many made essays in pure morphology.
+
+His chief follower was Serres, who is mentioned indeed in the
+_Philosophie anatomique_ as a fellow-worker. Serres was primarily a
+medical anatomist; his interest lay in human anatomy and embryology,
+normal and pathological.
+
+His best early work was an _Anatomie comparée du cerveau_ (1824-26),
+which met with a flattering reception from Cuvier.[130] He laid great
+stress upon the development of the brain and spinal cord in the
+different classes, and was quick to point out analogies not only between
+adult but also between embryonic structures. He paid much attention to
+cases of correlation, and noted a great many; he observed, for instance,
+a constant relation between the development of the spinal cord and of
+the corpora quadrigemina, and between the size of the corpora
+quadrigemina and the volume of the optic nerves and eyes. In this the
+influence of Cuvier is unmistakable.
+
+Serres' early theoretical views are to be found in a series of papers in
+the _Annales des Sciences naturelles_,[131] under the general title
+_Recherches d'Anatomie transcendante, sur les Lois de l'Organogénie
+appliquées à l'anatomie pathologique_, also published separately. We
+follow these papers in our exposé of Serres' doctrine, reserving for a
+future chapter (Chap. XII.) the consideration of his matured views of
+thirty years later.
+
+In the first of them he points out how neither position nor function has
+proved altogether sufficient to establish homologies. In the early days
+anatomists were guided by form; when form failed them, they traced an
+organ in its changes throughout the series of animals by considering its
+function. This method was satisfactory enough as regards the organs of
+the nutritive life. But in the organs of the life of relation, in the
+nervous system, the functions of the parts were difficult to discover,
+and their form very changeful. Hence a new principle was required, and
+Serres found it in the thought which he probably owed to the German
+transcendentalists (see Chap. VII.), that the permanent structure of the
+lower animals could be compared with phases in the development of the
+higher, and particularly of man, or, as he put it, that comparative
+anatomy was often only a fixed and permanent anthropogeny, and
+anthropogeny a fugitive and transitory comparative anatomy (xi., p.
+106).
+
+"In rising towards the first formations," he writes, "transcendental
+anatomy recognised that one and the same organ, however complicated its
+definitive form might be, repeated in its transitory states the organic
+simplicities of the lower classes. Thus the primitive heart of birds was
+first of all a canal, then a pocket or single cavity, then finally the
+complex organ of the class. Comparative anatomy was thus seen to be
+repeated and reproduced by embryogeny" (xii., p. 85).
+
+His explanation of the fact of repetition is that, "in animals belonging
+to the lower classes the _formative force_, whatever it may be, has a
+less energetic impulsion than in the higher animals, and hence the
+organs pass through only a part of the transformations which those of
+the higher forms undergo; and it is for this reason that they show
+permanently the organic dispositions which are only transitory in the
+embryo of man and the higher Vertebrates. Hence these double aortas,
+these double venæ cavæ which one observes more or less constantly among
+reptiles" (xxi., p. 48).
+
+The number of stages in embryogeny is proportionate to the complexity of
+the adult; the younger the embryo the simpler its organs--such is the
+general formula of the relation between the embryo and the adult. But
+here in Serres' doctrine of parallelism a complication enters. He
+observed that embryonic organs did not always develop in a piece, by
+simple growth, but often were formed by the union of separately formed
+parts or layers. Thus the kidney in man is formed by the fusion of a
+number of "little kidneys," and the spinal cord reaches its full
+development by the laying down of successive layers within it. He was
+greatly impressed with this fact, which, as a convinced believer in
+epigenesis, he used with great effect against the preformistic theories.
+"This method of isolated formation," he wrote, "is noticed in early
+stages in the thyroid, the liver, the heart, the aorta, the intestinal
+canal, the womb, the prostate, the clitoris, and the penis" (xi., p.
+69). So, too, in the development of the skeleton, ossification proceeds
+from separate centres, foramina are formed by the fusion of separate
+bones round them. In his memoir, _Lois d'Osteogénie_ (1819), Serres
+established several laws of ossification based upon this principle of
+separate formation.[132]
+
+How is the fact of multiple formation to be reconciled with the
+principle of repetition, according to which organs are simplest in the
+early embryo and in the lower animals? But observation shows that, as a
+rule, the further down the scale you go the more divided organs
+become--the more numerous the bones of the skull, for example. There is
+thus a parallel between multiple formation of organs in the embryos of
+the higher Vertebrates and their subdivided state in the lower. Take,
+for example, the kidney. In the genus _Felis_, and in birds, each kidney
+has two lobes, in the elephant four, in the otter ten, in the ox twelve
+to fourteen. The human kidney in its development starts with about a
+dozen lobes, and the number diminishes as the kidney grows. Thus the
+permanent state of the kidney in the animals mentioned is reproduced by
+the stages of its development in man (xii., p. 126).
+
+So, too, at the second or third month the uterus of the human embryo is
+bicornuate, and afterwards passes through stages comparable to the adult
+and permanent uterus of rodents, ruminants, and carnivores. There is
+indeed a time in the development of the human embryo when it resembles
+in many of its organs the adult stage of various lower animals. It is
+about this time that it possesses a tail.
+
+We note that Serres' theory of parallelism applies, strictly speaking,
+only to organs, not to organisms, although he, too, readily fell into
+the error of supposing that the organisation of an embryo could be
+compared as a whole with the adult organisation of an animal lower in
+the scale. Thus he wrote in one of his later papers[133]--"As our
+researches have made clear, an animal high in the organic scale only
+reaches this rank by passing through all the intermediate states which
+separate it from the animals placed below it. Man only becomes man after
+traversing transitional organisatory states which assimilate him first
+to fish, then to reptiles, then to birds and mammals." Serres was not
+altogether free from the besetting sin of the transcendentalists--hasty
+generalisation.
+
+The law of parallelism applied not only to Vertebrates but also to
+Invertebrates. In a short paper[134] of 1824 Serres attempted an
+explanation of the nervous system of Invertebrates. Invertebrates, he
+considered, lacked the cerebrospinal axis of Vertebrates, and their
+nervous system was the homologue of the sympathetic system of
+Vertebrates. The relation of the invertebrate to the vertebrate nervous
+system being thus fixed, can the nervous system of Invertebrates be
+reduced to one plan? It does not seem possible to establish a common
+plan for the adult nervous systems. But apply the principle of
+parallelism, which has proved so valuable within the limits of the
+vertebrate series. Taking insects as the highest class, we find that
+there are three stages in the development of their nervous system; in
+the first the nervous system is composed of two separate strands, in the
+second the strands unite round the oesophagus, in the third they unite
+also behind. Now in _Bulla aperta_, stage (1) is permanent; in _Clio_,
+_Doris_, _Aplysia_, _Tritonia_, _Sepia_, _Helix_, stage (2) is
+permanent, and in _Unio_ stage (3). In fact, all the varieties of the
+nervous system of molluscs fall into one or other of these three
+classes. "It follows, then, that as regards their nervous system, the
+Mollusca are more or less advanced larvæ of insects" (p. 380). The law
+of parallelism is here applied to single organ-systems, but in later
+years Serres applied it to whole organisations also, saying that the
+lower Invertebrates were permanent embryos of the higher.
+
+In the paper of 1834, already referred to, Serres pushed his
+speculations further and attempted to establish the unity of type of all
+animals, Vertebrates and Invertebrates alike--a favourite pastime of the
+transcendentalists. It is incontestable, he admits, that adult
+Invertebrates are quite different in structure from adult Vertebrates,
+"but if one regards them as what I take them to be, namely, _permanent
+embryos_, and if one compares their organisation with the embryogeny of
+Vertebrates, one sees the differences disappear, and from their
+analogies arise a crowd of unsuspected resemblances" (_loc. cit._, p.
+247).
+
+The last point of Serres' doctrine which calls for remark is his
+interpretation of abnormalities as being often comparable to grades of
+structure permanent in the lower animals. Thus the double aorta which
+may occur as an abnormality in man is the normal and permanent state in
+reptiles. This idea, of course, he got from Etienne Geoffroy St Hilaire.
+It is further developed in his "_Théorie des formations et des
+déformations organiques appliquée à l'anatomie comparée des
+monstruosités_ (1832), and in his final large memoir of 1860 (see below,
+p. 205).
+
+In 1816 appeared a fine piece of work by J. C. Savigny on the homologies
+of the appendages in Articulates. The standpoint was that of pure
+morphology. "I am convinced," he wrote, "that when a more complete
+examination has been made of the mouth of insects, properly so called,
+that is to say, having six legs and two antennæ, it will be found that
+whatever form it affects it is always essentially composed of the same
+elements.... The organ remains the same, only the function is modified
+or changed--such is Nature's constant plan."[135] In this the influence of
+Geoffroy can be traced; but the work was very free from the
+exaggerations of the transcendentalists, and many of Savigny's
+homologies are accepted even to-day. The first memoir dealt with the
+mouth-parts of insects; the second with the anterior appendages of
+Articulates generally. Savigny shows that the mouth-parts of insects can
+be reduced to the type shown in Orthoptera, where there are clearly two
+mandibles, two maxillæ, and a lower lip formed by the fusion of two
+second maxillæ. All other insects have these same mouth-parts, disposed
+in the same order, however much their form may have been modified in
+response to new functions. He goes on to compare the anterior set of
+appendages in a long series of Articulates, in _Julus_, _Scolopendra_,
+_Cancer_, _Gammarus_, _Cyamus_, _Nymphon_, _Phalangium_, _Apus_,
+_Caligus_, _Limulus_, and a few others. For Crustacea he established the
+homologies now accepted, of the mandibles with the mandibles of insects,
+of the first and second pairs of maxillæ with the parts so named in
+insects, and so on. He is quite clear that the maxillipedes of Crustacea
+are the homologues of the feet of Hexapoda. "Their disposition must lead
+one to think that the six anterior feet of _Julus_, that is to say, all
+the feet of the Hexapoda, are here transformed into jaws" (_loc. cit._,
+p. 48). In _Scolopendra_ also there is a similar transformation of two
+pairs of legs into auxiliary jaws. In _Gammarus_, where there is only
+the first pair of maxillipedes, the other two pairs have become
+"retransformed" into feet. We find him supporting his comparison of the
+three anterior pairs of legs in _Julus_ to the three pairs of legs in
+insects by an argument drawn from embryology; for only the first three
+pairs of feet are present in _Julus_ at birth (Degeer), "an observation,
+which, together with their position, should cause them to be considered
+as the representatives of the six thoracic feet of Hexapoda" (p. 44).
+
+His comparison of the Arachnid appendages with those of insects and
+Crustacea is very curious. As his starting-point he takes _Cyamus_,
+which has antennæ (two pairs) and mouth parts (four pairs) as in many
+Crustacea, and then seven pairs of legs; he compares with it _Nymphon_,
+which has in all seven pairs of appendages. These appendages he
+homologises with the seven pairs of legs of _Cyamus_, so that the first
+appendage in _Nymphon_ corresponds to the seventh appendage of _Cyamus_.
+This homology is extended to all Arachnids; their first two pairs of
+appendages, however they may be modified as "false" mandibles and
+"false" maxillæ, really correspond to the second and third maxillipedes
+in Crustacea, and to the second and third pairs of feet in insects. It
+is interesting to note that he treats _Limulus_ as an Arachnid, pointing
+out that there is as much difference between _Apus_ and _Limulus_ as
+between _Cancer_ and _Phalangium_. He describes the "gnathobases" in
+_Phalangium_ and _Limulus_. We may note that he had just an inkling of
+the modern doctrine that all the appendages of Articulates consist of a
+basal joint bearing an inner and an outer terminal piece, for he
+observes that the "cirri" of the maxillipedes of Crustacea give the
+appendage the same bifid appearance as the appendages of the abdomen and
+the thoracic legs of _Mysis_ (p. 50).
+
+V. Audouin, in his memoir, _Recherches anatomiques sur le thorax des
+animaux articulés_,[135] applied the principle of the unity of plan and
+composition to the exoskeleton of insects, Crustaceans, and Arachnids.
+His guiding ideas were, "(1) that the skeleton of articulated animals is
+formed of a definite number of pieces, which are either distinct or
+intimately fused with one another; (2) that in many cases, some pieces
+diminish or altogether disappear, while others reach an excessive
+development; (3) that the increase of one piece seems to exert on the
+neighbouring pieces a kind of influence which explains all the
+differences one finds between the individuals of each order, family and
+genus" (Sep. copy, p. 16). Geoffroy had already stated, without proof,
+that the parts of the Arthropod's skeleton, however they might change in
+shape and size, remained faithful to the principle of connections, at
+least at their points of insertion.[137] Audouin gave the detailed
+demonstration of this by his accurate and minute determination of the
+pieces of the arthropod skeleton. He recognised that the body of
+Arthropods was made up of a series of similar rings, and that even the
+compact head of insects consisted of fused segments. In each segment
+Audouin distinguished a fixed number of hard chitinous parts, the dorsal
+tergum, the ventral sternum, the lateral "flanc" of three pieces, all to
+be recognised by their positions relative to one another. Many of the
+names which he proposed are still in use; it was he who introduced the
+terms prothorax, mesothorax, and metathorax, for the three segments of
+the insect's thorax. He used Geoffroy's _Loi de balancement_ to explain
+cases of correlative development, such as the relation between the size
+of the front wings and the development of the mesothorax. In another
+paper Audouin compared the three pieces of the dorsal skeleton of
+Trilobites to the tergum and the upper part of the "flanc."[138] In a
+third paper of about the same time he tried to establish the homologies
+of the segments throughout the Articulate series--with less success than
+Savigny.
+
+Later on, in conjunction with Milne-Edwards, he demonstrated the unity
+of composition of the nervous system in Crustacea, showing how the
+concentrated system of the crab was formed by the same series of ganglia
+as in the Macrura.
+
+The entomologist Latreille also tackled the problem of the homologies of
+the segments in the different classes of Arthropods (Cuvier, _loc.
+cit._, p. cclxxii.). He thought he could find fifteen segments in all
+Arthropods. He made the retrograde step of likening the head of insects
+to a single segment. But some of his homologies showed morphological
+insight, _e.g._, his comparison of the "first jaws" of Arachnids to
+antennæ, because they were placed above the upper lip. It was he who
+first pointed out the resemblance of the leaf-like gills of Ephemerid
+larvæ to wings, and suggested that wings were "a sort of tracheal feet."
+
+He made also a rather hazy and speculative contribution on Okenian lines
+to the problem of the relation of Arthropods to Vertebrates, likening
+the carapace of Crustacea to an enormously developed hyoid, the
+appendages of the tail to the ventral and anal fins of fish. The
+masticatory organs of Arthropods were jaws disjointed at their
+symphysis; antennæ, nostrils turned outside in.
+
+Dugès also made a comparison of Articulates with Vertebrates.[139] He did
+not accept Geoffroy's vertebral theory of the Arthropod skeleton, though
+he admitted that in Arthropods the dorsal surface was turned towards the
+ground, basing this assumption on the position of the nervous system,
+and also, curiously enough, on the inverted position of the embryo on
+the lower surface of the yolk. He considered that the mandibles and
+first maxillæ of Arthropods were the homologues of the upper and lower
+jaws of Vertebrates, adducing as confirmatory evidence the fact that in
+snakes the rami are separate. The labium was the equivalent of the
+hyoid, the labial palps and maxillipedes the equivalent of the "hyoid"
+elements which form the branchial arches.
+
+But Dugès' main contribution to morphological method was his conception
+of the living organism as a colony of lesser units, which were
+themselves real "organisms." "By _organism_ the author means a complex
+of organs which taken together suffice to constitute, ideally or
+actually, a complete animal. An 'organism' is, as it were, an elementary
+or simple animal; several organisms combined form a complex animal" (p.
+255). Dugès hit upon this principle, which was first suggested to him by
+A. Moquin-Tandon's work on the leech (1827), as a great aid in
+demonstrating the unity of plan and composition throughout the animal
+kingdom.[140] According to his view there are three main types of
+animals--(1) Biserials, including bilaterally symmetrical animals,
+composed of two parallel series of "organisms"; (2) Radiates, composed
+of "organisms" arranged like the spokes of a wheel; and (3)
+Raceme-animals, in which the separate "organisms" were disposed more or
+less irregularly, in bunches (p. 257). The unitary "organism" is
+supposed to be the same in all, only the arrangement differing. Dugès of
+course admitted that the centralisation of the complete organism became
+greater the higher it stood in the scale, and that this held good also
+in individual development. The appendages of Articulates and Vertebrates
+were thought of as the members of as many separate organisms. He went so
+far as to suggest that the fingers of a man's hand were the free
+extremities of as many thoracic members.
+
+Dugès' conception of the organism has often been revived since in a
+saner form, _e.g._, by E. Perrier, and it has a certain validity. It has
+much affinity with the similar conceptions of Goethe and the German
+transcendentalists.
+
+    [130] _Mém. Acad. Sci._, iv., pp. cclxxxiv.-ccci., 1824.
+
+    [131] _Ann. Sci. Nat._, xi., xii., 1827; xvi., 1829; xxi., 1830.
+
+    [132] See Rádl, _loc. cit._, i., pp. 225-6.
+
+    [133] _Ann. Sci. nat._ (2), ii., p. 248, 1834.
+
+    [134] _Ann. Sci. nat._, iii., pp. 377-80, 1824.
+
+    [135] _Mémoires sur les Animaux sans Vertèbres_, Part I.,
+    p. 10, Paris, 1816.
+
+    [136] _Ann. Sci. Nat._, (1), i., pp. 97-135, 416-432,
+    1824.
+
+    [137] _Isis_, p. 456, 1820 (2).
+
+    [138] Cuvier, _Mém. Acad. Sci._, iv., p. cclxx., 1824.
+
+    [139] _Acad. Sci._ 18th Oct. 1831. Extract in _Ann. Sci.
+    Nat._, xxiv., pp. 254-60, 1831.
+
+    [140] His views were more fully elaborated in his _Mémoire
+    sur la conformité organique dans l'échelle animale_,
+    Montpellier, 1832.
+
+
+
+
+CHAPTER VII
+
+THE GERMAN TRANSCENDENTALISTS
+
+
+To complete our historical survey of the morphology of the early 19th
+century we have now to turn back some way and consider the curious
+development of morphological thought in Germany under the influence of
+the _Philosophy of Nature_. We have already seen many of these notions
+foreshadowed by Goethe, who had considerable affinity with the
+transcendentalists, but the full development of transcendental habits of
+thought comes a little later than the bulk of Goethe's scientific work,
+and owes more to Kielmeyer and Oken than to Goethe himself.
+
+A great wave of transcendentalism seems to have passed over biological
+thought in the early 19th century, arising mainly in Germany, but
+powerfully affecting, as we have seen, the thought of Geoffroy and his
+followers. Many ideas were common to the French and German schools of
+transcendental anatomy, the fundamental conception that there exists a
+unique plan of structure, the idea of the scale of beings, the notion of
+the parallelism between the development of the individual and the
+evolution of the race. It is difficult to disentangle the part played by
+each school and to determine which should have the credit for particular
+theories and discoveries. The philosophy seems to have come chiefly from
+Germany, the science from France. It must be borne in mind that German
+comparative anatomy was largely derivative from French, that the Paris
+Museum was the acknowledged anatomical centre, and that Cuvier was its
+acknowledged head.
+
+It is probably correct to say that the credit mainly belongs to the
+German transcendental school for the law of the parallelism between the
+stages of individual development and the stages of the scale of beings,
+and the theory of the repetition or multiplication of parts within the
+individual. The vertebral theory of the skull is a particular
+application of the second of these generalisations.
+
+The law of parallelism[141] seems to have been expressed first by
+Kielmeyer (1793),[142] who gave to it a physiological form, saying that
+the human embryo shows at first a purely vegetative life, then becomes
+like the lower animals, which move but have no sensation, and finally
+reaches the level of the animals that both feel and move.
+
+The idea was next taught by Autenrieth in 1797.[143]
+
+Oken (1779-1851) in his early tract _Die Zeugung_ (1805), and in his
+_Lehrbuch der Naturphilosophie_ (1809-11) elaborated the thought, and
+taught that every animal in its development passes through the classes
+immediately below it. "During its development the animal passes through
+all stages of the animal kingdom. The foetus is a representation of all
+animal classes in time."[144] The Insect, for example, is at first Worm,
+next Crab, then a perfect volant animal with limbs, a Fly (_ibid._, p.
+542).
+
+As Nature is "the representation of the individual activities of the
+spirit," so the animal kingdom is the representation of the activities
+or organs of man. The animal kingdom is therefore "a dismemberment of
+the highest animal, _i.e._, of Man" (p. 494). Now "animals are gradually
+perfected, entirely like the single animal body, by adding organ unto
+organ"--the way of evolution is the way of development. Hence "animals
+are only the persistent foetal stages or conditions of Man," who is the
+microcosm, and contains within himself all the animal kingdom.
+
+Oken was himself a careful student of embryology; von Baer[145] speaks of
+his work (published in Oken and Kieser, _Beiträge zur vergleichenden
+Zoologie, Anatomie und Physiologie_, 2 pts., 1806-7) as forming the
+turning-point in our understanding of the mammalian ovum. He had
+accordingly actually observed a resemblance in certain details of
+structure between the human foetus and the lower animals; but the
+peculiar form which the law took in his hands was a consequence of his
+hazy philosophy. He saw the relation of teratological to foetal
+structure, for he affirmed that "malformations are only persistent
+foetal conditions" (p. 492).
+
+The idea of comparing the embryo of higher animals with the adult of
+lower was widely spread at this time among German zoologists. We find,
+for example, in Tiedemann's brilliant little textbook[146] the statement
+that "Every animal, before reaching its full development, passes through
+the stage of organisation of one or more classes lower in the scale, or,
+every animal begins its metamorphosis with the simplest organisation"
+(p. 57).
+
+Thus the higher animals begin life as a kind of fluid animal jelly which
+resembles the substance of a polyp; the young mammal, like the lower
+Vertebrates, has only a simple circulation, and, like them, lives in
+water (the amniotic fluid); the frog is first like a worm, then develops
+gills and becomes like a fish (p. 57). In his work on the anatomy of the
+brain,[147] Tiedemann established the homology of the optic lobes in birds
+by comparing them with foetal corpora quadrigemina in man (see Serres,
+_Ann. Sci. nat._, xii., p. 112).
+
+J. F. Meckel, in 1811, devoted a long essay to a detailed proof of the
+parallelism between the embryonic states of the higher animals and the
+permanent states of the lower animals. In a previous memoir in the same
+collection[148] (i., 1, 1808) he had made some comparisons of this kind in
+dealing with the development of the human foetus; in this memoir (ii.,
+1, 1811) he brings together all the facts which seem to prove the
+parallelism.
+
+His collection of facts is a very heterogeneous one; he mingles
+morphological with physiological analogies, and makes the most
+far-fetched comparisons between organs belonging to animals of the most
+diverse groups. He compares, for instance, the placenta with the gills
+of fish, of molluscs and of worms, homologising the cotyledons with the
+separate tufts of gills in _Tethys, Scyllæa_ and _Arenicola_(p. 26).
+This is purely a physiological analogy. He compares the closed anus of
+the early human embryo with the permanent absence of an anus in
+Coelentera, and the embryo's lack of teeth with the absence of teeth in
+many reptiles and fish, in birds, and in many Cetacea (p. 46).[149] These
+are merely chance resemblances of no morphological importance. He
+considers bladderworms as animals which have never escaped from their
+amnion, and _Volvox_ as not having developed beyond the level of an egg
+(p. 7). He lays much stress upon likeness of shape and of relative size,
+comparing, for instance, the large multilobate liver of the human foetus
+with the many-lobed liver of lower Vertebrates and of Invertebrates. In
+general he shows himself, in his comparisons, lacking in morphological
+insight.
+
+His treatment of the vascular system affords perhaps the best example of
+his method (pp. 8-25). The simplest form of heart is the simple tubular
+organ in insects, and it is under this form that the heart first appears
+in the developing chick. The bent form of the embryonic heart recalls
+the heart of spiders; it lies at first free, as in the mollusc _Anomia_.
+The heart consists at first of one chamber only, recalling the
+one-chambered heart of Crustacea. A little later three chambers are
+developed, the auricle, ventricle, and aortic bulb; at this stage there
+is a resemblance to the heart of fish and amphibia. At the end of the
+fourth day the auricle becomes divided into two, affording a parallel
+with the adult heart of many reptiles.
+
+In his large text-book of a somewhat later date, the _System der
+vergleichenden Anatomie_ (i., 1821), he works out the idea again and
+gives to it a much wider theoretic sweep, hinting that the development
+of the individual is a repetition of the evolutionary history of the
+race. Meckel was a timid believer in evolution. He thought it quite
+possible that much of the variety of animal form was due to a process of
+evolution caused by forces inherent in the organism. "The
+transformations," he writes, "which have determined the most remarkable
+changes in the number and development of the instruments of organisation
+are incontestably much more the consequence of the tendency, inherent in
+organic matter, which leads it insensibly to rise to higher states of
+organisation, passing through a series of intermediate states."[150]
+
+His final enunciation of the law of parallelism in this same volume
+shows that he considered the development of the individual to be due to
+the same forces that rule evolution. "The development of the individual
+organism obeys the same laws as the development of the whole animal
+series; that is to say, the higher animal, in its gradual evolution,
+essentially passes through the permanent organic stages which lie below
+it; a circumstance which allows us to assume a close analogy between the
+differences which exist between the diverse stages of development, and
+between each of the animal classes" (p. 514).
+
+He was not, of course, able fully to prove his contention that the lower
+animals are the embryos of the higher, and we gather from the following
+passage that he could maintain it only in a somewhat modified form. "It
+is certain," he writes, "that if a given organ shows in the embryo of a
+higher animal a given form, identical with that shown throughout life by
+an animal belonging to a lower class, the embryo, in respect of this
+portion of its economy, belongs to the class in question" (p. 535). The
+embryo of a Vertebrate might at a certain stage of development, be
+called a mollusc, if for instance, it had the heart of a mollusc.
+
+He admits, too, that the highest animal of all does not pass through in
+his development the entire animal series. But the embryo of man always
+and necessarily passes through many animal stages, at least as regards
+its single organs and organ-systems, and this is enough in Meckel's eyes
+to justify the law of parallelism (p. 535).
+
+In his excellent discussion of teratology Meckel points out how the idea
+of parallelism throws light upon certain abnormalities which are found
+to be normal in other (lower) forms (p. 556).[151]
+
+We may refer to one other statement of the law of parallelism--by K. G.
+Carus in his _Lehrbuch der vergleichenden Anatomie_ (Leipzig, 1834). The
+standpoint is again that of _Naturphilosophie_. It is a general law of
+Nature, Carus thinks, that the higher formations include the lower; thus
+the animal includes the vegetable, for it possesses the "vegetative" as
+well as the "animal" organs. So it is, too, by a rational necessity that
+the development of a perfect animal repeats the series of antecedent
+formations.
+
+As we have said, the main credit for the enunciation of the law of
+parallelism belongs to the German transcendental school; but the law
+owes much also to Serres, who, with Meckel, worked out its implications.
+It might for convenience, and in order to distinguish it from the laws
+later enunciated by von Baer and Haeckel, be called the law of
+Meckel-Serres.
+
+Under the "theory of the repetition or multiplication of parts within
+the organism" may be included, first, generalisations on the serial
+homology of parts, and second, more or less confused attempts to
+demonstrate that the whole organisation is repeated in certain of the
+parts. The recognition of serial homologies constituted a real advance
+in morphology; the "philosophical" idea of the repetition of the whole
+in the parts led to many absurdities. It led Oken to assert that in the
+head the whole trunk is repeated, that the upper jaw corresponds to the
+arms, the lower to the legs, that in each jaw the same bony divisions
+exist as in the limbs, the teeth, for instance, corresponding to the
+claws (_loc. cit._, p. 408). It led him to distinguish "two animals" in
+every body--the cephalic and the sexual animal. Each of these has its
+own organs; thus "in the perfect animal there are two intestinal systems
+thoroughly distinct from each other, two intestines which belong to two
+different animals, the sexual and cephalic animal, or the plant and the
+animal" (p. 382). The intestine of the sexual animal is the large
+intestine; the lungs of the sexual animal are the kidneys, its glottis
+is the urethra, its mouth the anus. So, too, the mouth is the stomach of
+the head. On another line of thought the sternum is a ventral vertebral
+column. Limbs are connate ribs, the digits indicating the number of ribs
+included (_cf._ Dugès, _supra_, p. 88).
+
+J. F. Meckel[152] discusses "homologies" of this kind in the thorough and
+pedestrian way so characteristic of him. Not only, he says, are the
+right and left halves of the body comparable with one another, but also
+the upper and the lower, the dividing line being drawn at the level of
+the diaphragm. The lumbar complex corresponds to the skull, the anus to
+the mouth, the urino-genital opening to the nasal opening; in general,
+the urino-genital system corresponds to the respiratory, the kidneys to
+the lungs, the ureters to bronchi, the testes and ovaries to the thymus
+(he had observed the physiological relation between the development of
+the thymus and the state of the genital organs), the prostate and the
+uterus to the thyroid gland, and the penis and clitoris to the tongue.
+The fore-limbs and girdle correspond in detail with the hind limbs and
+the pelvis--a point already worked out by Vicq d'Azyr; the dorsal and
+ventral halves of the body are likewise comparable in some respects, the
+sternum, for example, answering in the arrangement of its bones, muscles
+and arteries to the vertebral column. The skeleton of each member is in
+some respects a repetition of the vertebral column.
+
+His brother, D. A. Meckel,[153] worked out an elaborate comparison between
+the alimentary canal and the genital organs, basing the legitimacy of
+the comparison upon early embryological relations and upon the state of
+things in Coelentera, where genital and digestive organs occupy the same
+cavity. In his view the uterus corresponded to the stomach, the vagina
+to the oesophagus, the fallopian tubes to the intestine, and so on.
+
+The vertebral theory of the skull took its origin from the same habit of
+thought. As part of the wider idea of the metameric repetition of parts
+it had some scientific worth, but the theory was pushed too far, and the
+facts were twisted to suit it. Among annulate animals the theory of
+repetition found ample scope; Oken was able to compare with justice the
+jaws of crabs and insects with their other limbs, as Savigny did later
+in a more scientific way. Among Vertebrates the application of the
+theory of serial repetition was not so obvious, except in the case of
+the vertebræ. Goethe seems to have been the first to hit upon the idea
+that the skull is composed of a number of vertebræ, serially homologous
+with those of the vertebral column. He tells us that the idea flashed
+into his mind when contemplating in the Jewish cemetery at Venice a
+dried sheep's skull. The discovery was made in 1790, but not published
+till 1820.[154]
+
+The idea seems to have been taught by Kielmeyer, one of the earliest of
+the "philosophers of nature," but it was not published by him.
+
+In a book (_Cours d'Études médicales_), published in 1803, Burdin
+assimilated the skull to the vertebral column.
+
+Oken, in an inaugural dissertation (Programm) _Ueber die Bedeutung der
+Schädelknochen_,[155] published in 1807, gave to the theory its necessary
+development. Autenrieth, also in 1807,[156] distinguishing separate
+ganglia in the brain, was not far from the hypothesis that each of these
+ganglia must have its separate vertebra.
+
+In 1808 Duméril read a paper to the Académie des Sciences in which he
+compared the skull to a gigantic vertebra, basing his hypothesis on the
+similarity existing between the crests and depressions on the hinder
+part of the skull and those on the posterior surfaces of the vertebræ.
+
+After Oken's work the vertebral theory was taken up generally by both
+the German and the French anatomists. Spix published in 1815 a large
+volume on the skull, entitled _Cephalogenesis_, distinguishing (as Oken
+did at first) three cranial vertebræ. Bojanus in his _Anatome testudinis
+europæae_ (1819), and in a series of papers in _Isis_ (1817-1819, and
+1821) established the existence of a fourth cranial vertebra, and this
+was accepted by Oken in the later editions of his _Lehrbuch_. Meckel and
+Carus among the Germans, de Blainville and E. Geoffroy among the French,
+contributed to the development of the theory. In England the theory was
+championed particularly by Richard Owen.
+
+It was one thing to assert in a moment of inspiration that the skull was
+composed of modified vertebræ; it was quite another to demonstrate the
+relation of the separate bones of the skull to the supposed vertebræ.
+Upon this much uncertainty reigned; there was not even unanimity as to
+the number of vertebræ to be distinguished. Goethe found six vertebræ in
+the skull; Spix, and at first Oken, three only, Geoffroy seven; the
+accepted orthodox number seems to have been four (Bojanus, Oken, Owen).
+
+As an example of the method of treatment adopted we may take Oken's
+matured account of the composition of the cranial vertebræ, as given in
+the English translation of his _Lehrbuch_. "To a perfect vertebra," he
+says, "belong at least five pieces, namely, the body, in front the two
+ribs, behind the two arches or spinous processes" (p. 370). In the
+cervical vertebræ the transverse processes represent the ribs. The skull
+consists of four vertebræ, the occipital, the parietal, the frontal and
+the nasal, or, named after the sense with which each is associated, the
+auditory, the lingual, the ocular and the olfactory. The "bodies" of
+these vertebræ are the body of the occipital (basioccipital), the two
+bodies of the sphenoid (basi- and pre-sphenoid), and the vomer. The
+transverse processes of each are the condyles of the occipitals
+(exoccipitals), the alæ of the two sphenoids (alisphenoids and
+orbitosphenoids) and the lateral surfaces of the vomer. The arches or
+spinous processes are the occipital crest, the parietals, the frontals,
+and the nasals.
+
+The cranium is thus composed of four rings of bone, each composed of the
+typical elements of a vertebra.
+
+The arbitrary nature of the comparison is obvious enough. As Cuvier
+pointed out in the posthumous edition of his _Leçons_, it is only the
+occipital segment that shows any real analogy with a vertebra--an
+analogy which Cuvier ascribed to similarity of function. He admitted a
+faint resemblance of the parietal segment to a vertebra:--"The body of
+the sphenoid does indeed look like a repetition of the basioccipital,
+but having a different function it takes on another form, especially
+above, by reason of its posterior clinoid apophyses."[157] He denied the
+resemblance of the frontal and nasal "vertebræ" to true vertebræ,
+pointing out that both parietals and frontals are bones specially
+developed for the purpose of roofing over and protecting the cerebrum.
+
+A very curious development was given to the vertebral theory by K. G.
+Carus, who seems to have taken as his text a saying of Oken's, that the
+whole skeleton is only a repeated vertebra.[158] His system is worthy of
+some consideration, for he tries to work out a geometry of the
+skeleton.[160]
+
+His method of deduction is a good example of pure _Naturphilosophie_.
+Life, he says, is the development of something determinate from
+something indeterminate. A finite indeterminate thing, that is, a
+liquid, must take a spherical form if it is to exist as an individual.
+Hence the sphere is the prototype of every organic body. Development
+takes place by antagonism, by polarity, typically by the division and
+multiplication of the sphere. In the course of development the sphere
+may change, by expansion into an egg-shaped body, or by contraction into
+a crystalline form, the changes due to expansion being typical of living
+things, those due to contraction being typical of dead. At the surface
+of the primitive living sphere is developed the protective
+_dermatoskeleton_, which naturally takes the shape of a hollow sphere;
+round the digestive cavity which is formed in the living sphere is
+developed the _splanchnoskeleton_; round the nervous system (which is,
+as it were, the animal within the animal) is developed the
+_neuroskeleton_. All skeletal formations belong to one or other of these
+systems.
+
+Carus defines his aim to be the discovery of the inner law which
+presides over the formation of the skeleton throughout the animal
+kingdom; he desires to know "how such and such a formation is realised
+in virtue of the eternal laws of reason" (iii., p. 93). Here we touch
+the kernel of _Naturphilosophie_--the search for rational laws which are
+active in Nature; the discontent with merely empirical laws.
+
+The thesis which Carus sustains is that all forms of skeleton, whether
+of dermatoskeleton, splanchnoskeleton, or neuroskeleton, can be deduced
+from the hollow sphere, which is the primary form of any skeleton
+whatsoever (p. 95). That means, put empirically, that every skeleton can
+be represented schematically by a number of hollow spheres, suitably
+modified in shape, and suitably arranged. The chief modification in
+shape exhibited by bones is one which is intermediate between the
+organic and the crystalline series of modifications of the sphere. The
+organic modifications are bounded by curved lines, the crystalline by
+straight; the intermediate partly by curved and partly by straight
+lines. They are the dicone (the shape of a diabolo) and the cylinder.
+These forms must necessarily be of importance for the skeleton, which is
+intermediate between the organic and the inorganic. "The dicone embodies
+the real significance of the bone," writes Carus. Each dicone and
+cylinder composing the skeleton is called by Carus a vertebra.
+
+We may expect then all skeletons to be composed of spheres, cylinders
+and dicones in diverse arrangements. Nature being infinite, all the
+possible types of arrangement of these elements must exist in the test
+or skeleton of some animal, living, fossil, or to come (p. 127). One
+conceives easily what the main types of skeleton must be. In some
+animals, _e.g._, sea-urchins, the skeleton is a simple sphere; in
+others, _e.g._, starfish, secondary rows of spheres radiate out from a
+central sphere or ring; in annulate animals the skeleton consists of a
+row of partially fused spheres.
+
+In Vertebrates the arrangement is more complex. There are first the
+protovertebral rings of the dermatoskeleton, these being principally the
+ribs, limb-girdles, and jaws. Round the central nervous system are
+developed the deutovertebral rings of the neuroskeleton (vertebræ in the
+ordinary sense). The apophyses and bodies of the vertebræ, and the bones
+of the members[160] are composed of columns of tritovertebræ, or vertebræ
+of the third order. Thus the whole vertebrate skeleton is a particular
+arrangement of vertebræ, which in their turn are modifications of the
+primary hollow sphere.
+
+The German transcendentalists were more or less contemporary with E.
+Geoffroy, and no doubt influenced him, especially in his later years, as
+they certainly did his follower Serres. Oken indeed wrote, in a note[161]
+appended to Geoffroy's paper on the vertebral column of insects, that
+"Mr Geoffroy [_sic_] is without a doubt the first to introduce in France
+_Naturphilosophie_ into comparative anatomy, that is to say, that
+philosophy one of whose doctrines it is to seek after the
+_signification_ of organs in the scale of organised beings." This is,
+however, an exaggeration, for Geoffroy was primarily a morphologist,
+whereas the morphology of the German transcendentalists was only a
+side-issue of their _Naturphilosophie_.
+
+Geoffroy, on his part, exercised some influence on the
+transcendentalists. He asserts[162] indeed that Spix got some of the ideas
+published in the _Cephalogenesis_ (1815) from attending his course of
+lectures in 1809. It is certainly the case that Spix published before
+Geoffroy the view that the opercular bones are homologous with the
+ear-ossicles, adopting, however, a different homology for the separate
+bones.[163]
+
+Some speculations seem to have been common to both schools--for
+instance, the law of Meckel-Serres, the vertebral theory of the skull,
+and the recognition of serial homology in the appendages of Arthropods
+(Savigny, Oken). Latreille and Dugès, as well as Serres, clearly show in
+their theoretical views the influence of Oken and the other
+transcendentalists. Geoffroy's principle of connections and law of
+compensation were recognised by some at least of the Germans.
+
+But whatever his actual historical relations may have been with the
+German school, Geoffroy was vastly their superior in the matter of pure
+morphology. He alone brought to clear consciousness the principles on
+which a pure morphology could be based: the Germans were transcendental
+philosophers first, and morphologists after.
+
+One understands from this how J. F. Meckel, who was in some ways the
+leading comparative anatomist in Germany at this time, could be at once
+a transcendentalist and an opponent of Geoffroy. Meckel had a curiously
+eclectic mind. A disciple of Cuvier, having studied in 1804-6 the rich
+collections at the Museum in Paris, the translator of Cuvier's _Leçons
+d'anatomie comparée_, he earned for himself the title of the "German
+Cuvier," partly through the publication of his comprehensive textbook
+(_System der vergl. Anatomie_, 5 vols.), partly by his extensive and
+many-sided research work, partly by his authoritative teaching. His
+_System_ shows in almost every page of its theoretical part the
+influence of Cuvier; and it is through having assimilated Cuvier's
+teaching as to the importance of function that Meckel combats Geoffroy's
+law of connections, at least in its rigorous form. He submits that the
+connections of bones and muscles must change in relation to functional
+requirements. He rejects Geoffroy's theory of the vertebrate nature of
+Articulates. Generally throughout his work the functional point of view
+is well to the fore.
+
+Yet at heart Meckel was a transcendentalist of the German school. His
+vagaries on the subject of "homologues" leave no doubt about that, and,
+in spite of Cuvier, he believed, though not very firmly, in the
+existence of one single type of structure.
+
+A Cuverian by training, his lack of morphological sense threw him into
+the ranks of the transcendentalists, to whom perhaps he belonged by
+nature.
+
+    [141] For a full account, see Kohlbrugge, _Zool. Annalen_,
+    xxxviii., 1911.
+
+    [142] _Rede über das Verhältnis der organischen Kräfte_,
+    Stuttgart u. Tübingen, 1793 (1814). See Rádl, _loc.
+    cit._, i., p. 261; ii., p. 57.
+
+    [143] _Supplem. ad historiam embryonis_, Tübingen, 1797.
+
+    [144] _Lehrbuch der Naturphilosophie_, Eng. trans., p.
+    491, 1847.
+
+    [145] _Ueber Entwickelungsgeschichte der Thiere_, i., p.
+    xvii., 1828.
+
+    [146] _Zoologie_, Landshut, i., 1808.
+
+    [147] _Anatomie u. Bildungsgeschichte des Gehirns im Fötus
+    des Menschen_, Nürnberg, 1816.
+
+    [148] _Beyträge zur vergleichende Anatomie_, Leipzig, i.,
+    1808-9, ii., 1811-2.
+
+    [149] Cetacea were generally considered at this time to be
+    mammals of low organisation.
+
+    [150] From the French trans., which appeared under the
+    title _Traité gén. d'Anat. comparée_, i., p. 449, 1828.
+
+    [151] _Cf._ Geoffroy (_supra_, p. 70).
+
+    [152] _Beyträge_, ii., 2, 1812. Also in his _System d.
+    vergl. Anat._, i., 1821.
+
+    [153] In J. F. Meckel's _Beyträge_, ii.
+
+    [154] _Zur Morphologie_, i., 2, p. 250, 1820; and ii., 2,
+    pp. 122-4, 1824.
+
+    [155] See translation, giving the gist of this paper, in
+    Huxley's _Lectures on the Elements of Comparative
+    Anatomy_, pp. 282-6, London, 1864.
+
+    [156] Reil's _Archiv. f. Physiol._, vii., 1807.
+
+    [157] _Leçons d'anatomie comparée_, 3rd ed., Brussels
+    reprint, i., p. 414, 1836.
+
+    [158] In his Programm, _U. d. Bedeut. d. Schädelknochen_,
+    1807.
+
+    [159] _Traité élémentaire d'anatomie comparée_ (French
+    trans.), vol. iii., Paris, 1835. First developed in his
+    volume _Von den Ur-Theilen des Knochen und
+    Schalen-Gerustes_, Leipzig, 1828.
+
+    [160] Dutrochet in 1821 had tried to prove that the bones
+    of the members belong to the type of the vertebra--the
+    dicone.
+
+    [161] _Isis_, pp. 552-9, 1820 (2).
+
+    [162] _Mém. Mus. d'Hist. nat._, ix., 1822.
+
+    [163] Cuvier and Valenciennes, _Hist. nat. Poissons_, i.,
+    p. 311, f.n.
+
+
+
+
+CHAPTER VIII
+
+TRANSCENDENTAL ANATOMY IN ENGLAND--RICHARD OWEN
+
+
+Richard Owen is the epigonos of transcendental morphology; in him its
+guiding ideas find clear expression, and in his writings are no
+half-truths struggling for utterance. But he was, though a staunch
+transcendentalist, an eclectic of the older ideas current in his time;
+for he picked out what was best in the older systems--Cuvier's
+teleology, Geoffroy's principle of connections, Oken's idea of the
+serial repetition of parts. In particular, he assimilated the teaching
+of Cuvier, the great opponent of the transcendentalists, and reconciled
+it in part with his own transcendentalism. His main theoretical views
+are to be found in his volume _On the Archetype and Homologies of the
+Vertebrate Skeleton_ (London, 1848). The master-idea of the book is that
+the vertebrate skeleton consists of a series of comparable segments,
+each of which Owen calls a vertebra. His definition of a vertebra is,
+"one of those segments of the endo-skeleton which constitute the axis of
+the body, and the protecting canals of the nervous and vascular trunks"
+(p. 81). The parts of a typical vertebra are shown in Fig. 4, which is
+copied from Owen's Fig. 14.
+
+                |||
+  zygapophysis  ||| -- neural spine
+             \  |||
+              *//^\\*
+diapophysis   //   \\ -- neurapohysis
+         \   //  o  \\
+          ===== --- =====
+              /     \
+       ===== |CENTRUM| O ===== -- peiurapophysis
+              \     /
+          ===== --- =====
+         /    \\   //
+parapophysis  *\\v//*
+             /  |||
+  zygapophysis  ||| -- hæmal spine
+                |||
+
+FIG. 4.--Ideal Typical Vertebra. (After Owen.)
+
+In Fig. 5 (page 103) is shown an actual vertebra, as Owen conceives it,
+the "vertebra" being that of a bird.
+
+[Illustration: FIG. 5.--Natural Typical Vertebra; Thorax of a Bird.
+(After Owen.)]
+
+A segment of sternum is included as the "hæmal spine" of the vertebra
+(_hs_); the vertebral rib is the "pleurapophysis" (_pl_); the sternal
+rib the "hæmapophysis" (_h_); the uncinate process of the vertebral rib
+is known as the "diverging appendage" (_a_). The whole vertebrate
+skeleton is composed of a series of vertebræ which show these typical
+parts. We arrive thus at the conception of an "Archetype" of the
+vertebrate skeleton, such as is represented in Fig. 6.
+
+The archetype is only a scheme of what is usually constant in the
+vertebrate skeleton, and both the number and the arrangement of the
+bones in any real Vertebrate are subject to variation. "It has been
+abundantly proved," Owen writes, towards the end of his volume, "that
+the idea of a natural segment (vertebra) of the endoskeleton does not
+necessarily involve the presence of a particular number of pieces, or
+even a determinate and unchangeable arrangement of them. The great
+object of my present labour has been to deduce ... the relative value
+and constancy of the different vertebral elements, and to trace the kind
+and extent of their variations within the limits of a plain and obvious
+maintenance of a typical character" (p. 146).
+
+It goes without saying that Owen considered the skull to be formed of
+vertebræ--the vertebral theory of the skull was, in his system, a
+deduction from the vertebral theory of the skeleton. He recognised four
+cranial vertebræ; the arrangement of them, and the relation of their
+constituent bones to the parts of the typical vertebra are shown in the
+table appearing on page 106. So far as their first three elements are
+concerned, these vertebræ are practically identical with the vertebræ
+distinguished in the classical vertebral theory of the skull, as
+enunciated by Oken. A divergence appears with the determination of the
+other elements of the vertebræ. The upper and lower jaws are associated
+with the nasal and frontal vertebræ respectively, not however as limbs
+of the head, but as constituent elements of these vertebræ. In the same
+way the hyoid apparatus is part and parcel of the parietal vertebra, and
+the pectoral girdle and fore-limbs part of the occipital vertebra.
+
+[Illustration: FIG. 6.--The Archetype of the Vertebrate Skeleton. (After
+Owen.)]
+
+Cranial Vertebræ.[164] (After Owen, 1848, p. 165.)
+
++---------------+---------------+----------------+---------------+-------------+
+|  Vertebræ.    |   Occipital.  |  Parietal.     |    Frontal.   |    Nasal.   |
++===============+===============+================+===============+=============+
+|Centra.        |Basioccipital. |Basisphenoid.   |Presphenoid.   |Vomer.       |
++---------------+---------------+----------------+---------------+-------------+
+|Neurapophyses. |Exoccipital.   |Alisphenoid.    |Orbitosphenoid.|Prefrontal.  |
++---------------+---------------+----------------+---------------+-------------+
+|Neural Spines. |Supraoccipital.|Parietal.       |Frontal.       |Nasal.       |
++---------------+---------------+----------------+---------------+-------------+
+|Parapophyses.  |Paroccipital.  |Mastoid.        |Postfrontal.   |None.        |
++---------------+---------------+----------------+---------------+-------------+
+|Pleurapophyses.|Scapular.      |Stylohyal.      |Tympanic.      |Palatal.     |
++---------------+---------------+----------------+---------------+-------------+
+|Hæmapophyses.  |Coracoid.      |Ceratohyal.     |Articular.     |Maxillary.   |
++---------------+---------------+----------------+---------------+-------------+
+|Hæmal Spines.  |Episternum.    |Basihyal.       |Dentary.       |Premaxillary.|
++---------------+---------------+----------------+---------------+-------------+
+|  Diverging    |Fore-limb or   |Branchiostegals.|Operculum.     |Pterygoid and|
+|  Appendage.   |  Fin.         |                |               |  Zygoma.    |
++---------------+---------------+----------------+---------------+-------------+
+
+Owen's reasons for considering the pectoral girdle and the fore-limb
+part of the occipital vertebra are as follows. In fish the pectoral
+girdle is slung to the skull by means of the post-temporal bone
+(supra-scapula, according to Owen) which abuts on the occipital arch. In
+_Lepidosiren_, whose skeleton resembles the archetype in many ways, the
+pectoral girdle is likewise attached to the occipital segment.
+
+In most other Vertebrates the pectoral girdle has shifted backwards
+along the vertebral column, by a "metastasis" (Geoffroy) similar to that
+by which the pelvic fins in many fish have shifted up close to the
+pectoral girdle. The scapula (with supra-scapula) is the pleurapophysis,
+the coracoid the hæmapophysis, of the occipital vertebra. The clavicle
+is homologised with the slender bone in fish now known as the
+post-clavicle, which shows a connection with the first or atlas vertebra
+of the vertebral column, forming, according to Owen, the hæmapophysis of
+the atlas. Owen considers it no objection to this view that in other
+Vertebrates the clavicle is anterior to the coracoid--"its anterior
+position to the coracoid in the air-breathing Vertebrata is no valid
+argument against the determination, since in these we have shown that
+the true scapular arch is displaced backwards" (_On the Nature of
+Limbs_, p. 63, London, 1849). In the pelvic girdle the ilium corresponds
+to the scapula, the ischium to the coracoid, the pubis to the clavicle.
+Hence the ilium is a pleurapophysis, the ischium and pubis are both
+hæmapophyses. The fore-limb is the developed "appendage" of the
+occipital vertebra, the hind-limb the developed "appendage" of the
+pelvic vertebra. They are serially homologous with, for example, the
+uncinate processes of the ribs in birds (see Figs. 5 and 6). The
+fore-limb is a simple filament in _Lepidosiren_, and presents few joints
+in _Proteus_ and _Amphiuma_; in other air-breathing Vertebrates it shows
+a more complete development, the humerus, radius and ulna, and the bones
+of the wrist and hand becoming differentiated out.
+
+As the fore-limb is equivalent to a single bone of the archetype, it is
+said to be, in its developed state, "teleologically compound" (p. 103).
+
+Since in the archetype every vertebra has its appendage, more than two
+pairs of locomotory limbs might have been developed. "Any given
+appendage might have been the seat of such developments as convert that
+of the pelvic arch into a locomotive limb; and the true insight into the
+general homology of limbs leads us to recognise many potential pairs in
+the typical endoskeleton. The possible and conceivable modifications of
+the vertebrate archetype are far from having been exhausted in the forms
+which have hitherto been recognised, from the primæval fishes of the
+palæozoic ocean of this planet up to the present time" (p. 102). It is
+not of the essence of the vertebrate type to be tetrapodal.
+
+In determining homologies Owen remained true to Geoffroy's principle of
+connections. Speaking of an attempt which had been made to determine
+homologies by the mode of development, he writes, "There exists
+doubtless a close general resemblance in the mode of development of
+homologous parts; but this is subject to modification, like the forms,
+proportions, functions, and very substance of such parts, without their
+essential homological relationships being thereby obliterated. These
+relationships are mainly, if not wholly, determined by the relative
+position and connection of the parts, and may exist independently of
+form, proportions, substance, function and similarity of development.
+But the connections must be sought for at every period of development,
+and the changes of relative position, if any, during growth, must be
+compared with the connections which the part presents in the classes
+where vegetative repetition is greatest and adaptive modification least"
+(p. 6). It is interesting to note that in Owen's opinion comparative
+anatomy explains embryology. Thus the scapula, which is the
+pleurapophysis of the occipital vertebra, is vertical on its first
+appearance in the embryo of tetrapoda, and lies close up to the head
+(_On the Nature of Limbs_, p. 49)--the embryo shows a greater
+resemblance to the archetype than the adult. "We perceive a return to
+it, as it were, in the early phases of development of the highest
+organised of the actually existing species, or we ought rather to say
+that development starts from the old point; and thus, in regard to the
+scapula, we can explain the constancy of its first appearance close to
+the head, whether in the human embryo or in that of the swan, also its
+vertical position to the axis of the spinal column, by its general
+homology as the rib or 'pleurapophysis' of the occipital
+vertebra" (_Limbs_, p. 56).
+
+We owe to Owen the first clear distinction between "homologous" and
+"analogous" organs; it was he who first proposed the terms "homologue"
+and "analogue," which he defined as follows:--"_Analogue_. A part or
+organ in one animal which has the same function as another part or organ
+in a different animal." "_Homologue_. The same organ in different
+animals under every variety of form and function."[165]
+
+He introduced also useful distinctions between Special, General, and
+Serial Homology. "The relations of homology," he writes, "are of three
+kinds: the first is that above defined, viz., the correspondency of a
+part or organ, determined by its relative position and connections, with
+a part or organ in a different animal; the determination of which
+homology indicates that such animals are constructed on a common type;
+when, for example, the correspondence of the basilar process of the
+human occipital bone with the distinct bone called 'basi-occipital' in a
+fish or crocodile is shown, the _special homology_ of that process is
+determined. A higher relation of homology is that in which a part or
+series of parts stands to the fundamental or general type, and its
+enunciation involves and implies a knowledge of the type on which a
+natural group of animals, the Vertebrate, for example, is constructed.
+Thus when the basilar process of the human occipital bone is determined
+to be the 'centrum' or 'body' of the last cranial vertebra, its _general
+homology_ is enunciated.
+
+"If it be admitted that the general type of the vertebrate endoskeleton
+is rightly represented by the idea of a series of essentially similar
+segments succeeding each other longitudinally from one end of the body
+to the other, such segments being for the most part composed of pieces
+similar in number and arrangement, and though sometimes extremely
+modified for special functions, yet never so as to wholly mask their
+typical character--then any given part of one segment may be repeated in
+the rest of the series, just as one bone may be reproduced in the
+skeletons of different species, and this kind of repetition or
+representative relation in the segments of the same skeleton I call
+'serial homology'" (p. 7). As an example of serial homology we might
+take the centra of the vertebræ--the vomer, the presphenoid, the
+basisphenoid, the basioccipital and the series of centra in the spinal
+column. Such serially repeated parts are called _homotypes_ (p. 8).
+
+Not all the bones of the vertebrate skeleton are included in the
+archetype as constituents of the vertebræ. Thus the branchial and
+pharyngeal arches are accounted part of the splanchnoskeleton, as
+belonging to the same category as the heart bone of some ruminants, and
+the ossicles of the stomach in the lobster (p. 70). The ossicles of the
+ear in mammals are "peculiar mammalian productions in relation to the
+exalted functions of a special organ of sense" (p. 140, f.n.). This
+recognition of a possible development of new organs to meet new
+functions shows unmistakably the influence of Cuvier. Owen was indeed
+well aware of the importance of the functional aspect of living things,
+and he often adopted the teleological point of view. As a true
+morphologist, however, he held that the principle of adaptation does not
+suffice to explain the existence of special homologies. The ossification
+of the bones of the skull from separate centres may be purposive in
+Eutheria, in that it prevents injury to the skull at birth; but how
+explain on teleological principles the similar ossification from
+separate centres in marsupials, birds and reptiles? How explain above
+all the fact that the centres are the same in number and relative
+position in all these groups? Surely we must accept the idea of an
+archetype "on which it has pleased the divine Architect to build up
+certain of his diversified living works" (p. 73).
+
+In his study of centres of ossification, Owen made in point of theory a
+distinct advance on his predecessors. We saw that Geoffroy recognised
+the importance of studying the ossification of the skeleton, and that
+Cuvier accepted such embryological evidence as an aid in determining
+homologies. Owen pointed out that it was necessary to distinguish
+between centres of ossification which were teleological in import and
+such as were purely indicative of homological relationships. Many bones,
+single in the adult, arise from separate centres of ossification, but we
+must distinguish between "those centres of ossification that have
+homological relations, and those that have only teleological ones;
+_i.e._, between the separate points of ossification of a human bone
+which typify vertebral elements, often permanently distinct bones in the
+lower animals; and the separate points which, without such
+signification, facilitate the progress of osteogeny, and have for their
+obvious final cause the well-being of the growing animal" (p. 105).
+There is, for example, a teleological reason why in mammals and leaping
+Amphibia (_e.g._, frogs), the long bones should ossify first at their
+ends, for the brain is thus protected from concussion; in reptiles that
+creep there is less danger of concussion, and the long bones ossify in
+the middle (p. 105). But there is no teleological reason why the
+coracoid process of the scapula should in all mammals develop from a
+separate centre. The coracoid is however a real vertebral element
+(hæmapophysis), and in monotremes, birds and reptiles it is in the adult
+a large and separate bone. Its ossification from a separate centre in
+mammals has therefore a homological significance. The scapula in mammals
+is an example of what Owen calls a "homologically compound" bone. All
+those bones which are formed by a coalescence of parts answering to
+distinct elements of the typical vertebra are "homologically compound"
+(p. 105). On the other hand, "All those bones which represent single
+vertebral elements are 'teleologically compound' when developed from
+more than one centre, whether such centres subsequently coalesce, or
+remain distinct, or even become the subject of individual adaptive
+modifications, with special joints, muscles, etc., for particular
+offices" (p. 106). The limb-skeleton, corresponding as it does to a
+single bone of the archetype, is the typical example of a teleologically
+compound bone. Owen in his definition of teleological compoundness has
+combined two kinds of adaptation--(1) temporary adaptation of bones to
+the exigencies of development, birth and growth (_e.g._, development of
+long bones from separate centres); (2) definitive adaptation of a
+skeletal part to the functions which it has to perform (_e.g._,
+teleological structure of limbs). Such adaptations are, so to speak,
+grafted on the archetype.
+
+Owen's general views on the nature of living things merit some
+attention. Organic forms, according to Owen, result from the
+antagonistic working of two principles, of which one brings about a
+vegetative repetition of structure, while the other, a teleological
+principle, shapes the living thing to its functions. The former
+principle is illustrated in the archetype of the vertebrate skeleton, in
+the segmentation of the Articulates, in the almost mathematical symmetry
+of Echinoderms, and the actually crystalline spicules of sponges. It is
+the same principle which causes repetition of the forms of crystals in
+the inorganic world. "The repetition of similar segments in a vertebral
+column, and of similar elements in a vertebral segment, is analogous to
+the repetition of similar crystals as the result of polarising force in
+the growth of an inorganic body" (p. 171). This "general polarising
+force" it is which mainly produces the similarity of forms, the
+repetition of parts, and generally the signs of the unity of
+organisation. The adaptive or "special organising force" or [Greek:
+idea], on the other hand, produces the diversity of organic beings. In
+every species these two forces are at work, and the extent to which the
+general polarising or "vegetative-repetition-force" is subdued by the
+teleological is an index of the grade of the species.
+
+This view is analogous to the Geoffroyan conception that the diversity
+of form is limited by the unity of plan. Owen thus ranges himself with
+Geoffroy against Cuvier, who considered that diversity of form is
+limited only by the principle of the adaptation of parts.
+
+    [164] Owen introduced most of the names of bones now
+    current.
+
+    [165] _Lectures on Invertebrate Animals_, pp. 374, 379,
+    1843.
+
+
+
+
+CHAPTER IX
+
+KARL ERNST VON BAER
+
+
+Von Baer was recognised as the founder of embryology even by his
+contemporaries. His predecessors, Aristotle,[166] Fabricius,[167]
+Harvey,[168] Malpighi,[169] Haller,[170] Wolff,[171] had made a
+beginning with the study of development; von Baer, by the thoroughness
+of his observation and the strength of his analysis, made embryology a
+science.
+
+It was to one of the German transcendentalists that von Baer owed the
+impulse to study development. Ignatius Döllinger, Professor in Würzburg,
+induced three of his pupils, Pander, d'Alton and von Baer, to devote
+themselves to embryological research. The development of animals was at
+this time little known, in spite of recent work by Meckel (1815 and
+1817), Tiedemann (_Anatomie u. Bildungsgeschichte des Gehirns_, 1816),
+by Oken (_loc. cit., supra_, p. 90), and some others.
+
+Pander, with whom apparently Döllinger and d'Alton collaborated, was the
+first to publish his results;[172] von Baer, who through absence from
+Würzburg had for a time dropped his embryological studies, started to
+work in 1819, after the publication of Pander's treatise, and produced
+in 1828 the first volume of his master-work, _Ueber
+Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion_
+(Königsberg, 1828). The second volume followed in 1837, but dates really
+from 1834, and was published in an incomplete form. This second volume
+is intended as an introduction to embryology for the use of doctors and
+science students. In it von Baer describes in full detail the
+development of many vertebrate types--chick, tortoise, snake, lizard,
+frog, fish, several mammals and man, basing his remarks largely upon his
+personal observations, but taking account also of all contemporary work.
+A separate account of the development of a fish (_Cyprinus blicca_)
+appeared in 1835.[173]
+
+We shall concentrate attention on the first volume. This volume contains
+the first full and adequate account of the development of the chick,
+followed by a masterly discussion of the laws of development in general.
+
+When we consider that von Baer worked chiefly with a simple microscope
+and dissecting needles, the minuteness and accuracy of his observations
+are astonishing. He described the main facts respecting the development
+of all the principal organs, and if, through lack of the proper means of
+observation, he erred in detail, he made up for it by his masterly
+understanding and profound analysis of the essential nature of
+development. His account of the development of the chick is a model of
+what a scientific memoir ought to be; the series of "Scholia" which
+follow contain the deductions he made from the data, and, in so far as
+they are direct generalisations from experience, they are valid for all
+time.
+
+The first Scholion is directed against the theory of preformation, and
+succeeds in refuting it on the ground of simple observation. The theme
+of the second Scholion is that the essential nature (_die Wesenheit_) of
+the animal determines its differentiation, that no stage of development
+is solely determined by the antecedent stage, but that throughout all
+stages the _Wesenheit_ or idea of the definitive whole exercises
+guidance. This guidance is shown most clearly in the regulatory
+processes of the germ, whereby the large individual variations commonly
+presented by the early embryo are compensated for or neutralised in the
+course of further development. Baer in this shows himself a vitalist.
+
+It is, however, the third and subsequent Scholia which must here
+particularly occupy our attention, for it is in these that von Baer
+comes to grips with morphological problems. Already in the second
+Scholion he had definitely enunciated the law which runs as a theme
+throughout the volume, the observational and the theoretical part alike,
+the law that development is essentially a process of differentiation by
+which the germ becomes ever more and more individualised. "The essential
+result of development," he writes, "when we consider it as a whole, is
+the increasing independence (_Selbständigkeit_) of the developing
+animal" (p. 148). In the third Scholion he elaborates this thought and
+shows that differentiation takes place in triple wise. The three
+processes of differentiation are "primary differentiation" or
+layer-formation, "histological differentiation" within the layers, and
+the "morphological differentiation" of primitive organs.
+
+The first of these differentiations in time is the formation of the
+germ-layers, which takes place by a splitting or separation of the
+blastoderm into a series of superimposed lamellæ. Baer's account of the
+process in the chick is as follows:--
+
+"First of all, the germ separates out into heterogeneous layers, which
+with advancing development acquire ever greater individuality, but even
+on their first appearance show rudiments of the structures which will
+characterise them later. Thus in the germ of the bird, so soon as it
+acquires consistency at the beginning of incubation, we can distinguish
+an upper smooth continuous surface and a lower more granular surface.
+The blastoderm separates thereupon into two distinct layers, of which
+the lower develops into the plastic body-parts of the embryo, the upper
+into the animal parts; the lower shows clearly a further division into
+two closely connected subsidiary layers--the mucous layer and the
+vessel-layer; the original upper layer also shows a division into two,
+which form respectively the skin and the parts which I have called the
+true ventral and dorsal plates--parts which contain in an
+undifferentiated state the skeletal and muscular systems, the connective
+tissues, and the nerves belonging to these. In order to have a
+convenient term for future use, I have named this layer the
+muscle-layer" (p. 153).
+
+The process of delamination results then in the formation of four
+layers, of which the upper two (composing the "animal" or "serous"
+layer) will give origin to the animal (neuromuscular) part of the body,
+the lower pair to the plastic or vegetative organs. The uppermost layer
+will form the external covering of the embryo, and also the amniotic
+folds; from it there differentiates out at a very early stage the
+rudiment of the central nervous system, forming a more or less
+independent layer. Below the outermost layer lies the layer from which
+are formed the muscular and skeletal systems, and beneath this
+"muscle-layer" comes the "vessel-layer," which gives origin to the main
+blood-vessels. The innermost layer of the four will form the mucous
+membrane of the alimentary canal and its dependencies; at the present
+stage, however, it is, like the other layers, a flat plate.
+
+From all these layers tubes are developed by the simple bending round of
+their edges. The outermost layer becomes the investing skin-tube of the
+embryo; the layer for the nervous system forms the tubular rudiment of
+the brain and spinal cord; the mucous layer curls round to form the
+alimentary tube; the muscle layer grows upwards and downwards to form
+the fleshy and osseous tube of the body wall; even the vessel layer
+forms a tube investing the alimentary canal, but a part of it goes to
+form the medial "Gekröse," or mesenterial complex, which departs
+considerably from the tubular form.
+
+When these tubes or "fundamental organs" are formed the process of
+primary differentiation is complete. The fundamental organs, however,
+have at no time actually the form of tubes; they exist as tubes only
+ideally, for morphological and histological differentiation go on
+concurrently with the process of primary differentiation.
+
+Through morphological differentiation the various parts of the
+fundamental organs become specialised, through unequal growth, first
+into the primitive organs and then into the functional organs of the
+body. "Single sections of the tubes originally formed from the layers
+develop individual forms, which later acquire special functions: these
+functions are in the most general way subordinate elements of the
+function of the whole tube, but yet differ from the functions of other
+sections. Thus the nerve-tube differentiates into sense-organs, brain
+and spinal cord, the alimentary tube into mouth cavity, oesophagus,
+stomach, intestine, respiratory apparatus, liver, bladder, etc. This
+specialisation in development is bound up with increased or diminished
+growth" (p. 155). Rapid growth concentrated at one point brings about an
+evagination; in this manner are formed the sense-organs from the
+nerve-tube, the liver and lungs from the alimentary tube. Or increased
+growth over a section of a tube causes it to swell out; in this wise the
+brain develops from the nerve-tube, the stomach from the alimentary
+tube. The segmentation which soon becomes so marked, particularly in the
+muscle layer, is also due to a process of morphological differentiation.
+
+At the same time that the organs of the body are being thus roughly
+blocked out and moulded from the germ-layers the third process of
+differentiation is actively going on. "In addition to the
+differentiation of the layers, there follows later another
+differentiation in the substance of the layers, whereby cartilage,
+muscle and nerve separate out, a part also of the mass becoming fluid
+and entering the bloodstream" (p. 154). Through histological
+differentiation the texture of the layers and incipient organs becomes
+individualised. In its earliest appearance the germ consists of an
+almost homogeneous mass, containing clear or dark globules suspended in
+its substance (ii., p. 92). This homogeneity gives place to
+heterogeneity; the structureless mass becomes fibrous to form muscles,
+hardens to form cartilage or bone, becomes liquid to form the blood,
+differentiates in a hundred other ways--into absorbing and secreting
+tissues, into nerves and ganglia, and so forth. It will be noticed that
+the concept of histological differentiation is independent of the
+cell-theory; it signifies that textural differentiation which leads to
+the formation of tissues in Bichat's sense. The tissues and the
+germ-layers stand in fairly close relation with one another, for while
+certain tissues are formed chiefly but not exclusively in one layer,
+others are formed only in one layer and never elsewhere. For example,
+peripheral nerves are for the most part formed in the muscle layer,
+though the bulk of the nervous tissue is formed in the walls of the
+nerve tube; similarly blood and blood-vessels may arise from almost any
+layer, though their chief seat of origin is the vessel-layer; on the
+other hand, bone is formed only in the muscle-layer (i., p. 155, ii.,
+pp. 92-3).
+
+This relation of tissue to germ-layer was more fully discussed and
+brought into greater prominence by Remak, from the standpoint of the
+cell-theory, and it will occupy us in a later chapter (Chap. XII.).
+
+The fourth Scholion elaborates the analysis of developmental processes
+still further, and discusses in particular the scheme of development
+which is shown by the Vertebrata. The characteristic structure of the
+vertebrate body is brought about by a "double symmetrical" rolling
+together of the germ-layers, whereby two main tubes are formed, one
+above and one below the axis of the body, which is the chorda. The
+dorsal tube is formed by the two animal layers, the ventral tube by all
+the layers combined (see Fig. 7).
+
+The process is indicated with sufficient clearness in the diagram. It
+will be seen that the real foundation and framework of the arrangement
+is the muscle-layer, with its two tubes, one surrounding the central
+nervous system and forming the "dorsal plates," the other surrounding
+the body cavity and forming the "ventral plates." In the dorsal plates,
+which early show metameric segmentation, the investing skeleton of the
+neural axis develops; in the ventral plates are formed the ribs, the
+ventral arches of the vertebræ, the hyoid, the lower jaw and other
+skeletal structures.
+
+The alimentary or "mucous" tube and the part of the vessel layer which
+invests it become so closely bound up with one another as to form a
+single primitive organ--the alimentary canal. The muscles of the
+alimentary canal are accordingly in all probability developed in the
+investing part of the vessel layer. From the "Gekröse," or remaining
+part of the vessel layer develop the Wolffian bodies (_Urnieren_,
+Pronephros), the kidneys, the sex glands, and the series of
+"blood-glands"--suprarenals, thyroid, thymus and spleen. Baer did not
+attach any special morphological significance to the peritoneal lining
+of the body cavity, as is done in more modern forms of the germ-layer
+theory. The gill-slits were largely formed by outgrowths from the
+alimentary canal.
+
+_a._ Chorda.
+_b._ Dorsal plates.
+_c._ Ventral plates.
+_d._ Spinal cord.
+_e._ Vessel-layer.
+_f._ Alimentary tube.
+_g._ Pronephros.
+_h._ Skin.
+_i._ Amnion.
+_k._ Serous membrane.
+_l._ Yolk-sac.
+
+In his germ-layer theory von Baer was influenced a good deal by
+Pander, to whom the actual discovery of the process of layer-formation
+is due. Pander, however, had distinguished only three germ-layers, an
+upper "serous" layer, a lower "mucous" layer and a middle
+"vessel-layer." He it was who introduced the terms "Keimhaut"
+(blastoderm) and "Keimblatt" (germ-layer).
+
+[Illustration: FIG. 7.--Ideal Transverse Section of a Vertebrate Embryo.
+(After von Baer.)]
+
+The honour of being the founder of the germ-layer theory is sometimes
+attributed to C. F. Wolff, notably by Kölliker and O. Hertwig. Wolff, it
+is true, in his memoir _De formatione intestinorum_ (1768-9) showed that
+the alimentary canal was first formed as a flat plate which folded round
+to form a tube, and in a somewhat vaguely worded passage he hinted that
+a similar mode of origin might be found to hold good for the other
+organ-systems. But it seems clear that Wolff had no definite conception
+of the process of layer-formation as the first and necessary step in all
+differentiation. This, at any rate, was von Baer's opinion, who assigns
+to Pander the glory of the discovery of the germ-layers. "You," he
+writes, "through your clearer recognition of the splitting of the
+germ--a process which remained dark to Wolff--have shed a light upon all
+forms of development" (p. xxi.).
+
+We have now seen, following von Baer's exposition, how development is
+essentially a process of differentiation, a progress from the general to
+the special, from the homogeneous to the heterogeneous; we have analysed
+the process into its three subordinate processes--primary, histological
+and morphological differentiation. So far we have considered development
+in general and the laws which govern it; we have now to consider the
+varieties of development which the animal kingdom offers in such
+profusion, in order to discover what relations exist between them. This
+is the problem set in the fifth Scholion. Baer at once brings us face to
+face with the solution of the problem attempted in the Meckel-Serres
+law. It is a generally received opinion, he writes, that the higher
+animals repeat in their development the adult stages of the lower, and
+this is held to be the essential law governing the relation of the
+variety of development to the variety of adult form. This opinion arose
+when there was little real knowledge of embryology; it threw light
+indeed upon certain cases of monstrous development, but it was pushed
+altogether too far. It complicated itself with a belief in a historical
+evolution;--"People gradually learnt to think of the different animal
+forms as developed one from another--and seemed, in some circles at
+least, determined to forget that this metamorphosis could only be
+conceptual" (p. 200). At the same time the theory of parallelism led men
+to rehabilitate the outworn conception of the scale of beings, to
+maintain that animals form one single series of increasing complexity, a
+scale which the higher members must mount step by step in their
+development--from which it followed that evolution, whether conceived as
+an ideal or as an historical process, could take place only along one
+line, could be only progressive or regressive. Not all the supporters of
+the theory of parallelism held these extreme views, but conclusions of
+this kind were natural and logical enough.
+
+Von Baer had soon found in the course of his embryological studies that
+the facts did not at all fit in with the doctrine of parallelism; the
+developing chick, for example, was at a very early stage demonstrably a
+Vertebrate, and did not recapitulate in its early stages the
+organisation of a polyp, a worm or a mollusc. He had published his
+doubts in 1823, but his final confutation of the theory of parallelism
+is found in this Scholion.
+
+If it were true, he says, that the essential thing in the development of
+an animal is this repetition of lower organisations, then certain
+deductions could be drawn, which one would expect to find confirmed in
+Nature. The first deduction would be that no structures should appear in
+the embryo of the higher animals that are not found in the lower
+animals. But this is not confirmed by the facts--no adult among the
+lower animals, for instance, has a yolk-sac like that of the chick
+embryo. Again, if the law of parallelism were true, the mammalian embryo
+would have to repeat the organisation of, among other groups, insects
+and birds. But the embryo _in utero_ is surrounded by fluid and cannot
+possibly breathe free air, so it cannot possibly repeat the structure of
+either insects or birds, which are pre-eminently air-organisms.
+Generally speaking, indeed, we find in all the higher embryos special
+structures which adapt them to the very special conditions of their
+development, and these we never find as permanent structures in the
+lower animals. The supporters of the theory of parallelism might,
+however, admit the existence of such special embryonic organs without
+greatly prejudicing their case, for these temporary organs stand to some
+extent outside the scope of the theory.
+
+But they would have to face a second and more important deduction from
+their views, namely, that the higher animals should repeat at every
+stage of their development the whole organisation of some lower animal,
+and not merely agree with them in isolated details of structure. The
+deduction is, however, not borne out by the facts. The embryo of a
+mammal resembles in many points, at different stages of its development,
+the adult state of a fish; it has gill-slits and complete aortic arches,
+a two-chambered heart, and so on. But at no time does it combine all the
+essential characters of a fish; nor has it ever the tail of a fish, nor
+the fins, nor the shape. Any recapitulation there may be is a
+recapitulation of single organs, there is never a repetition of the
+complete organisation of a fish. This is indeed the fundamental
+criticism of the theory of parallelism; and if it applies even within
+the limits of the vertebrate phylum, so much the more does it apply to
+comparisons between embryonic Vertebrates and adult Invertebrates.
+
+There are also some lesser arguments which might be urged against the
+theory of parallelism. If the theory were strictly true, no state which
+is permanent in a higher animal could be passed through by an animal
+lower in the scale. But birds, which are lower in the scale than
+mammals, pass through a stage in which they resemble mammals in certain
+respects much more than they do when adult, for in an embryonic
+condition they agree with mammals in having no feathers, no air sacs, no
+pneumatic sacs in the bones, no beak. Their brain also resembles that of
+mammals more in an earlier stage than it does later. So, too, myriapods
+and hydrachnids have at birth three pairs of feet, and resemble at this
+stage adult insects, which form a higher class.
+
+Again, were the analogy between the development of the individual and
+the evolution of the _Échelle des êtres_ complete, organs and
+organ-systems ought to develop in the individual in the order in which
+they appear in the scale of beings. But this is not always the case. In
+fish the hinder extremity develops only its terminal joint, while in the
+embryos of higher animals the basal joint is the first to appear.
+
+Another consequence one would expect to find realised, were the theory
+of parallelism correct, is the late appearance in development of parts
+which are confined to the higher animals. In the development of a
+Vertebrate accordingly one would not expect the vertebræ to appear
+before the embryo had passed through many Invertebrate stages. But
+experience shows the direct contrary, for in the chick the rudiments of
+the vertebral axis appear sooner than any other part.
+
+The theory of parallelism or recapitulation then is not borne out by the
+facts, and clearly cannot be the law which we are seeking. But what then
+is the true relation between the variety of development and the variety
+of adult structure? Before answering this question we must review the
+varied forms of adult organisation and consider in what relations they
+stand to one another. In particular we must enquire whether they belong
+to one type or to many. One point is here cardinal--we must distinguish
+between the _type_ of organisation and the _grade_ of differentiation.
+By "type" von Baer means the structural plan of the organism. "I call
+the _type_ the spatial relationship of the organic elements and organs"
+(p. 208). Each type of organisation characterises one of the big groups
+of animals; the lesser groups represent "grade" modifications of the
+type. "The product of the degree of differentiation and the type gives
+the several great groups of animals which are called classes" (p. 208).
+_Ausbildung_ (differentiation) takes place in one or other of several
+directions, in adaptation, for instance, to life in the water or to life
+in the air.
+
+There are, von Baer considers, four main types--(1) the peripheral or
+radiate type, (2) the longitudinal type, (3) the massive or molluscan
+type, (4) the vertebrate type. The radiate type is shown by discoid
+infusoria, by medusæ, by starfish and their allies. The longitudinal
+type characterises such genera as _Vibrio_, _Filaria_, _Gordius_, and
+all the annulate animals. Mollusca, rotifers, polyzoa, and such
+infusoria as are not included in types (1) and (2) belong to the massive
+type, in which the body and its parts form rounded masses. The
+longitudinal type is predominantly "animal," the massive type
+predominantly "plastic" (vegetative). The vertebrate type has both the
+"animal" and the "plastic" organs highly developed. In the symmetrical
+arrangement of the animal parts it resembles the longitudinal type; its
+plastic parts with their asymmetrical arrangement and rounded shape
+belong to the massive type.
+
+These types of von Baer inevitably recall the "Embranchements" of
+Cuvier, with which they more or less coincide. It seems that von Baer
+arrived at his types (from the study of adult structure) independently
+of Cuvier, though the priority of publication rests with Cuvier.[174]
+
+Now it is clear that the development of the individual, which is
+essentially an _Ausbildung_, a differentiation, is directly comparable
+with the grade-differentiation of forms within the type. And just as the
+type rules all its varied modifications, so does the development of the
+individual take place always within the bounds imposed by type. This is
+von Baer's chief contribution to the theory of embryonic
+relationships--the law that "the type of organisation determines the
+manner of development" (p. xxii.). Development is not merely from the
+general to the special--there are at least four distinct "general"
+types, from which the special is developed. The type is fixed in the
+very earliest stages of development--the embryo of a Vertebrate is from
+the very beginning a Vertebrate (p. 220), and it shows at no time any
+agreement in total organisation with any Invertebrate. The types are
+independent of one another; differentiation and development follow a
+different course in each of them. Not but what some analogies can be
+found between the very earliest stages of embryos of different type.
+Thus vertebrate and annulate embryos agree in certain points at the time
+of the formation of the primitive streak. And in the earliest stage of
+all, the egg-stage, there is probably agreement between all the types.
+In eggs with yolk, whether vertebrate or annulate, there is always a
+separation into an animal and a plastic layer. It seems, too, as if a
+hollow sphere were a constant stage in the development of all animals
+(pp. 224, 258). Apart from these analogies, development takes an
+entirely independent course in each of the four main types, and no
+embryo of one of the higher types repeats in its development the
+peculiar organisation of any adult of the lower types.
+
+If we consider now development within the type, which is the only
+legitimate thing to do, we arrive at certain laws governing the relation
+of embryos to one another. For instance, at a certain stage vertebrate
+embryos are uncommonly alike. Von Baer had two in spirit which he was
+unable to assign to their class among amniotes; they might have been
+lizard, bird, or mammal, he could not say definitely which.[175] Generally
+the farther back we go in the development of Vertebrates the more alike
+we find the embryos. The type-characters are first to appear, then the
+class characters, then the characters distinguishing the lesser
+classificatory groups. "From a more general type the special gradually
+emerges" (p. 221). The chick is first a Vertebrate, then a
+land-vertebrate, then a bird, then a land-bird, then a gallinaceous
+bird, and finally _Gallus domesticus_. Development within the type is a
+progress from the general to the special, a real evolution. The more
+divergent two adults are, the farther back we must go in their
+development to find an agreement between their embryos. We can sum up
+the case in the following laws:--
+
+"(1) _That the general characters of the big group to which the embryo
+belongs appear in development earlier than the special characters._ In
+agreement with this is the fact that the vesicular form is the most
+general form of all; for what is common in a greater degree to all
+animals than the opposition of an internal and an external surface?
+
+"(2) _The less general structural relations are formed after the more
+general, and so on until the most special appear._
+
+"(3) _The embryo of any given form, instead of passing through the state
+of other definite forms, on the contrary separates itself from them._
+
+"(4) _Fundamentally the embryo of a higher animal form never resembles
+the adult of another animal form, but only its embryo_" (p. 224).
+
+These laws relating to development within the limits of type are
+destructive of even a limited application of the theory of parallelism,
+for not even within the limits of the type is there a real scale which
+the higher forms must mount; each embryo develops for itself, and
+diverges sooner or later from the embryos of other species, the
+divergence coming earlier the greater the difference between the adult
+forms. It is only because the lower less-differentiated adult forms
+happen to be little divergent from the generalised or embryonic type,
+that they show a certain similarity with the embryos of the higher more
+differentiated members of the group. Such similarity, however, is due to
+no necessary law governing the development of the higher animals; it is,
+on the contrary, merely a consequence of the organisation of these lower
+animals (p. 224).
+
+Von Baer goes on to show what are the distinguishing embryological
+characters of the types and classes, working out a dichotomous schema of
+development, which each embryo must follow, branching off early or late
+to its terminal point, according to the lower or higher goal it has to
+reach.
+
+One important consequence for morphology results from von Baer's laws of
+differentiation within the type. If the embryo develops from the general
+to the special, then the state in which each organ or organ-system first
+appears must represent the general or typical state of that organ within
+the group. Embryology will therefore be of great assistance to
+comparative anatomy, whose chief aim it is to discover the generalised
+type, the common plan of structure, upon which the animals of each big
+group are built. And the surest way to determine the true homologies of
+parts will be to study their early development. "For since each organ
+becomes what it is only through the manner of its development, its true
+value can be recognised only from its method of formation. At present,
+we form our judgments by an undefined intuition, instead of regarding
+each organ merely as an isolated product of its fundamental organ, and
+discerning from this standpoint the correspondences and dissimilarities
+in the different types" (p. 233). Parts, therefore, which develop from
+the same "fundamental organ," and in the last resort from the same
+germ-layer, have a certain kinship, which may even reach the degree of
+exact homology.
+
+Now since the mode of development in each type is peculiar to that type,
+organs of the same name in different types must not necessarily be
+accounted homologous, even if they correspond exactly with one another
+in their general _functional_ relations to the rest of the organs. Thus
+the central nervous system of Arthropods must not be homologised with
+the central nervous system of Vertebrates, for it develops in a
+different manner. So, too, the brain of Arthropods or of Mollusca is not
+strictly comparable with the brain of Vertebrates. Again, the air-tubes
+or tracheæ of insects are, like the trachea and bronchi of many
+Vertebrates, air-breathing organs. But the two organs are not
+homologous, for the air-tubes of Vertebrates are developed from the
+alimentary tube ("fundamental organ" of the alimentary system, developed
+from the vegetative layer), while the air-tubes of insects arise either
+by histological differentiation, or by invagination of the skin (p.
+236). Organs can be homologous only within the limits of the big groups;
+there can be no question of homology between members of different types.
+
+The development of plants, like the development of animals, is
+essentially a progress from the general to the special (p. 242).
+Botanists have not been troubled by any recapitulation theory, and in
+founding their big groups, Acotyledons, Monocotyledons, and
+Dicotyledons, upon embryological characters, they were guided by true
+principles, which ought indeed to be followed in zoology. If we knew the
+development of all kinds of animals sufficiently well, then the best way
+to classify them would be according to the characters they show in their
+early development, for it is in early development that they show the
+characters of the type in their most generalised form. As it is, we have
+in our ignorance to establish the big groups by the study of adult
+structure, but we find, on putting together all we know of comparative
+embryology, that a classification of animals according to the mode of
+their development gives, as is only natural, the same four groups as
+does the study of adult structure. The four types of development are
+thus:--
+
+(1) The double-symmetrical, which is found in Vertebrates. It is called
+the double-symmetrical, because in Vertebrates development takes place
+from a central axis (notochord) in two directions, upwards and
+downwards, in such a way that two tubes are formed, one above and one
+below the axis. (2) The second type is the symmetrical, which is shown
+by Annulates. A primitive streak is formed on the ventral surface of the
+yolk; development proceeds symmetrically on both sides of the streak.
+(3) Radiate development is probably typical of the radiate structural
+type. (4) In the massive type, the development seems to be a spiral one.
+
+Common to most modes is a separation of the germ into animal and plastic
+layers, a separation which seems to be conditioned largely by the
+presence of yolk. A classification based upon embryological characters
+ought to be applied even to the lesser groups and would here prove
+itself of service. Embryology, for instance, fully supports de
+Blainville's separation of Batrachia from true reptiles,[176] for reptiles
+develop an amnion and Batrachia do not.
+
+We come now to the sixth and last Scholion. Development is a true
+evolution of the special from the general, so runs von Baer's most
+general law of all. This can be expressed in a slightly different way,
+and the words which he chooses in the sixth Scholion to express this
+final and most general result are these:--"The developmental history of
+the individual is the history of the growing individuality in every
+respect" (p. 263). The greatest modern treatise on embryology ends on a
+splendid note. One creative thought rules all the forms of life. And
+more--"It is this same thought that in cosmic space gathered the
+scattered masses into spheres and bound them together in the solar
+system, the same that from the weathered dust on the surface of the
+metallic planets brought forth the forms of life. And this thought is
+nought else but life itself, and the words and syllables in which life
+expresses itself are the varied forms of the living" (p. 264).
+
+Von Baer reminds one greatly of Cuvier. There is the same sheer
+intellectual power, the same sanity of mind, the same synthetic grip.
+Von Baer, like Cuvier, never forgot that he was working with living
+things; he was saturated, like Cuvier, with the sense of their
+functional adaptedness. In his paper on the external and internal
+skeleton[177] he gives a masterly analysis of the functional modifications
+of the limbs in Vertebrates, and the whole paper indeed, with its sober
+attack on transcendentalism, is a vindication as much of the functional
+point of view as of the importance of embryology.
+
+Both Cuvier and von Baer, by the very sanity of their views, found
+themselves in partial opposition to the theories current in their time.
+Cuvier was the critic of Geoffroy and the transcendentalists, of Lamarck
+and the believers in the _Échelle des êtres_, evolutionary or ideal. Von
+Baer also, though influenced greatly by _Naturphilosophie_, turned
+against the exaggerations of the transcendental school, and by his
+unanswerable criticism of the theory of parallelism took away the ground
+from those who too easily believed in an historical evolution.[178]
+
+We have seen what were von Baer's criticisms of the theory of
+parallelism. If we turn to the later writings of Cuvier we find the
+essential criticism expressed in similar terms. Speaking of an attempt
+which had been made to show that fish were molluscs developed to a
+higher degree, he wrote in 1828,[179] "Let us draw the conclusion that
+even if these animals can be spoken of as ennobled molluscs, as molluscs
+raised to a higher power, or if they are embryos of reptiles, the
+beginnings of reptiles, this can be true of them only in an abstract and
+metaphysical sense, and that even this abstract statement would be very
+far from giving an accurate idea of their organisation." From the fact
+that the respiratory and circulatory organs of fish greatly resemble
+those of tadpoles the conclusion has been drawn that fish are in a sense
+embryos of Amphibia (p. 547). But this manner of viewing things is none
+the less vicious, "for this reason ... that it considers only one or two
+points and neglects all the others" (p. 548), and is directly contrary
+to common sense. There is never a recapitulation of total organisations,
+only at the most of single organs.
+
+It will be remembered that Cuvier opposed and demolished the theory of
+the _Échelle des êtres_, not only by showing that there were in Nature
+four entirely different plans of animal structure, but also by
+demonstrating that even the animals of each single _Embranchement_ could
+not readily be arranged in one series, that a serial arrangement was
+really valid only for their separate organs. Von Baer also held that
+there are four distinct types of structure; he, too, combated the idea
+of gradation within the limits of the type. In so far as species
+represent successive stages in the development, the _Ausbildung_, of the
+type, so far can the idea of a scale of beings be applied. But the
+members of a type follow not one line of evolution but several diverging
+lines, in direct adaptation to different environmental conditions, so
+that a serial arrangement of them is not as a rule possible. It may be
+possible to establish a serial arrangement of single organs from the
+simplest to the most complex. But each organ or organ-system will
+require a different serial arrangement, for the different systems vary
+on different lines and an animal may be highly developed in respect of
+one system and little developed in respect of all the others. Man, for
+instance, is the highest animal only in respect of his nervous system.
+The idea of the scale of beings has therefore only a very limited
+application even within the limits of the type. Applied to the whole
+animal kingdom it becomes merely absurd.
+
+Another point of resemblance between Cuvier and von Baer was that
+Cuvier, though essentially a student of adult structure, did recognise
+the importance of embryology; following up some observations of
+Dutrochet he studied the foetal membrane of mammals and tried to
+establish their homologies.[180] And in his criticism of the vertebral
+theory of the skull he advanced as an argument against the
+basisphenoid being a vertebral centrum the fact (established
+by Kerkring, 1670), that it develops from two centres.[181] Von Baer's
+relation to transcendental anatomy was in some ways a close one, though
+he was a trenchant critic of the extreme views of the school.[182] He took
+from Oken the idea that a simple fundamental plan rules the organisation
+of all Vertebrates; "That jaws and limbs are modifications of one
+fundamental form is readily apparent, and, after Oken, the fact ought to
+be accepted by the majority of those naturalists who do not refuse to
+admit the existence of a general type from which the diversity of
+structure is developed" (i., p. 192). He accepted the vertebral theory
+of the skull in its main lines, and used his embryological knowledge to
+support the idea that jaws correspond to limbs--the latter point as part
+of the transcendental idea that the hind end of the body repeats the
+organisation of the anterior part (i., p. 192). The particular form
+which his theory of the relation of jaws to limbs took is shown in the
+following passage:--"The maxillary bone has ... the significance of an
+extremity and at the same time that of a rib or lower arch of a
+vertebra, just as the pelvic bones unite in themselves the signification
+of ribs and proximal members of the hinder extremity" (Meckel's
+_Archiv_, p. 367, 1826).
+
+He appreciated the morphological idea of the serial repetition of parts,
+and gave it accurate formulation. The whole vertebrate body, he
+considered, was composed of a longitudinal series of _morphological
+elements_, each of which was made up a section from each of the
+fundamental organs--a vertebra, a section of the nerve-cord, and so on
+(_Entwickelungsgeschichte_, ii., p. 53). Groups of these morphological
+elements formed _morphological divisions_, such as the vertebral
+segments of the head with their highly developed neural arches, or the
+segments of the neck with their undeveloped hæmal arches. The
+morphological elements are clearly shown only in the animal parts, but
+there are indications in the embryo of a segmentation also of the
+vegetative parts,--the gill-slits, for instance, and the vascular
+arches. The vegetative parts, however, develop on the whole
+unsymmetrically (_cf._ Bichat). These elements which von Baer
+distinguishes are morphological units, as he himself points out,
+contrasting them with organs which are not usually units in a
+morphological sense. "We call organ," he writes, "each part that has by
+reason of its form or its function a certain distinctiveness, but this
+concept is very indefinite, and possesses, from a morphological point of
+view, little value. For this reason it seems necessary to introduce into
+scientific morphology the concepts of morphological elements and
+divisions" (ii., p. 84).
+
+Von Baer exercised a very considerable influence upon the subsequent
+trend of morphological theory. By his criticism of the Meckel-Serres
+theory, he rid morphology for a time of an idea which was leading it
+astray; by his substitution of the law that development is always from
+the general to the special, he set morphologists looking for the
+archetype in the embryo, not in the adult alone, and made them realise
+that homologies could often best be sought in the earliest stages of
+development; by formulating the germ-layer theory he supplied
+morphologists with a new criterion of homology, based upon the special
+relations of the parts (germ-layers) which are first differentiated in
+all development. He made the study of development an essential part of
+morphology.
+
+    [166] _De generatione Animalium_.
+
+    [167] _De formato foetu_, ? 1600; _De formatione
+    foetus_, 1604.
+
+    [168] _Exercitationes de generatione animalium_, 1651.
+
+    [169] _De formatione pulli in ovo_, 1673; _De ovo
+    incubato_, 1686.
+
+    [170] _De formatione pulli in ovo_, 1757-8; _Sur la
+    formation du coeur dans le poulet_, 1758.
+
+    [171] _Theoria generatioinis_, 1759; _De formatione
+    intestinorum_, 1768-9.
+
+    [172] _Beiträge zur Entwickelung des Hühnchens im Ei._
+    Würzburg, 1818. Also in Latin in shorter form, 1817.
+
+    [173] _Untersuchungen ü. die Entwickelungsgeschichte der
+    Fische_; Leipzig, 1835.
+
+    [174] Cuvier, in 1812, _Ann. Mus. d'Hist. Nat._, xix.; von
+    Baer in 1816, _Nova Acta Acad. Nat. Cur._ See
+    _Entwickelungsgeschichte der Thiere_, i., p. vii., f.n.
+
+    [175] Compare a parallel passage in Prévost et Dumas:--"At
+    the very first sight one will be struck with the
+    resemblance between the forms of the very early embryos
+    of these two classes, a resemblance so extraordinary
+    that one cannot refuse to admit the conclusions
+    resulting from it. The resemblance is so striking that
+    one can defy the most experienced observer to
+    distinguish in any way the embryos of dog or rabbit ...
+    from those of fowls or ducks of a corresponding
+    age."--_Ann. Sci. nat._, iii., p. 132, 1824.
+
+    [176] _De l'organisation des Animaux_, i., p. 140, 1822.
+
+    [177] "Ueber das äussere und innere Skelet," Meckel's
+    _Archiv für Anat. u. Physiol._, pp. 327-76, 1826. See,
+    too, his _Entwickelungsgeschichte_, i., pp. 181, ff.
+
+    [178] Von Baer wrote an appreciative biography of Cuvier,
+    published posthumously in 1897, _Lebensgeschichte
+    Cuviers_, ed. L. Stieda. French trans. in _Ann. Sci.
+    Nat._ (_Zool._), ix., 1907.
+
+    [179] Cuvier et Valenciennes, _Histoire naturelle des
+    Poissons_, i., p. 550.
+
+    [180] _Mém. Mus. d'Hist. Nat._, iii., pp. 98-119, 1817.
+
+    [181] _Leçons d'Anatomie comparée_, 3rd ed., vol. i., p.
+    414, Bruxelles, 1836.
+
+    [182] In the aforementioned paper in Müller's _Archiv_ he
+    criticises Carus vigorously and is sarcastic on
+    Geoffroy.
+
+
+
+
+CHAPTER X
+
+THE EMBRYOLOGICAL CRITERION
+
+
+Pander's work of 1817 was the forerunner of an embryological period in
+which men's hopes and interest centred round the study of development.
+"With bewilderment we saw ourselves transported to the strange soil of a
+new world," wrote Pander, and many shared his hopeful enthusiasm. K. E.
+von Baer's _Entwickelungsgeschichte_ was by far the greatest product of
+this time, but it stands in a measure apart; we have in this chapter to
+consider the lesser men who were Baer's contemporaries, friends,
+followers or critics.
+
+It was largely a German science, this new embryology, and its leaders
+were all personally acquainted. Pander, von Baer and Rathke were on
+friendly terms with one another; von Baer dedicated his master-work to
+Pander; Rathke dedicated the second volume of his _Abhandlungen_ to von
+Baer. Interest in the new science was, however, not confined to Germany.
+In Italy, Rusconi commenced in 1817 his pioneer researches on the
+development of the Amphibia with a _Descrizione anatomica degli organi
+della circolazione delle larve delle Salamandre aquatiche_ (Pavia), in
+which he traced the metamorphoses of the aortic arches. This was
+followed in 1822 by his _Amours des Salamandres aquatiques_ (Milan), and
+in 1826 by his memoir _Du développement de la grenouille_ (Milan). In
+this last paper he described how the dark upper hemisphere of the frog's
+egg grows down over the lower white hemisphere and leaves free only the
+yolk plug; he observed the segmentation cavity and the archenteron, but
+thought that the former became the alimentary canal; he observed and
+interpreted rightly the formation of the medullary folds. The circular
+blastopore in the frog in later years often went by the name of the anus
+of Rusconi.
+
+In France Dutrochet[183] investigated the foetal membranes in various
+vertebrate classes; Prévost and Dumas studied the very earliest stages
+of development in birds, mammals and amphibia (_Ann. Sci. nat._, ii.,
+iii., 1824, xii., 1827).
+
+A little later came Dugès' studies of the osteology and myology of
+developing amphibia (1834),[184] and Coste's careful researches into the
+early developmental history of mammals.[185]
+
+[Illustration: FIG. 8.--Gill-slits of the Pig Embryo. (After Rathke.)]
+
+It was in 1825 that Heinrich Rathke (1793-1860), published his famous
+discovery of gill-slits in the embryo of a mammal,[186] a discovery which
+aroused considerable interest, and greatly stimulated embryological
+research. He describes how in a young embryo of a pig he saw four slits
+in the region of the neck, going right through into the oesophagus. They
+were separated by partitions which he called _Kiemenbogen_
+(gill-arches), and immediately in front of the first gill-slit lay the
+developing lower jaw. He compared these gill-slits with those of a
+dogfish. We reproduce his drawing of the pig-embryo (_Isis_, Pl. IV.,
+fig. 1).
+
+Later in the same year Rathke discovered gill-slits in the chick,[187] in
+this case finding only three. He described growing out from in front of
+the first slit a structure which he compared to the operculum or
+gill-cover of a fish.
+
+These discoveries were confirmed and extended for the chick[188] by the
+embryologist Huschke, a pupil of Oken. Like Rathke, he found only three
+indubitable gill-slits, but he noticed that the body-wall in front of
+the first gill-slit was really composed of two arches, which were on the
+whole similar to the gill-arches. The hinder of these two seemed to him
+to be a horn of the hyoid, the front one, which was bent at an angle, to
+be the rudiment of the upper and lower jaws (p. 401). Between these two
+arches he found an opening, just as between two gill-arches a gill-slit.
+This opening led into the mouth-cavity, and according to Huschke it
+became the external ear-passage. He discovered also three pairs of
+aortic arches in close relation with the gill-arches, so close indeed,
+that he did not hesitate to call them gill-arteries, and to recognise
+their resemblance with the aortic arches of fish. He traced, in part at
+least, the metamorphosis which these aortic arches undergo. This part of
+his discovery he developed in fuller detail in a paper of 1828,[189] in
+which he gave some excellent figures.
+
+Shortly after Huschke's first paper, von Baer published his views and
+observations on this subject in a short memoir in Meckel's _Archiv_.[190]
+In this paper he confirmed Rathke's discovery, and described the slits
+and arches in the dog and the chick. Both Rathke and he found gill-slits
+in the human embryo about this time (p. 557). There were generally
+present, he found, four gill-slits, and, as Rathke had suggested, the
+first gill-arch became the lower jaw. Von Baer also confirmed Rathke's
+discovery of the operculum, assigning it, however, to the second
+gill-arch. He refused to accept Huschke's derivation of the auditory
+meatus from the first gill-slit. Von Baer saw what had escaped Rathke
+and Huschke, that there were, not three nor four, but as many as five
+aortic arches.
+
+In his view of the metamorphosis of the aortic arches in the chick the
+first two pairs disappeared completely, the third pair gave rise to the
+arteries of the head and the fore-limbs, the right side of the fourth
+arch became the aorta, the left half of the fourth and the right half of
+the fifth arch became the pulmonary arteries, while the left half of the
+fifth arch disappeared. This schema, which for the last three arches was
+the same as Huschke's, von Baer upheld for the chick even in the second
+volume of his _Entwickelungsgeschichte_ (p. 116); he rectified it,
+however, for mammals in the same volume (p. 212), deriving both
+pulmonary arteries from the fifth arch, and the aorta from the fourth
+left. He fully recognised the great analogy of the embryonic arrangement
+of gill-arches and gill-arteries in Tetrapoda with their arrangement in
+fish (i., pp. 53, 73).
+
+Huschke, in a paper of 1832,[191] chiefly devoted to the development of
+the eye, figured and described the developing upper and lower jaws, and
+maintained against von Baer that the first slit turns into the auditory
+meatus and the Eustachian tube.
+
+These were the first papers of the embryological period. Before going on
+to discuss the principles which guided embryological research during the
+next ten or twenty years it is convenient to note what were the main
+lines of work characterising the period.
+
+The typical figure of the period is Rathke, who produced a great deal of
+first-class embryological work. He was, even more than von Baer, a
+comparative embryologist, and there were few groups of animals that he
+did not study. His first large publication, the _Beiträge zur Geschichte
+der Thierwelt_ (i.-iv., Halle, 1820-27), contained much anatomical work
+in addition to the purely embryological; he commenced here his series of
+papers on the development of the genital and urinary organs, continued
+in the _Abhandlungen zur Bildungsund Entwickelungs-Geschichte des
+Menschen und der Thiere_ (i., ii., Leipzig, 1832-3). A fellow-worker in
+this line was Johannes Müller, whose _Bildungsgeschichte der Genitalien_
+(Düsseldorf) appeared in 1830.
+
+In a memoir on the development of the crayfish which appeared in
+1829,[192] Rathke found in an Invertebrate confirmation of the germ-layer
+theory propounded by Pander and von Baer. He was greatly struck by the
+inverted position of the embryo with respect to the yolk. In following
+out the development of the appendages he noticed how much alike were
+jaws and legs in their earliest stage, and how this supported Savigny's
+contention that the limbs of Arthropods belonged to one single type of
+structure. In his paper (1832) on the development of the fresh-water
+Isopod, _Asellus_,[193] Rathke returns to this point. Commenting on the
+original similarity in development of antennæ, jaws and legs, he writes,
+"Whatever the doubts one may have reserved as to the intimate relation
+existing between the jaws and feet of articulate animals after the
+researches of Savigny on this subject and mine on developing crayfish,
+they must all fall to the ground when one examines with care the
+development of the fresh-water Asellus" (p. 147 of French translation).
+
+Further comparative work by Rathke is found in the two volumes of
+_Abhandlungen_ and in a book, _Zur Morphologie, Reisebemerkungen aus
+Taurien_ (1837), which contains embryological studies of many different
+types, including a study of the uniform plan of arthropod limbs. Later
+on Rathke devoted himself more to vertebrate embryology, producing among
+other works his classical papers on the development of the adder (1839),
+of the tortoise (1848), and of the crocodile (1866). He laid the
+foundations of all subsequent knowledge of the development of the
+blood-vascular system in a series of papers of various dates from 1838
+to 1856. The diagrams in his paper on the aortic arches of reptiles
+(1856) were for long copied in every text-book.
+
+Rathke was a foremost worker in another important line of embryological
+work, the study of the development of the skeleton and particularly of
+the skull. We shall discuss the history of the embryological study of
+the skull in some detail below; meantime, we note the two other
+important lines of research which characterise this period. One is the
+intensive study of the development of the human embryo, a study pursued
+by, among others, Pockels, Seiler, Breschet, Velpeau, Bischoff, Weber,
+Müller, and Wharton Jones.[194] The other important line--the early
+development of the Mammalia--was worked chiefly by Valentin,[195]
+Coste,[196] and, above all, by Bischoff, whose series of papers[197] was
+justly recognised as classical.
+
+What interests us chiefly in the work of this embryological period is,
+of course, the relation of embryology to comparative anatomy and to pure
+morphology. The embryologists were not slow to see that their work threw
+much light upon questions of homology, and upon the problem of the unity
+of plan. Von Baer, we have seen, recognised this clearly in 1828;
+Rathke, in one of his most brilliant papers, the
+_Anatomisch-philosophische Untersuchungen über den Kiemenapparat und das
+Zungenbein_ (Riga and Dorpat, 1832), used the facts of development with
+great effect to show the homology of the gill-arches and hyoid
+throughout the vertebrate series; Johannes Müller made great use of
+embryology in his classical _Vergleichende Anatomie der Myxinoiden_ (i.
+Theil, 1836), and, according to his pupil Reichert, firmly held the
+opinion that embryology was the final court of appeal in disputed points
+of comparative anatomy;[198] Reichert himself in a book of 1838
+(_Vergleichende Entwickelungsgeschichte des Kopfes der nackten
+Amphibien_) discussed the two different methods of arriving at the
+"Type"--the anatomical method of comparing adults, and the embryological
+method of comparing embryogenies. Of the embryological method, he says,
+"Its aim is to distinguish during the formation of the organism the
+originally given, the essence of the type, and to classify and interpret
+what is added or altered in the further course of development.
+Embryologists watch the gradual building up of the organism from its
+foundations, and distinguish the fundament, the primordial form, the
+type, from the individual developments; they reach thus, following
+Nature in a certain measure, the essential structure of the organism,
+and demonstrate the laws that manifest themselves during embryogeny" (p.
+vi.). The embryologists, influenced in this greatly by von Baer,
+gradually felt their way to substituting for the "Archetype" of pure
+morphology what one may perhaps best call the _embryological archetype_.
+How the transition was made we can best see by following out the course
+of discovery in one particular line. We choose for this purpose the
+development of the skull, a subject which excited much interest at this
+time and upon which much quite fundamental work was done, particularly
+by Rathke and Reichert.
+
+Following up his discovery of gill-slits and arches in the embryos of
+birds and mammals, Rathke in two papers of 1832[199] and 1833[200] worked
+out the detailed homologies of the gill-arches in the higher
+Vertebrates. He describes how in the embryo of the Blenny there is a
+short, thick arch between the first gill-slit and the mouth. A furrow
+appears down the middle of the arch dividing it incompletely into two.
+In the anterior halves a cartilaginous rod is developed which is
+connected with the skull; these rods become on either side the lower jaw
+and "quadrate." In the posterior halves two similar rods are formed
+which develop into the hyoid. The hyoid is at first connected with the
+skull, but afterwards frees itself and becomes slung to the "quadrate."
+From the hinder edge of the hyoid arch grows out the membranous
+operculum, in which develop later the opercular bones and branchiostegal
+rays. The upper jaw is an independent outgrowth of the serous layer.
+
+The serial homology of the lower jaw and quadrate with the hyoid and
+with the true gill-arches was thus established in fish, and Rathke had
+little difficulty in demonstrating a similar origin of lower jaw and
+hyoid in the embryos of higher Vertebrates. He could even, as we have
+noted before, find the homologue of the operculum in a flap which grows
+out from the hyoid arch in the embryo of birds.
+
+But Rathke could not altogether shake himself free from the
+transcendental notion of the homology of jaws with ribs, and this led
+him to draw a certain distinction between the first two and the
+remaining gill-arches, by which the homology of the former with the ribs
+was asserted and the homology of the latter denied. He thought he could
+show that the skeletal structures (lower jaw, "quadrate," and hyoid) of
+the first two arches were formed in the serous layer, just like true
+ribs, and like them in close connection with the vertebral skeletal
+axis. The other, "true," gill-arches appeared to him to be formed in the
+mucous layer, in the lining of the alimentary canal. They had no direct
+connection with the vertebral column, and seemed therefore to belong to
+what Carus[201] had called the visceral or splanchno-skeleton. He did not,
+however, let this distinction hinder him from asserting the substantial
+homology of all the gill-arches _inter se_, the first two included.
+
+Rathke's discoveries relative to the development of the jaws, the hyoid
+and the operculum, enabled him to make short work of the homologies
+proposed for them by the transcendentalists. He could prove from
+embryology that the jaws were not the equivalent of limbs, as so many
+Okenians believed. He could reject, with a mere reference to the facts
+of development, Geoffroy's comparison of the hyoid and the
+branchiostegal rays in fish with sternum and ribs. He could show the
+emptiness of the attempts made by Carus, Treviranus, de Blainville and
+Geoffroy, to establish by anatomical comparison the homologies of the
+opercular bones, for he could show that these bones were peculiar to
+fish, and were scarcely indicated, and that only temporarily, in the
+development of other Vertebrates.[202] He did not, however, himself
+realise the relation of the ear-ossicles to the gill-arches, though he
+knew that Spix and Geoffroy were quite wrong in homologising them with
+the opercular bones in fish. He described, it is true, the development
+of the external meatus of the ear and the Eustachian tube from the slit
+which appears between the first and the second arch, as Huschke had done
+before him; he described, in confirmation of Meckel, the "Meckelian
+process" of the hammer running down inside the lower jaw; but the
+discovery of the true homologies of the ear-ossicles was not made until
+a year or two later by Reichert.
+
+In his further study of the development of _Blennius viviparus_, Rathke
+observed some important facts about the development of the vertebral
+column and skull. He found that the vertebral centra were first formed
+as rings in the chorda-sheath, which give off neural and hæmal
+processes. The vertebra later ossifies from four centres. The chorda
+(notochord) is prolonged some little way into the head, and the base of
+the cranium is formed by the expanded sheath, which reaches forward in
+front of the end of the notochord. This cranial basis shows a division
+into three segments, in which Rathke was inclined to see an indication
+of three cranial vertebræ. (It turned out that this division into three
+segments did not really exist, and Rathke later acknowledged that he had
+made an error of observation.) The side walls of the skull grow out from
+this base and form a fibrous capsule for the brain. The cranial section
+of the chorda itself shows no sign of segmentation; but later on the
+cranial portion of the chorda-sheath ossifies, like the vertebræ, from
+several centres. The vomer, which, in the classical form of the
+vertebral theory of the skull, was the centrum of the fourth, or
+foremost, cranial vertebra, does not, according to Rathke, develop in
+continuity with the cranial basis and the chorda sheath, but develops
+separately in the facial region.
+
+Von Baer, like Rathke at this time, was also to some extent a believer
+in the vertebral theory of the skull. In his second volume (1834, pub.
+1837) he holds that the development of the skull, as the sum of the
+anterior vertebral arches, is in general the same as that of the other
+neural arches, and is modified only by the great bulk of the brain
+(_Entwickelungsgeschichte_, ii., p. 99). He had, however, some doubts as
+to the entire correctness of the vertebral theory, doubts suggested by a
+study of the developing skull. "In the course of the formation of the
+head in the higher animals, something additional is introduced which
+does not originally belong to the cranial vertebræ. At first we see the
+vertebration in the hinder region of the skull very clearly. Afterwards
+it becomes suddenly indistinct, as if some new formation overlaid it"
+(i., p. 194).
+
+Even more clearly is his doubt expressed in his paper on _Cyprinus_.
+"Upon the formation of the vertebral column only this need be said, that
+at this stage the notochord is very clearly seen, and the upper and
+lower arches and spinous processes are visible right to the end of the
+tail, but the separation into vertebræ ceases abruptly where the back
+passes into the head. I do not hesitate to assert _that bony fish, too,
+have at this stage an unsegmented cartilaginous cranium_ (as
+cartilaginous fish have all their life), the prominences and hollows of
+which constitute its only resemblance with the vertebral type" (1835, p.
+19).
+
+A convinced supporter of the vertebral theory was Johannes Müller, who,
+in his classical memoir on the Myxinoids,[203] discussed at some length
+the relation between the development of the vertebræ and the development
+of the skull. His memoir is principally devoted to comparative anatomy,
+but in treating of the skeleton he pays much attention to development.
+He describes the formation of the vertebræ in elasmobranch embryos; for
+the facts regarding other Vertebrates he relies largely on work by
+Rathke (_Blennius_, 1833) and Dugès (1834). He recognises as the basis
+of his comparisons the homology of the notochord in all vertebrate
+embryos with the persistent notochord which forms the chief part or the
+whole of the vertebral column in the Cyclostomes. The notochord
+possesses an inner and an outer sheath and the outer sheath is
+continuous with the _basis cranii_ (p. 92). It is in the outer sheath
+that the vertebræ develop--from four separate pieces, in fish at least,
+plus an additional element which helps to form the centrum. The skull of
+Vertebrates consists, according to Müller, of three vertebræ, whose
+centra are the basioccipital, the basisphenoid and the presphenoid.
+Other bones besides those belonging to the vertebræ are present, but
+this formation out of three vertebræ gives the essential schema for the
+skull. Now the brain capsule, like the sheath of the spinal cord, is a
+development from the outer sheath of the notochord. If the skull
+consists of vertebræ we should expect the centra of the skull-vertebræ
+to develop in the outer sheath at the sides of the cranial section of
+the notochord as two separate halves, just as do the bodies of the
+vertebræ; we should expect further the cartilaginous side-walls of the
+cranium to develop in the membranous brain-sheath just as the neural
+arches develop in the membranous sheath of the spinal column. In
+Rathke's discovery (!) of a segmentation of the _basis cranii_ into
+three parts, and of the isolated formation of the vomer, Müller sees a
+confirmation of his view that the skull is composed of three and not
+four vertebræ. But there is nothing in Rathke's observations to support
+the idea that the centra of the cranial vertebræ are formed from
+separate halves. Müller has to be content with a reference to the state
+of things in _Ammocoetes_ (which, by the way, he did not know to be the
+young of _Petromyzon_). In the simple skull of _Ammocoetes_ the base is
+formed chiefly by two cartilaginous bars lying more or less parallel
+with the longitudinal axis of the skull and embracing with their hinder
+ends the cranial portion of the notochord.
+
+These bars, declares Müller, are clearly the still separate halves of
+the _pars basilaris cranii_, and represent the divided centra of the two
+hinder cranial vertebræ. To complete the parallel between the
+development of the skull and of the vertebræ, it would have been
+necessary to show that the side walls of the cranium developed in a
+similar manner from separate pieces. Müller could not prove this point
+from the available embryological data, and indeed the facts which he did
+use had to be twisted to suit his theory. A curious apparent
+confirmation of his idea that the centra of the cranial vertebræ are
+formed from separate halves was supplied in 1839 by Rathke's discovery
+of the trabeculæ in the embryonic skull of the adder.
+
+The next big step in the study of the development of the skull was
+taken by a pupil of Müller, C. B. Reichert, who showed in his work
+very distinct traces of his master's influence. Reichert's first and
+most important contribution to the subject was his paper on the
+metamorphosis of the gill, or, as he called them, the visceral arches
+in Vertebrates,[204] particularly in the two higher classes. Reichert
+describes the similar origin in embryo of bird and mammal (pig) of
+three "visceral" arches. These arches stand in close relation to the
+three cranial vertebræ which Reichert, like Müller, distinguishes. He
+makes the retrograde step of admitting only three aortic arches, and
+he is not inclined to consider the three visceral arches as equivalent
+to the gill-arches of fish--in his opinion they have more analogy with
+ribs, though differing somewhat from ribs in their later
+modifications. The visceral arches are processes of the visceral
+plates (von Baer), which grow downwards and meet in the middle line,
+leaving between one another and the undivided body wall three visceral
+slits opening into the pharynx. The first visceral process is
+different in shape from the others, for it sends forward, parallel
+with the head and at right angles to its downward portion, an upper
+portion in which later the upper jaw is formed. The other two
+processes are straight. From the hinder edge of the second visceral
+arch there develops, as Rathke had seen, a fold which is comparable
+with the operculum of fish. The first slit develops externally into
+the ear-passage, internally into the Eustachian tube, and in the
+middle a partition forms the tympanic ring and tympanum. Inside each
+of the visceral processes on either side a cartilaginous rod develops.
+In the first process this rod shows three segments, of which the first
+lies inside that portion of the process which is parallel with the
+head. This upper segment forms the foundation for the bones of the
+upper jaw. The lowest segment of the cartilaginous rod becomes
+Meckel's cartilage, and on the outer side of this the bones of the
+lower jaw are formed. The middle segment becomes in mammals the incus
+(one of the ear-ossicles), and in birds the quadrate. Meckel's
+cartilage, which was discovered by Meckel[205] in fish, amphibians and
+birds, is a long strip of cartilage which runs from the ear-ossicle
+known as the hammer in mammals,[206] to the inside of the mandible.
+Reichert shows how this relation comes about. The hammer, according to
+his observations on the embryo of the pig, is simply the proximal end
+of Meckel's cartilage, which later becomes separated off from the long
+distal portion (see Fig. 9). The third ear-ossicle of mammals, the
+stapes, comes not from the first arch but from the second. The
+cartilaginous rod of the second arch segments like the first into
+three pieces. Of these the uppermost disappears, the middle one, which
+lies close up to the labyrinth of the ear, becomes the stapes, and the
+lowest becomes the anterior horn of the hyoid. The stapes forms a
+close connection with the hammer and the incus. In birds, where there
+is a single ear-ossicle, the columella, the middle piece of arch I
+forms, as we have seen, the quadrate, by means of which the lower jaw
+is joined to the skull. The proximal end of Meckel's cartilage, which
+in mammals forms the hammer, here gives the articular surface between
+the lower jaw and the quadrate. The columella is formed from the
+middle piece of the three into which the cartilage of the second arch
+segments. It is, therefore, the homologue of the stapes in mammals.
+The third arch takes a varying share, together with the second, in the
+formation of the hyoid apparatus.
+
+[Illustration: FIG. 9.--Meckel's Cartilage and Ear-ossicles in Embryo
+of Pig. (After Reichert.)]
+
+In this paper Reichert made a distinct advance on the previous workers
+in the same field--Rathke, Huschke, von Baer, Martin St Ange, Dugès.
+Huschke was indeed the first to suggest that both upper and lower jaws
+were formed in the first gill-arch. But both von Baer and Rathke[207] held
+that the upper jaw developed as a special process independent of the
+lower jaw rudiment, and the actual proof that the upper jaw is a
+derivative of the first visceral arch seems to have been first supplied
+by Reichert. His brilliant work on the development of the ear-ossicles
+founded what we may justly call the classical theory of their
+homologies. His views were attacked and in some points rectified, but
+the main homologies he established are even now accepted by many,
+perhaps the majority of morphologists.
+
+In a paper of 1838 on the comparative embryology of the skull in
+Amphibia,[208] Reichert added to his results for mammals and birds an
+account of the fate of the first and second visceral arches in Anura and
+Urodela.
+
+The first visceral arch, he found, gave in Amphibia practically the same
+structures as in the higher Vertebrates. Its skeleton segmented, as in
+mammals and birds, into three parts; the upper part gave rise to the
+palatine and pterygoid in Anura, but seemed to disappear in Urodeles,
+where the so-called palatine and pterygoid developed in the mucous
+membrane of the mouth; the middle part gave, as in birds, the quadrate,
+which formed a suspensorium for both arches; the lower part, as Meckel's
+cartilage, formed a foundation for the bones of the lower jaw. Of arch
+II., the lower part became the horn of the hyoid, the upper part had a
+varying fate. In some Anura it formed the ossicle of the ear (homologue
+of the columella of birds and the stapes of mammals), in others it
+disappeared. In reptiles the upper segment of the second arch formed, as
+in birds, the columella.
+
+The account of the metamorphoses of the visceral arches in Amphibia
+forms only a small part of Reichert's memoir of 1838, the chief object
+of which was to discover the general "typus" of the vertebrate skull,
+and to follow out its modifications in the different classes. Von Baer
+had shown that the generalised type appeared most clearly in the early
+embryo; Reichert therefore sought the archetype of the skull in the
+developing embryo. He brought to his task the preconceived notion that
+the skull could be reduced to an assemblage of vertebræ, but he saw that
+comparative anatomy alone could not effect this reduction; he had
+recourse, therefore, to embryology, hoping to find in the simplified
+structure of the embryo clear indications of three primitive cranial
+vertebræ (p. 121, 1837).
+
+In the head he distinguished two tubes, the upper formed by the dorsal
+plates, the lower by the ventral or visceral plates. Both of these tubes
+were derived from the serous or animal layer (_cf._ von Baer, _supra_,
+p. 118). The walls of the lower tube were formed by the visceral
+processes, within which later the skeleton of the visceral arches
+developed. The walls of the upper tube formed the bones and muscles of
+the cranium proper. The facial part of the head was formed by elements
+from both upper and lower tubes. The dorsal tube showed signs of a
+division into three cranial vertebræ (_Urwirbeln_, primitive vertebræ).
+In mammals and birds, as Reichert had shown in his 1837 paper, the three
+cranial vertebræ were indicated by transverse furrows on the ventral
+surface of the still membranous skull (see Fig. 10, p. 148).
+
+Even in mammals and birds, however, the positions of the eye, the
+ear-labyrinth, and the three visceral arches were the safest guides to
+the delimitation of the cranial vertebræ (pp. 134-138, 1837). In
+Amphibia generally there were no definite lines of separation on the
+skull itself. "At this stage," he writes of the cartilaginous cranium of
+the frog, "we find no trace of a veritable division into vertebræ in the
+cartilaginous trough formed by the _basis cranii_ and the side parts. On
+the contrary, it is quite continuous, as it is also in the higher
+Vertebrates during the process of chondrification" (p. 44, 1838). The
+vertebræ in the membranous or cartilaginous skull could be delimited in
+Amphibia by the help of the eye and the ear-labyrinth, which lie more or
+less between the first and second, and the second and third vertebræ,
+but, above all, by the vesicles of the brain.
+
+As in the higher Vertebrates, the visceral arches are associated with
+the cranial vertebræ as their ventral extensions, being equivalent to
+the visceral plates which form the ventral portion of the "primitive
+vertebræ" or primitive segments of the trunk.
+
+[Illustration: FIG. 10.--Cranial Vertebræ and Visceral Arches in Embryo
+of Pig. Ventral Aspect. (After Reichert.)]
+
+If the three cranial vertebræ are not very distinct in the early stages
+of development when the skull is still membranous or cartilaginous, they
+become clearly delimited when ossification sets in. Three rings of bone
+forming three more or less complete vertebræ are the final result of
+ossification. The composition of these rings is as follows:--
+
++-------------------------------------------------------------------+
+|                |   Base.       |    Sides.       |  Top.          |
+|----------------+---------------+-----------------+----------------|
+|First vertebra  |Presphenoid    |Orbitosphenoids  |Frontals        |
+|                |               |                 |                |
+|Second vertebra |Basisphenoid   |Alisphenoids     |Parietals       |
+|                |               |                 |                |
+|Third vertebra  |Basioccipital  |Exoccipitals     |Supraoccipital  |
++-------------------------------------------------------------------+
+
+The other bones of the skull are not included in the vertebræ, and this
+is in large part due to the fact that the sense capsules are formed
+separately from the cranium (p. 29, 1838). The ear-labyrinth, it is
+true, fuses indissolubly with the cranium at a later period, but the
+bones which develop in its capsule are not for all that integral parts
+of the primitive cranial vertebræ. This point, it is interesting to
+note, had already been made by Oken in his _Programm_ (1807). But many
+of the bones developed in relation to the sense organs can find their
+place in the generalised embryonic schema or archetype of the vertebrate
+skull, for they are of very constant occurrence during early
+development.
+
+Having arrived at a generalised embryonic type for the vertebrate skull,
+of which the fundamental elements are the three cranial vertebræ and
+their arches, Reichert goes on to discuss the particular forms under
+which the skull appears in adult Vertebrates. He accepts in general von
+Baer's law that the characters of the large groups appear earlier in
+embryogeny than the characters of the lesser classificatory divisions.
+"When we observe new and not originally present rudiments in very early
+embryonic stages, as, for instance, that for the lacrymals, the
+probability is that they belong to the distinctive development of one of
+the _larger_ vertebrate groups. From these are to be carefully
+distinguished such rudiments as arise later during ossification, mostly
+as _ossa intercalaria_, in order to give greater strength to the skull
+in view of the greater development of the brain, etc.; the latter give
+their individual character to the _smaller_ vertebrate groups, and
+comprise such bones as the _vomer_, the _Wormian bones_, the lowermost
+turbinal, etc." (p. 63, 1838).
+
+He did not accept the Meckel-Serres law of parallelism. He recognised
+the great similarity between the unsegmented cartilaginous cranium of
+Elasmobranchs, and the primordial cranium of the embryos of the higher
+Vertebrates, but he did not think that the cranium of Elasmobranchs was
+simply an undeveloped or embryonic stage of the skulls of the higher
+forms. Rather "do the _Holocephala_, _Plagiostomata_, and _Cyclostomata_
+appear to us to be lower developmental stages individually
+differentiated, so that the other fully differentiated Vertebrates
+cannot easily be referred directly to their type" (p. 152, 1838). The
+skull of these lower fishes is itself a specialised one; it is an
+individualised modification of a simple type of skull. And this holds
+good in general of the skulls of the lower Vertebrates--they are
+individualised exemplars of a simple general type, not merely unmodified
+embryonic stages of the greatly differentiated skulls of the higher
+Vertebrates (p. 250, 1838). Differentiation within the vertebrate phylum
+is therefore not uniserial, but takes place in several directions.
+Reichert describes two sorts of modifications of the typical
+skull--class modifications and functional modifications. The causes of
+the modifications which characterise classificatory groups are unknown;
+the second class of modifications occur in response to adaptational
+requirements.
+
+Reichert's two papers are of considerable importance, and Müller's
+remark in his review[209] of them is on the whole justified. "These
+praiseworthy investigations supply from the realm of embryology new and
+welcome foundations for comparative anatomy" (p. clxxxvii.).
+
+The development of the skull was, however, more thoroughly worked out by
+Rathke, and with less theoretical bias, in his classical paper on the
+adder.[210] This memoir of Rathke's is an exhaustive one and deals with
+the development of all the principal organ-systems, but particularly of
+the skeletal and vascular. He confirmed in its essentials Reichert's
+account of the metamorphoses of the first two visceral arches,
+describing how the rudiment of the skeleton of the first arch appears as
+a forked process of the cranial basis, the upper prong developing into
+the palatine and pterygoid, the lower forming Meckel's cartilage, while
+the quadrate develops from the angle of the fork. The actual bone of the
+upper jaw (maxillary) develops outside and separate from the
+palato-pterygoid bar. The cartilaginous rod supporting the second
+visceral arch divides into three pieces on each side, of which the lower
+two form the hyoid, the uppermost the columella. Like Reichert he held
+the visceral arches to be parts of the visceral plates, containing,
+however, elements from all three germ-layers--the serous, mucous, and
+vessel layers.
+
+The first gill-slit, or, as Rathke here prefers to call it, pharyngeal
+slit, closes completely in snakes and in Urodeles. It forms the
+Eustachian tube in all other Tetrapoda. As regards the vertebræ, Rathke
+describes them as being formed in the sheath of the chorda from paired
+rudiments, each of which sends two branches upwards, and two branches
+downwards. The two inner pairs of processes coalesce round the chorda,
+and later form the centrum; the upper outer pair meet above the spinal
+column; the lower outer pair form ribs. The odontoid process of the axis
+vertebra is the centrum of the atlas (p. 120). The formation of
+vertebral rudiments begins close behind the ear-labyrinth, but in front
+of this the chorda-sheath gives origin to a flat membranous plate which
+afterwards becomes cartilaginous. This plate reaches forward below the
+third cerebral vesicle as far as the infundibulum. The notochord ends in
+this plate, which is the _basis cranii_, just at the level of the
+ear-labyrinth. In no Vertebrate does the notochord extend farther
+forward (p. 122). The _basis cranii_ gives off three trabeculæ. The
+middle one is small and sticks up behind the infundibulum; it is absent
+in fish and Amphibia, and soon disappears during the development of the
+higher forms. The lateral trabeculæ are long bars which curve round the
+infundibulum and reach nearly to the front end of the head. Together
+they are lyre-shaped. The cranial basis and the trabeculæ are formed,
+like the vertebræ, in the sheath of the notochord, and the only
+differences between the two in the early stage of their development are
+that the formative mass for the cranial basis is much greater in amount
+than that for the vertebræ, and that the cranial basis by means of its
+processes, the trabeculæ, reaches well in front of the terminal portion
+of the notochord (p. 36). The capsule for the ear-labyrinth develops
+quite independently of the cranial basis and the notochord. It resembles
+on its first appearance, in form, position, composition, and
+connections, the ear-capsule of Cyclostomes, and so do the ear-capsules
+of all embryonic Vertebrates (p. 39). It manifests clearly the embryonic
+archetype, ... "there exists one single and original plan of formation,
+as we may suppose, upon which is built the labyrinth of Vertebrates in
+general" (p. 40). When ossification sets in, the ear-capsule forms three
+bones, of which two fuse with the supraoccipital and exoccipitals.
+
+[Illustration: FIG. 11.--Embryonic Cranium of the Adder. Ventral Aspect.
+(After Rathke.)]
+
+During the formation of the ear-capsule the cranial basis develops from
+a plate to a trench, for in its hinder section the side parts grow up to
+form the side walls of the brain, in exactly the same way as the
+processes of the vertebral rudiments grow up to enclose the spinal
+column (pp. 122, 192). The foundations of the skull are now complete,
+and ossification gradually sets in. The basioccipital is formed
+in the posterior part of the _basis cranii_, and the exoccipitals in the
+side walls of the trench in continuity with the fundament of the
+basioccipital (see Fig. 11). The supraoccipital is formed in cartilage
+above the exoccipitals. The basisphenoid develops, like the
+basioccipital, in the flat _basis cranii_, but towards its anterior
+edge, between the large foramen (_h_) and the pituitary space (_i_). It
+is formed from two centres, each of which is originally a ring round the
+carotid foramen. The presphenoid develops in isolation between the
+lateral trabeculæ, just behind the point where they fuse. The side parts
+of the basisphenoid and presphenoid (forming the alisphenoids and the
+orbitosphenoids respectively) develop in cartilage separately from the
+cranial basis, not like the exoccipitals in continuity with it. The
+hinder parts of the trabeculæ become enclosed by two processes of the
+basisphenoid; their front parts remain in a vestigial and cartilaginous
+state alongside the presphenoid. The frontals and parietals show a
+peculiar mode of origin in the adder, differing from their origin in
+other Vertebrates. The frontals develop in continuity with the
+orbitosphenoids, the parietals in continuity with the alisphenoids, and
+so have much resemblance with the vertebral neural arches which surround
+the spinal column (p. 195).
+
+Through Rathke's work the real embryonic archetype of the vertebrate
+skull was for the first time disclosed. Rathke discussed this archetype
+and its relation to the vertebral theory of the skull in another paper
+of the same year (1839), but before going on to this paper, we shall
+quote from the paper on the adder the following passage, remarkable for
+the clear way in which the idea of the embryological archetype is
+expressed. "Whatever differences may appear in the development of
+Vertebrates, there yet exists for the different classes and orders a
+universally valid idea (plan, schema, or type) ruling the first
+formation of their separate parts. This idea must first be worked out,
+though possibly with modifications, before more special ideas can find
+play. The result of the latter process, however, is that what was formed
+by the first idea is not so much hidden as partially or wholly
+destroyed" (p. 135).
+
+Rathke's general paper on the development of the skull in Vertebrates[211]
+treats the matter on a broader comparative basis than his paper on the
+adder, and takes into account all the vertebrate classes, in so far as
+their development was then known. He here makes the interesting
+suggestion, later entirely confirmed, that the _basis cranii_ or basilar
+plate is first laid down as two strips, one on each side of the
+chorda--the structures now known as parachordals (pp. 6, 27). For this
+supposition, he thinks, speaks the structure of the skull in
+_Ammocoetes_, which in this respect is the simplest of all Vertebrates
+(pp. 6, 22). In _Ammocoetes_, as Johannes Müller had shown, the
+foundation of the skull is formed by two long cartilaginous bars,
+between the hinder portions of which the notochord ends. In these Rathke
+was inclined to see the homologues of his trabeculæ, and of the
+parachordals which he was ready to assume from his embryological
+observations.
+
+Müller was, of course, very ready to accept Rathke's opinions on this
+subject, for he considered that they supported his own theory of the
+vertebral nature of the skull. After describing in his _Handbuch der
+Physiologie_ the cartilaginous bands in _Ammocoetes_ and their highly
+differentiated homologues in the Myxinoids, he writes in the later
+editions, "Hence we see that in the cranium, as in the spinal column,
+there are at first developed at the sides of the chorda dorsalis two
+symmetrical elements, which subsequently coalesce, and may wholly
+enclose the chorda. Rathke has recently observed, in the embryos of
+serpents and other animals, before the formation of the proper cranial
+vertebræ, two symmetrical bands of cartilage, similar to those which I
+discovered as a persistent structure in _Ammocoetes_.... At a later
+period the _basis cranii_ of vertebrate animals contains three parts
+analogous to the bodies of vertebræ, the most anterior of which, in the
+majority of animals, is generally small, and its development frequently
+abortive, whilst in man and mammiferous animals the three are very
+distinct. These parts are developed by the formation of three distinct
+points of ossification, one behind the other, in the basilar
+cartilage."[212]
+
+Rathke was very cautious about accepting the vertebral theory of the
+skull; he saw that the facts of development were not altogether
+favourable to the theory, and he gave his adherence with many
+reservations and saving clauses. His general attitude may be summed up
+as follows.[213]
+
+The chorda sheath is the common matrix of the vertebræ and of a large
+part of the skull. The basilar plate and the trabeculæ, which are
+developed from the chorda sheath, give origin to three bones, which
+might possibly be considered equivalent to vertebral centra--the
+basioccipital, the basisphenoid, and the _Riechbein_ (ethmoid). The
+_Riechbein_ develops from the fused ends of the trabeculæ. The
+presphenoid might also be considered as a vertebral body, but it
+develops independently of the basilar plate and trabeculæ.
+
+Now of these bones, the basioccipital is in every way equivalent to a
+vertebral centrum, for it develops in the basilar plate round the
+notochord. With the exoccipitals, which arise just like neural arches,
+it forms a true vertebra. The supraoccipital is an accessory bone
+developed in relation to bigger brains. The basisphenoid appears in the
+basilar plate, but in front of the notochord, nor does it arise in
+exactly the same way as the centrum of a vertebra. The basisphenoid with
+the alisphenoids, which develop independently in the side walls of the
+brain, may, however, still be considered as forming a vertebra, though
+the resemblance is not so great as in the case of the occipital ring.
+The presphenoid, being long and pointed, is very unlike a vertebral
+body. The orbitosphenoids develop separately from it. The ethmoid also
+differs from a vertebra, for it surrounds not the whole nervous axis as
+the two hinder "vertebræ" do, but only two prolongations of it, the
+olfactory lobes. In its development and final form it shows no
+particular resemblance to a vertebra. Its body, the _pars
+perpendicularis_ (mesethmoid) shows no similarity with a vertebral
+centrum. Completing the three hinder cranial "vertebræ" and roofing in
+the brain are the supraoccipital, the parietals and the frontals. The
+premaxillaries, vomer, and nasals do not belong to the cranial scheme;
+they are covering bones connected with the ethmoid. So, too, the
+ear-capsule is not part of the cranial vertebræ, but is rather to be
+compared to the intercalary bones in the vertebral column of certain
+fish. Summing up as regards the cranial vertebræ Rathke writes, "We find
+that the four different groups of bones, consisting of the basioccipital
+with its intercalary (the supraoccipital), the basisphenoid with its
+intercalaries (parietals), the presphenoid with its intercalaries
+(frontals), and the ethmoid with its outgrowths (turbinals and
+cribriform plate), taking them in order from behind forwards, show an
+increasing divergence from the plan according to which vertebræ as
+commonly understood develop, so that the basioccipital shows the
+greatest resemblance to a vertebra, the ethmoid the least" (p. 30).
+
+In a posthumous volume published in 1861 the same opinion is put
+forward. "In the head, too," he writes, "some vertebræ can be
+recognised, although in a more or less modified form. Yet at most only
+four cranial vertebræ can be assumed, and these differ from ordinary
+well-developed vertebræ in their manner of formation the more the
+farther forward they lie."[214]
+
+Rathke was an able and careful critic of the vertebral theory of the
+skull, but he accepted it in the main. Actual attack on the theory upon
+embryological grounds was begun by C. Vogt, in his work on the
+development of _Coregonus_,[215] and in his paper on the development of
+_Alytes_.[216] He described for _Coregonus_ an origin of the skull in the
+main similar to that established by Rathke for the adder. There was a
+"nuchal plate" in which the front end of the notochord was imbedded; the
+notochord ended at the level of the labyrinth; there were two lateral
+bands, comparable to Rathke's lateral trabeculæ; a "facial plate" was
+also formed, which seems on the whole equivalent to the plate formed by
+the fused anterior ends of the trabeculæ. A little later the cranium
+formed a complete cartilaginous box surrounding the brain, very similar
+to the adult cranium of a shark.
+
+In his criticism of the vertebral theory of the skull, Vogt started by
+defining the vertebra as a ring formed round the chorda. Now since only
+the occipital segment of the skull is formed actually round the
+notochord, the parts of the skull lying in front of this cannot
+themselves be vertebræ, though they may be considered as prolongations
+of the occipital or nuchal vertebra. "We must regard the nuchal plate as
+a true vertebra, modified, it is true, in its formation and development
+by its particular functions. Now, since the notochord ends with the
+nuchal plate we can no longer regard as vertebræ the parts of the skull
+that lie beyond, such as the lateral processes of the cranium and the
+facial plate, for they have no relation with the notochord" (p. 123).
+
+To support this view he adduced the fact that the vertebral divisions
+(primitive vertebræ) visible in the trunk do not extend into the head.
+He used precisely the same arguments in his paper on _Alytes_ to destroy
+the vertebral theory of the skull. We quote the following passage
+translated by Huxley (1864, p. 295) from this paper. "It has therefore
+become my distinct persuasion that the occipital vertebra is indeed a
+true vertebra, but that everything which lies before it is not fashioned
+upon the vertebrate type at all, and that efforts to interpret it in
+such a way are vain; that, therefore, if we except that vertebra
+(occipital) which ends the spinal column anteriorly, there are no
+cranial vertebræ at all."
+
+L. Agassiz, himself a pupil of Döllinger, in the general part (1844) of
+his _Recherches sur les Poissons fossiles_ (Neuchâtel, 1833-43), repeats
+in the main his pupil Vogt's criticism of the vertebral theory (vol. i.,
+pp. 125-9).
+
+These arguments of Vogt and Agassiz were not considered by Müller to
+dispose of the theory,[217] which maintained a firm hold even upon
+embryologists. It was still upheld by Reichert, and Kölliker in 1849
+showed himself convinced of its general validity.
+
+A useful step in the analysis of the concept "vertebra" was taken by
+Remak,[218] who showed what a complex affair the formation of vertebræ
+really is, involving as it does a complete resegmentation
+(_Neugliederung_) of the vertebral column, whereby the original
+vertebral bodies were replaced by the secondary definitive bodies (p.
+143). Remak showed, as he thought, that the protovertebral segmentation
+of the dorsal muscle-plates did not extend into the head, and he denied
+Reichert's assertion (1837) that the cranial basis in mammals showed
+transverse grooves delimiting three cranial vertebræ (p. 36). The
+gill-slits, he considered, could not possibly be regarded as marking the
+limits of head vertebræ.
+
+In 1858 appeared Huxley's well-known Croonian Lecture, _On the Theory of
+the Vertebrate Skull_,[219] in which he stated with great clearness and
+force the case for the embryological method of determining homologies,
+and criticised with vigour the vertebral theory of the skull. By this
+time the two rival methods in morphology had become clearly
+differentiated, and Huxley was able to contrast them, or at least to
+show how necessary the new embryological method was as a corrective and
+a supplement to the older anatomical, or, as he calls it, "gradation"
+method. Applied to the "Theory of the Skull," the gradation method
+consists in comparing the parts of the skull and vertebral column in
+adult animals with respect to their form and connections. "Using the
+other method, the investigator traces back skull and vertebral column to
+their earliest embryonic states and determines the identity of parts by
+their developmental relations" (p. 541). This second method is the final
+and ultimate. "The study of the gradations of structure presented by a
+series of living beings may have the utmost value in suggesting
+homologies, but the study of development alone can finally demonstrate
+them" (p. 541). As an example of the utility and, indeed, the necessity
+of applying the embryological method Huxley takes the case of the
+quadrate bone in birds. This bone had been generally regarded by
+anatomists as the equivalent of the tympanic of mammals, on account of
+its connection with the tympanum; but Reichert showed (1837) that the
+same segment of the first visceral arch developed into the incus in
+mammals, and into the quadrate in birds, and that therefore the quadrate
+was homologous with the incus. Similarly, on developmental grounds, the
+malleus or hammer of mammals is the homologue of the articular of birds,
+since both are developed from a portion of Meckel's cartilage identical
+in form and connections in the two groups. The homologies of the bones
+connected with the jaws in bony fishes had long been a subject of
+contention among comparative anatomists; Huxley shows from his personal
+observations how the development of the visceral arches throws light
+upon these difficulties. The mandibular arch in the developing fish is
+abruptly angled, as in the embryo of Tetrapoda; the upper prong of it
+ossifies into the palatine and pterygoid; at the angle is formed the
+quadrate (jugal, Cuvier), and to the quadrate is articulated the lower
+jaw, which ossifies round the lower prong or Meckel's cartilage. The
+scheme of development of the jaws is accordingly similar in fish to what
+it is in other Vertebrates, and this similarity of development enables
+Huxley to recognise what are the true homologues of the quadrate, the
+palatine and the pterygoid in adult bony fish, and to prove that the
+symplectic and the metapterygoid (tympanal, Cuvier) are bones peculiar
+to fish. In developing Amphibia Huxley found a suspensorium of hyoid and
+mandibular arches similar to the hyomandibular of fish.
+
+Tackling his main problem of the unity of plan of the vertebrate skull,
+Huxley shows, by a careful discussion of the anatomical relationships of
+the chief bones in typical examples of all vertebrate classes, that
+there is on the whole unity of plan as regards the osseous skull. This
+unity of composition can be established, on the gradation method, by
+considering the connections of the bones of the skull with one another,
+their relations to the parts of the brain and to the foramina of the
+principal cranial nerves. The assistance of the embryological method is,
+however, necessary in determining many points with regard to the bones
+developed in relation to the visceral arches. But there is a further
+step to be taken. "Admitting ... that a general unity of plan pervades
+the organisation of the ossified skull, the important fact remains that
+many vertebrated animals--all those fishes, in fact, which are known as
+_Elasmobranchii_, _Marsipobranchii_, _Pharyngobranchii_ and _Dipnoi_
+have no bony skull at all, at least in the sense in which the words have
+hitherto been used" (p. 571). The membranous or cartilaginous skull of
+these fishes shows a general resemblance in its main features to the
+ossified skull of other Vertebrates; the relations of the ear to the
+vagus and trigeminal nerves are, for instance, the same in both; the
+main regions of the cartilaginous skull can be homologised with definite
+bones or groups of bones in the bony skull; but discrepancies occur. It
+is again to development that we must turn to discover the true
+relationship of the cartilaginous to the ossified skull. "The study of
+the development of the ossified vertebrate skull ... satisfactorily
+proves that the adult crania of the lower _Vertebrata_ are but special
+developments[220] of conditions through which the embryonic crania of
+the highest members of the sub-kingdom pass" (p. 573). It is with the
+embryonic cranium of higher Vertebrates that the adult skull of the
+lower fishes must be compared, and the comparison will show a
+substantial though not a complete agreement between them. Thus, speaking
+of the development of the frog's skull, Huxley writes:--"If, bearing in
+mind the changes which are undergone by the palatosuspensorial
+apparatus, ... we now compare the stages of development of the frog's
+skull with the persistent conditions of the skull in the _Amphioxus_,
+the lamprey, and the shark, we shall discover the model and type of the
+latter in the former. The skull of the _Amphioxus_ presents a
+modification of that plan which is exhibited by the frog's skull when
+its walls are still membranous and the notochord is not yet embedded in
+cartilage. The skull of the lamprey is readily reducible to the same
+plan of structure as that which is exhibited by the tadpole when its
+gills are still external and its blood colourless. And finally, the
+skull of the shark is at once intelligible when we have studied the
+cranium in further advanced larvæ, or its cartilaginous basis in the
+adult frog" (p. 577). Development, therefore, proves what comparative
+anatomy could only foreshadow--the unity of plan of all vertebrate
+skulls, ossified and unossified alike. "We have thus attained to a
+theory or general expression of the laws of structure of the skull. All
+vertebrate skulls are originally alike; in all (save _Amphioxus_?) the
+base of the primitive cranium undergoes the mesocephalic flexure, behind
+which the notochord terminates, while immediately in front of it the
+pituitary body is developed;[221] in all, the cartilaginous cranium has
+primarily the same structure--a basal plate enveloping the end of the
+notochord and sending forth three processes, of which one is short and
+median, while the other two, the lateral trabeculæ, pass on each side of
+the space on which the pituitary body rests, and unite in front of it;
+in all, the mandibular arch is primarily attached behind the level of
+the pituitary space, and the auditory capsules are enveloped by a
+cartilaginous mass, continuous with the basal plate between them. The
+amount of further development to which the primary skull may attain
+varies, and no distinct ossifications at all may take place in it; but
+when such ossification does occur, the same bones are developed in
+similar relations to the primitive cartilaginous skull" (p. 578).
+
+In a word, there is a general plan or primordial type which is
+manifested in the higher forms most clearly in their earliest
+development--an embryological archetype therefore.
+
+Huxley now goes on to consider the relation of this general plan or type
+of the skull to the structure and development of the vertebral column.
+Does the skull in its development show any signs of a composition out of
+several vertebræ? The vertebral column develops as a segmented structure
+round the notochord; the skull develops first as an unsegmented plate
+extending far beyond the notochord. The processes of this basilar plate,
+the trabeculæ, are quite unlike anything in the vertebral column. It is
+true that when the process of ossification begins, separate bones are
+differentiated in the basilar plate one in front of the other, giving an
+appearance of segmentation. The hindmost of these bones, the
+basioccipital, ossifies round the notochord, quite like a vertebral
+centrum, and its side parts which form the occipital arch develop in a
+"remotely similar" way to the neural arches of the vertebræ. The next
+bone, however, the basisphenoid, develops in front of the notochord, and
+shows very little analogy with a vertebral body. The analogy is even
+more far-fetched when applied to the axial bones in front of the
+basisphenoid. The cranium might indeed be divided upon ossification into
+a series of segments bearing a more or less remote analogy with
+vertebræ. "In the process of ossification there is a certain analogy
+between the spinal column and the cranium, but that analogy becomes
+weaker and weaker as we proceed towards the anterior end of the skull"
+(p. 585). The best way to state the facts is to say that both skull and
+vertebral column start in their development from the same point, but
+immediately begin to diverge. The clear indications of segmentation
+which fully ossified adult skulls undoubtedly show are, therefore,
+secondary, and the vertebral theory of the skull, which was originally
+based upon the appearance of such fully ossified crania, is on the whole
+negatived by embryology.
+
+We have now to turn back a few years in order to follow up another line
+of discovery which had an important bearing upon the theory of the
+vertebrate skull--the working out of the distinction between membrane
+and cartilage bones.
+
+As early as 1731, R. Nesbitt,[222] in two lectures delivered to the Royal
+College of Surgeons, demonstrated that in the human foetus some bones
+were formed not in cartilage but directly in fibrous tissue, and this
+observation was confirmed by other human anatomists, particularly by
+Sharpey at a considerably later date. In 1822 Arendt[223] focussed
+attention upon the remarkable structure of the skull of the Pike, with
+its cartilaginous brain-box studded all over with bony plaques, an
+arrangement which had already attracted the interest of Cuvier and
+Meckel. K. E. von Baer[224] in 1826 discussed at some length the relation
+between the bony and the cartilaginous skull in fishes, with particular
+reference to the sturgeon, coming to the following just conclusion:--"If
+we consider the fibrous skeleton of _Ammocoetes_ as the first foundation
+of the skeleton of Vertebrates, we can form a series among the
+cartilaginous fishes, according as a cartilaginous skeleton penetrates
+more and more into this fibrous foundation. In the same way the process
+of ossification supplants the cartilaginous skeleton. So long as the
+ossifications lie in the skin, as in the sturgeon, they form corneous
+bones (_Hornknochen_), but when they lie under the skin, they form true
+bones, _e.g._, the bones of the skull in the pike" (p. 374).
+
+Embryologists soon become aware that a similar distinction between a
+primitive cartilaginous foundation and a secondary overlying
+ossification of the skull showed itself in the development of all
+Vertebrates. Dugès, in his _Recherches sur l'ostéologie et la myologie
+des Batraciens_ (1834), distinguished between such bones as are formed
+by direct ossification of the cartilaginous groundwork of the skull, and
+such as are developed in the periosteal fibrous tissue.
+
+Reichert in 1838[225] noted that several of the skull bones in Amphibia
+are formed without the intermediary of cartilage, such as the nasals,
+the maxillaries and the lacrymals. So, too, the frontals and parietals
+of Teleosts developed independently of the cartilaginous skull, and
+belonged to the skeletal system of the skin, not to the true vertebral
+axial skeleton (pp. 215-6). Even more interesting was his discovery,
+afterwards confirmed by Hertwig,[226] that in the newt several bones
+connected with the palate were formed in the mucous membrane of the
+mouth by the fusion of a number of little conical teeth (p. 97). Certain
+of these bones he considered to be the substitutes, not the equivalents,
+of the palatine and pterygoid of other Vertebrates, which are formed
+from the upper part of the first visceral arch, a part missing in the
+newt (p. 100). Owing to the difference of development he would not
+homologise these bones in the newt with the palatine and pterygoid of
+other Vertebrates. He recognised also that the bone now known as the
+parasphenoid was developed in the frog in the mucous membrane of the
+mouth, and had originally no connection with the cranial basis (p. 34).
+Rathke in 1839 also allowed the distinction between cartilage and
+membrane bone, but laid no stress upon it (_Entw. d. Natter._, p. 197).
+
+Jacobson in 1842[227] introduced the useful term, "primordial cranium,"
+for the primitive cartilaginous foundation of the skull, and drew a
+sharp distinction between cartilage bones and membrane bones.
+
+In his _Recherches sur les Poissons fossiles_,[228] L. Agassiz used Vogt's
+work on the development of _Coregonus_ to establish a classification of
+the bones of the skull in fish, a classification which had the merit of
+drawing a sharp distinction between the cartilaginous groundwork and
+the "protective plates" of the fish's skull. He recognised that the
+protective plates developed in a different way from the other bones of
+the skull. "We must distinguish," he writes, "two kinds of ossification;
+one which tends to transform the primitive parts of the embryonic
+cranium directly into bone, and another which leads to the deposition of
+protective plates round this core, which develop not only upon the upper
+surface, as has hitherto been supposed, but also on the lateral walls
+and on the lower surface of the cranium" (p. 112). In the skull of all
+fish there are three elements--(1) the cartilaginous base, including the
+nuchal plate, the trabeculæ and the facial plate, together with the
+auditory capsules; (2) the cartilaginous cerebral envelope; (3) the bony
+protective plates (absent in Elasmobranchs). The bones developed in
+relation to these cranial elements can be classified as follows:--(1)
+the basioccipital, exoccipitals (paroccipitals?), supraoccipital and
+"petrous" (_rocher_), developed from the nuchal plate; the ali- and
+orbito-sphenoids developed from the trabeculæ; the "cranial ethmoid"[229]
+developed from the facial plate; (2) the parietals, frontals and nasals
+formed from the "superior" protective plate; the "anterior" and
+"posterior" frontals and the temporal, from the "lateral" plates; the
+body of the sphenoid and the vomer from the "inferior" plates. The other
+element, the cartilaginous brain-box, does not ossify, and tends to
+become absorbed (p. 124).
+
+In 1849 Kölliker published a paper[230] dealing with the morphological
+significance of the distinction between membrane and cartilage bones,
+and in 1850[231] he defended his views against the criticisms of
+Reichert[232] in a further note entitled _Die Theorie des
+Primordialschädels festgehalten_. It is convenient to consider these
+papers together. Kölliker held that there was (1) a histological and (2)
+a morphological difference between the two categories of bones. The
+histological development of the two kinds was different, but this
+difference was not sufficient to establish a morphological distinction
+between them, a distinction in their anatomical _Bedeutung_. The true
+morphological distinction between them was their development in
+different skeleton-forming layers. Membrane bones were developed in
+fibrous tissue lying between the skin and the deep layer which formed
+the primordial cranium, and it was this formation in a separate layer
+that gave them a different morphological significance from the bones
+formed directly in the deep layer. Kölliker's distinction, therefore,
+was between the bones formed in the primordial cartilaginous cranium on
+the one hand, and the superficial ossifications in fibrous tissue on the
+other hand. The cartilaginous cranium in Kölliker's opinion was formed
+upon the vertebral type, and the membrane bones were accessory. This, at
+least, was his opinion in 1849. In 1850, after Stannius had shown that
+membrane bones occurred as integral parts of the vertebræ in certain
+fish, he modified his view of the membrane bones, and admitted them, at
+least in some cases, as constituents of the cranial vertebræ.
+
+On this morphological distinction of membrane and cartilage bones future
+comparative osteology was to be based:--
+
+"My sole aim is to state again the principle upon which comparative
+osteology is to be based and extended, and this is that first place
+should be assigned to anatomical considerations, and among these to the
+manner of origin of the whole bone in relation to the skeleton-forming
+layers" (1850, p. 290).
+
+The homologies established by this new principle might run counter to
+the homologies indicated by the study of adult structure. "Thus, for
+instance, although the lower jaw in position, function, form and shape,
+appears to be the same bone throughout, yet it must be admitted that it
+shows a difference in the different classes. In Mammals and Man it is an
+entirely secondary bone (an extremity according to Reichert), in Birds,
+Amphibia and Fishes only partially so, for its articular belongs to
+Meckel's cartilage and is accordingly analogous to a rib; indeed, in the
+Plagiostomes, etc., the whole lower jaw along with the articular is a
+persistent Meckel's cartilage" (p. 290, 1850).
+
+So, too, the supraoccipital in man cannot be fully homologised with the
+supraoccipital of many mammals, for its upper half arises at first in
+isolation as a secondary bone (p. 290).
+
+Reichert objected to the distinction drawn by Kölliker, and denied that
+there was either a histological or a morphological difference between
+membrane and cartilage bones. It was shown a few years later by H.
+Müller[233] that there was in truth no essential difference in
+histological development between the two categories of bone, that the
+cartilage cells were replaced by bone cells identical with those taking
+part in the formation of membrane bones. The morphological distinction
+continued however to be recognised, particularly by the embryologists.
+Rathke in his volume of 1861[234] classified the bones of the skull
+according to their origin from the primordial cranium or from the
+overlying fibrous layer, distinguishing as membrane bones, the
+parietals, frontals, nasals, lachrymals, maxillaries and premaxillaries,
+jugals, tympanic, parts of the "temporal," vomer, part of the
+supraoccipitals in some mammals, and the mandible (with the exception of
+the articular in such as have a quadrate bone). Huxley was also inclined
+in 1864[235] to recognise the distinction, but he writes with some
+reserve:--"Is there a clear line of demarcation between membrane bones
+and cartilage bones? Are certain bones always developed primarily from
+cartilage, while certain others as constantly originate in membrane? And
+further, if a membrane bone is found in the position ordinarily occupied
+by a cartilage bone, is it to be regarded merely as the analogue and not
+as the homologue of the latter?" (p. 296).
+
+We may note here that many comparative anatomists of the period were
+quite ready to decide Huxley's last question in a sense favourable to
+the older, purely anatomical, view of homology. Owen, for instance, held
+that difference of development did not disturb homologies established by
+form and connections. "Parts are homologous," he writes, "in the sense
+in which the term is used in this work, which are not always similarly
+developed: thus the 'pars occipitalis stricte dicta,' etc., of
+Soemmering is the special homologue of the supraoccipital bone of the
+cod, although it is developed out of pre-existing cartilage in the fish
+and out of aponeurotic membrane in the human subject."[236] Similarly he
+pointed to the diversities of development of the vertebral centrum in
+the different vertebrate classes as proof that development could not
+always be relied upon in deciding homologies (p. 89). But he could not
+deny that the archetype was better shown in the embryo than in the adult
+(_supra_, p. 108).
+
+J. V. Carus[237] likewise stood firm for the older method of determining
+homologies by comparison of adult structure. "We can regard as
+homologous," he writes, "only those parts which in the fully formed
+animal possess a like position and show the same topographical relations
+to the neighbouring parts" (p. 389). Parts homologous in this sense
+might develop in different ways, but no great importance was to be
+attached to such a circumstance. Membrane and cartilage bones developed
+in practically the same way, from the same skeleton-forming layer, and
+no morphological significance attached to their distinction (pp. 227,
+457). Embryology was of considerable value in helping to determine
+homologies, but the evidence that it supplied was contributory, not
+conclusive. Perhaps the greatest service which the study of development
+rendered was to disentangle, by a comparison of the earliest embryos,
+the generalised type (p. 389).
+
+We have now traced, by our historical study of the theory of the skull,
+the gradual evolution of the tendency to find in development the surest
+guide to determining homologies. We have seen how the embryological
+"type" came to be substituted, in whole or in part, for the anatomical
+"type" derived from the study of adult structure. But we have had to do
+only with a modification, not with a transformation, of the criterion of
+homology recognised by the anatomists. Homology is still determined by
+position, by connections, in the embryo as in the adult. "Similarity of
+development" has become the criterion of homology in the eyes of the
+embryologist, but "similarity of development" means, not identity of
+histological differentiation, but similarity of connections throughout
+the course of development. For the purposes of morphology, development
+has to be considered as an orderly sequence of successive forms, not in
+its real nature as a process essentially continuous. Morphology has to
+replace the living continuity by a kinematographic succession of stages.
+Since it is the earliest of these stages that manifest the simplest and
+most generalised structural relations of the parts, it is in the earlier
+stages that homologies can be most easily determined. But these
+homologies are still determined solely by the relative positions and
+connections of the parts, just as homologies are determined in the last
+of all the stages of development, the adult state. And since the
+generalised type is shown most clearly in the earliest stages and tends
+to become obscured by later differentiation, homologies observed in
+embryonic life are to be upheld even if the relations in adult life seem
+to indicate different interpretations.
+
+    [183] See review by Cuvier, _Mém. Mus. Hist, nat._, iii.,
+    pp. 82-97, 1817.
+
+    [184] _Mém. Savans étrangers_, vi. Extract in _Ann. Sci.
+    nat._ (2) i. (_Zool._), pp. 366-72, 1834.
+
+    [185] _Recherches sur la génération des Mammifères_, 1834.
+    _Embryogénie comparée_, 1837.
+
+    [186] "Kiemen bey Säugthieren," _Isis_, pp. 747-9, 1825.
+
+    [187] "Kiemen bey Vögeln," _Isis_, pp. 1100-1, 1825.
+
+    [188] "Ueber die Kiemenbogen und Kiemengefässe beym
+    bebrüteten Hühnchen," _Isis_, xx., pp. 401-3, 1827.
+    (Read in Sept. 1826 to the _Versammlung der deutschen
+    Naturforscher und Aerzte_, then recently founded by
+    Oken).
+
+    [189] _Isis_, pp. 160-4, Pl. II., 1828.
+
+    [190] "Ueber die Kiemen und Kiemengefässe in den Embryonen
+    der Wirbelthiere," Meckel's _Archiv_ for 1827, pp.
+    556-68. Also in _Ann. Sci. nat._, xv., pp. 266-80,
+    280-4, 1828.
+
+    [191] Meckel's _Archiv_, vi., pp. 1-47, 1832.
+
+    [192] _Untersuchungen über die Bildung und Entwickelung
+    der Fluss-Krebses_, Leipzig, folio, 1829. Preliminary
+    notice in _Isis_, pp. 1093-1100, 1825.
+
+    [193] "Untersuchungen über die Bildung und Entwickelung
+    der Wasser-Assel.," _Abh. z. Bild. u. Entwick.-Gesch._,
+    i., pp. 1-20, 1832. Translated in _Ann. Sci. nat._ (2),
+    ii., (_Zool._), pp. 139-57, 1834.
+
+    [194] Kölliker, _Entwickelungsgeschichte_, 2nd ed., p. 17,
+    Leipzig, 1879.
+
+    [195] _Handbuch der Entwickelungsgeschichte des Menschen
+    und ... der Säugethiere und Vögel_, Berlin, 1835.
+
+    [196] _Embryogénie comparée_, 1837; _Histoire générale du
+    développement des corps organisés_, 1847-49.
+
+    [197] _Entwickelungsgeschichte des Kaninchen-Eies_,
+    Braunschweig, 1842; _Entwickelungsgeschichte des
+    Hunde-Eies_, Braunschweig, 1845;
+    _Entwickelungsgeschichte des Meerschweinchens_, Giessen,
+    1852; _Entwickelungsgeschichte des Rehes_, Giessen,
+    1854.
+
+    [198] "It is the rôle of embryology, as my great teacher
+    says, to form the court of appeal for comparative
+    anatomy, and it is from embryology particularly, which
+    has in the last decades provided such signal instances
+    of the unravelling of obscure problems, that we have to
+    expect a definite clearing up of the problems relating
+    to the development of the head."--Müller's _Archiv_, p.
+    121, 1837.
+
+    [199] _Anat.-phil. Unters. ü. d. Kiemenapparat u. d. Zungenbein_, Riga
+    and Dorpat, 1832.
+
+    [200] "Bildungs- und Entwickelungs-geschichte des Blennius viviparus,"
+    _Abhandl. z. Bild. u. Entwick.-Gesch. des Menschen u. der Thiere_,
+    ii., pp. 1-68, Leipzig, 1833.
+
+    [201] _Von den Ur-Theilen des Knochen und
+    Schalen-Gerustes_, Leipzig, 1828.
+
+    [202] _Kiemenapparat_, pp. 107-118.
+
+    [203] _Vergleichende Anatomie der Myxinoiden_. Part I.
+    (Osteology and Myology). (_Abh. königl. Akad. Wiss.
+    Berlin_, for 1834, pp. 65-340, 9 pls., 1836.) Also
+    separately.
+
+    [204] "Ueber die Visceralbogen der Wirbelthiere in
+    Allgemeinen und deren Metamorphosen bei den Vögeln und
+    Säugethiere," Müller's _Archiv_, pp. 120-222, 1837.
+
+    [205] _Handbuch d. menschl. Anatomie_, iv., p. 47.
+
+    [206] This was shown by Serres (_Ann. Sci. nat._, xi., p.
+    54 f.n., 1827), who found in a human embryo a long
+    cartilaginous piece extending from the ear-ossicles to
+    the inside of the lower jaw, and suggested that it was
+    the foundation of the permanent mandible.
+
+    [207] _Abhandl._, i., p. 102, 1832; ii., p. 25, 1833. (_Blennius_
+    paper).
+
+    [208] _Vergleichende Entwickelungsgeschichte des Kopfes der nackten
+    Amphibien_, Königsberg, quarto, 276 pp., 1838.
+
+    [209] Müller's _Archiv_ for 1838.
+
+    [210] _Entwickelungsgeschichte der Natter_, Königsberg,
+    1839.
+
+    [211] _Bemerkungen über die Entwickelung des Schädels der
+    Wirbelthiere_, Königsberg, 1839.
+
+    [212] _Handbuch der Physiologie des Menschen_, Koblenz,
+    1835; Eng. trans. by W. Baly, ii., p. 1615, 1838.
+
+    [213] For a full statement of Rathke's conclusions, see
+    the translation given by Huxley in _Lectures on the
+    Elements of Comparative Anatomy_, London, 1864.
+
+    [214] _Entwickelungsgeschichte der Wirbelthiere_, p. 142,
+    1861.
+
+    [215] _Embryologie des Salmones_. A separate volume of L.
+    Agassiz's _Histoire naturelle des Poissons d'Eau douce
+    de l'Europe centrale_, Neuchâtel, 1842.
+
+    [216] _Untersuchungen über die Entwickelungsgeschichte der
+    Gebürtshelferkröte_, Solothurn, 1842.
+
+    [217] Müller's _Archiv_ for 1843, p. ccxlviii.
+
+    [218] _Untersuchtingen über die Entwickelung der
+    Wirbelthiere_, Berlin, 1850-55.
+
+    [219] Delivered 17th June 1858. Reprinted in _The
+    Scientific Memoirs of T. H. Huxley_, edited by M. Foster
+    and E. Ray Lankester, vol. i., pp. 538-606 (1898).
+
+    [220] _Cf._ Reichert, _supra_, p. 149.
+
+    [221] The origin of the pituitary body from the roof of
+    the mouth was first described by Rathke (1839).
+
+    [222] _Human Osteogeny explained in two Lectures_, London,
+    1736.
+
+    [223] _De capitis ossei Esocis lucii structura singulari.
+    Dissert. inaug._ Regiomonti, 1822.
+
+    [224] "Ueber das äussere und innere Skelet," Meckel's
+    _Archiv_, pp. 327-76, 1826.
+
+    [225] _Vergl. Entwick. d. Kopfes d. nackten Amphibien_ (p.
+    186).
+
+    [226] _Arch. f. mikr. Anat._, xi., Suppl., 1874.
+
+    [227] "Om Primordial-Craniet," _Förhandlingar Skand.
+    Naturf. Möle_, Stockholm, 1842.
+
+    [228] Vol. I., General part, pub. 1844.
+
+    [229] _Entosphenoid_, Owen.
+
+    [230] _Zweiter Bericht zootom. Anstalt zu Würzburg_, 1849.
+
+    [231] _Zeits. f. wiss. Zool._, ii., pp. 281-91.
+
+    [232] Müller's _Archiv_ for 1849, pp. 443-515.
+
+    [233] _Zeits. f. wiss Zool._, ix., 1858.
+
+    [234] _Entw. d. Wirbelthiere_, pp. 139-40, 1861.
+
+    [235] _Lectures on the Elements of Comparative Anatomy_.
+
+    [236] _On the Archetype of the Vertebrate Skeleton_, p. 5,
+    1848.
+
+    [237] _System der thierischen Morphologie_, Leipzig, 1853.
+
+
+
+
+CHAPTER XI
+
+THE CELL-THEORY.
+
+
+With the founding of the cell-theory by Schwann in 1839 an important
+step was taken in the analysis of the degrees of composition of the
+animal body. Aristotle had distinguished three--the unorganised
+material, itself compounded of the four primitive elements, earth and
+water, air and fire, the homogeneous parts or tissues and the
+heterogeneous parts or organs, and this conception was retained with
+little change even to the days of Cuvier and von Baer. Those of the old
+anatomists who speculated on the relations of organic elements to one
+another were dominated by Aristotle's simple and profound
+classification, and proposed schemes which differed from his only in
+detail. Bichat enlarged and deepened the concept of tissue, but the
+degree of composition below this was for him, as for all anatomists of
+his time, a fibrous or pulpy "cellulosity," living, indeed, but showing
+no uniform and elemental structure. It was Schwann's merit to interpose
+between the tissue and the mere unorganised material a new element of
+structure, the cell. And, as it happened, a few years before Schwann
+published his cell-theory, Dujardin hinted at another degree of
+composition which was later to take its place between the cell and the
+chemical elements--sarcode or protoplasm.
+
+As is well known, the concept of the cell arose first in botany. Robert
+Hooke discovered cells in cork and pith in 1667, and his discovery was
+followed up by Grew and Malpighi in 1671, and by Leeuenhoek in 1695. But
+they did not conceive the cell as a living, independent, structural
+unit. They were interested in the physiology of the plant as a whole,
+how it lived and nourished itself, and they studied cells and
+sieve-tubes, wood fibres and tracheæ with a view rather to finding out
+their functions and their significance for the life of the plant than to
+discovering the minutiæ of their structure. The same attitude was taken
+up by the few botanists who in the 18th century paid any heed to the
+microscopical anatomy of plants. For C. F. Wolff,[238] the formation of
+cells was a result of the secretion of drops of sap in the fundamental
+substance of the plant, this substance remaining as cell-walls when
+cell-formation was completed--no idea here of cells as units of
+structure.
+
+In the early 19th century, interest in plant anatomy revived somewhat,
+and much work was done by Treviranus, Mirbel, Moldenhawer, Meyen and von
+Mohl.[239] As a result of their work the fact was established that the
+tissues of plants are composed of elements which can, with few
+exceptions, be reduced to one simple fundamental form--the spherical
+closed cell. Thus the vessels of plants are formed by coalescence of
+cells, fibres by the elongation of cells and the thickening and
+toughening of their walls. At this time, interest was concentrated on
+the cell-wall, to the almost total neglect of the cell-contents; the
+"matured framework" of plant cells, to use Sach's convenient phrase, was
+the chief, almost the sole, object of study. And it was natural enough
+that the mere architecture of the plant should monopolise interest, that
+the composition of the tissues out of the cells, and the fitting
+together of the tissues to form the plant should awaken and hold the
+curiosity of the investigator; even the modifications of the cell-walls
+themselves, their rings and spiral thickenings and pits, offered a
+fascinating field of enquiry.
+
+The idea that the cell-contents might show a characteristic and
+individual structure had hardly dawned upon botanists when Schleiden
+published his famous paper, _Beiträge zur Phytogenesis_.[240] Schleiden's
+theme in this paper is the origin and development of the plant cell, a
+subject then very obscure, in spite of pioneer work by Mirbel. A few
+years before, Robert Brown had called attention to the presence in the
+epidermal cells of orchids and other plants of a characteristic spot
+which he called the areola or nucleus.[241] Schleiden saw the importance
+of this discovery, confirmed the constant presence of the nucleus in
+young cells, and held it to be an elementary organ of the cell. He named
+it the cytoblast because, in his opinion, it formed the cell. It was
+embedded in a peculiar gummy substance, the cytoblastem, which formed a
+lining to the cellulose cell-wall. Within the nucleus there was often a
+small dark spot or sphere--the nucleolus. The nucleus, Schleiden
+thought, originated as a minute granule in the cytoblastem which
+gradually increased in size, becoming first a nucleolus (_Kernchen_),
+and then, by further condensation of matter round it, a nucleus. Several
+nuclei might be formed in this way in a single cell. New cells took
+their origin directly from a full-grown nucleus, in a peculiar way which
+Schleiden describes as follows:--"As soon as the cytoblasts have reached
+their full size a delicate transparent vesicle arises on their surface;
+this is the young cell, which at first takes the shape of a very flat
+segment of a sphere, of which the plane surface is formed by the
+cytoblast, the convex side by the young cell itself, which lies upon the
+cytoblast like a watch-glass on a watch" (p. 145). The young cells
+increase in size and fill up the cavity of the old cell, which is in
+time resorbed. Cell-development always takes place within existing
+cells, and either one or many new cells may be formed within the
+mother-cell. Schleiden's views on cell-formation were drawn from some
+rather imperfect observations on the embryo-sac and pollen-tube, but he
+extended his theory to cell-formation in general. Though wrong in almost
+all respects the theory had at least the merit of fixing attention upon
+the really important constituents of the cell, the nucleus and the
+cell-plasma. To Schleiden, too, we owe the conception of the cell as a
+more or less independent living unity, whose life is not entirely
+identified with the life of the plant as a whole. "Each cell," he
+writes, "carries on a double life; one a quite independent and
+self-contained life, the other a dependent life in so far as the cell
+has become an integral part of the plant" (p. 138).
+
+So long as the definition of the plant cell embraced little more than
+the hardened cell-wall it was little wonder that "cells" in this sense
+were not recognised in animal tissues, except in a few exceptional
+cases--as in the notochord by Johannes Müller.[242] Careful observation of
+animal tissues discovered in some cases the existence of discontinuous
+units of structure, but these were not, as a rule, recognised before
+1838 as analogous to plant cells. Von Baer, for example, observed that
+the young chick embryo was composed partly of an albuminous mass and
+partly of _Kügelchen_ or little globules suspended in it
+(_Entwickelungsgeschichte_, i., pp. 19, 144). Since such _Kügelchen_
+disposed in a row formed the notochord (i., p. 145) it seems probable
+that his _Kügelchen_ were really cells. Similarly A. de Quatrefages[243]
+in 1834 saw and figured segmentation spheres in the developing egg of
+_Limnæa_, but he called them globules and did not recognise their
+analogy with the cells of plants. According to M'Kendrick,[244] Fontana,
+so far back as 1781,[245] described cells with nuclei in various tissues,
+and used acids and alkalis to bring out their structure more clearly.
+But it was not till 1836-7-8 that a fairly widespread occurrence of
+cells in animal tissues was recognised. The pioneer in this seems to
+have been Purkinje, who described cells in the choroidal plexus in
+1836,[246] and compared gland cells with the cells of plants in 1837.[247]
+Henle in 1837[248] and 1838[249] described various kinds of epithelial
+tissue, distinguishing them according to the kind of cell composing
+them; he also discovered the mode of growth of stratified epithelium.
+Valentin[250] appears to have seen cells in cartilage and epithelium even
+before Henle, and to have observed cells in the blastoderm of the chick.
+In his report on the progress of anatomy during 1838 Johannes Müller was
+able to refer to quite a number of papers dealing with the occurrence of
+cells in animal tissues. In addition to those already noted, he mentions
+work by Breschet and Gluge on the cells of the umbilical cord, by
+Dumortier on the cells in the liver of molluscs, by Remak and by
+Purkinje on nerve cells, by Donné on the cells of the conjuctiva, cornea
+and lens. He reports, too, that Turpin had compared the epithelial cells
+of the vagina with the cell-tissue of plants. Müller himself had not
+only recognised the cellular nature of the notochord, but had observed
+the cells of the vitreous humour, fat cells and pigment cells, and even
+the nuclei of cartilage cells. From Schwann (1839) we learn that C. H.
+Schults had followed back the corpuscles of the blood to their original
+state of nucleated cells, and that Werneck had recognised cells in the
+embryonic lens. A preliminary notice of Schwann's own work appeared in
+1838 (Froriep's _Notizen_, No. 91, 1838), the full memoir in 1839, under
+the title _Mikroskopische Untersuchungen über die Uebereinstimmung in
+der Struktur und dem Wachstume der Tiere und Pflanzen_.[251]
+
+Theodor Schwann was a pupil of Johannes Müller, and we know that Müller
+took much interest in the new histology. It is probably to his influence
+that we owe Schwann's brilliant work on the cell, which appeared just
+after Schwann left Berlin for Löwen. Schwann was himself, as his later
+work showed, more a physiologist than a morphologist; he did quite
+fundamental work on enzymes, discovering and isolating the pepsin of the
+gastric juice; he proved that yeast was not an inorganic precipitate but
+a mass of living cells; he carried out experiments directed to show that
+spontaneous generation does not occur. We shall see in his treatment of
+the cell-theory clear indications of his physiological turn of mind.
+Schwann was only twenty-nine when his master-work appeared, and the book
+is clearly the work of a young man. It has the clear structure, the
+logical finish, which the energy of youth imparts to its chosen work. So
+the work of Rathke's prime, the _Anatomische-philosophische
+Untersuchungen_ of 1832 shows more vigour and a more reasoned structure
+than his later papers. Schwann's book is indeed a model of construction
+and cumulative argument, and even for this reason alone justly deserves
+to rank as a classic.
+
+The first section of his book is devoted to a detailed study of the
+structure and development of cartilage cells and of the cells of the
+notochord, and to a comparison of these with plant cells. He accepts
+Schleiden's account of the origin and development of nuclei and cells as
+a standard of comparison; and he seeks to show that nucleus and
+nucleolus, cell-wall and cell-contents, show the same relations and
+behave in the same manner in these two types of animal cells as in the
+plant-cells studied by Schleiden. The types of cell which he chose for
+this comparison are the most plant-like of all animal cells, and he was
+even able to point to a thickening of the cell-wall in certain cartilage
+cells, analogous to the thickening which plays so important a part in
+the outward modification of plant-cells. The analogy indeed in structure
+and development between chorda and cartilage cells and the cells of
+plants seemed to him complete. The substance of the notochord consisted
+of polyhedral cells having attached to their wall an oval disc similar
+in all respects to the nucleus of the plant-cell, and like it containing
+one or more nucleoli. Inside the mother-cell were to be found young
+developing cells of spherical shape, lacking however a nucleus.
+Cartilage was even more like plant tissue. It was composed of cells,
+each with its cell membrane. The cells lay close to one another,
+separated only by their thickened cell-wall and the intercellular
+matrix, showing thus even the general appearance of the cellular tissue
+of plants. They contained a nucleus with one or two nucleoli, and the
+nucleus was often resorbed, as in plants, when the cell reached its full
+development. Other nuclei were in many cases present in the cell, round
+which young cells could be seen to develop, in exactly the same manner
+as in plants. These nuclei had accordingly the same significance as the
+nuclei of plants, and deserved the same name of cytoblasts or
+cell-generators. The true nucleus of the cartilage cell was probably in
+the same way the original generator of the mother-cell.
+
+Having proved the identity in structure and function of the cells of
+these selected tissues with the cells of plants, as conceived by
+Schleiden, Schwann had still to show that the generality of animal
+tissues consisted either in their adult or in their embryonic state of
+similar cells. This demonstration occupies the second and longest
+section of his book.
+
+His method is throughout genetic; he seeks to show, not so much that all
+animal tissues are actually in their finished state composed of cells
+and modifications of cells, as that all tissues, even the most complex,
+are developed from cells analogous in structure and growth with the
+cells of plants.
+
+All animals develop from an ovum; it was his first task to discover
+whether the ovum was or was not a cell. It happened that, some years
+before Schwann wrote, a good deal of work had been done on the minute
+structure of the ovum, particularly by Purkinje and von Baer. Purkinje
+in 1825[252] discovered and described in the unfertilised egg of the fowl
+a small vesicle containing granular matter, which he named the
+_Keimbläschen_ or germinal vesicle. It disappeared in the fertilised
+egg. As early as 1791 Poli had seen the germinal vesicle in the eggs of
+molluscs, but the first adequate account was given by Purkinje. In
+1827[253] von Baer discovered the true ova of mammals and cleared up a
+point which had been a stumbling block ever since the days of von Graaf,
+who had described as the ova the follicles now bearing his name.[254] Even
+von Graaf had noticed that the early uterine eggs were smaller than the
+supposed ovarian eggs; Prévost and Dumas[255] had observed the presence in
+the Graafian follicle of a minute spherical body, which, however, they
+hesitated to call the ovum; it was left to von Baer to elucidate the
+structure of the follicle and to prove that this small sphere was indeed
+the mammalian ovum. His discovery was confirmed by Sharpey and by Allen
+Thomson. Von Baer found the germinal vesicle in the eggs of frogs,
+snakes, molluscs, and worms, but not in the mammalian ovum; he
+considered the whole mammalian ovum to be the equivalent of the germinal
+vesicle of birds--a comparison rightly questioned by Purkinje (1834). In
+1834 Coste[256] discovered in the ovum of the rabbit a vesicle which he
+considered to be the germinal vesicle of Purkinje; he observed that it
+disappeared after fertilisation. Independently of Coste, and very little
+time after him, Wharton Jones[257] found the germinal vesicle in the
+mammalian ovum. Valentin in 1835,[258] Wagner in 1836,[259] and Krause in
+1837,[260] added considerably to the existing knowledge of the structure
+of the ovum. Wagner in his _Prodromus_ called attention to the
+widespread occurrence, within the germinal vesicle of a darker speck
+which he called the _Keimfleck_ or germinal spot, known sometimes as
+Wagner's spot. He recognised the _Keimfleck_ in the ova of many classes
+of animals from mammals to polyps. Frequently more than one _Keimfleck_
+occurred.
+
+Schwann had therefore a good deal of exact knowledge to go upon in
+discussing the significance of the ovum for the cell-theory. There were
+two possible interpretations. Either the ovum was a cell and the
+germinal vesicle its nucleus, or else the germinal vesicle was itself a
+cell within the larger cell of the ovum and the germinal spot was its
+nucleus. Schwann had some difficulty in deciding which of these views to
+adopt, but he finally inclined to the view that the ovum is a cell and
+the germinal vesicle its nucleus, basing his opinion largely upon
+observations by Wagner which tended to prove that the germinal vesicle
+was formed first and the ovum subsequently formed round it. But the ovum
+was not, in Schwann's view, a simple cell, for within it were contained
+yolk-granules, one set apparently containing a nucleus, the others not.
+Even the second set, those composing the yellow yolk, were considered by
+Schwann to deserve the name of cells, because, although a nucleus could
+not be observed in them, they had a definite membrane, distinct from
+their contents--a conception of the cell obviously dating from the
+earliest botanical notions of cells as little sacs. The yolk cells were
+not mere dead food material but living units which took part in the
+subsequent development of the egg. The relation between the unfertilised
+egg and the blastoderm which arises from it is not made altogether clear
+by Schwann. According to his account the cells of the blastoderm are
+formed actually in the ovum. Round the nucleus of the egg appears a
+_Niederschlag_ or precipitate which is the rudiment of the blastoderm
+(p. 68). When the egg leaves the ovary the nucleus disappears, leaving
+behind it this rudiment of the blastoderm, which rapidly grows and
+increases in size. The blastoderm of the chick before incubation is
+found to be composed of spherical anucleate bodies which Schwann
+considers to be cells, because they almost certainly develop into the
+cells of the incubated blastoderm, which are clearly recognisable as
+such after eight hours' incubation. The serous and mucous layers can be
+distinguished after sixteen hours' incubation, and it is found that the
+cells of the serous layer contain definite nuclei, though such seem to
+be absent in the cells of the mucous layer. Between the two layers other
+cells are formed belonging to the vessel layer, which is, however, in
+Schwann's opinion not a very definitely individualised layer.
+
+Schwann's next step is a detailed demonstration of the origin of each
+tissue from simple cells such as those composing the incubated
+blastoderm.
+
+"The foregoing investigation has taught us that the whole ovum shows
+nothing but a continual formation and differentiation of cells, from the
+moment of its appearance up to the time when, through the development of
+the serous and mucous layers of the blastoderm, the foundation is given
+for all the tissues subsequently appearing: we have found this common
+parent of all tissues itself to consist of cells; our next task must be
+to demonstrate not only in this general way that tissues originate from
+cells, but also that the special formative mass of each tissue is
+composed of cells, and that all tissues are either constituted by simple
+cells or by one or other of the manifold kinds of modified cells" (p.
+71). Five classes of tissue can be distinguished, according to the
+extent and manner of the modifications which the cells composing them
+have undergone. There are first of all independent and isolated cells,
+such as the corpuscles of the blood and lymph, not forming a coherent
+tissue in the ordinary sense. Next there are the assemblages of cells
+lying in contiguity with one another, but not in any way fused; examples
+of this class are the epidermal tissues and the lens of the eye. In the
+third class come tissues the cells of which have fused by their walls,
+but whose cell-cavities are not in continuity, such as osseous tissue
+and cartilage. In the tissues of the fourth class, comprising the most
+highly specialised of all, not only are the cell-walls continuous but
+also the cell-cavities; to this class belong muscle, nerve and capillary
+vessels. A fifth class, of rather a special nature, includes the fibrous
+tissues of all kinds. This is the first classification of tissues upon a
+cellular basis, and it marks the foundation of a new histology which
+took the place of the "general anatomy" of Bichat. The exhaustive
+account which Schwann gives of the structure and development of the
+tissues in this section of his book constitutes the first systematic
+treatise on histology in the modern sense, and it is still worth
+reading, in spite of many errors in detail.
+
+Schwann found it easy to demonstrate the cellular nature of the tissues
+of his first three classes. With the other two classes he had more
+difficulty. Fibres of all kinds, he considered, arose by an elongation
+of cells, which afterwards split longitudinally into long strips,
+forming as the case might be white or elastic fibrous tissue.
+Muscle-fibres and nerve-fibres were formed in a totally different way,
+by coalescence of cells; each separate muscle-fibre and nerve-fibre was
+thus a compound cell. Capillaries, Schwann held, were formed by cells
+hollowed out like drain-pipes, and set end to end--a mistaken view soon
+corrected by Vogt (_Embryologie des Salmones_, p. 206, 1842).
+
+In this detail part of his book Schwann accumulates material for a
+general theory of the cell which he develops in the third and last
+section. Taking up the physiological or dynamical standpoint, he points
+out that one process is common to all growth and development of tissues
+both in animals and plants, namely, the formation of cells, a process
+which he conceives to take place in the following manner. There is,
+first of all, a structureless substance, the cytoblastem, the matrix in
+which all cells originate. The cytoblastem may be either inside the
+cells, or, more usually, in the spaces between them. It is not a
+substance of definite chemical and physical properties, for the matrix
+of cartilage and the plasma of the blood alike come within the
+definition. It has largely the significance of food material for the
+developing cells. In plants, according to Schleiden, cells are never
+formed in the intercellular substance--the cytoblastem is within the
+cells; but extracellular cell formation seems to be the general rule in
+animals. An intracellular formation of cells occurs only in the ovum, in
+cartilage cells and chorda cells and in a few others, and even there it
+is not the exclusive method of formation; a formation of cells within
+cells never occurs in muscles and nerves, nor in fibrous tissue (p.
+204). In the cytoblastem granules appear, which gradually increase in
+size and take on the characteristic shape of nuclei; round each of these
+a young cell is formed. Sometimes the young cells appear to have no
+nuclei, as in the intracellular brood of chorda cells, but, as a rule, a
+nucleus is clearly visible. The nucleus is indeed the most
+characteristic constituent of the cell. "The most important and most
+constant criterion of the existence of a cell is the presence or absence
+of the nucleus," writes Schwann near the beginning of his book (p. 43).
+
+As a general rule the nucleolus is formed first, and round it by a sort
+of condensation or concretion the nucleus, which is frequently hollow,
+and round this again, by a somewhat similar process, the cell. "The
+whole process of the formation of a cell consists in the precipitation
+round a small previously formed corpuscle (the nucleolus) of first one
+layer (the nucleus) and then later round this a second layer (the cell
+substance)" (p. 213). The outermost layer of the cell usually thickens
+to form the membrane, but this membrane formation does not always occur,
+and the membrane is not present in all cells. The nucleus is formed in
+exactly the same manner as the cell, and it might with much truth itself
+be called a cell--a cell of the first order, while ordinary nucleated
+cells might be designated cells of the second order (p. 212). In
+anucleate cells there is probably only a single process of layer
+formation round an infinitely small nucleolus. In almost all nucleate
+cells the nucleus is resorbed when the cell reaches its full
+development, and it is larger and more important the younger the cell
+is.
+
+The cell was for Schwann not a morphological concept at all, but a
+physiological; the cell was a dynamical, not a statical unit.
+Cell-formation was the process at the back of all production of life,
+and cells were the centres of all vital activity. Each cell was itself
+an organism, and its life and activities were to some extent independent
+of the lives and activities of all the other cells. The multicellular
+organism was a colony of unicellular organisms, and its life was a sum
+of the lives of its constituent elements. This "theory of the organism,"
+which holds so important a place in biology even at the present day, is
+developed by Schwann in the concluding pages of his book.
+
+He begins by contrasting the teleological with the materialistic
+conception of living things. In the teleological view, a special force
+works in the living organism, guiding and directing its activities
+towards a purposeful end. According to the materialistic view there are
+no other forces at work in the living organism than those which act in
+the inorganic realm, or at least there are none but forces at one with
+these in their blindness and necessity. True, the purposiveness of
+living processes cannot be denied; but its ground lies, according to
+this view, not in a vital force which guides and rules the individual
+life, but in the original creation and collocation of matter according
+to a rational plan. The purposiveness of life is part of the
+purposiveness of the universe; just as the stars circle for ever in
+harmoniously adjusted paths, so do the processes of life work together
+towards a common end. Both are the inevitable result of the original
+distribution of matter in the primitive chaos, a distribution fixed by a
+rational and foreknowing Being (p. 222).
+
+Which of the two conceptions is to be adopted in biology? Teleological
+explanations have long been banished from the physical sciences, and in
+biology they are only a last resort when physical explanations have
+proved incomplete (p. 223). And if the ground of the purposiveness of
+living Nature is the same as the ground of the purposiveness of the
+universe, is it not reasonable to suppose that explanations which have
+proved satisfactory for inorganic things will in time with sufficient
+knowledge prove adequate also for organic things?
+
+The teleological conception, again, leads to difficulties particularly
+when it is applied to the facts of reproduction. If we suppose that a
+vital force unifies and coordinates the organism and is its very
+essence, we must also suppose that this force is divisible and that a
+part of it--separated in reproduction--can bring about the same results
+as the whole. If on the contrary the forces having play in the organism
+are the mere result of the particular combination of the matter
+composing it, the reconstruction of a particular combination of
+molecules in the ovum is all that is necessary to set development
+a-going along exactly the course taken by the ovum of the parent.
+Another argument against the teleological view is derived from the facts
+of the cell-theory. The cell-theory tells us that the molecules of the
+living body are not immediately built up in manifold combinations to
+form the organism, but are formed first into unit-constructions or
+cells, and that these units of composition are invariably formed in all
+development, of plants and animals alike, however diverse the goal of
+development may be. If there were a vital principle would we not expect
+to find that, scorning this roundabout way of reaching its goal, it went
+straight to the mark, taking a different and distinctive course for each
+individual development, building up the organism direct without the
+intermediary of cells? But since there is a universal principle of
+development, namely, the formation of cells, does it not seem that the
+cells must be the true organisms, that the whole "individual" organism
+must be an aggregate of cells, and that the concept of individuality
+applied to the organism is accordingly a logical fiction? And it is just
+upon this notion of the individuality of the organism that the
+teleological concept is based. The teleological view can perhaps not be
+completely refuted until the adequacy of materialistic explanations has
+been finally shown; but it is certain that the most promising method for
+research is the materialistic (p. 226).
+
+"We start out then from the assumption that the basis of the organism is
+not a force acting according to a definite plan; on the contrary, the
+organism arises through the action of blind and necessary laws, of
+forces which are as much implicit in matter as those of the inorganic
+world. Since the chemical elements in organic Nature differ in no way
+from those of inorganic Nature, the ground or cause of organic phenomena
+can consist only in a different mode of combination of matter, either in
+a peculiar mode of combination of the elementary atoms to form atoms of
+the second order, or in the particular arrangement of these compound
+molecules to form the separate morphological units of the organism or
+the whole organism itself" (p. 226). Accepting then the materialistic
+conception of the organism, we have to consider this further problem.
+Does the ground of organic processes lie in the whole organism or in its
+elementary parts? Translated into terms of metabolism--note the
+physiological point of view--the question runs, are metabolic processes
+the result of the molecular construction of the organism as a whole, or
+does the centre of metabolic activity lie in the cell? Is it the cell
+rather than the organism that is the immediate agent of assimilatory
+processes? In the first alternative the cause of the growth of the
+constituent parts lies in the totality of the organism; in the other
+alternative:--"Growth is not the result of a force having its ground in
+the organism as a whole, but each of the elementary parts possesses a
+force of its own, a life of its own, if you will; that is to say, in
+each elementary part the molecules are so combined as to set free a
+force whereby the cell is enabled to attract new molecules and so to
+grow, and the whole organism exists only through the reciprocal action
+of the single elementary parts.... In this eventuality it is the
+elementary parts that form the active element in nutrition, and the
+totality of the organism can be indeed a condition, but on this view it
+cannot be a cause" (p. 227).
+
+To help in the decision of this question, appeal must be made to the
+facts established as to the cellular nature of the organism and of its
+reproductive elements. We know that every organism is composed of cells,
+which are formed and grow according to the same laws wherever they are
+found, whose formation therefore is everywhere due to the same forces.
+If we find that certain of these cells--all of which we know to be
+essentially identical one with another--have the power when separated
+from the others of growing and developing into new organisms, we can
+infer that not only such cells but also all other cells have this
+assimilatory power. The ova of animals, the spores of plants, the
+isolated cells of lower organisms in general, all show the power of
+separate assimilation and development. "We must therefore, in general,
+ascribe to the cell an individual life, that is to say, the combination
+of the molecules in the single cell does suffice to produce the force
+whereby the cell is enabled to draw to itself new molecules. The ground
+of nutrition and growth lies not in the organism as a whole, but in the
+separate elementary parts, the cells. The fact that it is not every cell
+that can continue to grow when separated from the organism is not in
+itself an objection to this theory, any more than it is an objection to
+the individual life of a bee that it cannot continue to exist apart from
+the swarm. The activation of the forces existing within the cell depends
+on conditions which the cell encounters only in connection with the
+whole" (pp. 228-9).
+
+Schwann's next step is to discover what are the essential forces active
+in the cell, and here he enters the realm of hypothesis. He finds they
+can be reduced to two--an attractive force and a metabolic force. The
+attractive force is seen in the process of cell-formation, where first
+of all the nucleolus is formed by a concentration and precipitation of
+substances found free in the cytoblastem, and in the same way the
+nucleus and later the cell are laid down as concentric precipitates from
+the cytoblastem. Cell-formation also involves the second or metabolic
+force, by means of which the cell alters the chemical composition of the
+medium surrounding it so as to prepare it for assimilation. Schwann's
+attractive force brings about the actual taking up of the prepared
+substance; his metabolic force is the cause of the digestion of food
+substances, and is nearly identical with enzyme action. With what
+inorganic process, he now asks (p. 239), can the process of
+cell-formation be most nearly compared, and the answer obviously is,
+with the process of crystallisation. Cells are, it is true, quite
+different in shape and consistency from crystals, and they grow by
+intussusception, not by apposition--their plastic or attractive forces
+seem therefore to be different. A still more important difference is
+that the metabolic force is peculiar to the cell. Yet there are
+important analogies between crystals and cells. They agree in the
+important respect that they both grow in solutions at the cost of the
+dissolved substance, according to definite laws, and develop a definite
+and characteristic shape. It might even be maintained, Schwann thinks,
+that the attractive force of crystals is really identical with that of
+cells, and that the difference in result is due merely to the difference
+between the substance of the cell and the substance of the crystal. He
+points out how organic bodies are remarkable for their powers of
+imbibition, and he seeks to show that the cell is the form under which a
+body capable of imbibition must necessarily crystallise, and that the
+organism is an aggregate of such imbibition-crystals. The analogy
+between crystallisation and cell-formation he works out in the following
+manner:--"The substance of which cells are composed possesses the power
+of chemically transforming the substance with which it is in immediate
+contact, in somewhat the same way as the well-known preparation of
+platinum changes alcohol into acetic acid. Each part of the cell
+possesses this property. If now the cytoblastem is altered by an already
+formed cell in such a way that a substance is formed that cannot become
+part of the cell, it crystallises out first as the nucleolus of a new
+cell. This in its turn alters the composition of the cytoblastem. A part
+of the transfomed substance may remain in solution in the cytoblastem or
+may crystallise out as the beginning of a new cell; another part, the
+cell-substance, crystallises round the nucleolus. The cell-substance is
+either soluble in the cytoblastem and crystallises out only when the
+latter is saturated with it, or it is insoluble and crystallises as soon
+as it is formed, according to the aforementioned laws of the
+crystallisation of imbibition-bodies; it forms thus one or more layers
+round the nucleolus, etc. If one imagines cell-formation to take place
+in this way, one is led to think of the plastic force of the cell as
+identical with the force by means of which a crystal grows" (pp.
+249-50).
+
+Two difficulties have to be faced by this theory--(1) the origin of the
+metabolic power of the cells, (2) the reason why the cells arrange
+themselves so as to form an organism of complex and definite structure.
+Schwann tries to explain the origin of the "metabolic" action, the
+analogy of which with the contact-action of colloidal platinum he
+recognises, by attributing it to the peculiar structural arrangements of
+molecules. In attempting to account for the harmonious structure of the
+organism he points to the analogy of ordinary crystals, which often form
+complex and regular tree-like arrangements; plants in particular
+resemble these regularly shaped crystal-aggregates.
+
+The whole ingenious theory is offered merely as an hypothesis and a
+guide to research. It is interesting as one of the most carefully
+thought-out attempts ever made to give a thorough-going materialistic
+account of the origin and development of organic form, and it arose
+directly out of the cell-theory.
+
+Schleiden and Schwann started out from an erroneous theory of the origin
+and development of cells, which impaired to some extent the value of
+their results. It was not long, however, before their theory of the
+origin of cells by "crystallisation" from an intra- or extra-cellular
+cytoblastem was challenged and overthrown, and the generalisation that
+cells originate by division from pre-existing cells put in its place.
+
+This was established for plant cells by Meyen, Unger, von Mohl, Naegeli
+and Hofmeister in or about the forties.[261] Criticism of the
+Schwann-Schleiden theory from the zoological side was suggested by the
+study of the segmentation of the ovum--the developmental process in
+which the multiplication of cells is most easily observed. The
+segmentation of the ovum was well known to Schwann, for the process had
+been described in the frog by Prévost and Dumas in 1824,[262] in the frog
+and newt by Rusconi,[263] and an elaborate study of the process in the
+frog had been made by von Baer.[264] Schwann indeed suspected that there
+must be some connection between the segmentation of the ovum and the
+formation of cells, but he did not realise that the cellular blastoderm
+of the chick was formed by the division or segmentation of the egg-cell.
+
+Segmentation was soon found to be of widespread occurrence. Von Siebold
+in 1837 described the process in Entozoa,[265] and in the same year Lovén
+saw segmentation in _Campanularia_,[266] and Sars in the starfish and in
+Nudibranchs.[267]
+
+In 1838 Bischoff[268] observed segmentation in the mammalian ovum, and the
+whole course of segmentation in the ovum of the rabbit from the 2-celled
+to the morula stage was carefully described and figured by Barry[269] in
+1839. C. Vogt[270] in 1842 described segmentation in _Coregonus_ and
+_Alytes_. The discovery of segmentation in the ovum of birds was not
+made until 1847, by Bergmann,[271] confirmed independently by Coste[272]
+in 1850. By 1848 segmentation had been noted in _Hydra_ and various
+hydroids, in acalephs, in starfish, polyzoa, nematodes, rotifers,
+leeches, oligochætes, polychætes, in most groups of molluscs and
+arthropods, and in all the vertebrate classes.[273]
+
+The process was at first held to be merely one of yolk-division, or
+_Dotterfurchung_, and its details were by most interpreted in the light
+of the Schleiden-Schwann theory of cell-formation.
+
+The first steps towards a truer conception of the process seem to have
+been taken by Bergmann, who in 1841[274] called attention to the presence
+of nuclei in the segmentation-spheres of the frog's egg, and by Bagge in
+the same year, who observed that division of the nuclei preceded the
+multiplication of the segmentation spheres.[275] He considered the nuclei
+to be anucleate cells, and the same view was taken by Kölliker in
+1843.[276] Next year, however, in his classical paper on Cephalopod
+development[277] Kölliker came to the opinion that they were really
+nuclei. He showed that segmentation was brought about by cell-division,
+that between "total" and "partial" segmentation there was a difference
+of degree and not of kind, and that the cells of the body were formed by
+division of the segmentation spheres. He held, however, that the nuclei
+multiplied endogenously and not by division. The division of nuclei was
+observed by Coste in 1846.[278] Leydig in 1848[279] took the necessary step
+in advance and maintained that the nuclei as well as the cells increased
+always by division. He was supported by Remak, who in a paper of
+1852,[280] and more fully in his monumental _Untersuchungen über die
+Entwickelung der Wirbelthiere_ (Berlin, 1850-55), proved that in the
+frog's egg at least segmentation was a simple process of cell-division,
+initiated always by division of the nucleus.[281]
+
+One point Remak left undecided--the fate of the _Keimbläschen_ or
+egg-nucleus. It was generally held, even so late as the 'fifties, that
+the egg-nucleus disappeared just before segmentation began--Bischoff
+clung to this belief even in 1877.[282] Though Barry had held in 1839 that
+the egg-nucleus does not disappear in segmentation, J. Müller seems to
+have been the first actually to prove that it forms by division the
+nuclei of the first two segmentation spheres. He furnished the
+demonstration in the egg of _Entoconcha mirabilis_,[283] and his paper was
+known to Remak, who could not, however, observe a similar division of
+the egg-nucleus in the frog. Müller's discovery was confirmed for
+_Oceania armata_ by Gegenbaur,[284] and for _Notommata sieboldii_ by
+Leydig.[285]
+
+In 1854 Virchow,[286] previously a supporter of Schwann, crystallised the
+new views in the famous phrase--_Omnis cellula e cellula_--and gave wide
+publicity to them in his classical lectures on Cellular Pathology,
+delivered in 1858.[287] The new doctrine of cell-formation was also taught
+by Leydig[7] in his text-book of histology, published in 1857.
+
+The Schleiden-Schwann theory of the origin of cells by generation in a
+cytoblastem was now definitely overthrown.
+
+The importance of the protoplasmic content of the cell was brought into
+prominence through the work of Dujardin,[289] Purkinje,[290] Cohen[291] and
+Max Schultze.[292] The last-named in 1861 proposed a definition of the
+cell which might be accepted at the present day. "A cell," he wrote, "is
+a little blob of protoplasm containing a nucleus" (p. 11).
+
+    [238] _Theoria generationis_, Halae, 1759.
+
+    [239] See J. v. Sachs, _Geschichte der Botanik_, book ii.,
+    Eng. Trans., 2nd impr., 1906.
+
+    [240] Müller's _Archiv_, pp. 137-76, 1838.
+
+    [241] _Trans. Linnean Soc._, xvi., p. 710, 1833.
+
+    [242] _Myxinoiden_, i. Theil., p. 89, 1835.
+
+    [243] _Ann. Sci. nat._ (2) (_Zool._) ii., pp. 107-18, pl.
+    11, 1834.
+
+    [244] _Proc. Phil. Soc. Glasgow_, xix., pp. 71-125,
+    1887-8.
+
+    [245] _Traité sur le venin de la vipère_, 1781.
+
+    [246] Müller's _Archiv_, 1836.
+
+    [247] J. Müller, _Jahresbericht ü. d. Fortschritte der
+    anat.-physiol. Wissenschaften im Jahre_ 1838. Müller's
+    _Archiv_, 1838.
+
+    [248] _Symbolæ ad anatomiam villorum imprimis eorum
+    epithelii_, Berlin, 1837.
+
+    [249] _U. d. Ausbreitung des Epitheliums im menschlichen
+    Körper_. Müller's _Archiv_, 1838.
+
+    [250] See Schwann's _Bemerkungen_ at the end of his
+    _Mikroskopische Untersuchungen_.
+
+    [251] Republished in Ostwald's _Klassiker der exakten
+    Wissenschaften_, No. 176, Leipzig, 1910. References in
+    the text are to the original pagination.
+
+    [252] _Symbolæ ad ovi avium historiam_.
+
+    [253] _De ovi mammalium et hominis genesi_.
+
+    [254] _De mulierum organis_, 1672.
+
+    [255] _Ann. Sci. nat._, iii., p. 135, 1842.
+
+    [256] _Recherches sur la génération des Mammifères_.
+    Report by Academy Committee. _Ann. Sci. nat._ (2)
+    (_Zool._) ii., pp. 1-18, 1834; also _Embryogénie
+    comparée_, 1837.
+
+    [257] _Lond. and Edin. Phil. Mag._ (3) vii., 1835; _Phil.
+    Trans._ 1837.
+
+    [258] _Handbuch der Enfwickelungsgeschichte_, 1835, and
+    Müller's _Archiv_, 1836.
+
+    [259] _Prodromus historiæ generationis hominis atque
+    animalium_, Lipsiæ, 1836.
+
+    [260] Müller's _Archiv_, 1837.
+
+    [261] Sachs, _History of Botany_, Book ii.
+
+    [262] _Ann. Sci. nat._, i., pp. 110-14, 1824. Swammerdam
+    is said to have observed the 2-celled stage in the egg
+    of the frog (_Bibl. Nat._, 1752), and Rösel v. Rosenhof
+    the same stage in the tree-frog (_Hist. nat. ranarum
+    nostratium_, 1758).
+
+    [263] _Développement de la grenouille commune_, Milan,
+    1826. _Biblioteca italiana_, lxxix., 1836, and Müller's
+    _Archiv_, 1836. Agassiz is said by Vogt (1842) to have
+    seen segmentation in the Perch as early as 1831.
+
+    [264] Müller's _Archiv_, 1836.
+
+    [265] In Burdach, _Die Physiologie als
+    Erfahrungswissenschaft_, 2nd Ed., vol. ii.
+
+    [266] Wiegmann's _Archiv_, 1837.
+
+    [267] _Bericht Versamml. deutsch. Naturf. in Prag_, 1837.
+
+    [268] _Bericht Versamm. deutsch. Naturf. in Freiburg_,
+    1838. Later in his _Entw. d. Wirbelth_., and in his
+    papers on the development of the rabbit.
+
+    [269] _Phil. Trans._, 1839. See particularly Pl. vi.,
+    figs. 105-12.
+
+    [270] _Embryologie des Salmones_ 1842.
+
+    [271] Müller's _Archiv_, 1847.
+
+    [272] _C.R. Acad. Sci._, xxx., p. 638.
+
+    [273] See review by Leydig in _Isis_, 1848, pp. 161-193.
+
+    [274] Müller's _Archiv_, pp. 89-102, 1841.
+
+    [275] _De evolution Stronzyli auric. el Ascaridis acum._,
+    Erlangen, 1841.
+
+    [276] Müller's _Archiv_, pp. 66-141, 1843.
+
+    [277] _Entwickelungsgeschichte der Cephalopoden_, Zurich,
+    1844.
+
+    [278] _Froriep's Notizen_, No. 800, 1846.
+
+    [279] _Isis_, 1848.
+
+    [280] Müller's _Archiv_, p. 47, 1852, also 1854 and 1858.
+
+    [281] See particularly Plate IX., figs. 3-7.
+
+    [282] _Hist.-krit. Bemerkungen zu den neuesten
+    Mittheilungen ü. d. erste Entwickelung d.
+    Säugethiereier_, München, 1877.
+
+    [283] _Monatsber. Akad. Wiss. Berlin_, 1851.
+
+    [284] _Zur Lehre von Generationswechsel u. d. Fortpflanzen
+    d. Medusen u. Polypen_.
+
+    [285] _U. d. Bau u. d. system. Stellung d. Räderthiere_,
+    1854.
+
+    [286] _Arch f. path. Anat. Phys._, vii., pp. 1-39, 1854.
+    Also in his _Beiträge z. spec. Path. u. Therapie_.
+
+    [287] _Die Cellularpathologie_, Berlin, 1858.
+
+    [288] _Lehrbuch der Histologie_, 1857.
+
+    [289] _Ann, Sci. nat._ (2) iii., pp. 108-9 and pp. 312-4,
+    1835. Also iv, pp. 343-77.
+
+    [290] 1839 or 1840.
+
+    [2913] _Nova Acta Acad. Leop._, xxii., 1850. Trans. in 1853
+    for Ray Society.
+
+    [292] _Arch. f. Anat. u. Physiol._, pp. 1-27, 1861.
+
+
+
+
+CHAPTER XII
+
+THE CLOSE OF THE PRE-EVOLUTIONARY PERIOD
+
+
+The influence of the cell-theory on morphology was not altogether happy.
+The cell-theory was from the first physiological; cells were looked upon
+as centres of force rather than elements of form, and the explanation of
+all the activities of the organism was sought in the action of these
+separate dynamic centres. There resulted a certain loss of feeling for
+the problems of form. The organism was seen no longer as a cunningly
+constructed complex of organs, tissues and cells; it had become a mere
+cell-aggregate; the higher elements of form were disregarded and
+ignored.
+
+We have seen this physiological attitude expressed with the utmost
+clearness by the founder of the cell-theory himself; we shall see the
+same attitude taken up by most of his successors. Thus Vogt, who was
+later to become one of the protagonists of materialism in Germany,
+developed in his memoir on the embryology of _Coregonus_[293] the theory
+of the independent or individual life of the cell. "Each cell," he
+wrote, "represents in some measure a separate organism, and while their
+development necessarily conforms to the general plan and the particular
+tendencies of the parent organism, they nevertheless each follow their
+own particular tendency and do not lose their independence until, by
+reason of the metamorphoses which they undergo, they lose their cellular
+nature" (p. 275).
+
+And again, "... we are obliged to admit the existence in the cell of an
+independent life, which makes its development self-sufficient.... Each
+cell consequently represents a little independent organism, which
+assimilates foreign substances, builds them up, and rejects those that
+are useless; from this point of view the embryo can be compared up to a
+certain point with a zoophyte stock, of which each polyp, while living
+its own independent life, is yet incorporated in the common corm, which
+impresses its distinctive character upon every polyp" (p. 293).
+
+Classical expression was given to the "colonial theory" of the organism
+by Virchow in his lectures on "Cellular Pathology."[294] For Virchow the
+organism resolves itself into an assemblage of living centres, the
+cells; the organism has no real existence as a unity, for there is no
+one single centre from which its activities are ruled. Even the nervous
+system, which appears to act as a co-ordinating centre, is itself an
+aggregate of discrete cells. "A tree is a body of definite and orderly
+composition, the ultimate elements of which, in every part of it, in
+leaf and root, in stem and flower, are cellular elements--so also are
+animal forms. _Every animal is a sum of vital units_, each of which
+possesses the full characteristics of life. The character and the unity
+of life cannot be found in one definite point of a higher organisation,
+for example in the brain of man, but only in the definite, constantly
+recurring disposition shown individually by each single element. It
+follows that the composition of the major organism, the so-called
+individual, must be likened to a kind of social arrangement or society,
+in which a number of separate existences are dependent upon one another,
+in such a way, however, that each element possesses its own particular
+activity, and, although receiving the stimulus to activity from the
+other elements, carries out its own task by its own powers" (2nd ed.,
+pp. 12-13).
+
+Analysis, decomposition, or disintegration of the organism is here
+pushed to its extreme point, and the problem of recomposition, synthesis
+and co-ordination shirked or forgotten.
+
+The harmful influence of the cell-theory upon morphology did not pass
+unnoticed by the broader-minded zoologists of the day. Virchow's earlier
+paper[295] on the application of the cell-theory to physiology and
+pathology called forth a vigorous protest from Reichert,[296] who
+discussed in a very instructive way the contrast between the older
+"systematic" and the newer "atomistic" attitude to living Nature.
+
+Is it really true, he asks, that the cell is the dominant element in all
+organisation; is the cell comparable in importance to the atom of the
+chemists; or is it not rather the servant of a higher regulatory power?
+Johannes Müller, who was Reichert's master, had in his _Physiology_[297]
+argued splendidly for the existence of a creative force which guides and
+rules development, and brings to pass that unity and harmony of
+composition which distinguish living things from inorganic products.
+Reichert sought in vain in the writings of the biological "atomists" for
+any smallest recognition of these broader characteristics of living
+things upon which Müller had rightly laid stress. For the atomists the
+cell was the only element of form; they ignored the combination of cells
+to form tissues, of tissues to form organs, of organs to form an
+organism. For the morphologists the cell was one element among many, and
+the lowest of all.
+
+The difference of attitude is clearly shown if we consider from the two
+points of view a complicated organ-system such as the central nervous
+system. The atomist sees in this a mere aggregate of cells or at the
+most of groups of cells. "The morphologist," on the other hand, "sees in
+the central nervous system a _proximate_ element in the composition of
+the body--a primitive organ. From this point of view he apprehends and
+judges its morphological relations with, in the first place, the other
+co-ordinated primitive organs in the system as a whole; in all this the
+cells remain in the background, and have nothing to do directly with the
+determination of these morphological relations" (p. 6). Within the
+nervous system there are separate organs which stand to one another in a
+definite morphological and functional relationship. These organs are, it
+is true, composed of cells; but between the form and connections of
+these organs and the cells which compose them there is no direct and
+necessary relation (p. 6). It is true that the cell is the ultimate
+element of organic form, and that all development takes place by
+multiplication and form-change of cells. Yet is the cell in all this not
+independent of the unity of the developing embryo, and what the cells
+produce, they produce, so to speak, not of their own free will, nor by
+chance, but under the guiding influence of the unity of the whole, and
+in a certain measure as its agents (p. 7). The atomists will not admit
+the truth of this; they see in development nothing more than a process
+of the form-change and multiplication of cells. The full meaning of
+development escapes them, for they take no cognisance of the increasing
+complexity of the embryo, of the separating-out of tissues, of the
+moulding of organs, of the harmonious adaptation and adjustment of the
+parts to form a working whole.
+
+In general, the fault of the atomists is that they do not respect the
+limits which Nature herself has prescribed to the process of logical
+analysis and disintegration of the organism; they do not recognise the
+existence of natural and rational units or unities; they forget the one
+great principle of rational analysis, "that, by universally valid,
+inductive, logical method, natural objects must in all cases be accepted
+and dealt with in the combination and concatenation in which they are
+given" (p. 10).
+
+The atomists at least recognised one natural organic element, the cell;
+the materialistic physiologists of the time resolved even this unity
+into an aggregate of inorganic compounds, and regarded the organism
+itself as nothing but a vastly complicated physico-chemical mechanism.
+From this point of view morphology had no right of existence, and we
+find Ludwig, one of the foremost of the materialistic school,
+maintaining that morphology was of no scientific importance, that it was
+nothing more than an artistic game, interesting enough, but completely
+superseded and robbed of all value by the advance of materialistic
+physiology.[298]
+
+Naturally enough, morphologists did not accept this rather contemptuous
+estimate of their science, but held firmly to the morphological
+attitude. So Leuckart in his reply to Ludwig, so Rathke in a letter to
+Leuckart published in that reply, so Reichert in his _Bericht_, so J. V.
+Carus in his _System der thierischen Morphologie_,[299] upheld the
+validity, the independence, of morphological methods. Leuckart and
+Rathke called attention to the absolute impossibility of explaining by
+materialistic physiology the unity of plan underlying the diversity of
+animal form. J. V. Carus, who was convinced of the validity of
+physiological methods within their proper sphere, drew a sharp
+distinction between systematics and morphology on the one hand, and
+physiology on the other. Physiology had nothing to do with the problems
+of form at all; its business was to study the physical and chemical
+processes which lay at the base of all vital activities. Morphology, on
+its part, had to accept form as something given, and to study the
+abstract relations of forms to one another. "On this point," he writes,
+"stress is to be laid, that morphology has to do with animal form as
+something _given_ by Nature, that though it follows out the changes
+taking place during the development of an animal and tries to explain
+them, it does not enquire after the conditions whose necessary and
+physical consequence this form actually is" (p. 24). He expressed indeed
+a pious hope (p. 25) that physiology might one day be so far advanced
+that it could attempt with some hope of success to discover the
+physico-chemical determinism of form, but this remained with him merely
+a pious hope. Reichert, in his _Bericht_, applied to the rather wild
+theorisings of the physiologist Ludwig the same clear commonsense
+criticism that he bestowed on the other "atomists."
+
+It would take too long to describe the great development that
+materialistic physiology took at this time, and to show how the
+separation of morphology from physiology, which originally took place
+away back in the 17th century, had by this time become almost absolute.
+The years towards the end of the first half of the century marked indeed
+the beginning of the classical period as well of physiology as of
+dogmatic materialism. Moleschott and Buchner popularised materialism in
+Germany in the 'fifties, while Ludwig, du Bois Reymond and von Helmholtz
+began to apply the methods of physics to physiology. In France, Claude
+Bernard was at the height of his activity, rivalled by workers almost as
+great. The doctrine of the conservation of energy was established about
+this same time.
+
+Between the cell-theory on the one side, and physiology on the other, it
+was a wonder that morphology kept alive at all. The only thing that
+preserved it was the return to the sound Cuvierian tradition which had
+been made by many zoologists in the 'thirties and 'forties. It is a
+significant fact that this return to the functional attitude coincided
+in the main with the rise of marine zoology, and that the man who most
+typically preserved the Cuvierian attitude, H. Milne-Edwards, was also
+one of the first and most consistent of marine biologists. Milne-Edwards
+describes in his interesting _Rapport sur les Progrès récents des
+Sciences zoologiques en France_ (Paris) 1867, how "About the year 1826,
+two young naturalists, formed in the schools of Cuvier, Geoffroy and
+Majendie, considered that zoology, after having been purely descriptive
+or systematic and then anatomical, ought to take on a more physiological
+character; they considered that it was not enough to observe living
+objects in the repose of death, and that it was desirable to get to
+understand the organism in action, especially when the structure of
+these animals was so different from that of man that the notions
+acquired as to the special physiology of man could not properly be
+applied to them" (p. 17). The two young naturalists were H.
+Milne-Edwards and V. Audouin. In pursuance of these excellent ideas they
+set to work to study the animals of the seashore, producing in 1832-4
+two volumes of _Recherches pour servir à l'histoire naturelle du
+littoral de la France_. After Audouin's early death A. de Quatrefages
+was associated with Milne-Edwards in this pioneer work, and their
+valiant struggles with insufficient equipment and lack of all laboratory
+accommodation, and the rich harvest they reaped, may be read of in
+Quatrefage's fascinating account of their journeyings.[300] Note that
+though they called themselves physiologists they meant by physiology
+something very different from the mere physical and chemical study of
+living things. They were interested, as Cuvier was, primarily in the
+problems of form; they sought to penetrate the relation between form and
+function; their chief aim was, therefore, the study not of physiology[301]
+in the restricted sense, but physiological morphology. As a matter of
+fact they produced more taxanomic and anatomical work than work on
+physiological morphology, but this was only natural, since such a wealth
+of new forms was disclosed to their gaze. Milne-Edwards' masterly
+_Histoire Naturelle des Crustacés_[302] and A. de Quatrefage's _Histoire
+Naturelle des Annelés marins et d'eau douce_[303] were typical products of
+their activity.
+
+In the North, men like Sars and Lovén were starting to work on the
+littoral fauna of the fjords; in Britain, Edward Forbes was opening up
+new worlds by the use of the dredge; Johannes Müller was using the
+tow-net to gather material for his masterly papers on the metamorphoses
+of Echinoderms.[304] Work on the taxonomy and anatomy of marine animals
+was in general in full swing by the 'fifties and 'sixties.
+
+This return to Nature and to the sea had a very beneficial effect upon
+morphology, bringing it out from the laboratory to the open air and the
+seashore. It saved morphology from formalism and aridity, and in
+particular from a certain narrowness of outlook born of too close
+attention paid to the details of microscopical anatomy. It brought
+morphologists face to face again with the wonderful diversity of organic
+forms, with the unity of plan underlying that diversity, with the
+admirable adjustment of organ to function and of both to the life of the
+whole.
+
+Milne-Edwards' theoretical views, as expounded in his _Introduction à la
+zoologie générale_ (1851), well reflect this Cuvierian attitude.[305] He
+acknowledges himself the debt he owes to Cuvier; "the further I advance
+in the study of the sciences which he cultivated with so sure a hand,"
+he writes in 1867, "the more I venerate him."
+
+Milne-Edwards frankly takes up the teleological standpoint, and
+interprets organic forms on the assumption that they are purposive and
+rationally constructed. "To arrive at an understanding of the harmony of
+the organic creation," he writes, "it seemed to me that it would be well
+to accept the hypothesis that Nature has gone about her work as we would
+do ourselves according to the light of our own intelligence, if it were
+given us to produce a similar result. Comparing and studying living
+things as if they were machines created by the industry of man, I have
+tried to grasp the manner in which they might have been invented, and
+the principles whose application would have led to the production of
+such an assemblage of diversified instruments" (p. 435). The problem is
+to discover the laws which rule the diversity of organic forms. The
+first and most obvious of these laws is the "law of economy," or the law
+of unity of type. Nature, as Cuvier pointed out, has not had recourse to
+all the possible forms and combinations of organs; she appears to work
+with a limited number of types and to get the greatest possible
+diversity out of these by varying the proportions of the constitutive
+materials of structure. Within the limits of each type Nature has
+brought about diversity by raising her creatures to different degrees of
+perfection. This is the second law of organic form, and it is this law
+that Milne-Edwards chiefly elaborates. Degrees of perfection mean for
+him, as for Aristotle, primarily degrees of perfection of function, but
+since structure is necessarily in close relation with function,
+perfection of function brings in its train increased perfection of
+organisation. This can only be attained by a division of labour[306] among
+the organs and by their consequent differentiation. An animal is like a
+workshop where some complicated product is manufactured, and the organs
+are like the workmen. Each workman has his own special piece of work to
+do, at which he becomes thoroughly expert; and the finished product is
+manufactured more rapidly and efficiently by the co-operation of workers
+each skilled in one department than it would be if each workman had to
+produce the whole. Applied to the organism this principle of the
+division of labour means the differentiating out of the separate
+functions, their localisation in different parts of the organism, and
+their co-ordination to produce a combined result.
+
+This differentiation of functions implies a corresponding
+differentiation of organs, but it is functional differentiation which
+always takes the lead. "Where division of labour has not been introduced
+into the organism there must exist a great simplicity of structure. But
+just as uniformity in the functions of the different parts of the body
+implies a uniformity in their mode of constitution, so diversity in
+function must be accompanied by particularities in structure; and, in
+consequence also, the number of dissimilar parts must be augmented and
+the complication of the machine increased" (p. 463). Since function
+comes before form there is not always a special organ for every
+function. "It is a grave error to believe that a particular function can
+be performed only by one and the same organ. Nature can arrive at the
+desired result by various ways, and when we look down through the animal
+kingdom from the highest to the lowest forms we see that the function
+does not disappear even when the special instrument provided for the
+purpose in the higher types ceases to exist" (p 470).
+
+Nature, holding fast to the law of economy, does not even always create
+a new organ for a new function; she may simply adapt an undifferentiated
+part to special functions, or she may even convert to other uses an
+organ already specialised (p. 464). So, for example, the function of
+respiration is in the lowest animals diffused indifferently over the
+whole surface of the body, and only as organisation advances is it
+localised in special organs, such as gills. Now suppose that Nature
+wishes to adapt a fish, which breathes by gills, to life in the air; she
+does not create an organ specially for this purpose, but utilises the
+moist gill-chamber (_e.g._, in _Anabas scandens_), modifying it in
+certain ways so that the fish can take advantage of the oxygen it
+contains. But this gill-chamber lung is at best a makeshift, and when
+she comes to the more definitely terrestrial Amphibia Nature gives up
+the attempt to use the gill-chamber as a lung, and creates a new organ,
+the true vertebrate lung, specially adapted for breathing air (p. 475).
+
+But whatever means Nature adopts, her aim is always the same--to
+specialise, to differentiate, to produce diversity from uniformity.
+
+Differentiation not only raises the level of organisation; it usually
+also takes the direction of adaptation to particular habits of life, and
+this is perhaps the most fruitful cause of diversity. Everywhere we find
+animals specialised in adaptation to their environment--to life in air
+or water, or on land--and many of their most striking differences are
+due to this cause. But adaptation may also act in reducing diversity,
+for there necessarily occur many instances of parallel adaptation or
+convergence. So we get the extraordinary parallelism between the
+families of marsupials and the orders of placentals,[307] the remarkable
+similarity between the respiratory organs of land-crabs and
+air-breathing fish--to mention only two out of an immense range of
+analogous facts.
+
+The last cause of diversity that Milne-Edwards adduces is what he calls
+a "borrowing" of peculiarities of structure from another systematic
+group. Thus, "among reptiles, the tortoises seem to have borrowed from
+birds some of their characteristic features of organisation; and among
+the sauroid fishes the piscine type seems to have been influenced by the
+type from which reptiles are derived" (p. 479). So many riddles that, a
+little later on, stimulated the ingenuity of the evolutionists!
+
+Such, then, were the factors which Milne-Edwards considered adequate to
+explain the rich variety of animal forms. We cannot do better than quote
+his own summary of his doctrine:--"To sum up, then, the great
+differences introduced by Nature into the constitution of animals seem
+to depend essentially upon the existence of a certain number of general
+plans or distinct types, upon the perfecting in various degrees either
+of the whole or of parts of each of these structural plans, upon the
+adaptation of each type to varied conditions of existence, and upon the
+secondary imitation of foreign types by certain derivatives of each
+particular type" (p. 480).
+
+We have laid stress on the fact that Milne-Edwards put function before
+form, for this is the mark of the true Cuvierian. With it goes the
+belief that Nature forms new parts to meet new requirements, that she is
+not limited, as Geoffroy thought, to a definite number of "materials of
+organisation," but can produce others at need. Cuvier held, for example,
+that many of the muscles and even the bones of fish were peculiar to
+them, and without homologues in the other Vertebrates, having been
+created by Nature for special ends.[308] So, too, Johannes Müller, who in
+many ways and not least in his sane vitalism was a follower of the
+Cuvierian tradition, recognised that many of the complicated cartilages
+in the skull of Cyclostomes were specially formed for the important
+function of sucking, and had no equivalent in other fish.[309]
+
+So, too, the embryologists after Cuvier often came across instances of
+the special formation of parts to meet temporary needs. Thus Reichert
+interpreted the "palatine" and "pterygoid," which are formed in the
+mouth of the newt larva by a fusion of conical teeth, as special
+adaptations to enable the little larva to lead a carnivorous life.[310]
+
+Not many years after the publication of Milne-Edwards' _Introduction à
+la zoologie générale_ (1851) there appeared a book by H. G. Bronn in
+which was offered a very similar analysis of organic diversity. The
+curious thing was that Bronn approached the problem from quite a
+different standpoint, from the standpoint, indeed, of
+_Naturphilosophie_. Of this the title of the book is itself sufficient
+proof--_Morphologische Studien über die Gestaltungs-gesetze der
+Naturkörper überhaupt und der organischen insbesondere_ (Leipzig and
+Heidelberg, 1858).[311] The linking up of organic with inorganic form is
+characteristic; there is much talk, too, in the book of _Urstoffe_ and
+_Urkräfte_, but underlying the _Naturphilosophie_ we can trace the same
+Cuvierian treatment of form, and see crystallise out laws of progressive
+development that bear no small analogy with the laws established by
+Milne-Edwards.
+
+According to Bronn, the ideal fundamental form of the plant is an ovoid
+or strobiloid[312] body, for a plant reaches out in two directions in
+search of food--towards the sun and towards the earth. Animals differ
+from plants in being endowed with sensation and mobility (_cf._
+Aristotle and Cuvier), and it is this characteristic that gives them
+their distinctive form. The main types of animal form--the Amorphozoa,
+Actinozoa, and Hemisphenozoa--are essentially adaptations to particular
+modes of locomotion. Animals either are fixed, or they move in all
+directions without reference to any definite axis, or they move in one
+main direction.
+
+The Amorphozoa or shapeless animals include many of the Protozoa and
+sponges; they have no typical form, and most of them are sessile. The
+Actinozoa include such animals as the Coelentera, which are fixed, and
+the Echinoderms, which have a central point and move indifferently along
+any radial axis; their form differs from the strobiloid mainly in having
+radiate rather than spiral symmetry. The Hemisphenozoa, or bilaterally
+symmetrical animals, include all those that habitually move forward;
+they have a front end and a hind end, a dorsal surface and a ventral,
+and the mouth, sense-organs and "brain" are concentrated in the front
+end to form a head--all in direct adaptation to this forward movement;
+they make up the vast majority of animals.
+
+The fundamental forms of living things are, however, merely so many
+themes on which a multitude of further variations are woven, through the
+action of the laws which rule the detail of organic diversities. These
+further laws may be set down under four main heads. Under the first
+comes the law of the existence of certain fundamentally distinct
+structural types, which are distinguished from one another by their
+ground-form, by the number of organ-systems, and by the number of
+homotypic organs they possess, but principally by the relative position
+of the organs to one another (principle of connections). The form and
+connections of the nervous system are of particular importance in
+distinguishing the types (_cf._ Cuvier). The second factor in the
+diversity of organic form is the action of certain laws of progressive
+development[313] (_Entwickelungsgesetze_), which bear the same relation to
+the development of the animal kingdom as the laws of individual
+development bear to the development of the embryo, for organs appear in
+the different animal series in much the same order and manner as they
+develop in the individual. These laws are (1) progressive
+differentiation of functions and organs; (2) numerical reduction of
+serially repeated parts; (3) concentration of functions and their organs
+in particular parts of the body; (4) centralisation of organ-systems and
+parts of such, so that they come to depend upon one central organ; (5)
+internalisation of the "noblest" organs, unless these are necessarily
+external, and (6) increase in size of the whole or of parts. Of these
+the law of differentiation is by far the most important, and most of the
+others are in a sense merely special cases of this fundamental law. To
+this law of differentiation is due the increase in complexity or
+perfection of organisation which is shown by all the animal series.
+Bronn himself recognised the great similarity of this law of progressive
+differentiation to Milne-Edwards' principle of the division of labour;
+he seems, however, to have arrived at it independently.
+
+Bronn's third factor in the production of variety of form is adaptation
+to environment, or better, functional response to environment. Bronn
+gives an excellent account of adaptational modifications and calls
+attention, just as Milne-Edwards did, to the numerous analogies of
+structure which adaptation brings about. He works out the interesting
+view that there is some connection between classificatory groups and
+adaptational forms, especially such as are connected with the function
+of locomotion:--"Based upon a common characteristic method of locomotion
+are whole or nearly whole sub-phyla (Hexapoda), classes (mammals and
+reptiles, birds, fishes, gastropods, pteropods, brachiopods, Bryozoa,
+Rotifera, jelly-fish, polypes, sponges), sub-classes (mobile and
+immobile lamellibranchs, echinoderms, walking and swimming Crustacea,
+parasitic and free-living worms, and so on), often, however, only orders
+and quite small groups (snakes, eels, bats, sepias, medusæ, etc.)" (p.
+141).
+
+It was characteristic of the 'forties and 'fifties that transcendental
+anatomy, along with Nature-philosophy, went rather out of fashion, its
+false simplicities and premature generalisations being overwhelmed by
+the flood of new discoveries. A few stalwarts indeed upheld
+transcendental views. We have already discussed the morphological system
+built up by Richard Owen in the late 'forties, a system transcendental
+in its main lines. We have seen the vertebral theory of the skull still
+maintained in the 'fifties by such men as Reichert and Kölliker, and we
+find J. V. Carus in 1853[314] taking it as almost conclusively proved.[315]
+
+We may mention, too, as showing clear marks of the influence of
+transcendental ideas, L. Agassiz's work on the principles of
+classification.[316] And Serres, who was Geoffroy's chief disciple,
+recanted not a whit of his doctrine of recapitulation, but re-affirmed
+and expanded it from time to time, and particularly in a lengthy memoir
+published in 1860.[317] But in general we may say that pure morphology in
+the Geoffroyan or Okenian sense was becoming gradually discredited. A
+curious indication of this is seen in the fact that not only the idea
+but the very word "Archetype" came to be regarded with suspicion. Thus
+even J. V. Carus, who had much affinity with the transcendentalists,
+wrote of the vertebrate archetype (which he took over almost bodily from
+Owen)--"It may here be observed that this schema may be used as a
+methodological help, but it is not to be placed in the foreground"
+(_loc. cit._, p. 395). Huxley, who was definitely a follower of von
+Baer, was much more outspoken with regard to ideal types. In an
+important memoir on the general anatomy of the Gastropoda and
+Cephalopoda,[318] he set himself the task of reducing all their complex
+forms to one type. In summing up, he writes:--"From all that has been
+stated, I think that it is now possible to form a notion of the
+archetype of the Cephalous Mollusca, and I beg it to be understood that
+in using this term, I make no reference to any real or imaginary 'ideas'
+upon which animal forms are modelled. All that I mean is the conception
+of a form embodying the most general propositions that can be affirmed
+respecting the Cephalous Mollusca, standing in the same relation to them
+as the diagram to a geometrical theorem, and like it, at once imaginary
+and true" (i., p. 176). Again, in his Croonian lecture on the theory of
+the vertebrate skull, he remarks that a general diagram of the skull
+could easily be given. "There is no harm," he continues, "in calling
+such a convenient diagram the 'Archetype' of the skull, but I prefer to
+avoid a word whose connotation is so fundamentally opposed to the spirit
+of modern science" (_Sci. Memoirs_, vol. i., p. 571).
+
+It is instructive to find that between Serres and Milne-Edwards there
+existed the same antagonism as between von Baer and the German
+transcendentalists. Milne-Edwards was a constant critic of the law of
+parallelism which Serres continued to uphold with little modification
+for over thirty years, just as von Baer was a critic of that form of the
+doctrine which was current in the early part of the century. As early as
+1833, Milne-Edwards, through his studies of crustacean development,[319]
+had come to the conclusion, independently of von Baer, that development
+always proceeded from the general to the special; that class characters
+appeared before family characters, generic characters before specific.
+In an interesting paper published in 1844,[320] he discussed the relation
+of this law of development to the problems of classification, and
+arrived at results almost identical with those set forth by von Baer in
+his Fifth Scholion.
+
+Like von Baer he rejected completely the theory of parallelism and the
+doctrine of the scale of beings; like von Baer he held that the type of
+organisation--of which there are several--is manifested in the very
+earliest stages and becomes increasingly specialised throughout the
+course of further development; like von Baer, too, he sketched a
+classification based upon embryological characters.
+
+These views were further developed in his volume of 1851, and also in
+his _Rapport_ of 1867.
+
+They brought him into conflict with his confrere in the Academy of
+Sciences, Étienne Serres, who in a number of papers published in the
+'thirties and 'forties,[321] and particularly in his comprehensive memoir
+of 1860, still maintained the theory of parallelism and the doctrine of
+the absolute unity of type. His memoir of 1860 shows how completely
+Serres was under the domination of transcendental ideas. Much of it
+indeed goes back to Oken. "The animal kingdom," he writes, "may be
+considered in its entirety as a single ideal and complex being" (p.
+141). His views have become a little more complicated since his first
+exposition of them in 1827, and he has been forced to modify in some
+respects the rigour of his doctrine. But he still holds fast to the main
+thesis of transcendentalism--the absolute unity of plan of all animals,
+vertebrate and invertebrate alike,[322] the gradual perfecting of
+organisation from monad to man, the repetition in the embryogeny of the
+higher animals of the "zoogeny" of the lower.
+
+He recognised, however, that the idea of a simple scale of beings is
+only an abstraction, and that the true repetition is of organs rather
+than of organisms. He was willing even to admit, at least in the later
+pages of his memoir, that there might be not one animal series but
+several parallel series, as had been suggested by Isidore Geoffroy St
+Hilaire (p. 749). In general, his views are now less dogmatic than they
+were in his earlier writings, but they are not for all that changed in
+any essential. For, in summing up his main results, he writes, "The
+whole animal kingdom can in some measure be regarded ideally as a single
+animal, which, in the course of formation and metamorphosis in its
+diverse manifestations, here and there arrests its own development, and
+thus determines at each point of interruption, by the very state it has
+reached, the distinctive characters of the phyla, the classes, families,
+genera, and species" (p. 833).[323]
+
+To settle the dispute pending between two of its most illustrious
+members, the Academy proposed in 1853, as the subject of one of its
+prizes, "the positive determination of the resemblances and differences
+in the comparative development of Vertebrates and Invertebrates." A
+memoir was presented the next year by Lereboullet[324] which met with the
+approval of the Academy in so far as its statements of fact were
+concerned, but seemed to them to require amplification in its
+theoretical part. But even in this memoir Lereboullet was able to show
+that the balance of evidence was greatly in favour of Milne-Edwards'
+views, and his general conclusions in 1854 were that "in the presence of
+such fundamental differences, one is obliged to give up the idea of one
+single plan in the formation of animals; while, on the contrary, the
+existence of diverse plans or types is clearly demonstrated by all the
+facts" (p. 79). To fulfil the Academy's requirements, Lereboullet
+continued his work, and in 1861-63 he published a series of elaborate
+monographs[325] on the embryology of the trout, the lizard and the
+pond-snail _Lymnæa_, and rounded off his work with a full discussion[326]
+of the theoretical questions involved. In this considered and
+authoritative judgment he completely disposed of Serres' theories of the
+unity of plan and the unity of genetic formation. Except in the very
+earliest stages of oogenesis there is no real similarity between the
+development of a Zoophyte, a Mollusc, an Articulate and a Vertebrate,
+but each is stamped from the beginning with the characteristics of its
+type. The lower animals are not, and cannot possibly be the permanent
+embryos of the higher animals. "The results which I have obtained," he
+writes, "are diametrically opposed to the theory of the zoological
+series constituted by stages of increasing perfection, a theory which
+tries to demonstrate in the embryonic phases of the higher animals a
+repetition of the forms which characterise the lower animals, and which
+has led to the assertion that the latter are permanent embryos of the
+former. The embryo of a Vertebrate shows the vertebrate type from the
+very beginning, and retains this type throughout the whole course of its
+development; it never is, and never can be, either a Mollusc or an
+Articulate" (xx., p. 54).
+
+"We are led to establish ... as the general result of our researches,
+the existence of several types, and, consequently, of different plans,
+in the development of animals. These different types are manifested from
+the very beginning of embryonic life; the characters distinguishing them
+are therefore primordial, and we can say with M. Milne-Edwards that
+_everything goes to prove that the distinction established by Nature
+between animals belonging to different phyla is a primordial
+distinction_" (p. 58).
+
+In other directions also von Baer's work was confirmed and extended by
+later observers--those parts of it particularly that had reference to
+the germ-layer theory, and to the concept of histological
+differentiation. His germ-layer theory was accepted in its main lines by
+Rathke, Bischoff and Lereboullet, and applied by them to the multitude
+of new facts they discovered. Rathke, in particular, was a firm upholder
+of the doctrine, and made considerable use of it in his writings.[327]
+Even before the publication of von Baer's book he had interpreted in
+terms of the germ-layer theory sketched by his friend Pander the
+splitting of the blastoderm which occurs in the early development of
+_Astacus_, whereby there are formed a serous and a mucous layer, one
+inside the other--like the coats of an onion, to use his own expressive
+phrase.[328]
+
+An ingenious application of the Pander-Baer theory was made by Huxley,
+who compared the outer and inner cell-layers which form the groundwork
+of the Coelentera with the serous and mucous layers of the vertebrate
+germ.[329] He laid stress, it is true, rather on the physiological than on
+the morphological resemblance. "A complete identity of structure," he
+writes, "connects the 'foundation membranes' of the Medusæ with the
+corresponding organs in the rest of the series; and it is curious to
+remark, that throughout, the outer and inner membranes appear to bear
+the same physiological relation to one another as do the serous and
+mucous layers of the germ; the outer becoming developed into the
+muscular system, and giving rise to the organs of offence and defence;
+the inner, on the other hand, appearing to be more closely subservient
+to the purposes of nutrition and generation" (p. 24). Von Baer had
+already hinted at this homology in the second volume of his
+_Entwickelungsgeschichte_ (1837), where he says with reference to the
+separation of the blastoderm of the chick into two layers. "Yet
+originally there are not two distinct or even separable layers, it is
+rather the two surfaces of the germ which show this differentiation,
+just as polyps show the same contrast of an external surface and an
+internal digestive surface. In between the two layers there is in our
+germ as in the polyp an indifferent mass" (p. 67). The terms ectoderm
+and entoderm were introduced by Allman[330] in 1853 for the two
+cell-layers in the Hydrozoa.
+
+Remak is the second great name in the history of the germ-layer theory.
+He had the great advantage over von Baer of being able to make use of
+the cell-theory in interpreting the formation of the germ-layers.
+Microscopical technique also had been greatly improved since 1828.[331]
+
+Remak's greatest service was that he put the germ-layer theory in direct
+relation with the cell-theory by demonstrating the cellular continuity
+from egg-cell to tissue, and by showing that each germ-layer possessed
+distinctive histological characteristics. Hardly less important was his
+clear marking-off of the "middle layer" as a separate and distinct layer
+of the germ. He it was who introduced the modern conception of the
+mesoderm, and cleared up the confusion in which Pander and von Baer had
+left the organs formed between the serous and the mucous layer. Remak's
+middle layer was a different thing from Pander's ill-defined
+"vessel-layer"; it included and unified from a new point of view the
+"vessel" and "muscle" layers of von Baer.
+
+There are in the unincubated blastoderm of the chick, according to
+Remak,[332] two cell-layers, of which the undermost subsequently splits
+into two. Three layers are thus formed--the upper, middle and lower. The
+upper layer differentiates into a medullary plate and an epidermic plate
+(Remak's _Hornblatt_), and gives origin to the medullary tube with all
+its evaginations, and to the skin with all its derivatives and pockets.
+It forms such diverse structures as the brain, the spinal cord, the eye,
+the ear, the mouth, hairs, feathers, nails, sweat-glands, lacrymal
+glands, and so forth. All these parts are connected directly or
+indirectly with sensation, and the upper germ-layer may accordingly be
+called the _sensory_ layer. The lower layer gives rise to the epithelium
+and the proper tissue of the alimentary canal and its derivatives, as
+the liver, lungs, pancreas, kidneys, thyroid, thymus, etc. These parts
+are all concerned in the processes of assimilation and dissimilation,
+and the lower layer may accordingly be called the _trophic_ layer. Now
+between the upper or sensory layer and the lower or trophic layer there
+exists, in spite of their very different functions, a close histological
+likeness, for both are essentially epithelial layers. The resemblance is
+particularly strong if we compare the lower layer with the _Hornblatt_
+of the upper layer--both consist of epithelial tissue, and of its
+derivative, glandular tissue, and form neither vessels nor nerves. The
+middle layer, on the contrary, forms nerves and muscles, vessels and
+connective tissue, and little or no epithelium. It does not form all the
+blood-vessels without exception (and so cannot be called the
+vessel-layer), for the blood-vessels of the central nervous system are
+in all probability formed from the upper layer. So, too, it does not
+form all the nerves and muscles--the optic and auditory nerves and the
+nerves and muscles of the iris probably arise in the upper layer. But,
+in spite of these exceptions, its general histological character is so
+well defined that it may be contrasted with the other two as
+preeminently the layer that forms muscular, nervous, vascular and
+connective tissue. In view of its functional significance, it may be
+called the _motory_ layer, or better, since it forms also the sexual
+glands, the _motor-germinative_ layer. The middle layer, early in its
+history, shows a division into dorsal plates (_Urwirbelplatten_) and
+ventral plates (_Seitenplatten_). The former exhibit almost as soon as
+they are formed the characteristic proto-vertebral segmentation, the
+latter split to form the pleuro-peritoneal or body-cavity. Remak
+describes the latter process as follows:--"In the region of the trunk,
+where a greater independence of the fate of the alimentary canal and its
+annexes becomes necessary for the voluntary executive organs, the
+ventral plates undergo a process of splitting, leading to the formation
+of the sensitive part of the integument (the _Hautplatten_), the
+muscular part of the alimentary tube (the _Darmfaserplatten_), and the
+mother-tissue of the generative organs (the _Mittelplatten_). From the
+_Hautplatten_ there develops, without the dorsal plates seeming to take
+any part in the process, the rudiment of the extremities" (p. 79).
+
+[Illustration: FIG. 12.--Transverse Section of Chick Embryo. (After
+Remak.)]
+
+His _Darmfaserplatten_ form the nervous and muscular tissue of the
+alimentary canal and its dependencies, and also the heart; the
+_Hautplatten_ form the general body-wall (exclusive of the skin) and the
+appendages. In the embryo they line the amniotic cavity. The skeleton
+and peripheral nerves originate wholly within the middle layer.
+
+Remak's conception of the relations of the three germ-layers to one
+another and to the body-cavity is well illustrated in Fig. 12.
+
+In his germ-layer theory Remak's standpoint is histological rather than
+morphological. The distinction which he draws between the sensory and
+trophic layers on the one hand, and the motor-germinative layer on the
+other, is entirely a histological one. The greater part of his book,
+indeed, is devoted to a study of the histogenesis of the different
+organs of the body; he is bent chiefly upon unravelling the part which
+each germ-layer takes in the formation of each tissue and organ.
+
+His generalisation that two of the germ-layers give rise exclusively or
+almost exclusively to one kind of tissue excited great interest at the
+time, and gave the direction to histogenetic research for quite a number
+of years, though in the end it turned out to be insufficiently founded.
+
+Though Remak's germ-layer theory had thus principally a histological
+orientation, it laid down the main lines of the modern morphological
+treatment of the germ-layers.
+
+    [293] _Embryologie des Salmones_, 1842.
+
+    [294] _Die Cellularpathologie in ihrer Begründung auf
+    physiologische und pathologische Gewebelehre_, Berlin,
+    2nd ed. 1859; Eng. trans., by Chance, 1860.
+
+    [295] _Arch. path. Anat. Phys_., vii., pp. 1-39 (1854).
+
+    [296] _Bericht über die Fortschritte der mikroskopischen
+    Anatomie im jahre 1854._ Müller's _Archiv_, 1855. See
+    also 1856.
+
+    [297] _Hndb. d. Physiol._, i., 1835.
+
+    [298] See Leuckart's reply to Ludwig's criticism, in
+    _Zeit. f. wiss. Zool._, ii., p. 271, 1850.
+
+    [299] Leipzig, 1853.
+
+    [300] _Souvenirs d'un Naturaliste_, 2 vols., Paris, 1854.
+    Eng. Trans. as _Rambles of a Naturalist on the Coasts of
+    France, Spain, and Italy_, 2 vols., 1857.
+
+    [301] Milne-Edwards later published a classical textbook
+    on comparative anatomy and physiology--_Leçons sur la
+    Physiologie et l'Anatomie comparées_, 14 vols., Paris,
+    1857-80.
+
+    [302] Paris, 1834-40. Three volumes of the _Suites à
+    Buffon_.
+
+    [303] Paris, 1865. Two volumes of the _Suites à Buffon_.
+
+    [304] _U. d. Metamorphose der Ophiuren u. Seeigel._,
+    Berlin, 1848. _U. d. Metamorphose der Holothurien u.
+    Asterien._, Berlin, 1851.
+
+    [305] As I have been unable to obtain a copy of the
+    _Introduction_, the passages which follow are taken from
+    the _Rapport_ of 1867, where Milne-Edwards gives a
+    complete exposition of his doctrine, sometimes in the
+    words of the original.
+
+    [306] This principle was first developed by Milne-Edwards
+    in 1827, in the _Dictionnaire classique d'Hist.
+    naturelle_. It was probably suggested to him by his
+    studies on the Crustacea, among which the principle is
+    so beautifully exemplified in the concentration and
+    specialisation of the appendages and the ganglionic
+    chain.
+
+    [307] Studied by Isidore Geoffroy St Hilaire in his paper
+    _Classification parallélique des Mammifères, C. R. Acad.
+    Sci._, xx., 1845. Remarked upon by Cuvier, _Règne
+    animal_., i., p. 171, 1817, also by de Blainville.
+
+    [308] Cuvier et Valenciennes, _Hist. nat. des Poissons_,
+    i., p. 550, 1828.
+
+    [309] _Myxinoiden_, Th. I. _Abh. k. Akad. Wiss. Berlin_
+    for 1834, pp. 100, 110, 179, etc.
+
+    [310] _Vergl. Entw. Kopf. nackt. Amphibien_, p. 101, 1838.
+
+    [311] I have not seen the companion volume on
+    palæontological progression, _Unters. ü. d.
+    Entwickelungsgesetze der organischen Welt während der
+    Bildungszeit unserer Erdoberfläche_, Stuttgart, 1858.
+
+    [312] "Strobiloid" because of its spiral development. The
+    theory of the spiral growth of plants played an
+    important part in botanical morphology about this time.
+
+    [313] _Cf._ Meckel's Principle of progressive Evolution,
+    _supra_, p. 93.
+
+    [314] _System der thierischen Morphologie_, pp. 33, 457.
+    Also C. Bruch, _Die Wirbeltheorie des Schädels, am
+    Skelette des Lachses geprüft_, Frankfort-on-Main, 1862.
+
+    [315] In France the vertebral theory was advocated by
+    Lavocat in his _Nouvelle Ostéologie comparée de la tête
+    des animaux domestiques_, Toulouse, 1864. It seems also
+    that Lacaze-Duthiers held fast to it even in
+    1872--_Arch. zool. exp. gén._, i., p. 51, 1872.
+
+    [316] _An Essay on Classification_, Boston, 1857, London,
+    1859. He considered the classificatory categories to be
+    the categories of the Creator's thought, and hence
+    natural, and in no sense mere conventions.
+
+    [317] "Principes d'Embryogénie, de Zoogénie et de
+    Teratogénie," _Mém. Acad. Sci._, xxv., pp. 1-943, pls.
+    xxv., 1860.
+
+    [318] "On the Morphology of the Cephalous Mollusca,"
+    _Phil. Trans._, 1853, _Sci. Memoirs_, i., pp. 152-92.
+
+    [319] "Observations sur les changements de forme que les
+    divers Crustacés éprouvent," _Ann. Sci. nat._ (1) xxx.,
+    p. 360, 1833.
+
+    [320] "Considérations sur quelques principes relatifs à la
+    classification naturelle des animaux," _Ann. Sci. nat._
+    (3) i., p. 65, 1844.
+
+    [321] _Supra_, pp. 79-83. Also _Précis d'anatomie
+    transcendante, principes d'organogénie_, Paris, 1842.
+
+    [322] The inversion of the organs shown by Vertebrates as
+    compared with Invertebrates is due to the reversed
+    position of the embryo relatively to the yolk! (pp.
+    821-6).
+
+    [323] It is worth while recording that Serres enunciated a
+    "law of symmetry" according to which the embryo is
+    formed by the union of its two symmetrical halves--a law
+    which recalls the "concrescence theory" of His and some
+    modern embryologists.
+
+    [324] "Embryologie comparée du Brochet, de la Perche, et
+    de l'Ecrévisse," _Ann. Sci. nat._ (4), i., p. 237, 1854;
+    ii., p. 39, 1854. _Mém. Savans etrangers_, xvii.
+
+    [325] _Ann. Sci. nat._ (4) xvi., p. 113, 1861; xvii., p.
+    88, 1862; xviii., p. 5, 1862; xix., p. 5, 1863.
+
+    [326] xx., p. 5, 1863.
+
+    [327] Particularly in his _Blennius_ (1833) and _Natter_
+    (1839).
+
+    [328] In the "preliminary notice" of his Crayfish
+    paper--_Isis_, pp 1093-1100, 1825.
+
+    [329] "On the Anatomy and the Affinities of the Family of
+    the Medusæ," _Phil. Trans._, 1849; _Sci. Memoirs_, i.,
+    pp. 9-32.
+
+    [330] _Phil. Trans._, cxliii., p. 368, 1853.
+
+    [331] The principle of achromatism was discovered (by
+    Fraunhofer) and achromatic microscopes introduced in the
+    early part of the 19th century. The use of chemical
+    reagents, such as acetic acid, and various hardening
+    fluids, came into fashion not long after. J. Müller
+    seems to have been one of the first to realise their
+    importance. Remak himself invented one or two fixing and
+    hardening mixtures (pp. 87, 127, 1855), which enabled
+    him to cut excellent hand sections. Section-cutting
+    machines were not invented till later (V. Hensen, 1866,
+    His, 1870).
+
+    [332] _Untersuchungen über die Entwickelung der
+    Wirbelthiere_, folio, pp. xxxvii + 195, 12 plates,
+    Berlin, 1850-1855.
+
+
+
+
+CHAPTER XIII
+
+THE RELATION OF LAMARCK AND DARWIN TO MORPHOLOGY.
+
+
+It is a remarkable fact that morphology took but a very little part in
+the formation of evolution-theory. When one remembers what powerful
+arguments for evolution can be drawn from such facts as the unity of
+plan and composition and the law of parallelism, one is astonished to
+find that it was not the morphologists at all who founded the theory of
+evolution.
+
+It is true that the noticeable resemblances of animals to one another,
+the possibility of arranging them in a system, the vague perception of
+an all-pervading plan of structure, did suggest to many minds the
+thought that systematic affinities might be due to blood-relationship.
+Thus Leibniz considered that the cat tribe might possibly be descended
+from a common ancestor,[333] and another great philosopher, Immanuel Kant,
+was led by his perception of the unity of type to suggest as possible
+the derivation of the whole organic realm from one parent form, or even
+ultimately from inorganic matter. In the course of his masterly
+discussion of mechanism and teleology,[334] he writes, "The agreement of
+so many genera of animals in a certain common schema, which appears to
+be fundamental not only in the structure of their bones, but also in the
+disposition of their remaining parts--so that with an admirable
+simplicity of original outline, a great variety of species has been
+produced by the shortening of one member and the lengthening of another,
+the involution of this part and the evolution of that--allows a ray of
+hope, however faint, to penetrate into our minds, that here something
+may be accomplished by the aid of the principle of the mechanism of
+Nature (without which there can be no natural science in general). This
+analogy of forms, which with all their differences seem to have been
+produced according to a common original type, strengthens our suspicions
+of an actual relationship between them in their production from a common
+parent, through the gradual approximation of one animal-genus to
+another--from those in which the principle of purposes seems to be best
+authenticated, _i.e._, from man down to the polype, and again from this
+down to mosses and lichens, and finally to the lowest stage of Nature
+noticeable by us, viz., to crude matter."[335]
+
+So, too, Buffon's evolutionism was suggested by his study of the
+structural affinities of animals, and Erasmus Darwin in his _Zoonomia_
+(1794) brought forward as one of the strongest proofs of evolution, "the
+essential unity of plan in all warm-blooded animals."[336]
+
+But, as a matter of historical fact, no morphologist, not even Geoffroy,
+deduced from the facts of his science any comprehensive theory of
+evolution. The pre-Darwinian morphologists were comparatively little
+influenced by the evolution-theories current in their day, and it was in
+the anatomist Cuvier and the embryologist von Baer that the early
+evolutionists found their most uncompromising opponents.
+
+Speaking generally, and excepting for the moment the theory of Lamarck,
+we may say that the evolution-theories of the 18th and 19th centuries
+arose in connection with the transcendental notion of the _Échelle des
+êtres_, or scale of perfection. This notion, which plays so great a part
+in the philosophy of Leibniz, was very generally accepted about the
+middle of the 18th century, and received complete and even exaggerated
+expression from Bonnet and Robinet. Buffon also was influenced by it.
+Towards the beginning of the 19th century the idea was taken up eagerly
+by the transcendental school and by them given, in their theories of the
+"one animal," a more morphological turn. Their recapitulation theory was
+part and parcel of the same general idea.
+
+One understands how easily the notion of evolution could arise in minds
+filled with the thought of the ideal progression of the whole organic
+kingdom towards its crown and microcosm, man. Their theory of
+recapitulation led them to conceive evolution as the developmental
+history of the one great organism.[337] Many of them wavered between the
+conception of evolution as an ideal process, as a _Vorstellungsart_, and
+the conception of it as an historical process. Bonnet, Oken, and the
+majority of the transcendentalists seem to have chosen the former
+alternative; Robinet, Treviranus, Tiedemann, Meckel, and a few others
+held evolution to be a real process.
+
+We have already in previous chapters[338] briefly noticed the relation of
+one or two of the transcendental evolution-theories to morphology, and
+there is little more to be said about them here. They had as good as no
+influence upon morphological theory, nor indeed upon biology in
+general.[339] It is different with the theory of Lamarck, which, although
+it had little influence upon biological thought during and for long
+after the lifetime of its author, is still at the present day a living
+and developing doctrine.
+
+Lamarck's affinity with the transcendentalists was in many ways a close
+one, but he differed essentially in being before all a systematist. Nor
+is the direct influence of the German transcendentalists traceable in
+his work--his spiritual ancestors are the men of his own race, the
+materialists Condillac and Cabanis, and Buffon, whose friend he was. The
+idea of a gradation of all animals from the lowest to the highest was
+always present in Lamarck's mind, and links him up, perhaps through
+Buffon, with the school of Bonnet. The idea of the _Échelle des êtres_
+had for him much less a morphological orientation than it had even for
+the transcendentalists, for he was lacking almost completely in the
+sense for morphology. Lamarck's scientific, as distinguished from his
+speculative work, was exclusively systematic, and it was systematics of
+a very high order. He introduced many reforms into the general
+classification of animals. He was the first clearly to separate
+Crustacea (1799), and a little later (1800) Arachnids, from insects. He
+reduced to a certain orderliness the neglected tribes of the
+Invertebrates, and wrote what was for long the standard work on their
+systematics--the _Histoire naturelle des Animaux sans Vertèbres_
+(1816-22). His speculative work on biology is contained in three
+publications, the small book entitled _Considérations sur l'organisation
+des corps vivants_ (1802), the larger work of 1809, the _Philosophie
+zoologique_, and the introductory matter to his _Animaux sans Vertèbres_
+(vol. i., 1816).
+
+It is no easy matter to give in short compass an account of Lamarck's
+biological philosophy. He is an obscure writer, and often
+self-contradictory.
+
+In the first part of the _Philosophie zoologique_ Lamarck is largely
+pre-occupied with the problem of whether species are really distinct, or
+do not rather grade insensibly into one another. As a systematist of
+vast experience Lamarck knew how difficult it is in practice to
+distinguish species from varieties. "The more," he writes, "we collect
+the productions of Nature, the richer our collections become, the more
+do we see almost all the gaps filled up and the lines of separation
+effaced. We find ourselves reduced to an arbitrary determination, which
+sometimes leads us to seize upon the slightest differences of varieties,
+and form from them the distinctive character of what we call a species,
+and at other times leads us to consider as a variety of a certain
+species individuals a little bit different, which others regard as
+forming a separate species."[340]
+
+For Lamarck, as for Darwin later, the chief problem was not the
+evolution and differentiation of types of structure, but the mode of
+origin of species.
+
+Lamarck is at great pains to show how arbitrary are our determinations
+of species, and how artificial the classificatory groups which we
+distinguish in Nature. Strictly speaking, there are in Nature only
+individuals, "... this is certain, that among her products Nature has in
+reality formed neither classes, nor orders, nor families, nor genera,
+nor constant species, but only individuals which succeed one another and
+resemble those that produced them. Now, these individuals belong to
+infinitely diversified races, which shade into one another under all the
+forms and in all the degrees of organisation, and each of which
+maintains itself without change, so long as no cause of change acts upon
+it" (p. 41).
+
+But there is a natural order in the animal kingdom, a progression from
+the simpler to the more complex organisations, a natural _Échelle des
+êtres_.
+
+This order is shown by the relation to one another of the large
+classificatory groups, for they can be arranged in series from the
+simplest to the most complex, somewhat as follows:--
+
+1. Infusoria.
+2. Polyps.
+3. Radiates.
+4. Worms.
+5. Insects.
+6. Arachnids.
+7. Crustacea.
+8. Annelids.
+9. Cirripedes.
+10. Molluscs.
+11. Fishes.
+12. Reptiles.
+13. Birds.
+14. Mammals.
+
+But the order of Nature is essentially continuous, and the limits of
+even the best defined of these classes are in reality artificial--"if
+the order of Nature were perfectly known in a kingdom, the classes which
+we should be forced to establish in it would always constitute entirely
+artificial sections" (p. 45).
+
+In the same way the lesser classificatory groups represent smaller
+sections of the one unique order of Nature. Note that Lamarck's
+_Échelle_ is in no way a morphological one, and was not intended to be
+such. It is a scale of increasing physiological differentiation, and the
+stages of it are marked by the acquirement of this or that new organ
+(_cf._ Oken). "Observation of their state convinces one that in order to
+produce them successively Nature has proceeded gradually from the
+simpler to the more complex. Now Nature, having had in mind the
+realisation of a plan of organisation which would permit of the greatest
+perfecting (that of the Vertebrates), a plan very different from those
+which she has been obliged to form as a preliminary to reaching it, one
+understands that, among the multitude of animals, one must necessarily
+come across not a single system of organisation which has become
+progressively perfected, but diverse very distinct systems, each of
+which has come into existence at the moment when each primary organ
+first put in its appearance" (p. 171).
+
+For Lamarck this order of Nature was not merely ideal--Nature had
+actually formed the classes successively, proceeding from the simpler to
+the more complex; she had brought about this evolution by transforming
+the primitive species of animals, raising them to higher degrees of
+organisation, and modifying them in relation to the environment in which
+they found themselves.
+
+Lamarck's theory of evolution is worked out in great detail in his
+_Philosophie zoologique_, but the exposition is diffuse and
+disconnected; it is better in giving an account of it to follow the more
+concise, mature and general exposition which he gives in the
+Introduction to his _Histoire naturelle des Animaux sans Vertèbres_.[341]
+Near the beginning of the Introduction Lamarck gives us in a few short
+"Fundamental Principles" the main lines of his general philosophy. He is
+a confirmed materialist. Every fact and phenomenon is essentially
+physical and owes its existence or production entirely to material
+bodies or to relations between them. All change and all movement is in
+the last resort due to mechanical causes. Every fact or phenomenon
+observed in a living body is at once a physical fact or phenomenon and a
+product of organisation (p. 19). Life, thought and sensation are not
+properties of matter, but result from particular material combinations.
+
+His thorough-going materialism is most clearly shown in its relation to
+living things in the first three of the "Zoological Principles and
+Axioms," which are developed further on in the book.
+
+These are as follows:--"1. No kind or particle of matter can have in
+itself the power of moving, living, feeling, thinking, nor of having
+ideas; and if, outside of man, we observe bodies endowed with all or one
+of these faculties, we ought to consider these faculties as physical
+phenomena which Nature has been able to produce, not by employing some
+particular kind of matter which itself possesses one or other of these
+faculties, but by the order and state of things which she has
+constituted in each organisation and in each particular system of
+organs.
+
+"2. Every animal faculty, of whatever nature it may be, is an organic
+phenomenon, and results from a system of organs or an organ-apparatus
+which gives rise to it and upon which it is necessarily dependent.
+
+"3. The more highly a faculty is developed the more complex is the
+system of organs which produces it, and the higher the general
+organisation; the more difficult also does it become to grasp its
+mechanism. But the faculty is none the less a phenomenon of
+organisation, and for that reason purely physical" (p. 104).
+
+According to these "axioms" function is a direct and mechanical effect
+of structure.
+
+The curious thing is that in spite of his avowed materialism, Lamarck's
+conception of life and evolution is profoundly psychological, and from
+the conflict of his materialism and his vitalism (of which he was
+himself hardly conscious), arise most of the obscurities and the
+irreductible self-contradiction of his theory.
+
+Lamarck divided animals (psychologically!) into three great
+groups--apathetic or insensitive animals, animals endowed with
+sensation, and intelligent animals. The first group, which comprise all
+the lower Invertebrates, are distinguished from other animals by the
+fact that their actions are directly and mechanically due to the
+excitations of the environment; they have no principle of reaction to
+external influences, but passively prolong into action the excitations
+they receive from without. They are _irritable_ merely. The second group
+are distinguished from the first by their possessing, in addition to
+irritability, a power which Lamarck calls the _sentiment intérieur_. He
+has some difficulty in defining exactly what he means by it:--"I have no
+term to express this internal power possessed not only by intelligent
+animals but also by those that are endowed merely with the faculty of
+sensation; it is a power which, when set in action by the feeling of a
+need, causes the individual to act at once, _i.e._, in the very moment
+of the sensation it experiences; and if the individual is of those that
+are endowed with intelligence it nevertheless acts in such a case
+entirely without premeditation and before any mental operation has
+brought its _will_ into play" (p. 24).
+
+It is the power we call instinct in animals (p. 25), and it implies
+neither consciousness nor will. It acts by transforming external into
+internal excitations.
+
+To this second group of animals, possessing the _sentiment intérieur_,
+belong the higher Invertebrates, notably insects and molluscs. Only
+animals possessed of a more or less centralised nervous system can
+manifest this _sentiment_, or principle of (unconscious) reaction to
+external stimuli.
+
+The higher animals, or the four Vertebrate classes, form the group of
+"intelligent animals." In virtue of their more complex organisation they
+possess in addition to the _sentiment intérieur_ the faculties of
+intelligence and will.
+
+Now, broadly put, Lamarck's theory of evolution is that new organs are
+formed in direct reaction to needs (_besoins_) experienced by the
+_sentiment intérieur_. The _sentiment intérieur_ is therefore the cause
+not only of instinctive action but also of all morphogenetic processes.
+Will and intelligence (which are confined to a relatively small number
+of animals) have little or nothing to do directly with evolution.
+
+To understand the working-out of Lamarck's evolution-theory we must
+revert to his conception of the _Échelle des êtres_. What he wrote in
+the _Philosophie zoologique_ is here repeated in the work of 1816 with
+little modification.
+
+There is a real progression from the simpler to the more complex
+organisations; Nature has gradually complicated her creatures by giving
+them new organs and therefore new faculties.
+
+It is interesting to note that Lamarck expressly refers to Bonnet (p.
+110), but refuses to accept his view of an _Échelle_ extending down into
+the inorganic. Like Bonnet, however, and like the German
+transcendentalists, Lamarck makes man the goal of evolution (p. 116). He
+makes it quite clear that his _Échelle_ is a functional one, for he
+links Vertebrates to molluscs even while expressly admitting that they
+are not connected by any structural intermediates (p. 123). He does not
+fall into the error of the transcendentalists and assume that
+Vertebrates and Invertebrates alike are formed upon one common plan of
+structure.
+
+The progression of organisation shown by the animal kingdom has not been
+altogether regular and uninterrupted:--"The progression in complexity of
+organisation shows here and there, in the general animal series,
+anomalies induced by the influence of environment and by the influence
+of the habits contracted" (_Phil. zool._, i., p. 145).
+
+There are thus really two causes at work to produce the variety of
+organisation as it appears to us, one which tends to produce a regular
+increase in complexity, and one which disturbs and diversifies this
+regular advance.
+
+The first cause Lamarck calls the vital power (_pouvoir de la vie_); the
+other may be called the influence of circumstance (_Anim. s. Vert._, p.
+134). To the latter cause are due the lacunæ, the blind alleys, and the
+complications which the otherwise simple scale of perfection shows.
+
+To explain both these aspects of evolution Lamarck propounded in his
+volume of 1816 four laws, which read as follows:--
+
+"_First Law_.--Life, by its own forces, tends continually to increase
+the volume of every body possessing it, and to extend the dimensions of
+its parts, up to a limit which it brings about itself.
+
+"_Second Law_.--The production of a new organ in an animal body results
+from the arisal and continuance of a new need, and from the new movement
+which this need brings into being and sustains.
+
+"_Third Law_.--The degree of development of organs and their force of
+action are always proportionate to the use made of these organs.
+
+"_Fourth Law_.--All that has been acquired, imprinted or changed in the
+organisation of the individual during the course of its life is
+preserved by generation and transmitted to the new individuals that
+descend from the individual so modified" (pp. 151-2).
+
+It is mainly but not entirely by reason of the first of these laws that
+organisation tends to progress, and mainly by reason of the second and
+third that difference of environment brings about diversity of
+organisation. In virtue of the fourth law the acquirements of the
+individual become the property of the race.
+
+Lamarck's exposition of his first law, that life tends by its own powers
+to enlarge and extend its bodily instrument, is vague and difficult to
+understand. He has already explained some pages back how the first
+organisms arose by spontaneous generation in the form of minute
+gelatinous utricles (_cf._ Oken). He conceives that it is in the
+movements of the fluids proper to the organism that the power resides to
+enlarge and extend the body. Nutrition alone is not sufficient to bring
+about extension; a special force is required, acting from within
+outwards (p. 153). In the most primitive organisms the movements of the
+vital fluids are weak and slow, but in the course of evolution they
+gradually accelerate, and, becoming more rapid, trace out canals in the
+delicate tissue which contains them, and finally form organs.
+
+Subtle fluids play a great part in Lamarck's biology: they take the
+place of the soul or entelechy which the vitalists would postulate to
+explain organic happenings. Lamarck seems in this to follow certain of
+the old materialists, who conceived the soul to be formed of a matter
+more subtle than the ordinary.[342]
+
+In his second law Lamarck's essentially vitalistic attitude comes out
+very clearly, for it states that a psychological moment enters into all
+new production of form, that the ultimate cause of the development of
+new form is the need felt by the organism. This need is of course not a
+conscious one, it is a need perceived by the _sentiment intérieur_.
+
+In the large group of apathetic or insensitive animals, which do not
+possess this faculty, needs cannot be experienced; accordingly new
+organs are here formed directly and mechanically, by the movements of
+the vital fluids set in action by excitations from without--the
+evolution, like the behaviour, of these animals is due to the direct and
+physical action of the environment. "But this is not the case with the
+more highly organised animals which possess _feeling_. They experience
+needs, and each need felt, acting upon their 'inner feeling,'
+immediately directs the fluids and the forces to the part of the body
+where action can satisfy the need. Now, if there exists at this point an
+organ capable of performing the required action, it is quickly
+stimulated to act; and if the organ does not exist and the need is
+pressing and sustained, bit by bit the organ is produced and developed
+in proportion to the continuity and the energy of its use" (p. 155).
+
+In intelligent animals the _sentiment intérieur_ may be moved by thought
+or will.
+
+As an example of the way in which the law works Lamarck takes the
+hypothetical case of a gastropod mollusc, which as it creeps along
+experiences dimly the need to feel the objects in front of it. It makes
+an effort (unconscious, be it noted) to touch these objects with the
+anterior portions of its head, and sends forward continually to these
+parts a great volume of nervous and other fluids. From these efforts and
+the repeated afflux of fluids there must result a development of the
+nerves supplying these parts. And as, along with the nervous fluids,
+nutritive juices constantly flow to the parts, there must result the
+formation of two or four tentacles in the places to which these fluids
+are directed. A curious mixture of mechanistic "explanations" and
+vitalistic hypothesis!
+
+In his third law, that use and disuse are powerful to modify organs,
+Lamarck is upon more solid ground, and can point to many instances of
+the visible effect of these factors of change. It is of course rather
+closely bound up with his second law and may even be regarded as an
+extension of it.
+
+The law has reference to one of the most powerful means employed by
+Nature to diversify species, a means which comes into play whenever the
+environment changes. The cause of the great diversity shown by animal
+species is indeed ultimately to be sought in the environment. As the
+imperfect and earliest forms developed they spread over the earth and
+invaded the utmost corners of it:--"One can imagine what an enormous
+variety of habitats, stations, climates, available foods, environing
+media, etc., animals and plants have had to endure, as the existing
+species were forced to change their place of abode. And although these
+changes have taken place with extreme slowness ... their reality,
+necessitated by various causes, has none the less induced the species
+affected by them slowly to change their manner of life and their
+habitual actions. Through the effects of the second and third of the
+laws cited above, these induced activity-changes must have brought into
+being new organs, and must have been able to develop them further if
+more frequent use was made of them; they must in the same way have been
+capable of bringing about the degeneration and finally the complete
+disappearance of existing organs which had become useless" (p. 161).
+
+On the other hand, if the environment does not change, species remain
+constant.
+
+It is to be noted that change in environment is rather the occasion than
+the cause of modification; the environment induces the organism to
+change its habitual way of life; it sets up new needs, to satisfy which
+the organism must modify its structure. It is the organism that takes
+the active part in all this, the action of the environment is indirect.
+
+Of Lamarck's fourth law, which asserts the transmission of acquired
+characters, little need here be said in the way of exposition. Upon the
+truth of it depends of course Lamarck's whole theory. He himself never
+dreamed that anyone would ever dispute it.
+
+Lamarck sums up as follows:--"By the four laws which I have just
+enunciated all the facts of organisation seem to me to be easily
+explained; the progression in the complexity of organisation of animals,
+and in their faculties, seems to me easy to conceive; so, too, the means
+which Nature has employed to diversify animals, and bring them to the
+state in which we now see them, become easily determinable" (p. 168).
+
+It is never made quite clear, we may note in passing, how far his second
+and third laws tend to bring about an increase in complexity, in
+addition to diversifying animals.[343]
+
+"The function creates the organ," this would seem to be the kernel of
+Lamarck's doctrine. But how does he reconcile this essentially
+vitalistic conception with his strictly materialistic philosophy?
+
+We have seen that irritability, the _sentiment intérieur_, and
+intelligence itself, are the effects of organisation. We are told
+farther on that both the _sentiment_ and intelligence are caused by
+nervous fluids. A great part of both the _Philosophie zoologique_ and
+the introduction to the _Animaux sans Vertèbres_ is given up to the
+exposition of a materialistic psychology of animals and man, based
+entirely upon this hypothesis of nervous fluids. Thus habits are due to
+the fluids hollowing out definite paths for themselves.
+
+The _sentiment intérieur_ acts by directing the movements of the subtle
+fluids of the body (which are themselves modifications of the nervous
+fluids) upon the parts where a new organ is needed. But if it is itself
+only a result of the movement of nervous fluids? Again, how can a need
+be "felt" by a nervous fluid? This is an entirely psychological notion
+and cannot be applied to a purely material system. Whence arises the
+power of the _sentiment intérieur_ to canalise the energies of the
+organism, so to direct and co-ordinate them that they build up purposive
+structures, or effect purposive actions (as in all instinctive
+behaviour)? Either the _sentiment intérieur_ is a psychological faculty,
+or it is nothing.
+
+There is no doubt that, as expressed by Lamarck, the conception conceals
+a radical confusion of thought. It is not possible to be a
+thorough-going materialist, and at the same time to believe that new
+organs are formed in direct response to needs felt by the organism.
+Lamarck could never resolve this antinomy, and his speculations were
+thrown into confusion by it. To this cause is due the frequent obscurity
+of his writings.
+
+Should we be right in laying stress upon the psychological side of
+Lamarck's theory, and disregarding the materialistic dress in which,
+perhaps under the influence of the materialism current in his youth, he
+clothed his essentially vitalistic thought? Everything goes to prove
+it--his constant preoccupation with psychological questions, his tacit
+assimilation of organ-formation to instinctive behaviour, his constant
+insistence on the importance of _besoin_ and _habitude_.
+
+Let us not forget the profundity of his main idea, that, exception made
+for the lower forms, the animal is essentially active, that it always
+_reacts_ to the external world, is never passively acted upon. Let us
+not forget that he pointed out the essentially psychological moment
+implied in all processes of individual adaptation. With keen insight he
+realised that conscious intelligence counts for little in evolution, and
+focussed attention upon the unconscious but obscurely psychical
+processes of instinct and morphogenesis.
+
+Not without reason have the later schools of evolutionary thought, who
+developed the psychological and vitalistic side of his doctrine, called
+themselves Neo-Lamarckians.
+
+We shall say then that Lamarck, in spite of his materialism, was the
+founder of the "psychological" theory of evolution.
+
+Lamarck stood curiously aloof and apart from the scientific thought of
+his day.[344] He took no interest in the morphological problems that
+filled the minds of Cuvier and Geoffroy; he had indeed no feeling at all
+for morphology. He did not realise, like Cuvier, the _convenance des
+parties_, the marvellous co-ordination of parts to form a whole; he had
+little conception of what is really implied in the word "organism." He
+was not, like Geoffroy, imbued with a lively sense of the unity of plan
+and composition, and of the significance of vestigial organs as
+witnesses to that unity. He seems not to have known of the
+recapitulation theory, of which he might have made such good use as
+powerful evidence for evolution. Even with the German
+transcendentalists, with whom in the looseness of his generalisations he
+shows some affinity, he seems not to have been specially acquainted.
+
+He was interested more in the problems suggested to him by his daily
+work in the museum. He wanted to know why species graded so annoyingly
+into one another; he wanted to examine critically his haunting suspicion
+that species were really not distinct, and that classification was
+purely conventional. The question, too, of the adaptation of species to
+their environment, the problem of ecological adaptation, in distinction
+to that of functional adaptation which interested Cuvier so greatly,
+came vividly before him as he worked through the vast collections of the
+museum. He was the first systematist to occupy himself in a
+philosophical manner with the problems of general biology. He introduced
+new problems and a new way of looking at old. With Lamarck the problem
+of species and the problem of ecological adaptation enter into general
+biology.
+
+The one point in which he does definitely carry on the thought of his
+predecessors is his conception of the animal kingdom as forming a scale
+of (functional) perfection. He did not go to the same extreme as Bonnet;
+he did not even consider that the animal series was a continuation of
+the vegetable series; in his opinion they formed two diverging scales.
+He recognised, too, that among animals there was no simple and regular
+gradation from the lowest to the highest, but that the orderly
+progression was disturbed and diverted by the necessity of adaptation to
+different environments. It is interesting to note that in developing
+this idea he arrived at a roughly accurate distinction between
+homologous and analogous structures. More importance, he thought, was to
+be attributed in classifying animals to characters which appeared due to
+the "plan of Nature" than to such as were produced by an external
+modifying cause (p. 299). But he did not formulate the distinction in
+any strictly morphological way.
+
+As his ideas developed he laid less stress upon the simplicity and
+continuity of the scale; in his supplementary remarks to the
+Introduction of 1816 he admits that the series is really very much
+branched, and even that there may be two distinct series among animals
+instead of one. His last schema of the course of evolution shows no
+little analogy with the genealogical trees of Darwinian speculation. It
+is headed "The presumed _Order_ of the formation of Animals, showing two
+separate partly-branching series," and it reads as follows:--
+
+   I.--_Series of Non-articulated_    II.--_Series of Articulated_
+                 _Animals_.                      _Animals_.
+"
+I    ¦--         Infusoria.
+n    ¦               ¦
+s A  ¦             Polyps.
+e n  ¦               ¦
+n i  ¦        ----------------
+s m  ¦        ¦              ¦
+i a  ¦     Ascidians.     Radiates.                   Worms.
+t l  ¦        ¦                                         ¦
+i s  ¦        ¦                                  --------------
+v .  ¦        ¦                                  ¦            ¦
+e    ¦        ¦                                  ¦         Epizoa.
+"    ¦--      ¦                                  ¦            ¦
+              ¦                                  ¦            ¦
+"    ¦--      ¦                                  ¦            ¦
+S A  ¦     Acephala.                          Annelids.    Insects.
+e n  ¦        ¦                                               ¦
+n i  ¦        ¦                                               ¦
+s m  ¦     Molluscs.                                  -------------
+i a  ¦                                                ¦           ¦
+t l  ¦                                                ¦       Arachnids.
+i s  ¦                                            Crustacea.
+v .  ¦                                                ¦
+e    ¦                                                ¦
+"    ¦--                                          Cirripedes.
+
+I
+n    ¦--
+t A  ¦
+e n  ¦                          Fishes.
+l i  ¦                          Reptiles.
+l m  ¦                          Birds.
+i a  ¦                          Mammals.
+g l  ¦
+e s  ¦--
+n .
+t
+
+It is interesting to note that Vertebrates are placed between the two
+series, and are now not linked on directly to any Invertebrate group.
+
+Lamarck's theory had little success. There is evidence, however, that
+both Meckel and Geoffroy owed a good many of their evolutionary ideas to
+Lamarck, and Cuvier paid him at least the compliment of criticising his
+theory,[345] not distinguishing it, however, very clearly from the
+evolutionary theories of the transcendentalists. But, speaking
+generally, Lamarck's theory of evolution exercised very little influence
+upon his contemporaries. This was probably due partly to the obscurity
+and confusion of his thought, partly to his lack of sympathy with the
+biological thought of his day, which was preponderatingly morphological.
+
+It was not that men's minds were not ripe for evolution, for in the
+early decades of the 19th century evolution was in the air. There were
+few of von Baer's contemporaries who had not read Lamarck;[346] Erasmus
+Darwin's _Zoonomia_ ran through three editions, and was translated into
+German, French and Italian;[247] German philosophy was full of the idea of
+evolution.
+
+There was no unreadiness to accept the derivation of present-day species
+from a primordial form--if only some solid evidence for such derivation
+were forthcoming. Cuvier and von Baer, as we have seen, combated the
+current evolution theories on the ground that the evidence was
+insufficient, but von Baer at least had no rooted objection to
+evolution. In an essay of 1834, entitled _The Most General Law of Nature
+in all Development_,[348] von Baer expressed belief in a limited amount of
+evolution. In this paper he did not admit that all animals have
+developed from one parent form, and he refused to believe that man has
+descended from an ape; but, basing his supposition upon the facts of
+variability and upon the evidence of palæontology, he went so far as to
+maintain that many species have evolved from parent stocks. In the
+absence of conclusive proofs he did not commit himself to a belief in
+any extended or comprehensive process of evolution.
+
+Imbued as he was with the idea of development von Baer saw in evolution
+a process essentially of the same nature as the development of the
+individual. Evolution, like development, was due to a _Bildungskraft_ or
+formative force. The ultimate law of all becoming was that "the history
+of Nature is nothing but the history of the ever-advancing victory of
+spirit over matter" (p. 71). In a later essay (1835) in the same volume
+he says that all natural science is nothing but a long commentary on the
+single phrase _Es werde!_. (p. 86).
+
+As we shall see, von Baer adopted in later years the same attitude to
+Darwinism as he did to the evolution theories in vogue in his youth.
+
+Although in the twenty or thirty years before the publication of the
+_Origin of Species_ (1859) no evolution theory of any importance was
+published, and although the great majority of biologists believed in the
+constancy of species, there were not wanting some who, like von Baer,
+had an open mind on the subject, or even believed in the occurrence of
+evolutionary processes of small scope. Isidore Geoffroy St Hilaire, the
+son of the great Etienne Geoffroy St Hilaire, seems to have held that
+species might be formed from varieties. The law which L. Agassiz thought
+he could establish,[349] of the parallelism between palæontological
+succession, systematic rank, and embryological development, tended to
+help the progress of evolutionary ideas. J. V. Carus, who afterwards
+became a supporter of Darwin, seems already, in 1853, to have inferred
+from Agassiz's law the probability of evolution.[350]
+
+But no evolution theory was taken very seriously before 1859, when the
+_Origin of Species_ was published.
+
+Like Lamarck, Charles Darwin was, neither by inclination nor by
+training, a morphologist. In his youth he was a collector, a sportsman
+and a field geologist. His voyage round the world on the _Beagle_
+aroused in him keen interest in the problem of species--their variety,
+their variation according to place and time, their adaptedness to
+environment. The conviction gradually took possession of his mind that
+the puzzling facts of geographical range and geological succession which
+he observed wherever he went were explicable only on the hypothesis that
+species change. He was not satisfied with the theories of evolution that
+had been proposed by his grandfather, by Lamarck, and by E. Geoffroy St
+Hilaire--he did not indeed understand these theories any too well. He
+resolved to work out the problem in his own way, for his own
+satisfaction. He tells us all this very clearly in his autobiography.
+"During the voyage of the _Beagle_ I had been deeply impressed by
+discovering in the Pampean formation great fossil animals covered with
+armour like that on the existing armadillos; secondly, by the manner in
+which closely allied animals replace one another in proceeding
+southwards over the continent; and thirdly, by the South American
+character of most of the productions of the Galapagos archipelago, and
+more especially by the manner in which they differ slightly on each
+island of the group; some of the islands appearing to be very ancient in
+a geological sense.
+
+"It was evident that such facts as these, as well as many others, could
+only be explained on the supposition that species gradually become
+modified; and the subject haunted me. But it was equally evident that
+neither the action of the surrounding conditions, nor the will of the
+organisms (especially in the case of plants) could account for the
+innumerable cases in which organisms of every kind are beautifully
+adapted to their habits of life--for instance, a woodpecker or a
+tree-frog to climb trees, or a seed for dispersal by hooks or plumes. I
+had always been much struck by such adaptations, and until these could
+be explained it seemed to me almost useless to endeavour to prove by
+indirect evidence that species have been modified."[351]
+
+All Darwin's varied subsequent work revolved round these, for him,
+essential problems--How do species change, and how do they become
+adapted to their environment? He never ceased to be essentially a field
+naturalist, and his theory of natural selection would have been an empty
+and abstract thing if his vast knowledge and understanding of the "web
+of life" had not given it colour and form. He never lost touch with the
+living thing in its living, breathing reality--even plants he rightly
+regarded as active things, full of tricks and contrivances for making
+their way in the world. No one ever realised more vividly than he the
+delicacy and complexity of the adaptations to environment which are the
+necessary condition of success in the struggle for existence. Almost his
+greatest service to biology was that he made biologists realise as they
+never did before the vast importance of environment. He took biology
+into the open air, away from the museum and the dissecting-room.
+
+Naturally this attitude was not without its drawbacks. It led him to
+take only a lukewarm interest in the problems of morphology. It is true
+he used the facts of morphology with great effect as powerful arguments
+for evolution, but it was not from such facts that he deduced his theory
+to account for evolution. It is questionable indeed whether the theory
+of natural selection is properly applicable to the problems of form. It
+was invented to account for the evolution of specific differences and of
+ecological adaptations; it was not primarily intended as an explanation
+of the more wonderful and more mysterious facts of the _convenance des
+parties_ and the interaction of structure and function. Perhaps Darwin
+did not realise this inner aspect of adaptation quite so vividly as he
+did the more superficial adaptation of organisms to their environment.
+It was, perhaps, his lack of morphological training and experience that
+led him to disregard the problems of form, or at least to realise very
+insufficiently their difficulty.
+
+It is in any case very significant that only a small part of his _Origin
+of Species_ is devoted to the discussion of morphological
+questions--only one chapter out of the fourteen contained in the first
+edition.
+
+Though the theory of natural selection took little account of the
+problems of form, Darwin's masterly vindication of the theory of
+evolution was of immense service to morphology, and Darwin himself was
+the first to point out what a great light evolution threw upon all
+morphological problems. In a few pages of the _Origin_ he laid the
+foundations of evolutionary morphology.
+
+We have here to consider his interpretation of morphological facts and
+its relation to the current morphology of his time.
+
+The sketch of his theory, written in 1842,[352] shows a very significant
+division into two parts--the first dealing with the positive facts of
+variability and the theory of natural selection, the second with the
+general evidence for evolution. It is in the second part that the
+paragraphs on morphological matters occur. In paragraph 7, on affinities
+and classification, Darwin points out that on the theory of evolution
+homological relationship would be real relationship, and the natural
+system would really be genealogical. In the next paragraph he notes that
+evolution would account for the unity of type in the great classes, for
+the metamorphosis of organs, and for the close resemblance which early
+embryos show to one another. It is of special interest to note that he
+definitely rejects the Meckel-Serres theory of recapitulation. "It is
+not true," he writes, "that one passes through the form of a lower
+group, though no doubt fish more nearly related to foetal state" (p.
+42). The greater divergence which adults show seems to him to be due to
+the fact that selection acts more on the later than on the embryonic
+stages. He realises very clearly how illuminative the theory of
+evolution is when applied to the puzzling facts of embryonic
+development. "The less differences of foetus--this has obvious meaning
+on this view: otherwise how strange that a horse, a man, a bat should at
+one time of life have arteries, running in a manner which is only
+intelligibly useful in a fish! The natural system being on theory
+genealogical, we can at once see why foetus, retaining traces of the
+ancestral form, is of the highest value in classification" (p. 45).
+
+Abortive organs, too, gain significance on the evolutionary hypothesis.
+"The affinity of different groups, the unity of types of structure, the
+representative forms through which foetus passes, the metamorphosis of
+organs, the abortion of others, cease to be metaphorical expressions and
+become intelligible facts" (p. 50).
+
+In general, organisms can be understood only if we take into account the
+cardinal fact that they are historical beings. "We must look at every
+complicated mechanism and instinct as the summary of a long history of
+useful contrivances much like a work of art" (p. 51).[353]
+
+Already in 1842 Darwin had seized upon the main principles of
+evolutionary morphology: the indications then given are elaborated in
+the thirteenth chapter of the _Origin of Species_ (1st ed., 1859). A
+good part of this chapter is given up to a discussion of the principles
+of classification, only a few pages dealing with morphology proper. But,
+as Darwin rightly saw, the two things are inseparable.
+
+We note first that there is no hint of the "scale of beings"--Darwin
+conceives the genealogical tree as many branched. Animals can be classed
+in "groups under groups," and cannot be arranged in one single series.
+
+He discusses first what kind of characters have the greatest
+classificatory value. Certain empirical rules have been recognised, more
+or less consciously, by systematists--that analogical characters are
+less valuable than homological, that characters of great physiological
+importance are not always valuable for classificatory purposes, that
+rudimentary organs are often very useful, and so on. He finds that as a
+general rule "the less any part of the organisation is concerned with
+special habits, the more important it becomes for classification" (p.
+414), and adduces in support Owen's remark that the generative organs
+afford very clear indications of affinities, since they are unlikely to
+be modified by special habits. These rules of classification can be
+explained "on the view that the natural system is founded on descent
+with modification; that the characters which naturalists consider as
+showing true affinity ... are those which have been inherited from a
+common parent, and, in so far, all true classification is genealogical;
+that community of descent is the hidden bond which naturalists have been
+unconsciously seeking, and not some unknown plan of creation, or the
+enunciation of general propositions, and the mere putting together and
+separating objects more or less alike" (p. 420).
+
+In general, then, homological characters are more valuable for
+classificatory purposes because they have a longer pedigree than
+analogical characters, which represent recent acquirements of the race.
+
+Coming to morphology proper, Darwin takes up the question of the unity
+of type, and the homology of parts, for which the unity of type is but a
+general expression.
+
+He treats this on the same lines as E. Geoffroy St Hilaire, and Owen,
+referring indeed specifically to Geoffroy's law of connections. "What
+can be more curious," he asks, "than that the hand of a man, formed for
+grasping, that of a mole for digging, the leg of a horse, the paddle of
+the porpoise, and the wing of the bat, should all be constructed on the
+same pattern, and should include similar bones, in the same relative
+positions? Geoffroy St Hilaire has strongly insisted on the high
+importance of relative position or connection in homologous parts; they
+may differ to almost any extent in form and size, and yet remain
+connected together in the same invariable order" (p. 434).
+
+The unity of plan cannot be explained on teleological grounds, as Owen
+has admitted in his _Nature of Limbs_, nor is it explicable on the
+hypothesis of special creation (p. 435). It can be understood only on
+the theory that animals are descended from one another and retain for
+innumerable generations the essential organisation of their ancestors.
+"The explanation is to a large extent simple on the theory of the
+selection of successive slight modifications--each modification being
+profitable in some way to the modified form, but often affecting by
+correlation other parts of the organisation. In changes of this nature,
+there will be little or no tendency to alter the original pattern or to
+transpose the parts.... If we suppose that the ancient progenitor, the
+archetype as it may be called, of all animals, had its limbs constructed
+on the existing general pattern, for whatever purpose they served, we
+can at once perceive the plain significance of the homologous
+construction of the limbs throughout the whole class" (p. 435).
+
+We may note three important points in this passage--first, the
+identification of the archetype with the common progenitor; second, the
+view that progressive evolution is essentially adaptive, and dominated
+by natural selection; and third, the _petitio principii_ involved in the
+assumption that adaptive modification brings inevitably in its train the
+necessary correlative changes.
+
+In his section on morphology Darwin shows clearly the influence of Owen,
+and through him of the transcendental anatomists. He refers to the
+transcendental idea of "metamorphosis," as exemplified in the vertebral
+theory of the skull and the theory of the plant appendage, and shows
+how, on the hypothesis of descent with modification, "metamorphosis" may
+now be interpreted literally, and no longer figuratively merely (p.
+439).
+
+Very great interest attaches to Darwin's treatment of development, for
+post-Darwinian morphology was based to a very large extent on the
+presumed relation between the development of the individual and the
+evolution of the race. Just as he kept clear of the notion of the scale
+of beings, so he avoided the snare of the Meckel-Serres theory of
+recapitulation, according to which the embryo of the highest animal,
+man, during its development climbs the ladder upon the rungs of which
+the whole animal series is distributed, in its gradual progression from
+simplicity to complexity. The law of development which he adopts is that
+of von Baer, which states that development is essentially
+differentiation, and that as a result embryos belonging to the same
+group resemble one another the more the less advanced they are in
+development. There can be little doubt that he was indebted to von Baer
+for the idea, and in the later editions of the _Origin_ he acknowledges
+this by quoting the well-known passage in which von Baer tells how he
+had two embryos in spirit which he was unable to refer definitely to
+their proper class among Vertebrates.[354]
+
+Not only are embryos more alike than adults, because less
+differentiated, but it is in points not directly connected with the
+conditions of existence, not strictly adaptive, that their resemblance
+is strongest (p. 440)--think, for instance, of the arrangement of aortic
+arches common to all vertebrate embryos. Larval forms are to some extent
+exceptions to this rule, for they are often specially adapted to their
+particular mode of life, and convergence of structure may accordingly
+result. All these facts require an explanation. "How, then, can we
+explain these several facts in embryology--namely, the very general, but
+not universal, difference in structure between the embryo and the
+adult--of parts in the same individual embryo, which ultimately become
+very unlike and serve for different purposes, being at this early period
+of growth alike--of embryos of different species within the same class,
+generally but not universally, resembling each other--of the structure
+of the embryo not being closely related to its conditions of existence,
+except when the embryo becomes at any period of life active and has to
+provide for itself--of the embryo apparently having sometimes a higher
+organisation than the mature animal, into which it is developed" (pp.
+442-3). Obviously all these facts are formally explained by the doctrine
+of descent. But Darwin goes further, he tries to show exactly how it is
+that the embryos resemble one another more than the adults. He thinks
+that the phenomenon results from two principles--first, that
+modifications usually supervene late in the life of the individual; and
+second, that such modifications tend to be inherited by the offspring at
+a corresponding, not early, age (p. 444).
+
+Thus, applying these principles to a hypothetical case of the origin of
+new species of birds from a common stock, he writes:--"... from the many
+slight successive steps of variation having supervened at a rather late
+age and having been inherited at a corresponding age, the young of the
+new species of our supposed genus will manifestly tend to resemble each
+other much more closely than do the adults, just as we have seen in the
+case of pigeons"[355] (pp. 446-7).
+
+Since the embryo shows the generalised type, the structure of the embryo
+is useful for classificatory purposes. "For the embryo is the animal in
+its less modified state; and in so far it reveals the structure of its
+progenitor" (p. 449)--the embryological archetype reveals the ancestral
+form. "Embryology rises greatly in interest, when we thus look at the
+embryo as a picture, more or less complete, of the parent form of each
+great class of animals" (p. 450)--a prophetic remark, in view of the
+enormous subsequent development of phylogenetic speculation.
+
+We may sum up by saying that Darwin interpreted von Baer's law
+phylogenetically.
+
+The rest of the chapter is devoted to a discussion of abortive and
+vestigial organs, whose existence Darwin naturally turns to great
+advantage in his argument for evolution. Throughout the whole chapter
+Darwin's preoccupation with the problems of classification is clearly
+manifest.
+
+On the question as to whether descent was monophyletic or polyphyletic
+Darwin expressed no dogmatic opinion. "I believe that animals have
+descended from at most only four or five progenitors, and plants from an
+equal or lesser number.... I should infer from analogy that probably all
+the organic beings which have ever lived on this earth have descended
+from one primordial form, into which life was first breathed" (p. 484).
+
+Darwin rightly laid much stress upon the morphological evidence for
+evolution,[356] which he considered to be weighty. It probably contributed
+greatly to the success of his theory. Though he himself did little or no
+work in pure morphology, he was alive to the importance of such work,[357]
+and followed with interest the progress of evolutionary morphology,
+incorporating some of its results in later editions of the _Origin_, and
+in his _Descent of Man_ (1871).
+
+In his morphology Darwin was hardly up to date. He does not seem to have
+known at first hand the splendid work of the German morphologists, such
+as Rathke and Reichert; he pays no attention to the cell-theory, nor to
+the germ-layer theory. His sources are, in the main, Geoffroy St
+Hilaire, Owen, von Baer, Agassiz, Milne-Edwards, and Huxley.
+
+Perhaps his greatest omission was that he did not give any adequate
+treatment of the problem of functional adaptation and the correlation of
+parts. It is not too much to say that Darwin not only disregarded these
+problems almost entirely, but by his insistence upon ecological
+adaptation and upon certain superficial aspects of correlation,
+succeeded in giving to the words "adaptation" and "correlation" a new
+signification, whereby they lost to a large extent their true and
+original functional meaning.
+
+It is true that Darwin himself, as well as his successors, believed that
+natural selection was all-powerful to account for the evolution of the
+most complicated organs, but it may be questioned whether he realised
+all the conditions of the problem of which he thus easily disposed. He
+says, rightly, in an important passage, that "It is generally
+acknowledged that all organic beings have been formed on two great
+laws--Unity of Type, and the Conditions of Existence. By unity of type
+is meant that fundamental agreement in structure which we see in organic
+beings of the same class, and which is quite independent of their habits
+of life. On my theory, unity of type is explained by unity of descent.
+The expression of conditions of existence, so often insisted upon by the
+illustrious Cuvier, is fully embraced by the principle of natural
+selection. For natural selection acts by either now adapting _the
+varying parts of each being to its organic and inorganic conditions of
+life_:[358] or by having adapted them during past periods of time: the
+adaptations being aided in many cases by the increased use or disuse of
+parts, being affected by the direct action of the external conditions of
+life, and subjected in all cases to the several laws of growth and
+variation. Hence, in fact, the law of the Conditions of Existence is the
+higher law; as it includes, through the inheritance of former variations
+and adaptations, that of Unity of Type" (_Origin_, 6th ed., Pop.
+Impression, pp. 260-1). It is clear that Darwin took the phrase
+"Conditions of Existence" to mean the environmental conditions, and the
+law of the Conditions of Existence to mean the law of adaptation to
+environment. But that is not what Cuvier meant by the phrase: he
+understood by it the principle of the co-ordination of the parts to form
+the whole, the essential condition for the existence of any organism
+whatsoever (see above, Chap. III., p. 34).
+
+Of this thought there is in Darwin little trace, and that is why he did
+not sufficiently appreciate the weight of the argument brought against
+his theory that it did not account for the correlation of variations.
+
+Darwin's conception of correlation was singularly incomplete. As
+examples of correlation he advanced such trivial cases as the relation
+between albinism, deafness and blue eyes in cats, or between the
+tortoise-shell colour and the female sex. He used the word only in
+connection with what he called "correlated variation," meaning by this
+expression "that the whole organisation is so tied together during its
+growth and development, that when slight variations in any one part
+occur, and are accumulated through natural selection, other parts become
+modified" (6th ed., p. 177). He took it for granted that the "correlated
+variations" would be adapted to the original variation which was acted
+upon by natural selection, and he saw no difficulty in the gradual
+evolution of a complicated organ like the eye if only the steps were
+small enough. "It has been objected," he writes, "that in order to
+modify the eye and still preserve it as a perfect instrument, many
+changes would have to be effected simultaneously, which, it is assumed,
+could not be done through natural selection; but as I have attempted to
+show in my work on the variation of domestic animals, it is not
+necessary to suppose that the modifications were all simultaneous, if
+they were extremely slight and gradual" (6th ed., p. 226).
+
+In post-Darwinian speculation the difficulty of explaining correlated
+variation by natural selection alone became more acutely realised, and
+it was chiefly this difficulty that led Weismann to formulate his
+hypothesis of germinal selection as a necessary supplement to the
+general selection theory.
+
+The change in the conception of correlation which Darwin's influence
+brought about has been very clearly stated by E. von Hartmann,[359] from
+whom the following is taken:--"While the correlation of parts in the
+organism was before Darwin regarded exclusively from the standpoint of
+morphological systematics, Darwin tried to look at it from the
+standpoint of physiological and genealogical development, and in so
+doing he put the standpoint of morphological systematics in the shade.
+But the more we are now beginning to realise that systematic
+relationship does not necessarily imply genetic affinity the more must
+the correlation of parts come back into favour as a systematic
+principle. While Darwin only, as it were, against his will, relied on
+the law of correlation as a last resort when all other help failed, this
+law must be regarded, from the standpoint of the orderly inner
+determination of all organic form-change, as having the rank of the
+highest principle of all, a principle which rules parallel, divergent
+and convergent evolution" (pp. 47-8).
+
+Further on, following Rádl, he characterises Darwin's attitude to the
+law of correlation in these terms:--"Darwin's interest is entirely
+focussed on the variation, the function, the causes of form-production,
+in short, upon evolution. Accordingly he regards correlation essentially
+as correlative variation in the sense of a _departure_ from the given
+type. With morphological correlation in _different_ types Darwin
+troubles himself not at all, nor with correlation in the normal
+development of a type" (p. 49).
+
+Cuvier's conception of the _convenance des parties_, essential to all
+biology, remained on the whole foreign to Darwin's thought, and to the
+thought of his successors.
+
+It was indeed one of their boasts that they had finally eliminated all
+teleology from Nature. The great and immediate success which Darwinism
+had among the younger generation of biologists and among scientific men
+in general was due in large part to the fact that it fitted in well with
+the prevailing materialism of the day, and gave solid ground for the
+hope that in time a complete mechanistic explanation of life would be
+forthcoming. "Darwinismus" became the battle-cry of the militant spirits
+of that time.
+
+It was precisely this element in Darwinism that was repugnant to most of
+Darwin's opponents, in whose ranks were found the majority of the
+morphologists of the old school. They found it impossible to believe
+that evolution could have come about by fortuitous variation and
+fortuitous selection; they objected to Darwin that he had enunciated no
+real _Entwickelungsgesetz_, or law governing evolution. They were not
+unwilling to believe that evolution was a real process, though many drew
+the line at the derivation of man from apes, but they felt that if
+evolution had really taken place, it must have been under the guidance
+of some principle of development, that there must have been manifested
+in evolution some definite and orderly tendency towards perfection.[360]
+
+No one expressed this objection with greater force than did von Baer, in
+a series of masterly essays[361] which the Darwinians, through sheer
+inability to grasp his point of view, dismissed as the maunderings of
+old age. In these essays von Baer pointed out the necessity for the
+teleological point of view, at least as complementary to the
+mechanistic. His general position is that of the "statical"
+teleology--to use Driesch's term--of Kant and Cuvier. His attitude to
+Darwinism is determined by his teleology. He admits, just as in 1834, a
+limited amount of evolution; he criticises the evolution theory of
+Darwin on the same lines exactly as forty or fifty years previously he
+had criticised the recapitulation and evolution-theories of the
+transcendentalists--principally on the ground that their deductions far
+outrun the positive facts at their disposal. He rejects the theory of
+natural selection entirely, on the ground that evolution, like
+development, must have an end or purpose (_Ziel_)--"A becoming without a
+purpose is in general unthinkable" (p. 231); he points out, too, the
+difficulty of explaining the correlation of parts upon the Darwinian
+hypothesis. His own conception of the evolutionary process is that it is
+essentially _zielstrebig_ or guided by final causes, that it is a true
+_evolutio_ or differentiation, just as individual development is an
+orderly progress from the general to the special. He believed in
+saltatory evolution, in polyphyletic descent, and in the greater
+plasticity of the organism in earlier times.
+
+The idea of saltatory evolution he took from Kölliker, who shortly after
+the publication of the _Origin_ promulgated in a critical note on
+Darwinism a sketch of his theory of "heterogeneous generation."[362]
+
+Kölliker's attitude is typical of that taken up by many of the
+morphologists of the day.[363] He accepts evolution completely, but
+rejects Darwinism because it recognises no _Entwickelungsgesetz_, or
+principle of evolution. For the Darwinian theory of evolution through
+the selection of small fortuitous variations he would substitute the
+theory of evolution through sudden, large variations, brought about by
+the influence of a general law of evolution. This is his theory of
+heterogeneous generation. "The fundamental idea of this hypothesis is
+that under the influence of a general law of evolution creatures produce
+from their germs others which differ from them" (p. 181). It is to be
+noticed that Kölliker laid more stress upon the _Entwickelungsgesetz_
+than upon the saltatory nature of variation, for he says a few pages
+further on--"the notion at the base of my theory is that a great
+evolutionary plan underlies the development of the whole organised
+world, and urges on the simpler forms towards ever higher stages of
+complexity" (p. 184). Saltatory evolution was not the essential point of
+the theory:--"Another difference between the Darwinian hypothesis and
+mine is that I postulate many saltatory changes, but I will not and
+indeed cannot lay the chief stress upon this point, for I have not
+intended to maintain that the general law of evolution which I hold to
+be the cause of the creation of organisms, and which alone manifests
+itself in the activity of generation, cannot also so act that from one
+form others quite gradually arise" (p. 185). He put forward the
+hypothesis of saltatory variation because it seemed to him to lighten
+many of the difficulties of Darwinism--the lack of transition forms, the
+enormous time required for evolution, and so on. It should be noted that
+Kölliker regarded his principle of evolution as mechanical.
+
+It would take too long to show in detail how a belief in innate laws of
+evolution was held by the majority of Darwin's critics. A few further
+examples must suffice.
+
+Richard Owen, who in 1868[364] admitted the possibility of evolution, held
+that "a purposive route of development and change, of correlation and
+interdependence, manifesting intelligent Will, is as determinable in the
+succession of races as in the development and organisation of the
+individual. Generations do not vary accidentally, in any and every
+direction; but in pre-ordained, definite, and correlated courses" (p.
+808).
+
+He conceived change to have taken place by abrupt variation, independent
+of environment and habit, by "departures from parental type, probably
+sudden and seemingly monstrous, but adapting the progeny inheriting such
+modifications to higher purposes" (p. 797). He believed spontaneous
+generation to be a phenomenon constantly taking place, and constantly
+giving the possibility of new lines of evolution.
+
+E. von Hartmann in his _Philosophie des Unbewussten_ (1868) and in his
+valuable essay on _Wahrheit und Irrtum im Darwinismus_ (1874) criticised
+Darwinism in a most suggestive manner from the vitalistic standpoint. He
+drew attention to the importance of active adaptation, the necessity for
+assuming definite and correlated variability, and to the evidence for
+the existence of an immanent, purposive, but unconscious principle of
+evolution, active as well in phylogenetic as in individual development.
+
+In France H. Milne-Edwards[365] stated the problem thus:--"In the present
+state of science, ought we to attribute to modifications dependent on
+the action of known external agents the differences in the organic types
+manifested by the animals distributed over the surface of the globe
+either at the present day, or in past geological ages? Or must the
+origin of types transmissible by heredity be attributed to causes of
+another order, to forces whose effects are not apparent in the present
+state of things, to a creative power independent of the general
+properties of organisable matter such as we know them to-day?" (p. 426)
+
+He concluded that the action of environment, direct or indirect, was
+insufficient to account for the diversity of organic forms, and rejected
+Darwin's theory completely. He thought it likely that the successive
+faunas which palæontology discloses have originated from one another by
+descent. But he thought that the process by which they evolved should
+rightly be called "creation." The word was of course not to be taken in
+a crude sense. When the zoologist speaks of the "creation" of a new
+species, "he in no way means that the latter has arisen from the dust,
+rather than from a pre-existing animal whose mode of organisation was
+different; he merely means that the known properties of matter, whether
+inert or organic, are insufficient to bring about such a result, and
+that the intervention of a hidden cause, of a power of some higher
+order, seems to him necessary" (p. 429).
+
+The criticism of Darwinism exercised by the older currents of thought
+remained on the whole without influence. It was under the direct
+inspiration of the Darwinian theory that morphology developed during the
+next quarter of a century.
+
+    [333] Rádl, _loc. cit._, i., p. 71.
+
+    [334] _Kritik der Urtheilskraft_, 1790.
+
+    [335] Eng. Trans. by J. H. Bernard, p. 337, London, 1892.
+
+    [336] H. F. Osborn, _From the Greeks to Darwin_, p. 145,
+    New York and London, 1894.
+
+    [337] See Meckel, _supra_, p. 93; _cf._ Tiedemann,
+    _Zoologie_, p. 65, 1808. "Even as each individual
+    organism transforms itself, so the whole animal kingdom
+    is to be thought of as an organism in course of
+    metamorphosis." Also p. 73 of the same book.
+
+    [338] Chapters vii. and ix.
+
+    [339] On early evolution-theories see, in addition to
+    Osborn and Rádl, J. Arthur Thomson, _The Science of
+    Life_, 1899, and the opening essay in _Darwin and Modern
+    Science_, Cambridge, 1909.
+
+    [340] _Phil. zool._, ed. Ch. Martins, vol. i., p. 75,
+    1873.
+
+    [341] Quotations in the text are from the 2nd Edit.
+    (Deshayes and Milne-Edwards), i., Paris, 1835.
+
+    [342] For instance, Lucretius:--
+
+    "Is tibi nunc animus quali sit corpore et unde
+    constiterit pergam rationem reddere dictis. Principio
+    esse aio persubtilem atque minutis perquam corporibus
+    factum constare."
+
+    --_De Rerum Natura_, iii., vv. 177-80.
+
+    [343] Contrast Treviranus--"In every living being there
+    exists a capability of an endless variety of
+    form-assumption; each possesses the power to adapt its
+    organisation to the changes of the outer world, and it
+    is this power, put into action by the change of the
+    universe, that has raised the simple zoophytes of the
+    primitive world to continually higher stages of
+    organisation, and has introduced a countless variety of
+    species into animate Nature." Quoted by Haeckel in
+    _History of Creation_, i., p. 93, 1876.
+
+    [344] There is no evidence that he was influenced by
+    Erasmus Darwin, who forestalled his evolution theory, and
+    was indeed more aware of its vitalistic implications. See
+    S. Butler, _Evolution, Old and New_, London, 1879, for an
+    excellent account of Erasmus Darwin.
+
+    [345] As did also Lyell in his _Principles of Geology_,
+    1830.
+
+    [346] K. E. von Baer, _Reden_, i., p. 37, Petrograd, 1864.
+
+    [347] Rádl, _loc. cit._, i., p. 296.
+
+    [348] Reprinted in his _Reden_, i., 1864.
+
+    [349] See Huxley's criticism of it in a Royal Institution
+    lecture of 1851, republished in _Sci. Mem._, i., pp.
+    300-4. On its relation to Haeckel's biogenetic law, see
+    below, p. 255.
+
+    [350] _System der thierischen Morphologie_, p. 5, 1853.
+
+    [351] _Life and Letters of Charles Darwin_, ed. F. Darwin,
+    i., p. 82, 3rd ed., 1887.
+
+    [352] _The Foundations of the Origin of Species, a Sketch
+    written in 1842_. Ed. F. Darwin, Cambridge, 1909.
+
+    [353] _Cf._ a parallel passage in the _Origin_, 1st ed.,
+    pp. 485-6.
+
+    [354] In the 1st ed. (p. 439), Darwin makes the curious
+    mistake of attributing this story to Agassiz.
+
+    [355] In which nestlings of the different varieties are
+    much more alike than adults. Darwin attached much
+    importance to this idea, see _Life and Letters_, i., p.
+    88, and ii., p. 338.
+
+    [356] See his _Letters, passim_.
+
+    [357] Writing to Huxley on the subject of the latter's
+    work on the morphology of the Mollusca (1853), he
+    says:--"The discovery of the type or 'idea' (in your
+    sense, for I detest the word as used by Owen, Agassiz &
+    Co.) of each great class, I cannot doubt, is one of the
+    very highest ends of Natural History."--_More Letters_,
+    ed. F. Darwin and A. C. Seward, 1903, i., p. 73.
+
+    [358] Italics mine.
+
+    [359] _Das Problem des Lebens. Biologische Studien_. Bad
+    Sacha, 1906. See also E. Rádl, _Biol. Centralblatt_,
+    xxi., 1901.
+
+    [360] See the excellent treatment of the difference
+    between the "realism" of Darwin and the "rationalism" of
+    his critics, in Rádl, ii., particularly pp. 109, 135.
+    The most elaborate criticism of Darwinism from the older
+    standpoint was that given by A. Wigand in _Der
+    Darwinismus und die Naturforschung Newtons und Cuviers_,
+    3 vols., Braunschweig, 1872.
+
+    [361] In vol. ii. of his _Reden_, St Petersburg
+    (Petrograd), 1876--_Ueber den Zweck in den Vorgängen der
+    Natur; Ueber Zielstrebigkeit in den organischen Körpern
+    insbesondere_; and _Ueber Darwin's Lehre_.
+
+    [362] "Ueber die Darwinische Schöpfungstheorie," _Zeits.
+    f. wiss. Zool._, xiv., pp. 74-86, 1864. Elaborated in
+    _Anat. u. syst. Beschreibung d. Alcyonarien_, 1872.
+
+    [363] _Cf._ for instance Nägeli's theory of a perfecting
+    principle, first developed in his _Entstehung u. Begriff
+    der naturhistorischer Art_, München, 1865.
+
+    [364] _Anatomy of Vertebrates_, iii., 1868.
+
+    [365] _Rapport sur les Progrès récents des Sciences
+    zoologiques en France_. Paris, 1867.
+
+
+
+
+CHAPTER XIV
+
+ERNST HAECKEL AND CARL GEGENBAUR
+
+
+At the time when Darwin's work appeared there already existed, as we
+have seen, a fully formed morphology with set and definite principles.
+The aim of this pre-evolutionary morphology had been to discover and
+work out in detail the unity of plan underlying the diversity of forms,
+to disentangle the constant in animal form and distinguish from it the
+accessory and adaptive. The main principle upon which this work was
+based was the principle of connections, so clearly stated by Geoffroy.
+The principle of connections served as a guide in the search for the
+archetype, and this search was prosecuted in two directions--first, by
+the comparison of adult structure; and second, by the comparative study
+of developing embryos. It was found that the archetype was shown most
+clearly by the early embryo, and this embryological archetype came to be
+preferred before the archetype of comparative anatomy. It became
+apparent also that the parts first formed (germ-layers) were of primary
+importance for the establishing of homologies.
+
+While practically all morphologists were agreed as to the main
+principles of their science, they yet showed, as regards their general
+attitude to the problems of form, a fairly definite division into two
+groups, of which one laid stress upon the intimate relation existing
+between form and function, while the other disregarded function
+completely, and sought to build up a "pure" or abstract morphology. In
+opposition to both groups, in opposition really to morphology
+altogether, a movement had gained strength which tended towards the
+analysis and disintegration of the organism. This movement took its
+origin in the current materialism of the day, and found expression
+particularly in the cell-theory and in materialistic physiology.
+
+The separation between morphology as the science of form and physiology
+as the science of the physics and chemistry of the living body had by
+Darwin's day become well-nigh absolute.
+
+The morphology of the 'fifties lent itself readily to evolutionary
+interpretation. Darwin found it easy to give a formal solution of all
+the main problems which pre-evolutionary morphology had set--he was able
+to interpret the natural system of classification as being in reality
+genealogical, systematic relationship as being really
+blood-relationship; he was able to interpret homology and analogy in
+terms of heredity and adaptation; he was able to explain the unity of
+plan by descent from a common ancestor, and for the concept of
+"archetype" to substitute that of "ancestral form."
+
+The current morphology, Darwin found, could be taken over, lock, stock
+and barrel, to the evolutionary camp.
+
+In what follows we shall see that the coming of evolution made
+surprisingly little difference to morphology, that the same methods were
+consciously or unconsciously followed, the same mental attitudes taken
+up, after as before the publication of the _Origin of Species_.
+
+Darwin himself was not a professional morphologist; the conversion of
+morphology to evolutionary ideas was carried out principally by his
+followers, Ernst Haeckel and Carl Gegenbaur in Germany, Huxley,
+Lankester, and F. M. Balfour in England.
+
+It was in 1866 that Haeckel's chief work appeared, a _General Morphology
+of Organisms_,[366] which was intended by its author to bring all
+morphology under the sway and domination of evolution.
+
+It was a curious production, this first book of Haeckel's, and
+representative not so much of Darwinian as of pre-Darwinian thought. It
+was a medley of dogmatic materialism, idealistic morphology, and
+evolution theory; its sources were, approximately, Büchner, Theodor
+Schwann, Virchow, H. G. Bronn, and, of course, Charles Darwin.
+
+It was scarcely modern even on its first appearance, and many regarded
+it, not without reason, as a belated offshoot of _Naturphilosophie_.
+
+Its materialism is of the most intransigent character. The form and
+activities of living things are held to be merely the mechanical result
+of the physical and chemical composition of their bodies. The simplest
+living things, the Monera, are nothing more than homogeneous masses of
+protein substance. "They live, but without organs of life; all the
+phenomena of their life, nutrition and reproduction, movement and
+irritability, appear here as merely the immediate outcome of formless
+organic matter, itself an albumen compound" (p. 63, 1906).
+
+Teleology, the Achilles' heel of Kant's (otherwise sound!) philosophy,
+is to be regarded as a totally refuted and antiquated doctrine,
+definitely put out of court by Darwinism.
+
+Haeckel works out his materialistic philosophy of living things very
+much after the fashion of Schwann. There is the same talk of cells as
+organic crystals, of crystal trees, of the analogy between assimilation
+by the cell and the growth of crystals in a mother liquid. Heredity and
+adaptation are shown equally as well by crystals as by organisms; for
+heredity, or the internal _Bildungstrieb_ (!), is the mechanical effect
+of the material structure of the crystal or the germ, and adaptation, or
+the external _Bildungstrieb_, is a name for the modifications induced by
+the environment. Adaptation so defined comes to be synonymous with the
+fortuitous variation which plays so great a part in Darwin's theory of
+natural selection.
+
+It goes without saying that Haeckel allowed to the organism no other nor
+higher individuality than belongs to the crystal, and took no account at
+all of that harmonious interaction of the organs which Cuvier called the
+principle of the "conditions of existence." The concept of correlation
+had simply no meaning for Haeckel. The analysis and disintegration of
+the organism was pushed by him to its logical extreme, and in this also
+he was a child of his time.
+
+A no less important influence clearly visible in the _General
+Morphology_ is the idealistic morphology of men like K. G. Carus and H. G.
+Bronn. In previous chapters we have seen how K. G. Carus attempted to
+work out a geometry of the organism, and how Bronn tried in a modest way
+to found a stereometrical morphology, but had the grace not to push his
+stereometry _à l'outrance_, recognising very wisely that the greater
+part of organic form is functionally determined. Haeckel took over this
+idea[367] and pushed it to wild extremes, founding a new science of
+"Promorphology" of which he was the greatest--and only--exponent.[368]
+
+This "science" dealt with axes and planes, poles and angles, in a
+veritable orgy of barbarous technical terms. It was intended to be a
+"crystallography of the organic," and to lay the foundations of a
+mechanistic morphology, or morphography at least.
+
+How it was to be linked up with the physics and chemistry of living
+matter on the one hand and with the ordinary morphology of real animals
+on the other, was never made quite clear.
+
+The science of Promorphology has no historical significance; it is
+interesting only because it illustrates Haeckel's close affinity with
+the idealistic morphologists.
+
+Another abortive science of Haeckel's, the science of Tectology, was
+equally a heritage from idealistic morphology. Tectology is the science
+of the composition of organisms from individuals of different orders.
+There were six orders of individuals:--(1) Plastids (Cytodes and cells);
+(2) Organs (including cell-fusions, tissues, organs, organ-systems); (3)
+Antimeres (homotypic parts, _i.e._, halves or rays); (4) Metameres
+(homodynamic parts, _i.e._, segments); (5) Persons (individuals in the
+ordinary sense); (6) Corms (colonial animals).
+
+The thought is essentially transcendental, and recalls the "theory of
+the repetition of parts," of which so much use was made by the German
+transcendentalists, such as Goethe,[369] Oken, Meckel and K. G. Carus, as
+well as by Dugès.
+
+The third, and naturally the most important, ingredient in the _General
+Morphology_ was the doctrine of evolution, in the form given to it by
+Darwin. We have here no concern with Haeckel's evolutionary philosophy,
+with the way in which he combined his evolutionism and his materialism
+to form a queer Monism of his own. We are interested only in the way he
+applied evolution to morphology, what modifications he introduced into
+the principles of the science, and in general in what way he interpreted
+the facts and theories of morphology in the light of the new knowledge.
+
+We find that he repeats very much what Darwin said, giving, of course,
+more detail to the exposition, and elaborating, particularly in his
+recapitulation theory or "biogenetic law," certain doctrines not
+explicitly stated by Darwin.
+
+Like Darwin he held that the natural system is in reality genealogical.
+"There exists," he writes, "one single connected natural system of
+organisms, and this single natural system is the expression of real
+relations which actually exist between all organisms, alike those now in
+being on the earth and those that have existed there in some past time.
+The real relations which unite all living and extinct organisms in one
+or other of the principal groups of the natural system, are
+genealogical: their relationship in form is blood-relationship; the
+natural system is accordingly the genealogical tree of organisms, or
+their genealogema.... All organisms are in the last resort descendants
+of autogenous Monera, evolved as a consequence of the divergence of
+characters through natural selection. The different subordinate groups
+of the natural system, the categories of the class, order, family,
+genus, etc., are larger or smaller branches of the genealogical tree,
+and the degree of their divergence indicates the degree of genealogical
+affinity of the related organisms with one another and with the common
+ancestral form" (ii., p. 420).
+
+The degree of systematic relationship is thus the degree of genealogical
+affinity. It follows that the natural system of classification may be
+converted straightway into a genealogical tree, and this is actually
+what Haeckel does in the _General Morphology_. The genealogical trees
+depicted in the second volume (plates i.-viii.) are nothing more than
+graphic representations of the ordinary systematic relationships of
+organisms, with a few hypothetical ancestral groups or forms thrown in
+to give the whole a genealogical turn.
+
+If the genealogical tree is truly represented by the natural system, it
+would seem that for each genus a single ancestral form must be
+postulated, for each group of genera a single more primitive form, and
+so in general for each of the higher classificatory categories, right up
+to the phylum. Species of one genus must be descended from a generic
+ancestral form, genera of one family from a single family _Urform_, and
+so on for the higher categories.
+
+This consequence was explicitly recognised by Haeckel. "Genera and
+families," he writes, "as the next highest systematic grades, are
+extinct species which have resolved themselves into a divergent bunch of
+forms (_Formenbüschel_)" (ii., p. 420).
+
+The archetype of the genus, family, order, class and phylum was thus
+conceived to have had at some past time a real existence.
+
+The natural system of classification is based upon a proper appreciation
+of the distinction between homological and analogical characters.
+Haeckel, following Darwin, naturally interprets the former as due to
+inheritance, the latter as due to adaptation, using these words, we may
+note, in their accepted meaning and not in the abstract empty sense he
+had previously attributed to them.[370] Similarly the "type of
+organisation," in von Baer's sense, was due to heredity, the "grade of
+differentiation" to adaptation.
+
+So far Haeckel merely emphasised what Darwin had already said in the
+_Origin of Species_. But by his statement of the "biogenetic law," and
+particularly by the clever use he made of it, Haeckel went a step beyond
+Darwin, and exercised perhaps a more direct influence upon evolutionary
+morphology than Darwin himself.
+
+Haeckel was not the original discoverer of the law of recapitulation. It
+happened that a few years before the publication of Haeckel's _General
+Morphology_, a German doctor, Fritz Müller by name, stationed in Brazil,
+had been working on the development of Crustacea under the direct
+inspiration of Darwin's theory, and had published in 1864 a book[371] in
+which he showed that individual development gave a clue to ancestral
+history.
+
+He conceived that progressive evolution might take place in two
+different ways. "Descendants ... reach a new goal, either by deviating
+sooner or later whilst still on the way towards the form of their
+parents, or by passing along this course without deviation, but then
+instead of standing still advancing still farther" (Eng. trans., p.
+111). In the former case the developmental history of descendants agrees
+with that of the ancestors only up to a certain point and then diverges.
+"In the second case the entire development of the progenitors is also
+passed through by the descendants, and, therefore, so far as the
+production of a species depends upon this second mode of progress, the
+historical development of the species will be mirrored in its
+developmental history" (p. 112).
+
+Of course the recapitulation of ancestral history will be neither
+literal nor extended. "The historical record preserved in developmental
+history is gradually _effaced_ as the development strikes into a
+constantly straighter course from the egg to the perfect animal, and it
+is frequently _sophisticated_ by the struggle for existence which the
+free-living larvæ have to undergo" (p. 114).
+
+It follows that "the primitive history of a species will be preserved in
+its developmental history the more perfectly the longer the series of
+young stages through which it passes by uniform steps; and the more
+truly, the less the mode of life of the young departs from that of the
+adults, and the less the peculiarities of the individual young states
+can be conceived as transferred back from later ones in previous periods
+of life, or as independently acquired" (p. 121).
+
+Applying these principles to Crustacea, he concluded that the shrimp
+_Peneus_ with its long direct development gave the best and truest
+picture of the ancestral history of the Malacostraca, and that
+accordingly the nauplius and the zoaea larvæ represented important
+ancestral stages. He conceived it possible so to link up the various
+larval forms of Crustacea as to weave a picture of the primeval history
+of the class, and he made a plucky attempt to work out the phylogeny of
+the various groups.
+
+The thought that development repeats evolution was already implicit in
+the first edition of the _Origin_, but the credit for the first clear
+and detailed exposition of it belongs to F. Müller.
+
+In much the same form as it was propounded by Müller it was adopted by
+Haeckel, and made the corner-stone of his evolutionary embryology.
+Haeckel gave it more precise and more technical formulation, but added
+nothing essentially new to the idea.
+
+It is convenient to use his term for it--the biogenetic law
+(_Biogenetische Grundgesetz_)--to distinguish it from the laws of
+Meckel-Serres and von Baer, with which it is so often confused.
+
+Haeckel's statement of it may best be summarised in his own words,
+"Ontogeny, or the development of the organic individual, being the
+series of form-changes which each individual organism traverses during
+the whole time of its individual existence, is immediately conditioned
+by phylogeny, or the development of the organic stock (phylon) to which
+it belongs.
+
+"Ontogeny is the short and rapid recapitulation of phylogeny,
+conditioned by the physiological functions of heredity (reproduction)
+and adaptation (nutrition). The organic individual (as a morphological
+individual of the first to the sixth order) repeats during the rapid and
+short course of its individual development the most important of the
+form-changes which its ancestors traversed during the long and slow
+course of their palæontological evolution according to the laws of
+heredity and adaptation.
+
+"The complete and accurate repetition of phyletic by biontic development
+is obliterated and abbreviated by secondary contraction, as ontogeny
+strikes out for itself an ever straighter course; accordingly, the
+repetition is the more complete the longer the series of young stages
+successively passed through.
+
+"The complete and, accurate repetition of phyletic by biontic
+development is falsified and altered by secondary adaptation, in that
+the bion[372] during its individual development adapts itself to new
+conditions: accordingly the repetition is the more accurate the greater
+the resemblance between the conditions of existence under which
+respectively the bion and its ancestors developed" (ii., p. 300).
+
+The last two propositions, it will be observed, are taken over almost
+verbally from F. Müller.
+
+Now we have seen that the natural system of classification gives a true
+picture of the genealogical relationships of organisms, that the smaller
+and larger classificatory groups correspond to greater or lesser
+branches of the genealogical tree. If ontogeny is a recapitulation of
+phylogeny, we must expect to find the embryo repeating the organisation
+first of the ancestor of the phylum, then of the ancestor of the class,
+the order, the family and the genus to which it belongs. There must be a
+threefold parallelism between the natural system, ontogeny and phylogeny
+(ii., pp. 421-2).
+
+It will be observed that there is here implied an analogy between the
+biogenetic law and the law of von Baer, for both assert that development
+proceeds from the general to the special, that the farther back in
+development you go the more generalised do you find the structure of the
+embryo; both assert, too, that differentiation of structure takes place
+not in one progressive or regressive line, but in several diverging
+directions.
+
+But the analogy between the biogenetic law and the Meckel-Serres law is
+even more obvious, and the resemblance between the two is much more
+fundamental. It is a significant fact that in his theory of the
+threefold parallelism Haeckel merely resuscitated in an evolutionary
+form a doctrine widely discussed in the 'forties and 'fifties,[373] and
+championed particularly by L. Agassiz,[374] a doctrine which must be
+regarded as a development or expansion of the Meckel-Serres law.[375] It
+is the view that a parallelism exists between the natural system,
+embryonic development, and palæontological succession. Actually, as
+Agassiz stated it, the doctrine applied neither to types, nor as a
+general rule to classes, but merely to orders. It was well exemplified,
+he thought, in Crinoids:--"The successive stages of the embryonic growth
+of Crinoids typify, as it were, the principal forms of Crinoids which
+characterise the successive geological formations. First, it recalls the
+Cistoids of the palæozoic rocks, which are represented in its simple
+spheroidal head; next the few-plated Platycrinoids of the Carboniferous
+period; next the Pentacrinoids of the Lias and Oolite with their whorls
+of cirrhi; and finally, when freed from its stem, it stands as the
+highest Crinoid, as the prominent type of the family in the present
+period" (p. 171).
+
+The Meckel-Serres law, it will be remembered, expressed the idea that
+the higher animals repeat in their ontogeny the adult organisation of
+animals lower in the scale. Since Haeckel recognised clearly that a
+linear arrangement of the animal kingdom was a mere perversion of
+reality, and that a branching arrangement of groups more truly
+represented the real relations of animals to one another, he could not
+of course entertain the Meckel-Serres theory in its original form. But
+he accepted the main tenet of it when he asserted that each stage of
+ontogeny had its counterpart in an adult ancestral form. Such ancestral
+forms might or might not be in existence as real species at the present
+day; they might or might not be discoverable as fossils. That they had
+real existence either now or at some past epoch Haeckel never doubted.
+In his construction of phylogenetic trees he was so confident in the
+truth of his biogenetic law that he largely disregarded and consistently
+minimised the importance of the evidence from palæontology.
+
+The biogenetic law differed from the Meckel-Serres law chiefly in the
+circumstance that many of the adult lower forms whose organisation was
+supposed to be repeated in the development of the higher animals were
+purely hypothetical, being deduced directly from a study of ontogeny and
+systematic relationships. The hypothetical ancestral forms which the
+theory thus postulated naturally took their place in the natural system,
+for they were merely the concrete projections or archetypes of the
+classificatory groups.
+
+The transcendentalists, of course, conceived evolution, whether real or
+ideal, as a uniserial process, whereas Haeckel conceived it as
+multiserial and divergent. It is here that the superficial agreement of
+the biogenetic law with the law of von Baer comes in.
+
+We might almost sum up the relation of the biogenetic law to the laws of
+von Baer and Meckel-Serres by saying that it was the Meckel-Serres law
+applied to the divergent differentiation upheld by von Baer instead of
+to the uniserial progression believed in by the transcendentalists.
+
+How near in practice Haeckel's law came to the recapitulation theory of
+the transcendentalists may be seen in passages like the following, with
+its partial recognition of the _Échelle_ idea:[276]--"As so high and
+complicated an organism as that of man ... rises upwards from a simple
+cellular state, and as it progresses in its differentiating and
+perfecting, it passes through the same series of transformations which
+its animal progenitors have passed through, during immense spaces of
+time, inconceivable ages ago.... Certain very early and low stages in
+the development of man, and other vertebrate animals in general,
+correspond completely in many points of structure with conditions which
+last for life in the lower fishes. The next phase which follows on this
+presents us with a change of the fish-like being into a kind of
+amphibious animal. At a later period the mammal, with its special
+characteristics, develops out of the amphibian, and we can clearly see,
+in the successive stages of its later development, a series of steps of
+progressive transformation which evidently correspond with the
+differences of different mammalian orders and families."[377]
+
+The biogenetic law went beyond both the Meckel-Serres law and the law of
+von Baer in that it recognised that the ancestral history of the species
+accounts in part for the course which the development of the individual
+takes, that in a certain sense, though not in the crude way supposed by
+Haeckel, phylogeny is the cause of ontogeny. This thought, that the
+organism is before all an historical being, is of course implied in the
+evolution idea, is indeed the essential core of it. Take away this
+element from the biogenetic law--not a difficult matter--and it becomes
+merely a law of idealistic morphology, applicable to evolution
+considered as an ideal process, as the progressive development in the
+Divine thought of archetypal models.
+
+As a book, the _General Morphology_ suffers a good deal from the arid,
+schematic, almost scholastic manner of exposition adopted. Haeckel's
+Prussian mania for organisation, for absolute distinctions, for
+iron-bound formalism, is here given full scope. A treatment less
+adequate to the variety, fluidity and changeableness of living things
+could hardly be imagined.
+
+His doctrine, though it remains essentially unchanged, receives in his
+later works a less formal and more concrete expression, and, in
+particular, his views on the biogenetic law undergo some small
+modification.
+
+Even in the _General Morphology_ Haeckel had recognised that ontogeny is
+neither a complete nor an entirely accurate recapitulation of phylogeny;
+he had admitted, following F. Müller, that the true course of
+recapitulation was frequently modified by larval and foetal adaptations.
+As time went on, he was forced to hedge more and more on this point, and
+finally in his _Anthropogenie_ (1874) and his second paper on the
+Gastræa theory (1875),[378] he had to work out a distinction between
+palingenetic and cenogenetic characters, of which much use was made by
+subsequent writers.
+
+The distinction may be given in Haeckel's own words:--"Those ontogenetic
+processes," he writes, "which are to be referred immediately, in
+accordance with the biogenetic law, to an earlier completely developed
+_independent ancestral form_, and are transmitted from this by
+_heredity_, obviously possess _primary_ importance for the understanding
+of the casual-physiological relations; on the other hand, those
+developmental processes which appear subsequently through _adaptation_
+to the needs of embryonic or larval life, and accordingly can _not_ be
+regarded as repeating the organisation of an earlier independent
+ancestral form, can clearly have for the understanding of the ancestral
+history only a quite subordinate and _secondary_ importance.
+
+"The first I have named _palingenetic_, the second _cenogenetic_.
+Considered from this critical standpoint, the whole of ontogeny falls
+into two main parts:--First, _palingenesis_, or 'epitomised history'
+(_Auszugsgeschichte_), and second, _cenogenesis_, or 'counterfeit
+history' (_Fälschungsgeschichte_). The first is the true ontogenetic
+epitome or short recapitulation of past evolutionary history; the second
+is the exact contrary, a new foreign ingredient, a falsifying or
+concealing of the epitome of phylogeny."[379]
+
+As examples of palingenetic processes in the development of Amniotes,
+for instance, may be quoted the separation of two primary germ-layers,
+the formation of a simple notochord between medullary tube and
+alimentary canal, the appearance of a simple cartilaginous cranium, of
+the gill-arches and their vessels, of the primitive kidneys, the
+primitive tubular heart, the paired aortæ and the cardinal veins, the
+hermaphroditic rudiment of the gonads, and so on. Cenogenetic processes,
+on the other hand, include such phenomena as the formation of yolk and
+the embryonic membranes, the temporary allantoic circulation, the navel,
+the curved and contracted shape of the embryo, and the like.
+
+The most important phenomena to be included under the general heading of
+cenogenesis are, first, the occurrence of food-yolk, and second, those
+anomalies of development which are classed by Haeckel as heterochronies
+and heterotopies.
+
+It is to the influence of the different amounts of yolk present in the
+egg that are due the great differences in the segmentation and
+gastrulation processes, which almost mask their true significance.
+
+Heterochronic processes are such as arise through the dislocation of the
+proper phylogenetic order of succession: heterotopic processes in the
+same way are caused by a wandering of cells from one germ-layer to
+another. The two classes of phenomena are disturbances either of the
+proper spatial or of the proper temporal relation of the parts during
+development.
+
+Heterochrony shows itself, as a rule, either as an acceleration or as a
+retardation of developmental events, as compared with their relative
+time of occurrence during phylogeny. Thus the notochord, the brain, the
+eyes, the heart, appear earlier in the ontogenetic than in the
+phylogenetic series, while, on the other hand, the septum of the
+auricles appears in the development of the higher Vertebrates before the
+ventricular septum, which is undoubtedly a reversal of the phylogenetic
+order.
+
+Cases of heterotopy, or of organs being developed in a position or a
+germ-layer other than that in which they originally arose in phylogeny,
+are not so easy to find. According to Haeckel, the origin of the
+generative products in the mesoderm is a heterotopic phenomenon, for he
+considers that they must have originated phylogenetically in one of the
+two primary layers, ectoderm or endoderm.
+
+It is worthy of note that the help of comparative anatomy is admittedly
+required in deciding what processes are palingenetic and what
+cenogenetic (p. 412).
+
+Haeckel's morphological notions, and particularly his biogenetic law,
+excited a good deal of adverse criticism from men like His, Claus,
+Salensky, Semper and Goette. Nor was his principal work, the _General
+Morphology_, received with much favour. Nevertheless, since he did
+express, though in a crude, dogmatic and extreme manner, the main
+hypotheses upon which evolutionary morphology is founded, his historical
+importance is considerable. He cannot perhaps be regarded as typical of
+the morphologists of his time--he was too trenchantly materialistic, too
+much the populariser of a crude and commonplace philosophy of Nature. In
+point of concrete achievement in the field of pure research he fell
+notably behind many of his contemporaries.
+
+His friend, Carl Gegenbaur, who gained a great and well-deserved
+reputation by his masterly studies on vertebrate morphology,[380] was a
+sounder man, and probably exercised a wider and certainly a more
+wholesome influence upon the younger generation of professional
+morphologists than the more brilliant Haeckel. It is true that in his
+famous _Grundzüge der vergleichenden Anatomie_, the second edition of
+which, published in 1870, soon came to be regarded as the classical
+text-book of evolutionary morphology, Gegenbaur enunciated very much the
+same general principles as Haeckel, and referred to the _Generelle
+Morphologie_ as the chief and fundamental work on animal morphology. But
+in Gegenbaur's pages the Haeckelian doctrines are modified and subdued
+by the strong commonsense and thorough appreciation of the older
+classical or Cuvierian morphology that characterise Gegenbaur's work.
+According to Haeckel,[381] Gegenbaur was greatly influenced by J. Müller,
+who, as we know, laid as much stress on function as on form.
+
+The "General Part" of Gegenbaur's text-book is in many ways a
+significant document and deserves close attention.
+
+We note first of all that physiology and morphology are considered by
+Gegenbaur to be entirely distinct sciences, with different
+subject-matter and different methods. "The task of physiology is the
+investigation of the functions of the animal body or of its parts, the
+referring back of these functions to elementary processes and their
+explanation by general laws. The investigation of the material
+substratum of these functions, of the form of the body and its parts,
+and the explanation of this form, constitute the task of Morphology"
+(2nd ed., p. 3).
+
+Morphology falls naturally into two divisions--comparative anatomy and
+embryology. The method of comparative anatomy is _comparison_ (p. 6),
+and in employing this method account is to be taken of "the spatial
+relations of the parts to one another, their number, extent, structure,
+and texture." Through comparison one is enabled to arrange organs in
+continuous series, and it comes out very clearly during this proceeding
+"that the physiological value of an organ is by no means constant
+throughout the different form-states of the organ, that an organ,
+through the mere modification of its anatomical relations, can subserve
+very different functions. Exclusive regard for their physiological
+functions would place morphologically related organs in different
+categories. From this it follows that in comparative anatomy we should
+never in the first place consider the function of an organ. The
+physiological value comes only in the second place into consideration,
+when we have to reconstruct the relations to the organism as a whole of
+the modification which an organ has undergone as compared with another
+state of it. In this way comparative anatomy shows us how to arrange
+organs in series; within these series we meet with variations which
+sometimes are insignificant and sometimes greater in extent; they affect
+the extent, number, shape, and texture of the parts of an organ, and can
+even, though only in a slight degree, lead to alterations of position"
+(p. 6).
+
+Geoffroy St Hilaire would have subscribed to every word of this
+vindication of his "principle of connections."
+
+Between comparative anatomy and embryology there exists a close
+connection, for the one throws light on the other. "While in some cases
+the same organ shows only slight modifications in its development from
+its early beginnings to its perfect state, in other cases the organ is
+subjected to manifold modifications before it reaches its definitive
+form; we see parts appear in it which later disappear, we observe
+alterations in it in all its anatomical relations, alterations which may
+even affect its texture. This fact is of great importance, for those
+changes which an organ undergoes during its individual development lead
+through states which the organ in other cases permanently shows, or at
+the least the first appearance of the organ is the equivalent of a
+permanent state in another organism. If then the fully developed organ
+is in any special case so greatly modified that its proper relation to
+some organ-series is obscured, this relation may be cleared up by a
+knowledge of the organ's development. The earlier state indicated in
+this way enables one to find with ease the proper place for the organ
+and so insert it into an already known series. The relations which we
+observe in an organ-seriation are then the equivalent of processes which
+in certain cases take place in a similar manner during the individual
+development of an organ. Embryology enters therefore into the closest
+connection with comparative anatomy.... It teaches us to know organs in
+their earliest states, and connects them up with the permanent states of
+others, whereby they fill up the gaps which we meet with in the various
+series formed by the fully developed organs of the body" (pp. 6-7).
+
+This recognition of the parallelism between comparative anatomy and
+embryology is, of course, the kernel of the Meckel-Serres law. For
+Gegenbaur it had a very definite evolutionary meaning--he subscribed to
+the evolutionary form of it, the biogenetic law. How near his conception
+of the relation between ontogeny and phylogeny came to the old
+Meckel-Serres law may be gauged from the following passage, taken from a
+later work:--"Ontogeny thus represents, to a certain degree,
+palæontological development abbreviated or epitomised. The stages which
+are passed through by higher organisms in their ontogeny correspond to
+stages which are maintained in others as the definitive organisation.
+These embryonic stages may accordingly be explained by comparing them
+with the mature stages of lower organisms, since we regard them as forms
+inherited from ancestors belonging to such lower stages"[382] (p. 6).
+
+It is worth noting that in Gegenbaur's opinion comparative anatomy was
+prior in importance to embryology, that embryology could hardly exist as
+an independent science, since it must seek the interpretation of its
+facts always in the facts of comparative anatomy (_Grundzüge_, pp. 7-8).
+
+While Gegenbaur was at one with all "pure" morphologists, whether
+evolutionary or pre-evolutionary, in minimising as far as possible the
+importance of function in the study of form, he was too cautious and
+sober a thinker not to recognise the immense part which function really
+plays. Thus he classified organs, according to their function, into
+those that established relations with the external world and those that
+had to do with nutrition and reproduction, very much as Bichat had done
+before him.
+
+Like Darwin, Haeckel and most evolutionists, he interpreted the
+homological resemblances of animals as being due to heredity, their
+differences as due to adaptation,[383] but he did not adopt Haeckel's
+crude and shallow definition of these terms. For Gegenbaur heredity was
+a convenient expression for the fact of transmission, and was not
+explained offhand as the mere mechanical result of a certain material
+structure handed down from germ to germ. Adaptation he defined in a way
+which took the fullest account of function, and was as far as possible
+removed from Haeckel's definition of it as the direct mechanical effect
+of the environment upon the organism. "The organism is altered," writes
+Gegenbaur, "according to the conditions which influence it. The
+consequent _Adaptations_ are to be regarded as gradual, but steadily
+progressive, changes in the organisation, which are striven after during
+the individual life of the organism, preserved by transmission in a
+series of generations, and further developed by means of natural
+selection. What has been gained by the ancestor becomes the heritage of
+the descendant. Adaptation and Transmission are thus alternately
+effective, the former representing the modifying, the latter the
+conservative principle.... Adaptation is commenced by a change in the
+function of organs, so that the _physiological relations_ of organs play
+the most important part in it. Since adaptation is merely the material
+expression of this change of function, the modification of the function
+as much as its expression is to be regarded as a gradual process. In
+Adaptation, the closest connection between the function and the
+structure of an organ is thus indicated. Physiological functions govern,
+in a certain sense, structure; and so far what is morphological is
+subordinated to what is physiological" (_Elements_, pp. 8-9). Gegenbaur
+recognised also that morphological differentiation depended largely on
+the physiological division of labour (_Grundzüge_, p. 49).
+
+It is clear that Gegenbaur realised vividly the importance of function,
+and in this respect, as in others, he is far beyond Haeckel. The same
+thing comes out markedly in his treatment of correlation. Haeckel had no
+slightest feeling for the true meaning of correlation. For him, as for
+Darwin, it reduced itself to a law of correlative variation, according
+to which "actual adaptation not only changes those parts of the organism
+which are directly affected by its influence, but other parts also, not
+directly affected by it."[384] Such "correlative adaptation" was due to
+nutrition being a "connected, centralised activity."
+
+Gegenbaur, on the contrary, had a firm grasp of the Cuvierian
+conception, and expressed it in unmistakable terms. "As indeed follows
+from the conception of life as the harmonious expression of a sum of
+phenomena rigorously determining one another, no activity of an organ
+can in reality be thought of as existing for itself. Each kind of
+function (_Verrichtung_) presupposes a series of other functions, and
+accordingly every organ must possess close relations with, and be
+dependent on, all the others" (_Grundzüge_, p. 71). The organism must be
+regarded as an individual whole which is as much conditioned by its
+parts as one part is conditioned by the others. For an understanding of
+correlation a knowledge of functions, and of the functional relations of
+the organism to its environment, is clearly indispensable.
+
+Gegenbaur's morphological system was out-and-out evolutionary. "The most
+important part of the business of comparative anatomy," in Gegenbaur's
+eyes, "is to find indications of genetic connection in the organisation
+of the animal body" (_Elements_, p. 67).
+
+The most important clue to discovering this genetic connection is of
+course that given by homology; it is indeed the main principle of
+evolutionary morphology that what is common in organisation is due to
+common descent, what is divergent is due to adaptation. "Homology ...
+corresponds to the hypothetical genetic relationship. In the more or the
+less clear homology, we have the expression of the more or less intimate
+degree of relationship. Blood-relationship becomes dubious exactly in
+proportion as the proof of homologies is uncertain" (_Elements_, p. 63).
+
+It is worth noting that while Gegenbaur agrees with Haeckel generally
+that morphological relationships are really genealogical, that, for
+instance, each phylum has its ancestral form, he enters a caution
+against too hastily assuming the existence of a genetic relation between
+two forms on the basis of the comparison of one or two organs. "In
+treating comparative anatomy from the genealogical standpoint required
+by the evolution-theory," he writes, "we have to take into consideration
+the fact that the connections can almost never be discovered in the real
+genealogically related objects, for we have almost always to do with the
+divergent members of an evolutionary series. We derive, for instance,
+the circulatory system of insects from that of Crustacea ... but there
+exists neither a form that leads directly from Crustacea to insects nor
+any organisatory state (_Organisationszustand_), which as such shows the
+transition. Even when one point of organisation can be denoted as
+transitional, numerous other points prevent us from regarding the whole
+organism strictly in the same light" (_Grundzüge_, p. 75). The real
+ancestral forms cannot, as a rule, be discovered among living species,
+nor often as extinct. "When we arrange allied forms in series by means
+of comparison, and seek to derive the more complex from the simpler, we
+recognise in the lower and simpler forms only similarities with the
+ancestral form, which remains essentially hypothetical" (p. 75).
+
+The facts of development, Gegenbaur goes on to say, help us out greatly
+in our search for ancestral forms, for the early stages in the ontogeny
+of a highly organised animal give us some idea of the organisation of
+its original ancestor. Characters common to the early ontogeny of all
+the members of a large group are particularly important in this respect
+(_cf._ von Baer's law).
+
+Gegenbaur distinguishes homologous or morphologically equivalent
+structures from such as are analogous or physiologically equivalent,
+just as did Owen and the older anatomists. Like von Baer he recognises
+homologies, as a rule, only within the type.
+
+He contributed, however, to the common stock a useful analysis of the
+concept of homology, and established certain classes and degrees of it.
+He distinguished first between general and special homology, in quite a
+different sense from Owen.
+
+General homology, in Gegenbaur's sense, relates to resemblances of
+organs within the organism, and includes four kinds of resemblance,
+homotypy, homodynamy, homonomy and homonymy. Right and left organs are
+homotypic, metameric organs are homodynamic; homonomy is the relation
+exemplified by fin-rays or fingers, which are arranged with reference to
+a transverse axis of the body; homonymy is a sort of metamerism in
+secondary parts (not the main axis) of the body, and is shown by the
+various divisions of the appendages (_Grundzüge_, p. 80).
+
+Special homology, on the other hand, relates to resemblances between
+organs in different animals. The interesting thing is that Gegenbaur
+defines it genetically. Special homology is the name we give "to the
+relations which obtain between two organs which have had a common
+origin, and which have also a common embryonic history" (_Elements_, p.
+64). This is his definition; but, in practice, Gegenbaur establishes
+homologies by comparison just as the older anatomists did, and infers
+common descent from homology, not homology from common descent.
+
+"Special homology," he continues, "must be again separated into
+sub-divisions, according as the organs dealt with are essentially
+unchanged in their morphological characters, or are altered by the
+addition or removal of parts" (p. 65). In the former case the homology
+is said to be "complete," in the latter "incomplete." Thus the bones of
+the upper arm are completely homologous throughout all vertebrate
+classes from Amphibia upwards, while the heart of a fish is incompletely
+homologous with the heart of a mammal.
+
+Independently of Gegenbaur, Sir E. Ray Lankester proposed in 1870 a
+genetic definition of homology.[385] He proposed, indeed, to do away with
+the term homology altogether, on the ground that it included many
+resemblances which were obviously not due to common descent--as, for
+instance, the resemblance of metameres. So, too, organs which were
+homologous in the ordinary sense, as the heart of birds and mammals,
+might have arisen separately in evolution. He proposed, therefore, that
+"structures which are genetically related, in so far as they have a
+single representative in a common ancestor," should be called
+_homogenous_(p. 36). All other resemblances were to be called
+_homoplastic_. "Homoplasy includes all cases of close resemblance of
+form which are not traceable to homogeny, all details of agreement not
+homogenous, in structures which are broadly homogenous, as well as in
+structures having no genetic affinity" (p. 41). Serial homology, for
+instance, was a case of homoplasy.
+
+The term "analogy" was to be retained for cases of functional
+resemblance, whether homogenetic or not.
+
+The attempt was an interesting one, but most morphologists wisely
+adhered to the old concept of homology, in spite of Lankester's
+declaration that this belonged to an older "Platonic" philosophy, and
+ought to be superseded by a term more consonant with the new philosophy
+of evolution.
+
+    [366] _Generelle Morphologie der Organismen. Allgemeine
+    Grundzüge der organischen Formenwissenschaft, mechanisch
+    begründet durch die von Ch. Darwin reformierte
+    Descendenztheorie_. Berlin, 1866. Reprinted in part as
+    _Prinzipien der generellen Morphologie der Organismen_.
+    Berlin, 1906.
+
+    [367] He mentions as his predecessors in this field,
+    Bronn, J. Müller, Burmeister, and G. Jäger.
+
+    [368] In _Grundriss einer Allgemeinen Naturgeschichte der
+    Radiolarien_, Berlin, 1887, and _Kunstformen der Natur_,
+    Suppl. Heft, Leipzig.
+
+    [369] Haeckel had an intense admiration for Goethe's
+    morphological work. It is a curious coincidence that the
+    work of Goethe, Oken and Haeckel was closely associated
+    with the town of Jena.
+
+    [370] But he himself would not admit this! See _Gen.
+    Morph._, ii., p. 11.
+
+    [371] _Für Darwin_, 1864. Eng. trans, by Dallas as _Facts
+    and Arguments for Darwin_, London, 1869.
+
+    [372] The bion is the physiological, as the morphon is the
+    morphological, individual.
+
+    [373] See Vogt, _Embryologie des Salmones_, p. 259, 1842,
+    and _supra_, p. 230.
+
+    [374] _An Essay on Classification_, London, 1859.
+
+    [375] It was hinted at by Tiedemann. "It is clear that,
+    proceeding from the earlier to the more recent strata, a
+    gradation in fossil forms can be established from the
+    simplest organised animals, the polyps, up to the most
+    complex, the mammals, and that accordingly the animal
+    kingdom as a whole has its developmental periods just
+    like the single individual organism. The species and
+    genera which have become extinct during the evolutionary
+    process may be compared with the organs which disappear
+    during the development of the individual animal" (p. 73,
+    1808).
+
+    [376] _The History of Creation_, vol. i., p. 310, 1876.
+    Translation of the _Natürliche
+    Schöpfungsgeschichte_, 1868.
+
+    [377] _Cf._ a parallel passage from Serres, _supra_, p.
+    82.
+
+    [378] _Jenaische Zeitschrift_, ix., pp. 402-508, 1875.
+
+    [379] _Loc. cit._, ix., p. 409.
+
+    [380] _Untersuchungen zur vergl. Anatomie d.
+    Wirbelthiere_, Leipzig, i., 1864; ii., 1865; and iii.,
+    1872.
+
+    [381] "U. d. Biologie in Jena während des 19
+    Jahrhunderts," _Jenaische Zeitschrift_, xxxix., pp.
+    713-26, 1905.
+
+    [382] _Grundriss der vergl. Anatomie_, 1874, 2nd ed.,
+    1878. Trans. by F. Jeffrey Bell, revised by E. Ray
+    Lankester, as _Elements of Comparative Anatomy_, London,
+    1878.
+
+    [383] "This theory (evolution) shows that what was
+    formerly called 'structural plan' or 'type' is the sum
+    of the dispositions (_Einrichtungen_) of the animal
+    organisation which are perpetuated by heredity, while it
+    explains the modifications of these dispositions as
+    adaptive states. Heredity and adaptation are thus the
+    two important factors through which both the unity and
+    the variety of organisation can be understood"
+    (_Grundzüge_, p. 19).
+
+    [384] _History of Creation_, i., pp. 241-2.
+
+    [385] "On the use of the term Homology in Modern Zoology,
+    and the distinction between Homogenetic and Homoplastic
+    agreements," _Ann. Mag. Nat. Hist._ (4), vi., pp. 35-43,
+    1870.
+
+
+
+
+CHAPTER XV
+
+EARLY THEORIES ON THE ORIGIN OF VERTEBRATES
+
+
+Haeckel and Gegenbaur set the fashion for phylogenetic speculation, and
+up to the middle 'eighties, when the voice of the sceptics began to make
+itself heard, the chief concern of the younger morphologists was the
+construction of genealogical trees. The period from about 1865 to 1885
+might well be called the second speculative or transcendental period of
+morphology, differing only from the first period of transcendentalism by
+the greater bulk of its positive achievement. It must be remembered that
+the later workers (at least towards the end of this period) had immense
+advantages over their predecessors in the matter of equipment and
+technique; they possessed well-fitted laboratories in the university
+towns and by the sea; they had at their command perfected microscopes
+and microtomes; while the whole new technique of microscopical anatomy
+with its endless variety of stains and reagents made it possible for the
+tyro to confirm in a day what von Baer and Müller had taken weeks of
+painful endeavour to discover.[386] But the democratisation of morphology
+which followed upon the facilitation of its means of research left an
+evil heritage of detailed and unintelligent work to counterbalance the
+very great and real advances which technical improvements alone rendered
+possible.
+
+This period of rapid development, which set in soon after the coming of
+evolution and multiplied the concrete facts of morphology an
+hundredfold, may for our present purpose be conveniently divided into
+two somewhat overlapping periods, of which the second may be said to
+begin with the enunciation by Haeckel of his Gastræa theory. Within the
+first period fall the evolutionary speculations associated with the
+names of Kowalevsky, Dohrn, Semper, and others; the characteristic of
+the second period is the preponderating influence exercised upon
+phylogenetic speculations by the germ-layer doctrine in its two main
+evolutionary developments, the Gastræa and Coelom theories.
+
+In the first period we might again distinguish two main tendencies,
+according as speculations were based mainly upon anatomical or mainly
+upon embryological considerations, and it so happens that these two
+tendencies are very well illustrated by the various theories as to the
+origin of Vertebrates which began to appear towards the 'seventies. We
+shall accordingly, in this chapter, consider very briefly the history of
+the earlier views on the phylogeny of the vertebrate stock.
+
+In the early days, before the other claimants to the dignity of
+ancestral form to the Vertebrates--_Balanoglossus_, Nemertines and the
+rest--had put in an appearance, there were two main views on the
+subject, one upheld by Haeckel, Kowalevsky and others, to the effect
+that the proximate ancestor of Vertebrates was a form somewhat
+resembling the ascidian tadpole, the other supported principally by
+Dohrn and Semper that Vertebrates and Arthropods traced their descent to
+a common segmented annelid or pro-annelid ancestor. The former view is
+historically prior, and arose directly out of the brilliant
+embryological investigations of A. Kowalevsky, who proved himself to be
+a worthy successor of the great comparative embryologist Rathke. His
+work was indeed a true continuation of Rathke's. It was not directly
+inspired by evolution, though it supplied much useful confirmation of
+the theory--you may read Kowalevsky's earlier memoirs and not realise
+that they were written several years after the publication of the
+_Origin of Species_.
+
+His first paper of evolutionary importance was a note in Russian on the
+development of Amphioxus, published in 1865. This subject was followed
+up in two papers which appeared in 1867[387] and 1877.[388] In his
+papers on Amphioxus Kowalevsky made out the main features in the
+development of this primitive form, and showed that the chief organs
+were formed in essentially the same way as in Vertebrates; he described
+the formation of the archenteron by invagination, the appearance of the
+medullary folds, which coalesced to form the neural canal, the formation
+of the notochord and of the gill-slits. At first he made the mistake of
+supposing that the body-cavity arose from the segmentation-cavity, but
+in his later paper he rightly surmised that it was formed from the
+cavities of the "primitive vertebræ," or mesodermal segments. The origin
+of the notochord from the endoderm was also not made out by Kowalevsky
+in his paper of 1867.
+
+Although many important details remained to be discovered by later
+investigators,[389] Kowalevsky's work at once made the development of
+Amphioxus the key to vertebrate embryology, the typical ontogeny with
+which all others could be compared.
+
+Meanwhile, in 1866 and 1871, Kowalevsky had communicated memoirs of even
+greater interest,[390] in which he showed that the simple Ascidians
+developed in an extraordinarily similar way to Amphioxus and hence to
+Vertebrates in general. His proof that Ascidians also develop on the
+vertebrate type aroused great interest at the time, and was naturally
+acclaimed by the evolutionists as a striking piece of evidence in favour
+of their doctrine. The systematic position of the Ascidians was at that
+time quite uncertain; they were grouped, as a rule, with the Mollusca,
+and certainly no one suspected that their well-known tailed larvæ, first
+seen by Savigny, showed any but the most superficial analogy with the
+tadpoles of Amphibia. Kowalevsky's papers put a different complexion on
+the matter. In the first of them he showed how the nervous system of the
+simple Ascidian developed from ectodermal folds just as it did in
+Amphioxus and Vertebrates, how gill-slits were formed in the walls of
+the pharynx, and how there existed in the ascidian larva a structure
+which in position and mode of development was the strict homologue of
+the vertebrate notochord. In his second paper he entered into much more
+detail, and published some excellent figures, often reproduced since
+(see Fig. 13), but the proof of the affinity between Vertebrates and
+Ascidians was in all essentials complete in his paper of 1866.
+
+[Illustration: FIG. 13.--Development of the Ascidian Larva. (After
+Kowalevsky.)]
+
+Kowalevsky's results were accepted by Haeckel, Gegenbaur, Darwin,[391]
+and many others as conclusive evidence of the origin of Vertebrates
+from a form resembling the ascidian tadpole; they were extended and
+amplified by Kupffer[392] in 1870, later by van Beneden and Julin[393]
+and numerous other workers; they were adversely criticised by
+Metschnikoff[394] and von Baer,[395] as well as by H. de
+Lacaze-Duthiers and A. Giard.[396] Lacaze-Duthiers and von Baer both
+held fast to the old view that Ascidians were directly comparable with
+Lamellibranch molluscs; they denied the homology of the ascidian
+nervous system with that of Vertebrates, von Baer being at great pains
+to show that the ascidian nerve-centre was really ventral in position.
+He pointed out also that the "notochord" was confined to the tail of
+the ascidian larva. Giard's attitude was by no means so
+uncompromising, and the criticisms he passed on the Kowalevsky theory
+are both subtle and instructive. He admits that there exists a real
+homology between, for instance, the notochord of Vertebrates and that
+of Ascidians. "But," he adds, "it is too often forgotten that homology
+does not necessarily mean an immediate common origin or close
+relationship. There exist, doubtless, homologies of great atavistic
+importance--I consider as such, for example, the formation of the
+cavity of Rusconi [the archenteron] in Ascidians and lower
+Vertebrates. But there are also adaptive and purely analogical
+homologies, such as the interdigital palmation of aquatic birds,
+amphibians and mammals. These are not purely analogous organs, for
+they can be superposed one on another, which is not the case with
+simply analogous structures (the bat's wing, for example, cannot be
+superposed on the bird's wing); they are homologous formations,
+resulting from the adaptation of the same fundamental organs to
+identical functions. Such is, in my opinion, the nature of the
+homology existing between the tail of the ascidian tadpole and that of
+Amphioxus or of young amphibians. The ascidian larva, having no cilia
+and being necessarily motile, requires for the insertion of its
+muscles or contractile organs ... a central flexible axis, a true
+chorda dorsalis analogous to that of Vertebrates" (pp. 278-9). This
+point of view is strengthened by the fact that in _Molgula_, studied
+by Lacaze-Duthiers, the embryo is practically stationary, and forms no
+notochord, nor ever develops sense-organs in the cerebral vesicle.
+
+Giard's general conclusion is that "the true homology with Vertebrates
+ceases after the formation of the cavity of Rusconi and the medullary
+groove: the homologies established by Kowalevsky for the notochord and
+the relations of the digestive tube and nervous systems are not
+atavistic, but adaptive, homologies" (p. 282). There is accordingly no
+close genetic relationship between Ascidians and Vertebrates.
+
+Giard's criticisms did not avail to check the vogue of the new theory,
+which soon became an accepted article of faith in most morphological
+circles.[397] The fall of the Ascidians from their larval high estate
+provided the text for many a Darwinian sermon.
+
+Some years after the genetic relationship of Ascidians and Vertebrates
+had been established, a rival theory of the origin of Vertebrates made
+its appearance--a theory which was practically a rehabilitation in a
+somewhat altered form of the old Geoffroyan conception that Vertebrates
+are Arthropods walking on their backs. This was the so-called Annelid
+theory of Dohrn and Semper. Both Dohrn and Semper started out from the
+fact that Annelids and Vertebrates are alike segmented animals, and it
+was an essential part of their theory that this resemblance was due to
+descent from a common segmented ancestor. Both laid great stress on the
+fact that the main organs in Vertebrates are arranged in the same way as
+in an Annelid lying on its back, the nervous system being uppermost, the
+alimentary system coming next, and below this the vascular.
+
+Dohrn's earlier views are contained in the fascinating little book
+published in 1875, which bears the title _Der Ursprung der Wirbelthiere
+und das Princip des Functionswechsel_ (Leipzig). He followed this up by
+a long series of studies on vertebrate anatomy and embryology,[398] in
+which he modified his views in certain details. We shall confine our
+attention to the first sketch of his theory.
+
+If the Vertebrate is conceived to have evolved from a primitive Annelid
+which took to creeping or swimming ventral surface uppermost, a
+difficulty at once arises with regard to the relative positions of the
+"brain" and the mouth. In Vertebrates the brain, like the rest of the
+nervous system, is dorsal to the mouth and the alimentary canal; in an
+inverted Annelid, however, the brain is ventral to the mouth and is
+connected with the dorsal nerve cord by commissures passing round the
+oesophagus. It would seem, therefore, that the primitive Vertebrate must
+have acquired either a new brain or a new mouth. Dohrn took the latter
+view. He supposed that the original mouth of the primitive ancestor lay
+between the _crura cerebelli_ in the _fossa rhomboidea_, and that in
+Vertebrates this mouth has been replaced functionally by a new ventrally
+placed mouth, formed by the medial coalescence of a pair of
+gill-slits.[399] Probably the two mouths at one period co-existed, and the
+older one was ousted by the growing functional importance of the newer
+mouth.
+
+The gill-slits were considered by Dohrn to be derived from the segmental
+organs of Annelids, which were present originally in every segment of
+the primitive ancestor. The gills were at first external, like the gills
+of many Chætopods at the present day. For their support cartilaginous
+gill-arches naturally arose in the body-wall, and the superficial
+musculature became attached to these bars. "There existed in all the
+segments of the Annelid-ancestors of Vertebrates gills with
+cartilaginous skeleton and gill-arches in the body wall. Each gill had
+its veins and arteries, each had its branch of the ventral nerve-cord,
+and between each successive pair of gills a segmental organ opened to
+the exterior" (p. 14, 1875). The paired fins and limbs of the Vertebrate
+arose by the functional transformation of two pairs of these gills. The
+anterior gills became the definitive internal gills of the Vertebrate,
+for they gradually shifted into the mouths of the anterior segmental
+organs, which had already acquired an opening into the pharynx and had
+been transformed into true gill-slits. The posterior gills degenerated
+and disappeared, but their arches remained as ribs. Gill-arches and ribs
+were accordingly homologous structures and formed a _parietal_ skeleton.
+The vertebrate anus, like the mouth, was probably secondary and formed
+from a pair of gill-slits, the post-anal gut of vertebrate embryos
+hinting that the original anus was terminal as in Annelids. The unpaired
+fins of fish were originally paired and possibly arose from the
+coalescence of rows of parapodia. Dohrn assumed also that the primitive
+Annelid ancestor must have possessed a notochord to give support in
+swimming.
+
+If Vertebrates arose from primitive Annelid ancestors, how account for
+Amphioxus and the Ascidians, which seem to be the most primitive living
+Vertebrates and yet show no particular annelidan affinities? Dohrn tries
+to answer this awkward question by showing that these forms are not
+primitive but degenerate. He points out first that Cyclostomes are
+degenerate fish, half specialised and half degraded in adaptation to a
+parasitic mode of life. He thinks that if an _Ammocoetes_ were to become
+sexually mature and degenerate still further, forms would result which
+would resemble Amphioxus, and ultimately, if the process of degeneration
+went far enough, larval Ascidians. Amphioxus therefore might well be
+considered an extremely simplified and degenerate Cyclostome, and the
+ascidian larva the last term of this degeneration-series. Both Amphioxus
+and the Ascidians would accordingly be descended from fish, instead of
+fish being evolved from them.
+
+Dohrn conceived that the transformation of the Annelid into the
+Vertebrate took place mainly by reason of an important transforming
+principle, which he calls the principle of function-change. Each organ,
+Dohrn thinks, has besides its principal function a number of subsidiary
+functions which only await an opportunity to become active. "The
+transformation of an organ takes place by reason of the succession of
+the functions which one and the same organ possesses. Each function is a
+resultant of several components, of which one is the principal or
+primary function, while the others are the subsidiary or secondary
+functions. The weakening of the principal function and the strengthening
+of a subsidiary function alters the total function; the subsidiary
+function gradually becomes the chief function, the total function
+becomes quite different, and the consequence of the whole process is the
+transformation of the organ" (p. 60). Examples of function-change are
+not difficult to find. Thus the stomach in most Vertebrates performs
+both a chemical and a mechanical function, but in some forms a part of
+it specialises in the mechanical side of the work and becomes a gizzard,
+while the remaining part confines its energies to the secretion of the
+gastric juice. So, too, it is through function-change that certain of
+the ambulatory appendages of Arthropods have become transformed into
+jaws--their function as graspers of food has gradually prevailed over
+their main function as walking limbs. In the evolution of Vertebrates
+from Annelids the principle came into action in many connections--in the
+formation of a new mouth from gill-slits, in the transformation of gills
+into fins and limbs, of segmental organs into gill-slits, and so on.
+Dohrn tells us that the principle of function-change was suggested to
+him by Mivart's _Genesis of Species_ (1870), and he points out how it
+enables a partial reply to be made to the dangerous objection raised
+against the theory of natural selection that the first beginnings of new
+organs are necessarily useless in the struggle for existence.
+
+We may note in passing that a somewhat similar idea was later applied by
+Kleinenberg to the explanation of some of the ancestral features of
+development. He pointed out in his classical memoir on the embryology of
+the Annelid _Lopadorhynchus_[400] that many embryonic organs seem to be
+formed for the sole purpose of providing the necessary stimulus for the
+development of the definitive organs. Thus the notochord is the
+necessary forerunner of the vertebral column, cartilage the precursor of
+bone. "From this point of view," he writes, "many rudimentary organs
+appear in a different light. Their obstinate reappearance throughout
+long phylogenetic series would be hard to understand were they really no
+more than reminiscences of bygone and forgotten stages. Their
+significance in the processes of individual development may in truth be
+far greater than is generally recognised. When in the course of the
+phylogeny they have played their part as intermediary organs
+(_Vermittelungsorgane_) they assume the same function in the ontogeny.
+Through the stimulus or by the aid of these organs, now become
+rudimentary, the permanent parts of the embryo appear and are guided in
+their development; when these have attained a certain degree of
+independence, the intermediary organ, having played its part, may be
+placed upon the retired list."[401]
+
+Dohrn was well aware of the functional, or as he calls it, the
+physiological, orientation of his principle, and he rightly regarded
+this as one of its chief merits. He held that morphology became too
+abstract and one-sided if it disregarded physiology completely; he saw
+clearly that the evolution of function was quite as important a problem
+as the evolution of form, and that neither could be solved in isolation
+from the other. "The concept of function-change is purely
+physiological;" he writes, "it contains the elements out of which
+perhaps a history of the evolution of function may gradually arise, and
+for this very reason it will be of great utility in morphology, for the
+evolutionary history of structure is only the concrete projection of the
+content and course of the evolution of function, and cannot be
+comprehended apart from it" (p. 70).[402]
+
+It is very instructive in this connection to note that Dohrn was not,
+like so many of his contemporaries, a dogmatic materialist, but upheld
+the commonsense view that vital phenomena must, in the first instance at
+least, be accepted as they are. "It is for the time being irrelevant,"
+he writes, "to squabble over the question as to whether life is a result
+of physico-chemical processes or an original property (_Urqualität_) of
+all being.... Let us take it as given" (p. 75).
+
+Semper's speculations on the genetic affinity of Articulates and
+Vertebrates are contained in two papers[403] which appeared about the same
+time as Dohrn's. He openly acknowledges that his work is essentially a
+continuation of Geoffroy's transcendental speculations, and gives in his
+second paper a good historical account of the views of his great
+predecessor. It is a significant fact that evolutionary morphologists
+very generally held that Geoffroy was right in maintaining against
+Cuvier[404] the unity of plan of the whole animal kingdom, for they saw in
+this a strong argument for the monophyletic descent of all animals from
+one common ancestral form.
+
+In his first paper Semper does little more than break ground; he insists
+on the fact that both Annelids and Vertebrates are segmented animals,
+and he points out how close is the analogy between the nephridia or
+"segmental organs" of the former and the excretory (mesonephric) tubules
+of the latter, upon which he published in the same volume an extensive
+memoir. At this time he considered _Balanoglossus_--by reason of its
+gill-slits (its notochord he did not know)--to be the nearest living
+representative of the ancestral form of Vertebrates and Annelida.
+
+His second paper is a more exhaustive piece of work and deals with every
+aspect of the problem, both from an anatomical and from an embryological
+standpoint. It is consciously and admittedly an attempt to apply
+Geoffroy's principle of the unity of plan and composition to the three
+great metameric groups, the Annelida, Arthropoda, and Vertebrata. Semper
+follows Geoffroy's lead very closely in maintaining that it is not the
+position of the organs relative to the ground that must be taken into
+account in establishing their homologies, but solely their spatial
+relations one to another. He holds that dorsum and venter are terms of
+purely physiological import, and he proposes to substitute for them the
+terms neural and cardial (better, hæmal) surfaces, either of which may
+be either dorsal or ventral in position.
+
+Having established this primary principle, Semper has little difficulty
+in showing that the main organs of the body lie to one another in the
+same relative positions in Annelida, Arthropoda, and Vertebrata; and
+this, together with the metameric segmentation common to them all,
+constitutes his first great argument in favour of their genetic
+relationship. But he has still to show that Annelids possess at least
+the rudiments of certain organs which seem to be peculiar to
+Vertebrates, as the gill-slits, the notochord, and a nervous system
+developed from the ectoderm of the "dorsal" surface. He takes particular
+cognisance also of the old distinction drawn by von Baer, that
+Vertebrates show a "double-symmetrical" mode of development (_evolutio
+bigemina_), the dorsal muscle-plates forming a tube above the notochord,
+the ventral plates a tube below the notochord, whereas Articulates do
+not possess this axis, and form only one tube, namely, that round the
+"vegetative" organs (_evolutio gemina_). Semper is at pains to prove
+that _evolutio bigemina_ is characteristic also of Annelidan
+development.
+
+[Illustration: FIG. 14.--Transverse Section (Inverted) of the Worm
+_Nais_. (After Semper.)]
+
+He gets his facts from an elaborate study of the process of budding in
+the _Naidæ_, making the somewhat risky assumption that regeneration
+takes essentially the same course as embryonic development.
+
+He succeeds in showing--to his own satisfaction at least--that in the
+formation of new segments in _Nais_ and _Chætogaster_ a strand of cells
+appears between the alimentary canal and the nerve-cord, and that from
+this axial strand the hæmal muscle-plates grow out dorsally round the
+alimentary canal and the neural muscle-plates ventrally round the
+nerve-cord (see Fig. 14).
+
+This strand of cells, he concludes, must clearly be the notochord, and
+the type of development is obviously the double-symmetrical met with in
+Vertebrates.
+
+The nervous system Semper found to develop in the buds of _Nais_ and
+_Chætogaster_ by an ectodermal thickening, just as in some Vertebrates.
+The cerebral ganglion was formed by the ends of the nerve-cord growing
+up round the oesophagus and fusing with the paired "sense-plates" which
+develop from the ectoderm of the head. The cerebral ganglion is
+accordingly only secondarily hæmal in position, and there is no need
+therefore to seek in Vertebrates for the homologue of the oesophageal
+commissures of Annelids, as, for instance, Schneider did.
+
+Since the mouth opens on the neural surface in Annelids and on the hæmal
+surface in Vertebrates, Semper considers that they cannot be equivalent
+structures, and he finds the homologue of the Vertebrate mouth in a
+little pit on the hæmal surface of the head in the leech _Clepsine_ (also
+in the true mouth of Turbellaria and the proboscis-opening in
+Nemertines). The primitive Annelid mouth, however, does not appear in
+the embryogeny of Vertebrates, for the great development of the brain
+crowds it out of existence.
+
+The homologues of the gill-slits Semper finds in two little canals in
+the head of _Chætogaster_, which open from the pharynx to the exterior.
+In Sabellids he describes an elaborate system of gill-canals, with a
+supporting cartilaginous framework which forms a real _Kiemenkorb_ or
+gill-basket, comparable with that of Amphioxus.
+
+Gill-slits, notochord, relation of nervous system, mesonephric tubules,
+are thus common to Annelids and Vertebrates--what further proof could
+one desire of the close relationship of these groups? Yet Semper enters
+into refinements of comparison, seeing, for instance, in the lateral
+portions of the ventral ganglia (Fig. 14, _sp. g._) the homologues of
+the spinal ganglia of Vertebrates, and comparing the lateral line of
+sense organs in Annelids with the lateral line in Anamnia.
+
+He will not admit that Amphioxus and the Ascidians show a closer
+resemblance to Vertebrates than his beloved Annelids. Amphioxus, he
+thinks, is not a Vertebrate, and Ascidians, though sharing with Annelids
+the possession of a notochord, gill-slits, and a "dorsal" nervous
+system, yet are further removed from Vertebrates than the latter by
+reason of their lacking that essential characteristic of Vertebrates,
+metameric segmentation.
+
+Not content with establishing the unity of plan of Annelids, Arthropods,
+and Vertebrates, Semper tries to link on the Annelids, as the most
+primitive group of the three, to the unsegmented worms, and particularly
+to the Turbellaria. His speculations on this matter may be summed up
+somewhat as follows:--The common ancestor of all segmented animals is a
+segmented worm-like form, not quite like any existing type, resembling
+the Turbellaria in having two nerve strands on the dorsal side and no
+oesophageal ring, potentially able to develop either the Vertebrate or
+the Annelid mouth, and so to give origin both to the Articulate and to
+the Vertebrate series. The common ancestor alike of unsegmented worms
+and of all segmented types is probably the trochosphere larva, which in
+the Vertebrates is represented by the simple _Keimblase_ or blastula.
+
+The Annelid theory of Dohrn and Semper was perhaps not so widely
+accepted as the rival Ascidian theory, but it counted not a few
+adherents and gave a certain stimulus to comparative morphology. F. M.
+Balfour, who pointed out about the same time as Semper the analogy
+between the nephridia of Annelids and the mesonephric tubules of
+Vertebrates,[405] while not accepting the actual theories of Dohrn and
+Semper, took up a distinctly favourable attitude to the general idea
+that Annelids and Vertebrates were descended from a common segmented
+ancestor. Discussing this question in his classical work on the
+development of Elasmobranch fishes,[406] Balfour came to the conclusion
+"that we must look for the ancestors of the Chordata, not in allies of
+the present Chætopoda, but in a stock of segmented forms descended from
+the same unsegmented types as the Chætopoda, but in which two lateral
+nerve-cords, like those of Nemertines, coalesced dorsally instead of
+ventrally to form a median nervous cord. This group of forms, if my
+suggestion as to their existence is well founded, appears now to have
+perished."[407]
+
+He held that while there was much to be said for the interchange of
+dorsal and ventral surfaces postulated by Dohrn and Semper, the
+difficulties involved in the supposition were too great; he preferred,
+therefore, to assume that the present Vertebrate mouth was primitive,
+and not a secondary formation.
+
+His views as to the phylogeny of the Chordata and the genetic relation
+of the various classes to one another are exhibited in the following
+schema,[408] names of hypothetical groups being printed in capitals, names
+of degenerate groups in italics:--
+
+
+ Mammalia.                   Sauropsida.
+    |                            |
+    |____________________________|
+                    |
+               Proto-Amniota.         Amphibia.
+                    |                     |
+                    |_____________________|
+                             |
+                  Proto-Pentadactyloidei.
+                             |
+      Teleostei.             |
+          |                  |
+       Ganoidei.             |____________Dipnoi
+          |                  |
+          |__________________|
+                             |
+                       Proto-Ganoidei.
+                             |
+                             |____________Holocephali.
+                             |
+                             |____________Elasmobranchii.
+                             |
+                    Proto-Gnathostomata.
+                             |
+         ____________________|
+         |                   |
+   _Cyclostomata_.           |
+                             |
+                             |
+                     Proto-Vertebrata.
+                             |
+                             |
+                             |
+                             |
+         ____________________|______________________
+         |                                         |
+  _Cephalochorda_.    Protochordata.          _Urochorda_.
+
+
+The hypothetical ancestral forms (Protochordata) possessed a notochord,
+a ventral suctorial mouth and numerous gill-slits, and were presumably
+descended from the common ancestor of Annelids and Vertebrates.
+Amphioxus and the Ascidians found their place in this schema as
+degenerate offshoots of the ancestral Protochordates, while the
+Cyclostomes were in the same way the degenerate modern representatives
+of the ancestral Protovertebrates.
+
+Balfour's suggestion, that the nervous system in Annelids and
+Vertebrates might have arisen by the dorsal or ventral coalescence of
+the lateral nerve cords found in their common ancestor, bore fruit in
+the speculations of Hubrecht,[409] on the relation of Nemertines to
+Vertebrates.
+
+The Annelid theory was firmly supported by Eisig, who in his elaborate
+monograph on the _Capitellidæ_[410] maintained against Fürbringer the
+genetic identity of the Annelidan nephridia with the kidney tubules of
+Vertebrates. The independent discovery by E. Meyer[411] and J. T.
+Cunningham,[412] of an internal segmental duct in _Lanice_, into which
+several nephridia opened, seemed to strengthen this view.
+
+Following Ehlers,[413] Eisig found the homologue of the notochord in the
+accessory intestine of the _Capitellidæ_ and _Eunicidæ_, which he
+supposed might easily be transformed, according to the principle of
+function-change, from a respiratory to a supporting organ. He finally
+disposed of the alternative notion that the notochord was represented in
+Annelids by the "giant-fibres" or neurochordal strands which lie close
+above the nerve-cord, a view held by Kowalevsky,[414] and for a time by
+Semper. These strands were shown by Eisig, and by Spengel, to be the
+neurilemmar sheaths of thick nerve fibres which had in many cases
+degenerated. The view that the content of the neurochordal tubes was
+nervous in nature was first promulgated by Leydig in 1864.
+
+Much difference of opinion reigned as to the true homologies of the
+brain and mouth of Annelids and Vertebrates. Beard[415] and others got
+over the difficulty of the hæmal position of the cerebral ganglion in
+Annelids by supposing that it degenerated and disappeared altogether in
+the Annelidan ancestor of Vertebrates, and that accordingly it had no
+homologue in the Vertebrate nervous system. Beard put forward also the
+ingenious theory that the hypophysis represents the old Annelidan mouth.
+
+Van Beneden and Julin[416] assumed that in the ancestors of Vertebrates
+the oesophagus shifted forward between the still unconnected lobes of
+the brain to open on the hæmal surface.
+
+The fundamental assumption of the Annelid theory, that dorsal and
+ventral surfaces are morphologically interchangeable, seemed rather bold
+to many zoologists, and Gegenbaur[417] voiced a common opinion when he
+rejected as unscientific the comparison of the ventral nerve cord of
+Articulates with the dorsal nervous system of Vertebrates.
+
+The _Balanoglossus_ theory of Vertebrate descent also belongs, at least
+in its first form, to the earlier group of evolutionary speculations.
+The gill-slits of _Balanoglossus_ were discovered by Kowalevsky as early
+as 1866.[418] _Tornaria_ was discovered by J. Müller in 1850, but by him
+considered an Asterid larva; its true nature as the larva of
+_Balanoglossus_ was made out by Metschnikoff in 1870, who also remarked
+upon its extraordinary likeness to the larvæ of Echinoderms.[419] That it
+had some relationship with Vertebrates was recognised by Semper,
+Gegenbaur and others, but the full working-out of its Vertebrate
+affinities is due to Bateson.[420]
+
+Bateson broke completely with the Dohrn-Semper view that the metamerism
+of Articulates and Vertebrates must be put down to inheritance from a
+common ancestor. He held that metamerism was merely a special
+manifestation of the general property of repetition, common to all
+living things (_cf._ Owen's "vegetative force"), and that accordingly
+"however far back a segmented ancestor of a segmented descendant may
+possibly be found, yet ultimately the form has still to be sought for in
+which these repetitions had their origin" (p. 549). The meaning of the
+phenomenon was obscure, but he was convinced that the explanation was
+not to be found in ancestry. "This much alone is clear," he wrote, "that
+the meaning of cases of complex repetition will not be found in the
+search for an ancestral form, which, itself presenting this same
+character, may be twisted into a representation of its supposed
+descendant. Such forms there may be, but in finding them the real
+problem is not even resolved a single stage; for from whence was their
+repetition derived? The answer to this question can only come in a
+fuller understanding of the laws of growth and of variation, which are
+as yet merely terms" (pp. 548-9). It was in following up this line of
+thought that Bateson produced his monumental _Materials for the Study of
+Variation_ (1894).
+
+He found a strong positive argument for his theory that Vertebrates are
+descended from unsegmented forms in the fact that the notochord arises
+as an unsegmented structure. With the notochord he homologised the
+supporting rod in the proboscis of _Balanoglossus_, which like the
+notochord arises from the dorsal wall of the archenteron, and has a
+vacuolated structure. The gill-slits of _Balanoglossus_, with their
+close resemblance in detail to those of Amphioxus, Bateson also used as
+an argument in favour of the phylogenetic relationship of the
+Enteropneusta and Vertebrata, together with the formation from the
+ectoderm of a dorsal nerve tube.
+
+Bateson's views attracted considerable attention, and were thought by
+many to lighten appreciably the obscurity in which the origin of
+Vertebrates was wrapped. Thus Lankester wrote in his article on
+Vertebrates[421] in the _Encyclopedia Britannica_:--"It seems that in
+_Balanoglossus_ we at last find a form which, though no doubt
+specialised for its burrowing sand-life, and possibly to some extent
+degenerate, yet has not to any large extent fallen from an ancestral
+eminence. The ciliated epidermis, the long worm-like form, and the
+complete absence of segmentation of the body-muscles lead us to forms
+like the Nemertines. The great proboscis of _Balanoglossus_ may well be
+compared to the invaginable organ similarly placed in the Nemertines.
+The collar is the first commencement of a structure destined to assume
+great importance in _Cephalochorda_ and _Craniata_, and perhaps
+protective of a single gill-slit in _Balanoglossus_ before the number of
+those apertures had been extended. Borrowing, as we may, the nephridia
+from the Nemertines, and the lateral in addition to the dorsal nerve, we
+find that _Balanoglossus_ gives the most hopeful hypothetical solution
+of the pedigree of Vertebrates."
+
+Much doubt was cast upon the Chordate affinities of the Enteropneusta by
+Spengel in his monograph of the group,[422] but when the development of
+the coelom came to be more thoroughly worked out in _Balanoglossus_ and
+Amphioxus, the striking resemblance in this respect between the two
+forms gave additional support to the Batesonian view.[423]
+
+    [386] The stages in the development of microscopical
+    technique are well summarised by R. Burckhardt,
+    _Geschichte der Zoologie_, p. 121, Leipzig 1907.
+
+    [387] "Entwickelungsgeschichte des Amphioxus lanceolatus,"
+    _Mém. Acad. Sci. St Pétersbourg_ (Petrograd) (vii.),
+    xi., No. 4, 1867, 17 pp., 3 pls.
+
+    [388] "Weitere Studien ü. die Entwickelungsgeschichte des
+    Amphioxus lanceolatus," _Arch. für mikr. Anat._, xiii.,
+    pp. 181-204, 1877.
+
+    [389] Particularly by Hatschek (1881) and Boveri (1892).
+
+    [390] "Entwickelungsgeschichte der einfachen Ascidien,"
+    _Mém. Acad. Sci. St Pétersbourg_ (Petrograd), (vii.),
+    x., No. 15, 1866, 19 pp., 3 pls. "Weitere Studien ü. die
+    Entwicklung der einfachen Ascidien," _Arch. f. mikr.
+    Anat._, vii., pp. 101-130, 1871.
+
+    [391] _Descent of Man_, i., p. 205, 1871.
+
+    [392] _Arch. f. mikr. Anat._, vi., 1870, and viii., 1872.
+
+    [393] _Archives de Biologie_, 1884, 1885, and 1887.
+
+    [394] _Bull. Acad. Sci. St Pétersbourg_ (Petrograd) xiii.,
+    1869, and _Zeits. f. wiss. Zool._, xxii., 1872.
+
+    [395] _Mém. Acad. Sci. St Pétersbourg_(Petrograd)(7),
+    xix., 1873.
+
+    [396] Giard, _Arch. zool. expér. gén._, i., 1872, and
+    Lacaze-Duthiers, _ibid._, iii., 1874.
+
+    [397] For the later history of the Amphioxus-Ascidian
+    theory the reader may be referred to A. Willey's
+    well-known work, _Amphioxus and the Ancestry of the
+    Vertebrates_, New York and London, 1894, and to Delage
+    et Hérouard, _Traité de Zoologie concrète_, Tome viii.,
+    Paris, 1898.
+
+    [398] "Studien zur Urgeschichte des Wirbelthierkörpers,"
+    _Mittheil. Zool. Stat. Neapel_, 1882-1907.
+
+    [399] Leydig (_Vom Baue des thierischen Körpers_,
+    Tübingen, 1864), who, in a measure, forestalled Dohrn
+    and Semper by comparing Vertebrates with reversed
+    Arthropods, specially insects, supposed the old mouth to
+    pass between the _crura cerebri_.
+
+    [400] _Zeits. f. wiss. Zool._, xliv., 1886.
+
+    [401] Quoted by E. B. Wilson, _Wood's Holl Biological
+    Lectures for 1894_, p. 121.
+
+    [402] _Cf._ Metschnikoff, _Quart. Journ. Microsc. Sci._,
+    xxiv., pp. 89-111, 1884.
+
+    [403] "Die Stammesverwandschaft der Wirbelthiere und
+    Wirbellosen," _Arb. zool.-zoot. Instit. Würzburg_, ii.,
+    pp. 25-76, 1875; "Die Verwandschaftsbeziehungen der
+    gegliederten Thiere," _Ibid._, iii., pp. 115-404,
+    1876-7.
+
+    [404] Abuse of Cuvier also dates from the early days of
+    evolution, see Rádl, ii., pp. 12-17.
+
+    [405] "On the origin and history of the urino-genital
+    organs of Vertebrates," _Journ. Anat. Phys._, x., 1876.
+    The conclusions of Balfour and Semper were adversely
+    criticised by M. Fürbringer (_Morph. Jahrb._, iv.,
+    1878), and were negatived by later research.
+
+    [406] _A Monograph on the Development of Elasmobranch
+    Fishes_, London, 1878.
+
+    [407] _A Treatise on Comparative Embryology_, vol. ii., p.
+    311, London, 1881.
+
+    [408] _Loc. cit._, vol. ii., p. 327.
+
+    [409] "On the Ancestral Form of the Chordata," _Q.J.M.S._,
+    xxiii., 1883. "The Relation of the Nemertea to the
+    Vertebrata," _ibid._, xxvii., 1887. Hubrecht gives the
+    credit for the first indication of the relationship of
+    Nemertines and Vertebrates to Harting (_Leerboek van de
+    Grondbeginselen der Dierkunde_, 1874).
+
+    [410] "Monographie der Capitelliden des Golfes von
+    Neapel," _Fauna u. Flora des Golfes von Neapel_, Monog.
+    xvi., Berlin, 1887.
+
+    [411] _Mitt. Zool. Stat. Neapel_, vii., 1887.
+
+    [412] _Nature_, xxxvi., p. 162, 1887.
+
+    [413] "Nebendarm und Chorda dorsalis," _Nachr. Ges. Wiss.
+    Göttingen_, p. 390, 1885.
+
+    [414] "Embryologische Studien an Würmern u. Arthropoden,"
+    _Mém. Acad. Sci. St Pétersbourg_ (Petrograd), (7), xvi.,
+    1870. And in _Arch. f. mikr. Anat._, vii., p. 122, 1871.
+
+    [415] "The Old Mouth and the New," _Anat. Anz._, iii.,
+    1888. _Nature_, xxxix., 1889.
+
+    [416] "Recherches sur la Morphologie des Tuniciers,"
+    _Arch. de Biol._, vi., 1887.
+
+    [417] "Die Stellung u. Bedeutung der Morphologie," _Morph.
+    Jahrb._, i., pp. 1-19, 1876.
+
+    [418] "Anatomie des Balanoglossus," _Mém. Acad. Sci. St
+    Pétersbourg_ (Petrograd), (7), x., 1866.
+
+    [419] _Zeit. f. wiss. Zool._, xx., 1870. For a recent view
+    of the relation of the Enteropneusta to the Echinoderma,
+    see J. F. Gemmill, _Phil. Trans._ B., ccv., pp. 213-94,
+    1914.
+
+    [420] In a series of papers published in 1884-6, the
+    speculative results being discussed in his memoir on
+    "The Ancestry of the Chordata," _Q.J.M.S._ (n.s.), xxvi.,
+    pp. 535-71, 1886.
+
+    [421] Reprinted in _Zoological Articles_, London, 1891.
+
+    [422] "Die Enteropneusten des Golfes von Neapel," _Fauna
+    und Flora des Golfes von Neapel_, Monog. xviii., Berlin,
+    1893.
+
+    [423] See Macbride, "A Review of Prof. Spengel's Monograph
+    on Balanoglossus," _Q.J.M.S._, xxxvi., 1894, and "The
+    Early Development of Amphioxus," _Q.J.M.S._, xl., 1898.
+
+
+
+
+CHAPTER XVI
+
+THE GERM-LAYERS AND EVOLUTION
+
+
+In his papers of 1866 and 1867 Kowalevsky had remarked upon the
+widespread occurrence of a certain type or fundamental plan of early
+embryonic development, characterised by the formation, through
+invagination, of a two-layered sac, whose cavity became the alimentary
+canal. This developmental archetype was manifested in, for instance,
+_Sagitta_,[424] _Rana_,[425] _Lymnæa_,[426] _Astacus_,[427]
+_Phoronis_,[428] _Asterias_,[429] _Ascidia_,[428] the _Ctenophora_,[428]
+and _Amphioxus_.[428] He noticed also that the invagination-opening
+often became the definitive anus. Further instances of this mode of
+development were later observed by Metschnikoff[430] and by
+Kowalevsky[431] himself, but it was left to Haeckel to generalise these
+observations and build up from them his famous Gastræa theory. This was
+first enunciated in his monograph of the calcareous sponges,[432] and
+worked out in detail in a series of papers published in 1874-76.[433]
+
+Haeckel maintained that the "gastrula" stage occurred in the development
+of all Metazoa, and that it was typically formed, by invagination, from
+a hollow sphere of cells or "blastula." This typical formation might be
+masked by cenogenetic modifications caused chiefly by the presence of
+yolk. The gastrula stage was the palingenetic repetition of the
+ancestral form of all Metazoa, the Gastræa.
+
+From the Gastræa theory there followed at once two consequences, (1)
+that ectoderm and endoderm, invagination-cavity (_Urdarm_) and
+gastrula-mouth (_Urmund_ or _Protostoma_), were, with all their
+derivatives, homologous, because homogenous, throughout the Metazoa, and
+(2) that the descent of the Metazoa had been monophyletic, since all
+were derived from the ancestral Gastræa. Huxley's suggestion (_supra_,
+p. 208) that the outer and inner layers in Coelentera were homologous
+with the ectoderm and endoderm of the germ was thus fully confirmed and
+greatly extended.
+
+The great importance of the Gastræa theory lay in the fact that it
+linked up, by means of the biogenetic law, the germ-layer theory with
+the doctrine of evolution. It supplied an evolutionary interpretation of
+the earliest and most important of embryogenetic events, the process of
+layer-formation. Upon the Gastræa theory or its implications were
+founded most of the phylogenetic speculations which subsequently
+appeared.
+
+Upon the Gastræa theory Haeckel based a system of phylogenetic
+classification which was intended to replace Cuvier's and von Baer's
+doctrine of Types. This took the form of a monophyletic ancestral tree.
+Its main outlines are given on p. 290 in graphic form, combined and
+modified from the table on p. 53 of the 1874 paper and the genealogical
+tree given in the _Kalkschwämme_.[434]
+
+_Monophyletic Genealogical Tree of the Animal Kingdom, based upon the
+Gastræa Theory and the Homology of the Germ Layers_.
+
+_______________________________________________________________________
+|      |                                                       |    . |
+|      |                                                       |    m |
+|      |                               _Vertebrata_.           |    o |
+|    . |                                    |                  |    l |
+|    m |               _Arthropoda_.        |                  |    e |
+|    r |                     |              |                  |    o |
+|    e |                     |              |                  |    c |
+|    d |_Echinoderma_.       |              |  _Mollusca_.     |      |
+|    d |      |              |              |       |          |    a |
+|    n |      |              |   Sagitta.   \______ | ______/  |  |   |
+|    e |      |              |      |              \|/         | .  d |
+|      |      |              |      |               |          | a  n |
+|    y |      |              |      | Nematoda.   Himatega.    | i  a |
+|    b |      |              |      |     |            |       | r    |
+|      |      |              |      |     |            |       | a  d |
+|      |      |              |      |     |            |       | t  o |
+|      |      \______________|______|_  __|____________|_____/ | a  o |
+|      |                              \/                       | m  l |
+|      |                                                       | æ  b |
+|      |                            _Coelomati_                | H    |
+|      |                    (worms with body-cavity}.          |  | h |
+|      |                             \      /                  |    t |
+|      |                              \    /                   |    i |
+|      |                               \  /                    |    W |
+|      |________________________________\/_____________________|______|
+| .    |                                |                      |      |
+| )  d |                                |                      |    . |
+| s  e |    _Zoophyta_                  |      Plathelminthes. |    m |
+| l  n |   (Coe;enterata).              |               |      |    o |
+| a  i |                                |               |      |    l |
+| m  l |                Acalephæ.       \______________ |_____/|    e |
+| i    |                    |                          \/      |    o |
+| n  , |  Spongiæ.          |                  _Acoelomi_      |    c |
+| a  t |     |              |                (Worms without    |      |
+|    u | Archispongia.   Archydra.            body cavity).    |    o |
+| t  g |     |              |                         |        |    n |
+| u    |     |              |                         |        |      |
+| G  e |     \______  ______/                         |        |  | d |
+| (  u |            \/                                |        | a  n |
+|    r |         Protascus.                      Prothelmis.   | i  a |
+|    t |             |                                |        | r    |
+|      |             |                                |        | a  d |
+|    A |       Gastræa radialis            Gastræs bilateralis | æ  o |
+|  |   |          (sedens).                       (repens).    | n  o |
+| a  . |             |                                |        | A  l |
+| o  s |             |                                |        |  | b |
+| z  r |             \_______________  _______________/        |      |
+| a  e |                             \/                        |    o |
+| t  y |                         _Gastræa_                     |    N |
+| e  a |                   (Ontogeny : Gastrula).              |      |
+| M  l |                             |                         |      |
+|  |   |                             |                         |      |
+|    m |                             |                         |      |
+|    r |                             |                         |      |
+|    e |                             |                         |      |
+|    g |                             |                         |      |
+|      |                             |                         |      |
+|    y |                             |                         |      |
+|    r |                             |                         |      |
+|    a |                             |                         |      |
+|    m |                             |                         |      |
+|    i |                             |                         |      |
+|    r |                             |                         |      |
+|    P |                             |                         |      |
+|      |                             |                         |      |
+|    o |                             |                         |      |
+|    w |                             |                         |      |
+|    T |                             |                         |      |
+|______|                    _________|_________________________|______|
+|      |                    |                                         |
+|      |          __________|                                         |
+|      |          |                                                   |
+|    . |          |                                                   |
+|    t |      Planaeada               Acinetæ.             Ciliata.   |
+|    u | (Ontogeny : Planula).            |                    |      |
+|    g |          |                       \_________  _________/      |
+| >    |          |                                 \/                |
+| i  o |          |                             Infusoria.            |
+| /  n |          |                                  |                |
+| <    |          |                                  |                |
+| a  , |      Synamoebæ            Gregarinæ         |                |
+| o  s | (Ontogeny : Morula).          |             |                |
+| z  r |          |                    |             |                |
+| o  e |          |                    \_____  ______/                |
+| t  y |          |                          \/                       |
+| o  a |          |                        Amoebina.                  |
+| r  l |          |                           |                       |
+| P    |          \____________  _____________/                       |
+| >  m |                       \/                                     |
+| i  r |                    _Amoebæ_                    ?   ?   ?     |
+| <  e |              (Ontogeny : Ovulum).              |   |   |     |
+|    g |                        |                       |   |   |     |
+|      |                        |                       |   |   |     |
+|    o |                    _Monera_                     Monera.      |
+|    N |              (Ontogeny : Monerula).                          |
+|      |                                                              |
+|______|______________________________________________________________|
+
+
+The scheme is in many respects an interesting and important one. The
+great contrast between the Protozoa, or animals with neither gut nor
+germ-layers, and the Metazoa, which possess both structures, is for the
+first time clearly brought out. The derivation of all the Metazoa from a
+single ancestral form, the Gastræa, leads to the conclusion that the
+types are not distinct from one another as Cuvier and von Baer supposed,
+but agree in the one essential point, in the possession of an
+_archenteron_ (Lankester, 1875), and an ectoderm and endoderm which are
+homologous throughout all the Metazoan phyla. Finally, in the separation
+of the sponges, Coelenterata and Acoelomi as animals lacking a body
+cavity or coelom[435] from the four higher phyla, which are essentially
+Coelomati, there is contained the germ of a conception which later
+became of importance.
+
+Somewhat similar views as to the importance of the germ-layer theory for
+the phylogenetic classification of animals were published by Sir E. Ray
+Lankester in 1873.[436] He distinguished three grades of animals--the
+Homoblastica, Diploblastica, and Triploblastica. The first included the
+Protozoa, the second the Coelenterata, the third the other five phyla,
+distinguished by the possession of a third layer, the mesoderm, and a
+"blood-lymph" cavity enclosed therein. He used the germ-layer theory to
+prove the essential unity of type of all the Triploblastica.
+
+The Gastræa theory gave point and substance to the biogenetic law, and
+enabled Haeckel to state much more concretely the parallelism existing
+between ontogeny and phylogeny. He was able to assert that five
+primordial stages, each representing a primitive ancestral form,
+recurred with regularity in the very earliest development of all
+Metazoa.[437] These were the monerula, cytula, morula, blastula, and
+gastrula (see Fig. 15). The monerula was the fertilised ovum after the
+disappearance of the germinal vesicle;[438] it was the equivalent of
+the primordial anucleate Monera which are the ancestors of all
+animals. The ovum after the nucleus had been re-formed became the
+cytula, which was the ontogenetic counterpart of the amoeba. The
+morula, a compact mulberry-like congeries of segmentation-cells,
+corresponded to the synamoeba, or earliest association of
+undifferentiated amoeboid cells to form the first multicellular
+organism. The blastula, or hollow sphere of segmentation cells,
+usually ciliated, was reminiscent of the planæa, an ancestral
+free-swimming form whose nearest living relation is the spherical
+_Magosphæra_. The gastrula, finally, is the two-layered sac formed
+from the blastula, typically by invagination of its wall. It repeats
+the organisation of the gastræa, which is the common ancestor of all
+Metazoa, and finds its nearest living counterpart in the simple
+"sponges" _Haliphysema_ and _Gastrophysema_.[439] The ancestral line
+of all the higher animals begins with the five hypothetical forms of
+the moneron, amoeba, synamoeba, planæa, and gastræa.
+
+[Illustration: FIG. 15.--The Five Primary Stages of Ontogeny. (After
+Haeckel.) 1. Monerula. 2. Cytula. 3. Morula. 4. Blastula. 5. Gastrula.]
+
+We may take the following account[440] of the phylogeny of the human
+species, from the gastræa stage onwards, as typical of Haeckel's
+speculations on the evolution of the higher forms. The progenitors of
+man are, after the Gastræada:--
+
+
+1. Turbellaria.
+*2. Scolecida. (Worms with a coelom, probably represented
+        at the present day by _Balanoglossus_.)
+*3. Himatega. (Evolved from Scolecida by formation of
+        dorsal nerve-tube and chorda, and resembling tailed
+        larvæ of Ascidians.)
+4. Acrania. (With metameric segmentation. Including
+        Amphioxus.)
+5. Monorrhina. (Cyclostomes.)
+6. Selachia.
+7. Dipneusta.
+8. Sozobranchia. (Amphibia with permanent gills.)
+9. Sozura. (Tailed Amphibia.)
+*10. Protamnia.
+*11. Promammalia.
+12. Marsupialia.
+13. Prosimiæ.
+14. Menocerca. (Tailed apes.)
+15. Anthropoides.
+16. Pithecanthropi.
+17. Homines.
+
+It will be noticed that except for the hypothetical forms (marked with
+an asterisk), which are themselves generalised classificatory groups,
+the ancestral forms belong to long-recognised classes. The whole course
+of the evolution follows well-worn systematic lines. This is typical of
+Haeckel's phylogenetic speculations.
+
+A more abstractly morphological scheme of the evolution of Vertebrates
+is given in the _Systematic Phylogeny_ of 1895.[441] The ontogenetic and
+ancestral stages are arranged in parallel columns thus:--
+
+Cytula.                            Cytæa (Protozoa).
+Morula.                            Moræa (Coenobium of Protozoa).
+Blastula.                          Blastæa (_Volvocina_, etc.).
+Depula (invaginated blastula).     Depæa.
+Gastrula.                          Gastræa (cf. _Olynthus_, _Hydra_, and
+                                      primitive Coelentera).
+Coelomula (with one pair           Coelomæa (cf. _Sagitta_, _Ascidia_,
+  of coelom-pockets).                 and primitive Helminthes).
+Chordula (with medullary           Chordæa (_cf._ Ascidian larva and
+  tube and chorda).                   larva of Amphioxus).
+Spondula (with segmented           Prospondylus (Primitive Vertebrate).
+  mesoderm).
+
+This scheme differs from the earlier one chiefly in taking into account
+certain advances, notably as regards the cytology of the fertilised ovum
+and the true nature of the coelom, which had been made in the interval
+of some twenty years.
+
+Haeckel's Gastræa theory, though it exercised a great influence upon the
+subsequent trend of phylogenetic speculation, was by no means
+universally accepted _telle quelle_. Opinions differed considerably as
+to the primitive mode of origin of the two-layered sac which was very
+generally admitted to be of constant occurrence in early embryogeny. Ray
+Lankester, in his paper of 1873, and more fully in 1877,[442] propounded a
+"Planula" theory, according to which the ancestral form of the Metazoa
+was a two-layered closed sac formed typically by delamination, less
+often by invagination. He denied that the invagination opening (which he
+named the blastopore) represented the primitive mouth,[443] holding that
+this was typically formed by an "inruptive" process at the anterior end
+of the planula, which led to the formation of a "stomodæum." A similar
+process at the posterior end gave rise to the anus and the "proctodæum."
+
+The question as to whether delamination or invagination was to be
+considered the more primitive process was discussed in detail by
+Balfour,[444] without, however, any very definite conclusion being
+reached. He held that both processes could be proved in certain cases to
+be purely secondary or adaptive, and that accordingly there was nothing
+to show that either of them reproduced the original mode of transition
+from the Protozoa to the ancestral two-layered Metazoa (p. 342). He by
+no means rejected the theory that the Gastræa, "however evolved, was a
+primitive form of the Metazoa," but, having regard to the great
+variations shown in the relation of the blastopore to mouth and anus
+(pp. 340-1), he was inclined to think that if the gastrula had any
+ancestral characters at all, these could only be of the most general
+kind. Balfour's attitude perhaps best represents the general consensus
+of opinion with regard to the Gastræa theory.
+
+From the same origins as the Gastræa theory arose the theory of the
+coelom. The term dates back to Haeckel in 1872, and the observations
+which first led up to the theory were made by the men who supplied the
+foundations of the Gastræa theory--A. Agassiz, Metschnikoff and
+Kowalevsky. But it was not Haeckel himself who enunciated the coelom
+theory.
+
+It will be remembered that Remak introduced in 1855 the conception of
+the mesoderm as an independent layer derived from the endoderm. The
+pleuro-peritoneal or body-cavity was formed as a split in the "ventral
+plates" of the mesoderm. Haeckel's "coelom" corresponded to the
+"pleuro-peritoneal cavity" of Remak, but his view of the origin of the
+mesoderm brought him much closer to von Baer's conception of the origin
+of _two_ secondary layers from ectoderm and endoderm respectively than
+to Remak's conception of the mesoderm as a single independent layer.
+
+Much uncertainty reigned at the time as to the exact manner of origin of
+the mesoderm;[445] some held that it developed from the ectoderm, others
+that it originated in the endoderm, while still others, and among them
+Haeckel, considered that part of it came from the ectoderm and part from
+the endoderm (pp. 23-4, 1874).
+
+The solution of the problem came from those observations on the
+development of the lower forms to which we have just alluded.
+
+The early history of these discoveries and of the theory which grew out
+of them has been well summarised by Lankester,[446] and may conveniently
+be given in his own words:--
+
+"As far back as 1864 Alexander Agassiz ("Embryology of the Star-fish,"
+in _Contributions to the Natural History of the United States_, vol. v.,
+1864) showed in his account of the development of Echinoderma that the
+great body-cavity of those animals developed as a pouch-like outgrowth
+of the archenteron of the embryo, whilst a second outgrowth gave rise to
+their ambulacral system; and in 1869 Metschnikoff (_Mém. de l'Acad.
+impériale des Sciences de St Pétersbourg_, series vii., vol. xiv.,
+1869), confirmed the observations of Agassiz, and showed that in
+Tornaria (the larva of Balanoglossus) a similar formation of
+body-cavities by pouch-like outgrowths of the archenteron took place.
+Metschnikoff has further the credit of having, in 1874 (_Zeitsch. wiss.
+Zoologie_, vol. xxiv., p. 15, 1874), revived Leuckart's theory of the
+relationship of the coelenteric apparatus of the Enterocoela to the
+digestive canal and body-cavities of the higher animals. Leuckart had in
+1848 maintained that the alimentary canal and the body-cavity of higher
+animals were united in one system of cavities in the Enterocoela
+(_Verwandschaftsverhältnisse der wirbellosen Thiere_, Brunswick, 1848).
+Metschnikoff insisted upon such a correspondence when comparing the
+Echinoderm larva, with its still continuous enteron and coelom, to a
+Ctenophor, with its permanently continuous system of cavities and
+canals. Kowalevsky, in 1871, showed that the body-cavity of Sagitta was
+formed by a division of the archenteron into three parallel cavities,
+and in 1874 demonstrated the same fact for the Brachiopoda. In 1875
+(_Quart. Journ. Micr. Sci._, vol. xv., p. 52) Huxley proposed to
+distinguish three kinds of body-cavity: the schizocoel, formed by the
+splitting of the mesoblast, as in the chick's blastoderm; the
+enterocoel, formed by pouching of the archenteron, as in Echinoderms,
+Sagitta and Brachiopoda; and the epicoel.... Immediately after this I
+put forward the theory of the uniformity of origin of the coelom as an
+enterocoel (_Quart. Journ. Micr. Sci._, April, 1875).... My theory of
+the coelom as an enterocoel was accepted by Balfour and was greatly
+strengthened by his observations on the derivation of both notochord and
+mesoblastic somites from archenteron in the Elasmobranchs, and by the
+publication in 1877 by Kowalevsky of his second paper on the development
+of Amphioxus--in which the actual condition which I had supposed to
+exist in the Vertebrata was shown to occur, namely, the formation of the
+mesoblast as paired pouches in which a narrow lumen exists, but is
+practically obliterated on the nipping-off of the pouch from the
+archenteron, after which process it opens out again as coelom" (pp.
+16-18).
+
+The enterocoelic theory was taken up by O. and R. Hertwig as an
+essential part of their _Coelomtheorie_.[447] In a lengthy series of
+monographs these workers made a comparative study of the mode of
+formation of the middle layer, and arrived at a coherent theory of its
+origin. They distinguished in the middle layer two quite distinct
+elements, the mesoblast proper, formed by the evagination of the walls
+of the archenteron, and the mesenchyme, formed by free cells budded off
+from the germ-layers. The following passage gives a good idea of their
+views and of the phylogenetic implications involved:--"Ectoblast and
+entoblast are the two primary germ-layers which arise from the
+invagination of the blastula; they are always the first to be laid down,
+and they can be directly referred back to a simple ancestral form, the
+Gastræa; they form the limits of the organism towards the exterior and
+towards the archenteron. The parietal and visceral mesoblast, or the two
+middle layers, are always of later origin, and arise through evagination
+or plaiting of the entoblast, the remainder of which can now be
+distinguished as secondary entoblast from the primary. They form the
+walls of a new cavity, the enterocoel, which is to be regarded as a
+nipped-off diverticulum of the archenteron. Just as the two-layered
+animals can be derived from the Gastræa, so can the four-layered animals
+be derived from a Coelom form. Embryonic cells, which become singly
+detached from their epitheliar connections we consider to be something
+quite different from the germ-layers, and accordingly we call them by
+the special name of mesenchyme germs or primary cells of the mesenchyme.
+They may develop both in two-layered and in four-layered animals. Their
+function is to form between the epithelial limiting layers a secreted
+tissue (_Secretgewebe_) or connective tissue with scattered cells, which
+cells can undergo, like the epithelial elements, the most varied
+modifications.... This secreted tissue in its simple or in its
+differentiated state, with all its derivatives, we call the mesenchyme"
+(p. 122).
+
+The important point for us is that, just as all Metazoa were considered
+by Haeckel to be descended from the Gastræa, so all Coelomati were held
+by the Hertwigs to be derived from an original coelomate _Urform_. In
+both cases an embryological archetype becomes a hypothetical ancestral
+form.
+
+The Coelom theory was considerably modified, extended and developed by
+later workers, particularly as regards the relations to the coelom of
+the genital organs and ducts and the nephridia, but no special
+methodological interest attaches to these further developments.[448] We
+shall here focus attention upon one interesting line of speculation
+followed out in this country particularly by Sedgwick--the theory of the
+Actinozoan ancestry of segmented animals. Its relation to the Coelom
+theory lies in the fact that Sedgwick regarded the segmentation of the
+body as moulded upon the segmentation of the mesoblast, which in its
+turn, as Kowalevsky and Hatschek had shown, was a consequence of its
+mode of origin as a series of pouches of the archenteron. In other
+respects Sedgwick's speculations link on more closely to the Gastræa
+theory, for one of his main contentions is that the blastopore or
+_Urmund_ is homologous throughout at least the three metameric phyla. In
+following up Balfour's observations on the development of
+_Peripatus_,[449] Sedgwick was struck with the close resemblance existing
+between the elongated slit-like blastopore of this form (giving rise to
+both mouth and anus), with its border of nervous tissue, and the
+slit-like mouth of the Actinozoan (functioning both as mouth and anus),
+round which, as the Hertwigs had shown, there lies a special
+concentration of nerve cells and nerve fibres. He found another point of
+resemblance in the gastric pouches of the Actinozoa, which he
+homologised directly with the enterocoelic pouches of the Coelomati. He
+was led to enunciate the following theses:--[450] (1) that the mouth and
+anus of Vermes, Mollusca, Arthopoda, and probably Vertebrata, is derived
+from the elongated mouth of an ancestor resembling the Actinozoa; (2)
+that somites are derived from a series of archenteric pouches, like
+those of Actinozoa and Medusæ; (3) that excretory organs (nephridia,
+segmental organs) are derived from parts of these pouches which in the
+ancestral form, as in many polyps, were connected by a circular or
+longitudinal canal, and opened to the exterior by pores. This
+longitudinal canal was lost in Invertebrates, but persisted in
+Vertebrates as the pronephric duct, while the pores remained in
+Invertebrates and disappeared in Vertebrates; (4) that the tracheæ of
+Arthropods, as well as the canal of the central nervous system in
+Vertebrates, are to be traced back to certain ectodermal pits in the
+diploblastic ancestor comparable to the sub-genital pits of the
+Scyphomedusæ. These ectodermal pits were all originally respiratory
+organs. "The essence of all these propositions," he writes, "lies in the
+fact that the segmented animals are traced back not to a triploblastic
+unsegmented ancestor, but to a two-layered Coelenterate-like animal with
+a pouched gut, the pouching having arisen as a result of the necessity
+for an increase in the extent of the vegetative surfaces in a rapidly
+enlarging animal (for circulation and respiration)" (p. 47). "I have
+attempted to show," he writes further on, "that the majority of the
+Triploblastica ... are built upon a common plan, and that that plan is
+revealed by a careful examination of the anatomy of Coelenterata; that
+all the most important organ-systems of these Triploblastica are found
+in a rudimentary condition in the Coelenterata; and that all the
+Triploblastica referred to must be traced back to a diploblastic
+ancestor common to them and the Coelenterata" (p. 68). The main
+assumption was that the neural or blastoporal surface must be homologous
+throughout the Metazoa, though it was dorsal in the Chordata, ventral in
+the Annelida and Arthropoda. He derived the central nervous system of
+the Chordata from the circumoral ring of the common ancestor by means of
+the hypothesis that both the pre-blastoporal and the post-blastoporal
+parts of it disappeared.[451]
+
+The characteristic relation of the central nervous system to the
+blastopore in Annelida and Vertebrates had already been pointed out by
+Kowalevsky,[452] who had also sketched a theory of the common descent of
+these two phyla from an ancestral form in which the nervous system
+encircled the blastopore.
+
+In 1882, before the publication of Sedgwick's papers, A. Lang[453] had put
+forward the somewhat similar view that the stomach-diverticula of the
+Turbellaria, which he had found to be segmentally arranged in certain
+Triclads, were the morphological equivalents of the enterocoelic pouches
+of higher animals. This view, however, he soon gave up.[454] Sedgwick's
+views found a supporter in A. A. W. Hubrecht,[455] who utilised them in
+connection both with his speculations on the relation of Nemertines to
+Vertebrates, and with his exhaustive work on the early development of
+the Mammalia. He postulated as the far-back ancestor of Vertebrates, "an
+actinia-like, vermiform being, elongated in the direction of the
+mouth-slit" (p. 410, 1906), and derived the central nervous system from
+the circum-oral ring of this primitive form, the notochord from its
+stomodæum, and the coelom from the peripheral parts of the gastric
+cavity (p. 169, 1909).
+
+    [424] Gegenbaur, _Zeits. f. wiss. Zool._, v., 1853.
+
+    [425] Remak, _loc. cit._, p. 183, pl. xii.
+
+    [426] Lereboullet, _Ann. Sci. nat._ (4) xviii., pp. 118-9,
+    1862.
+
+    [527] Lereboullet, in Remak, p. 183 f.n.
+
+    [428] Kowalevsky, _Mém. Acad. Sci. St
+    Pétersbourg_ (Petrograd), (7), x. and xi., 1866 and 1867.
+
+    [429] A. Agassiz, _Contrib. Nat. Hist. United States_, v.,
+    1864.
+
+    [430] _Mém. Acad. Sci. St Pétersbourg_ (Petrograd), (7),
+    xiv., 1869.
+
+    [431] "Embryolog. Studien an Würmern u. Arthropoden,"
+    _Mém. Acad. Sci. St Pétersbourg_ (Petrograd), (7), xvi.,
+    1870.
+
+    [432] _Die Kalkschwämme_, 3 vols., Berlin, 1872. General
+    chapters translated in _Ann. Mag. Nat. Hist._ (4), xi.,
+    pp. 241-62, 421-30, 1873.
+
+    [433] "Die Gastræa-Theorie, die phylogenetische
+    Classification des Thierreichs und die Homologie der
+    Keimblätter." _Jenaische Zeitschrift_, viii., pp. 1-55,
+    1874. "Die Gastrula und die Eifurchung der Thiere,"
+    _ibid._, ix., pp. 402-508, 1875. "Die Physemarien,
+    Gastræaden der Gegenwart," and "Nachträge zur
+    Gastræa-Theorie," _ibid._, x., pp. 55-98, 1876.
+    Republished in _Biologische Studien_, 2nd part, _Studien
+    zur Gastræa-Theorie_, 270 pp., 14 pls., Jena, 1877.
+
+    [434] See _Ann. Mag. Nat. Hist._ (4), xi., p. 253.
+
+    [435] Term first introduced in _Die Kalkschwämme_, p. 468,
+    1872.
+
+    [436] "On the Primitive Cell-layers of the Embryo as the
+    Basis of Genealogical Classification of Animals, and on
+    the Origin of Vascular and Lymph Systems," _Ann. Mag.
+    Nat. Hist._ (4), xi., pp. 321-38, 1873.
+
+    [437] First distinguished in _Die Kalkschwämme_, i., p.
+    465.
+
+    [438] Even in the 'seventies it was still believed by many
+    that the egg-nucleus disappeared on fertilisation. The
+    true nature of the process was not fully made out till
+    1875, when O. Hertwig observed the fusion of egg- and
+    sperm-nuclei in _Toxopneustes (Morph. Jahrb._, i.,
+    1876).
+
+    [439] _Studien z. Gastræa-Theorie_, p. 214, 1877. These
+    forms were known even in 1870 (Carter, _Ann. Mag. Nat.
+    Hist._ (4), vi., pp. 346-7), to be Foraminifera. The
+    figures of supposed collar-cells, etc., do credit to
+    Haeckel's imagination.
+
+    [440] _History of Creation_, Eng. Trans., ii., pp. 278 ff.
+
+    [441] _Systematische Phylogenie_, iii., p. 41, Berlin,
+    1895.
+
+    [442] "Notes on the Embryology and Classification of the
+    Animal Kingdom," _Q.J.M.S._ (n.s.), xvii., pp. 399-454,
+    1877.
+
+    [443] It was "part of the non-historic mechanism of
+    growth" (_loc. cit._, p. 418).
+
+    [444] _Treatise on Comparative Embryology_, ii., chap.
+    xiii., 1881. For a modern discussion of this problem,
+    see Hubrecht, _Q.J.M.S._, xlix., 1906.
+
+    [445] See Balfour, _loc. cit._, Chapter xiii.
+
+    [446] _A Treatise on Zoology_, Pt. ii., 1900. Introduction
+    by Sir E. Ray Lankester.
+
+    [447] _Studien zur Blättertheorie_, Jena, 1879-80. "Die
+    Coelomtheorie, Versuch einer Erklärung des mittleren
+    Keimblattes," _Jenaische Zeitschrift_, xv., pp. 1-150,
+    1882.
+
+    [448] For an historical account of this work, see
+    Lankester, _loc. cit._, pp. 21-37.
+
+    [449] _Proc. Roy. Soc._, 1883, and _Q.J.M.S._, xxiii.,
+    1883.
+
+    [450] "Origin of Metameric Segmentation," _Q.J.M.S._,
+    xxiv., pp. 43-82 1884.
+
+    [451] See further the same author's article "Embryology"
+    in the _Ency. Brit._, vol. xi., 11th ed., Cambridge,
+    1910.
+
+    [452] _Arch. f. mikr. Anat._, xiii., pp. 181-204, 1877.
+
+    [453] "Der Bau von Gunda segmentata," _Mitth. Zool. Stat.
+    Neap._, iii., pp. 187-250, 1882.
+
+    [454] "Die Polycladen," _Fauna u. Flora des Golfes von
+    Neapel_, Monog. v., Leipzig, 1884, and "Beiträge zu
+    einer Trophocoeltheorie," _Jen. Zeits._, xxxviii., pp.
+    1-373, 1904 (which see for a modern account of theories
+    of metamerism).
+
+    [455] "Die Abstammung der Anneliden u. Chordaten," _ Jen.
+    Zeits._, xxxix., pp. 151-76, 1905. "The Gastrulation of
+    the Vertebrates," _Q.J.M.S._, xlix., pp. 403-19, 1906.
+    "Early Ontogenetic Phenomena in Mammals," _Q.J.M.S._,
+    liii., pp. 1-181, 1909.
+
+
+
+
+CHAPTER XVII
+
+THE ORGANISM AS AN HISTORICAL BEING
+
+
+"Of late the attempt to arrange genealogical trees involving
+hypothetical groups has come to be the subject of some ridicule, perhaps
+deserved. But since this is what modern morphological criticism in great
+measure aims at doing, it cannot be altogether profitless to follow this
+method to its logical conclusions. That the results of such criticism
+must be highly speculative, and often liable to grave error, is
+evident."
+
+The quotation is from Bateson's paper of 1886, and it is symptomatic of
+the change which was soon to come over morphological thought. New
+interests, new lines of work, began to usurp the place which pure
+morphology had held so long.
+
+This is accordingly a convenient stage at which to take stock of what
+has gone before, to consider the relation of evolutionary morphology to
+the transcendental and the Cuvierian schools of thought which preceded
+it, and to make clear what new element evolution-theory added to
+morphology.
+
+The close analogy between evolutionary and transcendental morphology has
+already been remarked upon and illustrated in the last three chapters.
+We have seen that the coming of evolution made comparatively little
+difference to pure morphology, that no new criteria of homology were
+introduced, and that so far as pure morphology was concerned, evolution
+might still have been conceived as an ideal process precisely as it was
+by the transcendentalists. The principle of connections still remained
+the guiding thread of morphological work; the search for archetypes,
+whether anatomical or embryological, still continued in the same way as
+before, and it was a point of subordinate importance that, under the
+influence of the evolution-theory, these were considered to represent
+real ancestral forms rather than purely abstract figments of the
+intelligence. The law of Meckel-Serres was revived in an altered shape
+as the law of the recapitulation of phylogeny by ontogeny; the natural
+system of classification was passively inherited, and, by a _petitio
+principii_, taken to represent the true course of evolution. It is true
+that the attempt was made to substitute for the concept of homology the
+purely genetic concept of homogeny, but no inkling was given of any
+possible method of recognising homogeny other than the well-worn methods
+generally employed in the search after homologies.
+
+There was a close spiritual affinity between the speculative
+evolutionists and the transcendentalists. Both showed the same
+subconscious craving for simplicist conceptions--the transcendentalists
+clung fast to the notion of the absolute unity of type, of the ideal
+existence of the "one animal," and the evolutionists did precisely the
+same thing when they blindly and instinctively accepted the doctrine of
+the monophyletic descent of all animals from one primeval form. Geoffroy
+persisted in regarding Arthropods as being built on the same plan as
+Vertebrates: Dohrn and Semper did nothing different when they derived
+both groups from an ancestor combining the main characters of both. The
+determination to link together all the main phyla of the animal kingdom
+and to force them all into a single mould was common to evolutionary and
+pre-evolutionary transcendentalists alike.
+
+From the fact that all Metazoa develop from an ovum which is a simple
+cell, the evolutionists inferred that all must have arisen from one
+primordial cell. From the fact that the next step in development is the
+segmentation of the ovum, they argued that the ancestral Metazoa came
+into being through the division of the primal Protozoon with aggregation
+of the division-products. From the fact that a gastrula stage is very
+commonly formed when segmentation has been completed, they assumed that
+all germ-layered animals were descended from an ancestral Gastræa.
+
+They quite ignored the possibility that a different explanation of the
+facts might be given; they seized upon the simplest and most obvious
+solution because it satisfied their overwhelming desire for
+simplification. But is the simplest explanation always the
+truest--especially when dealing with living things? One may be permitted
+to doubt it. It is easy to account for the structural resemblance of the
+members of a classificatory group, by the assumption that they are all
+descended from a common ancestral form; it is easy to postulate any
+number of hypothetical generalised types; but in the absence of positive
+evidence, such simplicist explanations must always remain doubtful. The
+evolutionists, however, had no such scruples.
+
+Phylogenetic method differed in no way from transcendental--except
+perhaps that it had learnt from von Baer and from Darwin to give more
+weight to embryology. The criticisms passed by Cuvier and von Baer upon
+the transcendentalists and their recapitulation theory might with equal
+justice be applied to the phylogenetic speculations which were based on
+the biogenetic law. There was the same tendency to fix upon isolated
+points of resemblance and disregard the rest of the organisation. Thus,
+on the ground of a presumed analogy of certain structures to the
+vertebrate notochord, several invertebrate groups, as the Enteropneusta,
+the Rhabdopleura, the Nemertea, were supposed to be, if not ancestral,
+at least offshoots from the direct line of vertebrate descent. And if
+other points of resemblance could in some of these cases be discovered,
+yet no successful attempt was made to show that the total organisation
+of any of these forms corresponded with that of the Vertebrate type.
+With the possible exception of the Ascidian theory, all the numerous
+theories of vertebrate descent suffered from this irremediable defect,
+and none carried complete conviction.
+
+In spite of the efforts of the evolutionists, as of those of the
+transcendentalists, the phyla or "types" remained distinct, or at best
+connected by the most general of bonds.
+
+The close affinity of transcendentalists and evolutionists is shown very
+clearly in their common contrast in habits of thought with the Cuvierian
+school. It is the cardinal principle of pure morphology that function
+must be excluded from consideration. This is a necessary and unavoidable
+simplification which must be carried out if there is to be a science of
+pure form at all. But this limitation of outlook, if carried over from
+morphology to general biology becomes harmful, since it wilfully ignores
+one whole side of life--and that the most important. The functional
+point of view is clearly indispensable for any general understanding of
+living things, and this is where the Cuvierian school has the advantage
+over the transcendental--its principles are applicable to biology in
+general.
+
+Geoffroy and Cuvier in pre-evolutionary times well typified the contrast
+between the formal and the functional standpoints. For Geoffroy form
+determined function, while for Cuvier function determined form. Geoffroy
+held that Nature formed nothing new, but adapted existing "materials of
+organisation" to meet new needs. Cuvier, on the other hand, was always
+ready to admit Nature's power to form entirely new organs in response to
+new functional requirements.
+
+The evolutionists followed Geoffroy rather than Cuvier. They laid great
+store by homological resemblances, and dismissed analogies of structure
+as of little interest. They were singularly unwilling to admit the
+existence of convergence or of parallel evolution, and they held very
+firmly the distinctively Geoffroyan view that Nature is so limited by
+the unity of composition that she can and does form no new organs.
+
+By no one has this underlying principle of evolutionary morphology been
+more explicitly recognised than by Hubrecht, who in his paper of 1887,
+after summarising the points of resemblance between Nemertines and
+Vertebrates which led him to assume a genetic connection between them,
+writes as follows:--"At the base of all the speculations contained in
+this chapter lies the conviction, so strongly insisted upon by Darwin,
+that new combinations or organs do not appear by the action of natural
+selection unless others have preceded, from which they are gradually
+derived by a slow change and differentiation.
+
+"That a notochord should develop out of the archenteric wall because a
+supporting axis would be beneficial to the animal may be a teleological
+assumption, but it is at the same time an evolutional heresy. It would
+never be fruitful to try to connect the different variations offered,
+_e.g._, by the nervous system throughout the animal kingdom, if similar
+assumptions were admitted, for there would be then quite as much to say
+for a repeated and independent origin of central nervous systems out of
+indifferent epiblast just as required in each special case. These would
+be steps that might bring us back a good way towards the doctrine of
+independent creations. The remembrance of Darwin's, Huxley's, and
+Gegenbaur's classical foundations, and of Balfour's and Weismann's
+brilliant superstructures, ought to warn us away from these dangerous
+regions" (p. 644).
+
+This same prejudice lies at the root of the idea of _Functionswechsel_,
+in spite of the general functional orientation of that idea.
+
+Dohrn's constant assumption is that Nature makes shift with old organs
+wherever possible, instead of forming new ones. He derives gill-slits
+from segmental organs, fins and limbs from gills, ribs from gill-arches,
+and so on, instead of admitting that these organs might quite as well
+have arisen independently. He objects on principle to the origin of
+organs _de novo_. Thus, rebutting the suggestion that certain organs
+which are not found in the lower Vertebrates might have arisen as new
+formations, he writes:--"Against this supposition the whole weight of
+all those objections can be directed that are to be brought in general
+against the method of explanation which consists in appealing without
+imperative necessity to the _Deus ex machina_, 'New formation,' which is
+neither better nor worse than _Generatio equivoca_" (p. 21).
+
+Of a similar nature was the objection to convergence.[456]
+
+Why, we may ask, were morphologists so unwilling to admit the creative
+power of life? Dohrn, for instance, was fully aware of the great
+transforming influence exerted by function upon form--his theory of
+_Functionswechsel_ regards as the most powerful agent of change the
+activity of the animal, its effort to make the best use of its organs,
+to apply them at need in new ways to meet new demands. Why then did he
+not go a step further and admit that the animal could by its own
+subconscious efforts form entirely new organs? Why did most
+morphologists join with him in belittling the organism's power of
+self-transformation?
+
+The reasons seem to have been several. There is first the fundamental
+reason, that the idea of an active creative organism is repugnant to the
+intelligence, and that we try by all means in our power to substitute
+for this some other conception. In so doing we instinctively fasten upon
+the relatively less living side of organisms--their routine habits and
+reflexes, their routine structure--and ignore the essential activity
+which they manifest both in behaviour and in form-change.
+
+We tend also to lay the causes of form-change, of evolution, as far as
+possible outside the living organism. With Darwin we seek the
+transforming factors in the environment rather than within the organism
+itself. We fight shy of the Lamarckian conception that the living thing
+obscurely works out its own salvation by blind and instinctive effort.
+We like to think of organisms as machines, as passive inventions[457]
+gradually perfected from generation to generation by some external
+agency, by environment or by natural selection, or what you will. All
+this makes us chary of believing that Nature is prodigal of new organs.
+
+Other causes of the unwillingness of morphologists to admit the new
+formation of organs are to be sought in the main principle of pure
+morphology itself, that the unity of plan imposes an iron limit upon
+adaptation, and in the powerful influence exercised at the time by
+materialistic habits of thought. Teleology had become a bugbear to the
+vast majority of biologists, and all real understanding of the Cuvierian
+attitude seems, in most cases, to have been lost, although, curiously
+enough, teleological conceptions were often unconsciously introduced in
+the course of discussions on the "utility" of organs in the struggle for
+existence.
+
+Evolutionary morphology, being for the most part a form of pure or
+non-functional morphology, agreed then in all essential respects with
+pre-evolutionary or transcendental morphology.
+
+But it contained the germ of a new conception which threw a new light
+upon the whole science of morphology. This was the conception of the
+organism as an historical being.
+
+We have seen this thought expressed with the utmost clearness by Darwin
+himself (_supra_, p. 233). In his eyes the structure and activities of
+the living thing were a heritage from a remote past, the organism was a
+living record of the achievements of its whole ancestral line. What a
+light this conception threw upon all biology! "When we no longer look at
+an organic being as a savage looks at a ship as something wholly beyond
+his comprehension; when we regard every production of Nature as one
+which has had a long history; when we contemplate every complex
+structure and instinct as the summing-up of many contrivances, each
+useful to the possessor, in the same way as any great mechanical
+invention is the summing-up of the labour, the experience, the reason,
+and even the blunders of numerous workmen; when we thus view each
+organic being, how far more interesting--I speak from experience--does
+the study of natural history become!" (_Origin_, 6th ed., pp. 665-6).
+
+Sedgwick expressed the same thing from the morphological point of view
+when he wrote, with reference to the ancestral significance of the
+blastopore:--"If there is anything in the theory of evolution, every
+change in the embryo must have had a counterpart in the history of the
+race, and it is our business as morphologists to find it out" (p. 49,
+1884).
+
+By the evolution-theory the problems of form were linked indissolubly
+with the problem of heredity. Unity of plan could no longer be explained
+idealistically as the manifestation of Divine archetypal ideas; it had a
+real historical basis, and was due to inheritance from a common
+ancestor. The evolution-theory gave meaning and intelligibility to the
+transcendental conception of the unity of plan; in particular it
+supplied a simple and satisfying explanation of those puzzling vestigial
+organs, whose existence was such a stumbling-block to the teleologists.
+It enabled the biogenetic law to be substituted for the laws of
+Meckel-Serres and von Baer, as being in some measure a combination and
+interpretation of both.
+
+Where the concept of evolution proved itself particularly useful was in
+the interpretation of structures which were not immediately conditioned
+by adaptation to present requirements, such as, for instance, the
+arrangement of gill-slits and aortic arches in the foetus of land
+Vertebrates. Such "heritage characters" could only be explained on the
+hypothesis that they had once had functional or adaptational meaning.
+Why, for instance, should the blastopore so often appear as a long slit,
+closing by concrescence, unless this had been the original method of its
+formation in remote Coelenterate ancestors?
+
+The point hardly requires elaboration, since it has become an integral
+part of all our thinking on biological problems. It may be as well,
+however, for the sake of continuity, to give one or two examples of the
+historical interpretation of animal structures. The first may
+conveniently be the phylogenetic interpretation of the contrast between
+"membrane" and "cartilage" bones.
+
+In his _Grundzüge_ of 1870, Gegenbaur made the suggestion that the
+investing or membrane bones were derived phylogenetically from
+integumentary ossifications, and this was worked out in detail a few
+years later by O. Hertwig.[458]
+
+Many years before, several observers--J. Müller, Williamson, and
+Steenstrup--had been struck with the resemblance existing between the
+placoid scales and the teeth of Elasmobranch fishes. Hertwig followed up
+this clue, and came to the conclusion not only that placoid scales and
+teeth were strictly homologous, but also that all membrane bones were
+derived phylogenetically from ossifications present in the skin or in
+the mucous membrane of the mouth, just as cartilage bones were derived
+from the cartilaginous skeletons of the primitive Vertebrates. In some
+cases this manner of derivation could even be observed in ontogeny, as
+Reichert had seen in the Newt, where certain bones in the roof of the
+mouth are actually formed by the concrescence of little teeth, (_supra_,
+p. 163). Hertwig considered that the following bones were originally
+formed by coalescence of teeth--parasphenoid, vomer, palatine,
+pterygoid, the tooth-bearing part of the pre-maxillary, the maxillary,
+the dentary and certain bones of the hyo-mandibular skeleton of
+Teleosts. All the investing bones (_Deckknochen_) of the skull were of
+common origin, and could be traced back to integumentary skeletal
+plates, which in the ancestral fish formed a dense carapace.
+
+These conclusions were accepted by Kölliker himself, who wrote in his
+_Entwickelungsgeschichte_ (1879)--"The distinction between the primary
+or primordial, and the investing or secondary bones is from the
+morphological standpoint sharp and definite. The former are
+ossifications of the (cartilaginous) primordial skeleton, the latter are
+formed outside this skeleton, and are probably all ossifications of the
+skin or the mucous membrane" (p. 464).
+
+Gegenbaur[459] consistently upheld the phylogenetic derivation of
+investing bones from dermal ossifications, and even went further and
+derived substitutionary bones as well from the integument, thus
+establishing a direct comparison between the skeletal formations of
+Vertebrates and Invertebrates. Investing bones were actual integumentary
+ossifications which had gradually sunk beneath the skin to become part
+of the internal skeleton; substitutionary bones were produced by cells
+(osteoblasts) which were ultimately derived from the integument.[460]
+
+A further instance of the historical interpretation of animal structure,
+taken from quite a different field, is afforded by the speculations of
+Dollo[461] on the ancestral history of the Marsupials. In a brilliant
+paper of 1880[462] Huxley made the suggestion that the ancestors of
+Marsupials were arboreal forms. "I think it probable," he wrote, "from
+the character of the pes, that the primitive forms, whence the existing
+Marsupialia have been derived, were arboreal animals; and it is not
+difficult, I conceive, to see that, with such habits, it may have been
+highly advantageous to an animal to get rid of its young from the
+interior of its body at as early a period of development as possible,
+and to supply it with nourishment during the later periods through the
+lacteal glands, rather than through an imperfect form of placenta" (p.
+655). Dollo followed up this suggestion, which had in the meantime been
+strengthened by Hill's discovery of a true allantoic placenta in
+_Perameles_, by demonstrating in the foot of present-day Marsupials
+certain features which could only be interpreted as inherited from a
+time when the ancestors of Marsupials were tree-living animals. These
+were the occurrence of an opposable big toe (when this was present at
+all), the great development of the fourth toe, the reduction and partial
+syndactylism of the second and third toes, and in some cases the
+regression of the nails. These characters were shown to be typical of
+arboreal Vertebrates, and their occurrence in forms not arboreal
+indicated that these were descended from tree-living ancestors. Traces
+of an arboreal ancestry could be demonstrated even in the marsupial mole
+_Notoryctes_.
+
+These are only two examples out of hundreds that might be given. Present
+day structure was interpreted in the light of past history; the common
+element in organic form was seen to be due to common descent; the
+existence of vestigial and non-functional organs was no longer a riddle.
+
+There was even a tendency to concentrate attention upon the historical
+side of structure, upon what the animal passively inherited rather than
+upon what it personally achieved. Homologies were considered more
+interesting than analogies, vestigial organs more interesting than
+foetal and larval adaptations. Convergence was anathema. The dead-weight
+of the past was appreciated at its full and more than its full value;
+and the essential vital activity of the living thing, so clearly shown
+in development and regeneration, was ignored or forgotten.
+
+But evolutionary morphology for all practical purposes was a development
+of pure or idealistic morphology, and was powerless to bring to fruit
+the new conception with which evolution-theory had enriched it. The
+reason is not far to seek. Pure morphology is essentially a science of
+comparison which seeks to disentangle the unity hidden beneath the
+diversity of organic form. It is not immediately concerned with the
+causes of organic diversity--that is rather the task of the sciences of
+the individual, heredity and development. To take an example--the
+recapitulation theory may legitimately be used as a law of pure
+morphology, as stating the abstract relation of ontogeny to phylogeny,
+and the probable line of descent of any organism may be deduced from it,
+as a mere matter of the ideal derivation of one form from another; but
+an explanation of the reason for the recapitulation of ancestral history
+during development can clearly not be given by pure morphology unaided.
+From the fact that the common starfish shows in the course of its
+development distinct traces of a stalk[463] it is possible to infer,
+taking other evidence also into consideration, that the ancestors of the
+starfish were at one stage of their existence stalked and sessile
+organisms. But this leaves unanswered the question as to how and why the
+starfish does still repeat after so many millions of years part of the
+organisation of one of its remote ancestors. Why is this feature
+retained, and by what means has it been conserved through countless
+generations? It is clear that the answer can be given only by a science
+of the causes of the production and retention of form, by a causal
+morphology, based upon a study of heredity and development.
+
+From the point of view of the pure morphologist the recapitulation
+theory is an instrument of research enabling him to reconstruct probable
+lines of descent; from the standpoint of the student of development and
+heredity the fact of recapitulation is a difficult problem whose
+solution would perhaps give the key to a true understanding of the real
+nature of heredity.
+
+To make full use of the conception of the organism as an historical
+being it is necessary then to understand the causal nexus between
+ontogeny and phylogeny.
+
+We shall see in the next chapter that the transformation of morphology
+from a comparative to a causal science did take place towards the end of
+the century, and that some progress was made towards an understanding of
+the relation between individual development and ancestral history,
+particularly by Roux and Samuel Butler, working with the fruitful
+Lamarckian conception of the transforming power of function.
+
+    [456] The importance of convergence came to be realised
+    after the vogue of phylogenetic speculation had
+    passed--see Friedmann, _Die Konvergenz der Organismen_,
+    Berlin, 1904, and A. Willey, _Convergence in Evolution_,
+    London, 1911. Also L. Vialleton, _Elements de
+    morphologie des Vertébrés_, Paris, 1912.
+
+    [457] From this point of view there is a very profound
+    analogy between artificial and natural selection. Upon
+    the theory of natural selection organisms are lifeless
+    constructs which are mechanically perfected by external
+    agency, just as machines are improved by a process of
+    conscious selection of the most successful among a
+    number of competing models. (_Cf._ passage quoted below,
+    on p. 308.)
+
+    [458] _Arch. f. mikr. Anat._, xi. (suppl.), 1874; _Morph.
+    Jahrb._, ii., 1876, v. 1879, and vii., 1882.
+
+    [459] _Vergleich. Anat. d. Wirbelthiere_, i., pp. 200-1,
+    1898.
+
+    [460] For a full historical account of work on membrane
+    and cartilage bones (as well as on the theory of the
+    skull) see E. Gaupp, "Altere und neuere Arbeiten über
+    den Wirbelthierschädel," _Ergeb. Anat. Entw._, x., 1901,
+    and "Die Entwickelung des Kopfskelettes," in Hertwig's
+    "_Handbuch vergl. exper. Entwickelungslehre d.
+    Wirbelthiere_," iii., 2, pp. 573-874, 1905.
+
+    [461] "Les Ancêtres des Marsupiaux étaient-ils
+    arboricoles?" _Trav. Stat. zool. Wimereux_, vii., pp.
+    188-203, pls. xi.-xii., 1899. See also Bensley, _Trans.
+    Linn. Soc._ (2) ix., pp. 83-214, 1903.
+
+    [462] _Proc. Zool. Soc._, pp. 649-62, 1880. _Sci. Mem._,
+    iv., pp. 457-72.
+
+    [463] J. F. Gemmill, _Phil. Trans. B_, ccv., p. 255, 1914.
+
+
+
+
+CHAPTER XVIII
+
+THE BEGINNINGS OF CAUSAL MORPHOLOGY
+
+
+Until well into the 'eighties animal morphology remained a purely
+descriptive science, content to state and summarise the relations
+between the coexistent and successive form-states of the same and of
+different animals. No serious attempt had been made to discover the
+causes which led to the production of form in the individual and in the
+race.
+
+It is true that evolution-theory had offered a simple solution of the
+great problem of the unity in diversity of animal forms, but this
+solution was formal merely, and went little beyond that abstract
+deduction of more complex from simpler forms, which had been the main
+operation of pre-evolutionary morphology. Little was known of the actual
+causes of ontogeny, and nothing at all of the causes of phylogeny; it
+was, for instance, mere rhetoric on Haeckel's part to proclaim that
+phylogeny was the mechanical cause of ontogeny.
+
+Animal physiology, on its side, had developed in complete isolation from
+morphology into a science of the functioning of the adult and finished
+animal, considered as a more or less stable physico-chemical mechanism.
+Since the days of Ludwig, Claude Bernard and E. du Bois Reymond, the
+physiologists' chief care had been to analyse vital activities into
+their component physical and chemical processes, and to trace out the
+interchange of matter and energy between the organism and its
+environment. Physiologists had left untouched, perhaps wisely, the much
+more difficult problem of the causes of the development of form. For all
+practical purposes they took the animal-machine as given, and did not
+trouble about its mode of origin. They held indeed that form-production
+was due to a complex of physico-chemical causes, which they hoped some
+day to unravel;[464] but this future physiology of development remained
+quite embryonic.
+
+Physiology then had not really come into contact with the problems of
+form, and it could give the morphologist no direct help when he turned
+to investigate the causes of form-production. It had, however, a
+determining influence upon the methods of those who first broke ground
+in this No Man's Land between morphology proper and physiology. But it
+is significant that it was a morphologist and not a physiologist that
+did the first spade-work.
+
+The pioneer in this field, both as investigator and as thinker, was W.
+Roux, who sketched in the 'eighties the main outlines of a new science
+of causal morphology, to which he gave the name of
+_Entwicklungsmechanik_. The choice of name was deliberate, and the word
+implied, first, that the new science was essentially an investigation of
+the development of form, not of the mode of action of a formed
+mechanism, and second, that the methods to be adopted were
+mechanistic.[465]
+
+Though Roux was the only begetter of the science of
+_Entwicklungsmechanik_, he was, of course, not the first to investigate
+experimentally the formative processes of animal life. Study of
+regeneration dates back to Trembley (1740-44), Réaumur (1742), Bonnet
+(1745), and Spallanzani (1768-82),[466] and in the years preceding Roux's
+activity good work was done by Philipeaux. A beginning had been made
+with experimental teratology by E. Geoffroy St Hilaire and others, and
+the work of C. Dareste[467] remains classical. Back in the 18th century,
+some of John Hunter's experiments had a bearing upon the problems of
+form; his work on transplantation was followed up in the 19th century by
+Flourens, P. Bert, Ollier and many others. In founding in 1872 the
+_Archives de Zoologie expérimentale et générale_ H. de Lacaze-Duthiers
+put forward in his introduction a powerful plea for the use of the
+experimental method in zoology.
+
+In some ways more directly connected with _Entwicklungsmechanik_ was
+His's attempt in 1874[468] to explain on mechanical principles the
+formation of certain of the embryonic organs by the bendings and
+foldings of tubes or plates of cells. "His compared the various layers
+of the chick embryo to elastic plates and tubes; out of these he
+suggested that some of the principal organs might be moulded by mere
+local inequalities of growth--the ventricles of the brain, for instance,
+the alimentary canal, the heart--and he further succeeded in imitating
+the formation of these organs by folding, pinching, and cutting
+india-rubber tubes and plates in various ways."[469]
+
+But Roux was undoubtedly the first to make a systematic survey of the
+problems to be solved and to work out an organised method of attack. His
+earliest work deals with the important problem of functional
+adaptation--its importance to the organism, and its possible mechanistic
+explanation. The first paper[470] was a study of the branching and
+distribution of the arteries in the human body (1878), and a second
+paper on the same subject followed in 1879.[471]
+
+In these papers Roux showed how the development of the blood-vascular
+system was largely determined by direct adaptation to functional
+requirements, and he inferred the existence in the vascular tissues of
+certain vital properties, in virtue of which the functional adaptation
+of the blood-vessels came about. Thus the intima or inner lining must
+possess the faculty of so reacting to the friction set up by the
+blood-current as to oppose the least possible resistance to its flow;
+the muscular coats must react to increased pressure by growing thicker,
+and so on.
+
+These papers were followed in 1881 by his well-known book, _Der Kampf
+der Theile im Organismus_, which contained the working-out of his
+mechanistic explanation of functional adaptation, and most of the
+elements of his general "causal-analytical" theory of form production.
+The significance of the book was popularly considered at the time to lie
+in its supposed application of the selection idea to the explanation of
+the internal adaptedness of animal structure--in the theory of "cellular
+selection," and the book owed its success to its fitting in so well with
+the prevalent Darwinism of the day. But its real importance, as a big
+step towards causal morphology, was naturally not so fully appreciated.
+
+During the next few years Roux continued his studies on functional
+adaptation,[472] and at the same time made a new departure by
+inaugurating, almost contemporaneously with the physiologist Pflüger,
+the study of experimental embryology. Isolated observations had
+previously been made upon the development of single blastomeres or parts
+of blastulæ, by Haeckel and Chun for instance,[473] but Roux[474] and
+Pflüger[475] were the first to investigate the subject systematically,
+choosing for their work the egg of the frog.[476] Roux continued for many
+years to follow up this line of work.[477]
+
+In 1890 he drew up a programme and manifesto[478] of
+_Entwicklungsmechanik_ as "an anatomical science of the future," and in
+1895 he founded the famous _Archiv für Entwicklungsmechanik_,[479]
+publishing in the same year the two large volumes of his collected
+papers,[480] of which the first volume dealt with functional adaptation,
+the second with experimental embryology.
+
+His subsequent work includes several important general papers;[481]
+besides a number of special memoirs dealing with the factors of
+development, and with his original subject, functional adaptation.[482]
+
+In our sketch of his views we shall have occasion to refer particularly
+to his publications of 1881, 1895 (the _Einleitung_), 1902, 1905, and
+1910.
+
+Although Roux's biological philosophy is out-and-out mechanistic, he yet
+recognises the difficulty, even the impossibility, of straightway
+reducing development to the physico-chemical level. He tries to steer a
+course midway between the simplicist conceptions of the materialists and
+the "metaphysics" of the neo-vitalist school, which the experimental
+study of development and regeneration soon brought into being. In 1895
+he writes:--"The too simple mechanistic conception on the one hand, and
+the metaphysical conception on the other represent the Scylla and
+Charybdis, between which to sail is indeed difficult, and so far by few
+satisfactorily accomplished; it cannot be denied that with the increase
+of knowledge the seduction of the second has lately notably increased"
+(p. 23).
+
+The _via media_ adopted by Roux is the analysis of development, not
+directly into simple physico-chemical processes, but into more complex
+organic processes dependent upon the fundamental properties of living
+matter. The aim of _Entwicklungsmechanik_ is defined by Roux to be the
+reduction of developmental events to the fewest and simplest
+_Wirkungsweisen_, or causal processes.[483] Two classes of causal
+processes may be distinguished, as "complex components" and "simple
+components" of development. The latter are directly explicable by the
+laws of physics and chemistry; the former, while in essence
+physico-chemical, are yet so very complicated that they cannot at
+present be reduced to physico-chemical terms. The ultimate aim of
+_Entwicklungsmechanik_ is to reduce development to its "simple
+components," but its main task at the present day and for many years to
+come is the analysis of development into its "complex components."
+
+These complex components must be accepted as having much of the validity
+of physical and chemical laws. They are mysterious in the sense that
+they cannot yet be explained mechanistically, but they are constant in
+their action, and under the same conditions produce always the same
+effect--hence they may be made the subject of strictly scientific study.
+They represent biological generalisations, in their way of equal
+validity with the generalisations of physics and chemistry.
+
+The principal "complex components" which Roux recognises are somewhat as
+follows:--First come the elementary cell-functions of assimilation and
+dissimilation, growth, reproduction and heredity, movement and
+self-division (as a special co-ordination of cell-movements). Then at a
+somewhat higher level, self-differentiation, and the trophic reaction to
+functional stimuli. Components of even greater complexity may also be
+distinguished, as, for instance, the biogenetic law. The various
+tropisms exhibited in development may be regarded as "directive" complex
+components. There must be added, not as being itself a component, but
+rather as a mode or peculiar property of all functioning, the
+omnipresent faculty of self-regulation.
+
+It will be noticed that Roux's "complex components" are simply the
+general properties or functions of organised matter.
+
+Expressing Roux's thought in another way, we might say that life can
+only be defined functionally, _i.e._, by an enumeration of the "complex
+components" or elementary functions which all living beings manifest,
+even down to the very simplest. "Living beings," writes Roux, "can at
+present be defined with any approach to completeness only functionally,
+that is to say, through characterisation of their activities, for we
+have an adequate acquaintance with their functions in a general way,
+though our knowledge of particulars is by no means complete" (p. 105,
+1905). Defined in the most general and abstract way, living things are
+material objects which persist in spite of their metabolism, and, by
+reason of their power of self-regulation, in spite also of the changes
+of the environment. This is the "functional minimum-definition of life"
+(pp. 106-7, 1905).
+
+We may now go on to consider the relation of function to form throughout
+the course of development. Roux distinguishes in all development two
+periods, in the first of which the organ is formed prior to and
+independent of its function, while in the second the differentiation and
+growth of the organ are dependent on its functioning. Latterly (1906 and
+1910) Roux has distinguished three periods, counting as the second the
+transition period when form is partly self-determined, partly determined
+by functioning. As this conception of Roux's is of the greatest
+importance we shall follow it out in some detail.
+
+The idea was first elaborated in the _Kampf der Theile_ (1881), where he
+wrote:--"There must be distinguished in the life of all the parts two
+periods, an embryonic in the broad sense, during which the parts
+develop, differentiate and grow of themselves, and a period of completer
+development, during which growth, and in many cases also the balance of
+assimilation over dissimilation, can come about only under the influence
+of stimuli" (p. 180). There is thus a period of self-differentiation in
+which the organs are roughly formed in anticipation of functioning, and
+a period of functional development in which the organs are perfected
+through functioning and only through functioning. The two periods cannot
+be sharply separated from one another, nor does the transition from the
+one to the other occur at the same time in the different tissues and
+organs.
+
+The conception is more fully expressed in 1905 as follows:--"This
+separation (of development into two periods) is intended only as a first
+beginning. The first period I called the embryonic period [Greek: kat'
+exochên] or the period of organ-rudiments. It includes the 'directly
+inherited' structures, _i.e._, the structures which are directly
+predetermined in the structure of the germ-plasm, as, for instance, the
+first differentiation of the germ, segmentation, the formation of the
+germ-layers and the organ-rudiments, as well as the next stage of
+'further differentiation,' and of _independent_ growth and maintenance,
+that is, of growth and maintenance which take place without the
+functioning of the organs.
+
+"This is accordingly the period of direct fashioning through the
+activity of the formative mechanism implicit in the germ-plasm, also the
+period of the self-conservation of the formed parts without active
+functioning.
+
+"The second period is the period of 'functional form-development.' It
+includes the further differentiation and the maintenance in their
+typical form of the organs laid down in the first period; and this is
+brought about by the exercise of the specific functions of the organs.
+This period adds the finishing touches to the finer functional
+differentiation of the organs, and so brings to pass the 'finer
+functional harmony' of all organs with the whole. The formative activity
+displayed during this period depends upon the circumstance that the
+functional stimulus, or rather the exercise by the organs of their
+specific functions, is accompanied by a subsidiary formative activity,
+which acts partly by producing new form and partly by maintaining that
+which is already formed.... Between the two periods lies presumably a
+transition period, an intermediary stage of varying duration in the
+different organs, in which both classes of causes are concerned in the
+further building-up of the already formed, those of the first period in
+gradually decreasing measure, those of the second in an increasing
+degree" (pp. 94-6, 1905).
+
+In the first period the organ forms or determines the function, in the
+second period the function forms the organ, or at least completes its
+differentiation. It is characteristic that in the first period
+functionally adapted structure appears in the complete absence of the
+functional stimulus.
+
+The explanation of the difference between the two periods is to be found
+in the different evolutionary history of the characters formed during
+each. First-period characters are _inherited_ characters, and taken
+together constitute the historical basis of the organism's form and
+activity; second-period characters are those of later acquirement which
+have not yet become incorporated in the racial heritage.
+
+Inherited characters appear in development in the absence of the
+stimulus that originally called them forth; acquired characters are
+those that have not yet freed themselves from this dependence upon the
+functional stimulus. First-period characters were originally, like
+second-period characters, entirely dependent for their development upon
+the functional stimuli in response to which they arose, and only
+gradually in the course of generations did they gain that independence
+of the functional stimulus which stamps them as true inherited
+characters. Speaking of the formative stimuli which are active in
+second-period development, Roux writes:--"These stimuli can also produce
+new structure, which if it is constantly formed throughout many
+generations finally becomes hereditary, _i.e._, develops in the
+descendants in the absence of the stimuli, becomes in our sense
+embryonic" (p. 180, 1881). Again, "form-characteristics which were
+originally acquired in post-embryonic life through functional adaptation
+may be developed in the embryo without the functional stimulus, and may
+in later development become more or less completely differentiated, and
+retain this differentiation without functional activity or with a
+minimum of it. But in the continued absence of functional activity they
+become atrophied ... and in the end disappear" (p. 201, 1881).
+
+This conception of the nature of hereditary transmission is an important
+one, and constitutes the first big step towards a real understanding of
+the historical element in organic form and activity. It supplies a
+practical criterion for the distinguishing of "heritage" characters from
+acquired characters, of palingenetic from cenogenetic--a criterion which
+descriptive morphology was unable to find.[484] The introduction of a
+functional moment into the concept of heredity was a methodological
+advance of the first importance, for it linked up in an understandable
+way the problems of embryology, and indirectly of all morphology, with
+the problem of hereditary transmission, and gave form and substance to
+the conception of the organism as an historical being.
+
+It is this element in Roux's theories that puts them so far in advance
+of those of Weismann. Weismann did not really tackle the big problem of
+the relation of form to function, and he left no place in his mechanical
+system of preformation for functional or second-period development; he
+conceived all development to be in Roux's sense embryonic, and due to
+the automatic unpacking of a complex germinal organisation. Roux himself
+was to a certain extent a preformationist, for the development of his
+first-period characters is conditioned by the inherited organisation of
+the germ-plasm, and is purely automatic. It was indeed his experiments
+on the frog's egg (1888) that supplied some of the strongest evidence in
+favour of the mosaic theory of development. The number of _Anlagen_
+which he postulates in the germ is however small, and the germ-plasm in
+his conception of it has a relatively simple structure (p. 103, 1905).
+
+The transmission of acquired characters forms, of course, an integral
+part of Roux's conception of heredity and development, for without this
+transmission second-stage characters could not be transformed into
+first-stage characters. He discusses this difficult question at some
+length in the _Kampf der Theile_, coming to the conclusion that such
+transmission takes place in small degree and gradually, and that many
+generations are required before a new character can become hereditary.
+He thinks that acquired characters are probably transmitted at the
+chemical level. It is conceivable that acquired form-changes are
+dependent on chemical changes, or are correlative with such, and that,
+since the germ-cells stand in close metabolic relations with the soma,
+these chemical changes may soak through to the germ-cells and so modify
+them that a predisposition will appear in the descendants towards
+similar form-changes.[485] From this point of view the problem of
+transmission might be merged in the broader problem of the production of
+form through chemical processes--the central problem of all development.
+
+Inherited characters develop by an automatic process of
+self-differentiation, and the separate parts of the embryo show during
+this first period a surprising functional independence of one another.
+But this state of things changes progressively as the second period is
+reached, until finally all form-production and maintenance and all
+correlation depend upon functioning. It is in the first period of
+automatic development through internal "determining" factors that the
+"developmental" functions in the strict sense, _e.g._ automatic growth,
+division and self-differentiation, are most clearly shown. In the second
+or "functional" period the formative influence of function upon
+structure comes into play, and development becomes largely a matter of
+"functional adaptation" to functional requirements.
+
+All structure, according to Roux, is either functional or
+non-functional. The former includes all structure that is adapted to
+subserve some function. "Such 'functional structures' are, for example,
+the composition of striated muscle fibres out of fibrillæ and these out
+of muscle-prisms, or again the length and thickness of the muscles, the
+static structure of the bones, the composition of the stomach and the
+blood-vessels out of longitudinal and circular fibres, the external
+shape of the vertebral centra and of the cuneiform bones of the foot"
+(p. 73, 1910). Indeed, as Cuvier had already pointed out, practically
+every organ in the body shows a functional structure which is accurately
+and minutely adjusted to the function it is intended to perform. Thus,
+to take some further examples, the arteries are admirably adapted as
+regards size of lumen, elasticity of wall, direction of branching, to
+conduct the blood to all parts of the body with the least possible waste
+of the propelling power through frictional resistance. So, too, the
+spongy substance of the long bones is arranged in lamellæ which take the
+direction of the principal stresses and strains which fall upon the
+bones in action.
+
+Functional structure may be formed either in the first or in the second
+period of development, may be either inherited or acquired, but it
+reaches its full differentiation only in the second period, _i.e._,
+under the influence of functioning. Practically speaking, functional
+structure is directly dependent for its full development and for its
+continued conservation upon the exercise of the particular function
+which it serves. In the second period, but not in the first, increased
+use leads to hypertrophy of the functional structure, disuse to atrophy.
+
+From functional structure is to be distinguished nonfunctional
+structure, which has no relation to the bodily functions--is neither
+adapted to perform any of these, nor has arisen as a by-product of
+functional activity. "To this category belong, for example, among
+typical structures, the triangular form of the cross-section of the
+tibia, the dolicocephalic or brachycephalic shape of the skull, most of
+the external characters distinguishing genera and species, many of the
+external features of the embryo which change in the course of
+development, besides most of the abnormal forms shown by monstrosities,
+tumours, etc." (p. 74, 1910). Non-functional structure is not affected
+by functional adaptation, and may accordingly be left out of
+consideration here.
+
+Now the influence of functioning upon the form and structure of an organ
+is twofold. There is first the immediate change brought about by the
+very act of functioning--for example, the shortening and thickening of
+skeletal muscles when they act. This is a purely temporary change, for
+the organ at once returns to its normal quiescent state as soon as it
+ceases to function. Such temporary functional change, brought about in
+the moment of functioning, is usually dependent for its initiation upon
+some neuro-muscular mechanism, though it may be elicited also by a
+chemical stimulus. It is thus always a phenomenon of "behaviour." "From
+such temporary changes are sharply to be distinguished all permanent
+alterations which first appear in perceptible fashion through
+oft-repeated or long-continued, enhanced functional activity. These
+produce a new and lasting internal equilibrium of the organ, consisting
+in an insertion of new molecules or a rearrangement of old. For this
+reason they outlast the periods of functional form-change, or, if as in
+the case of the muscles they themselves alter during functional
+activity, they regain their state when the organ ceases to function" (p.
+72, 1910). "Oft-repeated exercise or heightened exercise of the specific
+functions, or repeated action of the functional stimuli which determine
+them, produces, as we have said before, true form-changes as a
+by-product. These are of two kinds. In so far as these form-changes
+facilitate the repetition of the specific functions, I have called them
+_functional adaptations_.... Such as do not improve the functioning of
+the organ are indeed by-products of functioning, but without adaptive
+character; they do not belong to the class of functional adaptations at
+all" (p. 75, 1910).
+
+We may now enquire in what way functional adaptations can arise as
+by-products of functioning.
+
+It is clear that natural selection in the sense of individual or
+"personal" selection cannot adequately explain the origin of functional
+structure and the functional harmony of structure, for thousands of
+cells would have to vary together in a purposive way before any real
+advantage could be gained in the struggle for existence, and it is in
+the highest degree unlikely that this should come about by chance
+variation.[486] The development of purposive internal structure is only to
+be explained by the properties of the tissues concerned.
+
+In illustration and proof of the statement that functional adaptation is
+due to the properties of the tissues we may adduce the development and
+regulation of the blood-vascular system, which has been thoroughly
+studied from this point of view by Roux and Oppel (1910).
+
+It appears that only the very first rudiments of the vascular system are
+laid down in the short first period of automatic non-functional
+development. All the subsequent growth and differentiation of the
+blood-vessels falls into the second period, and is due wholly or in
+great part to direct functional adaptation to the requirements of the
+tissues. Thus from the rudiments formed in the first period there sprout
+out the definitive vessels in direct adaptation to the food-consumption
+of the tissues they are to supply. The size, direction and intimate
+structure of these vessels are accurately adjusted to the part they play
+in the economy of the whole, and this adjustment is brought about in
+virtue of the peculiar properties or reaction-capabilities of the
+different tissues of which the blood-vessels are composed.
+
+The properties which Roux finds himself compelled to postulate in the
+vascular tissues, after a thorough-going analysis of the different kinds
+of functional adaptation shown by the blood-vessels, are summarised by
+him as follows:--
+
+"(1) The faculty--depending on a direct sensibility possessed by the
+endothelium and perhaps also by the other layers of the intima--of
+yielding to the impact of the blood, so far as the external relations of
+the vessel permit. In this way the wall adapts itself to the
+hæmodynamically conditioned 'natural' shape of the blood-stream, and
+reaches this shape as nearly as possible." Through this faculty of the
+lining tissue of the blood-vessels, the size of the lumen and the
+direction of branching are so regulated as to oppose the least possible
+resistance to the flow of the blood.
+
+"(2) The faculty possessed by the endothelium of the capillaries of each
+organ of adapting itself qualitatively to the particular metabolism of
+the organ." This adaptedness of the capillaries is, however, more
+usually an inherited state, _i.e._, brought about in the first period of
+development.
+
+"(3) The faculty possessed by the capillary walls of being stimulated to
+sprout out and branch by increased functioning, _i.e._, by increased
+diffusion, and their power to exhibit a chemically conditioned
+cytotropism, which causes the sprouts to find one another and unite. A
+similar process can be directly observed in isolated segmentation-cells,
+which tend to unite in consequence of a power of mutual attraction.
+
+"(4) The faculty of developing normal arterial walls in response to
+strong intermittent pressure, and normal venous walls in response to
+continuous lesser pressure." It has been shown, for instance, by Fischer
+and Schmieden that in dogs a section of vein transplanted into an artery
+takes on an arterial structure, at least as regards the circular
+musculature, which doubles in thickness.
+
+"(5) The power to regulate the normal[487] length of the arteries and
+veins, in adaptation to the growth of the surrounding tissues, in such a
+way that the stretching action of the blood-stream brings the vessel to
+its proper functional length.
+
+"(6) The power to form, in response to slight increases in longitudinal
+tension, new structural parts which take their place alongside the
+existing longitudinal fibres.
+
+"(7) The power to regulate the width of the circular musculature
+according to the degree of food-consumption by the tissues, in response
+to nerve impulses initiated in these tissues.
+
+"(8) The power possessed by the circular musculature of responding to
+such continuous functional widening, by the formation of new structural
+parts in the circular musculature, and so of widening the vessel
+permanently or by this new formation of muscular fibres thickening the
+circular musculature.
+
+"(9) The faculty of being stimulated by increased blood-pressure to
+produce the same structural changes as mentioned in par. 8, though here
+the response is otherwise conditioned" (pp. 126-7, 1910).
+
+It is by virtue of the tissue-properties detailed above that the complex
+functional adaptations of the blood-vessels come about.
+
+The development of the vascular system is no mere automatic and
+mechanical production of form, apart from and independent of
+functioning; it implies a living and co-ordinated activity of the
+tissues and organs concerned, a power of active response to foreseen and
+unforeseen contingencies. Form is then not something fixed and
+congealed--it is the ever-changing manifestation of functional activity.
+"Since most of the structure and form of the blood-vessels arises in
+direct adaptation to function, the vessels of adult men and animals are
+no fixed structures, which, once formed, retain their form and
+structural build unchanged throughout life; on the contrary, they
+require even for their continued existence the stimulus of functional
+activity.... The fully formed blood-vessels are no static structures,
+such as they appear to be according to the teaching of normal histology,
+and such as they have long been taken to be. Observation and description
+of normal development never shows us anything but the visible side of
+organic happenings, the _products_ of activity, and leaves us ignorant
+of the real processes of form-development and form-conservation, and of
+their causes" (p. 125, 1910).
+
+The real thing in organisation is not form but activity. It is in this
+return to the Cuvierian or functional attitude to the problems of form
+that we hold Roux's greatest service to biology to consist. The
+attitude, however, seems to smack of vitalism, and Roux, as we have
+seen, is no vitalist. He holds that the marvellous and apparently
+purposive tissue-qualities which underlie all processes of functional
+adaptation have arisen "naturally," in the course of evolution, by the
+action of natural selection upon the various properties, useful and
+useless, which appeared fortuitously in the primary living organisms. He
+is, moreover, deeply imbued with the materialistic philosophy of his
+youth, and it is indeed one of the chief characteristics of his system
+that he states the fundamental properties or qualities of life in terms
+of metabolism. A vital quality is for Roux a special process or mode of
+assimilation. The faculty of "morphological assimilation" whereby form
+is imposed upon formless chemical processes is the ultimate term of
+Roux's analysis--"the most general, most essential, and most
+characteristic formative activity of life" (p. 631, 1902).
+
+We have now to consider very briefly the early results achieved by
+Roux's fellow-workers in the field of causal morphology. As D. Barfurth
+points out,[488] the years 1880-90 saw a general awakening of interest in
+experimental morphology, and it is hard to say whether Roux's work was
+cause or consequence. "There fall into this period," writes Barfurth,
+"the experimental investigations by Born and Pflüger on the sexual
+difference in frogs (1881), by Pflüger on the parthenogenetic
+segmentation of Amphibian ova, on crossing among the Amphibia, and on
+other important subjects (1882). In the following year (1883) appeared
+two papers of fundamental importance, by E. Pflüger and W. Roux: Pflüger
+publishing his researches on 'the influence of gravity on
+cell-division,' Roux his experimental investigations on 'the time of the
+determination of the chief planes in the frog-embryo.'... In the same
+year appeared A. Rauber's experimental studies 'on the influence of
+temperature, atmospheric pressure, and various substances on the
+development of animal ova,' which have brought many similar works in
+their train. The following year (1884) saw a lively controversy on
+Pflüger's gravity-experiments with animal eggs, in which took part
+Pflüger, Born, Roux, O. Hertwig and others, and in this year appeared
+work by Roux dealing with the experimental study of development, and in
+particular giving the results of the first definitely localised
+pricking-experiments on the frog's egg (in the _Schles. Gesell. f.
+vaterl. Kultur_, 15th Feb. 1884), also the important researches of M.
+Nussbaum and Gruber (followed up later by Verworn, Hofer and Balbiani)
+on Protozoa, and other experimental work" (pp. xi.-xii.).
+
+In 1888 appeared a famous paper by W. Roux,[489] in which he described how
+he had succeeded in killing by means of a hot needle one of the two
+first blastomeres of the frog's egg, and how a half-embryo had developed
+from the uninjured cell. Some years before[490] he had enunciated, at
+about the same time as Weismann, the view that development was brought
+about by a qualitative division of the germ-plasm contained in the
+nucleus, and that the complicated process of karyokinetic or mitotic
+division of the nucleus was essentially adapted to this end. He
+conceived that development proceeded by a mosaic-like distribution of
+potencies to the segmentation-cells, that, for instance, the first
+segmentation furrow separated off the material and potencies for the
+right half of the embryo from those for the left half. He had tried to
+show experimentally that the first furrow in the frog's egg coincided
+with the sagittal plane of the embryo,[491] and his later success in
+obtaining a half-embryo from one of the first two blastomeres seemed to
+establish the "mosaic theory" conclusively.
+
+Roux's needle-experiment aroused much interest, especially as Weismann's
+theory of heredity was then being keenly discussed. Chabry had published
+in 1887 some interesting results on the Ascidian egg,[492] which strongly
+supported the Roux-Weismann theory. Considerable astonishment was
+therefore caused by Driesch's announcement in 1891[493] that he had
+obtained complete larvæ from single blastomeres of the sea-urchin's egg
+isolated at the two-celled stage. He followed this up in the next
+year[493] by showing that whole embryos could be produced from one or more
+blastomeres isolated at the four-cell stage. Similar or even more
+striking results were obtained by E. B. Wilson on _Amphioxus_,[494] and
+Zoja on medusæ.[495] Driesch succeeded also in disturbing the normal
+course and order of segmentation by compressing the eggs of the
+sea-urchin between glass plates, and yet obtained normal embryos.
+Similar pressure-experiments were carried out on the frog by O.
+Hertwig,[496] and on _Nereis_ by E. B. Wilson,[497] with analogous results.
+
+In 1895 O. Schultze[498] showed that if the frog's egg is held between two
+plates and inverted at the two-celled stage there are formed two embryos
+instead of one. In the same year T. H. Morgan[499] repeated Roux's
+fundamental experiment of destroying one of the two blastomeres, but
+inverted the egg immediately after the operation--a whole embryo of half
+size resulted. A year or two later Herlitzka[500] found that if the first
+two blastomeres of the newt's egg were separated by constriction, two
+normal embryos of rather more than half normal size were formed.
+
+The main result of the first few years' work on the development of
+isolated blastomeres was to show that the mosaic theory was not strictly
+true, and that the hypothesis of a qualitative division of the nucleus
+was on the whole negatived by the facts.
+
+Evidence soon accumulated that the cytoplasm of the egg stood for much
+in the differentiation of the embryo. A number of years previously Chun
+had made the discovery that single blastomeres of the Ctenophore egg,
+isolated at the two-celled stage, gave half-embryos. This was in the
+main confirmed by Driesch and Morgan in 1896,[501] and they made the
+further interesting discovery that the same defective larvæ could be
+obtained by removing from the unsegmented egg a large amount of
+cytoplasm. Conclusive proof of the importance of the cytoplasm was
+obtained soon after by Crampton,[502] who removed the anucleate
+"yolk-lobe" from the egg of the mollusc _Ilyanassa_ at the two-celled
+stage, and obtained larvæ which lacked a mesoblast. This result was
+brilliantly confirmed and extended some years later by E. B. Wilson,[503]
+working on the egg of _Dentalium_. He found that if the similar
+anucleate "polar lobe" of this form is removed at the two-celled stage,
+deficient larvæ are formed, in which the post-trochal region and the
+apical organ are absent. He further showed that in the unsegmented but
+mature egg prelocalised cytoplasmic regions can be distinguished, which
+later become separated from one another through the segmentation of the
+egg. The segmentation-cells into which these cytoplasmic substances are
+thus segregated show a marked specificity of development, giving rise,
+even when isolated, to definite organs of the embryo. Wilson concluded
+that the cytoplasm of the egg contains a number of specific
+organ-forming stuffs, which have a definite topographical arrangement in
+the egg. Development is thus due in part to a qualitative division not
+of the nucleus but of the cytoplasm. Corroborative evidence of the
+existence of cytoplasmic organ-forming stuffs has been supplied for
+several other species, _e.g._, _Patella_ (Wilson), _Cynthia_ (Conklin),
+_Cerebratulus_ (Zeleny), and _Echinus_ (Boveri).
+
+It is interesting to recall that so long ago as 1874 W. His[504] put
+forward the theory that there exist in the blastoderm and even in the
+egg prelocalised areas, which contain the formative material for each
+organ of the embryo, and from which the embryo is developed by a simple
+process of unequal growth.
+
+The experimental study of form was prosecuted in many other directions
+besides that of experimental embryology. The study of regeneration and
+of regulatory processes attracted many workers, among whom may be
+mentioned T. H. Morgan, C. M. Child, and H. Driesch. In an interesting
+series of papers C. Herbst applied the principles of the physiology of
+stimulus to the interpretation of development.[505] The formative power of
+function was studied in Germany by Roux and his pupils, Fuld, O. Levy,
+Schepelmann and others, particularly by E. Babák. In France, F. Houssay
+inaugurated[506] an important series of memoirs by himself and his pupils
+on "dynamical morphology," the most important memoir being his own
+valuable discussion of the functional significance of form in fishes.[507]
+The principles of his dynamical morphology were first laid down in his
+book _La Forme et la Vie_ (1900).
+
+The famous experiments of Loeb, Delage and others on artificial
+parthenogenesis may also be mentioned, though their connection with
+morphology is somewhat remote.
+
+The period was characterised also by the lively discussion of first
+principles, in which Driesch took a leading part. Materialistic methods
+of interpretation were upheld by perhaps the majority of biologists, but
+vitalism found powerful support.
+
+    [464] See Carus's remark, referred to on p. 194, above.
+
+    [465] Roux, _Die Entwicklungsmechanik_, p. 26, Leipzig,
+    1905.
+
+    [466] T. H. Morgan, _Regeneration_, p. 1, New York and
+    London, 1901.
+
+    [467] _Recherches sur la production artificielle des
+    Monstruosités_, Paris, 1877, and many later papers.
+
+    [468] _Unsere Körperform und das physiologische Problem
+    ihrer Entstehung_, Leipzig, 1874.
+
+    [469] J. W. Jenkinson, _Experimental Embryology_, p. 3,
+    Oxford, 1909.
+
+    [470] "Ueber die Verzweigungen der Blutgefässe des
+    Menschen," _Jen. Zeit_., xii., 1878.
+
+    [471] "Ueber die Bedeutung der Ablenkung des
+    Arterienstammes bei der Astabgabe," _Jen. Zeit_., xiii.,
+    1879.
+
+    [472] "Beiträge zur Morphologie der funktionellen
+    Anpassung. I. Struktur eines hochdifferenzierten
+    bindgewebigen Organes (der Schwanzflosse des Delphin),"
+    _Arch. Anat. Physiol._ (_Anat. Abt._) for 1883. II.
+    "Ueber die Selbstregulation der 'morphologischen' Länge
+    der Skeletmuskeln des Menschen," _Jen. Zeit._, xvi.,
+    1883. III. "Beschreibung ... einer
+    Kniegelenkeknochenankylose," _Arch. Anat. Physiol._
+    (_Anat. Abt._) for 1885.
+
+    [473] In 1869 and 1877 respectively (Roux, p. 53, 1905).
+
+    [474] _Ueber die Zeit. der Bestimmung der Hauptrichtungen
+    des Froschembryo_, Leipzig, 1883.
+
+    [475] "Ueber den Einfluss der Schwerkraft auf die Teilung
+    der Zellen," Pflüger's _Archiv_, xxxi., 1883. Also
+    subsequent papers in same journal.
+
+    [476] For an account of the classical experiments on the
+    frog's egg, see T. H. Morgan, _The Development of the
+    Frog's Egg_, New York, 1897.
+
+    [477] In a series of "Beiträge zur Entwicklungsmechanik
+    des Embryo," published in various journals from 1884 to
+    1891, all dealing with the frog's egg. Also in many
+    papers in the _Archiv f. Entw. mech._, from 1895
+    onwards.
+
+    [478] _Die Entwicklungsmechanik der Organismen, eine
+    anatomische Wissenschaft der Zukunft_, Wien, 1890.
+
+    [479] The first volume contains the important _Einleitung_
+    or general Introduction.
+
+    [480] _Gesammelte Abhandlungen über Entwicklungsmechanik
+    der Organismen_, 2 vols., Leipzig, 1895.
+
+    [481] "Für unser Programm und seine Verwirklichung,"
+    _A.E.M._, v., pp. 1-80 and 219-342, 1897. "Ueber die
+    Selbstregulation der Lebewesen," _A.E.M._, xiii., pp.
+    610-5, 1902. "Die Entwicklungsmechanik, ein neuer Zweig
+    der biologischen Wissenschaft," Heft I. of the _Vorträge
+    u. Aufsätze über Entwicklungsmechanik der Organismen_,
+    Leipzig, 1905. Oppel and Roux, "Ueber die gestaltliche
+    Anpassung der Blutgefässe," Heft x., of the _Vorträge u.
+    Aufsätze_, Leipzig, 1910.
+
+    [482] "Ueber d. funkt. Anpassung des Muskelmagens der
+    Gans," _A.E.M._, xxi., pp. 461-99, 1906.
+
+    [483] The exact quantitative formulation of a
+    _Wirkungsweise_ constitutes a law. The word itself is
+    perhaps most conveniently rendered as "causal process."
+
+    [484] M. Fürbringer, perhaps under the influence of Roux,
+    emphasised the importance, from a morphological point of
+    view, of studying post-embryonic (functional)
+    development, _Unters. z. Morph. u. Syst. der Vögel_,
+    ii., Amsterdam, p. 925, 1888.
+
+    [485] See, for the development of this idea, Oppel, in
+    Roux-Oppel, 1910.
+
+    [486] _Cf._ the controversy between Herbert Spencer and
+    Weismann on the subject of "coadaptation" in the
+    _Contemporary Review_ for 1893 and 1894. See also
+    Weismann's paper in _Darwin and Modern Science_,
+    Cambridge, 1909.
+
+    [487] That is, the length they take up when separated from
+    the body.
+
+    [488] "Wilhelm Roux zum 60. Geburtstage," _Arch. f.
+    Entw.-Mech._, xxx. _Festschrift für Prof. Roux_, Pt. i,
+    1910.
+
+    [489] Virchow's _Archiv_, cxiv., 1888. First announced in
+    Sept. 1887.
+
+    [490] _Ueber die Bedeutung der Kernteilungsfiguren_,
+    Leipzig, 1883.
+
+    [491] _Bresl. ärtz. Zeitschr._, 1885.
+
+    [492] _Journ. de l'Anat. et de la Physiologie_, xxiii.,
+    1887.
+
+    [493] _Zeits. f. wiss. Zool._, liii., 1891 and 1892.
+
+    [494] _Journ. Morph._, viii., 1893.
+
+    [495] _Arch. f. Ent.-Mech._, i., 1895; ii., 1896.
+
+    [496] _Arch. f. mikr. Anat._, xliii., 1893.
+
+    [497] _Arch. f. Ent.-Mech._, iii., 1896.
+
+    [498] _Arch. f. Ent.-Mech._, i., 1895.
+
+    [499] _Anat. Anz._, x., 1895.
+
+    [500] _Arch. f. Ent.-Mech._, iv. 1897.
+
+    [501] _Arch. f. Ent.-Mech._, ii., 1896.
+
+    [502] _Arch. f. Ent.-Mech._, iii., 1896.
+
+    [503] _Journ. exper. Zool._, i., 1904.
+
+    [504] _Unsere Körperform_, p. 19, Leipzig, 1874.
+
+    [505] _Biolog. Centrlbl._, xiv., 1894, xv., 1895.
+    _Formative Reize in der thierischen Ontogenese_,
+    Leipzig, 1901.
+
+    [506] "La Morphologie dynamique," No. i. of the
+    _Collection de Morphologie dynamique_, Paris, 1911.
+
+    [507] "Forme, Puissance et Stabilité des Poissons," No.
+    iv. of the _Collection_, Paris, 1912.
+
+
+
+
+CHAPTER XIX
+
+SAMUEL BUTLER AND THE MEMORY THEORIES OF HEREDITY
+
+
+We have laid stress upon the distinction established by Roux between the
+two stages of development--the automatic and the functional--because of
+the light which it seems to throw upon the phylogenetic relation of form
+to function. We have pointed out, too, the paramount rôle that function
+plays in Roux's theories of development and heredity, and we have
+brought out the close kinship existing between his theory and that of
+Lamarck. For Roux, as for Lamarck, the function creates the organ, and
+it is only after long generations that the organ appears before the
+function.
+
+It so happened that just about the time when Roux's papers were
+beginning to appear a brilliant attempt was made by Samuel Butler to
+revive and complete the Lamarckian doctrine.
+
+A man of singular freshness and openness of mind, combining in an
+extraordinary degree extreme intellectual subtlety with a childlike
+simplicity of outlook, Butler was one of the most fascinating figures of
+the 19th century. He was not a professional biologist, and much of his
+biological work is, for that reason, imperfect. But he brought to bear
+upon the central problems of biology an unbiassed and powerful
+intelligence, and his attitude to these problems, just because it is
+that of a cultivated layman, is singularly illuminating.
+
+He was not well acquainted with biological literature; he seems to have
+hit upon the main ideas of his theory of life and habit in complete
+independence of Lamarck, and only later to have become aware that
+Lamarck had in a measure forestalled him. He puts this very beautifully
+in the following passage from his chief biological work _Life and Habit_
+(1877[508]):--"I admit that when I began to write upon my subject I did
+not seriously believe in it. I saw, as it were, a pebble upon the
+ground, with a sheen that pleased me; taking it up, I turned it over and
+over for my amusement, and found it always grow brighter and brighter
+the more I examined it. At length I became fascinated, and gave loose
+rein to self-illusion. The aspect of the world changed; the trifle which
+I had picked up idly had proved to be a talisman of inestimable value,
+and had opened a door through which I caught glimpses of a strange and
+interesting transformation. Then came one who told me that the stone was
+not mine, but that it had been dropped by Lamarck, to whom it belonged
+rightfully, but who had lost it; whereon I said I cared not who was the
+owner, if only I might use it and enjoy it. Now, therefore, having
+polished it with what art and care one who is no jeweller could bestow
+upon it, I return it, as best I may, to its possessor" (p. 306). In one
+of his later works, however, Butler made up for his first neglect of his
+predecessors by giving what is undeniably the best account in English
+literature of the work of Buffon, Lamarck, and Erasmus Darwin--in his
+_Evolution, Old and New_ (1879). Many of his facts he took from Charles
+Darwin, whose theory of natural selection he bitterly opposed, in the
+two books just mentioned and in _Unconscious Memory_ (1880) and _Luck or
+Cunning_ (1887).
+
+Butler's main thesis is that living things are active, intelligent
+agents, personally continuous with all their ancestors, possessing an
+intense but unconscious memory of all that their ancestors did and
+suffered, and moving through habit from the spontaneity of striving to
+the automatism of remembrance.
+
+The primary cause of all variation in structure is the active response
+of the organism to needs experienced by it, and the indispensable link
+between the outer world and the creature itself is that same "sense of
+need" upon which Lamarck insisted. "According to Lamarck, genera and
+species have been evolved, in the main, by exactly the same process as
+that by which human inventions and civilisations are now progressing;
+and this involves that intelligence, ingenuity, heroism, and all the
+elements of romance, should have had the main share in the development
+of every herb and living creature around us" (_Life and Habit_, p. 253).
+Variations are indubitably the raw material of evolution--"The question
+is as to the origin and character of these variations. We say they
+mainly originate in a creature through a sense of its needs, and vary
+through the varying surroundings which will cause those needs to vary,
+and through the opening-up of new desires in many creatures, as the
+consequence of the gratification of old ones; they depend greatly on
+differences of individual capacity and temperament; they are
+communicated, and in the course of time transmitted, as what we call
+hereditary habits or structures, though these are only, in truth,
+intense and epitomised memories of how certain creatures liked to deal
+with protoplasm" (p. 267).
+
+Butler's theory then is essentially a bold and enlightened Lamarckism,
+completed and rounded off by the conception that heredity too is a
+psychological process, of the same nature as memory.
+
+In seeking to establish a close analogy between memory and heredity
+Butler starts out from the fact of common experience, that actions which
+on their first performance require the conscious exercise of will and
+intelligence, and are then carried out with difficulty and hesitation,
+gradually through long-continued practice come to be performed easily
+and automatically, without the conscious exercise of intelligence or
+will.
+
+He tries to show that this is a general law--that knowledge and will
+become intense and perfect only when through long-continued exercise
+they become automatic and unconscious--and he applies this conception to
+the elucidation of development.
+
+Developmental processes, especially the early ones (of Roux's first
+stage) are automatic and unconscious, and yet imply the possession by
+the embryo of a wonderfully perfect knowledge of the processes to be
+gone through, and an assured power of will and judgment. Is it
+conceivable, says Butler, that the embryo can do all these things
+without knowing how to do them, and without having done them before?
+"Shall we say ... that a baby of a day old sucks (which involves the
+whole principle of the pump, and hence a profound practical knowledge of
+the laws of pneumatics and hydrostatics), digests, oxygenises its blood
+(millions of years before Sir Humphrey Davy discovered oxygen), sees and
+hears--all most difficult and complicated operations, involving a
+knowledge of the facts concerning optics and acoustics, compared with
+which the discoveries of Newton sink into utter insignificance? Shall we
+say that a baby can do all these things at once, doing them so well and
+so regularly, without being even able to direct its attention to them,
+and without mistake, and at the same time not know how to do them, and
+never have done them before?" (p. 54). Assuredly not.
+
+The only possible explanation is that the embryo's ancestors have done
+these things so often, throughout so many millions of generations, that
+the embryo's knowledge of how to do them has become unconscious and
+automatic by reason of this age-long practice. This implies that there
+is in a very real sense actual personal continuity between the embryo
+and all its ancestors, so that their experiences are his, their memory
+also his. "We must suppose the continuity of life and sameness between
+living beings, whether plants or animals, to be far closer than we have
+hitherto believed; so that the experience of one person is not enjoyed
+by his successor, so much as that the successor is _bona fide_ but a
+part of the life of his progenitor, imbued with all his memories,
+profiting by all his experiences--which are, in fact, his own--and only
+unconscious of the extent of his own memories and experiences owing to
+their vastness and already infinite repetitions" (p. 50). It is very
+suggestive in this connection, he continues--"I. That we are _most
+conscious of, and have most control over_, such habits as speech, the
+upright position, the arts and sciences, which are acquisitions peculiar
+to the human race, always acquired after birth, and not common to
+ourselves and any ancestor who had not become entirely human.
+
+"II. That we are _less conscious of, and have less control over_, eating
+and drinking, swallowing, breathing, seeing and hearing, which were
+acquisitions of our prehuman ancestry, and for which we had provided
+ourselves with all the necessary apparatus before we saw light, but
+which are, geologically speaking, recent, or comparatively recent.
+
+"III. That we are _most unconscious of, and have least control over_,
+our digestion and circulation, which belonged even to our invertebrate
+ancestry, and which are habits, geologically speaking, of extreme
+antiquity.... Does it not seem as though the older and more confirmed
+the habit, the more unquestioning the act of volition, till, in the case
+of the oldest habits, the practice of succeeding existences has so
+formulated the procedure, that, on being once committed to such and such
+a line beyond a certain point, the subsequent course is so clear as to
+be open to no further doubt, to admit of no alternative, till the very
+power of questioning is gone, and even the consciousness of volition"
+(pp. 51-2).
+
+The hypothesis then, that heredity and development are due to
+unconscious memory, finds much to support it--"the self-development of
+each new life in succeeding generations--the various stages through
+which it passes (as it would appear, at first sight, without rhyme or
+reason), the manner in which it prepares structures of the most
+surpassing intricacy and delicacy, for which it has no use at the time
+when it prepares them, and the many elaborate instincts which it
+exhibits immediately on, and indeed before, birth--all point in the
+direction of habit and memory, as the only causes which could produce
+them" (p. 125). The hypothesis explains, for instance, the fact of
+recapitulation:--"Why should the embryo of any animal go through so many
+stages--embryological allusions to forefathers of a widely different
+type? And why, again, should the germs of the same kind of creature
+always go through the same stages? If the germ of any animal now living
+is, in its simplest state, but part of the personal identity of one of
+the original germs of all life whatsoever, and hence, if any now living
+organism must be considered without quibble as being itself millions of
+years old, and as imbued with an intense though unconscious memory of
+all that it has done sufficiently often to have made a permanent
+impression; if this be so, we can answer the above questions perfectly
+well. The creature goes through so many intermediate stages between its
+earliest state as life at all, and its latest development, for the
+simplest of all reasons, namely, because this is the road by which it
+has always hitherto travelled to its present differentiation; this is
+the road it knows, and into every turn and up or down of which it has
+been guided by the force of circumstances and the balance of
+considerations" (pp. 125-6).
+
+The hypothesis explains also the way in which the orderly succession of
+stages in embryogeny is brought about, for we can readily understand
+that the embryo will not remember any stage until it has passed through
+the stage immediately preceding it. "Each step of normal development
+will lead the impregnated ovum up to, and remind it of, its next
+ordinary course of action, in the same way as we, when we recite a
+well-known passage, are led up to each successive sentence by the
+sentence which has immediately preceded it.... Though the ovum
+immediately after impregnation is instinct with all the memories of both
+parents, not one of these memories can normally become active till both
+the ovum itself and its surroundings are sufficiently like what they
+respectively were, when the occurrence now to be remembered last took
+place. The memory will then immediately return, and the creature will do
+as it did on the last occasion that it was in like case as now. This
+ensures that similarity of order shall be preserved in all the stages of
+development in successive generations" (pp. 297-8).
+
+Abnormal conditions of development will cause the embryo to pause and
+hesitate, as if at a loss what to do, having no ancestral experience to
+guide it. Abnormalities of development represent the embryo's attempt to
+make the best of an unexpected situation. Or, as Butler puts it, "When
+... events are happening to it which, if it has the kind of memory we
+are attributing to it, would baffle that memory, or which have rarely or
+never been included in the category of its recollections, _it acts
+precisely as a creature acts_ _when its recollection is disturbed, or
+when it is required to do something which it has never done before_" (p.
+132). "It is certainly noteworthy that the embryo is never at a loss,
+unless something happens to it which has not usually happened to its
+forefathers, and which in the nature of things it cannot remember" (p.
+132).
+
+Butler's teleological conception of organic evolution was of course
+completely antagonistic to the naturalistic conceptions current in his
+time. In one of his later books he repeats Paley's arguments in favour
+of design, and to the question, "Where, then, is your designer of beasts
+and birds, of fishes, and of plants?" he replies: "Our answer is simple
+enough; it is that we can and do point to a living tangible person with
+flesh, blood, eyes, nose, ears, organs, senses, dimensions, who did of
+his own cunning, after infinite proof of every kind of hazard and
+experiment, scheme out and fashion each organ of the human body. This is
+the person whom we claim as the designer and artificer of that body, and
+he is the one of all others the best fitted for the task by his
+antecedents, and his practical knowledge of the requirements of the
+case--for he is man himself. Not man, the individual of any given
+generation, but man in the entirety of his existence from the dawn of
+life onwards to the present moment" (_Evolution, Old and New_, p. 30,
+1879).
+
+Butler's theory of life and habit remained only a sketch, and he was
+perhaps not fully aware of its philosophical implications. Since
+Butler's time, a new complexion has been put upon biological philosophy
+by the profound speculations of Bergson.
+
+But it is not impossible that the future development of biological
+thought will follow some such lines as those which he tentatively laid
+down.
+
+Butler was not the first to suggest that there is a close connection
+between heredity and memory--it is a thought likely to occur to any
+unprejudiced thinker. The first enunciation of it which attracted
+general attention was that contained in Hering's famous lecture "On
+Memory as a general Function of organised Matter."[509] Butler was not
+aware of Hering's work when he published his _Life and Habit_, but in
+_Unconscious Memory_ (1880) he gave full credit to Hering as the first
+discoverer, and supplied an admirable translation of Hering's lecture.
+As far as the assimilation of heredity to memory is concerned Hering and
+Butler have much in common, but Hering did not share Butler's Lamarckian
+and vitalistic views, preferring to hold fast, for the practical
+purposes of physiology at all events, to the general accepted theory of
+the parallelism between psychical and physical processes. He was
+inclined to regard memory in the ordinary sense as a function of the
+brain, and memory in general as a function of all organised matter.
+Speaking of the psychical life, he says, "Thus the cause which produces
+the unity of all single phenomena of consciousness must be looked for in
+unconscious life. As we know nothing of this except what we learn from
+our investigations of matter, and since in a purely empirical
+consideration, matter and the unconscious must be regarded as identical,
+the physiologist may justly define memory in a wider sense to be a
+faculty of the brain, the results of which to a great extent belong to
+both consciousness and unconsciousness."[510] Hering's views were
+supported by Haeckel.[511]
+
+In 1893 an American, H. F. Orr,[512] tried to work out a theory of
+development and heredity based upon the fundamental idea "that the
+property which is the basis of bodily development in organisms is the
+same property which we recognise as the basis of psychic activity and
+psychic development." He tried also to explain the recapitulation of
+phylogeny by ontogeny as due to habit.
+
+The neo-Lamarckian school of American palæontologists were also in
+sympathy with the memory idea, and this was expressed most clearly
+perhaps by Cope.[513]
+
+In 1904 appeared the work on this subject which has attracted the most
+attention--R. Semon's _Die Mneme_.[514] This was an elaborate treatment of
+the question from the materialistic point of view, the main assumption
+of Semon's theory being that the action of a stimulus upon the organism
+leaves a more or less permanent material trace or "engramm," of such a
+nature as to modify the subsequent action of the organism.
+
+Applied to the explanation of heredity and development, Semon's theory
+comes to very much the same as Weismann's, with engramms substituted for
+determinants, but it has the great advantage of allowing for the
+transmission of acquired characters. The application of the concept of
+stimulus is valuable and suggestive, but it seems to us that the memory
+theory of heredity can be properly utilised only by adopting a frankly
+Lamarckian and vitalistic standpoint, and this standpoint Semon
+expressly combats. As Ward[515] points out in his illuminating lecture on
+heredity and memory--"Records or memoranda alone are not memory, for
+they presuppose it. _They_ may consist of physical traces, but memory,
+even when called 'unconscious,' suggests mind; for, as we have seen, the
+automatic character implied by this term 'unconscious' presupposes
+foregone experience.... The mnemic theory then, if it is to be worth
+anything, seems to me clearly to require not merely physical records or
+'engrams,' but living experience or tradition. The mnemic theory will
+work for those who can accept a monadistic or pampsychist interpretation
+of the beings that make up the world, who believe with Spinoza and
+Leibniz that 'all individual things are animated albeit in divers
+degree'" (pp. 55-6).
+
+Perhaps the best and most ingenious treatment of memory and heredity
+from a physical standpoint is that offered by E. Rignano in his book,
+_Sur la transmissibilité des caractères acquis_.[516] Rignano seeks to
+construct a physico-chemical "model" which will explain both heredity
+and memory.
+
+His system, which is based more firmly upon the facts of experimental
+embryology than Semon's, postulates the existence of "specific nervous
+accumulators." The essential hypothesis set up is that every functional
+stimulus is transformed into specific vital energy, and deposits in the
+nucleus of the cell a specific substance which is capable of
+discharging, in an inverse direction, the nervous current which has
+formed it, as soon as the dynamical equilibrium of the organism is
+restored to the state in which it was when the original stimulus acted
+upon it. These specific nuclear substances, different for each cell, are
+accumulated also in the nuclei of the germinal substance, constituting
+what Rignano calls the central zone of development. That is to say, each
+functional adaptation changes slightly the dynamical equilibrium of the
+organism, and this change in the system of distribution of the nervous
+currents leads to the deposit in the central zone of development of a
+new specific substance. In the development of the next individual this
+new specific element enters into activity, and reproduces the nervous
+current which has formed it, as soon as the organism reaches the same
+conditions of dynamical equilibrium as those obtaining when the stimulus
+acted on the parent.
+
+Development can thus be regarded as consisting of a number of stages, at
+each of which new specific elements enter automatically into play and
+lead the embryo from that stage to the stage succeeding. The germinal
+substance on this theory of Rignano's is to be regarded as being
+composed of a large number of specific elements, originally formed as a
+result of each new functional adaptation, but now forming part of the
+hereditary equipment.
+
+The theory represents an advance upon the more static conceptions of
+Semon. It owes much to Roux's influence.
+
+In this country, the mnemic theories have been championed particularly
+by M. Hartog[517] and Sir Francis Darwin.[518]
+
+    [508] The quotations are taken from the 1910 reprint,
+    London, Fifield.
+
+    [509] _Ueber das Gedächtnis als eine allgemeine Funktion
+    der organisierten Materie_, Wien, 1870.
+
+    [510] Eng. trans, in E. Hering, _Memory_, p. 9, Chicago
+    and London, 1913.
+
+    [511] _Die Perigenesis der Plastidule_, Jena, 1875.
+
+    [512] _A Theory of Development and Heredity_, New York,
+    1893.
+
+    [513] _The Primary Factors of Organic Evolution_, Chicago,
+    1896.
+
+    [514] _Die Mneme als erhaltendes Prinzip im Wechsel des
+    organischen Geschehens_, Leipzig, 1904; 2nd ed., 1908.
+
+    [515] _Heredity and Memory_, Cambridge, 1913.
+
+    [516] Paris, 1906. Also in Italian and German. Eng. trans.
+    by B. C. ,H. Harvey, Chicago, 1911.
+
+    [517] See _Problems of Life and Reproduction_, London,
+    1913.
+
+    [518] _Presidential Address to the British Association_,
+    1908.
+
+
+
+
+CHAPTER XX
+
+THE CLASSICAL TRADITION IN MODERN MORPHOLOGY
+
+
+To write a history of contemporary movements from a purely objective
+standpoint is well recognised to be an impossible task. It is difficult
+for those in the stream to see where the current is carrying them: the
+tendencies of the present will only become clear some twenty years in
+the future.
+
+I propose, therefore, in this concluding chapter to deal only with
+certain characteristics of modern work on the problems of form which
+seem to me to be derived directly from the older classical tradition of
+Cuvier and von Baer.
+
+The present time is essentially one of transition. Complete uncertainty
+reigns as to the main principles of biology. Many of us think that the
+materialistic and simplicist method has proved a complete failure, and
+that the time has come to strike out on entirely different lines. Just
+in what direction the new biology will grow out is hard to see at
+present, so many divergent beginnings have been made--the materialistic
+vitalism of Driesch, the profound intuitionalism of Bergson, the
+psychological biology of Delpino, Francé, Pauly, A. Wagner and W.
+Mackenzie. But if any of these are destined to give the future direction
+to biology, they will in a measure only be bringing biology back to its
+pre-materialistic tradition, the tradition of Aristotle, Cuvier, von
+Baer and J. Müller. It may well be that the intransigent materialism of
+the 19th century is merely an episode, an aberration rather, in the
+history of biology--an aberration brought about by the over-rapid
+development of a materialistic and luxurious civilisation, in which
+man's material means have outrun his mental and moral growth.
+
+Two movements seem significant in the morphology of the last decade or
+so of the 19th century--first, the experimental study of form, and
+second, the criticism of the concepts or prejudices of evolutionary
+morphology.
+
+The period was characterised also by the great interest taken in
+cytology, following upon the pioneer work of Hertwig, van Beneden and
+others on the behaviour of the nuclei in fertilisation and
+maturation.[519] This line of work gained added importance in connection
+with contemporary research and speculation on the nature of hereditary
+transmission, and it has in quite recent years received an additional
+stimulus from the re-discovery of Mendelian inheritance. Its importance,
+however, seems to lie rather in its possible relation to the problems of
+heredity than in any meaning it may have for the problems of form. More
+significant is the revolt against the cell-theory started by Sedgwick[520]
+and Whitman,[521] on the ground that the organism is something more than
+an aggregation of discrete, self-centred cells.
+
+The experimental work on the causes of the production and restoration of
+form infused new life into morphology. It opened men's eyes to the fact
+that the developing organism is very much a living, active, responsive
+thing, quite capable of relinquishing at need the beaten track of normal
+development which its ancestors have followed for countless generations,
+in order to meet emergencies with an immediate and purposive reaction.
+It was cases of this kind, cases of active regulation in development and
+regeneration, that led men like G. Wolff and H. Driesch to cast off the
+bonds of dogmatic Darwinism and declare boldly for vitalism and
+teleology.
+
+There was the famous case of the regeneration of the lens in Amphibia
+from the edge of the iris--an entirely novel mode of origin, not
+occurring in ontogeny. The fact seems to have been discovered first by
+Colucci in 1891, and independently by G. Wolff in 1895.[522] The
+experiment was later repeated and confirmed by Fischel and other
+workers. Wolff drew from this and other facts the conclusion that the
+organism possesses a faculty of "primary purposiveness" which cannot
+have arisen through natural selection.[523] And, as is well known, Driesch
+derived one of his most powerful arguments in favour of vitalism from
+the extraordinary regenerative processes shown by _Tubularia_ and
+_Clavellina_ in the course of which the organism actually demolishes and
+rebuilds a part or the whole of its structure. But under the influence
+of physiologists like Loeb many workers held fast to materialistic
+methods and conceptions.
+
+The great variety of regulative response of which the organism showed
+itself capable made it very difficult for the morphologist to uphold the
+generalisations which he had drawn from the facts of normal undisturbed
+development. The germ-layer theory was found inadequate to the new
+facts, and many reverted to the older criterion of homology based on
+destiny rather than origin. The trend of opinion was to reject the
+ontogenetic criterion of homology, and to refuse any morphological or
+phylogenetic value to the germ-layers.[524]
+
+The biogenetic law came more and more into disfavour, as the developing
+organism more and more showed itself to be capable of throwing off the
+dead-weight of the past, and working out its own salvation upon original
+and individual lines.[525] A. Giard in particular called attention to a
+remarkable group of facts which went to show that embryos or larvæ of
+the same or closely allied species might develop in most dissimilar ways
+according to the conditions in which they found themselves.[526] His
+classical case of "poecilogeny" was that of the shrimp _Palæmonetes
+varians_, the fresh-water form of which develops in an entirely
+different way from the salt-water form.
+
+Experimental workers indeed were inclined to rule the law out of
+account, to disregard completely the historical element in development,
+and this was perhaps the chief weakness of the neo-vitalist systems
+which took their origin in this experimental work.
+
+From the side also of descriptive morphology the biogenetic law
+underwent a critical revision. It was studied as a fact of embryology
+and without phylogenetic bias by men like Oppel, Keibel, Mehnert, O.
+Hertwig and Vialleton,[527] and they arrived at a critical estimate of it
+very similar to that of von Baer.
+
+Theoretical objections to the biogenetic law had been raised from time
+to time by many embryologists, but the positive testing of it by the
+comparison of embryos in respect of the degree of development of their
+different organs starts with Oppel's work of 1891.[528] He studied a large
+number of embryos of different species at different stages of their
+development, and determined the relative time of appearance of the
+principal organs and their relative size. His results are summarised in
+tabular form and have reference to all the more important organs. He was
+led to ascribe a certain validity to the biogenetic law, but he drew
+particular attention to the very considerable anomalies in the time of
+appearance which are shown by many organs, anomalies which had been
+classed by Haeckel under the name of heterochronies.
+
+Oppel's main conclusions were as follows:--"There are found in the
+developmental stages of different Vertebrates 'similar ontogenetic
+series,' that is to say, Vertebrates show at definite stages
+similarities with one another in the degree of development of the
+different organs. Early stages resemble one another, so also do later
+stages; equivalent stages of closely allied species resemble one
+another, and older stages of lower animals resemble younger stages of
+higher animals; young stages are more alike than old stages.... The
+differences which these similar series show (for which reason they
+cannot be regarded as identical) may be designated as temporal
+disturbances in the degree of development of the separate organs or
+organ-systems. Some organs show very considerable temporal dislocations,
+others a moderate amount, others again an inconsiderable amount. Among
+the developmental stages of various higher animals can be found some
+which correspond to the ancestral forms and also to the lower types
+which resemble these ancestral forms. On the basis of the tabulated data
+here given there can be distinguished with certainty in the ontogeny of
+Amniotes a pro-fish stage, a fish-stage, a land-animal stage, a
+pro-amniote stage, and following on these a fully developed reptile,
+bird or mammal stage."[529]
+
+Oppel's methods were employed by Keibel[530] in his investigations on the
+development of the pig, which formed the model for the well-known series
+of _Normentafeln_ of the ontogeny of Vertebrates which were issued in
+later years under Keibel's editorship. Keibel was more critical of the
+biogenetic law than Oppel, and he held that the ancestral stages
+distinguished by Oppel could not be satisfactorily established. He
+suggested an interesting explanation of heterochrony in development,
+according to which the premature or retarded appearance of organs in
+ontogeny stands in close relation with the time of their entering upon
+functional activity. Thus in many mammals the mesodermal part of the
+allantois often appears long before the endodermal part, though this is
+phylogenetically older. This Keibel ascribes to the fact that the
+endodermal part is almost functionless. "One can directly affirm," he
+writes, "that the time of appearance of an organ depends in an eminent
+degree upon the time when it has to enter upon functional activity. This
+moment is naturally dependent upon the external conditions. Among the
+highest Vertebrates, the mammals, the traces of phylogeny shown in
+ontogeny are to a great extent obliterated through the adaptation of
+ontogeny to the external conditions, and through the modifications which
+the germs of more highly organised animals necessarily exhibit from the
+very beginning as compared with germs which do not reach such a high
+level of development" (p. 754, 1897).
+
+Study of individual variation in the time of appearance of the organs in
+embryos of the same species was prosecuted with interesting results by
+Bonnet,[531] Mehnert,[532] and Fischel.[533] Fischel found that variability
+was greatest among the younger embryos, and became progressively less in
+later stages. Like von Baer (_supra_, p. 114) he inferred that
+regulatory processes were at work during development which brought
+divergent organs back to the normal and enabled them to play their part
+as correlated members of a functional whole.
+
+Important theoretical views were developed by Mehnert[534] in a series of
+publications appearing from 1891 to 1898. Like Keibel, Mehnert
+emphasised the importance of function in determining the late or early
+appearance of organs, but he conceived the influence of function to be
+exerted not only in ontogeny, but also throughout the whole course of
+phylogeny, by reason of the transmission to descendants of the effects
+of functioning in the individual life.
+
+In his paper of 1897 Mehnert details the results of an extensive
+examination of the development of the extremities throughout the Amniote
+series. He finds that in all cases a pentadactylate rudiment is formed,
+even in those forms in which only a few of the elements of the hand or
+foot come to full development. But whereas in forms with a normally
+developed hand, _e.g._ the tortoise and man, all the digits develop and
+differentiate at about the same rate, in forms which have in the adult
+reduced digits, _e.g._ the ostrich and the pig, these vestigial digits
+undergo a very slow and incomplete differentiation, while the others
+develop rapidly and completely. He draws a general distinction between
+organs that are phylogenetically progressive and such as are
+phylogenetically regressive, and seeks to prove that progressive organs
+show an ontogenetic acceleration and regressive organs a retardation.[535]
+The acceleration or retardation affects not only the mass-growth of the
+organs, but also their histological differentiation.
+
+Now between progression and functioning and between regression and
+functional atrophy there is obviously a close connection. Loss of
+function is well known to be one of the chief causes of the degeneration
+of organs in the individual life, and on the other hand, as Roux has
+pointed out, all post-embryonic development is ruled and guided by
+functioning. It is thus in the long run functioning that brings about
+phylogenetic progression, absence of functional activity that causes
+phylogenetic regression. This comes about through the transmission of
+acquired functional characters, a transmission which Mehnert conceives
+to be extraordinarily accurate and complete.
+
+In general Mehnert adopts the functional standpoint of Cuvier, von Baer,
+and Roux. His considered judgment as to the phylogenetic value of the
+biogenetic law closely resembles that formed by von Baer, for he admits
+recapitulation only as regards the single organs, not as regards the
+organism as a whole. He has, however, much more sympathy with the law
+than either Keibel or Oppel, though he agrees that it cannot be used for
+the construction of ancestral trees. But he ascribes to it as a fact of
+development considerable importance. The following passage gives a good
+summary of his view as to the scope and validity of the law. "The
+biogenetic law has not been shaken by the attacks of its opponents. The
+assertion is still true that individual organogenesis is exclusively
+dependent on phylogeny. But we must not expect to find that all the
+stages in the development of the separate organs, which coexisted in any
+member of the phylogenetic series, appear _at the same time_ in the
+individual ontogeny of the descendants, because each organ possesses its
+own specific rate of development. In this way it comes about naturally
+that organs which become differentiated rapidly, as, for example, the
+medullary tube, as a rule dominate earlier periods of ontogeny than do
+the organs of locomotion. For the same reason the cerebral hemispheres
+of man are almost as large in youth as in maturity. The picture which an
+embryo gives is not a repetition in detail of one and the same
+phylogenetic stage; it consists rather of an assemblage of organs, some
+of which are at a phyletically early stage of development, while others
+are at a phyletically older stage."[536]
+
+A different line of attack was that adopted by O. Hertwig in a series of
+papers, which contain also what is perhaps the best critical estimate of
+the present position and value of descriptive morphology.[537]
+
+It had not escaped the notice of many previous observers that quite
+early embryos not infrequently show specific characters even before the
+characters proper to their class, order and genus are developed--in
+direct contradiction of the law of von Baer. Thus L. Agassiz[538] had
+remarked in 1859 that specific characteristics were often developed
+precociously. "The Snapping Turtle, for instance, exhibits its small
+crosslike sternum, its long tail, its ferocious habits, even before it
+leaves the egg, before it breathes through lungs, before its derm is
+ossified to form a bony shield, etc.; nay, it snaps with its gaping jaws
+at anything brought near, when it is still surrounded by its amnion and
+allantois, and its yolk still exceeds in bulk its whole body" (p. 269).
+
+Wilhelm His,[539] in the course of an acute and damaging criticism of the
+biogenetic law as enunciated by Haeckel, showed clearly that by careful
+examination the very earliest embryos of a whole series of Vertebrates
+could be distinguished with certainty from one another. "An identity in
+external form of different animal embryos, despite the common
+affirmation to the contrary, does not exist. Even at early stages in
+their development embryos possess the characters of their class and
+order, nay, we can hardly doubt, of their species and sex, and even
+their individual characteristics" (201).
+
+This specificity of embryos was affirmed with even greater confidence by
+Sedgwick in a paper critical of von Baer's law.[540] He wrote:--"If v.
+Baer's law has any meaning at all, surely it must imply that animals so
+closely allied as the fowl and duck would be indistinguishable in the
+early stages of development; and that in two species so closely similar
+that I was long in doubt whether they were distinct species, viz.,
+_Peripatus capensis_ and _Balfouri_, it would be useless to look for
+embryonic differences; yet I can distinguish a fowl and a duck embryo on
+the second day by the inspection of a single transverse section through
+the trunk, and it was the embryonic differences between the Peripatuses
+which led me to establish without hesitation the two separate
+species.... I need only say ... that a species is distinct and
+distinguishable from its allies from the very earliest stages all
+through the development, although these embryonic differences do not
+necessarily implicate the same organs as do the adult differences" (p.
+39).
+
+Hertwig interprets this fact of the specific distinctness of closely
+allied embryos in the light of the preformistic conception of heredity.
+According to this view the whole adult organisation is represented in
+the structure of the germ-plasm contained in the fertilised ovum, from
+which it follows that the ova of two different species, and also their
+embryos at every stage of development, must be as distinct from one
+another as are the adults themselves, even though the differences may
+not be so obvious. If this be the case there can be no real
+recapitulation in ontogeny of the phylogeny of the race, for the
+egg-cell represents not the first term in phylogeny, but the last. The
+egg-cell _is_ the organism in an undeveloped state; it has a vastly more
+complicated structure than was possessed by the primordial cell from
+which its race has sprung, and it can in no way be considered the
+equivalent of this ancestral cell.
+
+Hertwig puts this vividly when he says that "the hen's egg is no more
+the equivalent of the first link in the phylogenetic chain than is the
+hen itself" (p. 160, 1906, b).
+
+If ontogeny is not a recapitulation of phylogeny, how is it that the
+early embryonic stages are so alike, even in animals of widely different
+organisation? Hertwig's answer to this is very interesting. He takes the
+view that many of the processes characterising early embryonic
+development are the means necessarily adopted for attaining certain
+ends. Such are the processes of segmentation, the formation of a
+blastula, of cell-layers, of medullary folds where the nervous system is
+a closed tube, the formation of the notochord as a necessary condition
+of the development of the vertebral column, and so on. "Looked at from
+this standpoint it cannot surprise us that in all animal phyla the
+earliest embryonic processes take place in similar fashion, so that we
+observe the occurrence both in Vertebrates and Invertebrates of a
+segmentation-process, a morula-stage, a blastula and a gastrula. If now
+these developmental processes do not depend on chance, but, on the
+contrary, are rooted in the nature of the animal cell itself, we have no
+reason for inferring from the recurrence of a similar
+segmentation-process, morula, blastula, and gastrula in all classes of
+the animal kingdom the common descent of all animals from one
+blastula-like or gastrula-like ancestral form. We recognise rather in
+the successive early stages of animal development only the manifestation
+of special laws, by which the shaping of animal forms (as distinct from
+plant forms) is brought about" (p. 178, 1906, b).
+
+"The principal reason why certain stages recur in ontogeny with such
+constancy and always in essentially the same manner is that they provide
+under all circumstances the necessary pre-conditions through which alone
+the later and higher stages of ontogeny can be realised. The unicellular
+organism can by its very nature transform itself into a multicellular
+organism only by the method of cell-division. Hence, in all Metazoa,
+ontogeny must start with a segmentation-process, and a similar statement
+could be made with regard to all the later stages" (p. 57, 1906, a).
+
+Similarities in early development are therefore no evidence of common
+descent, and in the same way the resemblances of adult animals, subsumed
+under the concepts of homology and the unity of plan, are not
+necessarily due to community of descent, but may also be brought about
+by the similarity or identity of the laws which govern the evolution of
+these animals. In the absence, therefore, of positive evidence as to the
+actual lines of descent (to be obtained only from palæontology),
+homological resemblance cannot be taken as proof of blood relationship,
+for homology is a wider concept than homogeny. The only valid definition
+of homology is that adopted in pre-evolutionary days, when those organs
+were considered homologous "which agree up to a certain point in
+structure and composition, in position, arrangement, and relation to the
+neighbouring organs, and accordingly possess identical functions and
+uses in the organism" (p. 151, 1906, b).
+
+The concept of homology has thus a value quite independent of any
+evolutionary interpretation which may be superadded to it. "Homology is
+a mental concept obtained by comparison, which under all circumstances
+retains its validity, whether the homology finds its explanation in
+common descent or in the common laws that rule organic development" (p.
+151, 1906, b). As A. Braun long ago pointed out, "It is not descent
+which decides in matters of morphology, but, on the contrary, morphology
+which has to decide as to the possibility of descent."[541]
+
+Hertwig, in a word, reverts to the pre-evolutionary conception of
+homology. "We see in homology," he writes, "only the expression of
+regularities (_Gesetzmässigkeiten_) in the organisation of the animals
+showing it, and we regard the question, how far this homology can be
+explained by common descent and how far by other principles, as for the
+present an open one, requiring for its solution investigations specially
+directed towards its elucidation" (p. 179, 1906, b).
+
+Holding, as he does, that no definite conclusions can be drawn from the
+facts of comparative anatomy and embryology as to the probable lines of
+descent of the animal kingdom, Hertwig accords very little value to
+phylogenetic speculation. It is, he admits, quite probable that the
+archetype of a class represents in a general sort of way the ancestral
+form, but this does not, in his opinion, justify us in assuming that
+such generalised types ever existed and gave origin to the present-day
+forms. "It is not legitimate to picture to ourselves the ancestral forms
+of the more highly organised animals in the guise of the lower animals
+of the present day--and that is just what we do when we speak of
+Proselachia, Proamphibia and Proreptilia" (p. 155, 1906, b).
+
+He rejects on the same general grounds the evolutionary dogma of
+monophyletic or almost monophyletic descent, and admits with Kölliker,
+von Baer, Wigand, Naegeli and others that evolution may quite well have
+started many times and from many different primordial cells.
+
+There is indeed a great similarity between the views developed by O.
+Hertwig and those held by the older critics of Darwinism--von Baer,
+Kölliker, Wigand, E. von Hartmann and others. It is true the
+philosophical standpoint is on the whole different, for while many of
+that older generation were vitalists Hertwig belongs to the mechanistic
+school.
+
+But both Hertwig and the older school agree in pointing out the _petitio
+principii_ involved in the assumption that the archetype represents the
+ancestral form; both reject the simplicist conception of a monophyletic
+evolution (which may be likened to the "one animal" idea of the
+transcendentalists); both admit the possibility that evolution has taken
+place along many separate and parallel lines, and explain the
+correspondences shown by these separate lines by the similarity of the
+intrinsic laws of evolution; finally, both emphasise the fact that we
+know nothing of the actual course of evolution save the few indications
+that are furnished by palæontology, and both insist upon the unique
+importance of the palæontological evidence.[542]
+
+It was a curious but very typical characteristic of evolutionary
+morphology that its devotees paid very little attention to the positive
+evidence accumulated by the palæontologists,[543] but shut themselves up
+in their tower of ivory and went on with their work of constructing
+ideal genealogies. It was perhaps fortunate for their peace of mind that
+they knew little of the advances made by palæontology, for the evidence
+acquired through the study of fossil remains was distinctly unfavourable
+to the pretty schemes they evolved.
+
+As Neumayr, Zittel, Depéret, Steinmann and others have pointed out, the
+palæontological record gives remarkably little support to the ideal
+genealogies worked out by morphologists. There is, for instance, a
+striking absence of transition forms between the great classificatory
+groups. A few types are known which go a little way towards bridging
+over the gaps--the famous _Archæopteryx_, for example--but these do not
+always represent the actual phylogenetic links. There is an almost
+complete absence of the archetypal ancestral forms which are postulated
+by evolutionary morphology. Amphibia do not demonstrably evolve from an
+archetypal Proamphibian, nor do mammals derive from a single generalised
+Promammalian type. Few of the hypothetical ancestral types imagined by
+Haeckel have ever been found as fossils. The great classificatory groups
+are almost as distinct in early fossiliferous strata as they are at the
+present day. As Depéret says in his admirable book,[544] in the course of
+a presentation of the matured views of the great Karl von Zittel, "We
+cannot forget that there exist a vast number of organisms which are not
+connected by any intermediate links, and that the relations between the
+great divisions of the animal and vegetable kingdoms are much less close
+than the theory [of evolution] demands. Even the Archæopteryx, the
+discovery of which made so much stir and appeared to establish a genetic
+relation between classes so distinct as Birds and Reptiles, fills up the
+gap only imperfectly, and does not indicate the point of bifurcation of
+these two classes. Intermediate links are lacking between Amphibia and
+Reptiles. Mammals, too, occupy an isolated position, and no zoologist
+can deny that they are clearly demarcated from other Vertebrates;
+indeed, no fossil mammal is certainly known which comes nearer to the
+lower Vertebrates than does Ornithorhynchus at the present day" (p.
+115).
+
+To take a parallel from the Invertebrata, B. B. Woodward,[545] after
+discussing the phylogeny of the Mollusca as worked out by the
+morphologists and comparing it with the probable actual course of the
+evolution of the group, as evidenced by fossil shells, sums up as
+follows:--"The lacunæ in our knowledge of the interrelationships of the
+members of the various families and orders of Mollusca are slight
+however, compared with the blank caused by the total absence from
+palæontological history of any hint of passage forms between the classes
+themselves, or between the Mollusca and their nearest allies. Nor is
+this hiatus confined to the Molluscan phylum; it is the same for all
+branches of the animal kingdom. There is circumstantial evidence that
+transitional forms must have existed, but of actual proof none whatever.
+All the classes of Mollusca appear fully fledged, as it were. No form
+has as yet been discovered of which it could be said that it in any way
+approached the hypothecated prorhipidoglossate mollusc, still less one
+linking all the classes" (p. 79).
+
+Pointing in the same direction as the absence of transitional forms is
+the undeniable fact that all the great groups of animals appear with all
+their typical characters at a very early geological epoch. Thus, in the
+Silurian age a very rich fauna has already developed, and
+representatives are found of all the main Invertebrate groups--sponges,
+corals, hydroid colonies, five types of Echinoderms, Bryozoa,
+Brachiopods, Worms, many types of Mollusca and Arthropoda. Of
+Vertebrates, at least two types of fish are present--Ganoids and
+Elasmobranchs. In the very earliest fossiliferous rocks of all, the
+Precambrian formation, there are remains of Molluscs, Trilobites and
+Gigantostraca, similar to those which flourished in Cambrian and
+Silurian times.
+
+The contributions of palæontology to the solution of the problems of
+descent posed by morphology are, however, not all of this negative
+character. The law of recapitulation is in some well-controlled cases
+triumphantly vindicated by palæontology. Thus Hyatt and others found
+that in Ammonites the first formed coils of the shell often reproduce
+the characters belonging to types known to be ancestral, and what is
+more they have demonstrated the actual occurrence of the phenomenon
+known as acceleration or tachygenesis, often postulated by speculative
+morphologists.[546] This is the tendency universally shown by embryos to
+reproduce the characters of their ancestors at earlier and earlier
+stages in their development.
+
+The most valuable contribution made by palæontologists to morphology and
+to the theory of evolution arose out of the careful and methodical study
+of the actual succession of fossil forms as exemplified in limited but
+richly represented groups. Classical examples were the researches of
+Hilgendorf[547] on the evolution of _Planorbis multiformis_ in the
+lacustrine deposits of Steinheim, those of Waagen[548] on the phylogeny of
+_Ammonites subradiatus_, and the work of Neumayr and Paul[549] on
+_Paludina_ (_Vivipara_).
+
+These investigations demonstrated that it was possible to follow out
+step by step in superjacent strata the actual evolution of fossil
+species and to establish the actual "phyletic series."
+
+To take an example from among the Vertebrates, Depéret has shown (_loc.
+cit._, pp. 184-9), that the European Proboscidea, belonging to the three
+different types of the Elephants, Mastodons and Dinotheria, have evolved
+since the Oligocene epoch along five distinct but continuous lines. The
+Dinotherian stock is represented at the beginning of the Miocene by the
+relatively small form _D. cuvieri_; this changes progressively
+throughout Miocene times into _D. laevius_, _D. giganteum_, and _D.
+gigantissimum_. Among the Mastodons two quite distinct phyletic series
+can be distinguished, the first commencing with _Palæomastodon
+beadnelli_ of the Oligocene, and evolving between the Miocene and
+Pliocene into _Mastodon arvernensis_, after traversing the forms _M.
+angustidens_ and _M. longirostris_, the second starting with the _M.
+turicensis_ of the Lower Miocene and evolving through _M. borsoni_ into
+the _M. americanus_ of the Quaternary. The phyletic series of the true
+elephants in Europe are relatively short, and go back only to the
+Quaternary, _Elephas antiquus_ giving origin to the Indian elephant, _E.
+priscus_ to the African.
+
+The careful study of phyletic series brought to light the significant
+fact that these lines of filiation tend to run for long stretches of
+time parallel to, and distinct from one another, without connecting
+forms. This is clearly exemplified in the case of the Proboscidea, and
+many other examples could be quoted. Almost all rich genera are
+polyphyletic in the sense that their component species evolve along
+separate and parallel lines of descent.[550] "Such great genera as the
+genus _Hoplites_ among the Ammonites, the genus _Cerithium_ among the
+Gastropoda, the genus _Pecten_ or the genus _Trigonia_ among the
+Lamellibranchs, each comprise perhaps more than twenty independent
+phyletic series" (Depéret, p. 200).
+
+Variation along the phyletic lines is gradual[551] and determinate, and
+appears to obey definite laws. The earliest members of a phyletic series
+are usually small in size and undifferentiated in structure, while the
+later members show a progressive increase in size and complexity. Rapid
+extinction often supervenes soon after the line has reached the maximum
+of its differentiation.
+
+The general picture which palæontology gives us of the evolution of the
+animal kingdom is accordingly that of an immense number of phyletic
+lines which evolve parallel to one another, and without coalescing,
+throughout longer or shorter periods of geological times. "Each of these
+lines culminates sooner or later in mutations of great size and highly
+specialised characters, which become extinct and leave no descendants.
+When one line disappears by extinction it hands the torch, so to speak,
+to another line which has hitherto evolved more slowly, and this line in
+its turn traverses the phases of maturity and old age which lead it
+inevitably to its doom. The species and genera of the present day belong
+to lines that have not reached the senile phase; but it may be surmised
+that some of them, _e.g._ elephants, whales, and ostriches, are
+approaching this final phase of their existence" (Depéret, p. 249).
+
+It is one of the paradoxes of biological history that the
+palæontologists have always laid more stress upon the functional side of
+living things than the morphologists, and have, as a consequence, shown
+much more sympathy for the Lamarckian theory of evolution. The American
+palæontologists in particular--Cope, Hyatt, Ryder, Dall, Packard,
+Osborn--have worked out a complete neo-Lamarckian theory based upon the
+fossil record.
+
+The functional point of view was well to the fore in the works of those
+great palæontologists, L. Rütimeyer (1825-1895) and V. O. Kowalevsky
+(1842-83), who seem to have carried on the splendid tradition of Cuvier.
+Speaking of Kowalevsky's classical memoir, _Versuch einer natürlichen
+Classification der fossilen Hufthiere_, Osborn[552] writes:--"This work is
+a model union of the detailed study of form and function with theory and
+the working hypothesis. It regards the fossil not as a petrified
+skeleton, but as having belonged to a moving and feeding animal; every
+joint and facet has a meaning, each cusp a certain significance. Rising
+to the philosophy of the matter, it brings the mechanical perfection and
+adaptiveness of different types into relation with environment, with
+changes of herbage, with the introduction of grass. In this survey of
+competition it speculates upon the causes of the rise, spread, and
+extinction of each animal group. In other words, the fossil quadrupeds
+are treated _biologically_--so far as is possible in the obscurity of
+the past" (p. 8). The same high praise might with justice be accorded to
+the work of Cope on the functional evolution of the various types of
+limb-skeleton in Vertebrates, and on the evolution of the teeth as well
+as to the work of other American palæontologists, including Osborn
+himself.
+
+Osborn's law of "adaptive radiation," which links on to Darwin's law of
+divergence,[553] constitutes a brilliant vindication of the functional
+point of view. "According to this law each isolated region, if large and
+sufficiently varied in its topography, soil, climate, and vegetation,
+will give rise to a diversified mammalian fauna. From primitive central
+types branches will spring off in all directions, with teeth and
+prehensile organs modified to take advantage of every possible
+opportunity of securing food, and in adaptation of the body, limbs and
+feet to habitats of every kind, as shown in the diagram [on p. 363]. The
+larger the region and the more diverse the conditions, the greater the
+variety of mammals which will result.
+
+"The most primitive mammals were probably small insectivorous or
+omnivorous forms, therefore with simple, short-crowned teeth, of
+slow-moving, ambulatory, terrestrial, or arboreal habit, and with short
+feet provided with claws. In seeking food and avoiding enemies in
+different habitats the limbs and feet radiate in four diverse
+directions; they either become _fossorial_ or adapted to digging habits,
+_natatorial_ or adapted to _amphibious_ and finally to _aquatic_
+habits, _cursorial_ or adapted to swift-moving, terrestrial progression,
+_arboreal_ or adapted to tree life. Tree life leads, as its final stage,
+into
+
+                        LIMBS AND FEET.
+                                                        Volant.
+                                                          /
+            Fossorial.                               Arboreal.
+                \                                       /
+        Short-limbed, plantigrade,      }       Ambulatory
+          pentadactyl, unguiculate      }           or
+          Stem.                         }       Terrestrial.
+                /                                       \
+          Natatorial.                                 Cursorial
+         Amphibious.                                 Digitigrade.
+             /                                             \
+         Aquatic                                       Unguligrade.
+
+
+                                  TEETH.
+                                Omnivorous.
+                                                              { Grass.
+                        { Fish.     |                         { Herb.
+Carnivorous             { Flesh.    |   Herbivorous           { Shrub.
+            \           { Carrion.  |        /                { Fruit.
+             \                      |       /                 { Root.
+              \                     |      /
+               \                    |     /   Myrmecophagous.
+                \                   |    /        / Dentition reduced.
+                 \                  |   /        /
+                  \                 |  /        /
+                   \                | /        /
+                    \               |/        /
+                        Stem: Insectivorous.
+
+
+the parachute types of the flying squirrels and phalangers, or into the
+true flying types of the bats.... Similarly in the case of the teeth,
+insectivorous and omnivorous types appear to be more central and ancient
+than either the exclusively carnivorous or herbivorous types. Thus the
+extremes of carnivorous adaptation, as in the case of the cats, of
+omnivorous adaptation, as in the case of the bears, of herbivorous
+adaptation, as in the case of the horses, or myrmecophagous adaptation,
+as in the case of the anteaters, are all secondary" (_loc. cit._, pp.
+23-4).
+
+We have now reached the end of our historical survey of the problems of
+form. What the future course of morphology will be no one can say. But
+one may hazard the opinion that the present century will see a return to
+a simpler and more humble attitude towards the great and unsolved
+problems of animal form. Dogmatic materialism and dogmatic theories of
+evolution have in the past tended to blind us to the complexity and
+mysteriousness of vital phenomena. We need to look at living things with
+new eyes and a truer sympathy. We shall then see them as active, living,
+passionate beings like ourselves, and we shall seek in our morphology to
+interpret as far as may be their form in terms of their activity.
+
+This is what Aristotle tried to do, and a succession of master-minds
+after him. We shall do well to get all the help from them we can.
+
+    [519] See E. B. Wilson's masterly book, _The Cell in
+    Development and Inheritance_, New York and London, 1900.
+
+    [520] _Q.J.M.S._, xxvi. 1886.
+
+    [521] _Wood's Holl Biological Lectures_ for 1893.
+
+    [522] _Arch. f. Ent.-Mech._, i., pp. 380-90, 1895.
+
+    [523] _Beiträge zur Kritik der Darwinschen Lehre_,
+    Leipzig, 1898.
+
+    [524] See E. B. Wilson, "The Embryological Criterion of
+    Homology," _Wood's Holl Biological Lectures_, Boston,
+    pp. 101-24, 1895; Braem, _Biol. Centrblt._, xv., 1895;
+    T. H. Morgan, _Arch. f. Ent.-Mech._, xviii.; J. W.
+    Jenkinson, _Mem. Manchester Lit. Phil. Soc._, 1906, and
+    _Vertebrate Embryology_, Oxford, 1913; A. Sedgwick,
+    article "Embryology" in _Ency. Brit._, p. 318, vol. xi.,
+    11th Ed. (1910).
+
+    [525] For a detailed treatment of this important point see
+    the remarkable volume of E. Schulz (Petrograd),
+    _Prinzipien der rationellen vergleichenden Embryologie_,
+    Leipzig, 1910.
+
+    [526] "La Poecilogonie," _Bull. Sci. France et Belgique_,
+    xxxix., pp. 153-87, 1905.
+
+    [527] _Un problème de l'évolution. La loi biogénétique
+    fondamentale_, Paris and Montpellier, 1908.
+
+    [528] _Vergleichung des Entwickelungsgrades der Organe zu
+    verschiedenen Entwickelungszeiten bei Wirbeltieren_,
+    Jena, 1891.
+
+    [529] Quoted by Keibel, _Ergebn. Anat. Entwick._, vii., p.
+    741.
+
+    [530] "Studien zur Entwickelungsgeschichte des Schweines,"
+    Schwalbe's _Morphol. Arbeiten_, iii., 1893, and v.,
+    1895.
+
+      _Normentafeln zur Entwickelungsgeschichte des
+      Schweines_, Jena, 1897.
+
+      "Das biogenetische Grundgesetz und die Cenogenese,"
+      _Ergebn. Anat. Entw._, vii., pp. 722-92, 1897.
+
+      "U. d. Entwickelungsgrad der Organe," _Handb. vergl.
+      exper. Entwick. der Wirbelthiere_, iii., 3, pp. 131-48,
+      1906.
+
+    [531] "Beiträge zur Embryologie der Wiederkäuer," _Arch.
+    Anat. Entw._, 1889.
+
+    [532] "Die individ. Variation d. Wirbeltierembryo,"
+    _Morph. Arbeit._, v., 1895.
+
+    [533] "U. Variabilität u. Wachstum d. embryonalen
+    Körpers," _Morph. Jahrb._, xxiv., 1896.
+
+    [534] "Gastrulation u. Keimblätterbildung der _Emys
+    lutaria taurica_," _Morph. Arbeit._, i., 1891.
+    "Kainogenese," _Morph. Arbeit._, vii., pp. 1-156, 1897,
+    and also separately. _Biomechanik, erschlossen aus dem
+    Prinzipe der Organogenese_, Jena, 1898.
+
+    [535] This law was foreshadowed by Reichert in 1837, when he
+    wrote:--"We notice in our investigation of embryos of different
+    animal forms that it is those organs, those systems, which in the
+    fully developed individual are peculiarly perfect, that in their
+    earliest rudiments and also throughout the whole course of their
+    development appear with the most striking distinctness" (Müller's
+    _Archiv_, p. 135, 1837). See also his _Entwick. Kopf. nackt.
+    Amphib._, p. 198, 1838. So, too, Rathke notes how the elongated
+    shape of the snake appears even in very early embryonic stages
+    (_Entwick. Natter._, p. 111, 1839).
+
+    [536] Quoted by Keibel (p. 790, 1897) from the
+    _Biomechanik_.
+
+    [537] _Die Zelle und die Gewebe_, Jena, 1898, and the
+    subsequent editions of this text-book, published under
+    the title of _Allgemeine Biologie. Die Entwickelung der
+    Biologie im neunzehnten Jahrhundert_, Jena, 1900, 2nd
+    ed., 1908. "Ueber die Stellung der vergl.
+    Entwickelungslehre zur vergl. Anatomie, zur Systematik
+    und Descendenztheorie," _Handb. vergl. exper.
+    Entwickelungslehre der Wirbeltiere_, iii., 3, pp.
+    149-80, Jena, 1906. (1906, b). Also in Pt. I. of Vol. I.
+    (1906, a).
+
+    [538] _An Essay on Classification_, London, 1859.
+
+    [539] _Unsere Körperform_, Leipzig, 1874.
+
+    [540] _Q.J.M.S._, xxxvi., pp. 35-52, 1894.
+
+    [541] Quoted by Hertwig. See also K. Goebel, "Die
+    Grundprobleme der heutigen Pflanzenmorphologie," _Biol.
+    Centrbl._, xxv., pp. 65-83, 1905.
+
+    [542] This is also emphasised by Fleischmann in his critical study of
+    evolutionary morphology entitled _Die Descendenztheorie_, Leipzig,
+    1901.
+
+    [543] The same remark applies to the bulk of speculation as to the
+    factors of evolution, with the exception of the contributions made
+    to evolution theory by the palæontologists by profession, such as
+    Cope.
+
+    [544] _Les Transformations du Monde animal_, Paris, 1907.
+
+    [545] "Malacology _versus_ Palæoconchology," _Proc.
+    Malacological Soc._, viii., pp. 66-83, 1908.
+
+    [546] Particularly by E. Perrier, "La Tachygenèse," _Ann.
+    Sci. nat._ (_Zool._) (8), xvi., 1903.
+
+    [547] _Monatsber. k. Akad. Wiss._, Berlin, pp. 474-504,
+    1866.
+
+    [548] _Geognost. u. Palæont. Beiträge_, ii., Heft 2, pp.
+    181-256, 1869.
+
+
+    [549] _Abhand. k.k. Geol. Reichsanstalt_, vii., Wien,
+    1875.
+
+    [550] The case for polyphyletism is very strongly put by
+    G. Steinmann in his book, _Die geologischen Grundlagen
+    der Abstammungslehre_, Leipzig, 1908.
+
+    [551] The steps in this chronological variation were
+    termed by Waagen "mutations."
+
+    [552] _The Age of Mammals in Europe, Asia, and North
+    America_, New York, 1910.
+
+    [553] _Origin of Species_, 6th ed., Chap. IV.
+
+
+
+
+INDEX
+
+
+ACTINOZOAN THEORY of Vertebrate Descent, 299-300
+
+Adaptation as Conservative Principle--
+  Cuvier, 39, 76
+
+Adaptation, Ecological--
+  Von Baer, 123
+  H. Milne-Edwards, 199
+  Lamarck, 221, 222, 223, 224, 227
+  Treviranus, 225 f.n.
+  C. Darwin, 231-2, 235, 239
+  Haeckel, 248, 263
+  Gegenbaur, 263
+  V. O. Kowalevsky, 362
+  Osborn, 362-4
+
+Adaptation, Ecological, and Classification--
+  Bronn, 203
+
+Adaptation of Parts. _See_ "Correlation, Functional," and "Conditions of
+  Existence"
+
+Adaptive Radiation (Osborn), 362-4
+
+Agassiz, A., 288 f.n., 295
+  On Coelom, 296
+
+Agassiz, L.--
+  Criticism of Vertebral Theory of Skull, 157
+  Membrane and Cartilage Bones, 164
+  Transcendentalism, 203
+  Classification, 203 f.n.
+  Three-fold Parallelism, 230, 255
+  Influence on Darwin, 238
+  Specific Distinctness of Embryos, 353
+
+Albertus Magnus, 17
+
+Alcmæon, 1
+
+Aldrovandus, 18
+
+Allman, 209
+
+Analogy. _See also_ Homology.
+  Aristotle, 8-10
+  Owen, 108
+  Haeckel, 251
+  Gegenbaur, 266
+  Lankester, 267
+
+Anaxagoras, 14
+
+Anaximander, 14
+
+Anaximenes, 1
+
+Animal and Vegetative Lives--
+  Aristotle, 16, 32
+  Buffon, 26-7
+  Bergson, 26 f.n.
+  Cuvier, 26, 32
+  Bichat, 27-9
+  Oken, 94
+  K. G. Carus, 94
+  Von Baer, 116, 123, 131
+  Remak (Sensory and trophic layers), 210
+  Gegenbaur, 263
+
+Annelid Theory of Vertebrate Descent, 274-85, 301
+
+Archetype, Anatomical, 246, 302-3
+  E. Geoffroy, 54, 67
+  Owen, 104-7, 110
+  J. V. Carus, Huxley, 204
+  C. Darwin, 238 f.n.
+
+Archetype, Anatomical, as Ancestral--
+  C. Darwin, 235, 247
+  Haeckel, 251
+  Gegenbaur, 265
+  Sedgwick, 300
+  Criticism of this idea--
+    O. Hertwig, 355-7
+
+Archetype, Embryological, 168, 246, 302-3
+  Von Baer, 126, 132
+  Reichert, 139, 147, 149
+  Rathke, 151, 153
+  Huxley, 159-61
+
+Archetype, Embryological, as Ancestral--
+  C. Darwin, 233, 236-7
+  Haeckel, 254, 289-91
+  Gegenbaur, 266
+  O. and R. Hertwig, 298
+  Sedgwick, 300
+  A. Kowalevsky, 300
+
+Arendt, 162
+
+Aristotle, 2-16, 17, 345, 364
+  _Historia Animalium_, 2
+  _De Partibus Animalium_, 2, 9
+  Knowledge of Animals, 3, 4
+  Comparative Embryology, 4
+  Classification of Animals, 4-6
+  Unity of Plan, 6-7, 10
+  Homology and Analogy, 7-10
+  Teleology and Correlation, 10-12
+  Law of Compensation, 11
+  Division of Labour, 12
+  Degrees of Composition--homogeneous and heterogeneous parts, 12-14, 169
+  Law of Development (Von Baer), 14
+  Scale of Beings, 14-16
+  Functional attitude, 15-16, 197
+  Animal and Vegetative Lives, 16, 32
+
+Ascidian Theory of Vertebrate Descent, 269-73, 304
+
+Atomists, 16
+
+Atomists, "Biological," 192-4
+
+Audouin, V.--
+  Unity of plan in Arthropods, 85-6
+  Law of Compensation, 86
+  Marine Zoology, 195
+
+Autenrieth, 90, 96
+
+Avicenna, 17
+
+
+BABÁK, E., 333
+
+Baer, K. E. von, 113-32, 133, 251, 304, 345, 356
+  Founder of Embryology, 113
+  _Entwickelungsgeschichte der Thiere_, 114
+  Regulation of Development, 114, 350
+  Development as Differentiation, 115, 128
+  Germ-Layer Theory, 115-6, 118-119, 208-9, 296
+  Morphological Differentiation, 116-7
+  Histological Differentiation, 117-8
+  Tissues and Germ-Layers, 118
+  Double symmetrical Development, 118, 279
+  Criticism of Meckel-Serres Law, 120-3, 304
+  Theory of Types, 123-4, 289, 291
+  Law of Development, 124-6
+  Embryological Criterion, 126-8, 132, 138
+  Embryological Archetype, 126, 132
+  Types of Development, 127-8
+  Von Baer and Cuvier, 128-30
+  Functional attitude, 129
+  Relation to Transcendentalists, 129, 131
+  Criticism of Scale of Beings, 130
+  Vertebral Theory of Skull, 131, 142
+  Serial Homology, 131-2
+  Gill-slits, Gill-arches and Aortic arches, 135-6, 146
+  Membrane and Cartilage Bones, 162-3
+  Degrees of Composition, 172
+  Ova of Mammals, 175-6
+  Segmentation of Ovum, 186
+  Criticism of Evolution Theory, 229, 242
+  Influence on Darwin, 236, 238
+  Criticism of Darwinism, 242
+  Teleology and Correlation, 242
+  On Ascidians, 271
+
+Baer's Law. _See_ "Development, Von Baer's Law"
+
+Bagge, 187
+
+_Balanoglossus_ Theory of Vertebrate Descent, 285-7
+
+Balbiani, 330
+
+Balfour, F. M., 247, 299
+  Annelid Theory, 282-4
+  Gastrulation and Gastræa Theory, 295
+  Mesoderm, 296 f.n.
+  Coelom, 297
+
+Barfurth, D., 330
+
+Barry, M., 186, 188
+
+Bateson, W.--
+  Metamerism, Vegetative Repetition, 286
+  _Balanoglossus_ Theory, 286-7
+  On Phylogenetic Speculation, 302
+
+Beard, J., 285
+
+Belon, 18
+
+Beneden van, and Julin, 271, 285, 346
+
+Bensley, A. B., 311 f.n.
+
+Bergmann, 187
+
+Bergson, H., 26 f.n., 341, 345
+
+Bernard, Claude, 195, 314
+
+Bert, P., 315
+
+Bichat, X., 27-30, 118, 132, 169, 178, 263
+  Animal and Vegetative Lives, 27-9
+  "General Anatomy," 29-30
+  _Vie propre_ of Tissues, 30
+
+Biogenetic Law. _See_"Development, Haeckel's Law"
+
+Bischoff, 138
+  Segmentation, 186, 188
+
+Blainville, de, 96, 128, 141, 199 f.n.
+
+Bojanus, 96, 97
+
+Bonnet, C.--
+  Scale of Beings, 22-3, 220, 227
+  Evolution, 215
+  Regeneration, 315
+
+Bonnet, R., 350
+
+Bonnier, G., on Albertus Magnus, 17
+
+Born, G., 330
+
+Boveri, T., 270 f.n., 333
+
+Braem, 347 f.n.
+
+Braun, A., 355
+
+Breschet, 138, 173
+
+Bronn, H. G., 200-3, 248
+  _Naturphilosophie_, 201
+  Functional attitude, 201-3
+  Geometry of Organism, 201, 249
+  Theory of Types, 202
+  Principle of Connections, 202
+  Intrinsic Laws of Evolution, 202
+  Division of Labour, 202
+  Ecological  Adaptation and Classification, 203
+
+Brown, R., 171
+
+Bruch, C., 203 f.n.
+
+Büchner, 194, 248
+
+Buffon, 24-7, 336
+  Scale of Beings, 24, 215
+  Unity of Plan, 24
+  Evolution, 24-5, 214
+  Classification, 25-6
+  Animal and Vegetative Lives, 26-7
+  Homology and Analogy, 27
+
+Burckhardt, R., 3 f.n., 268 f.n.
+
+Burdin, 96
+
+Burmeister, 249 f.n.
+
+Butler, S., 226 f.n., 313, 335-42
+  Relation to Lamarck, 335-7
+  Psychological Vitalism, 336-41
+  Heredity and Memory, 337-41
+  The Two Stages of Development, 337-9
+  Consciousness and Habit, 337-9
+  Recapitulation Theory, 339-40
+  Teleology, 341
+
+
+CABANIS, 215
+
+Camper, P., 45, 46
+
+Carter, 293 f.n.
+
+Carus, J. V.--
+  Criticism of Embryological Criterion, 167
+  Morphology and  Physiology, 194
+  Vertebral Theory of Skull, 203
+  On Archetype, 204
+  Evolution, 230
+
+Carus, K.G.--
+  Law of Parallelism, 94, 249
+  Vertebral Theory, 96
+  Geometry of Skeleton, 98-100
+  Splanchnoskeleton, 98, 140
+
+Causal Morphology, 312-3, 315-34
+
+Cell-Theory--
+  Schwann, 169, 173-86, 188
+  C. F. Wolff, 170
+  Schleiden, 170-2
+  Criticism of Schwann-Schleiden Theory, 185-8
+    Virchow, Leydig, 188
+
+Cell-Theory and Germ-Layer Theory--
+  Remak, 209-12
+
+Cell-Theory as Disintegrative--
+  Schwann, 180-5, 248
+  Vogt, 190-1
+  Virchow, 191
+  Haeckel, 248
+  Criticism of this idea--
+    Reichert, 192-3, 194
+    J. V. Carus, 194
+    Sedgwick, Whitman, 346
+
+Cell-Theory, Influence on Morphology, 190
+
+Cenogenesis, 258-9, 323
+
+Chabry, 331
+
+Child, C. M., 333
+
+Chun, C, 317, 332
+
+Classification of Animals--
+  Aristotle, 4-6
+  Rondeletius, Aldrovandus, Gesner, 18
+  Linnæus, 22
+  Buffon, 25-6
+  Cuvier, 39-41
+  E. Geoffroy, 60
+  L. Agassiz, 203 f.n.
+  Lamarck, 216-7, 227, 228
+
+Classification and Ecological Adaptation (Bronn), 203
+
+Classification as Genealogical--
+  Buffon, 24-5
+  Lamarck, 218, 228
+  C. Darwin, 233, 234, 247
+  Haeckel, 250-1, 254
+  Criticism of this idea, 303, 304,
+    O. Hertwig, 356
+
+Classification, Phylogenetic--
+  Haeckel's, 289-94
+
+Claus, 259
+
+Co-adaptation, 326 f.n.
+
+Coelom--
+  Remak, 211
+  A. Kowalevsky, 270, 295, 297
+  Haeckel, 291, 295, 296
+  Lankester, 291, 297
+
+Coelom, Theory of, 295-301
+
+Cohen, 189
+
+Coiter, 18
+
+Colucci, 346
+
+Compensation, Law of--
+  Aristotle, 11
+  Goethe, 49
+  E. Geoffroy, 72-3
+  Audouin, 86.
+  German Transcendentalists, 100
+
+Condillac, 215
+
+Conditions of Existence, Principle of--
+  Cuvier, 34, 75-6, 239
+  Gegenbaur, 263-4
+  Roux, 324, 326
+  Spencer, Weismann, 326 f.n.
+  Disregard for--
+    Lamarck, 226
+    C. Darwin, 232, 238-41
+    Haeckel, 248, 264
+
+Conklin, 333
+
+Connections, Principle of--
+  Goethe, 47
+  E. Geoffroy, 53-4, 62-3, 71, 74, 261
+  Audouin, 85
+
+Connections, Principle of--_contd._
+  German Transcendentalists, 100
+  J. F. Meckel, 101
+  Owen, 107-8
+  Bronn, 202
+  C. Darwin, 234-5
+  Gegenbaur, 261
+  Semper, 279
+  In Embryology, 168
+  Main Principle of Morphology, 246, 302
+
+Convergence--
+  Milne-Edwards, 199
+  I. Geoffroy St Hilaire, 199 f.n., 206
+  C. Darwin, 236
+  Friedmann, Willey, Vialleton, 306 f.n.
+
+Convergence, Rejected by Evolutionary Morphologists, 305, 312
+  Hubrecht, 305-6
+
+Cope, E. D., 342, 357 f.n., 361, 362
+
+Correlation, Functional--
+  Aristotle, 10-12
+  Cuvier, 35-8, 239, 241
+  E. Geoffroy, 77
+  Von Hartmann, 240-1
+  Rádl, 240 f.n., 241
+  Von Baer, 242
+  Gegenbaur, 264
+  Disregarded by--
+    C. Darwin, 235, 238-41
+    Haeckel, 248, 264
+
+Coste, 134, 138, 176, 187
+
+Crampton, 332
+
+Cunningham, J. T., 284
+
+Cuvier, 26, 31-44, 89, 196, 197, 199 f.n., 278, 345, 361
+  Functional attitude, 31-6, 65, 75-8, 200, 305
+  Animal and Vegetative Lives, 32
+  Degrees of Composition, 32-3
+  Teleology, 33-5
+  Functional Adaptedness, 33-5, 324
+  Principle of Conditions of Existence, 34, 75-6, 239
+  Correlation, 35-8, 239, 241
+  Metabolism, 38
+  Adaptation as Conservative Principle, 39, 76
+  Classification, 39-41
+  Principle of Subordination of Characters, 40
+  Criticism of Scale of Beings, 39-40, 130
+  Type Theory, 41, 124, 289, 291
+  Criticism of Evolution-Theory, 41-4, 129, 304
+  Variation, Limits of, 42
+  Palæontological Succession, 43
+  Polemic with Geoffroy, 64-5, 74-8
+  Criticism of Vertebral Theory of Skull, 97-8
+  Influence on J. F. Meckel, 101
+  Criticism of Meckel-Serres Law, 129-30, 304
+  As Embryologist, 130
+  Criticism of Lamarck, 228
+
+Cytology, 346
+
+Cytoplasm of Egg, Organ-forming Stuffs, 332-3
+
+
+DALL, 361
+
+D'Alton, 113
+
+Dareste, C., 315
+
+Darwin, Charles, 78, 230-41, 271, 304, 307, 336, 362
+  Systematist and Field Naturalist, 230, 231
+  Palæontological Succession, 231
+  Ecological Adaptation, 231-2, 235, 239
+  Species Problem, 231
+  Functional Adaptation, Disregard for, 232, 238-41
+  Classification as genealogical, 233, 234, 247
+  Unity of Plan due to Community of Descent, 233, 234-5, 239, 247
+  Embryological Archetype as ancestral, 233, 236-7
+  Rejects Meckel-Serres Law, 233, 236
+  Interpretation  of  Vestigial Organs, 233, 237
+  Organism as Historical Being, 233, 308
+  Rejects Scale of Beings, 234
+  Homology, 234-5, 247
+  Principle of Connections, 234-5
+  Anatomical Archetype as ancestral, 235, 247
+  Von Baer's Law interpreted phylogenetically, 236-7
+  Modifications inherited at corresponding age, 237
+  Monophyletism and Polyphyletism, 238
+  Causes of Success, 238, 241
+
+Darwin, Erasmus, 214, 226 f.n., 229, 336
+
+Darwin, Sir Francis, 344
+
+Daubenton, 26
+
+Degrees of Composition--
+  Aristotle, 12-14, 169
+  Glisson, 19
+  Malpighi, 20
+  Bichat, 29-30
+  Cuvier, 32-3,
+  Dujardin, 169, 188
+  Von Baer, 172
+  Effect of Invention of Microscope, 20
+  Relation to Cell-Theory, 169
+
+Delage, 333
+
+Delage and Hérouard, 273 f.n.
+
+Delpino, 345
+
+Demaillet, 44
+
+Democritus, 16
+
+Depéret, C, 357
+  On Cuvier, 43
+  Absence of intermediary forms in Palæontology, 358
+  Phyletic series and Polyphyletism, 360-1
+
+Development, Von Baer's Law--
+  Aristotle, 14
+  Von Baer, 124-6
+  Prévost and Dumas, 125 f.n.
+  Reichert, 149-50, 351 f.n.
+  Milne-Edwards, 205-8
+  Lereboullet, 206-8
+  Criticised by--
+    Agassiz, 352-3
+    His, 353
+    Sedgwick, 353
+    O. Hertwig, 354
+  Phylogenetic Interpretation of--
+    Darwin, 236-7
+    Gegenbaur, 266
+  Relation to Haeckel's Law, 254, 256, 257
+
+Development, Biogenetic Law (Haeckel)--
+  Haeckel, 251, 253-9, 291-4
+  F. Müller, 252-3, 254, 257
+  Gegenbaur, 262
+  Roux, 319
+  Butler, 339-40
+  Orr, 342
+  Criticism of--
+    Vialleton, 348
+    Oppel, 348-9
+    Keibel, 349-50
+    Mehnert, 350-2
+    O. Hertwig, 352, 354-5
+    His, 353
+  Relation to Laws of Meckel-Serres and Von Baer, 254, 256, 257, 303, 309
+  Relation to Heredity and Development, 312-3
+  Influence of Causal Morphology, 347-8
+  Palæontological Evidence for, 359
+
+Development, Meckel-Serres Law--
+  Harvey, 18
+  Hunter, 22
+  E. Geoffroy, 69-70, 72
+  Serres, 80-3, 94, 203-4, 205-6
+  Kielmeyer, Autenrieth, Oken, 90
+
+Development, Meckel-Serres Law-_contd._
+  Tiedemann, 91
+  J. F. Meckel, 91-3
+  K. G. Carus, 94
+  Criticism of--
+    Von Baer, 120-3, 304
+    Cuvier, 129-30, 304
+    Milne-Edwards, 205
+    Lereboullet, 206-8
+    C. Darwin, 233, 236
+  Analogy with Biogenetic Law, 254-7, 262, 303, 304, 309
+
+Development, Meckel-Serres Law, Theory of Three-fold Parallelism--
+  L. Agassiz, 230, 255
+  Tiedemann, Vogt, 255 f.n.
+  Haeckel, 254-5
+
+Development, The two periods of--
+  Roux, 320-4, 325, 327, 335
+  Butler, 337-9
+
+Diogenes of Apollonia, 1
+
+Disintegration. _See_ "Cell-Theory," and "Materialistic Attitude"
+
+Division of Labour, Principle of--
+  Aristotle, 12
+  Milne-Edwards, 197-8
+  Bronn, 202
+  Gegenbaur, 264
+
+Dohrn, A., 269, 274-8
+  Annelid Theory of Vertebrate Descent, 274-7, 303
+  Principle of Function-Change, 276-8, 307
+  Functional Attitude, 277-8, 307
+  Formal Attitude, 306
+
+Döllinger, I., 113, 157
+
+Dollo, 311
+
+Donné, 173
+
+D'Orbigny, 43
+
+Driesch, H., 242, 331, 332, 333, 334, 345, 346-7
+
+Dugès, A., 86-8, 100, 134, 142, 146
+  Unity of Plan, 87
+  Polyzoic conception of Organism, 87-8
+  Membrane and Cartilage Bones, 163
+
+Dujardin, 169, 188
+
+Dumas. _See_ Prévost and Dumas
+
+Duméril, 96
+
+Dumortier, 173
+
+Dutrochet, 99 f.n., 130, 134
+
+Duverney, 19
+
+
+EAR-OSSICLES, Homology of--
+  E. Geoffroy, 56
+  Spix, 100
+  Rathke, 141, 150
+  Reichert, 144-7
+
+_Échelle des êtres. See_ "Scale of Beings."
+
+Ehlers, 284
+
+Eisig, H., 284, 285
+
+Embryology, Comparative, Early Workers--
+  Aristotle, 4, 113
+  Fabricius, Harvey, 18, 113
+  Malpighi, 20, 113
+  Oken and Kieser, 90, 113
+  Haller, C. F. Wolff, J. F. Meckel, Tiedemann, 113
+
+Embryology, Experimental, 317, 318, 330-3
+
+Embryological Archetype. _See_ "Archetype, Embryological"
+
+Embryological Criterion of Homology, 133-168, 347
+  Goethe, 49
+  E. Geoffroy, 72, 110
+  Cuvier, 75, 110, 130
+  Owen, 110-1
+  Von Baer, 126-8, 132, 138
+  Rathke, 138, 140-1
+  J. Müller, 138
+  Reichert, 138-9, 144-7, 163
+  Vogt, 156-7
+  Huxley, 158-9, 166
+  Kölliker, 165-6
+  Criticised by--
+    Owen, J. V. Carus, 167
+
+Empedocles, 1, 15
+
+Engramm (Semon), 343
+
+_Entwicklungsgesetz._ _See_ "Evolution, Intrinsic Laws of"
+
+_Entwicklungsmechanik_, 315
+
+Erasistratus, 17
+
+Evolution Theory--
+  Lucretius, 16
+  Buffon, 24-5, 214
+  Cuvier's criticism, 41-4, 129, 304
+  E. Geoffroy, 66-9, 73, 228
+  J. F. Meckel, 92-3, 215, 228
+  Leibniz, 213
+  Kant, 213-4
+  Erasmus Darwin, 214, 229
+  C. Bonnet, Oken, Robinet, Treviranus, 215
+  Tiedemann, 215, 255 f.n.
+  Lamarck, 215-29
+  Von Baer, 229, 242
+  I. Geoffroy St Hilaire, J. V. Carus, 230
+  Charles Darwin, 230-41
+  Von Hartmann, 240-1, 244, 356
+  Kölliker, 243
+  Owen, 244
+  Milne-Edwards, 244-5
+  Haeckel, 250-9
+  Gegenbaur, 265
+  The Organism as an Historical Being, 308-13
+    C. Darwin, 233, 308
+    Haeckel, 252, 257
+    Sedgwick, 308
+    Roux, 313, 322-4
+    Butler, 313, 336-41
+
+Evolution-Theory, Influence on Morphology, 302-13
+
+Evolution, Intrinsic Laws of, 241
+  J. F. Meckel, 93
+  Bronn, 202
+  Von Baer, 229, 242, 356
+  Kölliker, Naegeei, 243, 356
+  Owen, 244
+  Von Hartmann, 244, 356
+  Milne-Edwards, 244-5
+  O. Hertwig, 354-5, 356-7
+  Wigand, 356
+  Depéret, 361
+
+
+FABRICIUS, 18, 113
+
+Fallopius, 18
+
+Fischel, 346, 350
+
+Fischer, 328
+
+Fleischmann, 357 f.n.
+
+Flourens, 46, 315
+
+Fontana, 172
+
+Forbes, E., 196
+
+Formal Attitude, 246, 305
+  Goethe, 49
+  E. Geoffroy, 62-3, 71, 75-8, 305
+  Haeckel, 249, 257, 260
+  Gegenbaur, 261, 263
+  Semper, 279
+  Adopted by Evolutionary Morphologists, 302-8, 311-2, 314
+  Hubrecht, 305-6
+  Dohrn, 306
+
+Francé, R., 345
+
+Friedmann, 306 f.n.
+
+Fuld, 333
+
+Functional Adaptation, 316-7, 318, 320-9, 333, 344, 351
+
+Functional Attitude--
+  Aristotle, 15-6, 197
+  Bichat, 27-9
+  Cuvier, 31-6, 65, 75-8, 200, 305
+  Goethe, 49-50
+  J. F. Meckel, 101
+  Owen, 109, 110, 111
+  Von Baer, 129
+  Milne-Edwards, 195, 197-200
+  J. Müller, Reichert, 200
+  Bronn, 201-3
+  Lamarck, 222-6, 307, 335
+  Gegenbaur, 260, 263-4
+  Dohrn, 277-8, 307
+  Roux, 320-9, 335
+  Houssay, 333
+  Butler, 336-41
+  G. Wolff, 346
+  Driesch, 346-7
+  Giard, 347
+  E. Schulz, 347 f.n.
+  Keibel, 349-50
+  Mehnert, 350-1
+  American Palæontologists, 361, 362
+  Rütimeyer, 361
+  V. O. Kowalevsky, 361-2
+  Osborn, 362-4
+
+Function-Change, Principle of--
+  Dohrn, 276-8, 306, 307
+  Eisig, 284
+
+Fürbringer, M., 282 f.n., 284, 323 f.n.
+
+
+GALEN, 17
+
+Gastræa Theory, 269, 288-95, 298, 299-3O1, 303
+
+Gastrula, Discovery of, 288
+
+Gaupp, E., 310 f.n.
+
+Gegenbaur, C, 247, 260-7, 271, 285, 286, 288 f.n.
+  Division of Egg-nucleus, 188
+  Functional Attitude, 260, 263-4
+  Formal Attitude, 261, 263
+  Principle of Connections, 261
+  Embryology and Comparative Anatomy, 261-2, 263
+  Biogenetic and Meckel-Serres Laws, 262
+  Homology, 261, 263, 265, 266-7
+  Adaptation and Correlation, 263-4
+  Archetype as ancestral, 263 f.n, 265
+  On Phylogenetic Speculation, 265-6
+  Embryological Archetype, 266
+  Membrane and Cartilage Bones, 309, 310
+
+Gemmill, J. F., 285 f.n., 312 f.n.
+
+Geoffroy, Etienne, St Hilaire, 40, 52-78, 141
+  Unity of Plan, 52-65, 70 ff., as conservative, 75, 78
+  Principle of Connections, 53-4, 62-3, 71, 74, 261
+  Unity of Composition, 54, 70-1, 75-6, 200, 305
+  Archetype, 54, 67
+  Metastasis, 55-6, 59, 74
+  Opercular Bones, 56
+  Unity of Composition of Sternum, 57-60
+  Classification, 60
+  Vertebrates and Articulates, 60-4, 274, 278-9, 303
+  Formal Attitude, 62-3, 65, 71, 75-8, 305
+  Cephalopods and Vertebrates, 64-5
+  Scale of Beings, 64
+  Polemic with Cuvier, 64-5, 74-8
+  Evolution, 66-9, 73, 228
+  Biogenetic Law, 69
+  Teratology, 69, 315
+  Meckel-Serres Law, 70, 72
+  Criteria of Homology, 71, 72, 110
+  Law of Compensation, 72-3
+  Criticism of his Principles, 74
+  Relation to German Transcendentalists, 89, 100-1
+  Vertebral Theory of Skull, 96, 97
+  Influence on Darwin, 234-5, 238
+
+Geoffroy, Isidore, St Hilaire, 65 f.n., 199 f.n., 230
+
+Geometry of the Organism, 33
+  K. G. Carus, 98-100, 249
+  Bronn, 201, 249
+  Haeckel, J. Müller, Burmeister, G. Jäger, 249
+
+Germinal Vesicle (Egg-nucleus), 175-7, 188, 291 f.n.
+
+Germ-Layer Theory--
+  Von Baer, 115-6, 118-9, 208-9, 296
+  Pander, 119-20, 209
+  C. F. Wolff, 119-20
+  Rathke, 136, 208
+  Lereboullet, Bischoff, 208
+  Huxley, 208, 289
+  Remak, 209-12, 296
+
+Germ-Layers and Gastræa Theory--
+  Haeckel, 289-95
+  Lankester, Balfour, 295
+
+Germ-Layer Theory, Influence of Causal Morphology on, 347
+
+Gesner, 18
+
+Giard, A.--
+  On Ascidian Theory, 271-3
+  Adaptive Homology, 273
+  Poecilogeny, 347-8
+
+Glisson, F., 19
+
+Gluge, 173
+
+Goebel, K., 356 f.n.
+
+Goethe, 45-51, 65, 89, 250
+  Unity of Plan, 45-7, 51
+  Homology, 47
+  Principle of Connections, 47
+  Formal and Functional Attitudes, 48-50
+  Teleology, 48
+  Metamorphosis of Plants, 48
+  Repetition of parts, 48-9
+  Vertebral Theory of Skull, 49, 96, 97
+  Law of Compensation, 49
+  Embryological Criterion, 49
+  Organisms as Nature's Works of Art, 50
+
+Goette, 259
+
+Graaf, von, 175
+
+Grew, N., 169
+
+Gruber, 330
+
+HAECKEL, Ernst, 247-60, 271, 314, 342, 353, 357
+  His sources, 248-50
+  Materialism, 248, 250
+  On Teleology, Heredity and Adaptation, 248, 263
+  Correlation, Disregard for, 248, 264
+  Geometry of the Organism (Promorphology), 249
+  Repetition of Parts (Tectology), 249-50
+  Classification as Genealogical, 250-1, 254
+  Archetype as ancestral, 251
+  Homology and Analogy, 251
+  Biogenetic  Law, 251, 253-9, 291-4
+  Three-fold parallelism, 254-5
+  Scale of Beings, 255, 256-7
+  Organism as an Historical Being, 257
+  Prussianism, 257
+  Palingenesis, 258
+  Cenogenesis, 258-9
+  Heterotopy, Heterochrony, 259
+  Gastræa Theory, 269, 288-95
+  Phylogenetic Classification, 289-94
+  Criticism of Theory of Types, Monophyletism, 289, 291
+  Gastræa Theory and Biogenetic Law, 291-4
+  Primary stages of Ontogeny and Phylogeny, 291-3
+  Coelom, 291, 295, 296
+  Experimental Embryology, 317
+
+Haller, 113
+
+Harting, 284 f.n.
+
+Hartmann, E. von--
+  On Darwin's conception of correlation, 240-1
+  Evolution, 244, 356
+
+Hartog, M., 344
+
+Harvey, 18, 113
+
+Hatschek, 270 f.n., 299
+
+Helmholtz, H. von, 195
+
+Henle, 172
+
+Hensen, V., 209 f.n.
+
+Herbst, C., 333
+
+Herder, 46
+
+Heredity and Memory, 336-44
+
+Hering, E., 341-2
+
+"Heritage" Characters, 309, 322
+
+Herlitzka, 332
+
+Herophilus, 17
+
+Hertwig, O., 163, 330, 331, 346
+  On C. F. Wolff, 119
+  Fertilisation, 291 f.n.
+  Membrane and Cartilage Bones, 309-10
+  Biogenetic Law, 352, 354-5
+  Von Baer's Law, 354
+  Intrinsic Laws of Evolution, 354-5, 356-7
+  Homology not necessarily Homogeny, 355-7
+  Unity of Plan not necessarily due to Community of Descent, 355-7
+  On Phylogenetic Speculation, 356
+
+Hertwig, O. and R.--
+  Coelom Theory, 297-8
+  Nervous System of Coelentera, 299
+
+Heterochrony, 259, 348, 349-52
+
+Heterogeneous Generation (Kölliker), 243
+
+Heterotopy, 259
+
+Hilgendorf, 359
+
+Hill, 311
+
+Hippocratic Treatises, 2
+
+His, W., 206 f.n., 209 f.n.
+  Causal Morphology, 316
+  Cytoplasm of Egg, Organ-forming Stuffs, 333
+  Specific Distinctness of Embryos, 353
+
+Histological Differentiation (von Baer), 117-8
+
+Histology. _See also_ "Cell-Theory"
+  Malpighi, 20
+  Stensen, 21
+  Bichat, 29-30, 169, 178
+  Von Baer, 117-8
+  Schwann, 178
+  Remak, 209-12
+
+Hofer, B., 330
+
+Hofmeister, 185
+
+Homogeny, 267, 303, 355
+
+Homology, 168, 303, 355-7. _See also_ "Connections, Principle of," and
+   "Embryological Criterion"
+  Aristotle, 7-10
+  Belon, 18
+  Buffon, 27
+  Goethe, 47
+  E. Geoffroy, 53, 71
+  Serres, 80
+  Owen, 107-9
+  Lamarck, 227
+  C. Darwin, 234-5, 247
+  Haeckel, 251
+  Gegenbaur, 261, 263, 265, 266-7
+  Giard, 273
+  Semper, 279
+  O. Hertwig, 355-7
+  Braun, 355
+
+Homology, Genetic Definition of--
+  Gegenbaur, 266
+  Lankester, 267
+  O. Hertwig's criticism, 355-7
+
+Homoplasy, 267
+
+Hooke, R., 20, 169
+
+Houssay, F., 19 f.n., 333
+
+Hubrecht, A. A. W., 284, 295 f.n., 301, 305-6
+
+Hunter, J., 22, 315
+
+Huschke, 134-5, 136, 141, 146
+
+Huxley, T. H., 157, 238, 247
+  On Rathke, 154 f.n.
+  Embryological Criterion, 158-9, 166
+  Embryological Archetype, 159-61
+  Criticism of Vertebral Theory of Skull, 161-2
+  Membrane and Cartilage Bones, 166-7
+  On Archetype, 204
+  Germ-Layer Theory, 208, 289
+  Criticism of Three-fold Parallelism, 230 f.n.
+  Coelom, 297
+  Ancestry of Marsupials, 311
+
+Hyatt, A., 359, 361
+
+
+INSTINCT and Morphogenesis, Analogy of, vi., 307, 312
+  Lamarck, 220, 226
+
+JACOBSON , 164
+
+Jäger, G., 249 f.n.
+
+_Jardin des Plantes_, Paris, 19
+
+Jenkinson, J. W., 347 f.n.
+  On His, 316
+
+Jones, Wharton, 138, 176
+
+Julin, C., 271, 285
+
+Jussieu, de, 40
+
+
+KANT, I.--
+  Teleology, 35, 213, 242
+  Unity of Plan, 46, 213-4
+  Evolution, 213-4
+
+Keibel, F., 348, 349-50
+
+Kerkring, 131
+
+Kielmeyer, 89, 90, 96
+
+Kieser, 90
+
+Kleinenberg, N., 277
+
+Kohlbrugge, J., 44 f.n., 65 f.n.
+
+Kölliker, A.--
+  On C. F. Wolff, 119
+  Vertebral Theory of Skull, 157
+  Membrane and Cartilage Bones, 164-6, 310
+  Embryological Criterion, 165-6
+  Cell-division, 187
+  Intrinsic Laws of Evolution, 243, 356
+  Saltatory Variation, 243
+
+Kowalevsky, A., 269-71, 284, 285, 299, 300
+  Development of Amphioxus, 270
+    Ascidians, 270-1
+  Coelom, 270, 295, 297
+  Gastrula, 288
+
+Kowalevsky, V. O., 361-2
+
+Krause, 176
+
+Kupffer, 271
+
+
+LACAZE-DUTHIERS, H. de, 203 f.n., 315-6
+  On Ascidians, 271, 273
+
+Lamarck, 44, 66, 78, 215-29
+  Relation to Buffon, 215
+  Scale of Beings, 215-8, 220-1, 227-8
+  As Evolutionary, 218, 220
+  Classification, 216-7, 227, 228
+  Species Problem, 216, 227
+  Materialism, 218-9, 222-3, 225-6
+  Psychological Vitalism, 219, 220-6, 307, 335
+  _Sentiment intérieur_, 219-20, 222-3, 225
+  Ecological Adaptation, 221, 222, 223, 224, 227
+  Laws of Evolution, 221-5
+  Transmission of Acquired Characters, 221-2, 224
+  Subtle Fluids, 222
+  Use and Disuse, 223-4
+  Independence of Current Thought, 226-7
+  Homology and Analogy, 227
+  Reception of his Theory, 228-9
+  Lamarck and Butler, 335-7
+
+Lang, A., 301
+
+Lankester, Sir E. Ray, 247
+  Homology, Homogeny, Homoplasy, and Analogy, 267
+  _Balanoglossus_ Theory of Vertebrate Descent, 287
+  Germ-Layer Theory and Phylogenetic Classification, 291
+  Planula Theory, 295
+  On Coelom Theory, 296-7, 299 f.n.
+
+Latreille, 86, 100
+
+Laurencet, 64
+
+Lavocat, 203 f.n.
+
+Leeuenhoek, 20, 21, 169
+
+Leibniz, 23, 213, 343
+
+Lereboullet--
+  Von Baer's Law, 206-8
+  Germ-layer Theory, 208
+  Gastrula, 288 f.n.
+
+Leucippus, 16
+
+Leuckart, 193 f.n., 194, 297
+
+Levy, O., 333
+
+Leydig, 187, 188, 275 f.n., 285
+
+Linnæus, 22
+
+Loeb, J., 333, 347
+
+_Loi de Balancement_. _See_ "Compensation, Law of"
+
+Lovén, 186, 196
+
+Lucretius, 16
+  On the Soul, 222 f.n.
+
+Ludwig, 193, 194, 314
+
+Lyell, Sir C., 228 f.n.
+
+Lyonnet, 22
+
+
+MACBRIDE, E. W., 287 f.n.
+
+M'Kendrick, J.--
+  On Fontana, 172
+
+Mackenzie, W., 345
+
+Malpighi, M., 20-1, 113, 169
+
+Marine Zoology, Rise of, 195-6
+
+Materialistic Attitude, 246-7, 345, 364
+  Schwann, 180-5
+  Vogt, 190-1
+  Virchow, 191
+  Ludwig, 193
+  Materialistic Physiology, 193-4, 314-5, 347
+  Lamarck, 218-9, 222-3, 225-6
+  The Darwinians, 241, 308
+  Haeckel, 248, 250
+  Roux, 315, 317, 318-9, 329
+  Semon, 343
+  Rignano, 344
+  Loeb, 347
+  Criticism of this attitude--
+    Reichert, 192-3
+
+Meckel, D. A., 95
+
+Meckel, J. F., 113
+  Meckel-Serres Law, 91-3
+  Evolution, 92-3, 215, 228
+  Teratology, 93-4
+  Repetition of Parts, 95
+  Vertebral Theory of Skull, 96
+  Eclecticism, 101
+
+Meckel's Cartilage, 141, 145
+
+Meckel-Serres Law. _See_ "Development, Meckel-Serres Law"
+
+Mehnert, E., 348, 350-2
+
+Membrane and Cartilage Bones, 162-7, 309-10
+
+Memory and Heredity, 336-44
+
+Mendelism, 346
+
+Mesenchyme, 298
+
+Mesoderm, 209-11, 296, 297, 298
+
+Metabolism--
+  Cuvier, 38
+  Schwann, 182-5
+  Roux, 324, 329
+
+Metamerism, 94, 95, 100, 109, 131-2, 266-7, 274-5, 279, 282, 286, 299, 301
+
+Metamorphosis of Plants, 48, 235
+
+Metastasis, Principle of--
+  E. Geoffroy, 55-6, 59, 74
+  Owen, 106
+
+Metschnikoff, E., 278 f.n., 285, 288
+  Criticism of Ascidian Theory, 271
+  Coelom, 295, 296, 297
+
+Meyen, 170, 185
+
+Meyer, E., 284
+
+Meyranx, 64
+
+Microscope, Invention of, 19
+
+Milne-Edwards, H., 12, 86, 238
+  Marine Zoology, 195
+  Functional Attitude, 195, 197-200
+  Unity of Plan, 197
+  Division of Labour, 197-8
+  Ecological Adaptation, Convergence, 199
+  Von Baer's Law, Polemic with Serres, 204-8
+  Evolution, 244-5
+
+Mirbel, 170, 171
+
+Mivart, St G., 277
+
+Mohl, von, 170, 185
+
+Moldenhawer, 170
+
+Moleschott, 194
+
+Moquin-Tandon, A., 87
+
+Morgan, T. H., 317 f.n., 332, 333, 347 f.n.
+
+Mosaic Theory of Development, 330-3
+
+Müller, F., Biogenetic Law, 252-3, 254, 257
+
+Müller, H., 166
+
+Müller, J., 136, 209 f.n., 260, 285, 309, 345
+  Embryological Criterion, 138
+  Vertebral Theory of Skull, 142-4, 154, 157
+  On Reichert, 150
+  Cell Theory, 172-3
+  Division of Egg-nucleus, 188
+  Vitalism, 192
+  Marine Zoology, 196
+  Functional Attitude, 200
+
+Mutations (Waagen), 361 f.n.
+
+
+NAEGELI, 185, 243 f.n., 356
+
+_Naturphilosophie._ _See_ "Philosophy of Nature"
+
+Nesbitt, R., 162
+
+Neumayr, 357, 360
+
+Nussbaum, M., 330
+
+
+OKEN, L., 89, 113, 131, 134, 149
+  Meckel-Serres Law, 90-1
+  Teratology, 91
+  Repetition of Parts, 94-5
+  Serial Homology, 95-6, 100
+  Vertebral Theory, 96, 97, 98
+  On Geoffroy, 100
+  Influence on Serres, 205
+  Evolution, 215
+
+Ollier, 315
+
+Oppel, A., 318 f.n., 324 f.n., 327, 348-9
+
+Orr, H. F., 342
+
+Osborn, H. F., 214 f.n., 361
+  On V. O. Kowalevsky, 362
+  Functional Attitude, 362-4
+  Law of Adaptive Radiation, 362-4
+
+Owen, R., 97, 102-12, 204
+  Eclecticism, 102
+  Vertebral Theory of Skeleton, 103-7
+  Archetype of Vertebrate Skeleton, 104-7, 110
+  Vertebral Theory of Skull, 104-6
+  Metastasis, 106
+  Principle of Connections, 107-8
+  Anatomy and Embryology, 108
+  Homology and Analogy, 108
+  Classes of Homology, 108-9, 266
+  Functional Attitude, 109, 110, 111
+  Embryological Criterion, 110, 167
+  Homological and Teleological Compoundedness, 110-1
+  Vegetative Repetition of Parts, 111, 286
+  Unity of Plan as Conservative Principle, 112
+  Influence on Darwin, 234, 235, 238
+  Evolution, 244
+
+
+PACKARD, 361
+
+Palæontological Record, 357-61
+  Absence of connecting forms, 357-9
+  Biogenetic Law, 359
+  Phyletic Series, 359-61
+
+Palæontological Succession--
+  Cuvier, 43
+  E. Geoffroy, 67
+  L. Agassiz, 230, 255
+  C. Darwin, 231
+  Milne-Edwards, 245
+  Tiedemann, 255 f.n.
+
+Paley, W., 341
+
+Palingenesis (Haeckel), 258, 323
+
+Pander, 113, 119-20, 133, 208, 209
+
+Parallelism, Theory of. _See_ "Development, Meckel-Serres Law"
+  Three-fold. _See_ "Development, Meckel-Serres Law"
+
+Paris Museum of Natural History, 19, 89, 101
+
+Paul, 360
+
+Pauly, A., 345
+
+Perrault, C., 19
+
+Perrier, E., 88, 359 f.n.
+
+Pflüger, E., 317, 330
+
+Philipeaux, 315
+
+"Philosophy of Nature," 89, 94, 98, 203, 248
+
+Phyletic Series, 359-61
+
+Physiology, Separation from Morphology, 194, 247, 260, 314
+
+Physiology of Development, 315
+
+Planula Theory (Lankester), 295
+
+Plato, 15
+
+Pockels, 138
+
+Poecilogeny (Giard), 347-8
+
+Poli, 175
+
+Polyphyletism--
+  Darwin, 238
+  Von Baer, 242, 356
+  Kölliker, Wigand, Naegeli, 356
+  Depéret, 360-1
+  Steinmann, 360 f.n.
+
+Polyzoic Conception of Organism--
+  Dugès, 87
+  Perrier, 88
+
+Prévost and Dumas, 125 f.n., 134, 175, 186
+
+Promorphology (Haeckel), 249
+
+Protoplasm, 169, 188-9
+
+Purkinje, 172, 173, 175, 176, 189
+
+
+QUATREFAGES, A. de, 172, 195-6
+
+
+RÁDL, E., on Goethe, 48
+  Correlation, 240 f.n., 241
+  On Darwin's Critics, 242 f.n.
+  On Cuvier's Critics, 278 f.n.
+
+Rathke, H., 133, 136-7, 174, 194, 269, 351 f.n.
+  Discovery of Gill-slits in Pig and Chick, 134
+  Discovery of Gill-slits in Man, 135
+  Germ-Layer Theory, 136, 208
+  Embryological Criterion, 138, 140-1
+  Homologies of Gill-arches, 139-41, 146, 150
+  Development of Skull, 141, 150-4
+  Vertebral Theory of Skull, 141, 154-6
+  Embryological Archetype, 151, 153
+  Membrane and Cartilage Bones, 163, 166
+
+Rauber, A., 330
+
+Réaumur, 22, 315
+
+Recapitulation Theory. _See_ "Development, Biogenetic Law"
+
+Regeneration, 315, 318, 333, 346
+
+Regulatory Processes in Development, 114, 319, 333, 346-7, 350
+
+Reichert, C. B., Embryological Criterion, 138-9, 144-7, 163
+  Archetype, 139, 147, 149
+  Homologies of Gill-arches and Ear-ossicles, 144-7
+  Vertebral Theory of Skull, 147-9, 157
+  Von Baer's Law, 149-50, 351 f.n.
+  Membrane and Cartilage Bones, 163, 165, 166, 310
+  Criticism of "Biological Atomists," 192-3, 194
+  Functional Attitude, 193, 200
+
+Remak, R., 118, 288 f.n.
+  On Vertebræ, 157
+  Cell Theory, 173, 187-8, 209
+  Microscopical Technique, 209 f.n.
+  Germ-Layer Theory, 209-12, 296
+  Cells, Tissues and Germ-Layers, 209-12
+  Mesoderm, 209-11
+  Coelom, 211, 296
+
+Repetition of Parts within the Organism, Theory of. _See also_
+   "Vertebral Theory of Skull"
+  Goethe, 48-9
+  Dugès, 87-8
+  Oken, 94-5
+  J. F. Meckel, D. A. Meckel, 95
+  Haeckel (Tectology), 249-50
+
+Reymond, E. du Bois, 194, 314
+
+Rignano, E., 343-4
+
+Robinet, 23, 215
+
+Rondeletius, 18
+
+Rosenhof, Rösel von, 22
+
+Roux, W., 313, 315-29, 344, 351
+  _Entwicklungsmechanik_, 315, 317-8
+  Materialistic Attitude, 315, 317, 318-9, 329
+  Functional Adaptation, 316-7, 318, 320-9, 333
+  Experimental Embryology, 317, 318, 330-1
+  Simple and Complex Components, 318-20
+  Functional Definition of Life, 320
+  Functional Attitude, 320-9, 335
+  The Two Periods of Development, 320-4, 325, 327, 335
+  Mosaic Theory of Development, 323, 330-1
+  Metabolism, 324, 329
+  Structure, Functional and Non-functional, 324-6
+  Functional Unity of Organism, 326
+  Functional Adaptation of Blood-vessels, 326-9
+  Form as manifestation of Activity, 329
+
+Ruini, C., 18
+
+Rusconi, 133-4, 186
+
+Rütimeyer, L., 361
+
+Ryder, 361
+
+
+SACHS, J. von, 170
+
+St Ange, M., 146
+
+Salensky, 259
+
+Saltatory Variation--
+  E. Geoffroy, 78
+  Von Baer, 242
+  Kölliker, 243
+  Owen, 244
+
+Sarcode, 169
+
+Sars, M., 186, 196
+
+Savigny, J. C., 83-5, 100, 137, 271
+
+Scale of Beings, 89, 206, 214-5
+  Aristotle, 14-6
+  Anaximander, Anaxagoras, 14
+  Empedocles, Plato, 15
+  Albertus Magnus, 17
+  C. Bonnet, 22-3
+  Robinet, 23
+  Buffon, 24
+  E. Geoffroy, 64
+  Lamarck, 215-8, 220-1, 227-8
+    As Evolutionary, 218, 220
+  Haeckel, 256-7
+  Criticism of this idea--
+    Cuvier, 39-40, 130
+    Von Baer, 130
+    Milne-Edwards, 205
+    Lereboullet, 207
+    Darwin, 234
+    Haeckel, 255
+    Relation to Evolution-Theory, 214-5
+
+Schepelmann, 333
+
+Schleiden, 170-2
+
+Schmieden, 328
+
+Schults, C. H., 173
+
+Schultze, Max, 189
+
+Schultze, O., 331
+
+Schulz, E., 347 f.n.
+
+Schwann, Theodor, 169, 173-86, 248
+  Physiological Standpoint, 173, 179, 180, 182
+  Development of Cells, 174-5, 179-80
+  Cellular Nature of Ovum, 175-7
+  Development of Tissues from Cells, 177-8
+  Histology, 178
+  Materialism and Teleology, 180-3, 185
+  Cell-metabolism, 182-5
+  Cells as organic Crystals, 184-5
+
+Sedgwick, A., 347 f.n.
+  Actinozoan Theory of Vertebrate Descent, 299-300
+  Metamerism, 299
+  Embryological Archetype, 300
+  Organism as Historical Being, 308
+  Cell-Theory, 346
+  Von Baer's Law, 353
+
+Segmentation of Ovum, 186-8
+
+Seiler, 138
+
+Selection, Natural and Artificial, 307 f.n.
+
+Self-Differentiation (Roux), 319, 320-1, 322, 323, 324, 327
+
+Self-Regulation (Roux), 319
+
+Semon, R., 342-3
+
+Semper, C., 259, 269, 278-82, 284, 286
+  Annelid Theory, 274, 278-82
+  Metamerism, 274, 279, 282
+  Follower of Geoffroy, 278
+  Unity of Plan and Composition, 279, 303
+  Principle of Connections, 279
+  Formal Attitude, 279
+
+_Sentiment intérieur_ (Lamarck), 219-20, 222-3, 225
+
+Serial Homology. _See_ "Metamerism"
+
+Serres, E., 79-83, 91, 100, 205-6, 257 f.n.
+  Criteria of Homology, 80
+  Law of parallelism, 80-3, 94, 203-4, 205-6
+  Law of Multiple Formation, 80-1
+  Unity of Plan, 83, 205, 206
+  Teratology, 83
+  Meckel's Cartilage, 145 f.n.
+  Transcendentalism, 205-6
+  Concrescence Theory, 206 f.n.
+
+Severino, 18
+
+Sharpey, 162, 176
+
+Siebold, von, 186
+
+Skull, Development of, 139-62.
+  _See also_ "Vertebral Theory"
+
+Spallanzani, 315
+
+Species-Problem--
+  Cuvier, 42
+  Lamarck, 216, 227
+  Darwin, 231
+
+Spencer, H., 326 f.n.
+
+Spengel, 285, 287
+
+Spinoza, 343
+
+Spix, 96, 97, 100, 141
+
+Stannius, 165
+
+Steenstrup, 309
+
+Steinmann, G., 357, 360 f.n.
+
+Stensen (Steno), 21
+
+Swammerdam, 20, 21-2
+
+
+TACHYGENESIS, 359
+
+Technique, Microscopical, 209 f.n., 268
+
+Tectology (Haeckel), 249
+
+Teleology--
+  Aristotle, 10
+  Cuvier, 33-5
+  Kant, 35, 213, 242
+  Von Baer, 242
+  Owen, Von Hartmann, 244
+  Butler, 341
+  G. Wolff, Driesch, 346
+  Criticism of--
+    Goethe, 48
+    Schwann, 180-2
+    The Darwinians, 241
+    Haeckel, 248
+    Evolutionary Morphologists, 308
+
+Teratology, 69, 83, 91, 93, 315
+
+Thienemann, 23 f.n.
+
+Thompson, D'Arcy W., 2 f.n.
+
+Thomson, A., 176
+
+Thomson, J. Arthur, 215 f.n.
+
+Tiedemann, 91, 113, 215, 255 f.n.
+
+Tissues and Germ-Layers, 118, 209-12
+
+Transcendental Anatomy, Relation to Evolutionary Morphology, 302-8, 312
+
+Transcendentalism, French and German Schools, 89, 100
+
+Trembley, 22, 315
+
+Treviranus, 141, 170, 215, 225 f.n.
+
+Turpin, 173
+
+Types, Theory of (Cuvier and Von Baer)--
+  Cuvier, 41, 124, 289, 291
+  Von Baer, 123-4, 289, 291
+  Bronn, 202
+  Lereboullet, 207
+
+Types, Theory of (Cuvier and Von Baer)--_contd._
+  Criticised by--
+    E. Geoffroy, 60
+    Haeckel, 289, 291
+    Lankester, 291
+
+Type-Theory and Evolution, 304
+
+
+UNGER, 185
+
+Unity of Composition, Principle of, Geoffroy, 54, 70-2, 75-6, 200, 305
+
+Unity of Plan, 88, 241, 278-9, 303, 312. _See also_ "Archetype"
+  Aristotle, 6-7, 10
+  Belon, Severino, 18
+  Perrault, 19
+  Robinet, 23
+  Buffon, 24
+  Cuvier, 41
+  Goethe, 45-7, 51
+  Vicq D'Azyr, 45
+  Camper, 45, 46
+  Herder, 46
+  Kant, 46, 213-4
+  E. Geoffroy, 52-65, 70 ff.
+  Serres, 83, 205, 206
+  Savigny, 83
+  Audouin, 85-6
+  Latreille, 86
+  Dugès, 86-7
+  J. F. Meckel, 101
+  Milne-Edwards, 197
+  Semper, 279
+  Haeckel, 289, 291
+  Lankester, 291
+
+Unity of Plan as due to Community of Descent--
+  Darwin, 233, 234-5, 239, 247
+  Haeckel, 250-1
+  Gegenbaur, 263 f.n., 265
+  Criticism of this idea--
+    O. Hertwig, 355-7
+
+Unity of Plan as Conservative Principle--
+  E. Geoffroy, 75, 78
+  Owen, 112
+  Gegenbaur, 263-4
+  Evolutionary Morphologists, 307
+
+
+VALENTIN, 138, 173, 176
+
+Variation, Limits of, Cuvier, 42
+
+Vegetative Repetition of Parts--
+  Owen, 111, 286
+  Bateson, 286
+
+Velpeau, 138
+
+Vertebral Theory of Skull, 49, 96-9, 104-6, 131, 141-4, 147-9, 154-7,
+  161-2, 165, 203, 235, 310 f.n.
+
+Vertebrate Descent, 269-87, 299-301, 304
+
+Verworn, M., 330
+
+Vesalius, 18
+
+Vestigial Organs, 233, 237, 309, 312
+
+Vialleton, L., 306 f.n., 348
+
+Vicq d'Azyr, 45, 95
+
+Virchow, R., 188, 191
+
+Vitalism, Psychological--
+  Lamarck, 219, 220-6, 307, 335
+  Butler, 336-41
+  Orr, Cope, 342
+  Ward, 343
+  Delpino, Francé, Pauly, A. Wagner, Mackenzie, 345
+
+Vogt, C.--
+  Criticism of Vertebral Theory, 156-7
+  Capillaries, 179
+  Segmentation, 186
+  Materialistic Attitude, 190-1
+  Threefold Parallelism, 255 f.n.
+
+
+WAAGEN, 359, 361 f.n.
+
+Wagner, A., 345
+
+Wagner, R., 176
+
+Ward, J., 343
+
+Weber, 138
+
+Weismann, A., 240, 323, 326 f.n., 330-1, 343
+
+Werneck, 173
+
+Whitman, C. O., 346
+
+Wigand, A., 242 f.n., 356
+
+Willey, A., 273 f.n., 306 f.n.
+
+Williamson, 309
+
+Willis, 19
+
+Wilson, E. B., 331, 332-3, 346 f.n., 347 f.n.
+
+Wolff, C. F., 113
+  Germ-layer Theory, 119-20
+  Cells, 170
+
+Wolff, G., 346-7
+
+Woodward, B. B., 358
+
+Wotton, E., 17
+
+
+ZELENY, 333
+
+Zittel, K. von, 357, 358
+
+Zoja, 331
+
+       *       *       *       *       *
+
+PRINTED BY
+
+OLIVER AND BOYD,
+
+EDINBURGH, SCOTLAND
+
+       *       *       *       *       *
+
+HEREDITY. By J. Arthur Thompson, M.A., LL.D., Regius Professor
+  of Natural History in the University of Aberdeen. With numerous
+  Illustrations. 9s. _net_.
+
+CONTENTS
+
+Heredity and Inheritance: Defined and Illustrated--The Physical Basis
+  of Inheritance--Heredity and Variation--Common Modes of
+  Inheritance--Reversion and allied Phenomena--Telegony and other
+  Dispute Questions--The Transmission of Acquired Characters--Heredity
+  and Disease--Statistical Study of Inheritance--Experimental Study of
+  Inheritance--History of Theories of Heredity and Inheritance--Heredity
+  and Development--Heredity and Sex--Social Aspects of Biological
+  Results--Bibliography--Subject-Index to Bibliography--Index.
+
+"We all know books of science which we ought to read with pleasure,
+but to which we turn with shrinking. Full, perhaps, of new facts and
+ideas, they are so expressed as to bore consumedly. 'Heredity' belongs
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+
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+
+"May be regarded as the standard work of reference on this subject. As
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+
+
+RECENT ADVANCES IN THE STUDY OF VARIATION, HEREDITY AND EVOLUTION.
+  By Robert H. Lock, M.A., sometime Fellow of Gonville and Caius
+  College, Cambridge, late Assistant-Director of the Royal Botanic
+  Gardens, Ceylon. With Portraits and other Illustrations. Revised by L.
+  Doncaster, D.Sc., F.R.S. With a Biographical Note by Bella Sidnay
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+
+
+MICROSCOPY. The Construction, Theory, and Use of the Microscope. By
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+
+
+CONVERGENCE IN EVOLUTION. By Arthur Willey, D.Sc. (Lond.); Hon. M.A.
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+
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+
+THE HEREDITY OF ACQUIRED CHARACTERS IN PLANTS. An aspect of the true
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+A HANDBOOK OF PHYSIOLOGY. By W.D. Halliburton, M.D., F.R.S., Professor
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+
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+
+
+THE BACTERIOLOGY OF MILK. By Harold Swithinbank, of the
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+BACTERIOLOGY AND THE PUBLIC HEALTH. By Sir George Newman, M.D., F.R.S.
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+
+
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+
+Professor T.G. Bonney says in _Nature_, 12th Feb. 1914:--"But we must
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+will be a great boon to students, for it is a veritable encyclopædia,
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+
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+MECHANISM, LIFE AND PERSONALITY. An Examination of the Mechanistic
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+<pre>
+
+Project Gutenberg's Form and Function, by E. S. (Edward Stuart) Russell
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Form and Function
+       A Contribution to the History of Animal Morphology
+
+Author: E. S. (Edward Stuart) Russell
+
+Release Date: January 23, 2007 [EBook #20426]
+
+Language: English
+
+Character set encoding: UTF-8
+
+*** START OF THIS PROJECT GUTENBERG EBOOK FORM AND FUNCTION ***
+
+
+
+
+Produced by Suzanne Lybarger, Turgut Dincer and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
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+
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+
+
+
+
+</pre>
+
+
+
+<p>&nbsp;</p>
+
+<h1>FORM AND FUNCTION</h1>
+
+<h3>A CONTRIBUTION TO THE</h3>
+
+<h3>HISTORY OF ANIMAL MORPHOLOGY</h3>
+
+<h3>&nbsp;</h3>
+
+<h3>By E. S. RUSSELL,</h3>
+
+<h4>M.A., B.Sc., F.Z.S.</h4>
+
+<h3>&nbsp;</h3>
+
+<h4>ILLUSTRATED</h4>
+
+<h3>&nbsp;</h3>
+
+<h4>LONDON</h4>
+
+<h4>JOHN MURRAY, ALBEMARLE STREET, W.</h4>
+
+<h4>1916</h4>
+
+<h5><i>All rights reserved</i></h5>
+
+<div class="trans-note">Transcriber's Note: Obvious printer errors have been corrected, all other inconsistencies
+in spelling and punctuation are as in the original.</div>
+
+<hr style="width: 65%;" />
+
+<h3>PREFACE</h3>
+
+<p><span class="smcap">This</span> 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.</p>
+
+<p>The main currents of morphological thought are to my mind
+three&mdash;the functional or synthetic, the formal or
+transcendental, and the materialistic or disintegrative.</p>
+
+<p>The first is associated with the great names of Aristotle,
+Cuvier, and von Baer, and leads easily to the more open vitalism of
+Lamarck and Samuel Butler. The typical representative of the second
+attitude is E. Geoffroy St. Hilaire, and this habit of thought has
+greatly influenced the development of evolutionary morphology.</p>
+
+<p>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&mdash;organisation or activity?</p>
+
+<p>The materialistic attitude is not distinctively biological, but
+is common to practically all fields of thought. It dates back to
+the Greek atomists, and the triumph of mechanical science in the
+19th century has induced many to accept materialism as the only
+possible scientific method. In biology it is more akin to the
+formal than to the functional attitude.</p>
+
+<p>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 <a name="pgvi" id=
+"pgvi"></a>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&mdash;which many of us have had drilled out of
+us&mdash;that they are active, purposeful agents, not mere
+complicated aggregations of protein and other substances. Such an
+attitude is probably quite as sound philosophically as the opposing
+one, but I have not in this place attempted any justification of
+it. I have touched very lightly upon the controversy between
+vitalism and materialism which has been revived with the early
+years of the present century. It hardly lends itself as yet to
+historical treatment, and I could hardly hope to maintain with
+regard to it that objective attitude which should characterise the
+historian.</p>
+
+<p>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.</p>
+
+<p>I owe a debt of gratitude to my friends Dr James F. Gemmill and
+Prof. J. Arthur Thomson for much kindly encouragement and helpful
+criticism. The credit for the illustrations is due to my wife, Mrs
+Jehanne A. Russell. One is from Nature; the others are drawn from
+the original figures.</p>
+
+<p class="right">E. S. R.</p>
+
+<p><span class="smcap"><small>&nbsp;&nbsp;Chelsea,
+1916.</small></span></p>
+
+<hr style="width: 65%;" />
+
+<h2>CONTENTS</h2>
+
+<table summary="TOC" border="0" cellpadding="2" cellspacing="10">
+<tr>
+<td class="cell_lt0"><small>CHAP.</small></td>
+<td class="cell_mid0"></td>
+<td class="cell_rt0"><span class="smcap">Page</span></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">I.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">The Beginnings of
+Comparative Anatomy</span></p></td>
+<td class="cell_rt0"><a href="#pg001">1</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">II.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Comparative Anatomy
+before Cuvier</span></p></td>
+<td class="cell_rt0"><a href="#pg017">17</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">III.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Cuvier</span></p></td>
+<td class="cell_rt0"><a href="#pg031">31</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">IV.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Goethe</span></p></td>
+<td class="cell_rt0"><a href="#pg045">45</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">V.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Etienne Geoffroy St
+Hilaire</span></p></td>
+<td class="cell_rt0"><a href="#pg052">52</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">VI.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">The Followers of Etienne
+Geoffroy St Hilaire</span></p></td>
+<td class="cell_rt0"><a href="#pg079">79</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">VII.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">The German
+Transcendentalists</span></p></td>
+<td class="cell_rt0"><a href="#pg089">89</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">VIII.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Transcendental Anatomy in
+England&mdash;Richard Oven</span></p></td>
+<td class="cell_rt0"><a href="#pg102">102</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">IX.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Karl Ernst von
+Baer</span></p></td>
+<td class="cell_rt0"><a href="#pg113">113</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">X.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">The Embryological
+Criterion</span></p></td>
+<td class="cell_rt0"><a href="#pg133">133</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">XI.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">The
+Cell-Theory</span></p></td>
+<td class="cell_rt0"><a href="#pg169">169</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">XII.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">The Close of the
+Pre-evolutionary Period</span></p></td>
+<td class="cell_rt0"><a href="#pg190">190</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">XIII.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">The Relation of Lamarck
+and Darwin to Morphology</span></p></td>
+<td class="cell_rt0"><a href="#pg213">213</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">XIV.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Ernst Haeckel and Carl
+Gegenbaur</span></p></td>
+<td class="cell_rt0"><a href="#pg246">246</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">XV.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Early Theories on the
+Origin of Vertebrates</span></p></td>
+<td class="cell_rt0"><a href="#pg268">268</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">XVI.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">The Germ-layers and
+Evolution</span></p></td>
+<td class="cell_rt0"><a href="#pg288">288</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">XVII.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">The Organism as an
+Historical Being</span></p></td>
+<td class="cell_rt0"><a href="#pg302">302</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">XVIII.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">The Beginnings of Causal
+Morphology</span></p></td>
+<td class="cell_rt0"><a href="#pg314">314</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">XIX.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Samuel Butler and the
+Memory Theories of Heredity</span></p></td>
+<td class="cell_rt0"><a href="#pg335">335</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">XX.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">The Classical Tradition
+in Modern Morphology</span></p></td>
+<td class="cell_rt0"><a href="#pg345">345</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0"></td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Index</span></p></td>
+<td class="cell_rt0"><a href="#pg365">365</a></td>
+</tr>
+</table>
+
+<hr style="width: 65%;" />
+<h2>ILLUSTRATIONS</h2>
+
+<table summary="Illustrations" border="0" cellpadding="2"
+cellspacing="10">
+<tbody>
+<tr>
+<td class="cell_lt0"><small>FIG.</small></td>
+<td class="cell_mid0">&nbsp;</td>
+<td class="cell_rt0"><span class="smcap">Page</span></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">1.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Hyoid Arch of the Conger.
+(Original.)</span></p></td>
+<td class="cell_rt0"><a href="#pg058">58</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">2.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">"Vertebra" of a
+Pleuronectid. (Geoffroy.)</span></p></td>
+<td class="cell_rt0"><a href="#pg061">61</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">3.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Abdominal Segment of the
+Lobster. (Geoffroy.)</span></p></td>
+<td class="cell_rt0"><a href="#pg063">63</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">4.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Ideal Typical Vertebra.
+(Owen.)</span></p></td>
+<td class="cell_rt0"><a href="#pg102">102</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">5.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Natural Typical Vertebra.
+(Owen.)</span></p></td>
+<td class="cell_rt0"><a href="#pg103">103</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">6.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">The Archetype of the
+Vertebrate Skeleton. (Owen.)</span></p></td>
+<td class="cell_rt0"><a href="#pg105">105</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">7.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Ideal Transverse Section
+of a Vertebrate Embryo. (Von Baer.)</span></p></td>
+<td class="cell_rt0"><a href="#pg119">119</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">8.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Gill-slits of the Pig
+Embryo. (Rathke.)</span></p></td>
+<td class="cell_rt0"><a href="#pg134">134</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">9.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Meckel's Cartilage and
+Ear-ossicles in Embryo of Pig. (Reichert.)</span></p></td>
+<td class="cell_rt0"><a href="#pg145">145</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">10.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Cranial Vertebr&aelig;
+and Visceral Arches in Embryo of Pig. (Reichert.)</span></p></td>
+<td class="cell_rt0"><a href="#pg148">148</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">11.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Embryonic Cranium of the
+Adder. (Rathke.)</span></p></td>
+<td class="cell_rt0"><a href="#pg152">152</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">12.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Transverse Section of
+Chick Embryo. (Remak.)</span></p></td>
+<td class="cell_rt0"><a href="#pg211">211</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">13.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Development of the
+Ascidian Larva (Kowalevsky.)</span></p></td>
+<td class="cell_rt0"><a href="#pg272">272</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">14.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">Transverse Section of the
+Worm <i>Nais</i>. (Semper.)</span></p></td>
+<td class="cell_rt0"><a href="#pg280">280</a></td>
+</tr>
+
+<tr>
+<td class="cell_lt0">15.</td>
+<td class="cell_mid0"><p class="one"><span class="smcap">The Five Primary Stages
+of Ontogeny. (Haeckel.)</span></p></td>
+<td class="cell_rt0"><a href="#pg292">292</a></td>
+</tr>
+</tbody>
+</table>
+
+
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg001" id=
+"pg001">001</a></span></p>
+
+<h2>FORM AND FUNCTION</h2>
+
+<h3>CHAPTER I</h3>
+
+<h4>THE BEGINNINGS OF COMPARATIVE ANATOMY</h4>
+
+<p><span class="smcap">The</span> first name of which the history
+of anatomy keeps record is that of Alcmaeon, a contemporary of
+Pythagoras (6th century <span class="smcap">B.C.</span>). 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.<a name="FNanchor_1" id=
+"FNanchor_1" /><a href="#Footnote_1" class="fnanchor">[1]</a>
+Both show his bias towards looking at the functional side of living
+things. The latter comparison reappears in Aristotle.</p>
+
+<p>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.<a name="FNanchor_2" id=
+"FNanchor_2" /><a href="#Footnote_2" class=
+"fnanchor">[2]</a></p>
+
+<p>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."<a name="FNanchor_3" id=
+"FNanchor_3" /><a href="#Footnote_3" class=
+"fnanchor">[3]</a></p>
+
+<p><span class="pagenum"><a name="pg002" id=
+"pg002">002</a></span>In the collected writings of Hippocrates and
+his school, the <i>Corpus Hippocraticum</i>, 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."<a name="FNanchor_4"
+id="FNanchor_4" /><a href="#Footnote_4" class=
+"fnanchor">[4]</a></p>
+
+<p>The author of the treatise "On the Joints," which Littr&eacute;
+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.</p>
+
+<p>Aristotle (384-322 <span class="smcap">B.C.</span>)<a name="FNanchor_5" id=
+"FNanchor_5" /><a href="#Footnote_5" class="fnanchor">[5]</a>
+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&mdash;in the <i>Historia Animalium</i>, the <i>De Partibus
+Animalium</i>, and the <i>De Generatione Animalium</i>.</p>
+
+<p>The <i>Historia Animalium</i> is a most comprehensive work, in
+some ways the finest text-book of Zoology ever written. Certainly
+few modern text-books take such a broad and sane view of living
+creatures. Aristotle never forgets that form and structure are but
+one of the many properties of living things; he takes quite as much
+interest in their behaviour, their ecology, distribution,
+comparative physiology. He takes a special interest in the
+comparative physiology of reproduction. The <i>Historia
+Animalium</i> contains a description of the form and structure of
+man and of as many animals as Aristotle was acquainted
+with&mdash;and he was acquainted with an astonishingly large
+number. The later <i>De Partibus Animalium</i> is a treatise on
+the causes of the form and <span class="pagenum"><a name="pg003"
+id="pg003">003</a></span>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.</p>
+
+<p>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.</p>
+
+<p>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 (<i>De Gen.</i>, ii.
+7).</p>
+
+<p>Speaking generally, however, we find in Aristotle no purely
+morphological concepts. What then does morphology owe to Aristotle?
+It owes to him, <i>first</i>, a great mass of facts about the
+structure of animals; <i>second</i>, the first scientific
+classification of animals;<a name="FNanchor_6" id=
+"FNanchor_6" /><a href="#Footnote_6" class="fnanchor">[6]</a>
+<i>third</i>, a clear enunciation of the fact of community of plan
+within each of the big groups; <i>fourth</i>, an attempt to explain
+certain instances of the correlation of parts; <i>fifth</i>, a
+pregnant distinction between homogeneous and heterogeneous parts;
+<i>sixth</i>, a generalisation on the succession of forms in
+development; and <i>seventh</i>, the first enunciation of the idea
+of the <i>&Eacute;chelle des &ecirc;tres</i>.</p>
+
+<p>(1) What surprises the modern reader of the <i>Historia
+Animalium</i> perhaps more than anything else is the extent and
+variety of Aristotle's knowledge of animals. He <span class=
+"pagenum"><a name="pg004" id="pg004">004</a></span>describes more
+than 500 kinds.<a name="FNanchor_7" id="FNanchor_7" /><a href=
+"#Footnote_7" class="fnanchor">[7]</a> 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 (<i>Palinurus</i>), 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 larv&aelig;. 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 (<i>Hist.
+Anim.</i>, ii.), or his description of the structure of cuttlefish
+(<i>Hist. Anim.</i>, iv), or that touch in the description of the
+hermit crab (<i>Hist. Anim.</i>, iv.)&mdash;"Two large eyes ...
+not ... turned on one side like those of crabs, but straight
+forward"&mdash;without being convinced that Aristotle is speaking
+of what he has seen. Naturally he could not make much of the
+anatomy of small insects and snails, and, to tell the truth, he
+does not seem to have cared greatly about the minuti&aelig; of
+structure. He was too much of a Greek and an aristocrat to care
+about laborious detail.</p>
+
+<p>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.</p>
+
+<p>(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 <span class=
+"pagenum"><a name="pg005" id="pg005">005</a></span>might, in
+addition to structural classifications, divide animals into
+gregarious, solitary and social, or land animals into troglodytes,
+surface-dwellers, and burrowers (<i>Hist. Anim.</i>, i.).</p>
+
+<p>He knew that dichotomous classifications were of little use for
+animals (<i>De Partibus</i>, i. 3) and he explicitly and in so many
+words accepted the principle of all "natural" classification, that
+affinities must be judged by comparing not one but the sum total of
+characters. As everyone knows, he was the first to distinguish the
+big groups of animals, many of which were already distinguished
+roughly by the common usages of speech. Among his Sanguinea he did
+little more than define with greater exactitude the limits of the
+groups established by the popular classification. Among the
+"exsanguineous" animals, however, corresponding to our
+Invertebrates, he established a much more definite classification
+than the popular, which is apt to call them indiscriminately
+"shellfish," "insects," or "creeping things." He went beyond the
+superficialities of popular classification, too, in clearly
+separating Cetacea from fishes. He had some notion of species and
+genera in our sense. He distinguished many species of
+cuttlefish&mdash;<i>Octopus (Polypus)</i> of which there were many
+kinds, <i>Eledone (Moschites)</i> which he knew to have only one
+row of suckers while <i>Octopus</i> has two, <i>Argonauta,
+Nautilus, Sepia</i>, and apparently <i>Loligo media</i> (= his
+Teuthis) and <i>L. vulgaris</i> (or <i>forbesii</i>) 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" (<i>Hist. Anim.</i>,
+Cresswell's trans., p.&nbsp;147, London, 1862).</p>
+
+<p>The groups he distinguished were&mdash;man, viviparous
+quadrupeds, oviparous quadrupeds, birds, fishes, Cetacea,
+Cephalopoda, Malacostraca (= higher Crustacea), Insecta (= annulose
+animals), Testacea (= molluscs, echinoderms, ascidians). A class of
+Acaleph&aelig;, including sea-anemones and <span class="pagenum"><a
+name="pg006" id="pg006">006</a></span>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.</p>
+
+<p>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, <i>loc. cit.</i>, p.&nbsp;90).</p>
+
+<p>(3) Aristotle's greatest service to morphology is his clear
+recognition of the unity of plan holding throughout each of the
+great groups.</p>
+
+<p>He recognises this most clearly in the case of man and the
+viviparous quadrupeds, with whose structure he was best acquainted.
+In the <i>Historia Animalium</i> 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, <i>loc. cit.</i>, 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, mamm&aelig;,
+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 mamm&aelig;. Instead of hair they have
+scales.</p>
+
+<p>Speaking generally, the Sanguinea differ from man and from one
+another in their parts, which may be present or absent, or exhibit
+differences in "excess and defect," or in form. Unity of plan
+extends to all the principal systems of organs. "All sanguineous
+animals have either a bony or a spinous column. The remainder of
+the bones exist in some animals; but not in others, for if they
+have the limbs they have the bones belonging to them" (Cresswell,
+<i>loc. cit.</i>, p.&nbsp;60). "Viviparous animals with blood and feet
+do not differ much in their bones, but rather by analogy, in
+hardness, softness, and size" (Cresswell, <i>loc. cit.</i>, p.&nbsp;59).
+<span class="pagenum"><a name="pg007" id="pg007">007</a></span>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, <i>loc.
+cit.</i>, p.&nbsp;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&mdash;between limbs and bones, and
+between blood-vessels and the parts to which they go.</p>
+
+<p>Sanguineous animals all possess certain organs&mdash;heart,
+liver, spleen, kidneys, and so on. Other organs occur in most of
+the classes&mdash;the &oelig;sophagus and the lungs. "The position
+which these parts occupy is the same in all animals [sc.
+Sanguinea]" (Cresswell, <i>loc. cit.</i>, p.&nbsp;39).</p>
+
+<p>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
+(<i>Palinurus</i>), and the astacus (<i>Homarus</i>), 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, <i>loc. cit.</i>, p.&nbsp;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, <i>loc. cit.</i>, p.&nbsp;85). In the last
+phrase we may perhaps read the first recognition of the
+embryological criterion.</p>
+
+<p>With the recognition of unity of plan within each group
+necessarily goes the recognition of what later morphology calls the
+homology of parts. The parts of a horse can be compared one by one
+with the parts of another viviparous quadruped; in all the animals
+belonging to the same class the parts are the same, only they
+differ in excess or defect&mdash;these remarks are placed in the
+forefront of the <span class="pagenum"><a name="pg008" id=
+"pg008">008</a></span><i>Historia Animalium</i>. 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
+(<i>Hist. Anim.</i>, 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.</p>
+
+<p>While Aristotle certainly recognised the existence of
+homologies, and even had a feeling for them, he did not clearly
+distinguish homology from analogy. He comes pretty near the
+distinction in the following passage. After explaining that in
+animals belonging to the same class the parts are the same,
+differing only in excess or defect, he says, "But some animals
+agree with each other in their parts neither in form nor in excess
+and defect, but have only an analogous likeness, such as a bone
+bears to a spine, a nail to a hoof, a hand to a crab's claw, the
+scale of a fish to the feather of a bird, for that which is a
+feather in the bird is a scale in the fish" (Cresswell, <i>loc.
+cit.</i>, p.&nbsp;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.</p>
+
+<p>In the <i>De Partibus Animalium</i> there occurs the following
+passage:&mdash;"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, <span class="pagenum"><a name="pg009" id=
+"pg009">009</a></span>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."<a name="FNanchor_8" id=
+"FNanchor_8" /><a href="#Footnote_8" class="fnanchor">[8]</a>
+It is thus similarity in form and structure which determines the
+formation of the main groups. Within each group the parts differ
+only in degree, in largeness or smallness, softness and hardness,
+smoothness or roughness, and the like (<i>loc. cit.</i>, i., 4,
+644<sup>b</sup>). 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 <i>organs</i>, in functioning parts, not in the
+mere spatial relationship of parts.</p>
+
+<p>This comes out clearly in his treatise <i>On the Parts of
+Animals</i>, which is subsequent to, and the complement of, his
+<i>History of Animals</i>. 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 <i>De Partibus
+Animalium</i> as follows:&mdash;"We have, then, first to describe
+the common functions, common, that is, to the whole animal kingdom,
+or to certain large groups, or to members of a species. In other
+words, we have to describe the attributes common to all animals, or
+to assemblages, like the class of Birds, of closely allied groups
+differentiated by gradation, or to groups like Man not
+differentiated into subordinate groups. In the first case the
+common attributes may be called analogous, in the second generic,
+in the third specific" (i, 5, 645<sup>b</sup>, 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, <i>loc. cit.</i>, p.&nbsp;6).</p>
+
+<p>The <i>De Partibus Animalium</i> becomes in form a comparative
+organography, but the emphasis is always on function and community
+of function. Thus he treats of bone, "fish-spine," and cartilage
+together (<i>De Partibus</i>, ii., 9, 655<sup>a</sup>), because
+they have the same function, though he says <span class=
+"pagenum"><a name="pg010" id="pg010">010</a></span>elsewhere that
+they are only analogous structures (ii., 8, 653<sup>b</sup>). In
+the same connection he describes also the supporting tissues of
+Invertebrates&mdash;the hard exoskeleton of Crustacea and Insects,
+the shell of Testacea, the "bone" of <i>Sepia</i> (ii., 8,
+654<sup>a</sup>). Aristotle took much more interest in analogies,
+in organs of similar function, than in homologies. He did recognise
+the existence of homologies, but rather <i>malgr&eacute; lui</i>,
+because the facts forced it upon him.</p>
+
+<p>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 (<i>De Partibus</i>, iv., 9, 684<sup>b</sup>). 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
+Acad&eacute;mie des Sciences, which formed the starting-point of
+the famous controversy between Cuvier and E. Geoffroy St Hilaire
+(see <a href="#pg052">Chap.&nbsp;V.</a>, below).</p>
+
+<p>Moreover, Aristotle did not so much compare a Cephalopod with a
+doubled-up Vertebrate as contrast Cephalopods (and also Testacea)
+with all other animals. Other animals have their organs in a
+straight line; Cephalopods and Testacea alone show this peculiar
+doubling up of the body.</p>
+
+<p>(4) Aristotle was much struck with certain facts of correlation,
+of the interdependence of two organs which are not apparently in
+functional dependence on one another. Such correlation may be
+positive or negative; the presence of one organ may either entail
+the presence of the other, or it may entail its absence. Aristotle
+has various ways of explaining facts of correlation. He observed
+that no animal has both tusks and horns, but this fact could easily
+be explained on the principle that Nature never makes anything
+superfluous or in vain. If an animal is protected by the possession
+of tusks it does not require horns, and <i>vice versa</i>. The
+correlation of a multiple stomach with deficient <span class=
+"pagenum"><a name="pg011" id="pg011">011</a></span>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! (<i>De Partibus</i>, iii., 14, 674<sup>b</sup>.) 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" (<i>De Partibus</i>, iii., 2,
+664<sup>a</sup>, trans. Ogle). A similar kind of explanation is
+offered of the fact that Selachia have cartilage instead of bone,
+"in these Selachia Nature has used all the earthy matter on the
+skin [<i>i.e.</i>, on the placoid scales]; and she is unable to
+allot to many different parts one and the same superfluity of
+material" (<i>De Partibus</i>, ii., 9, 655<sup>a</sup>, trans.
+Ogle). Speaking generally, "Nature invariably gives to one part
+what she subtracts from another" (<i>loc. cit.</i>, ii., 14,
+658<sup>a</sup>).</p>
+
+<p>This thought reappears again in the 19th century in E. Geoffroy
+St Hilaire's <i>loi de balancement</i> 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 (<i>loc. cit.</i>,
+iii., 2, 663<sup>b</sup>).</p>
+
+<p>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 f&oelig;tal membranes,
+and that most animals which have front teeth in both jaws and no
+horns have no cotyledons (<i>De Generatione</i>, ii., 7). He offers
+no explanation of this, but accepts it as a fact.</p>
+
+<p>We may conveniently refer here to one or two other ideas of
+Aristotle regarding the causes of form. He makes the <span class=
+"pagenum"><a name="pg012" id="pg012">012</a></span>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 (<i>De Partibus</i>, ii, 12). The fact of the
+inverse correlation is certain; the explanation is, though very
+vague, probably correct.</p>
+
+<p>In one passage of the <i>De Partibus</i> Aristotle clearly
+enunciates the principle of the division of labour, afterwards
+emphasised by H. Milne-Edwards. In some insects, he says, the
+proboscis combines the functions of a tongue and a sting, in others
+the tongue and the sting are quite separate. "Now it is better," he
+goes on, "that one and the same instrument shall not be made to
+serve several dissimilar ends; but that there shall be one organ to
+serve as a weapon, which can then be very sharp, and a distinct one
+to serve as a tongue, which can then be of spongy texture and fit
+to absorb nutriment. Whenever, therefore, Nature is able to provide
+two separate instruments for two separate uses, without the one
+hampering the other, she does so, instead of acting like a
+coppersmith who for cheapness makes a spit and lampholder in one"
+(iv., 6, 683<sup>a</sup>).</p>
+
+<p>(5) The first sentence of the <i>Historia Animalium</i>
+formulates, with that simplicity and directness which is so
+characteristic of Aristotle, the distinction between homogeneous
+and heterogeneous parts, in the mass the distinction between
+tissues and organs. "Some parts of animals are simple, and these
+can be divided into like parts, as flesh into pieces of flesh;
+others are compound, and cannot be divided into like parts, as the
+hand cannot be divided into hands, nor the face into faces. All the
+compound parts also are made up of simple parts&mdash;the hand, for
+example, of flesh and sinew and bone" (Cresswell, <i>loc. cit.</i>,
+p.&nbsp;1).</p>
+
+<p>In the <i>De Partibus Animalium</i> he broadens the conception
+by adding another form of composition. "Now there are," he says,
+"three degrees of composition; and of these the first in order, as
+all will allow, is composition out of what some call the elements,
+such as earth, air, water, fire.... <span class="pagenum"><a name=
+"pg013" id="pg013">013</a></span>The second degree of composition
+is that by which the homogeneous parts of animals, such as bone,
+flesh, and the like, are constituted out of the primary substances.
+The third and last stage is the composition which forms the
+heterogeneous parts, such as face, hand, and the rest" (ii., 1,
+646<sup>a</sup>, trans. Ogle).</p>
+
+<p>In the <i>Historia Animalium</i> the homogeneous parts are
+divided into (1) the soft and moist (or fluid), such as blood,
+serum, flesh, fat, suet, marrow, semen, gall, milk, phlegm,
+f&aelig;ces 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 <i>De
+Partibus Animalium</i> Aristotle, while still holding to the
+distinction set forth above, is alive to the fact that his simple
+parts include several different sorts of substances. He
+distinguishes among the homogeneous parts three sets. The first of
+these comprises the tissues out of which the heterogeneous parts
+are constructed, <i>e.g.</i>, flesh and bone; the second set form
+the nutriment of the parts, and are invariably fluid; while the
+third set are the residue of the second and constitute the residual
+excretions of the body (ii., 2, 647<sup>b</sup>). 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, 647<sup>b</sup>). Similarly for the heart and the other
+viscera. "The heart, like the other viscera, is one of the
+homogeneous parts; for, if cut up, its pieces are homogeneous in
+substance with each other. But it is at the same time heterogeneous
+in virtue of its definite configuration" (ii., 1, 647<sup>a</sup>,
+trans. Ogle).</p>
+
+<p>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 (<i>Historia Animalium</i>, iii., <span class=
+"pagenum"><a name="pg014" id="pg014">014</a></span>second half) is
+largely a comparative topographical anatomy; in it, for instance,
+he describes the venous and skeletal systems.</p>
+
+<p>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&mdash;"The capacity of action resides in
+the compound parts" (Cresswell, <i>loc. cit.</i>, p.&nbsp;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 (<i>De
+Generatione</i>, i., 18), but certain of them had other than purely
+physical properties, they were the organs of touch (<i>De
+Partibus</i>, ii., 1, 647<sup>a</sup>).</p>
+
+<p>(6) In a passage in the <i>De Generatione</i> (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 <a
+href="#pg113">Chap.&nbsp;IX.</a> below).</p>
+
+<p>He considers also that tissues arise before organs. The
+homogeneous parts are anterior genetically to the heterogeneous
+parts and posterior to the elementary material (<i>De Partibus</i>,
+ii., 1, 646<sup>b</sup>).</p>
+
+<p>(7) We meet in Aristotle an idea which later acquired
+considerable vogue, that of the <i>&Eacute;chelle des
+&ecirc;tres</i>(or "scale of beings"), that organisms, or even all
+objects organic or inorganic, can be arranged in a single ascending
+series. The idea is a common one; its first literary expression is
+found perhaps in primitive creation-myths, in which inorganic
+things are created before organic, and plants before animals. It
+may be recognised also in Anaximander's theory that land animals
+arose from aquatic animals, more clearly still in Anaxagoras'
+theory that life took its origin on this globe from vegetable germs
+which fell to earth with the rain. Anaxagoras considered animals
+higher in the scale than plants, for while the latter participated
+in pleasure (when they <span class="pagenum"><a name="pg015" id=
+"pg015">015</a></span>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
+<i>Timaeus</i>, 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.</p>
+
+<p>Aristotle seems not to have believed in any transformation of
+species, but he saw that Nature passes gradually from inanimate to
+animate things without a clear dividing line. "The race of plants
+succeeds immediately that of inanimate objects" (Cresswell, <i>loc.
+cit.</i>, p.&nbsp;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.</p>
+
+<p>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 <i>Hist.
+Animalium</i>, v.) he reverses this order. In the <i>De
+Generatione</i> (Book ii., 1) 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:&mdash;</p>
+
+<table class="two" summary="arrangement of animals" width="80%"
+border="0" cellpadding="0" cellspacing="0">
+<tbody>
+<tr>
+<td class="cell_lt"><p class="one">1. Internally viviparous Sanguinea</p></td>
+<td class="cell_mid" rowspan="2"><img src="images/para4.jpg" alt=
+"para" /></td>
+<td class="cell_rt" rowspan="2"><p class="one">producing a perfect animal</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt" ><p class="one">2. Externally viviparous Sanguinea</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt" colspan="3"><p class="one">3. Oviparous
+Sanguinea&mdash;producing a perfect egg.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt" colspan="3"><p class="one">4. Animals producing an imperfect
+egg (one which increases in size after being laid).</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt" colspan="3"><p class="one">5. Insects, producing a scolex (or
+grub).</p></td>
+</tr>
+</tbody>
+</table>
+
+<p>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, <span
+class="pagenum"><a name="pg016" id="pg016">016</a></span>growth,
+and reproduction; they possess only the nutritive soul. Animals
+possess in addition sensation and the sensitive or perceptive
+soul&mdash;"their manner of life differs in their having pleasure
+in sexual intercourse, in their mode of parturition and rearing
+their young" (<i>Hist. Anim.</i>, viii., trans. Cresswell, p.&nbsp;195).
+Man alone has the rational soul in addition to the two lower
+kinds.</p>
+
+<p>As it is put in the <i>De Partibus</i> (ii., 10,
+656<sup>a</sup>, 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."</p>
+
+<p>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&mdash;Leucippus,
+Democritus, and their great disciple Lucretius, who in his
+magnificent poem "De Natura Rerum" gave impassioned expression to
+the materialistic conception of the universe. But the full effect
+of materialism upon morphology does not become apparent till the
+rise of physiology in the 17th and 18th centuries, and reaches its
+culmination in the 19th century. The evolutionary ideas of
+Lucretius exercised no immediate influence upon the development of
+morphology.</p>
+
+<div class="footnote">
+<p><a name="Footnote_1" id="Footnote_1" /><a href=
+"#FNanchor_1"><span class="label">[1]</span></a> E. Zeller,
+<i>Greek Philosophy</i>, Eng. trans., i., 522 f.n., London 1881.
+Other particulars as to Alcmaeon in T. Gomperz, <i>Greek
+Thinkers</i>, Eng. trans., i., London, 1901.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_2" id="Footnote_2" /><a href=
+"#FNanchor_2"><span class="label">[2]</span></a> Zeller, <i>loc.
+cit.</i>, i., p.&nbsp;297.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_3" id="Footnote_3" /><a href=
+"#FNanchor_3"><span class="label">[3]</span></a> Gomperz, <i>loc.
+cit.</i>, i., p.&nbsp;244.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_4" id="Footnote_4" /><a href=
+"#FNanchor_4"><span class="label">[4]</span></a> R. Burckhardt,
+<i>Biologie u. Humanismus</i>, p.&nbsp;85, Jena, 1907.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_5" id="Footnote_5" /><a href=
+"#FNanchor_5"><span class="label">[5]</span></a> See the
+interesting account of Aristotle's biological work in Prof. D'Arcy
+W. Thompson's Herbert Spencer lecture (1913) and his translation of
+the <i>Historia Animalium</i> in the Oxford series.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_6" id="Footnote_6" /><a href=
+"#FNanchor_6"><span class="label">[6]</span></a> On Aristotle's
+forerunners, see R. Burckhardt, "Das ko&iuml;sche Tiersystem, eine
+Vorstufe des zoologischen Systematik des Aristoteles." <i>Verh.
+Naturf. Ges. Basel</i>, xx., 1904.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_7" id="Footnote_7" /><a href=
+"#FNanchor_7"><span class="label">[7]</span></a> T. E. Lones,
+<i>Aristotle's Researches in Natural Science</i>, pp.&nbsp;82-3, London,
+1912.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_8" id="Footnote_8" /><a href=
+"#FNanchor_8"><span class="label">[8]</span></a> <i>De Partibus
+Animalium</i>, i., 4, 644<sup>a</sup> trans. W. Ogle, Oxford,
+1911.</p>
+</div>
+
+
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg017" id=
+"pg017">017</a></span></p>
+
+<h3>CHAPTER II</h3>
+
+<h4>COMPARATIVE ANATOMY BEFORE CUVIER</h4>
+
+<p>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.</p>
+
+<p>During the Middle Ages, particularly after the introduction to
+Europe in the 13th century of the Arab texts and commentaries,
+Aristotle dominated men's thoughts of Nature. The commentary of
+Albertus Magnus, based upon that of Avicenna, did much to impose
+Aristotle upon the learned world. Albertus seems to have contented
+himself with following closely in the footsteps of his master.
+There are noted, however, by Bonnier certain improvements made by
+Albertus on Aristotle's view of the seriation of living things. "He
+is the first," writes Bonnier, "to take the correct view that fungi
+are lower plants allied to the most lowly organised animals. From
+this point there start, for Albertus Magnus, two series of living
+creatures, and he regards the plant series as culminating in the
+trees which have well-developed flowers."<a name="FNanchor_9" id=
+"FNanchor_9" /><a href="#Footnote_9" class=
+"fnanchor">[9]</a></p>
+
+<p>Aristotle's influence is predominant also in the work of Edward
+Wotton (1492-1555), who in his book <i>De differentiis
+animalium</i> adopted a classification similar to that proposed by
+Aristotle. He too laid stress upon the gradation shown from the
+lower to the higher forms.</p>
+
+<p>In the 16th century, two groups of men helped to lay foundations
+for a future science of comparative anatomy&mdash;the <span class=
+"pagenum"><a name="pg018" id="pg018">018</a></span>great Italian
+anatomists Vesalius, Fallopius and Fabricius, and the first
+systematists (though their "systems" were little more than
+catalogues) Rondeletius, Aldrovandus and Gesner.</p>
+
+<p>The anatomists, however, took little interest in problems of
+pure morphology; the anatomy of the human body was for them simply
+the necessary preliminary of the discovery of the functions of the
+parts&mdash;they were quite as much physiologists as
+anatomists.</p>
+
+<p>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.<a name=
+"FNanchor_10" id="FNanchor_10" /><a href="#Footnote_10"
+class="fnanchor">[10]</a> In his philosophy and habit of thought
+Harvey was a follower of Aristotle. It is worth noting that in his
+<i>Exercitationes anatomicae de motu cordis</i> (1628) there is a
+passage which dimly foreshadows the law of recapitulation in
+development which later had so much vogue.<a name="FNanchor_11"
+id="FNanchor_11" /><a href="#Footnote_11" class=
+"fnanchor">[11]</a></p>
+
+<p>A stimulating contribution to comparative anatomy was made by
+Belon,<a name="FNanchor_12" id="FNanchor_12" /><a href=
+"#Footnote_12" class="fnanchor">[12]</a> who published in 1555 a
+<i>Histoire de la nature des Oyseaux</i>, in which he showed
+opposite one another a skeleton of a bird and of a mammal, giving
+the same names to homologous bones. The anatomy of animals other
+than man was indeed not altogether neglected at this time. Coiter
+(1535-1600) studied the anatomy of Vertebrates, discovering among
+other things the fibrous structure of the brain. Carlo Ruini of
+Bologna wrote in 1598 a book on the anatomy of the horse.<a name=
+"FNanchor_13" id="FNanchor_13" /><a href="#Footnote_13"
+class="fnanchor">[13]</a> Somewhat later Severino, professor at
+Naples, dissected many animals and came to the conclusion <span
+class="pagenum"><a name="pg019" id="pg019">019</a></span>that they
+were built upon the same plan as man.<a name="FNanchor_14" id=
+"FNanchor_14" /><a href="#Footnote_14" class="fnanchor">[14]</a>
+Willis, of Oxford and London, in his <i>Cerebri Anatome</i> (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!<a name="FNanchor_15" id="FNanchor_15" /><a href=
+"#Footnote_15" class="fnanchor">[15]</a> He described the anatomy
+of the oyster and the crayfish. He had, however, not much feeling
+for morphology.</p>
+
+<p>The foundation of the Jardin des Plantes at Paris in 1626 and
+the subsequent addition to it of a Museum of Natural History and a
+menagerie gave a great impulse to the study of comparative anatomy
+by supplying a rich material for dissection. Advantage was taken of
+these facilities, particularly by Claude Perrault and Duverney.<a
+name="FNanchor_16" id="FNanchor_16" /><a href="#Footnote_16"
+class="fnanchor">[16]</a> In a volume entitled <i>De la
+M&eacute;canique des Animaux</i>, 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.<a name="FNanchor_17" id="FNanchor_17" /><a href=
+"#Footnote_17" class="fnanchor">[17]</a></p>
+
+<p>The beginning of the 17th century saw the invention of the
+microscope, which was to have such an enormous influence upon the
+development of biological studies. It did not come into scientific
+use until well on in the middle of the century. Just before it came
+into use Francis Glisson (1597-1677), an Englishman, gave in the
+introduction to his treatise on the liver an account of the notions
+then current on the structure of organic bodies. He classifies the
+parts as "similar" and "organic," the former determined by their
+material, the latter by the form which they assume. The similar
+parts are divided into the sanguineous or rich in blood and the
+spermatic. Both sets are further subdivided according to their
+physical characters,<a name="FNanchor_18" id="FNanchor_18" /><a
+href="#Footnote_18" class="fnanchor">[18]</a> 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.</p>
+
+<p><span class="pagenum"><a name="pg020" id=
+"pg020">020</a></span>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, <i>De viscerum structura</i> (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."<a
+name="FNanchor_19" id="FNanchor_19" /><a href="#Footnote_19"
+class="fnanchor">[19]</a> So he wrote in the <i>De viscerum
+structura</i>, and accordingly he studied the liver first in the
+snail, then in fishes, reptiles, mammals, and finally man. In the
+introduction to his <i>Anatome plantarum</i> (1675), in which he
+laid the foundations of plant histology, he vindicates the
+comparative method in the following words:&mdash;"In the enthusiasm
+of youth I applied myself to Anatomy, and although I was interested
+in particular problems, yet I dared to pry into them in the higher
+animals. But since these matters enveloped in peculiar mystery
+still lie in obscurity, they require the comparison of simpler
+conditions, and so the investigation of insects<a name=
+"FNanchor_20" id="FNanchor_20" /><a href="#Footnote_20"
+class="fnanchor">[20]</a> <span class="pagenum"><a name="pg021" id=
+"pg021">021</a></span>at once attracted me; finally, since this
+also has its own difficulties I applied my mind to the study of
+plants, intending after prolonged occupation with this domain, to
+retrace my steps by way of the vegetable kingdom, and get back to
+my former studies. But perhaps not even this will be sufficient;
+since the simpler world of minerals and the elements should have
+been taken first. In this case, however, the undertaking becomes
+enormous and far beyond my powers."<a name="FNanchor_21" id=
+"FNanchor_21" /><a href="#Footnote_21" class="fnanchor">[21]</a>
+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.</p>
+
+<p>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.</p>
+
+<p>To the microscope we owe not only histology but the comparative
+anatomy of the lower animals. Throughout the 17th and 18th
+centuries the discovery of structure in the lower animals went on
+continuously, as may be read in any history of Zoology.<a name=
+"FNanchor_22" id="FNanchor_22" /><a href="#Footnote_22"
+class="fnanchor">[22]</a> We content ourselves here with mentioning
+only some representative names.</p>
+
+<p>In the 17th century Leeuenhoek, applying the microscope almost
+at random, discovered fact after fact, his most famous discovery
+being that of the "spermatic animalcules."</p>
+
+<p>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 <span class="pagenum"><a name=
+"pg022" id="pg022">022</a></span>homologises the stages of the
+frog's development with the Egg, the Worm, and the Nymph of insects
+(<i>Book of Nature</i>, p.&nbsp;104, Eng. trans., 1785). He even speaks
+of the human embryo as being at a certain stage a Man-Vermicle.</p>
+
+<p>In the 18th century, R&eacute;aumur and Bonnet continued the
+minute study of insects, laying more stress, however, on their
+habits and physiology than upon their anatomy. Lyonnet made a most
+laborious investigation of the anatomy of the willow-caterpillar
+(1762). John Hunter (1728-93) dissected all kinds of animals, from
+holothurians to whales. His interest was, however, that of the
+physiologist, and he was not specially interested in problems of
+form. It is interesting to note a formulation in somewhat confused
+language of the recapitulation theory. The passage occurs in his
+description of the drawings he made to illustrate the development
+of the chick. It is quoted in full by Owen (J. Hunter,
+<i>Observations on certain Parts of the Animal &OElig;conomy</i>, with
+Notes by Richard Owen. London, 1837. Preface, p.&nbsp;xxvi). We give
+here the last and clearest sentence&mdash;"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."</p>
+
+<p>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 Linn&aelig;us (1707-1778). The former tendency is
+well represented by R&eacute;aumur (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 R&ouml;sel von Rosenhof's <i>Insektenbelustigungen</i>
+(1746-1761).</p>
+
+<p>Bonnet (1720-1793) deserves special mention here, since in his
+<i>Trait&eacute; d'Insectologie</i> (1745), and more fully in his
+<i>Contemplation de la Nature</i> (1764), he gives the most
+complete expression to the idea of the <i>&Eacute;chelle des
+&ecirc;tres</i>.</p>
+
+<p>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 <i>Trait&eacute;</i> <span class=
+"pagenum"><a name="pg023" id=
+"pg023">023</a></span><i>d'Insectologie</i> (&OElig;uvres, i.,
+1779) he gives a long table, headed "Id&eacute;e d'une
+&Eacute;chelle des &ecirc;tres naturels," and rather resembling a
+ladder, on the rungs of which the following names
+appear:&mdash;</p>
+
+<table summary="The Scale of Beings" width="100%" border="0"
+cellpadding="0" cellspacing="0">
+<tbody>
+<tr>
+<td class="cell_lt3"><p class="one"><span class="smcap">Man.</span></p></td>
+<td class="cell_lt3"><p class="one"><span class="smcap">Shell Fish.</span></p></td>
+<td class="cell_lt3"><p class="one"><span class="smcap">Stones.</span></p></td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one">Orang-utan.</p></td>
+<td class="cell_lt3"><p class="one">Tube-worms.</p></td>
+<td class="cell_lt3"><p class="one">Figured stones.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one">Ape.</p></td>
+<td class="cell_lt3"><p class="one">Clothes-worms.</p></td>
+<td class="cell_lt3"><p class="one">Crystals.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt3">&nbsp;</td>
+<td class="cell_lt3">&nbsp;</td>
+<td class="cell_lt3">&nbsp;</td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one"><span class="smcap">Quadrupeds.</span></p></td>
+<td class="cell_lt3"><p class="one"><span class="smcap">Insectes.</span></p></td>
+<td class="cell_lt3"><p class="one"><span class="smcap">Salts.</span></p></td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one">Flying squirrel.</p></td>
+<td class="cell_lt3"><p class="one">Gall insectes.</p></td>
+<td class="cell_lt3"><p class="one">Vitriols.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one">Bat.</p></td>
+<td class="cell_lt3"><p class="one">Taenia.</p></td>
+<td class="cell_lt3">&nbsp;</td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one">Ostrich.</p></td>
+<td class="cell_lt3"><p class="one">Polyps.</p></td>
+<td class="cell_lt3"><p class="one"><span class="smcap">Metals.</span></p></td>
+</tr>
+
+<tr>
+<td class="cell_lt3">&nbsp;</td>
+<td class="cell_lt3"><p class="one">Sea Nettles.</p></td>
+<td class="cell_lt3">&nbsp;</td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one"><span class="smcap">Birds.</span></p></td>
+<td class="cell_lt3"><p class="one">Sensitive plant.</p></td>
+<td class="cell_lt3"><p class="one"><span class="smcap">Half-metals.</span></p></td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one">Aquatic birds.</p></td>
+<td class="cell_lt3">&nbsp;</td>
+<td class="cell_lt3">&nbsp;</td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one">Amphibious birds.</p></td>
+<td class="cell_lt3"><p class="one"><span class="smcap">Plants.</span></p></td>
+<td class="cell_lt3"><p class="one"><span class="smcap">Sulphurs.</span></p></td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one">Flying Fish.</p></td>
+<td class="cell_lt3"><p class="one">Lichens.</p></td>
+<td class="cell_lt3"><p class="one">Bitumens.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt3">&nbsp;</td>
+<td class="cell_lt3"><p class="one">Moulds.</p></td>
+<td class="cell_lt3">&nbsp;</td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one"><span class="smcap">Fish.</span></p></td>
+<td class="cell_lt3"><p class="one">Fungi, Agarics.</p></td>
+<td class="cell_lt3"><p class="one"><span class="smcap">Earths.</span></p></td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one">Creeping fish.</p></td>
+<td class="cell_lt3"><p class="one">Truffles.</p></td>
+<td class="cell_lt3"><p class="one">Pure earth.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one">Eels.</p></td>
+<td class="cell_lt3"><p class="one">Corals, and Coralloids.</p></td>
+<td class="cell_lt3">&nbsp;</td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one">Water sepents.</p></td>
+<td class="cell_lt3"><p class="one">Lithophytes.</p></td>
+<td class="cell_lt3"><p class="one"><span class="smcap">Water.</span></p></td>
+</tr>
+
+<tr>
+<td class="cell_lt3">&nbsp;</td>
+<td class="cell_lt3"><p class="one">Asbestos.</p></td>
+<td class="cell_lt3">&nbsp;</td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one"><span class="smcap">Serpents.</span></p></td>
+<td class="cell_lt3"><p class="one">Talc, Gypsums.</p></td>
+<td class="cell_lt3"><p class="one"><span class="smcap">Air.</span></p></td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one">Slugs.</p></td>
+<td class="cell_lt3"><p class="one">Selenites, Slates.</p></td>
+<td class="cell_lt3">&nbsp;</td>
+</tr>
+
+<tr>
+<td class="cell_lt3"><p class="one">Snails.</p></td>
+<td class="cell_lt3">&nbsp;</td>
+<td class="cell_lt3"><p class="one"><span class="smcap">Fire.</span></p></td>
+</tr>
+
+<tr>
+<td class="cell_lt3">&nbsp;</td>
+<td class="cell_lt3">&nbsp;</td>
+<td class="cell_lt3">&nbsp;</td>
+</tr>
+
+<tr>
+<td class="cell_lt3">&nbsp;</td>
+<td class="cell_lt3">&nbsp;</td>
+<td class="cell_lt3"><p class="one">More subtile matter.</p></td>
+</tr>
+
+</tbody>
+</table>
+<p>&nbsp;</p>
+<p>The nature of the transitional forms which he inserts between
+his principal classes show very clearly his entire lack of
+morphological insight&mdash;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.</p>
+
+<p>Robinet, in the fifth volume of his book <i>De la nature</i>
+(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."<a name="FNanchor_23"
+id="FNanchor_23" /><a href="#Footnote_23" class=
+"fnanchor">[23]</a></p>
+
+<p><span class="pagenum"><a name="pg024" id=
+"pg024">024</a></span>The idea of a gradation of beings appears
+also in Buffon (1707-1788), but here it takes more definitely its
+true character as a functional gradation.<a name="FNanchor_24"
+id="FNanchor_24" /><a href="#Footnote_24" class=
+"fnanchor">[24]</a> "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."<a name="FNanchor_25" id="FNanchor_25" /><a href=
+"#Footnote_25" class="fnanchor">[25]</a></p>
+
+<p>He is quite well aware that the groups of Invertebrates are
+different in structural plan from the Vertebrates&mdash;"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."<a name="FNanchor_26" id=
+"FNanchor_26" /><a href="#Footnote_26" class=
+"fnanchor">[26]</a></p>
+
+<p>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.<a name="FNanchor_27" id=
+"FNanchor_27" /><a href="#Footnote_27" class="fnanchor">[27]</a>
+What is new in Buffon is his interpretation of the unity of plan.
+For the first time we find clearly expressed the thought that unity
+of plan is to be explained by community of origin.</p>
+
+<p>Buffon's utterances on this point are, as is well known,
+somewhat vacillating. The famous passage, however, which occurs in
+his account of the Ass shows pretty clearly that Buffon saw no
+theoretical objection to the descent of all the varied species of
+animals from one single form. Once admit, he argues, that within
+the bounds of a single family one species may originate from the
+type species by "degeneration," then one might reasonably suppose
+that from a single being Nature could in time produce all the other
+organised beings.<a name="FNanchor_28" id="FNanchor_28" /><a
+href="#Footnote_28" class="fnanchor">[28]</a> Elsewhere,
+<i>e.g.</i>, in the discourse <i>De la D&eacute;g&eacute;neration
+des Animaux</i>,<a name="FNanchor_29" id="FNanchor_29" /><a
+href="#Footnote_29" class="fnanchor">[29]</a> Buffon expresses
+himself with more caution. He finds that it is possible to reduce
+the two hundred species of quadrupeds which he has described to
+<span class="pagenum"><a name="pg025" id=
+"pg025">025</a></span>quite a small number of families "from which
+it is not impossible that all the rest are derived."<a name=
+"FNanchor_30" id="FNanchor_30" /><a href="#Footnote_30"
+class="fnanchor">[30]</a> 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
+<i>&Eacute;chelle des &ecirc;tres</i>.<a name="FNanchor_31" id=
+"FNanchor_31" /><a href="#Footnote_31" class=
+"fnanchor">[31]</a></p>
+
+<p>It is a mistake to suppose that Buffon was par excellence a
+maker of hypotheses. On the contrary he saw things very sanely and
+with a very open mind. He expressly mentions the great difficulties
+which one encounters in supposing that one species may arise from
+another by "degeneration." How does it happen that two individuals
+"degenerate" just in the right direction and to the right stage so
+as to be capable of breeding together? How is it that one does not
+find intermediate links between species? One is reminded of the
+objections, not altogether without validity, which were made to the
+Darwinian theory in its early days. I cannot agree with those who
+think that Buffon was an out-and-out evolutionist, who concealed
+his opinions for fear of the Church. No doubt he did trim his
+sails&mdash;the palpably insincere "Mais non, il est certain, par
+la r&eacute;v&eacute;lation, que tous les animaux ont
+&eacute;galement particip&eacute; &agrave; la grace de la
+cr&eacute;ation,"<a name="FNanchor_32" id="FNanchor_32" /><a
+href="#Footnote_32" class="fnanchor">[32]</a> following hard upon
+the too bold hypothesis of the origin of all species from a single
+one, is proof of it. But he was too sane and matter-of-fact a
+thinker to go much beyond his facts, and his evolution doctrine
+remained always tentative. One thing, however, he was sure of, that
+evolution would give a rational foundation to the classification
+which, almost in spite of himself, he recognised in Nature. If, and
+only if, the species of one family originated from a single type
+species, could families, be founded rationally, <i>avec
+raison</i>.</p>
+
+<p>Buffon was, curiously enough, rather unwilling to recognise any
+systematic unit higher than the species. Strictly speaking there
+are only individuals in Nature; but there <span class="pagenum"><a
+name="pg026" id="pg026">026</a></span>are also groups of
+individuals which resemble one another from generation to
+generation and are able to breed together. These are
+species&mdash;Buffon adheres to the genetic definition of
+species&mdash;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,<a name=
+"FNanchor_33" id="FNanchor_33" /><a href="#Footnote_33"
+class="fnanchor">[33]</a> and this is the only discontinuity which
+Nature shows us. Buffon put his views into practice in his
+<i>Histoire Naturelle</i>, where he describes species after
+species, never uniting them into larger groups. We have seen,
+however, how the facts forced upon him the conception of the
+"family."</p>
+
+<p>Buffon was no morphologist. He left to Daubenton what one might
+call the "dirty work" of his book, the dissection and minute
+description of the animals treated.</p>
+
+<p>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 <i>Discours sur la Nature des Animaux</i><a name=
+"FNanchor_34" id="FNanchor_34" /><a href="#Footnote_34"
+class="fnanchor">[34]</a> a distinction, which Bichat and Cuvier
+later developed with much profit, between the "animal" and the
+"vegetative" part of animals.<a name="FNanchor_35" id=
+"FNanchor_35" /><a href="#Footnote_35" class="fnanchor">[35]</a>
+The vegetative or organic functions go on continuously, even in
+sleep, and are performed by the internal organs, of which the heart
+is the central one. The active waking life of the animal, that part
+of its life which distinguishes it from the plant, involves the
+external parts&mdash;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."<a
+name="FNanchor_36" id="FNanchor_36" /><a href="#Footnote_36"
+class="fnanchor">[36]</a> Animals may differ from one another
+enormously in the external parts, particularly in the appendicular
+skeleton, without showing any great difference in the plan and
+arrangement of their internal organs. <span class="pagenum"><a
+name="pg027" id="pg027">027</a></span>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&mdash;the external parts will change infinitely more, and
+you will get another animal, an animal of a totally different
+nature. Thus an insect has a most singular internal economy, and,
+in consequence, you find it is in every point different from any
+vertebrate animal.</p>
+
+<p>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.</p>
+
+<p>To Xavier Bichat (1771-1802) belongs the credit of working out
+in detail the distinction drawn by Aristotle and Buffon between the
+animal and the vegetative functions. Bichat was not a comparative
+anatomist; his interest lay in human anatomy, normal and
+pathological. So his views are drawn chiefly from the consideration
+of human structure.</p>
+
+<p>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.<a name="FNanchor_37" id="FNanchor_37" /><a href=
+"#Footnote_37" class="fnanchor">[37]</a> "I call <i>animal
+life</i> that order of functions which connects us with surrounding
+bodies; signifying thereby that this order belongs only to animals"
+(p.&nbsp;lxxviii.). Its organs are the afferent and efferent nerves, the
+brain, the sense-organs and the voluntary muscles; the brain is its
+central organ. "Digestion, circulation, respiration, exhalation,
+absorption, secretion, nutrition, calorification, or production of
+animal heat, compose organic life, whose principal and central
+organ is the heart" (p.&nbsp;lxxix.).</p>
+
+<p>The contrast of the animal and the organic life runs <span
+class="pagenum"><a name="pg028" id="pg028">028</a></span>through
+all Bichat's work; it receives classical expression in his
+<i>Recherches Physiologiques sur la Vie et la Mort</i> (1800). The
+plant and the animal stand for two different modes of living. The
+plant lives within itself, and has with the external world only
+relations of nutrition; the animal adds to this organic life a life
+of active relation with surrounding things (3rd ed., 1805, p.&nbsp;2).
+"One might almost say that the plant is the framework, the
+foundation of the animal, and that to form the animal it sufficed
+to cover this foundation with a system of organs fitted to
+establish relations with the world outside. It follows that the
+functions of the animal form two quite distinct classes. One class
+consists in a continual succession of assimilation and excretion;
+through these functions the animal incessantly transforms into its
+own substance the molecules of surrounding bodies, later to reject
+these molecules when they have become heterogeneous to it. Through
+this first class of functions the animal exists only within itself;
+through the other class it exists outside; it is an inhabitant of
+the world, and not, like the plant, of the place which saw its
+birth. The animal feels and perceives its surroundings, reflects
+its sensations, moves of its own will under their influence, and,
+as a rule, can communicate by its voice its desires and its fears,
+its pleasures or its pains. I call organic life the sum of the
+functions of the former class, for all organised creatures, plants
+or animals, possess them to a more or less marked degree, and
+organised structure is the sole condition necessary to their
+exercise. The combined functions of the second class form the
+'animal' life, so named because it is the exclusive attribute of
+the animal kingdom" (pp.&nbsp;2-3).</p>
+
+<p>In both lives there is a double movement, in the animal life
+from the periphery to the centre and from the centre to the
+periphery, in the organic life also from the exterior to the
+interior and back again, but here a movement of composition and
+decomposition. As the brain mediates between sensation and motion,
+so the vascular system is the go-between of the organs of
+assimilation and the organs of dissimilation.</p>
+
+<p>The most essential structural difference between the organs of
+animal life and the organs of organic life is, in man and the higher
+animals at least, the symmetry of <span class="pagenum"><a name=
+"pg029" id="pg029">029</a></span>the one set and the irregularity
+of the other&mdash;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.</p>
+
+<p>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 (<i>loc. cit.</i>, p.&nbsp;127).</p>
+
+<p>Bichat was the founder of what was known for a time as General
+Anatomy&mdash;the study of the constituent tissues of the body in
+health and disease. His classification of tissues was macroscopical
+and physiological; he relied upon texture and function in
+distinguishing them rather than upon microscopical structure. The
+tissues he distinguished are as follows:&mdash;<a name=
+"FNanchor_38" id="FNanchor_38" /><a href="#Footnote_38"
+class="fnanchor">[38]</a></p>
+
+<table class="three" summary="arrangement of animals" width="80%"
+border="0" cellpadding="0" cellspacing="0">
+<tbody>
+<tr>
+<td class="cell_lt293a">1.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">The cellular membrane.</p></td>
+<td class="cell_lt293a">12.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Fibro-cartilage.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt293a">2.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Nerves of animal life.</p></td>
+<td class="cell_lt293a">13.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Muscles of organic life.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt293a">3.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Nerves of organic life.</p></td>
+<td class="cell_lt293a">14.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Muscles of animal life.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt293a">4.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Arteries.</p></td>
+<td class="cell_lt293a">15.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Mucous membrane.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt293a">5.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Veins.</p></td>
+<td class="cell_lt293a">16.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Serous membrane.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt293a">6.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Exhalants.</p></td>
+<td class="cell_lt293a">17.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Synovial membrane.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt293a">7.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Absorbents and glands.</p></td>
+<td class="cell_lt293a">18.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">The Glands.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt293a">8.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Bones.</p></td>
+<td class="cell_lt293a">19.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">The Dermis.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt293a">9.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Medulla.</p></td>
+<td class="cell_lt293a">20.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Epidermis.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt293a">10.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Cartilage.</p></td>
+<td class="cell_lt293a">21.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Cutis.</p></td>
+</tr>
+
+<tr>
+<td class="cell_lt293a">11.&nbsp;</td>
+<td class="cell_lt293c"><p class="one">Fibrous tissue.</p></td>
+<td class="cell_lt293a">&nbsp;</td>
+<td class="cell_lt293c"><p class="one">&nbsp;</p></td>
+</tr>
+</tbody>
+</table>
+
+<p>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.</p>
+
+<p>In Bichat's eyes this resolution of the organism into <span
+class="pagenum"><a name="pg030" id="pg030">030</a></span>tissues
+had a deeper significance than any separation into organs, for to
+each tissue must be attributed a <i>vie propre</i>, 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."<a name=
+"FNanchor_39" id="FNanchor_39" /><a href="#Footnote_39"
+class="fnanchor">[39]</a> The tissues have, too, a great importance
+for pathology, for diseases are often diseases of tissues rather
+than of organs.<a name="FNanchor_40" id="FNanchor_40" /><a
+href="#Footnote_40" class="fnanchor">[40]</a></p>
+
+<div class="footnote">
+<p><a name="Footnote_9" id="Footnote_9" /><a href=
+"#FNanchor_9"><span class="label">[9]</span></a> <i>Le Monde
+v&eacute;g&eacute;tal</i>, p.&nbsp;41, Paris, 1907.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_10" id="Footnote_10" /><a href=
+"#FNanchor_10"><span class="label">[10]</span></a>
+<i>Exercitationes de generatione animalium</i>, 1651. For an account
+of Harvey's work on generation and development, see Em.
+R&aacute;dl's masterly <i>Geschichte der biologischen Theorien</i>,
+i., pp.&nbsp;31-8, Leipzig, 1905.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_11" id="Footnote_11" /><a href=
+"#FNanchor_11"><span class="label">[11]</span></a> The passage
+runs:&mdash;"Sic natura perfecta et divina nihil faciens frustra,
+nec quipiam animali cor addidit, ubi non erat opus, neque priusquam
+esset ejus usus, fecit; sed iisdem gradibus in formatione
+cujuscumque animalis, transiens per omnium animalium constitutiones
+(ut ita dicam) ovum, vermem, f&oelig;tum, perfectionem in singulis
+acquirit."</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_12" id="Footnote_12" /><a href=
+"#FNanchor_12"><span class="label">[12]</span></a> See I. Geoffroy
+St Hilaire, <i>Essais de Zoologie g&eacute;n&eacute;rale</i>, p.
+71, Paris, 1841.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_13" id="Footnote_13" /><a href=
+"#FNanchor_13"><span class="label">[13]</span></a> M. Foster,
+<i>Lectures on the History of Physiology</i>, Cambridge, p.&nbsp;53,
+1901.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_14" id="Footnote_14" /><a href=
+"#FNanchor_14"><span class="label">[14]</span></a> <i>Zootomia
+democritea</i>, Nuremberg, 1645; <i>Antiperipatias, seu de
+respiratione piscium</i>, Amsterdam, 1661.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_15" id="Footnote_15" /><a href=
+"#FNanchor_15"><span class="label">[15]</span></a> R&aacute;dl,
+<i>loc. cit.</i>, i., p.&nbsp;50.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_16" id="Footnote_16" /><a href=
+"#FNanchor_16"><span class="label">[16]</span></a> Perrault et
+Duverney, <i>M&eacute;moires pour servir &agrave; l'histoire des
+Animaux</i>, Paris, 1699.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_17" id="Footnote_17" /><a href=
+"#FNanchor_17"><span class="label">[17]</span></a> F. Houssay,
+<i>Nature et Sciences naturelles</i>, Paris, p.&nbsp;76, n.d.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_18" id="Footnote_18" /><a href=
+"#FNanchor_18"><span class="label">[18]</span></a> Foster, <i>loc.
+cit.</i>, p.&nbsp;85.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_19" id="Footnote_19" /><a href=
+"#FNanchor_19"><span class="label">[19]</span></a> Trans. by
+Foster, <i>loc. cit.</i>, p.&nbsp;113.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_20" id="Footnote_20" /><a href=
+"#FNanchor_20"><span class="label">[20]</span></a> He made a
+careful study of the silkworm.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_21" id="Footnote_21" /><a href=
+"#FNanchor_21"><span class="label">[21]</span></a> "Etenim,
+ferventi actatis calore, Anatomica aggressus, licet circa
+peculiaria fuerim solicitus, in <i>perfectioribus</i> tamen haec
+rimari sum ausus. Verum, cum haec propriis tenebris obscura
+jaceant, simplicium analogismo egent; inde <i>insectorum</i> indago
+illico arrisit; quae cum et ipsa suas habeat difficultates ad
+Plantarum perquisitionem animum <i>postremo</i> adjeci, ut diu hoc
+lustrato mundo gressu retroacto Vegetantis Naturae gradu, ad prima
+studia iter mihi aperirem. Sed nec forte hoc ipsum sufficiet cum
+simplicior <i>Mineralium Elementorumque</i> mundus praeire debeat.
+At in immensum excrescit opus, et meis viribus omnino impar,"
+<i>Opera Omnia</i>, i., p.&nbsp;1, London, 1686.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_22" id="Footnote_22" /><a href=
+"#FNanchor_22"><span class="label">[22]</span></a> See
+particularly E. R&aacute;dl, <i>loc. cit.</i>. 1 Teil. J. V.. Carus,
+<i>Geschichte der Zoologie</i>, M&uuml;nchen, 1872.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_23" id="Footnote_23" /><a href=
+"#FNanchor_23"><span class="label">[23]</span></a> For a good
+historical account of the gradation theories see Thienemann's paper
+in the <i>Zoologische Annalen</i> (W&uuml;rzburg) iii., pp.&nbsp;185-274,
+1910, from which the quotation from Robinet is taken.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_24" id="Footnote_24" /><a href=
+"#FNanchor_24"><span class="label">[24]</span></a> <i>Histoire
+naturelle</i>, i., p.&nbsp;13; ii, p.&nbsp;9; iv., p.&nbsp;101; and xiv., pp.
+28-9, 1749 and later.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_25" id="Footnote_25" /><a href=
+"#FNanchor_25"><span class="label">[25]</span></a> No translation
+can render the beauty of the original&mdash;"Comme tout se fait et
+que tout est par nuance dans la Nature ..." (iv., p.&nbsp;101).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_26" id="Footnote_26" /><a href=
+"#FNanchor_26"><span class="label">[26]</span></a> <i>Hist.
+nat.</i>, iv., p.&nbsp;5.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_27" id="Footnote_27" /><a href=
+"#FNanchor_27"><span class="label">[27]</span></a> See
+particularly his comparison of the skeleton of the horse with that
+of man. <i>Hist. Nat.</i>, iv., p.&nbsp;381, also p.&nbsp;13.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_28" id="Footnote_28" /><a href=
+"#FNanchor_28"><span class="label">[28]</span></a> <i>Loc.
+cit.</i>, p.&nbsp;382.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_29" id="Footnote_29" /><a href=
+"#FNanchor_29"><span class="label">[29]</span></a> Tome xiv., pp.
+311-374.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_30" id="Footnote_30" /><a href=
+"#FNanchor_30"><span class="label">[30]</span></a> Tome xiv., p.
+358.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_31" id="Footnote_31" /><a href=
+"#FNanchor_31"><span class="label">[31]</span></a> See also
+"Oiseaux," Tome i., pp.&nbsp;394, 395. Pallas in 1766 adopted for the
+whole animal kingdom this branching arrangement.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_32" id="Footnote_32" /><a href=
+"#FNanchor_32"><span class="label">[32]</span></a> "But this
+cannot be, for it is certain by revelation that all animals have
+equally participated in the grace of creation."</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_33" id="Footnote_33" /><a href=
+"#FNanchor_33"><span class="label">[33]</span></a> iv., p.385.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_34" id="Footnote_34" /><a href=
+"#FNanchor_34"><span class="label">[34]</span></a> iv., pp.
+3-110.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_35" id="Footnote_35" /><a href=
+"#FNanchor_35"><span class="label">[35]</span></a> It has been
+revived in our own days by Bergson, <i>Mati&egrave;re et
+M&eacute;moire</i>, p.&nbsp;57.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_36" id="Footnote_36" /><a href=
+"#FNanchor_36"><span class="label">[36]</span></a> iv., pp.
+7-15.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_37" id="Footnote_37" /><a href=
+"#FNanchor_37"><span class="label">[37]</span></a> <i>Anatomie
+G&eacute;n&eacute;rale</i>, Paris, 1801, Eng. trans. 1824.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_38" id="Footnote_38" /><a href=
+"#FNanchor_38"><span class="label">[38]</span></a> <i>Anatomie
+G&eacute;n&eacute;rale</i>, Eng. trans., i., p.&nbsp;lii.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_39" id="Footnote_39" /><a href=
+"#FNanchor_39"><span class="label">[39]</span></a> <i>Anatomie
+G&eacute;n&eacute;rale</i>, Eng. trans., i., p.&nbsp;lviii.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_40" id="Footnote_40" /><a href=
+"#FNanchor_40"><span class="label">[40]</span></a> <i>Loc
+cit.</i>, i., sect. vii.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg031" id=
+"pg031">031</a></span></p>
+
+<h3>CHAPTER III</h3>
+
+<h4>CUVIER</h4>
+
+<p><span class="smcap">Cuvier</span> was perhaps the greatest of
+comparative anatomists; his work is, in the best sense of the word,
+classical.</p>
+
+<p>Like all his predecessors, like Aristotle, like the Italian
+anatomists, Cuvier studied structure and function together, even
+gave function the primacy.</p>
+
+<p>Some functions, he says,<a name="FNanchor_41" id=
+"FNanchor_41" /><a href="#Footnote_41" class="fnanchor">[41]</a>
+are common to all organised bodies&mdash;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&mdash;lungs or gills. <span class=
+"pagenum"><a name="pg032" id="pg032">032</a></span>Plants, and
+animals without a circulation, breathe by their whole surface.</p>
+
+<p>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.</p>
+
+<p>The Animal Functions not only determine the character of the
+Vital Functions, but influence also the primary faculty of
+generation, for animals' power of movement has rendered their mode
+of fecundation more simple, has therefore had an effect on their
+organs of generation.</p>
+
+<p>This division into "Animal" and "Vital" functions recalls
+Buffon's and Bichat's distinction of the "animal" and the
+"vegetative" lives. Cuvier apparently took this idea from Buffon,
+for he says that a plant is an animal that sleeps.<a name=
+"FNanchor_42" id="FNanchor_42" /><a href="#Footnote_42"
+class="fnanchor">[42]</a> But the idea is as old as Aristotle, who
+discusses the "sleep" of embryos and of plants in the last book of
+the <i>De Generatione animalium</i>. The distinction between animal
+and vegetative life is, of course, based for Aristotle in the
+difference between the &psi;&upsilon;&chi;&#942;
+&#7936;&iota;&sigma;&theta;&eta;&tau;&iota;&kappa;&#942; and the
+&psi;&upsilon;&chi;&#942;
+&theta;&rho;&#941;&pi;&tau;&iota;&kappa;&#942;. Cuvier, like
+Aristotle, Buffon, and Bichat, makes the heart the centre of
+the "vegetative" organs.</p>
+
+<p>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."<a name="FNanchor_43" id="FNanchor_43" /><a href=
+"#Footnote_43" class="fnanchor">[43]</a> It is only after having
+discussed and classified functions that Cuvier goes on to examine
+organs.</p>
+
+<p>First his views on the composition of the animal body. Aristotle
+distinguished three degrees of composition&mdash;the "elements,"
+the homogeneous parts, and the heterogeneous parts or organs.
+Cuvier does the same. Some small advance has been made in the two
+thousand years' interval, due in the first place to the progress of
+chemistry, and in the second to the invention of the microscope. To
+the first circumstance Cuvier owes his knowledge that the inorganic
+<span class="pagenum"><a name="pg033" id=
+"pg033">033</a></span>substances forming the first degree of
+composition are principally C, N, H, O, and P, combined to form
+albumen, fibrine, and the like, which are in their turn combined to
+form the solids and fluids of the body. To the latter circumstance
+Cuvier owes the statement that the finest fragments into which
+mechanical division can resolve the organism are little flakes and
+filaments, which, joined up loosely together, form a "cellulosity."
+The discovery of the true cellular nature of animal tissues did not
+come till much later, till some years after Cuvier's death in 1832.
+Knowledge of histological detail was, however, considerable by the
+beginning of the 19th century. Cuvier knew, for example, that each
+muscle fibre has its own nerve fibre. But he gives no elaborate
+account of the homogeneous parts, no detailed histology. On the
+other hand his treatment of the heterogeneous parts or organs is
+detailed and masterly.<a name="FNanchor_44" id=
+"FNanchor_44" /><a href="#Footnote_44" class=
+"fnanchor">[44]</a></p>
+
+<p>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
+(<i>rapproch&eacute;s</i>), 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.</p>
+
+<p>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 <span class="pagenum"><a name="pg034" id=
+"pg034">034</a></span>organism, is the fundamental one at the base
+of all Cuvier's work. Before him men had recognised more or less
+clearly the harmony of structure and function, and had based much
+of their work upon this unanalysed assumption. Cuvier was the first
+naturalist to raise this thought to the level of a principle
+peculiar to natural history. "It is on this mutual dependence of
+the functions and the assistance which they lend one to another
+that are founded the laws that determine the relations of their
+organs; these laws are as inevitable as the laws of metaphysics and
+mathematics, for it is evident that a proper harmony between organs
+that act one upon another is a necessary condition of the existence
+of the being to which they belong."<a name="FNanchor_45" id=
+"FNanchor_45" /><a href="#Footnote_45" class=
+"fnanchor">[45]</a></p>
+
+<p>This rational principle, peculiar to natural history, Cuvier
+calls the principle of the conditions of existence, for the
+following reason:&mdash;"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."<a name="FNanchor_46" id="FNanchor_46" /><a href=
+"#Footnote_46" class="fnanchor">[46]</a></p>
+
+<p>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&mdash;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.</p>
+
+<p>The principle of the adaptedness of parts may be used as an
+explanatory principle, enabling the naturalist to trace out in
+detail the interdependence of functions and their organs. When you
+have discovered how one organ is adapted to another and to the
+whole, you have gone a certain way towards understanding it. That
+is <span class="pagenum"><a name="pg035" id=
+"pg035">035</a></span>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 <i>Kritik der
+Urtheilskraft</i>, which appeared ten years before the publication
+of the <i>Le&ccedil;ons d'Anatomie Compar&eacute;e</i>. Teleology
+in Kant's sense is and will always be a necessary postulate of
+biology. It does not supply an explanation of organic forms and
+activities, but without it one cannot even begin to understand
+living things. Adaptedness is the most general fact of life, and
+innumerable lesser facts can be grouped as particular cases of it,
+can be, so far, understood.</p>
+
+<p>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."<a name=
+"FNanchor_47" id="FNanchor_47" /><a href="#Footnote_47"
+class="fnanchor">[47]</a> The logical basis of the principle is
+sound. The functions of the parts are all intimately bound up with
+one another, and one function cannot vary without bringing in its
+train corresponding modifications in the others. Structure and
+function are bound up together; every modification of a function
+entails therefore the modification of an organ. Hence from the
+shape of one organ you can infer the shape of the other
+organs&mdash;if you have sufficiently extensive empirical knowledge
+of functions, and of the relation of structure to function in each
+kind of organ. Given an alimentary canal capable of digesting only
+flesh, and possessing therefore a certain form, you know that the
+other functions must be adapted to this particular function of the
+alimentary canal. The animal must have keen sight, fine smell,
+speed, agility, and strength in paws and jaws. These particular
+functions must have correspondingly modified organs, <span class=
+"pagenum"><a name="pg036" id="pg036">036</a></span>well-developed
+eyes and ears, claws and teeth. Further, you know from experience
+that such and such definitely modified organs are invariably found
+with the carnivorous habit, carnassial teeth, for example, and
+reduced clavicles. From a "carnivorous" alimentary canal, then, you
+can infer with certainty that the animal possessed carnassial teeth
+and the other structural peculiarities of carnivorous animals,
+<i>e.g.</i>, 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."<a name="FNanchor_48" id=
+"FNanchor_48" /><a href="#Footnote_48" class=
+"fnanchor">[48]</a></p>
+
+<p>Similarly the great respiratory power of birds is correlated
+with their great muscular strength, and renders necessary great
+digestive powers. Hence the correlated structure of lungs, muscles
+and their attachments, and alimentary canal, in birds.</p>
+
+<p>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.</p>
+
+<p>We deduce the necessity, the constancy, of these co-existences
+of organs from the observed reciprocal influence of their
+functions. That being established, we can argue from observed
+constancy of relation between two organs an action of one upon the
+other, and so be led to a discovery of their functions. But even if
+we do not discover the functional interdependencies of the parts,
+we can use the established fact of the constant co-existence of two
+parts as proof of a functional correlation between them.</p>
+
+<p>Correlation is either a rational or an empirical principle,
+according as we know or do not know the interdependence of function
+of which it is the expression. Even when we apply the rational
+principle of correlation it would be useless in our hands if we had
+not extensive empirical knowledge; when we use an empirical rule of
+correlation we depend entirely upon observation. "There are a great
+many cases," writes <span class="pagenum"><a name="pg037" id=
+"pg037">037</a></span>Cuvier,<a name="FNanchor_49" id=
+"FNanchor_49" /><a href="#Footnote_49" class="fnanchor">[49]</a>
+"where our theoretical knowledge of the relations of forms would
+not suffice, if it were not filled out by observation," that is to
+say, there are many cases of correlation not yet explicable in
+terms of function. From a hoof you can deduce the main characters
+of herbivores (with a certain amount of assistance from your
+empirical knowledge of herbivores), but could you from a cloven
+hoof deduce that the animal is a ruminant, unless you had observed
+the constancy of relation, not directly explicable in terms of
+function, between cloven hoofs and chewing the cud? Or could you
+deduce from the existence of frontal horns that the animal
+ruminates? "Nevertheless, since these relations are constant, they
+must necessarily have a sufficient cause; but as we are ignorant of
+this cause, observation must supplement theory; observation
+establishes empirical laws which become almost as certain as the
+rational laws, when they are based upon a sufficient number of
+observations.... But that there exist all the same hidden reasons
+for all these relations is partly revealed by observation itself,
+independently of general philosophy."<a name="FNanchor_50" id=
+"FNanchor_50" /><a href="#Footnote_50" class="fnanchor">[50]</a>
+That is to say, even correlations for which no explanation in terms
+of function can be supplied are probably in reality functional
+correlations. This may, in some cases, be inferred from the graded
+correspondence of two sets of organs. For example, ungulates which
+do not ruminate, and have not a cloven hoof, have a more perfect
+dentition and more bones in the foot than the true cloven-hoofed
+ruminants. There is a correlation between the state of development
+of the teeth and of the foot. This correlation is a graded one, for
+camels, which have a more perfect dentition than other ruminants,
+have also a bone more in their tarsus. It seems probable,
+therefore, that there is some reason, that is, some explanation in
+terms of function, for this case of correlation.</p>
+
+<p>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 <span class="pagenum"><a
+name="pg038" id="pg038">038</a></span>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.</p>
+
+<p>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 <i>tourbillon vital</i>.
+"This general movement, universal in every part, is so unmistakably
+the very essence of life that parts separated from a living body
+straightway die."<a name="FNanchor_51" id="FNanchor_51" /><a
+href="#Footnote_51" class="fnanchor">[51]</a> The organisation of
+the body, the arrangement of its solids and liquids, is adapted to
+further the <i>tourbillon vital</i>. "Each part contributes to this
+general movement its own particular action and is affected by it in
+particular ways, with the result that, in every being, life is a
+unity which results from the mutual action and reaction of all its
+parts."<a name="FNanchor_52" id="FNanchor_52" /><a href=
+"#Footnote_52" class="fnanchor">[52]</a></p>
+
+<p>Cuvier, however, did not resolve life into metabolism, nor
+reduce vital happenings to the chemical level. The form of
+organised bodies is more essential than the matter of which they
+are composed, for the matter changes ceaselessly while the form
+remains unchanged. It is in form that we must seek the differences
+between species, and not in the combinations of matter, which are
+almost the same in all.<a name="FNanchor_53" id=
+"FNanchor_53" /><a href="#Footnote_53" class="fnanchor">[53]</a>
+The differences are to be sought at the level of the second and
+third degrees of composition.</p>
+
+<p>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>i.e.</i>, of
+<span class="pagenum"><a name="pg039" id="pg039">039</a></span>such
+a character that it does not destroy the harmony of the whole."<a
+name="FNanchor_54" id="FNanchor_54" /><a href="#Footnote_54"
+class="fnanchor">[54]</a> 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.</p>
+
+<p>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,<a name=
+"FNanchor_55" id="FNanchor_55" /><a href="#Footnote_55"
+class="fnanchor">[55]</a> a curious forestalment, in the rough, of
+the modern division of Metazoa into C&oelig;lentera and
+C&oelig;lomata.</p>
+
+<p>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 <i>&Eacute;chelle des
+&ecirc;tres</i> or graded series; and you can get even this kind
+of gradation only within each of the big groups formed on a common
+plan of structure; you can never grade, for example, from
+Invertebrates to Vertebrates through intermediate forms<a name=
+"FNanchor_56" id="FNanchor_56" /><a href="#Footnote_56"
+class="fnanchor">[56]</a> (which is perfectly true, in spite of
+Amphioxus and Balanoglossus!).</p>
+
+<p>In the <i>R&egrave;gne Animal</i> Cuvier restricts the
+application of the idea of the <i>&Eacute;chelle</i> 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
+<span class="pagenum"><a name="pg040" id="pg040">040</a></span>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."<a name="FNanchor_57" id=
+"FNanchor_57" /><a href="#Footnote_57" class="fnanchor">[57]</a>
+This remark, which is after all only just, perfectly expresses
+Cuvier's attitude to the transcendental theories, and was probably
+a protest against the sweeping generalisations of his colleague,
+Etienne Geoffroy St Hilaire.</p>
+
+<p>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
+<i>Le&ccedil;ons</i>, Article V., "Division des animaux
+d'apr&egrave;s l'ensemble de leur organisation." The scheme of
+classification actually given in the <i>Le&ccedil;ons</i> 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&mdash;Mammals, Birds, Reptiles, Fishes, Molluscs,
+Crustacea, Insects, Worms, Zoophytes (including Echinoderms and
+C&oelig;lenterates).</p>
+
+<p>A maturer theory and practice of classification is given in the
+<i>R&egrave;gne Animal</i> 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
+<i>Genera Plantarum</i>, 1789,<a name="FNanchor_58" id=
+"FNanchor_58" /><a href="#Footnote_58" class=
+"fnanchor">[58]</a>) is more clearly recognised. The properties or
+peculiarities of structure which have the greatest number of
+relations of incompatibility and coexistence, and therefore
+influence the whole in the greatest degree, are the important or
+dominating characters, to which the others must be subordinated in
+classification. These dominant characters are also the most
+constant.<a name="FNanchor_59" id="FNanchor_59" /><a href=
+"#Footnote_59" class="fnanchor">[59]</a> In deciding which
+characters are the most important Cuvier makes use of his
+fundamental classification of functions and organs into two main
+sets. "The heart and the organs of circulation are <span class=
+"pagenum"><a name="pg041" id="pg041">041</a></span>a kind of centre
+for the vegetative functions, as the brain and the spinal cord are
+for the animal functions."<a name="FNanchor_60" id=
+"FNanchor_60" /><a href="#Footnote_60" class="fnanchor">[60]</a>
+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,<a name=
+"FNanchor_61" id="FNanchor_61" /><a href="#Footnote_61"
+class="fnanchor">[61]</a> of which all the others are but
+modifications. These four types are Vertebrates, Molluscs,
+Articulates, and Radiates. The first three have bilateral, the last
+has radial symmetry. Vertebrates and Molluscs have blood-vessels,
+but Articulates show a functional transition from the blood-vessel
+to the tracheal system. Radiates approach the homogeneity of
+plants; they appear to lack a distinct nervous system and sense
+organs, and the lowest of them show only a homogeneous pulp which
+is mobile and sensitive. All four classes are principally
+distinguished from one another by the broad structural relations of
+their neuromuscular system, of the organs of the animal functions.
+Vertebrates have a spinal cord and brain, an internal skeleton
+built on a definite plan, with an axis and appendages; in Molluscs
+the muscles are attached to the skin and the shell, and the nervous
+system consists of separate masses; Articulates have a hard
+external skeleton and jointed limbs, and their nervous system
+consists of two long ventral cords; Radiates have ill-defined
+nervous and muscular systems, and in their lowest forms possess the
+animal functions without the animal organs.</p>
+
+<p>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.</p>
+
+<p>Cuvier has become known as the greatest champion of the fixity
+of species, but it is not often recognised that his <span class=
+"pagenum"><a name="pg042" id="pg042">042</a></span>attitude to this
+problem is at least as scientific as that of the evolutionists of
+his own and later times. No doubt he became dogmatic in his
+rejection of evolution-theory, but he was on sure ground in
+maintaining that the evolutionists of his day went beyond their
+facts. He considered that certain forms (species) have reproduced
+themselves from the origin of things without exceeding the limits
+of variation. His definition of a species was, "the individuals
+descended from one another or from common parents, together with
+those that resemble them as much as they resemble one another."<a
+name="FNanchor_62" id="FNanchor_62" /><a href="#Footnote_62"
+class="fnanchor">[62]</a> "These forms are neither produced nor do
+they change of themselves; life presupposes their existence, for it
+cannot arise save in organisations ready prepared for it."<a name=
+"FNanchor_63" id="FNanchor_63" /><a href="#Footnote_63"
+class="fnanchor">[63]</a></p>
+
+<p>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.<a name="FNanchor_64" id=
+"FNanchor_64" /><a href="#Footnote_64" class="fnanchor">[64]</a>
+Pal&aelig;ontology 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."<a
+name="FNanchor_65" id="FNanchor_65" /><a href="#Footnote_65"
+class="fnanchor">[65]</a> 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.</p>
+
+<p>Cuvier's first objection was of course determined to some extent
+by the imperfection of the pal&aelig;ontological 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 <i>Paludina</i> series, <span class=
+"pagenum"><a name="pg043" id="pg043">043</a></span>yet at any one
+geological level the species, the lines of descent, are all
+distinct from one another.<a name="FNanchor_66" id=
+"FNanchor_66" /><a href="#Footnote_66" class=
+"fnanchor">[66]</a></p>
+
+<p>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 Dep&eacute;ret puts it, "Cuvier not
+only demonstrated the presence in the sedimentary strata of a
+series of terrestrial faunas superimposed and distinct, but he was
+the first to express, and that very clearly, the idea of the
+gradual increase in complexity of these faunas from the oldest to
+the most recent" (p.&nbsp;10).</p>
+
+<p>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."<a name="FNanchor_67" id="FNanchor_67" /><a href=
+"#Footnote_67" class="fnanchor">[67]</a> It was left to d'Orbigny
+to teach the doctrine of successive creations, of which he
+distinguished twenty-seven (<i>Cours &eacute;l&eacute;mentaire de
+palaeontologie stratigraphique</i>, 1849).</p>
+
+<p>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 <i>Recherches sur les Ossemens
+Fossiles</i> 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 <span class="pagenum"><a name="pg044" id=
+"pg044">044</a></span>to a civilised state.<a name="FNanchor_68"
+id="FNanchor_68" /><a href="#Footnote_68" class=
+"fnanchor">[68]</a> He refused to accept the extravagant
+evolutionary theory of Demaillet and the somewhat confused theory
+of Lamarck (whom he joins with Demaillet),<a name="FNanchor_69"
+id="FNanchor_69" /><a href="#Footnote_69" class=
+"fnanchor">[69]</a> just as he rejected the transcendental theories
+of Geoffroy St Hilaire, because they seemed to him not based upon
+facts.</p>
+
+<div class="footnote">
+<p><a name="Footnote_41" id="Footnote_41" /><a href=
+"#FNanchor_41"><span class="label">[41]</span></a>
+<i>Le&ccedil;ons d'Anatomie Compar&eacute;e</i>, tome i., pp.&nbsp;10
+<i>et scq.</i>, 1800.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_42" id="Footnote_42" /><a href=
+"#FNanchor_42"><span class="label">[42]</span></a>
+<i>Le&ccedil;ons d'Anatomie Compar&eacute;e</i>, i., p.&nbsp;18.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_43" id="Footnote_43" /><a href=
+"#FNanchor_43"><span class="label">[43]</span></a> <i>Loc.
+cit.</i>, i., p.&nbsp;13.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_44" id="Footnote_44" /><a href=
+"#FNanchor_44"><span class="label">[44]</span></a>
+<i>Le&ccedil;ons d'Anatomie Compar&eacute;e</i>, tome i., Articles
+iii.-iv., 1800.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_45" id="Footnote_45" /><a href=
+"#FNanchor_45"><span class="label">[45]</span></a>
+<i>Le&ccedil;ons d'Anatomie Compar&eacute;e</i>, i., p.&nbsp;47.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_46" id="Footnote_46" /><a href=
+"#FNanchor_46"><span class="label">[46]</span></a> <i>Le
+R&egrave;gne Animal</i>, i., p.&nbsp;6, 1817.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_47" id="Footnote_47" /><a href=
+"#FNanchor_47"><span class="label">[47]</span></a> <i>Histoire des
+Progr&egrave;s des Sciences naturelles depuis 1789</i>, i., p.&nbsp;310,
+1826.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_48" id="Footnote_48" /><a href=
+"#FNanchor_48"><span class="label">[48]</span></a> <i>Recherches
+sur les Ossemens Fossiles</i>, i., p.&nbsp;60, 1812.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_49" id="Footnote_49" /><a href=
+"#FNanchor_49"><span class="label">[49]</span></a> <i>Ossemens
+fossiles</i>, i., p.&nbsp;60.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_50" id="Footnote_50" /><a href=
+"#FNanchor_50"><span class="label">[50]</span></a> <i>Loc.
+cit.</i>, i., p.&nbsp;63.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_51" id="Footnote_51" /><a href=
+"#FNanchor_51"><span class="label">[51]</span></a>
+<i>Le&ccedil;ons d'Anatomie Compar&eacute;e</i>, i., p.&nbsp;6.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_52" id="Footnote_52" /><a href=
+"#FNanchor_52"><span class="label">[52]</span></a> <i>Le
+R&egrave;gne Animal</i>, i., p.&nbsp;16.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_53" id="Footnote_53" /><a href=
+"#FNanchor_53"><span class="label">[53]</span></a> <i>Hist. Prog.
+Sci. Nat.</i>, i., p.&nbsp;187, 1826.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_54" id="Footnote_54" /><a href=
+"#FNanchor_54"><span class="label">[54]</span></a>
+<i>Le&ccedil;ons</i>, i., p.&nbsp;58.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_55" id="Footnote_55" /><a href=
+"#FNanchor_55"><span class="label">[55]</span></a> <i>Loc.
+cit.</i>, i., Article iii.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_56" id="Footnote_56" /><a href=
+"#FNanchor_56"><span class="label">[56]</span></a> <i>Loc.
+cit.</i>, i., p.&nbsp;60.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_57" id="Footnote_57" /><a href=
+"#FNanchor_57"><span class="label">[57]</span></a> <i>R&egrave;gne
+Animal</i>, i., p.&nbsp;xx.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_58" id="Footnote_58" /><a href=
+"#FNanchor_58"><span class="label">[58]</span></a> Cuvier,
+<i>Hist. Prog. Sci. Nat.</i>, i., p.&nbsp;288, 1826.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_59" id="Footnote_59" /><a href=
+"#FNanchor_59"><span class="label">[59]</span></a> <i>R&egrave;gne
+Animal</i>, i., p.&nbsp;10.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_60" id="Footnote_60" /><a href=
+"#FNanchor_60"><span class="label">[60]</span></a> <i>R&egrave;gne
+Animal</i>, p.&nbsp;55.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_61" id="Footnote_61" /><a href=
+"#FNanchor_61"><span class="label">[61]</span></a> First
+propounded by Cuvier in 1812, <i>Ann. Mus. d'Hist. Nat.</i>,
+xix.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_62" id="Footnote_62" /><a href=
+"#FNanchor_62"><span class="label">[62]</span></a> <i>R&egrave;gne
+Animal</i>, i., p.&nbsp;19.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_63" id="Footnote_63" /><a href=
+"#FNanchor_63"><span class="label">[63]</span></a> <i>Loc.
+cit.</i>, p.&nbsp;20.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_64" id="Footnote_64" /><a href=
+"#FNanchor_64"><span class="label">[64]</span></a> <i>Recherches
+sur les Ossemens Fossiles</i>, i., p.&nbsp;74, 1812.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_65" id="Footnote_65" /><a href=
+"#FNanchor_65"><span class="label">[65]</span></a> <i>Loc.
+cit.</i>, p.&nbsp;79.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_66" id="Footnote_66" /><a href=
+"#FNanchor_66"><span class="label">[66]</span></a> See C.
+Dep&eacute;ret, <i>Les transformations du Monde animal</i>, Paris,
+1907, and G. Steinmann, <i>Die geologischen Grundlagen der
+Abstammungslehre</i>, Leipzig, 1908.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_67" id="Footnote_67" /><a href=
+"#FNanchor_67"><span class="label">[67]</span></a>
+<i>Recherches</i>, i., p.&nbsp;81.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_68" id="Footnote_68" /><a href=
+"#FNanchor_68"><span class="label">[68]</span></a> <i>R&egrave;gne
+Animal</i>, i., p.&nbsp;91.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_69" id="Footnote_69" /><a href=
+"#FNanchor_69"><span class="label">[69]</span></a> <i>Ossemens
+Fossiles</i>, i., p.&nbsp;26.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg045" id=
+"pg045">045</a></span></p>
+
+<h3>CHAPTER IV</h3>
+
+<h4>GOETHE</h4>
+
+<p>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 formul&aelig;
+extracted from experience as intuitions imposed upon experience. So
+it was that Goethe, who was little more than a dilettante,<a name=
+"FNanchor_70" id="FNanchor_70" /><a href="#Footnote_70"
+class="fnanchor">[70]</a> seized upon the essential principles of a
+morphology some years before that morphology was accepted by the
+workers.</p>
+
+<p>Goethe is important in the history of morphological method
+because he was the first to bring to clear consciousness and to
+express in definite terms the idea on which comparative anatomy
+before him was based, the idea of the unity of plan. We have seen
+that this idea was familiar to Aristotle and that it was recognised
+implicitly by all who after him studied structure comparatively. In
+Goethe's time the idea had become ripe for expression. It was used
+as a guiding principle in Goethe's youth particularly by Vicq
+d'Azyr and by Camper. The former (1748-1794), who discovered<a
+name="FNanchor_71" id="FNanchor_71" /><a href="#Footnote_71"
+class="fnanchor">[71]</a> in the same year as Goethe (1784) the
+intermaxillary bone in man, pointed out the homology in structure
+between the fore limb and the hind limb, and interpreted certain
+rudimentary bones, the intermaxillaries and rudimentary clavicles,
+in the light of the theory that Vertebrates are built upon one
+single plan of structure.</p>
+
+<p>"Nature seems to operate always according to an original and
+general plan, from which she departs with regret and <span class=
+"pagenum"><a name="pg046" id="pg046">046</a></span>whose traces we
+come across everywhere" (Vicq d'Azyr, quoted by Flourens,
+<i>M&eacute;m. Acad. Sci.</i>, <span class="smcap">XXIII.</span>, p.&nbsp;xxxvi.).</p>
+
+<p>Peter Camper (1722-1789), we are told by Goethe himself in his
+<i>Ost&eacute;ologie</i>, was convinced of the unity of plan holding
+throughout Vertebrates; he compared in particular the brain of
+fishes with the brain of man.</p>
+
+<p>The idea of the unity of plan had not yet become limited and
+defined as a strictly scientific theory; it was an idea common to
+philosophy, to ordinary thought, and to anatomical science. We find
+it expressed by Herder (who perhaps got it from Kant) in his
+<i>Ideen zur Philosophie der Geschichte der Menschheit</i> (1784),
+and it is possible that Goethe became impressed with the importance
+of the idea through his conversations with Herder. Be that as it
+may, it is certain that Goethe sought for the intermaxillaries in
+man only because he was firmly convinced that the skeleton in all
+the higher animals was built upon one common plan and that
+accordingly bones such as the intermaxillaries, found well
+developed in some animals, must also be found in man. The idea was
+not drawn from the facts, but the facts were interpreted and even
+sought for in the light of the idea. "I eagerly worked upon a
+general osteological scheme, and had accordingly to assume that all
+the separate parts of the structure, in detail as in the whole,
+must be discoverable in all animals, because on this supposition is
+built the already long begun science of comparative anatomy."<a
+name="FNanchor_72" id="FNanchor_72" /><a href="#Footnote_72"
+class="fnanchor">[72]</a></p>
+
+<p>The principle comes to clear expression in his <i>Erster Entwurf
+einer allgemeinen Einleitung in die vergleichende Anatomie</i>
+(1795).<a name="FNanchor_73" id="FNanchor_73" /><a href=
+"#Footnote_73" class="fnanchor">[73]</a> He writes:&mdash;"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."<a name="FNanchor_74" id="FNanchor_74" /><a
+href="#Footnote_74" class="fnanchor">[74]</a> His aim is to
+discover a general scheme of the constant in organic parts, a
+scheme into which all animals will fit equally well, and no animal
+better than the rest. When we remember that the type to which
+anatomists before him had, consciously or unconsciously, referred
+all <span class="pagenum"><a name="pg047" id=
+"pg047">047</a></span>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.</p>
+
+<p>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:&mdash;"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."<a name="FNanchor_75" id="FNanchor_75" /><a href=
+"#Footnote_75" class="fnanchor">[75]</a> 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 <i>Entwurf</i> (1796),
+saying:&mdash;"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."<a
+name="FNanchor_76" id="FNanchor_76" /><a href="#Footnote_76"
+class="fnanchor">[76]</a> But in seeking for the intermaxillary bone
+in man he was guided by its position relative to the
+maxillaries&mdash;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.</p>
+
+<p>As a matter of fact Goethe's morphological views are neither
+very clearly expressed nor very consistent. This <span class=
+"pagenum"><a name="pg048" id="pg048">048</a></span>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."<a name="FNanchor_77" id="FNanchor_77" /><a href=
+"#Footnote_77" class="fnanchor">[77]</a> We are not to explain, he
+says, the tusks of the <i>Babirussa</i> 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
+R&aacute;dl<a name="FNanchor_78" id="FNanchor_78" /><a href=
+"#Footnote_78" class="fnanchor">[78]</a> is probably correct in
+saying that the following quotation with its double assertion
+represents most nearly Goethe's position:&mdash;</p>
+
+<div class="blockquot">
+<p>"Also bestimmt die Gestalt die Lebensweise des Thieres, Und die
+Weise zu leben, sie wirkt auf alle Gestalten M&auml;chtig
+zur&uuml;ck."<a name="FNanchor_79" id="FNanchor_79" /><a href=
+"#Footnote_79" class="fnanchor">[79]</a></p>
+</div>
+
+<p>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.</p>
+
+<p>What Goethe considered he had proved for the appendages of
+plants he extended to all living things. Every living thing is a
+complex of living independent beings, which "der <span class=
+"pagenum"><a name="pg049" id="pg049">049</a></span>Idee, der Anlage
+nach," are the same, but in appearance may be the same or similar,
+different or unlike.<a name="FNanchor_80" id="FNanchor_80" /><a
+href="#Footnote_80" class="fnanchor">[80]</a> 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 <i>inter
+se</i>. 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
+vertebr&aelig;. 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.</p>
+
+<p>Though Goethe's morphological views were lacking in definiteness
+he hit upon one or two ideas which proved useful. Thus he
+enunciated the "law of balance" long before Etienne Geoffroy St
+Hilaire, the law "that to no part can anything be added, without
+something being taken away from another part, and <i>vice
+versa</i>."<a name="FNanchor_81" id="FNanchor_81" /><a href=
+"#Footnote_81" class="fnanchor">[81]</a> 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.<a name="FNanchor_82" id="FNanchor_82" /><a href=
+"#Footnote_82" class="fnanchor">[82]</a> This also was a point to
+which the later transcendentalists gave considerable attention.</p>
+
+<p>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
+<i>Gestalt</i> or fixed form, <i>Bildung</i> or form change. He saw
+that <i>Gestalt</i> was but a momentary phase of <i>Bildung</i>,
+and could be considered apart and in itself only by an abstraction
+fatal to all understanding of the living thing. Mephistopheles
+<span class="pagenum"><a name="pg050" id=
+"pg050">050</a></span>scoffs at the scholars who would explain a
+living creature by anatomising it:</p>
+
+<div class="poem">
+<div class="stanza">
+<div class="i4">"Dann hat er die Theile in seiner Hand,</div>
+<div class="i4">&nbsp;Fehlt leider! nur das geistige Band."<a name=
+"FNanchor_83" id="FNanchor_83" /><a href="#Footnote_83"
+class="fnanchor">[83]</a></div>
+</div>
+</div>
+
+<p>Goethe kept clear of this mistake; he knew that the artist comes
+nearer to the truth than the analyst.</p>
+
+<p>In the fragment entitled <i>Bildung und Umbildung organischer
+Naturen</i> (1807), introductory to a reprint of his paper on the
+"Metamorphosis of Plants," we get an exposition of his general
+views on living things. He points out there how we try to
+understand things by separating them into their parts. We can, it
+is true, resolve the organism into its structural elements, but we
+cannot recompose it or endow it with life by joining up the parts.
+Hence we require some other means of understanding it. "In all ages
+even among scientific men there can be discerned a yearning to
+apprehend the living form as such, to grasp the connection of their
+external visible parts, to interpret them as indications of the
+inner activity, and so, in a certain measure, to master the whole
+conceptually." This science which should discover the inner meaning
+of organic <i>Bildung</i> is called Morphology.<a name=
+"FNanchor_84" id="FNanchor_84" /><a href="#Footnote_84"
+class="fnanchor">[84]</a> In Morphology we should not speak of
+<i>Gestalt</i> or fixed form, or if we do we should understand by
+it only a momentary phase of <i>Bildung</i>. 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 <i>bildende Kraft</i> or
+<i>Bildungstrieb</i>, 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&mdash;and so, incidentally, they
+require an artist to interpret them.</p>
+
+<p>This profound conception of the nature of life is applied not
+only to the growing changing individual but also to the whole
+changing world of organisms. They are all manifestations of a
+living shaping power which moulds them. This shaping power,
+immanent in all life, is conceived to work according to a general
+plan, and so we get an explanation of <span class="pagenum"><a
+name="pg051" id="pg051">051</a></span>the fact that living things
+seem simply varieties of one common type.</p>
+
+<p>"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...?"<a name="FNanchor_85" id=
+"FNanchor_85" /><a href="#Footnote_85" class=
+"fnanchor">[85]</a></p>
+
+<p>Such an interpretation of the unity of plan reaches perhaps
+beyond the bounds of science.</p>
+
+<div class="footnote">
+<p><a name="Footnote_70" id="Footnote_70" /><a href=
+"#FNanchor_70"><span class="label">[70]</span></a> <i>See</i>
+Kohlbrugge, "Hist. krit. Studien &uuml;ber Goethe als
+Naturforscher," <i>Zool. Annalen.</i> v., 1913, pp.&nbsp;83-231.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_71" id="Footnote_71" /><a href=
+"#FNanchor_71"><span class="label">[71]</span></a> Or
+re-discovered, according to Kohlbrugge.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_72" id="Footnote_72" /><a href=
+"#FNanchor_72"><span class="label">[72]</span></a> Cotta ed., vol.
+ix., p.&nbsp;448.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_73" id="Footnote_73" /><a href=
+"#FNanchor_73"><span class="label">[73]</span></a> "First Draft of
+a General Introduction to Comparative Anatomy."</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_74" id="Footnote_74" /><a href=
+"#FNanchor_74"><span class="label">[74]</span></a> Cotta ed., ix.,
+p.&nbsp;463.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_75" id="Footnote_75" /><a href=
+"#FNanchor_75"><span class="label">[75]</span></a> Cotta ed., p.
+478.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_76" id="Footnote_76" /><a href=
+"#FNanchor_76"><span class="label">[76]</span></a> <i>Loc.
+cit.</i>, p.&nbsp;491.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_77" id="Footnote_77" /><a href=
+"#FNanchor_77"><span class="label">[77]</span></a> <i>Entwurf</i>,
+Cotta ed., ix., p.&nbsp;465.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_78" id="Footnote_78" /><a href=
+"#FNanchor_78"><span class="label">[78]</span></a> <i>Geschichte
+der biologischen Theorien</i>, i., p.&nbsp;266.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_79" id="Footnote_79" /><a href=
+"#FNanchor_79"><span class="label">[79]</span></a> "So the form
+determines the manner of life of the animal, and the manner of life
+in its turn reacts powerfully upon all forms."</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_80" id="Footnote_80" /><a href=
+"#FNanchor_80"><span class="label">[80]</span></a> <i>Bildung und
+Umbildung organischer Naturen</i>, 1807.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_81" id="Footnote_81" /><a href=
+"#FNanchor_81"><span class="label">[81]</span></a> Cotta ed., ix.,
+p.&nbsp;466.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_82" id="Footnote_82" /><a href=
+"#FNanchor_82"><span class="label">[82]</span></a> <i>Loc.
+cit.</i>, pp.&nbsp;474-5.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_83" id="Footnote_83" /><a href=
+"#FNanchor_83"><span class="label">[83]</span></a> Then he has all
+the parts within his hand, excepting only, sad to say, the living
+bond.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_84" id="Footnote_84" /><a href=
+"#FNanchor_84"><span class="label">[84]</span></a> Goethe was the
+inventor of the word.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_85" id="Footnote_85" /><a href=
+"#FNanchor_85"><span class="label">[85]</span></a> Cotta ed., ix.,
+p.&nbsp;490.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg052" id=
+"pg052">052</a></span></p>
+
+<h3>CHAPTER V</h3>
+
+<h4>ETIENNE GEOFFROY SAINT-HILAIRE</h4>
+
+<p>E. Geoffrey made an experiment, unsuccessful but instructive. He
+tried to found a science of pure morphology; he failed: his failure
+showed, once and for all, that a pure morphology of organic forms
+is impracticable.</p>
+
+<p>Already, in 1796, in one of his earliest memoirs,<a name=
+"FNanchor_86" id="FNanchor_86" /><a href="#Footnote_86"
+class="fnanchor">[86]</a> 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.</p>
+
+<p>In a series of papers published in 1807, Geoffroy took a further
+step, and sought to establish homologies which were not
+obvious&mdash;homologies, too, not so much of organs as of
+parts.</p>
+
+<p>These memoirs (published in the <i>Annales du Mus&eacute;um
+d'Histoire naturelle</i>, 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 <span class=
+"pagenum"><a name="pg053" id="pg053">053</a></span>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 <i>Philosophie anatomique</i>, particularly the first
+volume (1818). This volume contains, beside the important "Discours
+pr&eacute;liminaire" and "Introduction" which we shall presently
+consider in detail, five memoirs, which deal with the various bones
+connected with the respiratory organs in fishes (the bones of the
+operculum, of the hyoid, of the branchial arches, of the pectoral
+girdle), and seek to discover their homologies with corresponding
+bones in air-breathing Vertebrates.</p>
+
+<p>"Can the organisation of vertebrated animals be referred to one
+uniform type?" This is the question with which the <i>Philosophie
+anatomique</i> 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?<a name=
+"FNanchor_87" id="FNanchor_87" /><a href="#Footnote_87"
+class="fnanchor">[87]</a></p>
+
+<p>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
+<i>connections</i>." 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.&nbsp;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.).</p>
+
+<p>It is this principle which enables one to follow out in detail
+the further fundamental conception that in every <span class=
+"pagenum"><a name="pg054" id="pg054">054</a></span>Vertebrate there
+are found the same "organic materials," or units of construction.
+This conception, which Geoffroy calls the <i>Th&eacute;orie des
+analogues</i> (p.&nbsp;xxxii.), is clearly one part of the old idea of
+the unity of type; it teaches the <i>unity of composition</i> of
+organic beings, while the <i>Principe des connexions</i> adds the
+<i>unity of plan</i>.</p>
+
+<p>Both conceptions are logically implicit in the vague notion of
+unity of type; Geoffroy disengaged them, and pushed each to its
+logical extreme.</p>
+
+<p>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.</p>
+
+<p>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.&nbsp;xxxv.).</p>
+
+<p>Comparative anatomists, with few exceptions, had hitherto taken
+man as the type, and referred all structure to his; Geoffroy's
+principles led him to give preference to no one animal in
+particular, but to seize upon each part in the species in which it
+reaches the maximum of its development (p.&nbsp;xxxvi.). He is thus led
+to refer all structures to a generalised abstract type. In this
+abstract type each organ exists at the maximum of its development,
+each organ shows all its potentialities realised. In a way,
+therefore, this type, this abstraction, gives the scheme of the
+possible transformations of each organ.</p>
+
+<p>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 <span class="pagenum"><a name=
+"pg055" id="pg055">055</a></span>speaking, only a single animal."<a
+name="FNanchor_88" id="FNanchor_88" /><a href="#Footnote_88"
+class="fnanchor">[88]</a> The "single animal" is simply the
+generalised type.</p>
+
+<p>Having laid down his two principles Geoffroy goes on to apply
+them to the difficult case of the comparison of the skeleton of
+fish with the skeleton of the higher Vertebrates. "My present task
+is to demonstrate that there is no part of the bony framework of
+fishes that cannot find its analogue in the other vertebrated
+animals."<a name="FNanchor_89" id="FNanchor_89" /><a href=
+"#Footnote_89" class="fnanchor">[89]</a> 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&mdash;the
+operculum, the branchiostegal rays, the branchial arches, and
+others. That the peculiar bones should be connected with the
+respiratory functions is only natural, for the contrast between
+fish and higher Vertebrates is essentially a contrast between
+water-breathing and air-breathing animals. Considering first the
+general form of the skeleton in fish, we are met at once with a
+difficulty; there is no obvious homologue in fishes of the neck,
+the trunk, and the abdomen of higher animals. What apparently
+corresponds to the trunk is in fishes crowded close up under the
+head. But, after all, it is not of the essence of the vertebrate
+type to have the trunk and the abdomen attached at definite and
+invariable distances along the vertebral column&mdash;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.&nbsp;9). Now, says Geoffroy, allow me this one
+hypothesis, that the trunk with its organs can, as it were, move
+bodily along the vertebral column, so as to be found in one class
+near the front end of the vertebral column, in another about the
+middle, and in a third near the end, then I can show you in detail
+that the constituent parts of this trunk are found in all classes
+to be invariably in the same positions relatively to one another
+(p.&nbsp;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 <span class="pagenum"><a name="pg056" id=
+"pg056">056</a></span>with his principal hypothesis of the
+invariability of connections, and that he, so to speak, gets a hold
+on his fish to apply his principle of connections only by admitting
+at the very outset an exception to his primary principle. A further
+application of the hypothesis of metastasis will be noticed below
+in connection with the determination of the sternum of fishes. We
+note here an interpretation of the first metastasis in terms of
+functional adaptation. "The constant and violent action of the
+tail, if it does not go so far as actually to displace and move
+forward the internal organs, at least fits in well with an
+arrangement in which the organs are so disposed" (p.&nbsp;99).</p>
+
+<p>The first memoir deals with the homologies of the opercular
+bones. Geoffroy considers that the external opening of the ear
+corresponds to the external opening of the gill-chamber, which lies
+between the operculum and the pectoral girdle. The ear communicates
+with the buccal cavity by the Eustachian tube, so does the
+branchial chamber by means of the gill-slits. The auditory chamber
+of higher Vertebrates is, therefore, the homologue of the branchial
+chamber in fish; the opercular bones in fish and the ossicles of
+the ear in other Vertebrates stand in close relation to this
+chamber; therefore the opercular bones are the homologues of the
+ossicles of the ear, the interoperculum corresponding to the
+malleus, the suboperculum to the lenticular, the minute lower part
+of the suboperculum to the incus, the operculum to the stapes, and
+the pre-operculum to the tympanic ring. In making these particular
+determinations Geoffroy professes to be led by his principle of
+connections. The pre-operculum has, he says, the same connections
+with neighbouring bones as the tympanic bone in other Vertebrates,
+and the other pieces of the gill-cover are homologised with
+particular ear-ossicles according to the order in which they stand
+to one another. The second memoir in the book deals with the
+sternum, and affords a very good example of Geoffroy's method of
+dealing with the facts of structure. We shall omit here any
+detailed reference to the other three memoirs, which deal with the
+hyoid, with the branchial arches and the structures which
+correspond in air-breathing Vertebrates, and with the bones of the
+shoulder-girdle.</p>
+
+<p><span class="pagenum"><a name="pg057" id=
+"pg057">057</a></span>In the memoir on the sternum Geoffroy's first
+care is to arrive at a definition of what a sternum is. He defines
+it partly by its functions, partly by its connections, as the
+system of bones which covers and protects the thorax, and gives
+attachment to certain groups of muscles.</p>
+
+<p>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 <i>nine elementary parts</i>" (p.
+105). These nine bones are in Geoffroy's nomenclature, the
+episternals, the hyosternals, the hyposternals, the xiphisternals,
+which are all paired bones, and the entosternal, which is unpaired.
+The arrangement of them is in the tortoise:&mdash;</p>
+
+<div class="figcenter">
+<img src="images/img057a.jpg" alt=
+"nine bones are in Geoffroy's nomenclature" /></div>
+
+<p>The articulations in the tortoise are indicated by the
+connecting lines. Geoffroy tries to show that the sternum in other
+animals is composed of these nine bones, or at least of a certain
+number of them, always in the same invariable relative positions.
+Thus in birds the sternum consists of five pieces, of a huge keeled
+entosternal, and of two "annexes" on either side, which are the
+hyo-and hyposternals. These are separate only in young birds.
+Occasionally, especially in <span class="pagenum"><a name="pg058"
+id="pg058">058</a></span>young birds, rudiments of episternals and
+xiphisternals also occur. The minuteness of the episternals and the
+xiphisternals</p>
+
+<div class="figcenter"><img src="images/fig01a.jpg"
+alt="Hyoid Arch of the Conger," /></div>
+
+<div class="center2"><span class="smcap">Fig</span> 1.&mdash;Hyoid Arch of the Conger.(Original)</div>
+
+<p>may be attributed to the gigantic size of the entosternal, in
+accordance with the <i>Loi de balancement</i>. In the other
+air-breathing Vertebrates the nine sternal elements can according
+to Geoffroy be discovered without <span class="pagenum"><a name=
+"pg059" id="pg059">059</a></span>great difficulty. But when we come
+to the determination of the sternum in fishes, difficulties abound,
+which Geoffroy solves in the following way. He points out that
+between the clavicles (<i>cleithra</i>) and the hyoid bone
+(<i>basihyal</i>) in fishes there is a long median bone
+(<i>urohyal</i>) which is attached in front by two strong tendons
+to the horns of the hyoid and is free behind (see <a href=
+"#pg058">Fig.&nbsp;1</a>). Gouan (1720) had seen in this bone the
+homologue of the sternum. Geoffroy adopts this view, but considers
+that this bone alone cannot represent the whole sternum. He finds
+the representatives of other bones of the sternum in the large
+bones (<i>epihyal</i> and <i>ceratohyal</i>, or the two pieces of
+the <i>ceratohyal</i>) which are comprised in the hyoid arch. But
+he is immediately met by the difficulty that this complex of bones
+is situated in front of the pectoral girdle, whereas the sternum in
+higher Vertebrates lies behind the pectoral girdle. He reflects,
+however, that the gills of fish, situated in front of the
+clavicles, are merely the lungs under another name. The gills have
+become shifted forward by a metastasis similar to that which
+brought the whole thoracic organs far forward in fish. This being
+so, their supporting elements, the sternum and the ribs, must have
+moved with them, and are hence to be found in front of the pectoral
+girdle.</p>
+
+<p>Geoffroy's next step is to point out that the only possible
+homologues of sternal ribs are the branchiostegal rays, which arise
+from the large bones of the hyoid arch. If these are sternal ribs,
+the bones to which they are attached must be the hyo- and
+hyposternals or "annexes," the bones from which in birds the ribs
+take their origin.</p>
+
+<p>The unpaired sternal bone (<i>urohyal</i>) cannot be homologous
+with the entosternal, for it has no connections with the annexes.
+He decides that it must represent the episternals, for in some
+young birds there is a two-headed episternal to which two strong
+tendons are attached, just in the same way as the unpaired piece in
+fish is bound to the bones of the hyoid by two tendons. "Thus it is
+not the sternum as a whole that has shifted in front of the
+clavicles and covered with its side pieces the gills placed there;
+it is a piece exclusively piscine, in the sense that it is only in
+the class of <span class="pagenum"><a name="pg060" id=
+"pg060">060</a></span>fishes that it reaches the <i>maximum</i> of
+its development" (p.&nbsp;83).</p>
+
+<p>To sum up, the sternum in all four vertebrate classes is
+composed of the same elements, arranged always in the same way.
+"One is ... led to the conception of an ideal type of sternum for
+all Vertebrates, which then, considered from a lower standpoint,
+resolves itself into several secondary forms according as the whole
+or the majority of the constituent materials are employed, or even
+as these elements come to change their respective dimensions or
+proportions" (p.&nbsp;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.&nbsp;132). What holds good for the sternum
+holds good for other organs&mdash;and accordingly the unity of plan
+and composition can be demonstrated for all the organs of
+Vertebrates.</p>
+
+<p>Soon after the publication of the <i>Philosophie anatomique</i>
+(1818) Geoffroy went further in his search for unity, and
+maintained that the structure of insects and Crustacea could be
+reduced to the vertebrate type.</p>
+
+<p>He proposed to replace Cuvier's classification of the animal
+kingdom into the four large groups, Vertebrata, Mollusca,
+Articulata, and Radiata by the following classification:&mdash;<a
+name="FNanchor_90" id="FNanchor_90" /><a href="#Footnote_90"
+class="fnanchor">[90]</a></p>
+
+<table width="90%" summary=
+"Vertebrata, Mollusca, Articulata, and Radiata" border="0">
+<tbody>
+<tr>
+<td class="cell_lt60a" rowspan="2">Vert&eacute;br&eacute;s</td>
+<td class="cell_lt60b" rowspan="2"><img src="images/para4a.jpg"
+height="100%" alt="para" /></td>
+<td class="cell_lt60c">Hauts-Vert&eacute;br&eacute;s (Vertebrata, Cuv.).</td>
+</tr>
+<tr>
+<td class="cell_lt60c">Dermo-Vert&eacute;br&eacute;s (Articulata, Cuv.).</td>
+</tr>
+
+<tr>
+<td class="cell_lt60a" rowspan="2">.Invert&eacute;br&eacute;s.</td>
+<td class="cell_lt60b" rowspan="2"><img src="images/para4a.jpg"
+height="100%" alt="para" /></td>
+<td class="cell_lt60c">Mollusques (Mollusca, Cuv.)</td>
+</tr>
+
+<tr>
+<td class="cell_lt60c">Rayonn&eacute;s (Radiata, Cuv.).</td>
+</tr>
+</tbody>
+</table>
+
+<p>The idea upon which is based the comparison of Articulates with
+Vertebrates is that each skeletal segment of Articulates is a
+vertebra. In the Hauts-vert&eacute;br&eacute;s the vertebr&aelig; are
+internal; in the Dermo-vert&eacute;br&eacute;s they are external.
+"<i>Every animal lives either outside or inside its vertebral
+column</i>."<a name="FNanchor_91" id="FNanchor_91" /><a href=
+"#Footnote_91" class="fnanchor">[91]</a> The essence of a vertebra
+is not its form, nor its function, but its composition from four
+<span class="pagenum"><a name="pg061" id=
+"pg061">061</a></span>elementary pieces which unite round a central
+space (<i>Isis, loc. cit.</i>, p.&nbsp;532). Serres had shown that in
+the higher animals every vertebra is formed from four centres of
+ossification, that the body of the vertebra is at first tubular,
+and that afterwards it becomes filled up. In lobsters and crabs
+each segment is composed of four elementary pieces, as may be seen
+most easily in young ones. "Accordingly each segment corresponds to
+a true vertebra in composition: there is the same number of
+'materials,' the same order in the course of ossification, the same
+kind of articulation, the same annular arrangement, the same empty
+space in the middle" (p.&nbsp;534). The only difference is that in
+Articulates the central space is very great and contains all the
+organs of the body, whereas in the higher Vertebrates the body of
+the vertebra becomes completely filled up. In the thoracic region
+of Crustacea it is not the whole segment with part of the carapace
+which corresponds to a vertebra, but merely the part round the
+ventral nerve-cord (endophragmal skeleton).</p>
+
+<div class="figright">
+             <img   src="images/fig02a.jpg"
+             alt="Fig. 2." /><br />
+         Fig.&nbsp;2.<br />"Vertebra" of a Pleuronectid.<br />(After Geoffroy.)
+    </div>
+
+
+<p>If the skeleton of the segment in Articulates corresponds to the
+body of a vertebra and is here external, then the appendages of the
+Articulate must correspond to ribs (p.&nbsp;538). The full development
+of this thought is found in a Memoir of 1822, "Sur la
+vert&egrave;bre."<a name="FNanchor_92" id="FNanchor_92" /><a
+href="#Footnote_92" class="fnanchor">[92]</a> He takes as the
+typical vertebra that of a Pleuronectid, probably the turbot. His
+original figure is reproduced (Fig.&nbsp;2).</p>
+
+<p>He includes as part of the vertebra not only the neural (e&#8242;, e&#8243;)
+and h&aelig;mal (o&#8242;, o&#8243;) arches, but also, above and below these,
+the radialia (a&#8243;, u&#8242;) and the fin-rays (a&#8242;, u&#8243;). (Neither the
+radialia nor the fin-rays are, <span class="pagenum"><a name=
+"pg062" id="pg062">062</a></span>by the way, in the same transverse
+plane as the body of the vertebra). Every vertebra, he considers,
+contains these nine pieces&mdash;the cycleal (or body), the two
+perials (e&#8242;, e&#8243;) and the two epials (a&#8242;, a&#8243;) above, the two paraals
+(o&#8242;, o&#8243;) and the two cataals (u&#8242;, u&#8243;) 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&mdash;the skull is formed of modified vertebr&aelig;&mdash;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.</p>
+
+<p>We have seen that in Arthropods the body of the vertebra
+(cycleal) forms the open ring of the segment, which lies
+immediately under the skin, the vertebral tube coinciding with the
+epidermal tube. The homologues of the other eight pieces of the
+vertebra must accordingly be sought in the external appendages. At
+first sight there seems here a contradiction of the principle of
+connections, for the appendages in Arthropods are lateral, whereas
+the paired bones of the vertebra are dorsal and ventral. But there
+is in reality no contradiction, for "what our law of connections
+absolutely requires is that all organs, whether internal or
+external, should stand to one another in the same relations; but it
+is all one whether the box (<i>coffre</i>) that encloses them lies
+with this or that side on the ground. What similarities in the
+organisation of man and the digitate mammals, and yet what
+differences between their attitudes when standing! The same holds
+true as regards the normal attitudes of the pleuronectids and the
+other fishes" (p.&nbsp;107).</p>
+
+<p>The exact way in which Geoffroy homologised the parts of
+the appendages in Arthropods with the paired pieces of the typical
+vertebra is best shown by the reproduction of his figure of an
+abdominal segment of the lobster (Fig.&nbsp;3), in
+which the parts homologous with those represented in the figure of
+the typical vertebra (<a href="#pg062">Fig.&nbsp;2</a>) are indicated by
+the same letters. The ingenuity of the comparison is
+astonishing.</p>
+
+<p><span class="pagenum"><a name="pg063" id=
+"pg063">063</a></span>The comparison of the Arthropod with the
+Vertebrate is extended also to the internal organs. The internal
+organs of the Arthropod are shown to stand in the same order to one
+another as in the Vertebrate, only the organs are inverted. Thus
+the nervous system is dorsal in the Vertebrate, ventral in the
+Arthropod. Turn the Arthropod on its back and the relative
+positions of the systems of organs are the same as in the
+Vertebrate. The relation of the organs to the external tube is of
+course different in Arthropods and Vertebrates, but this is no
+contradiction of the principle of connections. "Such a tube,
+although it is the organs essential to life that it contains, can
+yet behave in different ways with regard to the mass of these
+organs: the principle of connections demands only that all the
+organs maintain with one another fixed and definite relations; but
+the principle would be in no way invalidated if the whole mass had
+rotated inside the tube" (p.&nbsp;112).</p>
+
+<div class="figright">
+             <img   src="images/fig03a.jpg"
+             alt="Fig 3." /><br />
+         <span class="smcap">Fig.</span> 3.<br />Abdominal Segment of the Lobster. <br />(After Geoffroy.)
+    </div>
+
+
+<p>Geoffroy pushed the analogy between Arthropods and Vertebrates
+very far, for he asserted that every piece in the skeleton of an
+insect was homologous with some bone in Vertebrates, that it stood
+always in its proper place, and remained faithful to at least one
+of its connections.<a name="FNanchor_93" id="FNanchor_93" /><a
+href="#Footnote_93" class="fnanchor">[93]</a> 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, <i>Sur l'organisation des insectes</i>. Six segments are
+distinguished in an insect&mdash;the head, the three divisions
+<span class="pagenum"><a name="pg064" id="pg064">064</a></span>of
+the thorax, the abdomen, and the terminal segment of the abdomen
+(p.&nbsp;455).</p>
+
+<p>The skeleton of the insect's head is said to correspond to the
+bones of the face, to the bones of the cerebrum and to the hyoid of
+higher Vertebrates, the skeleton of the prothorax to the bones of
+the cerebellum, of the palate, and the pieces of the larynx, the
+skeleton of the mesothorax to the parietals, interparietals, and
+opercular bones, and that of the metathorax to the skeleton of the
+thorax of Vertebrates. The pieces of the abdomen and of the
+terminal segment correspond to the bones of the abdomen and coccyx
+(p.&nbsp;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.</p>
+
+<p>In 1830 two young naturalists, Meyranx and Laurencet, presented
+to the Acad&eacute;mie des Sciences a memoir in which they likened
+a Cephalopod to a Vertebrate bent back at the level of the
+umbilicus, saying that the Vertebrate in this position had all its
+organs in the same order as in the Cephalopod. Geoffroy took up
+this idea with enthusiasm, seeing in it a further application of
+his master-idea of the unity of plan and composition. By means of
+this comparison Mollusca definitely took their place in the
+<i>&Eacute;chelle des &ecirc;tres</i>, after the Articulata, just
+as Geoffroy had maintained in 1820, saying that crabs formed a link
+between the other Crustacea and the molluscs.<a name=
+"FNanchor_94" id="FNanchor_94" /><a href="#Footnote_94"
+class="fnanchor">[94]</a> The comparison brought him nearer to the
+end he had in view, the reference of all animal structure to one
+single type.</p>
+
+<p>But in championing the memoir of Meyranx and Laurencet, Geoffroy
+found himself in direct antagonism with Cuvier, who held that his
+four "Embranchements" had each a separate and distinct plan of
+structure. In a paper read to the Academy in February 1830,<a name=
+"FNanchor_95" id="FNanchor_95" /><a href="#Footnote_95"
+class="fnanchor">[95]</a> Cuvier easily demolished the crude
+comparison of the Cephalopod to the Vertebrate. He gave diagrams of
+the internal organs of a Cephalopod and of a Vertebrate bent back
+in the manner indicated by Meyranx and Laurencet, and he showed in
+<span class="pagenum"><a name="pg065" id=
+"pg065">065</a></span>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,<a name="FNanchor_96" id="FNanchor_96" /><a
+href="#Footnote_96" class="fnanchor">[96]</a> 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:&mdash;"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.&nbsp;257). Geoffroy could
+not accept this commonsense view of the matter, but made a fight
+for his transcendental theories. This was the beginning of the
+famous controversy between Geoffroy and Cuvier which so excited the
+interest of Goethe. It was a struggle between "comparative anatomy"
+and "morphology," between the commonsense teleological view of
+structure and the abstract, transcendental. Geoffroy brought
+forward all his theories on the homology of the skeleton of fish
+with the skeleton of higher Vertebrates, and tried to prove by them
+his great principle of the unity of plan and composition; Cuvier
+took Geoffroy's homologies one by one, and showed how very slight
+was their foundation. Cuvier was on sure ground in insisting upon
+the observable diversities of structural type, and his vast
+knowledge enabled him to score a decisive victory.<a name=
+"FNanchor_97" id="FNanchor_97" /><a href="#Footnote_97"
+class="fnanchor">[97]</a></p>
+
+<p>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.</p>
+
+<p><span class="pagenum"><a name="pg066" id=
+"pg066">066</a></span>In these Darwinian days Geoffroy has reaped a
+little posthumous glory as an early believer in evolution. That he
+did believe in evolution to a limited extent is certain; that his
+theory of evolution was, as it were, a by-product of his life-work,
+is also certain. Geoffroy was primarily a morphologist and a seeker
+after the unity hidden under the diversity of organic form. His
+theory of evolution had as good as no influence upon his
+morphology, for he did not to any extent interpret unity of plan as
+being due to community of descent. His morphological,
+non-evolutionary standpoint comes out quite clearly in several
+places in the <i>Philosophie anatomique</i>. 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.&nbsp;84). Again, with reference
+to the homology of the ossicles of the ear with the opercular bones
+in fish, "employing other resources equally hidden and rudimentary,
+Nature makes profitable use of the four tiny ossicles lodged in the
+auditory passage, and, raising them in fish to the greatest
+possible dimensions, forms from them these broad opercula...." (p.
+85). Or you may take it the other way about, and start from the
+organisation of fishes; opercular bones are of no use to
+air-breathing animals, so they dwindle away, and are pressed into
+the service of the ear, although they are of little use in hearing
+(p.&nbsp;46).</p>
+
+<p>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.<a name="FNanchor_98" id=
+"FNanchor_98" /><a href="#Footnote_98" class="fnanchor">[98]</a>
+In a memoir of 1828,<a name="FNanchor_99" id="FNanchor_99" /><a
+href="#Footnote_99" class="fnanchor">[99]</a> <span class=
+"pagenum"><a name="pg067" id="pg067">067</a></span>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.&nbsp;210). It is sufficiently clear from this
+quotation that Geoffroy was thinking only of a transformation of
+the antediluvian species created by God, and by no means of an
+evolution of all species from one primitive type. In matters of
+religion Geoffroy was orthodox. He goes on to point out how great a
+resemblance there is in essential structure between fossil and
+living species. All find their place in one scheme of
+classification; does it not seem that all are modifications "of one
+single being, of that abstract being or common type, which it is
+always possible to denote by the same name?" (p.&nbsp;211). This type is
+abstract, not actual, and it is certainly not conceived as an
+original ancestor of all animals.</p>
+
+<p>The fullest development of Geoffroy's views on evolution is
+found in his memoir "Le degr&eacute; d'influence du monde ambiant
+pour modifier les formes animales."<a name="FNanchor_100" id=
+"FNanchor_100" /><a href="#Footnote_100" class=
+"fnanchor">[100]</a> 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 <i>differences</i> between
+organic forms. The causes of these differences are to be found in
+the environment (pp.&nbsp;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.&nbsp;74), and that they have in the process
+become modified through external influences.</p>
+
+<p>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 <span class="pagenum"><a
+name="pg068" id="pg068">068</a></span>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.&nbsp;76). The respiratory milieu changes, the species change with
+it, or are eliminated (p.&nbsp;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.</p>
+
+<p>In the first geological period, we read in another Memoir of the
+same year,<a name="FNanchor_101" id="FNanchor_101" /><a href=
+"#Footnote_101" class="fnanchor">[101]</a> when ammonites and
+<i>Gryph&aelig;a</i> 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"<a name="FNanchor_102" id="FNanchor_102" /><a
+href="#Footnote_102" class="fnanchor">[102]</a> (p.&nbsp;58).</p>
+
+<p>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&mdash;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.</p>
+
+<p>From a passage in the memoir "Sur la Vert&egrave;bre" 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&mdash;"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.</p>
+
+<p>He believed in saltatory evolution, for he considered that the
+lower oviparous Vertebrates could not be transformed into birds by
+slow modification, but only by a sudden transformation of their
+lungs, which would bring about the other characteristics of birds
+(p.&nbsp;80). He considered, too, <span class="pagenum"><a name="pg069"
+id="pg069">069</a></span>that transformations could arise by means
+of monstrous development (p.&nbsp;86). In this connection the
+experiments which he made on the hen's egg<a name="FNanchor_103"
+id="FNanchor_103" /><a href="#Footnote_103" class=
+"fnanchor">[103]</a> in order to produce artificial monstrosities are
+significant, though his purpose was rather to obtain proof of the
+inadequacy of the preformation hypothesis.<a name="FNanchor_104"
+id="FNanchor_104" /><a href="#Footnote_104" class=
+"fnanchor">[104]</a></p>
+
+<p>It seems probable enough that if Geoffroy had developed his
+views on evolution he would finally have been led to interpret
+unity of plan in terms of genetic relationship. But as it was he
+remained at his morphological standpoint. He did not interpret
+rudimentary organs as useless heritages of the past; he preferred
+to think that Nature had prepared double means for the same
+function, one or other being predominant according as the animal
+lived in the water or on the land. "To the animal that lives
+exclusively in the air Nature has granted an organisation suited to
+this mode of respiration, without however suppressing the other
+corresponding means, that is to say, without depriving it of a
+second system which is applicable only to the mode of respiration
+by the intermediary of water, and <i>vice versa</i>."<a name=
+"FNanchor_105" id="FNanchor_105" /><a href="#Footnote_105"
+class="fnanchor">[105]</a></p>
+
+<p>He seems, in one instance at least, to have hit upon the
+root-idea of the biogenetic law, but he was far from appreciating
+its significance. He recognised that an amphibian in its
+development passed through a stage when it was in all essentials
+similar to a fish, and he saw in this visible transformation a
+picture of the evolutionary transformation. "An amphibian," he
+writes,<a name="FNanchor_106" id="FNanchor_106" /><a href=
+"#Footnote_106" class="fnanchor">[106]</a> "is at first a fish
+under the name of tadpole, and then a reptile [<i>sic</i>] 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
+<i>repetition</i> of its ancestral history.</p>
+
+<p>He went, however, a certain length towards recognising the main
+principle of a law which was a commonplace of <span class=
+"pagenum"><a name="pg070" id="pg070">070</a></span>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 f&oelig;tus of mammals. He compared
+also Articulates with embryonic Vertebrates in respect of their
+vertebr&aelig;, 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
+(<i>supra</i>, p.&nbsp;61). As regards their vertebr&aelig;, "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
+f&oelig;tal condition."<a name="FNanchor_107" id=
+"FNanchor_107" /><a href="#Footnote_107" class=
+"fnanchor">[107]</a></p>
+
+<p>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.<a name="FNanchor_108" id=
+"FNanchor_108" /><a href="#Footnote_108" class=
+"fnanchor">[108]</a></p>
+
+<p>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.</p>
+
+<p>The principle of unity of plan and composition is the keynote of
+Geoffroy's work. It states that the same materials of organisation
+are to be found in all animals, and that these materials stand
+always in the same general spatial relations to one another. The
+"materials of organisation" are not necessarily organs in the
+physiological sense, and indeed the principle of the unity of plan
+cannot be upheld if the unity has reference to organs only. This
+became clear to Geoffroy, especially in his later years. In 1835 he
+wrote, speaking of the principle of the unity of plan, "I have,
+moreover, regenerated this principle, and obtained for it
+universality of application, by showing that it is not always the
+organs as a whole, but merely the materials composing each <span
+class="pagenum"><a name="pg071" id="pg071">071</a></span>organ,
+that can be reduced to unity."<a name="FNanchor_109" id=
+"FNanchor_109" /><a href="#Footnote_109" class=
+"fnanchor">[109]</a> Even in the <i>Philosophie anatomique</i> he
+deals rather with parts than with organs; he deals, for instance,
+with the elementary parts of the sternum, not with the organ
+"sternum" in its totality. The functions of the sternum vary, and
+the primary protective function of the sternum may be assumed by
+quite other parts, <i>e.g.</i>, by the clavicles in fish, which
+protect the heart.<a name="FNanchor_110" id="FNanchor_110" /><a
+href="#Footnote_110" class="fnanchor">[110]</a></p>
+
+<p>True homologies can be established between materials of
+organisation but not always between organs, which may be composed
+of different "materials."</p>
+
+<p>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&mdash;but homology remains though form
+changes. Accordingly, form is one of the least important elements
+to be considered in determining a homology. "Nature," he wrote in
+one of his early papers, "tends to repeat the same organs in the
+same number and in the same relations, and varies to infinity only
+their form. In accordance with this principle I shall have to draw
+my conclusions, in the determining the bones of the fish's skull,
+not from a consideration of their form, but from a consideration of
+their connections."<a name="FNanchor_111" id=
+"FNanchor_111" /><a href="#Footnote_111" class=
+"fnanchor">[111]</a></p>
+
+<p>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."<a name="FNanchor_112" id="FNanchor_112" /><a
+href="#Footnote_112" class="fnanchor">[112]</a></p>
+
+<p><span class="pagenum"><a name="pg072" id=
+"pg072">072</a></span>Geoffroy's morphological units or materials
+of organisation were in the case of the skeleton&mdash;with which
+his researches principally deal&mdash;the single bones. But the
+interesting point is that he sought his skeleton-units in the
+embryo, and considered each separate centre of ossification as a
+separate bone. Coalescence of bones originally separate is one of
+the most usual events in development, and it is an occurrence
+which, more than any other, tends to obscure homologies. Because of
+its coalescence with the maxillaries, the intermaxillary in man was
+not discovered until Vicq d'Azyr and Goethe found it separate in
+the embryo. Apparently quite independently of Goethe, Geoffroy hit
+upon this plan of seeking in the embryo the primary elements or
+materials of organisation. In an early paper on the skull of
+Vertebrates,<a name="FNanchor_113" id="FNanchor_113" /><a href=
+"#Footnote_113" class="fnanchor">[113]</a> 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 f&oelig;tuses of mammals, and hence bear out the theory" (p.
+344). So, too, in dealing with the homologies of the sternal
+elements (<i>supra</i>, p.&nbsp;57) he treats as separate bones the
+"annexes" of the sternum in birds, though these are separate only
+in the young.</p>
+
+<p>If the same materials of organisation are present in all
+animals, and if they are arranged always in the same positions
+relatively to one another, how does it come about that animal forms
+are so varied, what explanation can be offered of the diversities
+of organic structure? Geoffroy's main answer to this question is
+his <i>Loi de balancement</i>. The law was enunciated by him
+already in 1807.<a name="FNanchor_114" id="FNanchor_114" /><a
+href="#Footnote_114" class="fnanchor">[114]</a> We take the
+following quotation, which represents his thought most nearly, from
+the <i>Cours de l'histoire naturelle des Mammif&egrave;res</i>
+(1829). "According to our manner of regarding the <span class=
+"pagenum"><a name="pg073" id="pg073">073</a></span>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., <i>Le&ccedil;on</i> 16, p.&nbsp;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."<a name="FNanchor_115" id="FNanchor_115" /><a href=
+"#Footnote_115" class="fnanchor">[115]</a> The nutritive material
+available is limited for each species; if one part gets more than
+its share the other parts must get less&mdash;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."<a name="FNanchor_116" id="FNanchor_116" /><a href=
+"#Footnote_116" class="fnanchor">[116]</a></p>
+
+<p>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 <i>&agrave; priori</i> 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.</p>
+
+<p>Now he found his theory to be in its pure form unworkable; he
+found, for example, that the skeleton of fishes <span class=
+"pagenum"><a name="pg074" id="pg074">074</a></span>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
+<i>metastases</i>, before he could bring the skeleton of fish into
+line. And these metastases are due to functional
+requirements&mdash;for example, the forward position of sternum and
+thoracic organs in fish is an adaptation to swimming.</p>
+
+<p>So he does not so much demonstrate the unity of plan of whole
+organisms as the unity of plan of particular corresponding parts of
+them. Thus he does not prove or attempt to prove that Articulates
+are in all points like Vertebrates, but simply that their skeleton
+is built upon the same plan as that of Vertebrates. The rest of the
+organs, while still comparable with the organs of Vertebrates,
+stand in different relations to the skeleton. An Articulate
+therefore, on his own showing, is not, <i>as a whole</i>, built
+upon the same general structural plan as a Vertebrate.</p>
+
+<p>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&mdash;a comparison which is purely
+physiological.</p>
+
+<p>Even with these concessions to the functional view of living
+things, Geoffroy was unable to make good his contention that all
+animals are built upon the same plan. His arguments failed to carry
+conviction to his contemporaries, and Cuvier in particular
+subjected them to destructive, and indeed final, criticism.</p>
+
+<p>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 <span class="pagenum"><a
+name="pg075" id="pg075">075</a></span>clearest way the radical
+opposition between the functional and the formal attitudes to
+living things.</p>
+
+<p>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&mdash;compare a polyp with a
+man!&mdash;on the other hand, if by unity is meant simply
+resemblance or homology, the statement is true within certain
+limits, but it has been employed as a principle since the days of
+Aristotle, and the theory of unity of composition is original only
+in so far as it is false. He admits, however, that Geoffroy has
+seized upon many hidden homologies, especially by his valuable
+discovery of the importance of f&oelig;tal structure. In all this
+Cuvier is undoubtedly right. Unity of plan and composition, as
+Geoffroy conceived it, simply does not exist. Cuvier goes on to say
+that this principle of Geoffroy's, in the greatly modified form in
+which it can be accepted, and has been accepted from the dawn of
+zoology, is not the sole and unique principle of the science. On
+the contrary, it is merely a subordinate principle, subordinate to
+a higher and more fruitful principle, that, namely, of the
+conditions of existence, of the adaptation (<i>convenance</i>) of
+the parts, of the co-ordination of the parts for the r&ocirc;le
+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.&nbsp;248).</p>
+
+<p>Geoffroy's position is the direct contrary. He holds that the
+principle of the unity of plan and composition is the true base of
+natural history,<a name="FNanchor_117" id="FNanchor_117" /><a
+href="#Footnote_117" class="fnanchor">[117]</a>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 <span class="pagenum"><a name=
+"pg076" id="pg076">076</a></span>manifest tendency to resemble one
+another, and to reproduce invariably the same primordial type."<a
+name="FNanchor_118" id="FNanchor_118" /><a href=
+"#Footnote_118" class="fnanchor">[118]</a> 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&mdash;those whose
+structure is adapted to their life. Therefore it is reasonable that
+these few types should be repeated in innumerable exemplars. One
+must remember, in order to appreciate Cuvier's view, that he was
+not obsessed, as we are, by the idea of evolution.</p>
+
+<p>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."<a name="FNanchor_119" id=
+"FNanchor_119" /><a href="#Footnote_119" class=
+"fnanchor">[119]</a> There are only a few kinds of organs, each
+adapted for a particular function, and these organs are necessarily
+repeated from class to class.&mdash;"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."<a name="FNanchor_120" id="FNanchor_120" /><a href=
+"#Footnote_120" class="fnanchor">[120]</a></p>
+
+<p>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
+<i>Mammif&egrave;res</i> (1829) to Cuvier's criticisms in the
+<i>Histoire naturelle des Poissons</i> (1828), he insists on the
+necessity of excluding function from consideration in any truly
+philosophical treatment of comparative anatomy (Discours
+pr&eacute;l., p.&nbsp;25). Cuvier held that function determined
+structure, or at least that the necessity <span class="pagenum"><a
+name="pg077" id="pg077">077</a></span>of adaptation ruled the
+transformations of form. Geoffroy considered that structure
+determined function, that changes of structure, however they might
+arise, caused changes of function. "Animals," he writes, "have no
+habits but those that result from the structure of their organs; if
+the latter varies, there vary in the same manner all their springs
+of action, all their faculties and all their actions."<a name=
+"FNanchor_121" id="FNanchor_121" /><a href="#Footnote_121"
+class="fnanchor">[121]</a></p>
+
+<p>Again, "a vegetarian r&eacute;gime is imposed upon the
+Quadrumana by their possession of a somewhat ample stomach, and
+intestines of moderate length."<a name="FNanchor_122" id=
+"FNanchor_122" /><a href="#Footnote_122" class=
+"fnanchor">[122]</a> The hand of the bat has become so modified as to
+constrain the bat to live in the air.<a name="FNanchor_123" id=
+"FNanchor_123" /><a href="#Footnote_123" class=
+"fnanchor">[123]</a></p>
+
+<p>The best example of Geoffroy's insistence upon the priority of
+structure to function, and so of his purely morphological attitude,
+is perhaps his interpretation, already alluded to, of the
+appendages of Articulates. The segments of the Articulate are, he
+says, the equivalents of the bodies of the vertebr&aelig; 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."<a name=
+"FNanchor_124" id="FNanchor_124" /><a href="#Footnote_124"
+class="fnanchor">[124]</a> The ribs become locomotory appendages.</p>
+
+<p>We may compare the similar thought that the ear ossicles are
+simply opercular bones reduced and turned to other uses.</p>
+
+<p>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.<a name="FNanchor_125" id=
+"FNanchor_125" /><a href="#Footnote_125" class=
+"fnanchor">[125]</a> 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 <span
+class="pagenum"><a name="pg078" id="pg078">078</a></span>of final
+causes."<a name="FNanchor_6_126" id="FNanchor_6_126" /><a href=
+"#Footnote_6_126" class="fnanchor">[126]</a> 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.<a name="FNanchor_127" id="FNanchor_127" /><a
+href="#Footnote_127" class="fnanchor">[127]</a> So too, although
+the other organs of the insect are all inside the body of the
+vertebr&aelig;, they are yet comparable with the organs of
+Vertebrates.<a name="FNanchor_128" id="FNanchor_128" /><a href=
+"#Footnote_128" class="fnanchor">[128]</a> The existence of
+rudimentary organs also seemed to him an argument against too
+strict a correlation of parts.</p>
+
+<p>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.</p>
+
+<p>Cuvier and Geoffroy are the greatest representatives of these
+opposing views. Which of them is right? Is there nothing more in
+the unity and diversity of organic forms than the results of
+functional adaptation, or is Geoffroy right in insisting upon an
+element of unity which cannot be explained in terms of adaptation?
+If there be an irreducible element of unity, is there any truth in
+Geoffroy's suggestion that this unity results from a power which is
+exercised in the world of atoms where are elements of inalterable
+character?<a name="FNanchor_129" id="FNanchor_129" /><a href=
+"#Footnote_129" class="fnanchor">[129]</a></p>
+
+<p>The problem as Geoffroy and Cuvier understood it was not an
+evolutionary one. But the problem exists unchanged for the
+evolutionist, and evolution-theory is essentially an attempt to
+solve it in the one direction or the other. Theories such as
+Darwin's, which assume a random variation which is not primarily a
+response to environmental changes, answer the problem in Geoffroy's
+sense. Theories such as Lamarck's, which postulate an active
+responsive self-adaptation of the organism, are essentially a
+continuation and completing of Cuvier's thought.</p>
+
+<div class="footnote">
+<p><a name="Footnote_86" id="Footnote_86" /><a href=
+"#FNanchor_86"><span class="label">[86]</span></a> "M&eacute;moire
+sur les rapports naturels des makis," <i>Magasin
+Encyclop&egrave;dique</i>, vii.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_87" id="Footnote_87" /><a href=
+"#FNanchor_87"><span class="label">[87]</span></a> Discours
+pr&eacute;liminaire, pp.&nbsp;xv.-xxiv.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_88" id="Footnote_88" /><a href=
+"#FNanchor_88"><span class="label">[88]</span></a>
+<i>&Eacute;tudes progressives d'un Naturaliste</i>, p.&nbsp;50, Paris,
+1835.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_89" id="Footnote_89" /><a href=
+"#FNanchor_89"><span class="label">[89]</span></a> <i>Philosophie
+Anatomique</i>., i., Introduction, p.&nbsp;1.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_90" id="Footnote_90" /><a href=
+"#FNanchor_90"><span class="label">[90]</span></a> "Sur une
+colonne vert&eacute;brale et ses c&ocirc;tes dans les insectes
+apiropodes," (<i>Acad. Sci.</i>, Feb. 12, 1820). Printed in
+<i>Isis</i>, pp.&nbsp;527-52, 1820 (2).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_91" id="Footnote_91" /><a href=
+"#FNanchor_91"><span class="label">[91]</span></a> "Sur
+l'organisation des insectes," p.&nbsp;458. <i>Isis</i>, pp.&nbsp;452-62, 1820
+(2).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_92" id="Footnote_92" /><a href=
+"#FNanchor_92"><span class="label">[92]</span></a> <i>M&eacute;m.
+Mus. d'Hist. nat.</i>, ix., pp.&nbsp;89-119, Pls. v-vii.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_93" id="Footnote_93" /><a href=
+"#FNanchor_93"><span class="label">[93]</span></a> <i>Sur
+l'organisation des insectes</i>, p.&nbsp;459.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_94" id="Footnote_94" /><a href=
+"#FNanchor_94"><span class="label">[94]</span></a> <i>Isis</i>, p.
+549.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_95" id="Footnote_95" /><a href=
+"#FNanchor_95"><span class="label">[95]</span></a> Published in
+<i>Ann. Sci. Nat.</i>, xix., pp.&nbsp;241-59, 1830.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_96" id="Footnote_96" /><a href=
+"#FNanchor_96"><span class="label">[96]</span></a> <i>Cf.</i>
+Aristotle (<i>supra</i>, p.&nbsp;10).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_97" id="Footnote_97" /><a href=
+"#FNanchor_97"><span class="label">[97]</span></a> For an account
+of the controversy reference may be made to I. Geoffroy St Hilaire,
+<i>Vie Travaux et Doctrine scientifique d'Etienne Geoffroy St
+Hilaire</i>, Paris, 1847; also Semper, <i>Arb. zool. zoot. Instit.
+W&uuml;rzburg</i>, iii., 1876-7, K. E. von Baer, <i>Lebensgeschichte
+Cuviers</i>, ed. L. Stieda, 1897, and J. Kohlbrugge, in <i>Zoolog.
+Annalen</i>, v., pp.&nbsp;143-95. 1913.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_98" id="Footnote_98" /><a href=
+"#FNanchor_98"><span class="label">[98]</span></a> "Recherches sur
+l'organisation des Gavials," <i>M&eacute;m. Mus. d'Hist. nat.</i>,
+xii., 1825.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_99" id="Footnote_99" /><a href=
+"#FNanchor_99"><span class="label">[99]</span></a> <i>M&eacute;m.
+Mus. d'Hist. nat.</i>, xvii., pp.&nbsp;209-29.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_100" id="Footnote_100" /><a href=
+"#FNanchor_100"><span class="label">[100]</span></a> <i>M&eacute;m.
+Acad. Sci.</i>, xii., pp.&nbsp;63-92, 1833.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_101" id="Footnote_101" /><a href=
+"#FNanchor_101"><span class="label">[101]</span></a> <i>M&eacute;m.
+Acad. Sci.</i>, xii., pp.&nbsp;43-61, 1833.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_102" id="Footnote_102" /><a href=
+"#FNanchor_102"><span class="label">[102]</span></a> Geoffroy's
+French style is at times incredibly bad, and more or less literal
+translations of his sentences are apt to read queerly!</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_103" id="Footnote_103" /><a href=
+"#FNanchor_103"><span class="label">[103]</span></a> <i>M&eacute;m.
+Mus. d'Hist. nat.</i>, xiii., p.&nbsp;289, 1826.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_104" id="Footnote_104" /><a href=
+"#FNanchor_104"><span class="label">[104]</span></a> <i>M&eacute;m.
+Mus. d'Hist. nat.</i>, xviii., p.&nbsp;221, 1828. His teratological work
+is important, and is chiefly contained in the second volume of the
+<i>Philosophie anatomique</i>.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_105" id="Footnote_105" /><a href=
+"#FNanchor_105"><span class="label">[105]</span></a> <i>Phil.
+anat.</i>, i., p.&nbsp;449.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_106" id="Footnote_106" /><a href=
+"#FNanchor_106"><span class="label">[106]</span></a> <i>M&eacute;m.
+Acad. Sci.</i>, xii., p.&nbsp;82, 1833.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_107" id="Footnote_107" /><a href=
+"#FNanchor_107"><span class="label">[107]</span></a> <i>M&eacute;m.
+Mus. d'Hist. nat.</i>, ix., p.&nbsp;101, 1822.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_108" id="Footnote_108" /><a href=
+"#FNanchor_108"><span class="label">[108]</span></a> <i>Cours de
+l'histoire naturelle des Mammif&egrave;res</i>, i., Le&ccedil;on 3,
+p.&nbsp;13, 1829.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_109" id="Footnote_109" /><a href=
+"#FNanchor_109"><span class="label">[109]</span></a>
+<i>&Eacute;tudes progressives d'un Naturaliste</i>, p.&nbsp;59, f.n.,
+Paris, 1835.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_110" id="Footnote_110" /><a href=
+"#FNanchor_110"><span class="label">[110]</span></a> <i>Phil.
+Anat.</i>, i., p.&nbsp;444.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_111" id="Footnote_111" /><a href=
+"#FNanchor_111"><span class="label">[111]</span></a> <i>Ann. Mus.
+d'Hist. nat.</i>, x., p.&nbsp;344, 1807.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_112" id="Footnote_112" /><a href=
+"#FNanchor_112"><span class="label">[112]</span></a> <i>Isis</i>,
+p.&nbsp;534, 1820 (2).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_113" id="Footnote_113" /><a href=
+"#FNanchor_113"><span class="label">[113]</span></a> <i>Ann. Mus.
+d'Hist. nat.</i>, x., pp.&nbsp;342-65, 1807.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_114" id="Footnote_114" /><a href=
+"#FNanchor_114"><span class="label">[114]</span></a> <i>loc.
+cit.</i>, x., p.&nbsp;343.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_115" id="Footnote_115" /><a href=
+"#FNanchor_115"><span class="label">[115]</span></a> <i>Phil.
+anat.</i>, i., 450, f.n. <i>Cf.</i> Aristotle (<i>supra</i>, p.
+11).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_116" id="Footnote_116" /><a href=
+"#FNanchor_116"><span class="label">[116]</span></a> <i>Loc.
+cit.</i>, p.&nbsp;136.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_117" id="Footnote_117" /><a href=
+"#FNanchor_117"><span class="label">[117]</span></a>
+<i>Mammif&egrave;res</i>, i., Discours pr&eacute;l., p.&nbsp;18.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_118" id="Footnote_118" /><a href=
+"#FNanchor_118"><span class="label">[118]</span></a> <i>Phil.
+anat.</i>, i., p.&nbsp;208.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_119" id="Footnote_119" /><a href=
+"#FNanchor_119"><span class="label">[119]</span></a> Cuvier and
+Valenciennes, <i>Hist. nat. Poissons</i>, i., p.&nbsp;550, 1828.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_120" id="Footnote_120" /><a href=
+"#FNanchor_120"><span class="label">[120]</span></a> Cuvier and
+Valenciennes, <i>loc. cit.</i>, p.&nbsp;544.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_121" id="Footnote_121" /><a href=
+"#FNanchor_121"><span class="label">[121]</span></a>
+<i>Mammif&egrave;res</i>, i., <i>Le&ccedil;on</i> 4, p.&nbsp;17.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_122" id="Footnote_122" /><a href=
+"#FNanchor_122"><span class="label">[122]</span></a> <i>Loc.
+cit.</i>, <i>Le&ccedil;on</i> 5, p.&nbsp;8.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_123" id="Footnote_123" /><a href=
+"#FNanchor_123"><span class="label">[123]</span></a> <i>Loc.
+cit.</i>, <i>Le&ccedil;on</i> 13, p.&nbsp;6.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_124" id="Footnote_124" /><a href=
+"#FNanchor_124"><span class="label">[124]</span></a> <i>Isis</i>,
+p.&nbsp;539, 1820 (2).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_125" id="Footnote_125" /><a href=
+"#FNanchor_125"><span class="label">[125]</span></a>
+<i>Mammif&egrave;res</i>, i., <i>Le&ccedil;on</i> 4, p.&nbsp;6.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_6_126" id="Footnote_6_126" /><a href=
+"#FNanchor_6_126"><span class="label">[126]</span></a>
+<i>Mammif&egrave;res</i>, Discours pr&eacute;l., p.&nbsp;7.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_127" id="Footnote_127" /><a href=
+"#FNanchor_127"><span class="label">[127]</span></a> <i>Isis</i>,
+p.&nbsp;460, 1820 (2).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_128" id="Footnote_128" /><a href=
+"#FNanchor_128"><span class="label">[128]</span></a> <i>M&eacute;m.
+Mus. d'Hist. nat.</i>, ix., p.&nbsp;102, 1822.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_129" id="Footnote_129" /><a href=
+"#FNanchor_129"><span class="label">[129]</span></a> <i>M&eacute;m.
+Acad. Sci.</i>., xii., p.&nbsp;76, 1833.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg079" id=
+"pg079">079</a></span></p>
+
+<h3>CHAPTER VI</h3>
+
+<h4>THE FOLLOWERS OF ETIENNE GEOFFROY SAINT-HILAIRE</h4>
+
+<p><span class="smcap">Geoffroy's</span> theories were not
+generally accepted by his contemporaries, but his methods had
+considerable influence, especially in France, where many made
+essays in pure morphology.</p>
+
+<p>His chief follower was Serres, who is mentioned indeed in the
+<i>Philosophie anatomique</i> as a fellow-worker. Serres was
+primarily a medical anatomist; his interest lay in human anatomy
+and embryology, normal and pathological.</p>
+
+<p>His best early work was an <i>Anatomie compar&eacute;e du
+cerveau</i> (1824-26), which met with a flattering reception from
+Cuvier.<a name="FNanchor_130" id="FNanchor_130" /><a href=
+"#Footnote_130" class="fnanchor">[130]</a> 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.</p>
+
+<p>Serres' early theoretical views are to be found in a series of
+papers in the <i>Annales des Sciences naturelles</i>,<a name=
+"FNanchor_131" id="FNanchor_131" /><a href="#Footnote_131"
+class="fnanchor">[131]</a> under the general title <i>Recherches
+d'Anatomie transcendante, sur les Lois de l'Organog&eacute;nie
+appliqu&eacute;es &agrave; l'anatomie pathologique</i>, also
+published separately. We follow these papers in our expos&eacute;
+of Serres' doctrine, reserving for a future chapter (<a href=
+"#pg190">Chap.&nbsp;XII.</a>) the consideration of his matured views of
+thirty years later.</p>
+
+<p><span class="pagenum"><a name="pg080" id=
+"pg080">080</a></span>In the first of them he points out how
+neither position nor function has proved altogether sufficient to
+establish homologies. In the early days anatomists were guided by
+form; when form failed them, they traced an organ in its changes
+throughout the series of animals by considering its function. This
+method was satisfactory enough as regards the organs of the
+nutritive life. But in the organs of the life of relation, in the
+nervous system, the functions of the parts were difficult to
+discover, and their form very changeful. Hence a new principle was
+required, and Serres found it in the thought which he probably owed
+to the German transcendentalists (see <a href="#pg089">Chap.
+VII.</a>), 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.&nbsp;106).</p>
+
+<p>"In rising towards the first formations," he writes,
+"transcendental anatomy recognised that one and the same organ,
+however complicated its definitive form might be, repeated in its
+transitory states the organic simplicities of the lower classes.
+Thus the primitive heart of birds was first of all a canal, then a
+pocket or single cavity, then finally the complex organ of the
+class. Comparative anatomy was thus seen to be repeated and
+reproduced by embryogeny" (xii., p.&nbsp;85).</p>
+
+<p>His explanation of the fact of repetition is that, "in animals
+belonging to the lower classes the <i>formative force</i>, 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 ven&aelig;
+cav&aelig; which one observes more or less constantly among
+reptiles" (xxi., p.&nbsp;48).</p>
+
+<p>The number of stages in embryogeny is proportionate to the
+complexity of the adult; the younger the embryo the simpler its
+organs&mdash;such is the general formula of the relation between
+the embryo and the adult. But here in Serres' <span class=
+"pagenum"><a name="pg081" id="pg081">081</a></span>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.&nbsp;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, <i>Lois d'Osteog&eacute;nie</i> (1819), Serres
+established several laws of ossification based upon this principle
+of separate formation.<a name="FNanchor_132" id=
+"FNanchor_132" /><a href="#Footnote_132" class=
+"fnanchor">[132]</a></p>
+
+<p>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&mdash;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 <i>Felis</i>, 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).</p>
+
+<p>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 <span class="pagenum"><a name=
+"pg082" id="pg082">082</a></span>in many of its organs the adult
+stage of various lower animals. It is about this time that it
+possesses a tail.</p>
+
+<p>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<a name="FNanchor_133" id="FNanchor_133" /><a href=
+"#Footnote_133" class="fnanchor">[133]</a>&mdash;"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&mdash;hasty generalisation.</p>
+
+<p>The law of parallelism applied not only to Vertebrates but also
+to Invertebrates. In a short paper<a name="FNanchor_134" id=
+"FNanchor_134" /><a href="#Footnote_134" class=
+"fnanchor">[134]</a> of 1824 Serres attempted an explanation of the
+nervous system of Invertebrates. Invertebrates, he considered,
+lacked the cerebrospinal axis of Vertebrates, and their nervous
+system was the homologue of the sympathetic system of Vertebrates.
+The relation of the invertebrate to the vertebrate nervous system
+being thus fixed, can the nervous system of Invertebrates be
+reduced to one plan? It does not seem possible to establish a
+common plan for the adult nervous systems. But apply the principle
+of parallelism, which has proved so valuable within the limits of
+the vertebrate series. Taking insects as the highest class, we find
+that there are three stages in the development of their nervous
+system; in the first the nervous system is composed of two separate
+strands, in the second the strands unite round the &#339;sophagus, in
+the third they unite also behind. Now in <i>Bulla aperta</i>, stage
+(1) is permanent; in <i>Clio</i>, <i>Doris</i>, <i>Aplysia</i>,
+<i>Tritonia</i>, <i>Sepia</i>, <i>Helix</i>, stage (2) is
+permanent, and in <i>Unio</i> 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 larv&aelig; of
+insects" (p.&nbsp;380). The law of parallelism <span class="pagenum"><a
+name="pg083" id="pg083">083</a></span>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.</p>
+
+<p>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&mdash;a
+favourite pastime of the transcendentalists. It is incontestable,
+he admits, that adult Invertebrates are quite different in
+structure from adult Vertebrates, "but if one regards them as what
+I take them to be, namely, <i>permanent embryos</i>, 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" (<i>loc. cit.</i>, p.&nbsp;247).</p>
+
+<p>The last point of Serres' doctrine which calls for remark is his
+interpretation of abnormalities as being often comparable to grades
+of structure permanent in the lower animals. Thus the double aorta
+which may occur as an abnormality in man is the normal and
+permanent state in reptiles. This idea, of course, he got from
+Etienne Geoffroy St Hilaire. It is further developed in his
+"<i>Th&eacute;orie des formations et des d&eacute;formations
+organiques appliqu&eacute;e &agrave; l'anatomie compar&eacute;e des
+monstruosit&eacute;s</i> (1832), and in his final large memoir of
+1860 (see below, p.&nbsp;<a href="#pg205">205</a>).</p>
+
+<p>In 1816 appeared a fine piece of work by J. C. Savigny on the
+homologies of the appendages in Articulates. The standpoint was
+that of pure morphology. "I am convinced," he wrote, "that when a
+more complete examination has been made of the mouth of insects,
+properly so called, that is to say, having six legs and two
+antenn&aelig;, 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&mdash;such is Nature's constant plan."<a name=
+"FNanchor_135" id="FNanchor_135" /><a href="#Footnote_135"
+class="fnanchor">[135]</a> 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 <span class="pagenum"><a name="pg084" id=
+"pg084">084</a></span>second with the anterior appendages of
+Articulates generally. Savigny shows that the mouth-parts of
+insects can be reduced to the type shown in Orthoptera, where there
+are clearly two mandibles, two maxill&aelig;, and a lower lip
+formed by the fusion of two second maxill&aelig;. 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 <i>Julus</i>, <i>Scolopendra</i>,
+<i>Cancer</i>, <i>Gammarus</i>, <i>Cyamus</i>, <i>Nymphon</i>,
+<i>Phalangium</i>, <i>Apus</i>, <i>Caligus</i>, <i>Limulus</i>, 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 maxill&aelig; 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
+<i>Julus</i>, that is to say, all the feet of the Hexapoda, are
+here transformed into jaws" (<i>loc. cit.</i>, p.&nbsp;48). In
+<i>Scolopendra</i> also there is a similar transformation of two
+pairs of legs into auxiliary jaws. In <i>Gammarus</i>, where there
+is only the first pair of maxillipedes, the other two pairs have
+become "retransformed" into feet. We find him supporting his
+comparison of the three anterior pairs of legs in <i>Julus</i> 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
+<i>Julus</i> 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.&nbsp;44).</p>
+
+<p>His comparison of the Arachnid appendages with those of insects
+and Crustacea is very curious. As his starting-point he takes
+<i>Cyamus</i>, which has antenn&aelig; (two pairs) and mouth parts
+(four pairs) as in many Crustacea, and then seven pairs of legs; he
+compares with it <i>Nymphon</i>, which has in all seven pairs of
+appendages. These appendages he homologises with the seven pairs of
+legs of <i>Cyamus</i>, so that the first appendage in
+<i>Nymphon</i> corresponds to the seventh appendage of
+<i>Cyamus</i>. This homology is extended to all Arachnids; their
+first two pairs of appendages, however <span class="pagenum"><a
+name="pg085" id="pg085">085</a></span>they may be modified as
+"false" mandibles and "false" maxill&aelig;, really correspond to
+the second and third maxillipedes in Crustacea, and to the second
+and third pairs of feet in insects. It is interesting to note that
+he treats <i>Limulus</i> as an Arachnid, pointing out that there is
+as much difference between <i>Apus</i> and <i>Limulus</i> as
+between <i>Cancer</i> and <i>Phalangium</i>. He describes the
+"gnathobases" in <i>Phalangium</i> and <i>Limulus</i>. 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 <i>Mysis</i> (p.&nbsp;50).</p>
+
+<p>V. Audouin, in his memoir, <i>Recherches anatomiques sur le
+thorax des animaux articul&eacute;s</i>,<a name="FNanchor_136"
+id="FNanchor_136" /><a href="#Footnote_136" class=
+"fnanchor">[136]</a> applied the principle of the unity of plan and
+composition to the exoskeleton of insects, Crustaceans, and
+Arachnids. His guiding ideas were, "(1) that the skeleton of
+articulated animals is formed of a definite number of pieces, which
+are either distinct or intimately fused with one another; (2) that
+in many cases, some pieces diminish or altogether disappear, while
+others reach an excessive development; (3) that the increase of one
+piece seems to exert on the neighbouring pieces a kind of influence
+which explains all the differences one finds between the
+individuals of each order, family and genus" (Sep. copy, p.&nbsp;16p.
+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.<a name="FNanchor_137" id=
+"FNanchor_137" /><a href="#Footnote_137" class=
+"fnanchor">[137]</a> 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 <span class=
+"pagenum"><a name="pg086" id="pg086">086</a></span>relative to one
+another. Many of the names which he proposed are still in use; it
+was he who introduced the terms prothorax, mesothorax, and
+metathorax, for the three segments of the insect's thorax. He used
+Geoffroy's <i>Loi de balancement</i> 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."<a name=
+"FNanchor_138" id="FNanchor_138" /><a href="#Footnote_138"
+class="fnanchor">[138]</a> In a third paper of about the same time he
+tried to establish the homologies of the segments throughout the
+Articulate series&mdash;with less success than Savigny.</p>
+
+<p>Later on, in conjunction with Milne-Edwards, he demonstrated the
+unity of composition of the nervous system in Crustacea, showing
+how the concentrated system of the crab was formed by the same
+series of ganglia as in the Macrura.</p>
+
+<p>The entomologist Latreille also tackled the problem of the
+homologies of the segments in the different classes of Arthropods
+(Cuvier, <i>loc. cit.</i>, p.&nbsp;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, <i>e.g.</i>, his
+comparison of the "first jaws" of Arachnids to antenn&aelig;,
+because they were placed above the upper lip. It was he who first
+pointed out the resemblance of the leaf-like gills of Ephemerid
+larv&aelig; to wings, and suggested that wings were "a sort of
+tracheal feet."</p>
+
+<p>He made also a rather hazy and speculative contribution on
+Okenian lines to the problem of the relation of Arthropods to
+Vertebrates, likening the carapace of Crustacea to an enormously
+developed hyoid, the appendages of the tail to the ventral and anal
+fins of fish. The masticatory organs of Arthropods were jaws
+disjointed at their symphysis; antenn&aelig;, nostrils turned
+outside in.</p>
+
+<p>Dug&egrave;s also made a comparison of Articulates with
+Vertebrates.<a name="FNanchor_139" id="FNanchor_139" /><a href=
+"#Footnote_139" class="fnanchor">[139]</a> He did not accept
+Geoffroy's vertebral theory <span class="pagenum"><a name="pg087"
+id="pg087">087</a></span>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 maxill&aelig; of Arthropods were the homologues
+of the upper and lower jaws of Vertebrates, adducing as
+confirmatory evidence the fact that in snakes the rami are
+separate. The labium was the equivalent of the hyoid, the labial
+palps and maxillipedes the equivalent of the "hyoid" elements which
+form the branchial arches.</p>
+
+<p>But Dug&egrave;s' main contribution to morphological method was
+his conception of the living organism as a colony of lesser units,
+which were themselves real "organisms." "By <i>organism</i> 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.&nbsp;255). Dug&egrave;s 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.<a name="FNanchor_140" id="FNanchor_140" /><a
+href="#Footnote_140" class="fnanchor">[140]</a> According to his
+view there are three main types of animals&mdash;(1) Biserials,
+including bilaterally symmetrical animals, composed of two parallel
+series of "organisms"; (2) Radiates, composed of "organisms"
+arranged like the spokes of a wheel; and (3) Raceme-animals, in
+which the separate "organisms" were disposed more or less
+irregularly, in bunches (p.&nbsp;257). The unitary "organism" is
+supposed to be the same in all, only the arrangement differing.
+Dug&egrave;s of course admitted that the centralisation of the
+complete organism became greater the higher it stood in the scale,
+and that this held good also in individual development. The
+appendages of Articulates and Vertebrates were thought of as the
+members of as many separate organisms. He went so far as to suggest
+that the <span class="pagenum"><a name="pg088" id=
+"pg088">088</a></span>fingers of a man's hand were the free
+extremities of as many thoracic members.</p>
+
+<p>Dug&egrave;s' conception of the organism has often been revived
+since in a saner form, <i>e.g.</i>, by E. Perrier, and it has a
+certain validity. It has much affinity with the similar conceptions
+of Goethe and the German transcendentalists.</p>
+
+<div class="footnote">
+<p><a name="Footnote_130" id="Footnote_130" /><a href=
+"#FNanchor_130"><span class="label">[130]</span></a> <i>Mem. Acad.
+Sci.</i>, iv., pp.&nbsp;cclxxxiv.-ccci., 1824.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_131" id="Footnote_131" /><a href=
+"#FNanchor_131"><span class="label">[131]</span></a> <i>Ann. Sci.
+Nat.</i>, xi., xii., 1827; xvi., 1829; xxi., 1830.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_132" id="Footnote_132" /><a href=
+"#FNanchor_132"><span class="label">[132]</span></a> See
+R&aacute;dl, <i>loc. cit.</i>, i., pp.&nbsp;225-6.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_133" id="Footnote_133" /><a href=
+"#FNanchor_133"><span class="label">[133]</span></a> <i>Ann. Sci.
+nat.</i> (2), ii., p.&nbsp;248, 1834.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_134" id="Footnote_134" /><a href=
+"#FNanchor_134"><span class="label">[134]</span></a> <i>Ann. Sci.
+nat.</i>, iii., pp.&nbsp;377-80, 1824.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_135" id="Footnote_135" /><a href=
+"#FNanchor_135"><span class="label">[135]</span></a>
+<i>M&eacute;moires sur les Animaux sans Vert&egrave;bres</i>, Part
+I., p.&nbsp;10, Paris, 1816.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_136" id="Footnote_136" /><a href=
+"#FNanchor_136"><span class="label">[136]</span></a> <i>Ann. Sci.
+Nat.</i>, (1), i., pp.&nbsp;97-135, 416-432, 1824.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_137" id="Footnote_137" /><a href=
+"#FNanchor_137"><span class="label">[137]</span></a> <i>Isis</i>,
+p.&nbsp;456, 1820 (2).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_138" id="Footnote_138" /><a href=
+"#FNanchor_138"><span class="label">[138]</span></a> Cuvier,
+<i>M&eacute;m. Acad. Sci.</i>, iv., p.&nbsp;cclxx., 1824.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_139" id="Footnote_139" /><a href=
+"#FNanchor_139"><span class="label">[139]</span></a> <i>Acad.
+Sci.</i> 18th Oct. 1831. Extract in <i>Ann. Sci. Nat.</i>, xxiv.,
+pp.&nbsp;254-60, 1831.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_140" id="Footnote_140" /><a href=
+"#FNanchor_140"><span class="label">[140]</span></a> His views were
+more fully elaborated in his <i>M&eacute;moire sur la
+conformit&eacute; organique dans l'&eacute;chelle animale</i>,
+Montpellier, 1832.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg089" id=
+"pg089">089</a></span></p>
+
+<h3>CHAPTER VII</h3>
+
+<h4>THE GERMAN TRANSCENDENTALISTS</h4>
+
+<p><span class="smcap">To</span> complete our historical survey of
+the morphology of the early 19th century we have now to turn back
+some way and consider the curious development of morphological
+thought in Germany under the influence of the <i>Philosophy of
+Nature</i>. We have already seen many of these notions foreshadowed
+by Goethe, who had considerable affinity with the
+transcendentalists, but the full development of transcendental
+habits of thought comes a little later than the bulk of Goethe's
+scientific work, and owes more to Kielmeyer and Oken than to Goethe
+himself.</p>
+
+<p>A great wave of transcendentalism seems to have passed over
+biological thought in the early 19th century, arising mainly in
+Germany, but powerfully affecting, as we have seen, the thought of
+Geoffroy and his followers. Many ideas were common to the French
+and German schools of transcendental anatomy, the fundamental
+conception that there exists a unique plan of structure, the idea
+of the scale of beings, the notion of the parallelism between the
+development of the individual and the evolution of the race. It is
+difficult to disentangle the part played by each school and to
+determine which should have the credit for particular theories and
+discoveries. The philosophy seems to have come chiefly from
+Germany, the science from France. It must be borne in mind that
+German comparative anatomy was largely derivative from French, that
+the Paris Museum was the acknowledged anatomical centre, and that
+Cuvier was its acknowledged head.</p>
+
+<p>It is probably correct to say that the credit mainly belongs to
+the German transcendental school for the law <span class=
+"pagenum"><a name="pg090" id="pg090">090</a></span>of the
+parallelism between the stages of individual development and the
+stages of the scale of beings, and the theory of the repetition or
+multiplication of parts within the individual. The vertebral theory
+of the skull is a particular application of the second of these
+generalisations.</p>
+
+<p>The law of parallelism<a name="FNanchor_141" id=
+"FNanchor_141" /><a href="#Footnote_141" class=
+"fnanchor">[141]</a> seems to have been expressed first by Kielmeyer
+(1793),<a name="FNanchor_142" id="FNanchor_142" /><a href=
+"#Footnote_142" class="fnanchor">[142]</a> 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.</p>
+
+<p>The idea was next taught by Autenrieth in 1797.<a name=
+"FNanchor_143" id="FNanchor_143" /><a href="#Footnote_143"
+class="fnanchor">[143]</a></p>
+
+<p>Oken (1779-1851) in his early tract <i>Die Zeugung</i> (1805),
+and in his <i>Lehrbuch der Naturphilosophie</i> (1809-11)
+elaborated the thought, and taught that every animal in its
+development passes through the classes immediately below it.
+"During its development the animal passes through all stages of the
+animal kingdom. The f&oelig;tus is a representation of all animal
+classes in time."<a name="FNanchor_144" id="FNanchor_144" /><a
+href="#Footnote_144" class="fnanchor">[144]</a> The Insect, for
+example, is at first Worm, next Crab, then a perfect volant animal
+with limbs, a Fly (<i>ibid.</i>, p.&nbsp;542).</p>
+
+<p>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>i.e.</i>, of Man" (p.&nbsp;494).
+Now "animals are gradually perfected, entirely like the single
+animal body, by adding organ unto organ"&mdash;the way of evolution
+is the way of development. Hence "animals are only the persistent
+f&oelig;tal stages or conditions of Man," who is the microcosm, and
+contains within himself all the animal kingdom.</p>
+
+<p>Oken was himself a careful student of embryology; von Baer<a
+name="FNanchor_145" id="FNanchor_145" /><a href=
+"#Footnote_145" class="fnanchor">[145]</a> speaks of his work
+(published in Oken and Kieser, <i>Beitr&auml;ge zur vergleichenden
+Zoologie, Anatomie und Physiologie</i>, <span class="pagenum"><a
+name="pg091" id="pg091">091</a></span>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 f&oelig;tus and
+the lower animals; but the peculiar form which the law took in his
+hands was a consequence of his hazy philosophy. He saw the relation
+of teratological to f&oelig;tal structure, for he affirmed that
+"malformations are only persistent f&oelig;tal conditions" (p.
+492).</p>
+
+<p>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<a name="FNanchor_146" id="FNanchor_146" /><a href=
+"#Footnote_146" class="fnanchor">[146]</a> 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.&nbsp;57).</p>
+
+<p>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.&nbsp;57).
+In his work on the anatomy of the brain,<a name="FNanchor_147"
+id="FNanchor_147" /><a href="#Footnote_147" class=
+"fnanchor">[147]</a> Tiedemann established the homology of the optic
+lobes in birds by comparing them with f&oelig;tal corpora
+quadrigemina in man (see Serres, <i>Ann. Sci. nat.</i>, xii., p.
+112).</p>
+
+<p>J. F. Meckel, in 1811, devoted a long essay to a detailed proof
+of the parallelism between the embryonic states of the higher
+animals and the permanent states of the lower animals. In a
+previous memoir in the same collection<a name="FNanchor_148" id=
+"FNanchor_148" /><a href="#Footnote_148" class=
+"fnanchor">[148]</a> (i., 1, 1808) he had made some comparisons of
+this kind in dealing with the development of the human f&oelig;tus;
+in this memoir (ii., 1, 1811) he brings together all the facts
+which seem to prove the parallelism.</p>
+
+<p>His collection of facts is a very heterogeneous one; he mingles
+morphological with physiological analogies, and makes the most
+far-fetched comparisons between organs <span class="pagenum"><a
+name="pg092" id="pg092">092</a></span>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 <i>Tethys,
+Scyll&aelig;a</i> and <i>Arenicola</i>(p.&nbsp;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
+C&oelig;lentera, 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).<a name="FNanchor_149" id="FNanchor_149" /><a href=
+"#Footnote_149" class="fnanchor">[149]</a> These are merely chance
+resemblances of no morphological importance. He considers
+bladderworms as animals which have never escaped from their amnion,
+and <i>Volvox</i> as not having developed beyond the level of an
+egg (p.&nbsp;7). He lays much stress upon likeness of shape and of
+relative size, comparing, for instance, the large multilobate liver
+of the human f&oelig;tus with the many-lobed liver of lower
+Vertebrates and of Invertebrates. In general he shows himself, in
+his comparisons, lacking in morphological insight.</p>
+
+<p>His treatment of the vascular system affords perhaps the best
+example of his method (pp.&nbsp;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 <i>Anomia</i>. 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.</p>
+
+<p>In his large text-book of a somewhat later date, the <i>System
+der vergleichenden Anatomie</i> (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 <span class="pagenum"><a name=
+"pg093" id="pg093">093</a></span>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."<a
+name="FNanchor_150" id="FNanchor_150" /><a href=
+"#Footnote_150" class="fnanchor">[150]</a></p>
+
+<p>His final enunciation of the law of parallelism in this same
+volume shows that he considered the development of the individual
+to be due to the same forces that rule evolution. "The development
+of the individual organism obeys the same laws as the development
+of the whole animal series; that is to say, the higher animal, in
+its gradual evolution, essentially passes through the permanent
+organic stages which lie below it; a circumstance which allows us
+to assume a close analogy between the differences which exist
+between the diverse stages of development, and between each of the
+animal classes" (p.&nbsp;514).</p>
+
+<p>He was not, of course, able fully to prove his contention that
+the lower animals are the embryos of the higher, and we gather from
+the following passage that he could maintain it only in a somewhat
+modified form. "It is certain," he writes, "that if a given organ
+shows in the embryo of a higher animal a given form, identical with
+that shown throughout life by an animal belonging to a lower class,
+the embryo, in respect of this portion of its economy, belongs to
+the class in question" (p.&nbsp;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.</p>
+
+<p>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).</p>
+
+<p>In his excellent discussion of teratology Meckel points out how
+the idea of parallelism throws light upon certain <span class=
+"pagenum"><a name="pg094" id="pg094">094</a></span>abnormalities
+which are found to be normal in other (lower) forms (p.&nbsp;556).<a
+name="FNanchor_151" id="FNanchor_151" /><a href=
+"#Footnote_151" class="fnanchor">[151]</a></p>
+
+<p>We may refer to one other statement of the law of
+parallelism&mdash;by K. G. Carus in his <i>Lehrbuch der
+vergleichenden Anatomie</i> (Leipzig, 1834). The standpoint is
+again that of <i>Naturphilosophie</i>. 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.</p>
+
+<p>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.</p>
+
+<p>Under the "theory of the repetition or multiplication of parts
+within the organism" may be included, first, generalisations on the
+serial homology of parts, and second, more or less confused
+attempts to demonstrate that the whole organisation is repeated in
+certain of the parts. The recognition of serial homologies
+constituted a real advance in morphology; the "philosophical" idea
+of the repetition of the whole in the parts led to many
+absurdities. It led Oken to assert that in the head the whole trunk
+is repeated, that the upper jaw corresponds to the arms, the lower
+to the legs, that in each jaw the same bony divisions exist as in
+the limbs, the teeth, for instance, corresponding to the claws
+(<i>loc. cit.</i>, p.&nbsp;408). It led him to distinguish "two animals"
+in every body&mdash;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.&nbsp;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 <span class="pagenum"><a name="pg095"
+id="pg095">095</a></span>sternum is a ventral vertebral column.
+Limbs are connate ribs, the digits indicating the number of ribs
+included (<i>cf.</i> Dug&egrave;s, <i>supra</i>, p.&nbsp;88).</p>
+
+<p>J. F. Meckel<a name="FNanchor_152" id="FNanchor_152" /><a
+href="#Footnote_152" class="fnanchor">[152]</a> discusses
+"homologies" of this kind in the thorough and pedestrian way so
+characteristic of him. Not only, he says, are the right and left
+halves of the body comparable with one another, but also the upper
+and the lower, the dividing line being drawn at the level of the
+diaphragm. The lumbar complex corresponds to the skull, the anus to
+the mouth, the urino-genital opening to the nasal opening; in
+general, the urino-genital system corresponds to the respiratory,
+the kidneys to the lungs, the ureters to bronchi, the testes and
+ovaries to the thymus (he had observed the physiological relation
+between the development of the thymus and the state of the genital
+organs), the prostate and the uterus to the thyroid gland, and the
+penis and clitoris to the tongue. The fore-limbs and girdle
+correspond in detail with the hind limbs and the pelvis&mdash;a
+point already worked out by Vicq d'Azyr; the dorsal and ventral
+halves of the body are likewise comparable in some respects, the
+sternum, for example, answering in the arrangement of its bones,
+muscles and arteries to the vertebral column. The skeleton of each
+member is in some respects a repetition of the vertebral
+column.</p>
+
+<p>His brother, D. A. Meckel,<a name="FNanchor_153" id=
+"FNanchor_153" /><a href="#Footnote_153" class=
+"fnanchor">[153]</a> 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 C&oelig;lentera, where genital and digestive
+organs occupy the same cavity. In his view the uterus corresponded
+to the stomach, the vagina to the &oelig;sophagus, the fallopian
+tubes to the intestine, and so on.</p>
+
+<p>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 <span class="pagenum"><a name=
+"pg096" id="pg096">096</a></span>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
+vertebr&aelig;. Goethe seems to have been the first to hit upon the
+idea that the skull is composed of a number of vertebr&aelig;,
+serially homologous with those of the vertebral column. He tells us
+that the idea flashed into his mind when contemplating in the
+Jewish cemetery at Venice a dried sheep's skull. The discovery was
+made in 1790, but not published till 1820.<a name="FNanchor_154"
+id="FNanchor_154" /><a href="#Footnote_154" class=
+"fnanchor">[154]</a></p>
+
+<p>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.</p>
+
+<p>In a book (<i>Cours d'&Eacute;tudes m&eacute;dicales</i>),
+published in 1803, Burdin assimilated the skull to the vertebral
+column.</p>
+
+<p>Oken, in an inaugural dissertation (Programm) <i>Ueber die
+Bedeutung der Sch&auml;delknochen</i>,<a name="FNanchor_155" id=
+"FNanchor_155" /><a href="#Footnote_155" class=
+"fnanchor">[155]</a> published in 1807, gave to the theory its
+necessary development. Autenrieth, also in 1807,<a name=
+"FNanchor_156" id="FNanchor_156" /><a href="#Footnote_156"
+class="fnanchor">[156]</a> distinguishing separate ganglia in the
+brain, was not far from the hypothesis that each of these ganglia
+must have its separate vertebra.</p>
+
+<p>In 1808 Dum&eacute;ril read a paper to the Acad&eacute;mie des
+Sciences in which he compared the skull to a gigantic vertebra,
+basing his hypothesis on the similarity existing between the crests
+and depressions on the hinder part of the skull and those on the
+posterior surfaces of the vertebr&aelig;.</p>
+
+<p>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 <i>Cephalogenesis</i>,
+distinguishing (as Oken did at first) three cranial vertebr&aelig;.
+Bojanus in his <i>Anatome testudinis europ&aelig;ae</i> (1819), and
+in a series of papers in <i>Isis</i> (1817-1819, and 1821)
+established the existence of a fourth cranial vertebra, and this
+was accepted by Oken in the later editions of his <i>Lehrbuch</i>.
+Meckel and Carus among the Germans, de Blainville and E. Geoffroy
+among the French, contributed to <span class="pagenum"><a name=
+"pg097" id="pg097">097</a></span>the development of the theory. In
+England the theory was championed particularly by Richard Owen.</p>
+
+<p>It was one thing to assert in a moment of inspiration that the
+skull was composed of modified vertebr&aelig;; it was quite another
+to demonstrate the relation of the separate bones of the skull to
+the supposed vertebr&aelig;. Upon this much uncertainty reigned;
+there was not even unanimity as to the number of vertebr&aelig; to
+be distinguished. Goethe found six vertebr&aelig; in the skull;
+Spix, and at first Oken, three only, Geoffroy seven; the accepted
+orthodox number seems to have been four (Bojanus, Oken, Owen).</p>
+
+<p>As an example of the method of treatment adopted we may take
+Oken's matured account of the composition of the cranial
+vertebr&aelig;, as given in the English translation of his
+<i>Lehrbuch</i>. "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.&nbsp;370). In the cervical
+vertebr&aelig; the transverse processes represent the ribs. The
+skull consists of four vertebr&aelig;, 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 vertebr&aelig; 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 al&aelig; 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.</p>
+
+<p>The cranium is thus composed of four rings of bone, each
+composed of the typical elements of a vertebra.</p>
+
+<p>The arbitrary nature of the comparison is obvious enough. As
+Cuvier pointed out in the posthumous edition of his
+<i>Le&ccedil;ons</i>, it is only the occipital segment that shows
+any real analogy with a vertebra&mdash;an analogy which Cuvier
+ascribed to similarity of function. He admitted a faint resemblance
+of the parietal segment to a vertebra:&mdash;"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 <span class=
+"pagenum"><a name="pg098" id="pg098">098</a></span>clinoid
+apophyses."<a name="FNanchor_157" id="FNanchor_157" /><a href=
+"#Footnote_157" class="fnanchor">[157]</a> He denied the
+resemblance of the frontal and nasal "vertebr&aelig;" to true
+vertebr&aelig;, pointing out that both parietals and frontals are
+bones specially developed for the purpose of roofing over and
+protecting the cerebrum.</p>
+
+<p>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.<a name=
+"FNanchor_158" id="FNanchor_158" /><a href="#Footnote_158"
+class="fnanchor">[158]</a> His system is worthy of some
+consideration, for he tries to work out a geometry of the
+skeleton.<a name="FNanchor_159" id="FNanchor_159" /><a href=
+"#Footnote_159" class="fnanchor">[159]</a></p>
+
+<p>His method of deduction is a good example of pure
+<i>Naturphilosophie</i>. 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 <i>dermatoskeleton</i>, which naturally
+takes the shape of a hollow sphere; round the digestive cavity
+which is formed in the living sphere is developed the
+<i>splanchnoskeleton</i>; round the nervous system (which is, as it
+were, the animal within the animal) is developed the
+<i>neuroskeleton</i>. All skeletal formations belong to one or
+other of these systems.</p>
+
+<p>Carus defines his aim to be the discovery of the inner law which
+presides over the formation of the skeleton throughout the animal
+kingdom; he desires to know "how such and such a formation is
+realised in virtue of the eternal laws of reason" (iii., p.&nbsp;93).
+Here we touch the kernel of <i>Naturphilosophie</i>&mdash;the
+search for rational laws which are active in Nature; the discontent
+with merely empirical laws.</p>
+
+<p><span class="pagenum"><a name="pg099" id=
+"pg099">099</a></span>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.&nbsp;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.</p>
+
+<p>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.&nbsp;127). One conceives easily what the main types of
+skeleton must be. In some animals, <i>e.g.</i>, sea-urchins, the
+skeleton is a simple sphere; in others, <i>e.g.</i>, 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.</p>
+
+<p>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 (vertebr&aelig; in the ordinary sense). The apophyses
+and bodies of the vertebr&aelig;, and the bones of the members<a
+name="FNanchor_160" id="FNanchor_160" /><a href=
+"#Footnote_160" class="fnanchor">[160]</a> are composed of columns
+of tritovertebr&aelig;, or vertebr&aelig; of the third order. Thus
+the whole vertebrate skeleton is a particular arrangement of
+vertebr&aelig;, which <span class="pagenum"><a name="pg100" id=
+"pg100">100</a></span>in their turn are modifications of the
+primary hollow sphere.</p>
+
+<p>The German transcendentalists were more or less contemporary
+with E. Geoffroy, and no doubt influenced him, especially in his
+later years, as they certainly did his follower Serres. Oken indeed
+wrote, in a note<a name="FNanchor_161" id="FNanchor_161" /><a
+href="#Footnote_161" class="fnanchor">[161]</a> appended to
+Geoffroy's paper on the vertebral column of insects, that "Mr
+Geoffroy [<i>sic</i>] is without a doubt the first to introduce in
+France <i>Naturphilosophie</i> into comparative anatomy, that is to
+say, that philosophy one of whose doctrines it is to seek after the
+<i>signification</i> of organs in the scale of organised beings."
+This is, however, an exaggeration, for Geoffroy was primarily a
+morphologist, whereas the morphology of the German
+transcendentalists was only a side-issue of their
+<i>Naturphilosophie</i>.</p>
+
+<p>Geoffroy, on his part, exercised some influence on the
+transcendentalists. He asserts<a name="FNanchor_162" id=
+"FNanchor_162" /><a href="#Footnote_162" class=
+"fnanchor">[162]</a> indeed that Spix got some of the ideas published
+in the <i>Cephalogenesis</i> (1815) from attending his course of
+lectures in 1809. It is certainly the case that Spix published
+before Geoffroy the view that the opercular bones are homologous
+with the ear-ossicles, adopting, however, a different homology for
+the separate bones.<a name="FNanchor_163" id=
+"FNanchor_163" /><a href="#Footnote_163" class=
+"fnanchor">[163]</a></p>
+
+<p>Some speculations seem to have been common to both
+schools&mdash;for instance, the law of Meckel-Serres, the vertebral
+theory of the skull, and the recognition of serial homology in the
+appendages of Arthropods (Savigny, Oken). Latreille and
+Dug&egrave;s, as well as Serres, clearly show in their theoretical
+views the influence of Oken and the other transcendentalists.
+Geoffroy's principle of connections and law of compensation were
+recognised by some at least of the Germans.</p>
+
+<p>But whatever his actual historical relations may have been with
+the German school, Geoffroy was vastly their superior in the matter
+of pure morphology. He alone brought to clear consciousness the
+principles on which a pure morphology could be based: the Germans
+were transcendental philosophers first, and morphologists
+after.</p>
+
+<p><span class="pagenum"><a name="pg101" id=
+"pg101">101</a></span>One understands from this how J. F. Meckel,
+who was in some ways the leading comparative anatomist in Germany
+at this time, could be at once a transcendentalist and an opponent
+of Geoffroy. Meckel had a curiously eclectic mind. A disciple of
+Cuvier, having studied in 1804-6 the rich collections at the Museum
+in Paris, the translator of Cuvier's <i>Le&ccedil;ons d'anatomie
+compar&eacute;e</i>, he earned for himself the title of the "German
+Cuvier," partly through the publication of his comprehensive
+textbook (<i>System der vergl. Anatomie</i>, 5 vols.), partly by
+his extensive and many-sided research work, partly by his
+authoritative teaching. His <i>System</i> shows in almost every
+page of its theoretical part the influence of Cuvier; and it is
+through having assimilated Cuvier's teaching as to the importance
+of function that Meckel combats Geoffroy's law of connections, at
+least in its rigorous form. He submits that the connections of
+bones and muscles must change in relation to functional
+requirements. He rejects Geoffroy's theory of the vertebrate nature
+of Articulates. Generally throughout his work the functional point
+of view is well to the fore.</p>
+
+<p>Yet at heart Meckel was a transcendentalist of the German
+school. His vagaries on the subject of "homologues" leave no doubt
+about that, and, in spite of Cuvier, he believed, though not very
+firmly, in the existence of one single type of structure.</p>
+
+<p>A Cuverian by training, his lack of morphological sense threw
+him into the ranks of the transcendentalists, to whom perhaps he
+belonged by nature.</p>
+
+<div class="footnote">
+<p><a name="Footnote_141" id="Footnote_141" /><a href=
+"#FNanchor_141"><span class="label">[141]</span></a> For a full
+account, see Kohlbrugge, <i>Zool. Annalen</i>, xxxviii., 1911.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_142" id="Footnote_142" /><a href=
+"#FNanchor_142"><span class="label">[142]</span></a> <i>Rede
+&uuml;ber das Verh&auml;ltnis der organischen Kr&auml;fte</i>,
+Stuttgart u. T&uuml;bingen, 1793 (1814). See R&aacute;dl, <i>loc.
+cit.</i>, i., p.&nbsp;261; ii., p.&nbsp;57.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_143" id="Footnote_143" /><a href=
+"#FNanchor_143"><span class="label">[143]</span></a> <i>Supplem. ad
+historiam embryonis</i>, T&uuml;bingen, 1797.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_144" id="Footnote_144" /><a href=
+"#FNanchor_144"><span class="label">[144]</span></a> <i>Lehrbuch
+der Naturphilosophie</i>, Eng. trans., p.&nbsp;491, 1847.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_145" id="Footnote_145" /><a href=
+"#FNanchor_145"><span class="label">[145]</span></a> <i>Ueber
+Entwickelungsgeschichte der Thiere</i>, i., p.&nbsp;xvii., 1828.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_146" id="Footnote_146" /><a href=
+"#FNanchor_146"><span class="label">[146]</span></a>
+<i>Zoologie</i>, Landshut, i., 1808.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_147" id="Footnote_147" /><a href=
+"#FNanchor_147"><span class="label">[147]</span></a> <i>Anatomie u.
+Bildungsgeschichte des Gehirns im F&ouml;tus des Menschen</i>,
+N&uuml;rnberg, 1816.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_148" id="Footnote_148" /><a href=
+"#FNanchor_148"><span class="label">[148]</span></a>
+<i>Beytr&auml;ge zur vergleichende Anatomie</i>, Leipzig, i.,
+1808-9, ii., 1811-2.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_149" id="Footnote_149" /><a href=
+"#FNanchor_149"><span class="label">[149]</span></a> Cetacea were
+generally considered at this time to be mammals of low
+organisation.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_150" id="Footnote_150" /><a href=
+"#FNanchor_150"><span class="label">[150]</span></a> From the
+French trans., which appeared under the title <i>Trait&eacute;
+g&eacute;n. d'Anat. compar&eacute;e</i>, i., p.&nbsp;449, 1828.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_151" id="Footnote_151" /><a href=
+"#FNanchor_151"><span class="label">[151]</span></a> <i>Cf.</i>
+Geoffroy (<i>supra</i>, p.&nbsp;70).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_152" id="Footnote_152" /><a href=
+"#FNanchor_152"><span class="label">[152]</span></a>
+<i>Beytr&auml;ge</i>, ii., 2, 1812. Also in his <i>System d. vergl.
+Anat.</i>, i., 1821.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_153" id="Footnote_153" /><a href=
+"#FNanchor_153"><span class="label">[153]</span></a> In J. F.
+Meckel's <i>Beytr&auml;ge</i>, ii.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_154" id="Footnote_154" /><a href=
+"#FNanchor_154"><span class="label">[154]</span></a> <i>Zur
+Morphologie</i>, i., 2, p.&nbsp;250, 1820; and ii., 2, pp.&nbsp;122-4,
+1824.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_155" id="Footnote_155" /><a href=
+"#FNanchor_155"><span class="label">[155]</span></a> See
+translation, giving the gist of this paper, in Huxley's <i>Lectures
+on the Elements of Comparative Anatomy</i>, pp.&nbsp;282-6, London,
+1864.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_156" id="Footnote_156" /><a href=
+"#FNanchor_156"><span class="label">[156]</span></a> Reil's
+<i>Archiv. f. Physiol.</i>, vii., 1807.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_157" id="Footnote_157" /><a href=
+"#FNanchor_157"><span class="label">[157]</span></a>
+<i>Le&ccedil;ons d'anatomie compar&eacute;e</i>, 3rd ed., Brussels
+reprint, i., p.&nbsp;414, 1836.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_158" id="Footnote_158" /><a href=
+"#FNanchor_158"><span class="label">[158]</span></a> In his
+Programm, <i>U. d. Bedeut. d. Sch&auml;delknochen</i>, 1807.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_159" id="Footnote_159" /><a href=
+"#FNanchor_159"><span class="label">[159]</span></a>
+<i>Trait&eacute; &eacute;l&eacute;mentaire d'anatomie
+compar&eacute;e</i> (French trans.), vol. iii., Paris, 1835. First
+developed in his volume <i>Von den Ur-Theilen des Knochen und
+Schalen-Gerustes</i>, Leipzig, 1828.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_160" id="Footnote_160" /><a href=
+"#FNanchor_160"><span class="label">[160]</span></a> Dutrochet in
+1821 had tried to prove that the bones of the members belong to the
+type of the vertebra&mdash;the dicone.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_161" id="Footnote_161" /><a href=
+"#FNanchor_161"><span class="label">[161]</span></a> <i>Isis</i>,
+pp.&nbsp;552-9, 1820 (2).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_162" id="Footnote_162" /><a href=
+"#FNanchor_162"><span class="label">[162]</span></a> <i>M&eacute;m.
+Mus. d'Hist. nat.</i>, ix., 1822.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_163" id="Footnote_163" /><a href=
+"#FNanchor_163"><span class="label">[163]</span></a> Cuvier and
+Valenciennes, <i>Hist. nat. Poissons</i>, i., p.&nbsp;311, f.n.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg102" id=
+"pg102">102</a></span></p>
+
+<h3>CHAPTER VIII</h3>
+
+<h4>TRANSCENDENTAL ANATOMY IN ENGLAND&mdash;RICHARD OWEN</h4>
+
+<p><span class="smcap">Richard Owen</span> is the epigonos of
+transcendental morphology; in him its guiding ideas find clear
+expression, and in his writings are no half-truths struggling for
+utterance.</p>
+
+<div class="figcenter">
+<img
+src="images/fig04a.jpg"
+alt="Ideal
+Typical Vertebra. (After Owen.)" /></div>
+
+<p class="center2"><span class="smcap">Fig.</span> 4.&mdash;Ideal
+Typical Vertebra. (After Owen.)</p>
+
+<p> But he was, though a staunch transcendentalist, an
+eclectic of the older ideas current in his time; for he picked out what was
+best in the older systems&mdash;Cuvier's teleology, Geoffroy's
+principle of connections, Oken's idea of the serial repetition of
+parts. In particular, he assimilated the teaching of Cuvier, the
+great opponent of the transcendentalists, and reconciled it <span
+class="pagenum"><a name="pg103" id="pg103">103</a></span>in part
+with his own transcendentalism. His main theoretical views are to
+be found in his volume <i>On the Archetype and Homologies of the
+Vertebrate Skeleton</i> (London, 1848). The master-idea of the book
+is that the vertebrate skeleton consists of a series of comparable
+segments, each of which Owen calls a vertebra.</p>
+
+<div class="figcenter">
+<a href="images/fig05a.jpg"><img
+src="images/fig05a-tb.jpg"
+alt=" 5.&mdash;Natural
+Typical Vertebra; Thorax of a Bird. (After Owen.)" /></a></div>
+
+<div class="center2"><span class="smcap">Fig</span> 5.&mdash;Natural
+Typical Vertebra; Thorax of a Bird. (After Owen.)</div>
+
+<p>His definition of a vertebra is,
+"one of those segments of the endo-skeleton which constitute the
+axis of the body, and the protecting canals of the nervous and
+vascular trunks" (p.&nbsp;81). The parts of a typical vertebra are shown
+in <a href="#pg102">Fig.&nbsp;4</a>, which is copied from Owen's Fig.
+14.</p>
+
+<p><span class="pagenum"><a name="pg104" id=
+"pg104">104</a></span>In Fig.&nbsp;5 (page 103) is
+shown an actual vertebra, as Owen conceives it, the "vertebra"
+being that of a bird.</p>
+
+<p>A segment of sternum is included as the "h&aelig;mal spine" of
+the vertebra (<i>hs</i>); the vertebral rib is the "pleurapophysis"
+(<i>pl</i>); the sternal rib the "h&aelig;mapophysis" (<i>h</i>);
+the uncinate process of the vertebral rib is known as the
+"diverging appendage" (<i>a</i>). The whole vertebrate skeleton is
+composed of a series of vertebr&aelig; which show these typical parts.
+We arrive thus at the conception of an "Archetype" of the
+vertebrate skeleton, such as is represented in <a href=
+"#pg105">Fig.&nbsp;6.</a></p>
+
+<p>The archetype is only a scheme of what is usually constant in
+the vertebrate skeleton, and both the number and the arrangement of
+the bones in any real Vertebrate are subject to variation. "It has
+been abundantly proved," Owen writes, towards the end of his
+volume, "that the idea of a natural segment (vertebra) of the
+endoskeleton does not necessarily involve the presence of a
+particular number of pieces, or even a determinate and unchangeable
+arrangement of them. The great object of my present labour has been
+to deduce ... the relative value and constancy of the different
+vertebral elements, and to trace the kind and extent of their
+variations within the limits of a plain and obvious maintenance of
+a typical character" (p.&nbsp;146).</p>
+
+<p>It goes without saying that Owen considered the skull to be
+formed of vertebr&aelig;&mdash;the vertebral theory of the skull
+was, in his system, a deduction from the vertebral theory of the
+skeleton. He recognised four cranial vertebr&aelig;; the arrangement of
+them, and the relation of their constituent bones to the parts of
+the typical vertebra are shown in the table appearing on page 106.
+So far as their first three elements are concerned, these
+vertebr&aelig; are practically identical with the vertebr&aelig;
+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 vertebr&aelig;. The upper and lower jaws
+are associated with the nasal and frontal vertebr&aelig;
+respectively, not however as limbs of the head, but as constituent
+elements of these vertebr&aelig;. In the same way the hyoid
+apparatus is part and parcel of the parietal vertebra, and the
+pectoral girdle and fore-limbs part of the occipital vertebra.</p>
+
+<p><span class="pagenum"><a name="pg105" id=
+"pg105">105</a></span></p>
+
+<div class="figcenter">
+<a href="images/fig06a.jpg"><img
+src="images/fig06a-tb.jpg"
+alt="The Archetype of the Vertebrate Skeleton. (After Owen.)" /></a></div>
+
+<p class="center2"><span class="smcap">Fig.</span> 6.&mdash;The
+Archetype of the Vertebrate Skeleton. (After Owen.)</p>
+
+<p><span class="pagenum"><a name="pg106" id=
+"pg106">106</a></span></p>
+
+<p class="center2">Cranial Vertebr&aelig;.<a name="FNanchor_164"
+id="FNanchor_164" /><a href="#Footnote_164" class=
+"fnanchor">[164]</a> (After Owen, 1848, p.&nbsp;165.)</p>
+
+<table summary=
+"Cranial Vertebr&aelig;. (After Owen, 1848, p.&nbsp;165.)" border="1"
+cellpadding="5" cellspacing="0">
+<tbody>
+<tr>
+<td class="cell_lt5">Vertebr&aelig;.</td>
+<td class="cell_lt5">Occipital.</td>
+<td class="cell_lt5">Parietal.</td>
+<td class="cell_lt5">Frontal.</td>
+<td class="cell_lt5">Nasal.</td>
+</tr>
+
+<tr>
+<td class="cell_lt6">Centra.</td>
+<td class="cell_lt6">Basioccipital.</td>
+<td class="cell_lt6">Basisphenoid.</td>
+<td class="cell_lt6">Presphenoid.</td>
+<td class="cell_lt6">Vomer.</td>
+</tr>
+
+<tr>
+<td class="cell_lt6">Neurapopbyses.</td>
+<td class="cell_lt6">Exoccipital.</td>
+<td class="cell_lt6">Alisphenoid.</td>
+<td class="cell_lt6">Orbitosphenoid.</td>
+<td class="cell_lt6">Prefrontal.</td>
+</tr>
+
+<tr>
+<td class="cell_lt6">Neural Spines.</td>
+<td class="cell_lt6">Supraoccipital.</td>
+<td class="cell_lt6">Parietal.</td>
+<td class="cell_lt6">Frontal.</td>
+<td class="cell_lt6">Nasal.</td>
+</tr>
+
+<tr>
+<td class="cell_lt6">Parapopbyses.</td>
+<td class="cell_lt6">Paroccipital.</td>
+<td class="cell_lt6">Mastoid.</td>
+<td class="cell_lt6">Postfrontal.</td>
+<td class="cell_lt6">None.</td>
+</tr>
+
+<tr>
+<td class="cell_lt6">Pleurapophyses.</td>
+<td class="cell_lt6">Scapular.</td>
+<td class="cell_lt6">Stylohyal.</td>
+<td class="cell_lt6">Tympanic.</td>
+<td class="cell_lt6">Palatal.</td>
+</tr>
+
+<tr>
+<td class="cell_lt6">H&aelig;mapophyses.</td>
+<td class="cell_lt6">Coracoid.</td>
+<td class="cell_lt6">Ceratohyal.</td>
+<td class="cell_lt6">Articular.</td>
+<td class="cell_lt6">Maxillary.</td>
+</tr>
+
+<tr>
+<td class="cell_lt6">H&aelig;mal Spines.</td>
+<td class="cell_lt6">Episternum.</td>
+<td class="cell_lt6">Basihyal.</td>
+<td class="cell_lt6">Dentary.</td>
+<td class="cell_lt6">Premaxillary.</td>
+</tr>
+
+<tr>
+<td class="cell_lt6">Diverging Appendage.</td>
+<td class="cell_lt6">Fore-limb or Fin.</td>
+<td class="cell_lt6">Branchio-stegals.</td>
+<td class="cell_lt6">Operculum.</td>
+<td class="cell_lt6">Pterygoid and Zygoma.</td>
+</tr>
+</tbody>
+</table>
+
+<p>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 <i>Lepidosiren</i>, whose skeleton
+resembles the archetype in many ways, the pectoral girdle is
+likewise attached to the occipital segment.</p>
+
+<p>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&aelig;mapophysis, 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&aelig;mapophysis of the
+atlas. Owen considers it no objection to this view that in other
+Vertebrates the <span class="pagenum"><a name="pg107" id=
+"pg107">107</a></span>clavicle is anterior to the
+coracoid&mdash;"its anterior position to the coracoid in the
+air-breathing Vertebrata is no valid argument against the
+determination, since in these we have shown that the true scapular
+arch is displaced backwards" (<i>On the Nature of Limbs</i>, p.&nbsp;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
+h&aelig;mapophyses. 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 <i>Lepidosiren</i>,
+and presents few joints in <i>Proteus</i> and <i>Amphiuma</i>; 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.</p>
+
+<p>As the fore-limb is equivalent to a single bone of the
+archetype, it is said to be, in its developed state,
+"teleologically compound" (p.&nbsp;103).</p>
+
+<p>Since in the archetype every vertebra has its appendage, more
+than two pairs of locomotory limbs might have been developed. "Any
+given appendage might have been the seat of such developments as
+convert that of the pelvic arch into a locomotive limb; and the
+true insight into the general homology of limbs leads us to
+recognise many potential pairs in the typical endoskeleton. The
+possible and conceivable modifications of the vertebrate archetype
+are far from having been exhausted in the forms which have hitherto
+been recognised, from the prim&aelig;val fishes of the
+pal&aelig;ozoic ocean of this planet up to the present time" (p.
+102). It is not of the essence of the vertebrate type to be
+tetrapodal.</p>
+
+<p>In determining homologies Owen remained true to Geoffroy's
+principle of connections. Speaking of an attempt which had been
+made to determine homologies by the mode of development, he writes,
+"There exists doubtless a close general resemblance in the mode of
+development of homologous parts; but this is subject to
+modification, like the forms, proportions, functions, and very
+substance of such <span class="pagenum"><a name="pg108" id=
+"pg108">108</a></span>parts, without their essential homological
+relationships being thereby obliterated. These relationships are
+mainly, if not wholly, determined by the relative position and
+connection of the parts, and may exist independently of form,
+proportions, substance, function and similarity of development. But
+the connections must be sought for at every period of development,
+and the changes of relative position, if any, during growth, must
+be compared with the connections which the part presents in the
+classes where vegetative repetition is greatest and adaptive
+modification least" (p.&nbsp;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 (<i>On the Nature of Limbs</i>, p.
+49)&mdash;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" (<i>Limbs</i>, p.&nbsp;56).</p>
+
+<p>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:&mdash;"<i>Analogue</i>. A part or organ in one animal
+which has the same function as another part or organ in a different
+animal." "<i>Homologue</i>. The same organ in different animals
+under every variety of form and function."<a name="FNanchor_165"
+id="FNanchor_165" /><a href="#Footnote_165" class=
+"fnanchor">[165]</a></p>
+
+<p>He introduced also useful distinctions between Special, General,
+and Serial Homology. "The relations of homology," he writes, "are
+of three kinds: the first is that above defined, viz., the
+correspondency of a part or organ, determined by its relative
+position and connections, with a part or organ in a different
+animal; the determination of which homology indicates that such
+animals are constructed on a common <span class="pagenum"><a name=
+"pg109" id="pg109">109</a></span>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 <i>special homology</i> 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 <i>general homology</i> is enunciated.</p>
+
+<p>"If it be admitted that the general type of the vertebrate
+endoskeleton is rightly represented by the idea of a series of
+essentially similar segments succeeding each other longitudinally
+from one end of the body to the other, such segments being for the
+most part composed of pieces similar in number and arrangement, and
+though sometimes extremely modified for special functions, yet
+never so as to wholly mask their typical character&mdash;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.&nbsp;7). As an example of serial homology we might take
+the centra of the vertebr&aelig;&mdash;the vomer, the presphenoid,
+the basisphenoid, the basioccipital and the series of centra in the
+spinal column. Such serially repeated parts are called
+<i>homotypes</i> (p.&nbsp;8).</p>
+
+<p>Not all the bones of the vertebrate skeleton are included in the
+archetype as constituents of the vertebr&aelig;. 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.&nbsp;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.&nbsp;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 <span class="pagenum"><a name="pg110" id=
+"pg110">110</a></span>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.&nbsp;73).</p>
+
+<p>In his study of centres of ossification, Owen made in point of
+theory a distinct advance on his predecessors. We saw that Geoffroy
+recognised the importance of studying the ossification of the
+skeleton, and that Cuvier accepted such embryological evidence as
+an aid in determining homologies. Owen pointed out that it was
+necessary to distinguish between centres of ossification which were
+teleological in import and such as were purely indicative of
+homological relationships. Many bones, single in the adult, arise
+from separate centres of ossification, but we must distinguish
+between "those centres of ossification that have homological
+relations, and those that have only teleological ones; <i>i.e.</i>,
+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 (<i>e.g.</i>, frogs), the long bones should
+ossify first at their ends, for the brain is thus protected from
+concussion; in reptiles that creep there is less danger of
+concussion, and the long bones ossify in the middle (p.&nbsp;105). But
+there is no teleological reason why the coracoid process of the
+scapula should in all mammals develop from a separate centre. The
+coracoid is however a real vertebral element (h&aelig;mapophysis),
+and in monotremes, birds and reptiles it is in the adult a large
+and separate bone. Its ossification from a separate <span class=
+"pagenum"><a name="pg111" id="pg111">111</a></span>centre in
+mammals has therefore a homological significance. The scapula in
+mammals is an example of what Owen calls a "homologically compound"
+bone. All those bones which are formed by a coalescence of parts
+answering to distinct elements of the typical vertebra are
+"homologically compound" (p.&nbsp;105). On the other hand, "All those
+bones which represent single vertebral elements are 'teleologically
+compound' when developed from more than one centre, whether such
+centres subsequently coalesce, or remain distinct, or even become
+the subject of individual adaptive modifications, with special
+joints, muscles, etc., for particular offices" (p.&nbsp;106). The
+limb-skeleton, corresponding as it does to a single bone of the
+archetype, is the typical example of a teleologically compound
+bone. Owen in his definition of teleological compoundness has
+combined two kinds of adaptation&mdash;(1) temporary adaptation of
+bones to the exigencies of development, birth and growth
+(<i>e.g.</i>, development of long bones from separate centres); (2)
+definitive adaptation of a skeletal part to the functions which it
+has to perform (<i>e.g.</i>, teleological structure of limbs). Such
+adaptations are, so to speak, grafted on the archetype.</p>
+
+<p>Owen's general views on the nature of living things merit some
+attention. Organic forms, according to Owen, result from the
+antagonistic working of two principles, of which one brings about a
+vegetative repetition of structure, while the other, a teleological
+principle, shapes the living thing to its functions. The former
+principle is illustrated in the archetype of the vertebrate
+skeleton, in the segmentation of the Articulates, in the almost
+mathematical symmetry of Echinoderms, and the actually crystalline
+spicules of sponges. It is the same principle which causes
+repetition of the forms of crystals in the inorganic world. "The
+repetition of similar segments in a vertebral column, and of
+similar elements in a vertebral segment, is analogous to the
+repetition of similar crystals as the result of polarising force in
+the growth of an inorganic body" (p.&nbsp;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
+&#7984;&delta;&#941;&alpha;, on the other hand, produces the
+diversity of organic <span class="pagenum"><a name="pg112" id=
+"pg112">112</a></span>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.</p>
+
+<p>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.</p>
+
+<div class="footnote">
+<p><a name="Footnote_164" id="Footnote_164" /><a href=
+"#FNanchor_164"><span class="label">[164]</span></a> Owen
+introduced most of the names of bones now current.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_165" id="Footnote_165" /><a href=
+"#FNanchor_165"><span class="label">[165]</span></a> <i>Lectures on
+Invertebrate Animals</i>, pp.&nbsp;374, 379, 1843.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg113" id=
+"pg113">113</a></span></p>
+
+<h3>CHAPTER IX</h3>
+
+<h4>KARL ERNST VON BAER</h4>
+
+<p><span class="smcap">Von Baer</span> was recognised as the
+founder of embryology even by his contemporaries. His predecessors,
+Aristotle,<a name="FNanchor_166" id="FNanchor_166" /><a href=
+"#Footnote_166" class="fnanchor">[166]</a> Fabricius,<a name=
+"FNanchor_167" id="FNanchor_167" /><a href="#Footnote_167"
+class="fnanchor">[167]</a> Harvey,<a name="FNanchor_168" id=
+"FNanchor_168" /><a href="#Footnote_168" class=
+"fnanchor">[168]</a> Malpighi,<a name="FNanchor_169" id=
+"FNanchor_169" /><a href="#Footnote_169" class=
+"fnanchor">[169]</a> Haller,<a name="FNanchor_170" id=
+"FNanchor_170" /><a href="#Footnote_170" class=
+"fnanchor">[170]</a> Wolff,<a name="FNanchor_171" id=
+"FNanchor_171" /><a href="#Footnote_171" class=
+"fnanchor">[171]</a> 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.</p>
+
+<p>It was to one of the German transcendentalists that von Baer
+owed the impulse to study development. Ignatius D&ouml;llinger,
+Professor in W&uuml;rzburg, 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
+(<i>Anatomie u. Bildungsgeschichte des Gehirns</i>, 1816), by Oken
+(<i>loc. cit., supra</i>, p.&nbsp;90), and some others.</p>
+
+<p>Pander, with whom apparently D&ouml;llinger and d'Alton
+collaborated, was the first to publish his results;<a name=
+"FNanchor_172" id="FNanchor_172" /><a href="#Footnote_172"
+class="fnanchor">[172]</a> von Baer, who through absence from
+W&uuml;rzburg 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,
+<i>Ueber Entwickelungsgeschichte</i> <span class="pagenum"><a name=
+"pg114" id="pg114">114</a></span><i>der Thiere. Beobachtung und
+Reflexion</i> (K&ouml;nigsberg, 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&mdash;chick, tortoise, snake, lizard, frog, fish,
+several mammals and man, basing his remarks largely upon his
+personal observations, but taking account also of all contemporary
+work. A separate account of the development of a fish (<i>Cyprinus
+blicca</i>) appeared in 1835.<a name="FNanchor_173" id=
+"FNanchor_173" /><a href="#Footnote_173" class=
+"fnanchor">[173]</a></p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>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 (<i>die Wesenheit</i>) of the animal determines its
+differentiation, that no stage of development is solely determined
+by the antecedent stage, but that throughout all stages the
+<i>Wesenheit</i> 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 <span class="pagenum"><a name="pg115" id=
+"pg115">115</a></span>early embryo are compensated for or
+neutralised in the course of further development. Baer in this
+shows himself a vitalist.</p>
+
+<p>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
+(<i>Selbst&auml;ndigkeit</i>) of the developing animal" (p.&nbsp;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.</p>
+
+<p>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 lamell&aelig;. Baer's
+account of the process in the chick is as follows:&mdash;</p>
+
+<p>"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&mdash;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 <span class="pagenum"><a name=
+"pg116" id="pg116">116</a></span>plates&mdash;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.&nbsp;153).</p>
+
+<p>The process of delamination results then in the formation of
+four layers, of which the upper two (composing the "animal" or
+"serous" layer) will give origin to the animal (neuromuscular) part
+of the body, the lower pair to the plastic or vegetative organs.
+The uppermost layer will form the external covering of the embryo,
+and also the amniotic folds; from it there differentiates out at a
+very early stage the rudiment of the central nervous system,
+forming a more or less independent layer. Below the outermost layer
+lies the layer from which are formed the muscular and skeletal
+systems, and beneath this "muscle-layer" comes the "vessel-layer,"
+which gives origin to the main blood-vessels. The innermost layer
+of the four will form the mucous membrane of the alimentary canal
+and its dependencies; at the present stage, however, it is, like
+the other layers, a flat plate.</p>
+
+<p>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
+"Gekr&ouml;se," or mesenterial complex, which departs considerably
+from the tubular form.</p>
+
+<p>When these tubes or "fundamental organs" are formed the process
+of primary differentiation is complete. The fundamental organs,
+however, have at no time actually the form of tubes; they exist as
+tubes only ideally, for morphological and histological
+differentiation go on concurrently with the process of primary
+differentiation.</p>
+
+<p>Through morphological differentiation the various parts of the
+fundamental organs become specialised, through <span class=
+"pagenum"><a name="pg117" id="pg117">117</a></span>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, &oelig;sophagus, stomach, intestine, respiratory
+apparatus, liver, bladder, etc. This specialisation in development
+is bound up with increased or diminished growth" (p.&nbsp;155). Rapid
+growth concentrated at one point brings about an evagination; in
+this manner are formed the sense-organs from the nerve-tube, the
+liver and lungs from the alimentary tube. Or increased growth over
+a section of a tube causes it to swell out; in this wise the brain
+develops from the nerve-tube, the stomach from the alimentary tube.
+The segmentation which soon becomes so marked, particularly in the
+muscle layer, is also due to a process of morphological
+differentiation.</p>
+
+<p>At the same time that the organs of the body are being thus
+roughly blocked out and moulded from the germ-layers the third
+process of differentiation is actively going on. "In addition to
+the differentiation of the layers, there follows later another
+differentiation in the substance of the layers, whereby cartilage,
+muscle and nerve separate out, a part also of the mass becoming
+fluid and entering the bloodstream" (p.&nbsp;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.&nbsp;92). This homogeneity
+gives place to heterogeneity; the structureless mass becomes
+fibrous to form muscles, hardens to form cartilage or bone, becomes
+liquid to form the blood, differentiates in a hundred other
+ways&mdash;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 <span class="pagenum"><a name="pg118" id=
+"pg118">118</a></span>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.&nbsp;155, ii., pp.&nbsp;92-3).</p>
+
+<p>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 (<a
+href="#pg190">Chap.&nbsp;XII.</a>).</p>
+
+<p>The fourth Scholion elaborates the analysis of developmental
+processes still further, and discusses in particular the scheme of
+development which is shown by the Vertebrata. The characteristic
+structure of the vertebrate body is brought about by a "double
+symmetrical" rolling together of the germ-layers, whereby two main
+tubes are formed, one above and one below the axis of the body,
+which is the chorda. The dorsal tube is formed by the two animal
+layers, the ventral tube by all the layers combined (see <a href=
+"#pg119">Fig.&nbsp;7</a>).</p>
+
+<p>The process is indicated with sufficient clearness in the
+diagram. It will be seen that the real foundation and framework of
+the arrangement is the muscle-layer, with its two tubes, one
+surrounding the central nervous system and forming the "dorsal
+plates," the other surrounding the body cavity and forming the
+"ventral plates." In the dorsal plates, which early show metameric
+segmentation, the investing skeleton of the neural axis develops;
+in the ventral plates are formed the ribs, the ventral arches of
+the vertebr&aelig;, the hyoid, the lower jaw and other skeletal
+structures.</p>
+
+<p>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&mdash;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
+"Gekr&ouml;se," or remaining part of the vessel layer develop the
+Wolffian bodies (<i>Urnieren</i>, <span class="pagenum"><a name=
+"pg119" id="pg119">119</a></span>Pronephros), the kidneys, the sex
+glands, and the series of "blood-glands"&mdash;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.</p>
+
+<div class="figcenter">
+<img
+src="images/fig07a.jpg"
+alt="Ideal Transverse Section of a Vertebrate Embryo. (After von Baer.)" /></div>
+
+<p class="center2"><span class="smcap">Fig.</span> 7.&mdash;Ideal
+Transverse Section of a Vertebrate Embryo. (After von Baer.)</p>
+
+<table width="80%" summary=
+"Ideal Transverse Section of a Vertebrate Embryo." border="0"
+cellpadding="2" cellspacing="2">
+<tbody>
+<tr>
+<td class="cell_lt217b"><i>a</i>.&nbsp;Chorda.</td>
+<td class="cell_lt217b"><i>e</i>.&nbsp;Vessel-layer.</td>
+<td class="cell_lt217b"><i>i</i>.&nbsp;Amnion.</td>
+</tr>
+
+<tr>
+<td class="cell_lt217b"><i>b</i>.&nbsp;Dorsal plates.</td>
+<td class="cell_lt217b"><i>f</i>.&nbsp;Alimentary tube.</td>
+<td class="cell_lt217b"><i>k</i>.&nbsp;Serous membrane.</td>
+</tr>
+
+<tr>
+<td class="cell_lt217b"><i>c.</i>&nbsp;Ventral plates.</td>
+<td class="cell_lt217b"><i>g.</i>&nbsp;Pronephros.</td>
+<td class="cell_lt217b"><i>l.</i>&nbsp;Tolk sac.</td>
+</tr>
+
+<tr>
+<td class="cell_lt217b"><i>d.</i>&nbsp;Spinal cord.</td>
+<td class="cell_lt217b"><i>h.</i>&nbsp;Skin.</td>
+<td class="cell_lt217b">&nbsp;</td>
+</tr>
+</tbody>
+</table>
+
+<p>In his germ-layer theory von Baer was influenced a good deal by
+Pander, to whom the actual discovery of the process of
+layer-formation is due. Pander, however, had distinguished only
+three germ-layers, an upper "serous" layer, a lower "mucous" layer
+and a middle "vessel-layer." He it was who introduced the terms
+"Keimhaut" (blastoderm) and "Keimblatt" (germ-layer).</p>
+
+<p>The honour of being the founder of the germ-layer theory is
+sometimes attributed to C. F. Wolff, notably by K&ouml;lliker and O.
+Hertwig. Wolff, it is true, in his memoir <i>De formatione
+intestinorum</i> (1768-9) showed that the alimentary canal was
+<span class="pagenum"><a name="pg120" id=
+"pg120">120</a></span>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&mdash;a process
+which remained dark to Wolff&mdash;have shed a light upon all forms
+of development" (p.&nbsp;xxi.).</p>
+
+<p>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&mdash;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;&mdash;"People gradually learnt to think of the different
+animal forms as developed one from another&mdash;and seemed, in
+some circles at least, determined to forget that this metamorphosis
+could only be conceptual" (p.&nbsp;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 <span class="pagenum"><a name="pg121" id=
+"pg121">121</a></span>development&mdash;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.</p>
+
+<p>Von Baer had soon found in the course of his embryological
+studies that the facts did not at all fit in with the doctrine of
+parallelism; the developing chick, for example, was at a very early
+stage demonstrably a Vertebrate, and did not recapitulate in its
+early stages the organisation of a polyp, a worm or a mollusc. He
+had published his doubts in 1823, but his final confutation of the
+theory of parallelism is found in this Scholion.</p>
+
+<p>If it were true, he says, that the essential thing in the
+development of an animal is this repetition of lower organisations,
+then certain deductions could be drawn, which one would expect to
+find confirmed in Nature. The first deduction would be that no
+structures should appear in the embryo of the higher animals that
+are not found in the lower animals. But this is not confirmed by
+the facts&mdash;no adult among the lower animals, for instance, has
+a yolk-sac like that of the chick embryo. Again, if the law of
+parallelism were true, the mammalian embryo would have to repeat
+the organisation of, among other groups, insects and birds. But the
+embryo <i>in utero</i> is surrounded by fluid and cannot possibly
+breathe free air, so it cannot possibly repeat the structure of
+either insects or birds, which are pre-eminently air-organisms.
+Generally speaking, indeed, we find in all the higher embryos
+special structures which adapt them to the very special conditions
+of their development, and these we never find as permanent
+structures in the lower animals. The supporters of the theory of
+parallelism might, however, admit the existence of such special
+embryonic organs without greatly prejudicing their case, for these
+temporary organs stand to some extent outside the scope of the
+theory.</p>
+
+<p>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 <span class="pagenum"><a
+name="pg122" id="pg122">122</a></span>with them in isolated details
+of structure. The deduction is, however, not borne out by the
+facts. The embryo of a mammal resembles in many points, at
+different stages of its development, the adult state of a fish; it
+has gill-slits and complete aortic arches, a two-chambered heart,
+and so on. But at no time does it combine all the essential
+characters of a fish; nor has it ever the tail of a fish, nor the
+fins, nor the shape. Any recapitulation there may be is a
+recapitulation of single organs, there is never a repetition of the
+complete organisation of a fish. This is indeed the fundamental
+criticism of the theory of parallelism; and if it applies even
+within the limits of the vertebrate phylum, so much the more does
+it apply to comparisons between embryonic Vertebrates and adult
+Invertebrates.</p>
+
+<p>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.</p>
+
+<p>Again, were the analogy between the development of the
+individual and the evolution of the <i>&Eacute;chelle des
+&ecirc;tres</i> 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.</p>
+
+<p>Another consequence one would expect to find realised, were the
+theory of parallelism correct, is the late appearance in
+development of parts which are confined to the higher animals. In
+the development of a Vertebrate accordingly <span class=
+"pagenum"><a name="pg123" id="pg123">123</a></span>one would not
+expect the vertebr&aelig; 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.</p>
+
+<p>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&mdash;we must distinguish between
+the <i>type</i> of organisation and the <i>grade</i> of
+differentiation. By "type" von Baer means the structural plan of
+the organism. "I call the <i>type</i> the spatial relationship of
+the organic elements and organs" (p.&nbsp;208). Each type of
+organisation characterises one of the big groups of animals; the
+lesser groups represent "grade" modifications of the type. "The
+product of the degree of differentiation and the type gives the
+several great groups of animals which are called classes" (p.&nbsp;208).
+<i>Ausbildung</i> (differentiation) takes place in one or other of
+several directions, in adaptation, for instance, to life in the
+water or to life in the air.</p>
+
+<p>There are, von Baer considers, four main types&mdash;(1) the
+peripheral or radiate type, (2) the longitudinal type, (3) the
+massive or molluscan type, (4) the vertebrate type. The radiate
+type is shown by discoid infusoria, by medus&aelig;, by starfish
+and their allies. The longitudinal type characterises such genera
+as <i>Vibrio</i>, <i>Filaria</i>, <i>Gordius</i>, and all the
+annulate animals. Mollusca, rotifers, polyzoa, and such infusoria
+as are not included in types (1) and (2) belong to the massive
+type, in which the body and its parts form rounded masses. The
+longitudinal type is predominantly "animal," the massive type
+predominantly "plastic" (vegetative). The vertebrate type has both
+the "animal" and the "plastic" organs highly developed. In the
+symmetrical arrangement of the animal parts it resembles the
+longitudinal type; its <span class="pagenum"><a name="pg124" id=
+"pg124">124</a></span>plastic parts with their asymmetrical
+arrangement and rounded shape belong to the massive type.</p>
+
+<p>These types of von Baer inevitably recall the "Embranchements"
+of Cuvier, with which they more or less coincide. It seems that von
+Baer arrived at his types (from the study of adult structure)
+independently of Cuvier, though the priority of publication rests
+with Cuvier.<a name="FNanchor_174" id="FNanchor_174" /><a href=
+"#Footnote_174" class="fnanchor">[174]</a></p>
+
+<p>Now it is clear that the development of the individual, which is
+essentially an <i>Ausbildung</i>, 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&mdash;the law that "the type
+of organisation determines the manner of development" (p.&nbsp;xxii.).
+Development is not merely from the general to the
+special&mdash;there are at least four distinct "general" types,
+from which the special is developed. The type is fixed in the very
+earliest stages of development&mdash;the embryo of a Vertebrate is
+from the very beginning a Vertebrate (p.&nbsp;220), and it shows at no
+time any agreement in total organisation with any Invertebrate. The
+types are independent of one another; differentiation and
+development follow a different course in each of them. Not but what
+some analogies can be found between the very earliest stages of
+embryos of different type. Thus vertebrate and annulate embryos
+agree in certain points at the time of the formation of the
+primitive streak. And in the earliest stage of all, the egg-stage,
+there is probably agreement between all the types. In eggs with
+yolk, whether vertebrate or annulate, there is always a separation
+into an animal and a plastic layer. It seems, too, as if a hollow
+sphere were a constant stage in the development of all animals (pp.
+224, 258). Apart from these analogies, development takes an
+entirely independent course in each of the four main types, and no
+embryo of one of the higher types repeats in its development the
+peculiar organisation of any adult of the lower types.</p>
+
+<p><span class="pagenum"><a name="pg125" id=
+"pg125">125</a></span>If we consider now development within the
+type, which is the only legitimate thing to do, we arrive at
+certain laws governing the relation of embryos to one another. For
+instance, at a certain stage vertebrate embryos are uncommonly
+alike. Von Baer had two in spirit which he was unable to assign to
+their class among amniotes; they might have been lizard, bird, or
+mammal, he could not say definitely which.<a name="FNanchor_175"
+id="FNanchor_175" /><a href="#Footnote_175" class=
+"fnanchor">[175]</a> Generally the farther back we go in the
+development of Vertebrates the more alike we find the embryos. The
+type-characters are first to appear, then the class characters,
+then the characters distinguishing the lesser classificatory
+groups. "From a more general type the special gradually emerges"
+(p.&nbsp;221). The chick is first a Vertebrate, then a land-vertebrate,
+then a bird, then a land-bird, then a gallinaceous bird, and
+finally <i>Gallus domesticus</i>. 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:&mdash;</p>
+
+<p>"(1) <i>That the general characters of the big group to which
+the embryo belongs appear in development earlier than the special
+characters.</i> 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?</p>
+
+<p>"(2) <i>The less general structural relations are formed after
+the more general, and so on until the most special appear.</i></p>
+
+<p>"(3) <i>The embryo of any given form, instead of passing through
+the state of other definite forms, on the contrary separates itself
+from them.</i></p>
+
+<p>"(4) <i>Fundamentally the embryo of a higher animal form</i>
+<span class="pagenum"><a name="pg126" id=
+"pg126">126</a></span><i>never resembles the adult of another
+animal form, but only its embryo</i>" (p.&nbsp;224).</p>
+
+<p>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.&nbsp;224).</p>
+
+<p>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.</p>
+
+<p>One important consequence for morphology results from von Baer's
+laws of differentiation within the type. If the embryo develops
+from the general to the special, then the state in which each organ
+or organ-system first appears must represent the general or typical
+state of that organ within the group. Embryology will therefore be
+of great assistance to comparative anatomy, whose chief aim it is
+to discover the generalised type, the common plan of structure,
+upon which the animals of each big group are built. And the surest
+way to determine the true homologies of parts will be to study
+their early development. "For since each organ becomes what it is
+only through the manner of its development, its true value can be
+recognised only from its method of formation. At present, we form
+our judgments by an undefined intuition, instead of regarding each
+organ merely as an isolated product of its fundamental organ, and
+discerning from this standpoint the correspondences and
+dissimilarities in the different types" (p.&nbsp;233). Parts, therefore,
+which <span class="pagenum"><a name="pg127" id=
+"pg127">127</a></span>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.</p>
+
+<p>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 <i>functional</i>
+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 trache&aelig; 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.&nbsp;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.</p>
+
+<p>The development of plants, like the development of animals, is
+essentially a progress from the general to the special (p.&nbsp;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 <span class=
+"pagenum"><a name="pg128" id="pg128">128</a></span>groups as does
+the study of adult structure. The four types of development are
+thus:&mdash;</p>
+
+<p>(1) The double-symmetrical, which is found in Vertebrates. It is
+called the double-symmetrical, because in Vertebrates development
+takes place from a central axis (notochord) in two directions,
+upwards and downwards, in such a way that two tubes are formed, one
+above and one below the axis. (2) The second type is the
+symmetrical, which is shown by Annulates. A primitive streak is
+formed on the ventral surface of the yolk; development proceeds
+symmetrically on both sides of the streak. (3) Radiate development
+is probably typical of the radiate structural type. (4) In the
+massive type, the development seems to be a spiral one.</p>
+
+<p>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,<a name="FNanchor_176" id="FNanchor_176" /><a href=
+"#Footnote_176" class="fnanchor">[176]</a> for reptiles develop an
+amnion and Batrachia do not.</p>
+
+<p>We come now to the sixth and last Scholion. Development is a
+true evolution of the special from the general, so runs von Baer's
+most general law of all. This can be expressed in a slightly
+different way, and the words which he chooses in the sixth Scholion
+to express this final and most general result are these:&mdash;"The
+developmental history of the individual is the history of the
+growing individuality in every respect" (p.&nbsp;263). The greatest
+modern treatise on embryology ends on a splendid note. One creative
+thought rules all the forms of life. And more&mdash;"It is this
+same thought that in cosmic space gathered the scattered masses
+into spheres and bound them together in the solar system, the same
+that from the weathered dust on the surface of the metallic planets
+brought forth the forms of life. And this thought is nought else
+but life itself, and the words and syllables in which life
+expresses itself are the varied forms of the living" (p.&nbsp;264).</p>
+
+<p>Von Baer reminds one greatly of Cuvier. There is <span class=
+"pagenum"><a name="pg129" id="pg129">129</a></span>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<a name="FNanchor_177" id="FNanchor_177" /><a
+href="#Footnote_177" class="fnanchor">[177]</a> 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.</p>
+
+<p>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
+<i>&Eacute;chelle des &ecirc;tres</i>, evolutionary or ideal. Von
+Baer also, though influenced greatly by <i>Naturphilosophie</i>,
+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.<a name="FNanchor_178" id="FNanchor_178" /><a href=
+"#Footnote_178" class="fnanchor">[178]</a></p>
+
+<p>We have seen what were von Baer's criticisms of the theory of
+parallelism. If we turn to the later writings of Cuvier we find the
+essential criticism expressed in similar terms. Speaking of an
+attempt which had been made to show that fish were molluscs
+developed to a higher degree, he wrote in 1828,<a name=
+"FNanchor_179" id="FNanchor_179" /><a href="#Footnote_179"
+class="fnanchor">[179]</a> "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 <span class="pagenum"><a name="pg130"
+id="pg130">130</a></span>in a sense embryos of Amphibia (p.&nbsp;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.&nbsp;548), and is directly contrary to
+common sense. There is never a recapitulation of total
+organisations, only at the most of single organs.</p>
+
+<p>It will be remembered that Cuvier opposed and demolished the
+theory of the <i>&Eacute;chelle des &ecirc;tres</i>, 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 <i>Embranchement</i> could not readily be arranged
+in one series, that a serial arrangement was really valid only for
+their separate organs. Von Baer also held that there are four
+distinct types of structure; he, too, combated the idea of
+gradation within the limits of the type. In so far as species
+represent successive stages in the development, the
+<i>Ausbildung</i>, of the type, so far can the idea of a scale of
+beings be applied. But the members of a type follow not one line of
+evolution but several diverging lines, in direct adaptation to
+different environmental conditions, so that a serial arrangement of
+them is not as a rule possible. It may be possible to establish a
+serial arrangement of single organs from the simplest to the most
+complex. But each organ or organ-system will require a different
+serial arrangement, for the different systems vary on different
+lines and an animal may be highly developed in respect of one
+system and little developed in respect of all the others. Man, for
+instance, is the highest animal only in respect of his nervous
+system. The idea of the scale of beings has therefore only a very
+limited application even within the limits of the type. Applied to
+the whole animal kingdom it becomes merely absurd.</p>
+
+<p>Another point of resemblance between Cuvier and von Baer was
+that Cuvier, though essentially a student of adult structure, did
+recognise the importance of embryology; following up some
+observations of Dutrochet he studied the f&oelig;tal membrane of
+mammals and tried to establish their homologies.<a name=
+"FNanchor_180" id="FNanchor_180" /><a href="#Footnote_180"
+class="fnanchor">[180]</a> And in his criticism of the vertebral
+theory of the skull he advanced as an argument against the
+basisphenoid <span class="pagenum"><a name="pg131" id=
+"pg131">131</a></span>being a vertebral centrum the fact
+(established by Kerkring, 1670), that it develops from two
+centres.<a name="FNanchor_181" id="FNanchor_181" /><a href=
+"#Footnote_181" class="fnanchor">[181]</a> 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.<a name=
+"FNanchor_182" id="FNanchor_182" /><a href="#Footnote_182"
+class="fnanchor">[182]</a> 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.&nbsp;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&mdash;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.&nbsp;192).
+The particular form which his theory of the relation of jaws to
+limbs took is shown in the following passage:&mdash;"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 <i>Archiv</i>, p.&nbsp;367,
+1826).</p>
+
+<p>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
+<i>morphological elements</i>, each of which was made up a section
+from each of the fundamental organs&mdash;a vertebra, a section of
+the nerve-cord, and so on (<i>Entwickelungsgeschichte</i>, ii., p.
+53). Groups of these morphological elements formed <i>morphological
+divisions</i>, such as the vertebral segments of the head with
+their highly developed neural arches, or the segments of the neck
+with their undeveloped h&aelig;mal arches. The morphological
+elements are clearly shown only in the animal parts, but there are
+indications in the embryo of a segmentation also of the vegetative
+parts,&mdash;the gill-slits, for instance, and the vascular arches.
+<span class="pagenum"><a name="pg132" id="pg132">132</a></span>The
+vegetative parts, however, develop on the whole unsymmetrically
+(<i>cf.</i> Bichat). These elements which von Baer distinguishes
+are morphological units, as he himself points out, contrasting them
+with organs which are not usually units in a morphological sense.
+"We call organ," he writes, "each part that has by reason of its
+form or its function a certain distinctiveness, but this concept is
+very indefinite, and possesses, from a morphological point of view,
+little value. For this reason it seems necessary to introduce into
+scientific morphology the concepts of morphological elements and
+divisions" (ii., p.&nbsp;84).</p>
+
+<p>Von Baer exercised a very considerable influence upon the
+subsequent trend of morphological theory. By his criticism of the
+Meckel-Serres theory, he rid morphology for a time of an idea which
+was leading it astray; by his substitution of the law that
+development is always from the general to the special, he set
+morphologists looking for the archetype in the embryo, not in the
+adult alone, and made them realise that homologies could often best
+be sought in the earliest stages of development; by formulating the
+germ-layer theory he supplied morphologists with a new criterion of
+homology, based upon the special relations of the parts
+(germ-layers) which are first differentiated in all development. He
+made the study of development an essential part of morphology.</p>
+
+<div class="footnote">
+<p><a name="Footnote_166" id="Footnote_166" /><a href=
+"#FNanchor_166"><span class="label">[166]</span></a> <i>De
+generatione Animalium</i>.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_167" id="Footnote_167" /><a href=
+"#FNanchor_167"><span class="label">[167]</span></a> <i>De formato
+f&oelig;tu</i>, ? 1600; <i>De formatione f&oelig;tus</i>, 1604.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_168" id="Footnote_168" /><a href=
+"#FNanchor_168"><span class="label">[168]</span></a>
+<i>Exercitationes de generatione animalium</i>, 1651.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_169" id="Footnote_169" /><a href=
+"#FNanchor_169"><span class="label">[169]</span></a> <i>De
+formatione pulli in ovo</i>, 1673; <i>De ovo incubato</i>, 1686.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_170" id="Footnote_170" /><a href=
+"#FNanchor_170"><span class="label">[170]</span></a> <i>De
+formatione pulli in ovo</i>, 1757-8; <i>Sur la formation du
+c&oelig;ur dans le poulet</i>, 1758.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_171" id="Footnote_171" /><a href=
+"#FNanchor_171"><span class="label">[171]</span></a> <i>Theoria
+generatioinis</i>, 1759; <i>De formatione
+intestinorum</i>, 1768-9.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_172" id="Footnote_172" /><a href=
+"#FNanchor_172"><span class="label">[172]</span></a>
+<i>Beitr&auml;ge zur Entwickelung des H&uuml;hnchens im Ei.</i>
+W&uuml;rzburg, 1818. Also in Latin in shorter form, 1817.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_173" id="Footnote_173" /><a href=
+"#FNanchor_173"><span class="label">[173]</span></a>
+<i>Untersuchungen &uuml;. die Entwickelungsgeschichte der
+Fische</i>; Leipzig, 1835.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_174" id="Footnote_174" /><a href=
+"#FNanchor_174"><span class="label">[174]</span></a> Cuvier, in
+1812, <i>Ann. Mus. d'Hist. Nat.</i>, xix.; von Baer in 1816,
+<i>Nova Acta Acad. Nat. Cur.</i> See <i>Entwickelungsgeschichte der
+Thiere</i>, i., p.&nbsp;vii., f.n.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_175" id="Footnote_175" /><a href=
+"#FNanchor_175"><span class="label">[175]</span></a> Compare a
+parallel passage in Pr&eacute;vost et Dumas:&mdash;"At the very
+first sight one will be struck with the resemblance between the
+forms of the very early embryos of these two classes, a resemblance
+so extraordinary that one cannot refuse to admit the conclusions
+resulting from it. The resemblance is so striking that one can defy
+the most experienced observer to distinguish in any way the embryos
+of dog or rabbit ... from those of fowls or ducks of a
+corresponding age."&mdash;<i>Ann. Sci. nat.</i>, iii., p.&nbsp;132,
+1824.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_176" id="Footnote_176" /><a href=
+"#FNanchor_176"><span class="label">[176]</span></a> <i>De
+l'organisation des Animaux</i>, i., p.&nbsp;140, 1822.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_177" id="Footnote_177" /><a href=
+"#FNanchor_177"><span class="label">[177]</span></a> "Ueber das
+&auml;ussere und innere Skelet," Meckel's <i>Archiv f&uuml;r Anat.
+u. Physiol.</i>, pp.&nbsp;327-76, 1826. See, too, his
+<i>Entwickelungsgeschichte</i>, i., pp.&nbsp;181, ff.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_178" id="Footnote_178" /><a href=
+"#FNanchor_178"><span class="label">[178]</span></a> Von Baer wrote
+an appreciative biography of Cuvier, published posthumously in
+1897, <i>Lebensgeschichte Cuviers</i>, ed. L. Stieda. French trans.
+in <i>Ann. Sci. Nat.</i> (<i>Zool.</i>), ix., 1907.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_179" id="Footnote_179" /><a href=
+"#FNanchor_179"><span class="label">[179]</span></a> Cuvier et
+Valenciennes, <i>Histoire naturelle des Poissons</i>, i., p.
+550.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_180" id="Footnote_180" /><a href=
+"#FNanchor_180"><span class="label">[180]</span></a> <i>M&eacute;m.
+Mus. d'Hist. Nat.</i>, iii., pp.&nbsp;98-119, 1817.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_181" id="Footnote_181" /><a href=
+"#FNanchor_181"><span class="label">[181]</span></a>
+<i>Le&ccedil;ons d'Anatomie compar&eacute;e</i>, 3rd ed., vol. i.,
+p.&nbsp;414, Bruxelles, 1836.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_182" id="Footnote_182" /><a href=
+"#FNanchor_182"><span class="label">[182]</span></a> In the
+aforementioned paper in M&uuml;ller's <i>Archiv</i> he criticises Carus
+vigorously and is sarcastic on Geoffroy.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg133" id=
+"pg133">133</a></span></p>
+
+<h3>CHAPTER X</h3>
+
+<h4>THE EMBRYOLOGICAL CRITERION</h4>
+
+<p><span class="smcap">Pander's</span> work of 1817 was the
+forerunner of an embryological period in which men's hopes and
+interest centred round the study of development. "With bewilderment
+we saw ourselves transported to the strange soil of a new world,"
+wrote Pander, and many shared his hopeful enthusiasm. K. E. von
+Baer's <i>Entwickelungsgeschichte</i> 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.</p>
+
+<p>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
+<i>Abhandlungen</i> 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 <i>Descrizione anatomica degli organi della circolazione delle
+larve delle Salamandre aquatiche</i> (Pavia), in which he traced
+the metamorphoses of the aortic arches. This was followed in 1822
+by his <i>Amours des Salamandres aquatiques</i> (Milan), and in
+1826 by his memoir <i>Du d&eacute;veloppement de la grenouille</i>
+(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 <span class="pagenum"><a name="pg134" id=
+"pg134">134</a></span>the alimentary canal; he observed and
+interpreted rightly the formation of the medullary folds. The
+circular blastopore in the frog in later years often went by the
+name of the anus of Rusconi.</p>
+
+<p>In France Dutrochet<a name="FNanchor_183" id=
+"FNanchor_183" /><a href="#Footnote_183" class=
+"fnanchor">[183]</a> investigated the f&oelig;tal membranes in
+various vertebrate classes; Pr&eacute;vost and Dumas studied the
+very earliest stages of development in birds, mammals and amphibia
+(<i>Ann. Sci. nat.</i>, ii., iii., 1824, xii., 1827).</p>
+
+<div class="figleft">
+             <img src="images/fig08a.jpg"
+             alt="Fig 8." /><br />
+         <span class="smcap">Fig.</span> 8.<br />Gill-slits of the Pig Embryo. <br />(After Rathke.)
+    </div>
+
+<p>A little later came Dug&egrave;s' studies of the osteology and
+myology of developing amphibia (1834),<a name="FNanchor_184" id=
+"FNanchor_184" /><a href="#Footnote_184" class=
+"fnanchor">[184]</a> and Coste's careful
+researches into the early developmental history of mammals.<a name=
+"FNanchor_185" id="FNanchor_185" /><a href="#Footnote_185"
+class="fnanchor">[185]</a></p>
+
+
+<p>It was in 1825 that Heinrich Rathke (1793-1860), published his
+famous discovery of gill-slits in the embryo of a mammal,<a name=
+"FNanchor_186" id="FNanchor_186" /><a href="#Footnote_186"
+class="fnanchor">[186]</a> a discovery which aroused considerable
+interest, and greatly stimulated embryological research. He
+describes how in a young embryo of a pig he saw four slits in the
+region of the neck, going right through into the &oelig;sophagus.
+They were separated by partitions which he called
+<i>Kiemenbogen</i> (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 (<i>Isis</i>, Pl. IV., fig. 1).</p>
+
+<p>Later in the same year Rathke discovered gill-slits in the
+chick,<a name="FNanchor_187" id="FNanchor_187" /><a href=
+"#Footnote_187" class="fnanchor">[187]</a> 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.</p>
+
+<p>These discoveries were confirmed and extended for the <span
+class="pagenum"><a name="pg135" id="pg135">135</a></span>chick<a
+name="FNanchor_188" id="FNanchor_188" /><a href=
+"#Footnote_188" class="fnanchor">[188]</a> 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.&nbsp;401). Between these two arches he found an opening, just
+as between two gill-arches a gill-slit. This opening led into the
+mouth-cavity, and according to Huschke it became the external
+ear-passage. He discovered also three pairs of aortic arches in
+close relation with the gill-arches, so close indeed, that he did
+not hesitate to call them gill-arteries, and to recognise their
+resemblance with the aortic arches of fish. He traced, in part at
+least, the metamorphosis which these aortic arches undergo. This
+part of his discovery he developed in fuller detail in a paper of
+1828,<a name="FNanchor_189" id="FNanchor_189" /><a href=
+"#Footnote_189" class="fnanchor">[189]</a> in which he gave some
+excellent figures.</p>
+
+<p>Shortly after Huschke's first paper, von Baer published his
+views and observations on this subject in a short memoir in
+Meckel's <i>Archiv</i>.<a name="FNanchor_190" id=
+"FNanchor_190" /><a href="#Footnote_190" class=
+"fnanchor">[190]</a> 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.&nbsp;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.</p>
+
+<p><span class="pagenum"><a name="pg136" id=
+"pg136">136</a></span>In his view of the metamorphosis of the
+aortic arches in the chick the first two pairs disappeared
+completely, the third pair gave rise to the arteries of the head
+and the fore-limbs, the right side of the fourth arch became the
+aorta, the left half of the fourth and the right half of the fifth
+arch became the pulmonary arteries, while the left half of the
+fifth arch disappeared. This schema, which for the last three
+arches was the same as Huschke's, von Baer upheld for the chick
+even in the second volume of his <i>Entwickelungsgeschichte</i> (p.
+116); he rectified it, however, for mammals in the same volume (p.
+212), deriving both pulmonary arteries from the fifth arch, and the
+aorta from the fourth left. He fully recognised the great analogy
+of the embryonic arrangement of gill-arches and gill-arteries in
+Tetrapoda with their arrangement in fish (i., pp.&nbsp;53, 73).</p>
+
+<p>Huschke, in a paper of 1832,<a name="FNanchor_191" id=
+"FNanchor_191" /><a href="#Footnote_191" class=
+"fnanchor">[191]</a> chiefly devoted to the development of the eye,
+figured and described the developing upper and lower jaws, and
+maintained against von Baer that the first slit turns into the
+auditory meatus and the Eustachian tube.</p>
+
+<p>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.</p>
+
+<p>The typical figure of the period is Rathke, who produced a great
+deal of first-class embryological work. He was, even more than von
+Baer, a comparative embryologist, and there were few groups of
+animals that he did not study. His first large publication, the
+<i>Beitr&auml;ge zur Geschichte der Thierwelt</i> (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
+<i>Abhandlungen zur Bildungsund Entwickelungs-Geschichte des
+Menschen und der Thiere</i> (i., ii., Leipzig, 1832-3). A
+fellow-worker in this line was Johannes M&uuml;ller, whose
+<i>Bildungsgeschichte der Genitalien</i> (D&uuml;sseldorf) appeared
+in 1830.</p>
+
+<p>In a memoir on the development of the crayfish which <span
+class="pagenum"><a name="pg137" id="pg137">137</a></span>appeared
+in 1829,<a name="FNanchor_192" id="FNanchor_192" /><a href=
+"#Footnote_192" class="fnanchor">[192]</a> Rathke found in an
+Invertebrate confirmation of the germ-layer theory propounded by
+Pander and von Baer. He was greatly struck by the inverted position
+of the embryo with respect to the yolk. In following out the
+development of the appendages he noticed how much alike were jaws
+and legs in their earliest stage, and how this supported Savigny's
+contention that the limbs of Arthropods belonged to one single type
+of structure. In his paper (1832) on the development of the
+fresh-water Isopod, <i>Asellus</i>,<a name="FNanchor_193" id=
+"FNanchor_193" /><a href="#Footnote_193" class=
+"fnanchor">[193]</a> Rathke returns to this point. Commenting on the
+original similarity in development of antenn&aelig;, 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).</p>
+
+<p>Further comparative work by Rathke is found in the two volumes
+of <i>Abhandlungen</i> and in a book, <i>Zur Morphologie,
+Reisebemerkungen aus Taurien</i> (1837), which contains
+embryological studies of many different types, including a study of
+the uniform plan of arthropod limbs. Later on Rathke devoted
+himself more to vertebrate embryology, producing among other works
+his classical papers on the development of the adder (1839), of the
+tortoise (1848), and of the crocodile (1866). He laid the
+foundations of all subsequent knowledge of the development of the
+blood-vascular system in a series of papers of various dates from
+1838 to 1856. The diagrams in his paper on the aortic arches of
+reptiles (1856) were for long copied in every text-book.</p>
+
+<p>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 <span class=
+"pagenum"><a name="pg138" id="pg138">138</a></span>history of the
+embryological study of the skull in some detail below; meantime, we
+note the two other important lines of research which characterise
+this period. One is the intensive study of the development of the
+human embryo, a study pursued by, among others, Pockels, Seiler,
+Breschet, Velpeau, Bischoff, Weber, M&uuml;ller, and Wharton
+Jones.<a name="FNanchor_194" id="FNanchor_194" /><a href=
+"#Footnote_194" class="fnanchor">[194]</a> The other important
+line&mdash;the early development of the Mammalia&mdash;was worked
+chiefly by Valentin,<a name="FNanchor_195" id=
+"FNanchor_195" /><a href="#Footnote_195" class=
+"fnanchor">[195]</a> Coste,<a name="FNanchor_196" id=
+"FNanchor_196" /><a href="#Footnote_196" class=
+"fnanchor">[196]</a> and, above all, by Bischoff, whose series of
+papers<a name="FNanchor_197" id="FNanchor_197" /><a href=
+"#Footnote_197" class="fnanchor">[197]</a> was justly recognised as
+classical.</p>
+
+<p>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 <i>Anatomische-philosophische Untersuchungen
+&uuml;ber den Kiemenapparat und das Zungenbein</i> (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 M&uuml;ller made great use of
+embryology in his classical <i>Vergleichende Anatomie der
+Myxinoiden</i> (i. Theil, 1836), and, according to his pupil
+Reichert, firmly held the opinion that embryology was the final
+court of appeal in disputed points of comparative anatomy;<a name=
+"FNanchor_198" id="FNanchor_198" /><a href="#Footnote_198"
+class="fnanchor">[198]</a> Reichert himself in a book of 1838
+(<i>Vergleichende <span class="pagenum"><a name="pg139" id=
+"pg139">139</a></span>Entwickelungsgeschichte des Kopfes der
+nackten Amphibien</i>) discussed the two different methods of
+arriving at the "Type"&mdash;the anatomical method of comparing
+adults, and the embryological method of comparing embryogenies. Of
+the embryological method, he says, "Its aim is to distinguish
+during the formation of the organism the originally given, the
+essence of the type, and to classify and interpret what is added or
+altered in the further course of development. Embryologists watch
+the gradual building up of the organism from its foundations, and
+distinguish the fundament, the primordial form, the type, from the
+individual developments; they reach thus, following Nature in a
+certain measure, the essential structure of the organism, and
+demonstrate the laws that manifest themselves during embryogeny"
+(p.&nbsp;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
+<i>embryological archetype</i>. 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.</p>
+
+<p>Following up his discovery of gill-slits and arches in the
+embryos of birds and mammals, Rathke in two papers of 1832<a name=
+"FNanchor_199" id="FNanchor_199" /><a href="#Footnote_199"
+class="fnanchor">[199]</a> and 1833<a name="FNanchor_200" id=
+"FNanchor_200" /><a href="#Footnote_200" class=
+"fnanchor">[200]</a> 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, <span
+class="pagenum"><a name="pg140" id="pg140">140</a></span>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.</p>
+
+<p>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.</p>
+
+<p>But Rathke could not altogether shake himself free from the
+transcendental notion of the homology of jaws with ribs, and this
+led him to draw a certain distinction between the first two and the
+remaining gill-arches, by which the homology of the former with the
+ribs was asserted and the homology of the latter denied. He thought
+he could show that the skeletal structures (lower jaw, "quadrate,"
+and hyoid) of the first two arches were formed in the serous layer,
+just like true ribs, and like them in close connection with the
+vertebral skeletal axis. The other, "true," gill-arches appeared to
+him to be formed in the mucous layer, in the lining of the
+alimentary canal. They had no direct connection with the vertebral
+column, and seemed therefore to belong to what Carus<a name=
+"FNanchor_201" id="FNanchor_201" /><a href="#Footnote_201"
+class="fnanchor">[201]</a> had called the visceral or
+splanchno-skeleton. He did not, however, let this distinction
+hinder him from asserting the substantial homology of all the
+gill-arches <i>inter se</i>, the first two included.</p>
+
+<p>Rathke's discoveries relative to the development of the jaws,
+the hyoid and the operculum, enabled him to make short work of the
+homologies proposed for them by the transcendentalists. He could
+prove from embryology that the jaws were not the equivalent of
+limbs, as so many Okenians believed. He could reject, with a mere
+reference to the facts of development, Geoffroy's comparison of the
+hyoid and the branchiostegal rays in fish with sternum and ribs. He
+could show the emptiness of the attempts made <span class=
+"pagenum"><a name="pg141" id="pg141">141</a></span>by Carus,
+Treviranus, de Blainville and Geoffroy, to establish by anatomical
+comparison the homologies of the opercular bones, for he could show
+that these bones were peculiar to fish, and were scarcely
+indicated, and that only temporarily, in the development of other
+Vertebrates.<a name="FNanchor_202" id="FNanchor_202" /><a href=
+"#Footnote_202" class="fnanchor">[202]</a> He did not, however,
+himself realise the relation of the ear-ossicles to the
+gill-arches, though he knew that Spix and Geoffroy were quite wrong
+in homologising them with the opercular bones in fish. He
+described, it is true, the development of the external meatus of
+the ear and the Eustachian tube from the slit which appears between
+the first and the second arch, as Huschke had done before him; he
+described, in confirmation of Meckel, the "Meckelian process" of
+the hammer running down inside the lower jaw; but the discovery of
+the true homologies of the ear-ossicles was not made until a year
+or two later by Reichert.</p>
+
+<p>In his further study of the development of <i>Blennius
+viviparus</i>, 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 h&aelig;mal 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 vertebr&aelig;. (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 vertebr&aelig;, 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.</p>
+
+<p><span class="pagenum"><a name="pg142" id=
+"pg142">142</a></span>Von Baer, like Rathke at this time, was also
+to some extent a believer in the vertebral theory of the skull. In
+his second volume (1834, pub. 1837) he holds that the development
+of the skull, as the sum of the anterior vertebral arches, is in
+general the same as that of the other neural arches, and is
+modified only by the great bulk of the brain
+(<i>Entwickelungsgeschichte</i>, ii., p.&nbsp;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
+vertebr&aelig;. At first we see the vertebration in the hinder
+region of the skull very clearly. Afterwards it becomes suddenly
+indistinct, as if some new formation overlaid it" (i., p.&nbsp;194).</p>
+
+<p>Even more clearly is his doubt expressed in his paper on
+<i>Cyprinus</i>. "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
+vertebr&aelig; ceases abruptly where the back passes into the head.
+I do not hesitate to assert <i>that bony fish, too, have at this
+stage an unsegmented cartilaginous cranium</i> (as cartilaginous
+fish have all their life), the prominences and hollows of which
+constitute its only resemblance with the vertebral type" (1835, p.
+19).</p>
+
+<p>A convinced supporter of the vertebral theory was Johannes
+M&uuml;ller, who, in his classical memoir on the Myxinoids,<a name=
+"FNanchor_203" id="FNanchor_203" /><a href="#Footnote_203"
+class="fnanchor">[203]</a> discussed at some length the relation
+between the development of the vertebr&aelig; and the development
+of the skull. His memoir is principally devoted to comparative
+anatomy, but in treating of the skeleton he pays much attention to
+development. He describes the formation of the vertebr&aelig; in
+elasmobranch embryos; for the facts regarding other Vertebrates he
+relies largely on work by Rathke (<i>Blennius</i>, 1833) and
+Dug&egrave;s (1834). He recognises as the basis of his comparisons
+the homology of the notochord <span class="pagenum"><a name="pg143"
+id="pg143">143</a></span>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 <i>basis cranii</i> (p.&nbsp;92). It is in the outer sheath that the
+vertebr&aelig; develop&mdash;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 M&uuml;ller, of
+three vertebr&aelig;, whose centra are the basioccipital, the
+basisphenoid and the presphenoid. Other bones besides those
+belonging to the vertebr&aelig; are present, but this formation out
+of three vertebr&aelig; 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 vertebr&aelig; we should expect the centra of the
+skull-vertebr&aelig; 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 vertebr&aelig;; 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 <i>basis cranii</i> into three parts, and
+of the isolated formation of the vomer, M&uuml;ller sees a
+confirmation of his view that the skull is composed of three and
+not four vertebr&aelig;. But there is nothing in Rathke's
+observations to support the idea that the centra of the cranial
+vertebr&aelig; are formed from separate halves. M&uuml;ller has to
+be content with a reference to the state of things in
+<i>Ammocoetes</i> (which, by the way, he did not know to be the
+young of <i>Petromyzon</i>). In the simple skull of
+<i>Ammocoetes</i> 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.</p>
+
+<p>These bars, declares M&uuml;ller, are clearly the still separate
+halves of the <i>pars basilaris cranii</i>, and represent the
+divided centra of the two hinder cranial vertebr&aelig;. To
+complete the parallel between the development of the skull and of
+the vertebr&aelig;, it would have been necessary to show that the
+side walls of the cranium developed in a similar manner from <span
+class="pagenum"><a name="pg144" id="pg144">144</a></span>separate
+pieces. M&uuml;ller 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 vertebr&aelig; are formed from
+separate halves was supplied in 1839 by Rathke's discovery of the
+trabecul&aelig; in the embryonic skull of the adder.</p>
+
+<p>The next big step in the study of the development of the skull
+was taken by a pupil of M&uuml;ller, 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,<a name="FNanchor_204" id=
+"FNanchor_204" /><a href="#Footnote_204" class=
+"fnanchor">[204]</a> 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 vertebr&aelig; which Reichert, like M&uuml;ller,
+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&mdash;in
+his opinion they have more analogy with ribs, though differing
+somewhat from ribs in their later modifications. The visceral
+arches are processes of the visceral plates (von Baer), which grow
+downwards and meet in the middle line, leaving between one another
+and the undivided body wall three visceral slits opening into the
+pharynx. The first visceral process is different in shape from the
+others, for it sends forward, parallel with the head and at right
+angles to its downward portion, an upper portion in which later the
+upper jaw is formed. The other two processes are straight. From the
+hinder edge of the second visceral arch there develops, as Rathke
+had seen, a fold which is comparable with the operculum of fish.
+The first slit develops externally into the ear-passage, internally
+into the Eustachian tube, and in the middle a partition forms the
+tympanic ring and tympanum. Inside each of the visceral processes
+on either side a cartilaginous rod develops. In <span class=
+"pagenum"><a name="pg145" id="pg145">145</a></span>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<a name="FNanchor_205" id=
+"FNanchor_205" /><a href="#Footnote_205" class=
+"fnanchor">[205]</a> in fish, amphibians and birds, is a long strip
+of cartilage which runs from the ear-ossicle known as the hammer in
+mammals,<a name="FNanchor_206" id="FNanchor_206" /><a href=
+"#Footnote_206" class="fnanchor">[206]</a> to the inside of the
+mandible. Reichert shows how this relation comes about.</p>
+
+<div class="figcenter">
+<a href="images/fig09a.jpg"><img src="images/fig09a-tb.jpg"
+alt="Fig.&nbsp;9.&mdash;Meckel's Cartilage and Ear-ossicles in Embryo of Pig.
+(After Reichert.)" /></a></div>
+
+<p class="center2"><span class="smcap">Fig.</span>
+9.&mdash;Meckel's Cartilage and Ear-ossicles in Embryo of Pig.
+(After Reichert.)</p>
+
+<table width="80%" summary="Meckel's Cartilage"
+border="0" cellpadding="2" cellspacing="2">
+<tbody>
+<tr>
+<td class="cell_lt217b"><i>a</i>.&nbsp;Mandible.</td>
+<td class="cell_lt217b"><i>h</i>.&nbsp;Hammer.</td>
+<td class="cell_lt217b"><i>k</i>.&nbsp;Incus.</td>
+</tr>
+
+<tr>
+<td class="cell_lt217b"><i>g</i>.&nbsp;Meckel's cartilage.</td>
+<td class="cell_lt217b"><i>i</i>.&nbsp;Handle of Hammer.</td>
+<td class="cell_lt217b"><i>n</i>.&nbsp;Stapes.</td>
+</tr>
+</tbody>
+</table>
+
+<p>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.&nbsp;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 <span class=
+"pagenum"><a name="pg146" id="pg146">146</a></span>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.</p>
+
+<p>In this paper Reichert made a distinct advance on the previous
+workers in the same field&mdash;Rathke, Huschke, von Baer, Martin
+St Ange, Dug&egrave;s. Huschke was indeed the first to suggest that
+both upper and lower jaws were formed in the first gill-arch. But
+both von Baer and Rathke<a name="FNanchor_207" id=
+"FNanchor_207" /><a href="#Footnote_207" class=
+"fnanchor">[207]</a> held that the upper jaw developed as a special
+process independent of the lower jaw rudiment, and the actual proof
+that the upper jaw is a derivative of the first visceral arch seems
+to have been first supplied by Reichert. His brilliant work on the
+development of the ear-ossicles founded what we may justly call the
+classical theory of their homologies. His views were attacked and
+in some points rectified, but the main homologies he established
+are even now accepted by many, perhaps the majority of
+morphologists.</p>
+
+<p>In a paper of 1838 on the comparative embryology of the skull in
+Amphibia,<a name="FNanchor_208" id="FNanchor_208" /><a href=
+"#Footnote_208" class="fnanchor">[208]</a> 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.</p>
+
+<p>The first visceral arch, he found, gave in Amphibia practically
+the same structures as in the higher Vertebrates. Its skeleton
+segmented, as in mammals and birds, into three parts; the upper
+part gave rise to the palatine and pterygoid in Anura, but seemed
+to disappear in Urodeles, where the so-called palatine and
+pterygoid developed in the mucous membrane of the mouth; the middle
+part gave, as in birds, <span class="pagenum"><a name="pg147" id=
+"pg147">147</a></span>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.</p>
+
+<p>The account of the metamorphoses of the visceral arches in
+Amphibia forms only a small part of Reichert's memoir of 1838, the
+chief object of which was to discover the general "typus" of the
+vertebrate skull, and to follow out its modifications in the
+different classes. Von Baer had shown that the generalised type
+appeared most clearly in the early embryo; Reichert therefore
+sought the archetype of the skull in the developing embryo. He
+brought to his task the preconceived notion that the skull could be
+reduced to an assemblage of vertebr&aelig;, 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 vertebr&aelig; (p.&nbsp;121, 1837).</p>
+
+<div class="figleft">
+             <a href="images/fig10a.jpg"><img
+             src="images/fig10a-tb.jpg"
+             alt="Fig 10." /></a><br />
+         <span class="smcap">Fig.</span> 10.<br />Cranial Vertebr&aelig; and Visceral Arches<br />in Embryo of Pig. Ventral Aspect.<br />(After Reichert.)
+    </div>
+
+<p>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
+(<i>cf.</i> von Baer, <i>supra</i>, p.&nbsp;118). The walls of the lower
+tube were formed by the visceral processes, within which later the
+skeleton of the visceral arches developed. The walls of the upper
+tube formed the bones and muscles of the cranium proper. The facial
+part of the head was formed by elements from both upper and lower
+tubes. The dorsal tube showed signs of a division into three
+cranial vertebr&aelig; (<i>Urwirbeln</i>, primitive
+vertebr&aelig;). In mammals and birds, as Reichert had shown in his
+1837 paper, the three cranial vertebr&aelig; were indicated by
+transverse furrows on the ventral surface of the still membranous
+skull (see Fig.&nbsp;10, p.&nbsp;148).</p>
+
+<p>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 vertebr&aelig; <span
+class="pagenum"><a name="pg148" id="pg148">148</a></span>(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 vertebr&aelig; in the cartilaginous trough
+formed by the <i>basis cranii</i> 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.&nbsp;44, 1838).
+The vertebr&aelig; in the membranous or cartilaginous skull could
+be delimited in Amphibia by the help of the eye and the
+ear-labyrinth, which lie more or less between the first and second,
+and the second and third vertebr&aelig;, but, above all, by the
+vesicles of the brain.</p>
+
+<p>As in the higher Vertebrates, the visceral arches are associated
+with the cranial vertebr&aelig; as their ventral extensions, being
+equivalent to the visceral plates which form the ventral portion of
+the "primitive vertebr&aelig;" or primitive segments of the
+trunk.</p>
+
+<p>If the three cranial vertebr&aelig; are not very distinct in the
+early stages of development when the skull is still membranous or
+cartilaginous, they become clearly delimited when ossification sets
+in. Three rings of bone forming three more or less complete
+vertebr&aelig; are the final result of ossification. The
+composition of these rings is as follows:&mdash;</p>
+
+<table width="100%" summary="Three rings of bone" border="1"
+cellpadding="2" cellspacing="2">
+<tbody>
+<tr>
+<td class="cell_lt148a"></td>
+<td class="cell_lt148b">Base.</td>
+<td class="cell_lt148b">Sides.</td>
+<td class="cell_lt148b">Top.</td>
+</tr>
+
+<tr>
+<td class="cell_lt148c">First vertebra</td>
+<td class="cell_lt148d">Presphenoid</td>
+<td class="cell_lt148d">Orbitosphenoids</td>
+<td class="cell_lt148d">Frontals</td>
+</tr>
+
+<tr>
+<td class="cell_lt148c">Second vertebra</td>
+<td class="cell_lt148d">Basisphenoid</td>
+<td class="cell_lt148d">Alisphenoids</td>
+<td class="cell_lt148d">Parietals</td>
+</tr>
+
+<tr>
+<td class="cell_lt148c">Third vertebra</td>
+<td class="cell_lt148d">Basioccipital</td>
+<td class="cell_lt148d">Exoccipitals</td>
+<td class="cell_lt148d">Supraoccipital</td>
+</tr>
+</tbody>
+</table>
+
+<p><span class="pagenum"><a name="pg149" id=
+"pg149">149</a></span>The other bones of the skull are not included
+in the vertebr&aelig;, 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 vertebr&aelig;. This point, it is interesting to note, had
+already been made by Oken in his <i>Programm</i> (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.</p>
+
+<p>Having arrived at a generalised embryonic type for the
+vertebrate skull, of which the fundamental elements are the three
+cranial vertebr&aelig; 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 <i>larger</i> vertebrate groups. From these are to be
+carefully distinguished such rudiments as arise later during
+ossification, mostly as <i>ossa intercalaria</i>, 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
+<i>smaller</i> vertebrate groups, and comprise such bones as the
+<i>vomer</i>, the <i>Wormian bones</i>, the lowermost turbinal,
+etc." (p.&nbsp;63, 1838).</p>
+
+<p>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
+<i>Holocephala</i>, <i>Plagiostomata</i>, and <i>Cyclostomata</i>
+appear to us to be lower developmental stages individually
+differentiated, so that the other fully differentiated Vertebrates
+cannot easily be referred directly <span class="pagenum"><a name=
+"pg150" id="pg150">150</a></span>to their type" (p.&nbsp;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&mdash;they are individualised exemplars of a simple
+general type, not merely unmodified embryonic stages of the greatly
+differentiated skulls of the higher Vertebrates (p.&nbsp;250, 1838).
+Differentiation within the vertebrate phylum is therefore not
+uniserial, but takes place in several directions. Reichert
+describes two sorts of modifications of the typical
+skull&mdash;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.</p>
+
+<p>Reichert's two papers are of considerable importance, and
+M&uuml;ller's remark in his review<a name="FNanchor_209" id=
+"FNanchor_209" /><a href="#Footnote_209" class=
+"fnanchor">[209]</a> 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.).</p>
+
+<p>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.<a name="FNanchor_210" id=
+"FNanchor_210" /><a href="#Footnote_210" class=
+"fnanchor">[210]</a> This memoir of Rathke's is an exhaustive one and
+deals with the development of all the principal organ-systems, but
+particularly of the skeletal and vascular. He confirmed in its
+essentials Reichert's account of the metamorphoses of the first two
+visceral arches, describing how the rudiment of the skeleton of the
+first arch appears as a forked process of the cranial basis, the
+upper prong developing into the palatine and pterygoid, the lower
+forming Meckel's cartilage, while the quadrate develops from the
+angle of the fork. The actual bone of the upper jaw (maxillary)
+develops outside and separate from the palato-pterygoid bar. The
+cartilaginous rod supporting the second visceral arch divides into
+three pieces on each side, of which the lower two form the hyoid,
+the uppermost the columella. Like Reichert he held the visceral
+arches to be parts of the visceral plates, containing, however,
+elements from all three germ-layers&mdash;the serous, mucous, and
+vessel layers.</p>
+
+<p><span class="pagenum"><a name="pg151" id=
+"pg151">151</a></span>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 vertebr&aelig;, 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.&nbsp;120).
+The formation of vertebral rudiments begins close behind the
+ear-labyrinth, but in front of this the chorda-sheath gives origin
+to a flat membranous plate which afterwards becomes cartilaginous.
+This plate reaches forward below the third cerebral vesicle as far
+as the infundibulum. The notochord ends in this plate, which is the
+<i>basis cranii</i>, just at the level of the ear-labyrinth. In no
+Vertebrate does the notochord extend farther forward (p.&nbsp;122). The
+<i>basis cranii</i> gives off three trabecul&aelig;. 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 trabecul&aelig; 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
+trabecul&aelig; are formed, like the vertebr&aelig;, 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
+vertebr&aelig;, and that the cranial basis by means of its
+processes, the trabecul&aelig;, reaches well in front of the
+terminal portion of the notochord (p.&nbsp;36). The capsule for the
+ear-labyrinth develops quite independently of the cranial basis and
+the notochord. It resembles on its first appearance, in form,
+position, composition, and connections, the ear-capsule of
+Cyclostomes, and so do the ear-capsules of all embryonic
+Vertebrates (p.&nbsp;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" <span class="pagenum"><a name="pg152" id=
+"pg152">152</a></span>(p.&nbsp;40). When ossification sets in, the
+ear-capsule forms three bones, of which two fuse with the
+supraoccipital and exoccipitals.</p>
+
+<p>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.&nbsp;122, 192). The foundations of the
+skull are now complete, and ossification gradually sets in.</p>
+
+<div class="figcenter">
+<img src="images/fig11a.jpg"
+alt="Embryonic Cranium of the Adder. Ventral Aspect. (After Rathke.)" /></div>
+
+<p class="center2"><span class="smcap">Fig.
+11.</span>&mdash;Embrionic Cranium of the Adder. Ventral Aspect.
+(After Rathke.)</p>
+
+<table width="80%" summary=
+"Embrionic Cranium of the Adder. Ventral Aspect. (After Rathke.)"
+border="0" cellpadding="2" cellspacing="2">
+<tbody>
+<tr>
+<td class="cell_lt217b"><i>a</i>.&nbsp;Basioccipital.</td>
+<td class="cell_lt217b"><i>d</i>.&nbsp;Basisphenoid.</td>
+<td class="cell_lt217b"><i>g</i>.&nbsp;Trabecul&aelig;.</td>
+</tr>
+
+<tr>
+<td class="cell_lt217b"><i>b</i>.&nbsp;Exoccipital.</td>
+<td class="cell_lt217b"><i>c</i>.&nbsp;Alisphenoid.</td>
+<td class="cell_lt217b"><i>h</i>.&nbsp;Foramen.</td>
+</tr>
+
+<tr>
+<td class="cell_lt217b"><i>c</i>.&nbsp;Ear capsule.</td>
+<td class="cell_lt217b"><i>f</i>.&nbsp;Orbitosphenoid.</td>
+<td class="cell_lt217b"><i>i</i>.&nbsp;Ptuitary space.</td>
+</tr>
+</tbody>
+</table>
+
+<p>The basioccipital is formed in the posterior part of the <i>basis cranii</i>, and the
+exoccipitals in the side walls of the trench in continuity with the
+fundament of the basioccipital (see <a href="#pg152">Fig.&nbsp;11</a>).
+The supraoccipital is formed in cartilage above the exoccipitals.
+The basisphenoid develops, like the basioccipital, in the flat
+<i>basis cranii</i>, but towards its anterior edge, between the
+large foramen (<i>h</i>) and the pituitary space (<i>i</i>). It is
+formed from two centres, each of which is originally a ring round
+the carotid foramen. The presphenoid develops in isolation between
+the lateral trabecul&aelig;, just behind the point where <span
+class="pagenum"><a name="pg153" id="pg153">153</a></span>they fuse.
+The side parts of the basisphenoid and presphenoid (forming the
+alisphenoids and the orbitosphenoids respectively) develop in
+cartilage separately from the cranial basis, not like the
+exoccipitals in continuity with it. The hinder parts of the
+trabecul&aelig; 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).</p>
+
+<p>Through Rathke's work the real embryonic archetype of the
+vertebrate skull was for the first time disclosed. Rathke discussed
+this archetype and its relation to the vertebral theory of the
+skull in another paper of the same year (1839), but before going on
+to this paper, we shall quote from the paper on the adder the
+following passage, remarkable for the clear way in which the idea
+of the embryological archetype is expressed. "Whatever differences
+may appear in the development of Vertebrates, there yet exists for
+the different classes and orders a universally valid idea (plan,
+schema, or type) ruling the first formation of their separate
+parts. This idea must first be worked out, though possibly with
+modifications, before more special ideas can find play. The result
+of the latter process, however, is that what was formed by the
+first idea is not so much hidden as partially or wholly destroyed"
+(p.&nbsp;135).</p>
+
+<p>Rathke's general paper on the development of the skull in
+Vertebrates<a name="FNanchor_211" id="FNanchor_211" /><a href=
+"#Footnote_211" class="fnanchor">[211]</a> 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 <i>basis cranii</i>
+or basilar plate is first laid down as two strips, one on each side
+of the chorda&mdash;the structures now known as parachordals (pp.
+6, 27). For this supposition, he thinks, speaks the structure of the
+<span class="pagenum"><a name="pg154" id=
+"pg154">154</a></span>skull in <i>Ammocoetes</i>, which in this
+respect is the simplest of all Vertebrates (pp.&nbsp;6, 22). In
+<i>Ammocoetes</i>, as Johannes M&uuml;ller 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 trabecul&aelig;,
+and of the parachordals which he was ready to assume from his
+embryological observations.</p>
+
+<p>M&uuml;ller 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 <i>Handbuch der Physiologie</i> the cartilaginous bands in
+<i>Ammocoetes</i> 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 vertebr&aelig;, two
+symmetrical bands of cartilage, similar to those which I discovered
+as a persistent structure in <i>Ammocoetes</i>.... At a later
+period the <i>basis cranii</i> of vertebrate animals contains three
+parts analogous to the bodies of vertebr&aelig;, 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."<a name="FNanchor_212" id=
+"FNanchor_212" /><a href="#Footnote_212" class=
+"fnanchor">[212]</a></p>
+
+<p>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.<a name="FNanchor_213" id="FNanchor_213" /><a
+href="#Footnote_213" class="fnanchor">[213]</a></p>
+
+<p><span class="pagenum"><a name="pg155" id=
+"pg155">155</a></span>The chorda sheath is the common matrix of the
+vertebr&aelig; and of a large part of the skull. The basilar plate
+and the trabecul&aelig;, which are developed from the chorda
+sheath, give origin to three bones, which might possibly be
+considered equivalent to vertebral centra&mdash;the basioccipital,
+the basisphenoid, and the <i>Riechbein</i> (ethmoid). The
+<i>Riechbein</i> develops from the fused ends of the
+trabecul&aelig;. The presphenoid might also be considered as a
+vertebral body, but it develops independently of the basilar plate
+and trabecul&aelig;.</p>
+
+<p>Now of these bones, the basioccipital is in every way equivalent
+to a vertebral centrum, for it develops in the basilar plate round
+the notochord. With the exoccipitals, which arise just like neural
+arches, it forms a true vertebra. The supraoccipital is an
+accessory bone developed in relation to bigger brains. The
+basisphenoid appears in the basilar plate, but in front of the
+notochord, nor does it arise in exactly the same way as the centrum
+of a vertebra. The basisphenoid with the alisphenoids, which
+develop independently in the side walls of the brain, may, however,
+still be considered as forming a vertebra, though the resemblance
+is not so great as in the case of the occipital ring. The
+presphenoid, being long and pointed, is very unlike a vertebral
+body. The orbitosphenoids develop separately from it. The ethmoid
+also differs from a vertebra, for it surrounds not the whole
+nervous axis as the two hinder "vertebr&aelig;" do, but only two
+prolongations of it, the olfactory lobes. In its development and
+final form it shows no particular resemblance to a vertebra. Its
+body, the <i>pars perpendicularis</i> (mesethmoid) shows no
+similarity with a vertebral centrum. Completing the three hinder
+cranial "vertebr&aelig;" 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 vertebr&aelig;, but is rather to be
+compared to the intercalary bones in the vertebral column of
+certain fish. Summing up as regards the cranial vertebr&aelig;
+Rathke writes, "We find that the four different groups of bones,
+consisting of the basioccipital with its intercalary (the
+supraoccipital), the basisphenoid with its intercalaries <span
+class="pagenum"><a name="pg156" id=
+"pg156">156</a></span>(parietals), the presphenoid with its
+intercalaries (frontals), and the ethmoid with its outgrowths
+(turbinals and cribriform plate), taking them in order from behind
+forwards, show an increasing divergence from the plan according to
+which vertebr&aelig; as commonly understood develop, so that the
+basioccipital shows the greatest resemblance to a vertebra, the
+ethmoid the least" (p.&nbsp;30).</p>
+
+<p>In a posthumous volume published in 1861 the same opinion is put
+forward. "In the head, too," he writes, "some vertebr&aelig; can be
+recognised, although in a more or less modified form. Yet at most
+only four cranial vertebr&aelig; can be assumed, and these differ
+from ordinary well-developed vertebr&aelig; in their manner of
+formation the more the farther forward they lie."<a name=
+"FNanchor_214" id="FNanchor_214" /><a href="#Footnote_214"
+class="fnanchor">[214]</a></p>
+
+<p>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 <i>Coregonus</i>,<a name="FNanchor_215" id=
+"FNanchor_215" /><a href="#Footnote_215" class=
+"fnanchor">[215]</a> and in his paper on the development of
+<i>Alytes</i>.<a name="FNanchor_216" id="FNanchor_216" /><a
+href="#Footnote_216" class="fnanchor">[216]</a> He described for
+<i>Coregonus</i> 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 trabecul&aelig;; a "facial plate"
+was also formed, which seems on the whole equivalent to the plate
+formed by the fused anterior ends of the trabecul&aelig;. A little
+later the cranium formed a complete cartilaginous box surrounding
+the brain, very similar to the adult cranium of a shark.</p>
+
+<p>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 <span class=
+"pagenum"><a name="pg157" id="pg157">157</a></span>lying in front
+of this cannot themselves be vertebr&aelig;, 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 vertebr&aelig; 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).</p>
+
+<p>To support this view he adduced the fact that the vertebral
+divisions (primitive vertebr&aelig;) visible in the trunk do not
+extend into the head. He used precisely the same arguments in his
+paper on <i>Alytes</i> to destroy the vertebral theory of the
+skull. We quote the following passage translated by Huxley (1864,
+p.&nbsp;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 vertebr&aelig; at all."</p>
+
+<p>L. Agassiz, himself a pupil of D&ouml;llinger, in the general
+part (1844) of his <i>Recherches sur les Poissons fossiles</i>
+(Neuch&acirc;tel, 1833-43), repeats in the main his pupil Vogt's
+criticism of the vertebral theory (vol. i., pp.&nbsp;125-9).</p>
+
+<p>These arguments of Vogt and Agassiz were not considered by
+M&uuml;ller to dispose of the theory,<a name="FNanchor_217" id=
+"FNanchor_217" /><a href="#Footnote_217" class=
+"fnanchor">[217]</a> which maintained a firm hold even upon
+embryologists. It was still upheld by Reichert, and K&ouml;lliker
+in 1849 showed himself convinced of its general validity.</p>
+
+<p>A useful step in the analysis of the concept "vertebra" was
+taken by Remak,<a name="FNanchor_218" id="FNanchor_218" /><a
+href="#Footnote_218" class="fnanchor">[218]</a> who showed what a
+complex affair the formation of vertebr&aelig; really is, involving
+as it does a complete resegmentation (<i>Neugliederung</i>) of the
+vertebral column, whereby the original vertebral bodies were
+replaced by the secondary definitive bodies (p.&nbsp;143). Remak showed,
+as he thought, that the protovertebral segmentation of the dorsal
+<span class="pagenum"><a name="pg158" id=
+"pg158">158</a></span>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
+vertebr&aelig; (p.&nbsp;36). The gill-slits, he considered, could not
+possibly be regarded as marking the limits of head
+vertebr&aelig;.</p>
+
+<p>In 1858 appeared Huxley's well-known Croonian Lecture, <i>On the
+Theory of the Vertebrate Skull</i>,<a name="FNanchor_219" id=
+"FNanchor_219" /><a href="#Footnote_219" class=
+"fnanchor">[219]</a> 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.&nbsp;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.&nbsp;541). As an example of the utility and, indeed, the necessity
+of applying the embryological method Huxley takes the case of the
+quadrate bone in birds. This bone had been generally regarded by
+anatomists as the equivalent of the tympanic of mammals, on account
+of its connection with the tympanum; but Reichert showed (1837)
+that the same segment of the first visceral arch developed into the
+incus in mammals, and into the quadrate in birds, and that
+therefore the quadrate was homologous with the incus. Similarly, on
+developmental grounds, the malleus or hammer of mammals is the
+homologue of the articular of birds, since both are developed from
+a portion <span class="pagenum"><a name="pg159" id=
+"pg159">159</a></span>of Meckel's cartilage identical in form and
+connections in the two groups. The homologies of the bones
+connected with the jaws in bony fishes had long been a subject of
+contention among comparative anatomists; Huxley shows from his
+personal observations how the development of the visceral arches
+throws light upon these difficulties. The mandibular arch in the
+developing fish is abruptly angled, as in the embryo of Tetrapoda;
+the upper prong of it ossifies into the palatine and pterygoid; at
+the angle is formed the quadrate (jugal, Cuvier), and to the
+quadrate is articulated the lower jaw, which ossifies round the
+lower prong or Meckel's cartilage. The scheme of development of the
+jaws is accordingly similar in fish to what it is in other
+Vertebrates, and this similarity of development enables Huxley to
+recognise what are the true homologues of the quadrate, the
+palatine and the pterygoid in adult bony fish, and to prove that
+the symplectic and the metapterygoid (tympanal, Cuvier) are bones
+peculiar to fish. In developing Amphibia Huxley found a
+suspensorium of hyoid and mandibular arches similar to the
+hyomandibular of fish.</p>
+
+<p>Tackling his main problem of the unity of plan of the vertebrate
+skull, Huxley shows, by a careful discussion of the anatomical
+relationships of the chief bones in typical examples of all
+vertebrate classes, that there is on the whole unity of plan as
+regards the osseous skull. This unity of composition can be
+established, on the gradation method, by considering the
+connections of the bones of the skull with one another, their
+relations to the parts of the brain and to the foramina of the
+principal cranial nerves. The assistance of the embryological
+method is, however, necessary in determining many points with
+regard to the bones developed in relation to the visceral arches.
+But there is a further step to be taken. "Admitting ... that a
+general unity of plan pervades the organisation of the ossified
+skull, the important fact remains that many vertebrated
+animals&mdash;all those fishes, in fact, which are known as
+<i>Elasmobranchii</i>, <i>Marsipobranchii</i>, <i>Pharyngobranchii</i> and
+<i>Dipnoi</i> have no bony skull at all, at least in the sense in
+which the words have hitherto been used" (p.&nbsp;571). The membranous
+or cartilaginous skull of these fishes shows a general resemblance
+in its main <span class="pagenum"><a name="pg160" id=
+"pg160">160</a></span>features to the ossified skull of other
+Vertebrates; the relations of the ear to the vagus and trigeminal
+nerves are, for instance, the same in both; the main regions of the
+cartilaginous skull can be homologised with definite bones or
+groups of bones in the bony skull; but discrepancies occur. It is
+again to development that we must turn to discover the true
+relationship of the cartilaginous to the ossified skull. "The study
+of the development of the ossified vertebrate skull ...
+satisfactorily proves that the adult crania of the lower
+<i>Vertebrata</i> are but special developments<a name=
+"FNanchor_220" id="FNanchor_220" /><a href="#Footnote_220"
+class="fnanchor">[220]</a> of conditions through which the embryonic
+crania of the highest members of the sub-kingdom pass" (p.&nbsp;573). It
+is with the embryonic cranium of higher Vertebrates that the adult
+skull of the lower fishes must be compared, and the comparison will
+show a substantial though not a complete agreement between them.
+Thus, speaking of the development of the frog's skull, Huxley
+writes:&mdash;"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 <i>Amphioxus</i>, the lamprey, and the shark,
+we shall discover the model and type of the latter in the former.
+The skull of the <i>Amphioxus</i> presents a modification of that
+plan which is exhibited by the frog's skull when its walls are
+still membranous and the notochord is not yet embedded in
+cartilage. The skull of the lamprey is readily reducible to the
+same plan of structure as that which is exhibited by the tadpole
+when its gills are still external and its blood colourless. And
+finally, the skull of the shark is at once intelligible when we
+have studied the cranium in further advanced larv&aelig;, or its
+cartilaginous basis in the adult frog" (p.&nbsp;577). Development,
+therefore, proves what comparative anatomy could only
+foreshadow&mdash;the unity of plan of all vertebrate skulls,
+ossified and unossified alike. "We have thus attained to a theory
+or general expression of the laws of structure of the skull. All
+vertebrate skulls are originally alike; in all (save
+<i>Amphioxus</i>?) the base of the primitive cranium undergoes the
+mesocephalic flexure, behind which the notochord terminates, while
+immediately in front of it <span class="pagenum"><a name="pg161"
+id="pg161">161</a></span>the pituitary body is developed;<a name=
+"FNanchor_221" id="FNanchor_221" /><a href="#Footnote_221"
+class="fnanchor">[221]</a> in all, the cartilaginous cranium has
+primarily the same structure&mdash;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 trabecul&aelig;,
+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.&nbsp;578).</p>
+
+<p>In a word, there is a general plan or primordial type which is
+manifested in the higher forms most clearly in their earliest
+development&mdash;an embryological archetype therefore.</p>
+
+<p>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 vertebr&aelig;? 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
+trabecul&aelig;, 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 vertebr&aelig;. 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 <span class="pagenum"><a name="pg162" id=
+"pg162">162</a></span>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 vertebr&aelig;.
+"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.</p>
+
+<p>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&mdash;the working out of the
+distinction between membrane and cartilage bones.</p>
+
+<p>As early as 1731, R. Nesbitt,<a name="FNanchor_222" id=
+"FNanchor_222" /><a href="#Footnote_222" class=
+"fnanchor">[222]</a> in two lectures delivered to the Royal College
+of Surgeons, demonstrated that in the human f&oelig;tus 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<a name="FNanchor_223" id="FNanchor_223" /><a href=
+"#Footnote_223" class="fnanchor">[223]</a> 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<a name="FNanchor_224" id=
+"FNanchor_224" /><a href="#Footnote_224" class=
+"fnanchor">[224]</a> 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:&mdash;"If we consider the fibrous skeleton of
+<i>Ammocoetes</i> as the first foundation of the skeleton of
+Vertebrates, we can form a <span class="pagenum"><a name="pg163"
+id="pg163">163</a></span>series among the cartilaginous fishes,
+according as a cartilaginous skeleton penetrates more and more into
+this fibrous foundation. In the same way the process of
+ossification supplants the cartilaginous skeleton. So long as the
+ossifications lie in the skin, as in the sturgeon, they form
+corneous bones (<i>Hornknochen</i>), but when they lie under the
+skin, they form true bones, <i>e.g.</i>, the bones of the skull in
+the pike" (p.&nbsp;374).</p>
+
+<p>Embryologists soon become aware that a similar distinction
+between a primitive cartilaginous foundation and a secondary
+overlying ossification of the skull showed itself in the
+development of all Vertebrates. Dug&egrave;s, in his <i>Recherches
+sur l'ost&eacute;ologie et la myologie des Batraciens</i> (1834),
+distinguished between such bones as are formed by direct
+ossification of the cartilaginous groundwork of the skull, and such
+as are developed in the periosteal fibrous tissue.</p>
+
+<p>Reichert in 1838<a name="FNanchor_225" id=
+"FNanchor_225" /><a href="#Footnote_225" class=
+"fnanchor">[225]</a> 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.&nbsp;215-6). Even
+more interesting was his discovery, afterwards confirmed by
+Hertwig,<a name="FNanchor_226" id="FNanchor_226" /><a href=
+"#Footnote_226" class="fnanchor">[226]</a> 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.&nbsp;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.&nbsp;34). Rathke in
+1839 also allowed the distinction between cartilage and membrane
+bone, but laid no stress upon it (<i>Entw. d. Natter.</i>, p.
+197).</p>
+
+<p><span class="pagenum"><a name="pg164" id=
+"pg164">164</a></span>Jacobson in 1842<a name="FNanchor_227" id=
+"FNanchor_227" /><a href="#Footnote_227" class=
+"fnanchor">[227]</a> 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.</p>
+
+<p>In his <i>Recherches sur les Poissons fossiles</i>,<a name=
+"FNanchor_228" id="FNanchor_228" /><a href="#Footnote_228"
+class="fnanchor">[228]</a> L. Agassiz used Vogt's work on the
+development of <i>Coregonus</i> 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.&nbsp;112). In the skull of all fish
+there are three elements&mdash;(1) the cartilaginous base,
+including the nuchal plate, the trabecul&aelig; 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:&mdash;(1) the basioccipital,
+exoccipitals (paroccipitals?), supraoccipital and "petrous"
+(<i>rocher</i>), developed from the nuchal plate; the ali- and
+orbito-sphenoids developed from the trabecul&aelig;; the "cranial
+ethmoid"<a name="FNanchor_229" id="FNanchor_229" /><a href=
+"#Footnote_229" class="fnanchor">[229]</a> developed from the
+facial plate; (2) the parietals, frontals and nasals formed from
+the "superior" protective plate; the "anterior" and "posterior"
+frontals and the temporal, from the "lateral" plates; the body of
+the sphenoid and the vomer from the "inferior" plates. The other
+element, the cartilaginous brain-box, does not ossify, and tends to
+become absorbed (p.&nbsp;124).</p>
+
+<p>In 1849 K&ouml;lliker published a paper<a name="FNanchor_230"
+id="FNanchor_230" /><a href="#Footnote_230" class=
+"fnanchor">[230]</a> dealing with the <span class="pagenum"><a name=
+"pg165" id="pg165">165</a></span>morphological significance of the
+distinction between membrane and cartilage bones, and in 1850<a
+name="FNanchor_231" id="FNanchor_231" /><a href=
+"#Footnote_231" class="fnanchor">[231]</a> he defended his views
+against the criticisms of Reichert<a name="FNanchor_232" id=
+"FNanchor_232" /><a href="#Footnote_232" class=
+"fnanchor">[232]</a> in a further note entitled <i>Die Theorie des
+Primordialsch&auml;dels festgehalten</i>. It is convenient to
+consider these papers together. K&ouml;lliker held that there was
+(1) a histological and (2) a morphological difference between the
+two categories of bones. The histological development of the two
+kinds was different, but this difference was not sufficient to
+establish a morphological distinction between them, a distinction
+in their anatomical <i>Bedeutung</i>. 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. K&ouml;lliker'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 K&ouml;lliker'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
+vertebr&aelig; in certain fish, he modified his view of the
+membrane bones, and admitted them, at least in some cases, as
+constituents of the cranial vertebr&aelig;.</p>
+
+<p>On this morphological distinction of membrane and cartilage
+bones future comparative osteology was to be based:&mdash;</p>
+
+<p>"My sole aim is to state again the principle upon which
+comparative osteology is to be based and extended, and this is that
+first place should be assigned to anatomical considerations, and
+among these to the manner of origin of the whole bone in relation
+to the skeleton-forming layers" (1850, p.&nbsp;290).</p>
+
+<p>The homologies established by this new principle might <span
+class="pagenum"><a name="pg166" id="pg166">166</a></span>run
+counter to the homologies indicated by the study of adult
+structure. "Thus, for instance, although the lower jaw in position,
+function, form and shape, appears to be the same bone throughout,
+yet it must be admitted that it shows a difference in the different
+classes. In Mammals and Man it is an entirely secondary bone (an
+extremity according to Reichert), in Birds, Amphibia and Fishes
+only partially so, for its articular belongs to Meckel's cartilage
+and is accordingly analogous to a rib; indeed, in the Plagiostomes,
+etc., the whole lower jaw along with the articular is a persistent
+Meckel's cartilage" (p.&nbsp;290, 1850).</p>
+
+<p>So, too, the supraoccipital in man cannot be fully homologised
+with the supraoccipital of many mammals, for its upper half arises
+at first in isolation as a secondary bone (p.&nbsp;290).</p>
+
+<p>Reichert objected to the distinction drawn by K&ouml;lliker, 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&uuml;ller<a name="FNanchor_233" id=
+"FNanchor_233" /><a href="#Footnote_233" class=
+"fnanchor">[233]</a> 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<a
+name="FNanchor_234" id="FNanchor_234" /><a href=
+"#Footnote_234" class="fnanchor">[234]</a> classified the bones of
+the skull according to their origin from the primordial cranium or
+from the overlying fibrous layer, distinguishing as membrane bones,
+the parietals, frontals, nasals, lachrymals, maxillaries and
+premaxillaries, jugals, tympanic, parts of the "temporal," vomer,
+part of the supraoccipitals in some mammals, and the mandible (with
+the exception of the articular in such as have a quadrate bone).
+Huxley was also inclined in 1864<a name="FNanchor_235" id=
+"FNanchor_235" /><a href="#Footnote_235" class=
+"fnanchor">[235]</a> to recognise the distinction, but he writes with
+some reserve:&mdash;"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, <span class=
+"pagenum"><a name="pg167" id="pg167">167</a></span>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.&nbsp;296).</p>
+
+<p>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."<a name="FNanchor_236"
+id="FNanchor_236" /><a href="#Footnote_236" class=
+"fnanchor">[236]</a> 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.&nbsp;89). But he could not deny that the
+archetype was better shown in the embryo than in the adult
+(<i>supra</i>, p.&nbsp;108).</p>
+
+<p>J. V. Carus<a name="FNanchor_237" id="FNanchor_237" /><a
+href="#Footnote_237" class="fnanchor">[237]</a> 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.&nbsp;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.&nbsp;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).</p>
+
+<p><span class="pagenum"><a name="pg168" id=
+"pg168">168</a></span>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.</p>
+
+<div class="footnote">
+<p><a name="Footnote_183" id="Footnote_183" /><a href=
+"#FNanchor_183"><span class="label">[183]</span></a> See review by
+Cuvier, <i>M&eacute;m. Mus. Hist, nat.</i>, iii., pp.&nbsp;82-97,
+1817.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_184" id="Footnote_184" /><a href=
+"#FNanchor_184"><span class="label">[184]</span></a> <i>M&eacute;m.
+Savans &eacute;trangers</i>, vi. Extract in <i>Ann. Sci. nat.</i>
+(2) i. (<i>Zool.</i>), pp.&nbsp;366-72, 1834.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_185" id="Footnote_185" /><a href=
+"#FNanchor_185"><span class="label">[185]</span></a> <i>Recherches
+sur la g&eacute;n&eacute;ration des Mammif&egrave;res</i>, 1834.
+<i>Embryog&eacute;nie compar&eacute;e</i>, 1837.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_186" id="Footnote_186" /><a href=
+"#FNanchor_186"><span class="label">[186]</span></a> "Kiemen bey
+S&auml;ugthieren," <i>Isis</i>, pp.&nbsp;747-9, 1825.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_187" id="Footnote_187" /><a href=
+"#FNanchor_187"><span class="label">[187]</span></a> "Kiemen bey
+V&ouml;geln," <i>Isis</i>, pp.&nbsp;1100-1, 1825.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_188" id="Footnote_188" /><a href=
+"#FNanchor_188"><span class="label">[188]</span></a> "Ueber die
+Kiemenbogen und Kiemengef&auml;sse beym bebr&uuml;teten
+H&uuml;hnchen," <i>Isis</i>, xx., pp.&nbsp;401-3, 1827. (Read in Sept.
+1826 to the <i>Versammlung der deutschen Naturforscher und
+Aerzte</i>, then recently founded by Oken).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_189" id="Footnote_189" /><a href=
+"#FNanchor_189"><span class="label">[189]</span></a> <i>Isis</i>,
+pp.&nbsp;160-4, Pl. II., 1828.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_190" id="Footnote_190" /><a href=
+"#FNanchor_190"><span class="label">[190]</span></a> "Ueber die
+Kiemen und Kiemengef&auml;sse in den Embryonen der Wirbelthiere,"
+Meckel's <i>Archiv</i> for 1827, pp.&nbsp;556-68. Also in <i>Ann. Sci.
+nat.</i>, xv., pp.&nbsp;266-80, 280-4, 1828.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_191" id="Footnote_191" /><a href=
+"#FNanchor_191"><span class="label">[191]</span></a> Meckel's
+<i>Archiv</i>, vi., pp.&nbsp;1-47, 1832.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_192" id="Footnote_192" /><a href=
+"#FNanchor_192"><span class="label">[192]</span></a>
+<i>Untersuchungen &uuml;ber die Bildung und Entwickelung der
+Fluss-Krebses</i>, Leipzig, folio, 1829. Preliminary notice in
+<i>Isis</i>, pp.&nbsp;1093-1100, 1825.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_193" id="Footnote_193" /><a href=
+"#FNanchor_193"><span class="label">[193]</span></a>
+"Untersuchungen &uuml;ber die Bildung und Entwickelung der
+Wasser-Assel.," <i>Abh. z. Bild. u. Entwick.-Gesch.</i>, i., pp.
+1-20, 1832. Translated in <i>Ann. Sci. nat.</i> (2), ii.,
+(<i>Zool.</i>), pp.&nbsp;139-57, 1834.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_194" id="Footnote_194" /><a href=
+"#FNanchor_194"><span class="label">[194]</span></a> K&ouml;lliker,
+<i>Entwickelungsgeschichte</i>, 2nd ed., p.&nbsp;17, Leipzig, 1879.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_195" id="Footnote_195" /><a href=
+"#FNanchor_195"><span class="label">[195]</span></a> <i>Handbuch
+der Entwickelungsgeschichte des Menschen und ... der
+S&auml;ugethiere und V&ouml;gel</i>, Berlin, 1835.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_196" id="Footnote_196" /><a href=
+"#FNanchor_196"><span class="label">[196]</span></a>
+<i>Embryog&eacute;nie compar&eacute;e</i>, 1837; <i>Histoire
+g&eacute;n&eacute;rale du d&eacute;veloppement des corps
+organis&eacute;s</i>, 1847-49.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_197" id="Footnote_197" /><a href=
+"#FNanchor_197"><span class="label">[197]</span></a>
+<i>Entwickelungsgeschichte des Kaninchen-Eies</i>, Braunschweig,
+1842; <i>Entwickelungsgeschichte des Hunde-Eies</i>, Braunschweig,
+1845; <i>Entwickelungsgeschichte des Meerschweinchens</i>, Giessen,
+1852; <i>Entwickelungsgeschichte des Rehes</i>, Giessen, 1854.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_198" id="Footnote_198" /><a href=
+"#FNanchor_198"><span class="label">[198]</span></a> "It is the
+r&ocirc;le of embryology, as my great teacher says, to form the
+court of appeal for comparative anatomy, and it is from embryology
+particularly, which has in the last decades provided such signal
+instances of the unravelling of obscure problems, that we have to
+expect a definite clearing up of the problems relating to the
+development of the head."&mdash;M&uuml;ller's <i>Archiv</i>, p.
+121, 1837.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_199" id="Footnote_199" /><a href=
+"#FNanchor_199"><span class="label">[199]</span></a> <i>Anat.-phil.
+Unters. &uuml;. d. Kiemenapparat u. d. Zungenbein</i>, Riga and Dorpat,
+1832.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_200" id="Footnote_200" /><a href=
+"#FNanchor_200"><span class="label">[200]</span></a> "Bildungs- und
+Entwickelungs-geschichte des Blennius viviparus," <i>Abhandl. z.
+Bild. u. Entwick.-Gesch. des Menschen u. der Thiere</i>, ii., pp.
+1-68, Leipzig, 1833.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_201" id="Footnote_201" /><a href=
+"#FNanchor_201"><span class="label">[201]</span></a> <i>Von den
+Ur-Theilen des Knochen und Schalen-Gerustes</i>, Leipzig, 1828.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_202" id="Footnote_202" /><a href=
+"#FNanchor_202"><span class="label">[202]</span></a>
+<i>Kiemenapparat</i>, pp.&nbsp;107-118.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_203" id="Footnote_203" /><a href=
+"#FNanchor_203"><span class="label">[203]</span></a>
+<i>Vergleichende Anatomie der Myxinoiden</i>. Part I. (Osteology
+and Myology). (<i>Abh. k&ouml;nigl. Akad. Wiss. Berlin</i>, for
+1834, pp.&nbsp;65-340, 9 pls., 1836.) Also separately.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_204" id="Footnote_204" /><a href=
+"#FNanchor_204"><span class="label">[204]</span></a> "Ueber die
+Visceralbogen der Wirbelthiere in Allgemeinen und deren
+Metamorphosen bei den V&ouml;geln und S&auml;ugethiere,"
+M&uuml;ller's <i>Archiv</i>, pp.&nbsp;120-222, 1837.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_205" id="Footnote_205" /><a href=
+"#FNanchor_205"><span class="label">[205]</span></a> <i>Handbuch d.
+menschl. Anatomie</i>, iv., p.&nbsp;47.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_206" id="Footnote_206" /><a href=
+"#FNanchor_206"><span class="label">[206]</span></a> This was shown
+by Serres (<i>Ann. Sci. nat.</i>, xi., p.&nbsp;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.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_207" id="Footnote_207" /><a href=
+"#FNanchor_207"><span class="label">[207]</span></a>
+<i>Abhandl.</i>, i., p.&nbsp;102, 1832; ii., p.&nbsp;25, 1833.
+(<i>Blennius</i> paper).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_208" id="Footnote_208" /><a href=
+"#FNanchor_208"><span class="label">[208]</span></a>
+<i>Vergleichende Entwickelungsgeschichte des Kopfes der nackten
+Amphibien</i>, K&ouml;nigsberg, quarto, 276 pp., 1838.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_209" id="Footnote_209" /><a href=
+"#FNanchor_209"><span class="label">[209]</span></a> M&uuml;ller's
+<i>Archiv</i> for 1838.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_210" id="Footnote_210" /><a href=
+"#FNanchor_210"><span class="label">[210]</span></a>
+<i>Entwickelungsgeschichte der Natter</i>, K&ouml;nigsberg,
+1839.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_211" id="Footnote_211" /><a href=
+"#FNanchor_211"><span class="label">[211]</span></a> <i>Bemerkungen
+&uuml;ber die Entwickelung des Sch&auml;dels der Wirbelthiere</i>,
+K&ouml;nigsberg, 1839.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_212" id="Footnote_212" /><a href=
+"#FNanchor_212"><span class="label">[212]</span></a> <i>Handbuch
+der Physiologie des Menschen</i>, Koblenz, 1835; Eng. trans. by W.
+Baly, ii., p.&nbsp;1615, 1838.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_213" id="Footnote_213" /><a href=
+"#FNanchor_213"><span class="label">[213]</span></a> For a full
+statement of Rathke's conclusions, see the translation given by
+Huxley in <i>Lectures on the Elements of Comparative Anatomy</i>,
+London, 1864.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_214" id="Footnote_214" /><a href=
+"#FNanchor_214"><span class="label">[214]</span></a>
+<i>Entwickelungsgeschichte der Wirbelthiere</i>, p.142, 1861.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_215" id="Footnote_215" /><a href=
+"#FNanchor_215"><span class="label">[215]</span></a> <i>Embryologie
+des Salmones</i>. A separate volume of L. Agassiz's <i>Histoire
+naturelle des Poissons d'Eau douce de l'Europe centrale</i>,
+Neuch&acirc;tel, 1842.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_216" id="Footnote_216" /><a href=
+"#FNanchor_216"><span class="label">[216]</span></a>
+<i>Untersuchungen &uuml;ber die Entwickelungsgeschichte der
+Geb&uuml;rtshelferkr&ouml;te</i>, Solothurn, 1842.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_217" id="Footnote_217" /><a href=
+"#FNanchor_217"><span class="label">[217]</span></a> M&uuml;ller's
+<i>Archiv</i> for 1843, p.&nbsp;ccxlviii.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_218" id="Footnote_218" /><a href=
+"#FNanchor_218"><span class="label">[218]</span></a>
+<i>Untersuchtingen &uuml;ber die Entwickelung der Wirbelthiere</i>,
+Berlin, 1850-55.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_219" id="Footnote_219" /><a href=
+"#FNanchor_219"><span class="label">[219]</span></a> Delivered 17th
+June 1858. Reprinted in <i>The Scientific Memoirs of T. H.
+Huxley</i>, edited by M. Foster and E. Ray Lankester, vol. i., pp.
+538-606 (1898).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_220" id="Footnote_220" /><a href=
+"#FNanchor_220"><span class="label">[220]</span></a> <i>Cf.</i>
+Reichert, <i>supra</i>, p.&nbsp;149.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_221" id="Footnote_221" /><a href=
+"#FNanchor_221"><span class="label">[221]</span></a> The origin of
+the pituitary body from the roof of the mouth was first described
+by Rathke (1839).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_222" id="Footnote_222" /><a href=
+"#FNanchor_222"><span class="label">[222]</span></a> <i>Human
+Osteogeny explained in two Lectures</i>, London, 1736.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_223" id="Footnote_223" /><a href=
+"#FNanchor_223"><span class="label">[223]</span></a> <i>De capitis
+ossei Esocis lucii structura singulari. Dissert. inaug.</i>
+Regiomonti, 1822.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_224" id="Footnote_224" /><a href=
+"#FNanchor_224"><span class="label">[224]</span></a> "Ueber das
+&auml;ussere und innere Skelet," Meckel's <i>Archiv</i>, pp.
+327-76, 1826.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_225" id="Footnote_225" /><a href=
+"#FNanchor_225"><span class="label">[225]</span></a> <i>Vergl.
+Entwick. d. Kopfes d. nackten Amphibien</i> (p.&nbsp;186).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_226" id="Footnote_226" /><a href=
+"#FNanchor_226"><span class="label">[226]</span></a> <i>Arch. f.
+mikr. Anat.</i>, xi., Suppl., 1874.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_227" id="Footnote_227" /><a href=
+"#FNanchor_227"><span class="label">[227]</span></a> "Om
+Primordial-Craniet," <i>F&ouml;rhandlingar Skand. Naturf.
+M&ouml;le</i>, Stockholm, 1842.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_228" id="Footnote_228" /><a href=
+"#FNanchor_228"><span class="label">[228]</span></a> Vol. I.,
+General part, pub. 1844.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_229" id="Footnote_229" /><a href=
+"#FNanchor_229"><span class="label">[229]</span></a>
+<i>Entosphenoid</i>, Owen.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_230" id="Footnote_230" /><a href=
+"#FNanchor_230"><span class="label">[230]</span></a> <i>Zweiter
+Bericht zootom. Anstalt zu W&uuml;rzburg</i>, 1849.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_231" id="Footnote_231" /><a href=
+"#FNanchor_231"><span class="label">[231]</span></a> <i>Zeits. f.
+wiss. Zool.</i>, ii., pp.&nbsp;281-91.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_232" id="Footnote_232" /><a href=
+"#FNanchor_232"><span class="label">[232]</span></a> M&uuml;ller's
+<i>Archiv</i> for 1849, pp.&nbsp;443-515.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_233" id="Footnote_233" /><a href=
+"#FNanchor_233"><span class="label">[233]</span></a> <i>Zeits. f.
+wiss Zool.</i>, ix., 1858.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_234" id="Footnote_234" /><a href=
+"#FNanchor_234"><span class="label">[234]</span></a> <i>Entw. d.
+Wirbelthiere</i>, pp.&nbsp;139-40, 1861.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_235" id="Footnote_235" /><a href=
+"#FNanchor_235"><span class="label">[235]</span></a> <i>Lectures on
+the Elements of Comparative Anatomy</i>.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_236" id="Footnote_236" /><a href=
+"#FNanchor_236"><span class="label">[236]</span></a> <i>On the
+Archetype of the Vertebrate Skeleton</i>, p.&nbsp;5, 1848.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_237" id="Footnote_237" /><a href=
+"#FNanchor_237"><span class="label">[237]</span></a> <i>System der
+thierischen Morphologie</i>, Leipzig, 1853.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg169" id=
+"pg169">169</a></span></p>
+
+<h3>CHAPTER XI</h3>
+
+<h4>THE CELL-THEORY.</h4>
+
+<p>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&mdash;the unorganised material, itself compounded of the four
+primitive elements, earth and water, air and fire, the homogeneous
+parts or tissues and the heterogeneous parts or organs, and this
+conception was retained with little change even to the days of
+Cuvier and von Baer. Those of the old anatomists who speculated on
+the relations of organic elements to one another were dominated by
+Aristotle's simple and profound classification, and proposed
+schemes which differed from his only in detail. Bichat enlarged and
+deepened the concept of tissue, but the degree of composition below
+this was for him, as for all anatomists of his time, a fibrous or
+pulpy "cellulosity," living, indeed, but showing no uniform and
+elemental structure. It was Schwann's merit to interpose between
+the tissue and the mere unorganised material a new element of
+structure, the cell. And, as it happened, a few years before
+Schwann published his cell-theory, Dujardin hinted at another
+degree of composition which was later to take its place between the
+cell and the chemical elements&mdash;sarcode or protoplasm.</p>
+
+<p>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 <span class="pagenum"><a name="pg170" id=
+"pg170">170</a></span>as a whole, how it lived and nourished
+itself, and they studied cells and sieve-tubes, wood fibres and
+trache&aelig; with a view rather to finding out their functions and
+their significance for the life of the plant than to discovering
+the minuti&aelig; of their structure. The same attitude was taken
+up by the few botanists who in the 18th century paid any heed to
+the microscopical anatomy of plants. For C. F. Wolff,<a name=
+"FNanchor_238" id="FNanchor_238" /><a href="#Footnote_238"
+class="fnanchor">[238]</a> 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&mdash;no idea here of cells as units of
+structure.</p>
+
+<p>In the early 19th century, interest in plant anatomy revived
+somewhat, and much work was done by Treviranus, Mirbel,
+Moldenhawer, Meyen and von Mohl.<a name="FNanchor_239" id=
+"FNanchor_239" /><a href="#Footnote_239" class=
+"fnanchor">[239]</a> 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&mdash;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.</p>
+
+<p>The idea that the cell-contents might show a characteristic and
+individual structure had hardly dawned upon botanists when
+Schleiden published his famous paper, <i>Beitr&auml;ge zur
+Phytogenesis</i>.<a name="FNanchor_240" id="FNanchor_240" /><a
+href="#Footnote_240" class="fnanchor">[240]</a> Schleiden's theme
+in this paper is the origin <span class="pagenum"><a name="pg171"
+id="pg171">171</a></span>and development of the plant cell, a
+subject then very obscure, in spite of pioneer work by Mirbel. A
+few years before, Robert Brown had called attention to the presence
+in the epidermal cells of orchids and other plants of a
+characteristic spot which he called the areola or nucleus.<a name=
+"FNanchor_241" id="FNanchor_241" /><a href="#Footnote_241"
+class="fnanchor">[241]</a> 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&mdash;the nucleolus.
+The nucleus, Schleiden thought, originated as a minute granule in
+the cytoblastem which gradually increased in size, becoming first a
+nucleolus (<i>Kernchen</i>), 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:&mdash;"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.&nbsp;145). The
+young cells increase in size and fill up the cavity of the old
+cell, which is in time resorbed. Cell-development always takes
+place within existing cells, and either one or many new cells may
+be formed within the mother-cell. Schleiden's views on
+cell-formation were drawn from some rather imperfect observations
+on the embryo-sac and pollen-tube, but he extended his theory to
+cell-formation in general. Though wrong in almost all respects the
+theory had at least the merit of fixing attention upon the really
+important constituents of the cell, the nucleus and the
+cell-plasma. To Schleiden, too, we owe the conception of the cell
+as a more or less independent living unity, whose life is not
+entirely identified with the life of the plant as a whole. "Each
+cell," he writes, "carries on a double life; one a quite
+independent and self-contained <span class="pagenum"><a name=
+"pg172" id="pg172">172</a></span>life, the other a dependent life
+in so far as the cell has become an integral part of the plant" (p.
+138).</p>
+
+<p>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&mdash;as in the notochord by Johannes
+M&uuml;ller.<a name="FNanchor_242" id="FNanchor_242" /><a href=
+"#Footnote_242" class="fnanchor">[242]</a> 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 <i>K&uuml;gelchen</i> or little
+globules suspended in it (<i>Entwickelungsgeschichte</i>, i., pp.
+19, 144). Since such <i>K&uuml;gelchen</i> disposed in a row formed
+the notochord (i., p.&nbsp;145) it seems probable that his
+<i>K&uuml;gelchen</i> were really cells. Similarly A. de
+Quatrefages<a name="FNanchor_243" id="FNanchor_243" /><a href=
+"#Footnote_243" class="fnanchor">[243]</a> in 1834 saw and figured
+segmentation spheres in the developing egg of <i>Limn&aelig;a</i>,
+but he called them globules and did not recognise their analogy
+with the cells of plants. According to M'Kendrick,<a name=
+"FNanchor_244" id="FNanchor_244" /><a href="#Footnote_244"
+class="fnanchor">[244]</a> Fontana, so far back as 1781,<a name=
+"FNanchor_245" id="FNanchor_245" /><a href="#Footnote_245"
+class="fnanchor">[245]</a> described cells with nuclei in various
+tissues, and used acids and alkalis to bring out their structure
+more clearly. But it was not till 1836-7-8 that a fairly widespread
+occurrence of cells in animal tissues was recognised. The pioneer
+in this seems to have been Purkinje, who described cells in the
+choroidal plexus in 1836,<a name="FNanchor_246" id=
+"FNanchor_246" /><a href="#Footnote_246" class=
+"fnanchor">[246]</a> and compared gland cells with the cells of
+plants in 1837.<a name="FNanchor_247" id="FNanchor_247" /><a
+href="#Footnote_247" class="fnanchor">[247]</a> Henle in 1837<a
+name="FNanchor_248" id="FNanchor_248" /><a href=
+"#Footnote_248" class="fnanchor">[248]</a> and 1838<a name=
+"FNanchor_249" id="FNanchor_249" /><a href="#Footnote_249"
+class="fnanchor">[249]</a> 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. <span class="pagenum"><a name="pg173" id=
+"pg173">173</a></span>Valentin<a name="FNanchor_250" id=
+"FNanchor_250" /><a href="#Footnote_250" class=
+"fnanchor">[250]</a> 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 M&uuml;ller 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 Donn&eacute; on the cells of the conjuctiva, cornea
+and lens. He reports, too, that Turpin had compared the epithelial
+cells of the vagina with the cell-tissue of plants. M&uuml;ller
+himself had not only recognised the cellular nature of the
+notochord, but had observed the cells of the vitreous humour, fat
+cells and pigment cells, and even the nuclei of cartilage cells.
+From Schwann (1839) we learn that C. H. Schults had followed back
+the corpuscles of the blood to their original state of nucleated
+cells, and that Werneck had recognised cells in the embryonic lens.
+A preliminary notice of Schwann's own work appeared in 1838
+(Froriep's <i>Notizen</i>, No. 91, 1838), the full memoir in 1839,
+under the title <i>Mikroskopische Untersuchungen &uuml;ber die
+Uebereinstimmung in der Struktur und dem Wachstume der Tiere und
+Pflanzen</i>.<a name="FNanchor_251" id="FNanchor_251" /><a
+href="#Footnote_251" class="fnanchor">[251]</a></p>
+
+<p>Theodor Schwann was a pupil of Johannes M&uuml;ller, and we know
+that M&uuml;ller took much interest in the new histology. It is
+probably to his influence that we owe Schwann's brilliant work on
+the cell, which appeared just after Schwann left Berlin for
+L&ouml;wen. Schwann was himself, as his later work showed, more a
+physiologist than a morphologist; he did quite fundamental work on
+enzymes, discovering and isolating the pepsin of the gastric juice;
+he proved that yeast was not an inorganic precipitate but a mass of
+living cells; he carried out experiments directed to show that
+spontaneous generation does not occur. We shall see in his
+treatment of the cell-theory clear indications of his physiological
+<span class="pagenum"><a name="pg174" id="pg174">174</a></span>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
+<i>Anatomische-philosophische Untersuchungen</i> 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.</p>
+
+<p>The first section of his book is devoted to a detailed study of
+the structure and development of cartilage cells and of the cells
+of the notochord, and to a comparison of these with plant cells. He
+accepts Schleiden's account of the origin and development of nuclei
+and cells as a standard of comparison; and he seeks to show that
+nucleus and nucleolus, cell-wall and cell-contents, show the same
+relations and behave in the same manner in these two types of
+animal cells as in the plant-cells studied by Schleiden. The types
+of cell which he chose for this comparison are the most plant-like
+of all animal cells, and he was even able to point to a thickening
+of the cell-wall in certain cartilage cells, analogous to the
+thickening which plays so important a part in the outward
+modification of plant-cells. The analogy indeed in structure and
+development between chorda and cartilage cells and the cells of
+plants seemed to him complete. The substance of the notochord
+consisted of polyhedral cells having attached to their wall an oval
+disc similar in all respects to the nucleus of the plant-cell, and
+like it containing one or more nucleoli. Inside the mother-cell
+were to be found young developing cells of spherical shape, lacking
+however a nucleus. Cartilage was even more like plant tissue. It
+was composed of cells, each with its cell membrane. The cells lay
+close to one another, separated only by their thickened cell-wall
+and the intercellular matrix, showing thus even the general
+appearance of the cellular tissue of plants. They contained a
+nucleus with one or two nucleoli, and the nucleus was often
+resorbed, as in plants, when the cell reached its full development.
+Other nuclei were in many cases present in the cell, round which
+young cells could be <span class="pagenum"><a name="pg175" id=
+"pg175">175</a></span>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.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>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<a name="FNanchor_252" id=
+"FNanchor_252" /><a href="#Footnote_252" class=
+"fnanchor">[252]</a> discovered and described in the unfertilised egg
+of the fowl a small vesicle containing granular matter, which he
+named the <i>Keimbl&auml;schen</i> or germinal vesicle. It
+disappeared in the fertilised egg. As early as 1791 Poli had seen
+the germinal vesicle in the eggs of molluscs, but the first
+adequate account was given by Purkinje. In 1827<a name=
+"FNanchor_253" id="FNanchor_253" /><a href="#Footnote_253"
+class="fnanchor">[253]</a> 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.<a name="FNanchor_254" id=
+"FNanchor_254" /><a href="#Footnote_254" class=
+"fnanchor">[254]</a> Even von Graaf had noticed that the early
+uterine eggs were smaller than the supposed ovarian eggs;
+Pr&eacute;vost and Dumas<a name="FNanchor_255" id=
+"FNanchor_255" /><a href="#Footnote_255" class=
+"fnanchor">[255]</a> 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 <span class="pagenum"><a
+name="pg176" id="pg176">176</a></span>that this small sphere was
+indeed the mammalian ovum. His discovery was confirmed by Sharpey
+and by Allen Thomson. Von Baer found the germinal vesicle in the
+eggs of frogs, snakes, molluscs, and worms, but not in the
+mammalian ovum; he considered the whole mammalian ovum to be the
+equivalent of the germinal vesicle of birds&mdash;a comparison
+rightly questioned by Purkinje (1834). In 1834 Coste<a name=
+"FNanchor_256" id="FNanchor_256" /><a href="#Footnote_256"
+class="fnanchor">[256]</a> discovered in the ovum of the rabbit a
+vesicle which he considered to be the germinal vesicle of Purkinje;
+he observed that it disappeared after fertilisation. Independently
+of Coste, and very little time after him, Wharton Jones<a name=
+"FNanchor_257" id="FNanchor_257" /><a href="#Footnote_257"
+class="fnanchor">[257]</a> found the germinal vesicle in the
+mammalian ovum. Valentin in 1835,<a name="FNanchor_258" id=
+"FNanchor_258" /><a href="#Footnote_258" class=
+"fnanchor">[258]</a> Wagner in 1836,<a name="FNanchor_259" id=
+"FNanchor_259" /><a href="#Footnote_259" class=
+"fnanchor">[259]</a> and Krause in 1837,<a name="FNanchor_260" id=
+"FNanchor_260" /><a href="#Footnote_260" class=
+"fnanchor">[260]</a> added considerably to the existing knowledge of
+the structure of the ovum. Wagner in his <i>Prodromus</i> called
+attention to the widespread occurrence, within the germinal vesicle
+of a darker speck which he called the <i>Keimfleck</i> or germinal
+spot, known sometimes as Wagner's spot. He recognised the
+<i>Keimfleck</i> in the ova of many classes of animals from mammals
+to polyps. Frequently more than one <i>Keimfleck</i> occurred.</p>
+
+<p>Schwann had therefore a good deal of exact knowledge to go upon
+in discussing the significance of the ovum for the cell-theory.
+There were two possible interpretations. Either the ovum was a cell
+and the germinal vesicle its nucleus, or else the germinal vesicle
+was itself a cell within the larger cell of the ovum and the
+germinal spot was its nucleus. Schwann had some difficulty in
+deciding which of these views to adopt, but he finally inclined to
+the view that the ovum is a cell and the germinal vesicle its
+nucleus, basing his opinion largely upon observations by Wagner
+which tended to prove that the germinal vesicle was formed <span
+class="pagenum"><a name="pg177" id="pg177">177</a></span>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&mdash;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 <i>Niederschlag</i> or precipitate which is the
+rudiment of the blastoderm (p.&nbsp;68). When the egg leaves the ovary
+the nucleus disappears, leaving behind it this rudiment of the
+blastoderm, which rapidly grows and increases in size. The
+blastoderm of the chick before incubation is found to be composed
+of spherical anucleate bodies which Schwann considers to be cells,
+because they almost certainly develop into the cells of the
+incubated blastoderm, which are clearly recognisable as such after
+eight hours' incubation. The serous and mucous layers can be
+distinguished after sixteen hours' incubation, and it is found that
+the cells of the serous layer contain definite nuclei, though such
+seem to be absent in the cells of the mucous layer. Between the two
+layers other cells are formed belonging to the vessel layer, which
+is, however, in Schwann's opinion not a very definitely
+individualised layer.</p>
+
+<p>Schwann's next step is a detailed demonstration of the origin of
+each tissue from simple cells such as those composing the incubated
+blastoderm.</p>
+
+<p>"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 <span class="pagenum"><a name=
+"pg178" id="pg178">178</a></span>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.&nbsp;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.</p>
+
+<p>Schwann found it easy to demonstrate the cellular nature of the
+tissues of his first three classes. With the other two classes he
+had more difficulty. Fibres of all kinds, he considered, arose by
+an elongation of cells, which afterwards split longitudinally into
+long strips, forming as the case might be white or elastic fibrous
+tissue. Muscle-fibres and nerve-fibres were formed in a totally
+different way, by coalescence of cells; each separate muscle-fibre
+and nerve-fibre was thus a compound cell. Capillaries, Schwann
+held, were formed by cells hollowed out like drain-pipes, and <span
+class="pagenum"><a name="pg179" id="pg179">179</a></span>set end to
+end&mdash;a mistaken view soon corrected by Vogt (<i>Embryologie
+des Salmones</i>, p.&nbsp;206, 1842).</p>
+
+<p>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&mdash;the cytoblastem
+is within the cells; but extracellular cell formation seems to be
+the general rule in animals. An intracellular formation of cells
+occurs only in the ovum, in cartilage cells and chorda cells and in
+a few others, and even there it is not the exclusive method of
+formation; a formation of cells within cells never occurs in
+muscles and nerves, nor in fibrous tissue (p.&nbsp;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.&nbsp;43).</p>
+
+<p>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 <span class=
+"pagenum"><a name="pg180" id="pg180">180</a></span>(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&mdash;a cell of the first order, while
+ordinary nucleated cells might be designated cells of the second
+order (p.&nbsp;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.</p>
+
+<p>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.</p>
+
+<p>He begins by contrasting the teleological with the materialistic
+conception of living things. In the teleological view, a special
+force works in the living organism, guiding and directing its
+activities towards a purposeful end. According to the materialistic
+view there are no other forces at work in the living organism than
+those which act in the inorganic realm, or at least there are none
+but forces at one with these in their blindness and necessity.
+True, the purposiveness of living processes cannot be denied; but
+its ground lies, according to this view, not in a vital force which
+guides and rules the individual life, but in the original creation
+and collocation of matter according to a rational plan. The
+purposiveness of life is part of the purposiveness of the universe;
+just as the stars circle for ever in harmoniously adjusted paths,
+so do the processes of life work together <span class="pagenum"><a
+name="pg181" id="pg181">181</a></span>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.&nbsp;222).</p>
+
+<p>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.&nbsp;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?</p>
+
+<p>The teleological conception, again, leads to difficulties
+particularly when it is applied to the facts of reproduction. If we
+suppose that a vital force unifies and coordinates the organism and
+is its very essence, we must also suppose that this force is
+divisible and that a part of it&mdash;separated in
+reproduction&mdash;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 <span class="pagenum"><a name="pg182" id=
+"pg182">182</a></span>organisms, that the whole "individual"
+organism must be an aggregate of cells, and that the concept of
+individuality applied to the organism is accordingly a logical
+fiction? And it is just upon this notion of the individuality of
+the organism that the teleological concept is based. The
+teleological view can perhaps not be completely refuted until the
+adequacy of materialistic explanations has been finally shown; but
+it is certain that the most promising method for research is the
+materialistic (p.&nbsp;226).</p>
+
+<p>"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.&nbsp;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&mdash;note the physiological point of view&mdash;the
+question runs, are metabolic processes the result of the molecular
+construction of the organism as a whole, or does the centre of
+metabolic activity lie in the cell? Is it the cell rather than the
+organism that is the immediate agent of assimilatory processes? In
+the first alternative the cause of the growth of the constituent
+parts lies in the totality of the organism; in the other
+alternative:&mdash;"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
+<span class="pagenum"><a name="pg183" id=
+"pg183">183</a></span>molecules and so to grow, and the whole
+organism exists only through the reciprocal action of the single
+elementary parts.... In this eventuality it is the elementary parts
+that form the active element in nutrition, and the totality of the
+organism can be indeed a condition, but on this view it cannot be a
+cause" (p.&nbsp;227).</p>
+
+<p>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&mdash;all of which we know to be essentially identical one
+with another&mdash;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.&nbsp;228-9).</p>
+
+<p>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&mdash;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 <span class="pagenum"><a name="pg184"
+id="pg184">184</a></span>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.&nbsp;239), can the
+process of cell-formation be most nearly compared, and the answer
+obviously is, with the process of crystallisation. Cells are, it is
+true, quite different in shape and consistency from crystals, and
+they grow by intussusception, not by apposition&mdash;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:&mdash;"The substance of which cells are
+composed possesses the power of chemically transforming the
+substance with which it is in immediate contact, in somewhat the
+same way as the well-known preparation of platinum changes alcohol
+into acetic acid. Each part of the cell possesses this property. If
+now the cytoblastem is altered by an already formed cell in such a
+way that a substance is formed that cannot become part of the cell,
+it crystallises out first as the nucleolus of a new cell. This in
+its turn alters the composition of the cytoblastem. A part of the
+transfomed <span class="pagenum"><a name="pg185" id=
+"pg185">185</a></span>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).</p>
+
+<p>Two difficulties have to be faced by this theory&mdash;(1) the
+origin of the metabolic power of the cells, (2) the reason why the
+cells arrange themselves so as to form an organism of complex and
+definite structure. Schwann tries to explain the origin of the
+"metabolic" action, the analogy of which with the contact-action of
+colloidal platinum he recognises, by attributing it to the peculiar
+structural arrangements of molecules. In attempting to account for
+the harmonious structure of the organism he points to the analogy
+of ordinary crystals, which often form complex and regular
+tree-like arrangements; plants in particular resemble these
+regularly shaped crystal-aggregates.</p>
+
+<p>The whole ingenious theory is offered merely as an hypothesis
+and a guide to research. It is interesting as one of the most
+carefully thought-out attempts ever made to give a thorough-going
+materialistic account of the origin and development of organic
+form, and it arose directly out of the cell-theory.</p>
+
+<p>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.</p>
+
+<p>This was established for plant cells by Meyen, Unger, von Mohl,
+Naegeli and Hofmeister in or about the <span class="pagenum"><a
+name="pg186" id="pg186">186</a></span>forties.<a name=
+"FNanchor_261" id="FNanchor_261" /><a href="#Footnote_261"
+class="fnanchor">[261]</a> Criticism of the Schwann-Schleiden theory
+from the zoological side was suggested by the study of the
+segmentation of the ovum&mdash;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 Pr&eacute;vost and Dumas in 1824,<a
+name="FNanchor_262" id="FNanchor_262" /><a href=
+"#Footnote_262" class="fnanchor">[262]</a> in the frog and newt by
+Rusconi,<a name="FNanchor_263" id="FNanchor_263" /><a href=
+"#Footnote_263" class="fnanchor">[263]</a> and an elaborate study
+of the process in the frog had been made by von Baer.<a name=
+"FNanchor_264" id="FNanchor_264" /><a href="#Footnote_264"
+class="fnanchor">[264]</a> 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.</p>
+
+<p>Segmentation was soon found to be of widespread occurrence. Von
+Siebold in 1837 described the process in Entozoa,<a name=
+"FNanchor_265" id="FNanchor_265" /><a href="#Footnote_265"
+class="fnanchor">[265]</a> and in the same year Lov&eacute;n saw
+segmentation in <i>Campanularia</i>,<a name="FNanchor_266" id=
+"FNanchor_266" /><a href="#Footnote_266" class=
+"fnanchor">[266]</a> and Sars in the starfish and in Nudibranchs.<a
+name="FNanchor_267" id="FNanchor_267" /><a href=
+"#Footnote_267" class="fnanchor">[267]</a></p>
+
+<p>In 1838 Bischoff<a name="FNanchor_268" id=
+"FNanchor_268" /><a href="#Footnote_268" class=
+"fnanchor">[268]</a> 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<a name="FNanchor_269" id="FNanchor_269" /><a href=
+"#Footnote_269" class="fnanchor">[269]</a> in 1839. C. Vogt<a name=
+"FNanchor_270" id="FNanchor_270" /><a href="#Footnote_270"
+class="fnanchor">[270]</a> in 1842 described segmentation in
+<i>Coregonus</i> and <i>Alytes</i>. The discovery of segmentation
+in the ovum of <span class="pagenum"><a name="pg187" id=
+"pg187">187</a></span>birds was not made until 1847, by Bergmann,<a
+name="FNanchor_271" id="FNanchor_271" /><a href=
+"#Footnote_271" class="fnanchor">[271]</a> confirmed
+independently by Coste<a name="FNanchor_272" id=
+"FNanchor_272" /><a href="#Footnote_272" class=
+"fnanchor">[272]</a> in 1850. By 1848 segmentation had been noted in
+<i>Hydra</i> and various hydroids, in acalephs, in starfish,
+polyzoa, nematodes, rotifers, leeches, oligoch&aelig;tes,
+polych&aelig;tes, in most groups of molluscs and arthropods, and in
+all the vertebrate classes.<a name="FNanchor_273" id=
+"FNanchor_273" /><a href="#Footnote_273" class=
+"fnanchor">[273]</a></p>
+
+<p>The process was at first held to be merely one of yolk-division,
+or <i>Dotterfurchung</i>, and its details were by most interpreted
+in the light of the Schleiden-Schwann theory of cell-formation.</p>
+
+<p>The first steps towards a truer conception of the process seem
+to have been taken by Bergmann, who in 1841<a name="FNanchor_274"
+id="FNanchor_274" /><a href="#Footnote_274" class=
+"fnanchor">[274]</a> called attention to the presence of nuclei in
+the segmentation-spheres of the frog's egg, and by Bagge in the
+same year, who observed that division of the nuclei preceded the
+multiplication of the segmentation spheres.<a name="FNanchor_275"
+id="FNanchor_275" /><a href="#Footnote_275" class=
+"fnanchor">[275]</a> He considered the nuclei to be anucleate cells,
+and the same view was taken by K&ouml;lliker in 1843.<a name=
+"FNanchor_276" id="FNanchor_276" /><a href="#Footnote_276"
+class="fnanchor">[276]</a> Next year, however, in his classical paper
+on Cephalopod development<a name="FNanchor_277" id=
+"FNanchor_277" /><a href="#Footnote_277" class=
+"fnanchor">[277]</a> K&ouml;lliker came to the opinion that they were
+really nuclei. He showed that segmentation was brought about by
+cell-division, that between "total" and "partial" segmentation
+there was a difference of degree and not of kind, and that the
+cells of the body were formed by division of the segmentation
+spheres. He held, however, that the nuclei multiplied endogenously
+and not by division. The division of nuclei was observed by Coste
+in 1846.<a name="FNanchor_278" id="FNanchor_278" /><a href=
+"#Footnote_278" class="fnanchor">[278]</a> Leydig in 1848<a name=
+"FNanchor_279" id="FNanchor_279" /><a href="#Footnote_279"
+class="fnanchor">[279]</a> took the necessary step in advance and
+maintained that the nuclei as well as the cells increased always by
+division. He was supported by Remak, who in a paper of 1852,<a
+name="FNanchor_280" id="FNanchor_280" /><a href=
+"#Footnote_280" class="fnanchor">[280]</a> and more fully in his
+monumental <span class="pagenum"><a name="pg188" id=
+"pg188">188</a></span><i>Untersuchungen &uuml;ber die Entwickelung
+der Wirbelthiere</i> (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.<a name=
+"FNanchor_281" id="FNanchor_281" /><a href="#Footnote_281"
+class="fnanchor">[281]</a></p>
+
+<p>One point Remak left undecided&mdash;the fate of the
+<i>Keimbl&auml;schen</i> or egg-nucleus. It was generally held,
+even so late as the 'fifties, that the egg-nucleus disappeared just
+before segmentation began&mdash;Bischoff clung to this belief even
+in 1877.<a name="FNanchor_282" id="FNanchor_282" /><a href=
+"#Footnote_282" class="fnanchor">[282]</a> Though Barry had held in
+1839 that the egg-nucleus does not disappear in segmentation, J.
+M&uuml;ller 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 <i>Entoconcha
+mirabilis</i>,<a name="FNanchor_283" id="FNanchor_283" /><a
+href="#Footnote_283" class="fnanchor">[283]</a> and his paper was
+known to Remak, who could not, however, observe a similar division
+of the egg-nucleus in the frog. M&uuml;ller's discovery was
+confirmed for <i>Oceania armata</i> by Gegenbaur,<a name=
+"FNanchor_284" id="FNanchor_284" /><a href="#Footnote_284"
+class="fnanchor">[284]</a> and for <i>Notommata sieboldii</i> by
+Leydig.<a name="FNanchor_285" id="FNanchor_285" /><a href=
+"#Footnote_285" class="fnanchor">[285]</a></p>
+
+<p>In 1854 Virchow,<a name="FNanchor_286" id=
+"FNanchor_286" /><a href="#Footnote_286" class=
+"fnanchor">[286]</a> previously a supporter of Schwann, crystallised
+the new views in the famous phrase&mdash;<i>Omnis cellula e
+cellula</i>&mdash;and gave wide publicity to them in his classical
+lectures on Cellular Pathology, delivered in 1858.<a name=
+"FNanchor_287" id="FNanchor_287" /><a href="#Footnote_287"
+class="fnanchor">[287]</a> The new doctrine of cell-formation was
+also taught by Leydig<a name="FNanchor_288" id=
+"FNanchor_288" /><a href="#Footnote_288" class=
+"fnanchor">[288]</a> in his text-book of histology, published in
+1857.</p>
+
+<p>The Schleiden-Schwann theory of the origin of cells by
+generation in a cytoblastem was now definitely overthrown.</p>
+
+<p>The importance of the protoplasmic content of the cell was
+brought into prominence through the work of Dujardin,<a name=
+"FNanchor_289" id="FNanchor_289" /><a href="#Footnote_289"
+class="fnanchor">[289]</a> <span class="pagenum"><a name="pg189" id=
+"pg189">189</a></span>Purkinje,<a name="FNanchor_290" id=
+"FNanchor_290" /><a href="#Footnote_290" class=
+"fnanchor">[290]</a> Cohen<a name="FNanchor_291" id=
+"FNanchor_291" /><a href="#Footnote_291" class=
+"fnanchor">[291]</a> and Max Schultze.<a name="FNanchor_292" id=
+"FNanchor_292" /><a href="#Footnote_292" class=
+"fnanchor">[292]</a> The last-named in 1861 proposed a definition of
+the cell which might be accepted at the present day. "A cell," he
+wrote, "is a little blob of protoplasm containing a nucleus" (p.
+11).</p>
+
+<div class="footnote">
+<p><a name="Footnote_238" id="Footnote_238" /><a href=
+"#FNanchor_238"><span class="label">[238]</span></a> <i>Theoria
+generationis</i>, Halae, 1759.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_239" id="Footnote_239" /><a href=
+"#FNanchor_239"><span class="label">[239]</span></a> See J. v.
+Sachs, <i>Geschichte der Botanik</i>, book ii., Eng. Trans., 2nd
+impr., 1906.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_240" id="Footnote_240" /><a href=
+"#FNanchor_240"><span class="label">[240]</span></a> M&uuml;ller's
+<i>Archiv</i>, pp.&nbsp;137-76, 1838.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_241" id="Footnote_241" /><a href=
+"#FNanchor_241"><span class="label">[241]</span></a> <i>Trans.
+Linnean Soc.</i>, xvi., p.&nbsp;710, 1833.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_242" id="Footnote_242" /><a href=
+"#FNanchor_242"><span class="label">[242]</span></a>
+<i>Myxinoiden</i>, i. Theil., p.&nbsp;89, 1835.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_243" id="Footnote_243" /><a href=
+"#FNanchor_243"><span class="label">[243]</span></a> <i>Ann. Sci.
+nat.</i> (2) (<i>Zool.</i>) ii., pp.&nbsp;107-18, pl. 11, 1834.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_244" id="Footnote_244" /><a href=
+"#FNanchor_244"><span class="label">[244]</span></a> <i>Proc. Phil.
+Soc. Glasgow</i>, xix., pp.&nbsp;71-125, 1887-8.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_245" id="Footnote_245" /><a href=
+"#FNanchor_245"><span class="label">[245]</span></a>
+<i>Trait&eacute; sur le venin de la vip&egrave;re</i>, 1781.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_246" id="Footnote_246" /><a href=
+"#FNanchor_246"><span class="label">[246]</span></a> M&uuml;ller's
+<i>Archiv</i>, 1836.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_247" id="Footnote_247" /><a href=
+"#FNanchor_247"><span class="label">[247]</span></a> J.
+M&uuml;ller, <i>Jahresbericht &uuml;. d. Fortschritte der
+anat.-physiol. Wissenschaften im Jahre</i> 1838. M&uuml;ller's
+<i>Archiv</i>, 1838.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_248" id="Footnote_248" /><a href=
+"#FNanchor_248"><span class="label">[248]</span></a>
+<i>Symbol&aelig; ad anatomiam villorum imprimis eorum
+epithelii</i>, Berlin, 1837.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_249" id="Footnote_249" /><a href=
+"#FNanchor_249"><span class="label">[249]</span></a> <i>U. d.
+Ausbreitung des Epitheliums im menschlichen K&ouml;rper</i>.
+M&uuml;ller's <i>Archiv</i>, 1838.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_250" id="Footnote_250" /><a href=
+"#FNanchor_250"><span class="label">[250]</span></a> See Schwann's
+<i>Bemerkungen</i> at the end of his <i>Mikroskopische
+Untersuchungen</i>.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_251" id="Footnote_251" /><a href=
+"#FNanchor_251"><span class="label">[251]</span></a> Republished
+in Ostwald's <i>Klassiker der exakten Wissenschaften</i>, No. 176,
+Leipzig, 1910. References in the text are to the original
+pagination.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_252" id="Footnote_252" /><a href=
+"#FNanchor_252"><span class="label">[252]</span></a> <i>Symbol&aelig; ad
+ovi avium historiam</i>.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_253" id="Footnote_253" /><a href=
+"#FNanchor_253"><span class="label">[253]</span></a> <i>De ovi
+mammalium et hominis genesi</i>.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_254" id="Footnote_254" /><a href=
+"#FNanchor_254"><span class="label">[254]</span></a> <i>De mulierum
+organis</i>, 1672.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_255" id="Footnote_255" /><a href=
+"#FNanchor_255"><span class="label">[255]</span></a> <i>Ann. Sci.
+nat.</i>, iii., p.&nbsp;135, 1842.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_256" id="Footnote_256" /><a href=
+"#FNanchor_256"><span class="label">[256]</span></a> <i>Recherches
+sur la g&eacute;n&eacute;ration des Mammif&egrave;res</i>. Report
+by Academy Committee. <i>Ann. Sci. nat.</i> (2) (<i>Zool.</i>) ii.,
+pp.&nbsp;1-18, 1834; also <i>Embryog&eacute;nie compar&eacute;e</i>,
+1837.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_257" id="Footnote_257" /><a href=
+"#FNanchor_257"><span class="label">[257]</span></a> <i>Lond. and
+Edin. Phil. Mag.</i> (3) vii., 1835; <i>Phil. Trans.</i> 1837.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_258" id="Footnote_258" /><a href=
+"#FNanchor_258"><span class="label">[258]</span></a> <i>Handbuch
+der Enfwickelungsgeschichte</i>, 1835, and M&uuml;ller's
+<i>Archiv</i>, 1836.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_259" id="Footnote_259" /><a href=
+"#FNanchor_259"><span class="label">[259]</span></a> <i>Prodromus
+histori&aelig; generationis hominis atque animalium</i>,
+Lipsi&aelig;, 1836.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_260" id="Footnote_260" /><a href=
+"#FNanchor_260"><span class="label">[260]</span></a> M&uuml;ller's
+<i>Archiv</i>, 1837.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_261" id="Footnote_261" /><a href=
+"#FNanchor_261"><span class="label">[261]</span></a> Sachs,
+<i>History of Botany</i>, Book ii.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_262" id="Footnote_262" /><a href=
+"#FNanchor_262"><span class="label">[262]</span></a> <i>Ann. Sci.
+nat.</i>, i., pp.&nbsp;110-14, 1824. Swammerdam is said to have observed
+the 2-celled stage in the egg of the frog (<i>Bibl. Nat.</i>,
+1752), and R&ouml;sel v. Rosenhof the same stage in the tree-frog
+(<i>Hist. nat. ranarum nostratium</i>, 1758).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_263" id="Footnote_263" /><a href=
+"#FNanchor_263"><span class="label">[263]</span></a>
+<i>D&eacute;veloppement de la grenouille commune</i>, Milan, 1826.
+<i>Biblioteca italiana</i>, lxxix., 1836, and M&uuml;ller's
+<i>Archiv</i>, 1836. Agassiz is said by Vogt (1842) to have seen
+segmentation in the Perch as early as 1831.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_264" id="Footnote_264" /><a href=
+"#FNanchor_264"><span class="label">[264]</span></a> M&uuml;ller's
+<i>Archiv</i>, 1836.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_265" id="Footnote_265" /><a href=
+"#FNanchor_265"><span class="label">[265]</span></a> In Burdach,
+<i>Die Physiologie als Erfahrungswissenschaft</i>, 2nd Ed., vol.
+ii.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_266" id="Footnote_266" /><a href=
+"#FNanchor_266"><span class="label">[266]</span></a> Wiegmann's
+<i>Archiv</i>, 1837.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_267" id="Footnote_267" /><a href=
+"#FNanchor_267"><span class="label">[267]</span></a> <i>Bericht
+Versamml. deutsch. Naturf. in Prag</i>, 1837.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_268" id="Footnote_268" /><a href=
+"#FNanchor_268"><span class="label">[268]</span></a> <i>Bericht
+Versamm. deutsch. Naturf. in Freiburg</i>, 1838. Later in his
+<i>Entw. d. Wirbelth.</i>, and in his papers on the development of
+the rabbit.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_269" id="Footnote_269" /><a href=
+"#FNanchor_269"><span class="label">[269]</span></a> <i>Phil.
+Trans.</i>, 1839. See particularly Pl. vi., figs. 105-12.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_270" id="Footnote_270" /><a href=
+"#FNanchor_270"><span class="label">[270]</span></a>
+<i>Embryologie des Salmones</i> 1842.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_271" id="Footnote_271" /><a href=
+"#FNanchor_271"><span class="label">[271]</span></a>
+M&uuml;ller's <i>Archiv</i>, 1847.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_272" id="Footnote_272" /><a href=
+"#FNanchor_272"><span class="label">[272]</span></a> <i>C.R.
+Acad. Sci.</i>, xxx., p.&nbsp;638.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_273" id="Footnote_273" /><a href=
+"#FNanchor_273"><span class="label">[273]</span></a> See review
+by Leydig in <i>Isis</i>, 1848, pp.&nbsp;161-193.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_274" id="Footnote_274" /><a href=
+"#FNanchor_274"><span class="label">[274]</span></a> M&uuml;ller's
+<i>Archiv</i>, pp.&nbsp;89-102, 1841.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_275" id="Footnote_275" /><a href=
+"#FNanchor_275"><span class="label">[275]</span></a> <i>De
+evolution Stronzyli auric. el Ascaridis acum.</i>, Erlangen,
+1841.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_276" id="Footnote_276" /><a href=
+"#FNanchor_276"><span class="label">[276]</span></a> M&uuml;ller's
+<i>Archiv</i>, pp.&nbsp;66-141, 1843.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_277" id="Footnote_277" /><a href=
+"#FNanchor_277"><span class="label">[277]</span></a>
+<i>Entwickelungsgeschichte der Cephalopoden</i>, Zurich, 1844.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_278" id="Footnote_278" /><a href=
+"#FNanchor_278"><span class="label">[278]</span></a> <i>Froriep's
+Notizen</i>, No. 800, 1846.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_279" id="Footnote_279" /><a href=
+"#FNanchor_279"><span class="label">[279]</span></a> <i>Isis</i>,
+1848.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_280" id="Footnote_280" /><a href=
+"#FNanchor_280"><span class="label">[280]</span></a> M&uuml;ller's
+<i>Archiv</i>, p.&nbsp;47, 1852, also 1854 and 1858.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_281" id="Footnote_281" /><a href=
+"#FNanchor_281"><span class="label">[281]</span></a> See
+particularly Plate IX., figs. 3-7.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_282" id="Footnote_282" /><a href=
+"#FNanchor_282"><span class="label">[282]</span></a> <i>Hist.-krit.
+Bemerkungen zu den neuesten Mittheilungen &uuml;. d. erste
+Entwickelung d. S&auml;ugethiereier</i>, M&uuml;nchen, 1877.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_283" id="Footnote_283" /><a href=
+"#FNanchor_283"><span class="label">[283]</span></a> <i>Monatsber.
+Akad. Wiss. Berlin</i>, 1851.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_284" id="Footnote_284" /><a href=
+"#FNanchor_284"><span class="label">[284]</span></a> <i>Zur Lehre
+von Generationswechsel u. d. Fortpflanzen d. Medusen u.
+Polypen</i>.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_285" id="Footnote_285" /><a href=
+"#FNanchor_285"><span class="label">[285]</span></a> <i>U. d. Bau
+u. d. system. Stellung d. R&auml;derthiere</i>, 1854.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_286" id="Footnote_286" /><a href=
+"#FNanchor_286"><span class="label">[286]</span></a> <i>Arch f.
+path. Anat. Phys.</i>, vii., pp.&nbsp;1-39, 1854. Also in his
+<i>Beitr&auml;ge z. spec. Path. u. Therapie</i>.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_287" id="Footnote_287" /><a href=
+"#FNanchor_287"><span class="label">[287]</span></a> <i>Die
+Cellularpathologie</i>, Berlin, 1858.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_288" id="Footnote_288" /><a href=
+"#FNanchor_288"><span class="label">[288]</span></a> <i>Lehrbuch
+der Histologie</i>, 1857.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_289" id="Footnote_289" /><a href=
+"#FNanchor_289"><span class="label">[288]</span></a> <i>Ann, Sci.
+nat.</i> (2) iii., pp.&nbsp;108-9 and pp.&nbsp;312-4, 1835. Also iv, pp
+343-77.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_290" id="Footnote_290" /><a href=
+"#FNanchor_290"><span class="label">[290]</span></a> 1839 or
+1840.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_291" id="Footnote_291" /><a href=
+"#FNanchor_291"><span class="label">[291]</span></a> <i>Nova Acta
+Acad. Leop.</i>, xxii., 1850. Trans. in 1853 for Ray Society.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_292" id="Footnote_292" /><a href=
+"#FNanchor_292"><span class="label">[292]</span></a> <i>Arch. f.
+Anat. u. Physiol.</i>, pp.&nbsp;1-27, 1861.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg190" id=
+"pg190">190</a></span></p>
+
+<h3>CHAPTER XII</h3>
+
+<h4>THE CLOSE OF THE PRE-EVOLUTIONARY PERIOD</h4>
+
+<p>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.</p>
+
+<p>We have seen this physiological attitude expressed with the
+utmost clearness by the founder of the cell-theory himself; we
+shall see the same attitude taken up by most of his successors.
+Thus Vogt, who was later to become one of the protagonists of
+materialism in Germany, developed in his memoir on the embryology
+of <i>Coregonus</i><a name="FNanchor_293" id=
+"FNanchor_293" /><a href="#Footnote_293" class=
+"fnanchor">[293]</a> 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.&nbsp;275).</p>
+
+<p>And again, "... we are obliged to admit the existence in the
+cell of an independent life, which makes its development
+self-sufficient.... Each cell consequently represents a little
+independent organism, which assimilates foreign substances, builds
+them up, and rejects those that are useless; <span class=
+"pagenum"><a name="pg191" id="pg191">191</a></span>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.&nbsp;293).</p>
+
+<p>Classical expression was given to the "colonial theory" of the
+organism by Virchow in his lectures on "Cellular Pathology."<a
+name="FNanchor_294" id="FNanchor_294" /><a href=
+"#Footnote_294" class="fnanchor">[294]</a> 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&mdash;so also are animal forms. <i>Every animal is a sum
+of vital units</i>, 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.&nbsp;12-13).</p>
+
+<p>Analysis, decomposition, or disintegration of the organism is
+here pushed to its extreme point, and the problem of recomposition,
+synthesis and co-ordination shirked or forgotten.</p>
+
+<p>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<a name="FNanchor_295" id=
+"FNanchor_295" /><a href="#Footnote_295" class=
+"fnanchor">[295]</a> on the application of the <span class=
+"pagenum"><a name="pg192" id="pg192">192</a></span>cell-theory to
+physiology and pathology called forth a vigorous protest from
+Reichert,<a name="FNanchor_296" id="FNanchor_296" /><a href=
+"#Footnote_296" class="fnanchor">[286]</a> who discussed in a very
+instructive way the contrast between the older "systematic" and the
+newer "atomistic" attitude to living Nature.</p>
+
+<p>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&uuml;ller, who was Reichert's
+master, had in his <i>Physiology</i><a name="FNanchor_297" id=
+"FNanchor_297" /><a href="#Footnote_297" class=
+"fnanchor">[297]</a> 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&uuml;ller 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.</p>
+
+<p>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
+<i>proximate</i> element in the composition of the body&mdash;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.&nbsp;6).
+Within the nervous system there are separate organs which stand to
+one another in a definite morphological and functional
+relationship. These organs are, it is true, composed of cells; but
+between the form and connections of <span class="pagenum"><a name=
+"pg193" id="pg193">193</a></span>these organs and the cells which
+compose them there is no direct and necessary relation (p.&nbsp;6). It
+is true that the cell is the ultimate element of organic form, and
+that all development takes place by multiplication and form-change
+of cells. Yet is the cell in all this not independent of the unity
+of the developing embryo, and what the cells produce, they produce,
+so to speak, not of their own free will, nor by chance, but under
+the guiding influence of the unity of the whole, and in a certain
+measure as its agents (p.&nbsp;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.</p>
+
+<p>In general, the fault of the atomists is that they do not
+respect the limits which Nature herself has prescribed to the
+process of logical analysis and disintegration of the organism;
+they do not recognise the existence of natural and rational units
+or unities; they forget the one great principle of rational
+analysis, "that, by universally valid, inductive, logical method,
+natural objects must in all cases be accepted and dealt with in the
+combination and concatenation in which they are given" (p.&nbsp;10).</p>
+
+<p>The atomists at least recognised one natural organic element,
+the cell; the materialistic physiologists of the time resolved even
+this unity into an aggregate of inorganic compounds, and regarded
+the organism itself as nothing but a vastly complicated
+physico-chemical mechanism. From this point of view morphology had
+no right of existence, and we find Ludwig, one of the foremost of
+the materialistic school, maintaining that morphology was of no
+scientific importance, that it was nothing more than an artistic
+game, interesting enough, but completely superseded and robbed of
+all value by the advance of materialistic physiology.<a name=
+"FNanchor_298" id="FNanchor_298" /><a href="#Footnote_298"
+class="fnanchor">[298]</a></p>
+
+<p>Naturally enough, morphologists did not accept this rather
+contemptuous estimate of their science, but held <span class=
+"pagenum"><a name="pg194" id="pg194">194</a></span>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 <i>Bericht</i>, so J. V. Carus in his <i>System der
+thierischen Morphologie</i>,<a name="FNanchor_299" id=
+"FNanchor_299" /><a href="#Footnote_299" class=
+"fnanchor">[299]</a> 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 <i>given</i> 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.&nbsp;24). He expressed indeed a pious hope (p.&nbsp;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 <i>Bericht</i>, applied to the rather wild
+theorisings of the physiologist Ludwig the same clear commonsense
+criticism that he bestowed on the other "atomists."</p>
+
+<p>It would take too long to describe the great development that
+materialistic physiology took at this time, and to show how the
+separation of morphology from physiology, which originally took
+place away back in the 17th century, had by this time become almost
+absolute. The years towards the end of the first half of the
+century marked indeed the beginning of the classical period as well
+of physiology as of dogmatic materialism. Moleschott and Buchner
+popularised materialism in Germany in the 'fifties, while Ludwig,
+du Bois <span class="pagenum"><a name="pg195" id=
+"pg195">195</a></span>Reymond and von Helmholtz began to apply the
+methods of physics to physiology. In France, Claude Bernard was at
+the height of his activity, rivalled by workers almost as great.
+The doctrine of the conservation of energy was established about
+this same time.</p>
+
+<p>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 <i>Rapport sur les Progr&egrave;s r&eacute;cents
+des Sciences zoologiques en France</i> (Paris) 1867, how "About the
+year 1826, two young naturalists, formed in the schools of Cuvier,
+Geoffroy and Majendie, considered that zoology, after having been
+purely descriptive or systematic and then anatomical, ought to take
+on a more physiological character; they considered that it was not
+enough to observe living objects in the repose of death, and that
+it was desirable to get to understand the organism in action,
+especially when the structure of these animals was so different
+from that of man that the notions acquired as to the special
+physiology of man could not properly be applied to them" (p.&nbsp;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
+<i>Recherches pour servir &agrave; l'histoire naturelle du littoral
+de la France</i>. 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.<a
+name="FNanchor_300" id="FNanchor_300" /><a href=
+"#Footnote_300" class="fnanchor">[300]</a> Note that though they
+called themselves <span class="pagenum"><a name="pg196" id=
+"pg196">196</a></span>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<a name="FNanchor_301" id="FNanchor_301" /><a href=
+"#Footnote_301" class="fnanchor">[301]</a> in the restricted sense,
+but physiological morphology. As a matter of fact they produced
+more taxanomic and anatomical work than work on physiological
+morphology, but this was only natural, since such a wealth of new
+forms was disclosed to their gaze. Milne-Edwards' masterly
+<i>Histoire Naturelle des Crustac&eacute;s</i><a name=
+"FNanchor_302" id="FNanchor_302" /><a href="#Footnote_302"
+class="fnanchor">[302]</a> and A. de Quatrefage's <i>Histoire
+Naturelle des Annel&eacute;s marins et d'eau douce</i><a name=
+"FNanchor_303" id="FNanchor_303" /><a href="#Footnote_303"
+class="fnanchor">[303]</a> were typical products of their
+activity.</p>
+
+<p>In the North, men like Sars and Lov&eacute;n 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&uuml;ller was using the tow-net to gather material for his
+masterly papers on the metamorphoses of Echinoderms.<a name=
+"FNanchor_304" id="FNanchor_304" /><a href="#Footnote_304"
+class="fnanchor">[304]</a> Work on the taxonomy and anatomy of marine
+animals was in general in full swing by the 'fifties and
+'sixties.</p>
+
+<p>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.</p>
+
+<p>Milne-Edwards' theoretical views, as expounded in his
+<i>Introduction &agrave; la zoologie g&eacute;n&eacute;rale</i>
+(1851), well reflect this Cuvierian attitude.<a name=
+"FNanchor_305" id="FNanchor_305" /><a href="#Footnote_305"
+class="fnanchor">[305]</a> He acknowledges himself the debt he <span
+class="pagenum"><a name="pg197" id="pg197">197</a></span>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."</p>
+
+<p>Milne-Edwards frankly takes up the teleological standpoint, and
+interprets organic forms on the assumption that they are purposive
+and rationally constructed. "To arrive at an understanding of the
+harmony of the organic creation," he writes, "it seemed to me that
+it would be well to accept the hypothesis that Nature has gone
+about her work as we would do ourselves according to the light of
+our own intelligence, if it were given us to produce a similar
+result. Comparing and studying living things as if they were
+machines created by the industry of man, I have tried to grasp the
+manner in which they might have been invented, and the principles
+whose application would have led to the production of such an
+assemblage of diversified instruments" (p.&nbsp;435). The problem is to
+discover the laws which rule the diversity of organic forms. The
+first and most obvious of these laws is the "law of economy," or
+the law of unity of type. Nature, as Cuvier pointed out, has not
+had recourse to all the possible forms and combinations of organs;
+she appears to work with a limited number of types and to get the
+greatest possible diversity out of these by varying the proportions
+of the constitutive materials of structure. Within the limits of
+each type Nature has brought about diversity by raising her
+creatures to different degrees of perfection. This is the second
+law of organic form, and it is this law that Milne-Edwards chiefly
+elaborates. Degrees of perfection mean for him, as for Aristotle,
+primarily degrees of perfection of function, but since structure is
+necessarily in close relation with function, perfection of function
+brings in its train increased perfection of organisation. This can
+only be attained by a division of labour<a name="FNanchor_306"
+id="FNanchor_306" /><a href="#Footnote_306" class=
+"fnanchor">[306]</a> <span class="pagenum"><a name="pg198" id=
+"pg198">198</a></span>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.</p>
+
+<p>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.&nbsp;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).</p>
+
+<p>Nature, holding fast to the law of economy, does not even always
+create a new organ for a new function; she may simply adapt an
+undifferentiated part to special functions, or she may even convert
+to other uses an organ already specialised (p.&nbsp;464). So, for
+example, the function of respiration is in the lowest animals
+diffused indifferently over the whole surface of the body, and only
+as organisation advances is it localised in special organs, such as
+gills. Now <span class="pagenum"><a name="pg199" id=
+"pg199">199</a></span>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 (<i>e.g.</i>, in <i>Anabas scandens</i>), 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.&nbsp;475).</p>
+
+<p>But whatever means Nature adopts, her aim is always the
+same&mdash;to specialise, to differentiate, to produce diversity
+from uniformity.</p>
+
+<p>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&mdash;to life in air or water, or on land&mdash;and
+many of their most striking differences are due to this cause. But
+adaptation may also act in reducing diversity, for there
+necessarily occur many instances of parallel adaptation or
+convergence. So we get the extraordinary parallelism between the
+families of marsupials and the orders of placentals,<a name=
+"FNanchor_307" id="FNanchor_307" /><a href="#Footnote_307"
+class="fnanchor">[307]</a> the remarkable similarity between the
+respiratory organs of land-crabs and air-breathing fish&mdash;to
+mention only two out of an immense range of analogous facts.</p>
+
+<p>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.&nbsp;479). So many riddles that, a little later on,
+stimulated the ingenuity of the evolutionists!</p>
+
+<p>Such, then, were the factors which Milne-Edwards <span class=
+"pagenum"><a name="pg200" id="pg200">200</a></span>considered
+adequate to explain the rich variety of animal forms. We cannot do
+better than quote his own summary of his doctrine:&mdash;"To sum
+up, then, the great differences introduced by Nature into the
+constitution of animals seem to depend essentially upon the
+existence of a certain number of general plans or distinct types,
+upon the perfecting in various degrees either of the whole or of
+parts of each of these structural plans, upon the adaptation of
+each type to varied conditions of existence, and upon the secondary
+imitation of foreign types by certain derivatives of each
+particular type" (p.&nbsp;480).</p>
+
+<p>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.<a name="FNanchor_308" id="FNanchor_308" /><a
+href="#Footnote_308" class="fnanchor">[308]</a> So, too, Johannes
+M&uuml;ller, 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.<a name="FNanchor_309" id=
+"FNanchor_309" /><a href="#Footnote_309" class=
+"fnanchor">[309]</a></p>
+
+<p>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.<a name="FNanchor_310" id=
+"FNanchor_310" /><a href="#Footnote_310" class=
+"fnanchor">[310]</a></p>
+
+<p>Not many years after the publication of Milne-Edwards'
+<i>Introduction &agrave; la zoologie g&eacute;n&eacute;rale</i>
+(1851) there appeared a book by H. G. Bronn in which was offered a
+very similar analysis of organic diversity. The curious thing was
+that <span class="pagenum"><a name="pg201" id=
+"pg201">201</a></span>Bronn approached the problem from quite a
+different standpoint, from the standpoint, indeed, of
+<i>Naturphilosophie</i>. Of this the title of the book is itself
+sufficient proof&mdash;<i>Morphologische Studien &uuml;ber die
+Gestaltungs-gesetze der Naturk&ouml;rper &uuml;berhaupt und der
+organischen insbesondere</i> (Leipzig and Heidelberg, 1858).<a
+name="FNanchor_311" id="FNanchor_311" /><a href=
+"#Footnote_311" class="fnanchor">[311]</a> The linking up of
+organic with inorganic form is characteristic; there is much talk,
+too, in the book of <i>Urstoffe</i> and <i>Urkr&auml;fte</i>, but
+underlying the <i>Naturphilosophie</i> we can trace the same
+Cuvierian treatment of form, and see crystallise out laws of
+progressive development that bear no small analogy with the laws
+established by Milne-Edwards.</p>
+
+<p>According to Bronn, the ideal fundamental form of the plant is
+an ovoid or strobiloid<a name="FNanchor_312" id=
+"FNanchor_312" /><a href="#Footnote_312" class=
+"fnanchor">[312]</a> body, for a plant reaches out in two directions
+in search of food&mdash;towards the sun and towards the earth.
+Animals differ from plants in being endowed with sensation and
+mobility (<i>cf.</i> Aristotle and Cuvier), and it is this
+characteristic that gives them their distinctive form. The main
+types of animal form&mdash;the Amorphozoa, Actinozoa, and
+Hemisphenozoa&mdash;are essentially adaptations to particular modes
+of locomotion. Animals either are fixed, or they move in all
+directions without reference to any definite axis, or they move in
+one main direction.</p>
+
+<p>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 C&oelig;lentera,
+which are fixed, and the Echinoderms, which have a central point
+and move indifferently along any radial axis; their form differs
+from the strobiloid mainly in having radiate rather than spiral
+symmetry. The Hemisphenozoa, or bilaterally symmetrical animals,
+include all those that habitually move forward; they have a front
+end and a hind end, a dorsal surface and a ventral, and the mouth,
+sense-organs and "brain" are concentrated <span class="pagenum"><a
+name="pg202" id="pg202">202</a></span>in the front end to form a
+head&mdash;all in direct adaptation to this forward movement; they
+make up the vast majority of animals.</p>
+
+<p>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 (<i>cf.</i> Cuvier). The second factor in the diversity of
+organic form is the action of certain laws of progressive
+development<a name="FNanchor_313" id="FNanchor_313" /><a href=
+"#Footnote_313" class="fnanchor">[313]</a>
+(<i>Entwickelungsgesetze</i>), which bear the same relation to the
+development of the animal kingdom as the laws of individual
+development bear to the development of the embryo, for organs
+appear in the different animal series in much the same order and
+manner as they develop in the individual. These laws are (1)
+progressive differentiation of functions and organs; (2) numerical
+reduction of serially repeated parts; (3) concentration of
+functions and their organs in particular parts of the body; (4)
+centralisation of organ-systems and parts of such, so that they
+come to depend upon one central organ; (5) internalisation of the
+"noblest" organs, unless these are necessarily external, and (6)
+increase in size of the whole or of parts. Of these the law of
+differentiation is by far the most important, and most of the
+others are in a sense merely special cases of this fundamental law.
+To this law of differentiation is due the increase in complexity or
+perfection of organisation which is shown by all the animal series.
+Bronn himself recognised the great similarity of this law of
+progressive differentiation to Milne-Edwards' principle of the
+division of labour; he seems, however, to have arrived at it
+independently.</p>
+
+<p><span class="pagenum"><a name="pg203" id=
+"pg203">203</a></span>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:&mdash;"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, medus&aelig;,
+etc.)" (p.&nbsp;141).</p>
+
+<p>It was characteristic of the 'forties and 'fifties that
+transcendental anatomy, along with Nature-philosophy, went rather
+out of fashion, its false simplicities and premature
+generalisations being overwhelmed by the flood of new discoveries.
+A few stalwarts indeed upheld transcendental views. We have already
+discussed the morphological system built up by Richard Owen in the
+late 'forties, a system transcendental in its main lines. We have
+seen the vertebral theory of the skull still maintained in the
+'fifties by such men as Reichert and K&ouml;lliker, and we find
+J. V.. Carus in 1853<a name="FNanchor_314" id="FNanchor_314" /><a
+href="#Footnote_314" class="fnanchor">[314]</a> taking it as almost
+conclusively proved.<a name="FNanchor_315" id=
+"FNanchor_315" /><a href="#Footnote_315" class=
+"fnanchor">[315]</a></p>
+
+<p>We may mention, too, as showing clear marks of the influence of
+transcendental ideas, L. Agassiz's work on the principles of
+classification.<a name="FNanchor_316" id="FNanchor_316" /><a
+href="#Footnote_316" class="fnanchor">[316]</a> And Serres, who was
+Geoffroy's <span class="pagenum"><a name="pg204" id=
+"pg204">204</a></span>chief disciple, recanted not a whit of his
+doctrine of recapitulation, but re-affirmed and expanded it from
+time to time, and particularly in a lengthy memoir published in
+1860.<a name="FNanchor_317" id="FNanchor_317" /><a href=
+"#Footnote_317" class="fnanchor">[317]</a> 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)&mdash;"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" (<i>loc. cit.</i>, p.&nbsp;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,<a name="FNanchor_318" id=
+"FNanchor_318" /><a href="#Footnote_318" class=
+"fnanchor">[318]</a> he set himself the task of reducing all their
+complex forms to one type. In summing up, he writes:&mdash;"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.&nbsp;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" (<i>Sci. Memoirs</i>, vol.
+i., p.&nbsp;571).</p>
+
+<p>It is instructive to find that between Serres and Milne-Edwards
+there existed the same antagonism as between von <span class=
+"pagenum"><a name="pg205" id="pg205">205</a></span>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,<a name="FNanchor_319" id=
+"FNanchor_319" /><a href="#Footnote_319" class=
+"fnanchor">[319]</a> had come to the conclusion, independently of von
+Baer, that development always proceeded from the general to the
+special; that class characters appeared before family characters,
+generic characters before specific. In an interesting paper
+published in 1844,<a name="FNanchor_320" id="FNanchor_320" /><a
+href="#Footnote_320" class="fnanchor">[320]</a> 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.</p>
+
+<p>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&mdash;of which there are several&mdash;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.</p>
+
+<p>These views were further developed in his volume of 1851, and
+also in his <i>Rapport</i> of 1867.</p>
+
+<p>They brought him into conflict with his confrere in the Academy
+of Sciences, &Eacute;tienne Serres, who in a number of papers
+published in the 'thirties and 'forties,<a name="FNanchor_321"
+id="FNanchor_321" /><a href="#Footnote_321" class=
+"fnanchor">[321]</a> 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.&nbsp;141). His views have become a little more
+complicated since his first exposition of them in 1827, <span
+class="pagenum"><a name="pg206" id="pg206">206</a></span>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&mdash;the absolute unity of plan of all animals,
+vertebrate and invertebrate alike,<a name="FNanchor_322" id=
+"FNanchor_322" /><a href="#Footnote_322" class=
+"fnanchor">[322]</a> the gradual perfecting of organisation from
+monad to man, the repetition in the embryogeny of the higher
+animals of the "zoogeny" of the lower.</p>
+
+<p>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.&nbsp;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.&nbsp;833).<a name="FNanchor_323"
+id="FNanchor_323" /><a href="#Footnote_323" class=
+"fnanchor">[323]</a></p>
+
+<p>To settle the dispute pending between two of its most
+illustrious members, the Academy proposed in 1853, as the subject
+of one of its prizes, "the positive determination of the
+resemblances and differences in the comparative development of
+Vertebrates and Invertebrates." A memoir was presented the next
+year by Lereboullet<a name="FNanchor_324" id=
+"FNanchor_324" /><a href="#Footnote_324" class=
+"fnanchor">[324]</a> which met with the approval of the Academy in so
+far as its statements of fact were concerned, but seemed to them to
+require amplification <span class="pagenum"><a name="pg207" id=
+"pg207">207</a></span>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.&nbsp;79). To fulfil the Academy's requirements, Lereboullet
+continued his work, and in 1861-63 he published a series of
+elaborate monographs<a name="FNanchor_325" id=
+"FNanchor_325" /><a href="#Footnote_325" class=
+"fnanchor">[325]</a> on the embryology of the trout, the lizard and
+the pond-snail <i>Lymn&aelig;a</i>, and rounded off his work with a
+full discussion<a name="FNanchor_326" id="FNanchor_326" /><a
+href="#Footnote_326" class="fnanchor">[326]</a> of the theoretical
+questions involved. In this considered and authoritative judgment
+he completely disposed of Serres' theories of the unity of plan and
+the unity of genetic formation. Except in the very earliest stages
+of oogenesis there is no real similarity between the development of
+a Zoophyte, a Mollusc, an Articulate and a Vertebrate, but each is
+stamped from the beginning with the characteristics of its type.
+The lower animals are not, and cannot possibly be the permanent
+embryos of the higher animals. "The results which I have obtained,"
+he writes, "are diametrically opposed to the theory of the
+zoological series constituted by stages of increasing perfection, a
+theory which tries to demonstrate in the embryonic phases of the
+higher animals a repetition of the forms which characterise the
+lower animals, and which has led to the assertion that the latter
+are permanent embryos of the former. The embryo of a Vertebrate
+shows the vertebrate type from the very beginning, and retains this
+type throughout the whole course of its development; it never is,
+and never can be, either a Mollusc or an Articulate" (xx., p.
+54).</p>
+
+<p>"We are led to establish ... as the general result of our
+researches, the existence of several types, and, consequently, of
+different plans, in the development of animals. These different
+types are manifested from the very beginning of embryonic life; the
+characters distinguishing them are therefore <span class=
+"pagenum"><a name="pg208" id="pg208">208</a></span>primordial, and
+we can say with M. Milne-Edwards that <i>everything goes to prove
+that the distinction established by Nature between animals
+belonging to different phyla is a primordial distinction</i>" (p.
+58).</p>
+
+<p>In other directions also von Baer's work was confirmed and
+extended by later observers&mdash;those parts of it particularly
+that had reference to the germ-layer theory, and to the concept of
+histological differentiation. His germ-layer theory was accepted in
+its main lines by Rathke, Bischoff and Lereboullet, and applied by
+them to the multitude of new facts they discovered. Rathke, in
+particular, was a firm upholder of the doctrine, and made
+considerable use of it in his writings.<a name="FNanchor_327" id=
+"FNanchor_327" /><a href="#Footnote_327" class=
+"fnanchor">[327]</a> 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 <i>Astacus</i>, whereby there are formed a
+serous and a mucous layer, one inside the other&mdash;like the
+coats of an onion, to use his own expressive phrase.<a name=
+"FNanchor_328" id="FNanchor_328" /><a href="#Footnote_328"
+class="fnanchor">[328]</a></p>
+
+<p>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 C&oelig;lentera with the serous and mucous layers of
+the vertebrate germ.<a name="FNanchor_329" id=
+"FNanchor_329" /><a href="#Footnote_329" class=
+"fnanchor">[329]</a> 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 Medus&aelig; with the corresponding organs in the
+rest of the series; and it is curious to remark, that throughout,
+the outer and inner membranes appear to bear the same physiological
+relation to one another as do the serous and mucous layers of the
+germ; the outer becoming developed into the muscular system, and
+giving rise to the organs of offence and defence; the inner, on the
+other hand, appearing to be more closely subservient to the
+purposes of nutrition and generation" (p.&nbsp;24). Von Baer had already
+hinted at this homology <span class="pagenum"><a name="pg209" id=
+"pg209">209</a></span>in the second volume of his
+<i>Entwickelungsgeschichte</i> (1837), where he says with reference
+to the separation of the blastoderm of the chick into two layers.
+"Yet originally there are not two distinct or even separable
+layers, it is rather the two surfaces of the germ which show this
+differentiation, just as polyps show the same contrast of an
+external surface and an internal digestive surface. In between the
+two layers there is in our germ as in the polyp an indifferent
+mass" (p.&nbsp;67). The terms ectoderm and entoderm were introduced by
+Allman<a name="FNanchor_330" id="FNanchor_330" /><a href=
+"#Footnote_330" class="fnanchor">[330]</a> in 1853 for the two
+cell-layers in the Hydrozoa.</p>
+
+<p>Remak is the second great name in the history of the germ-layer
+theory. He had the great advantage over von Baer of being able to
+make use of the cell-theory in interpreting the formation of the
+germ-layers. Microscopical technique also had been greatly improved
+since 1828.<a name="FNanchor_331" id="FNanchor_331" /><a href=
+"#Footnote_331" class="fnanchor">[331]</a></p>
+
+<p>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.</p>
+
+<p>There are in the unincubated blastoderm of the chick, according
+to Remak,<a name="FNanchor_332" id="FNanchor_332" /><a href=
+"#Footnote_332" class="fnanchor">[332]</a> two cell-layers, of
+which the undermost <span class="pagenum"><a name="pg210" id=
+"pg210">210</a></span>subsequently splits into two. Three layers
+are thus formed&mdash;the upper, middle and lower. The upper layer
+differentiates into a medullary plate and an epidermic plate
+(Remak's <i>Hornblatt</i>), and gives origin to the medullary tube
+with all its evaginations, and to the skin with all its derivatives
+and pockets. It forms such diverse structures as the brain, the
+spinal cord, the eye, the ear, the mouth, hairs, feathers, nails,
+sweat-glands, lacrymal glands, and so forth. All these parts are
+connected directly or indirectly with sensation, and the upper
+germ-layer may accordingly be called the <i>sensory</i> 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 <i>trophic</i> 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 <i>Hornblatt</i> of the upper layer&mdash;both
+consist of epithelial tissue, and of its derivative, glandular
+tissue, and form neither vessels nor nerves. The middle layer, on
+the contrary, forms nerves and muscles, vessels and connective
+tissue, and little or no epithelium. It does not form all the
+blood-vessels without exception (and so cannot be called the
+vessel-layer), for the blood-vessels of the central nervous system
+are in all probability formed from the upper layer. So, too, it
+does not form all the nerves and muscles&mdash;the optic and
+auditory nerves and the nerves and muscles of the iris probably
+arise in the upper layer. But, in spite of these exceptions, its
+general histological character is so well defined that it may be
+contrasted with the other two as preeminently the layer that forms
+muscular, nervous, vascular and connective tissue. In view of its
+functional significance, it may be called the <i>motory</i> layer,
+or better, since it forms also the sexual glands, the
+<i>motor-germinative</i> layer. The middle layer, early in its
+history, shows a division into dorsal plates
+(<i>Urwirbelplatten</i>) and ventral plates (<i>Seitenplatten</i>).
+The former exhibit almost as soon as they are <span class=
+"pagenum"><a name="pg211" id="pg211">211</a></span>formed the
+characteristic proto-vertebral segmentation, the latter split to
+form the pleuro-peritoneal or body-cavity. Remak describes the
+latter process as follows:&mdash;"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
+<i>Hautplatten</i>), the muscular part of the alimentary tube (the
+<i>Darmfaserplatten</i>), and the mother-tissue of the generative
+organs (the <i>Mittelplatten</i>). From the <i>Hautplatten</i>
+there develops, without the dorsal plates seeming to take any part
+in the process, the rudiment of the extremities" (p.&nbsp;79).</p>
+
+<p>His <i>Darmfaserplatten</i> form the nervous and muscular tissue
+of the alimentary canal and its dependencies, and also the heart;
+the <i>Hautplatten</i> 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.</p>
+
+<p>Remak's conception of the relations of the three germ-layers to
+one another and to the body-cavity is well illustrated in Fig.&nbsp;12.</p>
+
+<div class="figcenter"><a href="images/fig12a.jpg"><img src="images/fig12a-tb.jpg" alt="Transverse Section of Chick Embryo. (After Remak.)" title="" /></a></div>
+
+
+<p class="center2"><span class="smcap">Fig.</span>
+12.&mdash;Transverse Section of Chick Embryo. (After Remak.)</p>
+
+<table width="90%" summary=
+"Transverse Section of Chick Embryo. (After Remak.)" border="0"
+cellpadding="2" cellspacing="2">
+<tbody>
+<tr>
+<td class="cell_a"><i>h.</i></td>
+<td class="cell_b">Epidermis.</td>
+<td class="cell_a"><i>hp.</i></td>
+<td class="cell_a2" rowspan="3"><img src="images/para3.jpg" height=
+"100" alt="para" /></td>
+<td class="cell_c" rowspan="3">"Hautplatte"</td>
+<td class="cell_a"><i>x.</i></td>
+<td class="cell_b">Edge of the smniotic fold.</td>
+</tr>
+
+<tr>
+<td class="cell_a"><i>m.</i></td>
+<td class="cell_b">Spinal cord.</td>
+<td class="cell_a">and</td>
+<td class="cell_a"><i>ph.</i></td>
+<td class="cell_b">Pleuro-Peritonial cavity.</td>
+</tr>
+
+<tr>
+<td class="cell_a"><i>mu.</i></td>
+<td class="cell_b">Dorsal plate.</td>
+<td class="cell_a"><i>um.</i></td>
+<td></td>
+<td></td>
+</tr>
+
+<tr>
+<td class="cell_a"><i>ug.</i></td>
+<td class="cell_b">Pronephric duct.</td>
+<td class="cell_a"><i>mp.</i></td>
+<td class="cell_b" colspan="2">"Mittelplatte"</td>
+<td class="cell_a"><i>d.</i></td>
+<td class="cell_b">Epithelium of alimentary canal.</td>
+</tr>
+
+<tr>
+<td class="cell_a"><i>pa.</i></td>
+<td class="cell_b">Aortic root.</td>
+<td class="cell_a"><i>df.</i></td>
+<td class="cell_b" colspan="2">"Darmfaser platte."</td>
+<td></td>
+<td></td>
+</tr>
+</tbody>
+</table>
+
+<p>In his germ-layer theory Remak's standpoint is histological
+<span class="pagenum"><a name="pg212" id=
+"pg212">212</a></span>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.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<div class="footnote">
+<p><a name="Footnote_293" id="Footnote_293" /><a href=
+"#FNanchor_293"><span class="label">[293]</span></a> <i>Embryologie
+des Salmones</i>, 1842.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_294" id="Footnote_294" /><a href=
+"#FNanchor_294"><span class="label">[294]</span></a> <i>Die
+Cellularpathologie in ihrer Begr&uuml;ndung auf physiologische und
+pathologische Gewebelehre</i>, Berlin, 2nd ed. 1859; Eng. trans.,
+by Chance, 1860.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_295" id="Footnote_295" /><a href=
+"#FNanchor_295"><span class="label">[295]</span></a> <i>Arch. path.
+Anat. Phys</i>., vii., pp.&nbsp;1-39 (1854).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_296" id="Footnote_296" /><a href=
+"#FNanchor_296"><span class="label">[296]</span></a> <i>Bericht
+&uuml;ber die Fortschritte der mikroskopischen Anatomie im jahre
+1854.</i> M&uuml;ller's <i>Archiv</i>, 1855. See also 1856.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_297" id="Footnote_297" /><a href=
+"#FNanchor_297"><span class="label">[297]</span></a> <i>Hndb. d.
+Physiol.</i>, i., 1835.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_298" id="Footnote_298" /><a href=
+"#FNanchor_298"><span class="label">[298]</span></a> See Leuckart's
+reply to Ludwig's criticism, in <i>Zeit. f. wiss. Zool.</i>, ii.,
+p.&nbsp;271, 1850.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_299" id="Footnote_299" /><a href=
+"#FNanchor_299"><span class="label">[299]</span></a> Leipzig,
+1853.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_300" id="Footnote_300" /><a href=
+"#FNanchor_300"><span class="label">[300]</span></a> <i>Souvenirs
+d'un Naturaliste</i>, 2 vols., Paris, 1854. Eng. Trans. as
+<i>Rambles of a Naturalist on the Coasts of France, Spain, and
+Italy</i>, 2 vols., 1857.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_301" id="Footnote_301" /><a href=
+"#FNanchor_301"><span class="label">[301]</span></a> Milne-Edwards
+later published a classical textbook on comparative anatomy and
+physiology&mdash;<i>Le&ccedil;ons sur la Physiologie et l'Anatomie
+compar&eacute;es</i>, 14 vols., Paris, 1857-80.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_302" id="Footnote_302" /><a href=
+"#FNanchor_302"><span class="label">[302]</span></a> Paris,
+1834-40. Three volumes of the <i>Suites &agrave; Buffon</i>.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_303" id="Footnote_303" /><a href=
+"#FNanchor_303"><span class="label">[303]</span></a> Paris, 1865.
+Two volumes of the <i>Suites &agrave; Buffon</i>.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_304" id="Footnote_304" /><a href=
+"#FNanchor_304"><span class="label">[304]</span></a> <i>U. d.
+Metamorphose der Ophiuren u. Seeigel.</i>, Berlin, 1848. <i>U. d.
+Metamorphose der Holothurien u. Asterien</i>., Berlin, 1851.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_305" id="Footnote_305" /><a href=
+"#FNanchor_305"><span class="label">[305]</span></a> As I have been
+unable to obtain a copy of the <i>Introduction</i>, the passages
+which follow are taken from the <i>Rapport</i> of 1867, where
+Milne-Edwards gives a complete exposition of his doctrine,
+sometimes in the words of the original.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_306" id="Footnote_306" /><a href=
+"#FNanchor_306"><span class="label">[306]</span></a> This principle
+was first developed by Milne-Edwards in 1827, in the
+<i>Dictionnaire classique d'Hist. naturelle</i>. It was probably
+suggested to him by his studies on the Crustacea, among which the
+principle is so beautifully exemplified in the concentration and
+specialisation of the appendages and the ganglionic chain.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_307" id="Footnote_307" /><a href=
+"#FNanchor_307"><span class="label">[307]</span></a> Studied by
+Isidore Geoffroy St Hilaire in his paper <i>Classification
+parall&eacute;lique des Mammif&egrave;res, C. R. Acad. Sci.</i>,
+xx., 1845. Remarked upon by Cuvier, <i>R&egrave;gne animal</i>.,
+i., p.&nbsp;171, 1817, also by de Blainville.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_308" id="Footnote_308" /><a href=
+"#FNanchor_308"><span class="label">[308]</span></a> Cuvier et
+Valenciennes, <i>Hist. nat. des Poissons</i>, i., p.&nbsp;550, 1828.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_309" id="Footnote_309" /><a href=
+"#FNanchor_309"><span class="label">[309]</span></a>
+<i>Myxinoiden</i>, Th. I. <i>Abh. k. Akad. Wiss. Berlin</i> for
+1834, pp.&nbsp;100, 110, 179, etc.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_310" id="Footnote_310" /><a href=
+"#FNanchor_310"><span class="label">[310]</span></a> <i>Vergl.
+Entw. Kopf. nackt. Amphibien</i>, p.&nbsp;101, 1838.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_311" id="Footnote_311" /><a href=
+"#FNanchor_311"><span class="label">[311]</span></a> I have not
+seen the companion volume on pal&aelig;ontological progression,
+<i>Unters. &uuml;. d. Entwickelungsgesetze der organischen Welt
+w&auml;hrend der Bildungszeit unserer Erdoberfl&auml;che</i>,
+Stuttgart, 1858.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_312" id="Footnote_312" /><a href=
+"#FNanchor_312"><span class="label">[312]</span></a> "Strobiloid"
+because of its spiral development. The theory of the spiral growth
+of plants played an important part in botanical morphology about
+this time.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_313" id="Footnote_313" /><a href=
+"#FNanchor_313"><span class="label">[313]</span></a> <i>Cf.</i>
+Meckel's Principle of progressive Evolution, <i>supra</i>, p.
+93.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_314" id="Footnote_314" /><a href=
+"#FNanchor_314"><span class="label">[314]</span></a> <i>System der
+thierischen Morphologie</i>, pp.&nbsp;33, 457. Also C. Bruch, <i>Die
+Wirbeltheorie des Sch&auml;dels, am Skelette des Lachses
+gepr&uuml;ft</i>, Frankfort-on-Main, 1862.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_315" id="Footnote_315" /><a href=
+"#FNanchor_315"><span class="label">[315]</span></a> In France the
+vertebral theory was advocated by Lavocat in his <i>Nouvelle
+Ost&eacute;ologie compar&eacute;e de la t&ecirc;te des animaux
+domestiques</i>, Toulouse, 1864. It seems also that Lacaze-Duthiers
+held fast to it even in 1872&mdash;<i>Arch. zool. exp.
+g&eacute;n.</i>, i., p.&nbsp;51, 1872.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_316" id="Footnote_316" /><a href=
+"#FNanchor_316"><span class="label">[316]</span></a> <i>An Essay on
+Classification</i>, 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.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_317" id="Footnote_317" /><a href=
+"#FNanchor_317"><span class="label">[317]</span></a> "Principes
+d'Embryog&eacute;nie, de Zoog&eacute;nie et de Teratog&eacute;nie,"
+<i>M&eacute;m. Acad. Sci.</i>, xxv., pp.&nbsp;1-943, pls. xxv.,
+1860.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_318" id="Footnote_318" /><a href=
+"#FNanchor_318"><span class="label">[318]</span></a> "On the
+Morphology of the Cephalous Mollusca," <i>Phil. Trans.</i>, 1853,
+<i>Sci. Memoirs</i>, i., pp.&nbsp;152-92.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_319" id="Footnote_319" /><a href=
+"#FNanchor_319"><span class="label">[319]</span></a> "Observations
+sur les changements de forme que les divers Crustac&eacute;s
+&eacute;prouvent," <i>Ann. Sci. nat.</i> (1) xxx., p.&nbsp;360,
+1833.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_320" id="Footnote_320" /><a href=
+"#FNanchor_320"><span class="label">[320]</span></a>
+"Consid&eacute;rations sur quelques principes relatifs &agrave; la
+classification naturelle des animaux," <i>Ann. Sci. nat.</i> (3)
+i., p.&nbsp;65, 1844.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_321" id="Footnote_321" /><a href=
+"#FNanchor_321"><span class="label">[321]</span></a> <i>Supra</i>,
+pp.&nbsp;79-83. Also <i>Pr&eacute;cis d'anatomie transcendante,
+principes d'organog&eacute;nie</i>, Paris, 1842.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_322" id="Footnote_322" /><a href=
+"#FNanchor_322"><span class="label">[322]</span></a> 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.&nbsp;821-6).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_323" id="Footnote_323" /><a href=
+"#FNanchor_323"><span class="label">[323]</span></a> It is worth
+while recording that Serres enunciated a "law of symmetry"
+according to which the embryo is formed by the union of its two
+symmetrical halves&mdash;a law which recalls the "concrescence
+theory" of His and some modern embryologists.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_324" id="Footnote_324" /><a href=
+"#FNanchor_324"><span class="label">[324]</span></a> "Embryologie
+compar&eacute;e du Brochet, de la Perche, et de
+l'Ecr&eacute;visse," <i>Ann. Sci. nat.</i> (4), i., p.&nbsp;237, 1854;
+ii., p.&nbsp;39, 1854. <i>M&eacute;m. Savans etrangers</i>, xvii.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_325" id="Footnote_325" /><a href=
+"#FNanchor_325"><span class="label">[325]</span></a> <i>Ann. Sci.
+nat.</i> (4) xvi., p.&nbsp;113, 1861; xvii., p.&nbsp;88, 1862; xviii., p.&nbsp;5,
+1862; xix., p.&nbsp;5, 1863.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_326" id="Footnote_326" /><a href=
+"#FNanchor_326"><span class="label">[326]</span></a> xx., p.&nbsp;5,
+1863.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_327" id="Footnote_327" /><a href=
+"#FNanchor_327"><span class="label">[327]</span></a> Particularly
+in his <i>Blennius</i> (1833) and <i>Natter</i> (1839).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_328" id="Footnote_328" /><a href=
+"#FNanchor_328"><span class="label">[328]</span></a> In the
+"preliminary notice" of his Crayfish paper&mdash;<i>Isis</i>, pp
+1093-1100, 1825.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_329" id="Footnote_329" /><a href=
+"#FNanchor_329"><span class="label">[329]</span></a> "On the
+Anatomy and the Affinities of the Family of the Medus&aelig;,"
+<i>Phil. Trans.</i>, 1849; <i>Sci. Memoirs</i>, i., pp.&nbsp;9-32.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_330" id="Footnote_330" /><a href=
+"#FNanchor_330"><span class="label">[330]</span></a> <i>Phil.
+Trans.</i>, cxliii., p.&nbsp;368, 1853.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_331" id="Footnote_331" /><a href=
+"#FNanchor_331"><span class="label">[331]</span></a> The principle
+of achromatism was discovered (by Fraunhofer) and achromatic
+microscopes introduced in the early part of the 19th century. The
+use of chemical reagents, such as acetic acid, and various
+hardening fluids, came into fashion not long after. J. M&uuml;ller
+seems to have been one of the first to realise their importance.
+Remak himself invented one or two fixing and hardening mixtures
+(pp.&nbsp;87, 127, 1855), which enabled him to cut excellent hand
+sections. Section-cutting machines were not invented till later (V.
+Hensen, 1866, His, 1870).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_332" id="Footnote_332" /><a href=
+"#FNanchor_332"><span class="label">[332]</span></a>
+<i>Untersuchungen &uuml;ber die Entwickelung der Wirbelthiere</i>,
+folio, pp.&nbsp;xxxvii + 195, 12 plates, Berlin, 1850-1855.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg213" id=
+"pg213">213</a></span></p>
+
+<h3>CHAPTER XIII</h3>
+
+<h4>THE RELATION OF LAMARCK AND DARWIN TO MORPHOLOGY.</h4>
+
+<p>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.</p>
+
+<p>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,<a name=
+"FNanchor_333" id="FNanchor_333" /><a href="#Footnote_333"
+class="fnanchor">[333]</a> 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,<a name=
+"FNanchor_334" id="FNanchor_334" /><a href="#Footnote_334"
+class="fnanchor">[334]</a> 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&mdash;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&mdash;allows a ray of hope, however faint, to
+penetrate into our minds, that here <span class="pagenum"><a name=
+"pg214" id="pg214">214</a></span>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&mdash;from those in which the principle of
+purposes seems to be best authenticated, <i>i.e.</i>, 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."<a name="FNanchor_335" id="FNanchor_335" /><a
+href="#Footnote_335" class="fnanchor">[335]</a></p>
+
+<p>So, too, Buffon's evolutionism was suggested by his study of the
+structural affinities of animals, and Erasmus Darwin in his
+<i>Zoonomia</i> (1794) brought forward as one of the strongest
+proofs of evolution, "the essential unity of plan in all
+warm-blooded animals."<a name="FNanchor_336" id=
+"FNanchor_336" /><a href="#Footnote_336" class=
+"fnanchor">[336]</a></p>
+
+<p>But, as a matter of historical fact, no morphologist, not even
+Geoffroy, deduced from the facts of his science any comprehensive
+theory of evolution. The pre-Darwinian morphologists were
+comparatively little influenced by the evolution-theories current
+in their day, and it was in the anatomist Cuvier and the
+embryologist von Baer that the early evolutionists found their most
+uncompromising opponents.</p>
+
+<p>Speaking generally, and excepting for the moment the theory of
+Lamarck, we may say that the evolution-theories of the 18th and
+19th centuries arose in connection with the transcendental notion
+of the <i>&Eacute;chelle des &ecirc;tres</i>, or scale of
+perfection. This notion, which plays so great a part in the
+philosophy of Leibniz, was very generally accepted about the middle
+of the 18th century, and received complete and even exaggerated
+expression from Bonnet and Robinet. Buffon also was influenced by
+it. Towards the beginning of the 19th century the idea was taken up
+eagerly by the transcendental school and by them given, in their
+theories of the <span class="pagenum"><a name="pg215" id=
+"pg215">215</a></span>"one animal," a more morphological turn.
+Their recapitulation theory was part and parcel of the same general
+idea.</p>
+
+<p>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.<a name=
+"FNanchor_337" id="FNanchor_337" /><a href="#Footnote_337"
+class="fnanchor">[337]</a> Many of them wavered between the
+conception of evolution as an ideal process, as a
+<i>Vorstellungsart</i>, and the conception of it as an historical
+process. Bonnet, Oken, and the majority of the transcendentalists
+seem to have chosen the former alternative; Robinet, Treviranus,
+Tiedemann, Meckel, and a few others held evolution to be a real
+process.</p>
+
+<p>We have already in previous chapters<a name="FNanchor_338" id=
+"FNanchor_338" /><a href="#Footnote_338" class=
+"fnanchor">[338]</a> 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.<a name="FNanchor_339" id="FNanchor_339" /><a href=
+"#Footnote_339" class="fnanchor">[339]</a> 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.</p>
+
+<p>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&mdash;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
+<i>&Eacute;chelle des &ecirc;tres</i> had for him much less a <span
+class="pagenum"><a name="pg216" id=
+"pg216">216</a></span>morphological orientation than it had even
+for the transcendentalists, for he was lacking almost completely in
+the sense for morphology. Lamarck's scientific, as distinguished
+from his speculative work, was exclusively systematic, and it was
+systematics of a very high order. He introduced many reforms into
+the general classification of animals. He was the first clearly to
+separate Crustacea (1799), and a little later (1800) Arachnids,
+from insects. He reduced to a certain orderliness the neglected
+tribes of the Invertebrates, and wrote what was for long the
+standard work on their systematics&mdash;the <i>Histoire naturelle
+des Animaux sans Vert&egrave;bres</i> (1816-22). His speculative
+work on biology is contained in three publications, the small book
+entitled <i>Consid&eacute;rations sur l'organisation des corps
+vivants</i> (1802), the larger work of 1809, the <i>Philosophie
+zoologique</i>, and the introductory matter to his <i>Animaux sans
+Vert&egrave;bres</i> (vol. i., 1816).</p>
+
+<p>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.</p>
+
+<p>In the first part of the <i>Philosophie zoologique</i> Lamarck
+is largely pre-occupied with the problem of whether species are
+really distinct, or do not rather grade insensibly into one
+another. As a systematist of vast experience Lamarck knew how
+difficult it is in practice to distinguish species from varieties.
+"The more," he writes, "we collect the productions of Nature, the
+richer our collections become, the more do we see almost all the
+gaps filled up and the lines of separation effaced. We find
+ourselves reduced to an arbitrary determination, which sometimes
+leads us to seize upon the slightest differences of varieties, and
+form from them the distinctive character of what we call a species,
+and at other times leads us to consider as a variety of a certain
+species individuals a little bit different, which others regard as
+forming a separate species."<a name="FNanchor_340" id=
+"FNanchor_340" /><a href="#Footnote_340" class=
+"fnanchor">[340]</a></p>
+
+<p>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.</p>
+
+<p>Lamarck is at great pains to show how arbitrary are our <span
+class="pagenum"><a name="pg217" id=
+"pg217">217</a></span>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.&nbsp;41).</p>
+
+<p>But there is a natural order in the animal kingdom, a
+progression from the simpler to the more complex organisations, a
+natural <i>&Eacute;chelle des &ecirc;tres</i>.</p>
+
+<p>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:&mdash;</p>
+
+<table summary="&Eacute;chelle des &ecirc;tres." width="90%"
+border="0" cellpadding="2">
+<tbody>
+<tr>
+<td class="cell_lt217">1. Infusoria.</td>
+<td class="cell_lt217">6. Arachnids.</td>
+<td class="cell_lt217">11. Fishes.</td>
+</tr>
+
+<tr>
+<td class="cell_lt217">2. Polyps.</td>
+<td class="cell_lt217">7. Crustacea.</td>
+<td class="cell_lt217">12. Reptiles.</td>
+</tr>
+
+<tr>
+<td class="cell_lt217">3. Radiates.</td>
+<td class="cell_lt217">8. Annelids.</td>
+<td class="cell_lt217">13. Birds.</td>
+</tr>
+
+<tr>
+<td class="cell_lt217">4. Worms.</td>
+<td class="cell_lt217">9. Cirripedes.</td>
+<td class="cell_lt217">14. Mammals.</td>
+</tr>
+
+<tr>
+<td class="cell_lt217">5. Insects.</td>
+<td class="cell_lt217">10. Molluscs.</td>
+<td class="cell_lt217">&nbsp;</td>
+</tr>
+</tbody>
+</table>
+
+<p>But the order of Nature is essentially continuous, and the
+limits of even the best defined of these classes are in reality
+artificial&mdash;"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.&nbsp;45).</p>
+
+<p>In the same way the lesser classificatory groups represent
+smaller sections of the one unique order of Nature. Note that
+Lamarck's <i>&Eacute;chelle</i> is in no way a morphological one,
+and was not intended to be such. It is a scale of increasing
+physiological differentiation, and the stages of it are marked by
+the acquirement of this or that new organ (<i>cf.</i> 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 <span class="pagenum"><a name="pg218" id=
+"pg218">218</a></span>which would permit of the greatest perfecting
+(that of the Vertebrates), a plan very different from those which
+she has been obliged to form as a preliminary to reaching it, one
+understands that, among the multitude of animals, one must
+necessarily come across not a single system of organisation which
+has become progressively perfected, but diverse very distinct
+systems, each of which has come into existence at the moment when
+each primary organ first put in its appearance" (p.&nbsp;171).</p>
+
+<p>For Lamarck this order of Nature was not merely
+ideal&mdash;Nature had actually formed the classes successively,
+proceeding from the simpler to the more complex; she had brought
+about this evolution by transforming the primitive species of
+animals, raising them to higher degrees of organisation, and
+modifying them in relation to the environment in which they found
+themselves.</p>
+
+<p>Lamarck's theory of evolution is worked out in great detail in
+his <i>Philosophie zoologique</i>, 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 <i>Histoire naturelle des Animaux sans
+Vert&egrave;bres</i>.<a name="FNanchor_341" id=
+"FNanchor_341" /><a href="#Footnote_341" class=
+"fnanchor">[341]</a> 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.&nbsp;19). Life, thought and sensation are not
+properties of matter, but result from particular material
+combinations.</p>
+
+<p>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.</p>
+
+<p>These are as follows:&mdash;"1. No kind or particle of matter
+<span class="pagenum"><a name="pg219" id="pg219">219</a></span>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.</p>
+
+<p>"2. Every animal faculty, of whatever nature it may be, is an
+organic phenomenon, and results from a system of organs or an
+organ-apparatus which gives rise to it and upon which it is
+necessarily dependent.</p>
+
+<p>"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.&nbsp;104).</p>
+
+<p>According to these "axioms" function is a direct and mechanical
+effect of structure.</p>
+
+<p>The curious thing is that in spite of his avowed materialism,
+Lamarck's conception of life and evolution is profoundly
+psychological, and from the conflict of his materialism and his
+vitalism (of which he was himself hardly conscious), arise most of
+the obscurities and the irreductible self-contradiction of his
+theory.</p>
+
+<p>Lamarck divided animals (psychologically!) into three great
+groups&mdash;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
+<i>irritable</i> merely. The second group are distinguished from
+the first by their possessing, in addition to irritability, a power
+which Lamarck calls the <i>sentiment int&eacute;rieur</i>. He has
+some difficulty in defining exactly what he means by it:&mdash;"I
+<span class="pagenum"><a name="pg220" id="pg220">220</a></span>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>i.e.</i>, in the very moment of the sensation it experiences;
+and if the individual is of those that are endowed with
+intelligence it nevertheless acts in such a case entirely without
+premeditation and before any mental operation has brought its
+<i>will</i> into play" (p.&nbsp;24).</p>
+
+<p>It is the power we call instinct in animals (p.&nbsp;25), and it
+implies neither consciousness nor will. It acts by transforming
+external into internal excitations.</p>
+
+<p>To this second group of animals, possessing the <i>sentiment
+int&eacute;rieur</i>, belong the higher Invertebrates, notably
+insects and molluscs. Only animals possessed of a more or less
+centralised nervous system can manifest this <i>sentiment</i>, or
+principle of (unconscious) reaction to external stimuli.</p>
+
+<p>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 <i>sentiment
+int&eacute;rieur</i> the faculties of intelligence and will.</p>
+
+<p>Now, broadly put, Lamarck's theory of evolution is that new
+organs are formed in direct reaction to needs (<i>besoins</i>)
+experienced by the <i>sentiment int&eacute;rieur</i>. The
+<i>sentiment int&eacute;rieur</i> is therefore the cause not only
+of instinctive action but also of all morphogenetic processes. Will
+and intelligence (which are confined to a relatively small number
+of animals) have little or nothing to do directly with
+evolution.</p>
+
+<p>To understand the working-out of Lamarck's evolution-theory we
+must revert to his conception of the <i>&Eacute;chelle des
+&ecirc;tres</i>. What he wrote in the <i>Philosophie zoologique</i>
+is here repeated in the work of 1816 with little modification.</p>
+
+<p>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.</p>
+
+<p>It is interesting to note that Lamarck expressly refers to
+Bonnet (p.&nbsp;110), but refuses to accept his view of an
+<i>&Eacute;chelle</i> extending down into the inorganic. Like
+Bonnet, however, <span class="pagenum"><a name="pg221" id=
+"pg221">221</a></span>and like the German transcendentalists,
+Lamarck makes man the goal of evolution (p.&nbsp;116). He makes it quite
+clear that his <i>&Eacute;chelle</i> is a functional one, for he
+links Vertebrates to molluscs even while expressly admitting that
+they are not connected by any structural intermediates (p.&nbsp;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.</p>
+
+<p>The progression of organisation shown by the animal kingdom has
+not been altogether regular and uninterrupted:&mdash;"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"
+(<i>Phil. zool.</i>, i., p.&nbsp;145).</p>
+
+<p>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.</p>
+
+<p>The first cause Lamarck calls the vital power (<i>pouvoir de la
+vie</i>); the other may be called the influence of circumstance
+(<i>Anim. s. Vert.</i>, p.&nbsp;134). To the latter cause are due the
+lacun&aelig;, the blind alleys, and the complications which the
+otherwise simple scale of perfection shows.</p>
+
+<p>To explain both these aspects of evolution Lamarck propounded in
+his volume of 1816 four laws, which read as follows:&mdash;</p>
+
+<p>"<i>First Law</i>.&mdash;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.</p>
+
+<p>"<i>Second Law</i>.&mdash;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.</p>
+
+<p>"<i>Third Law</i>.&mdash;The degree of development of organs and
+their force of action are always proportionate to the use made of
+these organs.</p>
+
+<p>"<i>Fourth Law</i>.&mdash;All that has been acquired, imprinted
+or changed in the organisation of the individual during the course
+of its life is preserved by generation and transmitted <span class=
+"pagenum"><a name="pg222" id="pg222">222</a></span>to the new
+individuals that descend from the individual so modified" (pp.
+151-2).</p>
+
+<p>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.</p>
+
+<p>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 (<i>cf.</i> 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.&nbsp;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.</p>
+
+<p>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.<a name=
+"FNanchor_342" id="FNanchor_342" /><a href="#Footnote_342"
+class="fnanchor">[342]</a></p>
+
+<p>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 <i>sentiment int&eacute;rieur</i>.</p>
+
+<p><span class="pagenum"><a name="pg223" id=
+"pg223">223</a></span>In the large group of apathetic or
+insensitive animals, which do not possess this faculty, needs
+cannot be experienced; accordingly new organs are here formed
+directly and mechanically, by the movements of the vital fluids set
+in action by excitations from without&mdash;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 <i>feeling</i>. They
+experience needs, and each need felt, acting upon their 'inner
+feeling,' immediately directs the fluids and the forces to the part
+of the body where action can satisfy the need. Now, if there exists
+at this point an organ capable of performing the required action,
+it is quickly stimulated to act; and if the organ does not exist
+and the need is pressing and sustained, bit by bit the organ is
+produced and developed in proportion to the continuity and the
+energy of its use" (p.&nbsp;155).</p>
+
+<p>In intelligent animals the <i>sentiment int&eacute;rieur</i> may
+be moved by thought or will.</p>
+
+<p>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!</p>
+
+<p>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.</p>
+
+<p>The law has reference to one of the most powerful means <span
+class="pagenum"><a name="pg224" id="pg224">224</a></span>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:&mdash;"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.&nbsp;161).</p>
+
+<p>On the other hand, if the environment does not change, species
+remain constant.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>Lamarck sums up as follows:&mdash;"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 <span class="pagenum"><a name="pg225" id=
+"pg225">225</a></span>state in which we now see them, become easily
+determinable" (p.&nbsp;168).</p>
+
+<p>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.<a name=
+"FNanchor_343" id="FNanchor_343" /><a href="#Footnote_343"
+class="fnanchor">[343]</a></p>
+
+<p>"The function creates the organ," this would seem to be the
+kernel of Lamarck's doctrine. But how does he reconcile this
+essentially vitalistic conception with his strictly materialistic
+philosophy?</p>
+
+<p>We have seen that irritability, the <i>sentiment
+int&eacute;rieur</i>, and intelligence itself, are the effects of
+organisation. We are told farther on that both the <i>sentiment</i>
+and intelligence are caused by nervous fluids. A great part of both
+the <i>Philosophie zoologique</i> and the introduction to the
+<i>Animaux sans Vert&egrave;bres</i> 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.</p>
+
+<p>The <i>sentiment int&eacute;rieur</i> acts by directing the
+movements of the subtle fluids of the body (which are themselves
+modifications of the nervous fluids) upon the parts where a new
+organ is needed. But if it is itself only a result of the movement
+of nervous fluids? Again, how can a need be "felt" by a nervous
+fluid? This is an entirely psychological notion and cannot be
+applied to a purely material system. Whence arises the power of the
+<i>sentiment int&eacute;rieur</i> 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 <i>sentiment
+int&eacute;rieur</i> is a psychological faculty, or it is
+nothing.</p>
+
+<p>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 <span class="pagenum"><a
+name="pg226" id="pg226">226</a></span>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.</p>
+
+<p>Should we be right in laying stress upon the psychological side
+of Lamarck's theory, and disregarding the materialistic dress in
+which, perhaps under the influence of the materialism current in
+his youth, he clothed his essentially vitalistic thought?
+Everything goes to prove it&mdash;his constant preoccupation with
+psychological questions, his tacit assimilation of organ-formation
+to instinctive behaviour, his constant insistence on the importance
+of <i>besoin</i> and <i>habitude</i>.</p>
+
+<p>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 <i>reacts</i> 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.</p>
+
+<p>Not without reason have the later schools of evolutionary
+thought, who developed the psychological and vitalistic side of his
+doctrine, called themselves Neo-Lamarckians.</p>
+
+<p>We shall say then that Lamarck, in spite of his materialism, was
+the founder of the "psychological" theory of evolution.</p>
+
+<p>Lamarck stood curiously aloof and apart from the scientific
+thought of his day.<a name="FNanchor_344" id=
+"FNanchor_344" /><a href="#Footnote_344" class=
+"fnanchor">[344]</a> He took no interest in the morphological
+problems that filled the minds of Cuvier and Geoffroy; he had
+indeed no feeling at all for morphology. He did not realise, like
+Cuvier, the <i>convenance des parties</i>, the marvellous
+co-ordination of parts to form a whole; he had little conception of
+what is really implied in the word "organism." He was not, like
+Geoffroy, imbued with a lively sense of the unity of plan and
+composition, and of the significance of vestigial <span class=
+"pagenum"><a name="pg227" id="pg227">227</a></span>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.</p>
+
+<p>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.</p>
+
+<p>The one point in which he does definitely carry on the thought
+of his predecessors is his conception of the animal kingdom as
+forming a scale of (functional) perfection. He did not go to the
+same extreme as Bonnet; he did not even consider that the animal
+series was a continuation of the vegetable series; in his opinion
+they formed two diverging scales. He recognised, too, that among
+animals there was no simple and regular gradation from the lowest
+to the highest, but that the orderly progression was disturbed and
+diverted by the necessity of adaptation to different environments.
+It is interesting to note that in developing this idea he arrived
+at a roughly accurate distinction between homologous and analogous
+structures. More importance, he thought, was to be attributed in
+classifying animals to characters which appeared due to the "plan
+of Nature" than to such as were produced by an external modifying
+cause (p.&nbsp;299). But he did not formulate the distinction in any
+strictly morphological way.</p>
+
+<p>As his ideas developed he laid less stress upon the simplicity
+<span class="pagenum"><a name="pg228" id="pg228">228</a></span>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 <i>Order</i> of the
+formation of Animals, showing two separate partly-branching
+series," and it reads as follows:&mdash;</p>
+
+<div class="figcenter">
+<img src="images/img228a.jpg" alt=
+"formation of animals" /></div>
+
+<p>It is interesting to note that Vertebrates are placed between
+the two series, and are now not linked on directly to any
+Invertebrate group.</p>
+
+<p>Lamarck's theory had little success. There is evidence, however,
+that both Meckel and Geoffroy owed a good many of their
+evolutionary ideas to Lamarck, and Cuvier paid him at least the
+compliment of criticising his theory,<a name="FNanchor_345" id=
+"FNanchor_345" /><a href="#Footnote_345" class=
+"fnanchor">[345]</a> not distinguishing it, however, very clearly
+from the evolutionary theories of the transcendentalists. But,
+speaking generally, Lamarck's theory of evolution exercised very
+little influence upon his <span class="pagenum"><a name="pg229" id=
+"pg229">229</a></span>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.</p>
+
+<p>It was not that men's minds were not ripe for evolution, for in
+the early decades of the 19th century evolution was in the air.
+There were few of von Baer's contemporaries who had not read
+Lamarck;<a name="FNanchor_346" id="FNanchor_346" /><a href=
+"#Footnote_346" class="fnanchor">[346]</a> Erasmus Darwin's
+<i>Zoonomia</i> ran through three editions, and was translated into
+German, French and Italian;<a name="FNanchor_347" id=
+"FNanchor_347" /><a href="#Footnote_347" class=
+"fnanchor">[347]</a> German philosophy was full of the idea of
+evolution.</p>
+
+<p>There was no unreadiness to accept the derivation of present-day
+species from a primordial form&mdash;if only some solid evidence
+for such derivation were forthcoming. Cuvier and von Baer, as we
+have seen, combated the current evolution theories on the ground
+that the evidence was insufficient, but von Baer at least had no
+rooted objection to evolution. In an essay of 1834, entitled <i>The
+Most General Law of Nature in all Development</i>,<a name=
+"FNanchor_348" id="FNanchor_348" /><a href="#Footnote_348"
+class="fnanchor">[348]</a> 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
+pal&aelig;ontology, he went so far as to maintain that many species have
+evolved from parent stocks. In the absence of conclusive proofs he
+did not commit himself to a belief in any extended or comprehensive
+process of evolution.</p>
+
+<p>Imbued as he was with the idea of development von Baer saw in
+evolution a process essentially of the same nature as the
+development of the individual. Evolution, like development, was due
+to a <i>Bildungskraft</i> 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 <i>Es werde!</i>. (p.&nbsp;86).</p>
+
+<p>As we shall see, von Baer adopted in later years the same <span
+class="pagenum"><a name="pg230" id="pg230">230</a></span>attitude
+to Darwinism as he did to the evolution theories in vogue in his
+youth.</p>
+
+<p>Although in the twenty or thirty years before the publication of
+the <i>Origin of Species</i> (1859) no evolution theory of any
+importance was published, and although the great majority of
+biologists believed in the constancy of species, there were not
+wanting some who, like von Baer, had an open mind on the subject,
+or even believed in the occurrence of evolutionary processes of
+small scope. Isidore Geoffroy St Hilaire, the son of the great
+Etienne Geoffroy St Hilaire, seems to have held that species might
+be formed from varieties. The law which L. Agassiz thought he could
+establish,<a name="FNanchor_349" id="FNanchor_349" /><a href=
+"#Footnote_349" class="fnanchor">[349]</a> of the parallelism
+between pal&aelig;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.<a name="FNanchor_350" id=
+"FNanchor_350" /><a href="#Footnote_350" class=
+"fnanchor">[350]</a></p>
+
+<p>But no evolution theory was taken very seriously before 1859,
+when the <i>Origin of Species</i> was published.</p>
+
+<p>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
+<i>Beagle</i> aroused in him keen interest in the problem of
+species&mdash;their variety, their variation according to place and
+time, their adaptedness to environment. The conviction gradually
+took possession of his mind that the puzzling facts of geographical
+range and geological succession which he observed wherever he went
+were explicable only on the hypothesis that species change. He was
+not satisfied with the theories of evolution that had been proposed
+by his grandfather, by Lamarck, and by E. Geoffroy St
+Hilaire&mdash;he did not indeed understand these theories any too
+well. He resolved to work out the problem in his own way, for his
+own satisfaction. He tells us all this very clearly in his
+autobiography. "During the voyage <span class="pagenum"><a name=
+"pg231" id="pg231">231</a></span>of the <i>Beagle</i> 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.</p>
+
+<p>"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&mdash;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."<a name=
+"FNanchor_351" id="FNanchor_351" /><a href="#Footnote_351"
+class="fnanchor">[351]</a></p>
+
+<p>All Darwin's varied subsequent work revolved round these, for
+him, essential problems&mdash;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&mdash;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 <span
+class="pagenum"><a name="pg232" id="pg232">232</a></span>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.</p>
+
+<p>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 <i>convenance des
+parties</i> and the interaction of structure and function. Perhaps
+Darwin did not realise this inner aspect of adaptation quite so
+vividly as he did the more superficial adaptation of organisms to
+their environment. It was, perhaps, his lack of morphological
+training and experience that led him to disregard the problems of
+form, or at least to realise very insufficiently their
+difficulty.</p>
+
+<p>It is in any case very significant that only a small part of his
+<i>Origin of Species</i> is devoted to the discussion of
+morphological questions&mdash;only one chapter out of the fourteen
+contained in the first edition.</p>
+
+<p>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
+<i>Origin</i> he laid the foundations of evolutionary
+morphology.</p>
+
+<p>We have here to consider his interpretation of morphological
+facts and its relation to the current morphology of his time.</p>
+
+<p>The sketch of his theory, written in 1842,<a name=
+"FNanchor_352" id="FNanchor_352" /><a href="#Footnote_352"
+class="fnanchor">[352]</a> shows a very significant division into two
+parts&mdash;the first dealing with the positive facts of
+variability and the theory of natural selection, <span class=
+"pagenum"><a name="pg233" id="pg233">233</a></span>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 f&oelig;tal state" (p.&nbsp;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
+f&oelig;tus&mdash;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 f&oelig;tus, retaining traces of the
+ancestral form, is of the highest value in classification" (p.
+45).</p>
+
+<p>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 f&oelig;tus
+passes, the metamorphosis of organs, the abortion of others, cease
+to be metaphorical expressions and become intelligible facts" (p.
+50).</p>
+
+<p>In general, organisms can be understood only if we take into
+account the cardinal fact that they are historical beings. "We must
+look at every complicated mechanism and instinct as the summary of
+a long history of useful contrivances much like a work of art" (p.
+51).<a name="FNanchor_353" id="FNanchor_353" /><a href=
+"#Footnote_353" class="fnanchor">[353]</a></p>
+
+<p>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<span class="pagenum"><a
+name="pg234" id="pg234">234</a></span> <i>Origin of Species</i> (1st ed.,
+1859). A good part of this chapter is given up to a discussion of
+the principles of classification, only a few pages dealing with
+morphology proper. But, as Darwin rightly saw, the two things are
+inseparable.</p>
+
+<p>We note first that there is no hint of the "scale of
+beings"&mdash;Darwin conceives the genealogical tree as many
+branched. Animals can be classed in "groups under groups," and
+cannot be arranged in one single series.</p>
+
+<p>He discusses first what kind of characters have the greatest
+classificatory value. Certain empirical rules have been recognised,
+more or less consciously, by systematists&mdash;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.&nbsp;414), and adduces in
+support Owen's remark that the generative organs afford very clear
+indications of affinities, since they are unlikely to be modified
+by special habits. These rules of classification can be explained
+"on the view that the natural system is founded on descent with
+modification; that the characters which naturalists consider as
+showing true affinity ... are those which have been inherited from
+a common parent, and, in so far, all true classification is
+genealogical; that community of descent is the hidden bond which
+naturalists have been unconsciously seeking, and not some unknown
+plan of creation, or the enunciation of general propositions, and
+the mere putting together and separating objects more or less
+alike" (p.&nbsp;420).</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>He treats this on the same lines as E. Geoffroy St Hilaire, and
+Owen, referring indeed specifically to Geoffroy's law of
+connections. "What can be more curious," he asks, <span class=
+"pagenum"><a name="pg235" id="pg235">235</a></span>"than that the
+hand of a man, formed for grasping, that of a mole for digging, the
+leg of a horse, the paddle of the porpoise, and the wing of the
+bat, should all be constructed on the same pattern, and should
+include similar bones, in the same relative positions? Geoffroy St
+Hilaire has strongly insisted on the high importance of relative
+position or connection in homologous parts; they may differ to
+almost any extent in form and size, and yet remain connected
+together in the same invariable order" (p.&nbsp;434).</p>
+
+<p>The unity of plan cannot be explained on teleological grounds,
+as Owen has admitted in his <i>Nature of Limbs</i>, nor is it
+explicable on the hypothesis of special creation (p.&nbsp;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&mdash;each modification being profitable in some way
+to the modified form, but often affecting by correlation other
+parts of the organisation. In changes of this nature, there will be
+little or no tendency to alter the original pattern or to transpose
+the parts.... If we suppose that the ancient progenitor, the
+archetype as it may be called, of all animals, had its limbs
+constructed on the existing general pattern, for whatever purpose
+they served, we can at once perceive the plain significance of the
+homologous construction of the limbs throughout the whole class"
+(p.&nbsp;435).</p>
+
+<p>We may note three important points in this passage&mdash;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
+<i>petitio principii</i> involved in the assumption that adaptive
+modification brings inevitably in its train the necessary
+correlative changes.</p>
+
+<p>In his section on morphology Darwin shows clearly the influence
+of Owen, and through him of the transcendental anatomists. He
+refers to the transcendental idea of "metamorphosis," as
+exemplified in the vertebral theory of the skull and the theory of
+the plant appendage, and shows how, on the hypothesis of descent
+with modification, "metamorphosis" <span class="pagenum"><a name=
+"pg236" id="pg236">236</a></span>may now be interpreted literally,
+and no longer figuratively merely (p.&nbsp;439).</p>
+
+<p>Very great interest attaches to Darwin's treatment of
+development, for post-Darwinian morphology was based to a very
+large extent on the presumed relation between the development of
+the individual and the evolution of the race. Just as he kept clear
+of the notion of the scale of beings, so he avoided the snare of
+the Meckel-Serres theory of recapitulation, according to which the
+embryo of the highest animal, man, during its development climbs
+the ladder upon the rungs of which the whole animal series is
+distributed, in its gradual progression from simplicity to
+complexity. The law of development which he adopts is that of von
+Baer, which states that development is essentially differentiation,
+and that as a result embryos belonging to the same group resemble
+one another the more the less advanced they are in development.
+There can be little doubt that he was indebted to von Baer for the
+idea, and in the later editions of the <i>Origin</i> 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.<a name=
+"FNanchor_354" id="FNanchor_354" /><a href="#Footnote_354"
+class="fnanchor">[354]</a></p>
+
+<p>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.&nbsp;440)&mdash;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&mdash;namely, the very general, but not
+universal, difference in structure between the embryo and the
+adult&mdash;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&mdash;of embryos of
+different species within the same class, generally but not
+universally, <span class="pagenum"><a name="pg237" id=
+"pg237">237</a></span>resembling each other&mdash;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&mdash;of the embryo apparently
+having sometimes a higher organisation than the mature animal, into
+which it is developed" (pp.&nbsp;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&mdash;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.&nbsp;444).</p>
+
+<p>Thus, applying these principles to a hypothetical case of the
+origin of new species of birds from a common stock, he
+writes:&mdash;"... from the many slight successive steps of
+variation having supervened at a rather late age and having been
+inherited at a corresponding age, the young of the new species of
+our supposed genus will manifestly tend to resemble each other much
+more closely than do the adults, just as we have seen in the case
+of pigeons"<a name="FNanchor_355" id="FNanchor_355" /><a href=
+"#Footnote_355" class="fnanchor">[355]</a> (pp.&nbsp;446-7).</p>
+
+<p>Since the embryo shows the generalised type, the structure of
+the embryo is useful for classificatory purposes. "For the embryo
+is the animal in its less modified state; and in so far it reveals
+the structure of its progenitor" (p.&nbsp;449)&mdash;the embryological
+archetype reveals the ancestral form. "Embryology rises greatly in
+interest, when we thus look at the embryo as a picture, more or
+less complete, of the parent form of each great class of animals"
+(p.&nbsp;450)&mdash;a prophetic remark, in view of the enormous
+subsequent development of phylogenetic speculation.</p>
+
+<p>We may sum up by saying that Darwin interpreted von Baer's law
+phylogenetically.</p>
+
+<p>The rest of the chapter is devoted to a discussion of abortive
+and vestigial organs, whose existence Darwin <span class=
+"pagenum"><a name="pg238" id="pg238">238</a></span>naturally turns
+to great advantage in his argument for evolution. Throughout the
+whole chapter Darwin's preoccupation with the problems of
+classification is clearly manifest.</p>
+
+<p>On the question as to whether descent was monophyletic or
+polyphyletic Darwin expressed no dogmatic opinion. "I believe that
+animals have descended from at most only four or five progenitors,
+and plants from an equal or lesser number.... I should infer from
+analogy that probably all the organic beings which have ever lived
+on this earth have descended from one primordial form, into which
+life was first breathed" (p.&nbsp;484).</p>
+
+<p>Darwin rightly laid much stress upon the morphological evidence
+for evolution,<a name="FNanchor_356" id="FNanchor_356" /><a
+href="#Footnote_356" class="fnanchor">[356]</a> 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,<a name=
+"FNanchor_357" id="FNanchor_357" /><a href="#Footnote_357"
+class="fnanchor">[357]</a> and followed with interest the progress of
+evolutionary morphology, incorporating some of its results in later
+editions of the <i>Origin</i>, and in his <i>Descent of Man</i>
+(1871).</p>
+
+<p>In his morphology Darwin was hardly up to date. He does not seem
+to have known at first hand the splendid work of the German
+morphologists, such as Rathke and Reichert; he pays no attention to
+the cell-theory, nor to the germ-layer theory. His sources are, in
+the main, Geoffroy St Hilaire, Owen, von Baer, Agassiz,
+Milne-Edwards, and Huxley.</p>
+
+<p>Perhaps his greatest omission was that he did not give any
+adequate treatment of the problem of functional adaptation and the
+correlation of parts. It is not too much to say that Darwin not
+only disregarded these problems almost entirely, but by his
+insistence upon ecological adaptation and upon certain superficial
+aspects of correlation, succeeded in giving to the words
+"adaptation" and "correlation" <span class="pagenum"><a name=
+"pg239" id="pg239">239</a></span>a new signification, whereby they
+lost to a large extent their true and original functional
+meaning.</p>
+
+<p>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&mdash;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 <i>the varying parts of each being to its organic and
+inorganic conditions of life</i>:<a name="FNanchor_358" id=
+"FNanchor_358" /><a href="#Footnote_358" class=
+"fnanchor">[358]</a> 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" (<i>Origin</i>, 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, <a href="#pg034">Chap.&nbsp;III., p.
+34</a>).</p>
+
+<p>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.</p>
+
+<p><span class="pagenum"><a name="pg240" id=
+"pg240">240</a></span>Darwin's conception of correlation was
+singularly incomplete. As examples of correlation he advanced such
+trivial cases as the relation between albinism, deafness and blue
+eyes in cats, or between the tortoise-shell colour and the female
+sex. He used the word only in connection with what he called
+"correlated variation," meaning by this expression "that the whole
+organisation is so tied together during its growth and development,
+that when slight variations in any one part occur, and are
+accumulated through natural selection, other parts become modified"
+(6th ed., p.&nbsp;177). He took it for granted that the "correlated
+variations" would be adapted to the original variation which was
+acted upon by natural selection, and he saw no difficulty in the
+gradual evolution of a complicated organ like the eye if only the
+steps were small enough. "It has been objected," he writes, "that
+in order to modify the eye and still preserve it as a perfect
+instrument, many changes would have to be effected simultaneously,
+which, it is assumed, could not be done through natural selection;
+but as I have attempted to show in my work on the variation of
+domestic animals, it is not necessary to suppose that the
+modifications were all simultaneous, if they were extremely slight
+and gradual" (6th ed., p.&nbsp;226).</p>
+
+<p>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.</p>
+
+<p>The change in the conception of correlation which Darwin's
+influence brought about has been very clearly stated by E. von
+Hartmann,<a name="FNanchor_359" id="FNanchor_359" /><a href=
+"#Footnote_359" class="fnanchor">[359]</a> from whom the following
+is taken:&mdash;"While the correlation of parts in the organism was
+before Darwin regarded exclusively from the standpoint of
+morphological systematics, Darwin tried to look at it from the
+standpoint of physiological and genealogical development, and in so
+doing he put the standpoint of morphological systematics in the
+shade. But the more we are now beginning to realise that systematic
+relationship does not necessarily <span class="pagenum"><a name=
+"pg241" id="pg241">241</a></span>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.&nbsp;47-8).</p>
+
+<p>Further on, following R&aacute;dl, he characterises Darwin's
+attitude to the law of correlation in these terms:&mdash;"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 <i>departure</i> from the given type. With morphological
+correlation in <i>different</i> types Darwin troubles himself not
+at all, nor with correlation in the normal development of a type"
+(p.&nbsp;49).</p>
+
+<p>Cuvier's conception of the <i>convenance des parties</i>,
+essential to all biology, remained on the whole foreign to Darwin's
+thought, and to the thought of his successors.</p>
+
+<p>It was indeed one of their boasts that they had finally
+eliminated all teleology from Nature. The great and immediate
+success which Darwinism had among the younger generation of
+biologists and among scientific men in general was due in large
+part to the fact that it fitted in well with the prevailing
+materialism of the day, and gave solid ground for the hope that in
+time a complete mechanistic explanation of life would be
+forthcoming. "Darwinismus" became the battle-cry of the militant
+spirits of that time.</p>
+
+<p>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 <i>Entwickelungsgesetz</i>, 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 <span class=
+"pagenum"><a name="pg242" id="pg242">242</a></span>of development,
+that there must have been manifested in evolution some definite and
+orderly tendency towards perfection.<a name="FNanchor_360" id=
+"FNanchor_360" /><a href="#Footnote_360" class=
+"fnanchor">[360]</a></p>
+
+<p>No one expressed this objection with greater force than did von
+Baer, in a series of masterly essays<a name="FNanchor_361" id=
+"FNanchor_361" /><a href="#Footnote_361" class=
+"fnanchor">[361]</a> 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&mdash;to use Driesch's term&mdash;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&mdash;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 (<i>Ziel</i>)&mdash;"A becoming without a purpose
+is in general unthinkable" (p.&nbsp;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 <i>zielstrebig</i> or guided by
+final causes, that it is a true <i>evolutio</i> 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.</p>
+
+<p>The idea of saltatory evolution he took from K&ouml;lliker, who
+shortly after the publication of the <i>Origin</i> promulgated
+<span class="pagenum"><a name="pg243" id="pg243">243</a></span>in a
+critical note on Darwinism a sketch of his theory of "heterogeneous
+generation."<a name="FNanchor_362" id="FNanchor_362" /><a href=
+"#Footnote_362" class="fnanchor">[362]</a></p>
+
+<p>K&ouml;lliker's attitude is typical of that taken up by many of
+the morphologists of the day.<a name="FNanchor_363" id=
+"FNanchor_363" /><a href="#Footnote_363" class=
+"fnanchor">[363]</a> He accepts evolution completely, but rejects
+Darwinism because it recognises no <i>Entwickelungsgesetz</i>, 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.&nbsp;181). It is to be noticed that
+K&ouml;lliker laid more stress upon the <i>Entwickelungsgesetz</i>
+than upon the saltatory nature of variation, for he says a few
+pages further on&mdash;"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.&nbsp;184). Saltatory evolution was not the
+essential point of the theory:&mdash;"Another difference between
+the Darwinian hypothesis and mine is that I postulate many
+saltatory changes, but I will not and indeed cannot lay the chief
+stress upon this point, for I have not intended to maintain that
+the general law of evolution which I hold to be the cause of the
+creation of organisms, and which alone manifests itself in the
+activity of generation, cannot also so act that from one form
+others quite gradually arise" (p.&nbsp;185). He put forward the
+hypothesis of saltatory variation because it seemed to him to
+lighten many of the difficulties of Darwinism&mdash;the lack of
+transition forms, the enormous time required for evolution, and so
+on. It should be noted that K&ouml;lliker regarded his principle of
+evolution as mechanical.</p>
+
+<p><span class="pagenum"><a name="pg244" id=
+"pg244">244</a></span>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.</p>
+
+<p>Richard Owen, who in 1868<a name="FNanchor_364" id=
+"FNanchor_364" /><a href="#Footnote_364" class=
+"fnanchor">[364]</a> 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.&nbsp;808).</p>
+
+<p>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.&nbsp;797).
+He believed spontaneous generation to be a phenomenon constantly
+taking place, and constantly giving the possibility of new lines of
+evolution.</p>
+
+<p>E. von Hartmann in his <i>Philosophie des Unbewussten</i> (1868)
+and in his valuable essay on <i>Wahrheit und Irrtum im
+Darwinismus</i> (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.</p>
+
+<p>In France H. Milne-Edwards<a name="FNanchor_365" id=
+"FNanchor_365" /><a href="#Footnote_365" class=
+"fnanchor">[365]</a> stated the problem thus:&mdash;"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 <span class="pagenum"><a name="pg245" id=
+"pg245">245</a></span>independent of the general properties of
+organisable matter such as we know them to-day?" (p.&nbsp;426)</p>
+
+<p>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 pal&aelig;ontology discloses have originated
+from one another by descent. But he thought that the process by
+which they evolved should rightly be called "creation." The word
+was of course not to be taken in a crude sense. When the zoologist
+speaks of the "creation" of a new species, "he in no way means that
+the latter has arisen from the dust, rather than from a
+pre-existing animal whose mode of organisation was different; he
+merely means that the known properties of matter, whether inert or
+organic, are insufficient to bring about such a result, and that
+the intervention of a hidden cause, of a power of some higher
+order, seems to him necessary" (p.&nbsp;429).</p>
+
+<p>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.</p>
+
+<div class="footnote">
+<p><a name="Footnote_333" id="Footnote_333" /><a href=
+"#FNanchor_333"><span class="label">[333]</span></a> R&aacute;dl,
+<i>loc. cit.</i>, i., p.&nbsp;71.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_334" id="Footnote_334" /><a href=
+"#FNanchor_334"><span class="label">[334]</span></a> <i>Kritik der
+Urtheilskraft</i>, 1790.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_335" id="Footnote_335" /><a href=
+"#FNanchor_335"><span class="label">[335]</span></a> Eng. Trans. by
+J. H. Bernard, p.&nbsp;337, London, 1892.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_336" id="Footnote_336" /><a href=
+"#FNanchor_336"><span class="label">[336]</span></a> H. F. Osborn,
+<i>From the Greeks to Darwin</i>, p.&nbsp;145, New York and London,
+1894.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_337" id="Footnote_337" /><a href=
+"#FNanchor_337"><span class="label">[337]</span></a> See Meckel,
+<i>supra</i>, p.&nbsp;93; <i>cf.</i> Tiedemann, <i>Zoologie</i>, p.&nbsp;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.&nbsp;73 of the same book.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_338" id="Footnote_338" /><a href=
+"#FNanchor_338"><span class="label">[338]</span></a> Chapters vii.
+and ix.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_339" id="Footnote_339" /><a href=
+"#FNanchor_339"><span class="label">[339]</span></a> On early
+evolution-theories see, in addition to Osborn and R&aacute;dl, J.
+Arthur Thomson, <i>The Science of Life</i>, 1899, and the opening
+essay in <i>Darwin and Modern Science</i>, Cambridge, 1909.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_340" id="Footnote_340" /><a href=
+"#FNanchor_340"><span class="label">[340]</span></a> <i>Phil.
+zool.</i>, ed. Ch. Martins, vol. i., p.&nbsp;75, 1873.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_341" id="Footnote_341" /><a href=
+"#FNanchor_341"><span class="label">[341]</span></a> Quotations in
+the text are from the 2nd Edit. (Deshayes and Milne-Edwards), i.,
+Paris, 1835.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_342" id="Footnote_342" /><a href=
+"#FNanchor_342"><span class="label">[342]</span></a> For instance,
+Lucretius:&mdash;</p>
+<p>"Is tibi nunc animus quali sit corpore et unde constiterit
+pergam rationem reddere dictis. Principio esse aio persubtilem
+atque minutis perquam corporibus factum constare."</p>
+
+<p>&mdash;<i>De Rerum Natura</i>, iii., vv. 177-80.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_343" id="Footnote_343" /><a href=
+"#FNanchor_343"><span class="label">[343]</span></a> Contrast
+Treviranus&mdash;"In every living being there exists a capability
+of an endless variety of form-assumption; each possesses the power
+to adapt its organisation to the changes of the outer world, and it
+is this power, put into action by the change of the universe, that
+has raised the simple zoophytes of the primitive world to
+continually higher stages of organisation, and has introduced a
+countless variety of species into animate Nature." Quoted by
+Haeckel in <i>History of Creation</i>, i., p.&nbsp;93, 1876.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_344" id="Footnote_344" /><a href=
+"#FNanchor_344"><span class="label">[344]</span></a> There is no
+evidence that he was influenced by Erasmus Darwin, who forestalled
+his evolution theory, and was indeed more aware of its vitalistic
+implications. See S. Butler, <i>Evolution, Old and New</i>, London,
+1879, for an excellent account of Erasmus Darwin.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_345" id="Footnote_345" /><a href=
+"#FNanchor_345"><span class="label">[345]</span></a> As did also
+Lyell in his <i>Principles of Geology</i>, 1830.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_346" id="Footnote_346" /><a href=
+"#FNanchor_346"><span class="label">[346]</span></a> K. E. von Baer,
+<i>Reden</i>, i., p.&nbsp;37, Petrograd, 1864.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_347" id="Footnote_347" /><a href=
+"#FNanchor_347"><span class="label">[347]</span></a> R&aacute;dl,
+<i>loc. cit.</i>, i., p.&nbsp;296.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_348" id="Footnote_348" /><a href=
+"#FNanchor_348"><span class="label">[348]</span></a> Reprinted in
+his <i>Reden</i>, i., 1864.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_349" id="Footnote_349" /><a href=
+"#FNanchor_349"><span class="label">[349]</span></a> See Huxley's
+criticism of it in a Royal Institution lecture of 1851, republished
+in <i>Sci. Mem.</i>, i., pp.&nbsp;300-4. On its relation to Haeckel's
+biogenetic law, see below, p.&nbsp;<a href="#pg255">255</a>.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_350" id="Footnote_350" /><a href=
+"#FNanchor_350"><span class="label">[350]</span></a> <i>System der
+thierischen Morphologie</i>, p.&nbsp;5, 1853.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_351" id="Footnote_351" /><a href=
+"#FNanchor_351"><span class="label">[351]</span></a> <i>Life and
+Letters of Charles Darwin</i>, ed. F. Darwin, i., p.&nbsp;82, 3rd ed.,
+1887.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_352" id="Footnote_352" /><a href=
+"#FNanchor_352"><span class="label">[352]</span></a> <i>The
+Foundations of the Origin of Species, a Sketch written in 1842</i>.
+Ed. F. Darwin, Cambridge, 1909.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_353" id="Footnote_353" /><a href=
+"#FNanchor_353"><span class="label">[353]</span></a> <i>Cf.</i> a
+parallel passage in the <i>Origin</i>, 1st ed., pp.&nbsp;485-6.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_354" id="Footnote_354" /><a href=
+"#FNanchor_354"><span class="label">[354]</span></a> In the 1st ed.
+(p.&nbsp;439), Darwin makes the curious mistake of attributing this
+story to Agassiz.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_355" id="Footnote_355" /><a href=
+"#FNanchor_355"><span class="label">[355]</span></a> In which
+nestlings of the different varieties are much more alike than
+adults. Darwin attached much importance to this idea, see <i>Life
+and Letters</i>, i., p.&nbsp;88, and ii., p.&nbsp;338.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_356" id="Footnote_356" /><a href=
+"#FNanchor_356"><span class="label">[356]</span></a> See his
+<i>Letters, passim</i>.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_357" id="Footnote_357" /><a href=
+"#FNanchor_357"><span class="label">[357]</span></a> Writing to
+Huxley on the subject of the latter's work on the morphology of the
+Mollusca (1853), he says:&mdash;"The discovery of the type or
+'idea' (in your sense, for I detest the word as used by Owen,
+Agassiz &amp; Co.) of each great class, I cannot doubt, is one of
+the very highest ends of Natural History."&mdash;<i>More
+Letters</i>, ed. F. Darwin and A. C. Seward, 1903, i., p.&nbsp;73.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_358" id="Footnote_358" /><a href=
+"#FNanchor_358"><span class="label">[358]</span></a> Italics
+mine.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_359" id="Footnote_359" /><a href=
+"#FNanchor_359"><span class="label">[359]</span></a> <i>Das Problem
+des Lebens. Biologische Studien</i>. Bad Sacha, 1906. See also E.
+R&aacute;dl, <i>Biol. Centralblatt</i>, xxi., 1901.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_360" id="Footnote_360" /><a href=
+"#FNanchor_360"><span class="label">[360]</span></a> See the
+excellent treatment of the difference between the "realism" of
+Darwin and the "rationalism" of his critics, in R&aacute;dl, ii.,
+particularly pp.&nbsp;109, 135. The most elaborate criticism of
+Darwinism from the older standpoint was that given by A. Wigand in
+<i>Der Darwinismus und die Naturforschung Newtons und Cuviers</i>,
+3 vols., Braunschweig, 1872.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_361" id="Footnote_361" /><a href=
+"#FNanchor_361"><span class="label">[361]</span></a> In vol. ii. of
+his <i>Reden</i>, St Petersburg (Petrograd), 1876&mdash;<i>Ueber den
+Zweck in den Vorg&auml;ngen der Natur; Ueber Zielstrebigkeit in den
+organischen K&ouml;rpern insbesondere</i>; and <i>Ueber Darwin's
+Lehre</i>.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_362" id="Footnote_362" /><a href=
+"#FNanchor_362"><span class="label">[362]</span></a> "Ueber die
+Darwinische Sch&ouml;pfungstheorie," <i>Zeits. f. wiss. Zool.</i>,
+xiv., pp.&nbsp;74-86, 1864. Elaborated in <i>Anat. u. syst. Beschreibung
+d. Alcyonarien</i>, 1872.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_363" id="Footnote_363" /><a href=
+"#FNanchor_363"><span class="label">[363]</span></a> <i>Cf.</i> for
+instance N&auml;geli's theory of a perfecting principle, first
+developed in his <i>Entstehung u. Begriff der naturhistorischer
+Art</i>, M&uuml;nchen, 1865.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_364" id="Footnote_364" /><a href=
+"#FNanchor_364"><span class="label">[364]</span></a> <i>Anatomy of
+Vertebrates</i>, iii., 1868.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_365" id="Footnote_365" /><a href=
+"#FNanchor_365"><span class="label">[365]</span></a> <i>Rapport sur
+les Progr&egrave;s r&eacute;cents des Sciences zoologiques en
+France</i>. Paris, 1867.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg246" id=
+"pg246">246</a></span></p>
+
+<h3>CHAPTER XIV</h3>
+
+<h4>ERNST HAECKEL AND CARL GEGENBAUR</h4>
+
+<p><span class="smcap">At</span> 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&mdash;first, by the comparison of adult structure; and
+second, by the comparative study of developing embryos. It was
+found that the archetype was shown most clearly by the early
+embryo, and this embryological archetype came to be preferred
+before the archetype of comparative anatomy. It became apparent
+also that the parts first formed (germ-layers) were of primary
+importance for the establishing of homologies.</p>
+
+<p>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 <span
+class="pagenum"><a name="pg247" id=
+"pg247">247</a></span>materialism of the day, and found expression
+particularly in the cell-theory and in materialistic
+physiology.</p>
+
+<p>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.</p>
+
+<p>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&mdash;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."</p>
+
+<p>The current morphology, Darwin found, could be taken over, lock,
+stock and barrel, to the evolutionary camp.</p>
+
+<p>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
+<i>Origin of Species</i>.</p>
+
+<p>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.</p>
+
+<p>It was in 1866 that Haeckel's chief work appeared, a <i>General
+Morphology of Organisms</i>,<a name="FNanchor_366" id=
+"FNanchor_366" /><a href="#Footnote_366" class=
+"fnanchor">[366]</a> which was intended by its author to bring all
+morphology under the sway and domination of evolution.</p>
+
+<p>It was a curious production, this first book of Haeckel's, and
+representative not so much of Darwinian as of pre-Darwinian <span
+class="pagenum"><a name="pg248" id="pg248">248</a></span>thought.
+It was a medley of dogmatic materialism, idealistic morphology, and
+evolution theory; its sources were, approximately, B&uuml;chner,
+Theodor Schwann, Virchow, H. G. Bronn, and, of course, Charles
+Darwin.</p>
+
+<p>It was scarcely modern even on its first appearance, and many
+regarded it, not without reason, as a belated offshoot of
+<i>Naturphilosophie</i>.</p>
+
+<p>Its materialism is of the most intransigent character. The form
+and activities of living things are held to be merely the
+mechanical result of the physical and chemical composition of their
+bodies. The simplest living things, the Monera, are nothing more
+than homogeneous masses of protein substance. "They live, but
+without organs of life; all the phenomena of their life, nutrition
+and reproduction, movement and irritability, appear here as merely
+the immediate outcome of formless organic matter, itself an albumen
+compound" (p.&nbsp;63, 1906).</p>
+
+<p>Teleology, the Achilles' heel of Kant's (otherwise sound!)
+philosophy, is to be regarded as a totally refuted and antiquated
+doctrine, definitely put out of court by Darwinism.</p>
+
+<p>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
+<i>Bildungstrieb</i> (!), is the mechanical effect of the material
+structure of the crystal or the germ, and adaptation, or the
+external <i>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.</p>
+
+<p>It goes without saying that Haeckel allowed to the organism no
+other nor higher individuality than belongs to the crystal, and
+took no account at all of that harmonious interaction of the organs
+which Cuvier called the principle of the "conditions of existence."
+The concept of correlation had simply no meaning for Haeckel. The
+analysis and <span class="pagenum"><a name="pg249" id=
+"pg249">249</a></span>disintegration of the organism was pushed by
+him to its logical extreme, and in this also he was a child of his
+time.</p>
+
+<p>A no less important influence clearly visible in the <i>General
+Morphology</i> is the idealistic morphology of men like K. G. Carus
+and H. G. Bronn. In previous chapters we have seen how K. G. Carus
+attempted to work out a geometry of the organism, and how Bronn
+tried in a modest way to found a stereometrical morphology, but had
+the grace not to push his stereometry <i>&agrave; l'outrance</i>,
+recognising very wisely that the greater part of organic form is
+functionally determined. Haeckel took over this idea<a name=
+"FNanchor_367" id="FNanchor_367" /><a href="#Footnote_367"
+class="fnanchor">[367]</a> and pushed it to wild extremes, founding a
+new science of "Promorphology" of which he was the
+greatest&mdash;and only&mdash;exponent.<a name="FNanchor_368" id=
+"FNanchor_368" /><a href="#Footnote_368" class=
+"fnanchor">[368]</a></p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>The science of Promorphology has no historical significance; it
+is interesting only because it illustrates Haeckel's close affinity
+with the idealistic morphologists.</p>
+
+<p>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:&mdash;(1)
+Plastids (Cytodes and cells); (2) Organs (including cell-fusions,
+tissues, organs, organ-systems); (3) Antimeres (homotypic parts,
+<i>i.e.</i>, halves or rays); (4) Metameres (homodynamic parts,
+<i>i.e.</i>, segments); (5) Persons (individuals in the ordinary
+sense); (6) Corms (colonial animals).</p>
+
+<p>The thought is essentially transcendental, and recalls <span
+class="pagenum"><a name="pg250" id="pg250">250</a></span>the
+"theory of the repetition of parts," of which so much use was made
+by the German transcendentalists, such as Goethe,<a name=
+"FNanchor_369" id="FNanchor_369" /><a href="#Footnote_369"
+class="fnanchor">[369]</a> Oken, Meckel and K. G. Carus, as well as by
+Dug&egrave;s.</p>
+
+<p>The third, and naturally the most important, ingredient in the
+<i>General Morphology</i> was the doctrine of evolution, in the
+form given to it by Darwin. We have here no concern with Haeckel's
+evolutionary philosophy, with the way in which he combined his
+evolutionism and his materialism to form a queer Monism of his own.
+We are interested only in the way he applied evolution to
+morphology, what modifications he introduced into the principles of
+the science, and in general in what way he interpreted the facts
+and theories of morphology in the light of the new knowledge.</p>
+
+<p>We find that he repeats very much what Darwin said, giving, of
+course, more detail to the exposition, and elaborating,
+particularly in his recapitulation theory or "biogenetic law,"
+certain doctrines not explicitly stated by Darwin.</p>
+
+<p>Like Darwin he held that the natural system is in reality
+genealogical. "There exists," he writes, "one single connected
+natural system of organisms, and this single natural system is the
+expression of real relations which actually exist between all
+organisms, alike those now in being on the earth and those that
+have existed there in some past time. The real relations which
+unite all living and extinct organisms in one or other of the
+principal groups of the natural system, are genealogical: their
+relationship in form is blood-relationship; the natural system is
+accordingly the genealogical tree of organisms, or their
+genealogema.... All organisms are in the last resort descendants of
+autogenous Monera, evolved as a consequence of the divergence of
+characters through natural selection. The different subordinate
+groups of the natural system, the categories of the class, order,
+family, genus, etc., are larger or smaller branches of the
+genealogical tree, and the degree of their divergence indicates the
+degree of genealogical affinity of the <span class="pagenum"><a
+name="pg251" id="pg251">251</a></span>related organisms with one
+another and with the common ancestral form" (ii., p.&nbsp;420).</p>
+
+<p>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 <i>General
+Morphology</i>. The genealogical trees depicted in the second
+volume (plates i.-viii.) are nothing more than graphic
+representations of the ordinary systematic relationships of
+organisms, with a few hypothetical ancestral groups or forms thrown
+in to give the whole a genealogical turn.</p>
+
+<p>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 <i>Urform</i>, and so on for the
+higher categories.</p>
+
+<p>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 (<i>Formenb&uuml;schel</i>)" (ii., p.&nbsp;420).</p>
+
+<p>The archetype of the genus, family, order, class and phylum was
+thus conceived to have had at some past time a real existence.</p>
+
+<p>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.<a
+name="FNanchor_370" id="FNanchor_370" /><a href=
+"#Footnote_370" class="fnanchor">[370]</a> Similarly the "type of
+organisation," in von Baer's sense, was due to heredity, the "grade
+of differentiation" to adaptation.</p>
+
+<p>So far Haeckel merely emphasised what Darwin had already said in
+the <i>Origin of Species</i>. But by his statement of the
+"biogenetic law," and particularly by the clever use <span class=
+"pagenum"><a name="pg252" id="pg252">252</a></span>he made of it,
+Haeckel went a step beyond Darwin, and exercised perhaps a more
+direct influence upon evolutionary morphology than Darwin
+himself.</p>
+
+<p>Haeckel was not the original discoverer of the law of
+recapitulation. It happened that a few years before the publication
+of Haeckel's <i>General Morphology</i>, a German doctor, Fritz
+M&uuml;ller by name, stationed in Brazil, had been working on the
+development of Crustacea under the direct inspiration of Darwin's
+theory, and had published in 1864 a book<a name="FNanchor_371"
+id="FNanchor_371" /><a href="#Footnote_371" class=
+"fnanchor">[371]</a> in which he showed that individual development
+gave a clue to ancestral history.</p>
+
+<p>He conceived that progressive evolution might take place in two
+different ways. "Descendants ... reach a new goal, either by
+deviating sooner or later whilst still on the way towards the form
+of their parents, or by passing along this course without
+deviation, but then instead of standing still advancing still
+farther" (Eng. trans., p.&nbsp;111). In the former case the
+developmental history of descendants agrees with that of the
+ancestors only up to a certain point and then diverges. "In the
+second case the entire development of the progenitors is also
+passed through by the descendants, and, therefore, so far as the
+production of a species depends upon this second mode of progress,
+the historical development of the species will be mirrored in its
+developmental history" (p.&nbsp;112).</p>
+
+<p>Of course the recapitulation of ancestral history will be
+neither literal nor extended. "The historical record preserved in
+developmental history is gradually <i>effaced</i> as the
+development strikes into a constantly straighter course from the
+egg to the perfect animal, and it is frequently
+<i>sophisticated</i> by the struggle for existence which the
+free-living larv&aelig; have to undergo" (p.&nbsp;114).</p>
+
+<p>It follows that "the primitive history of a species will be
+preserved in its developmental history the more perfectly the
+longer the series of young stages through which it passes by
+uniform steps; and the more truly, the less the mode of life of the
+young departs from that of the adults, and the less the
+peculiarities of the individual young states can be conceived <span
+class="pagenum"><a name="pg253" id="pg253">253</a></span>as
+transferred back from later ones in previous periods of life, or as
+independently acquired" (p.&nbsp;121).</p>
+
+<p>Applying these principles to Crustacea, he concluded that the
+shrimp <i>Peneus</i> with its long direct development gave the best
+and truest picture of the ancestral history of the Malacostraca,
+and that accordingly the nauplius and the zoaea larv&aelig;
+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.</p>
+
+<p>The thought that development repeats evolution was already
+implicit in the first edition of the <i>Origin</i>, but the credit
+for the first clear and detailed exposition of it belongs to F.
+M&uuml;ller.</p>
+
+<p>In much the same form as it was propounded by M&uuml;ller 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.</p>
+
+<p>It is convenient to use his term for it&mdash;the biogenetic law
+(<i>Biogenetische Grundgesetz</i>)&mdash;to distinguish it from the
+laws of Meckel-Serres and von Baer, with which it is so often
+confused.</p>
+
+<p>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.</p>
+
+<p>"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
+pal&aelig;ontological evolution according to the laws of heredity
+and adaptation.</p>
+
+<p><span class="pagenum"><a name="pg254" id=
+"pg254">254</a></span>"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.</p>
+
+<p>"The complete and accurate repetition of phyletic by biontic
+development is falsified and altered by secondary adaptation, in
+that the bion<a name="FNanchor_372" id="FNanchor_372" /><a
+href="#Footnote_372" class="fnanchor">[372]</a> during its
+individual development adapts itself to new conditions: accordingly
+the repetition is the more accurate the greater the resemblance
+between the conditions of existence under which respectively the
+bion and its ancestors developed" (ii., p.&nbsp;300).</p>
+
+<p>The last two propositions, it will be observed, are taken over
+almost verbally from F. M&uuml;ller.</p>
+
+<p>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).</p>
+
+<p>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.</p>
+
+<p>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 <span class="pagenum"><a name="pg255" id=
+"pg255">255</a></span>parallelism Haeckel merely resuscitated in an
+evolutionary form a doctrine widely discussed in the 'forties and
+'fifties,<a name="FNanchor_373" id="FNanchor_373" /><a href=
+"#Footnote_373" class="fnanchor">[373]</a> and championed
+particularly by L. Agassiz,<a name="FNanchor_374" id=
+"FNanchor_374" /><a href="#Footnote_374" class=
+"fnanchor">[374]</a> a doctrine which must be regarded as a
+development or expansion of the Meckel-Serres law.<a name=
+"FNanchor_375" id="FNanchor_375" /><a href="#Footnote_375"
+class="fnanchor">[375]</a> It is the view that a parallelism exists
+between the natural system, embryonic development, and
+pal&aelig;ontological 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:&mdash;"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 pal&aelig;ozoic 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.&nbsp;171).</p>
+
+<p>The Meckel-Serres law, it will be remembered, expressed the idea
+that the higher animals repeat in their ontogeny the adult
+organisation of animals lower in the scale. Since Haeckel
+recognised clearly that a linear arrangement of the animal kingdom
+was a mere perversion of reality, and that a branching arrangement
+of groups more truly represented the real relations of animals to
+one another, he could not of course entertain the Meckel-Serres
+theory in its original form. But he accepted the main tenet of it
+when he asserted that each stage of ontogeny had its counterpart in
+an adult ancestral form. Such ancestral forms might or might not be
+in existence as <span class="pagenum"><a name="pg256" id=
+"pg256">256</a></span>real species at the present day; they might
+or might not be discoverable as fossils. That they had real
+existence either now or at some past epoch Haeckel never doubted.
+In his construction of phylogenetic trees he was so confident in
+the truth of his biogenetic law that he largely disregarded and
+consistently minimised the importance of the evidence from
+pal&aelig;ontology.</p>
+
+<p>The biogenetic law differed from the Meckel-Serres law chiefly
+in the circumstance that many of the adult lower forms whose
+organisation was supposed to be repeated in the development of the
+higher animals were purely hypothetical, being deduced directly
+from a study of ontogeny and systematic relationships. The
+hypothetical ancestral forms which the theory thus postulated
+naturally took their place in the natural system, for they were
+merely the concrete projections or archetypes of the classificatory
+groups.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>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 <i>&Eacute;chelle</i>
+idea:<a name="FNanchor_376" id="FNanchor_376" /><a href=
+"#Footnote_376" class="fnanchor">[376]</a>&mdash;"As so high and
+complicated an organism as that of man ... rises upwards from a
+simple cellular state, and as it progresses in its differentiating
+and perfecting, it passes through the same series of
+transformations which its animal progenitors have passed through,
+during immense spaces of time, inconceivable ages ago.... Certain
+very early and low stages in the development of man, and other
+vertebrate animals in general, correspond completely in many points
+of structure with conditions which <span class="pagenum"><a name=
+"pg257" id="pg257">257</a></span>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."<a name="FNanchor_377"
+id="FNanchor_377" /><a href="#Footnote_377" class=
+"fnanchor">[377]</a></p>
+
+<p>The biogenetic law went beyond both the Meckel-Serres law and
+the law of von Baer in that it recognised that the ancestral
+history of the species accounts in part for the course which the
+development of the individual takes, that in a certain sense,
+though not in the crude way supposed by Haeckel, phylogeny is the
+cause of ontogeny. This thought, that the organism is before all an
+historical being, is of course implied in the evolution idea, is
+indeed the essential core of it. Take away this element from the
+biogenetic law&mdash;not a difficult matter&mdash;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.</p>
+
+<p>As a book, the <i>General Morphology</i> suffers a good deal
+from the arid, schematic, almost scholastic manner of exposition
+adopted. Haeckel's Prussian mania for organisation, for absolute
+distinctions, for iron-bound formalism, is here given full scope. A
+treatment less adequate to the variety, fluidity and changeableness
+of living things could hardly be imagined.</p>
+
+<p>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.</p>
+
+<p>Even in the <i>General Morphology</i> Haeckel had recognised
+that ontogeny is neither a complete nor an entirely accurate
+recapitulation of phylogeny; he had admitted, following F.
+M&uuml;ller, that the true course of recapitulation was frequently
+modified by larval and f&oelig;tal adaptations. As time went on, he
+was forced to hedge more and more on this point, and finally in his
+<i>Anthropogenie</i> (1874) and his second <span class="pagenum"><a
+name="pg258" id="pg258">258</a></span>paper on the Gastr&aelig;a
+theory (1875),<a name="FNanchor_378" id="FNanchor_378" /><a
+href="#Footnote_378" class="fnanchor">[378]</a> he had to work out
+a distinction between palingenetic and cenogenetic characters, of
+which much use was made by subsequent writers.</p>
+
+<p>The distinction may be given in Haeckel's own
+words:&mdash;"Those ontogenetic processes," he writes, "which are
+to be referred immediately, in accordance with the biogenetic law,
+to an earlier completely developed <i>independent ancestral
+form</i>, and are transmitted from this by <i>heredity</i>,
+obviously possess <i>primary</i> importance for the understanding
+of the casual-physiological relations; on the other hand, those
+developmental processes which appear subsequently through
+<i>adaptation</i> to the needs of embryonic or larval life, and
+accordingly can <i>not</i> be regarded as repeating the
+organisation of an earlier independent ancestral form, can clearly
+have for the understanding of the ancestral history only a quite
+subordinate and <i>secondary</i> importance.</p>
+
+<p>"The first I have named <i>palingenetic</i>, the second
+<i>cenogenetic</i>. Considered from this critical standpoint, the
+whole of ontogeny falls into two main parts:&mdash;First,
+<i>palingenesis</i>, or 'epitomised history'
+(<i>Auszugsgeschichte</i>), and second, <i>cenogenesis</i>, or
+'counterfeit history' (<i>F&auml;lschungsgeschichte</i>). 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."<a name="FNanchor_379" id="FNanchor_379" /><a href=
+"#Footnote_379" class="fnanchor">[379]</a></p>
+
+<p>As examples of palingenetic processes in the development of
+Amniotes, for instance, may be quoted the separation of two primary
+germ-layers, the formation of a simple notochord between medullary
+tube and alimentary canal, the appearance of a simple cartilaginous
+cranium, of the gill-arches and their vessels, of the primitive
+kidneys, the primitive tubular heart, the paired aort&aelig; 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.</p>
+
+<p>The most important phenomena to be included under <span class=
+"pagenum"><a name="pg259" id="pg259">259</a></span>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.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>Heterochrony shows itself, as a rule, either as an acceleration
+or as a retardation of developmental events, as compared with their
+relative time of occurrence during phylogeny. Thus the notochord,
+the brain, the eyes, the heart, appear earlier in the ontogenetic
+than in the phylogenetic series, while, on the other hand, the
+septum of the auricles appears in the development of the higher
+Vertebrates before the ventricular septum, which is undoubtedly a
+reversal of the phylogenetic order.</p>
+
+<p>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.</p>
+
+<p>It is worthy of note that the help of comparative anatomy is
+admittedly required in deciding what processes are palingenetic and
+what cenogenetic (p.&nbsp;412).</p>
+
+<p>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
+<i>General Morphology</i>, received with much favour. Nevertheless,
+since he did express, <span class="pagenum"><a name="pg260" id=
+"pg260">260</a></span>though in a crude, dogmatic and extreme
+manner, the main hypotheses upon which evolutionary morphology is
+founded, his historical importance is considerable. He cannot
+perhaps be regarded as typical of the morphologists of his
+time&mdash;he was too trenchantly materialistic, too much the
+populariser of a crude and commonplace philosophy of Nature. In
+point of concrete achievement in the field of pure research he fell
+notably behind many of his contemporaries.</p>
+
+<p>His friend, Carl Gegenbaur, who gained a great and well-deserved
+reputation by his masterly studies on vertebrate morphology,<a
+name="FNanchor_380" id="FNanchor_380" /><a href=
+"#Footnote_380" class="fnanchor">[380]</a> 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
+<i>Grundz&uuml;ge der vergleichenden Anatomie</i>, the second
+edition of which, published in 1870, soon came to be regarded as
+the classical text-book of evolutionary morphology, Gegenbaur
+enunciated very much the same general principles as Haeckel, and
+referred to the <i>Generelle Morphologie</i> as the chief and
+fundamental work on animal morphology. But in Gegenbaur's pages the
+Haeckelian doctrines are modified and subdued by the strong
+commonsense and thorough appreciation of the older classical or
+Cuvierian morphology that characterise Gegenbaur's work. According
+to Haeckel,<a name="FNanchor_381" id="FNanchor_381" /><a href=
+"#Footnote_381" class="fnanchor">[381]</a> Gegenbaur was greatly
+influenced by J. M&uuml;ller, who, as we know, laid as much stress
+on function as on form.</p>
+
+<p>The "General Part" of Gegenbaur's text-book is in many ways a
+significant document and deserves close attention.</p>
+
+<p>We note first of all that physiology and morphology are
+considered by Gegenbaur to be entirely distinct sciences, with
+different subject-matter and different methods. "The task of
+physiology is the investigation of the functions of the animal body
+or of its parts, the referring back of these functions to
+elementary processes and their explanation by <span class=
+"pagenum"><a name="pg261" id="pg261">261</a></span>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.&nbsp;3).</p>
+
+<p>Morphology falls naturally into two divisions&mdash;comparative
+anatomy and embryology. The method of comparative anatomy is
+<i>comparison</i> (p.&nbsp;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.&nbsp;6).</p>
+
+<p>Geoffroy St Hilaire would have subscribed to every word of this
+vindication of his "principle of connections."</p>
+
+<p>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
+<span class="pagenum"><a name="pg262" id="pg262">262</a></span>fact
+is of great importance, for those changes which an organ undergoes
+during its individual development lead through states which the
+organ in other cases permanently shows, or at the least the first
+appearance of the organ is the equivalent of a permanent state in
+another organism. If then the fully developed organ is in any
+special case so greatly modified that its proper relation to some
+organ-series is obscured, this relation may be cleared up by a
+knowledge of the organ's development. The earlier state indicated
+in this way enables one to find with ease the proper place for the
+organ and so insert it into an already known series. The relations
+which we observe in an organ-seriation are then the equivalent of
+processes which in certain cases take place in a similar manner
+during the individual development of an organ. Embryology enters
+therefore into the closest connection with comparative anatomy....
+It teaches us to know organs in their earliest states, and connects
+them up with the permanent states of others, whereby they fill up
+the gaps which we meet with in the various series formed by the
+fully developed organs of the body" (pp.&nbsp;6-7).</p>
+
+<p>This recognition of the parallelism between comparative anatomy
+and embryology is, of course, the kernel of the Meckel-Serres law.
+For Gegenbaur it had a very definite evolutionary meaning&mdash;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:&mdash;"Ontogeny thus represents,
+to a certain degree, pal&aelig;ontological 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"<a name="FNanchor_382"
+id="FNanchor_382" /><a href="#Footnote_382" class=
+"fnanchor">[382]</a> (p.&nbsp;6).</p>
+
+<p>It is worth noting that in Gegenbaur's opinion comparative <span
+class="pagenum"><a name="pg263" id="pg263">263</a></span>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 (<i>Grundz&uuml;ge</i>, pp.&nbsp;7-8).</p>
+
+<p>While Gegenbaur was at one with all "pure" morphologists,
+whether evolutionary or pre-evolutionary, in minimising as far as
+possible the importance of function in the study of form, he was
+too cautious and sober a thinker not to recognise the immense part
+which function really plays. Thus he classified organs, according
+to their function, into those that established relations with the
+external world and those that had to do with nutrition and
+reproduction, very much as Bichat had done before him.</p>
+
+<p>Like Darwin, Haeckel and most evolutionists, he interpreted the
+homological resemblances of animals as being due to heredity, their
+differences as due to adaptation,<a name="FNanchor_383" id=
+"FNanchor_383" /><a href="#Footnote_383" class=
+"fnanchor">[383]</a> but he did not adopt Haeckel's crude and shallow
+definition of these terms. For Gegenbaur heredity was a convenient
+expression for the fact of transmission, and was not explained
+offhand as the mere mechanical result of a certain material
+structure handed down from germ to germ. Adaptation he defined in a
+way which took the fullest account of function, and was as far as
+possible removed from Haeckel's definition of it as the direct
+mechanical effect of the environment upon the organism. "The
+organism is altered," writes Gegenbaur, "according to the
+conditions which influence it. The consequent <i>Adaptations</i>
+are to be regarded as gradual, but steadily progressive, changes in
+the organisation, which are striven after during the individual
+life of the organism, preserved by transmission in a series of
+generations, and further developed by means of natural selection.
+What has been gained by the ancestor becomes the heritage of the
+descendant. Adaptation and Transmission are thus alternately
+effective, the former representing the modifying, the latter the
+conservative <span class="pagenum"><a name="pg264" id=
+"pg264">264</a></span>principle.... Adaptation is commenced by a
+change in the function of organs, so that the <i>physiological
+relations</i> of organs play the most important part in it. Since
+adaptation is merely the material expression of this change of
+function, the modification of the function as much as its
+expression is to be regarded as a gradual process. In Adaptation,
+the closest connection between the function and the structure of an
+organ is thus indicated. Physiological functions govern, in a
+certain sense, structure; and so far what is morphological is
+subordinated to what is physiological" (<i>Elements</i>, pp.&nbsp;8-9).
+Gegenbaur recognised also that morphological differentiation
+depended largely on the physiological division of labour
+(<i>Grundz&uuml;ge</i>, p.&nbsp;49).</p>
+
+<p>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."<a name="FNanchor_384" id="FNanchor_384" /><a
+href="#Footnote_384" class="fnanchor">[384]</a> Such "correlative
+adaptation" was due to nutrition being a "connected, centralised
+activity."</p>
+
+<p>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 (<i>Verrichtung</i>) presupposes a series of
+other functions, and accordingly every organ must possess close
+relations with, and be dependent on, all the others"
+(<i>Grundz&uuml;ge</i>, p.&nbsp;71). The organism must be regarded as an
+individual whole which is as much conditioned by its parts as one
+part is conditioned by the others. For an understanding of
+correlation a knowledge of functions, and of the functional
+relations of the organism to its environment, is clearly
+indispensable.</p>
+
+<p><span class="pagenum"><a name="pg265" id=
+"pg265">265</a></span>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" (<i>Elements</i>, p.&nbsp;67).</p>
+
+<p>The most important clue to discovering this genetic connection
+is of course that given by homology; it is indeed the main
+principle of evolutionary morphology that what is common in
+organisation is due to common descent, what is divergent is due to
+adaptation. "Homology ... corresponds to the hypothetical genetic
+relationship. In the more or the less clear homology, we have the
+expression of the more or less intimate degree of relationship.
+Blood-relationship becomes dubious exactly in proportion as the
+proof of homologies is uncertain" (<i>Elements</i>, p.&nbsp;63).</p>
+
+<p>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 (<i>Organisationszustand</i>), 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"
+(<i>Grundz&uuml;ge</i>, p.&nbsp;75). The real ancestral forms cannot, as
+a rule, be discovered among living species, nor often as extinct.
+"When we arrange allied forms in series by means of comparison, and
+seek to derive the more complex from the simpler, we recognise in
+the lower and simpler forms only similarities <span class=
+"pagenum"><a name="pg266" id="pg266">266</a></span>with the
+ancestral form, which remains essentially hypothetical" (p.
+75).</p>
+
+<p>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 (<i>cf.</i> von Baer's law).</p>
+
+<p>Gegenbaur distinguishes homologous or morphologically equivalent
+structures from such as are analogous or physiologically
+equivalent, just as did Owen and the older anatomists. Like von
+Baer he recognises homologies, as a rule, only within the type.</p>
+
+<p>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.</p>
+
+<p>General homology, in Gegenbaur's sense, relates to resemblances
+of organs within the organism, and includes four kinds of
+resemblance, homotypy, homodynamy, homonomy and homonymy. Right and
+left organs are homotypic, metameric organs are homodynamic;
+homonomy is the relation exemplified by fin-rays or fingers, which
+are arranged with reference to a transverse axis of the body;
+homonymy is a sort of metamerism in secondary parts (not the main
+axis) of the body, and is shown by the various divisions of the
+appendages (<i>Grundz&uuml;ge</i>, p.&nbsp;80).</p>
+
+<p>Special homology, on the other hand, relates to resemblances
+between organs in different animals. The interesting thing is that
+Gegenbaur defines it genetically. Special homology is the name we
+give "to the relations which obtain between two organs which have
+had a common origin, and which have also a common embryonic
+history" (<i>Elements</i>, p.&nbsp;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.</p>
+
+<p>"Special homology," he continues, "must be again separated <span
+class="pagenum"><a name="pg267" id="pg267">267</a></span>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.&nbsp;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.</p>
+
+<p>Independently of Gegenbaur, Sir E. Ray Lankester proposed in
+1870 a genetic definition of homology.<a name="FNanchor_385" id=
+"FNanchor_385" /><a href="#Footnote_385" class=
+"fnanchor">[385]</a> He proposed, indeed, to do away with the term
+homology altogether, on the ground that it included many
+resemblances which were obviously not due to common
+descent&mdash;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 <i>homogenous</i>(p.&nbsp;36).
+All other resemblances were to be called <i>homoplastic</i>.
+"Homoplasy includes all cases of close resemblance of form which
+are not traceable to homogeny, all details of agreement not
+homogenous, in structures which are broadly homogenous, as well as
+in structures having no genetic affinity" (p.&nbsp;41). Serial homology,
+for instance, was a case of homoplasy.</p>
+
+<p>The term "analogy" was to be retained for cases of functional
+resemblance, whether homogenetic or not.</p>
+
+<p>The attempt was an interesting one, but most morphologists
+wisely adhered to the old concept of homology, in spite of
+Lankester's declaration that this belonged to an older "Platonic"
+philosophy, and ought to be superseded by a term more consonant
+with the new philosophy of evolution.</p>
+
+<div class="footnote">
+<p><a name="Footnote_366" id="Footnote_366" /><a href=
+"#FNanchor_366"><span class="label">[366]</span></a> <i>Generelle
+Morphologie der Organismen. Allgemeine Grundz&uuml;ge der
+organischen Formenwissenschaft, mechanisch begr&uuml;ndet durch die
+von Ch. Darwin reformierte Descendenztheorie</i>. Berlin, 1866.
+Reprinted in part as <i>Prinzipien der generellen Morphologie der
+Organismen</i>. Berlin, 1906.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_367" id="Footnote_367" /><a href=
+"#FNanchor_367"><span class="label">[367]</span></a> He mentions as
+his predecessors in this field, Bronn, J. M&uuml;ller, Burmeister,
+and G. J&auml;ger.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_368" id="Footnote_368" /><a href=
+"#FNanchor_368"><span class="label">[368]</span></a> In
+<i>Grundriss einer Allgemeinen Naturgeschichte der Radiolarien</i>,
+Berlin, 1887, and <i>Kunstformen der Natur</i>, Suppl. Heft,
+Leipzig.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_369" id="Footnote_369" /><a href=
+"#FNanchor_369"><span class="label">[369]</span></a> Haeckel had an
+intense admiration for Goethe's morphological work. It is a curious
+coincidence that the work of Goethe, Oken and Haeckel was closely
+associated with the town of Jena.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_370" id="Footnote_370" /><a href=
+"#FNanchor_370"><span class="label">[370]</span></a> But he himself
+would not admit this! See <i>Gen. Morph.</i>, ii., p.&nbsp;11.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_371" id="Footnote_371" /><a href=
+"#FNanchor_371"><span class="label">[371]</span></a> <i>F&uuml;r
+Darwin</i>, 1864. Eng. trans, by Dallas as <i>Facts and Arguments
+for Darwin</i>, London, 1869.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_372" id="Footnote_372" /><a href=
+"#FNanchor_372"><span class="label">[372]</span></a> The bion is
+the physiological, as the morphon is the morphological,
+individual.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_373" id="Footnote_373" /><a href=
+"#FNanchor_373"><span class="label">[373]</span></a> See Vogt,
+<i>Embryologie des Salmones</i>, p.&nbsp;259, 1842, and <i>supra</i>, p.
+230.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_374" id="Footnote_374" /><a href=
+"#FNanchor_374"><span class="label">[374]</span></a> <i>An Essay on
+Classification</i>, London, 1859.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_375" id="Footnote_375" /><a href=
+"#FNanchor_375"><span class="label">[375]</span></a> It was hinted
+at by Tiedemann. "It is clear that, proceeding from the earlier to
+the more recent strata, a gradation in fossil forms can be
+established from the simplest organised animals, the polyps, up to
+the most complex, the mammals, and that accordingly the animal
+kingdom as a whole has its developmental periods just like the
+single individual organism. The species and genera which have
+become extinct during the evolutionary process may be compared with
+the organs which disappear during the development of the individual
+animal" (p.&nbsp;73, 1808).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_376" id="Footnote_376" /><a href=
+"#FNanchor_376"><span class="label">[376]</span></a> <i>The History
+of Creation</i>, vol. i., p.&nbsp;310, 1876. Translation of the
+<i>Nat&uuml;rliche Sch&ouml;pfungsgeschichte</i>, 1868.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_377" id="Footnote_377" /><a href=
+"#FNanchor_377"><span class="label">[377]</span></a> <i>Cf.</i> a
+parallel passage from Serres, <i>supra</i>, p.&nbsp;82.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_378" id="Footnote_378" /><a href=
+"#FNanchor_378"><span class="label">[378]</span></a> <i>Jenaische
+Zeitschrift</i>, ix., pp.&nbsp;402-508, 1875.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_379" id="Footnote_379" /><a href=
+"#FNanchor_379"><span class="label">[379]</span></a> <i>Loc.
+cit.</i>, ix., p.&nbsp;409.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_380" id="Footnote_380" /><a href=
+"#FNanchor_380"><span class="label">[380]</span></a>
+<i>Untersuchungen zur vergl. Anatomie d. Wirbelthiere</i>, Leipzig,
+i., 1864; ii., 1865; and iii., 1872.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_381" id="Footnote_381" /><a href=
+"#FNanchor_381"><span class="label">[381]</span></a> "U. d.
+Biologie in Jena w&auml;hrend des 19 Jahrhunderts," <i>Jenaische
+Zeitschrift</i>, xxxix., pp.&nbsp;713-26, 1905.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_382" id="Footnote_382" /><a href=
+"#FNanchor_382"><span class="label">[382]</span></a> <i>Grundriss
+der vergl. Anatomie</i>, 1874, 2nd ed., 1878. Trans. by F. Jeffrey
+Bell, revised by E. Ray Lankester, as <i>Elements of Comparative
+Anatomy</i>, London, 1878.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_383" id="Footnote_383" /><a href=
+"#FNanchor_383"><span class="label">[383]</span></a> "This theory
+(evolution) shows that what was formerly called 'structural plan'
+or 'type' is the sum of the dispositions (<i>Einrichtungen</i>) 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" (<i>Grundz&uuml;ge</i>, p.&nbsp;19).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_384" id="Footnote_384" /><a href=
+"#FNanchor_384"><span class="label">[384]</span></a> <i>History of
+Creation</i>, i., pp.&nbsp;241-2.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_385" id="Footnote_385" /><a href=
+"#FNanchor_385"><span class="label">[385]</span></a> "On the use of
+the term Homology in Modern Zoology, and the distinction between
+Homogenetic and Homoplastic agreements," <i>Ann. Mag. Nat.
+Hist.</i> (4), vi., pp.&nbsp;35-43, 1870.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg268" id=
+"pg268">268</a></span></p>
+
+<h3>CHAPTER XV</h3>
+
+<h4>EARLY THEORIES ON THE ORIGIN OF VERTEBRATES</h4>
+
+<p><span class="smcap">Haeckel</span> and Gegenbaur set the fashion
+for phylogenetic speculation, and up to the middle 'eighties, when
+the voice of the sceptics began to make itself heard, the chief
+concern of the younger morphologists was the construction of
+genealogical trees. The period from about 1865 to 1885 might well
+be called the second speculative or transcendental period of
+morphology, differing only from the first period of
+transcendentalism by the greater bulk of its positive achievement.
+It must be remembered that the later workers (at least towards the
+end of this period) had immense advantages over their predecessors
+in the matter of equipment and technique; they possessed
+well-fitted laboratories in the university towns and by the sea;
+they had at their command perfected microscopes and microtomes;
+while the whole new technique of microscopical anatomy with its
+endless variety of stains and reagents made it possible for the
+tyro to confirm in a day what von Baer and M&uuml;ller had taken
+weeks of painful endeavour to discover.<a name="FNanchor_386" id=
+"FNanchor_386" /><a href="#Footnote_386" class=
+"fnanchor">[386]</a> 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.</p>
+
+<p>This period of rapid development, which set in soon after the
+coming of evolution and multiplied the concrete <span class=
+"pagenum"><a name="pg269" id="pg269">269</a></span>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 Gastr&aelig;a 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 Gastr&aelig;a and C&oelig;lom
+theories.</p>
+
+<p>In the first period we might again distinguish two main
+tendencies, according as speculations were based mainly upon
+anatomical or mainly upon embryological considerations, and it so
+happens that these two tendencies are very well illustrated by the
+various theories as to the origin of Vertebrates which began to
+appear towards the 'seventies. We shall accordingly, in this
+chapter, consider very briefly the history of the earlier views on
+the phylogeny of the vertebrate stock.</p>
+
+<p>In the early days, before the other claimants to the dignity of
+ancestral form to the Vertebrates&mdash;<i>Balanoglossus</i>,
+Nemertines and the rest&mdash;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&mdash;you may read Kowalevsky's earlier
+memoirs and not realise that they were written several years after
+the publication of the <i>Origin of Species</i>.</p>
+
+<p>His first paper of evolutionary importance was a note in <span
+class="pagenum"><a name="pg270" id="pg270">270</a></span>Russian on
+the development of Amphioxus, published in 1865. This subject was
+followed up in two papers which appeared in 1867<a name=
+"FNanchor_387" id="FNanchor_387" /><a href="#Footnote_387"
+class="fnanchor">[387]</a> and 1877.<a name="FNanchor_388" id=
+"FNanchor_388" /><a href="#Footnote_388" class=
+"fnanchor">[388]</a> In his papers on Amphioxus Kowalevsky made out
+the main features in the development of this primitive form, and
+showed that the chief organs were formed in essentially the same
+way as in Vertebrates; he described the formation of the
+archenteron by invagination, the appearance of the medullary folds,
+which coalesced to form the neural canal, the formation of the
+notochord and of the gill-slits. At first he made the mistake of
+supposing that the body-cavity arose from the segmentation-cavity,
+but in his later paper he rightly surmised that it was formed from
+the cavities of the "primitive vertebr&aelig;," or mesodermal
+segments. The origin of the notochord from the endoderm was also
+not made out by Kowalevsky in his paper of 1867.</p>
+
+<p>Although many important details remained to be discovered by
+later investigators,<a name="FNanchor_389" id=
+"FNanchor_389" /><a href="#Footnote_389" class=
+"fnanchor">[389]</a> 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.</p>
+
+<p>Meanwhile, in 1866 and 1871, Kowalevsky had communicated memoirs
+of even greater interest,<a name="FNanchor_390" id=
+"FNanchor_390" /><a href="#Footnote_390" class=
+"fnanchor">[390]</a> in which he showed that the simple Ascidians
+developed in an extraordinarily similar way to Amphioxus and hence
+to Vertebrates in general. His proof that Ascidians also develop on
+the vertebrate type aroused great interest at the time, and was
+naturally acclaimed by the evolutionists as a striking piece of
+evidence in favour of their doctrine. The systematic position of
+the Ascidians was at that time quite uncertain; they were grouped,
+as a rule, with the Mollusca, and certainly no one suspected that
+their well-known <span class="pagenum"><a name="pg271" id=
+"pg271">271</a></span>tailed larv&aelig;, first seen by Savigny,
+showed any but the most superficial analogy with the tadpoles of
+Amphibia. Kowalevsky's papers put a different complexion on the
+matter. In the first of them he showed how the nervous system of
+the simple Ascidian developed from ectodermal folds just as it did
+in Amphioxus and Vertebrates, how gill-slits were formed in the
+walls of the pharynx, and how there existed in the ascidian larva a
+structure which in position and mode of development was the strict
+homologue of the vertebrate notochord. In his second paper he
+entered into much more detail, and published some excellent
+figures, often reproduced since (see <a href="#pg272">Fig.&nbsp;13</a>),
+but the proof of the affinity between Vertebrates and Ascidians was
+in all essentials complete in his paper of 1866.</p>
+
+<p><span class="pagenum"><a name="pg272" id=
+"pg272">272</a></span></p>
+
+<div class="figcenter">
+<img src="images/fig13a.jpg"
+alt="Development of the Ascidian Larva. (After Kowalevsky.)" /></div>
+
+<p class="center2"><span class="smcap">Fig.</span>
+13.&mdash;Development of the Ascidian Larva. (After
+Kowalevsky.)</p>
+
+<p>Kowalevsky's results were accepted by Haeckel, Gegenbaur,
+Darwin,<a name="FNanchor_391" id="FNanchor_391" /><a href=
+"#Footnote_391" class="fnanchor">[391]</a> and many others as
+conclusive evidence of the origin of Vertebrates from a form
+resembling the ascidian tadpole; they were extended and amplified
+by Kupffer<a name="FNanchor_392" id="FNanchor_392" /><a href=
+"#Footnote_392" class="fnanchor">[392]</a> in 1870, later by van
+Beneden and Julin<a name="FNanchor_393" id="FNanchor_393" /><a
+href="#Footnote_393" class="fnanchor">[393]</a> and numerous other
+workers; they were adversely criticised by Metschnikoff<a name=
+"FNanchor_394" id="FNanchor_394" /><a href="#Footnote_394"
+class="fnanchor">[394]</a> and von Baer,<a name="FNanchor_395" id=
+"FNanchor_395" /><a href="#Footnote_395" class=
+"fnanchor">[395]</a> as well as by H. de Lacaze-Duthiers and A.
+Giard.<a name="FNanchor_396" id="FNanchor_396" /><a href=
+"#Footnote_396" class="fnanchor">[396]</a> Lacaze-Duthiers and von
+Baer both held fast to the old view that Ascidians were directly
+comparable with Lamellibranch molluscs; they denied the homology of
+the ascidian nervous system with that of Vertebrates, von Baer
+being at great pains to show that the ascidian nerve-centre was
+really ventral in position. He pointed out also that the
+"notochord" was confined to the tail of the ascidian larva. Giard's
+attitude was by no means so uncompromising, and the criticisms he
+passed on the Kowalevsky theory are both subtle and instructive. He
+admits that there exists a real homology between, for instance, the
+notochord of Vertebrates and that of Ascidians. "But," he adds, "it
+is too often forgotten <span class="pagenum"><a name="pg273" id=
+"pg273">273</a></span>that homology does not necessarily mean an
+immediate common origin or close relationship. There exist,
+doubtless, homologies of great atavistic importance&mdash;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.&nbsp;278-9).
+This point of view is strengthened by the fact that in
+<i>Molgula</i>, studied by Lacaze-Duthiers, the embryo is
+practically stationary, and forms no notochord, nor ever develops
+sense-organs in the cerebral vesicle.</p>
+
+<p>Giard's general conclusion is that "the true homology with
+Vertebrates ceases after the formation of the cavity of Rusconi and
+the medullary groove: the homologies established by Kowalevsky for
+the notochord and the relations of the digestive tube and nervous
+systems are not atavistic, but adaptive, homologies" (p.&nbsp;282).
+There is accordingly no close genetic relationship between
+Ascidians and Vertebrates.</p>
+
+<p>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.<a name="FNanchor_397" id=
+"FNanchor_397" /><a href="#Footnote_397" class=
+"fnanchor">[397]</a> The fall of the Ascidians from their larval high
+estate provided the text for many a Darwinian sermon.</p>
+
+<p><span class="pagenum"><a name="pg274" id=
+"pg274">274</a></span>Some years after the genetic relationship of
+Ascidians and Vertebrates had been established, a rival theory of
+the origin of Vertebrates made its appearance&mdash;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.</p>
+
+<p>Dohrn's earlier views are contained in the fascinating little
+book published in 1875, which bears the title <i>Der Ursprung der
+Wirbelthiere und das Princip des Functionswechsel</i> (Leipzig). He
+followed this up by a long series of studies on vertebrate anatomy
+and embryology,<a name="FNanchor_398" id="FNanchor_398" /><a
+href="#Footnote_398" class="fnanchor">[398]</a> in which he
+modified his views in certain details. We shall confine our
+attention to the first sketch of his theory.</p>
+
+<p>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 &oelig;sophagus. 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
+<i>crura cerebelli</i> in the <i>fossa rhomboidea</i>, and that in
+Vertebrates this mouth has been replaced functionally by a new
+ventrally placed mouth, formed by the <span class="pagenum"><a
+name="pg275" id="pg275">275</a></span>medial coalescence of a pair
+of gill-slits.<a name="FNanchor_399" id="FNanchor_399" /><a
+href="#Footnote_399" class="fnanchor">[399]</a> Probably the two
+mouths at one period co-existed, and the older one was ousted by
+the growing functional importance of the newer mouth.</p>
+
+<p>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 Ch&aelig;topods at the present
+day. For their support cartilaginous gill-arches naturally arose in
+the body-wall, and the superficial musculature became attached to
+these bars. "There existed in all the segments of the
+Annelid-ancestors of Vertebrates gills with cartilaginous skeleton
+and gill-arches in the body wall. Each gill had its veins and
+arteries, each had its branch of the ventral nerve-cord, and
+between each successive pair of gills a segmental organ opened to
+the exterior" (p.&nbsp;14, 1875). The paired fins and limbs of the
+Vertebrate arose by the functional transformation of two pairs of
+these gills. The anterior gills became the definitive internal
+gills of the Vertebrate, for they gradually shifted into the mouths
+of the anterior segmental organs, which had already acquired an
+opening into the pharynx and had been transformed into true
+gill-slits. The posterior gills degenerated and disappeared, but
+their arches remained as ribs. Gill-arches and ribs were
+accordingly homologous structures and formed a <i>parietal</i>
+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.</p>
+
+<p>If Vertebrates arose from primitive Annelid ancestors, how
+account for Amphioxus and the Ascidians, which seem to <span class=
+"pagenum"><a name="pg276" id="pg276">276</a></span>be the most
+primitive living Vertebrates and yet show no particular annelidan
+affinities? Dohrn tries to answer this awkward question by showing
+that these forms are not primitive but degenerate. He points out
+first that Cyclostomes are degenerate fish, half specialised and
+half degraded in adaptation to a parasitic mode of life. He thinks
+that if an <i>Ammocoetes</i> were to become sexually mature and
+degenerate still further, forms would result which would resemble
+Amphioxus, and ultimately, if the process of degeneration went far
+enough, larval Ascidians. Amphioxus therefore might well be
+considered an extremely simplified and degenerate Cyclostome, and
+the ascidian larva the last term of this degeneration-series. Both
+Amphioxus and the Ascidians would accordingly be descended from
+fish, instead of fish being evolved from them.</p>
+
+<p>Dohrn conceived that the transformation of the Annelid into the
+Vertebrate took place mainly by reason of an important transforming
+principle, which he calls the principle of function-change. Each
+organ, Dohrn thinks, has besides its principal function a number of
+subsidiary functions which only await an opportunity to become
+active. "The transformation of an organ takes place by reason of
+the succession of the functions which one and the same organ
+possesses. Each function is a resultant of several components, of
+which one is the principal or primary function, while the others
+are the subsidiary or secondary functions. The weakening of the
+principal function and the strengthening of a subsidiary function
+alters the total function; the subsidiary function gradually
+becomes the chief function, the total function becomes quite
+different, and the consequence of the whole process is the
+transformation of the organ" (p.&nbsp;60). Examples of function-change
+are not difficult to find. Thus the stomach in most Vertebrates
+performs both a chemical and a mechanical function, but in some
+forms a part of it specialises in the mechanical side of the work
+and becomes a gizzard, while the remaining part confines its
+energies to the secretion of the gastric juice. So, too, it is
+through function-change that certain of the ambulatory appendages
+of Arthropods have become transformed into jaws&mdash;their
+function as graspers of food has gradually prevailed over their
+main <span class="pagenum"><a name="pg277" id=
+"pg277">277</a></span>function as walking limbs. In the evolution
+of Vertebrates from Annelids the principle came into action in many
+connections&mdash;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
+<i>Genesis of Species</i> (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.</p>
+
+<p>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 <i>Lopadorhynchus</i><a name=
+"FNanchor_400" id="FNanchor_400" /><a href="#Footnote_400"
+class="fnanchor">[400]</a> 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
+(<i>Vermittelungsorgane</i>) 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."<a name="FNanchor_401"
+id="FNanchor_401" /><a href="#Footnote_401" class=
+"fnanchor">[401]</a></p>
+
+<p>Dohrn was well aware of the functional, or as he calls <span
+class="pagenum"><a name="pg278" id="pg278">278</a></span>it, the
+physiological, orientation of his principle, and he rightly
+regarded this as one of its chief merits. He held that morphology
+became too abstract and one-sided if it disregarded physiology
+completely; he saw clearly that the evolution of function was quite
+as important a problem as the evolution of form, and that neither
+could be solved in isolation from the other. "The concept of
+function-change is purely physiological;" he writes, "it contains
+the elements out of which perhaps a history of the evolution of
+function may gradually arise, and for this very reason it will be
+of great utility in morphology, for the evolutionary history of
+structure is only the concrete projection of the content and course
+of the evolution of function, and cannot be comprehended apart from
+it" (p.&nbsp;70).<a name="FNanchor_402" id="FNanchor_402" /><a href=
+"#Footnote_402" class="fnanchor">[402]</a></p>
+
+<p>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 (<i>Urqualit&auml;t</i>) of all being.... Let us
+take it as given" (p.&nbsp;75).</p>
+
+<p>Semper's speculations on the genetic affinity of Articulates and
+Vertebrates are contained in two papers<a name="FNanchor_403" id=
+"FNanchor_403" /><a href="#Footnote_403" class=
+"fnanchor">[403]</a> which appeared about the same time as Dohrn's.
+He openly acknowledges that his work is essentially a continuation
+of Geoffroy's transcendental speculations, and gives in his second
+paper a good historical account of the views of his great
+predecessor. It is a significant fact that evolutionary
+morphologists very generally held that Geoffroy was right in
+maintaining against Cuvier<a name="FNanchor_404" id=
+"FNanchor_404" /><a href="#Footnote_404" class=
+"fnanchor">[404]</a> the unity of plan of the whole <span class=
+"pagenum"><a name="pg279" id="pg279">279</a></span>animal kingdom,
+for they saw in this a strong argument for the monophyletic descent
+of all animals from one common ancestral form.</p>
+
+<p>In his first paper Semper does little more than break ground; he
+insists on the fact that both Annelids and Vertebrates are
+segmented animals, and he points out how close is the analogy
+between the nephridia or "segmental organs" of the former and the
+excretory (mesonephric) tubules of the latter, upon which he
+published in the same volume an extensive memoir. At this time he
+considered <i>Balanoglossus</i>&mdash;by reason of its gill-slits
+(its notochord he did not know)&mdash;to be the nearest living
+representative of the ancestral form of Vertebrates and
+Annelida.</p>
+
+<p>His second paper is a more exhaustive piece of work and deals
+with every aspect of the problem, both from an anatomical and from
+an embryological standpoint. It is consciously and admittedly an
+attempt to apply Geoffroy's principle of the unity of plan and
+composition to the three great metameric groups, the Annelida,
+Arthropoda, and Vertebrata. Semper follows Geoffroy's lead very
+closely in maintaining that it is not the position of the organs
+relative to the ground that must be taken into account in
+establishing their homologies, but solely their spatial relations
+one to another. He holds that dorsum and venter are terms of purely
+physiological import, and he proposes to substitute for them the
+terms neural and cardial (better, h&aelig;mal) surfaces, either of
+which may be either dorsal or ventral in position.</p>
+
+<p>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 (<i>evolutio
+bigemina</i>), the <span class="pagenum"><a name="pg280" id=
+"pg280">280</a></span>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 (<i>evolutio
+gemina</i>). Semper is at pains to prove that <i>evolutio
+bigemina</i> is characteristic also of Annelidan development.</p>
+
+<div class="figcenter">
+<img
+src="images/fig14a.jpg"
+alt="Transverse Section (Inverted) of the Worm Nais. (After Semper.)" /></div>
+
+<p class="center2"><span class="smcap">Fig.</span>
+14.&mdash;Transverse Section (Inverted) of the Worm Nais. (After
+Semper.)</p>
+
+<table width="80%" summary="Meckel's Cartilage"
+border="0" cellpadding="2" cellspacing="2">
+<tbody>
+<tr>
+<td class="cell_lt217b"><i>a.c.</i>&nbsp;&nbsp;Alimentary&nbsp;canal.</td>
+<td class="cell_lt217b"><i>sp.g.</i>&nbsp;&nbsp;Spinal&nbsp;ganglion.</td>
+<td class="cell_lt217b"><i>d.p.</i>&nbsp;&nbsp;Neural&nbsp;muscle-plate.</td>
+</tr>
+
+<tr>
+<td class="cell_lt217b"><i>n.c.</i>&nbsp;&nbsp;Nerve&nbsp;cord.</td>
+<td class="cell_lt217b">
+<i>&nbsp;&nbsp;&nbsp;&nbsp;n.</i>&nbsp;&nbsp;Notechord.</td>
+<td class="cell_lt217b"><i>v.p.</i>&nbsp;&nbsp;Haemal&nbsp;muscle-plate.</td>
+</tr>
+</tbody>
+</table>
+
+<p>He gets his facts from an elaborate study of the process of
+budding in the <i>Naid&aelig;</i>, making the somewhat risky
+assumption that regeneration takes essentially the same course as
+embryonic development.</p>
+
+<p>He succeeds in showing&mdash;to his own satisfaction at
+least&mdash;that in the formation of new segments in <i>Nais</i>
+and <i>Ch&aelig;togaster</i> a strand of cells appears between the
+alimentary canal and the nerve-cord, and that from this axial
+strand the <span class="pagenum"><a name="pg281" id=
+"pg281">281</a></span>h&aelig;mal muscle-plates grow out dorsally
+round the alimentary canal and the neural muscle-plates ventrally
+round the nerve-cord (see <a href="#pg280">Fig.&nbsp;14</a>).</p>
+
+<p>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.</p>
+
+<p>The nervous system Semper found to develop in the buds of
+<i>Nais</i> and <i>Ch&aelig;togaster</i> 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
+&oelig;sophagus and fusing with the paired "sense-plates" which
+develop from the ectoderm of the head. The cerebral ganglion is
+accordingly only secondarily h&aelig;mal in position, and there is
+no need therefore to seek in Vertebrates for the homologue of the
+&oelig;sophageal commissures of Annelids, as, for instance,
+Schneider did.</p>
+
+<p>Since the mouth opens on the neural surface in Annelids and on
+the h&aelig;mal 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 h&aelig;mal surface of the
+head in the leech <i>Clepsine</i> (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.</p>
+
+<p>The homologues of the gill-slits Semper finds in two little
+canals in the head of <i>Ch&aelig;togaster</i>, 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 <i>Kiemenkorb</i> or gill-basket, comparable
+with that of Amphioxus.</p>
+
+<p>Gill-slits, notochord, relation of nervous system, mesonephric
+tubules, are thus common to Annelids and Vertebrates&mdash;what
+further proof could one desire of the close relationship of these
+groups? Yet Semper enters into refinements of comparison, seeing,
+for instance, in the lateral portions of the ventral ganglia (<a
+href="#pg280">Fig.&nbsp;14</a>, <i>sp.&nbsp;g.</i>) the homologues of the
+spinal ganglia of Vertebrates, and comparing the lateral line of
+sense organs in Annelids with the lateral line in Anamnia.</p>
+
+<p>He will not admit that Amphioxus and the Ascidians <span class=
+"pagenum"><a name="pg282" id="pg282">282</a></span>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.</p>
+
+<p>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:&mdash;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 &oelig;sophageal
+ring, potentially able to develop either the Vertebrate or the
+Annelid mouth, and so to give origin both to the Articulate and to
+the Vertebrate series. The common ancestor alike of unsegmented
+worms and of all segmented types is probably the trochosphere
+larva, which in the Vertebrates is represented by the simple
+<i>Keimblase</i> or blastula.</p>
+
+<p>The Annelid theory of Dohrn and Semper was perhaps not so widely
+accepted as the rival Ascidian theory, but it counted not a few
+adherents and gave a certain stimulus to comparative morphology.
+F. M. Balfour, who pointed out about the same time as Semper the
+analogy between the nephridia of Annelids and the mesonephric
+tubules of Vertebrates,<a name="FNanchor_405" id=
+"FNanchor_405" /><a href="#Footnote_405" class=
+"fnanchor">[405]</a> 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,<a name="FNanchor_406"
+id="FNanchor_406" /><a href="#Footnote_406" class=
+"fnanchor">[406]</a> Balfour came to the conclusion "that we must
+look for the ancestors of the Chordata, not in allies <span class=
+"pagenum"><a name="pg283" id="pg283">283</a></span>of the present
+Ch&aelig;topoda, but in a stock of segmented forms descended from
+the same unsegmented types as the Ch&aelig;topoda, but in which two
+lateral nerve-cords, like those of Nemertines, coalesced dorsally
+instead of ventrally to form a median nervous cord. This group of
+forms, if my suggestion as to their existence is well founded,
+appears now to have perished."<a name="FNanchor_407" id=
+"FNanchor_407" /><a href="#Footnote_407" class=
+"fnanchor">[407]</a></p>
+
+<p>He held that while there was much to be said for the interchange
+of dorsal and ventral surfaces postulated by Dohrn and Semper, the
+difficulties involved in the supposition were too great; he
+preferred, therefore, to assume that the present Vertebrate mouth
+was primitive, and not a secondary formation.</p>
+
+<p>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,<a name="FNanchor_408" id="FNanchor_408" /><a
+href="#Footnote_408" class="fnanchor">[408]</a> names of
+hypothetical groups being printed in capitals, names of degenerate
+groups in italics:&mdash;</p>
+
+<div class="figcenter">
+<img
+src="images/img283a.jpg"
+alt="phylogeny of the Chordata and the genetic relation of the various classes" /></div>
+
+<p><span class="pagenum"><a name="pg284" id=
+"pg284">284</a></span>The hypothetical ancestral forms
+(Protochordata) possessed a notochord, a ventral suctorial mouth
+and numerous gill-slits, and were presumably descended from the
+common ancestor of Annelids and Vertebrates. Amphioxus and the
+Ascidians found their place in this schema as degenerate offshoots
+of the ancestral Protochordates, while the Cyclostomes were in the
+same way the degenerate modern representatives of the ancestral
+Protovertebrates.</p>
+
+<p>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,<a name="FNanchor_409" id=
+"FNanchor_409" /><a href="#Footnote_409" class=
+"fnanchor">[409]</a> on the relation of Nemertines to
+Vertebrates.</p>
+
+<p>The Annelid theory was firmly supported by Eisig, who in his
+elaborate monograph on the <i>Capitellid&aelig;</i><a name=
+"FNanchor_410" id="FNanchor_410" /><a href="#Footnote_410"
+class="fnanchor">[410]</a> maintained against F&uuml;rbringer the
+genetic identity of the Annelidan nephridia with the kidney tubules
+of Vertebrates. The independent discovery by E. Meyer<a name=
+"FNanchor_411" id="FNanchor_411" /><a href="#Footnote_411"
+class="fnanchor">[411]</a> and J. T. Cunningham,<a name=
+"FNanchor_412" id="FNanchor_412" /><a href="#Footnote_412"
+class="fnanchor">[412]</a> of an internal segmental duct in
+<i>Lanice</i>, into which several nephridia opened, seemed to
+strengthen this view.</p>
+
+<p>Following Ehlers,<a name="FNanchor_413" id=
+"FNanchor_413" /><a href="#Footnote_413" class=
+"fnanchor">[413]</a> Eisig found the homologue of the notochord in
+the accessory intestine of the <i>Capitellid&aelig;</i> and
+<i>Eunicid&aelig;</i>, which he supposed might easily be
+transformed, according to the principle of function-change, from a
+respiratory to a supporting organ. He finally disposed of the
+alternative notion that the notochord was represented in Annelids
+by the "giant-fibres" or neurochordal strands which lie close above
+the nerve-cord, a view held by Kowalevsky,<a name="FNanchor_414"
+id="FNanchor_414" /><a href="#Footnote_414" class=
+"fnanchor">[414]</a> and for a time by Semper. These strands were
+<span class="pagenum"><a name="pg285" id=
+"pg285">285</a></span>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.</p>
+
+<p>Much difference of opinion reigned as to the true homologies of
+the brain and mouth of Annelids and Vertebrates. Beard<a name=
+"FNanchor_415" id="FNanchor_415" /><a href="#Footnote_415"
+class="fnanchor">[415]</a> and others got over the difficulty of the
+h&aelig;mal position of the cerebral ganglion in Annelids by
+supposing that it degenerated and disappeared altogether in the
+Annelidan ancestor of Vertebrates, and that accordingly it had no
+homologue in the Vertebrate nervous system. Beard put forward also
+the ingenious theory that the hypophysis represents the old
+Annelidan mouth.</p>
+
+<p>Van Beneden and Julin<a name="FNanchor_416" id=
+"FNanchor_416" /><a href="#Footnote_416" class=
+"fnanchor">[416]</a> assumed that in the ancestors of Vertebrates the
+&oelig;sophagus shifted forward between the still unconnected lobes
+of the brain to open on the h&aelig;mal surface.</p>
+
+<p>The fundamental assumption of the Annelid theory, that dorsal
+and ventral surfaces are morphologically interchangeable, seemed
+rather bold to many zoologists, and Gegenbaur<a name=
+"FNanchor_417" id="FNanchor_417" /><a href="#Footnote_417"
+class="fnanchor">[417]</a> 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.</p>
+
+<p>The <i>Balanoglossus</i> theory of Vertebrate descent also
+belongs, at least in its first form, to the earlier group of
+evolutionary speculations. The gill-slits of <i>Balanoglossus</i>
+were discovered by Kowalevsky as early as 1866.<a name=
+"FNanchor_418" id="FNanchor_418" /><a href="#Footnote_418"
+class="fnanchor">[418]</a> <i>Tornaria</i> was discovered by J.
+M&uuml;ller in 1850, but by him considered an Asterid larva; its
+true nature as the larva of <i>Balanoglossus</i> was made out by
+Metschnikoff in 1870, who also remarked upon its extraordinary
+likeness to the larv&aelig; of Echinoderms.<a name="FNanchor_419"
+id="FNanchor_419" /><a href="#Footnote_419" class=
+"fnanchor">[419]</a> <span class="pagenum"><a name="pg286" id=
+"pg286">286</a></span>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.<a
+name="FNanchor_420" id="FNanchor_420" /><a href=
+"#Footnote_420" class="fnanchor">[420]</a></p>
+
+<p>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 (<i>cf.</i> 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.&nbsp;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.&nbsp;548-9). It was in
+following up this line of thought that Bateson produced his
+monumental <i>Materials for the Study of Variation</i> (1894).</p>
+
+<p>He found a strong positive argument for his theory that
+Vertebrates are descended from unsegmented forms in the fact that
+the notochord arises as an unsegmented structure. With the
+notochord he homologised the supporting rod in the proboscis of
+<i>Balanoglossus</i>, which like the notochord arises from the
+dorsal wall of the archenteron, and has a vacuolated structure. The
+gill-slits of <i>Balanoglossus</i>, with their close resemblance in
+detail to those of Amphioxus, Bateson also used as an argument in
+favour of the phylogenetic relationship of the Enteropneusta and
+Vertebrata, <span class="pagenum"><a name="pg287" id=
+"pg287">287</a></span>together with the formation from the ectoderm
+of a dorsal nerve tube.</p>
+
+<p>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<a name="FNanchor_421" id=
+"FNanchor_421" /><a href="#Footnote_421" class=
+"fnanchor">[421]</a> in the <i>Encyclopedia Britannica</i>:&mdash;"It
+seems that in <i>Balanoglossus</i> 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 <i>Balanoglossus</i> 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 <i>Cephalochorda</i> and <i>Craniata</i>, and perhaps
+protective of a single gill-slit in <i>Balanoglossus</i> 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 <i>Balanoglossus</i> gives the most
+hopeful hypothetical solution of the pedigree of Vertebrates."</p>
+
+<p>Much doubt was cast upon the Chordate affinities of the
+Enteropneusta by Spengel in his monograph of the group,<a name=
+"FNanchor_422" id="FNanchor_422" /><a href="#Footnote_422"
+class="fnanchor">[422]</a> but when the development of the
+c&oelig;lom came to be more thoroughly worked out in
+<i>Balanoglossus</i> and Amphioxus, the striking resemblance in
+this respect between the two forms gave additional support to the
+Batesonian view.<a name="FNanchor_423" id="FNanchor_423" /><a
+href="#Footnote_423" class="fnanchor">[423]</a></p>
+
+<div class="footnote">
+<p><a name="Footnote_386" id="Footnote_386" /><a href=
+"#FNanchor_386"><span class="label">[386]</span></a> The stages in
+the development of microscopical technique are well summarised by
+R. Burckhardt, <i>Geschichte der Zoologie</i>, p.&nbsp;121, Leipzig
+1907.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_387" id="Footnote_387" /><a href=
+"#FNanchor_387"><span class="label">[387]</span></a>
+"Entwickelungsgeschichte des Amphioxus lanceolatus," <i>M&eacute;m.
+Acad. Sci. St P&eacute;tersbourg</i> (Petrograd) (vii.), xi., No.
+4, 1867, 17 pp., 3 pls.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_388" id="Footnote_388" /><a href=
+"#FNanchor_388"><span class="label">[388]</span></a> "Weitere
+Studien &uuml;. die Entwickelungsgeschichte des Amphioxus
+lanceolatus," <i>Arch. f&uuml;r mikr. Anat.</i>, xiii., pp.
+181-204, 1877.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_389" id="Footnote_389" /><a href=
+"#FNanchor_389"><span class="label">[389]</span></a> Particularly
+by Hatschek (1881) and Boveri (1892).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_390" id="Footnote_390" /><a href=
+"#FNanchor_390"><span class="label">[390]</span></a>
+"Entwickelungsgeschichte der einfachen Ascidien," <i>M&eacute;m.
+Acad. Sci. St P&eacute;tersbourg</i> (Petrograd), (vii.), x., No.
+15, 1866, 19 pp., 3 pls. "Weitere Studien &uuml;. die Entwicklung
+der einfachen Ascidien," <i>Arch. f. mikr. Anat.</i>, vii., pp.
+101-130, 1871.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_391" id="Footnote_391" /><a href=
+"#FNanchor_391"><span class="label">[391]</span></a> <i>Descent of
+Man</i>, i., p.&nbsp;205, 1871.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_392" id="Footnote_392" /><a href=
+"#FNanchor_392"><span class="label">[392]</span></a> <i>Arch. f.
+mikr. Anat.</i>, vi., 1870, and viii., 1872.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_393" id="Footnote_393" /><a href=
+"#FNanchor_393"><span class="label">[393]</span></a> <i>Archives de
+Biologie</i>, 1884, 1885, and 1887.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_394" id="Footnote_394" /><a href=
+"#FNanchor_394"><span class="label">[384]</span></a> <i>Bull. Acad.
+Sci. St P&eacute;tersbourg</i> (Petrograd) xiii., 1869, and
+<i>Zeits. f. wiss. Zool.</i>, xxii., 1872.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_395" id="Footnote_395" /><a href=
+"#FNanchor_395"><span class="label">[395]</span></a> <i>M&eacute;m.
+Acad. Sci. St P&eacute;tersbourg</i>(Petrograd)(7), xix., 1873.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_396" id="Footnote_396" /><a href=
+"#FNanchor_396"><span class="label">[396]</span></a> Giard,
+<i>Arch. zool. exp&eacute;r. g&eacute;n.</i>, i., 1872, and
+Lacaze-Duthiers, <i>ibid.</i>, iii., 1874.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_397" id="Footnote_397" /><a href=
+"#FNanchor_397"><span class="label">[397]</span></a> For the later
+history of the Amphioxus-Ascidian theory the reader may be referred
+to A. Willey's well-known work, <i>Amphioxus and the Ancestry of
+the Vertebrates</i>, New York and London, 1894, and to Delage et
+H&eacute;rouard, <i>Trait&eacute; de Zoologie concr&egrave;te</i>,
+Tome viii., Paris, 1898.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_398" id="Footnote_398" /><a href=
+"#FNanchor_398"><span class="label">[398]</span></a> "Studien zur
+Urgeschichte des Wirbelthierk&ouml;rpers," <i>Mittheil. Zool. Stat.
+Neapel</i>, 1882-1907.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_399" id="Footnote_399" /><a href=
+"#FNanchor_399"><span class="label">[399]</span></a> Leydig (<i>Vom
+Baue des thierischen K&ouml;rpers</i>, T&uuml;bingen, 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 <i>crura cerebri</i>.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_400" id="Footnote_400" /><a href=
+"#FNanchor_400"><span class="label">[400]</span></a> <i>Zeits. f.
+wiss. Zool.</i>, xliv., 1886.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_401" id="Footnote_401" /><a href=
+"#FNanchor_401"><span class="label">[401]</span></a> Quoted by E. B.
+Wilson, <i>Wood's Holl Biological Lectures for 1894</i>, p.
+121.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_402" id="Footnote_402" /><a href=
+"#FNanchor_402"><span class="label">[402]</span></a> <i>Cf.</i>
+Metschnikoff, <i>Quart. Journ. Microsc. Sci.</i>, xxiv., pp.
+89-111, 1884.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_403" id="Footnote_403" /><a href=
+"#FNanchor_403"><span class="label">[403]</span></a> "Die
+Stammesverwandschaft der Wirbelthiere und Wirbellosen," <i>Arb.
+zool.-zoot. Instit. W&uuml;rzburg</i>, ii., pp.&nbsp;25-76, 1875; "Die
+Verwandschaftsbeziehungen der gegliederten Thiere," <i>Ibid.</i>,
+iii., pp.&nbsp;115-404, 1876-7.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_404" id="Footnote_404" /><a href=
+"#FNanchor_404"><span class="label">[404]</span></a> Abuse of
+Cuvier also dates from the early days of evolution, see
+R&aacute;dl, ii., pp.&nbsp;12-17.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_405" id="Footnote_405" /><a href=
+"#FNanchor_405"><span class="label">[405]</span></a> "On the origin
+and history of the urino-genital organs of Vertebrates," <i>Journ.
+Anat. Phys.</i>, x., 1876. The conclusions of Balfour and Semper
+were adversely criticised by M. F&uuml;rbringer (<i>Morph.
+Jahrb.</i>, iv., 1878), and were negatived by later research.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_406" id="Footnote_406" /><a href=
+"#FNanchor_406"><span class="label">[406]</span></a> <i>A Monograph
+on the Development of Elasmobranch Fishes</i>, London, 1878.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_407" id="Footnote_407" /><a href=
+"#FNanchor_407"><span class="label">[407]</span></a> <i>A Treatise
+on Comparative Embryology</i>, vol. ii., p.&nbsp;311, London, 1881.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_408" id="Footnote_408" /><a href=
+"#FNanchor_408"><span class="label">[408]</span></a> <i>Loc.
+cit.</i>, vol. ii., p.&nbsp;327.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_409" id="Footnote_409" /><a href=
+"#FNanchor_409"><span class="label">[409]</span></a> "On the
+Ancestral Form of the Chordata," <i>Q.J.M.S.</i>, xxiii., 1883.
+"The Relation of the Nemertea to the Vertebrata," <i>ibid.</i>,
+xxvii., 1887. Hubrecht gives the credit for the first indication of
+the relationship of Nemertines and Vertebrates to Harting
+(<i>Leerboek van de Grondbeginselen der Dierkunde</i>, 1874).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_410" id="Footnote_410" /><a href=
+"#FNanchor_410"><span class="label">[410]</span></a> "Monographie
+der Capitelliden des Golfes von Neapel," <i>Fauna u. Flora des
+Golfes von Neapel</i>, Monog. xvi., Berlin, 1887.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_411" id="Footnote_411" /><a href=
+"#FNanchor_411"><span class="label">[411]</span></a> <i>Mitt. Zool.
+Stat. Neapel</i>, vii., 1887.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_412" id="Footnote_412" /><a href=
+"#FNanchor_412"><span class="label">[412]</span></a> <i>Nature</i>,
+xxxvi., p.&nbsp;162, 1887.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_413" id="Footnote_413" /><a href=
+"#FNanchor_413"><span class="label">[413]</span></a> "Nebendarm und
+Chorda dorsalis," <i>Nachr. Ges. Wiss. G&ouml;ttingen</i>, p.&nbsp;390,
+1885.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_414" id="Footnote_414" /><a href=
+"#FNanchor_414"><span class="label">[414]</span></a>
+"Embryologische Studien an W&uuml;rmern u. Arthropoden,"
+<i>M&eacute;m. Acad. Sci. St P&eacute;tersbourg</i> (Petrograd),
+(7), xvi., 1870. And in <i>Arch. f. mikr. Anat.</i>, vii., p.&nbsp;122,
+1871.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_415" id="Footnote_415" /><a href=
+"#FNanchor_415"><span class="label">[415]</span></a> "The Old Mouth
+and the New," <i>Anat. Anz.</i>, iii., 1888. <i>Nature</i>, xxxix.,
+1889.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_416" id="Footnote_416" /><a href=
+"#FNanchor_416"><span class="label">[416]</span></a> "Recherches
+sur la Morphologie des Tuniciers," <i>Arch. de Biol.</i>, vi.,
+1887.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_417" id="Footnote_417" /><a href=
+"#FNanchor_417"><span class="label">[417]</span></a> "Die Stellung
+u. Bedeutung der Morphologie," <i>Morph. Jahrb.</i>, i., pp.&nbsp;1-19,
+1876.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_418" id="Footnote_418" /><a href=
+"#FNanchor_418"><span class="label">[418]</span></a> "Anatomie des
+Balanoglossus," <i>M&eacute;m. Acad. Sci. St P&eacute;tersbourg</i>
+(Petrograd), (7), x., 1866.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_419" id="Footnote_419" /><a href=
+"#FNanchor_419"><span class="label">[419]</span></a> <i>Zeit. f.
+wiss. Zool.</i>, xx., 1870. For a recent view of the relation of
+the Enteropneusta to the Echinoderma, see J. F. Gemmill, <i>Phil.
+Trans.</i> B., ccv., pp.&nbsp;213-94, 1914.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_420" id="Footnote_420" /><a href=
+"#FNanchor_420"><span class="label">[420]</span></a> In a series of
+papers published in 1884-6, the speculative results being discussed
+in his memoir on "The Ancestry of the Chordata,"
+<i>Q.J.M.S.</i> (n.s.), xxvi., pp.&nbsp;535-71, 1886.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_421" id="Footnote_421" /><a href=
+"#FNanchor_421"><span class="label">[421]</span></a> Reprinted in
+<i>Zoological Articles</i>, London, 1891.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_422" id="Footnote_422" /><a href=
+"#FNanchor_422"><span class="label">[422]</span></a> "Die
+Enteropneusten des Golfes von Neapel," <i>Fauna und Flora des
+Golfes von Neapel</i>, Monog. xviii., Berlin, 1893.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_423" id="Footnote_423" /><a href=
+"#FNanchor_423"><span class="label">[423]</span></a> See Macbride,
+"A Review of Prof. Spengel's Monograph on Balanoglossus,"
+<i>Q.J.M.S.</i>, xxxvi., 1894, and "The Early Development of
+Amphioxus," <i>Q.J.M.S.</i>, xl., 1898.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg288" id=
+"pg288">288</a></span></p>
+
+<h3>CHAPTER XVI</h3>
+
+<h4>THE GERM-LAYERS AND EVOLUTION</h4>
+
+<p><span class="smcap">In</span> his papers of 1866 and 1867
+Kowalevsky had remarked upon the widespread occurrence of a certain
+type or fundamental plan of early embryonic development,
+characterised by the formation, through invagination, of a
+two-layered sac, whose cavity became the alimentary canal. This
+developmental archetype was manifested in, for instance,
+<i>Sagitta</i>,<a name="FNanchor_424" id="FNanchor_424" /><a
+href="#Footnote_424" class="fnanchor">[424]</a> <i>Rana</i>,<a name=
+"FNanchor_425" id="FNanchor_425" /><a href="#Footnote_425"
+class="fnanchor">[425]</a> <i>Lymn&aelig;a</i>,<a name=
+"FNanchor_426" id="FNanchor_426" /><a href="#Footnote_426"
+class="fnanchor">[426]</a> <i>Astacus</i>,<a name="FNanchor_427"
+id="FNanchor_427" /><a href="#Footnote_427" class=
+"fnanchor">[427]</a> <i>Phoronis</i>,<a href="#Footnote_428" class=
+"fnanchor">[428]</a> <i>Asterias</i>,<a name="FNanchor_429" id=
+"FNanchor_429" /><a href="#Footnote_429" class=
+"fnanchor">[429]</a> <i>Ascidia</i>,<a href="#Footnote_428" class=
+"fnanchor">[428]</a> the <i>Ctenophora</i>,<a href="#Footnote_428"
+class="fnanchor">[428]</a> and <i>Amphioxus</i>.<a href=
+"#Footnote_428" class="fnanchor">[428]</a> He noticed also that the
+invagination-opening often became the definitive anus. Further
+instances of this mode of development were later observed by
+Metschnikoff<a name="FNanchor_430" id="FNanchor_430" /><a href=
+"#Footnote_430" class="fnanchor">[430]</a> and by Kowalevsky<a
+name="FNanchor_431" id="FNanchor_431" /><a href=
+"#Footnote_431" class="fnanchor">[431]</a> himself, but it was left
+to Haeckel to generalise these observations and build up from them
+his famous Gastr&aelig;a theory. This was first enunciated in his
+monograph of the calcareous sponges,<a name="FNanchor_432" id=
+"FNanchor_432" /><a href="#Footnote_432" class=
+"fnanchor">[432]</a> and worked out in detail in a series of papers
+published in 1874-76.<a name="FNanchor_433" id=
+"FNanchor_433" /><a href="#Footnote_433" class=
+"fnanchor">[433]</a></p>
+
+<p><span class="pagenum"><a name="pg289" id=
+"pg289">289</a></span>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 Gastr&aelig;a.</p>
+
+<p>From the Gastr&aelig;a theory there followed at once two
+consequences, (1) that ectoderm and endoderm, invagination-cavity
+(<i>Urdarm</i>) and gastrula-mouth (<i>Urmund</i> or
+<i>Protostoma</i>), 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 Gastr&aelig;a. Huxley's suggestion
+(<i>supra</i>, p.&nbsp;208) that the outer and inner layers in
+C&oelig;lentera were homologous with the ectoderm and endoderm of
+the germ was thus fully confirmed and greatly extended.</p>
+
+<p>The great importance of the Gastr&aelig;a 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 Gastr&aelig;a
+theory or its implications were founded most of the phylogenetic
+speculations which subsequently appeared.</p>
+
+<p>Upon the Gastr&aelig;a 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.&nbsp;290
+in graphic form, combined and modified from the table on p.&nbsp;53 of
+the 1874 paper and the genealogical tree given in the
+<i>Kalkschw&auml;mme</i>.<a name="FNanchor_434" id=
+"FNanchor_434" /><a href="#Footnote_434" class=
+"fnanchor">[434]</a></p>
+
+<p><span class="pagenum"><a name="pg290" id=
+"pg290">290</a></span></p>
+
+<p class="two"><i>Monophyletic Genealogical Tree of the Animal
+Kingdom, based upon the Gastr&aelig;a Theory and the Homology of
+the Germ Layers</i>.</p>
+
+<div class="figcenter">
+<img
+src="images/img290a.jpg"
+alt="monophyletic ancestral tree" /></div>
+
+<p>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, <span class="pagenum"><a
+name="pg291" id="pg291">291</a></span>which possess both
+structures, is for the first time clearly brought out. The
+derivation of all the Metazoa from a single ancestral form, the
+Gastr&aelig;a, 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
+<i>archenteron</i> (Lankester, 1875), and an ectoderm and endoderm
+which are homologous throughout all the Metazoan phyla. Finally, in
+the separation of the sponges, C&oelig;lenterata and Ac&oelig;lomi
+as animals lacking a body cavity or c&oelig;lom<a name=
+"FNanchor_435" id="FNanchor_435" /><a href=
+"#Footnote_435" class="fnanchor">[435]</a> from the four higher
+phyla, which are essentially C&oelig;lomati, there is contained the
+germ of a conception which later became of importance.</p>
+
+<p>Somewhat similar views as to the importance of the germ-layer
+theory for the phylogenetic classification of animals were
+published by Sir E. Ray Lankester in 1873.<a name=
+"FNanchor_436" id="FNanchor_436" /><a href=
+"#Footnote_436" class="fnanchor">[436]</a> He distinguished three
+grades of animals&mdash;the Homoblastica, Diploblastica, and
+Triploblastica. The first included the Protozoa, the second the
+C&oelig;lenterata, 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.</p>
+
+<p>The Gastr&aelig;a 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.<a name="FNanchor_437" id=
+"FNanchor_437" /><a href="#Footnote_437" class=
+"fnanchor">[437]</a> These were the monerula, cytula, morula,
+blastula, and gastrula (see <a href="#pg292">Fig.&nbsp;15</a>). The
+monerula was the fertilised ovum after the disappearance of the
+germinal vesicle;<a name="FNanchor_438" id=
+"FNanchor_438" /><a href="#Footnote_438" class=
+"fnanchor">[438]</a> it was the equivalent of the primordial
+anucleate Monera which are the ancestors of all animals.</p>
+
+<p><span class="pagenum"><a name="pg292" id=
+"pg292">292</a></span></p>
+
+<div class="figcenter">
+<img
+src="images/fig15a.jpg"
+alt="The Five Primary Stages of Ontogeny. (After Haeckel.)" /></div>
+
+<p class="center2"><span class="smcap">Fig.</span> 15.&mdash;The
+Five Primary Stages of Ontogeny. (After Haeckel.)</p>
+
+<table width="80%" summary=
+"The Five Primary Stages of Ontogeny. (After Haeckel.)" border="0"
+cellpadding="5" cellspacing="0">
+<tbody>
+<tr>
+<td class="cell_lt6">1.&nbsp;Monerula.</td>
+<td class="cell_lt6">2.&nbsp;Cytula.</td>
+<td class="cell_lt6">3.&nbsp;Morula.</td>
+<td class="cell_lt6">4.&nbsp;Blastula.</td>
+<td class="cell_lt6">5.&nbsp;Gastrula.</td>
+</tr>
+</tbody>
+</table>
+
+<p><span class="pagenum"><a name="pg293" id=
+"pg293">293</a></span>The ovum after the nucleus had been re-formed became the cytula, which
+was the ontogenetic counterpart of the am&oelig;ba. The morula, a
+compact mulberry-like congeries of segmentation-cells, corresponded
+to the synam&oelig;ba, or earliest association of undifferentiated
+am&oelig;boid cells to form the first multicellular organism. The
+blastula, or hollow sphere of segmentation cells, usually ciliated,
+was reminiscent of the plan&aelig;a, an ancestral free-swimming
+form whose nearest living relation is the spherical
+<i>Magosph&aelig;ra</i>. The gastrula, finally, is the two-layered
+sac formed from the blastula, typically by invagination of its
+wall. It repeats the organisation of the gastr&aelig;a, which is
+the common ancestor of all Metazoa, and finds its nearest living
+counterpart in the simple "sponges" <i>Haliphysema</i> and
+<i>Gastrophysema</i>.<a name="FNanchor_439" id=
+"FNanchor_439" /><a href="#Footnote_439" class=
+"fnanchor">[439]</a> The ancestral line of all the higher animals
+begins with the five hypothetical forms of the moneron,
+am&oelig;ba, synam&oelig;ba, plan&aelig;a, and gastr&aelig;a.</p>
+
+<p>We may take the following account<a name="FNanchor_440" id=
+"FNanchor_440" /><a href="#Footnote_440" class=
+"fnanchor">[440]</a> of the phylogeny of the human species, from the
+gastr&aelig;a stage onwards, as typical of Haeckel's speculations
+on the evolution of the higher forms. The progenitors of man are,
+after the Gastr&aelig;ada:&mdash;</p>
+
+<p class="onex">&nbsp;&nbsp;&nbsp;&nbsp;1. Turbellaria.</p>
+
+<p class="onex">&nbsp;&nbsp;*2. Scolecida. (Worms with a
+c&oelig;lom, probably represented at the present day by
+<i>Balanoglossus</i>.)</p>
+
+<p class="onex">&nbsp;&nbsp;*3. Himatega. (Evolved from Scolecida by
+formation of dorsal nerve-tube and chorda, and resembling tailed
+larv&aelig; of Ascidians.)</p>
+
+<p class="onex">&nbsp;&nbsp;&nbsp;&nbsp;4. Acrania. (With metameric
+segmentation. Including Amphioxus.)</p>
+
+<p class="onex">&nbsp;&nbsp;&nbsp;&nbsp;5. Monorrhina.
+(Cyclostomes.)</p>
+
+<p class="onex">&nbsp;&nbsp;&nbsp;&nbsp;6. Selachia.</p>
+
+<p class="onex">&nbsp;&nbsp;&nbsp;&nbsp;7. Dipneusta.</p>
+
+<p class="onex">&nbsp;&nbsp;&nbsp;&nbsp;8. Sozobranchia. (Amphibia
+with permanent gills.)</p>
+
+<p class="onex">&nbsp;&nbsp;&nbsp;&nbsp;<span class="pagenum"><a
+name="pg294" id="pg294">294</a></span>9. Sozura. (Tailed
+Amphibia.)</p>
+
+<p class="onex">*10. Protamnia.</p>
+
+<p class="onex">*11. Promammalia.</p>
+
+<p class="onex">&nbsp;&nbsp;12. Marsupialia.</p>
+
+<p class="onex">&nbsp;&nbsp;13. Prosimi&aelig;.</p>
+
+<p class="onex">&nbsp;&nbsp;14. Menocerca. (Tailed apes.)</p>
+
+<p class="onex">&nbsp;&nbsp;15. Anthropoides.</p>
+
+<p class="onex">&nbsp;&nbsp;16. Pithecanthropi.</p>
+
+<p class="onex">&nbsp;&nbsp;17. Homines.</p>
+
+<p>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.</p>
+
+<p>A more abstractly morphological scheme of the evolution of
+Vertebrates is given in the <i>Systematic Phylogeny</i> of 1895.<a
+name="FNanchor_441" id="FNanchor_441" /><a href=
+"#Footnote_441" class="fnanchor">[441]</a> The ontogenetic and
+ancestral stages are arranged in parallel columns thus:&mdash;</p>
+
+<table width="100%" summary="scheme of the evolution" border="0"
+cellspacing="0" cellpadding="2">
+<tr>
+<td class="cell_lt294a">Cytula.</td>
+<td class="cell_lt294b">&nbsp;</td>
+<td class="cell_lt294c" colspan="2">Cyt&aelig;a (Protozoa).</td>
+</tr>
+
+<tr>
+<td class="cell_lt294a">Morula.</td>
+<td class="cell_lt294b">&nbsp;</td>
+<td class="cell_lt294c">Mor&aelig;a (C&oelig;nobium of
+Protozoa).</td>
+</tr>
+
+<tr>
+<td class="cell_lt294a">Blastula.</td>
+<td class="cell_lt294b">&nbsp;</td>
+<td class="cell_lt294c">Blast&aelig;a (<i>Volvocina</i>,
+etc.).</td>
+</tr>
+
+<tr>
+<td class="cell_lt294a">Depula (invaginated blastula).</td>
+<td class="cell_lt294b">&nbsp;</td>
+<td class="cell_lt294c">Dep&aelig;a.</td>
+</tr>
+
+<tr>
+<td class="cell_lt294a">Gastrula.</td>
+<td class="cell_lt294b">&nbsp;</td>
+<td class="cell_lt294c">Gastr&aelig;a (cf. <i>Olynthus</i>,
+<i>Hydra</i>, and primitive Coelentera).</td>
+</tr>
+
+<tr>
+<td class="cell_lt294a">C&oelig;lomula (with one pair of
+c&oelig;lom-pockets).</td>
+<td class="cell_lt294b">&nbsp;</td>
+<td class="cell_lt294c">C&oelig;lom&aelig;a (cf. <i>Sagitta</i>,
+<i>Ascidia</i>, and primitive Helminthes).</td>
+</tr>
+
+<tr>
+<td class="cell_lt294a">Chordula (with medullary tube and
+chorda).</td>
+<td class="cell_lt294b">&nbsp;</td>
+<td class="cell_lt294c">Chord&aelig;a (<i>cf.</i> Ascidian larva
+and larva of Amphioxus).</td>
+</tr>
+
+<tr>
+<td class="cell_lt294a">Spondula (with segmented mesoderm).</td>
+<td class="cell_lt294b">&nbsp;</td>
+<td class="cell_lt294c">Prospondylus (Primitive Vertebrate).</td>
+</tr>
+</table>
+
+<p>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 c&oelig;lom, which had
+been made in the interval of some twenty years.</p>
+
+<p>Haeckel's Gastr&aelig;a theory, though it exercised a great
+influence upon the subsequent trend of phylogenetic speculation,
+was by no means universally accepted <i>telle quelle</i>. Opinions
+differed considerably as to the primitive mode of <span class=
+"pagenum"><a name="pg295" id="pg295">295</a></span>origin of the
+two-layered sac which was very generally admitted to be of constant
+occurrence in early embryogeny. Ray Lankester, in his paper of
+1873, and more fully in 1877,<a name="FNanchor_442" id=
+"FNanchor_442" /><a href="#Footnote_442" class=
+"fnanchor">[442]</a> 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,<a name=
+"FNanchor_443" id="FNanchor_443" /><a href="#Footnote_443"
+class="fnanchor">[443]</a> holding that this was typically formed by
+an "inruptive" process at the anterior end of the planula, which
+led to the formation of a "stomod&aelig;um." A similar process at
+the posterior end gave rise to the anus and the
+"proctod&aelig;um."</p>
+
+<p>The question as to whether delamination or invagination was to
+be considered the more primitive process was discussed in detail by
+Balfour,<a name="FNanchor_444" id="FNanchor_444" /><a href=
+"#Footnote_444" class="fnanchor">[444]</a> 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.&nbsp;342). He by no
+means rejected the theory that the Gastr&aelig;a, "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.&nbsp;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 Gastr&aelig;a
+theory.</p>
+
+<p>From the same origins as the Gastr&aelig;a theory arose the
+theory of the c&oelig;lom. 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 Gastr&aelig;a
+theory&mdash;A. Agassiz, Metschnikoff and Kowalevsky. <span class=
+"pagenum"><a name="pg296" id="pg296">296</a></span>But it was not
+Haeckel himself who enunciated the c&oelig;lom theory.</p>
+
+<p>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
+"c&oelig;lom" 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 <i>two</i>
+secondary layers from ectoderm and endoderm respectively than to
+Remak's conception of the mesoderm as a single independent
+layer.</p>
+
+<p>Much uncertainty reigned at the time as to the exact manner of
+origin of the mesoderm;<a name="FNanchor_445" id=
+"FNanchor_445" /><a href="#Footnote_445" class=
+"fnanchor">[445]</a> 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.&nbsp;23-4, 1874).</p>
+
+<p>The solution of the problem came from those observations on the
+development of the lower forms to which we have just alluded.</p>
+
+<p>The early history of these discoveries and of the theory which
+grew out of them has been well summarised by Lankester,<a name=
+"FNanchor_446" id="FNanchor_446" /><a href="#Footnote_446"
+class="fnanchor">[446]</a> and may conveniently be given in his own
+words:&mdash;</p>
+
+<p>"As far back as 1864 Alexander Agassiz ("Embryology of the
+Star-fish," in <i>Contributions to the Natural History of the
+United States</i>, vol. v., 1864) showed in his account of the
+development of Echinoderma that the great body-cavity of those
+animals developed as a pouch-like outgrowth of the archenteron of
+the embryo, whilst a second outgrowth gave rise to their ambulacral
+system; and in 1869 Metschnikoff (<i>M&eacute;m. de l'Acad.
+imp&eacute;riale des Sciences de St P&eacute;tersbourg</i>, 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. <span class="pagenum"><a name="pg297" id=
+"pg297">297</a></span>Metschnikoff has further the credit of
+having, in 1874 (<i>Zeitsch. wiss. Zoologie</i>, vol. xxiv., p.&nbsp;15,
+1874), revived Leuckart's theory of the relationship of the
+c&oelig;lenteric apparatus of the Enteroc&oelig;la 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
+Enteroc&oelig;la (<i>Verwandschaftsverh&auml;ltnisse der
+wirbellosen Thiere</i>, Brunswick, 1848). Metschnikoff insisted
+upon such a correspondence when comparing the Echinoderm larva,
+with its still continuous enteron and c&oelig;lom, 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 (<i>Quart. Journ. Micr. Sci.</i>, vol. xv., p.
+52) Huxley proposed to distinguish three kinds of body-cavity: the
+schizoc&oelig;l, formed by the splitting of the mesoblast, as in
+the chick's blastoderm; the enteroc&oelig;l, formed by pouching of
+the archenteron, as in Echinoderms, Sagitta and Brachiopoda; and
+the epic&oelig;l.... Immediately after this I put forward the
+theory of the uniformity of origin of the c&oelig;lom as an
+enteroc&oelig;l (<i>Quart. Journ. Micr. Sci.</i>, April, 1875)....
+My theory of the c&oelig;lom as an enteroc&oelig;l 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&mdash;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 c&oelig;lom" (pp.&nbsp;16-18).</p>
+
+<p>The enteroc&oelig;lic theory was taken up by O. and R. Hertwig
+as an essential part of their <i>C&oelig;lomtheorie</i>.<a name=
+"FNanchor_447" id="FNanchor_447" /><a href="#Footnote_447"
+class="fnanchor">[447]</a> In <span class="pagenum"><a name="pg298"
+id="pg298">298</a></span>a lengthy series of monographs these
+workers made a comparative study of the mode of formation of the
+middle layer, and arrived at a coherent theory of its origin. They
+distinguished in the middle layer two quite distinct elements, the
+mesoblast proper, formed by the evagination of the walls of the
+archenteron, and the mesenchyme, formed by free cells budded off
+from the germ-layers. The following passage gives a good idea of
+their views and of the phylogenetic implications
+involved:&mdash;"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 Gastr&aelig;a; they
+form the limits of the organism towards the exterior and towards
+the archenteron. The parietal and visceral mesoblast, or the two
+middle layers, are always of later origin, and arise through
+evagination or plaiting of the entoblast, the remainder of which
+can now be distinguished as secondary entoblast from the primary.
+They form the walls of a new cavity, the enteroc&oelig;l, which is
+to be regarded as a nipped-off diverticulum of the archenteron.
+Just as the two-layered animals can be derived from the
+Gastr&aelig;a, so can the four-layered animals be derived from a
+C&oelig;lom 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 (<i>Secretgewebe</i>)
+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.&nbsp;122).</p>
+
+<p>The important point for us is that, just as all Metazoa were
+considered by Haeckel to be descended from the Gastr&aelig;a, so
+all C&oelig;lomati were held by the Hertwigs to be derived from an
+original c&oelig;lomate <i>Urform</i>. In both cases an
+embryological archetype becomes a hypothetical ancestral form.</p>
+
+<p>The C&oelig;lom theory was considerably modified, extended <span
+class="pagenum"><a name="pg299" id="pg299">299</a></span>and
+developed by later workers, particularly as regards the relations
+to the c&oelig;lom of the genital organs and ducts and the nephridia,
+but no special methodological interest attaches to these further
+developments.<a name="FNanchor_448" id="FNanchor_448" /><a
+href="#Footnote_448" class="fnanchor">[448]</a> We shall here focus
+attention upon one interesting line of speculation followed out in
+this country particularly by Sedgwick&mdash;the theory of the
+Actinozoan ancestry of segmented animals. Its relation to the
+C&oelig;lom theory lies in the fact that Sedgwick regarded the
+segmentation of the body as moulded upon the segmentation of the
+mesoblast, which in its turn, as Kowalevsky and Hatschek had shown,
+was a consequence of its mode of origin as a series of pouches of
+the archenteron. In other respects Sedgwick's speculations link on
+more closely to the Gastr&aelig;a theory, for one of his main
+contentions is that the blastopore or <i>Urmund</i> is homologous
+throughout at least the three metameric phyla. In following up
+Balfour's observations on the development of <i>Peripatus</i>,<a
+name="FNanchor_449" id="FNanchor_449" /><a href=
+"#Footnote_449" class="fnanchor">[449]</a> 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 enteroc&oelig;lic pouches of the C&oelig;lomati. He was
+led to enunciate the following theses:&mdash;<a name=
+"FNanchor_450" id="FNanchor_450" /><a href="#Footnote_450"
+class="fnanchor">[450]</a> (1) that the mouth and anus of Vermes,
+Mollusca, Arthopoda, and probably Vertebrata, is derived from the
+elongated mouth of an ancestor resembling the Actinozoa; (2) that
+somites are derived from a series of archenteric pouches, like
+those of Actinozoa and Medus&aelig;; (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 <span
+class="pagenum"><a name="pg300" id="pg300">300</a></span>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 trache&aelig; 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 Scyphomedus&aelig;. 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 C&oelig;lenterate-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.&nbsp;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 C&oelig;lenterata; that all the most
+important organ-systems of these Triploblastica are found in a
+rudimentary condition in the C&oelig;lenterata; and that all the
+Triploblastica referred to must be traced back to a diploblastic
+ancestor common to them and the C&oelig;lenterata" (p.&nbsp;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.<a
+name="FNanchor_451" id="FNanchor_451" /><a href=
+"#Footnote_451" class="fnanchor">[451]</a></p>
+
+<p>The characteristic relation of the central nervous system to the
+blastopore in Annelida and Vertebrates had already been pointed out
+by Kowalevsky,<a name="FNanchor_452" id="FNanchor_452" /><a
+href="#Footnote_452" class="fnanchor">[452]</a> 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.</p>
+
+<p><span class="pagenum"><a name="pg301" id=
+"pg301">301</a></span>In 1882, before the publication of Sedgwick's
+papers, A. Lang<a name="FNanchor_453" id="FNanchor_453" /><a
+href="#Footnote_453" class="fnanchor">[453]</a> 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
+enteroc&oelig;lic pouches of higher animals. This view, however, he
+soon gave up.<a name="FNanchor_454" id="FNanchor_454" /><a
+href="#Footnote_454" class="fnanchor">[454]</a> Sedgwick's views
+found a supporter in A. A. W. Hubrecht,<a name="FNanchor_455" id=
+"FNanchor_455" /><a href="#Footnote_455" class=
+"fnanchor">[455]</a> 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.&nbsp;410, 1906), and derived the central nervous system
+from the circum-oral ring of this primitive form, the notochord
+from its stomod&aelig;um, and the c&oelig;lom from the peripheral
+parts of the gastric cavity (p.&nbsp;169, 1909).</p>
+
+<div class="footnote">
+<p><a name="Footnote_424" id="Footnote_424" /><a href=
+"#FNanchor_424"><span class="label">[424]</span></a> Gegenbaur,
+<i>Zeits. f. wiss. Zool.</i>, v., 1853.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_425" id="Footnote_425" /><a href=
+"#FNanchor_425"><span class="label">[425]</span></a> Remak, <i>loc.
+cit.</i>, p.&nbsp;183, pl. xii.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_426" id="Footnote_426" /><a href=
+"#FNanchor_426"><span class="label">[426]</span></a> Lereboullet,
+<i>Ann. Sci. nat.</i> (4) xviii., pp.&nbsp;118-9, 1862.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_427" id="Footnote_427" /><a href=
+"#FNanchor_427"><span class="label">[427]</span></a> Lereboullet,
+in Remak, p.&nbsp;183 f.n.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_428" id="Footnote_428" /><span class=
+"label">[428]</span> Kowalevsky, <i>M&eacute;m. Acad. Sci. St
+P&eacute;tersbourg</i> (Petrograd), (7), x. and xi., 1866 and
+1867.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_429" id="Footnote_429" /><a href=
+"#FNanchor_429"><span class="label">[429]</span></a> A. Agassiz,
+<i>Contrib. Nat. Hist. United States</i>, v., 1864.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_430" id="Footnote_430" /><a href=
+"#FNanchor_430"><span class="label">[430]</span></a> <i>M&eacute;m.
+Acad. Sci. St P&eacute;tersbourg</i> (Petrograd), (7), xiv.,
+1869.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_431" id="Footnote_431" /><a href=
+"#FNanchor_431"><span class="label">[431]</span></a> "Embryolog.
+Studien an W&uuml;rmern u. Arthropoden," <i>M&eacute;m. Acad. Sci.
+St P&eacute;tersbourg</i> (Petrograd), (7), xvi., 1870.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_432" id="Footnote_432" /><a href=
+"#FNanchor_432"><span class="label">[432]</span></a> <i>Die
+Kalkschw&auml;mme</i>, 3 vols., Berlin, 1872. General chapters
+translated in <i>Ann. Mag. Nat. Hist.</i> (4), xi., pp.&nbsp;241-62,
+421-30, 1873.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_433" id="Footnote_433" /><a href=
+"#FNanchor_433"><span class="label">[433]</span></a> "Die
+Gastr&aelig;a-Theorie, die phylogenetische Classification des
+Thierreichs und die Homologie der Keimbl&auml;tter." <i>Jenaische
+Zeitschrift</i>, viii., pp.&nbsp;1-55, 1874. "Die Gastrula und die
+Eifurchung der Thiere," <i>ibid.</i>, ix., pp.&nbsp;402-508, 1875. "Die
+Physemarien, Gastr&aelig;aden der Gegenwart," and "Nachtr&auml;ge
+zur Gastr&aelig;a-Theorie," <i>ibid.</i>, x., pp.&nbsp;55-98, 1876.
+Republished in <i>Biologische Studien</i>, 2nd part, <i>Studien zur
+Gastr&aelig;a-Theorie</i>, 270 pp., 14 pls., Jena, 1877.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_434" id="Footnote_434" /><a href=
+"#FNanchor_434"><span class="label">[434]</span></a> See <i>Ann.
+Mag. Nat. Hist.</i> (4), xi., p.&nbsp;253.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_435" id="Footnote_435" /><a href=
+"#FNanchor_435"><span class="label">[435]</span></a> Term first
+introduced in <i>Die Kalkschw&auml;mme</i>, p.&nbsp;468, 1872.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_436" id="Footnote_436" /><a href=
+"#FNanchor_436"><span class="label">[436]</span></a>"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," <i>Ann. Mag. Nat. Hist.</i> (4), xi., pp.&nbsp;321-38,
+1873.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_437" id="Footnote_437" /><a href=
+"#FNanchor_437"><span class="label">[437]</span></a> First
+distinguished in <i>Die Kalkschw&auml;mme</i>, i., p.&nbsp;465.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_438" id="Footnote_438" /><a href=
+"#FNanchor_438"><span class="label">[438]</span></a>
+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 <i>Toxopneustes</i>
+(<i>Morph. Jahrb.</i>, i., 1876).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_439" id="Footnote_439" /><a href=
+"#FNanchor_439"><span class="label">[439]</span></a> <i>Studien z.
+Gastr&aelig;a-Theorie</i>, p.&nbsp;214, 1877. These forms were known
+even in 1870 (Carter, <i>Ann. Mag. Nat. Hist.</i> (4), vi., pp.
+346-7), to be Foraminifera. The figures of supposed collar-cells,
+etc., do credit to Haeckel's imagination.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_440" id="Footnote_440" /><a href=
+"#FNanchor_440"><span class="label">[440]</span></a> <i>History of
+Creation</i>, Eng. Trans., ii., pp.&nbsp;278 ff.</p>
+</div>
+
+<div class="footnote"><a name="Footnote_441" id=
+"Footnote_441" /><a href="#FNanchor_441"><span class=
+"label">[441]</span></a>
+<p><i>Systematische Phylogenie</i>, iii., p.&nbsp;41, Berlin, 1895.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_442" id="Footnote_442" /><a href=
+"#FNanchor_442"><span class="label">[442]</span></a> "Notes on the
+Embryology and Classification of the Animal Kingdom,"
+<i>Q.J.M.S.</i> (n.s.), xvii., pp.&nbsp;399-454, 1877.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_443" id="Footnote_443" /><a href=
+"#FNanchor_443"><span class="label">[443]</span></a> It was "part
+of the non-historic mechanism of growth" (<i>loc. cit.</i>, p.
+418).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_444" id="Footnote_444" /><a href=
+"#FNanchor_444"><span class="label">[444]</span></a> <i>Treatise on
+Comparative Embryology</i>, ii., chap.&nbsp;xiii., 1881. For a modern
+discussion of this problem, see Hubrecht, <i>Q.J.M.S.</i>, xlix.,
+1906.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_445" id="Footnote_445" /><a href=
+"#FNanchor_445"><span class="label">[445]</span></a> See Balfour,
+<i>loc. cit.</i>, Chapter xiii.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_446" id="Footnote_446" /><a href=
+"#FNanchor_446"><span class="label">[446]</span></a> <i>A Treatise
+on Zoology</i>, Pt. ii., 1900. Introduction by Sir E. Ray
+Lankester.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_447" id="Footnote_447" /><a href=
+"#FNanchor_447"><span class="label">[447]</span></a> <i>Studien zur
+Bl&auml;ttertheorie</i>, Jena, 1879-80. "Die C&oelig;lomtheorie,
+Versuch einer Erkl&auml;rung des mittleren Keimblattes,"
+<i>Jenaische Zeitschrift</i>, xv., pp.&nbsp;1-150, 1882.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_448" id="Footnote_448" /><a href=
+"#FNanchor_448"><span class="label">[448]</span></a> For an
+historical account of this work, see Lankester, <i>loc. cit.</i>,
+pp.&nbsp;21-37.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_449" id="Footnote_449" /><a href=
+"#FNanchor_449"><span class="label">[449]</span></a> <i>Proc. Roy.
+Soc.</i>, 1883, and <i>Q.J.M.S.</i>, xxiii., 1883.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_450" id="Footnote_450" /><a href=
+"#FNanchor_450"><span class="label">[450]</span></a> "Origin of
+Metameric Segmentation," <i>Q.J.M.S.</i>, xxiv., pp.&nbsp;43-82
+1884.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_451" id="Footnote_451" /><a href=
+"#FNanchor_451"><span class="label">[451]</span></a> See further
+the same author's article "Embryology" in the <i>Ency. Brit.</i>,
+vol. xi., 11th ed., Cambridge, 1910.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_452" id="Footnote_452" /><a href=
+"#FNanchor_452"><span class="label">[452]</span></a> <i>Arch. f.
+mikr. Anat.</i>, xiii., pp.&nbsp;181-204, 1877.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_453" id="Footnote_453" /><a href=
+"#FNanchor_453"><span class="label">[453]</span></a> "Der Bau von
+Gunda segmentata," <i>Mitth. Zool. Stat. Neap.</i>, iii., pp.
+187-250, 1882.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_454" id="Footnote_454" /><a href=
+"#FNanchor_454"><span class="label">[454]</span></a> "Die
+Polycladen," <i>Fauna u. Flora des Golfes von Neapel</i>, Monog.
+v., Leipzig, 1884, and "Beitr&auml;ge zu einer
+Trophoc&oelig;ltheorie," <i>Jen. Zeits.</i>, xxxviii., pp.&nbsp;1-373,
+1904 (which see for a modern account of theories of
+metamerism).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_455" id="Footnote_455" /><a href=
+"#FNanchor_455"><span class="label">[455]</span></a> "Die
+Abstammung der Anneliden u. Chordaten," <i>Jen. Zeits.</i>, xxxix.,
+pp.&nbsp;151-76, 1905. "The Gastrulation of the Vertebrates,"
+<i>Q.J.M.S.</i>, xlix., pp.&nbsp;403-19, 1906. "Early Ontogenetic
+Phenomena in Mammals," <i>Q.J.M.S.</i>, liii., pp.&nbsp;1-181, 1909.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg302" id=
+"pg302">302</a></span></p>
+
+<h3>CHAPTER XVII</h3>
+
+<h4>THE ORGANISM AS AN HISTORICAL BEING</h4>
+
+<p>"<span class="smcap">Of</span> 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."</p>
+
+<p>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.</p>
+
+<p>This is accordingly a convenient stage at which to take stock of
+what has gone before, to consider the relation of evolutionary
+morphology to the transcendental and the Cuvierian schools of
+thought which preceded it, and to make clear what new element
+evolution-theory added to morphology.</p>
+
+<p>The close analogy between evolutionary and transcendental
+morphology has already been remarked upon and illustrated in the
+last three chapters. We have seen that the coming of evolution made
+comparatively little difference to pure morphology, that no new
+criteria of homology were introduced, and that so far as pure
+morphology was concerned, evolution might still have been conceived
+as an ideal process precisely as it was by the transcendentalists.
+The principle of connections still remained the guiding thread of
+morphological work; the search for archetypes, whether anatomical
+<span class="pagenum"><a name="pg303" id="pg303">303</a></span>or
+embryological, still continued in the same way as before, and it
+was a point of subordinate importance that, under the influence of
+the evolution-theory, these were considered to represent real
+ancestral forms rather than purely abstract figments of the
+intelligence. The law of Meckel-Serres was revived in an altered
+shape as the law of the recapitulation of phylogeny by ontogeny;
+the natural system of classification was passively inherited, and,
+by a <i>petitio principii</i>, 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.</p>
+
+<p>There was a close spiritual affinity between the speculative
+evolutionists and the transcendentalists. Both showed the same
+subconscious craving for simplicist conceptions&mdash;the
+transcendentalists clung fast to the notion of the absolute unity
+of type, of the ideal existence of the "one animal," and the
+evolutionists did precisely the same thing when they blindly and
+instinctively accepted the doctrine of the monophyletic descent of
+all animals from one primeval form. Geoffroy persisted in regarding
+Arthropods as being built on the same plan as Vertebrates: Dohrn
+and Semper did nothing different when they derived both groups from
+an ancestor combining the main characters of both. The
+determination to link together all the main phyla of the animal
+kingdom and to force them all into a single mould was common to
+evolutionary and pre-evolutionary transcendentalists alike.</p>
+
+<p>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 Gastr&aelig;a.</p>
+
+<p><span class="pagenum"><a name="pg304" id=
+"pg304">304</a></span>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&mdash;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.</p>
+
+<p>Phylogenetic method differed in no way from
+transcendental&mdash;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.</p>
+
+<p>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.</p>
+
+<p>The close affinity of transcendentalists and evolutionists is
+shown very clearly in their common contrast in habits of <span
+class="pagenum"><a name="pg305" id="pg305">305</a></span>thought
+with the Cuvierian school. It is the cardinal principle of pure
+morphology that function must be excluded from consideration. This
+is a necessary and unavoidable simplification which must be carried
+out if there is to be a science of pure form at all. But this
+limitation of outlook, if carried over from morphology to general
+biology becomes harmful, since it wilfully ignores one whole side
+of life&mdash;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&mdash;its principles are
+applicable to biology in general.</p>
+
+<p>Geoffroy and Cuvier in pre-evolutionary times well typified the
+contrast between the formal and the functional standpoints. For
+Geoffroy form determined function, while for Cuvier function
+determined form. Geoffroy held that Nature formed nothing new, but
+adapted existing "materials of organisation" to meet new needs.
+Cuvier, on the other hand, was always ready to admit Nature's power
+to form entirely new organs in response to new functional
+requirements.</p>
+
+<p>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.</p>
+
+<p>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:&mdash;"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.</p>
+
+<p><span class="pagenum"><a name="pg306" id=
+"pg306">306</a></span>"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, <i>e.g.</i>, 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.&nbsp;644).</p>
+
+<p>This same prejudice lies at the root of the idea of
+<i>Functionswechsel</i>, in spite of the general functional
+orientation of that idea.</p>
+
+<p>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 <i>de novo</i>. Thus, rebutting
+the suggestion that certain organs which are not found in the lower
+Vertebrates might have arisen as new formations, he
+writes:&mdash;"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 <i>Deus ex machina</i>, 'New
+formation,' which is neither better nor worse than <i>Generatio
+equivoca</i>" (p.&nbsp;21).</p>
+
+<p>Of a similar nature was the objection to convergence.<a name=
+"FNanchor_456" id="FNanchor_456" /><a href="#Footnote_456"
+class="fnanchor">[456]</a></p>
+
+<p>Why, we may ask, were morphologists so unwilling to <span class=
+"pagenum"><a name="pg307" id="pg307">307</a></span>admit the
+creative power of life? Dohrn, for instance, was fully aware of the
+great transforming influence exerted by function upon
+form&mdash;his theory of <i>Functionswechsel</i> 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?</p>
+
+<p>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&mdash;their routine habits and reflexes, their routine
+structure&mdash;and ignore the essential activity which they
+manifest both in behaviour and in form-change.</p>
+
+<p>We tend also to lay the causes of form-change, of evolution, as
+far as possible outside the living organism. With Darwin we seek
+the transforming factors in the environment rather than within the
+organism itself. We fight shy of the Lamarckian conception that the
+living thing obscurely works out its own salvation by blind and
+instinctive effort. We like to think of organisms as machines, as
+passive inventions<a name="FNanchor_457" id="FNanchor_457" /><a
+href="#Footnote_457" class="fnanchor">[457]</a> 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.</p>
+
+<p>Other causes of the unwillingness of morphologists to admit the
+new formation of organs are to be sought in the main principle of
+pure morphology itself, that the unity of plan imposes an iron
+limit upon adaptation, and in the <span class="pagenum"><a name=
+"pg308" id="pg308">308</a></span>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.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>We have seen this thought expressed with the utmost clearness by
+Darwin himself (<i>supra</i>, p.&nbsp;233). In his eyes the structure
+and activities of the living thing were a heritage from a remote
+past, the organism was a living record of the achievements of its
+whole ancestral line. What a light this conception threw upon all
+biology! "When we no longer look at an organic being as a savage
+looks at a ship as something wholly beyond his comprehension; when
+we regard every production of Nature as one which has had a long
+history; when we contemplate every complex structure and instinct
+as the summing-up of many contrivances, each useful to the
+possessor, in the same way as any great mechanical invention is the
+summing-up of the labour, the experience, the reason, and even the
+blunders of numerous workmen; when we thus view each organic being,
+how far more interesting&mdash;I speak from experience&mdash;does
+the study of natural history become!" (<i>Origin</i>, 6th ed., pp.
+665-6).</p>
+
+<p>Sedgwick expressed the same thing from the morphological point
+of view when he wrote, with reference to the ancestral significance
+of the blastopore:&mdash;"If there is anything in the theory of
+evolution, every change in the embryo must have had a counterpart
+in the history of the race, and it is our business as morphologists
+to find it out" (p.&nbsp;49, 1884).</p>
+
+<p>By the evolution-theory the problems of form were linked
+indissolubly with the problem of heredity. Unity of plan <span
+class="pagenum"><a name="pg309" id="pg309">309</a></span>could no
+longer be explained idealistically as the manifestation of Divine
+archetypal ideas; it had a real historical basis, and was due to
+inheritance from a common ancestor. The evolution-theory gave
+meaning and intelligibility to the transcendental conception of the
+unity of plan; in particular it supplied a simple and satisfying
+explanation of those puzzling vestigial organs, whose existence was
+such a stumbling-block to the teleologists. It enabled the
+biogenetic law to be substituted for the laws of Meckel-Serres and
+von Baer, as being in some measure a combination and interpretation
+of both.</p>
+
+<p>Where the concept of evolution proved itself particularly useful
+was in the interpretation of structures which were not immediately
+conditioned by adaptation to present requirements, such as, for
+instance, the arrangement of gill-slits and aortic arches in the
+f&oelig;tus 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
+C&oelig;lenterate ancestors?</p>
+
+<p>The point hardly requires elaboration, since it has become an
+integral part of all our thinking on biological problems. It may be
+as well, however, for the sake of continuity, to give one or two
+examples of the historical interpretation of animal structures. The
+first may conveniently be the phylogenetic interpretation of the
+contrast between "membrane" and "cartilage" bones.</p>
+
+<p>In his <i>Grundz&uuml;ge</i> of 1870, Gegenbaur made the
+suggestion that the investing or membrane bones were derived
+phylogenetically from integumentary ossifications, and this was
+worked out in detail a few years later by O. Hertwig.<a name=
+"FNanchor_458" id="FNanchor_458" /><a href="#Footnote_458"
+class="fnanchor">[458]</a></p>
+
+<p>Many years before, several observers&mdash;J. M&uuml;ller,
+Williamson, and Steenstrup&mdash;had been struck with the
+resemblance existing between the placoid scales and the teeth of
+Elasmobranch fishes. Hertwig followed up this clue, and came to the
+conclusion not only that placoid scales and teeth were <span class=
+"pagenum"><a name="pg310" id="pg310">310</a></span>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, (<i>supra</i>, p.&nbsp;163). Hertwig considered that the
+following bones were originally formed by coalescence of
+teeth&mdash;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 (<i>Deckknochen</i>) 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.</p>
+
+<p>These conclusions were accepted by K&ouml;lliker himself, who
+wrote in his <i>Entwickelungsgeschichte</i> (1879)&mdash;"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.&nbsp;464).</p>
+
+<p>Gegenbaur<a name="FNanchor_459" id="FNanchor_459" /><a href=
+"#Footnote_459" class="fnanchor">[459]</a> consistently upheld the
+phylogenetic derivation of investing bones from dermal
+ossifications, and even went further and derived substitutionary
+bones as well from the integument, thus establishing a direct
+comparison between the skeletal formations of Vertebrates and
+Invertebrates. Investing bones were actual integumentary
+ossifications which had gradually sunk beneath the skin to become
+part of the internal skeleton; substitutionary bones were produced
+by cells (osteoblasts) which were ultimately derived from the
+integument.<a name="FNanchor_460" id="FNanchor_460" /><a href=
+"#Footnote_460" class="fnanchor">[460]</a></p>
+
+<p><span class="pagenum"><a name="pg311" id="pg311">311</a></span>A
+further instance of the historical interpretation of animal
+structure, taken from quite a different field, is afforded by the
+speculations of Dollo<a name="FNanchor_461" id=
+"FNanchor_461" /><a href="#Footnote_461" class=
+"fnanchor">[461]</a> on the ancestral history of the Marsupials. In a
+brilliant paper of 1880<a name="FNanchor_462" id=
+"FNanchor_462" /><a href="#Footnote_462" class=
+"fnanchor">[462]</a> Huxley made the suggestion that the ancestors of
+Marsupials were arboreal forms. "I think it probable," he wrote,
+"from the character of the pes, that the primitive forms, whence
+the existing Marsupialia have been derived, were arboreal animals;
+and it is not difficult, I conceive, to see that, with such habits,
+it may have been highly advantageous to an animal to get rid of its
+young from the interior of its body at as early a period of
+development as possible, and to supply it with nourishment during
+the later periods through the lacteal glands, rather than through
+an imperfect form of placenta" (p.&nbsp;655). Dollo followed up this
+suggestion, which had in the meantime been strengthened by Hill's
+discovery of a true allantoic placenta in <i>Perameles</i>, 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 <i>Notoryctes</i>.</p>
+
+<p>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.</p>
+
+<p>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 <span class=
+"pagenum"><a name="pg312" id="pg312">312</a></span>achieved.
+Homologies were considered more interesting than analogies,
+vestigial organs more interesting than f&oelig;tal 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.</p>
+
+<p>But evolutionary morphology for all practical purposes was a
+development of pure or idealistic morphology, and was powerless to
+bring to fruit the new conception with which evolution-theory had
+enriched it. The reason is not far to seek. Pure morphology is
+essentially a science of comparison which seeks to disentangle the
+unity hidden beneath the diversity of organic form. It is not
+immediately concerned with the causes of organic
+diversity&mdash;that is rather the task of the sciences of the
+individual, heredity and development. To take an example&mdash;the
+recapitulation theory may legitimately be used as a law of pure
+morphology, as stating the abstract relation of ontogeny to
+phylogeny, and the probable line of descent of any organism may be
+deduced from it, as a mere matter of the ideal derivation of one
+form from another; but an explanation of the reason for the
+recapitulation of ancestral history during development can clearly
+not be given by pure morphology unaided. From the fact that the
+common starfish shows in the course of its development distinct
+traces of a stalk<a name="FNanchor_463" id="FNanchor_463" /><a
+href="#Footnote_463" class="fnanchor">[463]</a> 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.</p>
+
+<p>From the point of view of the pure morphologist the
+recapitulation theory is an instrument of research enabling <span
+class="pagenum"><a name="pg313" id="pg313">313</a></span>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.</p>
+
+<p>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.</p>
+
+<p>We shall see in the next chapter that the transformation of
+morphology from a comparative to a causal science did take place
+towards the end of the century, and that some progress was made
+towards an understanding of the relation between individual
+development and ancestral history, particularly by Roux and Samuel
+Butler, working with the fruitful Lamarckian conception of the
+transforming power of function.</p>
+
+<div class="footnote">
+<p><a name="Footnote_456" id="Footnote_456" /><a href=
+"#FNanchor_456"><span class="label">[456]</span></a> The importance
+of convergence came to be realised after the vogue of phylogenetic
+speculation had passed&mdash;see Friedmann, <i>Die Konvergenz der
+Organismen</i>, Berlin, 1904, and A. Willey, <i>Convergence in
+Evolution</i>, London, 1911. Also L. Vialleton, <i>Elements de
+morphologie des Vert&eacute;br&eacute;s</i>, Paris, 1912.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_457" id="Footnote_457" /><a href=
+"#FNanchor_457"><span class="label">[457]</span></a> 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. (<i>Cf.</i> passage quoted below, on p.&nbsp;<a href="#pg308">308</a>.)</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_458" id="Footnote_458" /><a href=
+"#FNanchor_458"><span class="label">[458]</span></a> <i>Arch. f.
+mikr. Anat.</i>, xi. (suppl.), 1874; <i>Morph. Jahrb.</i>, ii.,
+1876, v. 1879, and vii., 1882.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_459" id="Footnote_459" /><a href=
+"#FNanchor_459"><span class="label">[459]</span></a> <i>Vergleich.
+Anat. d. Wirbelthiere</i>, i., pp.&nbsp;200-1, 1898.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_460" id="Footnote_460" /><a href=
+"#FNanchor_460"><span class="label">[460]</span></a> For a full
+historical account of work on membrane and cartilage bones (as well
+as on the theory of the skull) see E. Gaupp, "Altere und neuere
+Arbeiten &uuml;ber den Wirbelthiersch&auml;del," <i>Ergeb. Anat.
+Entw.</i>, x., 1901, and "Die Entwickelung des Kopfskelettes," in
+Hertwig's "<i>Handbuch vergl. exper. Entwickelungslehre d.
+Wirbelthiere</i>," iii., 2, pp.&nbsp;573-874, 1905.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_461" id="Footnote_461" /><a href=
+"#FNanchor_461"><span class="label">[461]</span></a> "Les
+Anc&ecirc;tres des Marsupiaux &eacute;taient-ils arboricoles?"
+<i>Trav. Stat. zool. Wimereux</i>, vii., pp.&nbsp;188-203, pls.
+xi.-xii., 1899. See also Bensley, <i>Trans. Linn. Soc.</i> (2) ix.,
+pp.&nbsp;83-214, 1903.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_462" id="Footnote_462" /><a href=
+"#FNanchor_462"><span class="label">[462]</span></a> <i>Proc. Zool.
+Soc.</i>, pp.&nbsp;649-62, 1880. <i>Sci. Mem.</i>, iv., pp.&nbsp;457-72.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_463" id="Footnote_463" /><a href=
+"#FNanchor_463"><span class="label">[463]</span></a> J. F. Gemmill,
+<i>Phil. Trans. B</i>, ccv., p.&nbsp;255, 1914.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg314" id=
+"pg314">314</a></span></p>
+
+<h3>CHAPTER XVIII</h3>
+
+<h4>THE BEGINNINGS OF CAUSAL MORPHOLOGY</h4>
+
+<p><span class="smcap">Until</span> 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.</p>
+
+<p>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.</p>
+
+<p>Animal physiology, on its side, had developed in complete
+isolation from morphology into a science of the functioning of the
+adult and finished animal, considered as a more or less stable
+physico-chemical mechanism. Since the days of Ludwig, Claude
+Bernard and E. du Bois Reymond, the physiologists' chief care had
+been to analyse vital activities into their component physical and
+chemical processes, and to trace out the interchange of matter and
+energy between the organism and its environment. Physiologists had
+left untouched, perhaps wisely, the much more difficult problem of
+the causes of the development of form. For all practical purposes
+they took the animal-machine as given, and did not trouble about
+its mode of origin. They held indeed <span class="pagenum"><a name=
+"pg315" id="pg315">315</a></span>that form-production was due to a
+complex of physico-chemical causes, which they hoped some day to
+unravel;<a name="FNanchor_464" id="FNanchor_464" /><a href=
+"#Footnote_464" class="fnanchor">[464]</a> but this future
+physiology of development remained quite embryonic.</p>
+
+<p>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.</p>
+
+<p>The pioneer in this field, both as investigator and as thinker,
+was W. Roux, who sketched in the 'eighties the main outlines of a
+new science of causal morphology, to which he gave the name of
+<i>Entwicklungsmechanik</i>. 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.<a name="FNanchor_465" id="FNanchor_465" /><a
+href="#Footnote_465" class="fnanchor">[465]</a></p>
+
+<p>Though Roux was the only begetter of the science of
+<i>Entwicklungsmechanik</i>, he was, of course, not the first to
+investigate experimentally the formative processes of animal life.
+Study of regeneration dates back to Trembley (1740-44), R&eacute;aumur
+(1742), Bonnet (1745), and Spallanzani (1768-82),<a name=
+"FNanchor_466" id="FNanchor_466" /><a href="#Footnote_466"
+class="fnanchor">[466]</a> and in the years preceding Roux's activity
+good work was done by Philipeaux. A beginning had been made with
+experimental teratology by E. Geoffroy St Hilaire and others, and
+the work of C. Dareste<a name="FNanchor_467" id=
+"FNanchor_467" /><a href="#Footnote_467" class=
+"fnanchor">[467]</a> remains classical. Back in the 18th century,
+some of John Hunter's experiments had a bearing upon the problems
+of form; his work on transplantation was followed up in the 19th
+century by Flourens, p.&nbsp;Bert, Ollier and many others. In founding
+in 1872 the <i>Archives de Zoologie exp&eacute;rimentale et
+g&eacute;n&eacute;rale</i> H. de Lacaze-Duthiers <span class=
+"pagenum"><a name="pg316" id="pg316">316</a></span>put forward in
+his introduction a powerful plea for the use of the experimental
+method in zoology.</p>
+
+<p>In some ways more directly connected with
+<i>Entwicklungsmechanik</i> was His's attempt in 1874<a name=
+"FNanchor_468" id="FNanchor_468" /><a href="#Footnote_468"
+class="fnanchor">[468]</a> 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&mdash;the ventricles
+of the brain, for instance, the alimentary canal, the
+heart&mdash;and he further succeeded in imitating the formation of
+these organs by folding, pinching, and cutting india-rubber tubes
+and plates in various ways."<a name="FNanchor_469" id=
+"FNanchor_469" /><a href="#Footnote_469" class=
+"fnanchor">[469]</a></p>
+
+<p>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&mdash;its importance to the organism, and its
+possible mechanistic explanation. The first paper<a name=
+"FNanchor_470" id="FNanchor_470" /><a href="#Footnote_470"
+class="fnanchor">[470]</a> was a study of the branching and
+distribution of the arteries in the human body (1878), and a second
+paper on the same subject followed in 1879.<a name="FNanchor_471"
+id="FNanchor_471" /><a href="#Footnote_471" class=
+"fnanchor">[471]</a></p>
+
+<p>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.</p>
+
+<p>These papers were followed in 1881 by his well-known <span
+class="pagenum"><a name="pg317" id="pg317">317</a></span>book,
+<i>Der Kampf der Theile im Organismus</i>, 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&mdash;in the theory of
+"cellular selection," and the book owed its success to its fitting
+in so well with the prevalent Darwinism of the day. But its real
+importance, as a big step towards causal morphology, was naturally
+not so fully appreciated.</p>
+
+<p>During the next few years Roux continued his studies on
+functional adaptation,<a name="FNanchor_472" id=
+"FNanchor_472" /><a href="#Footnote_472" class=
+"fnanchor">[472]</a> and at the same time made a new departure by
+inaugurating, almost contemporaneously with the physiologist
+Pfl&uuml;ger, the study of experimental embryology. Isolated
+observations had previously been made upon the development of
+single blastomeres or parts of blastul&aelig;, by Haeckel and Chun
+for instance,<a name="FNanchor_473" id="FNanchor_473" /><a
+href="#Footnote_473" class="fnanchor">[473]</a> but Roux<a name=
+"FNanchor_474" id="FNanchor_474" /><a href="#Footnote_474"
+class="fnanchor">[474]</a> and Pfl&uuml;ger<a name="FNanchor_475"
+id="FNanchor_475" /><a href="#Footnote_475" class=
+"fnanchor">[475]</a> were the first to investigate the subject
+systematically, choosing for their work the egg of the frog.<a
+name="FNanchor_476" id="FNanchor_476" /><a href=
+"#Footnote_476" class="fnanchor">[476]</a> Roux continued for many
+years to follow up this line of work.<a name="FNanchor_477" id=
+"FNanchor_477" /><a href="#Footnote_477" class=
+"fnanchor">[477]</a></p>
+
+<p>In 1890 he drew up a programme and manifesto<a name=
+"FNanchor_478" id="FNanchor_478" /><a href="#Footnote_478"
+class="fnanchor">[478]</a> of <i>Entwicklungsmechanik</i> as "an
+anatomical science of the <span class="pagenum"><a name="pg318" id=
+"pg318">318</a></span>future," and in 1895 he founded the famous
+<i>Archiv f&uuml;r Entwicklungsmechanik</i>,<a name=
+"FNanchor_479" id="FNanchor_479" /><a href="#Footnote_479"
+class="fnanchor">[479]</a> publishing in the same year the two large
+volumes of his collected papers,<a name="FNanchor_480" id=
+"FNanchor_480" /><a href="#Footnote_480" class=
+"fnanchor">[480]</a> of which the first volume dealt with functional
+adaptation, the second with experimental embryology.</p>
+
+<p>His subsequent work includes several important general papers;<a
+name="FNanchor_481" id="FNanchor_481" /><a href=
+"#Footnote_481" class="fnanchor">[481]</a> besides a number of
+special memoirs dealing with the factors of development, and with
+his original subject, functional adaptation.<a name=
+"FNanchor_482" id="FNanchor_482" /><a href="#Footnote_482"
+class="fnanchor">[482]</a></p>
+
+<p>In our sketch of his views we shall have occasion to refer
+particularly to his publications of 1881, 1895 (the
+<i>Einleitung</i>), 1902, 1905, and 1910.</p>
+
+<p>Although Roux's biological philosophy is out-and-out
+mechanistic, he yet recognises the difficulty, even the
+impossibility, of straightway reducing development to the
+physico-chemical level. He tries to steer a course midway between
+the simplicist conceptions of the materialists and the
+"metaphysics" of the neo-vitalist school, which the experimental
+study of development and regeneration soon brought into being. In
+1895 he writes:&mdash;"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.&nbsp;23).</p>
+
+<p>The <i>via media</i> adopted by Roux is the analysis of
+development, not directly into simple physico-chemical processes,
+but into more complex organic processes dependent <span class=
+"pagenum"><a name="pg319" id="pg319">319</a></span>upon the
+fundamental properties of living matter. The aim of
+<i>Entwicklungsmechanik</i> is defined by Roux to be the reduction
+of developmental events to the fewest and simplest
+<i>Wirkungsweisen</i>, or causal processes.<a name="FNanchor_483"
+id="FNanchor_483" /><a href="#Footnote_483" class=
+"fnanchor">[483]</a> 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
+<i>Entwicklungsmechanik</i> 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."</p>
+
+<p>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&mdash;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.</p>
+
+<p>The principal "complex components" which Roux recognises are
+somewhat as follows:&mdash;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.</p>
+
+<p>It will be noticed that Roux's "complex components" are <span
+class="pagenum"><a name="pg320" id="pg320">320</a></span>simply the
+general properties or functions of organised matter.</p>
+
+<p>Expressing Roux's thought in another way, we might say that life
+can only be defined functionally, <i>i.e.</i>, by an enumeration of
+the "complex components" or elementary functions which all living
+beings manifest, even down to the very simplest. "Living beings,"
+writes Roux, "can at present be defined with any approach to
+completeness only functionally, that is to say, through
+characterisation of their activities, for we have an adequate
+acquaintance with their functions in a general way, though our
+knowledge of particulars is by no means complete" (p.&nbsp;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.&nbsp;106-7, 1905).</p>
+
+<p>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.</p>
+
+<p>The idea was first elaborated in the <i>Kampf der Theile</i>
+(1881), where he wrote:&mdash;"There must be distinguished in the
+life of all the parts two periods, an embryonic in the broad sense,
+during which the parts develop, differentiate and grow of
+themselves, and a period of completer development, during which
+growth, and in many cases also the balance of assimilation over
+dissimilation, can come about only under the influence of stimuli"
+(p.&nbsp;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 <span
+class="pagenum"><a name="pg321" id="pg321">321</a></span> 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.</p>
+
+<p>The conception is more fully expressed in 1905 as
+follows:&mdash;"This separation (of development into two periods)
+is intended only as a first beginning. The first period I called
+the embryonic period &kappa;&alpha;&tau;'
+&#7952;&xi;&omicron;&chi;&#942;&nu;, or the period of
+organ-rudiments. It includes the 'directly inherited' structures,
+<i>i.e.</i>, 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 <i>independent</i> growth and
+maintenance, that is, of growth and maintenance which take place
+without the functioning of the organs.</p>
+
+<p>"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.</p>
+
+<p>"The second period is the period of 'functional
+form-development.' It includes the further differentiation and the
+maintenance in their typical form of the organs laid down in the
+first period; and this is brought about by the exercise of the
+specific functions of the organs. This period adds the finishing
+touches to the finer functional differentiation of the organs, and
+so brings to pass the 'finer functional harmony' of all organs with
+the whole. The formative activity displayed during this period
+depends upon the circumstance that the functional stimulus, or
+rather the exercise by the organs of their specific functions, is
+accompanied by a subsidiary formative activity, which acts partly
+by producing new form and partly by maintaining that which is
+already formed.... Between the two periods lies presumably a
+transition period, an intermediary stage of varying duration in the
+different organs, in which both classes of causes are concerned in
+the further building-up of the already formed, those of the first
+period in gradually decreasing measure, those of the second in an
+increasing degree" (pp.&nbsp;94-6, 1905).</p>
+
+<p>In the first period the organ forms or determines the <span
+class="pagenum"><a name="pg322" id="pg322">322</a></span>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.</p>
+
+<p>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 <i>inherited</i>
+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.</p>
+
+<p>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:&mdash;"These stimuli can also produce new structure, which
+if it is constantly formed throughout many generations finally
+becomes hereditary, <i>i.e.</i>, develops in the descendants in the
+absence of the stimuli, becomes in our sense embryonic" (p.&nbsp;180,
+1881). Again, "form-characteristics which were originally acquired
+in post-embryonic life through functional adaptation may be
+developed in the embryo without the functional stimulus, and may in
+later development become more or less completely differentiated,
+and retain this differentiation without functional activity or with
+a minimum of it. But in the continued absence of functional
+activity they become atrophied ... and in the end disappear" (p.
+201, 1881).</p>
+
+<p>This conception of the nature of hereditary transmission is an
+important one, and constitutes the first big step towards a real
+understanding of the historical element in organic form and
+activity. It supplies a practical criterion for the distinguishing
+of "heritage" characters from acquired <span class="pagenum"><a
+name="pg323" id="pg323">323</a></span>characters, of palingenetic
+from cenogenetic&mdash;a criterion which descriptive morphology was
+unable to find.<a name="FNanchor_484" id="FNanchor_484" /><a
+href="#Footnote_484" class="fnanchor">[484]</a> 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.</p>
+
+<p>It is this element in Roux's theories that puts them so far in
+advance of those of Weismann. Weismann did not really tackle the
+big problem of the relation of form to function, and he left no
+place in his mechanical system of preformation for functional or
+second-period development; he conceived all development to be in
+Roux's sense embryonic, and due to the automatic unpacking of a
+complex germinal organisation. Roux himself was to a certain extent
+a preformationist, for the development of his first-period
+characters is conditioned by the inherited organisation of the
+germ-plasm, and is purely automatic. It was indeed his experiments
+on the frog's egg (1888) that supplied some of the strongest
+evidence in favour of the mosaic theory of development. The number
+of <i>Anlagen</i> which he postulates in the germ is however small,
+and the germ-plasm in his conception of it has a relatively simple
+structure (p.&nbsp;103, 1905).</p>
+
+<p>The transmission of acquired characters forms, of course, an
+integral part of Roux's conception of heredity and development, for
+without this transmission second-stage characters could not be
+transformed into first-stage characters. He discusses this
+difficult question at some length in the <i>Kampf der Theile</i>,
+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 <span
+class="pagenum"><a name="pg324" id="pg324">324</a></span>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.<a name="FNanchor_485" id=
+"FNanchor_485" /><a href="#Footnote_485" class=
+"fnanchor">[485]</a> From this point of view the problem of
+transmission might be merged in the broader problem of the
+production of form through chemical processes&mdash;the central
+problem of all development.</p>
+
+<p>Inherited characters develop by an automatic process of
+self-differentiation, and the separate parts of the embryo show
+during this first period a surprising functional independence of
+one another. But this state of things changes progressively as the
+second period is reached, until finally all form-production and
+maintenance and all correlation depend upon functioning. It is in
+the first period of automatic development through internal
+"determining" factors that the "developmental" functions in the
+strict sense, <i>e.g.</i> 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.</p>
+
+<p>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
+fibrill&aelig; and these out of muscle-prisms, or again the length
+and thickness of the muscles, the static structure of the bones,
+the composition of the stomach and the blood-vessels out of
+longitudinal and circular fibres, the external shape of the
+vertebral centra and of the cuneiform bones of the foot" (p.&nbsp;73,
+1910). Indeed, as Cuvier had already pointed out, practically every
+organ in the body shows a functional structure which is accurately
+and minutely adjusted to the function it is intended to perform.
+Thus, to take some further examples, the arteries are admirably
+adapted as regards size of lumen, elasticity of wall, direction of
+branching, to conduct the blood to all parts of the body <span
+class="pagenum"><a name="pg325" id="pg325">325</a></span>with the
+least possible waste of the propelling power through frictional
+resistance. So, too, the spongy substance of the long bones is
+arranged in lamell&aelig; which take the direction of the principal
+stresses and strains which fall upon the bones in action.</p>
+
+<p>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>i.e.</i>, 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.</p>
+
+<p>From functional structure is to be distinguished nonfunctional
+structure, which has no relation to the bodily functions&mdash;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.&nbsp;74,
+1910). Non-functional structure is not affected by functional
+adaptation, and may accordingly be left out of consideration
+here.</p>
+
+<p>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&mdash;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."
+<span class="pagenum"><a name="pg326" id=
+"pg326">326</a></span>"From such temporary changes are sharply to
+be distinguished all permanent alterations which first appear in
+perceptible fashion through oft-repeated or long-continued,
+enhanced functional activity. These produce a new and lasting
+internal equilibrium of the organ, consisting in an insertion of
+new molecules or a rearrangement of old. For this reason they
+outlast the periods of functional form-change, or, if as in the
+case of the muscles they themselves alter during functional
+activity, they regain their state when the organ ceases to
+function" (p.&nbsp;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 <i>functional
+adaptations</i>.... 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.&nbsp;75, 1910).</p>
+
+<p>We may now enquire in what way functional adaptations can arise
+as by-products of functioning.</p>
+
+<p>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.<a name="FNanchor_486" id=
+"FNanchor_486" /><a href="#Footnote_486" class=
+"fnanchor">[486]</a> The development of purposive internal structure
+is only to be explained by the properties of the tissues
+concerned.</p>
+
+<p>In illustration and proof of the statement that functional
+adaptation is due to the properties of the tissues we may adduce
+the development and regulation of the blood-vascular <span class=
+"pagenum"><a name="pg327" id="pg327">327</a></span>system, which
+has been thoroughly studied from this point of view by Roux and
+Oppel (1910).</p>
+
+<p>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.</p>
+
+<p>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:&mdash;</p>
+
+<p>"(1) The faculty&mdash;depending on a direct sensibility
+possessed by the endothelium and perhaps also by the other layers
+of the intima&mdash;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 h&aelig;modynamically 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.</p>
+
+<p>"(2) The faculty possessed by the endothelium of the capillaries
+of each organ of adapting itself qualitatively to the particular
+metabolism of the organ." This adaptedness of the capillaries is,
+however, more usually an inherited state, <i>i.e.</i>, brought
+about in the first period of development.</p>
+
+<p>"(3) The faculty possessed by the capillary walls of being
+stimulated to sprout out and branch by increased functioning, <span
+class="pagenum"><a name="pg328" id=
+"pg328">328</a></span><i>i.e.</i>, 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.</p>
+
+<p>"(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.</p>
+
+<p>"(5) The power to regulate the normal<a name="FNanchor_487"
+id="FNanchor_487" /><a href="#Footnote_487" class=
+"fnanchor">[487]</a> 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.</p>
+
+<p>"(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.</p>
+
+<p>"(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.</p>
+
+<p>"(8) The power possessed by the circular musculature of
+responding to such continuous functional widening, by the formation
+of new structural parts in the circular musculature, and so of
+widening the vessel permanently or by this new formation of
+muscular fibres thickening the circular musculature.</p>
+
+<p>"(9) The faculty of being stimulated by increased blood-pressure
+to produce the same structural changes as mentioned in par. 8,
+though here the response is otherwise conditioned" (pp.&nbsp;126-7,
+1910).</p>
+
+<p>It is by virtue of the tissue-properties detailed above that the
+complex functional adaptations of the blood-vessels come about.</p>
+
+<p>The development of the vascular system is no mere automatic and
+mechanical production of form, apart from <span class="pagenum"><a
+name="pg329" id="pg329">329</a></span>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&mdash;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 <i>products</i> of activity, and leaves us ignorant
+of the real processes of form-development and form-conservation,
+and of their causes" (p.&nbsp;125, 1910).</p>
+
+<p>The real thing in organisation is not form but activity. It is
+in this return to the Cuvierian or functional attitude to the
+problems of form that we hold Roux's greatest service to biology to
+consist. The attitude, however, seems to smack of vitalism, and
+Roux, as we have seen, is no vitalist. He holds that the marvellous
+and apparently purposive tissue-qualities which underlie all
+processes of functional adaptation have arisen "naturally," in the
+course of evolution, by the action of natural selection upon the
+various properties, useful and useless, which appeared fortuitously
+in the primary living organisms. He is, moreover, deeply imbued
+with the materialistic philosophy of his youth, and it is indeed
+one of the chief characteristics of his system that he states the
+fundamental properties or qualities of life in terms of metabolism.
+A vital quality is for Roux a special process or mode of
+assimilation. The faculty of "morphological assimilation" whereby
+form is imposed upon formless chemical processes is the ultimate
+term of Roux's analysis&mdash;"the most general, most essential,
+and most characteristic formative activity of life" (p.&nbsp;631,
+1902).</p>
+
+<p>We have now to consider very briefly the early results <span
+class="pagenum"><a name="pg330" id="pg330">330</a></span>achieved
+by Roux's fellow-workers in the field of causal morphology. As D.
+Barfurth points out,<a name="FNanchor_488" id=
+"FNanchor_488" /><a href="#Footnote_488" class=
+"fnanchor">[488]</a> 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 Pfl&uuml;ger on the sexual difference in frogs (1881), by
+Pfl&uuml;ger 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. Pfl&uuml;ger and W. Roux:
+Pfl&uuml;ger publishing his researches on 'the influence of gravity
+on cell-division,' Roux his experimental investigations on 'the
+time of the determination of the chief planes in the
+frog-embryo.'... In the same year appeared A. Rauber's experimental
+studies 'on the influence of temperature, atmospheric pressure, and
+various substances on the development of animal ova,' which have
+brought many similar works in their train. The following year
+(1884) saw a lively controversy on Pfl&uuml;ger's
+gravity-experiments with animal eggs, in which took part
+Pfl&uuml;ger, Born, Roux, O. Hertwig and others, and in this year
+appeared work by Roux dealing with the experimental study of
+development, and in particular giving the results of the first
+definitely localised pricking-experiments on the frog's egg (in the
+<i>Schles. Gesell. f. vaterl. Kultur</i>, 15th Feb. 1884), also the
+important researches of M. Nussbaum and Gruber (followed up later
+by Verworn, Hofer and Balbiani) on Protozoa, and other experimental
+work" (pp.&nbsp;xi.-xii.).</p>
+
+<p>In 1888 appeared a famous paper by W. Roux,<a name=
+"FNanchor_489" id="FNanchor_489" /><a href="#Footnote_489"
+class="fnanchor">[489]</a> in which he described how he had succeeded
+in killing by means of a hot needle one of the two first
+blastomeres of the frog's egg, and how a half-embryo had developed
+from the uninjured cell. Some years before<a name="FNanchor_490"
+id="FNanchor_490" /><a href="#Footnote_490" class=
+"fnanchor">[490]</a> he had enunciated, at about the same time as
+Weismann, the view that development <span class="pagenum"><a name=
+"pg331" id="pg331">331</a></span>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,<a name="FNanchor_491" id=
+"FNanchor_491" /><a href="#Footnote_491" class=
+"fnanchor">[491]</a> and his later success in obtaining a half-embryo
+from one of the first two blastomeres seemed to establish the
+"mosaic theory" conclusively.</p>
+
+<p>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,<a name="FNanchor_492" id="FNanchor_492" /><a
+href="#Footnote_492" class="fnanchor">[492]</a> which strongly
+supported the Roux-Weismann theory. Considerable astonishment was
+therefore caused by Driesch's announcement in 1891<a href="#Footnote_493" class=
+"fnanchor">[493]</a> that he had
+obtained complete larv&aelig; from single blastomeres of the
+sea-urchin's egg isolated at the two-celled stage. He followed this
+up in the next year<a name="FNanchor_493" id=
+"FNanchor_493" /><a href="#Footnote_493" class=
+"fnanchor">[493]</a> 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 <i>Amphioxus</i>,<a name="FNanchor_494" id=
+"FNanchor_494" /><a href="#Footnote_494" class=
+"fnanchor">[494]</a> and Zoja on medus&aelig;.<a name=
+"FNanchor_495" id="FNanchor_495" /><a href="#Footnote_495"
+class="fnanchor">[495]</a> 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,<a name="FNanchor_496" id="FNanchor_496" /><a
+href="#Footnote_496" class="fnanchor">[496]</a> and on
+<i>Nereis</i> by E. B. Wilson,<a name="FNanchor_497" id=
+"FNanchor_497" /><a href="#Footnote_497" class=
+"fnanchor">[497]</a> with analogous results.</p>
+
+<p>In 1895 O. Schultze<a name="FNanchor_498" id=
+"FNanchor_498" /><a href="#Footnote_498" class=
+"fnanchor">[498]</a> showed that if the frog's egg is held between
+two plates and inverted at the two-celled stage <span class=
+"pagenum"><a name="pg332" id="pg332">332</a></span>there are formed
+two embryos instead of one. In the same year T. H. Morgan<a name=
+"FNanchor_499" id="FNanchor_499" /><a href="#Footnote_499"
+class="fnanchor">[499]</a> repeated Roux's fundamental experiment of
+destroying one of the two blastomeres, but inverted the egg
+immediately after the operation&mdash;a whole embryo of half size
+resulted. A year or two later Herlitzka<a name="FNanchor_500" id=
+"FNanchor_500" /><a href="#Footnote_500" class=
+"fnanchor">[500]</a> 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.</p>
+
+<p>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.</p>
+
+<p>Evidence soon accumulated that the cytoplasm of the egg stood
+for much in the differentiation of the embryo. A number of years
+previously Chun had made the discovery that single blastomeres of
+the Ctenophore egg, isolated at the two-celled stage, gave
+half-embryos. This was in the main confirmed by Driesch and Morgan
+in 1896,<a name="FNanchor_501" id="FNanchor_501" /><a href=
+"#Footnote_501" class="fnanchor">[501]</a> and they made the
+further interesting discovery that the same defective larv&aelig;
+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,<a name=
+"FNanchor_502" id="FNanchor_502" /><a href="#Footnote_502"
+class="fnanchor">[502]</a> who removed the anucleate "yolk-lobe" from
+the egg of the mollusc <i>Ilyanassa</i> at the two-celled stage,
+and obtained larv&aelig; which lacked a mesoblast. This result was
+brilliantly confirmed and extended some years later by E. B.
+Wilson,<a name="FNanchor_503" id="FNanchor_503" /><a href=
+"#Footnote_503" class="fnanchor">[503]</a> working on the egg of
+<i>Dentalium</i>. He found that if the similar anucleate "polar
+lobe" of this form is removed at the two-celled stage, deficient
+larv&aelig; are formed, in which the post-trochal region and the
+apical organ are absent. He further showed that in the unsegmented
+but mature egg prelocalised cytoplasmic regions can be
+distinguished, which later become separated from one another
+through the segmentation of the egg. The segmentation-cells into
+which these cytoplasmic substances are thus segregated show a
+marked specificity of development, <span class="pagenum"><a name=
+"pg333" id="pg333">333</a></span>giving rise, even when isolated,
+to definite organs of the embryo. Wilson concluded that the
+cytoplasm of the egg contains a number of specific organ-forming
+stuffs, which have a definite topographical arrangement in the egg.
+Development is thus due in part to a qualitative division not of
+the nucleus but of the cytoplasm. Corroborative evidence of the
+existence of cytoplasmic organ-forming stuffs has been supplied for
+several other species, <i>e.g.</i>, <i>Patella</i> (Wilson),
+<i>Cynthia</i> (Conklin), <i>Cerebratulus</i> (Zeleny), and
+<i>Echinus</i> (Boveri).</p>
+
+<p>It is interesting to recall that so long ago as 1874 W. His<a
+name="FNanchor_504" id="FNanchor_504" /><a href=
+"#Footnote_504" class="fnanchor">[504]</a> 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.</p>
+
+<p>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.<a name="FNanchor_505" id="FNanchor_505" /><a href=
+"#Footnote_505" class="fnanchor">[505]</a> The formative power of
+function was studied in Germany by Roux and his pupils, Fuld, O.
+Levy, Schepelmann and others, particularly by E. Bab&aacute;k. In
+France, F. Houssay inaugurated<a name="FNanchor_506" id=
+"FNanchor_506" /><a href="#Footnote_506" class=
+"fnanchor">[506]</a> 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.<a name="FNanchor_507" id="FNanchor_507" /><a
+href="#Footnote_507" class="fnanchor">[507]</a> The principles of
+his dynamical morphology were first laid down in his book <i>La
+Forme et la Vie</i> (1900).</p>
+
+<p>The famous experiments of Loeb, Delage and others on artificial
+parthenogenesis may also be mentioned, though their connection with
+morphology is somewhat remote.</p>
+
+<p><span class="pagenum"><a name="pg334" id=
+"pg334">334</a></span>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.</p>
+
+<div class="footnote">
+<p><a name="Footnote_464" id="Footnote_464" /><a href=
+"#FNanchor_464"><span class="label">[464]</span></a> See Carus's
+remark, referred to on p.&nbsp;<a href="#pg194">194</a>, above.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_465" id="Footnote_465" /><a href=
+"#FNanchor_465"><span class="label">[465]</span></a> Roux, <i>Die
+Entwicklungsmechanik</i>, p.&nbsp;26, Leipzig, 1905.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_466" id="Footnote_466" /><a href=
+"#FNanchor_466"><span class="label">[466]</span></a> T. H. Morgan,
+<i>Regeneration</i>, p.&nbsp;1, New York and London, 1901.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_467" id="Footnote_467" /><a href=
+"#FNanchor_467"><span class="label">[467]</span></a> <i>Recherches
+sur la production artificielle des Monstruosit&eacute;s</i>, Paris,
+1877, and many later papers.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_468" id="Footnote_468" /><a href=
+"#FNanchor_468"><span class="label">[468]</span></a> <i>Unsere
+K&ouml;rperform und das physiologische Problem ihrer
+Entstehung</i>, Leipzig, 1874.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_469" id="Footnote_469" /><a href=
+"#FNanchor_469"><span class="label">[469]</span></a> J. W.
+Jenkinson, <i>Experimental Embryology</i>, p.&nbsp;3, Oxford, 1909.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_470" id="Footnote_470" /><a href=
+"#FNanchor_470"><span class="label">[470]</span></a> "Ueber die
+Verzweigungen der Blutgef&auml;sse des Menschen," <i>Jen.
+Zeit</i>., xii., 1878.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_471" id="Footnote_471" /><a href=
+"#FNanchor_471"><span class="label">[471]</span></a> "Ueber die
+Bedeutung der Ablenkung des Arterienstammes bei der Astabgabe,"
+<i>Jen. Zeit</i>., xiii., 1879.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_472" id="Footnote_472" /><a href=
+"#FNanchor_472"><span class="label">[472]</span></a> "Beitr&auml;ge
+zur Morphologie der funktionellen Anpassung. I. Struktur eines
+hochdifferenzierten bindgewebigen Organes (der Schwanzflosse des
+Delphin)," <i>Arch. Anat. Physiol.</i> (<i>Anat. Abt.</i>) for
+1883. II. "Ueber die Selbstregulation der 'morphologischen'
+L&auml;nge der Skeletmuskeln des Menschen," <i>Jen. Zeit.</i>,
+xvi., 1883. III. "Beschreibung ... einer
+Kniegelenkeknochenankylose," <i>Arch. Anat. Physiol.</i> (<i>Anat.
+Abt.</i>) for 1885.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_473" id="Footnote_473" /><a href=
+"#FNanchor_473"><span class="label">[473]</span></a> In 1869 and
+1877 respectively (Roux, p.&nbsp;53, 1905).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_474" id="Footnote_474" /><a href=
+"#FNanchor_474"><span class="label">[474]</span></a> <i>Ueber die
+Zeit. der Bestimmung der Hauptrichtungen des Froschembryo</i>,
+Leipzig, 1883.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_475" id="Footnote_475" /><a href=
+"#FNanchor_475"><span class="label">[475]</span></a> "Ueber den
+Einfluss der Schwerkraft auf die Teilung der Zellen,"
+Pfl&uuml;ger's <i>Archiv</i>, xxxi., 1883. Also subsequent papers
+in same journal.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_476" id="Footnote_476" /><a href=
+"#FNanchor_476"><span class="label">[476]</span></a> For an account
+of the classical experiments on the frog's egg, see T. H. Morgan,
+<i>The Development of the Frog's Egg</i>, New York, 1897.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_477" id="Footnote_477" /><a href=
+"#FNanchor_477"><span class="label">[477]</span></a> In a series of
+"Beitr&auml;ge 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 <i>Archiv f. Entw. mech.</i>, from
+1895 onwards.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_478" id="Footnote_478" /><a href=
+"#FNanchor_478"><span class="label">[478]</span></a> <i>Die
+Entwicklungsmechanik der Organismen, eine anatomische Wissenschaft
+der Zukunft</i>, Wien, 1890.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_479" id="Footnote_479" /><a href=
+"#FNanchor_479"><span class="label">[479]</span></a> The first
+volume contains the important <i>Einleitung</i> or general
+Introduction.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_480" id="Footnote_480" /><a href=
+"#FNanchor_480"><span class="label">[480]</span></a> <i>Gesammelte
+Abhandlungen &uuml;ber Entwicklungsmechanik der Organismen</i>, 2
+vols., Leipzig, 1895.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_481" id="Footnote_481" /><a href=
+"#FNanchor_481"><span class="label">[481]</span></a> "F&uuml;r
+unser Programm und seine Verwirklichung," <i>A.E.M.</i>, v., pp.
+1-80 and 219-342, 1897. "Ueber die Selbstregulation der Lebewesen,"
+<i>A.E.M.</i>, xiii., pp.&nbsp;610-5, 1902. "Die Entwicklungsmechanik,
+ein neuer Zweig der biologischen Wissenschaft," Heft I. of the
+<i>Vortr&auml;ge u. Aufs&auml;tze &uuml;ber Entwicklungsmechanik
+der Organismen</i>, Leipzig, 1905. Oppel and Roux, "Ueber die
+gestaltliche Anpassung der Blutgef&auml;sse," Heft x., of the
+<i>Vortr&auml;ge u. Aufs&auml;tze</i>, Leipzig, 1910.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_482" id="Footnote_482" /><a href=
+"#FNanchor_482"><span class="label">[482]</span></a> "Ueber d.
+funkt. Anpassung des Muskelmagens der Gans," <i>A.E.M.</i>, xxi.,
+pp.&nbsp;461-99, 1906.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_483" id="Footnote_483" /><a href=
+"#FNanchor_483"><span class="label">[483]</span></a> The exact
+quantitative formulation of a <i>Wirkungsweise</i> constitutes a
+law. The word itself is perhaps most conveniently rendered as
+"causal process."</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_484" id="Footnote_484" /><a href=
+"#FNanchor_484"><span class="label">[484]</span></a> M.
+F&uuml;rbringer, perhaps under the influence of Roux, emphasised
+the importance, from a morphological point of view, of studying
+post-embryonic (functional) development, <i>Unters. z. Morph. u.
+Syst. der V&ouml;gel</i>, ii., Amsterdam, p.&nbsp;925, 1888.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_485" id="Footnote_485" /><a href=
+"#FNanchor_485"><span class="label">[485]</span></a> See, for the
+development of this idea, Oppel, in Roux-Oppel, 1910.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_486" id="Footnote_486" /><a href=
+"#FNanchor_486"><span class="label">[486]</span></a> <i>Cf.</i> the
+controversy between Herbert Spencer and Weismann on the subject of
+"coadaptation" in the <i>Contemporary Review</i> for 1893 and 1894.
+See also Weismann's paper in <i>Darwin and Modern Science</i>,
+Cambridge, 1909.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_487" id="Footnote_487" /><a href=
+"#FNanchor_487"><span class="label">[487]</span></a> That is, the
+length they take up when separated from the body.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_488" id="Footnote_488" /><a href=
+"#FNanchor_488"><span class="label">[488]</span></a> "Wilhelm Roux
+zum 60. Geburtstage," <i>Arch. f. Entw.-Mech.</i>, xxx.
+<i>Festschrift f&uuml;r Prof. Roux</i>, Pt. i, 1910.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_489" id="Footnote_489" /><a href=
+"#FNanchor_489"><span class="label">[489]</span></a> Virchow's
+<i>Archiv</i>, cxiv., 1888. First announced in Sept. 1887.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_490" id="Footnote_490" /><a href=
+"#FNanchor_490"><span class="label">[490]</span></a> <i>Ueber die
+Bedeutung der Kernteilungsfiguren</i>, Leipzig, 1883.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_491" id="Footnote_491" /><a href=
+"#FNanchor_491"><span class="label">[491]</span></a> <i>Bresl.
+&auml;rtz. Zeitschr.</i>, 1885.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_492" id="Footnote_492" /><a href=
+"#FNanchor_492"><span class="label">[492]</span></a> <i>Journ. de
+l'Anat. et de la Physiologie</i>, xxiii., 1887.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_493" id="Footnote_493" /><a href=
+"#FNanchor_493"><span class="label">[493]</span></a> <i>Zeits. f.
+wiss. Zool.</i>, liii., 1891 and 1892.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_494" id="Footnote_494" /><a href=
+"#FNanchor_494"><span class="label">[494]</span></a> <i>Journ.
+Morph.</i>, viii., 1893.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_495" id="Footnote_495" /><a href=
+"#FNanchor_495"><span class="label">[495]</span></a> <i>Arch. f.
+Ent.-Mech.</i>, i., 1895; ii., 1896.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_496" id="Footnote_496" /><a href=
+"#FNanchor_496"><span class="label">[496]</span></a> <i>Arch. f.
+mikr. Anat.</i>, xliii., 1893.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_497" id="Footnote_497" /><a href=
+"#FNanchor_497"><span class="label">[497]</span></a> <i>Arch. f.
+Ent.-Mech.</i>, iii., 1896.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_498" id="Footnote_498" /><a href=
+"#FNanchor_498"><span class="label">[498]</span></a> <i>Arch. f.
+Ent.-Mech.</i>, i., 1895.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_499" id="Footnote_499" /><a href=
+"#FNanchor_499"><span class="label">[499]</span></a> <i>Anat.
+Anz.</i>, x., 1895.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_500" id="Footnote_500" /><a href=
+"#FNanchor_500"><span class="label">[500]</span></a> <i>Arch. f.
+Ent.-Mech.</i>, iv. 1897.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_501" id="Footnote_501" /><a href=
+"#FNanchor_501"><span class="label">[501]</span></a> <i>Arch. f.
+Ent.-Mech.</i>, ii., 1896.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_502" id="Footnote_502" /><a href=
+"#FNanchor_502"><span class="label">[502]</span></a> <i>Arch. f.
+Ent.-Mech.</i>, iii., 1896.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_503" id="Footnote_503" /><a href=
+"#FNanchor_503"><span class="label">[503]</span></a> <i>Journ.
+exper. Zool.</i>, i., 1904.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_504" id="Footnote_504" /><a href=
+"#FNanchor_504"><span class="label">[504]</span></a> <i>Unsere
+K&ouml;rperform</i>, p.&nbsp;19, Leipzig, 1874.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_505" id="Footnote_505" /><a href=
+"#FNanchor_505"><span class="label">[505]</span></a> <i>Biolog.
+Centrlbl.</i>, xiv., 1894, xv., 1895. <i>Formative Reize in der
+thierischen Ontogenese</i>, Leipzig, 1901.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_506" id="Footnote_506" /><a href=
+"#FNanchor_506"><span class="label">[506]</span></a> "La
+Morphologie dynamique," No. i. of the <i>Collection de Morphologie
+dynamique</i>, Paris, 1911.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_507" id="Footnote_507" /><a href=
+"#FNanchor_507"><span class="label">[507]</span></a> "Forme,
+Puissance et Stabilit&eacute; des Poissons," No. iv. of the
+<i>Collection</i>, Paris, 1912.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg335" id=
+"pg335">335</a></span></p>
+
+<h3>CHAPTER XIX</h3>
+
+<h4>SAMUEL BUTLER AND THE MEMORY THEORIES OF HEREDITY</h4>
+
+<p><span class="smcap">We</span> have laid stress upon the
+distinction established by Roux between the two stages of
+development&mdash;the automatic and the functional&mdash;because of
+the light which it seems to throw upon the phylogenetic relation of
+form to function. We have pointed out, too, the paramount r&ocirc;le 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.</p>
+
+<p>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.</p>
+
+<p>A man of singular freshness and openness of mind, combining in
+an extraordinary degree extreme intellectual subtlety with a
+childlike simplicity of outlook, Butler was one of the most
+fascinating figures of the 19th century. He was not a professional
+biologist, and much of his biological work is, for that reason,
+imperfect. But he brought to bear upon the central problems of
+biology an unbiassed and powerful intelligence, and his attitude to
+these problems, just because it is that of a cultivated layman, is
+singularly illuminating.</p>
+
+<p>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 <span class="pagenum"><a
+name="pg336" id="pg336">336</a></span>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 <i>Life and Habit</i> (1877<a name="FNanchor_508" id=
+"FNanchor_508" /><a href="#Footnote_508" class=
+"fnanchor">[508]</a>):&mdash;"I admit that when I began to write upon
+my subject I did not seriously believe in it. I saw, as it were, a
+pebble upon the ground, with a sheen that pleased me; taking it up,
+I turned it over and over for my amusement, and found it always
+grow brighter and brighter the more I examined it. At length I
+became fascinated, and gave loose rein to self-illusion. The aspect
+of the world changed; the trifle which I had picked up idly had
+proved to be a talisman of inestimable value, and had opened a door
+through which I caught glimpses of a strange and interesting
+transformation. Then came one who told me that the stone was not
+mine, but that it had been dropped by Lamarck, to whom it belonged
+rightfully, but who had lost it; whereon I said I cared not who was
+the owner, if only I might use it and enjoy it. Now, therefore,
+having polished it with what art and care one who is no jeweller
+could bestow upon it, I return it, as best I may, to its possessor"
+(p.&nbsp;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&mdash;in his <i>Evolution, Old and
+New</i> (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 <i>Unconscious Memory</i> (1880) and
+<i>Luck or Cunning</i> (1887).</p>
+
+<p>Butler's main thesis is that living things are active,
+intelligent agents, personally continuous with all their ancestors,
+possessing an intense but unconscious memory of all that their
+ancestors did and suffered, and moving through habit from the
+spontaneity of striving to the automatism of remembrance.</p>
+
+<p>The primary cause of all variation in structure is the active
+response of the organism to needs experienced by it, and the
+indispensable link between the outer world and the creature itself
+is that same "sense of need" upon which <span class="pagenum"><a
+name="pg337" id="pg337">337</a></span>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" (<i>Life and Habit</i>, p.&nbsp;253).
+Variations are indubitably the raw material of evolution&mdash;"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.&nbsp;267).</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>He tries to show that this is a general law&mdash;that knowledge
+and will become intense and perfect only when through
+long-continued exercise they become automatic and
+unconscious&mdash;and he applies this conception to the elucidation
+of development.</p>
+
+<p>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 <span class=
+"pagenum"><a name="pg338" id="pg338">338</a></span>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&mdash;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.&nbsp;54). Assuredly not.</p>
+
+<p>The only possible explanation is that the embryo's ancestors
+have done these things so often, throughout so many millions of
+generations, that the embryo's knowledge of how to do them has
+become unconscious and automatic by reason of this age-long
+practice. This implies that there is in a very real sense actual
+personal continuity between the embryo and all its ancestors, so
+that their experiences are his, their memory also his. "We must
+suppose the continuity of life and sameness between living beings,
+whether plants or animals, to be far closer than we have hitherto
+believed; so that the experience of one person is not enjoyed by
+his successor, so much as that the successor is <i>bona fide</i>
+but a part of the life of his progenitor, imbued with all his
+memories, profiting by all his experiences&mdash;which are, in
+fact, his own&mdash;and only unconscious of the extent of his own
+memories and experiences owing to their vastness and already
+infinite repetitions" (p.&nbsp;50). It is very suggestive in this
+connection, he continues&mdash;"I. That we are <i>most conscious
+of, and have most control over</i>, 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 <span class="pagenum"><a name="pg339" id=
+"pg339">339</a></span>to ourselves and any ancestor who had not
+become entirely human.</p>
+
+<p>"II. That we are <i>less conscious of, and have less control
+over</i>, 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.</p>
+
+<p>"III. That we are <i>most unconscious of, and have least control
+over</i>, 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).</p>
+
+<p>The hypothesis then, that heredity and development are due to
+unconscious memory, finds much to support it&mdash;"the
+self-development of each new life in succeeding
+generations&mdash;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&mdash;all point in the
+direction of habit and memory, as the only causes which could
+produce them" (p.&nbsp;125). The hypothesis explains, for instance, the
+fact of recapitulation:&mdash;"Why should the embryo of any animal
+go through so many stages&mdash;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 <span class="pagenum"><a name="pg340" id=
+"pg340">340</a></span>old, and as imbued with an intense though
+unconscious memory of all that it has done sufficiently often to
+have made a permanent impression; if this be so, we can answer the
+above questions perfectly well. The creature goes through so many
+intermediate stages between its earliest state as life at all, and
+its latest development, for the simplest of all reasons, namely,
+because this is the road by which it has always hitherto travelled
+to its present differentiation; this is the road it knows, and into
+every turn and up or down of which it has been guided by the force
+of circumstances and the balance of considerations" (pp.
+125-6).</p>
+
+<p>The hypothesis explains also the way in which the orderly
+succession of stages in embryogeny is brought about, for we can
+readily understand that the embryo will not remember any stage
+until it has passed through the stage immediately preceding it.
+"Each step of normal development will lead the impregnated ovum up
+to, and remind it of, its next ordinary course of action, in the
+same way as we, when we recite a well-known passage, are led up to
+each successive sentence by the sentence which has immediately
+preceded it.... Though the ovum immediately after impregnation is
+instinct with all the memories of both parents, not one of these
+memories can normally become active till both the ovum itself and
+its surroundings are sufficiently like what they respectively were,
+when the occurrence now to be remembered last took place. The
+memory will then immediately return, and the creature will do as it
+did on the last occasion that it was in like case as now. This
+ensures that similarity of order shall be preserved in all the
+stages of development in successive generations" (pp.&nbsp;297-8).</p>
+
+<p>Abnormal conditions of development will cause the embryo to
+pause and hesitate, as if at a loss what to do, having no ancestral
+experience to guide it. Abnormalities of development represent the
+embryo's attempt to make the best of an unexpected situation. Or,
+as Butler puts it, "When ... events are happening to it which, if
+it has the kind of memory we are attributing to it, would baffle
+that memory, or which have rarely or never been included in the
+category of its recollections, <i>it acts precisely as a creature
+acts</i> <span class="pagenum"><a name="pg341" id=
+"pg341">341</a></span><i>when its recollection is disturbed, or
+when it is required to do something which it has never done
+before</i>" (p.&nbsp;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.&nbsp;132).</p>
+
+<p>Butler's teleological conception of organic evolution was of
+course completely antagonistic to the naturalistic conceptions
+current in his time. In one of his later books he repeats Paley's
+arguments in favour of design, and to the question, "Where, then,
+is your designer of beasts and birds, of fishes, and of plants?" he
+replies: "Our answer is simple enough; it is that we can and do
+point to a living tangible person with flesh, blood, eyes, nose,
+ears, organs, senses, dimensions, who did of his own cunning, after
+infinite proof of every kind of hazard and experiment, scheme out
+and fashion each organ of the human body. This is the person whom
+we claim as the designer and artificer of that body, and he is the
+one of all others the best fitted for the task by his antecedents,
+and his practical knowledge of the requirements of the
+case&mdash;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" (<i>Evolution, Old and
+New</i>, p.&nbsp;30, 1879).</p>
+
+<p>Butler's theory of life and habit remained only a sketch, and he
+was perhaps not fully aware of its philosophical implications.
+Since Butler's time, a new complexion has been put upon biological
+philosophy by the profound speculations of Bergson.</p>
+
+<p>But it is not impossible that the future development of
+biological thought will follow some such lines as those which he
+tentatively laid down.</p>
+
+<p>Butler was not the first to suggest that there is a close
+connection between heredity and memory&mdash;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."<a name="FNanchor_509" id="FNanchor_509" /><a href=
+"#Footnote_509" class="fnanchor">[509]</a> Butler was not <span
+class="pagenum"><a name="pg342" id="pg342">342</a></span>aware of
+Hering's work when he published his <i>Life and Habit</i>, but in
+<i>Unconscious Memory</i> (1880) he gave full credit to Hering as
+the first discoverer, and supplied an admirable translation of
+Hering's lecture. As far as the assimilation of heredity to memory
+is concerned Hering and Butler have much in common, but Hering did
+not share Butler's Lamarckian and vitalistic views, preferring to
+hold fast, for the practical purposes of physiology at all events,
+to the general accepted theory of the parallelism between psychical
+and physical processes. He was inclined to regard memory in the
+ordinary sense as a function of the brain, and memory in general as
+a function of all organised matter. Speaking of the psychical life,
+he says, "Thus the cause which produces the unity of all single
+phenomena of consciousness must be looked for in unconscious life.
+As we know nothing of this except what we learn from our
+investigations of matter, and since in a purely empirical
+consideration, matter and the unconscious must be regarded as
+identical, the physiologist may justly define memory in a wider
+sense to be a faculty of the brain, the results of which to a great
+extent belong to both consciousness and unconsciousness."<a name=
+"FNanchor_510" id="FNanchor_510" /><a href="#Footnote_510"
+class="fnanchor">[510]</a> Hering's views were supported by
+Haeckel.<a name="FNanchor_511" id="FNanchor_511" /><a href=
+"#Footnote_511" class="fnanchor">[511]</a></p>
+
+<p>In 1893 an American, H. F. Orr,<a name="FNanchor_512" id=
+"FNanchor_512" /><a href="#Footnote_512" class=
+"fnanchor">[512]</a> tried to work out a theory of development and
+heredity based upon the fundamental idea "that the property which
+is the basis of bodily development in organisms is the same
+property which we recognise as the basis of psychic activity and
+psychic development." He tried also to explain the recapitulation
+of phylogeny by ontogeny as due to habit.</p>
+
+<p>The neo-Lamarckian school of American pal&aelig;ontologists were
+also in sympathy with the memory idea, and this was expressed most
+clearly perhaps by Cope.<a name="FNanchor_513" id=
+"FNanchor_513" /><a href="#Footnote_513" class=
+"fnanchor">[513]</a></p>
+
+<p>In 1904 appeared the work on this subject which has attracted
+the most attention&mdash;R. Semon's <i>Die Mneme</i>.<a name=
+"FNanchor_514" id="FNanchor_514" /><a href="#Footnote_514"
+class="fnanchor">[514]</a> <span class="pagenum"><a name="pg343" id=
+"pg343">343</a></span>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.</p>
+
+<p>Applied to the explanation of heredity and development, Semon's
+theory comes to very much the same as Weismann's, with engramms
+substituted for determinants, but it has the great advantage of
+allowing for the transmission of acquired characters. The
+application of the concept of stimulus is valuable and suggestive,
+but it seems to us that the memory theory of heredity can be
+properly utilised only by adopting a frankly Lamarckian and
+vitalistic standpoint, and this standpoint Semon expressly combats.
+As Ward<a name="FNanchor_515" id="FNanchor_515" /><a href=
+"#Footnote_515" class="fnanchor">[515]</a> points out in his
+illuminating lecture on heredity and memory&mdash;"Records or
+memoranda alone are not memory, for they presuppose it. <i>They</i>
+may consist of physical traces, but memory, even when called
+'unconscious,' suggests mind; for, as we have seen, the automatic
+character implied by this term 'unconscious' presupposes foregone
+experience.... The mnemic theory then, if it is to be worth
+anything, seems to me clearly to require not merely physical
+records or 'engrams,' but living experience or tradition. The
+mnemic theory will work for those who can accept a monadistic or
+pampsychist interpretation of the beings that make up the world,
+who believe with Spinoza and Leibniz that 'all individual things
+are animated albeit in divers degree'" (pp.&nbsp;55-6).</p>
+
+<p>Perhaps the best and most ingenious treatment of memory and
+heredity from a physical standpoint is that offered by E. Rignano
+in his book, <i>Sur la transmissibilit&eacute; des
+caract&egrave;res acquis</i>.<a name="FNanchor_516" id=
+"FNanchor_516" /><a href="#Footnote_516" class=
+"fnanchor">[516]</a> Rignano seeks to construct a physico-chemical
+"model" which will explain both heredity and memory.</p>
+
+<p>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 <span class=
+"pagenum"><a name="pg344" id="pg344">344</a></span>hypothesis set
+up is that every functional stimulus is transformed into specific
+vital energy, and deposits in the nucleus of the cell a specific
+substance which is capable of discharging, in an inverse direction,
+the nervous current which has formed it, as soon as the dynamical
+equilibrium of the organism is restored to the state in which it
+was when the original stimulus acted upon it. These specific
+nuclear substances, different for each cell, are accumulated also
+in the nuclei of the germinal substance, constituting what Rignano
+calls the central zone of development. That is to say, each
+functional adaptation changes slightly the dynamical equilibrium of
+the organism, and this change in the system of distribution of the
+nervous currents leads to the deposit in the central zone of
+development of a new specific substance. In the development of the
+next individual this new specific element enters into activity, and
+reproduces the nervous current which has formed it, as soon as the
+organism reaches the same conditions of dynamical equilibrium as
+those obtaining when the stimulus acted on the parent.</p>
+
+<p>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.</p>
+
+<p>The theory represents an advance upon the more static
+conceptions of Semon. It owes much to Roux's influence.</p>
+
+<p>In this country, the mnemic theories have been championed
+particularly by M. Hartog<a name="FNanchor_517" id=
+"FNanchor_517" /><a href="#Footnote_517" class=
+"fnanchor">[517]</a> and Sir Francis Darwin.<a name="FNanchor_518"
+id="FNanchor_518" /><a href="#Footnote_518" class=
+"fnanchor">[518]</a></p>
+
+<div class="footnote">
+<p><a name="Footnote_508" id="Footnote_508" /><a href=
+"#FNanchor_508"><span class="label">[508]</span></a> The quotations
+are taken from the 1910 reprint, London, Fifield.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_509" id="Footnote_509" /><a href=
+"#FNanchor_509"><span class="label">[509]</span></a> <i>Ueber das
+Ged&auml;chtnis als eine allgemeine Funktion der organisierten
+Materie</i>, Wien, 1870.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_510" id="Footnote_510" /><a href=
+"#FNanchor_510"><span class="label">[510]</span></a> Eng. trans, in
+E. Hering, <i>Memory</i>, p.&nbsp;9, Chicago and London, 1913.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_511" id="Footnote_511" /><a href=
+"#FNanchor_511"><span class="label">[511]</span></a> <i>Die
+Perigenesis der Plastidule</i>, Jena, 1875.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_512" id="Footnote_512" /><a href=
+"#FNanchor_512"><span class="label">[512]</span></a> <i>A Theory of
+Development and Heredity</i>, New York, 1893.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_513" id="Footnote_513" /><a href=
+"#FNanchor_513"><span class="label">[513]</span></a> <i>The Primary
+Factors of Organic Evolution</i>, Chicago, 1896.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_514" id="Footnote_514" /><a href=
+"#FNanchor_514"><span class="label">[514]</span></a> <i>Die Mneme
+als erhaltendes Prinzip im Wechsel des organischen Geschehens</i>,
+Leipzig, 1904; 2nd ed., 1908.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_515" id="Footnote_515" /><a href=
+"#FNanchor_515"><span class="label">[515]</span></a> <i>Heredity
+and Memory</i>, Cambridge, 1913.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_516" id="Footnote_516" /><a href=
+"#FNanchor_516"><span class="label">[516]</span></a> Paris, 1906.
+Also in Italian and German. Eng. trans. by B. C. H. Harvey, Chicago,
+1911.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_517" id="Footnote_517" /><a href=
+"#FNanchor_517"><span class="label">[517]</span></a> See
+<i>Problems of Life and Reproduction</i>, London, 1913.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_518" id="Footnote_518" /><a href=
+"#FNanchor_518"><span class="label">[518]</span></a>
+<i>Presidential Address to the British Association</i>, 1908.</p>
+</div>
+
+<hr style="width: 65%;" />
+<p><span class="pagenum"><a name="pg345" id=
+"pg345">345</a></span></p>
+
+<h3>CHAPTER XX</h3>
+
+<h4>THE CLASSICAL TRADITION IN MODERN MORPHOLOGY</h4>
+
+<p><span class="smcap">To</span> 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.</p>
+
+<p>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.</p>
+
+<p>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&mdash;the materialistic vitalism of
+Driesch, the profound intuitionalism of Bergson, the psychological
+biology of Delpino, Franc&eacute;, 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. M&uuml;ller. It may well be that
+the intransigent materialism of the 19th century is merely an
+episode, an aberration rather, in the history of biology&mdash;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.</p>
+
+<p><span class="pagenum"><a name="pg346" id=
+"pg346">346</a></span>Two movements seem significant in the
+morphology of the last decade or so of the 19th
+century&mdash;first, the experimental study of form, and second,
+the criticism of the concepts or prejudices of evolutionary
+morphology.</p>
+
+<p>The period was characterised also by the great interest taken in
+cytology, following upon the pioneer work of Hertwig, van Beneden
+and others on the behaviour of the nuclei in fertilisation and
+maturation.<a name="FNanchor_519" id="FNanchor_519" /><a href=
+"#Footnote_519" class="fnanchor">[519]</a> This line of work gained
+added importance in connection with contemporary research and
+speculation on the nature of hereditary transmission, and it has in
+quite recent years received an additional stimulus from the
+re-discovery of Mendelian inheritance. Its importance, however,
+seems to lie rather in its possible relation to the problems of
+heredity than in any meaning it may have for the problems of form.
+More significant is the revolt against the cell-theory started by
+Sedgwick<a name="FNanchor_520" id="FNanchor_520" /><a href=
+"#Footnote_520" class="fnanchor">[520]</a> and Whitman,<a name=
+"FNanchor_521" id="FNanchor_521" /><a href="#Footnote_521"
+class="fnanchor">[521]</a> on the ground that the organism is
+something more than an aggregation of discrete, self-centred
+cells.</p>
+
+<p>The experimental work on the causes of the production and
+restoration of form infused new life into morphology. It opened
+men's eyes to the fact that the developing organism is very much a
+living, active, responsive thing, quite capable of relinquishing at
+need the beaten track of normal development which its ancestors
+have followed for countless generations, in order to meet
+emergencies with an immediate and purposive reaction. It was cases
+of this kind, cases of active regulation in development and
+regeneration, that led men like G. Wolff and H. Driesch to cast off
+the bonds of dogmatic Darwinism and declare boldly for vitalism and
+teleology.</p>
+
+<p>There was the famous case of the regeneration of the lens in
+Amphibia from the edge of the iris&mdash;an entirely novel mode of
+origin, not occurring in ontogeny. The fact seems to have been
+discovered first by Colucci in 1891, and independently by G. Wolff
+in 1895.<a name="FNanchor_522" id="FNanchor_522" /><a href=
+"#Footnote_522" class="fnanchor">[522]</a> The experiment was later
+repeated and confirmed by Fischel and other workers. <span class=
+"pagenum"><a name="pg347" id="pg347">347</a></span>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.<a name="FNanchor_523" id="FNanchor_523" /><a
+href="#Footnote_523" class="fnanchor">[523]</a> And, as is well
+known, Driesch derived one of his most powerful arguments in favour
+of vitalism from the extraordinary regenerative processes shown by
+<i>Tubularia</i> and <i>Clavellina</i> 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.</p>
+
+<p>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.<a name="FNanchor_524" id="FNanchor_524" /><a
+href="#Footnote_524" class="fnanchor">[524]</a></p>
+
+<p>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.<a name=
+"FNanchor_525" id="FNanchor_525" /><a href="#Footnote_525"
+class="fnanchor">[525]</a> A. Giard in particular called attention to
+a remarkable group of facts which went to show that embryos or
+larv&aelig; of the same or closely allied species might develop in
+most dissimilar ways according to the conditions in which they
+found themselves.<a name="FNanchor_526" id="FNanchor_526" /><a
+href="#Footnote_526" class="fnanchor">[526]</a> His classical case
+of <span class="pagenum"><a name="pg348" id=
+"pg348">348</a></span>"p&oelig;cilogeny" was that of the shrimp
+<i>Pal&aelig;monetes varians</i>, the fresh-water form of which
+develops in an entirely different way from the salt-water form.</p>
+
+<p>Experimental workers indeed were inclined to rule the law out of
+account, to disregard completely the historical element in
+development, and this was perhaps the chief weakness of the
+neo-vitalist systems which took their origin in this experimental
+work.</p>
+
+<p>From the side also of descriptive morphology the biogenetic law
+underwent a critical revision. It was studied as a fact of
+embryology and without phylogenetic bias by men like Oppel, Keibel,
+Mehnert, O. Hertwig and Vialleton,<a name="FNanchor_527" id=
+"FNanchor_527" /><a href="#Footnote_527" class=
+"fnanchor">[527]</a> and they arrived at a critical estimate of it
+very similar to that of von Baer.</p>
+
+<p>Theoretical objections to the biogenetic law had been raised
+from time to time by many embryologists, but the positive testing
+of it by the comparison of embryos in respect of the degree of
+development of their different organs starts with Oppel's work of
+1891.<a name="FNanchor_528" id="FNanchor_528" /><a href=
+"#Footnote_528" class="fnanchor">[528]</a> 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.</p>
+
+<p>Oppel's main conclusions were as follows:&mdash;"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 <span class="pagenum"><a name="pg349" id=
+"pg349">349</a></span>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."<a name="FNanchor_529" id=
+"FNanchor_529" /><a href="#Footnote_529" class=
+"fnanchor">[529]</a></p>
+
+<p>Oppel's methods were employed by Keibel<a name="FNanchor_530"
+id="FNanchor_530" /><a href="#Footnote_530" class=
+"fnanchor">[530]</a> in his investigations on the development of the
+pig, which formed the model for the well-known series of
+<i>Normentafeln</i> 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 <span class=
+"pagenum"><a name="pg350" id="pg350">350</a></span>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.&nbsp;754,
+1897).</p>
+
+<p>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,<a name="FNanchor_531" id=
+"FNanchor_531" /><a href="#Footnote_531" class=
+"fnanchor">[531]</a> Mehnert,<a name="FNanchor_532" id=
+"FNanchor_532" /><a href="#Footnote_532" class=
+"fnanchor">[532]</a> and Fischel.<a name="FNanchor_533" id=
+"FNanchor_533" /><a href="#Footnote_533" class=
+"fnanchor">[533]</a> Fischel found that variability was greatest
+among the younger embryos, and became progressively less in later
+stages. Like von Baer (<i>supra</i>, p.&nbsp;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.</p>
+
+<p>Important theoretical views were developed by Mehnert<a name=
+"FNanchor_534" id="FNanchor_534" /><a href="#Footnote_534"
+class="fnanchor">[534]</a> 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.</p>
+
+<p>In his paper of 1897 Mehnert details the results of an extensive
+examination of the development of the extremities throughout the
+Amniote series. He finds that in all cases a pentadactylate
+rudiment is formed, even in those forms in <span class="pagenum"><a
+name="pg351" id="pg351">351</a></span>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, <i>e.g.</i> the tortoise
+and man, all the digits develop and differentiate at about the same
+rate, in forms which have in the adult reduced digits, <i>e.g.</i>
+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.<a
+name="FNanchor_535" id="FNanchor_535" /><a href=
+"#Footnote_535" class="fnanchor">[535]</a> The acceleration or
+retardation affects not only the mass-growth of the organs, but
+also their histological differentiation.</p>
+
+<p>Now between progression and functioning and between regression
+and functional atrophy there is obviously a close connection. Loss
+of function is well known to be one of the chief causes of the
+degeneration of organs in the individual life, and on the other
+hand, as Roux has pointed out, all post-embryonic development is
+ruled and guided by functioning. It is thus in the long run
+functioning that brings about phylogenetic progression, absence of
+functional activity that causes phylogenetic regression. This comes
+about through the transmission of acquired functional characters, a
+transmission which Mehnert conceives to be extraordinarily accurate
+and complete.</p>
+
+<p>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 <span class="pagenum"><a name="pg352"
+id="pg352">352</a></span>law than either Keibel or Oppel, though he
+agrees that it cannot be used for the construction of ancestral
+trees. But he ascribes to it as a fact of development considerable
+importance. The following passage gives a good summary of his view
+as to the scope and validity of the law. "The biogenetic law has
+not been shaken by the attacks of its opponents. The assertion is
+still true that individual organogenesis is exclusively dependent
+on phylogeny. But we must not expect to find that all the stages in
+the development of the separate organs, which coexisted in any
+member of the phylogenetic series, appear <i>at the same time</i>
+in the individual ontogeny of the descendants, because each organ
+possesses its own specific rate of development. In this way it
+comes about naturally that organs which become differentiated
+rapidly, as, for example, the medullary tube, as a rule dominate
+earlier periods of ontogeny than do the organs of locomotion. For
+the same reason the cerebral hemispheres of man are almost as large
+in youth as in maturity. The picture which an embryo gives is not a
+repetition in detail of one and the same phylogenetic stage; it
+consists rather of an assemblage of organs, some of which are at a
+phyletically early stage of development, while others are at a
+phyletically older stage."<a name="FNanchor_536" id=
+"FNanchor_536" /><a href="#Footnote_536" class=
+"fnanchor">[536]</a></p>
+
+<p>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.<a name="FNanchor_537" id="FNanchor_537" /><a href=
+"#Footnote_537" class="fnanchor">[537]</a></p>
+
+<p>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&mdash;in direct contradiction of the law of von Baer.
+Thus L. Agassiz<a name="FNanchor_538" id="FNanchor_538" /><a
+href="#Footnote_538" class="fnanchor">[538]</a> had remarked <span
+class="pagenum"><a name="pg353" id="pg353">353</a></span>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.&nbsp;269).</p>
+
+<p>Wilhelm His,<a name="FNanchor_539" id="FNanchor_539" /><a
+href="#Footnote_539" class="fnanchor">[539]</a> in the course of an
+acute and damaging criticism of the biogenetic law as enunciated by
+Haeckel, showed clearly that by careful examination the very
+earliest embryos of a whole series of Vertebrates could be
+distinguished with certainty from one another. "An identity in
+external form of different animal embryos, despite the common
+affirmation to the contrary, does not exist. Even at early stages
+in their development embryos possess the characters of their class
+and order, nay, we can hardly doubt, of their species and sex, and
+even their individual characteristics" (201).</p>
+
+<p>This specificity of embryos was affirmed with even greater
+confidence by Sedgwick in a paper critical of von Baer's law.<a
+name="FNanchor_540" id="FNanchor_540" /><a href=
+"#Footnote_540" class="fnanchor">[540]</a> He wrote:&mdash;"If v.
+Baer's law has any meaning at all, surely it must imply that
+animals so closely allied as the fowl and duck would be
+indistinguishable in the early stages of development; and that in
+two species so closely similar that I was long in doubt whether
+they were distinct species, viz., <i>Peripatus capensis</i> and
+<i>Balfouri</i>, it would be useless to look for embryonic
+differences; yet I can distinguish a fowl and a duck embryo on the
+second day by the inspection of a single transverse section through
+the trunk, and it was the embryonic differences between the
+Peripatuses which led me to establish without hesitation the two
+separate species.... I need only say ... that a species is distinct
+and distinguishable from its allies from the very earliest stages
+all through the development, although these embryonic differences
+do not necessarily implicate the same organs as do the adult
+differences" (p.&nbsp;39).</p>
+
+<p><span class="pagenum"><a name="pg354" id=
+"pg354">354</a></span>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 <i>is</i> 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.</p>
+
+<p>Hertwig puts this vividly when he says that "the hen's egg is no
+more the equivalent of the first link in the phylogenetic chain
+than is the hen itself" (p.&nbsp;160, 1906, b).</p>
+
+<p>If ontogeny is not a recapitulation of phylogeny, how is it that
+the early embryonic stages are so alike, even in animals of widely
+different organisation? Hertwig's answer to this is very
+interesting. He takes the view that many of the processes
+characterising early embryonic development are the means
+necessarily adopted for attaining certain ends. Such are the
+processes of segmentation, the formation of a blastula, of
+cell-layers, of medullary folds where the nervous system is a
+closed tube, the formation of the notochord as a necessary
+condition of the development of the vertebral column, and so on.
+"Looked at from this standpoint it cannot surprise us that in all
+animal phyla the earliest embryonic processes take place in similar
+fashion, so that we observe the occurrence both in Vertebrates and
+Invertebrates of a segmentation-process, a morula-stage, a blastula
+and a gastrula. If now these developmental processes do not depend
+on chance, but, on the contrary, are rooted in the nature of the
+animal cell itself, we have no reason for inferring from the
+recurrence of a similar segmentation-process, morula, blastula, and
+gastrula in all classes of the animal kingdom the common descent of
+all animals from one <span class="pagenum"><a name="pg355" id=
+"pg355">355</a></span>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.&nbsp;178, 1906, b).</p>
+
+<p>"The principal reason why certain stages recur in ontogeny with
+such constancy and always in essentially the same manner is that
+they provide under all circumstances the necessary pre-conditions
+through which alone the later and higher stages of ontogeny can be
+realised. The unicellular organism can by its very nature transform
+itself into a multicellular organism only by the method of
+cell-division. Hence, in all Metazoa, ontogeny must start with a
+segmentation-process, and a similar statement could be made with
+regard to all the later stages" (p.&nbsp;57, 1906, a).</p>
+
+<p>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 pal&aelig;ontology), 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.&nbsp;151, 1906, b).</p>
+
+<p>The concept of homology has thus a value quite independent of
+any evolutionary interpretation which may be superadded to it.
+"Homology is a mental concept obtained by comparison, which under
+all circumstances retains its validity, whether the homology finds
+its explanation in common descent or in the common laws that rule
+organic development" (p.&nbsp;151, 1906, b). As A. Braun long ago
+pointed out, "It is not descent which decides in matters <span
+class="pagenum"><a name="pg356" id="pg356">356</a></span>of
+morphology, but, on the contrary, morphology which has to decide as
+to the possibility of descent."<a name="FNanchor_541" id=
+"FNanchor_541" /><a href="#Footnote_541" class=
+"fnanchor">[541]</a></p>
+
+<p>Hertwig, in a word, reverts to the pre-evolutionary conception
+of homology. "We see in homology," he writes, "only the expression
+of regularities (<i>Gesetzm&auml;ssigkeiten</i>) in the
+organisation of the animals showing it, and we regard the question,
+how far this homology can be explained by common descent and how
+far by other principles, as for the present an open one, requiring
+for its solution investigations specially directed towards its
+elucidation" (p.&nbsp;179, 1906, b).</p>
+
+<p>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&mdash;and that is just what we do when we speak of
+Proselachia, Proamphibia and Proreptilia" (p.&nbsp;155, 1906, b).</p>
+
+<p>He rejects on the same general grounds the evolutionary dogma of
+monophyletic or almost monophyletic descent, and admits with
+K&ouml;lliker, von Baer, Wigand, Naegeli and others that evolution
+may quite well have started many times and from many different
+primordial cells.</p>
+
+<p>There is indeed a great similarity between the views developed
+by O. Hertwig and those held by the older critics of
+Darwinism&mdash;von Baer, K&ouml;lliker, Wigand, E. von Hartmann
+and others. It is true the philosophical standpoint is on the whole
+different, for while many of that older generation were vitalists
+Hertwig belongs to the mechanistic school.</p>
+
+<p>But both Hertwig and the older school agree in pointing out the
+<i>petitio principii</i> involved in the assumption that the <span
+class="pagenum"><a name="pg357" id="pg357">357</a></span>archetype
+represents the ancestral form; both reject the simplicist
+conception of a monophyletic evolution (which may be likened to the
+"one animal" idea of the transcendentalists); both admit the
+possibility that evolution has taken place along many separate and
+parallel lines, and explain the correspondences shown by these
+separate lines by the similarity of the intrinsic laws of
+evolution; finally, both emphasise the fact that we know nothing of
+the actual course of evolution save the few indications that are
+furnished by pal&aelig;ontology, and both insist upon the unique
+importance of the pal&aelig;ontological evidence.<a name=
+"FNanchor_542" id="FNanchor_542" /><a href="#Footnote_542"
+class="fnanchor">[542]</a></p>
+
+<p>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 pal&aelig;ontologists,<a name=
+"FNanchor_543" id="FNanchor_543" /><a href="#Footnote_543"
+class="fnanchor">[543]</a> 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 pal&aelig;ontology, for
+the evidence acquired through the study of fossil remains was
+distinctly unfavourable to the pretty schemes they evolved.</p>
+
+<p>As Neumayr, Zittel, Dep&eacute;ret, Steinmann and others have
+pointed out, the pal&aelig;ontological 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&mdash;the famous <i>Arch&aelig;opteryx</i>, for
+example&mdash;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 <span class="pagenum"><a name="pg358" id=
+"pg358">358</a></span>been found as fossils. The great
+classificatory groups are almost as distinct in early fossiliferous
+strata as they are at the present day. As Dep&eacute;ret says in
+his admirable book,<a name="FNanchor_544" id=
+"FNanchor_544" /><a href="#Footnote_544" class=
+"fnanchor">[544]</a> 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
+Arch&aelig;opteryx, the discovery of which made so much stir and
+appeared to establish a genetic relation between classes so
+distinct as Birds and Reptiles, fills up the gap only imperfectly,
+and does not indicate the point of bifurcation of these two
+classes. Intermediate links are lacking between Amphibia and
+Reptiles. Mammals, too, occupy an isolated position, and no
+zoologist can deny that they are clearly demarcated from other
+Vertebrates; indeed, no fossil mammal is certainly known which
+comes nearer to the lower Vertebrates than does Ornithorhynchus at
+the present day" (p.&nbsp;115).</p>
+
+<p>To take a parallel from the Invertebrata, B. B. Woodward,<a name=
+"FNanchor_545" id="FNanchor_545" /><a href="#Footnote_545"
+class="fnanchor">[545]</a> after discussing the phylogeny of the
+Mollusca as worked out by the morphologists and comparing it with
+the probable actual course of the evolution of the group, as
+evidenced by fossil shells, sums up as follows:&mdash;"The
+lacun&aelig; 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
+pal&aelig;ontological history of any hint of passage forms between
+the classes themselves, or between the Mollusca and their nearest
+allies. Nor is this hiatus confined to the Molluscan phylum; it is
+the same for all branches of the animal kingdom. There is
+circumstantial evidence that transitional forms must have existed,
+but of actual proof none whatever. All the classes of Mollusca
+appear fully fledged, as it were. No form has as yet been
+discovered of which it could be said that it in any way approached
+the <span class="pagenum"><a name="pg359" id=
+"pg359">359</a></span>hypothecated prorhipidoglossate mollusc,
+still less one linking all the classes" (p.&nbsp;79).</p>
+
+<p>Pointing in the same direction as the absence of transitional
+forms is the undeniable fact that all the great groups of animals
+appear with all their typical characters at a very early geological
+epoch. Thus, in the Silurian age a very rich fauna has already
+developed, and representatives are found of all the main
+Invertebrate groups&mdash;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&mdash;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.</p>
+
+<p>The contributions of pal&aelig;ontology 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
+pal&aelig;ontology. 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.<a name="FNanchor_546" id="FNanchor_546" /><a
+href="#Footnote_546" class="fnanchor">[546]</a> This is the
+tendency universally shown by embryos to reproduce the characters
+of their ancestors at earlier and earlier stages in their
+development.</p>
+
+<p>The most valuable contribution made by pal&aelig;ontologists to
+morphology and to the theory of evolution arose out of the careful
+and methodical study of the actual succession of fossil forms as
+exemplified in limited but richly represented groups. Classical
+examples were the researches of Hilgendorf<a name="FNanchor_547"
+id="FNanchor_547" /><a href="#Footnote_547" class=
+"fnanchor">[547]</a> on the evolution of <i>Planorbis multiformis</i>
+in the lacustrine deposits of Steinheim, those of Waagen<a name=
+"FNanchor_548" id="FNanchor_548" /><a href="#Footnote_548"
+class="fnanchor">[548]</a> on the <span class="pagenum"><a name=
+"pg360" id="pg360">360</a></span>phylogeny of <i>Ammonites
+subradiatus</i>, and the work of Neumayr and Paul<a name=
+"FNanchor_549" id="FNanchor_549" /><a href="#Footnote_549"
+class="fnanchor">[549]</a> on <i>Paludina</i> (<i>Vivipara</i>).</p>
+
+<p>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."</p>
+
+<p>To take an example from among the Vertebrates, Dep&eacute;ret
+has shown (<i>loc. cit.</i>, pp.&nbsp;184-9), that the European
+Proboscidea, belonging to the three different types of the
+Elephants, Mastodons and Dinotheria, have evolved since the
+Oligocene epoch along five distinct but continuous lines. The
+Dinotherian stock is represented at the beginning of the Miocene by
+the relatively small form <i>D. cuvieri</i>; this changes
+progressively throughout Miocene times into <i>D. laevius</i>,
+<i>D. giganteum</i>, and <i>D. gigantissimum</i>. Among the
+Mastodons two quite distinct phyletic series can be distinguished,
+the first commencing with <i>Pal&aelig;omastodon beadnelli</i> of
+the Oligocene, and evolving between the Miocene and Pliocene into
+<i>Mastodon arvernensis</i>, after traversing the forms <i>M.
+angustidens</i> and <i>M. longirostris</i>, the second starting
+with the <i>M. turicensis</i> of the Lower Miocene and evolving
+through <i>M. borsoni</i> into the <i>M. americanus</i> of the
+Quaternary. The phyletic series of the true elephants in Europe are
+relatively short, and go back only to the Quaternary, <i>Elephas
+antiquus</i> giving origin to the Indian elephant, <i>E.
+priscus</i> to the African.</p>
+
+<p>The careful study of phyletic series brought to light the
+significant fact that these lines of filiation tend to run for long
+stretches of time parallel to, and distinct from one another,
+without connecting forms. This is clearly exemplified in the case
+of the Proboscidea, and many other examples could be quoted. Almost
+all rich genera are polyphyletic in the sense that their component
+species evolve along separate and parallel lines of descent.<a
+name="FNanchor_550" id="FNanchor_550" /><a href=
+"#Footnote_550" class="fnanchor">[550]</a> "Such great genera as
+the genus <i>Hoplites</i> among the Ammonites, the genus
+<i>Cerithium</i> among the Gastropoda, the genus <i>Pecten</i> or
+the genus <span class="pagenum"><a name="pg361" id=
+"pg361">361</a></span><i>Trigonia</i> among the Lamellibranchs,
+each comprise perhaps more than twenty independent phyletic series"
+(Dep&eacute;ret, p.&nbsp;200).</p>
+
+<p>Variation along the phyletic lines is gradual<a name=
+"FNanchor_551" id="FNanchor_551" /><a href="#Footnote_551"
+class="fnanchor">[551]</a> 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.</p>
+
+<p>The general picture which pal&aelig;ontology 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, <i>e.g.</i> elephants, whales, and
+ostriches, are approaching this final phase of their existence"
+(Dep&eacute;ret, p.&nbsp;249).</p>
+
+<p>It is one of the paradoxes of biological history that the
+pal&aelig;ontologists 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 pal&aelig;ontologists in
+particular&mdash;Cope, Hyatt, Ryder, Dall, Packard,
+Osborn&mdash;have worked out a complete neo-Lamarckian theory based
+upon the fossil record.</p>
+
+<p>The functional point of view was well to the fore in the works
+of those great pal&aelig;ontologists, L. R&uuml;timeyer (1825-1895)
+and V. O. Kowalevsky (1842-83), who seem to have carried on the
+splendid tradition of Cuvier. Speaking of <span class="pagenum"><a
+name="pg362" id="pg362">362</a></span>Kowalevsky's classical
+memoir, <i>Versuch einer nat&uuml;rlichen Classification der
+fossilen Hufthiere</i>, Osborn<a name="FNanchor_552" id=
+"FNanchor_552" /><a href="#Footnote_552" class=
+"fnanchor">[552]</a> writes:&mdash;"This work is a model union of the
+detailed study of form and function with theory and the working
+hypothesis. It regards the fossil not as a petrified skeleton, but
+as having belonged to a moving and feeding animal; every joint and
+facet has a meaning, each cusp a certain significance. Rising to
+the philosophy of the matter, it brings the mechanical perfection
+and adaptiveness of different types into relation with environment,
+with changes of herbage, with the introduction of grass. In this
+survey of competition it speculates upon the causes of the rise,
+spread, and extinction of each animal group. In other words, the
+fossil quadrupeds are treated <i>biologically</i>&mdash;so far as
+is possible in the obscurity of the past" (p.&nbsp;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 pal&aelig;ontologists, including Osborn
+himself.</p>
+
+<p>Osborn's law of "adaptive radiation," which links on to Darwin's
+law of divergence,<a name="FNanchor_553" id="FNanchor_553" /><a
+href="#Footnote_553" class="fnanchor">[553]</a> constitutes a
+brilliant vindication of the functional point of view. "According
+to this law each isolated region, if large and sufficiently varied
+in its topography, soil, climate, and vegetation, will give rise to
+a diversified mammalian fauna. From primitive central types
+branches will spring off in all directions, with teeth and
+prehensile organs modified to take advantage of every possible
+opportunity of securing food, and in adaptation of the body, limbs
+and feet to habitats of every kind, as shown in the diagram [on p.
+<a href="#pg363">363</a>]. The larger the region and the more diverse the conditions,
+the greater the variety of mammals which will result.</p>
+
+<p>"The most primitive mammals were probably small insectivorous or
+omnivorous forms, therefore with simple, short-crowned teeth, of
+slow-moving, ambulatory, terrestrial, or arboreal habit, and with
+short feet provided with claws. In seeking food and avoiding
+enemies in different habitats <span class="pagenum"><a name="pg363"
+id="pg363">363</a></span>the limbs and feet radiate in four diverse
+directions; they either become <i>fossorial</i> or adapted to
+digging habits, <i>natatorial</i> or adapted to <i>amphibious</i>
+and finally to <i>aquatic</i> habits, <i>cursorial</i> or adapted
+to swift-moving, terrestrial progression, <i>arboreal</i> or
+adapted to tree life. Tree life leads, as its final stage, into</p>
+
+<div class="figcenter">
+<img
+src="images/img363a.jpg"
+alt="LIMBS AND FEET." /></div>
+
+
+<p>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 <span class="pagenum"><a name="pg364"
+id="pg364">364</a></span>extremes of carnivorous adaptation, as in
+the case of the cats, of omnivorous adaptation, as in the case of
+the bears, of herbivorous adaptation, as in the case of the horses,
+or myrmecophagous adaptation, as in the case of the anteaters, are
+all secondary" (<i>loc. cit.</i>, pp.&nbsp;23-4).</p>
+
+<p>We have now reached the end of our historical survey of the
+problems of form. What the future course of morphology will be no
+one can say. But one may hazard the opinion that the present
+century will see a return to a simpler and more humble attitude
+towards the great and unsolved problems of animal form. Dogmatic
+materialism and dogmatic theories of evolution have in the past
+tended to blind us to the complexity and mysteriousness of vital
+phenomena. We need to look at living things with new eyes and a
+truer sympathy. We shall then see them as active, living,
+passionate beings like ourselves, and we shall seek in our
+morphology to interpret as far as may be their form in terms of
+their activity.</p>
+
+<p>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.</p>
+
+<div class="footnote">
+<p><a name="Footnote_519" id="Footnote_519" /><a href=
+"#FNanchor_519"><span class="label">[519]</span></a> See E. B.
+Wilson's masterly book, <i>The Cell in Development and
+Inheritance</i>, New York and London, 1900.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_520" id="Footnote_520" /><a href=
+"#FNanchor_520"><span class="label">[520]</span></a>
+<i>Q.J.M.S.</i>, xxvi. 1886.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_521" id="Footnote_521" /><a href=
+"#FNanchor_521"><span class="label">[521]</span></a> <i>Wood's Holl
+Biological Lectures</i> for 1893.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_522" id="Footnote_522" /><a href=
+"#FNanchor_522"><span class="label">[522]</span></a> <i>Arch. f.
+Ent.-Mech.</i>, i., pp.&nbsp;380-90, 1895.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_523" id="Footnote_523" /><a href=
+"#FNanchor_523"><span class="label">[523]</span></a>
+<i>Beitr&auml;ge zur Kritik der Darwinschen Lehre</i>, Leipzig,
+1898.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_524" id="Footnote_524" /><a href=
+"#FNanchor_524"><span class="label">[524]</span></a> See E. B.
+Wilson, "The Embryological Criterion of Homology," <i>Wood's Holl
+Biological Lectures</i>, Boston, pp.&nbsp;101-24, 1895; Braem, <i>Biol.
+Centrblt.</i>, xv., 1895; T. H. Morgan, <i>Arch. f. Ent.-Mech.</i>,
+xviii.; J. W. Jenkinson, <i>Mem. Manchester Lit. Phil. Soc.</i>,
+1906, and <i>Vertebrate Embryology</i>, Oxford, 1913; A. Sedgwick,
+article "Embryology" in <i>Ency. Brit.</i>, p.&nbsp;318, vol. xi., 11th
+Ed. (1910).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_525" id="Footnote_525" /><a href=
+"#FNanchor_525"><span class="label">[525]</span></a> For a detailed
+treatment of this important point see the remarkable volume of E.
+Schulz (Petrograd), <i>Prinzipien der rationellen vergleichenden
+Embryologie</i>, Leipzig, 1910.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_526" id="Footnote_526" /><a href=
+"#FNanchor_526"><span class="label">[526]</span></a> "La
+P&oelig;cilogonie," <i>Bull. Sci. France et Belgique</i>, xxxix.,
+pp.&nbsp;153-87, 1905.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_527" id="Footnote_527" /><a href=
+"#FNanchor_527"><span class="label">[527]</span></a> <i>Un
+probl&egrave;me de l'&eacute;volution. La loi biog&eacute;n&eacute;tique
+fondamentale</i>, Paris and Montpellier, 1908.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_528" id="Footnote_528" /><a href=
+"#FNanchor_528"><span class="label">[528]</span></a>
+<i>Vergleichung des Entwickelungsgrades der Organe zu verschiedenen
+Entwickelungszeiten bei Wirbeltieren</i>, Jena, 1891.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_529" id="Footnote_529" /><a href=
+"#FNanchor_529"><span class="label">[529]</span></a> Quoted by
+Keibel, <i>Ergebn. Anat. Entwick.</i>, vii., p.&nbsp;741.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_530" id="Footnote_530" /><a href=
+"#FNanchor_530"><span class="label">[530]</span></a> "Studien zur
+Entwickelungsgeschichte des Schweines," Schwalbe's <i>Morphol.
+Arbeiten</i>, iii., 1893, and v., 1895.</p>
+<p><i>Normentafeln zur Entwickelungsgeschichte des Schweines</i>,
+Jena, 1897.</p>
+
+<p>"Das biogenetische Grundgesetz und die Cenogenese," <i>Ergebn.
+Anat. Entw.</i>, vii., pp.&nbsp;722-92, 1897.</p>
+
+<p>"U. d. Entwickelungsgrad der Organe," <i>Handb. vergl. exper.
+Entwick. der Wirbelthiere</i>, iii., 3, pp.&nbsp;131-48, 1906.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_531" id="Footnote_531" /><a href=
+"#FNanchor_531"><span class="label">[531]</span></a> "Beitr&auml;ge
+zur Embryologie der Wiederk&auml;uer," <i>Arch. Anat. Entw.</i>,
+1889.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_532" id="Footnote_532" /><a href=
+"#FNanchor_532"><span class="label">[532]</span></a> "Die individ.
+Variation d. Wirbeltierembryo," <i>Morph. Arbeit.</i>, v.,
+1895.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_533" id="Footnote_533" /><a href=
+"#FNanchor_533"><span class="label">[533]</span></a> "U.
+Variabilit&auml;t u. Wachstum d. embryonalen K&ouml;rpers,"
+<i>Morph. Jahrb.</i>, xxiv., 1896.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_534" id="Footnote_534" /><a href=
+"#FNanchor_534"><span class="label">[534]</span></a> "Gastrulation
+u. Keimbl&auml;tterbildung der <i>Emys lutaria taurica</i>,"
+<i>Morph. Arbeit.</i>, i., 1891. "Kainogenese," <i>Morph.
+Arbeit.</i>, vii., pp.&nbsp;1-156, 1897, and also separately.
+<i>Biomechanik, erschlossen aus dem Prinzipe der Organogenese</i>,
+Jena, 1898.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_535" id="Footnote_535" /><a href=
+"#FNanchor_535"><span class="label">[535]</span></a> This law was
+foreshadowed by Reichert in 1837, when he wrote:&mdash;"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" (M&uuml;ller's <i>Archiv</i>,
+p.&nbsp;135, 1837). See also his <i>Entwick. Kopf. nackt. Amphib.</i>,
+p.&nbsp;198, 1838. So, too, Rathke notes how the elongated shape of the
+snake appears even in very early embryonic stages (<i>Entwick.
+Natter.</i>, p.&nbsp;111, 1839).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_536" id="Footnote_536" /><a href=
+"#FNanchor_536"><span class="label">[536]</span></a> Quoted by
+Keibel (p.&nbsp;790, 1897) from the <i>Biomechanik</i>.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_537" id="Footnote_537" /><a href=
+"#FNanchor_537"><span class="label">[537]</span></a> <i>Die Zelle
+und die Gewebe</i>, Jena, 1898, and the subsequent editions of this
+text-book, published under the title of <i>Allgemeine Biologie. Die
+Entwickelung der Biologie im neunzehnten Jahrhundert</i>, Jena,
+1900, 2nd ed., 1908. "Ueber die Stellung der vergl.
+Entwickelungslehre zur vergl. Anatomie, zur Systematik und
+Descendenztheorie," <i>Handb. vergl. exper. Entwickelungslehre der
+Wirbeltiere</i>, iii., 3, pp.&nbsp;149-80, Jena, 1906. (1906, b). Also
+in Pt. I. of Vol. I. (1906, a).</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_538" id="Footnote_538" /><a href=
+"#FNanchor_538"><span class="label">[538]</span></a> <i>An Essay on
+Classification</i>, London, 1859.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_539" id="Footnote_539" /><a href=
+"#FNanchor_539"><span class="label">[539]</span></a> <i>Unsere
+K&ouml;rperform</i>, Leipzig, 1874.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_540" id="Footnote_540" /><a href=
+"#FNanchor_540"><span class="label">[540]</span></a>
+<i>Q.J.M.S.</i>, xxxvi., pp.&nbsp;35-52, 1894.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_541" id="Footnote_541" /><a href=
+"#FNanchor_541"><span class="label">[541]</span></a> Quoted by
+Hertwig. See also K. Goebel, "Die Grundprobleme der heutigen
+Pflanzenmorphologie," <i>Biol. Centrbl.</i>, xxv., pp.&nbsp;65-83,
+1905.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_542" id="Footnote_542" /><a href=
+"#FNanchor_542"><span class="label">[542]</span></a> This is also
+emphasised by Fleischmann in his critical study of evolutionary
+morphology entitled <i>Die Descendenztheorie</i>, Leipzig,
+1901.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_543" id="Footnote_543" /><a href=
+"#FNanchor_543"><span class="label">[543]</span></a> 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 pal&aelig;ontologists by profession, such
+as Cope.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_544" id="Footnote_544" /><a href=
+"#FNanchor_544"><span class="label">[544]</span></a> <i>Les
+Transformations du Monde animal</i>, Paris, 1907.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_545" id="Footnote_545" /><a href=
+"#FNanchor_545"><span class="label">[545]</span></a> "Malacology
+<i>versus</i> Pal&aelig;oconchology," <i>Proc. Malacological
+Soc.</i>, viii., pp.&nbsp;66-83, 1908.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_546" id="Footnote_546" /><a href=
+"#FNanchor_546"><span class="label">[546]</span></a> Particularly
+by E. Perrier, "La Tachygen&egrave;se," <i>Ann. Sci. nat.</i>
+(<i>Zool.</i>) (8), xvi., 1903.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_547" id="Footnote_547" /><a href=
+"#FNanchor_547"><span class="label">[547]</span></a> <i>Monatsber.
+k. Akad. Wiss.</i>, Berlin, pp.&nbsp;474-504, 1866.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_548" id="Footnote_548" /><a href=
+"#FNanchor_548"><span class="label">[548]</span></a> <i>Geognost.
+u. Pal&aelig;ont. Beitr&auml;ge</i>, ii., Heft 2, pp.&nbsp;181-256,
+1869.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_549" id="Footnote_549" /><a href=
+"#FNanchor_549"><span class="label">[549]</span></a> <i>Abhand.
+k.k. Geol. Reichsanstalt</i>, vii., Wien, 1875.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_550" id="Footnote_550" /><a href=
+"#FNanchor_550"><span class="label">[550]</span></a> The case for
+polyphyletism is very strongly put by G. Steinmann in his book,
+<i>Die geologischen Grundlagen der Abstammungslehre</i>, Leipzig,
+1908.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_551" id="Footnote_551" /><a href=
+"#FNanchor_551"><span class="label">[551]</span></a> The steps in
+this chronological variation were termed by Waagen "mutations."</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_552" id="Footnote_552" /><a href=
+"#FNanchor_552"><span class="label">[552]</span></a> <i>The Age of
+Mammals in Europe, Asia, and North America</i>, New York, 1910.</p>
+</div>
+
+<div class="footnote">
+<p><a name="Footnote_553" id="Footnote_553" /><a href=
+"#FNanchor_553"><span class="label">[553]</span></a> <i>Origin of
+Species</i>, 6th ed., Chap.&nbsp;IV.</p>
+</div>
+
+<p><span class="pagenum"><a name="pg365" id=
+"pg365">365</a></span></p>
+
+<hr style="width: 65%;" />
+
+<h3>INDEX</h3>
+
+<ul class="IX">
+<li><span class="smcap">Actinozoan Theory</span> of Vertebrate
+Descent, <a href="#pg299">299</a>-300</li>
+
+<li>Adaptation as Conservative Principle&mdash;
+
+<ul>
+<li>Cuvier, <a href="#pg039">39</a>, <a href="#pg076">76</a></li>
+</ul>
+</li>
+
+<li>Adaptation, Ecological&mdash;
+
+<ul>
+<li>Von Baer, <a href="#pg123">123</a></li>
+
+<li>H. Milne-Edwards, <a href="#pg199">199</a></li>
+
+<li>Lamarck, <a href="#pg221">221</a>, <a href="#pg222">222</a>, <a
+href="#pg223">223</a>, <a href="#pg224">224</a>, <a href=
+"#pg227">227</a></li>
+
+<li>Treviranus, <a href="#pg225">225</a> f.n.</li>
+
+<li>C. Darwin, <a href="#pg231">231</a>-2, <a href=
+"#pg235">235</a>, <a href="#pg239">239</a></li>
+
+<li>Haeckel, <a href="#pg248">248</a>, <a href=
+"#pg263">263</a></li>
+
+<li>Gegenbaur, <a href="#pg263">263</a></li>
+
+<li>V.O. Kowalevsky, <a href="#pg362">362</a></li>
+
+<li>Osborn, <a href="#pg362">362</a>-4</li>
+</ul>
+</li>
+
+<li>Adaptation, Ecological, and Classification&mdash;
+
+<ul>
+<li>Bronn, <a href="#pg203">203</a></li>
+</ul>
+</li>
+
+<li>Adaptation of Parts. <i>See</i> "Correlation, Functional," and
+"Conditions of Existence"</li>
+
+<li>Adaptive Radiation (Osborn), <a href="#pg362">362</a>-4</li>
+
+<li>Agassiz, A., <a href="#pg288">288</a> f.n., <a href="#pg295">295</a>
+<ul>
+<li>On C&oelig;lom, <a href="#pg296">296</a></li>
+</ul></li>
+
+<li>Agassiz, L.&mdash;
+<ul>
+<li>Criticism of Vertebral Theory of Skull, <a href="#pg157">157</a></li>
+<li>Membrane and Cartilage Bones, <a href="#pg164">164</a></li>
+<li>Transcendentalism, <a href="#pg203">203</a></li>
+<li>Classification, <a href="#pg203">203</a> f.n.</li>
+<li>Three-fold Parallelism, <a href="#pg203">203</a>, <a href="#pg255">255</a></li>
+<li>Influence on Darwin, <a href="#pg238">238</a></li>
+<li>Specific Distinctness of Embryos, <a href="#pg353">353</a></li>
+</ul></li>
+<li>Albertus Magnus, <a href="#pg017">17</a></li>
+
+<li>Alcm&aelig;on, <a href="#pg001">1</a></li>
+
+<li>Aldrovandus, <a href="#pg018">18</a></li>
+
+<li>Allman, <a href="#pg209">209</a></li>
+
+<li>Analogy. _See also_ Homology.
+<ul>
+<li>Aristotle, <a href="#pg008">8</a>-10</li>
+<li>Owen, <a href="#pg108">108</a></li>
+<li>Haeckel, <a href="#pg251">251</a></li>
+<li>Gegenbaur, <a href="#pg266">266</a></li>
+<li>Lankester, <a href="#pg267">267</a></li>
+</ul></li>
+<li>Anaxagoras, <a href="#pg014">14</a></li>
+
+<li>Anaximander, <a href="#pg014">14</a></li>
+
+<li>Anaximenes, <a href="#pg001">1</a></li>
+
+<li>Animal and Vegetative Lives&mdash;
+<ul>
+<li>Aristotle, <a href="#pg016">16</a>, <a href="#pg032">32</a></li>
+<li>Buffon, <a href="#pg026">26</a>-7</li>
+<li>Bergson, <a href="#pg026">26</a> f.n.</li>
+<li>Cuvier, <a href="#pg026">26</a>26, <a href="#pg032">32</a></li>
+<li>Bichat, <a href="#pg027">27</a>-9</li>
+<li>Oken, <a href="#pg296">296</a>94</li>
+<li>K. G. Carus, <a href="#pg094">94</a></li>
+<li>Von Baer, <a href="#pg116">116</a>, <a href="#pg123">123</a>, <a href="#pg131">131</a></li>
+<li>Remak (Sensory and trophic layers), <a href="#pg210">210</a></li>
+<li>Gegenbaur, <a href="#pg263">263</a></li>
+</ul></li>
+<li>Annelid Theory of Vertebrate Descent, <a href="#pg274">274</a>-85, <a href="#pg301">301</a></li>
+
+<li>Archetype, Anatomical, <a href="#pg246">246</a>, <a href="#pg302">302</a>-3
+<ul>
+<li>E. Geoffroy, <a href="#pg054">54</a>, <a href="#pg067">67</a></li>
+<li>Owen, <a href="#pg104">104</a>-7, <a href="#pg110">110</a></li>
+<li>J. V. Carus, Huxley, <a href="#pg204">204</a></li>
+<li>C. Darwin, <a href="#pg238">238</a> f.n.</li>
+</ul></li>
+<li>Archetype, Anatomical, as Ancestral&mdash;
+<ul>
+<li>C. Darwin, <a href="#pg235">235</a>, <a href="#pg247">247</a></li>
+<li>Haeckel, <a href="#pg251">251</a></li>
+<li>Gegenbaur, <a href="#pg265">265</a></li>
+<li>Sedgwick, <a href="#pg300">300</a></li>
+<li>Criticism of this idea&mdash;
+<ul>
+<li>O. Hertwig, <a href="#pg355">355</a>-7</li>
+</ul></li>
+</ul></li>
+<li>Archetype, Embryological, <a href="#pg168">168</a>, <a href="#pg246">246</a>, <a href="#pg302">302</a>-3
+<ul>
+<li>Von Baer, <a href="#pg126">126</a>, <a href="#pg132">132</a></li>
+<li>Reichert, <a href="#pg139">139</a>, <a href="#pg147">147</a>, <a href="#pg149">149</a></li>
+<li>Rathke, <a href="#pg151">151</a>, <a href="#pg153">153</a></li>
+<li>Huxley, <a href="#pg159">159</a>-61</li>
+</ul></li>
+<li>Archetype, Embryological, as Ancestral&mdash;
+<ul>
+<li>C. Darwin, <a href="#pg233">233</a>, <a href="#pg236">236</a>-7</li>
+<li>Haeckel, <a href="#pg296">296</a>254, <a href="#pg289">289</a>-91</li>
+<li>Gegenbaur, <a href="#pg266">266</a></li>
+<li>O. and R. Hertwig, <a href="#pg298">298</a></li>
+<li>Sedgwick, <a href="#pg300">300</a></li>
+<li>A. Kowalevsky, <a href="#pg300">300</a></li>
+</ul></li>
+<li>Arendt, <a href="#pg162">162</a></li>
+
+<li>Aristotle, <a href="#pg002">2</a>-16, <a href="#pg017">17</a>, <a href="#pg345">345</a>, <a href="#pg364">364</a>
+<ul>
+<li>_Historia Animalium_, <a href="#pg002">2</a></li>
+<li>_De Partibus Animalium_, <a href="#pg002">2</a>, <a href="#pg009">9</a></li>
+<li>Knowledge of Animals, <a href="#pg003">3</a>, <a href="#pg004">4</a></li>
+<li>Comparative Embryology, <a href="#pg004">4</a></li>
+<li>Classification of Animals, <a href="#pg004">4</a>-6</li>
+<li>Unity of Plan, <a href="#pg006">6</a>-7, <a href="#pg010">10</a></li>
+<li>Homology and Analogy, <a href="#pg007">7</a>-10</li>
+<li>Teleology and Correlation, <a href="#pg010">10</a>-12</li>
+<li>Law of Compensation, <a href="#pg011">11</a></li>
+<li>Division of Labour, <a href="#pg012">12</a></li>
+<li>Degrees of Composition--homogeneous and heterogeneous parts, <a href="#pg012">12</a>-14, <a href="#pg169">169</a></li>
+<li>Law of Development (Von Baer), <a href="#pg014">14</a></li>
+<li>Scale of Beings, <a href="#pg014">14</a>-16</li>
+<li>Functional attitude, <a href="#pg015">15</a>-16, <a href="#pg197">197</a></li>
+<li>Animal and Vegetative Lives, <a href="#pg016">16</a>, <a href="#pg032">32</a></li>
+</ul></li>
+<li>Ascidian Theory of Vertebrate Descent, <a href="#pg269">269</a>-73, <a href="#pg304">304</a></li>
+
+<li>Atomists, <a href="#pg016">16</a></li>
+
+<li>Atomists, "Biological," <a href="#pg192">192</a>-4</li>
+
+<li>Audouin, V.&mdash;
+<ul>
+<li>Unity of plan in Arthropods, <a href="#pg085">85</a>-6</li>
+<li>Law of Compensation, <a href="#pg086">86</a></li>
+<li>Marine Zoology, <a href="#pg195">195</a></li>
+</ul></li>
+<li>Autenrieth, <a href="#pg090">90</a>, <a href="#pg096">96</a></li>
+
+<li>Avicenna, <a href="#pg017">17</a></li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Bab&aacute;k</span>, E., <a href=
+"#pg333">333</a></li>
+
+<li>Baer, K. E. von, <a href="#pg113">113</a>-32, <a href=
+"#pg133">133</a>, <a href="#pg251">251</a>, <a href=
+"#pg304">304</a>, <a href="#pg345">345</a>, <a href=
+"#pg356">356</a>
+
+<ul>
+<li>Founder of Embryology, <a href="#pg113">113</a></li>
+
+<li><i>Entwickelungsgeschichte der Thiere</i>, <a href=
+"#pg114">114</a></li>
+
+<li>Regulation of Development, <a href="#pg114">114</a>, <a href=
+"#pg350">350</a></li>
+
+<li>Development as Differentiation, <a href="#pg115">115</a>, <a
+href="#pg128">128</a></li>
+
+<li>Germ-Layer Theory, <a href="#pg115">115</a>-6, <a href=
+"#pg118">118</a>-119, <a href="#pg208">208</a>-9, <a href=
+"#pg296">296</a></li>
+
+<li>Morphological Differentiation, <a href="#pg116">116</a>-7</li>
+
+<li>Histological Differentiation, <a href="#pg117">117</a>-8</li>
+
+<li>Tissues and Germ-Layers, <a href="#pg118">118</a></li>
+
+<li>Double symmetrical Development, <a href="#pg118">118</a>, <a
+href="#pg279">279</a></li>
+
+<li>Criticism of Meckel-Serres Law, <a href="#pg120">120</a>-3, <a
+href="#pg304">304</a></li>
+
+<li>Theory of Types, <a href="#pg289">123</a>-4, <a href=
+"#pg289">289</a>, <a href="#pg291">291</a></li>
+
+<li>Law of Development, <a href="#pg124">124</a>-6</li>
+
+<li>Embryological Criterion, <a href="#pg126">126</a>-8, <a href=
+"#pg132">132</a>, <a href="#pg138">138</a></li>
+
+<li>Embryological Archetype, <a href="#pg126">126</a>, <a href=
+"#pg132">132</a></li>
+
+<li>Types of Development, <a href="#pg127">127</a>-8</li>
+
+<li>Von Baer and Cuvier, <a href="#pg128">128</a>-30</li>
+
+<li>Functional attitude, <a href="#pg129">129</a></li>
+
+<li>Relation to Transcendentalists, <a href="#pg129">129</a>, <a
+href="#pg131">131</a></li>
+
+<li>Criticism of Scale of Beings, <a href="#pg130">130</a></li>
+
+<li>Vertebral Theory of Skull, <a href="#pg131">131</a>, <a href=
+"#pg142">142</a></li>
+
+<li>Serial Homology, <a href="#pg131">131</a>-2</li>
+
+<li>Gill-slits, Gill-arches and Aortic arches, <a href=
+"#pg135">135</a>-6, <a href="#pg146">146</a></li>
+
+<li>Membrane and Cartilage Bones, <a href="#pg162">162</a>-3</li>
+
+<li>Degrees of Composition, <a href="#pg172">172</a></li>
+
+<li>Ova of Mammals, <a href="#pg175">175</a>-6</li>
+
+<li>Segmentation of Ovum, <a href="#pg186">186</a></li>
+
+<li>Criticism of Evolution Theory, <a href="#pg229">229</a>, <a
+href="#pg242">242</a></li>
+
+<li>Influence on Darwin, <a href="#pg236">236</a>, <a href=
+"#pg238">238</a></li>
+
+<li>Criticism of Darwinism, <a href="#pg242">242</a></li>
+
+<li><span class="pagenum"><a name="pg367" id=
+"pg367">367</a></span>Teleology and Correlation, <a href=
+"#pg242">242</a></li>
+
+<li>On Ascidians, <a href="#pg271">271</a></li>
+</ul>
+</li>
+
+<li>Baer's Law. <i>See</i> "Development, Von Baer's Law"</li>
+
+<li>Bagge, <a href="#pg187">187</a></li>
+
+<li><i>Balanoglossus</i> Theory of Vertebrate Descent, <a href=
+"#pg285">285</a>-7</li>
+
+<li>Balbiani, <a href="#pg330">330</a></li>
+
+<li>Balfour, F. M., <a href="#pg247">247</a>, <a href=
+"#pg299">299</a>
+
+<ul>
+<li>Annelid Theory, <a href="#pg282">282</a>-4</li>
+
+<li>Gastrulation and Gastr&aelig;a Theory, <a href=
+"#pg295">295</a></li>
+
+<li>Mesoderm, <a href="#pg296">296</a> f.n.</li>
+
+<li>C&oelig;lom, <a href="#pg297">297</a></li>
+</ul>
+</li>
+
+<li>Barfurth, D., <a href="#pg330">330</a></li>
+
+<li>Barry, M., <a href="#pg186">186</a>, <a href=
+"#pg188">188</a></li>
+
+<li>Bateson, W.&mdash;
+
+<ul>
+<li>Metamerism, Vegetative Repetition, <a href=
+"#pg286">286</a></li>
+
+<li><i>Balanoglossus</i> Theory, <a href="#pg286">286</a>-7</li>
+
+<li>On Phylogenetic Speculation, <a href="#pg302">302</a></li>
+</ul>
+</li>
+
+<li>Beard, J., <a href="#pg285">285</a></li>
+
+<li>Belon, <a href="#pg018">18</a></li>
+
+<li>Beneden van, and Julin, <a href="#pg271">271</a>, <a href=
+"#pg285">285</a>, <a href="#pg346">346</a></li>
+
+<li>Bensley, A. B., <a href="#pg311">311</a> f.n.</li>
+
+<li>Bergmann, <a href="#pg187">187</a></li>
+
+<li>Bergson, H., <a href="#pg026">26</a> f.n., <a href=
+"#pg341">341</a>, <a href="#pg345">345</a></li>
+
+<li>Bernard, Claude, <a href="#pg195">195</a>, <a href=
+"#pg314">314</a></li>
+
+<li>Bert, P., <a href="#pg315">315</a></li>
+
+<li>Bichat, X., <a href="#pg027">27</a>-30, <a href=
+"#pg118">118</a>, <a href="#pg132">132</a>, <a href=
+"#pg169">169</a>, <a href="#pg178">178</a>, <a href=
+"#pg263">263</a>
+
+<ul>
+<li>Animal and Vegetative Lives, <a href="#pg027">27</a>-9</li>
+
+<li>"General Anatomy," <a href="#pg029">29</a>-30</li>
+
+<li><i>Vie propre</i> of Tissues, <a href="#pg030">30</a></li>
+</ul>
+</li>
+
+<li>Biogenetic Law. <i>See</i> "Development, Haeckel's Law"</li>
+
+<li>Bischoff, <a href="#pg138">138</a>
+
+<ul>
+<li>Segmentation, <a href="#pg186">186</a>, <a href=
+"#pg188">188</a></li>
+</ul>
+</li>
+
+<li>Blainville, de, <a href="#pg096">96</a>, <a href=
+"#pg128">128</a>, <a href="#pg141">141</a>, <a href=
+"#pg199">199</a> f.n.</li>
+
+<li>Bojanus, <a href="#pg096">96</a>, <a href="#pg097">97</a></li>
+
+<li>Bonnet, C.&mdash;
+
+<ul>
+<li>Scale of Beings, <a href="#pg022">22</a>-3, <a href=
+"#pg220">220</a>, <a href="#pg227">227</a></li>
+
+<li>Evolution, <a href="#pg215">215</a></li>
+
+<li>Regeneration, <a href="#pg315">315</a></li>
+</ul>
+</li>
+
+<li>Bonnet, R., <a href="#pg350">350</a></li>
+
+<li>Bonnier, G., on Albertus Magnus, <a href="#pg017">17</a></li>
+
+<li>Born, G., <a href="#pg330">330</a></li>
+
+<li>Boveri, T., <a href="#pg270">270</a> f.n., <a href=
+"#pg333">333</a></li>
+
+<li>Braem, <a href="#pg347">347</a> f.n.</li>
+
+<li>Braun, A., <a href="#pg355">355</a></li>
+
+<li>Breschet, <a href="#pg138">138</a>, <a href=
+"#pg173">173</a></li>
+
+<li>Bronn, H. G., <a href="#pg200">200</a>-3, <a href=
+"#pg248">248</a>
+
+<ul>
+<li><i>Naturphilosophie</i>, <a href="#pg201">201</a></li>
+
+<li>Functional attitude, <a href="#pg201">201</a>-3</li>
+
+<li>Geometry of Organism, <a href="#pg201">201</a>, <a href=
+"#pg249">249</a></li>
+
+<li>Theory of Types, <a href="#pg202">202</a></li>
+
+<li>Principle of Connections, <a href="#pg202">202</a></li>
+
+<li>Intrinsic Laws of Evolution, <a href="#pg202">202</a></li>
+
+<li>Division of Labour, <a href="#pg202">202</a></li>
+
+<li>Ecological Adaptation and Classification, <a href=
+"#pg203">203</a></li>
+</ul>
+</li>
+
+<li>Brown, R., <a href="#pg171">171</a></li>
+
+<li>Bruch, C., <a href="#pg203">203</a> f.n.</li>
+
+<li>B&uuml;chner, <a href="#pg194">194</a>, <a href=
+"#pg248">248</a></li>
+
+<li>Buffon, <a href="#pg024">24</a>-7, <a href="#pg336">336</a>
+
+<ul>
+<li>Scale of Beings, <a href="#pg024">24</a>, <a href=
+"#pg215">215</a></li>
+
+<li>Unity of Plan, <a href="#pg024">24</a></li>
+
+<li>Evolution, <a href="#pg024">24</a>-5, <a href=
+"#pg214">214</a></li>
+
+<li>Classification, <a href="#pg025">25</a>-6</li>
+
+<li>Animal and Vegetative Lives, <a href="#pg026">26</a>-7</li>
+
+<li>Homology and Analogy, <a href="#pg027">27</a></li>
+</ul>
+</li>
+
+<li>Burckhardt, R., <a href="#pg003">3</a> f.n., <a href=
+"#pg268">268</a> f.n.</li>
+
+<li>Burdin, <a href="#pg096">96</a></li>
+
+<li>Burmeister, <a href="#pg249">249</a> f.n.</li>
+
+<li>Butler, S., <a href="#pg226">226</a> f.n., <a href=
+"#pg313">313</a>, <a href="#pg335">335</a>-42
+
+<ul>
+<li>Relation to Lamarck, <a href="#pg335">335</a>-7</li>
+
+<li>Psychological Vitalism, <a href="#pg336">336</a>-41</li>
+
+<li>Heredity and Memory, <a href="#pg337">337</a>-41</li>
+
+<li>The Two Stages of Development, <a href="#pg337">337</a>-9</li>
+
+<li>Consciousness and Habit, <a href="#pg337">337</a>-9</li>
+
+<li>Recapitulation Theory, <a href="#pg339">339</a>-40</li>
+
+<li>Teleology, <a href="#pg341">341</a></li>
+</ul>
+</li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Cabanis</span>, <a href=
+"#pg215">215</a></li>
+
+<li>Camper, P., <a href="#pg045">45</a>, <a href=
+"#pg046">46</a></li>
+
+<li>Carter, <a href="#pg293">293</a> f.n.</li>
+
+<li><span class="pagenum"><a name="pg368" id=
+"pg368">368</a></span>Carus, J. V..&mdash;
+
+<ul>
+<li>Criticism of Embryological Criterion, <a href=
+"#pg167">167</a></li>
+
+<li>Morphology and&nbsp; Physiology, <a href="#pg194">194</a></li>
+
+<li>Vertebral Theory of Skull, <a href="#pg203">203</a></li>
+
+<li>On Archetype, <a href="#pg204">204</a></li>
+
+<li>Evolution, <a href="#pg230">230</a></li>
+</ul>
+</li>
+
+<li>Carus, K. G.&mdash;
+
+<ul>
+<li>Law of Parallelism, <a href="#pg094">94</a>, <a href=
+"#pg249">249</a></li>
+
+<li>Vertebral Theory, <a href="#pg096">96</a></li>
+
+<li>Geometry of Skeleton, <a href="#pg098">98</a>-100</li>
+
+<li>Splanchnoskeleton, <a href="#pg098">98</a>, <a href=
+"#pg140">140</a></li>
+</ul>
+</li>
+
+<li>Causal Morphology, <a href="#pg312">312</a>-3, <a href=
+"#pg315">315</a>-34</li>
+
+<li>Cell-Theory&mdash;
+
+<ul>
+<li>Schwann, <a href="#pg169">169</a>, <a href="#pg173">173</a>-86,
+<a href="#pg188">188</a></li>
+
+<li>C. F. Wolff, <a href="#pg170">170</a></li>
+
+<li>Schleiden, <a href="#pg170">170</a>-2</li>
+
+<li>Criticism of Schwann-Schleiden Theory, <a href=
+"#pg185">185</a>-8
+
+<ul>
+<li>Virchow, Leydig, <a href="#pg188">188</a></li>
+</ul>
+</li>
+</ul>
+</li>
+
+<li>Cell-Theory and Germ-Layer Theory&mdash;
+
+<ul>
+<li>Remak, <a href="#pg209">209</a>-12</li>
+</ul>
+</li>
+
+<li>Cell-Theory as Disintegrative&mdash;
+
+<ul>
+<li>Schwann, <a href="#pg180">180</a>-5, <a href=
+"#pg248">248</a></li>
+
+<li>Vogt, <a href="#pg190">190</a>-1</li>
+
+<li>Virchow, <a href="#pg191">191</a></li>
+
+<li>Haeckel, <a href="#pg248">248</a></li>
+
+<li>Criticism of this idea&mdash;
+
+<ul>
+<li>Reichert, <a href="#pg192">192</a>-3, <a href=
+"#pg194">194</a></li>
+
+<li>J. V.. Carus, <a href="#pg194">194</a></li>
+
+<li>Sedgwick, Whitman, <a href="#pg346">346</a></li>
+</ul>
+</li>
+</ul>
+</li>
+
+<li>Cell-Theory, Influence on Morphology, <a href=
+"#pg190">190</a></li>
+
+<li>Cenogenesis, <a href="#pg258">258</a>-9, <a href=
+"#pg323">323</a></li>
+
+<li>Chabry, <a href="#pg331">331</a></li>
+
+<li>Child, C. M., <a href="#pg339">333</a></li>
+
+<li>Chun, C, <a href="#pg317">317</a>, <a href=
+"#pg332">332</a></li>
+
+<li>Classification of Animals&mdash;
+
+<ul>
+<li>Aristotle, <a href="#pg004">4</a>-6</li>
+
+<li>Rondeletius, Aldrovandus, Gesner, <a href="#pg018">18</a></li>
+
+<li>Linn&aelig;us, <a href="#pg022">22</a></li>
+
+<li>Buffon, <a href="#pg025">25</a>-6</li>
+
+<li>Cuvier, <a href="#pg039">39</a>-41</li>
+
+<li>E. Geoffroy, <a href="#pg060">60</a></li>
+
+<li>L. Agassiz, <a href="#pg203">203</a> f.n.</li>
+
+<li>Lamarck, <a href="#pg216">216</a>-7, <a href="#pg227">227</a>,
+<a href="#pg228">228</a></li>
+</ul>
+</li>
+
+<li>Classification and Ecological Adaptation (Bronn), <a href=
+"#pg203">203</a></li>
+
+<li>Classification as Genealogical&mdash;
+
+<ul>
+<li>Buffon, <a href="#pg024">24</a>-5</li>
+
+<li>Lamarck, <a href="#pg218">218</a>, <a href=
+"#pg228">228</a></li>
+
+<li>C. Darwin, <a href="#pg233">233</a>, <a href="#pg234">234</a>,
+<a href="#pg247">247</a></li>
+
+<li>Haeckel, <a href="#pg250">250</a>-1, <a href=
+"#pg254">254</a></li>
+
+<li>Criticism of this idea, <a href="#pg303">303</a>, <a href=
+"#pg304">304</a>,
+
+<ul>
+<li>O. Hertwig, <a href="#pg356">356</a></li>
+</ul>
+</li>
+</ul>
+</li>
+
+<li>Classification, Phylogenetic&mdash;
+
+<ul>
+<li>Haeckel's, <a href="#pg289">289</a>-94</li>
+</ul>
+</li>
+
+<li>Claus, <a href="#pg259">259</a></li>
+
+<li>Co-adaptation, <a href="#pg326">326</a> f.n.</li>
+
+<li>C&oelig;lom&mdash;
+
+<ul>
+<li>Remak, <a href="#pg211">211</a></li>
+
+<li>A. Kowalevsky, <a href="#pg270">270</a>, <a href=
+"#pg295">295</a>, <a href="#pg297">297</a></li>
+
+<li>Haeckel, <a href="#pg291">291</a>, <a href="#pg295">295</a>, <a
+href="#pg296">296</a></li>
+
+<li>Lankester, <a href="#pg291">291</a>, <a href=
+"#pg297">297</a></li>
+</ul>
+</li>
+
+<li>C&oelig;lom, Theory of, <a href="#pg295">295</a>-301</li>
+
+<li>Cohen, <a href="#pg189">189</a></li>
+
+<li>Coiter, <a href="#pg018">18</a></li>
+
+<li>Colucci, <a href="#pg346">346</a></li>
+
+<li>Compensation, Law of&mdash;
+
+<ul>
+<li>Aristotle, <a href="#pg011">11</a></li>
+
+<li>Goethe, <a href="#pg049">49</a></li>
+
+<li>E. Geoffroy, <a href="#pg072">72</a>-3</li>
+
+<li>Audouin, <a href="#pg086">86</a></li>
+
+<li>German Transcendentalists, <a href="#pg100">100</a></li>
+</ul>
+</li>
+
+<li>Condillac, <a href="#pg215">215</a></li>
+
+<li>Conditions of Existence, Principle of&mdash;
+
+<ul>
+<li>Cuvier, <a href="#pg034">34</a>, <a href="#pg075">75</a>-6, <a
+href="#pg239">239</a></li>
+
+<li>Gegenbaur, <a href="#pg263">263</a>-4</li>
+
+<li>Roux, <a href="#pg324">324</a>, <a href="#pg326">326</a></li>
+
+<li>Spencer, Weismann, <a href="#pg326">326</a> f.n.</li>
+
+<li>Disregard for&mdash;
+
+<ul>
+<li>Lamarck, <a href="#pg226">226</a></li>
+
+<li>C. Darwin, <a href="#pg232">232</a>, <a href=
+"#pg238">238</a>-41</li>
+
+<li>Haeckel, <a href="#pg248">248</a>, <a href=
+"#pg264">264</a></li>
+</ul>
+</li>
+</ul>
+</li>
+
+<li>Conklin, <a href="#pg333">333</a></li>
+
+<li>Connections, Principle of&mdash;
+
+<ul>
+<li>Goethe, <a href="#pg047">47</a></li>
+
+<li>E. Geoffroy, <a href="#pg053">53</a>-4, <a href=
+"#pg062">62</a>-3, <a href="#pg071">71</a>, <a href=
+"#pg074">74</a>, <a href="#pg261">261</a></li>
+
+<li>Audouin, <a href="#pg085">85</a></li>
+
+<li><span class="pagenum"><a name="pg369" id=
+"pg369">369</a></span>German Transcendentalists, <a href=
+"#pg100">100</a></li>
+
+<li>J. F. Meckel, <a href="#pg101">101</a></li>
+
+<li>Owen, <a href="#pg107">107</a>-8</li>
+
+<li>Bronn, <a href="#pg202">202</a></li>
+
+<li>C. Darwin, <a href="#pg234">234</a>-5</li>
+
+<li>Gegenbaur, <a href="#pg261">261</a></li>
+
+<li>Semper, <a href="#pg279">279</a></li>
+
+<li>In Embryology, <a href="#pg168">168</a></li>
+
+<li>Main Principle of Morphology, <a href="#pg246">246</a>, <a
+href="#pg302">302</a></li>
+</ul>
+</li>
+
+<li>Convergence&mdash;
+
+<ul>
+<li>Milne-Edwards, <a href="#pg199">199</a></li>
+
+<li>I. Geoffroy St Hilaire, <a href="#pg199">199</a> f.n., <a href=
+"#pg206">206</a></li>
+
+<li>C. Darwin, <a href="#pg236">236</a></li>
+
+<li>Friedmann, Willey, Vialleton, <a href="#pg306">306</a>
+f.n.</li>
+</ul>
+</li>
+
+<li>Convergence, Rejected by Evolutionary Morphologists, <a href=
+"#pg305">305</a>, <a href="#pg312">312</a>
+
+<ul>
+<li>Hubrecht, <a href="#pg305">305</a>-6</li>
+</ul>
+</li>
+
+<li>Cope, E. D., <a href="#pg342">342</a>, <a href="#pg357">357</a>
+f.n., <a href="#pg361">361</a>, <a href="#pg362">362</a></li>
+
+<li>Correlation, Functional&mdash;
+
+<ul>
+<li>Aristotle, <a href="#pg010">10</a>-12</li>
+
+<li>Cuvier, <a href="#pg035">35</a>-8, <a href="#pg239">239</a>, <a
+href="#pg241">241</a></li>
+
+<li>E. Geoffroy, <a href="#pg077">77</a></li>
+
+<li>Von Hartmann, <a href="#pg240">240</a>-1</li>
+
+<li>R&aacute;dl, <a href="#pg240">240</a> f.n., <a href=
+"#pg241">241</a></li>
+
+<li>Von Baer, <a href="#pg242">242</a></li>
+
+<li>Gegenbaur, <a href="#pg264">264</a></li>
+
+<li>Disregarded by&mdash;
+
+<ul>
+<li>C. Darwin, <a href="#pg235">235</a>, <a href=
+"#pg238">238</a>-41</li>
+
+<li>Haeckel, <a href="#pg248">248</a>, <a href=
+"#pg264">264</a></li>
+</ul>
+</li>
+</ul>
+</li>
+
+<li>Coste, <a href="#pg138">138</a>, <a href="#pg176">176</a>, <a
+href="#pg187">187</a></li>
+
+<li>Crampton, <a href="#pg322">332</a></li>
+
+<li>Cunningham, J. T., <a href="#pg284">284</a></li>
+
+<li>Cuvier, <a href="#pg026">26</a>, <a href="#pg031">31</a>-44, <a
+href="#pg089">89</a>, <a href="#pg196">196</a>, <a href=
+"#pg197">197</a>, <a href="#pg199">199</a> f.n., <a href=
+"#pg278">278</a>, <a href="#pg345">345</a>, <a href=
+"#pg361">361</a>
+<ul>
+<li>Functional attitude, <a href="#pg031">31</a>-6, <a href=
+"#pg065">65</a>, <a href="#pg075">75</a>-8, <a href=
+"#pg200">200</a>, <a href="#pg305">305</a></li>
+
+<li>Animal and Vegetative Lives, <a href="#pg032">32</a></li>
+
+<li>Degrees of Composition, <a href="#pg032">32</a>-3</li>
+
+<li>Teleology, <a href="#pg033">33</a>-5</li>
+
+<li>Functional Adaptedness, <a href="#pg033">33</a>-5, <a href=
+"#pg324">324</a></li>
+
+<li>Principle of Conditions of Existence, <a href="#pg034">34</a>,
+<a href="#pg075">75</a>-6, <a href="#pg239">239</a></li>
+
+<li>Correlation, <a href="#pg341">35</a>-8, <a href=
+"#pg239">239</a>, <a href="#pg241">241</a></li>
+
+<li>Metabolism, <a href="#pg341">38</a></li>
+
+<li>Adaptation as Conservative Principle, <a href=
+"#pg341">39</a>, <a href="#pg341">76</a></li>
+
+<li>Classification, <a href="#pg341">39</a>-41</li>
+
+<li>Principle of Subordination of Characters, <a href=
+"#pg040">40</a></li>
+
+<li>Criticism of Scale of Beings, <a href="#pg039">39</a>-40, <a
+href="#pg130">130</a></li>
+
+<li>Type Theory, <a href="#pg041">41</a>, <a href="#pg124">124</a>,
+<a href="#pg289">289</a>, <a href="#pg291">291</a></li>
+
+<li>Criticism of Evolution-Theory, <a href="#pg041">41</a>-4, <a
+href="#pg129">129</a>, <a href="#pg304">304</a></li>
+
+<li>Variation, Limits of, <a href="#pg341">42</a></li>
+
+<li>Pal&aelig;ontological Succession, <a href="#pg043">43</a></li>
+
+<li>Polemic with Geoffroy, <a href="#pg064">64</a>-5, <a href=
+"#pg074">74</a>-8</li>
+
+<li>Criticism of Vertebral Theory of Skull, <a href=
+"#pg097">97</a>-8</li>
+
+<li>Influence on J. F. Meckel, <a href="#pg101">101</a></li>
+
+<li>Criticism of Meckel-Serres Law, <a href="#pg129">129</a>-30, <a
+href="#pg304">304</a></li>
+
+<li>As Embryologist, <a href="#pg130">130</a></li>
+
+<li>Criticism of Lamarck, <a href="#pg228">228</a></li>
+</ul>
+</li>
+
+<li>Cytology, <a href="#pg346">346</a></li>
+
+<li>Cytoplasm of Egg, Organ-forming Stuffs, <a href=
+"#pg332">332</a>-3</li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Dall</span>, <a href="#pg361">361</a></li>
+
+<li>D'Alton, <a href="#pg113">113</a></li>
+
+<li>Dareste, C., <a href="#pg315">315</a></li>
+
+<li>Darwin, Charles, <a href="#pg078">78</a>, <a href=
+"#pg230">230</a>-41, <a href="#pg271">271</a>, <a href=
+"#pg304">304</a>, <a href="#pg307">307</a>, <a href=
+"#pg336">336</a>, <a href="#pg362">362</a>
+<ul>
+<li>Systematist and Field Naturalist, <a href="#pg230">230</a>, <a
+href="#pg231">231</a></li>
+
+<li>Pal&aelig;ontological Succession, <a href="#pg231">231</a></li>
+
+<li>Ecological Adaptation, <a href="#pg231">231</a>-2, <a href=
+"#pg235">235</a>, <a href="#pg239">239</a></li>
+
+<li>Species Problem, <a href="#pg231">231</a></li>
+
+<li>Functional Adaptation, Disregard for, <a href="#pg232">232</a>,
+<a href="#pg238">238</a>-41</li>
+
+<li>Classification as genealogical, <a href="#pg233">233</a>, <a
+href="#pg234">234</a>, <a href="#pg247">247</a></li>
+
+<li>Unity of Plan due to Community of Descent, <a href=
+"#pg233">233</a>, <a href="#pg234">234</a>-5, <a href=
+"#pg239">239</a>, <a href="#pg247">247</a></li>
+
+<li>Embryological Archetype as ancestral, <a href="#pg233">233</a>,
+<a href="#pg236">236</a>-7</li>
+
+<li><span class="pagenum"><a name="pg370" id="pg370">370</a></span>
+Rejects Meckel-Serres Law, <a href="#pg233">233</a>, <a href=
+"#pg236">236</a></li>
+
+<li>Interpretation&nbsp; of&nbsp; Vestigial Organs, <a href=
+"#pg233">233</a>, <a href="#pg237">237</a></li>
+
+<li>Organism as Historical Being, <a href="#pg233">233</a>, <a
+href="#pg308">308</a></li>
+
+<li>Rejects Scale of Beings, <a href="#pg234">234</a></li>
+
+<li>Homology, <a href="#pg234">234</a>-5, <a href=
+"#pg247">247</a></li>
+
+<li>Principle of Connections, <a href="#pg234">234</a>-5</li>
+
+<li>Anatomical Archetype as ancestral, <a href="#pg235">235</a>, <a
+href="#pg247">247</a></li>
+
+<li>Von Baer's Law interpreted phylogenetically, <a href=
+"#pg236">236</a>-7</li>
+
+<li>Modifications inherited at corresponding age, <a href=
+"#pg237">237</a></li>
+
+<li>Monophyletism and Polyphyletism, <a href="#pg238">238</a></li>
+
+<li>Causes of Success, <a href="#pg238">238</a>, <a href=
+"#pg241">241</a></li>
+</ul>
+</li>
+
+<li>Darwin, Erasmus, <a href="#pg214">214</a>, <a href=
+"#pg226">226</a> f.n., <a href="#pg229">229</a>, <a href=
+"#pg336">336</a></li>
+
+<li>Darwin, Sir Francis, <a href="#pg344">344</a></li>
+
+<li>Daubenton, <a href="#pg026">26</a></li>
+
+<li>Degrees of Composition&mdash;
+
+<ul>
+<li>Aristotle, <a href="#pg012">12</a>-14, <a href=
+"#pg169">169</a></li>
+
+<li>Glisson, <a href="#pg019">19</a></li>
+
+<li>Malpighi, <a href="#pg020">20</a></li>
+
+<li>Bichat, <a href="#pg029">29</a>-30</li>
+
+<li>Cuvier, <a href="#pg032">32</a>-3,</li>
+
+<li>Dujardin, <a href="#pg169">169</a>, <a href=
+"#pg188">188</a></li>
+
+<li>Von Baer, <a href="#pg172">172</a></li>
+
+<li>Effect of Invention of Microscope, <a href="#pg020">20</a></li>
+
+<li>Relation to Cell-Theory, <a href="#pg169">169</a></li>
+</ul>
+</li>
+
+<li>Delage, <a href="#pg333">333</a></li>
+
+<li>Delage and H&eacute;rouard, <a href="#pg273">273</a> f.n.</li>
+
+<li>Delpino, <a href="#pg345">345</a></li>
+
+<li>Demaillet, <a href="#pg044">44</a></li>
+
+<li>Democritus, <a href="#pg016">16</a></li>
+
+<li>Dep&eacute;ret, C., <a href="#pg357">357</a>
+<ul>
+<li>On Cuvier, <a href="#pg043">43</a></li>
+
+<li>Absence of intermediary forms in Pal&aelig;ontology, <a href=
+"#pg358">358</a></li>
+
+<li>Phyletic&nbsp; series and Polyphyletism, <a href=
+"#pg360">360</a>-1</li>
+</ul>
+</li>
+
+<li>Development, Von Baer's Law&mdash;
+
+<ul>
+<li>Aristotle, <a href="#pg014">14</a></li>
+
+<li>Von Baer, <a href="#pg124">124</a>-6</li>
+
+<li>Pr&eacute;vost and Dumas, <a href="#pg125">125</a> f.n.</li>
+
+<li>Reichert, <a href="#pg149">149</a>-50, <a href="#pg351">351</a>
+f.n.</li>
+
+<li>Milne-Edwards, <a href="#pg205">205</a>-8</li>
+
+<li>Lereboullet, <a href="#pg206">206</a>-8</li>
+
+<li>Criticised by&mdash;
+
+<ul>
+<li>Agassiz, <a href="#pg352">352</a>-3</li>
+
+<li>His, <a href="#pg353">353</a></li>
+
+<li>Sedgwick, <a href="#pg353">353</a></li>
+
+<li>O. Hertwig, <a href="#pg354">354</a></li>
+</ul>
+</li>
+
+<li>Phylogenetic Interpretation of&mdash;
+
+<ul>
+<li>Darwin, <a href="#pg236">236</a>-7</li>
+
+<li>Gegenbaur, <a href="#pg266">266</a></li>
+
+<li>Relation to Haeckel's Law, <a href="#pg254">254</a>, <a href=
+"#pg256">256</a>, <a href="#pg257">257</a></li>
+</ul>
+</li>
+</ul>
+</li>
+
+<li>Development, Biogenetic Law (Haeckel)&mdash;
+
+<ul>
+<li>Haeckel, <a href="#pg251">251</a>, <a href="#pg253">253</a>-9,
+<a href="#pg291">291</a>-4</li>
+
+<li>F. M&uuml;ller, <a href="#pg252">252</a>-3, <a href=
+"#pg254">254</a>, <a href="#pg257">257</a></li>
+
+<li>Gegenbaur, <a href="#pg262">262</a></li>
+
+<li>Roux, <a href="#pg319">319</a></li>
+
+<li>Butler, <a href="#pg339">339</a>-40</li>
+
+<li>Orr, <a href="#pg342">342</a></li>
+
+<li>Criticism of&mdash;
+<ul>
+<li>Vialleton, <a href="#pg348">348</a></li>
+
+<li>Oppel, <a href="#pg348">348</a>-9</li>
+
+<li>Keibel, <a href="#pg349">349</a>-50</li>
+
+<li>Mehnert, <a href="#pg350">350</a>-2</li>
+
+<li>O. Hertwig, <a href="#pg352">352</a>, <a href=
+"#pg354">354</a>-5</li>
+
+<li>His, <a href="#pg353">353</a></li>
+</ul>
+</li>
+<li>Relation to Laws of Meckel-Serres and Von Baer, <a href=
+"#pg254">254</a>, <a href="#pg256">256</a>, <a href=
+"#pg257">257</a>, <a href="#pg303">303</a>, <a href=
+"#pg309">309</a></li>
+
+<li>Relation to Heredity and Development, <a href=
+"#pg312">312</a>-3</li>
+
+<li>Influence of Causal Morphology, <a href="#pg347">347</a>-8</li>
+
+<li>Pal&aelig;ontological Evidence for, <a href=
+"#pg359">359</a></li>
+</ul>
+</li>
+
+<li>Development, Meckel-Serres Law&mdash;
+
+<ul>
+<li>Harvey, <a href="#pg018">18</a></li>
+
+<li>Hunter, <a href="#pg022">22</a></li>
+
+<li>E. Geoffroy, <a href="#pg069">69</a>-70, <a href=
+"#pg072">72</a></li>
+
+<li>Serres, <a href="#pg080">80</a>-3, <a href="#pg094">94</a>, <a
+href="#pg203">203</a>-4, <a href="#pg205">205</a>-6</li>
+
+<li>Kielmeyer, Autenrieth, Oken, <a href="#pg090">90</a></li>
+
+<li><span class="pagenum"><a name="pg371" id="pg371">371</a></span>
+Tiedemann, <a href="#pg091">91</a></li>
+
+<li>J. F. Meckel, <a href="#pg091">91</a>-3</li>
+
+<li>K. G. Carus, <a href="#pg094">94</a></li>
+
+<li>Criticism of&mdash;
+
+<ul>
+<li>Von Baer, <a href="#pg120">120</a>-3, <a href=
+"#pg304">304</a></li>
+
+<li>Cuvier, <a href="#pg129">129</a>-30, <a href=
+"#pg304">304</a></li>
+
+<li>Milne-Edwards, <a href="#pg205">205</a></li>
+
+<li>Lereboullet, <a href="#pg206">206</a>-8</li>
+
+<li>C. Darwin, <a href="#pg233">233</a>, <a href=
+"#pg236">236</a></li>
+</ul>
+</li>
+
+<li>Analogy with Biogenetic Law, <a href="#pg254">254</a>-7, <a
+href="#pg262">262</a>, <a href="#pg303">303</a>, <a href=
+"#pg304">304</a>, <a href="#pg309">309</a></li>
+</ul>
+</li>
+
+<li>Development, Meckel-Serres Law, Theory of Three-fold
+Parallelism&mdash;
+
+<ul>
+<li>L. Agassiz, <a href="#pg230">230</a>, <a href=
+"#pg255">255</a></li>
+
+<li>Tiedemann, Vogt, <a href="#pg255">255</a> f.n.</li>
+
+<li>Haeckel, <a href="#pg254">254</a>-5</li>
+</ul>
+</li>
+
+<li>Development, The two periods of&mdash;
+
+<ul>
+<li>Roux, <a href="#pg320">320</a>-4, <a href="#pg325">325</a>, <a
+href="#pg327">327</a>, <a href="#pg335">335</a></li>
+
+<li>Butler, <a href="#pg337">337</a>-9</li>
+</ul>
+</li>
+
+<li>Diogenes of Apollonia, 1</li>
+
+<li>Disintegration. <i>See</i> "Materialistic
+Attitude"</li>
+
+<li>Division of Labour, Principle of&mdash;
+
+<ul>
+<li>Aristotle, <a href="#pg012">12</a></li>
+
+<li>Milne-Edwards, <a href="#pg197">197</a>-8</li>
+
+<li>Bronn, <a href="#pg202">202</a></li>
+
+<li>Gegenbaur, <a href="#pg264">264</a></li>
+</ul>
+</li>
+
+<li>Dohrn, A., <a href="#pg269">269</a>, <a href="#pg274">274</a>-8
+
+<ul>
+<li>Annelid Theory of Vertebrate Descent, <a href=
+"#pg274">274</a>-7, <a href="#pg303">303</a></li>
+
+<li>Principle of Function-Change, <a href="#pg276">276</a>-8, <a
+href="#pg307">307</a></li>
+
+<li>Functional Attitude, <a href="#pg277">277</a>-8, <a href=
+"#pg307">307</a></li>
+
+<li>Formal Attitude, <a href="#pg306">306</a></li>
+</ul>
+</li>
+
+<li>D&ouml;llinger, I., <a href="#pg113">113</a>, <a href=
+"#pg157">157</a></li>
+
+<li>Dollo, <a href="#pg311">311</a></li>
+
+<li>Donn&eacute;, <a href="#pg173">173</a></li>
+
+<li>D'Orbigny, <a href="#pg043">43</a></li>
+
+<li>Driesch, H., <a href="#pg242">242</a>, <a href=
+"#pg331">331</a>, <a href="#pg332">332</a>, <a href=
+"#pg333">333</a>, <a href="#pg334">334</a>, <a href=
+"#pg345">345</a>, <a href="#pg346">346</a>-7</li>
+
+<li>Dug&egrave;s, A., <a href="#pg086">86</a>-8, <a href=
+"#pg100">100</a>, <a href="#pg134">134</a>, <a href=
+"#pg142">142</a>, <a href="#pg146">146</a>
+<ul>
+<li>Unity of Plan, <a href="#pg087">87</a></li>
+
+<li>Polyzoic conception of Organism, <a href="#pg087">87</a>-8</li>
+
+<li>Membrane and Cartilage Bones, <a href="#pg163">163</a></li>
+</ul>
+</li>
+
+<li>Dujardin, <a href="#pg169">169</a>, <a href=
+"#pg188">188</a></li>
+
+<li>Dumas. <i>See</i> Pr&eacute;vost and Dumas</li>
+
+<li>Dum&eacute;ril, <a href="#pg096">96</a></li>
+
+<li>Dumortier, <a href="#pg173">173</a></li>
+
+<li>Dutrochet, <a href="#pg099">99</a> f.n., <a href=
+"#pg130">130</a>, <a href="#pg134">134</a></li>
+
+<li>Duverney, <a href="#pg019">19</a></li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Ear-Ossicles</span>, Homology of&mdash;
+
+<ul>
+<li>E. Geoffroy, <a href="#pg056">56</a></li>
+
+<li>Spix, <a href="#pg100">100</a></li>
+
+<li>Rathke, <a href="#pg141">141</a>, <a href="#pg150">150</a></li>
+
+<li>Reichert, <a href="#pg144">144</a>-7</li>
+</ul>
+</li>
+
+<li><i>&Eacute;chelle des &ecirc;tres. See</i> "Scale of
+Beings."</li>
+
+<li>Ehlers, <a href="#pg284">284</a></li>
+
+<li>Eisig, H., <a href="#pg284">284</a>, <a href=
+"#pg285">285</a></li>
+
+<li>Embryology, Comparative, Early Workers&mdash;
+
+<ul>
+<li>Aristotle, 4, <a href="#pg113">113</a></li>
+
+<li>Fabricius, Harvey, <a href="#pg018">18</a>, <a href=
+"#pg113">113</a></li>
+
+<li>Malpighi, <a href="#pg020">20</a>, <a href=
+"#pg113">113</a></li>
+
+<li>Oken and Kieser, <a href="#pg090">90</a>, <a href=
+"#pg113">113</a></li>
+
+<li>Haller, C. F. Wolff, J. F. Meckel, Tiedemann, <a href=
+"#pg113">113</a></li>
+</ul>
+</li>
+
+<li>Embryology, Experimental, <a href="#pg317">317</a>, <a href=
+"#pg318">318</a>, <a href="#pg330">330</a>-3</li>
+
+<li>Embryological Archetype. <i>See</i> "Archetype,
+Embryological"</li>
+
+<li>Embryological Criterion of Homology, <a href=
+"#pg133">133</a>-168, <a href="#pg347">347</a>
+<ul>
+<li>Goethe, <a href="#pg049">49</a></li>
+
+<li>E. Geoffroy, <a href="#pg072">72</a>, <a href=
+"#pg110">110</a></li>
+
+<li>Cuvier, <a href="#pg075">75</a>, <a href="#pg110">110</a>, <a
+href="#pg130">130</a></li>
+
+<li>Owen, <a href="#pg110">110</a>-1</li>
+
+<li>Von Baer, <a href="#pg126">126</a>-8, <a href="#pg132">132</a>,
+<a href="#pg138">138</a></li>
+
+<li>Rathke, <a href="#pg138">138</a>, <a href=
+"#pg140">140</a>-1</li>
+
+<li>J. M&uuml;ller, <a href="#pg138">138</a></li>
+
+<li>Reichert, <a href="#pg138">138</a>-9, <a href=
+"#pg144">144</a>-7, <a href="#pg163">163</a></li>
+
+<li>Vogt, <a href="#pg156">156</a>-7</li>
+
+<li>Huxley, <a href="#pg158">158</a>-9, <a href=
+"#pg166">166</a></li>
+
+<li>K&ouml;lliker, <a href="#pg165">165</a>-6</li>
+
+<li>Criticised by&mdash;
+
+<ul>
+<li>Owen, J. V. Carus, <a href="#pg167">167</a></li>
+</ul>
+</li>
+</ul>
+</li>
+
+<li>Empedocles, 1, <a href="#pg015">15</a></li>
+
+<li><span class="pagenum"><a name="pg372" id=
+"pg372">372</a></span>Engramm (Semon), <a href=
+"#pg343">343</a></li>
+
+<li><i>Entwicklungsgesetz. See</i> "Evolution, Intrinsic Laws
+of"</li>
+
+<li><i>Entwicklungsmechanik</i>, <a href="#pg315">315</a></li>
+
+<li>Erasistratus, <a href="#pg017">17</a></li>
+
+<li>Evolution Theory&mdash;
+
+<ul>
+<li>Lucretius, <a href="#pg016">16</a></li>
+
+<li>Buffon, <a href="#pg024">24</a>-5, <a href=
+"#pg214">214</a></li>
+
+<li>Cuvier's criticism, <a href="#pg041">41</a>-4, <a href=
+"#pg129">129</a>, <a href="#pg304">304</a></li>
+
+<li>E. Geoffroy, <a href="#pg066">66</a>-9, <a href=
+"#pg073">73</a>, <a href="#pg228">228</a></li>
+
+<li>J. F. Meckel, <a href="#pg092">92</a>-3, <a href=
+"#pg215">215</a>, <a href="#pg228">228</a></li>
+
+<li>Leibniz, <a href="#pg213">213</a></li>
+
+<li>Kant, <a href="#pg213">213</a>-4</li>
+
+<li>Erasmus Darwin, <a href="#pg214">214</a>, <a href=
+"#pg229">229</a></li>
+
+<li>C. Bonnet, Oken, Robinet, Treviranus, <a href=
+"#pg215">215</a></li>
+
+<li>Tiedemann, <a href="#pg215">215</a>, <a href="#pg255">255</a>
+f.n.</li>
+
+<li>Lamarck, <a href="#pg215">215</a>-29</li>
+
+<li>Von Baer, <a href="#pg229">229</a>, <a href=
+"#pg242">242</a></li>
+
+<li>I. Geoffroy St Hilaire, J. V.. Carus, <a href=
+"#pg230">230</a></li>
+
+<li>Charles Darwin, <a href="#pg230">230</a>-41</li>
+
+<li>Von Hartmann, <a href="#pg240">240</a>-1, <a href=
+"#pg244">244</a>, <a href="#pg356">356</a></li>
+
+<li>K&ouml;lliker, <a href="#pg243">243</a></li>
+
+<li>Owen, <a href="#pg244">244</a></li>
+
+<li>Milne-Edwards, <a href="#pg244">244</a>-5</li>
+
+<li>Haeckel, <a href="#pg250">250</a>-9</li>
+
+<li>Gegenbaur, <a href="#pg265">265</a></li>
+
+<li>The Organism as an Historical Being, <a href=
+"#pg308">308</a>-13</li>
+
+<li>C. Darwin, <a href="#pg233">233</a>, <a href=
+"#pg308">308</a></li>
+
+<li>Haeckel, <a href="#pg252">252</a>, <a href=
+"#pg257">257</a></li>
+
+<li>Sedgwick, <a href="#pg308">308</a></li>
+
+<li>Roux, <a href="#pg313">313</a>, <a href="#pg322">322</a>-4</li>
+
+<li>Butler, <a href="#pg313">313</a>, <a href=
+"#pg336">336</a>-41</li>
+</ul>
+</li>
+
+<li>Evolution-Theory, Influence on Morphology, <a href=
+"#pg302">302</a>-13</li>
+
+<li>Evolution, Intrinsic Laws of, <a href="#pg241">241</a>
+<ul>
+<li>J. F. Meckel, <a href="#pg093">93</a></li>
+
+<li>Bronn, <a href="#pg202">202</a></li>
+
+<li>Von Baer, <a href="#pg229">229</a>, <a href="#pg242">242</a>,
+<a href="#pg356">356</a></li>
+
+<li>K&ouml;lliker, Naegeei, <a href="#pg243">243</a>, <a href=
+"#pg356">356</a></li>
+
+<li>Owen, <a href="#pg244">244</a></li>
+
+<li>Von Hartmann, <a href="#pg244">244</a>, <a href=
+"#pg356">356</a></li>
+
+<li>Milne-Edwards, <a href="#pg244">244</a>-5</li>
+
+<li>O. Hertwig, <a href="#pg354">354</a>-5, <a href=
+"#pg356">356</a>-7</li>
+
+<li>Wigand, <a href="#pg356">356</a></li>
+
+<li>Dep&eacute;ret, <a href="#pg361">361</a></li>
+</ul>
+</li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Fabricius</span>, <a href="#pg018">18</a>,
+<a href="#pg113">113</a></li>
+
+<li>Fallopius, <a href="#pg018">18</a></li>
+
+<li>Fischel, <a href="#pg346">346</a>, <a href=
+"#pg350">350</a></li>
+
+<li>Fischer, <a href="#pg328">328</a></li>
+
+<li>Fleischmann, <a href="#pg357">357</a> f.n.</li>
+
+<li>Flourens, <a href="#pg046">46</a>, <a href=
+"#pg315">315</a></li>
+
+<li>Fontana, <a href="#pg172">172</a></li>
+
+<li>Forbes, E., <a href="#pg196">196</a></li>
+
+<li>Formal Attitude, <a href="#pg246">246</a>, <a href=
+"#pg305">305</a>
+<ul>
+<li>Goethe, <a href="#pg049">49</a></li>
+
+<li>E. Geoffroy, <a href="#pg062">62</a>-3, <a href=
+"#pg071">71</a>, <a href="#pg075">75</a>-8, <a href=
+"#pg305">305</a></li>
+
+<li>Haeckel, <a href="#pg249">249</a>, <a href="#pg257">257</a>, <a
+href="#pg260">260</a></li>
+
+<li>Gegenbaur, <a href="#pg261">261</a>, <a href=
+"#pg263">263</a></li>
+
+<li>Semper, <a href="#pg279">279</a></li>
+
+<li>Adopted by Evolutionary Morphologists, <a href=
+"#pg302">302</a>-8, <a href="#pg311">311</a>-2, <a href=
+"#pg314">314</a></li>
+
+<li>Hubrecht, <a href="#pg305">305</a>-6</li>
+
+<li>Dohrn, <a href="#pg306">306</a></li>
+</ul>
+</li>
+
+<li>Franc&eacute;, R., <a href="#pg345">345</a></li>
+
+<li>Friedmann, <a href="#pg306">306</a> f.n.</li>
+
+<li>Fuld, <a href="#pg333">333</a></li>
+
+<li>Functional Adaptation, <a href="#pg316">316</a>-7, <a href=
+"#pg318">318</a>, <a href="#pg320">320</a>-9, <a href=
+"#pg333">333</a>, <a href="#pg344">344</a>, <a href=
+"#pg351">351</a></li>
+
+<li>Functional Attitude&mdash;
+
+<ul>
+<li>Aristotle, <a href="#pg015">15</a>-6, <a href=
+"#pg197">197</a></li>
+
+<li>Bichat, <a href="#pg027">27</a>-9</li>
+
+<li>Cuvier, <a href="#pg031">31</a>-6, <a href="#pg065">65</a>, <a
+href="#pg075">75</a>-8, <a href="#pg200">200</a>, <a href=
+"#pg305">305</a></li>
+
+<li>Goethe, <a href="#pg049">49</a>-50</li>
+
+<li>J. F. Meckel, <a href="#pg101">101</a></li>
+
+<li>Owen, <a href="#pg109">109</a>, <a href="#pg110">110</a>, <a
+href="#pg111">111</a></li>
+
+<li>Von Baer, <a href="#pg129">129</a></li>
+
+<li>Milne-Edwards, <a href="#pg195">195</a>, <a href=
+"#pg197">197</a>-200</li>
+
+<li>J. M&uuml;ller, Reichert, <a href="#pg200">200</a></li>
+
+<li>Bronn, <a href="#pg201">201</a>-3</li>
+
+<li>Lamarck, <a href="#pg222">222</a>-6, <a href="#pg307">307</a>,
+<a href="#pg335">335</a></li>
+
+<li>Gegenbaur, <a href="#pg260">260</a>, <a href=
+"#pg263">263</a>-4</li>
+
+<li>Dohrn, <a href="#pg277">277</a>-8, <a href=
+"#pg307">307</a></li>
+
+<li>Roux, <a href="#pg320">320</a>-9, <a href="#pg335">335</a></li>
+
+<li>Houssay, <a href="#pg333">333</a></li>
+
+<li>Butler, <a href="#pg336">336</a>-41</li>
+
+<li>G. Wolff, <a href="#pg346">346</a></li>
+
+<li>Driesch, <a href="#pg346">346</a>-7</li>
+
+<li>Giard, <a href="#pg347">347</a></li>
+
+<li><span class="pagenum"><a name="pg373" id="pg373">373</a></span>
+E. Schulz, <a href="#pg347">347</a> f.n.</li>
+
+<li>Keibel, <a href="#pg349">349</a>-50</li>
+
+<li>Mehnert, <a href="#pg350">350</a>-1</li>
+
+<li>American Pal&aelig;ontologists, <a href="#pg361">361</a>, <a
+href="#pg362">362</a></li>
+
+<li>R&uuml;timeyer, <a href="#pg361">361</a></li>
+
+<li>V. O. Kowalevsky, <a href="#pg361">361</a>-2</li>
+
+<li>Osborn, <a href="#pg362">362</a>-4</li>
+</ul>
+</li>
+
+<li>Function-Change, Principle of&mdash;
+
+<ul>
+<li>Dohrn, <a href="#pg276">276</a>-8, <a href="#pg306">306</a>, <a
+href="#pg307">307</a></li>
+
+<li>Eisig, <a href="#pg284">284</a></li>
+</ul>
+</li>
+
+<li>F&uuml;rbringer, M., <a href="#pg282">282</a> f.n., <a href=
+"#pg284">284</a>, <a href="#pg323">323</a> f.n.</li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Galen</span>, <a href="#pg017">17</a></li>
+
+<li>Gastr&aelig;a Theory, <a href="#pg269">269</a>, <a href=
+"#pg288">288</a>-95, <a href="#pg298">298</a>, <a href="#pg299">299</a>-301, <a href=
+"#pg303">303</a></li>
+
+<li>Gastrula, Discovery of, <a href="#pg288">288</a></li>
+
+<li>Gaupp, E., <a href="#pg310">310</a> f.n.</li>
+
+<li>Gegenbaur, C., <a href="#pg247">247</a>, <a href=
+"#pg260">260</a>-7, <a href="#pg271">271</a>, <a href=
+"#pg285">285</a>, <a href="#pg286">286</a>, <a href=
+"#pg288">288</a> f.n.
+
+<ul>
+<li>Division of Egg-nucleus, <a href="#pg188">188</a></li>
+
+<li>Functional Attitude, <a href="#pg260">260</a>, <a href=
+"#pg263">263</a>-4</li>
+
+<li>Formal Attitude, <a href="#pg261">261</a>, <a href=
+"#pg263">263</a></li>
+
+<li>Principle of Connections, <a href="#pg261">261</a></li>
+
+<li>Embryology and Comparative Anatomy, <a href="#pg261">261</a>-2,
+<a href="#pg263">263</a></li>
+
+<li>Biogenetic and Meckel-Serres Laws, <a href=
+"#pg262">262</a></li>
+
+<li>Homology, <a href="#pg261">261</a>, <a href="#pg263">263</a>,
+<a href="#pg265">265</a>, <a href="#pg266">266</a>-7</li>
+
+<li>Adaptation and Correlation, <a href=
+"#pg263">263</a>-4</li>
+
+<li>Archetype as ancestral, <a href="#pg263">263</a> f.n, <a href=
+"#pg265">265</a></li>
+
+<li>On Phylogenetic Speculation, <a href="#pg265">265</a>-6</li>
+
+<li>Embryological Archetype, <a href="#pg266">266</a></li>
+
+<li>Membrane and Cartilage Bones, <a href="#pg309">309</a>, <a
+href="#pg310">310</a></li>
+</ul>
+</li>
+
+<li>Gemmill, J. F., <a href="#pg285">285</a> f.n., <a href=
+"#pg312">312</a> f.n.</li>
+
+<li>Geoffroy, Etienne, St Hilaire, <a href="#pg040">40</a>, <a
+href="#pg052">52</a>-78, <a href="#pg141">141</a>
+<ul>
+<li>Unity of Plan, <a href="#pg052">52</a>-65, <a href=
+"#pg070">70</a> ff., as conservative, <a href="#pg075">75</a>, <a
+href="#pg078">78</a></li>
+
+<li>Principle of Connections, <a href="#pg053">53</a>-4, <a href=
+"#pg062">62</a>-3, <a href="#pg071">71</a>, <a href=
+"#pg074">74</a>, <a href="#pg261">261</a></li>
+
+<li>Unity of Composition, <a href="#pg054">54</a>, <a href=
+"#pg070">70</a>-1, <a href="#pg075">75</a>-6, <a href=
+"#pg200">200</a>, <a href="#pg305">305</a></li>
+
+<li>Archetype, <a href="#pg054">54</a>, <a href=
+"#pg067">67</a></li>
+
+<li>Metastasis, <a href="#pg055">55</a>-6, <a href="#pg059">59</a>,
+<a href="#pg074">74</a></li>
+
+<li>Opercular Bones, <a href="#pg056">56</a></li>
+
+<li>Unity of Composition of Sternum, <a href=
+"#pg057">57</a>-60</li>
+
+<li>Classification, <a href="#pg060">60</a></li>
+
+<li>Vertebrates and Articulates, <a href="#pg060">60</a>-4,
+<a href="#pg274">274</a>, <a href="#pg278">278</a>-9, <a href=
+"#pg303">303</a></li>
+
+<li>Formal Attitude, <a href="#pg062">62</a>-3, <a href=
+"#pg065">65</a>, <a href="#pg071">71</a>, <a href=
+"#pg075">75</a>-8, <a href="#pg305">305</a></li>
+
+<li>Cephalopods and Vertebrates, <a href="#pg064">64</a>-5</li>
+
+<li>Scale of Beings, <a href="#pg064">64</a></li>
+
+<li>Polemic with Cuvier, <a href="#pg064">64</a>-5, <a href=
+"#pg074">74</a>-8</li>
+
+<li>Evolution, <a href="#pg066">66</a>-9, <a href="#pg073">73</a>,
+<a href="#pg228">228</a></li>
+
+<li>Biogenetic Law, <a href="#pg069">69</a></li>
+
+<li>Teratology, <a href="#pg069">69</a>, <a href=
+"#pg315">315</a></li>
+
+<li>Meckel-Serres Law, <a href="#pg070">70</a>, <a href=
+"#pg072">72</a></li>
+
+<li>Criteria of Homology, <a href="#pg071">71</a>, <a href=
+"#pg072">72</a>, <a href="#pg110">110</a></li>
+
+<li>Law of Compensation, <a href="#pg072">72</a>-3</li>
+
+<li>Criticism of his Principles, <a href="#pg074">74</a></li>
+
+<li>Relation to German Transcendentalists, <a href="#pg089">89</a>,
+<a href="#pg100">100</a>-1</li>
+
+<li>Vertebral Theory of Skull, <a href="#pg096">96</a>, <a href=
+"#pg097">97</a></li>
+
+<li>Influence on Darwin, <a href="#pg234">234</a>-5, <a href=
+"#pg238">238</a></li>
+</ul>
+</li>
+
+<li>Geoffroy, Isidore, St Hilaire, <a href="#pg065">65</a> f.n., <a
+href="#pg199">199</a> f.n., <a href="#pg230">230</a></li>
+
+<li>Geometry of the Organism, <a href="#pg033">33</a>
+<ul>
+<li>K. G. Carus, <a href="#pg098">98</a>-100, <a href=
+"#pg249">249</a></li>
+
+<li>Bronn, <a href="#pg201">201</a>, <a href="#pg249">249</a></li>
+
+<li>Haeckel, J. M&uuml;ller, Burmeister, G. J&auml;ger, <a href=
+"#pg249">249</a></li>
+</ul>
+</li>
+
+<li>Germinal Vesicle (Egg-nucleus), <a href="#pg175">175</a>-7, <a
+href="#pg188">188</a>, <a href="#pg291">291</a> f.n.</li>
+
+<li>Germ-Layer Theory&mdash;
+
+<ul>
+<li>Von Baer, <a href="#pg115">115</a>-6, <a href=
+"#pg118">118</a>-9, <a href="#pg208">208</a>-9, <a href=
+"#pg296">296</a></li>
+
+<li>Pander, <a href="#pg119">119</a>-20, <a href=
+"#pg209">209</a></li>
+
+<li>C. F. Wolff, <a href="#pg119">119</a>-20</li>
+
+<li>Rathke, <a href="#pg136">136</a>, <a href="#pg208">208</a></li>
+
+<li>Lereboullet, Bischoff, <a href="#pg208">208</a></li>
+
+<li>Huxley, <a href="#pg208">208</a>, <a href="#pg289">289</a></li>
+
+<li>Remak, <a href="#pg209">209</a>-12, <a href=
+"#pg296">296</a></li>
+</ul>
+</li>
+
+<li><span class="pagenum"><a name="pg374" id=
+"pg374">374</a></span>Germ-Layers and Gastr&aelig;a Theory&mdash;
+
+<ul>
+<li>Haeckel, <a href="#pg289">289</a>-95</li>
+
+<li>Lankester, Balfour, <a href="#pg295">295</a></li>
+</ul>
+</li>
+
+<li>Germ-Layer Theory, Influence of Causal Morphology on, <a href=
+"#pg347">347</a></li>
+
+<li>Gesner, <a href="#pg018">18</a></li>
+
+<li>Giard, A.&mdash;
+
+<ul>
+<li>On Ascidian Theory, <a href="#pg271">271</a>-3</li>
+
+<li>Adaptive Homology, <a href="#pg273">273</a></li>
+
+<li>P&oelig;cilogeny, <a href="#pg347">347</a>-8</li>
+</ul>
+</li>
+
+<li>Glisson, F., <a href="#pg019">19</a></li>
+
+<li>Gluge, <a href="#pg173">173</a></li>
+
+<li>Goebel, K., <a href="#pg356">356</a> f.n.</li>
+
+<li>Goethe, <a href="#pg045">45</a>-51, <a href="#pg065">65</a>, <a
+href="#pg089">89</a>, <a href="#pg250">250</a>
+<ul>
+<li>Unity of Plan, <a href="#pg045">45</a>-7, <a href=
+"#pg051">51</a></li>
+
+<li>Homology, <a href="#pg047">47</a></li>
+
+<li>Principle of Connections, 47</li>
+
+<li>Formal and Functional Attitudes, <a href=
+"#pg048">48</a>-50</li>
+
+<li>Teleology, <a href="#pg048">48</a></li>
+
+<li>Metamorphosis of Plants, <a href="#pg048">48</a></li>
+
+<li>Repetition of parts, <a href="#pg048">48</a>-9</li>
+
+<li>Vertebral Theory of Skull, <a href="#pg049">49</a>, <a href=
+"#pg096">96</a>, <a href="#pg097">97</a></li>
+
+<li>Law of Compensation, <a href="#pg049">49</a></li>
+
+<li>Embryological Criterion, <a href="#pg049">49</a></li>
+
+<li>Organisms as Nature's Works of Art, <a href=
+"#pg050">50</a></li>
+</ul>
+</li>
+
+<li>Goette, <a href="#pg259">259</a></li>
+
+<li>Graaf, von, <a href="#pg175">175</a></li>
+
+<li>Grew, N., <a href="#pg169">169</a></li>
+
+<li>Gruber, <a href="#pg330">330</a></li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Haeckel</span>, Ernst, <a href=
+"#pg247">247</a>-60, <a href="#pg271">271</a>, <a href=
+"#pg314">314</a>, <a href="#pg342">342</a>, <a href=
+"#pg353">353</a>, <a href="#pg357">357</a>
+<ul>
+<li>His sources, <a href="#pg248">248</a>-50</li>
+
+<li>Materialism, <a href="#pg248">248</a>, <a href=
+"#pg250">250</a></li>
+
+<li>On Teleology, Heredity and Adaptation, <a href=
+"#pg248">248</a>, <a href="#pg263">263</a></li>
+
+<li>Correlation, Disregard for, <a href="#pg248">248</a>, <a href=
+"#pg264">264</a></li>
+
+<li>Geometry of the Organism (Promorphology), <a href=
+"#pg249">249</a></li>
+
+<li>Repetition of Parts (Tectology), <a href=
+"#pg249">249</a>-50</li>
+
+<li>Classification as Genealogical, <a href="#pg250">250</a>-1, <a
+href="#pg254">254</a></li>
+
+<li>Archetype as ancestral, <a href="#pg251">251</a></li>
+
+<li>Homology and Analogy, <a href="#pg251">251</a></li>
+
+<li>Biogenetic&nbsp; Law, <a href="#pg251">251</a>, <a href=
+"#pg253">253</a>-9, <a href="#pg291">291</a>-4</li>
+
+<li>Three-fold parallelism, <a href="#pg254">254</a>-5</li>
+
+<li>Scale of Beings, <a href="#pg255">255</a>, <a href=
+"#pg256">256</a>-7</li>
+
+<li>Organism as an Historical Being, <a href="#pg257">257</a></li>
+
+<li>Prussianism, <a href="#pg257">257</a></li>
+
+<li>Palingenesis, <a href="#pg258">258</a></li>
+
+<li>Cenogenesis, <a href="#pg258">258</a>-9</li>
+
+<li>Heterotopy, Heterochrony, <a href="#pg259">259</a></li>
+
+<li>Gastr&aelig;a Theory, <a href="#pg269">269</a>, <a href=
+"#pg288">288</a>-95</li>
+
+<li>Phylogenetic Classification, <a href="#pg289">289</a>-94</li>
+
+<li>Criticism of Theory of Types, Monophyletism, <a href=
+"#pg289">289</a>, <a href="#pg291">291</a></li>
+
+<li>Gastr&aelig;a Theory and Biogenetic Law, <a href=
+"#pg291">291</a>-4</li>
+
+<li>Primary stages of Ontogeny and Phylogeny, <a href=
+"#pg291">291</a>-3</li>
+
+<li>C&oelig;lom, <a href="#pg291">291</a>, <a href=
+"#pg295">295</a>, <a href="#pg296">296</a></li>
+
+<li>Experimental Embryology, <a href="#pg317">317</a></li>
+</ul>
+</li>
+
+<li>Haller, <a href="#pg113">113</a></li>
+
+<li>Harting, <a href="#pg284">284</a> f.n.</li>
+
+<li>Hartmann, E. von&mdash;
+
+<ul>
+<li>On Darwin's conception of correlation, <a href=
+"#pg240">240</a>-1</li>
+
+<li>Evolution, <a href="#pg244">244</a>, <a href=
+"#pg356">356</a></li>
+</ul>
+</li>
+
+<li>Hartog, M., <a href="#pg344">344</a></li>
+
+<li>Harvey, <a href="#pg018">18</a>, <a href="#pg113">113</a></li>
+
+<li>Hatschek, <a href="#pg270">270</a> f.n., <a href=
+"#pg299">299</a></li>
+
+<li>Helmholtz, H. von, <a href="#pg195">195</a></li>
+
+<li>Henle, <a href="#pg172">172</a></li>
+
+<li>Hensen, V., <a href="#pg209">209</a> f.n.</li>
+
+<li>Herbst, C., <a href="#pg333">333</a></li>
+
+<li>Herder, <a href="#pg046">46</a></li>
+
+<li>Heredity and Memory, <a href="#pg336">336</a>-44</li>
+
+<li>Hering, E., <a href="#pg341">341</a>-2</li>
+
+<li>"Heritage" Characters, <a href="#pg309">309</a>, <a href=
+"#pg322">322</a></li>
+
+<li>Herlitzka, <a href="#pg332">332</a></li>
+
+<li>Herophilus, <a href="#pg017">17</a></li>
+
+<li>Hertwig, O., <a href="#pg163">163</a>, <a href=
+"#pg330">330</a>, <a href="#pg331">331</a>, <a href=
+"#pg346">346</a>
+<ul>
+<li>On C. F. Wolff, <a href="#pg119">119</a></li>
+
+<li><span class="pagenum"><a name="pg375" id="pg375">375</a></span>
+Fertilisation, <a href="#pg291">291</a> f.n.</li>
+
+<li>Membrane and Cartilage Bones, <a href="#pg309">309</a>-10</li>
+
+<li>Biogenetic Law, <a href="#pg352">352</a>, <a href=
+"#pg354">354</a>-5</li>
+
+<li>Von Baer's Law, <a href="#pg354">354</a></li>
+
+<li>Intrinsic Laws of Evolution, <a href="#pg354">354</a>-5, <a
+href="#pg356">356</a>-7</li>
+
+<li>Homology not necessarily Homogeny, <a href=
+"#pg355">355</a>-7</li>
+
+<li>Unity of Plan not necessarily due to Community of Descent, <a
+href="#pg355">355</a>-7</li>
+
+<li>On Phylogenetic Speculation, <a href="#pg356">356</a></li>
+</ul>
+</li>
+
+<li>Hertwig, O. and R.&mdash;
+
+<ul>
+<li>C&oelig;lom Theory, <a href="#pg297">297</a>-8</li>
+
+<li>Nervous System of C&oelig;lentera, <a href=
+"#pg299">299</a></li>
+</ul>
+</li>
+
+<li>Heterochrony, <a href="#pg259">259</a>, <a href=
+"#pg348">348</a>, <a href="#pg349">349</a>-52</li>
+
+<li>Heterogeneous Generation (K&ouml;lliker), <a href=
+"#pg243">243</a></li>
+
+<li>Heterotopy, <a href="#pg259">259</a></li>
+
+<li>Hilgendorf, <a href="#pg359">359</a></li>
+
+<li>Hill, <a href="#pg311">311</a></li>
+
+<li>Hippocratic Treatises, <a href="#pg002">2</a></li>
+
+<li>His, W., <a href="#pg206">206</a> f.n., <a href=
+"#pg209">209</a> f.n.
+
+<ul>
+<li>Causal Morphology, <a href="#pg316">316</a></li>
+
+<li>Cytoplasm of Egg, Organ-forming Stuffs, <a href=
+"#pg333">333</a></li>
+
+<li>Specific Distinctness of Embryos, <a href="#pg353">353</a></li>
+</ul>
+</li>
+
+<li>Histological Differentiation (von Baer), <a href=
+"#pg117">117</a>-8</li>
+
+<li>Histology. <i>See also</i> "Cell-Theory"
+
+<ul>
+<li>Malpighi, <a href="#pg020">20</a></li>
+
+<li>Stensen, <a href="#pg021">21</a></li>
+
+<li>Bichat, <a href="#pg029">29</a>-30, <a href="#pg169">169</a>,
+<a href="#pg178">178</a></li>
+
+<li>Von Baer, <a href="#pg117">117</a>-8</li>
+
+<li>Schwann, <a href="#pg178">178</a></li>
+
+<li>Remak, <a href="#pg209">209</a>-12</li>
+</ul>
+</li>
+
+<li>Hofer, B., <a href="#pg330">330</a></li>
+
+<li>Hofmeister, <a href="#pg185">185</a></li>
+
+<li>Homogeny, <a href="#pg267">267</a>, <a href="#pg303">303</a>,
+<a href="#pg355">355</a></li>
+
+<li>Homology, <a href="#pg168">168</a>, <a href="#pg303">303</a>,
+<a href="#pg355">355</a>-7. <i>See also</i> "Connections, Principle
+of," and "Embryological Criterion"
+
+<ul>
+<li>Aristotle, <a href="#pg007">7</a>-10</li>
+
+<li>Belon, <a href="#pg018">18</a></li>
+
+<li>Buffon, <a href="#pg027">27</a></li>
+
+<li>Goethe, <a href="#pg047">47</a></li>
+
+<li>E. Geoffroy, <a href="#pg053">53</a>, <a href=
+"#pg071">71</a></li>
+
+<li>Serres, <a href="#pg080">80</a></li>
+
+<li>Owen, <a href="#pg107">107</a>-9</li>
+
+<li>Lamarck, <a href="#pg227">227</a></li>
+
+<li>C. Darwin, <a href="#pg234">234</a>-5, <a href=
+"#pg247">247</a></li>
+
+<li>Haeckel, <a href="#pg251">251</a></li>
+
+<li>Gegenbaur, <a href="#pg261">261</a>, <a href="#pg263">263</a>,
+<a href="#pg265">265</a>, <a href="#pg266">266</a>-7</li>
+
+<li>Giard, <a href="#pg273">273</a></li>
+
+<li>Semper, <a href="#pg279">279</a></li>
+
+<li>O. Hertwig, <a href="#pg355">355</a>-7</li>
+
+<li>Braun, <a href="#pg355">355</a></li>
+</ul>
+</li>
+
+<li>Homology, Genetic Definition of&mdash;
+
+<ul>
+<li>Gegenbaur, <a href="#pg266">266</a></li>
+
+<li>Lankester, <a href="#pg267">267</a></li>
+
+<li>O. Hertwig's criticism, <a href="#pg355">355</a>-7</li>
+</ul>
+</li>
+
+<li>Homoplasy, <a href="#pg267">267</a></li>
+
+<li>Hooke, R., <a href="#pg020">20</a>, <a href=
+"#pg169">169</a></li>
+
+<li>Houssay, F., <a href="#pg019">19</a> f.n., <a href=
+"#pg333">333</a></li>
+
+<li>Hubrecht, A. A. W., <a href="#pg284">284</a>, <a href=
+"#pg295">295</a> f.n., <a href="#pg301">301</a>, <a href=
+"#pg305">305</a>-6</li>
+
+<li>Hunter, J., <a href="#pg022">22</a>, <a href=
+"#pg315">315</a></li>
+
+<li>Huschke, <a href="#pg134">134</a>-5, <a href="#pg136">136</a>,
+<a href="#pg141">141</a>, <a href="#pg146">146</a></li>
+
+<li>Huxley, T. H., <a href="#pg157">157</a>, <a href=
+"#pg238">238</a>, <a href="#pg247">247</a>
+<ul>
+<li>On Rathke, <a href="#pg154">154</a> f.n.</li>
+
+<li>Embryological Criterion, <a href="#pg158">158</a>-9, <a href=
+"#pg166">166</a></li>
+
+<li>Embryological Archetype, <a href="#pg159">159</a>-61</li>
+
+<li>Criticism of Vertebral Theory of Skull, <a href=
+"#pg161">161</a>-2</li>
+
+<li>Membrane and Cartilage Bones, <a href="#pg166">166</a>-7</li>
+
+<li>On Archetype, <a href="#pg204">204</a></li>
+
+<li>Germ-Layer Theory, <a href="#pg208">208</a>, <a href=
+"#pg289">289</a></li>
+
+<li>Criticism of Three-fold Parallelism, <a href="#pg230">230</a>
+f.n.</li>
+
+<li>C&oelig;lom, <a href="#pg297">297</a></li>
+
+<li>Ancestry of Marsupials, <a href="#pg311">311</a></li>
+</ul>
+</li>
+
+<li>Hyatt, A., <a href="#pg359">359</a>, <a href=
+"#pg361">361</a></li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Instinct</span> and Morphogenesis, Analogy
+of, <a href="#pgvi">vi</a>., <a href="#pg307">307</a>, <a href="#pg312">312</a>
+<ul>
+<li>Lamarck, <a href="#pg220">220</a>, <a href=
+"#pg226">226</a></li>
+</ul>
+</li>
+<li>&nbsp;</li>
+<li><span class="pagenum"><a name="pg376" id=
+"pg376">376</a></span><span class="smcap">Jacobson</span> , <a
+href="#pg164">164</a></li>
+
+<li>J&auml;ger, G., <a href="#pg249">249</a> f.n.</li>
+
+<li><i>Jardin des Plantes</i>, Paris, <a href="#pg019">19</a></li>
+
+<li>Jenkinson, J. W., <a href="#pg347">347</a> f.n.
+
+<ul>
+<li>On His, <a href="#pg316">316</a></li>
+</ul>
+</li>
+
+<li>Jones, Wharton, <a href="#pg138">138</a>, <a href=
+"#pg176">176</a></li>
+
+<li>Julin, C., <a href="#pg271">271</a>, <a href=
+"#pg285">285</a></li>
+
+<li>Jussieu, de, <a href="#pg040">40</a></li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Kant</span>, I.&mdash;
+
+<ul>
+<li>Teleology, <a href="#pg035">35</a>, <a href="#pg213">213</a>,
+<a href="#pg242">242</a></li>
+
+<li>Unity of Plan, <a href="#pg046">46</a>, <a href=
+"#pg213">213</a>-4</li>
+
+<li>Evolution, <a href="#pg213">213</a>-4</li>
+</ul>
+</li>
+
+<li>Keibel, F., <a href="#pg348">348</a>, <a href=
+"#pg349">349</a>-50</li>
+
+<li>Kerkring, <a href="#pg131">131</a></li>
+
+<li>Kielmeyer, <a href="#pg089">89</a>, <a href="#pg090">90</a>, <a
+href="#pg096">96</a></li>
+
+<li>Kieser, <a href="#pg090">90</a></li>
+
+<li>Kleinenberg, N., <a href="#pg277">277</a></li>
+
+<li>Kohlbrugge, J., <a href="#pg044">44</a> f.n., <a href=
+"#pg065">65</a> f.n.</li>
+
+<li>K&ouml;lliker, A.&mdash;
+
+<ul>
+<li>On C. F. Wolff, <a href="#pg119">119</a></li>
+
+<li>Vertebral Theory of Skull, <a href="#pg157">157</a></li>
+
+<li>Membrane and Cartilage Bones, <a href="#pg164">164</a>-6, <a
+href="#pg310">310</a></li>
+
+<li>Embryological Criterion, <a href="#pg165">165</a>-6</li>
+
+<li>Cell-division, <a href="#pg187">187</a></li>
+
+<li>Intrinsic Laws of Evolution, <a href="#pg243">243</a>, <a href=
+"#pg356">356</a></li>
+
+<li>Saltatory Variation, <a href="#pg243">243</a></li>
+</ul>
+</li>
+
+<li>Kowalevsky, A., <a href="#pg269">269</a>-71, <a href=
+"#pg284">284</a>, <a href="#pg285">285</a>, <a href=
+"#pg299">299</a>, <a href="#pg300">300</a>
+<ul>
+<li>Development of Amphioxus, <a href="#pg270">270</a>
+<ul>
+<li>Ascidians, <a href="#pg270">270</a>-1</li>
+</ul></li>
+<li>C&oelig;lom, <a href="#pg270">270</a>, <a href="#pg295">295</a>, <a
+href="#pg297">297</a></li>
+
+<li>Gastrula, <a href="#pg288">288</a></li>
+</ul>
+</li>
+
+<li>Kowalevsky, V. O., <a href="#pg361">361</a>-2</li>
+
+<li>Krause, <a href="#pg176">176</a></li>
+
+<li>Kupffer, <a href="#pg271">271</a></li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Lacaze-Duthiers, H.</span> de, <a href=
+"#pg203">203</a> f.n., <a href="#pg315">315</a>-6
+
+<ul>
+<li>On Ascidians, <a href="#pg271">271</a>, <a href=
+"#pg273">273</a></li>
+</ul>
+</li>
+
+<li>Lamarck, <a href="#pg044">44</a>, <a href="#pg066">66</a>, <a
+href="#pg078">78</a>, <a href="#pg215">215</a>-29
+
+<ul>
+<li>Relation to Buffon, <a href="#pg215">215</a></li>
+
+<li>Scale of Beings, <a href="#pg215">215</a>-8, <a href=
+"#pg220">220</a>-1, <a href="#pg227">227</a>-8</li>
+
+<li>As Evolutionary, <a href="#pg218">218</a>, <a href=
+"#pg220">220</a></li>
+
+<li>Classification, <a href="#pg216">216</a>-7, <a href=
+"#pg227">227</a>, <a href="#pg228">228</a></li>
+
+<li>Species Problem, <a href="#pg216">216</a>, <a href=
+"#pg227">227</a></li>
+
+<li>Materialism, <a href="#pg218">218</a>-9, <a href=
+"#pg222">222</a>-3, <a href="#pg225">225</a>-6</li>
+
+<li>Psychological Vitalism, <a href="#pg219">219</a>, <a href=
+"#pg220">220</a>-6, <a href="#pg307">307</a>, <a href=
+"#pg335">335</a></li>
+
+<li><i>Sentiment int&eacute;rieur</i>, <a href="#pg219">219</a>-20,
+<a href="#pg222">222</a>-3, <a href="#pg225">225</a></li>
+
+<li>Ecological Adaptation, <a href="#pg221">221</a>, <a href=
+"#pg222">222</a>, <a href="#pg223">223</a>, <a href=
+"#pg224">224</a>, <a href="#pg227">227</a></li>
+
+<li>Laws of Evolution, <a href="#pg221">221</a>-5</li>
+
+<li>Transmission of Acquired Characters, <a href=
+"#pg221">221</a>-2, <a href="#pg224">224</a></li>
+
+<li>Subtle Fluids, <a href="#pg222">222</a></li>
+
+<li>Use and Disuse, <a href="#pg223">223</a>-4</li>
+
+<li>Independence of Current Thought, <a href=
+"#pg226">226</a>-7</li>
+
+<li>Homology and Analogy, <a href="#pg227">227</a></li>
+
+<li>Reception of his Theory, <a href="#pg228">228</a>-9</li>
+
+<li>Lamarck and Butler, <a href="#pg335">335</a>-7</li>
+</ul>
+</li>
+
+<li>Lang, A., <a href="#pg301">301</a></li>
+
+<li>Lankester, Sir E. Ray, <a href="#pg247">247</a>
+<ul>
+<li>Homology, Homogeny, Homoplasy, and Analogy, <a href=
+"#pg267">267</a></li>
+
+<li><i>Balanoglossus</i> Theory of Vertebrate Descent, <a href=
+"#pg287">287</a></li>
+
+<li>Germ-Layer Theory and Phylogenetic Classification, <a href=
+"#pg291">291</a></li>
+
+<li>Planula Theory, <a href="#pg295">295</a></li>
+
+<li>On C&oelig;lom Theory, <a href="#pg296">296</a>-7, <a href=
+"#pg299">299</a> f.n.</li>
+</ul>
+</li>
+
+<li>Latreille, <a href="#pg086">86</a>, <a href=
+"#pg100">100</a></li>
+
+<li>Laurencet, <a href="#pg064">64</a></li>
+
+<li>Lavocat, <a href="#pg203">203</a> f.n.</li>
+
+<li>Leeuenhoek, <a href="#pg020">20</a>, <a href="#pg021">21</a>,
+<a href="#pg169">169</a></li>
+
+<li>Leibniz, <a href="#pg023">23</a>, <a href="#pg213">213</a>, <a
+href="#pg343">343</a></li>
+
+<li>Lereboullet&mdash;
+
+<ul>
+<li>Von Baer's Law, <a href="#pg206">206</a>-8</li>
+
+<li>Germ-layer Theory, <a href="#pg208">208</a></li>
+
+<li>Gastrula, <a href="#pg288">288</a> f.n.</li>
+</ul>
+</li>
+
+<li>Leucippus, <a href="#pg016">16</a></li>
+
+<li>Leuckart, <a href="#pg193">193</a> f.n., <a href=
+"#pg194">194</a>, <a href="#pg297">297</a></li>
+
+<li>Levy, O., <a href="#pg333">333</a></li>
+
+<li>Leydig, <a href="#pg187">187</a>, <a href="#pg188">188</a>, <a
+href="#pg275">275</a> f.n., <a href="#pg285">285</a></li>
+
+<li>Linn&aelig;us, <a href="#pg022">22</a></li>
+
+<li>Loeb, J., <a href="#pg333">333</a>, <a href=
+"#pg347">347</a></li>
+
+<li><span class="pagenum"><a name="pg377" id=
+"pg377">377</a></span><i>Loi de Balancement</i>. <i>See</i>
+"Compensation, Law of"</li>
+
+<li>Lov&eacute;n, <a href="#pg186">186</a>, <a href=
+"#pg196">196</a></li>
+
+<li>Lucretius, <a href="#pg016">16</a>
+<ul>
+<li>On the Soul, <a href="#pg222">222</a> f.n.</li>
+</ul>
+</li>
+
+<li>Ludwig, <a href="#pg193">193</a>, <a href="#pg194">194</a>, <a
+href="#pg314">314</a></li>
+
+<li>Lyell, Sir C., <a href="#pg228">228</a> f.n.</li>
+
+<li>Lyonnet, <a href="#pg022">22</a></li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Macbride, E. W.,</span> <a href="#pg287">287</a> f.n.</li>
+
+<li>M'Kendrick, J.&mdash;
+
+<ul>
+<li>On Fontana, <a href="#pg172">172</a></li>
+</ul>
+</li>
+
+<li>Mackenzie, W., <a href="#pg345">345</a></li>
+
+<li>Malpighi, M., <a href="#pg020">20</a>-1, <a href=
+"#pg113">113</a>, <a href="#pg169">169</a></li>
+
+<li>Marine Zoology, Rise of, <a href="#pg195">195</a>-6</li>
+
+<li>Materialistic Attitude, <a href="#pg246">246</a>-7, <a href=
+"#pg345">345</a>, <a href="#pg364">364</a>
+<ul>
+<li>Schwann, <a href="#pg180">180</a>-5</li>
+
+<li>Vogt, <a href="#pg190">190</a>-1</li>
+
+<li>Virchow, <a href="#pg191">191</a></li>
+
+<li>Ludwig, <a href="#pg193">193</a></li>
+
+<li>Materialistic Physiology, <a href="#pg193">193</a>-4, <a href=
+"#pg314">314</a>-5, <a href="#pg347">347</a></li>
+
+<li>Lamarck, <a href="#pg218">218</a>-9, <a href=
+"#pg222">222</a>-3, <a href="#pg225">225</a>-6</li>
+
+<li>The Darwinians, <a href="#pg241">241</a>, <a href=
+"#pg308">308</a></li>
+
+<li>Haeckel, <a href="#pg248">248</a>, <a href=
+"#pg250">250</a></li>
+
+<li>Roux, <a href="#pg315">315</a>, <a href="#pg317">317</a>, <a
+href="#pg318">318</a>-9, <a href="#pg329">329</a></li>
+
+<li>Semon, <a href="#pg343">343</a></li>
+
+<li>Rignano, <a href="#pg344">344</a></li>
+
+<li>Loeb, <a href="#pg347">347</a></li>
+
+<li>Criticism of this attitude&mdash;
+<ul>
+<li>Reichert, <a href="#pg192">192</a>-3</li>
+</ul>
+</li>
+</ul>
+</li>
+
+<li>Meckel, D. A., <a href="#pg095">95</a></li>
+
+<li>Meckel, J. F., <a href="#pg113">113</a>
+<ul>
+<li>Meckel-Serres Law, <a href="#pg091">91</a>-3</li>
+
+<li>Evolution, <a href="#pg092">92</a>-3, <a href="#pg215">215</a>,
+<a href="#pg228">228</a></li>
+
+<li>Teratology, <a href="#pg093">93</a>-4</li>
+
+<li>Repetition of Parts, <a href="#pg095">95</a></li>
+
+<li>Vertebral Theory of Skull, <a href="#pg096">96</a></li>
+
+<li>Eclecticism, <a href="#pg101">101</a></li>
+</ul>
+</li>
+
+<li>Meckel's Cartilage, <a href="#pg141">141</a>, <a href=
+"#pg145">145</a></li>
+
+<li>Meckel-Serres Law. <i>See</i> "Development, Meckel-Serres
+Law"</li>
+
+<li>Mehnert, E., <a href="#pg348">348</a>, <a href=
+"#pg350">350</a>-2</li>
+
+<li>Membrane and Cartilage Bones, <a href="#pg162">162</a>-7, <a
+href="#pg309">309</a>-10</li>
+
+<li>Memory and Heredity, <a href="#pg336">336</a>-44</li>
+
+<li>Mendelism, <a href="#pg346">346</a></li>
+
+<li>Mesenchyme, <a href="#pg298">298</a></li>
+
+<li>Mesoderm, <a href="#pg209">209</a>-11, <a href=
+"#pg296">296</a>, <a href="#pg297">297</a>, <a href=
+"#pg298">298</a></li>
+
+<li>Metabolism&mdash;
+
+<ul>
+<li>Cuvier, <a href="#pg038">38</a></li>
+
+<li>Schwann, <a href="#pg182">182</a>-5</li>
+
+<li>Roux, <a href="#pg324">324</a>, <a href="#pg329">329</a></li>
+</ul>
+</li>
+
+<li>Metamerism, <a href="#pg094">94</a>, <a href="#pg095">95</a>,
+<a href="#pg100">100</a>, <a href="#pg109">109</a>, <a href=
+"#pg131">131</a>-2, <a href="#pg266">266</a>-7, <a href=
+"#pg274">274</a>-5, <a href="#pg279">279</a>, <a href=
+"#pg282">282</a>, <a href="#pg286">286</a>, <a href=
+"#pg299">299</a>, <a href="#pg301">301</a></li>
+
+<li>Metamorphosis of Plants, <a href="#pg048">48</a>, <a href=
+"#pg235">235</a></li>
+
+<li>Metastasis, Principle of&mdash;
+
+<ul>
+<li>E. Geoffroy, <a href="#pg055">55</a>-6, <a href=
+"#pg059">59</a>, <a href="#pg074">74</a></li>
+
+<li>Owen, <a href="#pg106">106</a></li>
+</ul>
+</li>
+
+<li>Metschnikoff, E., <a href="#pg278">278</a> f.n., <a href=
+"#pg285">285</a>, <a href="#pg288">288</a>
+<ul>
+<li>Criticism of Ascidian Theory, <a href="#pg271">271</a></li>
+
+<li>C&oelig;lom, <a href="#pg295">295</a>, <a href="#pg296">296</a>, <a
+href="#pg297">297</a></li>
+</ul>
+</li>
+
+<li>Meyen, <a href="#pg170">170</a>, <a href="#pg185">185</a></li>
+
+<li>Meyer, E., <a href="#pg284">284</a></li>
+
+<li>Meyranx, <a href="#pg064">64</a></li>
+
+<li>Microscope, Invention of, <a href="#pg019">19</a></li>
+
+<li>Milne-Edwards, H., <a href="#pg012">12</a>, <a href=
+"#pg086">86</a>, <a href="#pg238">238</a>
+<ul>
+<li>Marine Zoology, <a href="#pg195">195</a></li>
+
+<li>Functional Attitude, <a href="#pg195">195</a>, <a href=
+"#pg197">197</a>-200</li>
+
+<li>Unity of Plan, <a href="#pg197">197</a></li>
+
+<li>Division of Labour, <a href="#pg197">197</a>-8</li>
+
+<li>Ecological Adaptation, Convergence, <a href=
+"#pg199">199</a></li>
+
+<li>Von Baer's Law, Polemic with Serres, <a href=
+"#pg204">204</a>-8</li>
+
+<li>Evolution, <a href="#pg244">244</a>-5</li>
+</ul>
+</li>
+
+<li>Mirbel, <a href="#pg170">170</a>, <a href="#pg171">171</a></li>
+
+<li>Mivart, St G., <a href="#pg277">277</a></li>
+
+<li>Mohl, von, <a href="#pg170">170</a>, <a href=
+"#pg185">185</a></li>
+
+<li>Moldenhawer, <a href="#pg170">170</a></li>
+
+<li>Moleschott, <a href="#pg194">194</a></li>
+
+<li>Moquin-Tandon, A., <a href="#pg087">87</a></li>
+
+<li>Morgan, T. H., <a href="#pg317">317</a> f.n., <a href=
+"#pg332">332</a>, <a href="#pg333">333</a>, <a href=
+"#pg347">347</a> f.n.</li>
+
+<li>Mosaic Theory of Development, <a href="#pg330">330</a>-3</li>
+
+<li>M&uuml;ller, F., Biogenetic Law, <a href="#pg252">252</a>-3, <a
+href="#pg254">254</a>, <a href="#pg257">257</a></li>
+
+<li>M&uuml;ller, H., <a href="#pg166">166</a></li>
+
+<li><span class="pagenum"><a name="pg378" id=
+"pg378">378</a></span>M&uuml;ller, J., <a href="#pg136">136</a>, <a
+href="#pg209">209</a> f.n., <a href="#pg260">260</a>, <a href=
+"#pg285">285</a>, <a href="#pg309">309</a>, <a href=
+"#pg345">345</a>
+<ul>
+<li>Embryological Criterion, <a href="#pg138">138</a></li>
+
+<li>Vertebral Theory of Skull, <a href="#pg142">142</a>-4, <a href=
+"#pg154">154</a>, <a href="#pg157">157</a></li>
+
+<li>On Reichert, <a href="#pg150">150</a></li>
+
+<li>Cell Theory, <a href="#pg172">172</a>-3</li>
+
+<li>Division of Egg-nucleus, <a href="#pg188">188</a></li>
+
+<li>Vitalism, <a href="#pg192">192</a></li>
+
+<li>Marine Zoology, <a href="#pg196">196</a></li>
+
+<li>Functional Attitude, <a href="#pg200">200</a></li>
+</ul>
+</li>
+
+<li>Mutations (Waagen), <a href="#pg361">361</a> f.n.</li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Naegeli</span>, <a href="#pg185">185</a>,
+<a href="#pg243">243</a> f.n., <a href="#pg356">356</a></li>
+
+<li><i>Naturphilosophie. See</i> "Philosophy of Nature"</li>
+
+<li>Nesbitt, R., <a href="#pg162">162</a></li>
+
+<li>Neumayr, <a href="#pg357">357</a>, <a href=
+"#pg360">360</a></li>
+
+<li>Nussbaum, M., <a href="#pg330">330</a></li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Oken</span>, L., <a href="#pg089">89</a>,
+<a href="#pg113">113</a>, <a href="#pg131">131</a>, <a href=
+"#pg134">134</a>, <a href="#pg149">149</a>
+<ul>
+<li>Meckel-Serres Law, <a href="#pg090">90</a>-1</li>
+
+<li>Teratology, <a href="#pg091">91</a></li>
+
+<li>Repetition of Parts, <a href="#pg094">94</a>-5</li>
+
+<li>Serial Homology, <a href="#pg095">95</a>-6, <a href=
+"#pg100">100</a></li>
+
+<li>Vertebral Theory, <a href="#pg096">96</a>, <a href=
+"#pg097">97</a>, <a href="#pg098">98</a></li>
+
+<li>On Geoffroy, <a href="#pg100">100</a></li>
+
+<li>Influence on Serres, <a href="#pg205">205</a></li>
+
+<li>Evolution, <a href="#pg215">215</a></li>
+</ul>
+</li>
+
+<li>Ollier, <a href="#pg315">315</a></li>
+
+<li>Oppel, A., <a href="#pg318">318</a> f.n., <a href=
+"#pg324">324</a> f.n., <a href="#pg327">327</a>, <a href=
+"#pg348">348</a>-9</li>
+
+<li>Orr, H. F., <a href="#pg342">342</a></li>
+
+<li>Osborn, H. F., <a href="#pg214">214</a> f.n., <a href=
+"#pg361">361</a>
+<ul>
+<li>On V. O. Kowalevsky, <a href="#pg362">362</a></li>
+
+<li>Functional Attitude, <a href="#pg362">362</a>-4</li>
+
+<li>Law of Adaptive Radiation, <a href="#pg362">362</a>-4</li>
+</ul>
+</li>
+
+<li>Owen, R., <a href="#pg097">97</a>, <a href="#pg102">102</a>-12,
+<a href="#pg204">204</a>
+<ul>
+<li>Eclecticism, <a href="#pg102">102</a></li>
+
+<li>Vertebral Theory of Skeleton, <a href="#pg103">103</a>-7</li>
+
+<li>Archetype of Vertebrate Skeleton, <a href="#pg104">104</a>-7,
+<a href="#pg110">110</a></li>
+
+<li>Vertebral Theory of Skull, <a href="#pg104">104</a>-6</li>
+
+<li>Metastasis, <a href="#pg106">106</a></li>
+
+<li>Principle of Connections, <a href="#pg107">107</a>-8</li>
+
+<li>Anatomy and Embryology, <a href="#pg108">108</a></li>
+
+<li>Homology and Analogy, <a href="#pg108">108</a></li>
+
+<li>Classes of Homology, <a href="#pg108">108</a>-9, <a href=
+"#pg266">266</a></li>
+
+<li>Functional Attitude, <a href="#pg109">109</a>, <a href=
+"#pg110">110</a>, <a href="#pg111">111</a></li>
+
+<li>Embryological Criterion, <a href="#pg110">110</a>, <a href=
+"#pg167">167</a></li>
+
+<li>Homological and Teleological Compoundedness, <a href=
+"#pg110">110</a>-1</li>
+
+<li>Vegetative Repetition of Parts, <a href="#pg111">111</a>, <a
+href="#pg286">286</a></li>
+
+<li>Unity of Plan as Conservative Principle, <a href=
+"#pg112">112</a></li>
+
+<li>Influence on Darwin, <a href="#pg234">234</a>, <a href=
+"#pg235">235</a>, <a href="#pg238">238</a></li>
+
+<li>Evolution, <a href="#pg244">244</a></li>
+</ul>
+</li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Packard</span>, <a href=
+"#pg361">361</a></li>
+
+<li>Pal&aelig;ontological Record, <a href="#pg357">357</a>-61
+
+<ul>
+<li>Absence of connecting forms, <a href="#pg357">357</a>-9</li>
+
+<li>Biogenetic Law, <a href="#pg359">359</a></li>
+
+<li>Phyletic Series, <a href="#pg359">359</a>-61</li>
+</ul>
+</li>
+
+<li>Pal&aelig;ontological Succession&mdash;
+
+<ul>
+<li>Cuvier, <a href="#pg043">43</a></li>
+
+<li>E. Geoffroy, <a href="#pg067">67</a></li>
+
+<li>L. Agassiz, <a href="#pg230">230</a>, <a href=
+"#pg255">255</a></li>
+
+<li>C. Darwin, <a href="#pg231">231</a></li>
+
+<li>Milne-Edwards, <a href="#pg245">245</a></li>
+
+<li>Tiedemann, <a href="#pg255">255</a> f.n.</li>
+</ul>
+</li>
+
+<li>Paley, W., <a href="#pg341">341</a></li>
+
+<li>Palingenesis (Haeckel), <a href="#pg258">258</a>, <a href=
+"#pg323">323</a></li>
+
+<li>Pander, <a href="#pg113">113</a>, <a href="#pg119">119</a>-20,
+<a href="#pg133">133</a>, <a href="#pg208">208</a>, <a href=
+"#pg209">209</a></li>
+
+<li>Parallelism, Theory of. <i>See</i> "Development, Meckel-Serres
+Law"
+
+<ul>
+<li>Three-fold. <i>See</i> "Development, Meckel-Serres Law"</li>
+</ul>
+</li>
+
+<li>Paris Museum of Natural History, <a href="#pg019">19</a>, <a
+href="#pg089">89</a>, <a href="#pg101">101</a></li>
+
+<li>Paul, <a href="#pg360">360</a></li>
+
+<li>Pauly, A., <a href="#pg345">345</a></li>
+
+<li>Perrault, C., <a href="#pg019">19</a></li>
+
+<li>Perrier, E., <a href="#pg088">88</a>, <a href="#pg359">359</a>
+f.n.</li>
+
+<li>Pfl&uuml;ger, E., <a href="#pg317">317</a>, <a href=
+"#pg330">330</a></li>
+
+<li>Philipeaux, <a href="#pg315">315</a></li>
+
+<li><span class="pagenum"><a name="pg379" id=
+"pg379">379</a></span>"Philosophy of Nature," <a href=
+"#pg089">89</a>, <a href="#pg094">94</a>, <a href="#pg098">98</a>,
+<a href="#pg203">203</a>, <a href="#pg248">248</a></li>
+
+<li>Phyletic Series, <a href="#pg359">359</a>-61</li>
+
+<li>Physiology, Separation from Morphology, <a href=
+"#pg194">194</a>, <a href="#pg247">247</a>, <a href=
+"#pg260">260</a>, <a href="#pg314">314</a></li>
+
+<li>Physiology of Development, <a href="#pg315">315</a></li>
+
+<li>Planula Theory (Lankester), <a href="#pg295">295</a></li>
+
+<li>Plato, <a href="#pg015">15</a></li>
+
+<li>Pockels, <a href="#pg138">138</a></li>
+
+<li>P&oelig;cilogeny (Giard), <a href="#pg347">347</a>-8</li>
+
+<li>Poli, <a href="#pg175">175</a></li>
+
+<li>Polyphyletism&mdash;
+
+<ul>
+<li>Darwin, <a href="#pg238">238</a></li>
+
+<li>Von Baer, <a href="#pg242">242</a>, <a href=
+"#pg356">356</a></li>
+
+<li>K&ouml;lliker, Wigand, Naegeli, <a href="#pg356">356</a></li>
+
+<li>Dep&eacute;ret, <a href="#pg360">360</a>-1</li>
+
+<li>Steinmann, <a href="#pg360">360</a> f.n.</li>
+</ul>
+</li>
+
+<li>Polyzoic Conception of Organism&mdash;
+
+<ul>
+<li>Dug&egrave;s, <a href="#pg087">87</a></li>
+
+<li>Perrier, <a href="#pg088">88</a></li>
+</ul>
+</li>
+
+<li>Pr&eacute;vost and Dumas, <a href="#pg125">125</a> f.n., <a
+href="#pg134">134</a>, <a href="#pg175">175</a>, <a href=
+"#pg186">186</a></li>
+
+<li>Promorphology (Haeckel), <a href="#pg249">249</a></li>
+
+<li>Protoplasm, <a href="#pg169">169</a>, <a href=
+"#pg188">188</a>-9</li>
+
+<li>Purkinje, <a href="#pg172">172</a>, <a href="#pg173">173</a>,
+<a href="#pg175">175</a>, <a href="#pg176">176</a>, <a href=
+"#pg189">189</a></li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Quatrefages</span>, A. de, <a href=
+"#pg172">172</a>, <a href="#pg195">195</a>-6</li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">R&aacute;dl</span>, E., on Goethe, <a href=
+"#pg048">48</a>
+<ul>
+<li>Correlation, <a href="#pg240">240</a> f.n., <a href=
+"#pg241">241</a></li>
+
+<li>On Darwin's Critics, <a href="#pg242">242</a> f.n.</li>
+
+<li>On Cuvier's Critics, <a href="#pg278">278</a> f.n.</li>
+</ul>
+</li>
+
+<li>Rathke, H., <a href="#pg133">133</a>, <a href=
+"#pg136">136</a>-7, <a href="#pg174">174</a>, <a href=
+"#pg194">194</a>, <a href="#pg269">269</a>, <a href=
+"#pg351">351</a> f.n.
+
+<ul>
+<li>Discovery of Gill-slits in Pig and Chick, <a href=
+"#pg134">134</a></li>
+
+<li>Discovery of Gill-slits in Man, <a href="#pg135">135</a></li>
+
+<li>Germ-Layer Theory, <a href="#pg136">136</a>, <a href=
+"#pg208">208</a></li>
+
+<li>Embryological Criterion, <a href="#pg138">138</a>, <a href=
+"#pg140">140</a>-1</li>
+
+<li>Homologies of Gill-arches, <a href="#pg139">139</a>-41, <a
+href="#pg146">146</a>, <a href="#pg150">150</a></li>
+
+<li>Development of Skull, <a href="#pg141">141</a>, <a href=
+"#pg150">150</a>-4</li>
+
+<li>Vertebral Theory of Skull, <a href="#pg141">141</a>, <a href=
+"#pg154">154</a>-6</li>
+
+<li>Embryological Archetype, <a href="#pg151">151</a>, <a href=
+"#pg153">153</a></li>
+
+<li>Membrane and Cartilage Bones, <a href="#pg163">163</a>, <a
+href="#pg166">166</a></li>
+</ul>
+</li>
+
+<li>Rauber, A., <a href="#pg330">330</a></li>
+
+<li>R&eacute;aumur, <a href="#pg022">22</a>, <a href=
+"#pg315">315</a></li>
+
+<li>Recapitulation Theory. <i>See</i> "Development, Biogenetic
+Law"</li>
+
+<li>Regeneration, <a href="#pg315">315</a>, <a href=
+"#pg318">318</a>, <a href="#pg333">333</a>, <a href=
+"#pg346">346</a></li>
+
+<li>Regulatory Processes in Development, <a href="#pg114">114</a>,
+<a href="#pg319">319</a>, <a href="#pg333">333</a>, <a href=
+"#pg346">346</a>-7, <a href="#pg350">350</a></li>
+
+<li>Reichert, C. B., Embryological Criterion, <a href=
+"#pg138">138</a>-9, <a href="#pg144">144</a>-7, <a href=
+"#pg163">163</a> Archetype, <a href="#pg139">139</a>, <a href=
+"#pg147">147</a>, <a href="#pg149">149</a>
+<ul>
+<li>Homologies of Gill-arches and Ear-ossicles, <a href=
+"#pg144">144</a>-7</li>
+
+<li>Vertebral Theory of Skull, <a href="#pg147">147</a>-9, <a href=
+"#pg157">157</a></li>
+
+<li>Von Baer's Law, <a href="#pg149">149</a>-50, <a href=
+"#pg351">351</a> f.n.</li>
+
+<li>Membrane and Cartilage Bones, <a href="#pg163">163</a>, <a
+href="#pg165">165</a>, <a href="#pg166">166</a>, <a href=
+"#pg310">310</a></li>
+
+<li>Criticism of "Biological Atomists," <a href="#pg192">192</a>-3,
+<a href="#pg194">194</a></li>
+
+<li>Functional Attitude, <a href="#pg193">193</a>, <a href=
+"#pg200">200</a></li>
+</ul>
+</li>
+
+<li>Remak, R., <a href="#pg118">118</a>, <a href="#pg288">288</a>
+f.n.
+
+<ul>
+<li>On Vertebr&aelig;, <a href="#pg157">157</a></li>
+
+<li>Cell Theory, <a href="#pg173">173</a>, <a href=
+"#pg187">187</a>-8, <a href="#pg209">209</a></li>
+
+<li>Microscopical Technique, <a href="#pg209">209</a> f.n.</li>
+
+<li>Germ-Layer Theory, <a href="#pg209">209</a>-12, <a href=
+"#pg296">296</a></li>
+
+<li>Cells, Tissues and Germ-Layers, <a href=
+"#pg209">209</a>-12</li>
+
+<li>Mesoderm, <a href="#pg209">209</a>-11</li>
+
+<li>C&oelig;lom, <a href="#pg211">211</a>, <a href=
+"#pg296">296</a></li>
+</ul>
+</li>
+
+<li>Repetition of Parts within the Organism, Theory of. <i>See
+also</i> "Vertebral Theory of Skull"
+
+<ul>
+<li>Goethe, <a href="#pg048">48</a>-9</li>
+
+<li>Dug&egrave;s, <a href="#pg087">87</a>-8</li>
+
+<li>Oken, <a href="#pg094">94</a>-5</li>
+
+<li>J. F. Meckel, D. A. Meckel, <a href="#pg095">95</a></li>
+
+<li>Haeckel (Tectology), <a href="#pg249">249</a>-50</li>
+</ul>
+</li>
+
+<li>Reymond, E. du Bois, <a href="#pg194">194</a>, <a href=
+"#pg314">314</a></li>
+
+<li>Rignano, E., <a href="#pg343">343</a>-4</li>
+
+<li>Robinet, <a href="#pg023">23</a>, <a href="#pg215">215</a></li>
+
+<li>Rondeletius, <a href="#pg018">18</a></li>
+
+<li>Rosenhof, R&ouml;sel von, <a href="#pg022">22</a></li>
+
+<li><span class="pagenum"><a name="pg380" id=
+"pg380">380</a></span>Roux, W., <a href="#pg313">313</a>, <a href=
+"#pg315">315</a>-29, <a href="#pg344">344</a>, <a href=
+"#pg351">351</a>
+<ul>
+<li><i>Entwicklungsmechanik</i>, <a href="#pg315">315</a>, <a href=
+"#pg317">317</a>-8</li>
+
+<li>Materialistic Attitude, <a href="#pg315">315</a>, <a href=
+"#pg317">317</a>, <a href="#pg318">318</a>-9, <a href=
+"#pg329">329</a></li>
+
+<li>Functional Adaptation, <a href="#pg316">316</a>-7, <a href=
+"#pg318">318</a>, <a href="#pg320">320</a>-9, <a href=
+"#pg333">333</a></li>
+
+<li>Experimental Embryology, <a href="#pg317">317</a>, <a href=
+"#pg318">318</a>, <a href="#pg330">330</a>-1</li>
+
+<li>Simple and Complex Components, <a href="#pg318">318</a>-20</li>
+
+<li>Functional Definition of Life, <a href="#pg320">320</a></li>
+
+<li>Functional Attitude, <a href="#pg320">320</a>-9, <a href=
+"#pg335">335</a></li>
+
+<li>The Two Periods of Development, <a href="#pg320">320</a>-4, <a
+href="#pg325">325</a>, <a href="#pg327">327</a>, <a href=
+"#pg335">335</a></li>
+
+<li>Mosaic Theory of Development, <a href="#pg323">323</a>, <a
+href="#pg330">330</a>-1</li>
+
+<li>Metabolism, <a href="#pg324">324</a>, <a href=
+"#pg329">329</a></li>
+
+<li>Structure, Functional and Non-functional, <a href=
+"#pg324">324</a>-6</li>
+
+<li>Functional Unity of Organism, <a href="#pg326">326</a></li>
+
+<li>Functional Adaptation of Blood-vessels, <a href=
+"#pg326">326</a>-9</li>
+
+<li>Form as manifestation of Activity, <a href=
+"#pg329">329</a></li>
+</ul>
+</li>
+
+<li>Ruini, C., <a href="#pg018">18</a></li>
+
+<li>Rusconi, <a href="#pg133">133</a>-4, <a href=
+"#pg186">186</a></li>
+
+<li>R&uuml;timeyer, L., <a href="#pg361">361</a></li>
+
+<li>Ryder, <a href="#pg361">361</a></li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Sachs</span>, J. von, <a href=
+"#pg170">170</a></li>
+
+<li>St Ange, M., <a href="#pg146">146</a></li>
+
+<li>Salensky, <a href="#pg259">259</a></li>
+
+<li>Saltatory Variation&mdash;
+
+<ul>
+<li>E. Geoffroy, <a href="#pg078">78</a></li>
+
+<li>Von Baer, <a href="#pg242">242</a></li>
+
+<li>K&ouml;lliker, <a href="#pg243">243</a></li>
+
+<li>Owen, <a href="#pg244">244</a></li>
+</ul>
+</li>
+
+<li>Sarcode, <a href="#pg169">169</a></li>
+
+<li>Sars, M., <a href="#pg186">186</a>, <a href=
+"#pg196">196</a></li>
+
+<li>Savigny, J. C., <a href="#pg083">83</a>-5, <a href=
+"#pg100">100</a>, <a href="#pg137">137</a>, <a href=
+"#pg271">271</a></li>
+
+<li>Scale of Beings, <a href="#pg089">89</a>, <a href=
+"#pg206">206</a>, <a href="#pg214">214</a>-5
+
+<ul>
+<li>Aristotle, <a href="#pg014">14</a>-6</li>
+
+<li>Anaximander, Anaxagoras, <a href="#pg014">14</a></li>
+
+<li>Empedocles, Plato, <a href="#pg015">15</a></li>
+
+<li>Albertus Magnus, <a href="#pg017">17</a></li>
+
+<li>C. Bonnet, <a href="#pg022">22</a>-3</li>
+
+<li>Robinet, <a href="#pg023">23</a></li>
+
+<li>Buffon, <a href="#pg024">24</a></li>
+
+<li>E. Geoffroy, <a href="#pg064">64</a></li>
+
+<li>Lamarck, <a href="#pg215">215</a>-8, <a href=
+"#pg220">220</a>-1, <a href="#pg227">227</a>-8</li>
+
+<li>As Evolutionary, <a href="#pg218">218</a>, <a href=
+"#pg220">220</a></li>
+
+<li>Haeckel, <a href="#pg256">256</a>-7</li>
+
+<li>Criticism of this idea&mdash;
+<ul>
+<li>Cuvier, <a href="#pg039">39</a>-40, <a href=
+"#pg130">130</a></li>
+
+<li>Von Baer, <a href="#pg130">130</a></li>
+
+<li>Milne-Edwards, <a href="#pg205">205</a></li>
+
+<li>Lereboullet, <a href="#pg207">207</a></li>
+
+<li>Darwin, <a href="#pg234">234</a></li>
+
+<li>Haeckel, <a href="#pg255">255</a></li>
+
+<li>Relation to Evolution-Theory, <a href="#pg214">214</a>-5</li>
+</ul>
+</li>
+</ul>
+</li>
+
+<li>Schepelmann, <a href="#pg333">333</a></li>
+
+<li>Schleiden, <a href="#pg170">170</a>-2</li>
+
+<li>Schmieden, <a href="#pg328">328</a></li>
+
+<li>Schults, C. H., <a href="#pg173">173</a></li>
+
+<li>Schultze, Max, <a href="#pg189">189</a></li>
+
+<li>Schultze, O., <a href="#pg331">331</a></li>
+
+<li>Schulz, E., <a href="#pg347">347</a> f.n.</li>
+
+<li>Schwann, Theodor, <a href="#pg169">169</a>, <a href=
+"#pg173">173</a>-86, <a href="#pg248">248</a>
+<ul>
+<li>Physiological Standpoint, <a href="#pg173">173</a>, <a href=
+"#pg179">179</a>, <a href="#pg180">180</a>, <a href=
+"#pg182">182</a></li>
+
+<li>Development of Cells, <a href="#pg174">174</a>-5, <a href=
+"#pg179">179</a>-80</li>
+
+<li>Cellular Nature of Ovum, <a href="#pg175">175</a>-7</li>
+
+<li>Development of Tissues from Cells, <a href=
+"#pg177">177</a>-8</li>
+
+<li>Histology, <a href="#pg178">178</a></li>
+
+<li>Materialism and Teleology, <a href="#pg180">180</a>-3, <a href=
+"#pg185">185</a></li>
+
+<li>Cell-metabolism, <a href="#pg182">182</a>-5</li>
+
+<li>Cells as organic Crystals, <a href="#pg184">184</a>-5</li>
+</ul>
+</li>
+
+<li>Sedgwick, A., <a href="#pg347">347</a> f.n.
+
+<ul>
+<li>Actinozoan Theory of Vertebrate Descent, <a href=
+"#pg299">299</a>-300</li>
+
+<li>Metamerism, <a href="#pg299">299</a></li>
+
+<li>Embryological Archetype, <a href="#pg300">300</a></li>
+
+<li>Organism as Historical Being, <a href="#pg308">308</a></li>
+
+<li>Cell-Theory, <a href="#pg346">346</a></li>
+
+<li>Von Baer's Law, <a href="#pg353">353</a></li>
+</ul>
+</li>
+
+<li><span class="pagenum"><a name="pg381" id=
+"pg381">381</a></span>Segmentation of Ovum, <a href=
+"#pg186">186</a>-8</li>
+
+<li>Seiler, <a href="#pg138">138</a></li>
+
+<li>Selection, Natural and Artificial, <a href="#pg307">307</a>
+f.n.</li>
+
+<li>Self-Differentiation (Roux), <a href="#pg319">319</a>, <a href=
+"#pg320">320</a>-1, <a href="#pg322">322</a>, <a href=
+"#pg323">323</a>, <a href="#pg324">324</a>, <a href=
+"#pg327">327</a></li>
+
+<li>Self-Regulation (Roux), <a href="#pg319">319</a></li>
+
+<li>Semon, R., <a href="#pg342">342</a>-3</li>
+
+<li>Semper, C., <a href="#pg259">259</a>, <a href="#pg269">269</a>,
+<a href="#pg278">278</a>-82, <a href="#pg284">284</a>, <a href=
+"#pg286">286</a>
+<ul>
+<li>Annelid Theory, <a href="#pg274">274</a>, <a href=
+"#pg278">278</a>-82</li>
+
+<li>Metamerism, <a href="#pg274">274</a>, <a href="#pg279">279</a>,
+<a href="#pg282">282</a></li>
+
+<li>Follower of Geoffroy, <a href="#pg278">278</a></li>
+
+<li>Unity of Plan and Composition, <a href="#pg279">279</a>, <a
+href="#pg303">303</a></li>
+
+<li>Principle of Connections, <a href="#pg279">279</a></li>
+
+<li>Formal Attitude, <a href="#pg279">279</a></li>
+</ul>
+</li>
+
+<li><i>Sentiment int&eacute;rieur</i> (Lamarck), <a href=
+"#pg219">219</a>-20, <a href="#pg222">222</a>-3, <a href=
+"#pg225">225</a></li>
+
+<li>Serial Homology. <i>See</i> "Metamerism"</li>
+
+<li>Serres, E., <a href="#pg079">79</a>-83, <a href=
+"#pg091">91</a>, <a href="#pg100">100</a>, <a href=
+"#pg205">205</a>-6, <a href="#pg257">257</a> f.n.
+
+<ul>
+<li>Criteria of Homology, <a href="#pg080">80</a></li>
+
+<li>Law of parallelism, <a href="#pg080">80</a>-3, <a href=
+"#pg094">94</a>, <a href="#pg203">203</a>-4, <a href=
+"#pg205">205</a>-6</li>
+
+<li>Law of Multiple Formation, <a href="#pg080">80</a>-1</li>
+
+<li>Unity of Plan, <a href="#pg083">83</a>, <a href=
+"#pg205">205</a>, <a href="#pg206">206</a></li>
+
+<li>Teratology, <a href="#pg083">83</a></li>
+
+<li>Meckel's Cartilage, <a href="#pg145">145</a> f.n.</li>
+
+<li>Transcendentalism, <a href="#pg205">205</a>-6</li>
+
+<li>Concrescence Theory, <a href="#pg206">206</a> f.n.</li>
+</ul>
+</li>
+
+<li>Severino, <a href="#pg018">18</a></li>
+
+<li>Sharpey, <a href="#pg162">162</a>, <a href=
+"#pg176">176</a></li>
+
+<li>Siebold, von, <a href="#pg186">186</a></li>
+
+<li>Skull, Development of, <a href="#pg139">139</a>-62. <i>See
+also</i> "Vertebral Theory"</li>
+
+<li>Spallanzani, <a href="#pg315">315</a></li>
+
+<li>Species-Problem&mdash;
+
+<ul>
+<li>Cuvier, <a href="#pg042">42</a></li>
+
+<li>Lamarck, <a href="#pg216">216</a>, <a href=
+"#pg227">227</a></li>
+
+<li>Darwin, <a href="#pg231">231</a></li>
+</ul>
+</li>
+
+<li>Spencer, H., <a href="#pg326">326</a> f.n.</li>
+
+<li>Spengel, <a href="#pg285">285</a>, <a href=
+"#pg287">287</a></li>
+
+<li>Spinoza, <a href="#pg343">343</a></li>
+
+<li>Spix, <a href="#pg096">96</a>, <a href="#pg097">97</a>, <a
+href="#pg100">100</a>, <a href="#pg141">141</a></li>
+
+<li>Stannius, <a href="#pg165">165</a></li>
+
+<li>Steenstrup, <a href="#pg309">309</a></li>
+
+<li>Steinmann, G., <a href="#pg357">357</a>, <a href=
+"#pg360">360</a> f.n.</li>
+
+<li>Stensen (Steno), <a href="#pg021">21</a></li>
+
+<li>Swammerdam, <a href="#pg020">20</a>, <a href=
+"#pg021">21</a>-2</li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Tachygenesis</span>, <a href=
+"#pg359">359</a></li>
+
+<li>Technique, Microscopical, <a href="#pg209">209</a> f.n., <a
+href="#pg268">268</a></li>
+
+<li>Tectology (Haeckel), <a href="#pg249">249</a></li>
+
+<li>Teleology&mdash;
+
+<ul>
+<li>Aristotle, <a href="#pg010">10</a></li>
+
+<li>Cuvier, <a href="#pg033">33</a>-5</li>
+
+<li>Kant, <a href="#pg035">35</a>, <a href="#pg213">213</a>, <a
+href="#pg242">242</a></li>
+
+<li>Von Baer, <a href="#pg242">242</a></li>
+
+<li>Owen, Von Hartmann, <a href="#pg244">244</a></li>
+
+<li>Butler, <a href="#pg341">341</a></li>
+
+<li>G. Wolff, Driesch, <a href="#pg346">346</a></li>
+
+<li>Criticism of&mdash;
+<ul>
+<li>Goethe, <a href="#pg048">48</a></li>
+
+<li>Schwann, <a href="#pg180">180</a>-2</li>
+
+<li>The Darwinians, <a href="#pg241">241</a></li>
+
+<li>Haeckel, <a href="#pg248">248</a></li>
+
+<li>Evolutionary Morphologists, <a href="#pg308">308</a></li>
+</ul>
+</li>
+</ul>
+</li>
+
+<li>Teratology, <a href="#pg069">69</a>, <a href="#pg083">83</a>,
+<a href="#pg091">91</a>, <a href="#pg093">93</a>, <a href=
+"#pg315">315</a></li>
+
+<li>Thienemann, <a href="#pg023">23</a> f.n.</li>
+
+<li>Thompson, D'Arcy W., <a href="#pg002">2</a> f.n.</li>
+
+<li>Thomson, A., <a href="#pg176">176</a></li>
+
+<li>Thomson, J. Arthur, <a href="#pg215">215</a> f.n.</li>
+
+<li>Tiedemann, <a href="#pg091">91</a>, <a href="#pg113">113</a>,
+<a href="#pg215">215</a>, <a href="#pg255">255</a> f.n.</li>
+
+<li>Tissues and Germ-Layers, <a href="#pg118">118</a>, <a href=
+"#pg209">209</a>-12</li>
+
+<li>Transcendental Anatomy, Relation to Evolutionary Morphology, <a
+href="#pg302">302</a>-8, <a href="#pg312">312</a></li>
+
+<li>Transcendentalism, French and German Schools, <a href=
+"#pg089">89</a>, <a href="#pg100">100</a></li>
+
+<li>Trembley, <a href="#pg022">22</a>, <a href=
+"#pg315">315</a></li>
+
+<li>Treviranus, <a href="#pg141">141</a>, <a href="#pg170">170</a>,
+<a href="#pg215">215</a>, <a href="#pg225">225</a> f.n.</li>
+
+<li>Turpin, <a href="#pg173">173</a></li>
+
+<li>Types, Theory of (Cuvier and Von Baer)&mdash;
+
+<ul>
+<li>Cuvier, <a href="#pg041">41</a>, <a href="#pg124">124</a>, <a
+href="#pg289">289</a>, <a href="#pg291">291</a></li>
+
+<li>Von Baer, <a href="#pg123">123</a>-4, <a href="#pg289">289</a>,
+<a href="#pg291">291</a></li>
+
+<li>Bronn, <a href="#pg202">202</a></li>
+
+<li>Lereboullet, <a href="#pg207">207</a></li>
+
+<li><span class="pagenum"><a name="pg382" id="pg382">382</a></span>
+Criticised by&mdash;
+<ul>
+<li>E. Geoffroy, <a href="#pg060">60</a></li>
+
+<li>Haeckel, <a href="#pg289">289</a>, <a href=
+"#pg291">291</a></li>
+
+<li>Lankester, <a href="#pg291">291</a></li>
+</ul>
+</li>
+</ul>
+</li>
+
+<li>Type-Theory and Evolution, <a href="#pg304">304</a></li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Unger</span>, <a href="#pg185">185</a></li>
+
+<li>Unity of Composition, Principle of, Geoffroy, <a href=
+"#pg054">54</a>, <a href="#pg070">70</a>-2, <a href=
+"#pg075">75</a>-6, <a href="#pg200">200</a>, <a href=
+"#pg305">305</a></li>
+
+<li>Unity of Plan, <a href="#pg088">88</a>, <a href=
+"#pg241">241</a>, <a href="#pg278">278</a>-9, <a href=
+"#pg303">303</a>, <a href="#pg312">312</a>. <i>See also</i>
+"Archetype"
+
+<ul>
+<li>Aristotle, 6-7, <a href="#pg010">10</a></li>
+
+<li>Belon, Severino, <a href="#pg018">18</a></li>
+
+<li>Perrault, <a href="#pg019">19</a></li>
+
+<li>Robinet, <a href="#pg023">23</a></li>
+
+<li>Buffon, <a href="#pg024">24</a></li>
+
+<li>Cuvier, <a href="#pg041">41</a></li>
+
+<li>Goethe, <a href="#pg045">45</a>-7, <a href="#pg051">51</a></li>
+
+<li>Vicq D'Azyr, <a href="#pg045">45</a></li>
+
+<li>Camper, <a href="#pg045">45</a>, <a href="#pg046">46</a></li>
+
+<li>Herder, <a href="#pg046">46</a></li>
+
+<li>Kant, <a href="#pg046">46</a>, <a href="#pg213">213</a>-4</li>
+
+<li>E. Geoffroy, <a href="#pg052">52</a>-65, <a href=
+"#pg070">70</a> ff.</li>
+
+<li>Serres, <a href="#pg083">83</a>, <a href="#pg205">205</a>, <a
+href="#pg206">206</a></li>
+
+<li>Savigny, <a href="#pg083">83</a></li>
+
+<li>Audouin, <a href="#pg085">85</a>-6</li>
+
+<li>Latreille, <a href="#pg086">86</a></li>
+
+<li>Dug&egrave;s, <a href="#pg086">86</a>-7</li>
+
+<li>J. F. Meckel, <a href="#pg101">101</a></li>
+
+<li>Milne-Edwards, <a href="#pg197">197</a></li>
+
+<li>Semper, <a href="#pg279">279</a></li>
+
+<li>Haeckel, <a href="#pg289">289</a>, <a href=
+"#pg291">291</a></li>
+
+<li>Lankester, <a href="#pg291">291</a></li>
+</ul>
+</li>
+
+<li>Unity of Plan as due to Community of Descent&mdash;
+
+<ul>
+<li>Darwin, <a href="#pg233">233</a>, <a href="#pg234">234</a>-5,
+<a href="#pg239">239</a>, <a href="#pg247">247</a></li>
+
+<li>Haeckel, <a href="#pg250">250</a>-1</li>
+
+<li>Gegenbaur, <a href="#pg263">263</a> f.n., <a href=
+"#pg265">265</a></li>
+
+<li>Criticism of this idea&mdash;
+<ul>
+<li>O. Hertwig, <a href="#pg355">355</a>-7</li>
+</ul>
+</li>
+</ul>
+</li>
+
+<li>Unity of Plan as Conservative Principle&mdash;
+
+<ul>
+<li>E. Geoffroy, <a href="#pg075">75</a>, <a href=
+"#pg078">78</a></li>
+
+<li>Owen, <a href="#pg112">112</a></li>
+
+<li>Gegenbaur, <a href="#pg263">263</a>-4</li>
+
+<li>Evolutionary Morphologists, <a href="#pg307">307</a></li>
+</ul>
+</li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Valentin</span>, <a href="#pg138">138</a>,
+<a href="#pg173">173</a>, <a href="#pg176">176</a></li>
+
+<li>Variation, Limits of, Cuvier, <a href="#pg042">42</a></li>
+
+<li>Vegetative Repetition of Parts&mdash;
+
+<ul>
+<li>Owen, <a href="#pg111">111</a>, <a href="#pg286">286</a></li>
+
+<li>Bateson, <a href="#pg286">286</a></li>
+</ul>
+</li>
+
+<li>Velpeau, <a href="#pg138">138</a></li>
+
+<li>Vertebral Theory of Skull, <a href="#pg049">49</a>, <a href=
+"#pg096">96</a>-9, <a href="#pg104">104</a>-6, <a href=
+"#pg131">131</a>, <a href="#pg141">141</a>-4, <a href=
+"#pg147">147</a>-9, <a href="#pg154">154</a>-7, <a href=
+"#pg161">161</a>-2, <a href="#pg165">165</a>, <a href=
+"#pg203">203</a>, <a href="#pg235">235</a>, <a href=
+"#pg310">310</a> f.n.</li>
+
+<li>Vertebrate Descent, <a href="#pg269">269</a>-87, <a href=
+"#pg299">299</a>-301, <a href="#pg304">304</a></li>
+
+<li>Verworn, M., <a href="#pg330">330</a></li>
+
+<li>Vesalius, <a href="#pg018">18</a></li>
+
+<li>Vestigial Organs, <a href="#pg233">233</a>, <a href=
+"#pg237">237</a>, <a href="#pg309">309</a>, <a href=
+"#pg312">312</a></li>
+
+<li>Vialleton, L., <a href="#pg306">306</a> f.n., <a href=
+"#pg348">348</a></li>
+
+<li>Vicq d'Azyr, <a href="#pg045">45</a>, <a href=
+"#pg095">95</a></li>
+
+<li>Virchow, R., <a href="#pg188">188</a>, <a href=
+"#pg191">191</a></li>
+
+<li>Vitalism, Psychological&mdash;
+
+<ul>
+<li>Lamarck, <a href="#pg219">219</a>, <a href="#pg220">220</a>-6,
+<a href="#pg307">307</a>, <a href="#pg335">335</a></li>
+
+<li>Butler, <a href="#pg336">336</a>-41</li>
+
+<li>Orr, Cope, <a href="#pg342">342</a></li>
+
+<li>Ward, <a href="#pg343">343</a></li>
+
+<li>Delpino, Franc&eacute;, Pauly, A. Wagner, Mackenzie, <a href=
+"#pg345">345</a></li>
+</ul>
+</li>
+
+<li>Vogt, C.&mdash;
+
+<ul>
+<li>Criticism of Vertebral Theory, <a href="#pg156">156</a>-7</li>
+
+<li>Capillaries, <a href="#pg179">179</a></li>
+
+<li>Segmentation, <a href="#pg186">186</a></li>
+
+<li>Materialistic Attitude, <a href="#pg190">190</a>-1</li>
+
+<li>Threefold Parallelism, <a href="#pg255">255</a> f.n.</li>
+</ul>
+</li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Waagen</span>, <a href="#pg359">359</a>, <a
+href="#pg361">361</a> f.n.</li>
+
+<li>Wagner, A., <a href="#pg345">345</a></li>
+
+<li>Wagner, R., <a href="#pg176">176</a></li>
+
+<li>Ward, J., <a href="#pg343">343</a></li>
+
+<li>Weber, <a href="#pg138">138</a></li>
+
+<li>Weismann, A., <a href="#pg240">240</a>, <a href=
+"#pg323">323</a>, <a href="#pg326">326</a> f.n., <a href=
+"#pg330">330</a>-1, <a href="#pg343">343</a></li>
+
+<li>Werneck, <a href="#pg173">173</a></li>
+
+<li>Whitman, C. O., <a href="#pg346">346</a></li>
+
+<li>Wigand, A., <a href="#pg242">242</a> f.n., <a href=
+"#pg356">356</a></li>
+
+<li><span class="pagenum"><a name="pg383" id=
+"pg383">383</a></span>Willey, A., <a href="#pg273">273</a> f.n., <a
+href="#pg306">306</a> f.n.</li>
+
+<li>Williamson, <a href="#pg309">309</a></li>
+
+<li>Willis, <a href="#pg019">19</a></li>
+
+<li>Wilson, E. B., <a href="#pg331">331</a>, <a href=
+"#pg332">332</a>-3, <a href="#pg346">346</a> f.n., <a href=
+"#pg347">347</a> f.n.</li>
+
+<li>Wolff, C. F., <a href="#pg113">113</a>
+<ul>
+<li>Germ-layer Theory, <a href="#pg119">119</a>-20</li>
+
+<li>Cells, <a href="#pg170">170</a></li>
+</ul>
+</li>
+
+<li>Wolff, G., <a href="#pg346">346</a>-7</li>
+
+<li>Woodward, B. B., <a href="#pg358">358</a></li>
+
+<li>Wotton, E., <a href="#pg017">17</a></li>
+
+<li>&nbsp;</li>
+
+<li><span class="smcap">Zeleny</span>, <a href=
+"#pg333">333</a></li>
+
+<li>Zittel, K. von, <a href="#pg357">357</a>, <a href=
+"#pg358">358</a></li>
+
+<li>Zoja, <a href="#pg331">331</a></li>
+</ul>
+
+<hr style="width: 65%;" />
+
+<p><span class="pagenum"><a name="pg384" id=
+"pg384">384</a></span></p>
+
+<h5>PRINTED BY</h5>
+<h6>OLIVER AND BOYD,</h6>
+
+<h6>EDINBURGH, SCOTLAND</h6>
+
+<hr style="width: 65%;" />
+
+<p class="two"><b>HEREDITY.</b> By <span class="smcap">J. Arthur Thompson,</span> M.A., LL.D., Regius
+Professor of Natural History in the University of Aberdeen. With
+numerous Illustrations. <b>9s.</b> <i>net</i>.</p>
+
+<div class="center"><div class="smcap">Contents</div></div>
+
+<p class="small">Heredity and Inheritance: Defined and Illustrated--The Physical Basis
+of Inheritance--Heredity and Variation--Common Modes of
+Inheritance--Reversion and allied Phenomena--Telegony and other
+Dispute Questions--The Transmission of Acquired Characters--Heredity
+and Disease--Statistical Study of Inheritance--Experimental Study of
+Inheritance--History of Theories of Heredity and Inheritance--Heredity
+and Development--Heredity and Sex--Social Aspects of Biological
+Results--Bibliography--Subject-Index to Bibliography--Index.</p>
+
+<p class="small2">"We all know books of science which we ought to read with pleasure,
+but to which we turn with shrinking. Full, perhaps, of new facts and
+ideas, they are so expressed as to bore consumedly. 'Heredity' belongs
+to another category. He who runs may read, even if he be a beginner,
+and he who reads will probably not cease to run until he has traversed
+the last page."--<i>Nature</i>.</p>
+
+<p class="small2">"This is certainly the best modern book on heredity to recommend to
+the student and the intelligently curious."--<i>Science Progress</i>.</p>
+
+<p class="small2">"May be regarded as the standard work of reference on this subject. As
+a judicial summary of an exceedingly difficult and controversial
+subject it is masterly, while in the matter of clearness of exposition
+it has no rival."--<i>Knowledge</i>.</p>
+
+
+<p class="two"><b>RECENT ADVANCES IN THE STUDY OF VARIATION, HEREDITY AND EVOLUTION.</b> By
+<span class="smcap">Robert H. Lock,</span> M.A., sometime Fellow of Gonville and Caius College,
+Cambridge, late Assistant-Director of the Royal Botanic Gardens,
+Ceylon. With Portraits and other Illustrations. Revised by L.
+Doncaster, D.Sc., F.R.S. With a Biographical Note by Bella Sidnay
+Woolf (Mrs R. H. Lock). Crown 8vo.</p>
+
+
+<p class="two"><b>MICROSCOPY.</b> The Construction, Theory, and Use of the Microscope. By
+<span class="smcap">Edmund J. Spitta,</span> F.R.A.S., F.R.M.S., etc. With numerous Diagrams and
+Illustrations. Second Edition. <b>12s. 6d.</b> <i>net</i>.</p>
+
+<p class="small2">"Let us hasten to urge every student of the microscope who wishes to
+gain a thorough understanding of its principles and possibilities and
+its defects, and every user of the instrument who desires a work of
+reference to which he may turn for an explanation of some unexplained
+optical phenomenon, or for particulars of up-to-date apparatus, to
+procure a copy of Mr Spitta's book without delay."--<i>Nature</i>.</p>
+
+<p class="two"><b>CONVERGENCE IN EVOLUTION.</b> By <span class="smcap">Arthur
+Willey</span>, D.Sc. (Lond.); Hon. M.A. (Cantab.); F.R.S. With
+Diagrams. Demy 8vo. <b>7s. 6d.</b> <i>net</i>.</p>
+
+<p class="small2">This work brings together some scattered facts of parallel development
+of outward form and internal structure in the Animal kingdom, introducing
+new cases and fresh interpretations. It is, taken as a whole, an
+original contribution to the theory of organic evolution, with special reference
+to the forms of Animal life.</p>
+
+
+<p class="two"><b>THE HEREDITY OF ACQUIRED CHARACTERS IN PLANTS.</b> An aspect of the true
+Darwinism based on Personal Observations and Experiments. By the Rev.
+Prof. <span class="smcap">George Henslow</span>. With Illustrations. Demy 8vo. <b>6s.</b> <i>net</i>.</p>
+
+
+<p class="two"><b>A HANDBOOK OF PHYSIOLOGY.</b> By <span class="smcap">W.D.
+Halliburton</span>, M.D., F.R.S., Professor of Physiology, King's
+College, London. Eleventh Edition, being the Twenty-fourth
+of Kirkes'. With nearly Seven Hundred Illustrations, including
+some Coloured Plates. Large Crown 8vo. <b>15s.</b> <i>net</i>.</p>
+
+<p class="small2">"One of the best manuals for the student which we possess ... the book is
+an eminently trustworthy one, and will prove a valuable foundation for, and
+introduction to, the large treatises on physiology."&mdash;<i>Lancet</i>.</p>
+
+
+<p class="two"><b>THE BACTERIOLOGY OF MILK.</b> By <span class="smcap">Harold
+Swithinbank</span>, of the Bacteriological Research Laboratory,
+Durham, and Sir <span class="smcap">George Newman</span>, M.D., F.R.S.E., D.P.H.,
+Chief Medical Officer, Board of Education. With Special
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+and the Control of the Milk Supply. With numerous Illustrations.
+Royal 8vo. <b>25s.</b> <i>net</i>.</p>
+
+<p class="small2">"Ought to find a place in the library of every medical officer of health and
+of every milk-producer. Scientific in method and lucid in exposition, the authors
+have given us a really invaluable text-book."&mdash;<i>Spectator</i>.</p>
+
+
+<p class="two"><b>BACTERIOLOGY AND THE PUBLIC HEALTH.</b> By Sir <span class="smcap">George Newman</span>, M.D., F.R.S.
+(Edin.), D.P.H., Chief Medical Officer, Board of Education.
+With Illustrations. Medium 8vo. <b>21s.</b> <i>net</i>.</p>
+
+<p class="small2">"The present work, though nominally a third edition of 'Bacteria in
+Relation to the Economy of Nature, Industrial Processes, and the Public
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+that is necessary for the student of hygiene and the officer of health to know,
+so far as every-day problems of sanitation and preventive medicine demand....
+Dr Newman has done a good work in producing a treatise which places at
+the service of the community what is known about all these topics."&mdash;<i>Daily
+Telegraph</i>.</p>
+
+<p class="two"><b>THE RECENT DEVELOPMENT OF PHYSICAL
+SCIENCE.</b> By <span class="smcap">W.C.D. Whetham</span>, M.A.,
+F.R.S. Illustrated. Large Crown 8vo. <b>5s.</b> <i>net</i>.</p>
+
+<p><span class="smcap">The Philosophical Basis of Physical Science&mdash;The Liquefaction
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+Problems of Solution&mdash;The Conduction of Electricity
+Through Gases&mdash;Radio-Activity&mdash;Atoms and &AElig;ther&mdash;Astro-Physics&mdash;Index.</span></p>
+
+
+<p class="two"><b>THE REALM OF NATURE.</b> An Outline of
+Physiography. By <span class="smcap">H.R. Mill</span>, D.SC, LL.D., Director of
+the British Rainfall Organisation. Second Edition. Revised
+and entirely reset. With 19 coloured Maps and 73 Illustrations
+in the text. Crown 8vo. <b>5s.</b> <i>net</i>.</p>
+
+<p class="small2">"Dr Mill is to be congratulated on having now brought his information, so
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+are its comprehensiveness and conciseness.... It would, indeed, be difficult to
+point to any other English work on physiography giving so much trustworthy
+matter in equally condensed form, yet so readable."&mdash;<i>Athen&aelig;um</i>.</p>
+
+
+<p class="two"><b>NATURE AND ORIGIN OF FIORDS.</b> By
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+Heart of Australia." With Illustrations. Demy 8vo. <b>16s.</b> <i>net</i>.</p>
+
+<p class="small2">Professor <span class="smcap">T.G. Bonney</span> says in <i>Nature</i>, 12th Feb. 1914:&mdash;"But we must
+conclude, and do this by expressing our hearty thanks to him for this admirable
+history of fiords and other forms of inlets of the sea. It will be a great boon to
+students, for it is a veritable encyclop&aelig;dia, full of important facts."</p>
+
+
+<p class="two"><b>MECHANISM, LIFE AND PERSONALITY.</b>
+An Examination of the Mechanistic Theory of Life and Mind.
+By <span class="smcap">J.S. Haldane</span>, M.D., LL.D., F.R.S., Fellow of New
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+
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+
+<hr style='width: 45%;' />
+
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+
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+
+<p class="two">RIVER DEVELOPMENT. As Illustrated by the Rivers
+of North America. By Professor <span class="smcap">I.C. Russell</span>. Illustrated. <i>6s. net</i>.</p>
+
+<p class="two">VOLCANOES: Their Structure and Significance. By
+Professor <span class="smcap">Bonney</span>, D.SC, F.R.S. Illustrated. <i>6s. net</i>.</p>
+
+<p class="two">EARTHQUAKES, In the Light of the New Seismology.
+By <span class="smcap">Clarence E. Dutton</span>, Major U.S.A.  Illustrated.  <i>6s. net</i>.</p>
+
+<hr style='width: 45%;' />
+
+<h4>Life and Works of</h4>
+
+<h3>CHARLES DARWIN</h3>
+
+<hr style='width: 45%;' />
+
+<p class="two">THE ORIGIN OF SPECIES BY MEANS OF
+NATURAL SELECTION. <i>6s. net.</i> Popular Edition. <i>2s. 6d. net.</i> Also in
+Paper Covers, <i>1s. net.</i></p>
+
+<p class="two">DESCENT OF MAN, AND SELECTION IN
+RELATION TO SEX. With Illustrations. <i>3s. 6d. net.</i></p>
+
+<p class="two">VARIATION OF ANIMALS AND PLANTS
+UNDER DOMESTICATION. Woodcuts. 2 vols. <i>15s. net.</i> Popular
+Edition. <i>7s. 6d. net.</i></p>
+
+<p class="two">EXPRESSION OF THE EMOTIONS IN MAN
+AND ANIMALS. With Illustrations. <i>9s. net.</i> Popular Edition. <i>3s. 6d. net.</i></p>
+
+<p class="two">VARIOUS CONTRIVANCES BY WHICH ORCHIDS
+ARE FERTILIZED BY INSECTS. Woodcuts. <i>7s. 6d. net.</i>
+Popular Edition. <i>3s. 6d. net.</i></p>
+
+<p class="two">MOVEMENTS AND HABITS OF CLIMBING
+PLANTS. Popular Edition. <i>3s. 6d. net.</i></p>
+
+<p class="two">INSECTIVOROUS PLANTS. Popular Edition. <i>3s. 6d. net.</i></p>
+
+<p class="two">CROSS AND SELF-FERTILIZATION IN THE
+VEGETABLE KINGDOM. <i>9s. net.</i></p>
+
+<p class="two">DIFFERENT FORMS OF FLOWERS ON
+PLANTS OF THE SAME SPECIES. <i>7s. 6d. net.</i></p>
+
+<p class="two">FORMATION OF VEGETABLE MOULD
+THROUGH THE ACTION OF WORMS. Illustrations. <i>6s. net.</i> Popular
+Edition. <i>3s. 6d. net.</i></p>
+
+<p class="two">JOURNAL OF A NATURALIST DURING A
+VOYAGE ROUND THE WORLD IN H.M.S. "BEAGLE." With 100
+Illustrations. Medium 8vo. <i>21s. net.</i> Popular Edition. With 16 full-page
+Plates. <i>2s. 6d. net.</i></p>
+
+<hr style='width: 45%;' />
+
+<p class="two">LIFE and LETTERS of CHARLES DARWIN.
+With an Autobiographical Chapter. Edited by FRANCIS DARWIN, F.R.S.
+With 3 Portraits and Illustrations. 3 vols. 8vo. <i>36s.</i></p>
+
+<p class="two">CHARLES DARWIN: An Autobiography. With
+Selections from his Letters by FRANCIS DARWIN. Portrait, <i>7s. 6d. net.</i> Popular
+Edition. <i>2s. 6d. net.</i></p>
+
+<p class="two">MORE LETTERS OF CHARLES DARWIN.
+A Record of his Work in a Series of hitherto Unpublished Letters. Edited by
+FRANCIS DARWIN and A.C. SEWARD. With Portraits. 2 vols. Demy 8vo.
+<i>32s. net.</i></p>
+
+<hr style='width: 45%;' />
+
+<p class="two"><b>THERAPEUTICS OF THE CIRCULATION.</b>
+By Sir <span class="smcap">T. Lauder Brunton</span>, Bart., M.D., LL.D., F.R.C.P.,
+F.R.S., Consulting Physician to St Bartholomew's Hospital.
+New and Revised Edition. With Illustrations. <b>5s.</b> <i>net</i>.</p>
+
+<p class="small2">In this new edition so much matter has been added that the book has been
+practically re-written. It is intended to supplement, and not to replace, the usual
+text books dealing with circulation.</p>
+
+
+<p class="two"><b>THE PREVENTION OF MALARIA.</b> By Sir
+<span class="smcap">Ronald Ross</span>, K.C.B., F.R.S., etc. With Contributions
+by twenty of the Leading Experts. With Illustrations.
+Demy 8vo. <b>21s.</b> <i>net</i>.</p>
+
+<p class="small2">"A thoroughly sound and comprehensive treatise; Sir Ronald Ross and
+his colleagues have turned out work worthy of their high reputations. The
+student of malaria in all respects will find in this work the most complete
+exposition of the subject in medical literature."&mdash;<i>Lancet</i>.</p>
+
+
+<p class="two"><b>THE HOUSE-FLY:</b> Disease Carrier. An Account of
+its dangerous activities and of the means of destroying it.
+By <span class="smcap">Leland O. Howard</span>, Ph.D. With numerous Illustrations.
+<b>6s.</b> <i>net</i>.</p>
+
+<p class="small2">In this book the Chief of the U.S. Bureau of Entomology sets forth complete
+information about the fly.</p>
+
+<p class="small2">After describing the nature of the common house-fly, its habits and methods
+of breeding, he proves his case against it as a carrier of disease, and goes on to
+what will be the most interesting section to most readers&mdash;that on remedies and
+preventive measures. A special point is made or the possibilities of action by
+communities, with suggestions as to organisation, publicity, interesting the
+children, and the work of Boards of Health.</p>
+
+<hr style='width: 45%;' />
+
+<h3>WORKS BY EDWARD HALFORD ROSS,</h3>
+<h4>M.R.C.S. (Eng.), L.R.C.P. (Lond.).</h4>
+
+<p>Of the John Howard McFadden Researches; the Lister Institute
+of Preventive Medicine, London; and sometime Health Officer,
+Port Said, the Suez Canal District and Cairo.</p>
+
+
+<p class="two"><b>THE REDUCTION OF DOMESTIC MOSQUITOS.</b>
+Instructions for the use of Municipalities,
+Town Councils, Health Officers, Sanitary Inspectors, and
+Residents in Warm Climates. Demy 8vo. <b>5s.</b> <i>net</i>.</p>
+
+
+<p class="two"><b>THE REDUCTION OF DOMESTIC FLIES.</b>
+With Illustrations. Demy 8vo. <b>5s.</b> <i>net</i>.</p>
+
+<hr style='width: 45%;' />
+
+<h4>LONDON: JOHN MURRAY, ALBEMARLE STREET, W.</h4>
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
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+</pre>
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+</body>
+</html>
+
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+Project Gutenberg's Form and Function, by E. S. (Edward Stuart) Russell
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Form and Function
+       A Contribution to the History of Animal Morphology
+
+Author: E. S. (Edward Stuart) Russell
+
+Release Date: January 23, 2007 [EBook #20426]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK FORM AND FUNCTION ***
+
+
+
+
+Produced by Suzanne Lybarger, Turgut Dincer and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive/Canadian Libraries)
+
+
+
+
+
+
+
+
+
+FORM AND FUNCTION
+
+A CONTRIBUTION TO THE
+HISTORY OF ANIMAL MORPHOLOGY
+
+By E.S. RUSSELL,
+M.A., B.Sc., F.Z.S.
+
+ILLUSTRATED
+
+LONDON
+
+JOHN MURRAY, ALBEMARLE STREET, W.
+
+1916
+
+_All rights reserved_
+
++---------------------------------------+
+| Transcriber's Note: Obvious printer   |
+| errors have been corrected, all other |
+| inconsistencies in spelling and       |
+| punctuation are as in the original.   |
++---------------------------------------+
+
+
+PREFACE
+
+
+This book is not intended to be a full or detailed history of animal
+morphology: a complete account is given neither of morphological
+discoveries nor of morphological theories. My aim has been rather to
+call attention to the existence of diverse typical attitudes to the
+problems of form, and to trace the interplay of the theories that have
+arisen out of them.
+
+The main currents of morphological thought are to my mind three--the
+functional or synthetic, the formal or transcendental, and the
+materialistic or disintegrative.
+
+The first is associated with the great names of Aristotle, Cuvier, and
+von Baer, and leads easily to the more open vitalism of Lamarck and
+Samuel Butler. The typical representative of the second attitude is E.
+Geoffroy St. Hilaire, and this habit of thought has greatly influenced
+the development of evolutionary morphology.
+
+The main battle-ground of these two opposing tendencies is the problem
+of the relation of function to form. Is function the mechanical result
+of form, or is form merely the manifestation of function or activity?
+What is the essence of life--organisation or activity?
+
+The materialistic attitude is not distinctively biological, but is
+common to practically all fields of thought. It dates back to the
+Greek atomists, and the triumph of mechanical science in the 19th
+century has induced many to accept materialism as the only possible
+scientific method. In biology it is more akin to the formal than to
+the functional attitude.
+
+In the course of this book I have not hidden my own sympathy with the
+functional attitude. It appears to me probable that more insight will
+be gained into the real nature of life and organisation by
+concentrating on the active response of the animal, as manifested both
+in behaviour and in morphogenesis, particularly in the post-embryonic
+stages, than by giving attention exclusively to the historical aspect
+of structure, as is the custom of "pure morphology." I believe we
+shall only make progress in this direction if we frankly adopt the
+simple everyday conception of living things--which many of us have had
+drilled out of us--that they are active, purposeful agents, not mere
+complicated aggregations of protein and other substances. Such an
+attitude is probably quite as sound philosophically as the opposing
+one, but I have not in this place attempted any justification of it. I
+have touched very lightly upon the controversy between vitalism and
+materialism which has been revived with the early years of the present
+century. It hardly lends itself as yet to historical treatment, and I
+could hardly hope to maintain with regard to it that objective
+attitude which should characterise the historian.
+
+The main result I hope to have achieved with this book is the
+demonstration, tentative and incomplete as it is, of the essential
+continuity of animal morphology from the days of Aristotle down to our
+own time. It is unfortunately true that modern biology, perhaps in
+consequence of the great advances it has made in certain directions,
+has to a considerable extent lost its historical consciousness, and if
+this book helps in any degree to counteract this tendency so far as
+animal morphology is concerned, it will have served its purpose.
+
+I owe a debt of gratitude to my friends Dr James F. Gemmill and Prof.
+J. Arthur Thomson for much kindly encouragement and helpful criticism.
+The credit for the illustrations is due to my wife, Mrs Jehanne A.
+Russell. One is from Nature; the others are drawn from the original
+figures.
+
+E.S.R.
+
+CHELSEA, 1916.
+
+
+
+
+CONTENTS
+
+
+CHAP.                                                    PAGE
+
+I. THE BEGINNINGS OF COMPARATIVE ANATOMY                    1
+
+II. COMPARATIVE ANATOMY BEFORE CUVIER                      17
+
+III. CUVIER                                                31
+
+IV. GOETHE                                                 45
+
+V. ETIENNE GEOFFROY ST HILAIRE                             52
+
+VI. THE FOLLOWERS OF ETIENNE GEOFFROY ST HILAIRE           79
+
+VII. THE GERMAN TRANSCENDENTALISTS                         89
+
+VIII. TRANSCENDENTAL ANATOMY IN ENGLAND--RICHARD OWEN     102
+
+IX. KARL ERNST VON BAER                                   113
+
+X. THE EMBRYOLOGICAL CRITERION                            133
+
+XI. THE CELL-THEORY                                       169
+
+XII. THE CLOSE OF THE PRE-EVOLUTIONARY PERIOD             190
+
+XIII. THE RELATION OF LAMARCK AND DARWIN TO MORPHOLOGY    213
+
+XIV. ERNST HAECKEL AND CARL GEGENBAUR                     246
+
+XV. EARLY THEORIES ON THE ORIGIN OF VERTEBRATES           268
+
+XVI. THE GERM-LAYERS AND EVOLUTION                        288
+
+XVII. THE ORGANISM AS AN HISTORICAL BEING                 302
+
+XVIII. THE BEGINNINGS OF CAUSAL MORPHOLOGY                314
+
+XIX. SAMUEL BUTLER AND THE MEMORY THEORIES OF HEREDITY    335
+
+XX. THE CLASSICAL TRADITION IN MODERN MORPHOLOGY          345
+
+INDEX                                                     365
+
+
+
+
+ILLUSTRATIONS
+
+
+FIG.                                                     PAGE
+
+1. HYOID ARCH OF THE CONGER. (ORIGINAL.)                   58
+
+2. "VERTEBRA" OF A PLEURONECTID. (GEOFFROY.)               61
+
+3. ABDOMINAL SEGMENT OF THE LOBSTER. (GEOFFROY.)           63
+
+4. IDEAL TYPICAL VERTEBRA. (OWEN.)                        102
+
+5. NATURAL TYPICAL VERTEBRA. (OWEN.)                      103
+
+6. THE ARCHETYPE OF THE VERTEBRATE SKELETON. (OWEN.)      105
+
+7. IDEAL TRANSVERSE SECTION OF A VERTEBRATE EMBRYO.
+     (VON BAER.)                                          119
+
+8. GILL-SLITS OF THE PIG EMBRYO. (RATHKE.)                134
+
+9. MECKEL'S CARTILAGE AND EAR-OSSICLES IN EMBRYO OF
+     PIG. (REICHERT.)                                     145
+
+10. CRANIAL VERTEBRAE AND VISCERAL ARCHES IN EMBRYO
+      OF PIG. (REICHERT.)                                 148
+
+11. EMBRYONIC CRANIUM OF THE ADDER. (RATHKE.)             152
+
+12. TRANSVERSE SECTION OF CHICK EMBRYO. (REMAK.)          211
+
+13. DEVELOPMENT OF THE ASCIDIAN LARVA (KOWALEVSKY.)       272
+
+14. TRANSVERSE SECTION OF THE WORM _NAIS_. (SEMPER.)      280
+
+15. THE FIVE PRIMARY STAGES OF ONTOGENY. (HAECKEL.)       292
+
+
+FORM AND FUNCTION
+
+
+
+
+CHAPTER I
+
+THE BEGINNINGS OF COMPARATIVE ANATOMY
+
+
+The first name of which the history of anatomy keeps record is that of
+Alcmaeon, a contemporary of Pythagoras (6th century B.C.). His
+interests appear to have been rather physiological than anatomical. He
+traced the chief nerves of sense to the brain, which he considered to
+be the seat of the soul, and he made some good guesses at the
+mechanism of the organs of special sense. He showed that, contrary to
+the received opinion, the seminal fluid did not originate in the
+spinal cord. Two comparisons are recorded of his, one that puberty is
+the equivalent of the flowering time in plants, the other that milk is
+the equivalent of white of egg.[1] Both show his bias towards looking
+at the functional side of living things. The latter comparison
+reappears in Aristotle.
+
+A century later Diogenes of Apollonia gave a description of the venous
+system. He too placed the seat of sensation in the brain. He assumed a
+vital air in all living things, being in this influenced by Anaximenes
+whose primitive matter was infinite air. In following out this thought
+he tried to prove that both fishes and oysters have the power of
+breathing.[2]
+
+A more strictly morphological note is struck by a curious saying of
+Empedocles (4th century B.C.), that "hair and foliage and the thick
+plumage of birds are one."[3]
+
+In the collected writings of Hippocrates and his school, the _Corpus
+Hippocraticum_, of which no part is later than the end of the 5th
+century, there are recorded many anatomical facts. The author of the
+treatise "On the Muscles" knew, for instance, that the spinal marrow
+is different from ordinary marrow and has membranes continuous with
+those of the brain. Embryos of seven days (!) have all the parts of
+the body plainly visible. Work on comparative embryology is contained
+in the treatise "On the Development of the Child."[4]
+
+The author of the treatise "On the Joints," which 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 B.C.)[5] may fairly be said to be the founder of
+comparative anatomy, not because he was specially interested in
+problems of "pure morphology," but because he described the structure
+of many animals and classified them in a scientific way. We shall
+discuss here the morphological ideas which occur in his writings upon
+animals--in the _Historia Animialium_, the _De Partibus Animalium_,
+and the _De Generatione Animalium_.
+
+The _Historia Animalium_ is a most comprehensive work, in some ways
+the finest text-book of Zoology ever written. Certainly few modern
+text-books take such a broad and sane view of living creatures.
+Aristotle never forgets that form and structure are but one of the
+many properties of living things; he takes quite as much interest in
+their behaviour, their ecology, distribution, comparative physiology.
+He takes a special interest in the comparative physiology of
+reproduction. The _Historia Animalium_ contains a description of the
+form and structure of man and of as many animals as Aristotle was
+acquainted with--and he was acquainted with an astonishingly large
+number. The later _De Partibus Animalium_ is a treatise on the causes
+of the form and structure of animals. Owing to the importance which
+Aristotle ascribed to the final cause this work became really a
+treatise on the functions of the parts, a discussion of the problems
+of the relation of form to function, and the adaptedness of structure.
+
+Aristotle was quite well aware that each of the big groups of animals
+was built upon one plan of structure, which showed endless variations
+"in excess and defect" in the different members of the group. But he
+did not realise that this fact of community of plan constituted a
+problem in itself. His interest was turned towards the functional side
+of living things, form was for him a secondary result of function.
+
+Yet he was not unaware of facts of form for which he could not quite
+find a place in his theory of organic form, facts of form which were
+not, at first sight at least, facts of function. Thus he was aware of
+certain facts of "correlation," which could not be explained off-hand
+as due to correlation of the functions of the parts. He knew, for
+instance, that all animals without front teeth in the upper jaw have
+cotyledons, while most that have front teeth on both jaws and no horns
+have no cotyledons (_De Gen._, ii. 7).
+
+Speaking generally, however, we find in Aristotle no purely
+morphological concepts. What then does morphology owe to Aristotle? It
+owes to him, _first_, a great mass of facts about the structure of
+animals; _second_, the first scientific classification of animals;[6]
+_third_, a clear enunciation of the fact of community of plan within
+each of the big groups; _fourth_, an attempt to explain certain
+instances of the correlation of parts; _fifth_, a pregnant distinction
+between homogeneous and heterogeneous parts; _sixth_, a generalisation
+on the succession of forms in development; and _seventh_, the first
+enunciation of the idea of the _Echelle des etres_.
+
+(1) What surprises the modern reader of the _Historia Animalium_
+perhaps more than anything else is the extent and variety of
+Aristotle's knowledge of animals. He describes more than 500 kinds.[7]
+Not only does he know the ordinary beasts, birds, and fishes with
+which everyone is acquainted, but he knows a great deal about
+cuttlefish, snails and oysters, about crabs, crawfish (_Palinurus_),
+lobsters, shrimps, and hermit crabs, about sea-urchins and starfish,
+sea-anemones and sponges, about ascidians (which seem to have puzzled
+him not a little!). He has noticed even fish-lice and intestinal
+worms, both flat and round. Of the smaller land animals, he knows a
+great many insects and their 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. Anim._, ii.), or his description of
+the structure of cuttlefish (_Hist. Anim._, iv), or that touch in the
+description of the hermit crab (_Hist. Anim._, iv.)--" Two large eyes
+... not ... turned on one side like those of crabs, but straight
+forward"--without being convinced that Aristotle is speaking of what
+he has seen. Naturally he could not make much of the anatomy of small
+insects and snails, and, to tell the truth, he does not seem to have
+cared greatly about the minutiae of structure. He was too much of a
+Greek and an aristocrat to care about laborious detail.
+
+Not only did he lay a foundation for comparative anatomy, but he made
+a real start with comparative embryology. Medical men before him had
+known many facts about human development; Aristotle seems to have been
+the first to study in any detail the development of the chick. He
+describes this as it appears to the naked eye, the position of the
+embryo on the yolk, the palpitating spot at the third day, the
+formation of the body and of the large sightless eyes, the veins on
+the yolk, the embryonic membranes, of which he distinguished two.
+
+(2) Aristotle had various systems of classifying animals. They could
+be classified, he thought, according to their structure, their manner
+of reproduction, their manner of life, their mode of locomotion, their
+food, and so on. Thus you might, in addition to structural
+classifications, divide animals into gregarious, solitary and social,
+or land animals into troglodytes, surface-dwellers, and burrowers
+(_Hist. Anim._, i.).
+
+He knew that dichotomous classifications were of little use for
+animals (_De Partibus_, i. 3) and he explicitly and in so many words
+accepted the principle of all "natural" classification, that
+affinities must be judged by comparing not one but the sum total of
+characters. As everyone knows, he was the first to distinguish the big
+groups of animals, many of which were already distinguished roughly by
+the common usages of speech. Among his Sanguinea he did little more
+than define with greater exactitude the limits of the groups
+established by the popular classification. Among the "exsanguineous"
+animals, however, corresponding to our Invertebrates, he established a
+much more definite classification than the popular, which is apt to
+call them indiscriminately "shellfish," "insects," or "creeping
+things." He went beyond the superficialities of popular
+classification, too, in clearly separating Cetacea from fishes. He had
+some notion of species and genera in our sense. He distinguished many
+species of cuttlefish--_Octopus (Polypus)_ of which there were many
+kinds, _Eledone (Moschites)_ which he knew to have only one row of
+suckers while _Octopus_ has two, _Argonauta, Nautilus, Sepia_, and
+apparently _Loligo media_ (= his Teuthis) and _L. vulgaris_(or
+_forbesii_) which seems to be his Teuthos. He had a grasp of the
+principles which should be followed in judging of the natural
+affinities of species. For example, he knew that the cuckoo resembles
+a hawk. "But," he says, "the hawk has crooked talons, which the cuckoo
+has not, nor does it resemble the hawk in the form of its head, but in
+these respects is more like the pigeon than the hawk, which it
+resembles in nothing but its colour; the markings, however, upon the
+hawk are like lines, while the cuckoo is spotted" (_Hist. Anim._,
+Cresswell's trans., p. 147, London, 1862).
+
+The groups he distinguished were--man, viviparous quadrupeds,
+oviparous quadrupeds, birds, fishes, Cetacea, Cephalopoda,
+Malacostraca (= higher Crustacea), Insecta (= annulose animals),
+Testacea (= molluscs, echinoderms, ascidians). A class of Acalephae,
+including sea-anemones and sponges, was grouped with the Testacea. The
+first five groups were classed together as sanguineous, the others as
+exsanguineous, from the presence or absence of red blood.
+
+Besides these classes "there are," he says, "many other creatures in
+the sea which it is not possible to arrange in any class from their
+scarcity" (Creswell, _loc. cit._, p. 90).
+
+(3) Aristotle's greatest service to morphology is his clear
+recognition of the unity of plan holding throughout each of the great
+groups.
+
+He recognises this most clearly in the case of man and the viviparous
+quadrupeds, with whose structure he was best acquainted. In the
+_Historia Animalium_ he takes man as a standard, and describes his
+external and internal parts in detail, then considers viviparous
+quadrupeds and compares them with man. "Whatever parts a man has
+before, a quadruped has beneath; those that are behind in man form the
+quadruped's back" (Cresswell, _loc. cit._, p. 26). Apes, monkeys, and
+Cynocephali combine the characteristics of man and quadrupeds. He
+notices that all viviparous quadrupeds have hair. Oviparous quadrupeds
+resemble the viviparous, but they lack some organs, such as ears with
+an external pinna, 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 mammae. Instead of hair they have scales.
+
+Speaking generally, the Sanguinea differ from man and from one another
+in their parts, which may be present or absent, or exhibit differences
+in "excess and defect," or in form. Unity of plan extends to all the
+principal systems of organs. "All sanguineous animals have either a
+bony or a spinous column. The remainder of the bones exist in some
+animals; but not in others, for if they have the limbs they have the
+bones belonging to them" (Cresswell, _loc. cit._, p. 60). "Viviparous
+animals with blood and feet do not differ much in their bones, but
+rather by analogy, in hardness, softness, and size" (Cresswell, _loc.
+cit._, p. 59). The venous system, too, is built upon the same general
+plan throughout the Sanguinea. "In all sanguineous animals, the nature
+and origin of the principal veins are the same, but the multitude of
+smaller veins is not alike in all, for neither are the parts of the
+same nature, nor do all possess the same parts" (Cresswell, _loc.
+cit._, p. 56). It will be noticed in the first and last of these three
+quotations that Aristotle recognises the fact of correlation between
+systems of organs,--between limbs and bones, and between blood-vessels
+and the parts to which they go.
+
+Sanguineous animals all possess certain organs--heart, liver, spleen,
+kidneys, and so on. Other organs occur in most of the classes--the
+oesophagus and the lungs. "The position which these parts occupy is
+the same in all animals [sc. Sanguinea]" (Cresswell, _loc. cit._, p.
+39).
+
+Unity of plan is observable not only in the Sanguinea, but also within
+each of the other large groups. Aristotle recognises that all his
+cuttlefish are alike in structure. Among his Malacostraca he compares
+point by point the external parts of the carabus (_Palinurus_), and
+the astacus (_Homarus_), and he compares also the general internal
+anatomy of the various "genera" he distinguishes. As regards Testacea,
+he writes, "The nature of their internal structure is similar in all,
+especially in the turbinated animals, for they differ in size and in
+the relations of excess; the univalves and bivalves do not exhibit
+many differences" (Cresswell, _loc. cit._, p. 83). There is an
+interesting remark about "the creature called carcinium"
+(hermit-crab), that it "resembles both the Malacostraca and the
+Testacea, for this in its nature is similar to the animals that are
+like carabi, and it is born naked" (Cresswell, _loc. cit._, p. 85). In
+the last phrase we may perhaps read the first recognition of the
+embryological criterion.
+
+With the recognition of unity of plan within each group necessarily
+goes the recognition of what later morphology calls the homology of
+parts. The parts of a horse can be compared one by one with the parts
+of another viviparous quadruped; in all the animals belonging to the
+same class the parts are the same, only they differ in excess or
+defect--these remarks are placed in the forefront of the _Historia
+Animalium_. Generally speaking, parts which bear the same name are for
+Aristotle homologous throughout the class. But he goes further and
+notes the essential resemblance underlying the differences of certain
+parts. He classes together nails and claws, the spines of the
+hedgehog, and hair, as being homologous structures. He says that teeth
+are allied to bones, whereas horns are more nearly allied to skin
+(_Hist. Anim._, iii.). This is an astonishingly happy guess,
+considering that all he had to go upon was the observation that in
+black animals the horns are black but the teeth white. One cannot but
+admire the way in which Aristotle fixes upon apparently trivial and
+commonplace facts, and draws from them far-reaching consequences. He
+often goes wrong, it is true, but he always errs in the grand manner.
+
+While Aristotle certainly recognised the existence of homologies, and
+even had a feeling for them, he did not clearly distinguish homology
+from analogy. He comes pretty near the distinction in the following
+passage. After explaining that in animals belonging to the same class
+the parts are the same, differing only in excess or defect, he says,
+"But some animals agree with each other in their parts neither in form
+nor in excess and defect, but have only an analogous likeness, such as
+a bone bears to a spine, a nail to a hoof, a hand to a crab's claw,
+the scale of a fish to the feather of a bird, for that which is a
+feather in the bird is a scale in the fish" (Cresswell, _loc. cit._,
+p. 2). One of these comparisons is, however, a homology not an
+analogy, and the last phrase throws a little doubt upon the whole
+question, for it is not made clear whether it is position or function
+that determines what are equivalent organs.
+
+In the _De Partibus Animalium_ there occurs the following
+passage:--"Groups that only differ in degree, and in the more or less
+of an identical element that they possess, are aggregated under a
+single class; groups whose attributes are not identical but analogous
+are separated. For instance, bird differs from bird by gradation, or
+by excess and defect; some birds have long feathers, others short
+ones, but all are feathered. Bird and Fish are more remote and only
+agree in having analogous organs; for what in the bird is feather, in
+the fish is scale. Such analogies can scarcely, however, serve
+universally as indications for the formation of groups, for almost all
+animals present analogies in their corresponding parts."[8] It is thus
+similarity in form and structure which determines the formation of the
+main groups. Within each group the parts differ only in degree, in
+largeness or smallness, softness and hardness, smoothness or
+roughness, and the like (_loc. cit._, i., 4, 644^b). These passages
+show that Aristotle had some conception of homology as distinct from
+analogy. He did not, however, develop the idea. What Aristotle sought
+in the variety of animal structure, and what he found, were not
+homologies, but rather communities of function, parts with the same
+attributes. His interest was all in _organs_, in functioning parts,
+not in the mere spatial relationship of parts.
+
+This comes out clearly in his treatise _On the Parts of Animals_,
+which is subsequent to, and the complement of, his _History of
+Animals_. The latter is a description of the variety of animal form,
+the former is a treatise on the functions of the parts. He describes
+the plan of the _De Partibus Animalium_ as follows:--"We have, then,
+first to describe the common functions, common, that is, to the whole
+animal kingdom, or to certain large groups, or to members of a
+species. In other words, we have to describe the attributes common to
+all animals, or to assemblages, like the class of Birds, of closely
+allied groups differentiated by gradation, or to groups like Man not
+differentiated into subordinate groups. In the first case the common
+attributes may be called analogous, in the second generic, in the
+third specific" (i, 5, 645^b, trans. Ogle). The alimentary canal is a
+good example of a part which is "analogous" throughout the animal
+kingdom, for "all animals possess in common those parts by which they
+take in food, and into which they receive it" (Cresswell, _loc. cit._,
+p. 6).
+
+The _De Partibus Animalium_ becomes in form a comparative
+organography, but the emphasis is always on function and community of
+function. Thus he treats of bone, "fish-spine," and cartilage together
+(_De Partibus_, ii., 9, 655^a), because they have the same function,
+though he says elsewhere that they are only analogous structures (ii.,
+8, 653^b). In the same connection he describes also the supporting
+tissues of Invertebrates--the hard exoskeleton of Crustacea and
+Insects, the shell of Testacea, the "bone" of _Sepia_ (ii., 8,
+654^a). Aristotle took much more interest in analogies, in organs of
+similar function, than in homologies. He did recognise the existence
+of homologies, but rather _malgre lui_, because the facts forced it
+upon him.
+
+His only excursion into the realm of "transcendental anatomy" is his
+comparison of a Cephalopod to a doubled-up Vertebrate whose legs have
+become adherent to its head, whose alimentary canal has doubled upon
+itself in such a way as to bring the anus near the mouth (_De
+Partibus_, iv., 9, 684^b). It is clear, however, that Aristotle did
+not seek to establish by this comparison any true homologies of parts,
+but merely analogies, thus avoiding the error into which Meyranx and
+Laurencet fell more than two thousand years later in their paper
+communicated to the Academie des Sciences, which formed the
+starting-point of the famous controversy between Cuvier and E.
+Geoffroy St Hilaire (see Chap. V., below).
+
+Moreover, Aristotle did not so much compare a Cephalopod with a
+doubled-up Vertebrate as contrast Cephalopods (and also Testacea) with
+all other animals. Other animals have their organs in a straight line;
+Cephalopods and Testacea alone show this peculiar doubling up of the
+body.
+
+(4) Aristotle was much struck with certain facts of correlation, of
+the interdependence of two organs which are not apparently in
+functional dependence on one another. Such correlation may be positive
+or negative; the presence of one organ may either entail the presence
+of the other, or it may entail its absence. Aristotle has various ways
+of explaining facts of correlation. He observed that no animal has
+both tusks and horns, but this fact could easily be explained on the
+principle that Nature never makes anything superfluous or in vain. If
+an animal is protected by the possession of tusks it does not require
+horns, and _vice versa_. The correlation of a multiple stomach with
+deficient development of the teeth (as in Ruminants) is accounted for
+by saying that the animal needs its complex stomach to make up for the
+shortcomings of its teeth! (_De Partibus_, iii., 14, 674^b.) Other
+examples of correlation were not susceptible of this explanation in
+terms of final causes. He lays stress on the fact, in the main true,
+of the inverse development of horns and front teeth in the upper jaw,
+exemplified in Ruminants. He explains the fact in this way. Teeth and
+horns are formed from earthy matter in the body and there is not
+enough to form both teeth and horns, so "Nature by subtracting from
+the teeth adds to the horns; the nutriment which in most animals goes
+to the former being here spent on the augmentation of the latter" (_De
+Partibus_, iii., 2, 664^a, trans. Ogle). A similar kind of explanation
+is offered of the fact that Selachia have cartilage instead of bone,
+"in these Selachia Nature has used all the earthy matter on the skin
+[_i.e._, on the placoid scales]; and she is unable to allot to many
+different parts one and the same superfluity of material" (_De
+Partibus_, ii., 9, 655^a, trans. Ogle). Speaking generally, "Nature
+invariably gives to one part what she subtracts from another" (_loc.
+cit._, ii., 14, 658^a).
+
+This thought reappears again in the 19th century in E. Geoffroy St
+Hilaire's _loi de balancement_ and also in Goethe's writings on
+morphology. For Aristotle it meant that Nature was limited by the
+nature of her means, that finality was limited by necessity. Thus in
+the larger animals there is an excess of earthy matter, as a necessary
+result of the material nature of the animal; this excess is turned by
+Nature to good account, but there is not enough to serve both for
+teeth and for horns (_loc. cit._, iii., 2, 663^b).
+
+But there are other instances of correlation which seem to have taxed
+even Aristotle's ingenuity beyond its powers. Thus he knew that all
+animals (meaning viviparous quadrupeds) with no front teeth in the
+upper jaw have cotyledons on their foetal membranes, and that most
+animals which have front teeth in both jaws and no horns have no
+cotyledons (_De Generatione_, ii., 7). He offers no explanation of
+this, but accepts it as a fact.
+
+We may conveniently refer here to one or two other ideas of Aristotle
+regarding the causes of form. He makes the profound remark that the
+possible range of form of an organ is limited to some extent by its
+existing differentiation. Thus he explains the absence of external
+(projecting) ears in birds and reptiles by the fact that their skin is
+hard and does not easily take on the form of an external ear (_De
+Partibus_, ii, 12). The fact of the inverse correlation is certain;
+the explanation is, though very vague, probably correct.
+
+In one passage of the _De Partibus_ Aristotle clearly enunciates the
+principle of the division of labour, afterwards emphasised by H.
+Milne-Edwards. In some insects, he says, the proboscis combines the
+functions of a tongue and a sting, in others the tongue and the sting
+are quite separate. "Now it is better," he goes on, "that one and the
+same instrument shall not be made to serve several dissimilar ends;
+but that there shall be one organ to serve as a weapon, which can then
+be very sharp, and a distinct one to serve as a tongue, which can then
+be of spongy texture and fit to absorb nutriment. Whenever, therefore,
+Nature is able to provide two separate instruments for two separate
+uses, without the one hampering the other, she does so, instead of
+acting like a coppersmith who for cheapness makes a spit and
+lampholder in one" (iv., 6, 683^a).
+
+(5) The first sentence of the _Historia Animalium_ formulates, with
+that simplicity and directness which is so characteristic of
+Aristotle, the distinction between homogeneous and heterogeneous
+parts, in the mass the distinction between tissues and organs. "Some
+parts of animals are simple, and these can be divided into like parts,
+as flesh into pieces of flesh; others are compound, and cannot be
+divided into like parts, as the hand cannot be divided into hands, nor
+the face into faces. All the compound parts also are made up of simple
+parts--the hand, for example, of flesh and sinew and bone" (Cresswell,
+_loc. cit_., p. I).
+
+In the _De Partibus Animalium_ he broadens the conception by adding
+another form of composition. "Now there are," he says, "three degrees
+of composition; and of these the first in order, as all will allow, is
+composition out of what some call the elements, such as earth, air,
+water, fire.... The second degree of composition is that by which the
+homogeneous parts of animals, such as bone, flesh, and the like, are
+constituted out of the primary substances. The third and last stage is
+the composition which forms the heterogeneous parts, such as face,
+hand, and the rest" (ii., 1, 646^a, trans. Ogle).
+
+In the _Historia Animalium_ the homogeneous parts are divided into (1)
+the soft and moist (or fluid), such as blood, serum, flesh, fat, suet,
+marrow, semen, gall, milk, phlegm, 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 Animalium_ Aristotle, while still holding to
+the distinction set forth above, is alive to the fact that his simple
+parts include several different sorts of substances. He distinguishes
+among the homogeneous parts three sets. The first of these comprises
+the tissues out of which the heterogeneous parts are constructed,
+_e.g._, flesh and bone; the second set form the nutriment of the
+parts, and are invariably fluid; while the third set are the residue
+of the second and constitute the residual excretions of the body (ii.,
+2, 647^b). He sees clearly the difficulty of calling vein or
+blood-vessel a simple part, for while a bloodvessel and a part of it
+are both blood-vessel, as we should say vascular tissue, yet a part of
+a blood-vessel is not a bloodvessel. There is form superadded to
+homogeneity of structure (ii., 2, 647^b). Similarly for the heart and
+the other viscera. "The heart, like the other viscera, is one of the
+homogeneous parts; for, if cut up, its pieces are homogeneous in
+substance with each other. But it is at the same time heterogeneous in
+virtue of its definite configuration" (ii., 1, 647^a, trans. Ogle).
+
+Aristotle, therefore, came very near our conception of tissue. He was
+of course not a histologist; he describes not the structure of
+tissues, which he could not know, but rather their distribution within
+the organism; his section on the homogeneous parts of Sanguinea
+(_Historia Animalium_, iii., second half) is largely a comparative
+topographical anatomy; in it, for instance, he describes the venous
+and skeletal systems.
+
+This distinction which Aristotle drew plays an important part in all
+his writings on animals, particularly in his theory of development. It
+was a distinction of immense value, and is full of meaning even at the
+present day. No one has ever given a better definition of organ than
+is implied in Aristotle's description of the heterogeneous parts--"The
+capacity of action resides in the compound parts" (Cresswell, _loc.
+cit._, p. 7). The heterogeneous parts were distinguished by the
+faculty of doing something, they were the active or executive parts.
+The homogeneous parts were distinguished mainly by physical characters
+(_De Generatione_, i., 18), but certain of them had other than purely
+physical properties, they were the organs of touch (_De Partibus_,
+ii., 1, 647^a).
+
+(6) In a passage in the _De Generatione_ (ii, 3) Aristotle says that
+the embryo is an animal before it is a particular animal, that the
+general characters appear before the special. This is a foreshadowing
+of the essential point in von Baer's law (see Chap. IX. below).
+
+He considers also that tissues arise before organs. The homogeneous
+parts are anterior genetically to the heterogeneous parts and
+posterior to the elementary material (_De Partibus_, ii., 1, 646^b).
+
+(7) We meet in Aristotle an idea which later acquired considerable
+vogue, that of the _Echelle des etres_(or "scale of beings"), that
+organisms, or even all objects organic or inorganic, can be arranged
+in a single ascending series. The idea is a common one; its first
+literary expression is found perhaps in primitive creation-myths, in
+which inorganic things are created before organic, and plants before
+animals. It may be recognised also in Anaximander's theory that land
+animals arose from aquatic animals, more clearly still in Anaxagoras'
+theory that life took its origin on this globe from vegetable germs
+which fell to earth with the rain. Anaxagoras considered animals
+higher in the scale than plants, for while the latter participated in
+pleasure (when they grew) and pain (when they lost their leaves),
+animals had in addition "Nous." In Empedocles' theory of evolution,
+the vegetable world preceded the animal. Plato, in the _Timaeus_,
+describes the whole organic world as being formed by degradation from
+man, who is created first. Man sinks first into woman, then into brute
+form, traversing all the stages from the higher to the lower animals,
+and becoming finally a plant. This is a reversal of the more usual
+notion, but the idea of gradation is equally present.
+
+Aristotle seems not to have believed in any transformation of species,
+but he saw that Nature passes gradually from inanimate to animate
+things without a clear dividing line. "The race of plants succeeds
+immediately that of inanimate objects" (Cresswell, _loc. cit._, p.
+94). Within the organic realm the passage from plants to animals is
+gradual. Some creatures, for example, the sea-anemones and sponges,
+might belong to either class.
+
+Aristotle recognised also a natural series among the groups of
+animals, a series of increasing complexity of structure. He begins his
+study of structure with man, who is the most intricate, and then takes
+up in turn viviparous and oviparous quadrupeds, then birds, then
+fishes. After the Sanguinea he considers the Exsanguinea, and of the
+latter first the most highly organised, the Cephalopods, and last the
+simplest, the lower members of his class of the Testacea. In treating
+of generation (in _Hist. Animalium_, v.) he reverses this order. In
+the _De Generatione_ (Book ii., I) there is given another serial
+arrangement of animals, this time in relation to their manner of
+reproduction. There is a gradation, he says, of the following kind:--
+
+1. Internally viviparous Sanguinea  } producing a perfect
+2. Externally viviparous Sanguinea  }       animal.
+3. Oviparous Sanguinea--producing a perfect egg.
+4. Animals producing an imperfect egg (one which
+     increases in size after being laid).
+5. Insects, producing a scolex (or grub).
+
+In Aristotle's view the gradation of organic forms is the consequence,
+not the cause, of the gradation observable in their activities. Plants
+have no work to do beside nutrition, growth, and reproduction; they
+possess only the nutritive soul. Animals possess in addition sensation
+and the sensitive or perceptive soul--"their manner of life differs in
+their having pleasure in sexual intercourse, in their mode of
+parturition and rearing their young" (_Hist. Anim._, viii., trans.
+Cresswell, p. 195). Man alone has the rational soul in addition to the
+two lower kinds.
+
+As it is put in the _De Partibus_ (ii., 10, 656^a, trans. Ogle),
+"Plants, again, inasmuch as they are without locomotion, present no
+great variety in their heterogeneous parts. For, where the functions
+are but few, few also are the organs required to effect them....
+Animals, however, that not only live but feel, present a greater
+multiformity of parts, and this diversity is greater in some animals
+than in others, being most varied in those to whose share has fallen
+not mere life but life of high degree. Now such an animal is man."
+
+With the great exception of Aristotle, the philosophers of Greece and
+Rome made little contribution to morphological theory. Passing mention
+may be made of the Atomists--Leucippus, Democritus, and their great
+disciple Lucretius, who in his magnificent poem "De Natura Rerum" gave
+impassioned expression to the materialistic conception of the
+universe. But the full effect of materialism upon morphology does not
+become apparent till the rise of physiology in the 17th and 18th
+centuries, and reaches its culmination in the 19th century. The
+evolutionary ideas of Lucretius exercised no immediate influence upon
+the development of morphology.
+
+    [1] E. Zeller, _Greek Philosophy_, Eng. trans., i., 522
+    f.n., London 1881. Other particulars as to Alcmaeon in
+    T. Gomperz, _Greek Thinkers_, Eng. trans., i., London,
+    1901.
+
+    [2] Zeller, _loc. cit._, i., p. 297.
+
+    [3] Gomperz, _loc. cit._, i., p. 244.
+
+    [4] R. Burckhardt, _Biologie u. Humanismus_, p. 85,
+    Jena, 1907.
+
+    [5] See the interesting account of Aristotle's
+    biological work in Prof. D'Arcy W. Thompson's Herbert
+    Spencer lecture (1913) and his translation of the
+    _Historia Animalium_ in the Oxford series.
+
+    [6] On Aristotle's forerunners, see R. Burckhardt, "Das
+    koische Tiersystem, eine Vorstufe des zoologischen
+    Systematik des Aristoteles." _Verh. Naturf. Ges. Basel_,
+    xx., 1904.
+
+    [7] T.E. Lones, _Aristotle's Researches in Natural
+    Science_, pp. 82-3, London, 1912.
+
+    [8] _De Partibus Animalium_, i., 4, 644^a trans. W.
+    Ogle, Oxford, 1911.
+
+
+
+
+CHAPTER II
+
+COMPARATIVE ANATOMY BEFORE CUVIER
+
+
+For two thousand years after Aristotle little advance was made upon
+his comparative anatomy. Knowledge of the human body was increased not
+long after his death by Herophilus and Erasistratus, but not even
+Galen more than four centuries later made any essential additions to
+Aristotle's anatomy.
+
+During the Middle Ages, particularly after the introduction to Europe
+in the 13th century of the Arab texts and commentaries, Aristotle
+dominated men's thoughts of Nature. The commentary of Albertus Magnus,
+based upon that of Avicenna, did much to impose Aristotle upon the
+learned world. Albertus seems to have contented himself with following
+closely in the footsteps of his master. There are noted, however, by
+Bonnier certain improvements made by Albertus on Aristotle's view of
+the seriation of living things. "He is the first," writes Bonnier, "to
+take the correct view that fungi are lower plants allied to the most
+lowly organised animals. From this point there start, for Albertus
+Magnus, two series of living creatures, and he regards the plant
+series as culminating in the trees which have well-developed
+flowers."[9]
+
+Aristotle's influence is predominant also in the work of Edward Wotton
+(1492-1555), who in his book _De differentiis animalium_ adopted a
+classification similar to that proposed by Aristotle. He too laid
+stress upon the gradation shown from the lower to the higher forms.
+
+In the 16th century, two groups of men helped to lay foundations for a
+future science of comparative anatomy--the great Italian anatomists
+Vesalius, Fallopius and Fabricius, and the first systematists (though
+their "systems" were little more than catalogues) Rondeletius,
+Aldrovandus and Gesner.
+
+The anatomists, however, took little interest in problems of pure
+morphology; the anatomy of the human body was for them simply the
+necessary preliminary of the discovery of the functions of the
+parts--they were quite as much physiologists as anatomists.
+
+One of them, Fabricius, made observations on the development of the
+chick (1615). Harvey, who was a pupil of Fabricius, likewise published
+an account of the embryology of the chick.[10] In his philosophy and
+habit of thought Harvey was a follower of Aristotle. It is worth
+noting that in his _Exercitationes anatomicae de motu cordis_ (1628)
+there is a passage which dimly foreshadows the law of recapitulation
+in development which later had so much vogue.[11]
+
+A stimulating contribution to comparative anatomy was made by
+Belon,[12] who published in 1555 a _Histoire de la nature des Oyseaux_,
+in which he showed opposite one another a skeleton of a bird and of a
+mammal, giving the same names to homologous bones. The anatomy of
+animals other than man was indeed not altogether neglected at this
+time. Coiter (1535-1600) studied the anatomy of Vertebrates,
+discovering among other things the fibrous structure of the brain.
+Carlo Ruini of Bologna wrote in 1598 a book on the anatomy of the
+horse.[13] Somewhat later Severino, professor at Naples, dissected many
+animals and came to the conclusion that they were built upon the same
+plan as man.[14] Willis, of Oxford and London, in his _Cerebri Anatome_
+(1659) recognised the necessity for comparative study of the structure
+of the brain. He found out that the brain of man is very like that of
+other mammals, the brain of birds, on the contrary, like that of
+fishes![15] He described the anatomy of the oyster and the crayfish. He
+had, however, not much feeling for morphology.
+
+The foundation of the Jardin des Plantes at Paris in 1626 and the
+subsequent addition to it of a Museum of Natural History and a
+menagerie gave a great impulse to the study of comparative anatomy by
+supplying a rich material for dissection. Advantage was taken of these
+facilities, particularly by Claude Perrault and Duverney.[16] In a
+volume entitled _De la Mecanique des Animaux_, Perrault recognises
+clearly the idea of unity of type, and even pushes it too far, seeking
+to prove that in plants there exists an arterial system and veins
+provided with valves.[17]
+
+The beginning of the 17th century saw the invention of the microscope,
+which was to have such an enormous influence upon the development of
+biological studies. It did not come into scientific use until well on
+in the middle of the century. Just before it came into use Francis
+Glisson (1597-1677), an Englishman, gave in the introduction to his
+treatise on the liver an account of the notions then current on the
+structure of organic bodies. He classifies the parts as "similar" and
+"organic," the former determined by their material, the latter by the
+form which they assume. The similar parts are divided into the
+sanguineous or rich in blood and the spermatic. Both sets are further
+subdivided according to their physical characters,[18] the latter, for
+instance, into the hard, soft, and tensile tissues. The classification
+resembles greatly that propounded by Aristotle, though it is notably
+inferior in the details of its working out.
+
+For Aristotle, as for all anatomists before the days of the
+microscope, the tissues were not much more than inorganic substances,
+differing from one another in texture, in hardness, and other physical
+properties. They possessed indeed properties, such as contractility,
+which were not inorganic, but as far as their visible structure was
+concerned there was little to raise them above the inorganic level.
+The application of the microscope changed all that, for it revealed in
+the tissues an organic structure as complex in its grade as the gross
+and visible structure of the whole organism. Of the four men who first
+made adequate use of the new aid, Malpighi, Hooke, Leeuenhoek, and
+Swammerdam, the first-named contributed the most to make current the
+new conceptions of organic structure. He studied in some detail the
+development of the chick. He described the minute structure of the
+lungs (1661), demonstrating for the first time, by his discovery of
+the capillaries, the connection of the arteries with the veins. In his
+work, _De viscerum structura_ (1666), he describes the histology of
+the spleen, the kidney, the liver, and the cortex of the brain,
+establishing among other things the fact that the liver was really a
+conglomerate gland, and discovering the Malpighian bodies in the
+kidney. This work was done on a broad comparative basis. "Since in the
+higher, more perfect, red-blooded animals, the simplicity of their
+structure is wont to be involved by many obscurities, it is necessary
+that we should approach the subject by the observation of the lower,
+imperfect animals."[19] So he wrote in the _De viscerum structura_, and
+accordingly he studied the liver first in the snail, then in fishes,
+reptiles, mammals, and finally man. In the introduction to his
+_Anatome plantarum_ (1675), in which he laid the foundations of plant
+histology, he vindicates the comparative method in the following
+words:--"In the enthusiasm of youth I applied myself to Anatomy, and
+although I was interested in particular problems, yet I dared to pry
+into them in the higher animals. But since these matters enveloped in
+peculiar mystery still lie in obscurity, they require the comparison
+of simpler conditions, and so the investigation of insects[20] at once
+attracted me; finally, since this also has its own difficulties I
+applied my mind to the study of plants, intending after prolonged
+occupation with this domain, to retrace my steps by way of the
+vegetable kingdom, and get back to my former studies. But perhaps not
+even this will be sufficient; since the simpler world of minerals and
+the elements should have been taken first. In this case, however, the
+undertaking becomes enormous and far beyond my powers."[21] There is
+something fine in this life of broad outlines, devoted whole-heartedly
+to an idea, to a plan of research, which required a lifetime to carry
+out.
+
+An important histological discovery dating from this time is that of
+the finer structure of muscle, made by Stensen (or Steno) in 1664. He
+described the structure of muscle-fibres, resolving them into their
+constituent fibrils.
+
+To the microscope we owe not only histology but the comparative
+anatomy of the lower animals. Throughout the 17th and 18th centuries
+the discovery of structure in the lower animals went on continuously,
+as may be read in any history of Zoology.[22] We content ourselves here
+with mentioning only some representative names.
+
+In the 17th century Leeuenhoek, applying the microscope almost at
+random, discovered fact after fact, his most famous, discovery being
+that of the "spermatic animalcules."
+
+Swammerdam studied the metamorphoses of insects and made wonderfully
+minute dissections of all sorts of animals, snails and insects
+particularly. He described also the development of the frog. It is
+curious to see what a grip his conception of metamorphosis had upon
+him when he homologises the stages of the frog's development with the
+Egg, the Worm, and the Nymph of insects (_Book of Nature_, p. 104,
+Eng. trans., 1785). He even speaks of the human embryo as being at a
+certain stage a Man-Vermicle.
+
+In the 18th century, 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 recapitulation
+theory. The passage occurs in his description of the drawings he made
+to illustrate the development of the chick. It is quoted in full by
+Owen (J. Hunter, _Observations on certain Parts of the Animal
+OEconomy_, with Notes by Richard Owen. London, 1837. Preface, p.
+xxvi). We give here the last and clearest sentence--"If we were to
+take a series of animals from the more imperfect to the perfect, we
+should probably find an imperfect animal corresponding with some stage
+of the most perfect."
+
+The tendency of the time was not towards morphology, but rather to
+general natural history and to systematics, the latter under the
+powerful influence of 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 Roesel von
+Rosenhof's _Insektenbelustigungen_ (1746-1761).
+
+Bonnet (1720-1793) deserves special mention here, since in his _Traite
+d'Insectologie_ (1745), and more fully in his _Contemplation de la
+Nature_ (1764), he gives the most complete expression to the idea of
+the _Echelle des etres_.
+
+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 _Traite_ _d'Insectologie_ (OEuvres, i., 1779) he
+gives a long table, headed "Idee d'une Echelle des etres naturels,"
+and rather resembling a ladder, on the rungs of which the following
+names appear:--
+
+MAN.
+Orang-utan.
+Ape.
+
+QUADRUPEDS.
+Flying squirrel.
+Bat.
+Ostrich.
+
+BIRDS.
+Aquatic birds.
+Amphibious birds.
+Flying Fish.
+
+FISH.
+Creeping fish.
+Eels.
+Water serpents.
+
+SERPENTS.
+Slugs.
+Snails.
+
+SHELL FISH.
+Tube-worms.
+Clothes-moths.
+
+INSECTS.
+Gall insects.
+Taenia.
+Polyps.
+Sea Nettles.
+Sensitive plant.
+
+PLANTS.
+Lichens.
+Moulds.
+Fungi, Agarics.
+Truffles.
+Corals, and Coralloids.
+Lithophytes.
+Asbestos.
+Talcs, Gypsums.
+Selenites, Slates.
+
+STONES.
+Figured stones.
+Crystals.
+
+SALTS.
+Vitriols.
+
+METALS.
+
+HALF-METALS.
+
+SULPHURS.
+Bitumens.
+
+EARTHS.
+Pure earth.
+
+WATER.
+
+AIR.
+
+FIRE.
+
+More subtile matter.
+
+The nature of the transitional forms which he inserts between his
+principal classes show very clearly his entire lack of morphological
+insight--the transitions are functional. The positions assigned to
+clothes-moths and corals are very curious! The whole scheme, so
+fantastic in its details, was largely influenced by Leibniz's
+continuity philosophy, and is in no way an improvement on the older
+and saner Aristotelian scheme.
+
+Robinet, in the fifth volume of his book _De la nature_ (1761-6),
+foreshadows the somewhat similar views of the German
+transcendentalists. "All beings," he writes, "have been conceived and
+formed on one single plan, of which they are the endlessly graduated
+variations: this prototype is the human form, the metamorphoses of
+which are to be considered as so many steps towards the most excellent
+form of being."[23]
+
+The idea of a gradation of beings appears also in Buffon (1707-1788),
+but here it takes more definitely its true character as a functional
+gradation.[24] "Since everything in Nature shades into everything
+else," he says, "it is possible to establish a scale for judging of
+the degrees of the intrinsic qualities of every animal."[25]
+
+He is quite well aware that the groups of Invertebrates are different
+in structural plan from the Vertebrates--"The animal kingdom includes
+various animated beings, whose organisation is very different from our
+own and from that of the animals whose body is similarly constructed
+to ours."[26]
+
+He limits himself to a consideration of the Vertebrates, deeming that
+the economy of an oyster ought not to form part of his subject matter!
+He has a clear perception of the unity of plan which reigns throughout
+the vertebrate series.[27] What is new in Buffon is his interpretation
+of the unity of plan. For the first time we find clearly expressed the
+thought that unity of plan is to be explained by community of origin.
+
+Buffon's utterances on this point are, as is well known, somewhat
+vacillating. The famous passage, however, which occurs in his account
+of the Ass shows pretty clearly that Buffon saw no theoretical
+objection to the descent of all the varied species of animals from one
+single form. Once admit, he argues, that within the bounds of a single
+family one species may originate from the type species by
+"degeneration," then one might reasonably suppose that from a single
+being Nature could in time produce all the other organised beings.[28]
+Elsewhere, _e.g._, in the discourse _De la Degeneration des
+Animaux_,[29] Buffon expresses himself with more caution. He finds that
+it is possible to reduce the two hundred species of quadrupeds which
+he has described to quite a small number of families "from which it is
+not impossible that all the rest are derived."[30] Within each of the
+families the species branch off from a parent or type species. This we
+may note is a great advance on the linear arrangement implied in the
+idea of an _Echelle des etres_.[31]
+
+It is a mistake to suppose that Buffon was par excellence a maker of
+hypotheses. On the contrary he saw things very sanely and with a very
+open mind. He expressly mentions the great difficulties which one
+encounters in supposing that one species may arise from another by
+"degeneration." How does it happen that two individuals "degenerate"
+just in the right direction and to the right stage so as to be capable
+of breeding together? How is it that one does not find intermediate
+links between species? One is reminded of the objections, not
+altogether without validity, which were made to the Darwinian theory
+in its early days. I cannot agree with those who think that Buffon was
+an out-and-out evolutionist, who concealed his opinions for fear of
+the Church. No doubt he did trim his sails--the palpably insincere
+"Mais non, il est certain, par la revelation, que tous les animaux ont
+egalement participe a la grace de la creation,"[32] following hard upon
+the too bold hypothesis of the origin of all species from a single
+one, is proof of it. But he was too sane and matter-of-fact a thinker
+to go much beyond his facts, and his evolution doctrine remained
+always tentative. One thing, however, he was sure of, that evolution
+would give a rational foundation to the classification which, almost
+in spite of himself, he recognised in Nature. If, and only if, the
+species of one family originated from a single type species, could
+families, be founded rationally, _avec raison_.
+
+Buffon was, curiously enough, rather unwilling to recognise any
+systematic unit higher than the species. Strictly; speaking there are
+only individuals in Nature; but there are also groups of individuals
+which resemble one another from generation to generation and are able
+to breed together. These are species--Buffon adheres to the genetic
+definition of species--and the species is a much more definite unit
+than the genus, the order, the class, which are not divisions imposed
+by us upon Nature. Species are definitely discontinuous,[33] and this
+is the only discontinuity which Nature shows us. Buffon put his views
+into practice in his _Histoire Naturelle_, where he describes species
+after species, never uniting them into larger groups. We have seen,
+however, how the facts forced upon him the conception of the "family."
+
+Buffon was no morphologist. He left to Daubenton what one might call
+the "dirty work" of his book, the dissection and minute description of
+the animals treated.
+
+But Buffon was a man of genius, and accordingly his ideas on
+morphology are fresh and illuminating. Few naturalists have been so
+free from the prejudices and traditions of their trade. He makes in
+the _Discours sur la Nature des Animaux_[34] a distinction, which
+Bichat and Cuvier later developed with much profit, between the
+"animal" and the "vegetative" part of animals.[35] The vegetative or
+organic functions go on continuously, even in sleep, and are performed
+by the internal organs, of which the heart is the central one. The
+active waking life of the animal, that part of its life which
+distinguishes it from the plant, involves the external parts--the
+sense-organs and the extremities. An animal is, as it were, made up of
+a complex of organs performing the vegetative functions, assimilation,
+growth, and reproduction, surrounded by an envelope formed by the
+limbs, the sense-organs, the nerves and the brain, which is the centre
+of this "envelope."[36] Animals may differ from one another enormously
+in the external parts, particularly in the appendicular skeleton,
+without showing any great difference in the plan and arrangement of
+their internal organs. Quadrupeds, Cetacea, birds, amphibians and fish
+are as unlike as possible in external form and in the shape of their
+limbs; but they all resemble one another in their internal organs. Let
+the internal organs change, however--the external parts will change
+infinitely more, and you will get another animal, an animal of a
+totally different nature. Thus an insect has a most singular internal
+economy, and, in consequence, you find it is in every point different
+from any vertebrate animal.
+
+In this contrast, on the whole justified, between the importance of
+variations in the "vegetative" and variations in the "animal" parts,
+one may see without doing violence to Buffon's thought, an indication
+of the difference between homology and analogy. It is usually in the
+external parts, in the organs by which the animal adapts itself to its
+environment, that one meets with the greatest number of analogical
+resemblances. This contrast of vegetative and animal parts and their
+relative importance for the discovery of affinities was at any rate a
+considerable step towards an analysis of the concept of unity of plan.
+
+To Xavier Bichat (1771-1802) belongs the credit of working out in
+detail the distinction drawn by Aristotle and Buffon between the
+animal and the vegetative functions. Bichat was not a comparative
+anatomist; his interest lay in human anatomy, normal and pathological.
+So his views are drawn chiefly from the consideration of human
+structure.
+
+He classifies functions into those relating to the individual and
+those relating to the species. The functions pertaining to the
+individual may be divided into those of the animal and those of the
+organic life.[37] "I call _animal life_ that order of functions which
+connects us with surrounding bodies; signifying thereby that this
+order belongs only to animals" (p. lxxviii.). Its organs are the
+afferent and efferent nerves, the brain, the sense-organs and the
+voluntary muscles; the brain is its central organ. "Digestion,
+circulation, respiration, exhalation, absorption, secretion,
+nutrition, calorification, or production of animal heat, compose
+organic life, whose principal and central organ is the heart" (p.
+lxxix.).
+
+The contrast of the animal and the organic life runs through all
+Bichat's work; it receives classical expression in his _Recherches
+Physiologiques sur la Vie et la Mort_ (1800). The plant and the animal
+stand for two different modes of living. The plant lives within
+itself, and has with the external world only relations of nutrition;
+the animal adds to this organic life a life of active relation with
+surrounding things (3rd ed., 1805, p. 2). "One might almost say that
+the plant is the framework, the foundation of the animal, and that to
+form the animal it sufficed to cover this foundation with a system of
+organs fitted to establish relations with the world outside. It
+follows that the functions of the animal form two quite distinct
+classes. One class consists in a continual succession of assimilation
+and excretion; through these functions the animal incessantly
+transforms into its own substance the molecules of surrounding bodies,
+later to reject these molecules when they have become heterogeneous to
+it. Through this first class of functions the animal exists only
+within itself; through the other class it exists outside; it is an
+inhabitant of the world, and not, like the plant, of the place which
+saw its birth. The animal feels and perceives its surroundings,
+reflects its sensations, moves of its own will under their influence,
+and, as a rule, can communicate by its voice its desires and its
+fears, its pleasures or its pains. I call organic life the sum of the
+functions of the former class, for all organised creatures, plants or
+animals, possess them to a more or less marked degree, and organised
+structure is the sole condition necessary to their exercise. The
+combined functions of the second class form the 'animal' life, so
+named because it is the exclusive attribute of the animal kingdom"
+(pp. 2-3).
+
+In both lives there is a double movement, in the animal life from the
+periphery to the centre and from the centre to the periphery, in the
+organic life also from the exterior to the interior and back again,
+but here a movement of composition and decomposition. As the brain
+mediates between sensation and motion, so the vascular system is the
+go-between of the organs of assimilation and the organs of
+dissimilation.
+
+The most essential structural difference between the organs of animal
+life and the organs of organic life is in man and the higher animals
+at least, the symmetry of the one set and the irregularity of the
+other--compare the symmetry of the nerves and muscles of the animal
+life with the asymmetrical disposition of the visceral muscles and the
+sympathetic nerves, which belong to the organic life.
+
+Noteworthy differences exist between the two lives with respect to the
+influence of habit. Everything in the animal life is under the
+dominion of habit. Habit dulls sensation, habit strengthens the
+judgment. In the organic life, on the contrary, habit exercises no
+influence. The difference comes out clearly in the development of the
+individual. The organs of the organic life attain their full
+perfection independently of use; the organs of the animal life require
+an education, and without education they do not reach perfection
+(_Loc. cit._, p. 127).
+
+Bichat was the founder of what was known for a time as General
+Anatomy--the study of the constituent tissues of the body in health
+and disease. His classification of tissues was macroscopical and
+physiological; he relied upon texture and function in distinguishing
+them rather than upon microscopical structure. The tissues he
+distinguished are as follows:--[38]
+
+1. The cellular membrane.
+2. Nerves of animal life.
+3. Nerves of organic life.
+4. Arteries.
+5. Veins.
+6. Exhalants.
+7. Absorbents and glands.
+8. Bones.
+9. Medulla.
+10. Cartilage.
+11. Fibrous tissue.
+12. Fibro-cartilage.
+13. Muscles of organic life.
+14. Muscles of animal life.
+15. Mucous membrane.
+16. Serous membrane.
+17. Synovial membrane.
+18. The Glands.
+19. The Dermis.
+20. Epidermis.
+21. Cutis.
+
+The "cellular membrane" seems to mean undifferentiated connective
+tissue; "exhalants" are imperceptible tubes arising from the
+capillaries and secreting fat, serum, marrow, etc.; the "absorbents
+and glands" are the lymphatics and the lymphatic glands.
+
+In Bichat's eyes this resolution of the organism into tissues had a
+deeper significance than any separation into organs, for to each
+tissue must be attributed a _vie propre_, an individual and peculiar
+life. "When we study a function we must consider the complicated organ
+which performs it in a general way; but if we would be instructed in
+the properties and life of that organ we must absolutely resolve it
+into its constituent parts."[39] The tissues have, too, a great
+importance for pathology, for diseases are often diseases of tissues
+rather than of organs.[40]
+
+    [9] _Le Monde vegetal_, p. 41, Paris, 1907.
+
+    [10] _Exercitationes de generatione animalium_,1651. For
+    an account of Harvey's work on generation and
+    development, see Em. Radl's masterly _Geschichte der
+    biologischen Theorien_, i., pp. 31-8, Leipzig, 1905.
+
+    [11] The passage runs:--"Sic natura perfecta et divina
+    nihil faciens frustra, nec quipiam animali cor addidit,
+    ubi non erat opus, neque priusquam esset ejus usus,
+    fecit; sed iisdem gradibus in formatione cujuscumque
+    animalis, transiens per omnium animalium constitutiones
+    (ut ita dicam) ovum, vermem, foetum, perfectionem in
+    singulis acquirit."
+
+    [12] See I. Geoffroy St Hilaire, _Essais de Zoologie
+    generale_, p. 71, Paris, 1841.
+
+    [13] M. Foster, _Lectures on the History of Physiology_,
+    Cambridge, p. 53, 1901.
+
+    [14] _Zootomia democritea_, Nuremberg, 1645;
+    _Antiperipatias, seu de respiratione piscium_,
+    Amsterdam, 1661.
+
+    [15] Radl, _loc. cit._, i., p. 50.
+
+    [16] Perrault et Duverney, _Memoires pour servir a
+    l'histoire des Animaux_, Paris, 1699.
+
+    [17] F. Houssay, _Nature et Sciences naturelles_, Paris,
+    p. 76, n.d.
+
+    [18] Foster, _loc. cit._, p. 85.
+
+    [19] Trans, by Foster, _loc. cit._, p. 113.
+
+    [20] He made a careful study of the silkworm.
+
+    [21] "Etenim, ferventi aetatis calore, Anatomica
+    aggressus, licet circa peculiaria fuerim solicitus, in
+    _perfectioribus_ tamen haec rimari sum ausus. Verum, cum
+    haec propriis tenebris obscura jaceant, simplicium
+    analogismo egent; inde _insectorum_ indago illico
+    arrisit; quae cum et ipsa suas habeat difficultates ad
+    Plantarum perquisitionem animum _postremo_ adjeci, ut
+    diu hoc lustrato mundo gressu retroacto Vegetantis
+    Naturae gradu, ad prima studia iter mihi aperirem. Sed
+    nec forte hoc ipsum sufficiet cum simplicior _Mineralium
+    Elementorumque_ mundus praeire debeat. At in immensum
+    excrescit opus, et meis viribus omnino impar," _Opera
+    Omnia_, i., p. 1, London, 1686.
+
+    [22] See particularly E. Radl, _loc. cit._. I Teil. J. V.
+    Carus, _Geschichte der Zoologie_, Muenchen, 1872.
+
+    [23] For a good historical account of the gradation
+    theories see Thienemann's paper in the _Zoologische
+    Annalen_(Wuerzburg) iii., pp. 185-274, 1910, from which
+    the quotation from Robinet is taken.
+
+    [24] _Histoire naturelle_, i., p. 13; ii, p. 9; iv., p.
+    101; and xiv., pp. 28-9, 1749 and later.
+
+    [25] No translation can render the beauty of the
+    original--"Comme tout se fait et que tout est par nuance
+    dans la Nature ..." (iv., p. 101).
+
+    [26] _Hist. nat._, iv., p. 5.
+
+    [27] See particularly his comparison of the skeleton of
+    the horse with that of man. _Hist. Nat._, iv., p. 381,
+    also p. 13.
+
+    [28] _Loc. cit._, p. 382.
+
+    [29] Tome xiv., pp. 311-374.
+
+    [30] Tome xiv., p. 358.
+
+    [31] See also "Oiseaux," Tome i., pp. 394, 395. Pallas in
+    1766 adopted for the whole animal kingdom this branching
+    arrangement.
+
+    [32] "But this cannot be, for it is certain by revelation
+    that all animals have equally participated in the grace
+    of creation."
+
+    [33] iv., p. 385.
+
+    [34] iv., pp. 3-110.
+
+    [35] It has been revived in our own days by Bergson,
+    _Matiere et Memoire_, p. 57.
+
+    [36] iv., pp. 7-15.
+
+    [37] _Anatomie Generale_, Paris, 1801, Eng. trans. 1824.
+
+    [38] _Anatomie Generale_, Eng. trans., i., p. lii.
+
+    [39] _Anatomie Generale_, Eng. trans., i., p. lviii.
+
+    [40] _Loc cit._, i., sect. vii.
+
+
+
+
+CHAPTER III
+
+CUVIER
+
+
+Cuvier was perhaps the greatest of comparative anatomists; his work
+is, in the best sense of the word, classical.
+
+Like all his predecessors, like Aristotle, like the Italian
+anatomists, Cuvier studied structure and function together, even gave
+function the primacy.
+
+Some functions, he says,[41] are common to all organised bodies--origin
+by generation, growth by nutrition, end by death. There are also
+secondary functions. Of these the most important, in animals at least,
+are the faculties of feeling and moving. These two faculties are
+necessarily bound up together; if Nature has given animals sensation
+she must also have given them the power of movement, the power to flee
+from what is harmful and draw near to what is good. These two
+faculties determine all the others. A creature that feels and moves
+requires a stomach to carry food in. Food requires instruments to
+divide it, liquids to digest it. Plants, which do not feel and do not
+move, have no need of a stomach, but have roots instead. Thus the
+"Animal Functions" of feeling and moving determine the character of
+the organs of the second order, the organs of digestion. These in
+their turn are prior to the organs of circulation, which are a means
+to the end of distributing the nutrient fluid or blood to all parts of
+the body. These organs of the third order are not only dependent on
+those of the second order, but are also not even necessary, for many
+animals are without them. Only animals with a circulatory system can
+have definite breathing organs--lungs or gills. Plants, and animals
+without a circulation, breathe by their whole surface.
+
+There is accordingly a rational order of functions, and therefore of
+the systems of organs which perform them. The most important are the
+Animal Functions, with their great organ-system, the neuro-muscular
+mechanism. Then come the digestive functions, and after them, and in a
+sense accessory to them, the functions and organs of circulation and
+respiration. The last three may be grouped as the Vital Functions.
+
+The Animal Functions not only determine the character of the Vital
+Functions, but influence also the primary faculty of generation, for
+animals' power of movement has rendered their mode of fecundation more
+simple, has therefore had an effect on their organs of generation.
+
+This division into "Animal" and "Vital" functions recalls Buffon's and
+Bichat's distinction of the "animal" and the "vegetative" lives.
+Cuvier apparently took this idea from Buffon, for he says that a plant
+is an animal that sleeps.[42] But the idea is as old as Aristotle, who
+discusses the "sleep" of embryos and of plants in the last book of the
+_De Generatione animalium_. The distinction between animal and
+vegetative life is, of course, based for Aristotle in the difference
+between the [Greek: psyche aisthetike] and the [Greek: psyche
+threptike]. Cuvier, like Aristotle, Buffon, and Bichat, makes the
+heart the centre of the "vegetative" organs.
+
+It is important to note that Cuvier puts function before structure,
+and infers from function what the organ will be. "Plants," he writes,
+"having few faculties, have a very simple organisation."[43] It is only
+after having discussed and classified functions that Cuvier goes on to
+examine organs.
+
+First his views on the composition of the animal body. Aristotle
+distinguished three degrees of composition--the "elements," the
+homogeneous parts, and the heterogeneous parts or organs. Cuvier does
+the same. Some small advance has been made in the two thousand years'
+interval, due in the first place to the progress of chemistry, and in
+the second to the invention of the microscope. To the first
+circumstance Cuvier owes his knowledge that the inorganic substances
+forming the first degree of composition are principally C, N, H, O,
+and P, combined to form albumen, fibrine, and the like, which are in
+their turn combined to form the solids and fluids of the body. To the
+latter circumstance Cuvier owes the statement that the finest
+fragments into which mechanical division can resolve the organism are
+little flakes and filaments, which, joined up loosely together, form a
+"cellulosity." The discovery of the true cellular nature of animal
+tissues did not come till much later, till some years after Cuvier's
+death in 1832. Knowledge of histological detail was, however,
+considerable by the beginning of the 19th century. Cuvier knew, for
+example, that each muscle fibre has its own nerve fibre. But he gives
+no elaborate account of the homogeneous parts, no detailed histology.
+On the other hand his treatment of the heterogeneous parts or organs
+is detailed and masterly.[44]
+
+The main systems of organs are, in order of importance, the nervous
+and muscular, the digestive, the circulatory, and the respiratory.
+Each organ or system of organs may have many forms. If any form of any
+organ could exist in combination with any form of all the others there
+would be an enormous number of combinations theoretically possible.
+But these combinations do not all exist in Nature, for organs are not
+merely assembled (_rapproche's_), but act upon one another, and act
+all together for a common end. Accordingly only the combinations that
+fulfil these conditions exist in Nature. Cuvier thus dismisses the
+question of a science of possible organic forms and considers only the
+forms or combinations actually existing. This question of the
+possibility of a "theoretical" morphology of living things, after the
+fashion of the morphology of crystals with their sixteen possible
+types, was raised in later years by K. G. Carus, Bronn, and Haeckel.
+
+Organisms, then, are harmonious combinations of organs, and the
+harmony is primarily a harmony of functions. Every function depends
+upon every other, and all are necessary. The harmony of organs and
+their mutual dependence are the results of the interdependence of
+function. This thought, the recognition of the functional unity of the
+organism, is the fundamental one at the base of all Cuvier's work.
+Before him men had recognised more or less clearly the harmony of
+structure and function, and had based much of their work upon this
+unanalysed assumption. Cuvier was the first naturalist to raise this
+thought to the level of a principle peculiar to natural history. "It
+is on this mutual dependence of the functions and the assistance which
+they lend one to another that are founded the laws that determine the
+relations of their organs; these laws are as inevitable as the laws of
+metaphysics and mathematics, for it is evident that a proper harmony
+between organs that act one upon another is a necessary condition of
+the existence of the being to which they belong."[45]
+
+This rational principle, peculiar to natural history, Cuvier calls the
+principle of the conditions of existence, for the following
+reason:--"Since nothing can exist that does not fulfil the conditions
+which render its existence possible, the different parts of each being
+must be co-ordinated in such a way as to render possible the existence
+of the being as a whole, not only in itself, but also in its relations
+with other beings, and the analysis of these conditions often leads to
+general laws which are as certain as those which are derived from
+calculation or from experiment."[46]
+
+By "conditions of existence" he means something quite different from
+what is now commonly understood. The idea of the external conditions
+of existence, the environment, enters very little into his thought. He
+is intent on the adaptations of function and organ within the living
+creature--a point of view rather neglected nowadays, but essential for
+the understanding of living things. The very condition of existence of
+a living thing, and part of the essential definition of it, is that
+its parts work together for the good of the whole.
+
+The principle of the adaptedness of parts may be used as an
+explanatory principle, enabling the naturalist to trace out in detail
+the interdependence of functions and their organs. When you have
+discovered how one organ is adapted to another and to the whole, you
+have gone a certain way towards understanding it. That is using
+teleology as a regulative principle, in Kant's sense of the word.
+Cuvier was indeed a teleologist after the fashion of Kant, and there
+can be no doubt that he was influenced, at least in the exposition of
+his ideas, by Kant's _Kritik der Urtheilskraft_, which appeared ten
+years before the publication of the _Lecons d'Anatomie Comparee_.
+Teleology in Kant's sense is and will always be a necessary postulate
+of biology. It does not supply an explanation of organic forms and
+activities, but without it one cannot even begin to understand living
+things. Adaptedness is the most general fact of life, and innumerable
+lesser facts can be grouped as particular cases of it, can be, so far,
+understood.
+
+Cuvier's famous principle of correlation, the corner-stone of his
+work, is simply the practical application to the facts of structure of
+the principle of functional adaptedness. By the principle of
+correlation, from one part of an animal, given sufficient knowledge of
+the structure of its like, you can in a general way construct the
+whole. "This must necessarily be so: for all the organs of an animal
+form a single system, the parts of which hang together, and act and
+re-act upon one another; and no modifications can appear in one part
+without bringing about corresponding modifications in all the
+rest."[47] The logical basis of the principle is sound. The functions
+of the parts are all intimately bound up with one another, and one
+function cannot vary without bringing in its train corresponding
+modifications in the others. Structure and function are bound up
+together; every modification of a function entails therefore the
+modification of an organ. Hence from the shape of one organ you can
+infer the shape of the other organs--if you have sufficiently
+extensive empirical knowledge of functions, and of the relation of
+structure to function in each kind of organ. Given an alimentary canal
+capable of digesting only flesh, and possessing therefore a certain
+form, you know that the other functions must be adapted to this
+particular function of the alimentary canal. The animal must have keen
+sight, fine smell, speed, agility, and strength in paws and jaws.
+These particular functions must have correspondingly modified organs,
+well-developed eyes and ears, claws and teeth. Further, you know from
+experience that such and such definitely modified organs are
+invariably found with the carnivorous habit, carnassial teeth, for
+example, and reduced clavicles. From a "carnivorous" alimentary canal,
+then, you can infer with certainty that the animal possessed
+carnassial teeth and the other structural peculiarities of carnivorous
+animals, _e.g._, the peculiar coronoid process of the mandible. From
+the carnassial tooth you can infer the reduced clavicle, and so on.
+"In a word, the form of the tooth implies the form of the condyle;
+that of the shoulder blade that of the claws, just as the equation of
+a curve implies all its properties."[48]
+
+Similarly the great respiratory power of birds is correlated with
+their great muscular strength, and renders necessary great digestive
+powers. Hence the correlated structure of lungs, muscles and their
+attachments, and alimentary canal, in birds.
+
+Not only do systems of organs, by being adjusted to special
+modifications of function, influence one another, but so also do parts
+of the same organ. This is noticeably the case with the skeleton,
+where hardly a facet can vary without the others varying
+proportionately, so that from one bone you can up to a certain point
+deduce all the rest.
+
+We deduce the necessity, the constancy, of these co-existences of
+organs from the observed reciprocal influence of their functions. That
+being established, we can argue from observed constancy of relation
+between two organs an action of one upon the other, and so be led to a
+discovery of their functions. But even if we do not discover the
+functional interdependencies of the parts, we can use the established
+fact of the constant co-existence of two parts as proof of a
+functional correlation between them.
+
+Correlation is either a rational or an empirical principle, according
+as we know or do not know the interdependence of function of which it
+is the expression. Even when we apply the rational principle of
+correlation it would be useless in our hands if we had not extensive
+empirical knowledge; when we use an empirical rule of correlation we
+depend entirely upon observation. "There are a great many cases,"
+writes Cuvier,[49] "where our theoretical knowledge of the relations of
+forms would not suffice, if it were not filled out by observation,"
+that is to say, there are many cases of correlation not yet explicable
+in terms of function. From a hoof you can deduce the main characters
+of herbivores (with a certain amount of assistance from your empirical
+knowledge of herbivores), but could you from a cloven hoof deduce that
+the animal is a ruminant, unless you had observed the constancy of
+relation, not directly explicable in terms of function, between cloven
+hoofs and chewing the cud? Or could you deduce from the existence of
+frontal horns that the animal ruminates? "Nevertheless, since these
+relations are constant, they must necessarily have a sufficient cause;
+but as we are ignorant of this cause, observation must supplement
+theory; observation establishes empirical laws which become almost as
+certain as the rational laws, when they are based upon a sufficient
+number of observations.... But that there exist all the same hidden
+reasons for all these relations is partly revealed by observation
+itself, independently of general philosophy."[50] That is to say, even
+correlations for which no explanation in terms of function can be
+supplied are probably in reality functional correlations. This may, in
+some cases, be inferred from the graded correspondence of two sets of
+organs. For example, ungulates which do not ruminate, and have not a
+cloven hoof, have a more perfect dentition and more bones in the foot
+than the true cloven-hoofed ruminants. There is a correlation between
+the state of development of the teeth and of the foot. This
+correlation is a graded one, for camels, which have a more perfect
+dentition than other ruminants, have also a bone more in their tarsus.
+It seems probable, therefore, that there is some reason, that is, some
+explanation in terms of function, for this case of correlation.
+
+Nevertheless, the fact remains that many correlations are not
+explicable in terms of function, and the substitution of correlation
+as an empirical principle for correlation as a rational principle
+marks for Cuvier a step away from his functional comparative anatomy
+towards a pure morphology. It is significant that in later times the
+term correlation has come to be applied more especially to the purely
+empirical constancies of relation, and has lost most of its functional
+significance. But the correlation of the parts of an organism is no
+mere mathematical concept, to be expressed by a coefficient, but
+something deeper and more vital.
+
+Cuvier interpreted the functional dependence of the parts in terms of
+what we now call the general metabolism. He had a clear vision of the
+constant movement of molecules in the living tissue, combining and
+recombining, of the organism taking in and intercalating molecules
+from outside from the food and rejecting molecules in the excretions,
+a ceaseless _tourbillon vital_. "This general movement, universal in
+every part, is so unmistakably the very essence of life that parts
+separated from a living body straightway die."[51] The organisation of
+the body, the arrangement of its solids and liquids, is adapted to
+further the _tourbillon vital_. "Each part contributes to this general
+movement its own particular action and is affected by it in particular
+ways, with the result that, in every being, life is a unity which
+results from the mutual action and reaction of all its parts."[52]
+
+Cuvier, however, did not resolve life into metabolism, nor reduce
+vital happenings to the chemical level. The form of organised bodies
+is more essential than the matter of which they are composed, for the
+matter changes ceaselessly while the form remains unchanged. It is in
+form that we must seek the differences between species, and not in the
+combinations of matter, which are almost the same in all.[53] The
+differences are to be sought at the level of the second and third
+degrees of composition.
+
+The existence of differences of form introduces a new problem, the
+problem of diversity. There are only a few possible combinations of
+the principal organs, but as you get down to less important parts the
+possible scope of variation is greatly increased, and most of the
+possible variations do exist. Nature seems prodigal of form, of form
+which needs not to be useful in order to exist. "It needs only to be
+possible, _i.e._, of such a character that it does not, destroy the
+harmony of the whole."[54] We seize here the relation of the principle
+of the adaptedness of parts to the problem of the variety of form. The
+former is in a sense a regulative and conservative principle which
+lays down limits beyond which variation may not stray. In itself it is
+not a fountain of change; there must be another cause of change. This
+thought is of great importance for theories of descent.
+
+Cuvier has no theory to account for the variety of form: he contents
+himself with a classification. There are two main ways of classifying
+forms; you may classify according to single organs or according to the
+totality of organs. By the first method you can have as many
+classifications as you have organs, and the classifications will not
+necessarily coincide. Thus you can divide animals according to their
+organs of digestion into two classes, those in which the alimentary
+canal is a sac with one opening (zoophytes) and those in which the
+canal has two openings,[55] a curious forestalment, in the rough, of
+the modern division of Metazoa into Coelentera and Coelomata.
+
+It is only by taking single organs that you can arrange animals into
+long series, and you will have as many series as you take organs. Only
+in this way can you form any _Echelle des etres_ or graded series; and
+you can get even this kind of gradation only within each of the big
+groups formed on a common plan of structure; you can never grade, for
+example, from Invertebrates to Vertebrates through intermediate
+forms[56] (which is perfectly true, in spite of Amphioxus and
+Balanoglossus!).
+
+In the _Regne Animal_ Cuvier restricts the application of the idea of
+the _Echelle_ within even narrower limits, refusing to admit its
+validity within the bounds of the vertebrate phylum, or even within
+the vertebrate classes. This seems, however, to refer to a seriation
+of whole organisms and not of organs, so that the possibility of a
+seriation of organs within a class is not denied. Cuvier was, above
+all, a positive spirit, and he looked askance at all speculation which
+went beyond the facts. "The pretended scale of beings," he wrote, "is
+only an erroneous application to the totality of creation of partial
+observations, which have validity only when confined to the sphere
+within which they were made."[57] This remark, which is after all only
+just, perfectly expresses Cuvier's attitude to the transcendental
+theories, and was probably a protest against the sweeping
+generalisations of his colleague, Etienne Geoffroy St Hilaire.
+
+A true classification should be based upon the comparison of all
+organs, but all organs are not of equal value for classification, nor
+are all the variations of each organ equally important. In estimating
+the value of variations more stress should be laid on function than on
+form, for only those variations are important which affect the mode of
+functioning. These are the principles on which Cuvier bases the
+classification of animals given in the _Lecons_, Article V., "Division
+des animaux d'apres l'ensemble de leur organisation." The scheme of
+classification actually given in the _Lecons_ recalls curiously that
+of Aristotle, for there is the same broad division into Vertebrates,
+with red blood, and Invertebrates, almost all with white blood. Nine
+classes altogether are distinguished--Mammals, Birds, Reptiles,
+Fishes, Molluscs, Crustacea, Insects, Worms, Zoophytes (including
+Echinoderms and Coelenterates).
+
+A maturer theory and practice of classification is given in the _Regne
+Animal_ of seventeen years later. Here the principle of the
+subordination of characters (which seems to have been first explicitly
+stated by the younger de Jussieu in his _Genera Plantarum_, 1789,[58])
+is more clearly recognised. The properties or peculiarities of
+structure which have the greatest number of relations of
+incompatibility and coexistence, and therefore influence the whole in
+the greatest degree, are the important or dominating characters, to
+which the others must be subordinated in classification. These
+dominant characters are also the most constant.[59] In deciding which
+characters are the most important Cuvier makes use of his fundamental
+classification of functions and organs into two main sets. "The heart
+and the organs of circulation are a kind of centre for the vegetative
+functions, as the brain and the spinal cord are for the animal
+functions."[60] These two organ-systems vary in harmony, and their
+characters must form the basis for the delimitation of the great
+groups. Judged by this standard there are four principal types of
+form,[61] of which all the others are but modifications. These four
+types are Vertebrates, Molluscs, Articulates, and Radiates. The first
+three have bilateral, the last has radial symmetry. Vertebrates and
+Molluscs have blood-vessels, but Articulates show a functional
+transition from the blood-vessel to the tracheal system. Radiates
+approach the homogeneity of plants; they appear to lack a distinct
+nervous system and sense organs, and the lowest of them show only a
+homogeneous pulp which is mobile and sensitive. All four classes are
+principally distinguished from one another by the broad structural
+relations of their neuromuscular system, of the organs of the animal
+functions. Vertebrates have a spinal cord and brain, an internal
+skeleton built on a definite plan, with an axis and appendages; in
+Molluscs the muscles are attached to the skin and the shell, and the
+nervous system consists of separate masses; Articulates have a hard
+external skeleton and jointed limbs, and their nervous system consists
+of two long ventral cords; Radiates have ill-defined nervous and
+muscular systems, and in their lowest forms possess the animal
+functions without the animal organs.
+
+This well-rounded classification of animal forms is in a sense the
+crown of Cuvier's work, for the principle of the subordination of
+characters, in the interpretation which he gives to it, is a direct
+application of his principle of functional correlation. Each of the
+great groups is built upon one plan. The idea of the unity of plan has
+become for Cuvier a commonplace of his thought, and it is tacitly
+recognised in all his anatomical work. But he never takes it as a
+hard-and-fast principle which must at all costs be imposed upon the
+facts.
+
+Cuvier has become known as the greatest champion of the fixity of
+species, but it is not often recognised that his attitude to this
+problem is at least as scientific as that of the evolutionists of his
+own and later times. No doubt he became dogmatic in his rejection of
+evolution-theory, but he was on sure ground in maintaining that the
+evolutionists of his day went beyond their facts. He considered that
+certain forms (species) have reproduced themselves from the origin of
+things without exceeding the limits of variation. His definition of a
+species was, "the individuals descended from one another or from
+common parents, together with those that resemble them as much as they
+resemble one another."[62] "These forms are neither produced nor do
+they change of themselves; life presupposes their existence, for it
+cannot arise save in organisations ready prepared for it."[63]
+
+He based his rejection of all theories of descent upon the absence of
+definite evidence for evolution. If species have gradually changed, he
+argued, one ought to find traces of these gradual modifications.[64]
+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."[65] This second
+objection is the objection of the morphologist. It would be an
+interesting study to compare Cuvier's views on variation with those of
+Darwin, who was essentially a systematist.
+
+Cuvier's first objection was of course determined to some extent by
+the imperfection of the 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
+_Paludina_ series, yet at any one geological level the species, the
+lines of descent, are all distinct from one another.[66]
+
+Cuvier recognised very clearly that there is a succession of forms in
+time, and that on the whole the most primitive forms are the earliest
+to appear. Mammals are later than reptiles, and fishes appear earlier
+than either. As Deperet puts it, "Cuvier not only demonstrated the
+presence in the sedimentary strata of a series of terrestrial faunas
+superimposed and distinct, but he was the first to express, and that
+very clearly, the idea of the gradual increase in complexity of these
+faunas from the oldest to the most recent" (p. 10).
+
+He did not believe that the fauna of one epoch was transformed into
+the fauna of the next. He explained the disappearance of the one by
+the hypothesis of sudden catastrophes, and the appearance of the next
+by the hypothesis of immigration. He nowhere advanced the hypothesis
+of successive new creations. "For the rest, when I maintain that the
+stony layers contain the bones of several genera and the earthy layers
+those of several species which no longer exist, I do not mean that a
+new creation has been necessary to produce the existing species, I
+merely say that they did not exist in the same localities and must
+have come thither from elsewhere."[67] It was left to d'Orbigny to
+teach the doctrine of successive creations, of which he distinguished
+twenty-seven (_Cours elementaire de palaeontologie stratigraphique_,
+1849).
+
+Cuvier, however, can hardly have believed that all species were
+present at the beginning, since he does admit a progression of forms.
+Probably he had no theory on the subject, for theories without facts
+had little interest for him. At any rate it is a mistake to think that
+Cuvier was a supporter of the theological doctrine of special
+creation. His philosophy of Nature was mechanistic, and he dedicated
+his _Recherches sur les Ossemens Fossiles_ to his friend Laplace. He
+admitted the idea of evolution at least so far as to conceive of a
+development of man from a savage to a civilised state.[68] He refused
+to accept the extravagant evolutionary theory of Demaillet and the
+somewhat confused theory of Lamarck (whom he joins with Demaillet),[69]
+just as he rejected the transcendental theories of Geoffroy St
+Hilaire, because they seemed to him not based upon facts.
+
+    [41] _Lecons d'Anatomie Comparee_, tome i., pp. 10 _et
+    scq._, 1800.
+
+    [42] _Lecons d'Anatomie Comparee_, i., p. 18.
+
+    [43] _Loc. cit._, i., p. 13.
+
+    [44] _Lecons d'Anatomie Comparee_, tome i., Articles
+    iii.-iv., 1800.
+
+    [45] _Lecons d'Anatomie Comparee_, i., p. 47.
+
+    [46] _Le Regne Animal_, i., p. 6, 1817.
+
+    [47] _Histoire des Progres des Sciences naturelles depuis
+    1789_, i., p. 310, 1826.
+
+    [48] _Recherches sur les Ossemens Fossiles_, i., p. 60,
+    1812.
+
+    [49] _Ossemens fossiles_, i., p. 60.
+
+    [50] _Loc. cit._, i., p. 63.
+
+    [51] _Lecons d'Anatomie Comparee_, i., p. 6.
+
+    [52] _Le Regne Animal_, i., p. 16.
+
+    [53] _Hist. Prog. Sci. Nat._, i., p. 187, 1826.
+
+    [54] _Lecons_, i., p. 58.
+
+    [55] _Loc. cit._, i., Article iii.
+
+    [56] _Loc. cit._, i., p. 60.
+
+    [57] _Regne Animal_, i., p. xx.
+
+    [58] Cuvier, _Hist. Prog. Sci. Nat._, i., p. 288, 1826.
+
+    [59] _Regne Animal_, i., p. 10.
+
+    [60] _Regne Animal_, p. 55.
+
+    [61] First propounded by Cuvier in 1812, _Ann. Mus.
+    d'Hist. Nat._, xix.
+
+    [62] _Regne Animal_, i., p. 19.
+
+    [63] _Loc. cit._, p. 20.
+
+    [64] _Recherches sur les Ossemens Fossiles_, i., p. 74,
+    1812.
+
+    [65] _Loc. cit._, p. 79.
+
+    [66] See C. Deperet, _Les transformations du Monde
+    animal_, Paris, 1907, and G. Steinmann, _Die
+    geologischen Grundlagen der Abstammungslehre_, Leipzig,
+    1908.
+
+    [67] _Recherches_, i., p. 81.
+
+    [68] _Regne Animal_, i., p. 91.
+
+    [69] _Ossemens Fossiles_, i., p. 26.
+
+
+
+
+CHAPTER IV
+
+GOETHE
+
+
+Science, in so far as it rises above the mere accumulation of facts,
+is a product of the mind's creative activity. Scientific theories are
+not so much formulae extracted from experience as intuitions imposed
+upon experience. So it was that Goethe, who was little more than a
+dilettante,[70] seized upon the essential principles of a morphology
+some years before that morphology was accepted by the workers.
+
+Goethe is important in the history of morphological method because he
+was the first to bring to clear consciousness and to express in
+definite terms the idea on which comparative anatomy before him was
+based, the idea of the unity of plan. We have seen that this idea was
+familiar to Aristotle and that it was recognised implicitly by all who
+after him studied structure comparatively. In Goethe's time the idea
+had become ripe for expression. It was used as a guiding principle in
+Goethe's youth particularly by Vicq d'Azyr and by Camper. The former
+(1748-1794), who discovered[71] in the same year as Goethe (1784) the
+intermaxillary bone in man, pointed out the homology in structure
+between the fore limb and the hind limb, and interpreted certain
+rudimentary bones, the intermaxillaries and rudimentary clavicles, in
+the light of the theory that Vertebrates are built upon one single
+plan of structure.
+
+"Nature seems to operate always according to an original and general
+plan, from which she departs with regret and whose traces we come
+across everywhere" (Vicq d'Azyr, quoted by Flourens, _Mem. Acad.
+Sei._, XXIII., 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 common to philosophy,
+to ordinary thought, and to anatomical science. We find it expressed
+by Herder (who perhaps got it from Kant) in his _Ideen sur Philosophie
+der Geschichte der Menschheit_ (1784), and it is possible that Goethe
+became impressed with the importance of the idea through his
+conversations with Herder. Be that as it may, it is certain that
+Goethe sought for the intermaxillaries in man only because he was
+firmly convinced that the skeleton in all the higher animals was built
+upon one common plan and that accordingly bones such as the
+intermaxillaries, found well developed in some animals, must also be
+found in man. The idea was not drawn from the facts, but the facts
+were interpreted and even sought for in the light of the idea. "I
+eagerly worked upon a general osteological scheme, and had accordingly
+to assume that all the separate parts of the structure, in detail as
+in the whole, must be discoverable in all animals, because on this
+supposition is built the already long begun science of comparative
+anatomy."[72]
+
+The principle comes to clear expression in his _Erster Entwurf einer
+allgemeinen Einleitung in die vergleichende Anatomie_ (1795).[73] He
+writes:--"On this account an attempt is here made to arrive at an
+anatomical type, a general picture in which the forms of all animals
+are contained in potentia, and by means of which we can describe each
+animal in an invariable order."[74] His aim is to discover a general
+scheme of the constant in organic parts, a scheme into which all
+animals will fit equally well, and no animal better than the rest.
+When we remember that the type to which anatomists before him had,
+consciously or unconsciously, referred all other structure was man
+himself, we see that in seeking after an abstract generalised type
+Goethe was reaching out to a new conception. The fact that only the
+structure of man and the higher animals was at all well-known in his
+time led Goethe to think that his general Typus would hold for the
+lower animals as well, though it was to be arrived at primarily from a
+study of the higher animals. All he could assert of the entire animal
+kingdom was that all animals agreed in having a head, a middle part,
+and an end part, with their characteristic organs, and that
+accordingly they might, in this respect at least, be reduced to one
+common Typus. Goethe's knowledge of the lower animals was not
+extensive.
+
+Though Goethe did not work out a criterion of the homology of parts
+with any great clearness, he had an inkling of the principle later
+developed by E. Geoffroy St Hilaire, and called by him the "Principle
+of Connections." According to this principle, the homology of a part
+is determined by its position relative to other parts. Goethe
+expresses it thus:--"On the other hand the most constant factor is the
+position in which the bone is invariably found, and the function to
+which it is adapted in the organic edifice."[75] But from this sentence
+it is not clear that Goethe understood the principle as one of form
+independent of function, for he seems to consider that the homology of
+an organ is partly determined by the function which it performs for
+the whole. He wavers between the purely formal or morphological
+interpretation of the principle of connections and the functional. We
+find him in the additions to the _Entwurf_ (1796), saying:--"We must
+take into consideration not merely the spatial relations of the parts,
+but also their living reciprocal influence, their dependence upon and
+action on one another." [76] But in seeking for the intermaxillary bone
+in man he was guided by its position relative to the maxillaries--it
+must be the bone between the anterior ends of the maxillaries, a bone
+whose limits are indicated in the adult only by surface grooves.
+
+As a matter of fact Goethe's morphological views are neither very
+clearly expressed nor very consistent. This comes out in his treatment
+of the relation between structure and function. Sometimes he takes the
+view that structure determines function. "The parts of the animal," he
+writes, "their reciprocal forms, their relations, their particular
+properties determine the life and habits of the creature."[77] We are
+not to explain, he says, the tusks of the _Babirussa_ by their
+possible use, but we must ask how it comes to have tusks. In the same
+way we must not suppose that a bull has horns in order to gore, but we
+must investigate the process by which it comes to have horns to gore
+with. This is the rigorous morphological view. On the other hand he
+admits elsewhere that function may influence form. Apparently he did
+not work out his ideas on this point to logical clearness, and Radl[78]
+is probably correct in saying that the following quotation with its
+double assertion represents most nearly Goethe's position:--
+
+"Also bestimmt die Gestalt die Lebensweise des Thieres, Und die Weise
+zu leben, sie wirkt auf alle Gestalten Maechtig zurueck."[79]
+
+His best piece of purely morphological work was his theory of the
+metamorphosis of plants. Stripped of its vaguer elements, and of the
+crude attempt to explain differences in the character of plant organs
+by differences in the degree of "refinement" of the sap supplied to
+them, the theory is that stem-leaves, sepals, petals, and stamens are
+all identical members or appendages. These appendages differ from one
+another only in shape and in degree of expansion, stem-leaves being
+expanded, sepals contracted, petals expanded, and so on alternately.
+It is equally correct to call a stamen a contracted petal, and a petal
+an expanded stamen, for no one of the organs is the type of the
+others, but all equally are varieties of a single abstract
+plant-appendage.
+
+What Goethe considered he had proved for the appendages of plants he
+extended to all living things. Every living thing is a complex of
+living independent beings, which "der Idee, der Anlage nach," are the
+same, but in appearance may be the same or similar, different or
+unlike.[80] Not only is there a primordial animal and a primordial
+plant, schematic forms to which all separate species are referable,
+but the parts of each are themselves units, which "der Idee nach," are
+identical _inter se_. This fantasy can hardly be taken seriously as a
+scientific theory; it seems, however, to have been what guided Goethe
+in his "discovery" of the vertebral nature of the skull. Just as the
+fore limb can be homologised with the hind limb, so, reasoning by
+analogy, the skull should be capable of being homologised with the
+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 Geoffroy St Hilaire, the law
+"that to no part can anything be added, without something being taken
+away from another part, and _vice versa_."[81] He saw, too, what a help
+to the interpretation of adult structure the study of the embryo would
+be, for many bones which are fused in the adult are separate in the
+embryo.[82] This also was a point to which the later transcendentalists
+gave considerable attention.
+
+So far we have spoken of Goethe as if he were merely the prophet of
+formal morphology; we have pointed out how he brought to clear
+expression the morphological principle implicit in the idea of unity
+of type, and how he seized upon some important guiding ideas, such as
+the principle of connections. But Goethe was not a formalist, and he
+was very far from the static conception of life which is at the base
+of pure morphology. His interest was not in _Gestalt_ or fixed form,
+_Bildung_ or form change. He saw that _Gestalt_ was but a momentary
+phase of _Bildung_, and could be considered apart and in itself only
+by an abstraction fatal to all understanding of the living thing.
+Mephistopheles scoffs at the scholars who would explain a living
+creature by anatomising it:
+
+ "Dann hat er die Theile in seiner Hand,
+  Fehlt leider! nur das geistige Band."[83]
+
+Goethe kept clear of this mistake; he knew that the artist comes
+nearer to the truth than the analyst.
+
+In the fragment entitled _Bildung und Umbildung organischer Naturen_
+(1807), introductory to a reprint of his paper on the "Metamorphosis
+of Plants," we get an exposition of his general views on living
+things. He points out there how we try to understand things by
+separating them into their parts. We can, it is true, resolve the
+organism into its structural elements, but we cannot recompose it or
+endow it with life by joining up the parts. Hence we require some
+other means of understanding it. "In all ages even among scientific
+men there can be discerned a yearning to apprehend the living form as
+such, to grasp the connection of their external visible parts, to
+interpret them as indications of the inner activity, and so, in a
+certain measure, to master the whole conceptually." This science which
+should discover the inner meaning of organic _Bildung_ is called
+Morphology.[84] In Morphology we should not speak of _Gestalt_ or fixed
+form, or if we do we should understand by it only a momentary phase of
+_Bildung_. Form is of interest not in itself but only as the
+manifestation of the inner activity of the living being. Over
+development, he says elsewhere, there presides a formative force, a
+_bildende Kraft_ or _Bildungstrieb_, which works out the idea of the
+organism. Living things, in his view of them, strive to manifest an
+idea. They are Nature's works of art--and so, incidentally, they
+require an artist to interpret them.
+
+This profound conception of the nature of life is applied not only to
+the growing changing individual but also to the whole changing world
+of organisms. They are all manifestations of a living shaping power
+which moulds them. This shaping power, immanent in all life, is
+conceived to work according to a general plan, and so we get an
+explanation of the fact that living things seem simply varieties of
+one common type.
+
+"If we once recognise," says Goethe, "that the creative spirit brings
+into being and shapes the evolution of the more perfect organic
+creatures according to a general scheme, is it altogether impossible
+to represent this original plan if not to the senses at least to the
+mind...?"[85]
+
+Such an interpretation of the unity of plan reaches perhaps beyond the
+bounds of science.
+
+    [70] _See_ Kohlbrugge, "Hist. krit. Studien ueber Goethe
+    als Naturforscher," _Zool. Annalen._ v., 1913, pp.
+    83-231.
+
+    [71] Or re-discovered, according to Kohlbrugge.
+
+    [72] Cotta ed., vol. ix., p. 448.
+
+    [73] "First Draft of a General Introduction to
+    Comparative Anatomy."
+
+    [74] Cotta ed., ix., p. 463.
+
+    [75] Cotta ed., p. 478.
+
+    [76] _Loc. cit._, p. 491.
+
+    [77] _Entwurf_, Cotta ed., ix., p. 465.
+
+    [78] _Geschichte der biologischen Theorien_, i., p. 266.
+
+    [79] "So the form determines the manner of life of the
+    animal, and the manner of life in its turn reacts
+    powerfully upon all forms."
+
+    [80] _Bildung und Umbildung organischer Naturen_, 1807.
+
+    [81] Cotta ed., ix., p. 466.
+
+    [82] _Loc. cit._, pp. 474-5.
+
+    [83] Then he has all the parts within his hand, excepting
+    only, sad to say, the living bond.
+
+    [84] Goethe was the inventor of the word.
+
+    [85] Cotta ed., ix., p. 490.
+
+
+
+
+CHAPTER V
+
+ETIENNE GEOFFROY SAINT-HILAIRE
+
+
+E. Geoffrey made an experiment, unsuccessful but instructive. He tried
+to found a science of pure morphology; he failed: his failure showed,
+once and for all, that a pure morphology of organic forms is
+impracticable.
+
+Already, in 1796, in one of his earliest memoirs,[86] Geoffroy was
+guided by the idea that Nature has formed all living things upon one
+plan. Organs which seem anomalous are merely modifications of the
+normal; the trunk of an elephant is formed by the excessively
+prolonged nostrils, the horn of a rhinoceros is simply a mass of
+adhering hairs. In general, however varied their form, all organs are
+simply variations of a common scheme; Nature employs no new organs.
+Organs which are rudimentary, such as the clavicles in the ostrich and
+the nictitating membrane in man, bear witness to the unity of plan. In
+this Geoffroy goes no further than his predecessors. They too had
+recognised homologies of organs; they too had interpreted rudimentary
+organs as vestiges of an original plan.
+
+In a series of papers published in 1807, Geoffroy took a further step,
+and sought to establish homologies which were not obvious--homologies,
+too, not so much of organs as of parts.
+
+These memoirs (published in the _Annales du Museum d'Histoire
+naturelle_, vols. ix. and x., 1807) dealt with the homology between
+the bones of the pectoral fin and girdle in fish and the bones of the
+arm and shoulder-girdle in higher Vertebrates, with the homologies of
+the bones of the sternum, and with the determination of the pieces of
+the skull, particularly in the crocodile. All Geoffroy's morphological
+doctrine is found in them, but for the full expression of his views we
+must take his chief work, the _Philosophie anatomique_, particularly
+the first volume (1818). This volume contains, beside the important
+"Discours preliminaire" and "Introduction" which we shall presently
+consider in detail, five memoirs, which deal with the various bones
+connected with the respiratory organs in fishes (the bones of the
+operculum, of the hyoid, of the branchial arches, of the pectoral
+girdle), and seek to discover their homologies with corresponding
+bones in air-breathing Vertebrates.
+
+"Can the organisation of vertebrated animals be referred to one
+uniform type?" This is the question with which the _Philosophie
+anatomique_ opens, the question to which the whole book is an answer.
+But is it not generally acknowledged by naturalists that Vertebrates
+are built upon one uniform plan, that, for instance, the fore limb may
+be modified for running, climbing, swimming, or flying, yet the
+arrangement of the bones remain the same? How else could there be a
+"natural method" of classification?[87]
+
+But the homologies so drawn repose upon a vague and confused feeling for
+likenesses; they are not based upon an explicit principle. What general
+principle can be applied? "Now it is evident that the sole general
+principle one can apply is given by the position, the relations, and the
+dependencies of the parts, that is to say, by what I name and include
+under the term of _connections_." For instance, the part known as the
+hand in man and generally as the fore foot in other Vertebrates, is the
+fourth part in order in the anterior member, and its homologue can
+always be recognised by this fact of its connections (p. xxvi.). The
+principle of connections serves as a guide in tracing an organ through
+all its functional transformations, for "an organ can be deteriorated,
+atrophied, annihilated, but not transposed" (p. xxx.).
+
+It is this principle which enables one to follow out in detail the
+further fundamental conception that in every Vertebrate there are found
+the same "organic materials," or units of construction. This conception,
+which Geoffroy calls the _Theorie des analogues_ (p. xxxii.), is clearly
+one part of the old idea of the unity of type; it teaches the _unity of
+composition_ of organic beings, while the _Principe des connexions_ adds
+the _unity of plan_.
+
+Both conceptions are logically implicit in the vague notion of unity of
+type; Geoffroy disengaged them, and pushed each to its logical extreme.
+
+Most of the ordinary homologies of structure in air-breathing
+Vertebrates have already been seized, he continues, for they are more or
+less obvious, and many intermediate states exist (p. xxxiv.). But
+ordinary methods of comparison fail when the attempt is made to
+homologise the structure of fishes with that of air-breathing
+Vertebrates, for the homologies are anything but obvious and no
+intermediate organs are found.
+
+Most air-breathing Vertebrates have a larynx, a trachea, and bronchi,
+which are absent in fish; and fish have many parts which seem to be
+absent in higher Vertebrates. But apply the "Theory of Analogues"; it
+teaches that there can be no organ peculiar to fish and not found in
+other Vertebrates; apply the "Principle of Connections," it will show
+which organs are homologous in the two types (p. xxxv.).
+
+Comparative anatomists, with few exceptions, had hitherto taken man as
+the type, and referred all structure to his; Geoffroy's principles led
+him to give preference to no one animal in particular, but to seize upon
+each part in the species in which it reaches the maximum of its
+development (p. xxxvi.). He is thus led to refer all structures to a
+generalised abstract type. In this abstract type each organ exists at
+the maximum of its development, each organ shows all its potentialities
+realised. In a way, therefore, this type, this abstraction, gives the
+scheme of the possible transformations of each organ.
+
+It is true Geoffroy does not refer to this "Archetype" in so many words,
+but it must always have been vaguely present in his mind. He has this
+idea in his head when he says in one of his later works, "There is,
+philosophically speaking, only a single animal."[88] The "single animal"
+is simply the generalised type.
+
+Having laid down his two principles Geoffroy goes on to apply them to
+the difficult case of the comparison of the skeleton of fish with the
+skeleton of the higher Vertebrates. "My present task is to demonstrate
+that there is no part of the bony framework of fishes that cannot find
+its analogue in the other vertebrated animals."[89] It seems at first
+sight that many bones are peculiar to fish, formed expressly for
+performing the functions which fish do not share with higher animals.
+These are the bones connected with respiration--the operculum, the
+branchiostegal rays, the branchial arches, and others. That the peculiar
+bones should be connected with the respiratory functions is only
+natural, for the contrast between fish and higher Vertebrates is
+essentially a contrast between water-breathing and air-breathing
+animals. Considering first the general form of the skeleton in fish, we
+are met at once with a difficulty; there is no obvious homologue in
+fishes of the neck, the trunk, and the abdomen of higher animals. What
+apparently corresponds to the trunk is in fishes crowded close up under
+the head. But, after all, it is not of the essence of the vertebrate
+type to have the trunk and the abdomen attached at definite and
+invariable distances along the vertebral column--that is a notion
+surviving from the anatomy which made man its type. The "trunk" differs
+in position according to the class, in quadrupeds, birds, and fishes (p.
+9). Now, says Geoffroy, allow me this one hypothesis, that the trunk
+with its organs can, as it were, move bodily along the vertebral column,
+so as to be found in one class near the front end of the vertebral
+column, in another about the middle, and in a third near the end, then I
+can show you in detail that the constituent parts of this trunk are
+found in all classes to be invariably in the same positions relatively
+to one another (p. 10). It is important to note this hypothesis of a
+"metastasis" which Geoffroy makes, for it is the key to the
+understanding of many of the far-fetched homologies which he tries to
+establish. It is, of course, clear that this hypothesis is in formal
+contradiction with his principal hypothesis of the invariability of
+connections, and that he, so to speak, gets a hold on his fish to apply
+his principle of connections only by admitting at the very outset an
+exception to his primary principle. A further application of the
+hypothesis of metastasis will be noticed below in connection with the
+determination of the sternum of fishes. We note here an interpretation
+of the first metastasis in terms of functional adaptation. "The constant
+and violent action of the tail, if it does not go so far as actually to
+displace and move forward the internal organs, at least fits in well
+with an arrangement in which the organs are so disposed" (p. 99).
+
+The first memoir deals with the homologies of the opercular bones.
+Geoffroy considers that the external opening of the ear corresponds to
+the external opening of the gill-chamber, which lies between the
+operculum and the pectoral girdle. The ear communicates with the buccal
+cavity by the Eustachian tube, so does the branchial chamber by means of
+the gill-slits. The auditory chamber of higher Vertebrates is,
+therefore, the homologue of the branchial chamber in fish; the opercular
+bones in fish and the ossicles of the ear in other Vertebrates stand in
+close relation to this chamber; therefore the opercular bones are the
+homologues of the ossicles of the ear, the interoperculum corresponding
+to the malleus, the suboperculum to the lenticular, the minute lower
+part of the suboperculum to the incus, the operculum to the stapes, and
+the pre-operculum to the tympanic ring. In making these particular
+determinations Geoffroy professes to be led by his principle of
+connections. The pre-operculum has, he says, the same connections with
+neighbouring bones as the tympanic bone in other Vertebrates, and the
+other pieces of the gill-cover are homologised with particular
+ear-ossicles according to the order in which they stand to one another.
+The second memoir in the book deals with the sternum, and affords a very
+good example of Geoffroy's method of dealing with the facts of
+structure. We shall omit here any detailed reference to the other three
+memoirs, which deal with the hyoid, with the branchial arches and the
+structures which correspond in air-breathing Vertebrates, and with the
+bones of the shoulder-girdle.
+
+In the memoir on the sternum Geoffroy's first care is to arrive at a
+definition of what a sternum is. He defines it partly by its functions,
+partly by its connections, as the system of bones which covers and
+protects the thorax, and gives attachment to certain groups of muscles.
+
+The most highly developed sternum (according to this definition) is the
+plastron of the tortoise, whose structure it dominates (p. 103). It is
+important, therefore, to determine of how many bones the plastron is
+composed, since the full number of elementary parts of which an organ is
+composed is best seen when the organ is at the maximum of its
+development. There are nine bones in the plastron of the tortoise. "The
+conclusion to be drawn from this is that every sternum, provided that it
+is not inhibited in its development by some obstacle, is composed of
+_nine elementary parts_" (p. 105). These nine bones are in Geoffroy's
+nomenclature, the episternals, the hyosternals, the hyposternals, the
+xiphisternals, which are all paired bones, and the entosternal, which is
+unpaired. The arrangement of them is in the tortoise:--
+
+Episternal---------------------------Episternal
+    |\__                               __/|
+    |   \__                         __/   |
+    |      \__                   __/      |
+    |         \__ Entosternal __/         |
+    |       __/                 \__       |
+    |    __/                       \__    |
+    | __/                             \__ |
+    |/                                   \|
+Hyosternal                           Hyosternal
+    |                                     |
+    |                                     |
+    |                                     |
+    |                                     |
+Hyposternal-------------------------Hyposternal
+    |                                     |
+    |                                     |
+    |                                     |
+    |                                     |
+Xiphisternal------------------------Xiphisternal.
+
+The articulations in the tortoise are indicated by the connecting
+lines. Geoffroy tries to show that the sternum in other animals is
+composed of these nine bones, or at least of a certain number of them,
+always in the same invariable relative positions. Thus in birds the
+sternum consists of five pieces, of a huge keeled entosternal, and of
+two "annexes" on either side, which are the hyo-and hyposternals.
+These are separate only in young birds. Occasionally, especially in
+young birds, rudiments of episternals and xiphisternals also occur.
+The minuteness of the episternals and the xiphisternals may be
+attributed to the gigantic size of the entosternal, in accordance with
+the _Loi de balancement_. In the other air-breathing Vertebrates the
+nine sternal elements can according to Geoffroy be discovered without
+great difficulty. But when we come to the determination of the sternum
+in fishes, difficulties abound, which Geoffroy solves in the following
+way. He points out that between the clavicles (_cleithra_) and the
+hyoid bone (_basihyal_) in fishes there is a long median bone
+(_urohyal_) which is attached in front by two strong tendons to the
+horns of the hyoid and is free behind (see Fig. 1). Gouan (1720) had
+seen in this bone the homologue of the sternum. Geoffroy adopts this
+view, but considers that this bone alone cannot represent the whole
+sternum. He finds the representatives of other bones of the sternum in
+the large bones (_epihyal_ and _ceratohyal_, or the two pieces of the
+_ceratohyal_) which are comprised in the hyoid arch. But he is
+immediately met by the difficulty that this complex of bones is
+situated in front of the pectoral girdle, whereas the sternum in
+higher Vertebrates lies behind the pectoral girdle. He reflects,
+however, that the gills of fish, situated in front of the clavicles,
+are merely the lungs under another name. The gills have become shifted
+forward by a metastasis similar to that which brought the whole
+thoracic organs far forward in fish. This being so, their supporting
+elements, the sternum and the ribs, must have moved with them, and are
+hence to be found in front of the pectoral girdle.
+
+[Illustration: FIG. 1.--Hyoid Arch of the Conger. (Original.)]
+
+Geoffroy's next step is to point out that the only possible homologues
+of sternal ribs are the branchiostegal rays, which arise from the large
+bones of the hyoid arch. If these are sternal ribs, the bones to which
+they are attached must be the hyo- and hyposternals or "annexes," the
+bones from which in birds the ribs take their origin.
+
+The unpaired sternal bone (_urohyal_) cannot be homologous with the
+entosternal, for it has no connections with the annexes. He decides that
+it must represent the episternals, for in some young birds there is a
+two-headed episternal to which two strong tendons are attached, just in
+the same way as the unpaired piece in fish is bound to the bones of the
+hyoid by two tendons. "Thus it is not the sternum as a whole that has
+shifted in front of the clavicles and covered with its side pieces the
+gills placed there; it is a piece exclusively piscine, in the sense that
+it is only in the class of fishes that it reaches the _maximum_ of its
+development" (p. 83).
+
+To sum up, the sternum in all four vertebrate classes is composed of the
+same elements, arranged always in the same way. "One is ... led to the
+conception of an ideal type of sternum for all Vertebrates, which then,
+considered from a lower standpoint, resolves itself into several
+secondary forms according as the whole or the majority of the
+constituent materials are employed, or even as these elements come to
+change their respective dimensions or proportions" (p. 134). As to the
+elementary constituents, "they give proof of individuality, and
+sometimes even, in certain abnormalities, of independence, and rise to
+the level of primary organisatory materials" (p. 132). What holds good
+for the sternum holds good for other organs--and accordingly the unity
+of plan and composition can be demonstrated for all the organs of
+Vertebrates.
+
+Soon after the publication of the _Philosophie anatomique_ (1818)
+Geoffroy went further in his search for unity, and maintained that the
+structure of insects and Crustacea could be reduced to the vertebrate
+type.
+
+He proposed to replace Cuvier's classification of the animal kingdom
+into the four large groups, Vertebrata, Mollusca, Articulata, and
+Radiata by the following classification:--[90]
+
+                Hauts-Vertebres (Vertebrata, Cuv.).
+     Vertebres /
+               \
+                Dermo-Vertebres (Articulata, Cuv.).
+
+
+                Mollusques (Mollusca, Cuv.).
+   Invertebres /
+               \
+                Rayonnes (Radiata, Cuv.).
+
+The idea upon which is based the comparison of Articulates with
+Vertebrates is that each skeletal segment of Articulates is a vertebra.
+In the Hauts-vertebres the vertebrae are internal; in the
+Dermo-vertebres they are external. "_Every animal lives either outside
+or inside its vertebral column_."[91] The essence of a vertebra is not
+its form, nor its function, but its composition from four elementary
+pieces which unite round a central space (_Isis, loc. cit._, p. 532).
+Serres had shown that in the higher animals every vertebra is formed
+from four centres of ossification, that the body of the vertebra is at
+first tubular, and that afterwards it becomes filled up. In lobsters and
+crabs each segment is composed of four elementary pieces, as may be seen
+most easily in young ones. "Accordingly each segment corresponds to a
+true vertebra in composition: there is the same number of 'materials,'
+the same order in the course of ossification, the same kind of
+articulation, the same annular arrangement, the same empty space in the
+middle" (p. 534). The only difference is that in Articulates the central
+space is very great and contains all the organs of the body, whereas in
+the higher Vertebrates the body of the vertebra becomes completely
+filled up. In the thoracic region of Crustacea it is not the whole
+segment with part of the carapace which corresponds to a vertebra, but
+merely the part round the ventral nerve-cord (endophragmal skeleton).
+
+If the skeleton of the segment in Articulates corresponds to the body of
+a vertebra and is here external, then the appendages of the Articulate
+must correspond to ribs (p. 538). The full development of this thought
+is found in a Memoir of 1822, "Sur la vertebre."[92] He takes as the
+typical vertebra that of a Pleuronectid, probably the turbot. His
+original figure is reproduced (Fig. 2).
+
+[Illustration: FIG. 2.--"Vertebra" of a Pleuronectid. (After Geoffroy.)]
+
+He includes as part of the vertebra not only the neural (e', e'') and
+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, 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, whether internal or
+external, should stand to one another in the same relations; but it is
+all one whether the box (_coffre_) that encloses them lies with this or
+that side on the ground. What similarities in the organisation of man
+and the digitate mammals, and yet what differences between their
+attitudes when standing! The same holds true as regards the normal
+attitudes of the pleuronectids and the other fishes" (p. 107).
+
+The exact way in which Geoffroy homologised the parts of the appendages
+in Arthropods with the paired pieces of the typical vertebra is best
+shown by the reproduction of his figure of an abdominal segment of the
+lobster (Fig. 3), in which the parts homologous with those represented
+in the figure of the typical vertebra (Fig. 2) are indicated by the same
+letters. The ingenuity of the comparison is astonishing.
+
+[Illustration: FIG. 3.--Abdominal Segment of the Lobster. (After
+Geoffroy.)]
+
+The comparison of the Arthropod with the Vertebrate is extended also to
+the internal organs. The internal organs of the Arthropod are shown to
+stand in the same order to one another as in the Vertebrate, only the
+organs are inverted. Thus the nervous system is dorsal in the
+Vertebrate, ventral in the Arthropod. Turn the Arthropod on its back and
+the relative positions of the systems of organs are the same as in the
+Vertebrate. The relation of the organs to the external tube is of course
+different in Arthropods and Vertebrates, but this is no contradiction of
+the principle of connections. "Such a tube, although it is the organs
+essential to life that it contains, can yet behave in different ways
+with regard to the mass of these organs: the principle of connections
+demands only that all the organs maintain with one another fixed and
+definite relations; but the principle would be in no way invalidated if
+the whole mass had rotated inside the tube" (p. 112).
+
+Geoffroy pushed the analogy between Arthropods and Vertebrates very far,
+for he asserted that every piece in the skeleton of an insect was
+homologous with some bone in Vertebrates, that it stood always in its
+proper place, and remained faithful to at least one of its
+connections.[93] It does not appear that he attempted to prove in detail
+this very big assumption, but the beginnings of a detailed comparison
+are found in the paper of 1820, _Sur l'organisation des insectes_. Six
+segments are distinguished in an insect--the head, the three divisions
+of the thorax, the abdomen, and the terminal segment of the abdomen (p.
+455).
+
+The skeleton of the insect's head is said to correspond to the bones of
+the face, to the bones of the cerebrum and to the hyoid of higher
+Vertebrates, the skeleton of the prothorax to the bones of the
+cerebellum, of the palate, and the pieces of the larynx, the skeleton of
+the mesothorax to the parietals, interparietals, and opercular bones,
+and that of the metathorax to the skeleton of the thorax of Vertebrates.
+The pieces of the abdomen and of the terminal segment correspond to the
+bones of the abdomen and coccyx (p. 458). It does not need the
+subsequent likening of the hind wings of insects to the air bladder of
+fish, and of the stigmata to the pores of the lateral line, to convince
+one finally of the fancifulness of the whole comparison.
+
+In 1830 two young naturalists, Meyranx and Laurencet, presented to the
+Academie des Sciences a memoir in which they likened a Cephalopod to a
+Vertebrate bent back at the level of the umbilicus, saying that the
+Vertebrate in this position had all its organs in the same order as in
+the Cephalopod. Geoffroy took up this idea with enthusiasm, seeing in it
+a further application of his master-idea of the unity of plan and
+composition. By means of this comparison Mollusca definitely took their
+place in the _Echelle des etres_, after the Articulata, just as Geoffroy
+had maintained in 1820, saying that crabs formed a link between the
+other Crustacea and the molluscs.[94] The comparison brought him nearer
+to the end he had in view, the reference of all animal structure to one
+single type.
+
+But in championing the memoir of Meyranx and Laurencet, Geoffroy found
+himself in direct antagonism with Cuvier, who held that his four
+"Embranchements" had each a separate and distinct plan of structure. In
+a paper read to the Academy in February 1830,[95] Cuvier easily
+demolished the crude comparison of the Cephalopod to the Vertebrate. He
+gave diagrams of the internal organs of a Cephalopod and of a Vertebrate
+bent back in the manner indicated by Meyranx and Laurencet, and he
+showed in detail that the arrangement of the main organs was quite
+different, that the likeness would have been much greater if the
+Cephalopod had been likened to a Vertebrate doubled up the other way,[96]
+but that even then the arrangement of the organs would not be the same.
+The organs, too, of the Cephalopod are differently constructed. He sums
+up his criticism by saying:--"I give true and summary expression to all
+these facts when I say that Cephalopods have several organs in common
+with Vertebrates, which fulfil in either case similar functions, but
+that these organs are differently arranged with respect to one another,
+and often constructed in a different way; that they are in Cephalopods
+accompanied by several other organs which Vertebrates do not possess,
+whilst the latter on their side have many organs which Cephalopods lack"
+(p. 257). Geoffroy could not accept this commonsense view of the matter,
+but made a fight for his transcendental theories. This was the beginning
+of the famous controversy between Geoffroy and Cuvier which so excited
+the interest of Goethe. It was a struggle between "comparative anatomy"
+and "morphology," between the commonsense teleological view of structure
+and the abstract, transcendental. Geoffroy brought forward all his
+theories on the homology of the skeleton of fish with the skeleton of
+higher Vertebrates, and tried to prove by them his great principle of
+the unity of plan and composition; Cuvier took Geoffroy's homologies one
+by one, and showed how very slight was their foundation. Cuvier was on
+sure ground in insisting upon the observable diversities of structural
+type, and his vast knowledge enabled him to score a decisive victory.[97]
+
+The controversy was not, as we are sometimes told, a controversy between
+a believer in evolution and an upholder of the fixity of species,
+although it raised a question upon which evolution theory was to throw
+some light.
+
+In these Darwinian days Geoffroy has reaped a little posthumous glory as
+an early believer in evolution. That he did believe in evolution to a
+limited extent is certain; that his theory of evolution was, as it were,
+a by-product of his life-work, is also certain. Geoffroy was primarily a
+morphologist and a seeker after the unity hidden under the diversity of
+organic form. His theory of evolution had as good as no influence upon
+his morphology, for he did not to any extent interpret unity of plan as
+being due to community of descent. His morphological, non-evolutionary
+standpoint comes out quite clearly in several places in the _Philosophie
+anatomique_. He does not derive the structure of the higher Vertebrates
+from the simpler structure of the lower, but when he finds in fish a
+part at the maximum of its development, he speaks of the same part,
+rudimentary in the higher forms, as being, as it were, held in reserve
+for use in the fish. Thus, speaking of the episternal in fish which
+forms the central piece of its sternum, he says, "it is a bone that is
+rudimentary in birds (one might almost add a bone that is held in
+reserve in birds for this fate) which is destined to form in the centre
+the principal keel of this new machine" (p. 84). Again, with reference
+to the homology of the ossicles of the ear with the opercular bones in
+fish, "employing other resources equally hidden and rudimentary, Nature
+makes profitable use of the four tiny ossicles lodged in the auditory
+passage, and, raising them in fish to the greatest possible dimensions,
+forms from them these broad opercula...." (p. 85). Or you may take it
+the other way about, and start from the organisation of fishes;
+opercular bones are of no use to air-breathing animals, so they dwindle
+away, and are pressed into the service of the ear, although they are of
+little use in hearing (p. 46).
+
+There is here no thought of evolution; in later years, however, his
+researches upon fossil crocodilians led him to consider the possibility
+that the living species were descended from the antediluvian. For the
+factors of the transformation he refers to Lamarck's hypotheses.[98] In a
+memoir of 1828,[99] dealing with the possible genetic relation of living
+to fossil species, he still regards the question as more or less open.
+Although fossil species are mostly different from living species are we
+therefore to conclude, he asks, that they are not the ancestors of the
+present day forms? "The contrary idea arises more naturally in the mind;
+for otherwise the six-days' creation would have had to be repeated and
+new beings produced by a fresh creation. Now this proposition, contrary
+as it is to the most ancient historical traditions, is inadmissible" (p.
+210). It is sufficiently clear from this quotation that Geoffroy was
+thinking only of a transformation of the antediluvian species created by
+God, and by no means of an evolution of all species from one primitive
+type. In matters of religion Geoffroy was orthodox. He goes on to point
+out how great a resemblance there is in essential structure between
+fossil and living species. All find their place in one scheme of
+classification; does it not seem that all are modifications "of one
+single being, of that abstract being or common type, which it is always
+possible to denote by the same name?" (p. 211). This type is abstract,
+not actual, and it is certainly not conceived as an original ancestor of
+all animals.
+
+The fullest development of Geoffroy's views on evolution is found in his
+memoir "Le degre d'influence du monde ambiant pour modifier les formes
+animales."[100] Here the relation of his evolution-theory to his
+morphology is pointed out. The principle of unity of plan and
+composition cannot be the final goal of zoology; there must follow on it
+a philosophical study of the _differences_ between organic forms. The
+causes of these differences are to be found in the environment (pp.
+66-7). Geoffroy seems here to be moving from a pure to a causal
+morphology. It is probable, he continues, that living species have
+descended by uninterrupted generation from the antediluvian species (p.
+74), and that they have in the process become modified through external
+influences.
+
+Now of all functions respiration is the most important, and upon
+respiration everything is regulated. "If it be admitted that the slow
+progression of the centuries has brought in its train successive changes
+in the proportion of the different elements of the atmosphere, it
+follows as a rigorously necessary consequence that the organisation has
+been proportionately influenced by them" (p. 76). The respiratory milieu
+changes, the species change with it, or are eliminated (p. 79). We may
+see, perhaps, in the stress which Geoffroy lays upon respiration and the
+respiratory milieu a result of his constant obsession with the
+comparison of fish with air-breathing Vertebrates.
+
+In the first geological period, we read in another Memoir of the same
+year,[101] when ammonites and _Gryphaea_ flourished, hot-blooded animals
+with lungs could not exist. "A lung constructed like that of mammals and
+birds would not have been adapted to the essence of the respiratory
+element such as in my conception of it the system of the environing air
+used to be"[102] (p. 58).
+
+Geoffroy does not tell us exactly how the milieu is to act upon the
+organism; the whole theory is little more than a sketch and a pointing
+out of the way for future research--and in this prophetic enough. The
+action of external agents was apparently considered as physical, and no
+power of active adaptation was ascribed to the organism.
+
+From a passage in the memoir "Sur la Vertebre" we may perhaps infer that
+he believed increasing complexity of structure to be due to a
+realisation of potentialities, to the development of parts present in
+the lower animals only in potency--"the organisation ... only awaits
+favourable conditions to rise, by addition of parts, from the simplicity
+of the first formations to the complication of the creatures at the head
+of the scale" (p. 112). Evolution takes place as the environment allows,
+and in a sense in opposition to the environment.
+
+He believed in saltatory evolution, for he considered that the lower
+oviparous Vertebrates could not be transformed into birds by slow
+modification, but only by a sudden transformation of their lungs, which
+would bring about the other characteristics of birds (p. 80). He
+considered, too, that transformations could arise by means of monstrous
+development (p. 86). In this connection the experiments which he made on
+the hen's egg[103] in order to produce artificial monstrosities are
+significant, though his purpose was rather to obtain proof of the
+inadequacy of the preformation hypothesis.[104]
+
+It seems probable enough that if Geoffroy had developed his views on
+evolution he would finally have been led to interpret unity of plan in
+terms of genetic relationship. But as it was he remained at his
+morphological standpoint. He did not interpret rudimentary organs as
+useless heritages of the past; he preferred to think that Nature had
+prepared double means for the same function, one or other being
+predominant according as the animal lived in the water or on the land.
+"To the animal that lives exclusively in the air Nature has granted an
+organisation suited to this mode of respiration, without however
+suppressing the other corresponding means, that is to say, without
+depriving it of a second system which is applicable only to the mode of
+respiration by the intermediary of water, and _vice versa_."[105]
+
+He seems, in one instance at least, to have hit upon the root-idea of
+the biogenetic law, but he was far from appreciating its significance.
+He recognised that an amphibian in its development passed through a
+stage when it was in all essentials similar to a fish, and he saw in
+this visible transformation a picture of the evolutionary
+transformation. "An amphibian," he writes,[106] "is at first a fish under
+the name of tadpole, and then a reptile [_sic_] under that of frog....
+In this observed fact is realised what we have above represented as an
+hypothesis, the transformation of one organic stage into the stage
+immediately superior." But it is not clear that he considered the
+development of the amphibian to be a _repetition_ of its ancestral
+history.
+
+He went, however, a certain length towards recognising the main
+principle of a law which was a commonplace of German transcendental
+thought, and was developed later by his disciple E. Serres, the law that
+the higher animals repeat during their development the main features of
+the adult organisation of animals lower in the scale. Thus he compared
+fish as regards certain parts of their structure with the foetus of
+mammals. He compared also Articulates with embryonic Vertebrates in
+respect of their 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."[107]
+
+This idea was destined to exercise a great influence upon the
+development of morphology. A further development of the thought is that
+certain abnormalities in the higher animals, resulting from arrest of
+development, represent states of organisation which are permanent in the
+lower animals.[108]
+
+So far we have considered Geoffroy's theories in their application to
+the facts. We go on to discuss the theories themselves, and the general
+conception of living things which underlies them.
+
+The principle of unity of plan and composition is the keynote of
+Geoffroy's work. It states that the same materials of organisation are
+to be found in all animals, and that these materials stand always in the
+same general spatial relations to one another. The "materials of
+organisation" are not necessarily organs in the physiological sense, and
+indeed the principle of the unity of plan cannot be upheld if the unity
+has reference to organs only. This became clear to Geoffroy, especially
+in his later years. In 1835 he wrote, speaking of the principle of the
+unity of plan, "I have, moreover, regenerated this principle, and
+obtained for it universality of application, by showing that it is not
+always the organs as a whole, but merely the materials composing each
+organ, that can be reduced to unity."[109] Even in the _Philosophie
+anatomique_ he deals rather with parts than with organs; he deals, for
+instance, with the elementary parts of the sternum, not with the organ
+"sternum" in its totality. The functions of the sternum vary, and the
+primary protective function of the sternum may be assumed by quite other
+parts, _e.g._, by the clavicles in fish, which protect the heart.[110]
+
+True homologies can be established between materials of organisation but
+not always between organs, which may be composed of different
+"materials."
+
+Almost as a corollary to this comes the further view that form is of
+little importance in determining homologies. An organ is essentially an
+instrument for doing a particular kind of work, and its form is
+determined by its function. Organs which perform the same function are
+usually similar in form though the elementary materials composing them
+may be different. This is seen in many cases of convergence. Organs,
+therefore, which perform the same function and are similar in external
+form are not necessary homologous. Conversely, the same complex of
+materials, say a fore limb, may take on the most varied shapes according
+as the function of the organ changes--but homology remains though form
+changes. Accordingly, form is one of the least important elements to be
+considered in determining a homology. "Nature," he wrote in one of his
+early papers, "tends to repeat the same organs in the same number and in
+the same relations, and varies to infinity only their form. In
+accordance with this principle I shall have to draw my conclusions, in
+the determining the bones of the fish's skull, not from a consideration
+of their form, but from a consideration of their connections."[111]
+
+Again, after comparing a vertebra of the Aurochs with an abdominal
+segment of the crab, he says, "I have insisted upon an identity which
+has extended to the least important relation of all, that of form."[112]
+
+Geoffroy's morphological units or materials of organisation were in the
+case of the skeleton--with which his researches principally deal--the
+single bones. But the interesting point is that he sought his
+skeleton-units in the embryo, and considered each separate centre of
+ossification as a separate bone. Coalescence of bones originally
+separate is one of the most usual events in development, and it is an
+occurrence which, more than any other, tends to obscure homologies.
+Because of its coalescence with the maxillaries, the intermaxillary in
+man was not discovered until Vicq d'Azyr and Goethe found it separate in
+the embryo. Apparently quite independently of Goethe, Geoffroy hit upon
+this plan of seeking in the embryo the primary elements or materials of
+organisation. In an early paper on the skull of Vertebrates,[113] where he
+is concerned with showing that each bone of the fish's skull has its
+homologue in the skull of higher Vertebrates, he is faced with the
+difficulty that the skull of the fish has more bones than the skull of
+higher Vertebrates. "Having had the inspiration," he writes, "to reckon
+as many bones as there are distinct centres of ossification, and having
+made a consistent trial of this method, I have been able to appreciate
+the correctness of the idea: fish, in their earliest stages, are in the
+same conditions relatively to their development as the foetuses of
+mammals, and hence bear out the theory" (p. 344). So, too, in dealing
+with the homologies of the sternal elements (_supra_, p. 57) he treats
+as separate bones the "annexes" of the sternum in birds, though these
+are separate only in the young.
+
+If the same materials of organisation are present in all animals, and if
+they are arranged always in the same positions relatively to one
+another, how does it come about that animal forms are so varied, what
+explanation can be offered of the diversities of organic structure?
+Geoffroy's main answer to this question is his _Loi de balancement_. The
+law was enunciated by him already in 1807.[114] We take the following
+quotation, which represents his thought most nearly, from the _Cours de
+l'histoire naturelle des Mammiferes_ (1829). "According to our manner of
+regarding the organisation of mammals, there is only a single animal
+modified by the inverse reciprocal variation of all or some of its
+parts. Now, from the fact that there is only one single general animal,
+it follows that for each section of its components or for each of its
+organs there is available only a given quantity of formative materials.
+Now suppose that the distribution of these materials has not been made
+in such a way as to ensure an exact equilibrium between all the parts
+concerned, one organ will get more than its share, another less. My law
+of the compensation of organs is founded on these principles" (i.,
+_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."[115] The nutritive material available is limited
+for each species; if one part gets more than its share the other parts
+must get less--that is all the law means. As an example, take the
+minuteness of the episternals and xiphisternals in birds, as contrasted
+with the huge size of the entosternal. "The minuteness of the
+episternals and xiphisternals might be imputed to this gigantic piece
+diverting to its own profit the nutritive fluid, since the bigger it is
+the smaller these are."[116]
+
+One has constantly to remember in dealing with Geoffroy's theories that
+he was not an evolutionist, but purely a morphologist. It is therefore,
+perhaps, to ask too much to require of him an explanation of the causes
+of diversity. The morphologist describes, classifies, generalises; he
+does not seek for causes. But we must leave this question aside in order
+to discuss how far Geoffroy's theory of the unity of plan and
+composition fits the facts. As Geoffroy himself admitted on several
+occasions, his theory was an _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 unworkable; he found, for
+example, that the skeleton of fishes could not be compared directly,
+bone for bone, with the skeleton of higher Vertebrates; he had to admit
+differences of position of whole sets of organs in the two groups, he
+had to admit various _metastases_, before he could bring the skeleton of
+fish into line. And these metastases are due to functional
+requirements--for example, the forward position of sternum and thoracic
+organs in fish is an adaptation to swimming.
+
+So he does not so much demonstrate the unity of plan of whole organisms
+as the unity of plan of particular corresponding parts of them. Thus he
+does not prove or attempt to prove that Articulates are in all points
+like Vertebrates, but simply that their skeleton is built upon the same
+plan as that of Vertebrates. The rest of the organs, while still
+comparable with the organs of Vertebrates, stand in different relations
+to the skeleton. An Articulate therefore, on his own showing, is not,
+_as a whole_, built upon the same general structural plan as a
+Vertebrate.
+
+Further, he does not always remain true to his principles, for he does
+not establish homologies of parts entirely by their connections but
+sometimes by their functions as well. Thus the sternum, or rather the
+complex of sternal elements, is defined and discovered in particular
+cases not by its connections only but also by its functions. The
+framework of the gills is homologised part by part with the framework of
+the lungs, not because the relations of the framework to the rest of the
+skeleton are the same in fish and air-breathing Vertebrates, but simply
+because gills are considered the equivalents of lungs--a comparison
+which is purely physiological.
+
+Even with these concessions to the functional view of living things,
+Geoffroy was unable to make good his contention that all animals are
+built upon the same plan. His arguments failed to carry conviction to
+his contemporaries, and Cuvier in particular subjected them to
+destructive, and indeed final, criticism.
+
+The paper, already referred to, in which Cuvier disposed of the
+transcendentalists' comparison of Cephalopods and Vertebrates is of
+great significance, for it states in the clearest way the radical
+opposition between the functional and the formal attitudes to living
+things.
+
+Cuvier points out that if by unity of composition is meant identity,
+then the statement that all animals show the same composition is simply
+not true--compare a polyp with a man!--on the other hand, if by unity is
+meant simply resemblance or homology, the statement is true within
+certain limits, but it has been employed as a principle since the days
+of Aristotle, and the theory of unity of composition is original only in
+so far as it is false. He admits, however, that Geoffroy has seized upon
+many hidden homologies, especially by his valuable discovery of the
+importance of foetal structure. In all this Cuvier is undoubtedly right.
+Unity of plan and composition, as Geoffroy conceived it, simply does not
+exist. Cuvier goes on to say that this principle of Geoffroy's, in the
+greatly modified form in which it can be accepted, and has been accepted
+from the dawn of zoology, is not the sole and unique principle of the
+science. On the contrary, it is merely a subordinate principle,
+subordinate to a higher and more fruitful principle, that, namely, of
+the conditions of existence, of the adaptation (_convenance_) of the
+parts, of the co-ordination of the parts for the 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).
+
+Geoffroy's position is the direct contrary. He holds that the principle
+of the unity of plan and composition is the true base of natural
+history,[117] and that this unity limits the possible transformations of
+the organism. Thus, speaking of the influence of the respiratory medium,
+he says, "All the same this influence of the external world, if it has
+ever become a cause which disturbed organisation, must necessarily have
+been confined within fairly narrow limits; animals must have opposed to
+it certain conditions inherent to their nature, the existence of the
+same materials composing them, and a manifest tendency to resemble one
+another, and to reproduce invariably the same primordial type."[118] Unity
+of plan and composition is, on this view, prior to adaptation and limits
+adaptation. Cuvier's view, on the contrary, is that the necessity of
+functional and ecological adaptation accounts for the repetition of the
+same types of structure. There are, of all the possible combinations of
+organs, only a few viable types--those whose structure is adapted to
+their life. Therefore it is reasonable that these few types should be
+repeated in innumerable exemplars. One must remember, in order to
+appreciate Cuvier's view, that he was not obsessed, as we are, by the
+idea of evolution.
+
+Cuvier thought in terms of organs, not in terms of "materials of
+organisation." He held that the resemblances between the organs of one
+class of animals and the organs of another were due to the similarity of
+their functions. "Let us conclude, then, that if there are resemblances
+between the organs of fish and those of other classes, it is only in the
+measure that there is a resemblance between their functions."[119] There
+are only a few kinds of organs, each adapted for a particular function,
+and these organs are necessarily repeated from class to class.--"As the
+animal kingdom has received only a limited number of organs, it is
+inevitable that some at least of these organs should be common to
+several classes."[120]
+
+Geoffroy thought in terms of "materials," of parts of indefinite
+function, parts which might take on any function. He insists upon the
+necessity of disregarding function when tracing out the unity of
+composition. He considers, in direct opposition to Cuvier's
+interpretation of structural resemblance as due to similarity of
+function, that unity of composition is the primary fact, and similarity
+of function subsidiary. In his reply in the _Mammiferes_ (1829) to
+Cuvier's criticisms in the _Histoire naturelle des Poissons_ (1828), he
+insists on the necessity of excluding function from consideration in any
+truly philosophical treatment of comparative anatomy (Discours prel., p.
+25). Cuvier held that function determined structure, or at least that
+the necessity of adaptation ruled the transformations of form. Geoffroy
+considered that structure determined function, that changes of
+structure, however they might arise, caused changes of function.
+"Animals," he writes, "have no habits but those that result from the
+structure of their organs; if the latter varies, there vary in the same
+manner all their springs of action, all their faculties and all their
+actions."[121]
+
+Again, "a vegetarian regime is imposed upon the Quadrumana by their
+possession of a somewhat ample stomach, and intestines of moderate
+length."[122] The hand of the bat has become so modified as to constrain
+the bat to live in the air.[123]
+
+The best example of Geoffroy's insistence upon the priority of structure
+to function, and so of his purely morphological attitude, is perhaps his
+interpretation, already alluded to, of the appendages of Articulates.
+The segments of the Articulate are, he says, the equivalents of the
+bodies of the 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."[124] The ribs become
+locomotory appendages.
+
+We may compare the similar thought that the ear ossicles are simply
+opercular bones reduced and turned to other uses.
+
+Geoffroy could not but recognise the correlation of structure to
+function, for this is a fact which imposes itself upon every observer.
+He recognised also correlation between functions, as when he pointed out
+the connection between increased respiration and enhanced muscular
+activity in birds.[125] He interpreted structure at times in terms of
+function, the short, strong clavicle of the mole as an adaptation to
+digging, the keeled sternum of birds as an adaptation to flying, and so
+on. But we may say that his whole tendency was to disregard function, to
+look upon it as subsidiary. He protests against arguing from function
+and habits to structure, as an "abuse of final causes."[126] He was not so
+convinced as Cuvier was of the all-importance of functional correlation;
+in this view he was probably confirmed by his work on teratology. It did
+not surprise him that Insects, in which lungs, heart and circulation
+have disappeared(!), should yet have a skeleton built upon the same plan
+as the skeleton of Vertebrates, which possess these organs; the
+correlation of organ-systems is not so close as to prevent this.[127] So
+too, although the other organs of the insect are all inside the body of
+the vertebrae, they are yet comparable with the organs of Vertebrates.[128]
+The existence of rudimentary organs also seemed to him an argument
+against too strict a correlation of parts.
+
+The contrast between the teleological attitude, with its insistence upon
+the priority of function to structure, and the morphological attitude,
+with its conviction of the priority of structure to function, is one of
+the most fundamental in biology.
+
+Cuvier and Geoffroy are the greatest representatives of these opposing
+views. Which of them is right? Is there nothing more in the unity and
+diversity of organic forms than the results of functional adaptation, or
+is Geoffroy right in insisting upon an element of unity which cannot be
+explained in terms of adaptation? If there be an irreducible element of
+unity, is there any truth in Geoffroy's suggestion that this unity
+results from a power which is exercised in the world of atoms where are
+elements of inalterable character?[129]
+
+The problem as Geoffroy and Cuvier understood it was not an evolutionary
+one. But the problem exists unchanged for the evolutionist, and
+evolution-theory is essentially an attempt to solve it in the one
+direction or the other. Theories such as Darwin's, which assume a random
+variation which is not primarily a response to environmental changes,
+answer the problem in Geoffroy's sense. Theories such as Lamarck's,
+which postulate an active responsive self-adaptation of the organism,
+are essentially a continuation and completing of Cuvier's thought.
+
+    [86] "Memoire sur les rapports naturels des makis,"
+    _Magasin Encyclopedique_, vii.
+
+    [87] Discours preliminaire, pp. xv.-xxiv.
+
+    [88] _Etudes progressives d'un Naturaliste_, p. 50,
+    Paris, 1835.
+
+    [89] _Philosophie Anatomique_., i., Introduction, p. 1.
+
+    [90] "Sur une colonne vertebrale et ses cotes dans les
+    insectes apiropodes," (_Acad. Sci._, Feb. 12, 1820).
+    Printed in _Isis_, pp. 527-52, 1820 (2).
+
+    [91] "Sur l'organisation des insectes," p. 458. _Isis_,
+    pp. 452-62, 1820 (2).
+
+    [92] _Mem. Mus. d'Hist. nat._, ix., pp. 89-119, Pls.
+    v-vii.
+
+    [93] _Sur l'organisation des insectes_, p. 459.
+
+    [94] _Isis_, p. 549.
+
+    [95] Published in _Ann. Sci. Nat._, xix., pp. 241-59,
+    1830.
+
+    [96] _Cf._ Aristotle (_supra_, p. 10).
+
+    [97] For an account of the controversy reference may be
+    made to I. Geoffroy St Hilaire, _Vie Travaux et Doctrine
+    scientifique d'Etienne Geoffroy St Hilaire_, Paris,
+    1847; also Semper, _Arb. zool. zoot. Instit. Wuerzburg_,
+    iii., 1876-7, K. E. von Baer, _Lebensgeschichte Cuviers_,
+    ed. L. Stieda, 1897, and J. Kohlbrugge, in _Zoolog.
+    Annalen_, v., pp. 143-95. 1913.
+
+    [98] "Recherches sur l'organisation des Gavials," _Mem.
+    Mus. d'Hist. nat._, xii., 1825.
+
+    [99] _Mem. Mus. d'Hist. nat._, xvii., pp. 209-29.
+
+    [100] _Mem. Acad. Sci._, xii., pp. 63-92, 1833.
+
+    [101] _Mem. Acad. Sci._, xii., pp. 43-61, 1833.
+
+    [102] Geoffroy's French style is at times incredibly bad,
+    and more or less literal translations of his sentences
+    are apt to read queerly!
+
+    [103] _Mem. Mus. d'Hist. nat._, xiii., p. 289, 1826.
+
+    [104] _Mem. Mus. d'Hist. nat._, xviii., p. 221, 1828. His
+    teratological work is important, and is chiefly
+    contained in the second volume of the _Philosophie
+    anatomique_.
+
+    [105] _Phil. anat._, i., p. 449.
+
+    [106] _Mem. Acad. Sci._, xii., p. 82, 1833.
+
+    [107] _Mem. Mus. d'Hist. nat._, ix., p. 101, 1822.
+
+    [108] _Cours de l'histoire naturelle des Mammiferes_, i.,
+    Lecon 3, p. 13, 1829.
+
+    [109] _Etudes progressives d'un Naturaliste_, p. 59, f.n.,
+    Paris, 1835.
+
+    [110] _Phil. Anat._, i., p. 444.
+
+    [111] _Ann. Mus. d'Hist. nat._, x., p. 344, 1807.
+
+    [112] _Isis_, p. 534, 1820 (2).
+
+    [113] _Ann. Mus. d'Hist. nat._, x., pp. 342-65, 1807.
+
+    [114] _loc. cit._, x., p. 343.
+
+    [115] _Phil. anat._, i., 450, f.n. _Cf._ Aristotle
+    (_supra_, p. 11).
+
+    [116] _Loc. cit._, p. 136.
+
+    [117] _Mammiferes_, i., Discours prel., p. 18.
+
+    [118] _Phil. anat._, i., p. 208.
+
+    [119] Cuvier and Valenciennes, _Hist. nat. Poissons_, i.,
+    p. 550, 1828.
+
+    [120] Cuvier and Valenciennes, _loc. cit._, p. 544.
+
+    [121] _Mammiferes_, i., _Lecon_ 4, p. 17.
+
+    [122] _Loc. cit._, _Lecon_ 5, p. 8.
+
+    [123] _Loc. cit._, _Lecon_ 13, p. 6.
+
+    [124] _Isis_, p. 539, 1820 (2).
+
+    [125] _Mammiferes_, i., _Lecon_ 4, p. 6.
+
+    [126] _Mammiferes_, Discours prel., p. 7.
+
+    [127] _Isis_, p. 460, 1820 (2).
+
+    [128] _Mem. Mus. d'Hist. nat._, ix., p. 102, 1822.
+
+    [129] _Mem. Acad. Sci._., xii., p. 76, 1833.
+
+
+
+
+CHAPTER VI
+
+THE FOLLOWERS OF ETIENNE GEOFFROY SAINT-HILAIRE
+
+
+Geoffroy's theories were not generally accepted by his contemporaries,
+but his methods had considerable influence, especially in France, where
+many made essays in pure morphology.
+
+His chief follower was Serres, who is mentioned indeed in the
+_Philosophie anatomique_ as a fellow-worker. Serres was primarily a
+medical anatomist; his interest lay in human anatomy and embryology,
+normal and pathological.
+
+His best early work was an _Anatomie comparee du cerveau_ (1824-26),
+which met with a flattering reception from Cuvier.[130] He laid great
+stress upon the development of the brain and spinal cord in the
+different classes, and was quick to point out analogies not only between
+adult but also between embryonic structures. He paid much attention to
+cases of correlation, and noted a great many; he observed, for instance,
+a constant relation between the development of the spinal cord and of
+the corpora quadrigemina, and between the size of the corpora
+quadrigemina and the volume of the optic nerves and eyes. In this the
+influence of Cuvier is unmistakable.
+
+Serres' early theoretical views are to be found in a series of papers in
+the _Annales des Sciences naturelles_,[131] under the general title
+_Recherches d'Anatomie transcendante, sur les Lois de l'Organogenie
+appliquees a l'anatomie 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.
+
+In the first of them he points out how neither position nor function has
+proved altogether sufficient to establish homologies. In the early days
+anatomists were guided by form; when form failed them, they traced an
+organ in its changes throughout the series of animals by considering its
+function. This method was satisfactory enough as regards the organs of
+the nutritive life. But in the organs of the life of relation, in the
+nervous system, the functions of the parts were difficult to discover,
+and their form very changeful. Hence a new principle was required, and
+Serres found it in the thought which he probably owed to the German
+transcendentalists (see Chap. VII.), that the permanent structure of the
+lower animals could be compared with phases in the development of the
+higher, and particularly of man, or, as he put it, that comparative
+anatomy was often only a fixed and permanent anthropogeny, and
+anthropogeny a fugitive and transitory comparative anatomy (xi., p.
+106).
+
+"In rising towards the first formations," he writes, "transcendental
+anatomy recognised that one and the same organ, however complicated its
+definitive form might be, repeated in its transitory states the organic
+simplicities of the lower classes. Thus the primitive heart of birds was
+first of all a canal, then a pocket or single cavity, then finally the
+complex organ of the class. Comparative anatomy was thus seen to be
+repeated and reproduced by embryogeny" (xii., p. 85).
+
+His explanation of the fact of repetition is that, "in animals belonging
+to the lower classes the _formative force_, whatever it may be, has a
+less energetic impulsion than in the higher animals, and hence the
+organs pass through only a part of the transformations which those of
+the higher forms undergo; and it is for this reason that they show
+permanently the organic dispositions which are only transitory in the
+embryo of man and the higher Vertebrates. Hence these double aortas,
+these double venae cavae which one observes more or less constantly among
+reptiles" (xxi., p. 48).
+
+The number of stages in embryogeny is proportionate to the complexity of
+the adult; the younger the embryo the simpler its organs--such is the
+general formula of the relation between the embryo and the adult. But
+here in Serres' doctrine of parallelism a complication enters. He
+observed that embryonic organs did not always develop in a piece, by
+simple growth, but often were formed by the union of separately formed
+parts or layers. Thus the kidney in man is formed by the fusion of a
+number of "little kidneys," and the spinal cord reaches its full
+development by the laying down of successive layers within it. He was
+greatly impressed with this fact, which, as a convinced believer in
+epigenesis, he used with great effect against the preformistic theories.
+"This method of isolated formation," he wrote, "is noticed in early
+stages in the thyroid, the liver, the heart, the aorta, the intestinal
+canal, the womb, the prostate, the clitoris, and the penis" (xi., p.
+69). So, too, in the development of the skeleton, ossification proceeds
+from separate centres, foramina are formed by the fusion of separate
+bones round them. In his memoir, _Lois d'Osteogenie_ (1819), Serres
+established several laws of ossification based upon this principle of
+separate formation.[132]
+
+How is the fact of multiple formation to be reconciled with the
+principle of repetition, according to which organs are simplest in the
+early embryo and in the lower animals? But observation shows that, as a
+rule, the further down the scale you go the more divided organs
+become--the more numerous the bones of the skull, for example. There is
+thus a parallel between multiple formation of organs in the embryos of
+the higher Vertebrates and their subdivided state in the lower. Take,
+for example, the kidney. In the genus _Felis_, and in birds, each kidney
+has two lobes, in the elephant four, in the otter ten, in the ox twelve
+to fourteen. The human kidney in its development starts with about a
+dozen lobes, and the number diminishes as the kidney grows. Thus the
+permanent state of the kidney in the animals mentioned is reproduced by
+the stages of its development in man (xii., p. 126).
+
+So, too, at the second or third month the uterus of the human embryo is
+bicornuate, and afterwards passes through stages comparable to the adult
+and permanent uterus of rodents, ruminants, and carnivores. There is
+indeed a time in the development of the human embryo when it resembles
+in many of its organs the adult stage of various lower animals. It is
+about this time that it possesses a tail.
+
+We note that Serres' theory of parallelism applies, strictly speaking,
+only to organs, not to organisms, although he, too, readily fell into
+the error of supposing that the organisation of an embryo could be
+compared as a whole with the adult organisation of an animal lower in
+the scale. Thus he wrote in one of his later papers[133]--"As our
+researches have made clear, an animal high in the organic scale only
+reaches this rank by passing through all the intermediate states which
+separate it from the animals placed below it. Man only becomes man after
+traversing transitional organisatory states which assimilate him first
+to fish, then to reptiles, then to birds and mammals." Serres was not
+altogether free from the besetting sin of the transcendentalists--hasty
+generalisation.
+
+The law of parallelism applied not only to Vertebrates but also to
+Invertebrates. In a short paper[134] of 1824 Serres attempted an
+explanation of the nervous system of Invertebrates. Invertebrates, he
+considered, lacked the cerebrospinal axis of Vertebrates, and their
+nervous system was the homologue of the sympathetic system of
+Vertebrates. The relation of the invertebrate to the vertebrate nervous
+system being thus fixed, can the nervous system of Invertebrates be
+reduced to one plan? It does not seem possible to establish a common
+plan for the adult nervous systems. But apply the principle of
+parallelism, which has proved so valuable within the limits of the
+vertebrate series. Taking insects as the highest class, we find that
+there are three stages in the development of their nervous system; in
+the first the nervous system is composed of two separate strands, in the
+second the strands unite round the oesophagus, in the third they unite
+also behind. Now in _Bulla aperta_, stage (1) is permanent; in _Clio_,
+_Doris_, _Aplysia_, _Tritonia_, _Sepia_, _Helix_, stage (2) is
+permanent, and in _Unio_ stage (3). In fact, all the varieties of the
+nervous system of molluscs fall into one or other of these three
+classes. "It follows, then, that as regards their nervous system, the
+Mollusca are more or less advanced larvae of insects" (p. 380). The law
+of parallelism is here applied to single organ-systems, but in later
+years Serres applied it to whole organisations also, saying that the
+lower Invertebrates were permanent embryos of the higher.
+
+In the paper of 1834, already referred to, Serres pushed his
+speculations further and attempted to establish the unity of type of all
+animals, Vertebrates and Invertebrates alike--a favourite pastime of the
+transcendentalists. It is incontestable, he admits, that adult
+Invertebrates are quite different in structure from adult Vertebrates,
+"but if one regards them as what I take them to be, namely, _permanent
+embryos_, and if one compares their organisation with the embryogeny of
+Vertebrates, one sees the differences disappear, and from their
+analogies arise a crowd of unsuspected resemblances" (_loc. cit._, p.
+247).
+
+The last point of Serres' doctrine which calls for remark is his
+interpretation of abnormalities as being often comparable to grades of
+structure permanent in the lower animals. Thus the double aorta which
+may occur as an abnormality in man is the normal and permanent state in
+reptiles. This idea, of course, he got from Etienne Geoffroy St Hilaire.
+It is further developed in his "_Theorie des formations et des
+deformations organiques appliquee a l'anatomie 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 convinced," he wrote, "that when a more complete
+examination has been made of the mouth of insects, properly so called,
+that is to say, having six legs and two 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."[135] In this the influence of
+Geoffroy can be traced; but the work was very free from the
+exaggerations of the transcendentalists, and many of Savigny's
+homologies are accepted even to-day. The first memoir dealt with the
+mouth-parts of insects; the second with the anterior appendages of
+Articulates generally. Savigny shows that the mouth-parts of insects can
+be reduced to the type shown in Orthoptera, where there are clearly two
+mandibles, two 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 _Julus_, _Scolopendra_,
+_Cancer_, _Gammarus_, _Cyamus_, _Nymphon_, _Phalangium_, _Apus_,
+_Caligus_, _Limulus_, and a few others. For Crustacea he established the
+homologies now accepted, of the mandibles with the mandibles of insects,
+of the first and second pairs of 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 _Julus_, that is to say, all
+the feet of the Hexapoda, are here transformed into jaws" (_loc. cit._,
+p. 48). In _Scolopendra_ also there is a similar transformation of two
+pairs of legs into auxiliary jaws. In _Gammarus_, where there is only
+the first pair of maxillipedes, the other two pairs have become
+"retransformed" into feet. We find him supporting his comparison of the
+three anterior pairs of legs in _Julus_ to the three pairs of legs in
+insects by an argument drawn from embryology; for only the first three
+pairs of feet are present in _Julus_ at birth (Degeer), "an observation,
+which, together with their position, should cause them to be considered
+as the representatives of the six thoracic feet of Hexapoda" (p. 44).
+
+His comparison of the Arachnid appendages with those of insects and
+Crustacea is very curious. As his starting-point he takes _Cyamus_,
+which has antennae (two pairs) and mouth parts (four pairs) as in many
+Crustacea, and then seven pairs of legs; he compares with it _Nymphon_,
+which has in all seven pairs of appendages. These appendages he
+homologises with the seven pairs of legs of _Cyamus_, so that the first
+appendage in _Nymphon_ corresponds to the seventh appendage of _Cyamus_.
+This homology is extended to all Arachnids; their first two pairs of
+appendages, however they may be modified as "false" mandibles and
+"false" maxillae, really correspond to the second and third maxillipedes
+in Crustacea, and to the second and third pairs of feet in insects. It
+is interesting to note that he treats _Limulus_ as an Arachnid, pointing
+out that there is as much difference between _Apus_ and _Limulus_ as
+between _Cancer_ and _Phalangium_. He describes the "gnathobases" in
+_Phalangium_ and _Limulus_. We may note that he had just an inkling of
+the modern doctrine that all the appendages of Articulates consist of a
+basal joint bearing an inner and an outer terminal piece, for he
+observes that the "cirri" of the maxillipedes of Crustacea give the
+appendage the same bifid appearance as the appendages of the abdomen and
+the thoracic legs of _Mysis_ (p. 50).
+
+V. Audouin, in his memoir, _Recherches anatomiques sur le thorax des
+animaux articules_,[135] applied the principle of the unity of plan and
+composition to the exoskeleton of insects, Crustaceans, and Arachnids.
+His guiding ideas were, "(1) that the skeleton of articulated animals is
+formed of a definite number of pieces, which are either distinct or
+intimately fused with one another; (2) that in many cases, some pieces
+diminish or altogether disappear, while others reach an excessive
+development; (3) that the increase of one piece seems to exert on the
+neighbouring pieces a kind of influence which explains all the
+differences one finds between the individuals of each order, family and
+genus" (Sep. copy, p. 16). Geoffroy had already stated, without proof,
+that the parts of the Arthropod's skeleton, however they might change in
+shape and size, remained faithful to the principle of connections, at
+least at their points of insertion.[137] Audouin gave the detailed
+demonstration of this by his accurate and minute determination of the
+pieces of the arthropod skeleton. He recognised that the body of
+Arthropods was made up of a series of similar rings, and that even the
+compact head of insects consisted of fused segments. In each segment
+Audouin distinguished a fixed number of hard chitinous parts, the dorsal
+tergum, the ventral sternum, the lateral "flanc" of three pieces, all to
+be recognised by their positions relative to one another. Many of the
+names which he proposed are still in use; it was he who introduced the
+terms prothorax, mesothorax, and metathorax, for the three segments of
+the insect's thorax. He used Geoffroy's _Loi de balancement_ to explain
+cases of correlative development, such as the relation between the size
+of the front wings and the development of the mesothorax. In another
+paper Audouin compared the three pieces of the dorsal skeleton of
+Trilobites to the tergum and the upper part of the "flanc."[138] In a
+third paper of about the same time he tried to establish the homologies
+of the segments throughout the Articulate series--with less success than
+Savigny.
+
+Later on, in conjunction with Milne-Edwards, he demonstrated the unity
+of composition of the nervous system in Crustacea, showing how the
+concentrated system of the crab was formed by the same series of ganglia
+as in the Macrura.
+
+The entomologist Latreille also tackled the problem of the homologies of
+the segments in the different classes of Arthropods (Cuvier, _loc.
+cit._, p. cclxxii.). He thought he could find fifteen segments in all
+Arthropods. He made the retrograde step of likening the head of insects
+to a single segment. But some of his homologies showed morphological
+insight, _e.g._, his comparison of the "first jaws" of Arachnids to
+antennae, because they were placed above the upper lip. It was he who
+first pointed out the resemblance of the leaf-like gills of Ephemerid
+larvae to wings, and suggested that wings were "a sort of tracheal feet."
+
+He made also a rather hazy and speculative contribution on Okenian lines
+to the problem of the relation of Arthropods to Vertebrates, likening
+the carapace of Crustacea to an enormously developed hyoid, the
+appendages of the tail to the ventral and anal fins of fish. The
+masticatory organs of Arthropods were jaws disjointed at their
+symphysis; antennae, nostrils turned outside in.
+
+Duges also made a comparison of Articulates with Vertebrates.[139] He did
+not accept Geoffroy's vertebral theory of the Arthropod skeleton, though
+he admitted that in Arthropods the dorsal surface was turned towards the
+ground, basing this assumption on the position of the nervous system,
+and also, curiously enough, on the inverted position of the embryo on
+the lower surface of the yolk. He considered that the mandibles and
+first 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 equivalent 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.[140] According to his view there are three main types of
+animals--(1) Biserials, including bilaterally symmetrical animals,
+composed of two parallel series of "organisms"; (2) Radiates, composed
+of "organisms" arranged like the spokes of a wheel; and (3)
+Raceme-animals, in which the separate "organisms" were disposed more or
+less irregularly, in bunches (p. 257). The unitary "organism" is
+supposed to be the same in all, only the arrangement differing. Duges of
+course admitted that the centralisation of the complete organism became
+greater the higher it stood in the scale, and that this held good also
+in individual development. The appendages of Articulates and Vertebrates
+were thought of as the members of as many separate organisms. He went so
+far as to suggest that the fingers of a man's hand were the free
+extremities of as many thoracic members.
+
+Duges' conception of the organism has often been revived since in a
+saner form, _e.g._, by E. Perrier, and it has a certain validity. It has
+much affinity with the similar conceptions of Goethe and the German
+transcendentalists.
+
+    [130] _Mem. Acad. Sci._, iv., pp. cclxxxiv.-ccci., 1824.
+
+    [131] _Ann. Sci. Nat._, xi., xii., 1827; xvi., 1829; xxi., 1830.
+
+    [132] See Radl, _loc. cit._, i., pp. 225-6.
+
+    [133] _Ann. Sci. nat._ (2), ii., p. 248, 1834.
+
+    [134] _Ann. Sci. nat._, iii., pp. 377-80, 1824.
+
+    [135] _Memoires sur les Animaux sans Vertebres_, Part I.,
+    p. 10, Paris, 1816.
+
+    [136] _Ann. Sci. Nat._, (1), i., pp. 97-135, 416-432,
+    1824.
+
+    [137] _Isis_, p. 456, 1820 (2).
+
+    [138] Cuvier, _Mem. Acad. Sci._, iv., p. cclxx., 1824.
+
+    [139] _Acad. Sci._ 18th Oct. 1831. Extract in _Ann. Sci.
+    Nat._, xxiv., pp. 254-60, 1831.
+
+    [140] His views were more fully elaborated in his _Memoire
+    sur la conformite organique dans l'echelle animale_,
+    Montpellier, 1832.
+
+
+
+
+CHAPTER VII
+
+THE GERMAN TRANSCENDENTALISTS
+
+
+To complete our historical survey of the morphology of the early 19th
+century we have now to turn back some way and consider the curious
+development of morphological thought in Germany under the influence of
+the _Philosophy of Nature_. We have already seen many of these notions
+foreshadowed by Goethe, who had considerable affinity with the
+transcendentalists, but the full development of transcendental habits of
+thought comes a little later than the bulk of Goethe's scientific work,
+and owes more to Kielmeyer and Oken than to Goethe himself.
+
+A great wave of transcendentalism seems to have passed over biological
+thought in the early 19th century, arising mainly in Germany, but
+powerfully affecting, as we have seen, the thought of Geoffroy and his
+followers. Many ideas were common to the French and German schools of
+transcendental anatomy, the fundamental conception that there exists a
+unique plan of structure, the idea of the scale of beings, the notion of
+the parallelism between the development of the individual and the
+evolution of the race. It is difficult to disentangle the part played by
+each school and to determine which should have the credit for particular
+theories and discoveries. The philosophy seems to have come chiefly from
+Germany, the science from France. It must be borne in mind that German
+comparative anatomy was largely derivative from French, that the Paris
+Museum was the acknowledged anatomical centre, and that Cuvier was its
+acknowledged head.
+
+It is probably correct to say that the credit mainly belongs to the
+German transcendental school for the law of the parallelism between the
+stages of individual development and the stages of the scale of beings,
+and the theory of the repetition or multiplication of parts within the
+individual. The vertebral theory of the skull is a particular
+application of the second of these generalisations.
+
+The law of parallelism[141] seems to have been expressed first by
+Kielmeyer (1793),[142] who gave to it a physiological form, saying that
+the human embryo shows at first a purely vegetative life, then becomes
+like the lower animals, which move but have no sensation, and finally
+reaches the level of the animals that both feel and move.
+
+The idea was next taught by Autenrieth in 1797.[143]
+
+Oken (1779-1851) in his early tract _Die Zeugung_ (1805), and in his
+_Lehrbuch der Naturphilosophie_ (1809-11) elaborated the thought, and
+taught that every animal in its development passes through the classes
+immediately below it. "During its development the animal passes through
+all stages of the animal kingdom. The foetus is a representation of all
+animal classes in time."[144] The Insect, for example, is at first Worm,
+next Crab, then a perfect volant animal with limbs, a Fly (_ibid._, p.
+542).
+
+As Nature is "the representation of the individual activities of the
+spirit," so the animal kingdom is the representation of the activities
+or organs of man. The animal kingdom is therefore "a dismemberment of
+the highest animal, _i.e._, of Man" (p. 494). Now "animals are gradually
+perfected, entirely like the single animal body, by adding organ unto
+organ"--the way of evolution is the way of development. Hence "animals
+are only the persistent foetal stages or conditions of Man," who is the
+microcosm, and contains within himself all the animal kingdom.
+
+Oken was himself a careful student of embryology; von Baer[145] speaks of
+his work (published in Oken and Kieser, _Beitraege zur vergleichenden
+Zoologie, Anatomie und Physiologie_, 2 pts., 1806-7) as forming the
+turning-point in our understanding of the mammalian ovum. He had
+accordingly actually observed a resemblance in certain details of
+structure between the human foetus and the lower animals; but the
+peculiar form which the law took in his hands was a consequence of his
+hazy philosophy. He saw the relation of teratological to foetal
+structure, for he affirmed that "malformations are only persistent
+foetal conditions" (p. 492).
+
+The idea of comparing the embryo of higher animals with the adult of
+lower was widely spread at this time among German zoologists. We find,
+for example, in Tiedemann's brilliant little textbook[146] the statement
+that "Every animal, before reaching its full development, passes through
+the stage of organisation of one or more classes lower in the scale, or,
+every animal begins its metamorphosis with the simplest organisation"
+(p. 57).
+
+Thus the higher animals begin life as a kind of fluid animal jelly which
+resembles the substance of a polyp; the young mammal, like the lower
+Vertebrates, has only a simple circulation, and, like them, lives in
+water (the amniotic fluid); the frog is first like a worm, then develops
+gills and becomes like a fish (p. 57). In his work on the anatomy of the
+brain,[147] Tiedemann established the homology of the optic lobes in birds
+by comparing them with foetal corpora quadrigemina in man (see Serres,
+_Ann. Sci. nat._, xii., p. 112).
+
+J. F. Meckel, in 1811, devoted a long essay to a detailed proof of the
+parallelism between the embryonic states of the higher animals and the
+permanent states of the lower animals. In a previous memoir in the same
+collection[148] (i., 1, 1808) he had made some comparisons of this kind in
+dealing with the development of the human foetus; in this memoir (ii.,
+1, 1811) he brings together all the facts which seem to prove the
+parallelism.
+
+His collection of facts is a very heterogeneous one; he mingles
+morphological with physiological analogies, and makes the most
+far-fetched comparisons between organs belonging to animals of the most
+diverse groups. He compares, for instance, the placenta with the gills
+of fish, of molluscs and of worms, homologising the cotyledons with the
+separate tufts of gills in _Tethys, Scyllaea_ and _Arenicola_(p. 26).
+This is purely a physiological analogy. He compares the closed anus of
+the early human embryo with the permanent absence of an anus in
+Coelentera, and the embryo's lack of teeth with the absence of teeth in
+many reptiles and fish, in birds, and in many Cetacea (p. 46).[149] These
+are merely chance resemblances of no morphological importance. He
+considers bladderworms as animals which have never escaped from their
+amnion, and _Volvox_ as not having developed beyond the level of an egg
+(p. 7). He lays much stress upon likeness of shape and of relative size,
+comparing, for instance, the large multilobate liver of the human foetus
+with the many-lobed liver of lower Vertebrates and of Invertebrates. In
+general he shows himself, in his comparisons, lacking in morphological
+insight.
+
+His treatment of the vascular system affords perhaps the best example of
+his method (pp. 8-25). The simplest form of heart is the simple tubular
+organ in insects, and it is under this form that the heart first appears
+in the developing chick. The bent form of the embryonic heart recalls
+the heart of spiders; it lies at first free, as in the mollusc _Anomia_.
+The heart consists at first of one chamber only, recalling the
+one-chambered heart of Crustacea. A little later three chambers are
+developed, the auricle, ventricle, and aortic bulb; at this stage there
+is a resemblance to the heart of fish and amphibia. At the end of the
+fourth day the auricle becomes divided into two, affording a parallel
+with the adult heart of many reptiles.
+
+In his large text-book of a somewhat later date, the _System der
+vergleichenden Anatomie_ (i., 1821), he works out the idea again and
+gives to it a much wider theoretic sweep, hinting that the development
+of the individual is a repetition of the evolutionary history of the
+race. Meckel was a timid believer in evolution. He thought it quite
+possible that much of the variety of animal form was due to a process of
+evolution caused by forces inherent in the organism. "The
+transformations," he writes, "which have determined the most remarkable
+changes in the number and development of the instruments of organisation
+are incontestably much more the consequence of the tendency, inherent in
+organic matter, which leads it insensibly to rise to higher states of
+organisation, passing through a series of intermediate states."[150]
+
+His final enunciation of the law of parallelism in this same volume
+shows that he considered the development of the individual to be due to
+the same forces that rule evolution. "The development of the individual
+organism obeys the same laws as the development of the whole animal
+series; that is to say, the higher animal, in its gradual evolution,
+essentially passes through the permanent organic stages which lie below
+it; a circumstance which allows us to assume a close analogy between the
+differences which exist between the diverse stages of development, and
+between each of the animal classes" (p. 514).
+
+He was not, of course, able fully to prove his contention that the lower
+animals are the embryos of the higher, and we gather from the following
+passage that he could maintain it only in a somewhat modified form. "It
+is certain," he writes, "that if a given organ shows in the embryo of a
+higher animal a given form, identical with that shown throughout life by
+an animal belonging to a lower class, the embryo, in respect of this
+portion of its economy, belongs to the class in question" (p. 535). The
+embryo of a Vertebrate might at a certain stage of development, be
+called a mollusc, if for instance, it had the heart of a mollusc.
+
+He admits, too, that the highest animal of all does not pass through in
+his development the entire animal series. But the embryo of man always
+and necessarily passes through many animal stages, at least as regards
+its single organs and organ-systems, and this is enough in Meckel's eyes
+to justify the law of parallelism (p. 535).
+
+In his excellent discussion of teratology Meckel points out how the idea
+of parallelism throws light upon certain abnormalities which are found
+to be normal in other (lower) forms (p. 556).[151]
+
+We may refer to one other statement of the law of parallelism--by K. G.
+Carus in his _Lehrbuch der vergleichenden Anatomie_ (Leipzig, 1834). The
+standpoint is again that of _Naturphilosophie_. It is a general law of
+Nature, Carus thinks, that the higher formations include the lower; thus
+the animal includes the vegetable, for it possesses the "vegetative" as
+well as the "animal" organs. So it is, too, by a rational necessity that
+the development of a perfect animal repeats the series of antecedent
+formations.
+
+As we have said, the main credit for the enunciation of the law of
+parallelism belongs to the German transcendental school; but the law
+owes much also to Serres, who, with Meckel, worked out its implications.
+It might for convenience, and in order to distinguish it from the laws
+later enunciated by von Baer and Haeckel, be called the law of
+Meckel-Serres.
+
+Under the "theory of the repetition or multiplication of parts within
+the organism" may be included, first, generalisations on the serial
+homology of parts, and second, more or less confused attempts to
+demonstrate that the whole organisation is repeated in certain of the
+parts. The recognition of serial homologies constituted a real advance
+in morphology; the "philosophical" idea of the repetition of the whole
+in the parts led to many absurdities. It led Oken to assert that in the
+head the whole trunk is repeated, that the upper jaw corresponds to the
+arms, the lower to the legs, that in each jaw the same bony divisions
+exist as in the limbs, the teeth, for instance, corresponding to the
+claws (_loc. cit._, p. 408). It led him to distinguish "two animals" in
+every body--the cephalic and the sexual animal. Each of these has its
+own organs; thus "in the perfect animal there are two intestinal systems
+thoroughly distinct from each other, two intestines which belong to two
+different animals, the sexual and cephalic animal, or the plant and the
+animal" (p. 382). The intestine of the sexual animal is the large
+intestine; the lungs of the sexual animal are the kidneys, its glottis
+is the urethra, its mouth the anus. So, too, the mouth is the stomach of
+the head. On another line of thought the sternum is a ventral vertebral
+column. Limbs are connate ribs, the digits indicating the number of ribs
+included (_cf._ Duges, _supra_, p. 88).
+
+J. F. Meckel[152] discusses "homologies" of this kind in the thorough and
+pedestrian way so characteristic of him. Not only, he says, are the
+right and left halves of the body comparable with one another, but also
+the upper and the lower, the dividing line being drawn at the level of
+the diaphragm. The lumbar complex corresponds to the skull, the anus to
+the mouth, the urino-genital opening to the nasal opening; in general,
+the urino-genital system corresponds to the respiratory, the kidneys to
+the lungs, the ureters to bronchi, the testes and ovaries to the thymus
+(he had observed the physiological relation between the development of
+the thymus and the state of the genital organs), the prostate and the
+uterus to the thyroid gland, and the penis and clitoris to the tongue.
+The fore-limbs and girdle correspond in detail with the hind limbs and
+the pelvis--a point already worked out by Vicq d'Azyr; the dorsal and
+ventral halves of the body are likewise comparable in some respects, the
+sternum, for example, answering in the arrangement of its bones, muscles
+and arteries to the vertebral column. The skeleton of each member is in
+some respects a repetition of the vertebral column.
+
+His brother, D. A. Meckel,[153] worked out an elaborate comparison between
+the alimentary canal and the genital organs, basing the legitimacy of
+the comparison upon early embryological relations and upon the state of
+things in Coelentera, where genital and digestive organs occupy the same
+cavity. In his view the uterus corresponded to the stomach, the vagina
+to the oesophagus, the fallopian tubes to the intestine, and so on.
+
+The vertebral theory of the skull took its origin from the same habit of
+thought. As part of the wider idea of the metameric repetition of parts
+it had some scientific worth, but the theory was pushed too far, and the
+facts were twisted to suit it. Among annulate animals the theory of
+repetition found ample scope; Oken was able to compare with justice the
+jaws of crabs and insects with their other limbs, as Savigny did later
+in a more scientific way. Among Vertebrates the application of the
+theory of serial repetition was not so obvious, except in the case of
+the vertebrae. Goethe seems to have been the first to hit upon the idea
+that the skull is composed of a number of vertebrae, serially homologous
+with those of the vertebral column. He tells us that the idea flashed
+into his mind when contemplating in the Jewish cemetery at Venice a
+dried sheep's skull. The discovery was made in 1790, but not published
+till 1820.[154]
+
+The idea seems to have been taught by Kielmeyer, one of the earliest of
+the "philosophers of nature," but it was not published by him.
+
+In a book (_Cours d'Etudes medicales_), published in 1803, Burdin
+assimilated the skull to the vertebral column.
+
+Oken, in an inaugural dissertation (Programm) _Ueber die Bedeutung der
+Schaedelknochen_,[155] published in 1807, gave to the theory its necessary
+development. Autenrieth, also in 1807,[156] distinguishing separate
+ganglia in the brain, was not far from the hypothesis that each of these
+ganglia must have its separate vertebra.
+
+In 1808 Dumeril read a paper to the Academie des Sciences in which he
+compared the skull to a gigantic vertebra, basing his hypothesis on the
+similarity existing between the crests and depressions on the hinder
+part of the skull and those on the posterior surfaces of the 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 Oken
+did at first) three cranial vertebrae. Bojanus in his _Anatome testudinis
+europaeae_ (1819), and in a series of papers in _Isis_ (1817-1819, and
+1821) established the existence of a fourth cranial vertebra, and this
+was accepted by Oken in the later editions of his _Lehrbuch_. Meckel and
+Carus among the Germans, de Blainville and E. Geoffroy among the French,
+contributed to the development of the theory. In England the theory was
+championed particularly by Richard Owen.
+
+It was one thing to assert in a moment of inspiration that the skull was
+composed of modified 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 _Lehrbuch_. "To a perfect vertebra," he
+says, "belong at least five pieces, namely, the body, in front the two
+ribs, behind the two arches or spinous processes" (p. 370). In the
+cervical vertebrae the transverse processes represent the ribs. The skull
+consists of four vertebrae, 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 clinoid apophyses."[157] He denied the
+resemblance of the frontal and nasal "vertebrae" 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.[158] His system is worthy of
+some consideration, for he tries to work out a geometry of the
+skeleton.[160]
+
+His method of deduction is a good example of pure _Naturphilosophie_.
+Life, he says, is the development of something determinate from
+something indeterminate. A finite indeterminate thing, that is, a
+liquid, must take a spherical form if it is to exist as an individual.
+Hence the sphere is the prototype of every organic body. Development
+takes place by antagonism, by polarity, typically by the division and
+multiplication of the sphere. In the course of development the sphere
+may change, by expansion into an egg-shaped body, or by contraction into
+a crystalline form, the changes due to expansion being typical of living
+things, those due to contraction being typical of dead. At the surface
+of the primitive living sphere is developed the protective
+_dermatoskeleton_, which naturally takes the shape of a hollow sphere;
+round the digestive cavity which is formed in the living sphere is
+developed the _splanchnoskeleton_; round the nervous system (which is,
+as it were, the animal within the animal) is developed the
+_neuroskeleton_. All skeletal formations belong to one or other of these
+systems.
+
+Carus defines his aim to be the discovery of the inner law which
+presides over the formation of the skeleton throughout the animal
+kingdom; he desires to know "how such and such a formation is realised
+in virtue of the eternal laws of reason" (iii., p. 93). Here we touch
+the kernel of _Naturphilosophie_--the search for rational laws which are
+active in Nature; the discontent with merely empirical laws.
+
+The thesis which Carus sustains is that all forms of skeleton, whether
+of dermatoskeleton, splanchnoskeleton, or neuroskeleton, can be deduced
+from the hollow sphere, which is the primary form of any skeleton
+whatsoever (p. 95). That means, put empirically, that every skeleton can
+be represented schematically by a number of hollow spheres, suitably
+modified in shape, and suitably arranged. The chief modification in
+shape exhibited by bones is one which is intermediate between the
+organic and the crystalline series of modifications of the sphere. The
+organic modifications are bounded by curved lines, the crystalline by
+straight; the intermediate partly by curved and partly by straight
+lines. They are the dicone (the shape of a diabolo) and the cylinder.
+These forms must necessarily be of importance for the skeleton, which is
+intermediate between the organic and the inorganic. "The dicone embodies
+the real significance of the bone," writes Carus. Each dicone and
+cylinder composing the skeleton is called by Carus a vertebra.
+
+We may expect then all skeletons to be composed of spheres, cylinders
+and dicones in diverse arrangements. Nature being infinite, all the
+possible types of arrangement of these elements must exist in the test
+or skeleton of some animal, living, fossil, or to come (p. 127). One
+conceives easily what the main types of skeleton must be. In some
+animals, _e.g._, sea-urchins, the skeleton is a simple sphere; in
+others, _e.g._, starfish, secondary rows of spheres radiate out from a
+central sphere or ring; in annulate animals the skeleton consists of a
+row of partially fused spheres.
+
+In Vertebrates the arrangement is more complex. There are first the
+protovertebral rings of the dermatoskeleton, these being principally the
+ribs, limb-girdles, and jaws. Round the central nervous system are
+developed the deutovertebral rings of the neuroskeleton (vertebrae in the
+ordinary sense). The apophyses and bodies of the vertebrae, and the bones
+of the members[160] are composed of columns of tritovertebrae, or vertebrae
+of the third order. Thus the whole vertebrate skeleton is a particular
+arrangement of vertebrae, which in their turn are modifications of the
+primary hollow sphere.
+
+The German transcendentalists were more or less contemporary with E.
+Geoffroy, and no doubt influenced him, especially in his later years, as
+they certainly did his follower Serres. Oken indeed wrote, in a note[161]
+appended to Geoffroy's paper on the vertebral column of insects, that
+"Mr Geoffroy [_sic_] is without a doubt the first to introduce in France
+_Naturphilosophie_ into comparative anatomy, that is to say, that
+philosophy one of whose doctrines it is to seek after the
+_signification_ of organs in the scale of organised beings." This is,
+however, an exaggeration, for Geoffroy was primarily a morphologist,
+whereas the morphology of the German transcendentalists was only a
+side-issue of their _Naturphilosophie_.
+
+Geoffroy, on his part, exercised some influence on the
+transcendentalists. He asserts[162] indeed that Spix got some of the ideas
+published in the _Cephalogenesis_ (1815) from attending his course of
+lectures in 1809. It is certainly the case that Spix published before
+Geoffroy the view that the opercular bones are homologous with the
+ear-ossicles, adopting, however, a different homology for the separate
+bones.[163]
+
+Some speculations seem to have been common to both schools--for
+instance, the law of Meckel-Serres, the vertebral theory of the skull,
+and the recognition of serial homology in the appendages of Arthropods
+(Savigny, Oken). Latreille and Duges, as well as Serres, clearly show in
+their theoretical views the influence of Oken and the other
+transcendentalists. Geoffroy's principle of connections and law of
+compensation were recognised by some at least of the Germans.
+
+But whatever his actual historical relations may have been with the
+German school, Geoffroy was vastly their superior in the matter of pure
+morphology. He alone brought to clear consciousness the principles on
+which a pure morphology could be based: the Germans were transcendental
+philosophers first, and morphologists after.
+
+One understands from this how J. F. Meckel, who was in some ways the
+leading comparative anatomist in Germany at this time, could be at once
+a transcendentalist and an opponent of Geoffroy. Meckel had a curiously
+eclectic mind. A disciple of Cuvier, having studied in 1804-6 the rich
+collections at the Museum in Paris, the translator of Cuvier's _Lecons
+d'anatomie comparee_, he earned for himself the title of the "German
+Cuvier," partly through the publication of his comprehensive textbook
+(_System der vergl. Anatomie_, 5 vols.), partly by his extensive and
+many-sided research work, partly by his authoritative teaching. His
+_System_ shows in almost every page of its theoretical part the
+influence of Cuvier; and it is through having assimilated Cuvier's
+teaching as to the importance of function that Meckel combats Geoffroy's
+law of connections, at least in its rigorous form. He submits that the
+connections of bones and muscles must change in relation to functional
+requirements. He rejects Geoffroy's theory of the vertebrate nature of
+Articulates. Generally throughout his work the functional point of view
+is well to the fore.
+
+Yet at heart Meckel was a transcendentalist of the German school. His
+vagaries on the subject of "homologues" leave no doubt about that, and,
+in spite of Cuvier, he believed, though not very firmly, in the
+existence of one single type of structure.
+
+A Cuverian by training, his lack of morphological sense threw him into
+the ranks of the transcendentalists, to whom perhaps he belonged by
+nature.
+
+    [141] For a full account, see Kohlbrugge, _Zool. Annalen_,
+    xxxviii., 1911.
+
+    [142] _Rede ueber das Verhaeltnis der organischen Kraefte_,
+    Stuttgart u. Tuebingen, 1793 (1814). See Radl, _loc.
+    cit._, i., p. 261; ii., p. 57.
+
+    [143] _Supplem. ad historiam embryonis_, Tuebingen, 1797.
+
+    [144] _Lehrbuch der Naturphilosophie_, Eng. trans., p.
+    491, 1847.
+
+    [145] _Ueber Entwickelungsgeschichte der Thiere_, i., p.
+    xvii., 1828.
+
+    [146] _Zoologie_, Landshut, i., 1808.
+
+    [147] _Anatomie u. Bildungsgeschichte des Gehirns im Foetus
+    des Menschen_, Nuernberg, 1816.
+
+    [148] _Beytraege zur vergleichende Anatomie_, Leipzig, i.,
+    1808-9, ii., 1811-2.
+
+    [149] Cetacea were generally considered at this time to be
+    mammals of low organisation.
+
+    [150] From the French trans., which appeared under the
+    title _Traite gen. d'Anat. comparee_, i., p. 449, 1828.
+
+    [151] _Cf._ Geoffroy (_supra_, p. 70).
+
+    [152] _Beytraege_, ii., 2, 1812. Also in his _System d.
+    vergl. Anat._, i., 1821.
+
+    [153] In J. F. Meckel's _Beytraege_, ii.
+
+    [154] _Zur Morphologie_, i., 2, p. 250, 1820; and ii., 2,
+    pp. 122-4, 1824.
+
+    [155] See translation, giving the gist of this paper, in
+    Huxley's _Lectures on the Elements of Comparative
+    Anatomy_, pp. 282-6, London, 1864.
+
+    [156] Reil's _Archiv. f. Physiol._, vii., 1807.
+
+    [157] _Lecons d'anatomie comparee_, 3rd ed., Brussels
+    reprint, i., p. 414, 1836.
+
+    [158] In his Programm, _U. d. Bedeut. d. Schaedelknochen_,
+    1807.
+
+    [159] _Traite elementaire d'anatomie comparee_ (French
+    trans.), vol. iii., Paris, 1835. First developed in his
+    volume _Von den Ur-Theilen des Knochen und
+    Schalen-Gerustes_, Leipzig, 1828.
+
+    [160] Dutrochet in 1821 had tried to prove that the bones
+    of the members belong to the type of the vertebra--the
+    dicone.
+
+    [161] _Isis_, pp. 552-9, 1820 (2).
+
+    [162] _Mem. Mus. d'Hist. nat._, ix., 1822.
+
+    [163] Cuvier and Valenciennes, _Hist. nat. Poissons_, i.,
+    p. 311, f.n.
+
+
+
+
+CHAPTER VIII
+
+TRANSCENDENTAL ANATOMY IN ENGLAND--RICHARD OWEN
+
+
+Richard Owen is the epigonos of transcendental morphology; in him its
+guiding ideas find clear expression, and in his writings are no
+half-truths struggling for utterance. But he was, though a staunch
+transcendentalist, an eclectic of the older ideas current in his time;
+for he picked out what was best in the older systems--Cuvier's
+teleology, Geoffroy's principle of connections, Oken's idea of the
+serial repetition of parts. In particular, he assimilated the teaching
+of Cuvier, the great opponent of the transcendentalists, and reconciled
+it in part with his own transcendentalism. His main theoretical views
+are to be found in his volume _On the Archetype and Homologies of the
+Vertebrate Skeleton_ (London, 1848). The master-idea of the book is that
+the vertebrate skeleton consists of a series of comparable segments,
+each of which Owen calls a vertebra. His definition of a vertebra is,
+"one of those segments of the endo-skeleton which constitute the axis of
+the body, and the protecting canals of the nervous and vascular trunks"
+(p. 81). The parts of a typical vertebra are shown in Fig. 4, which is
+copied from Owen's Fig. 14.
+
+                |||
+  zygapophysis  ||| -- neural spine
+             \  |||
+              *//^\\*
+diapophysis   //   \\ -- neurapohysis
+         \   //  o  \\
+          ===== --- =====
+              /     \
+       ===== |CENTRUM| O ===== -- peiurapophysis
+              \     /
+          ===== --- =====
+         /    \\   //
+parapophysis  *\\v//*
+             /  |||
+  zygapophysis  ||| -- haemal spine
+                |||
+
+FIG. 4.--Ideal Typical Vertebra. (After Owen.)
+
+In Fig. 5 (page 103) is shown an actual vertebra, as Owen conceives it,
+the "vertebra" being that of a bird.
+
+[Illustration: FIG. 5.--Natural Typical Vertebra; Thorax of a Bird.
+(After Owen.)]
+
+A segment of sternum is included as the "haemal spine" of the vertebra
+(_hs_); the vertebral rib is the "pleurapophysis" (_pl_); the sternal
+rib the "haemapophysis" (_h_); the uncinate process of the vertebral rib
+is known as the "diverging appendage" (_a_). The whole vertebrate
+skeleton is composed of a series of 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 arrangement of them. The great
+object of my present labour has been to deduce ... the relative value
+and constancy of the different vertebral elements, and to trace the kind
+and extent of their variations within the limits of a plain and obvious
+maintenance of a typical character" (p. 146).
+
+It goes without saying that Owen considered the skull to be formed of
+vertebrae--the vertebral theory of the skull was, in his system, a
+deduction from the vertebral theory of the skeleton. He recognised four
+cranial vertebrae; the arrangement of them, and the relation of their
+constituent bones to the parts of the typical vertebra are shown in the
+table appearing on page 106. So far as their first three elements are
+concerned, these vertebrae are practically identical with the vertebrae
+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 vertebrae. The upper and lower jaws are associated
+with the nasal and frontal vertebrae respectively, not however as limbs
+of the head, but as constituent elements of these vertebrae. In the same
+way the hyoid apparatus is part and parcel of the parietal vertebra, and
+the pectoral girdle and fore-limbs part of the occipital vertebra.
+
+[Illustration: FIG. 6.--The Archetype of the Vertebrate Skeleton. (After
+Owen.)]
+
+Cranial Vertebrae.[164] (After Owen, 1848, p. 165.)
+
++---------------+---------------+----------------+---------------+-------------+
+|  Vertebrae.    |   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.     |
++---------------+---------------+----------------+---------------+-------------+
+|Haemapophyses.  |Coracoid.      |Ceratohyal.     |Articular.     |Maxillary.   |
++---------------+---------------+----------------+---------------+-------------+
+|Haemal Spines.  |Episternum.    |Basihyal.       |Dentary.       |Premaxillary.|
++---------------+---------------+----------------+---------------+-------------+
+|  Diverging    |Fore-limb or   |Branchiostegals.|Operculum.     |Pterygoid and|
+|  Appendage.   |  Fin.         |                |               |  Zygoma.    |
++---------------+---------------+----------------+---------------+-------------+
+
+Owen's reasons for considering the pectoral girdle and the fore-limb
+part of the occipital vertebra are as follows. In fish the pectoral
+girdle is slung to the skull by means of the post-temporal bone
+(supra-scapula, according to Owen) which abuts on the occipital arch. In
+_Lepidosiren_, whose skeleton resembles the archetype in many ways, the
+pectoral girdle is likewise attached to the occipital segment.
+
+In most other Vertebrates the pectoral girdle has shifted backwards
+along the vertebral column, by a "metastasis" (Geoffroy) similar to that
+by which the pelvic fins in many fish have shifted up close to the
+pectoral girdle. The scapula (with supra-scapula) is the pleurapophysis,
+the coracoid the haemapophysis, 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 haemapophysis of
+the atlas. Owen considers it no objection to this view that in other
+Vertebrates the clavicle is anterior to the coracoid--"its anterior
+position to the coracoid in the air-breathing Vertebrata is no valid
+argument against the determination, since in these we have shown that
+the true scapular arch is displaced backwards" (_On the Nature of
+Limbs_, p. 63, London, 1849). In the pelvic girdle the ilium corresponds
+to the scapula, the ischium to the coracoid, the pubis to the clavicle.
+Hence the ilium is a pleurapophysis, the ischium and pubis are both
+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 _Amphiuma_; in other air-breathing Vertebrates it shows
+a more complete development, the humerus, radius and ulna, and the bones
+of the wrist and hand becoming differentiated out.
+
+As the fore-limb is equivalent to a single bone of the archetype, it is
+said to be, in its developed state, "teleologically compound" (p. 103).
+
+Since in the archetype every vertebra has its appendage, more than two
+pairs of locomotory limbs might have been developed. "Any given
+appendage might have been the seat of such developments as convert that
+of the pelvic arch into a locomotive limb; and the true insight into the
+general homology of limbs leads us to recognise many potential pairs in
+the typical endoskeleton. The possible and conceivable modifications of
+the vertebrate archetype are far from having been exhausted in the forms
+which have hitherto been recognised, from the 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 Geoffroy's principle of
+connections. Speaking of an attempt which had been made to determine
+homologies by the mode of development, he writes, "There exists
+doubtless a close general resemblance in the mode of development of
+homologous parts; but this is subject to modification, like the forms,
+proportions, functions, and very substance of such parts, without their
+essential homological relationships being thereby obliterated. These
+relationships are mainly, if not wholly, determined by the relative
+position and connection of the parts, and may exist independently of
+form, proportions, substance, function and similarity of development.
+But the connections must be sought for at every period of development,
+and the changes of relative position, if any, during growth, must be
+compared with the connections which the part presents in the classes
+where vegetative repetition is greatest and adaptive modification least"
+(p. 6). It is interesting to note that in Owen's opinion comparative
+anatomy explains embryology. Thus the scapula, which is the
+pleurapophysis of the occipital vertebra, is vertical on its first
+appearance in the embryo of tetrapoda, and lies close up to the head
+(_On the Nature of Limbs_, p. 49)--the embryo shows a greater
+resemblance to the archetype than the adult. "We perceive a return to
+it, as it were, in the early phases of development of the highest
+organised of the actually existing species, or we ought rather to say
+that development starts from the old point; and thus, in regard to the
+scapula, we can explain the constancy of its first appearance close to
+the head, whether in the human embryo or in that of the swan, also its
+vertical position to the axis of the spinal column, by its general
+homology as the rib or 'pleurapophysis' of the occipital
+vertebra" (_Limbs_, p. 56).
+
+We owe to Owen the first clear distinction between "homologous" and
+"analogous" organs; it was he who first proposed the terms "homologue"
+and "analogue," which he defined as follows:--"_Analogue_. A part or
+organ in one animal which has the same function as another part or organ
+in a different animal." "_Homologue_. The same organ in different
+animals under every variety of form and function."[165]
+
+He introduced also useful distinctions between Special, General, and
+Serial Homology. "The relations of homology," he writes, "are of three
+kinds: the first is that above defined, viz., the correspondency of a
+part or organ, determined by its relative position and connections, with
+a part or organ in a different animal; the determination of which
+homology indicates that such animals are constructed on a common type;
+when, for example, the correspondence of the basilar process of the
+human occipital bone with the distinct bone called 'basi-occipital' in a
+fish or crocodile is shown, the _special homology_ of that process is
+determined. A higher relation of homology is that in which a part or
+series of parts stands to the fundamental or general type, and its
+enunciation involves and implies a knowledge of the type on which a
+natural group of animals, the Vertebrate, for example, is constructed.
+Thus when the basilar process of the human occipital bone is determined
+to be the 'centrum' or 'body' of the last cranial vertebra, its _general
+homology_ is enunciated.
+
+"If it be admitted that the general type of the vertebrate endoskeleton
+is rightly represented by the idea of a series of essentially similar
+segments succeeding each other longitudinally from one end of the body
+to the other, such segments being for the most part composed of pieces
+similar in number and arrangement, and though sometimes extremely
+modified for special functions, yet never so as to wholly mask their
+typical character--then any given part of one segment may be repeated in
+the rest of the series, just as one bone may be reproduced in the
+skeletons of different species, and this kind of repetition or
+representative relation in the segments of the same skeleton I call
+'serial homology'" (p. 7). As an example of serial homology we might
+take the centra of the vertebrae--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 the teleological point of view. As a true
+morphologist, however, he held that the principle of adaptation does not
+suffice to explain the existence of special homologies. The ossification
+of the bones of the skull from separate centres may be purposive in
+Eutheria, in that it prevents injury to the skull at birth; but how
+explain on teleological principles the similar ossification from
+separate centres in marsupials, birds and reptiles? How explain above
+all the fact that the centres are the same in number and relative
+position in all these groups? Surely we must accept the idea of an
+archetype "on which it has pleased the divine Architect to build up
+certain of his diversified living works" (p. 73).
+
+In his study of centres of ossification, Owen made in point of theory a
+distinct advance on his predecessors. We saw that Geoffroy recognised
+the importance of studying the ossification of the skeleton, and that
+Cuvier accepted such embryological evidence as an aid in determining
+homologies. Owen pointed out that it was necessary to distinguish
+between centres of ossification which were teleological in import and
+such as were purely indicative of homological relationships. Many bones,
+single in the adult, arise from separate centres of ossification, but we
+must distinguish between "those centres of ossification that have
+homological relations, and those that have only teleological ones;
+_i.e._, between the separate points of ossification of a human bone
+which typify vertebral elements, often permanently distinct bones in the
+lower animals; and the separate points which, without such
+signification, facilitate the progress of osteogeny, and have for their
+obvious final cause the well-being of the growing animal" (p. 105).
+There is, for example, a teleological reason why in mammals and leaping
+Amphibia (_e.g._, frogs), the long bones should ossify first at their
+ends, for the brain is thus protected from concussion; in reptiles that
+creep there is less danger of concussion, and the long bones ossify in
+the middle (p. 105). But there is no teleological reason why the
+coracoid process of the scapula should in all mammals develop from a
+separate centre. The coracoid is however a real vertebral element
+(haemapophysis), and in monotremes, birds and reptiles it is in the adult
+a large and separate bone. Its ossification from a separate centre in
+mammals has therefore a homological significance. The scapula in mammals
+is an example of what Owen calls a "homologically compound" bone. All
+those bones which are formed by a coalescence of parts answering to
+distinct elements of the typical vertebra are "homologically compound"
+(p. 105). On the other hand, "All those bones which represent single
+vertebral elements are 'teleologically compound' when developed from
+more than one centre, whether such centres subsequently coalesce, or
+remain distinct, or even become the subject of individual adaptive
+modifications, with special joints, muscles, etc., for particular
+offices" (p. 106). The limb-skeleton, corresponding as it does to a
+single bone of the archetype, is the typical example of a teleologically
+compound bone. Owen in his definition of teleological compoundness has
+combined two kinds of adaptation--(1) temporary adaptation of bones to
+the exigencies of development, birth and growth (_e.g._, development of
+long bones from separate centres); (2) definitive adaptation of a
+skeletal part to the functions which it has to perform (_e.g._,
+teleological structure of limbs). Such adaptations are, so to speak,
+grafted on the archetype.
+
+Owen's general views on the nature of living things merit some
+attention. Organic forms, according to Owen, result from the
+antagonistic working of two principles, of which one brings about a
+vegetative repetition of structure, while the other, a teleological
+principle, shapes the living thing to its functions. The former
+principle is illustrated in the archetype of the vertebrate skeleton, in
+the segmentation of the Articulates, in the almost mathematical symmetry
+of Echinoderms, and the actually crystalline spicules of sponges. It is
+the same principle which causes repetition of the forms of crystals in
+the inorganic world. "The repetition of similar segments in a vertebral
+column, and of similar elements in a vertebral segment, is analogous to
+the repetition of similar crystals as the result of polarising force in
+the growth of an inorganic body" (p. 171). This "general polarising
+force" it is which mainly produces the similarity of forms, the
+repetition of parts, and generally the signs of the unity of
+organisation. The adaptive or "special organising force" or [Greek:
+idea], on the other hand, produces the diversity of organic beings. In
+every species these two forces are at work, and the extent to which the
+general polarising or "vegetative-repetition-force" is subdued by the
+teleological is an index of the grade of the species.
+
+This view is analogous to the Geoffroyan conception that the diversity
+of form is limited by the unity of plan. Owen thus ranges himself with
+Geoffroy against Cuvier, who considered that diversity of form is
+limited only by the principle of the adaptation of parts.
+
+    [164] Owen introduced most of the names of bones now
+    current.
+
+    [165] _Lectures on Invertebrate Animals_, pp. 374, 379,
+    1843.
+
+
+
+
+CHAPTER IX
+
+KARL ERNST VON BAER
+
+
+Von Baer was recognised as the founder of embryology even by his
+contemporaries. His predecessors, Aristotle,[166] Fabricius,[167]
+Harvey,[168] Malpighi,[169] Haller,[170] Wolff,[171] had made a
+beginning with the study of development; von Baer, by the thoroughness
+of his observation and the strength of his analysis, made embryology a
+science.
+
+It was to one of the German transcendentalists that von Baer owed the
+impulse to study development. Ignatius Doellinger, Professor in Wuerzburg,
+induced three of his pupils, Pander, d'Alton and von Baer, to devote
+themselves to embryological research. The development of animals was at
+this time little known, in spite of recent work by Meckel (1815 and
+1817), Tiedemann (_Anatomie u. Bildungsgeschichte des Gehirns_, 1816),
+by Oken (_loc. cit., supra_, p. 90), and some others.
+
+Pander, with whom apparently Doellinger and d'Alton collaborated, was the
+first to publish his results;[172] von Baer, who through absence from
+Wuerzburg had for a time dropped his embryological studies, started to
+work in 1819, after the publication of Pander's treatise, and produced
+in 1828 the first volume of his master-work, _Ueber
+Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion_
+(Koenigsberg, 1828). The second volume followed in 1837, but dates really
+from 1834, and was published in an incomplete form. This second volume
+is intended as an introduction to embryology for the use of doctors and
+science students. In it von Baer describes in full detail the
+development of many vertebrate types--chick, tortoise, snake, lizard,
+frog, fish, several mammals and man, basing his remarks largely upon his
+personal observations, but taking account also of all contemporary work.
+A separate account of the development of a fish (_Cyprinus blicca_)
+appeared in 1835.[173]
+
+We shall concentrate attention on the first volume. This volume contains
+the first full and adequate account of the development of the chick,
+followed by a masterly discussion of the laws of development in general.
+
+When we consider that von Baer worked chiefly with a simple microscope
+and dissecting needles, the minuteness and accuracy of his observations
+are astonishing. He described the main facts respecting the development
+of all the principal organs, and if, through lack of the proper means of
+observation, he erred in detail, he made up for it by his masterly
+understanding and profound analysis of the essential nature of
+development. His account of the development of the chick is a model of
+what a scientific memoir ought to be; the series of "Scholia" which
+follow contain the deductions he made from the data, and, in so far as
+they are direct generalisations from experience, they are valid for all
+time.
+
+The first Scholion is directed against the theory of preformation, and
+succeeds in refuting it on the ground of simple observation. The theme
+of the second Scholion is that the essential nature (_die Wesenheit_) of
+the animal determines its differentiation, that no stage of development
+is solely determined by the antecedent stage, but that throughout all
+stages the _Wesenheit_ or idea of the definitive whole exercises
+guidance. This guidance is shown most clearly in the regulatory
+processes of the germ, whereby the large individual variations commonly
+presented by the early embryo are compensated for or neutralised in the
+course of further development. Baer in this shows himself a vitalist.
+
+It is, however, the third and subsequent Scholia which must here
+particularly occupy our attention, for it is in these that von Baer
+comes to grips with morphological problems. Already in the second
+Scholion he had definitely enunciated the law which runs as a theme
+throughout the volume, the observational and the theoretical part alike,
+the law that development is essentially a process of differentiation by
+which the germ becomes ever more and more individualised. "The essential
+result of development," he writes, "when we consider it as a whole, is
+the increasing independence (_Selbstaendigkeit_) 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 plates--parts which contain in an
+undifferentiated state the skeletal and muscular systems, the connective
+tissues, and the nerves belonging to these. In order to have a
+convenient term for future use, I have named this layer the
+muscle-layer" (p. 153).
+
+The process of delamination results then in the formation of four
+layers, of which the upper two (composing the "animal" or "serous"
+layer) will give origin to the animal (neuromuscular) part of the body,
+the lower pair to the plastic or vegetative organs. The uppermost layer
+will form the external covering of the embryo, and also the amniotic
+folds; from it there differentiates out at a very early stage the
+rudiment of the central nervous system, forming a more or less
+independent layer. Below the outermost layer lies the layer from which
+are formed the muscular and skeletal systems, and beneath this
+"muscle-layer" comes the "vessel-layer," which gives origin to the main
+blood-vessels. The innermost layer of the four will form the mucous
+membrane of the alimentary canal and its dependencies; at the present
+stage, however, it is, like the other layers, a flat plate.
+
+From all these layers tubes are developed by the simple bending round of
+their edges. The outermost layer becomes the investing skin-tube of the
+embryo; the layer for the nervous system forms the tubular rudiment of
+the brain and spinal cord; the mucous layer curls round to form the
+alimentary tube; the muscle layer grows upwards and downwards to form
+the fleshy and osseous tube of the body wall; even the vessel layer
+forms a tube investing the alimentary canal, but a part of it goes to
+form the medial "Gekroese," or mesenterial complex, which departs
+considerably from the tubular form.
+
+When these tubes or "fundamental organs" are formed the process of
+primary differentiation is complete. The fundamental organs, however,
+have at no time actually the form of tubes; they exist as tubes only
+ideally, for morphological and histological differentiation go on
+concurrently with the process of primary differentiation.
+
+Through morphological differentiation the various parts of the
+fundamental organs become specialised, through unequal growth, first
+into the primitive organs and then into the functional organs of the
+body. "Single sections of the tubes originally formed from the layers
+develop individual forms, which later acquire special functions: these
+functions are in the most general way subordinate elements of the
+function of the whole tube, but yet differ from the functions of other
+sections. Thus the nerve-tube differentiates into sense-organs, brain
+and spinal cord, the alimentary tube into mouth cavity, oesophagus,
+stomach, intestine, respiratory apparatus, liver, bladder, etc. This
+specialisation in development is bound up with increased or diminished
+growth" (p. 155). Rapid growth concentrated at one point brings about an
+evagination; in this manner are formed the sense-organs from the
+nerve-tube, the liver and lungs from the alimentary tube. Or increased
+growth over a section of a tube causes it to swell out; in this wise the
+brain develops from the nerve-tube, the stomach from the alimentary
+tube. The segmentation which soon becomes so marked, particularly in the
+muscle layer, is also due to a process of morphological differentiation.
+
+At the same time that the organs of the body are being thus roughly
+blocked out and moulded from the germ-layers the third process of
+differentiation is actively going on. "In addition to the
+differentiation of the layers, there follows later another
+differentiation in the substance of the layers, whereby cartilage,
+muscle and nerve separate out, a part also of the mass becoming fluid
+and entering the bloodstream" (p. 154). Through histological
+differentiation the texture of the layers and incipient organs becomes
+individualised. In its earliest appearance the germ consists of an
+almost homogeneous mass, containing clear or dark globules suspended in
+its substance (ii., p. 92). This homogeneity gives place to
+heterogeneity; the structureless mass becomes fibrous to form muscles,
+hardens to form cartilage or bone, becomes liquid to form the blood,
+differentiates in a hundred other ways--into absorbing and secreting
+tissues, into nerves and ganglia, and so forth. It will be noticed that
+the concept of histological differentiation is independent of the
+cell-theory; it signifies that textural differentiation which leads to
+the formation of tissues in Bichat's sense. The tissues and the
+germ-layers stand in fairly close relation with one another, for while
+certain tissues are formed chiefly but not exclusively in one layer,
+others are formed only in one layer and never elsewhere. For example,
+peripheral nerves are for the most part formed in the muscle layer,
+though the bulk of the nervous tissue is formed in the walls of the
+nerve tube; similarly blood and blood-vessels may arise from almost any
+layer, though their chief seat of origin is the vessel-layer; on the
+other hand, bone is formed only in the muscle-layer (i., p. 155, ii.,
+pp. 92-3).
+
+This relation of tissue to germ-layer was more fully discussed and
+brought into greater prominence by Remak, from the standpoint of the
+cell-theory, and it will occupy us in a later chapter (Chap. XII.).
+
+The fourth Scholion elaborates the analysis of developmental processes
+still further, and discusses in particular the scheme of development
+which is shown by the Vertebrata. The characteristic structure of the
+vertebrate body is brought about by a "double symmetrical" rolling
+together of the germ-layers, whereby two main tubes are formed, one
+above and one below the axis of the body, which is the chorda. The
+dorsal tube is formed by the two animal layers, the ventral tube by all
+the layers combined (see Fig. 7).
+
+The process is indicated with sufficient clearness in the diagram. It
+will be seen that the real foundation and framework of the arrangement
+is the muscle-layer, with its two tubes, one surrounding the central
+nervous system and forming the "dorsal plates," the other surrounding
+the body cavity and forming the "ventral plates." In the dorsal plates,
+which early show metameric segmentation, the investing skeleton of the
+neural axis develops; in the ventral plates are formed the ribs, the
+ventral arches of the vertebrae, 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 "Gekroese," or remaining
+part of the vessel layer develop the Wolffian bodies (_Urnieren_,
+Pronephros), the kidneys, the sex glands, and the series of
+"blood-glands"--suprarenals, thyroid, thymus and spleen. Baer did not
+attach any special morphological significance to the peritoneal lining
+of the body cavity, as is done in more modern forms of the germ-layer
+theory. The gill-slits were largely formed by outgrowths from the
+alimentary canal.
+
+_a._ Chorda.
+_b._ Dorsal plates.
+_c._ Ventral plates.
+_d._ Spinal cord.
+_e._ Vessel-layer.
+_f._ Alimentary tube.
+_g._ Pronephros.
+_h._ Skin.
+_i._ Amnion.
+_k._ Serous membrane.
+_l._ Yolk-sac.
+
+In his germ-layer theory von Baer was influenced a good deal by
+Pander, to whom the actual discovery of the process of layer-formation
+is due. Pander, however, had distinguished only three germ-layers, an
+upper "serous" layer, a lower "mucous" layer and a middle
+"vessel-layer." He it was who introduced the terms "Keimhaut"
+(blastoderm) and "Keimblatt" (germ-layer).
+
+[Illustration: FIG. 7.--Ideal Transverse Section of a Vertebrate Embryo.
+(After von Baer.)]
+
+The honour of being the founder of the germ-layer theory is sometimes
+attributed to C. F. Wolff, notably by Koelliker and O. Hertwig. Wolff, it
+is true, in his memoir _De formatione intestinorum_ (1768-9) showed that
+the alimentary canal was first formed as a flat plate which folded round
+to form a tube, and in a somewhat vaguely worded passage he hinted that
+a similar mode of origin might be found to hold good for the other
+organ-systems. But it seems clear that Wolff had no definite conception
+of the process of layer-formation as the first and necessary step in all
+differentiation. This, at any rate, was von Baer's opinion, who assigns
+to Pander the glory of the discovery of the germ-layers. "You," he
+writes, "through your clearer recognition of the splitting of the
+germ--a process which remained dark to Wolff--have shed a light upon all
+forms of development" (p. xxi.).
+
+We have now seen, following von Baer's exposition, how development is
+essentially a process of differentiation, a progress from the general to
+the special, from the homogeneous to the heterogeneous; we have analysed
+the process into its three subordinate processes--primary, histological
+and morphological differentiation. So far we have considered development
+in general and the laws which govern it; we have now to consider the
+varieties of development which the animal kingdom offers in such
+profusion, in order to discover what relations exist between them. This
+is the problem set in the fifth Scholion. Baer at once brings us face to
+face with the solution of the problem attempted in the Meckel-Serres
+law. It is a generally received opinion, he writes, that the higher
+animals repeat in their development the adult stages of the lower, and
+this is held to be the essential law governing the relation of the
+variety of development to the variety of adult form. This opinion arose
+when there was little real knowledge of embryology; it threw light
+indeed upon certain cases of monstrous development, but it was pushed
+altogether too far. It complicated itself with a belief in a historical
+evolution;--"People gradually learnt to think of the different animal
+forms as developed one from another--and seemed, in some circles at
+least, determined to forget that this metamorphosis could only be
+conceptual" (p. 200). At the same time the theory of parallelism led men
+to rehabilitate the outworn conception of the scale of beings, to
+maintain that animals form one single series of increasing complexity, a
+scale which the higher members must mount step by step in their
+development--from which it followed that evolution, whether conceived as
+an ideal or as an historical process, could take place only along one
+line, could be only progressive or regressive. Not all the supporters of
+the theory of parallelism held these extreme views, but conclusions of
+this kind were natural and logical enough.
+
+Von Baer had soon found in the course of his embryological studies that
+the facts did not at all fit in with the doctrine of parallelism; the
+developing chick, for example, was at a very early stage demonstrably a
+Vertebrate, and did not recapitulate in its early stages the
+organisation of a polyp, a worm or a mollusc. He had published his
+doubts in 1823, but his final confutation of the theory of parallelism
+is found in this Scholion.
+
+If it were true, he says, that the essential thing in the development of
+an animal is this repetition of lower organisations, then certain
+deductions could be drawn, which one would expect to find confirmed in
+Nature. The first deduction would be that no structures should appear in
+the embryo of the higher animals that are not found in the lower
+animals. But this is not confirmed by the facts--no adult among the
+lower animals, for instance, has a yolk-sac like that of the chick
+embryo. Again, if the law of parallelism were true, the mammalian embryo
+would have to repeat the organisation of, among other groups, insects
+and birds. But the embryo _in utero_ is surrounded by fluid and cannot
+possibly breathe free air, so it cannot possibly repeat the structure of
+either insects or birds, which are pre-eminently air-organisms.
+Generally speaking, indeed, we find in all the higher embryos special
+structures which adapt them to the very special conditions of their
+development, and these we never find as permanent structures in the
+lower animals. The supporters of the theory of parallelism might,
+however, admit the existence of such special embryonic organs without
+greatly prejudicing their case, for these temporary organs stand to some
+extent outside the scope of the theory.
+
+But they would have to face a second and more important deduction from
+their views, namely, that the higher animals should repeat at every
+stage of their development the whole organisation of some lower animal,
+and not merely agree with them in isolated details of structure. The
+deduction is, however, not borne out by the facts. The embryo of a
+mammal resembles in many points, at different stages of its development,
+the adult state of a fish; it has gill-slits and complete aortic arches,
+a two-chambered heart, and so on. But at no time does it combine all the
+essential characters of a fish; nor has it ever the tail of a fish, nor
+the fins, nor the shape. Any recapitulation there may be is a
+recapitulation of single organs, there is never a repetition of the
+complete organisation of a fish. This is indeed the fundamental
+criticism of the theory of parallelism; and if it applies even within
+the limits of the vertebrate phylum, so much the more does it apply to
+comparisons between embryonic Vertebrates and adult Invertebrates.
+
+There are also some lesser arguments which might be urged against the
+theory of parallelism. If the theory were strictly true, no state which
+is permanent in a higher animal could be passed through by an animal
+lower in the scale. But birds, which are lower in the scale than
+mammals, pass through a stage in which they resemble mammals in certain
+respects much more than they do when adult, for in an embryonic
+condition they agree with mammals in having no feathers, no air sacs, no
+pneumatic sacs in the bones, no beak. Their brain also resembles that of
+mammals more in an earlier stage than it does later. So, too, myriapods
+and hydrachnids have at birth three pairs of feet, and resemble at this
+stage adult insects, which form a higher class.
+
+Again, were the analogy between the development of the individual and
+the evolution of the _Echelle des etres_ complete, organs and
+organ-systems ought to develop in the individual in the order in which
+they appear in the scale of beings. But this is not always the case. In
+fish the hinder extremity develops only its terminal joint, while in the
+embryos of higher animals the basal joint is the first to appear.
+
+Another consequence one would expect to find realised, were the theory
+of parallelism correct, is the late appearance in development of parts
+which are confined to the higher animals. In the development of a
+Vertebrate accordingly one would not expect the 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 organisation characterises one of the big groups
+of animals; the lesser groups represent "grade" modifications of the
+type. "The product of the degree of differentiation and the type gives
+the several great groups of animals which are called classes" (p. 208).
+_Ausbildung_ (differentiation) takes place in one or other of several
+directions, in adaptation, for instance, to life in the water or to life
+in the air.
+
+There are, von Baer considers, four main types--(1) the peripheral or
+radiate type, (2) the longitudinal type, (3) the massive or molluscan
+type, (4) the vertebrate type. The radiate type is shown by discoid
+infusoria, by medusae, by starfish and their allies. The longitudinal
+type characterises such genera as _Vibrio_, _Filaria_, _Gordius_, and
+all the annulate animals. Mollusca, rotifers, polyzoa, and such
+infusoria as are not included in types (1) and (2) belong to the massive
+type, in which the body and its parts form rounded masses. The
+longitudinal type is predominantly "animal," the massive type
+predominantly "plastic" (vegetative). The vertebrate type has both the
+"animal" and the "plastic" organs highly developed. In the symmetrical
+arrangement of the animal parts it resembles the longitudinal type; its
+plastic parts with their asymmetrical arrangement and rounded shape
+belong to the massive type.
+
+These types of von Baer inevitably recall the "Embranchements" of
+Cuvier, with which they more or less coincide. It seems that von Baer
+arrived at his types (from the study of adult structure) independently
+of Cuvier, though the priority of publication rests with Cuvier.[174]
+
+Now it is clear that the development of the individual, which is
+essentially an _Ausbildung_, a differentiation, is directly comparable
+with the grade-differentiation of forms within the type. And just as the
+type rules all its varied modifications, so does the development of the
+individual take place always within the bounds imposed by type. This is
+von Baer's chief contribution to the theory of embryonic
+relationships--the law that "the type of organisation determines the
+manner of development" (p. xxii.). Development is not merely from the
+general to the special--there are at least four distinct "general"
+types, from which the special is developed. The type is fixed in the
+very earliest stages of development--the embryo of a Vertebrate is from
+the very beginning a Vertebrate (p. 220), and it shows at no time any
+agreement in total organisation with any Invertebrate. The types are
+independent of one another; differentiation and development follow a
+different course in each of them. Not but what some analogies can be
+found between the very earliest stages of embryos of different type.
+Thus vertebrate and annulate embryos agree in certain points at the time
+of the formation of the primitive streak. And in the earliest stage of
+all, the egg-stage, there is probably agreement between all the types.
+In eggs with yolk, whether vertebrate or annulate, there is always a
+separation into an animal and a plastic layer. It seems, too, as if a
+hollow sphere were a constant stage in the development of all animals
+(pp. 224, 258). Apart from these analogies, development takes an
+entirely independent course in each of the four main types, and no
+embryo of one of the higher types repeats in its development the
+peculiar organisation of any adult of the lower types.
+
+If we consider now development within the type, which is the only
+legitimate thing to do, we arrive at certain laws governing the relation
+of embryos to one another. For instance, at a certain stage vertebrate
+embryos are uncommonly alike. Von Baer had two in spirit which he was
+unable to assign to their class among amniotes; they might have been
+lizard, bird, or mammal, he could not say definitely which.[175] Generally
+the farther back we go in the development of Vertebrates the more alike
+we find the embryos. The type-characters are first to appear, then the
+class characters, then the characters distinguishing the lesser
+classificatory groups. "From a more general type the special gradually
+emerges" (p. 221). The chick is first a Vertebrate, then a
+land-vertebrate, then a bird, then a land-bird, then a gallinaceous
+bird, and finally _Gallus domesticus_. Development within the type is a
+progress from the general to the special, a real evolution. The more
+divergent two adults are, the farther back we must go in their
+development to find an agreement between their embryos. We can sum up
+the case in the following laws:--
+
+"(1) _That the general characters of the big group to which the embryo
+belongs appear in development earlier than the special characters._ In
+agreement with this is the fact that the vesicular form is the most
+general form of all; for what is common in a greater degree to all
+animals than the opposition of an internal and an external surface?
+
+"(2) _The less general structural relations are formed after the more
+general, and so on until the most special appear._
+
+"(3) _The embryo of any given form, instead of passing through the state
+of other definite forms, on the contrary separates itself from them._
+
+"(4) _Fundamentally the embryo of a higher animal form never resembles
+the adult of another animal form, but only its embryo_" (p. 224).
+
+These laws relating to development within the limits of type are
+destructive of even a limited application of the theory of parallelism,
+for not even within the limits of the type is there a real scale which
+the higher forms must mount; each embryo develops for itself, and
+diverges sooner or later from the embryos of other species, the
+divergence coming earlier the greater the difference between the adult
+forms. It is only because the lower less-differentiated adult forms
+happen to be little divergent from the generalised or embryonic type,
+that they show a certain similarity with the embryos of the higher more
+differentiated members of the group. Such similarity, however, is due to
+no necessary law governing the development of the higher animals; it is,
+on the contrary, merely a consequence of the organisation of these lower
+animals (p. 224).
+
+Von Baer goes on to show what are the distinguishing embryological
+characters of the types and classes, working out a dichotomous schema of
+development, which each embryo must follow, branching off early or late
+to its terminal point, according to the lower or higher goal it has to
+reach.
+
+One important consequence for morphology results from von Baer's laws of
+differentiation within the type. If the embryo develops from the general
+to the special, then the state in which each organ or organ-system first
+appears must represent the general or typical state of that organ within
+the group. Embryology will therefore be of great assistance to
+comparative anatomy, whose chief aim it is to discover the generalised
+type, the common plan of structure, upon which the animals of each big
+group are built. And the surest way to determine the true homologies of
+parts will be to study their early development. "For since each organ
+becomes what it is only through the manner of its development, its true
+value can be recognised only from its method of formation. At present,
+we form our judgments by an undefined intuition, instead of regarding
+each organ merely as an isolated product of its fundamental organ, and
+discerning from this standpoint the correspondences and dissimilarities
+in the different types" (p. 233). Parts, therefore, which develop from
+the same "fundamental organ," and in the last resort from the same
+germ-layer, have a certain kinship, which may even reach the degree of
+exact homology.
+
+Now since the mode of development in each type is peculiar to that type,
+organs of the same name in different types must not necessarily be
+accounted homologous, even if they correspond exactly with one another
+in their general _functional_ relations to the rest of the organs. Thus
+the central nervous system of Arthropods must not be homologised with
+the central nervous system of Vertebrates, for it develops in a
+different manner. So, too, the brain of Arthropods or of Mollusca is not
+strictly comparable with the brain of Vertebrates. Again, the air-tubes
+or 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 groups as
+does the study of adult structure. The four types of development are
+thus:--
+
+(1) The double-symmetrical, which is found in Vertebrates. It is called
+the double-symmetrical, because in Vertebrates development takes place
+from a central axis (notochord) in two directions, upwards and
+downwards, in such a way that two tubes are formed, one above and one
+below the axis. (2) The second type is the symmetrical, which is shown
+by Annulates. A primitive streak is formed on the ventral surface of the
+yolk; development proceeds symmetrically on both sides of the streak.
+(3) Radiate development is probably typical of the radiate structural
+type. (4) In the massive type, the development seems to be a spiral one.
+
+Common to most modes is a separation of the germ into animal and plastic
+layers, a separation which seems to be conditioned largely by the
+presence of yolk. A classification based upon embryological characters
+ought to be applied even to the lesser groups and would here prove
+itself of service. Embryology, for instance, fully supports de
+Blainville's separation of Batrachia from true reptiles,[176] for reptiles
+develop an amnion and Batrachia do not.
+
+We come now to the sixth and last Scholion. Development is a true
+evolution of the special from the general, so runs von Baer's most
+general law of all. This can be expressed in a slightly different way,
+and the words which he chooses in the sixth Scholion to express this
+final and most general result are these:--"The developmental history of
+the individual is the history of the growing individuality in every
+respect" (p. 263). The greatest modern treatise on embryology ends on a
+splendid note. One creative thought rules all the forms of life. And
+more--"It is this same thought that in cosmic space gathered the
+scattered masses into spheres and bound them together in the solar
+system, the same that from the weathered dust on the surface of the
+metallic planets brought forth the forms of life. And this thought is
+nought else but life itself, and the words and syllables in which life
+expresses itself are the varied forms of the living" (p. 264).
+
+Von Baer reminds one greatly of Cuvier. There is the same sheer
+intellectual power, the same sanity of mind, the same synthetic grip.
+Von Baer, like Cuvier, never forgot that he was working with living
+things; he was saturated, like Cuvier, with the sense of their
+functional adaptedness. In his paper on the external and internal
+skeleton[177] he gives a masterly analysis of the functional modifications
+of the limbs in Vertebrates, and the whole paper indeed, with its sober
+attack on transcendentalism, is a vindication as much of the functional
+point of view as of the importance of embryology.
+
+Both Cuvier and von Baer, by the very sanity of their views, found
+themselves in partial opposition to the theories current in their time.
+Cuvier was the critic of Geoffroy and the transcendentalists, of Lamarck
+and the believers in the _Echelle des etres_, evolutionary or ideal. Von
+Baer also, though influenced greatly by _Naturphilosophie_, turned
+against the exaggerations of the transcendental school, and by his
+unanswerable criticism of the theory of parallelism took away the ground
+from those who too easily believed in an historical evolution.[178]
+
+We have seen what were von Baer's criticisms of the theory of
+parallelism. If we turn to the later writings of Cuvier we find the
+essential criticism expressed in similar terms. Speaking of an attempt
+which had been made to show that fish were molluscs developed to a
+higher degree, he wrote in 1828,[179] "Let us draw the conclusion that
+even if these animals can be spoken of as ennobled molluscs, as molluscs
+raised to a higher power, or if they are embryos of reptiles, the
+beginnings of reptiles, this can be true of them only in an abstract and
+metaphysical sense, and that even this abstract statement would be very
+far from giving an accurate idea of their organisation." From the fact
+that the respiratory and circulatory organs of fish greatly resemble
+those of tadpoles the conclusion has been drawn that fish are in a sense
+embryos of Amphibia (p. 547). But this manner of viewing things is none
+the less vicious, "for this reason ... that it considers only one or two
+points and neglects all the others" (p. 548), and is directly contrary
+to common sense. There is never a recapitulation of total organisations,
+only at the most of single organs.
+
+It will be remembered that Cuvier opposed and demolished the theory of
+the _Echelle des etres_, not only by showing that there were in Nature
+four entirely different plans of animal structure, but also by
+demonstrating that even the animals of each single _Embranchement_ could
+not readily be arranged in one series, that a serial arrangement was
+really valid only for their separate organs. Von Baer also held that
+there are four distinct types of structure; he, too, combated the idea
+of gradation within the limits of the type. In so far as species
+represent successive stages in the development, the _Ausbildung_, of the
+type, so far can the idea of a scale of beings be applied. But the
+members of a type follow not one line of evolution but several diverging
+lines, in direct adaptation to different environmental conditions, so
+that a serial arrangement of them is not as a rule possible. It may be
+possible to establish a serial arrangement of single organs from the
+simplest to the most complex. But each organ or organ-system will
+require a different serial arrangement, for the different systems vary
+on different lines and an animal may be highly developed in respect of
+one system and little developed in respect of all the others. Man, for
+instance, is the highest animal only in respect of his nervous system.
+The idea of the scale of beings has therefore only a very limited
+application even within the limits of the type. Applied to the whole
+animal kingdom it becomes merely absurd.
+
+Another point of resemblance between Cuvier and von Baer was that
+Cuvier, though essentially a student of adult structure, did recognise
+the importance of embryology; following up some observations of
+Dutrochet he studied the foetal membrane of mammals and tried to
+establish their homologies.[180] And in his criticism of the vertebral
+theory of the skull he advanced as an argument against the
+basisphenoid being a vertebral centrum the fact (established
+by Kerkring, 1670), that it develops from two centres.[181] Von Baer's
+relation to transcendental anatomy was in some ways a close one, though
+he was a trenchant critic of the extreme views of the school.[182] He took
+from Oken the idea that a simple fundamental plan rules the organisation
+of all Vertebrates; "That jaws and limbs are modifications of one
+fundamental form is readily apparent, and, after Oken, the fact ought to
+be accepted by the majority of those naturalists who do not refuse to
+admit the existence of a general type from which the diversity of
+structure is developed" (i., p. 192). He accepted the vertebral theory
+of the skull in its main lines, and used his embryological knowledge to
+support the idea that jaws correspond to limbs--the latter point as part
+of the transcendental idea that the hind end of the body repeats the
+organisation of the anterior part (i., p. 192). The particular form
+which his theory of the relation of jaws to limbs took is shown in the
+following passage:--"The maxillary bone has ... the significance of an
+extremity and at the same time that of a rib or lower arch of a
+vertebra, just as the pelvic bones unite in themselves the signification
+of ribs and proximal members of the hinder extremity" (Meckel's
+_Archiv_, p. 367, 1826).
+
+He appreciated the morphological idea of the serial repetition of parts,
+and gave it accurate formulation. The whole vertebrate body, he
+considered, was composed of a longitudinal series of _morphological
+elements_, each of which was made up a section from each of the
+fundamental organs--a vertebra, a section of the nerve-cord, and so on
+(_Entwickelungsgeschichte_, ii., p. 53). Groups of these morphological
+elements formed _morphological divisions_, such as the vertebral
+segments of the head with their highly developed neural arches, or the
+segments of the neck with their undeveloped haemal arches. The
+morphological elements are clearly shown only in the animal parts, but
+there are indications in the embryo of a segmentation also of the
+vegetative parts,--the gill-slits, for instance, and the vascular
+arches. The vegetative parts, however, develop on the whole
+unsymmetrically (_cf._ Bichat). These elements which von Baer
+distinguishes are morphological units, as he himself points out,
+contrasting them with organs which are not usually units in a
+morphological sense. "We call organ," he writes, "each part that has by
+reason of its form or its function a certain distinctiveness, but this
+concept is very indefinite, and possesses, from a morphological point of
+view, little value. For this reason it seems necessary to introduce into
+scientific morphology the concepts of morphological elements and
+divisions" (ii., p. 84).
+
+Von Baer exercised a very considerable influence upon the subsequent
+trend of morphological theory. By his criticism of the Meckel-Serres
+theory, he rid morphology for a time of an idea which was leading it
+astray; by his substitution of the law that development is always from
+the general to the special, he set morphologists looking for the
+archetype in the embryo, not in the adult alone, and made them realise
+that homologies could often best be sought in the earliest stages of
+development; by formulating the germ-layer theory he supplied
+morphologists with a new criterion of homology, based upon the special
+relations of the parts (germ-layers) which are first differentiated in
+all development. He made the study of development an essential part of
+morphology.
+
+    [166] _De generatione Animalium_.
+
+    [167] _De formato foetu_, ? 1600; _De formatione
+    foetus_, 1604.
+
+    [168] _Exercitationes de generatione animalium_, 1651.
+
+    [169] _De formatione pulli in ovo_, 1673; _De ovo
+    incubato_, 1686.
+
+    [170] _De formatione pulli in ovo_, 1757-8; _Sur la
+    formation du coeur dans le poulet_, 1758.
+
+    [171] _Theoria generatioinis_, 1759; _De formatione
+    intestinorum_, 1768-9.
+
+    [172] _Beitraege zur Entwickelung des Huehnchens im Ei._
+    Wuerzburg, 1818. Also in Latin in shorter form, 1817.
+
+    [173] _Untersuchungen ue. die Entwickelungsgeschichte der
+    Fische_; Leipzig, 1835.
+
+    [174] Cuvier, in 1812, _Ann. Mus. d'Hist. Nat._, xix.; von
+    Baer in 1816, _Nova Acta Acad. Nat. Cur._ See
+    _Entwickelungsgeschichte der Thiere_, i., p. vii., f.n.
+
+    [175] Compare a parallel passage in 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 extraordinary
+    that one cannot refuse to admit the conclusions
+    resulting from it. The resemblance is so striking that
+    one can defy the most experienced observer to
+    distinguish in any way the embryos of dog or rabbit ...
+    from those of fowls or ducks of a corresponding
+    age."--_Ann. Sci. nat._, iii., p. 132, 1824.
+
+    [176] _De l'organisation des Animaux_, i., p. 140, 1822.
+
+    [177] "Ueber das aeussere und innere Skelet," Meckel's
+    _Archiv fuer Anat. u. Physiol._, pp. 327-76, 1826. See,
+    too, his _Entwickelungsgeschichte_, i., pp. 181, ff.
+
+    [178] Von Baer wrote an appreciative biography of Cuvier,
+    published posthumously in 1897, _Lebensgeschichte
+    Cuviers_, ed. L. Stieda. French trans. in _Ann. Sci.
+    Nat._ (_Zool._), ix., 1907.
+
+    [179] Cuvier et Valenciennes, _Histoire naturelle des
+    Poissons_, i., p. 550.
+
+    [180] _Mem. Mus. d'Hist. Nat._, iii., pp. 98-119, 1817.
+
+    [181] _Lecons d'Anatomie comparee_, 3rd ed., vol. i., p.
+    414, Bruxelles, 1836.
+
+    [182] In the aforementioned paper in Mueller's _Archiv_ he
+    criticises Carus vigorously and is sarcastic on
+    Geoffroy.
+
+
+
+
+CHAPTER X
+
+THE EMBRYOLOGICAL CRITERION
+
+
+Pander's work of 1817 was the forerunner of an embryological period in
+which men's hopes and interest centred round the study of development.
+"With bewilderment we saw ourselves transported to the strange soil of a
+new world," wrote Pander, and many shared his hopeful enthusiasm. K. E.
+von Baer's _Entwickelungsgeschichte_ was by far the greatest product of
+this time, but it stands in a measure apart; we have in this chapter to
+consider the lesser men who were Baer's contemporaries, friends,
+followers or critics.
+
+It was largely a German science, this new embryology, and its leaders
+were all personally acquainted. Pander, von Baer and Rathke were on
+friendly terms with one another; von Baer dedicated his master-work to
+Pander; Rathke dedicated the second volume of his _Abhandlungen_ to von
+Baer. Interest in the new science was, however, not confined to Germany.
+In Italy, Rusconi commenced in 1817 his pioneer researches on the
+development of the Amphibia with a _Descrizione anatomica degli organi
+della circolazione delle larve delle Salamandre aquatiche_ (Pavia), in
+which he traced the metamorphoses of the aortic arches. This was
+followed in 1822 by his _Amours des Salamandres aquatiques_ (Milan), and
+in 1826 by his memoir _Du developpement de la grenouille_ (Milan). In
+this last paper he described how the dark upper hemisphere of the frog's
+egg grows down over the lower white hemisphere and leaves free only the
+yolk plug; he observed the segmentation cavity and the archenteron, but
+thought that the former became the alimentary canal; he observed and
+interpreted rightly the formation of the medullary folds. The circular
+blastopore in the frog in later years often went by the name of the anus
+of Rusconi.
+
+In France Dutrochet[183] investigated the foetal membranes in various
+vertebrate classes; Prevost and Dumas studied the very earliest stages
+of development in birds, mammals and amphibia (_Ann. Sci. nat._, ii.,
+iii., 1824, xii., 1827).
+
+A little later came Duges' studies of the osteology and myology of
+developing amphibia (1834),[184] and Coste's careful researches into the
+early developmental history of mammals.[185]
+
+[Illustration: FIG. 8.--Gill-slits of the Pig Embryo. (After Rathke.)]
+
+It was in 1825 that Heinrich Rathke (1793-1860), published his famous
+discovery of gill-slits in the embryo of a mammal,[186] a discovery which
+aroused considerable interest, and greatly stimulated embryological
+research. He describes how in a young embryo of a pig he saw four slits
+in the region of the neck, going right through into the oesophagus. They
+were separated by partitions which he called _Kiemenbogen_
+(gill-arches), and immediately in front of the first gill-slit lay the
+developing lower jaw. He compared these gill-slits with those of a
+dogfish. We reproduce his drawing of the pig-embryo (_Isis_, Pl. IV.,
+fig. 1).
+
+Later in the same year Rathke discovered gill-slits in the chick,[187] in
+this case finding only three. He described growing out from in front of
+the first slit a structure which he compared to the operculum or
+gill-cover of a fish.
+
+These discoveries were confirmed and extended for the chick[188] by the
+embryologist Huschke, a pupil of Oken. Like Rathke, he found only three
+indubitable gill-slits, but he noticed that the body-wall in front of
+the first gill-slit was really composed of two arches, which were on the
+whole similar to the gill-arches. The hinder of these two seemed to him
+to be a horn of the hyoid, the front one, which was bent at an angle, to
+be the rudiment of the upper and lower jaws (p. 401). Between these two
+arches he found an opening, just as between two gill-arches a gill-slit.
+This opening led into the mouth-cavity, and according to Huschke it
+became the external ear-passage. He discovered also three pairs of
+aortic arches in close relation with the gill-arches, so close indeed,
+that he did not hesitate to call them gill-arteries, and to recognise
+their resemblance with the aortic arches of fish. He traced, in part at
+least, the metamorphosis which these aortic arches undergo. This part of
+his discovery he developed in fuller detail in a paper of 1828,[189] in
+which he gave some excellent figures.
+
+Shortly after Huschke's first paper, von Baer published his views and
+observations on this subject in a short memoir in Meckel's _Archiv_.[190]
+In this paper he confirmed Rathke's discovery, and described the slits
+and arches in the dog and the chick. Both Rathke and he found gill-slits
+in the human embryo about this time (p. 557). There were generally
+present, he found, four gill-slits, and, as Rathke had suggested, the
+first gill-arch became the lower jaw. Von Baer also confirmed Rathke's
+discovery of the operculum, assigning it, however, to the second
+gill-arch. He refused to accept Huschke's derivation of the auditory
+meatus from the first gill-slit. Von Baer saw what had escaped Rathke
+and Huschke, that there were, not three nor four, but as many as five
+aortic arches.
+
+In his view of the metamorphosis of the aortic arches in the chick the
+first two pairs disappeared completely, the third pair gave rise to the
+arteries of the head and the fore-limbs, the right side of the fourth
+arch became the aorta, the left half of the fourth and the right half of
+the fifth arch became the pulmonary arteries, while the left half of the
+fifth arch disappeared. This schema, which for the last three arches was
+the same as Huschke's, von Baer upheld for the chick even in the second
+volume of his _Entwickelungsgeschichte_ (p. 116); he rectified it,
+however, for mammals in the same volume (p. 212), deriving both
+pulmonary arteries from the fifth arch, and the aorta from the fourth
+left. He fully recognised the great analogy of the embryonic arrangement
+of gill-arches and gill-arteries in Tetrapoda with their arrangement in
+fish (i., pp. 53, 73).
+
+Huschke, in a paper of 1832,[191] chiefly devoted to the development of
+the eye, figured and described the developing upper and lower jaws, and
+maintained against von Baer that the first slit turns into the auditory
+meatus and the Eustachian tube.
+
+These were the first papers of the embryological period. Before going on
+to discuss the principles which guided embryological research during the
+next ten or twenty years it is convenient to note what were the main
+lines of work characterising the period.
+
+The typical figure of the period is Rathke, who produced a great deal of
+first-class embryological work. He was, even more than von Baer, a
+comparative embryologist, and there were few groups of animals that he
+did not study. His first large publication, the _Beitraege zur Geschichte
+der Thierwelt_ (i.-iv., Halle, 1820-27), contained much anatomical work
+in addition to the purely embryological; he commenced here his series of
+papers on the development of the genital and urinary organs, continued
+in the _Abhandlungen zur Bildungsund Entwickelungs-Geschichte des
+Menschen und der Thiere_ (i., ii., Leipzig, 1832-3). A fellow-worker in
+this line was Johannes Mueller, whose _Bildungsgeschichte der Genitalien_
+(Duesseldorf) appeared in 1830.
+
+In a memoir on the development of the crayfish which appeared in
+1829,[192] Rathke found in an Invertebrate confirmation of the germ-layer
+theory propounded by Pander and von Baer. He was greatly struck by the
+inverted position of the embryo with respect to the yolk. In following
+out the development of the appendages he noticed how much alike were
+jaws and legs in their earliest stage, and how this supported Savigny's
+contention that the limbs of Arthropods belonged to one single type of
+structure. In his paper (1832) on the development of the fresh-water
+Isopod, _Asellus_,[193] Rathke returns to this point. Commenting on the
+original similarity in development of 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 Morphologie, Reisebemerkungen aus
+Taurien_ (1837), which contains embryological studies of many different
+types, including a study of the uniform plan of arthropod limbs. Later
+on Rathke devoted himself more to vertebrate embryology, producing among
+other works his classical papers on the development of the adder (1839),
+of the tortoise (1848), and of the crocodile (1866). He laid the
+foundations of all subsequent knowledge of the development of the
+blood-vascular system in a series of papers of various dates from 1838
+to 1856. The diagrams in his paper on the aortic arches of reptiles
+(1856) were for long copied in every text-book.
+
+Rathke was a foremost worker in another important line of embryological
+work, the study of the development of the skeleton and particularly of
+the skull. We shall discuss the history of the embryological study of
+the skull in some detail below; meantime, we note the two other
+important lines of research which characterise this period. One is the
+intensive study of the development of the human embryo, a study pursued
+by, among others, Pockels, Seiler, Breschet, Velpeau, Bischoff, Weber,
+Mueller, and Wharton Jones.[194] The other important line--the early
+development of the Mammalia--was worked chiefly by Valentin,[195]
+Coste,[196] and, above all, by Bischoff, whose series of papers[197] was
+justly recognised as classical.
+
+What interests us chiefly in the work of this embryological period is,
+of course, the relation of embryology to comparative anatomy and to pure
+morphology. The embryologists were not slow to see that their work threw
+much light upon questions of homology, and upon the problem of the unity
+of plan. Von Baer, we have seen, recognised this clearly in 1828;
+Rathke, in one of his most brilliant papers, the
+_Anatomisch-philosophische Untersuchungen ueber den Kiemenapparat und das
+Zungenbein_ (Riga and Dorpat, 1832), used the facts of development with
+great effect to show the homology of the gill-arches and hyoid
+throughout the vertebrate series; Johannes Mueller made great use of
+embryology in his classical _Vergleichende Anatomie der Myxinoiden_ (i.
+Theil, 1836), and, according to his pupil Reichert, firmly held the
+opinion that embryology was the final court of appeal in disputed points
+of comparative anatomy;[198] Reichert himself in a book of 1838
+(_Vergleichende Entwickelungsgeschichte des Kopfes der nackten
+Amphibien_) discussed the two different methods of arriving at the
+"Type"--the anatomical method of comparing adults, and the embryological
+method of comparing embryogenies. Of the embryological method, he says,
+"Its aim is to distinguish during the formation of the organism the
+originally given, the essence of the type, and to classify and interpret
+what is added or altered in the further course of development.
+Embryologists watch the gradual building up of the organism from its
+foundations, and distinguish the fundament, the primordial form, the
+type, from the individual developments; they reach thus, following
+Nature in a certain measure, the essential structure of the organism,
+and demonstrate the laws that manifest themselves during embryogeny" (p.
+vi.). The embryologists, influenced in this greatly by von Baer,
+gradually felt their way to substituting for the "Archetype" of pure
+morphology what one may perhaps best call the _embryological archetype_.
+How the transition was made we can best see by following out the course
+of discovery in one particular line. We choose for this purpose the
+development of the skull, a subject which excited much interest at this
+time and upon which much quite fundamental work was done, particularly
+by Rathke and Reichert.
+
+Following up his discovery of gill-slits and arches in the embryos of
+birds and mammals, Rathke in two papers of 1832[199] and 1833[200] worked
+out the detailed homologies of the gill-arches in the higher
+Vertebrates. He describes how in the embryo of the Blenny there is a
+short, thick arch between the first gill-slit and the mouth. A furrow
+appears down the middle of the arch dividing it incompletely into two.
+In the anterior halves a cartilaginous rod is developed which is
+connected with the skull; these rods become on either side the lower jaw
+and "quadrate." In the posterior halves two similar rods are formed
+which develop into the hyoid. The hyoid is at first connected with the
+skull, but afterwards frees itself and becomes slung to the "quadrate."
+From the hinder edge of the hyoid arch grows out the membranous
+operculum, in which develop later the opercular bones and branchiostegal
+rays. The upper jaw is an independent outgrowth of the serous layer.
+
+The serial homology of the lower jaw and quadrate with the hyoid and
+with the true gill-arches was thus established in fish, and Rathke had
+little difficulty in demonstrating a similar origin of lower jaw and
+hyoid in the embryos of higher Vertebrates. He could even, as we have
+noted before, find the homologue of the operculum in a flap which grows
+out from the hyoid arch in the embryo of birds.
+
+But Rathke could not altogether shake himself free from the
+transcendental notion of the homology of jaws with ribs, and this led
+him to draw a certain distinction between the first two and the
+remaining gill-arches, by which the homology of the former with the ribs
+was asserted and the homology of the latter denied. He thought he could
+show that the skeletal structures (lower jaw, "quadrate," and hyoid) of
+the first two arches were formed in the serous layer, just like true
+ribs, and like them in close connection with the vertebral skeletal
+axis. The other, "true," gill-arches appeared to him to be formed in the
+mucous layer, in the lining of the alimentary canal. They had no direct
+connection with the vertebral column, and seemed therefore to belong to
+what Carus[201] had called the visceral or splanchno-skeleton. He did not,
+however, let this distinction hinder him from asserting the substantial
+homology of all the gill-arches _inter se_, the first two included.
+
+Rathke's discoveries relative to the development of the jaws, the hyoid
+and the operculum, enabled him to make short work of the homologies
+proposed for them by the transcendentalists. He could prove from
+embryology that the jaws were not the equivalent of limbs, as so many
+Okenians believed. He could reject, with a mere reference to the facts
+of development, Geoffroy's comparison of the hyoid and the
+branchiostegal rays in fish with sternum and ribs. He could show the
+emptiness of the attempts made by Carus, Treviranus, de Blainville and
+Geoffroy, to establish by anatomical comparison the homologies of the
+opercular bones, for he could show that these bones were peculiar to
+fish, and were scarcely indicated, and that only temporarily, in the
+development of other Vertebrates.[202] He did not, however, himself
+realise the relation of the ear-ossicles to the gill-arches, though he
+knew that Spix and Geoffroy were quite wrong in homologising them with
+the opercular bones in fish. He described, it is true, the development
+of the external meatus of the ear and the Eustachian tube from the slit
+which appears between the first and the second arch, as Huschke had done
+before him; he described, in confirmation of Meckel, the "Meckelian
+process" of the hammer running down inside the lower jaw; but the
+discovery of the true homologies of the ear-ossicles was not made until
+a year or two later by Reichert.
+
+In his further study of the development of _Blennius viviparus_, Rathke
+observed some important facts about the development of the vertebral
+column and skull. He found that the vertebral centra were first formed
+as rings in the chorda-sheath, which give off neural and 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.
+
+Von Baer, like Rathke at this time, was also to some extent a believer
+in the vertebral theory of the skull. In his second volume (1834, pub.
+1837) he holds that the development of the skull, as the sum of the
+anterior vertebral arches, is in general the same as that of the other
+neural arches, and is modified only by the great bulk of the brain
+(_Entwickelungsgeschichte_, ii., p. 99). He had, however, some doubts as
+to the entire correctness of the vertebral theory, doubts suggested by a
+study of the developing skull. "In the course of the formation of the
+head in the higher animals, something additional is introduced which
+does not originally belong to the cranial vertebrae. At first we see the
+vertebration in the hinder region of the skull very clearly. Afterwards
+it becomes suddenly indistinct, as if some new formation overlaid it"
+(i., p. 194).
+
+Even more clearly is his doubt expressed in his paper on _Cyprinus_.
+"Upon the formation of the vertebral column only this need be said, that
+at this stage the notochord is very clearly seen, and the upper and
+lower arches and spinous processes are visible right to the end of the
+tail, but the separation into vertebrae ceases abruptly where the back
+passes into the head. I do not hesitate to assert _that bony fish, too,
+have at this stage an unsegmented cartilaginous cranium_ (as
+cartilaginous fish have all their life), the prominences and hollows of
+which constitute its only resemblance with the vertebral type" (1835, p.
+19).
+
+A convinced supporter of the vertebral theory was Johannes Mueller, who,
+in his classical memoir on the Myxinoids,[203] discussed at some length
+the relation between the development of the vertebrae and the development
+of the skull. His memoir is principally devoted to comparative anatomy,
+but in treating of the skeleton he pays much attention to development.
+He describes the formation of the vertebrae in elasmobranch embryos; for
+the facts regarding other Vertebrates he relies largely on work by
+Rathke (_Blennius_, 1833) and Duges (1834). He recognises as the basis
+of his comparisons the homology of the notochord in all vertebrate
+embryos with the persistent notochord which forms the chief part or the
+whole of the vertebral column in the Cyclostomes. The notochord
+possesses an inner and an outer sheath and the outer sheath is
+continuous with the _basis cranii_ (p. 92). It is in the outer sheath
+that the vertebrae 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 Mueller, 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, Mueller 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. Mueller has to be content with a reference to the state
+of things in _Ammocoetes_ (which, by the way, he did not know to be the
+young of _Petromyzon_). In the simple skull of _Ammocoetes_ the base is
+formed chiefly by two cartilaginous bars lying more or less parallel
+with the longitudinal axis of the skull and embracing with their hinder
+ends the cranial portion of the notochord.
+
+These bars, declares Mueller, 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 separate pieces. Mueller 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 trabeculae 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 Mueller, C. B. Reichert, who showed in his work
+very distinct traces of his master's influence. Reichert's first and
+most important contribution to the subject was his paper on the
+metamorphosis of the gill, or, as he called them, the visceral arches
+in Vertebrates,[204] particularly in the two higher classes. Reichert
+describes the similar origin in embryo of bird and mammal (pig) of
+three "visceral" arches. These arches stand in close relation to the
+three cranial vertebrae which Reichert, like Mueller, distinguishes. He
+makes the retrograde step of admitting only three aortic arches, and
+he is not inclined to consider the three visceral arches as equivalent
+to the gill-arches of fish--in his opinion they have more analogy with
+ribs, though differing somewhat from ribs in their later
+modifications. The visceral arches are processes of the visceral
+plates (von Baer), which grow downwards and meet in the middle line,
+leaving between one another and the undivided body wall three visceral
+slits opening into the pharynx. The first visceral process is
+different in shape from the others, for it sends forward, parallel
+with the head and at right angles to its downward portion, an upper
+portion in which later the upper jaw is formed. The other two
+processes are straight. From the hinder edge of the second visceral
+arch there develops, as Rathke had seen, a fold which is comparable
+with the operculum of fish. The first slit develops externally into
+the ear-passage, internally into the Eustachian tube, and in the
+middle a partition forms the tympanic ring and tympanum. Inside each
+of the visceral processes on either side a cartilaginous rod develops.
+In the first process this rod shows three segments, of which the first
+lies inside that portion of the process which is parallel with the
+head. This upper segment forms the foundation for the bones of the
+upper jaw. The lowest segment of the cartilaginous rod becomes
+Meckel's cartilage, and on the outer side of this the bones of the
+lower jaw are formed. The middle segment becomes in mammals the incus
+(one of the ear-ossicles), and in birds the quadrate. Meckel's
+cartilage, which was discovered by Meckel[205] in fish, amphibians and
+birds, is a long strip of cartilage which runs from the ear-ossicle
+known as the hammer in mammals,[206] to the inside of the mandible.
+Reichert shows how this relation comes about. The hammer, according to
+his observations on the embryo of the pig, is simply the proximal end
+of Meckel's cartilage, which later becomes separated off from the long
+distal portion (see Fig. 9). The third ear-ossicle of mammals, the
+stapes, comes not from the first arch but from the second. The
+cartilaginous rod of the second arch segments like the first into
+three pieces. Of these the uppermost disappears, the middle one, which
+lies close up to the labyrinth of the ear, becomes the stapes, and the
+lowest becomes the anterior horn of the hyoid. The stapes forms a
+close connection with the hammer and the incus. In birds, where there
+is a single ear-ossicle, the columella, the middle piece of arch I
+forms, as we have seen, the quadrate, by means of which the lower jaw
+is joined to the skull. The proximal end of Meckel's cartilage, which
+in mammals forms the hammer, here gives the articular surface between
+the lower jaw and the quadrate. The columella is formed from the
+middle piece of the three into which the cartilage of the second arch
+segments. It is, therefore, the homologue of the stapes in mammals.
+The third arch takes a varying share, together with the second, in the
+formation of the hyoid apparatus.
+
+[Illustration: FIG. 9.--Meckel's Cartilage and Ear-ossicles in Embryo
+of Pig. (After Reichert.)]
+
+In this paper Reichert made a distinct advance on the previous workers
+in the same field--Rathke, Huschke, von Baer, Martin St Ange, 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 Rathke[207] held
+that the upper jaw developed as a special process independent of the
+lower jaw rudiment, and the actual proof that the upper jaw is a
+derivative of the first visceral arch seems to have been first supplied
+by Reichert. His brilliant work on the development of the ear-ossicles
+founded what we may justly call the classical theory of their
+homologies. His views were attacked and in some points rectified, but
+the main homologies he established are even now accepted by many,
+perhaps the majority of morphologists.
+
+In a paper of 1838 on the comparative embryology of the skull in
+Amphibia,[208] Reichert added to his results for mammals and birds an
+account of the fate of the first and second visceral arches in Anura and
+Urodela.
+
+The first visceral arch, he found, gave in Amphibia practically the same
+structures as in the higher Vertebrates. Its skeleton segmented, as in
+mammals and birds, into three parts; the upper part gave rise to the
+palatine and pterygoid in Anura, but seemed to disappear in Urodeles,
+where the so-called palatine and pterygoid developed in the mucous
+membrane of the mouth; the middle part gave, as in birds, the quadrate,
+which formed a suspensorium for both arches; the lower part, as Meckel's
+cartilage, formed a foundation for the bones of the lower jaw. Of arch
+II., the lower part became the horn of the hyoid, the upper part had a
+varying fate. In some Anura it formed the ossicle of the ear (homologue
+of the columella of birds and the stapes of mammals), in others it
+disappeared. In reptiles the upper segment of the second arch formed, as
+in birds, the columella.
+
+The account of the metamorphoses of the visceral arches in Amphibia
+forms only a small part of Reichert's memoir of 1838, the chief object
+of which was to discover the general "typus" of the vertebrate skull,
+and to follow out its modifications in the different classes. Von Baer
+had shown that the generalised type appeared most clearly in the early
+embryo; Reichert therefore sought the archetype of the skull in the
+developing embryo. He brought to his task the preconceived notion that
+the skull could be reduced to an assemblage of 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
+vertebrae (p. 121, 1837).
+
+In the head he distinguished two tubes, the upper formed by the dorsal
+plates, the lower by the ventral or visceral plates. Both of these tubes
+were derived from the serous or animal layer (_cf._ von Baer, _supra_,
+p. 118). The walls of the lower tube were formed by the visceral
+processes, within which later the skeleton of the visceral arches
+developed. The walls of the upper tube formed the bones and muscles of
+the cranium proper. The facial part of the head was formed by elements
+from both upper and lower tubes. The dorsal tube showed signs of a
+division into three cranial vertebrae (_Urwirbeln_, primitive vertebrae).
+In mammals and birds, as Reichert had shown in his 1837 paper, the three
+cranial vertebrae 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 (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 _basis cranii_ and the side parts. On
+the contrary, it is quite continuous, as it is also in the higher
+Vertebrates during the process of chondrification" (p. 44, 1838). The
+vertebrae in the membranous or cartilaginous skull could be delimited in
+Amphibia by the help of the eye and the ear-labyrinth, which lie more or
+less between the first and second, and the second and third vertebrae,
+but, above all, by the vesicles of the brain.
+
+As in the higher Vertebrates, the visceral arches are associated with
+the cranial vertebrae as their ventral extensions, being equivalent to
+the visceral plates which form the ventral portion of the "primitive
+vertebrae" or primitive segments of the trunk.
+
+[Illustration: FIG. 10.--Cranial Vertebrae and Visceral Arches in Embryo
+of Pig. Ventral Aspect. (After Reichert.)]
+
+If the three cranial vertebrae are not very distinct in the early stages
+of development when the skull is still membranous or cartilaginous, they
+become clearly delimited when ossification sets in. Three rings of bone
+forming three more or less complete vertebrae are the final result of
+ossification. The composition of these rings is as follows:--
+
++-------------------------------------------------------------------+
+|                |   Base.       |    Sides.       |  Top.          |
+|----------------+---------------+-----------------+----------------|
+|First vertebra  |Presphenoid    |Orbitosphenoids  |Frontals        |
+|                |               |                 |                |
+|Second vertebra |Basisphenoid   |Alisphenoids     |Parietals       |
+|                |               |                 |                |
+|Third vertebra  |Basioccipital  |Exoccipitals     |Supraoccipital  |
++-------------------------------------------------------------------+
+
+The other bones of the skull are not included in the 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 _Programm_ (1807). But many
+of the bones developed in relation to the sense organs can find their
+place in the generalised embryonic schema or archetype of the vertebrate
+skull, for they are of very constant occurrence during early
+development.
+
+Having arrived at a generalised embryonic type for the vertebrate skull,
+of which the fundamental elements are the three cranial 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 _Wormian bones_, the lowermost
+turbinal, etc." (p. 63, 1838).
+
+He did not accept the Meckel-Serres law of parallelism. He recognised
+the great similarity between the unsegmented cartilaginous cranium of
+Elasmobranchs, and the primordial cranium of the embryos of the higher
+Vertebrates, but he did not think that the cranium of Elasmobranchs was
+simply an undeveloped or embryonic stage of the skulls of the higher
+forms. Rather "do the _Holocephala_, _Plagiostomata_, and _Cyclostomata_
+appear to us to be lower developmental stages individually
+differentiated, so that the other fully differentiated Vertebrates
+cannot easily be referred directly to their type" (p. 152, 1838). The
+skull of these lower fishes is itself a specialised one; it is an
+individualised modification of a simple type of skull. And this holds
+good in general of the skulls of the lower Vertebrates--they are
+individualised exemplars of a simple general type, not merely unmodified
+embryonic stages of the greatly differentiated skulls of the higher
+Vertebrates (p. 250, 1838). Differentiation within the vertebrate phylum
+is therefore not uniserial, but takes place in several directions.
+Reichert describes two sorts of modifications of the typical
+skull--class modifications and functional modifications. The causes of
+the modifications which characterise classificatory groups are unknown;
+the second class of modifications occur in response to adaptational
+requirements.
+
+Reichert's two papers are of considerable importance, and Mueller's
+remark in his review[209] of them is on the whole justified. "These
+praiseworthy investigations supply from the realm of embryology new and
+welcome foundations for comparative anatomy" (p. clxxxvii.).
+
+The development of the skull was, however, more thoroughly worked out by
+Rathke, and with less theoretical bias, in his classical paper on the
+adder.[210] This memoir of Rathke's is an exhaustive one and deals with
+the development of all the principal organ-systems, but particularly of
+the skeletal and vascular. He confirmed in its essentials Reichert's
+account of the metamorphoses of the first two visceral arches,
+describing how the rudiment of the skeleton of the first arch appears as
+a forked process of the cranial basis, the upper prong developing into
+the palatine and pterygoid, the lower forming Meckel's cartilage, while
+the quadrate develops from the angle of the fork. The actual bone of the
+upper jaw (maxillary) develops outside and separate from the
+palato-pterygoid bar. The cartilaginous rod supporting the second
+visceral arch divides into three pieces on each side, of which the lower
+two form the hyoid, the uppermost the columella. Like Reichert he held
+the visceral arches to be parts of the visceral plates, containing,
+however, elements from all three germ-layers--the serous, mucous, and
+vessel layers.
+
+The first gill-slit, or, as Rathke here prefers to call it, pharyngeal
+slit, closes completely in snakes and in Urodeles. It forms the
+Eustachian tube in all other Tetrapoda. As regards the vertebrae, Rathke
+describes them as being formed in the sheath of the chorda from paired
+rudiments, each of which sends two branches upwards, and two branches
+downwards. The two inner pairs of processes coalesce round the chorda,
+and later form the centrum; the upper outer pair meet above the spinal
+column; the lower outer pair form ribs. The odontoid process of the axis
+vertebra is the centrum of the atlas (p. 120). The formation of
+vertebral rudiments begins close behind the ear-labyrinth, but in front
+of this the chorda-sheath gives origin to a flat membranous plate which
+afterwards becomes cartilaginous. This plate reaches forward below the
+third cerebral vesicle as far as the infundibulum. The notochord ends in
+this plate, which is the _basis cranii_, just at the level of the
+ear-labyrinth. In no Vertebrate does the notochord extend farther
+forward (p. 122). The _basis cranii_ gives off three 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
+connections, the ear-capsule of Cyclostomes, and so do the ear-capsules
+of all embryonic Vertebrates (p. 39). It manifests clearly the embryonic
+archetype, ... "there exists one single and original plan of formation,
+as we may suppose, upon which is built the labyrinth of Vertebrates in
+general" (p. 40). When ossification sets in, the ear-capsule forms three
+bones, of which two fuse with the supraoccipital and exoccipitals.
+
+[Illustration: FIG. 11.--Embryonic Cranium of the Adder. Ventral Aspect.
+(After Rathke.)]
+
+During the formation of the ear-capsule the cranial basis develops from
+a plate to a trench, for in its hinder section the side parts grow up to
+form the side walls of the brain, in exactly the same way as the
+processes of the vertebral rudiments grow up to enclose the spinal
+column (pp. 122, 192). The foundations of the skull are now complete,
+and ossification gradually sets in. The basioccipital is formed
+in the posterior part of the _basis cranii_, and the exoccipitals in the
+side walls of the trench in continuity with the fundament of the
+basioccipital (see Fig. 11). The supraoccipital is formed in cartilage
+above the exoccipitals. The basisphenoid develops, like the
+basioccipital, in the flat _basis cranii_, but towards its anterior
+edge, between the large foramen (_h_) and the pituitary space (_i_). It
+is formed from two centres, each of which is originally a ring round the
+carotid foramen. The presphenoid develops in isolation between the
+lateral trabeculae, just behind the point where they fuse. The side parts
+of the basisphenoid and presphenoid (forming the alisphenoids and the
+orbitosphenoids respectively) develop in cartilage separately from the
+cranial basis, not like the exoccipitals in continuity with it. The
+hinder parts of the 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 hidden as partially or wholly
+destroyed" (p. 135).
+
+Rathke's general paper on the development of the skull in Vertebrates[211]
+treats the matter on a broader comparative basis than his paper on the
+adder, and takes into account all the vertebrate classes, in so far as
+their development was then known. He here makes the interesting
+suggestion, later entirely confirmed, that the _basis cranii_ or basilar
+plate is first laid down as two strips, one on each side of the
+chorda--the structures now known as parachordals (pp. 6, 27). For this
+supposition, he thinks, speaks the structure of the skull in
+_Ammocoetes_, which in this respect is the simplest of all Vertebrates
+(pp. 6, 22). In _Ammocoetes_, as Johannes Mueller 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 trabeculae, and of the
+parachordals which he was ready to assume from his embryological
+observations.
+
+Mueller was, of course, very ready to accept Rathke's opinions on this
+subject, for he considered that they supported his own theory of the
+vertebral nature of the skull. After describing in his _Handbuch der
+Physiologie_ the cartilaginous bands in _Ammocoetes_ and their highly
+differentiated homologues in the Myxinoids, he writes in the later
+editions, "Hence we see that in the cranium, as in the spinal column,
+there are at first developed at the sides of the chorda dorsalis two
+symmetrical elements, which subsequently coalesce, and may wholly
+enclose the chorda. Rathke has recently observed, in the embryos of
+serpents and other animals, before the formation of the proper cranial
+vertebrae, two symmetrical bands of cartilage, similar to those which I
+discovered as a persistent structure in _Ammocoetes_.... At a later
+period the _basis cranii_ of vertebrate animals contains three parts
+analogous to the bodies of 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."[212]
+
+Rathke was very cautious about accepting the vertebral theory of the
+skull; he saw that the facts of development were not altogether
+favourable to the theory, and he gave his adherence with many
+reservations and saving clauses. His general attitude may be summed up
+as follows.[213]
+
+The chorda sheath is the common matrix of the 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 basisphenoid, and the _Riechbein_ (ethmoid). The
+_Riechbein_ develops from the fused ends of the trabeculae. The
+presphenoid 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 independently in the side walls of the
+brain, may, however, still be considered as forming a vertebra, though
+the resemblance is not so great as in the case of the occipital ring.
+The presphenoid, being long and pointed, is very unlike a vertebral
+body. The orbitosphenoids develop separately from it. The ethmoid also
+differs from a vertebra, for it surrounds not the whole nervous axis as
+the two hinder "vertebrae" do, but only two prolongations of it, the
+olfactory lobes. In its development and final form it shows no
+particular resemblance to a vertebra. Its body, the _pars
+perpendicularis_ (mesethmoid) shows no similarity with a vertebral
+centrum. Completing the three hinder cranial "vertebrae" 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
+compared 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
+intercalaries (parietals), the presphenoid with its intercalaries
+(frontals), and the ethmoid with its outgrowths (turbinals and
+cribriform plate), taking them in order from behind forwards, show an
+increasing divergence from the plan according to which vertebrae 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 vertebrae can be
+recognised, although in a more or less modified form. Yet at most only
+four cranial vertebrae can be assumed, and these differ from ordinary
+well-developed vertebrae in their manner of formation the more the
+farther forward they lie."[214]
+
+Rathke was an able and careful critic of the vertebral theory of the
+skull, but he accepted it in the main. Actual attack on the theory upon
+embryological grounds was begun by C. Vogt, in his work on the
+development of _Coregonus_,[215] and in his paper on the development of
+_Alytes_.[216] He described for _Coregonus_ an origin of the skull in the
+main similar to that established by Rathke for the adder. There was a
+"nuchal plate" in which the front end of the notochord was imbedded; the
+notochord ended at the level of the labyrinth; there were two lateral
+bands, comparable to Rathke's lateral trabeculae; a "facial plate" was
+also formed, which seems on the whole equivalent to the plate formed by
+the fused anterior ends of the trabeculae. A little later the cranium
+formed a complete cartilaginous box surrounding the brain, very similar
+to the adult cranium of a shark.
+
+In his criticism of the vertebral theory of the skull, Vogt started by
+defining the vertebra as a ring formed round the chorda. Now since only
+the occipital segment of the skull is formed actually round the
+notochord, the parts of the skull lying in front of this cannot
+themselves be 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 vertebrae) 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 Doellinger, in the general part (1844) of
+his _Recherches sur les Poissons fossiles_ (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 Mueller to
+dispose of the theory,[217] which maintained a firm hold even upon
+embryologists. It was still upheld by Reichert, and Koelliker in 1849
+showed himself convinced of its general validity.
+
+A useful step in the analysis of the concept "vertebra" was taken by
+Remak,[218] who showed what a complex affair the formation of vertebrae
+really is, involving as it does a complete resegmentation
+(_Neugliederung_) of the vertebral column, whereby the original
+vertebral bodies were replaced by the secondary definitive bodies (p.
+143). Remak showed, as he thought, that the protovertebral segmentation
+of the dorsal muscle-plates did not extend into the head, and he denied
+Reichert's assertion (1837) that the cranial basis in mammals showed
+transverse grooves delimiting three cranial 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, _On the Theory of
+the Vertebrate Skull_,[219] in which he stated with great clearness and
+force the case for the embryological method of determining homologies,
+and criticised with vigour the vertebral theory of the skull. By this
+time the two rival methods in morphology had become clearly
+differentiated, and Huxley was able to contrast them, or at least to
+show how necessary the new embryological method was as a corrective and
+a supplement to the older anatomical, or, as he calls it, "gradation"
+method. Applied to the "Theory of the Skull," the gradation method
+consists in comparing the parts of the skull and vertebral column in
+adult animals with respect to their form and connections. "Using the
+other method, the investigator traces back skull and vertebral column to
+their earliest embryonic states and determines the identity of parts by
+their developmental relations" (p. 541). This second method is the final
+and ultimate. "The study of the gradations of structure presented by a
+series of living beings may have the utmost value in suggesting
+homologies, but the study of development alone can finally demonstrate
+them" (p. 541). As an example of the utility and, indeed, the necessity
+of applying the embryological method Huxley takes the case of the
+quadrate bone in birds. This bone had been generally regarded by
+anatomists as the equivalent of the tympanic of mammals, on account of
+its connection with the tympanum; but Reichert showed (1837) that the
+same segment of the first visceral arch developed into the incus in
+mammals, and into the quadrate in birds, and that therefore the quadrate
+was homologous with the incus. Similarly, on developmental grounds, the
+malleus or hammer of mammals is the homologue of the articular of birds,
+since both are developed from a portion of Meckel's cartilage identical
+in form and connections in the two groups. The homologies of the bones
+connected with the jaws in bony fishes had long been a subject of
+contention among comparative anatomists; Huxley shows from his personal
+observations how the development of the visceral arches throws light
+upon these difficulties. The mandibular arch in the developing fish is
+abruptly angled, as in the embryo of Tetrapoda; the upper prong of it
+ossifies into the palatine and pterygoid; at the angle is formed the
+quadrate (jugal, Cuvier), and to the quadrate is articulated the lower
+jaw, which ossifies round the lower prong or Meckel's cartilage. The
+scheme of development of the jaws is accordingly similar in fish to what
+it is in other Vertebrates, and this similarity of development enables
+Huxley to recognise what are the true homologues of the quadrate, the
+palatine and the pterygoid in adult bony fish, and to prove that the
+symplectic and the metapterygoid (tympanal, Cuvier) are bones peculiar
+to fish. In developing Amphibia Huxley found a suspensorium of hyoid and
+mandibular arches similar to the hyomandibular of fish.
+
+Tackling his main problem of the unity of plan of the vertebrate skull,
+Huxley shows, by a careful discussion of the anatomical relationships of
+the chief bones in typical examples of all vertebrate classes, that
+there is on the whole unity of plan as regards the osseous skull. This
+unity of composition can be established, on the gradation method, by
+considering the connections of the bones of the skull with one another,
+their relations to the parts of the brain and to the foramina of the
+principal cranial nerves. The assistance of the embryological method is,
+however, necessary in determining many points with regard to the bones
+developed in relation to the visceral arches. But there is a further
+step to be taken. "Admitting ... that a general unity of plan pervades
+the organisation of the ossified skull, the important fact remains that
+many vertebrated animals--all those fishes, in fact, which are known as
+_Elasmobranchii_, _Marsipobranchii_, _Pharyngobranchii_ and _Dipnoi_
+have no bony skull at all, at least in the sense in which the words have
+hitherto been used" (p. 571). The membranous or cartilaginous skull of
+these fishes shows a general resemblance in its main features to the
+ossified skull of other Vertebrates; the relations of the ear to the
+vagus and trigeminal nerves are, for instance, the same in both; the
+main regions of the cartilaginous skull can be homologised with definite
+bones or groups of bones in the bony skull; but discrepancies occur. It
+is again to development that we must turn to discover the true
+relationship of the cartilaginous to the ossified skull. "The study of
+the development of the ossified vertebrate skull ... satisfactorily
+proves that the adult crania of the lower _Vertebrata_ are but special
+developments[220] of conditions through which the embryonic crania of
+the highest members of the sub-kingdom pass" (p. 573). It is with the
+embryonic cranium of higher Vertebrates that the adult skull of the
+lower fishes must be compared, and the comparison will show a
+substantial though not a complete agreement between them. Thus, speaking
+of the development of the frog's skull, Huxley writes:--"If, bearing in
+mind the changes which are undergone by the palatosuspensorial
+apparatus, ... we now compare the stages of development of the frog's
+skull with the persistent conditions of the skull in the _Amphioxus_,
+the lamprey, and the shark, we shall discover the model and type of the
+latter in the former. The skull of the _Amphioxus_ presents a
+modification of that plan which is exhibited by the frog's skull when
+its walls are still membranous and the notochord is not yet embedded in
+cartilage. The skull of the lamprey is readily reducible to the same
+plan of structure as that which is exhibited by the tadpole when its
+gills are still external and its blood colourless. And finally, the
+skull of the shark is at once intelligible when we have studied the
+cranium in further advanced 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 are originally alike; in all (save _Amphioxus_?) the
+base of the primitive cranium undergoes the mesocephalic flexure, behind
+which the notochord terminates, while immediately in front of it the
+pituitary body is developed;[221] in all, the cartilaginous cranium has
+primarily the same structure--a basal plate enveloping the end of the
+notochord and sending forth three processes, of which one is short and
+median, while the other two, the lateral 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. 578).
+
+In a word, there is a general plan or primordial type which is
+manifested in the higher forms most clearly in their earliest
+development--an embryological archetype therefore.
+
+Huxley now goes on to consider the relation of this general plan or type
+of the skull to the structure and development of the vertebral column.
+Does the skull in its development show any signs of a composition out of
+several 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 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
+vertebrae. "In the process of ossification there is a certain analogy
+between the spinal column and the cranium, but that analogy becomes
+weaker and weaker as we proceed towards the anterior end of the skull"
+(p. 585). The best way to state the facts is to say that both skull and
+vertebral column start in their development from the same point, but
+immediately begin to diverge. The clear indications of segmentation
+which fully ossified adult skulls undoubtedly show are, therefore,
+secondary, and the vertebral theory of the skull, which was originally
+based upon the appearance of such fully ossified crania, is on the whole
+negatived by embryology.
+
+We have now to turn back a few years in order to follow up another line
+of discovery which had an important bearing upon the theory of the
+vertebrate skull--the working out of the distinction between membrane
+and cartilage bones.
+
+As early as 1731, R. Nesbitt,[222] in two lectures delivered to the Royal
+College of Surgeons, demonstrated that in the human foetus some bones
+were formed not in cartilage but directly in fibrous tissue, and this
+observation was confirmed by other human anatomists, particularly by
+Sharpey at a considerably later date. In 1822 Arendt[223] focussed
+attention upon the remarkable structure of the skull of the Pike, with
+its cartilaginous brain-box studded all over with bony plaques, an
+arrangement which had already attracted the interest of Cuvier and
+Meckel. K. E. von Baer[224] in 1826 discussed at some length the relation
+between the bony and the cartilaginous skull in fishes, with particular
+reference to the sturgeon, coming to the following just conclusion:--"If
+we consider the fibrous skeleton of _Ammocoetes_ as the first foundation
+of the skeleton of Vertebrates, we can form a series among the
+cartilaginous fishes, according as a cartilaginous skeleton penetrates
+more and more into this fibrous foundation. In the same way the process
+of ossification supplants the cartilaginous skeleton. So long as the
+ossifications lie in the skin, as in the sturgeon, they form corneous
+bones (_Hornknochen_), but when they lie under the skin, they form true
+bones, _e.g._, the bones of the skull in the pike" (p. 374).
+
+Embryologists soon become aware that a similar distinction between a
+primitive cartilaginous foundation and a secondary overlying
+ossification of the skull showed itself in the development of all
+Vertebrates. Duges, in his _Recherches sur l'osteologie et la myologie
+des Batraciens_ (1834), distinguished between such bones as are formed
+by direct ossification of the cartilaginous groundwork of the skull, and
+such as are developed in the periosteal fibrous tissue.
+
+Reichert in 1838[225] noted that several of the skull bones in Amphibia
+are formed without the intermediary of cartilage, such as the nasals,
+the maxillaries and the lacrymals. So, too, the frontals and parietals
+of Teleosts developed independently of the cartilaginous skull, and
+belonged to the skeletal system of the skin, not to the true vertebral
+axial skeleton (pp. 215-6). Even more interesting was his discovery,
+afterwards confirmed by Hertwig,[226] that in the newt several bones
+connected with the palate were formed in the mucous membrane of the
+mouth by the fusion of a number of little conical teeth (p. 97). Certain
+of these bones he considered to be the substitutes, not the equivalents,
+of the palatine and pterygoid of other Vertebrates, which are formed
+from the upper part of the first visceral arch, a part missing in the
+newt (p. 100). Owing to the difference of development he would not
+homologise these bones in the newt with the palatine and pterygoid of
+other Vertebrates. He recognised also that the bone now known as the
+parasphenoid was developed in the frog in the mucous membrane of the
+mouth, and had originally no connection with the cranial basis (p. 34).
+Rathke in 1839 also allowed the distinction between cartilage and
+membrane bone, but laid no stress upon it (_Entw. d. Natter._, p. 197).
+
+Jacobson in 1842[227] introduced the useful term, "primordial cranium,"
+for the primitive cartilaginous foundation of the skull, and drew a
+sharp distinction between cartilage bones and membrane bones.
+
+In his _Recherches sur les Poissons fossiles_,[228] L. Agassiz used Vogt's
+work on the development of _Coregonus_ to establish a classification of
+the bones of the skull in fish, a classification which had the merit of
+drawing a sharp distinction between the cartilaginous groundwork and
+the "protective plates" of the fish's skull. He recognised that the
+protective plates developed in a different way from the other bones of
+the skull. "We must distinguish," he writes, "two kinds of ossification;
+one which tends to transform the primitive parts of the embryonic
+cranium directly into bone, and another which leads to the deposition of
+protective plates round this core, which develop not only upon the upper
+surface, as has hitherto been supposed, but also on the lateral walls
+and on the lower surface of the cranium" (p. 112). In the skull of all
+fish there are three elements--(1) the cartilaginous base, including the
+nuchal plate, the 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:--(1)
+the basioccipital, exoccipitals (paroccipitals?), supraoccipital and
+"petrous" (_rocher_), developed from the nuchal plate; the ali- and
+orbito-sphenoids developed from the trabeculae; the "cranial ethmoid"[229]
+developed from the facial plate; (2) the parietals, frontals and nasals
+formed from the "superior" protective plate; the "anterior" and
+"posterior" frontals and the temporal, from the "lateral" plates; the
+body of the sphenoid and the vomer from the "inferior" plates. The other
+element, the cartilaginous brain-box, does not ossify, and tends to
+become absorbed (p. 124).
+
+In 1849 Koelliker published a paper[230] dealing with the morphological
+significance of the distinction between membrane and cartilage bones,
+and in 1850[231] he defended his views against the criticisms of
+Reichert[232] in a further note entitled _Die Theorie des
+Primordialschaedels festgehalten_. It is convenient to consider these
+papers together. Koelliker held that there was (1) a histological and (2)
+a morphological difference between the two categories of bones. The
+histological development of the two kinds was different, but this
+difference was not sufficient to establish a morphological distinction
+between them, a distinction in their anatomical _Bedeutung_. The true
+morphological distinction between them was their development in
+different skeleton-forming layers. Membrane bones were developed in
+fibrous tissue lying between the skin and the deep layer which formed
+the primordial cranium, and it was this formation in a separate layer
+that gave them a different morphological significance from the bones
+formed directly in the deep layer. Koelliker'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 Koelliker'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 membrane 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 considerations, and among these to the
+manner of origin of the whole bone in relation to the skeleton-forming
+layers" (1850, p. 290).
+
+The homologies established by this new principle might run counter to
+the homologies indicated by the study of adult structure. "Thus, for
+instance, although the lower jaw in position, function, form and shape,
+appears to be the same bone throughout, yet it must be admitted that it
+shows a difference in the different classes. In Mammals and Man it is an
+entirely secondary bone (an extremity according to Reichert), in Birds,
+Amphibia and Fishes only partially so, for its articular belongs to
+Meckel's cartilage and is accordingly analogous to a rib; indeed, in the
+Plagiostomes, etc., the whole lower jaw along with the articular is a
+persistent Meckel's cartilage" (p. 290, 1850).
+
+So, too, the supraoccipital in man cannot be fully homologised with the
+supraoccipital of many mammals, for its upper half arises at first in
+isolation as a secondary bone (p. 290).
+
+Reichert objected to the distinction drawn by Koelliker, 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.
+Mueller[233] that there was in truth no essential difference in
+histological development between the two categories of bone, that the
+cartilage cells were replaced by bone cells identical with those taking
+part in the formation of membrane bones. The morphological distinction
+continued however to be recognised, particularly by the embryologists.
+Rathke in his volume of 1861[234] classified the bones of the skull
+according to their origin from the primordial cranium or from the
+overlying fibrous layer, distinguishing as membrane bones, the
+parietals, frontals, nasals, lachrymals, maxillaries and premaxillaries,
+jugals, tympanic, parts of the "temporal," vomer, part of the
+supraoccipitals in some mammals, and the mandible (with the exception of
+the articular in such as have a quadrate bone). Huxley was also inclined
+in 1864[235] to recognise the distinction, but he writes with some
+reserve:--"Is there a clear line of demarcation between membrane bones
+and cartilage bones? Are certain bones always developed primarily from
+cartilage, while certain others as constantly originate in membrane? And
+further, if a membrane bone is found in the position ordinarily occupied
+by a cartilage bone, is it to be regarded merely as the analogue and not
+as the homologue of the latter?" (p. 296).
+
+We may note here that many comparative anatomists of the period were
+quite ready to decide Huxley's last question in a sense favourable to
+the older, purely anatomical, view of homology. Owen, for instance, held
+that difference of development did not disturb homologies established by
+form and connections. "Parts are homologous," he writes, "in the sense
+in which the term is used in this work, which are not always similarly
+developed: thus the 'pars occipitalis stricte dicta,' etc., of
+Soemmering is the special homologue of the supraoccipital bone of the
+cod, although it is developed out of pre-existing cartilage in the fish
+and out of aponeurotic membrane in the human subject."[236] Similarly he
+pointed to the diversities of development of the vertebral centrum in
+the different vertebrate classes as proof that development could not
+always be relied upon in deciding homologies (p. 89). But he could not
+deny that the archetype was better shown in the embryo than in the adult
+(_supra_, p. 108).
+
+J. V. Carus[237] likewise stood firm for the older method of determining
+homologies by comparison of adult structure. "We can regard as
+homologous," he writes, "only those parts which in the fully formed
+animal possess a like position and show the same topographical relations
+to the neighbouring parts" (p. 389). Parts homologous in this sense
+might develop in different ways, but no great importance was to be
+attached to such a circumstance. Membrane and cartilage bones developed
+in practically the same way, from the same skeleton-forming layer, and
+no morphological significance attached to their distinction (pp. 227,
+457). Embryology was of considerable value in helping to determine
+homologies, but the evidence that it supplied was contributory, not
+conclusive. Perhaps the greatest service which the study of development
+rendered was to disentangle, by a comparison of the earliest embryos,
+the generalised type (p. 389).
+
+We have now traced, by our historical study of the theory of the skull,
+the gradual evolution of the tendency to find in development the surest
+guide to determining homologies. We have seen how the embryological
+"type" came to be substituted, in whole or in part, for the anatomical
+"type" derived from the study of adult structure. But we have had to do
+only with a modification, not with a transformation, of the criterion of
+homology recognised by the anatomists. Homology is still determined by
+position, by connections, in the embryo as in the adult. "Similarity of
+development" has become the criterion of homology in the eyes of the
+embryologist, but "similarity of development" means, not identity of
+histological differentiation, but similarity of connections throughout
+the course of development. For the purposes of morphology, development
+has to be considered as an orderly sequence of successive forms, not in
+its real nature as a process essentially continuous. Morphology has to
+replace the living continuity by a kinematographic succession of stages.
+Since it is the earliest of these stages that manifest the simplest and
+most generalised structural relations of the parts, it is in the earlier
+stages that homologies can be most easily determined. But these
+homologies are still determined solely by the relative positions and
+connections of the parts, just as homologies are determined in the last
+of all the stages of development, the adult state. And since the
+generalised type is shown most clearly in the earliest stages and tends
+to become obscured by later differentiation, homologies observed in
+embryonic life are to be upheld even if the relations in adult life seem
+to indicate different interpretations.
+
+    [183] See review by Cuvier, _Mem. Mus. Hist, nat._, iii.,
+    pp. 82-97, 1817.
+
+    [184] _Mem. Savans etrangers_, vi. Extract in _Ann. Sci.
+    nat._ (2) i. (_Zool._), pp. 366-72, 1834.
+
+    [185] _Recherches sur la generation des Mammiferes_, 1834.
+    _Embryogenie comparee_, 1837.
+
+    [186] "Kiemen bey Saeugthieren," _Isis_, pp. 747-9, 1825.
+
+    [187] "Kiemen bey Voegeln," _Isis_, pp. 1100-1, 1825.
+
+    [188] "Ueber die Kiemenbogen und Kiemengefaesse beym
+    bebrueteten Huehnchen," _Isis_, xx., pp. 401-3, 1827.
+    (Read in Sept. 1826 to the _Versammlung der deutschen
+    Naturforscher und Aerzte_, then recently founded by
+    Oken).
+
+    [189] _Isis_, pp. 160-4, Pl. II., 1828.
+
+    [190] "Ueber die Kiemen und Kiemengefaesse in den Embryonen
+    der Wirbelthiere," Meckel's _Archiv_ for 1827, pp.
+    556-68. Also in _Ann. Sci. nat._, xv., pp. 266-80,
+    280-4, 1828.
+
+    [191] Meckel's _Archiv_, vi., pp. 1-47, 1832.
+
+    [192] _Untersuchungen ueber die Bildung und Entwickelung
+    der Fluss-Krebses_, Leipzig, folio, 1829. Preliminary
+    notice in _Isis_, pp. 1093-1100, 1825.
+
+    [193] "Untersuchungen ueber die Bildung und Entwickelung
+    der Wasser-Assel.," _Abh. z. Bild. u. Entwick.-Gesch._,
+    i., pp. 1-20, 1832. Translated in _Ann. Sci. nat._ (2),
+    ii., (_Zool._), pp. 139-57, 1834.
+
+    [194] Koelliker, _Entwickelungsgeschichte_, 2nd ed., p. 17,
+    Leipzig, 1879.
+
+    [195] _Handbuch der Entwickelungsgeschichte des Menschen
+    und ... der Saeugethiere und Voegel_, Berlin, 1835.
+
+    [196] _Embryogenie comparee_, 1837; _Histoire generale du
+    developpement des corps organises_, 1847-49.
+
+    [197] _Entwickelungsgeschichte des Kaninchen-Eies_,
+    Braunschweig, 1842; _Entwickelungsgeschichte des
+    Hunde-Eies_, Braunschweig, 1845;
+    _Entwickelungsgeschichte des Meerschweinchens_, Giessen,
+    1852; _Entwickelungsgeschichte des Rehes_, Giessen,
+    1854.
+
+    [198] "It is the 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 we have to
+    expect a definite clearing up of the problems relating
+    to the development of the head."--Mueller's _Archiv_, p.
+    121, 1837.
+
+    [199] _Anat.-phil. Unters. ue. d. Kiemenapparat u. d. Zungenbein_, Riga
+    and Dorpat, 1832.
+
+    [200] "Bildungs- und Entwickelungs-geschichte des Blennius viviparus,"
+    _Abhandl. z. Bild. u. Entwick.-Gesch. des Menschen u. der Thiere_,
+    ii., pp. 1-68, Leipzig, 1833.
+
+    [201] _Von den Ur-Theilen des Knochen und
+    Schalen-Gerustes_, Leipzig, 1828.
+
+    [202] _Kiemenapparat_, pp. 107-118.
+
+    [203] _Vergleichende Anatomie der Myxinoiden_. Part I.
+    (Osteology and Myology). (_Abh. koenigl. Akad. Wiss.
+    Berlin_, for 1834, pp. 65-340, 9 pls., 1836.) Also
+    separately.
+
+    [204] "Ueber die Visceralbogen der Wirbelthiere in
+    Allgemeinen und deren Metamorphosen bei den Voegeln und
+    Saeugethiere," Mueller's _Archiv_, pp. 120-222, 1837.
+
+    [205] _Handbuch d. menschl. Anatomie_, iv., p. 47.
+
+    [206] This was shown by Serres (_Ann. Sci. nat._, xi., p.
+    54 f.n., 1827), who found in a human embryo a long
+    cartilaginous piece extending from the ear-ossicles to
+    the inside of the lower jaw, and suggested that it was
+    the foundation of the permanent mandible.
+
+    [207] _Abhandl._, i., p. 102, 1832; ii., p. 25, 1833. (_Blennius_
+    paper).
+
+    [208] _Vergleichende Entwickelungsgeschichte des Kopfes der nackten
+    Amphibien_, Koenigsberg, quarto, 276 pp., 1838.
+
+    [209] Mueller's _Archiv_ for 1838.
+
+    [210] _Entwickelungsgeschichte der Natter_, Koenigsberg,
+    1839.
+
+    [211] _Bemerkungen ueber die Entwickelung des Schaedels der
+    Wirbelthiere_, Koenigsberg, 1839.
+
+    [212] _Handbuch der Physiologie des Menschen_, Koblenz,
+    1835; Eng. trans. by W. Baly, ii., p. 1615, 1838.
+
+    [213] For a full statement of Rathke's conclusions, see
+    the translation given by Huxley in _Lectures on the
+    Elements of Comparative Anatomy_, London, 1864.
+
+    [214] _Entwickelungsgeschichte der Wirbelthiere_, p. 142,
+    1861.
+
+    [215] _Embryologie des Salmones_. A separate volume of L.
+    Agassiz's _Histoire naturelle des Poissons d'Eau douce
+    de l'Europe centrale_, Neuchatel, 1842.
+
+    [216] _Untersuchungen ueber die Entwickelungsgeschichte der
+    Gebuertshelferkroete_, Solothurn, 1842.
+
+    [217] Mueller's _Archiv_ for 1843, p. ccxlviii.
+
+    [218] _Untersuchtingen ueber die Entwickelung der
+    Wirbelthiere_, Berlin, 1850-55.
+
+    [219] Delivered 17th June 1858. Reprinted in _The
+    Scientific Memoirs of T. H. Huxley_, edited by M. Foster
+    and E. Ray Lankester, vol. i., pp. 538-606 (1898).
+
+    [220] _Cf._ Reichert, _supra_, p. 149.
+
+    [221] The origin of the pituitary body from the roof of
+    the mouth was first described by Rathke (1839).
+
+    [222] _Human Osteogeny explained in two Lectures_, London,
+    1736.
+
+    [223] _De capitis ossei Esocis lucii structura singulari.
+    Dissert. inaug._ Regiomonti, 1822.
+
+    [224] "Ueber das aeussere und innere Skelet," Meckel's
+    _Archiv_, pp. 327-76, 1826.
+
+    [225] _Vergl. Entwick. d. Kopfes d. nackten Amphibien_ (p.
+    186).
+
+    [226] _Arch. f. mikr. Anat._, xi., Suppl., 1874.
+
+    [227] "Om Primordial-Craniet," _Foerhandlingar Skand.
+    Naturf. Moele_, Stockholm, 1842.
+
+    [228] Vol. I., General part, pub. 1844.
+
+    [229] _Entosphenoid_, Owen.
+
+    [230] _Zweiter Bericht zootom. Anstalt zu Wuerzburg_, 1849.
+
+    [231] _Zeits. f. wiss. Zool._, ii., pp. 281-91.
+
+    [232] Mueller's _Archiv_ for 1849, pp. 443-515.
+
+    [233] _Zeits. f. wiss Zool._, ix., 1858.
+
+    [234] _Entw. d. Wirbelthiere_, pp. 139-40, 1861.
+
+    [235] _Lectures on the Elements of Comparative Anatomy_.
+
+    [236] _On the Archetype of the Vertebrate Skeleton_, p. 5,
+    1848.
+
+    [237] _System der thierischen Morphologie_, Leipzig, 1853.
+
+
+
+
+CHAPTER XI
+
+THE CELL-THEORY.
+
+
+With the founding of the cell-theory by Schwann in 1839 an important
+step was taken in the analysis of the degrees of composition of the
+animal body. Aristotle had distinguished three--the unorganised
+material, itself compounded of the four primitive elements, earth and
+water, air and fire, the homogeneous parts or tissues and the
+heterogeneous parts or organs, and this conception was retained with
+little change even to the days of Cuvier and von Baer. Those of the old
+anatomists who speculated on the relations of organic elements to one
+another were dominated by Aristotle's simple and profound
+classification, and proposed schemes which differed from his only in
+detail. Bichat enlarged and deepened the concept of tissue, but the
+degree of composition below this was for him, as for all anatomists of
+his time, a fibrous or pulpy "cellulosity," living, indeed, but showing
+no uniform and elemental structure. It was Schwann's merit to interpose
+between the tissue and the mere unorganised material a new element of
+structure, the cell. And, as it happened, a few years before Schwann
+published his cell-theory, Dujardin hinted at another degree of
+composition which was later to take its place between the cell and the
+chemical elements--sarcode or protoplasm.
+
+As is well known, the concept of the cell arose first in botany. Robert
+Hooke discovered cells in cork and pith in 1667, and his discovery was
+followed up by Grew and Malpighi in 1671, and by Leeuenhoek in 1695. But
+they did not conceive the cell as a living, independent, structural
+unit. They were interested in the physiology of the plant as a whole,
+how it lived and nourished itself, and they studied cells and
+sieve-tubes, wood fibres and 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 18th century paid any heed to the
+microscopical anatomy of plants. For C. F. Wolff,[238] the formation of
+cells was a result of the secretion of drops of sap in the fundamental
+substance of the plant, this substance remaining as cell-walls when
+cell-formation was completed--no idea here of cells as units of
+structure.
+
+In the early 19th century, interest in plant anatomy revived somewhat,
+and much work was done by Treviranus, Mirbel, Moldenhawer, Meyen and von
+Mohl.[239] As a result of their work the fact was established that the
+tissues of plants are composed of elements which can, with few
+exceptions, be reduced to one simple fundamental form--the spherical
+closed cell. Thus the vessels of plants are formed by coalescence of
+cells, fibres by the elongation of cells and the thickening and
+toughening of their walls. At this time, interest was concentrated on
+the cell-wall, to the almost total neglect of the cell-contents; the
+"matured framework" of plant cells, to use Sach's convenient phrase, was
+the chief, almost the sole, object of study. And it was natural enough
+that the mere architecture of the plant should monopolise interest, that
+the composition of the tissues out of the cells, and the fitting
+together of the tissues to form the plant should awaken and hold the
+curiosity of the investigator; even the modifications of the cell-walls
+themselves, their rings and spiral thickenings and pits, offered a
+fascinating field of enquiry.
+
+The idea that the cell-contents might show a characteristic and
+individual structure had hardly dawned upon botanists when Schleiden
+published his famous paper, _Beitraege zur Phytogenesis_.[240] Schleiden's
+theme in this paper is the origin and development of the plant cell, a
+subject then very obscure, in spite of pioneer work by Mirbel. A few
+years before, Robert Brown had called attention to the presence in the
+epidermal cells of orchids and other plants of a characteristic spot
+which he called the areola or nucleus.[241] Schleiden saw the importance
+of this discovery, confirmed the constant presence of the nucleus in
+young cells, and held it to be an elementary organ of the cell. He named
+it the cytoblast because, in his opinion, it formed the cell. It was
+embedded in a peculiar gummy substance, the cytoblastem, which formed a
+lining to the cellulose cell-wall. Within the nucleus there was often a
+small dark spot or sphere--the nucleolus. The nucleus, Schleiden
+thought, originated as a minute granule in the cytoblastem which
+gradually increased in size, becoming first a nucleolus (_Kernchen_),
+and then, by further condensation of matter round it, a nucleus. Several
+nuclei might be formed in this way in a single cell. New cells took
+their origin directly from a full-grown nucleus, in a peculiar way which
+Schleiden describes as follows:--"As soon as the cytoblasts have reached
+their full size a delicate transparent vesicle arises on their surface;
+this is the young cell, which at first takes the shape of a very flat
+segment of a sphere, of which the plane surface is formed by the
+cytoblast, the convex side by the young cell itself, which lies upon the
+cytoblast like a watch-glass on a watch" (p. 145). The young cells
+increase in size and fill up the cavity of the old cell, which is in
+time resorbed. Cell-development always takes place within existing
+cells, and either one or many new cells may be formed within the
+mother-cell. Schleiden's views on cell-formation were drawn from some
+rather imperfect observations on the embryo-sac and pollen-tube, but he
+extended his theory to cell-formation in general. Though wrong in almost
+all respects the theory had at least the merit of fixing attention upon
+the really important constituents of the cell, the nucleus and the
+cell-plasma. To Schleiden, too, we owe the conception of the cell as a
+more or less independent living unity, whose life is not entirely
+identified with the life of the plant as a whole. "Each cell," he
+writes, "carries on a double life; one a quite independent and
+self-contained life, the other a dependent life in so far as the cell
+has become an integral part of the plant" (p. 138).
+
+So long as the definition of the plant cell embraced little more than
+the hardened cell-wall it was little wonder that "cells" in this sense
+were not recognised in animal tissues, except in a few exceptional
+cases--as in the notochord by Johannes Mueller.[242] Careful observation of
+animal tissues discovered in some cases the existence of discontinuous
+units of structure, but these were not, as a rule, recognised before
+1838 as analogous to plant cells. Von Baer, for example, observed that
+the young chick embryo was composed partly of an albuminous mass and
+partly of _Kuegelchen_ or little globules suspended in it
+(_Entwickelungsgeschichte_, i., pp. 19, 144). Since such _Kuegelchen_
+disposed in a row formed the notochord (i., p. 145) it seems probable
+that his _Kuegelchen_ were really cells. Similarly A. de Quatrefages[243]
+in 1834 saw and figured segmentation spheres in the developing egg of
+_Limnaea_, but he called them globules and did not recognise their
+analogy with the cells of plants. According to M'Kendrick,[244] Fontana,
+so far back as 1781,[245] described cells with nuclei in various tissues,
+and used acids and alkalis to bring out their structure more clearly.
+But it was not till 1836-7-8 that a fairly widespread occurrence of
+cells in animal tissues was recognised. The pioneer in this seems to
+have been Purkinje, who described cells in the choroidal plexus in
+1836,[246] and compared gland cells with the cells of plants in 1837.[247]
+Henle in 1837[248] and 1838[249] described various kinds of epithelial
+tissue, distinguishing them according to the kind of cell composing
+them; he also discovered the mode of growth of stratified epithelium.
+Valentin[250] appears to have seen cells in cartilage and epithelium even
+before Henle, and to have observed cells in the blastoderm of the chick.
+In his report on the progress of anatomy during 1838 Johannes Mueller 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 plants. Mueller himself had not
+only recognised the cellular nature of the notochord, but had observed
+the cells of the vitreous humour, fat cells and pigment cells, and even
+the nuclei of cartilage cells. From Schwann (1839) we learn that C. H.
+Schults had followed back the corpuscles of the blood to their original
+state of nucleated cells, and that Werneck had recognised cells in the
+embryonic lens. A preliminary notice of Schwann's own work appeared in
+1838 (Froriep's _Notizen_, No. 91, 1838), the full memoir in 1839, under
+the title _Mikroskopische Untersuchungen ueber die Uebereinstimmung in
+der Struktur und dem Wachstume der Tiere und Pflanzen_.[251]
+
+Theodor Schwann was a pupil of Johannes Mueller, and we know that Mueller
+took much interest in the new histology. It is probably to his influence
+that we owe Schwann's brilliant work on the cell, which appeared just
+after Schwann left Berlin for Loewen. Schwann was himself, as his later
+work showed, more a physiologist than a morphologist; he did quite
+fundamental work on enzymes, discovering and isolating the pepsin of the
+gastric juice; he proved that yeast was not an inorganic precipitate but
+a mass of living cells; he carried out experiments directed to show that
+spontaneous generation does not occur. We shall see in his treatment of
+the cell-theory clear indications of his physiological turn of mind.
+Schwann was only twenty-nine when his master-work appeared, and the book
+is clearly the work of a young man. It has the clear structure, the
+logical finish, which the energy of youth imparts to its chosen work. So
+the work of Rathke's prime, the _Anatomische-philosophische
+Untersuchungen_ of 1832 shows more vigour and a more reasoned structure
+than his later papers. Schwann's book is indeed a model of construction
+and cumulative argument, and even for this reason alone justly deserves
+to rank as a classic.
+
+The first section of his book is devoted to a detailed study of the
+structure and development of cartilage cells and of the cells of the
+notochord, and to a comparison of these with plant cells. He accepts
+Schleiden's account of the origin and development of nuclei and cells as
+a standard of comparison; and he seeks to show that nucleus and
+nucleolus, cell-wall and cell-contents, show the same relations and
+behave in the same manner in these two types of animal cells as in the
+plant-cells studied by Schleiden. The types of cell which he chose for
+this comparison are the most plant-like of all animal cells, and he was
+even able to point to a thickening of the cell-wall in certain cartilage
+cells, analogous to the thickening which plays so important a part in
+the outward modification of plant-cells. The analogy indeed in structure
+and development between chorda and cartilage cells and the cells of
+plants seemed to him complete. The substance of the notochord consisted
+of polyhedral cells having attached to their wall an oval disc similar
+in all respects to the nucleus of the plant-cell, and like it containing
+one or more nucleoli. Inside the mother-cell were to be found young
+developing cells of spherical shape, lacking however a nucleus.
+Cartilage was even more like plant tissue. It was composed of cells,
+each with its cell membrane. The cells lay close to one another,
+separated only by their thickened cell-wall and the intercellular
+matrix, showing thus even the general appearance of the cellular tissue
+of plants. They contained a nucleus with one or two nucleoli, and the
+nucleus was often resorbed, as in plants, when the cell reached its full
+development. Other nuclei were in many cases present in the cell, round
+which young cells could be seen to develop, in exactly the same manner
+as in plants. These nuclei had accordingly the same significance as the
+nuclei of plants, and deserved the same name of cytoblasts or
+cell-generators. The true nucleus of the cartilage cell was probably in
+the same way the original generator of the mother-cell.
+
+Having proved the identity in structure and function of the cells of
+these selected tissues with the cells of plants, as conceived by
+Schleiden, Schwann had still to show that the generality of animal
+tissues consisted either in their adult or in their embryonic state of
+similar cells. This demonstration occupies the second and longest
+section of his book.
+
+His method is throughout genetic; he seeks to show, not so much that all
+animal tissues are actually in their finished state composed of cells
+and modifications of cells, as that all tissues, even the most complex,
+are developed from cells analogous in structure and growth with the
+cells of plants.
+
+All animals develop from an ovum; it was his first task to discover
+whether the ovum was or was not a cell. It happened that, some years
+before Schwann wrote, a good deal of work had been done on the minute
+structure of the ovum, particularly by Purkinje and von Baer. Purkinje
+in 1825[252] discovered and described in the unfertilised egg of the fowl
+a small vesicle containing granular matter, which he named the
+_Keimblaeschen_ or germinal vesicle. It disappeared in the fertilised
+egg. As early as 1791 Poli had seen the germinal vesicle in the eggs of
+molluscs, but the first adequate account was given by Purkinje. In
+1827[253] von Baer discovered the true ova of mammals and cleared up a
+point which had been a stumbling block ever since the days of von Graaf,
+who had described as the ova the follicles now bearing his name.[254] Even
+von Graaf had noticed that the early uterine eggs were smaller than the
+supposed ovarian eggs; Prevost and Dumas[255] had observed the presence in
+the Graafian follicle of a minute spherical body, which, however, they
+hesitated to call the ovum; it was left to von Baer to elucidate the
+structure of the follicle and to prove that this small sphere was indeed
+the mammalian ovum. His discovery was confirmed by Sharpey and by Allen
+Thomson. Von Baer found the germinal vesicle in the eggs of frogs,
+snakes, molluscs, and worms, but not in the mammalian ovum; he
+considered the whole mammalian ovum to be the equivalent of the germinal
+vesicle of birds--a comparison rightly questioned by Purkinje (1834). In
+1834 Coste[256] discovered in the ovum of the rabbit a vesicle which he
+considered to be the germinal vesicle of Purkinje; he observed that it
+disappeared after fertilisation. Independently of Coste, and very little
+time after him, Wharton Jones[257] found the germinal vesicle in the
+mammalian ovum. Valentin in 1835,[258] Wagner in 1836,[259] and Krause in
+1837,[260] added considerably to the existing knowledge of the structure
+of the ovum. Wagner in his _Prodromus_ called attention to the
+widespread occurrence, within the germinal vesicle of a darker speck
+which he called the _Keimfleck_ or germinal spot, known sometimes as
+Wagner's spot. He recognised the _Keimfleck_ in the ova of many classes
+of animals from mammals to polyps. Frequently more than one _Keimfleck_
+occurred.
+
+Schwann had therefore a good deal of exact knowledge to go upon in
+discussing the significance of the ovum for the cell-theory. There were
+two possible interpretations. Either the ovum was a cell and the
+germinal vesicle its nucleus, or else the germinal vesicle was itself a
+cell within the larger cell of the ovum and the germinal spot was its
+nucleus. Schwann had some difficulty in deciding which of these views to
+adopt, but he finally inclined to the view that the ovum is a cell and
+the germinal vesicle its nucleus, basing his opinion largely upon
+observations by Wagner which tended to prove that the germinal vesicle
+was formed first and the ovum subsequently formed round it. But the ovum
+was not, in Schwann's view, a simple cell, for within it were contained
+yolk-granules, one set apparently containing a nucleus, the others not.
+Even the second set, those composing the yellow yolk, were considered by
+Schwann to deserve the name of cells, because, although a nucleus could
+not be observed in them, they had a definite membrane, distinct from
+their contents--a conception of the cell obviously dating from the
+earliest botanical notions of cells as little sacs. The yolk cells were
+not mere dead food material but living units which took part in the
+subsequent development of the egg. The relation between the unfertilised
+egg and the blastoderm which arises from it is not made altogether clear
+by Schwann. According to his account the cells of the blastoderm are
+formed actually in the ovum. Round the nucleus of the egg appears a
+_Niederschlag_ or precipitate which is the rudiment of the blastoderm
+(p. 68). When the egg leaves the ovary the nucleus disappears, leaving
+behind it this rudiment of the blastoderm, which rapidly grows and
+increases in size. The blastoderm of the chick before incubation is
+found to be composed of spherical anucleate bodies which Schwann
+considers to be cells, because they almost certainly develop into the
+cells of the incubated blastoderm, which are clearly recognisable as
+such after eight hours' incubation. The serous and mucous layers can be
+distinguished after sixteen hours' incubation, and it is found that the
+cells of the serous layer contain definite nuclei, though such seem to
+be absent in the cells of the mucous layer. Between the two layers other
+cells are formed belonging to the vessel layer, which is, however, in
+Schwann's opinion not a very definitely individualised layer.
+
+Schwann's next step is a detailed demonstration of the origin of each
+tissue from simple cells such as those composing the incubated
+blastoderm.
+
+"The foregoing investigation has taught us that the whole ovum shows
+nothing but a continual formation and differentiation of cells, from the
+moment of its appearance up to the time when, through the development of
+the serous and mucous layers of the blastoderm, the foundation is given
+for all the tissues subsequently appearing: we have found this common
+parent of all tissues itself to consist of cells; our next task must be
+to demonstrate not only in this general way that tissues originate from
+cells, but also that the special formative mass of each tissue is
+composed of cells, and that all tissues are either constituted by simple
+cells or by one or other of the manifold kinds of modified cells" (p.
+71). Five classes of tissue can be distinguished, according to the
+extent and manner of the modifications which the cells composing them
+have undergone. There are first of all independent and isolated cells,
+such as the corpuscles of the blood and lymph, not forming a coherent
+tissue in the ordinary sense. Next there are the assemblages of cells
+lying in contiguity with one another, but not in any way fused; examples
+of this class are the epidermal tissues and the lens of the eye. In the
+third class come tissues the cells of which have fused by their walls,
+but whose cell-cavities are not in continuity, such as osseous tissue
+and cartilage. In the tissues of the fourth class, comprising the most
+highly specialised of all, not only are the cell-walls continuous but
+also the cell-cavities; to this class belong muscle, nerve and capillary
+vessels. A fifth class, of rather a special nature, includes the fibrous
+tissues of all kinds. This is the first classification of tissues upon a
+cellular basis, and it marks the foundation of a new histology which
+took the place of the "general anatomy" of Bichat. The exhaustive
+account which Schwann gives of the structure and development of the
+tissues in this section of his book constitutes the first systematic
+treatise on histology in the modern sense, and it is still worth
+reading, in spite of many errors in detail.
+
+Schwann found it easy to demonstrate the cellular nature of the tissues
+of his first three classes. With the other two classes he had more
+difficulty. Fibres of all kinds, he considered, arose by an elongation
+of cells, which afterwards split longitudinally into long strips,
+forming as the case might be white or elastic fibrous tissue.
+Muscle-fibres and nerve-fibres were formed in a totally different way,
+by coalescence of cells; each separate muscle-fibre and nerve-fibre was
+thus a compound cell. Capillaries, Schwann held, were formed by cells
+hollowed out like drain-pipes, and set end to end--a mistaken view soon
+corrected by Vogt (_Embryologie des Salmones_, p. 206, 1842).
+
+In this detail part of his book Schwann accumulates material for a
+general theory of the cell which he develops in the third and last
+section. Taking up the physiological or dynamical standpoint, he points
+out that one process is common to all growth and development of tissues
+both in animals and plants, namely, the formation of cells, a process
+which he conceives to take place in the following manner. There is,
+first of all, a structureless substance, the cytoblastem, the matrix in
+which all cells originate. The cytoblastem may be either inside the
+cells, or, more usually, in the spaces between them. It is not a
+substance of definite chemical and physical properties, for the matrix
+of cartilage and the plasma of the blood alike come within the
+definition. It has largely the significance of food material for the
+developing cells. In plants, according to Schleiden, cells are never
+formed in the intercellular substance--the cytoblastem is within the
+cells; but extracellular cell formation seems to be the general rule in
+animals. An intracellular formation of cells occurs only in the ovum, in
+cartilage cells and chorda cells and in a few others, and even there it
+is not the exclusive method of formation; a formation of cells within
+cells never occurs in muscles and nerves, nor in fibrous tissue (p.
+204). In the cytoblastem granules appear, which gradually increase in
+size and take on the characteristic shape of nuclei; round each of these
+a young cell is formed. Sometimes the young cells appear to have no
+nuclei, as in the intracellular brood of chorda cells, but, as a rule, a
+nucleus is clearly visible. The nucleus is indeed the most
+characteristic constituent of the cell. "The most important and most
+constant criterion of the existence of a cell is the presence or absence
+of the nucleus," writes Schwann near the beginning of his book (p. 43).
+
+As a general rule the nucleolus is formed first, and round it by a sort
+of condensation or concretion the nucleus, which is frequently hollow,
+and round this again, by a somewhat similar process, the cell. "The
+whole process of the formation of a cell consists in the precipitation
+round a small previously formed corpuscle (the nucleolus) of first one
+layer (the nucleus) and then later round this a second layer (the cell
+substance)" (p. 213). The outermost layer of the cell usually thickens
+to form the membrane, but this membrane formation does not always occur,
+and the membrane is not present in all cells. The nucleus is formed in
+exactly the same manner as the cell, and it might with much truth itself
+be called a cell--a cell of the first order, while ordinary nucleated
+cells might be designated cells of the second order (p. 212). In
+anucleate cells there is probably only a single process of layer
+formation round an infinitely small nucleolus. In almost all nucleate
+cells the nucleus is resorbed when the cell reaches its full
+development, and it is larger and more important the younger the cell
+is.
+
+The cell was for Schwann not a morphological concept at all, but a
+physiological; the cell was a dynamical, not a statical unit.
+Cell-formation was the process at the back of all production of life,
+and cells were the centres of all vital activity. Each cell was itself
+an organism, and its life and activities were to some extent independent
+of the lives and activities of all the other cells. The multicellular
+organism was a colony of unicellular organisms, and its life was a sum
+of the lives of its constituent elements. This "theory of the organism,"
+which holds so important a place in biology even at the present day, is
+developed by Schwann in the concluding pages of his book.
+
+He begins by contrasting the teleological with the materialistic
+conception of living things. In the teleological view, a special force
+works in the living organism, guiding and directing its activities
+towards a purposeful end. According to the materialistic view there are
+no other forces at work in the living organism than those which act in
+the inorganic realm, or at least there are none but forces at one with
+these in their blindness and necessity. True, the purposiveness of
+living processes cannot be denied; but its ground lies, according to
+this view, not in a vital force which guides and rules the individual
+life, but in the original creation and collocation of matter according
+to a rational plan. The purposiveness of life is part of the
+purposiveness of the universe; just as the stars circle for ever in
+harmoniously adjusted paths, so do the processes of life work together
+towards a common end. Both are the inevitable result of the original
+distribution of matter in the primitive chaos, a distribution fixed by a
+rational and foreknowing Being (p. 222).
+
+Which of the two conceptions is to be adopted in biology? Teleological
+explanations have long been banished from the physical sciences, and in
+biology they are only a last resort when physical explanations have
+proved incomplete (p. 223). And if the ground of the purposiveness of
+living Nature is the same as the ground of the purposiveness of the
+universe, is it not reasonable to suppose that explanations which have
+proved satisfactory for inorganic things will in time with sufficient
+knowledge prove adequate also for organic things?
+
+The teleological conception, again, leads to difficulties particularly
+when it is applied to the facts of reproduction. If we suppose that a
+vital force unifies and coordinates the organism and is its very
+essence, we must also suppose that this force is divisible and that a
+part of it--separated in reproduction--can bring about the same results
+as the whole. If on the contrary the forces having play in the organism
+are the mere result of the particular combination of the matter
+composing it, the reconstruction of a particular combination of
+molecules in the ovum is all that is necessary to set development
+a-going along exactly the course taken by the ovum of the parent.
+Another argument against the teleological view is derived from the facts
+of the cell-theory. The cell-theory tells us that the molecules of the
+living body are not immediately built up in manifold combinations to
+form the organism, but are formed first into unit-constructions or
+cells, and that these units of composition are invariably formed in all
+development, of plants and animals alike, however diverse the goal of
+development may be. If there were a vital principle would we not expect
+to find that, scorning this roundabout way of reaching its goal, it went
+straight to the mark, taking a different and distinctive course for each
+individual development, building up the organism direct without the
+intermediary of cells? But since there is a universal principle of
+development, namely, the formation of cells, does it not seem that the
+cells must be the true organisms, that the whole "individual" organism
+must be an aggregate of cells, and that the concept of individuality
+applied to the organism is accordingly a logical fiction? And it is just
+upon this notion of the individuality of the organism that the
+teleological concept is based. The teleological view can perhaps not be
+completely refuted until the adequacy of materialistic explanations has
+been finally shown; but it is certain that the most promising method for
+research is the materialistic (p. 226).
+
+"We start out then from the assumption that the basis of the organism is
+not a force acting according to a definite plan; on the contrary, the
+organism arises through the action of blind and necessary laws, of
+forces which are as much implicit in matter as those of the inorganic
+world. Since the chemical elements in organic Nature differ in no way
+from those of inorganic Nature, the ground or cause of organic phenomena
+can consist only in a different mode of combination of matter, either in
+a peculiar mode of combination of the elementary atoms to form atoms of
+the second order, or in the particular arrangement of these compound
+molecules to form the separate morphological units of the organism or
+the whole organism itself" (p. 226). Accepting then the materialistic
+conception of the organism, we have to consider this further problem.
+Does the ground of organic processes lie in the whole organism or in its
+elementary parts? Translated into terms of metabolism--note the
+physiological point of view--the question runs, are metabolic processes
+the result of the molecular construction of the organism as a whole, or
+does the centre of metabolic activity lie in the cell? Is it the cell
+rather than the organism that is the immediate agent of assimilatory
+processes? In the first alternative the cause of the growth of the
+constituent parts lies in the totality of the organism; in the other
+alternative:--"Growth is not the result of a force having its ground in
+the organism as a whole, but each of the elementary parts possesses a
+force of its own, a life of its own, if you will; that is to say, in
+each elementary part the molecules are so combined as to set free a
+force whereby the cell is enabled to attract new molecules and so to
+grow, and the whole organism exists only through the reciprocal action
+of the single elementary parts.... In this eventuality it is the
+elementary parts that form the active element in nutrition, and the
+totality of the organism can be indeed a condition, but on this view it
+cannot be a cause" (p. 227).
+
+To help in the decision of this question, appeal must be made to the
+facts established as to the cellular nature of the organism and of its
+reproductive elements. We know that every organism is composed of cells,
+which are formed and grow according to the same laws wherever they are
+found, whose formation therefore is everywhere due to the same forces.
+If we find that certain of these cells--all of which we know to be
+essentially identical one with another--have the power when separated
+from the others of growing and developing into new organisms, we can
+infer that not only such cells but also all other cells have this
+assimilatory power. The ova of animals, the spores of plants, the
+isolated cells of lower organisms in general, all show the power of
+separate assimilation and development. "We must therefore, in general,
+ascribe to the cell an individual life, that is to say, the combination
+of the molecules in the single cell does suffice to produce the force
+whereby the cell is enabled to draw to itself new molecules. The ground
+of nutrition and growth lies not in the organism as a whole, but in the
+separate elementary parts, the cells. The fact that it is not every cell
+that can continue to grow when separated from the organism is not in
+itself an objection to this theory, any more than it is an objection to
+the individual life of a bee that it cannot continue to exist apart from
+the swarm. The activation of the forces existing within the cell depends
+on conditions which the cell encounters only in connection with the
+whole" (pp. 228-9).
+
+Schwann's next step is to discover what are the essential forces active
+in the cell, and here he enters the realm of hypothesis. He finds they
+can be reduced to two--an attractive force and a metabolic force. The
+attractive force is seen in the process of cell-formation, where first
+of all the nucleolus is formed by a concentration and precipitation of
+substances found free in the cytoblastem, and in the same way the
+nucleus and later the cell are laid down as concentric precipitates from
+the cytoblastem. Cell-formation also involves the second or metabolic
+force, by means of which the cell alters the chemical composition of the
+medium surrounding it so as to prepare it for assimilation. Schwann's
+attractive force brings about the actual taking up of the prepared
+substance; his metabolic force is the cause of the digestion of food
+substances, and is nearly identical with enzyme action. With what
+inorganic process, he now asks (p. 239), can the process of
+cell-formation be most nearly compared, and the answer obviously is,
+with the process of crystallisation. Cells are, it is true, quite
+different in shape and consistency from crystals, and they grow by
+intussusception, not by apposition--their plastic or attractive forces
+seem therefore to be different. A still more important difference is
+that the metabolic force is peculiar to the cell. Yet there are
+important analogies between crystals and cells. They agree in the
+important respect that they both grow in solutions at the cost of the
+dissolved substance, according to definite laws, and develop a definite
+and characteristic shape. It might even be maintained, Schwann thinks,
+that the attractive force of crystals is really identical with that of
+cells, and that the difference in result is due merely to the difference
+between the substance of the cell and the substance of the crystal. He
+points out how organic bodies are remarkable for their powers of
+imbibition, and he seeks to show that the cell is the form under which a
+body capable of imbibition must necessarily crystallise, and that the
+organism is an aggregate of such imbibition-crystals. The analogy
+between crystallisation and cell-formation he works out in the following
+manner:--"The substance of which cells are composed possesses the power
+of chemically transforming the substance with which it is in immediate
+contact, in somewhat the same way as the well-known preparation of
+platinum changes alcohol into acetic acid. Each part of the cell
+possesses this property. If now the cytoblastem is altered by an already
+formed cell in such a way that a substance is formed that cannot become
+part of the cell, it crystallises out first as the nucleolus of a new
+cell. This in its turn alters the composition of the cytoblastem. A part
+of the transfomed substance may remain in solution in the cytoblastem or
+may crystallise out as the beginning of a new cell; another part, the
+cell-substance, crystallises round the nucleolus. The cell-substance is
+either soluble in the cytoblastem and crystallises out only when the
+latter is saturated with it, or it is insoluble and crystallises as soon
+as it is formed, according to the aforementioned laws of the
+crystallisation of imbibition-bodies; it forms thus one or more layers
+round the nucleolus, etc. If one imagines cell-formation to take place
+in this way, one is led to think of the plastic force of the cell as
+identical with the force by means of which a crystal grows" (pp.
+249-50).
+
+Two difficulties have to be faced by this theory--(1) the origin of the
+metabolic power of the cells, (2) the reason why the cells arrange
+themselves so as to form an organism of complex and definite structure.
+Schwann tries to explain the origin of the "metabolic" action, the
+analogy of which with the contact-action of colloidal platinum he
+recognises, by attributing it to the peculiar structural arrangements of
+molecules. In attempting to account for the harmonious structure of the
+organism he points to the analogy of ordinary crystals, which often form
+complex and regular tree-like arrangements; plants in particular
+resemble these regularly shaped crystal-aggregates.
+
+The whole ingenious theory is offered merely as an hypothesis and a
+guide to research. It is interesting as one of the most carefully
+thought-out attempts ever made to give a thorough-going materialistic
+account of the origin and development of organic form, and it arose
+directly out of the cell-theory.
+
+Schleiden and Schwann started out from an erroneous theory of the origin
+and development of cells, which impaired to some extent the value of
+their results. It was not long, however, before their theory of the
+origin of cells by "crystallisation" from an intra- or extra-cellular
+cytoblastem was challenged and overthrown, and the generalisation that
+cells originate by division from pre-existing cells put in its place.
+
+This was established for plant cells by Meyen, Unger, von Mohl, Naegeli
+and Hofmeister in or about the forties.[261] Criticism of the
+Schwann-Schleiden theory from the zoological side was suggested by the
+study of the segmentation of the ovum--the developmental process in
+which the multiplication of cells is most easily observed. The
+segmentation of the ovum was well known to Schwann, for the process had
+been described in the frog by Prevost and Dumas in 1824,[262] in the frog
+and newt by Rusconi,[263] and an elaborate study of the process in the
+frog had been made by von Baer.[264] Schwann indeed suspected that there
+must be some connection between the segmentation of the ovum and the
+formation of cells, but he did not realise that the cellular blastoderm
+of the chick was formed by the division or segmentation of the egg-cell.
+
+Segmentation was soon found to be of widespread occurrence. Von Siebold
+in 1837 described the process in Entozoa,[265] and in the same year Loven
+saw segmentation in _Campanularia_,[266] and Sars in the starfish and in
+Nudibranchs.[267]
+
+In 1838 Bischoff[268] observed segmentation in the mammalian ovum, and the
+whole course of segmentation in the ovum of the rabbit from the 2-celled
+to the morula stage was carefully described and figured by Barry[269] in
+1839. C. Vogt[270] in 1842 described segmentation in _Coregonus_ and
+_Alytes_. The discovery of segmentation in the ovum of birds was not
+made until 1847, by Bergmann,[271] confirmed independently by Coste[272]
+in 1850. By 1848 segmentation had been noted in _Hydra_ and various
+hydroids, in acalephs, in starfish, polyzoa, nematodes, rotifers,
+leeches, oligochaetes, polychaetes, in most groups of molluscs and
+arthropods, and in all the vertebrate classes.[273]
+
+The process was at first held to be merely one of yolk-division, or
+_Dotterfurchung_, and its details were by most interpreted in the light
+of the Schleiden-Schwann theory of cell-formation.
+
+The first steps towards a truer conception of the process seem to have
+been taken by Bergmann, who in 1841[274] called attention to the presence
+of nuclei in the segmentation-spheres of the frog's egg, and by Bagge in
+the same year, who observed that division of the nuclei preceded the
+multiplication of the segmentation spheres.[275] He considered the nuclei
+to be anucleate cells, and the same view was taken by Koelliker in
+1843.[276] Next year, however, in his classical paper on Cephalopod
+development[277] Koelliker came to the opinion that they were really
+nuclei. He showed that segmentation was brought about by cell-division,
+that between "total" and "partial" segmentation there was a difference
+of degree and not of kind, and that the cells of the body were formed by
+division of the segmentation spheres. He held, however, that the nuclei
+multiplied endogenously and not by division. The division of nuclei was
+observed by Coste in 1846.[278] Leydig in 1848[279] took the necessary step
+in advance and maintained that the nuclei as well as the cells increased
+always by division. He was supported by Remak, who in a paper of
+1852,[280] and more fully in his monumental _Untersuchungen ueber die
+Entwickelung der Wirbelthiere_ (Berlin, 1850-55), proved that in the
+frog's egg at least segmentation was a simple process of cell-division,
+initiated always by division of the nucleus.[281]
+
+One point Remak left undecided--the fate of the _Keimblaeschen_ or
+egg-nucleus. It was generally held, even so late as the 'fifties, that
+the egg-nucleus disappeared just before segmentation began--Bischoff
+clung to this belief even in 1877.[282] Though Barry had held in 1839 that
+the egg-nucleus does not disappear in segmentation, J. Mueller seems to
+have been the first actually to prove that it forms by division the
+nuclei of the first two segmentation spheres. He furnished the
+demonstration in the egg of _Entoconcha mirabilis_,[283] and his paper was
+known to Remak, who could not, however, observe a similar division of
+the egg-nucleus in the frog. Mueller's discovery was confirmed for
+_Oceania armata_ by Gegenbaur,[284] and for _Notommata sieboldii_ by
+Leydig.[285]
+
+In 1854 Virchow,[286] previously a supporter of Schwann, crystallised the
+new views in the famous phrase--_Omnis cellula e cellula_--and gave wide
+publicity to them in his classical lectures on Cellular Pathology,
+delivered in 1858.[287] The new doctrine of cell-formation was also taught
+by Leydig[7] in his text-book of histology, published in 1857.
+
+The Schleiden-Schwann theory of the origin of cells by generation in a
+cytoblastem was now definitely overthrown.
+
+The importance of the protoplasmic content of the cell was brought into
+prominence through the work of Dujardin,[289] Purkinje,[290] Cohen[291] and
+Max Schultze.[292] The last-named in 1861 proposed a definition of the
+cell which might be accepted at the present day. "A cell," he wrote, "is
+a little blob of protoplasm containing a nucleus" (p. 11).
+
+    [238] _Theoria generationis_, Halae, 1759.
+
+    [239] See J. v. Sachs, _Geschichte der Botanik_, book ii.,
+    Eng. Trans., 2nd impr., 1906.
+
+    [240] Mueller's _Archiv_, pp. 137-76, 1838.
+
+    [241] _Trans. Linnean Soc._, xvi., p. 710, 1833.
+
+    [242] _Myxinoiden_, i. Theil., p. 89, 1835.
+
+    [243] _Ann. Sci. nat._ (2) (_Zool._) ii., pp. 107-18, pl.
+    11, 1834.
+
+    [244] _Proc. Phil. Soc. Glasgow_, xix., pp. 71-125,
+    1887-8.
+
+    [245] _Traite sur le venin de la vipere_, 1781.
+
+    [246] Mueller's _Archiv_, 1836.
+
+    [247] J. Mueller, _Jahresbericht ue. d. Fortschritte der
+    anat.-physiol. Wissenschaften im Jahre_ 1838. Mueller's
+    _Archiv_, 1838.
+
+    [248] _Symbolae ad anatomiam villorum imprimis eorum
+    epithelii_, Berlin, 1837.
+
+    [249] _U. d. Ausbreitung des Epitheliums im menschlichen
+    Koerper_. Mueller's _Archiv_, 1838.
+
+    [250] See Schwann's _Bemerkungen_ at the end of his
+    _Mikroskopische Untersuchungen_.
+
+    [251] Republished in Ostwald's _Klassiker der exakten
+    Wissenschaften_, No. 176, Leipzig, 1910. References in
+    the text are to the original pagination.
+
+    [252] _Symbolae ad ovi avium historiam_.
+
+    [253] _De ovi mammalium et hominis genesi_.
+
+    [254] _De mulierum organis_, 1672.
+
+    [255] _Ann. Sci. nat._, iii., p. 135, 1842.
+
+    [256] _Recherches sur la generation des Mammiferes_.
+    Report by Academy Committee. _Ann. Sci. nat._ (2)
+    (_Zool._) ii., pp. 1-18, 1834; also _Embryogenie
+    comparee_, 1837.
+
+    [257] _Lond. and Edin. Phil. Mag._ (3) vii., 1835; _Phil.
+    Trans._ 1837.
+
+    [258] _Handbuch der Enfwickelungsgeschichte_, 1835, and
+    Mueller's _Archiv_, 1836.
+
+    [259] _Prodromus historiae generationis hominis atque
+    animalium_, Lipsiae, 1836.
+
+    [260] Mueller's _Archiv_, 1837.
+
+    [261] Sachs, _History of Botany_, Book ii.
+
+    [262] _Ann. Sci. nat._, i., pp. 110-14, 1824. Swammerdam
+    is said to have observed the 2-celled stage in the egg
+    of the frog (_Bibl. Nat._, 1752), and Roesel v. Rosenhof
+    the same stage in the tree-frog (_Hist. nat. ranarum
+    nostratium_, 1758).
+
+    [263] _Developpement de la grenouille commune_, Milan,
+    1826. _Biblioteca italiana_, lxxix., 1836, and Mueller's
+    _Archiv_, 1836. Agassiz is said by Vogt (1842) to have
+    seen segmentation in the Perch as early as 1831.
+
+    [264] Mueller's _Archiv_, 1836.
+
+    [265] In Burdach, _Die Physiologie als
+    Erfahrungswissenschaft_, 2nd Ed., vol. ii.
+
+    [266] Wiegmann's _Archiv_, 1837.
+
+    [267] _Bericht Versamml. deutsch. Naturf. in Prag_, 1837.
+
+    [268] _Bericht Versamm. deutsch. Naturf. in Freiburg_,
+    1838. Later in his _Entw. d. Wirbelth_., and in his
+    papers on the development of the rabbit.
+
+    [269] _Phil. Trans._, 1839. See particularly Pl. vi.,
+    figs. 105-12.
+
+    [270] _Embryologie des Salmones_ 1842.
+
+    [271] Mueller's _Archiv_, 1847.
+
+    [272] _C.R. Acad. Sci._, xxx., p. 638.
+
+    [273] See review by Leydig in _Isis_, 1848, pp. 161-193.
+
+    [274] Mueller's _Archiv_, pp. 89-102, 1841.
+
+    [275] _De evolution Stronzyli auric. el Ascaridis acum._,
+    Erlangen, 1841.
+
+    [276] Mueller's _Archiv_, pp. 66-141, 1843.
+
+    [277] _Entwickelungsgeschichte der Cephalopoden_, Zurich,
+    1844.
+
+    [278] _Froriep's Notizen_, No. 800, 1846.
+
+    [279] _Isis_, 1848.
+
+    [280] Mueller's _Archiv_, p. 47, 1852, also 1854 and 1858.
+
+    [281] See particularly Plate IX., figs. 3-7.
+
+    [282] _Hist.-krit. Bemerkungen zu den neuesten
+    Mittheilungen ue. d. erste Entwickelung d.
+    Saeugethiereier_, Muenchen, 1877.
+
+    [283] _Monatsber. Akad. Wiss. Berlin_, 1851.
+
+    [284] _Zur Lehre von Generationswechsel u. d. Fortpflanzen
+    d. Medusen u. Polypen_.
+
+    [285] _U. d. Bau u. d. system. Stellung d. Raederthiere_,
+    1854.
+
+    [286] _Arch f. path. Anat. Phys._, vii., pp. 1-39, 1854.
+    Also in his _Beitraege z. spec. Path. u. Therapie_.
+
+    [287] _Die Cellularpathologie_, Berlin, 1858.
+
+    [288] _Lehrbuch der Histologie_, 1857.
+
+    [289] _Ann, Sci. nat._ (2) iii., pp. 108-9 and pp. 312-4,
+    1835. Also iv, pp. 343-77.
+
+    [290] 1839 or 1840.
+
+    [2913] _Nova Acta Acad. Leop._, xxii., 1850. Trans. in 1853
+    for Ray Society.
+
+    [292] _Arch. f. Anat. u. Physiol._, pp. 1-27, 1861.
+
+
+
+
+CHAPTER XII
+
+THE CLOSE OF THE PRE-EVOLUTIONARY PERIOD
+
+
+The influence of the cell-theory on morphology was not altogether happy.
+The cell-theory was from the first physiological; cells were looked upon
+as centres of force rather than elements of form, and the explanation of
+all the activities of the organism was sought in the action of these
+separate dynamic centres. There resulted a certain loss of feeling for
+the problems of form. The organism was seen no longer as a cunningly
+constructed complex of organs, tissues and cells; it had become a mere
+cell-aggregate; the higher elements of form were disregarded and
+ignored.
+
+We have seen this physiological attitude expressed with the utmost
+clearness by the founder of the cell-theory himself; we shall see the
+same attitude taken up by most of his successors. Thus Vogt, who was
+later to become one of the protagonists of materialism in Germany,
+developed in his memoir on the embryology of _Coregonus_[293] the theory
+of the independent or individual life of the cell. "Each cell," he
+wrote, "represents in some measure a separate organism, and while their
+development necessarily conforms to the general plan and the particular
+tendencies of the parent organism, they nevertheless each follow their
+own particular tendency and do not lose their independence until, by
+reason of the metamorphoses which they undergo, they lose their cellular
+nature" (p. 275).
+
+And again, "... we are obliged to admit the existence in the cell of an
+independent life, which makes its development self-sufficient.... Each
+cell consequently represents a little independent organism, which
+assimilates foreign substances, builds them up, and rejects those that
+are useless; from this point of view the embryo can be compared up to a
+certain point with a zoophyte stock, of which each polyp, while living
+its own independent life, is yet incorporated in the common corm, which
+impresses its distinctive character upon every polyp" (p. 293).
+
+Classical expression was given to the "colonial theory" of the organism
+by Virchow in his lectures on "Cellular Pathology."[294] For Virchow the
+organism resolves itself into an assemblage of living centres, the
+cells; the organism has no real existence as a unity, for there is no
+one single centre from which its activities are ruled. Even the nervous
+system, which appears to act as a co-ordinating centre, is itself an
+aggregate of discrete cells. "A tree is a body of definite and orderly
+composition, the ultimate elements of which, in every part of it, in
+leaf and root, in stem and flower, are cellular elements--so also are
+animal forms. _Every animal is a sum of vital units_, each of which
+possesses the full characteristics of life. The character and the unity
+of life cannot be found in one definite point of a higher organisation,
+for example in the brain of man, but only in the definite, constantly
+recurring disposition shown individually by each single element. It
+follows that the composition of the major organism, the so-called
+individual, must be likened to a kind of social arrangement or society,
+in which a number of separate existences are dependent upon one another,
+in such a way, however, that each element possesses its own particular
+activity, and, although receiving the stimulus to activity from the
+other elements, carries out its own task by its own powers" (2nd ed.,
+pp. 12-13).
+
+Analysis, decomposition, or disintegration of the organism is here
+pushed to its extreme point, and the problem of recomposition, synthesis
+and co-ordination shirked or forgotten.
+
+The harmful influence of the cell-theory upon morphology did not pass
+unnoticed by the broader-minded zoologists of the day. Virchow's earlier
+paper[295] on the application of the cell-theory to physiology and
+pathology called forth a vigorous protest from Reichert,[296] who
+discussed in a very instructive way the contrast between the older
+"systematic" and the newer "atomistic" attitude to living Nature.
+
+Is it really true, he asks, that the cell is the dominant element in all
+organisation; is the cell comparable in importance to the atom of the
+chemists; or is it not rather the servant of a higher regulatory power?
+Johannes Mueller, who was Reichert's master, had in his _Physiology_[297]
+argued splendidly for the existence of a creative force which guides and
+rules development, and brings to pass that unity and harmony of
+composition which distinguish living things from inorganic products.
+Reichert sought in vain in the writings of the biological "atomists" for
+any smallest recognition of these broader characteristics of living
+things upon which Mueller had rightly laid stress. For the atomists the
+cell was the only element of form; they ignored the combination of cells
+to form tissues, of tissues to form organs, of organs to form an
+organism. For the morphologists the cell was one element among many, and
+the lowest of all.
+
+The difference of attitude is clearly shown if we consider from the two
+points of view a complicated organ-system such as the central nervous
+system. The atomist sees in this a mere aggregate of cells or at the
+most of groups of cells. "The morphologist," on the other hand, "sees in
+the central nervous system a _proximate_ element in the composition of
+the body--a primitive organ. From this point of view he apprehends and
+judges its morphological relations with, in the first place, the other
+co-ordinated primitive organs in the system as a whole; in all this the
+cells remain in the background, and have nothing to do directly with the
+determination of these morphological relations" (p. 6). Within the
+nervous system there are separate organs which stand to one another in a
+definite morphological and functional relationship. These organs are, it
+is true, composed of cells; but between the form and connections of
+these organs and the cells which compose them there is no direct and
+necessary relation (p. 6). It is true that the cell is the ultimate
+element of organic form, and that all development takes place by
+multiplication and form-change of cells. Yet is the cell in all this not
+independent of the unity of the developing embryo, and what the cells
+produce, they produce, so to speak, not of their own free will, nor by
+chance, but under the guiding influence of the unity of the whole, and
+in a certain measure as its agents (p. 7). The atomists will not admit
+the truth of this; they see in development nothing more than a process
+of the form-change and multiplication of cells. The full meaning of
+development escapes them, for they take no cognisance of the increasing
+complexity of the embryo, of the separating-out of tissues, of the
+moulding of organs, of the harmonious adaptation and adjustment of the
+parts to form a working whole.
+
+In general, the fault of the atomists is that they do not respect the
+limits which Nature herself has prescribed to the process of logical
+analysis and disintegration of the organism; they do not recognise the
+existence of natural and rational units or unities; they forget the one
+great principle of rational analysis, "that, by universally valid,
+inductive, logical method, natural objects must in all cases be accepted
+and dealt with in the combination and concatenation in which they are
+given" (p. 10).
+
+The atomists at least recognised one natural organic element, the cell;
+the materialistic physiologists of the time resolved even this unity
+into an aggregate of inorganic compounds, and regarded the organism
+itself as nothing but a vastly complicated physico-chemical mechanism.
+From this point of view morphology had no right of existence, and we
+find Ludwig, one of the foremost of the materialistic school,
+maintaining that morphology was of no scientific importance, that it was
+nothing more than an artistic game, interesting enough, but completely
+superseded and robbed of all value by the advance of materialistic
+physiology.[298]
+
+Naturally enough, morphologists did not accept this rather contemptuous
+estimate of their science, but held firmly to the morphological
+attitude. So Leuckart in his reply to Ludwig, so Rathke in a letter to
+Leuckart published in that reply, so Reichert in his _Bericht_, so J. V.
+Carus in his _System der thierischen Morphologie_,[299] upheld the
+validity, the independence, of morphological methods. Leuckart and
+Rathke called attention to the absolute impossibility of explaining by
+materialistic physiology the unity of plan underlying the diversity of
+animal form. J. V. Carus, who was convinced of the validity of
+physiological methods within their proper sphere, drew a sharp
+distinction between systematics and morphology on the one hand, and
+physiology on the other. Physiology had nothing to do with the problems
+of form at all; its business was to study the physical and chemical
+processes which lay at the base of all vital activities. Morphology, on
+its part, had to accept form as something given, and to study the
+abstract relations of forms to one another. "On this point," he writes,
+"stress is to be laid, that morphology has to do with animal form as
+something _given_ by Nature, that though it follows out the changes
+taking place during the development of an animal and tries to explain
+them, it does not enquire after the conditions whose necessary and
+physical consequence this form actually is" (p. 24). He expressed indeed
+a pious hope (p. 25) that physiology might one day be so far advanced
+that it could attempt with some hope of success to discover the
+physico-chemical determinism of form, but this remained with him merely
+a pious hope. Reichert, in his _Bericht_, applied to the rather wild
+theorisings of the physiologist Ludwig the same clear commonsense
+criticism that he bestowed on the other "atomists."
+
+It would take too long to describe the great development that
+materialistic physiology took at this time, and to show how the
+separation of morphology from physiology, which originally took place
+away back in the 17th century, had by this time become almost absolute.
+The years towards the end of the first half of the century marked indeed
+the beginning of the classical period as well of physiology as of
+dogmatic materialism. Moleschott and Buchner popularised materialism in
+Germany in the 'fifties, while Ludwig, du Bois Reymond and von Helmholtz
+began to apply the methods of physics to physiology. In France, Claude
+Bernard was at the height of his activity, rivalled by workers almost as
+great. The doctrine of the conservation of energy was established about
+this same time.
+
+Between the cell-theory on the one side, and physiology on the other, it
+was a wonder that morphology kept alive at all. The only thing that
+preserved it was the return to the sound Cuvierian tradition which had
+been made by many zoologists in the 'thirties and 'forties. It is a
+significant fact that this return to the functional attitude coincided
+in the main with the rise of marine zoology, and that the man who most
+typically preserved the Cuvierian attitude, H. Milne-Edwards, was also
+one of the first and most consistent of marine biologists. Milne-Edwards
+describes in his interesting _Rapport sur les Progres recents des
+Sciences zoologiques en France_ (Paris) 1867, how "About the year 1826,
+two young naturalists, formed in the schools of Cuvier, Geoffroy and
+Majendie, considered that zoology, after having been purely descriptive
+or systematic and then anatomical, ought to take on a more physiological
+character; they considered that it was not enough to observe living
+objects in the repose of death, and that it was desirable to get to
+understand the organism in action, especially when the structure of
+these animals was so different from that of man that the notions
+acquired as to the special physiology of man could not properly be
+applied to them" (p. 17). The two young naturalists were H.
+Milne-Edwards and V. Audouin. In pursuance of these excellent ideas they
+set to work to study the animals of the seashore, producing in 1832-4
+two volumes of _Recherches pour servir a l'histoire naturelle du
+littoral de la France_. After Audouin's early death A. de Quatrefages
+was associated with Milne-Edwards in this pioneer work, and their
+valiant struggles with insufficient equipment and lack of all laboratory
+accommodation, and the rich harvest they reaped, may be read of in
+Quatrefage's fascinating account of their journeyings.[300] Note that
+though they called themselves physiologists they meant by physiology
+something very different from the mere physical and chemical study of
+living things. They were interested, as Cuvier was, primarily in the
+problems of form; they sought to penetrate the relation between form and
+function; their chief aim was, therefore, the study not of physiology[301]
+in the restricted sense, but physiological morphology. As a matter of
+fact they produced more taxanomic and anatomical work than work on
+physiological morphology, but this was only natural, since such a wealth
+of new forms was disclosed to their gaze. Milne-Edwards' masterly
+_Histoire Naturelle des Crustaces_[302] and A. de Quatrefage's _Histoire
+Naturelle des Anneles marins et d'eau douce_[303] 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 Mueller was using the
+tow-net to gather material for his masterly papers on the metamorphoses
+of Echinoderms.[304] Work on the taxonomy and anatomy of marine animals
+was in general in full swing by the 'fifties and 'sixties.
+
+This return to Nature and to the sea had a very beneficial effect upon
+morphology, bringing it out from the laboratory to the open air and the
+seashore. It saved morphology from formalism and aridity, and in
+particular from a certain narrowness of outlook born of too close
+attention paid to the details of microscopical anatomy. It brought
+morphologists face to face again with the wonderful diversity of organic
+forms, with the unity of plan underlying that diversity, with the
+admirable adjustment of organ to function and of both to the life of the
+whole.
+
+Milne-Edwards' theoretical views, as expounded in his _Introduction a la
+zoologie generale_ (1851), well reflect this Cuvierian attitude.[305] He
+acknowledges himself the debt he owes to Cuvier; "the further I advance
+in the study of the sciences which he cultivated with so sure a hand,"
+he writes in 1867, "the more I venerate him."
+
+Milne-Edwards frankly takes up the teleological standpoint, and
+interprets organic forms on the assumption that they are purposive and
+rationally constructed. "To arrive at an understanding of the harmony of
+the organic creation," he writes, "it seemed to me that it would be well
+to accept the hypothesis that Nature has gone about her work as we would
+do ourselves according to the light of our own intelligence, if it were
+given us to produce a similar result. Comparing and studying living
+things as if they were machines created by the industry of man, I have
+tried to grasp the manner in which they might have been invented, and
+the principles whose application would have led to the production of
+such an assemblage of diversified instruments" (p. 435). The problem is
+to discover the laws which rule the diversity of organic forms. The
+first and most obvious of these laws is the "law of economy," or the law
+of unity of type. Nature, as Cuvier pointed out, has not had recourse to
+all the possible forms and combinations of organs; she appears to work
+with a limited number of types and to get the greatest possible
+diversity out of these by varying the proportions of the constitutive
+materials of structure. Within the limits of each type Nature has
+brought about diversity by raising her creatures to different degrees of
+perfection. This is the second law of organic form, and it is this law
+that Milne-Edwards chiefly elaborates. Degrees of perfection mean for
+him, as for Aristotle, primarily degrees of perfection of function, but
+since structure is necessarily in close relation with function,
+perfection of function brings in its train increased perfection of
+organisation. This can only be attained by a division of labour[306] among
+the organs and by their consequent differentiation. An animal is like a
+workshop where some complicated product is manufactured, and the organs
+are like the workmen. Each workman has his own special piece of work to
+do, at which he becomes thoroughly expert; and the finished product is
+manufactured more rapidly and efficiently by the co-operation of workers
+each skilled in one department than it would be if each workman had to
+produce the whole. Applied to the organism this principle of the
+division of labour means the differentiating out of the separate
+functions, their localisation in different parts of the organism, and
+their co-ordination to produce a combined result.
+
+This differentiation of functions implies a corresponding
+differentiation of organs, but it is functional differentiation which
+always takes the lead. "Where division of labour has not been introduced
+into the organism there must exist a great simplicity of structure. But
+just as uniformity in the functions of the different parts of the body
+implies a uniformity in their mode of constitution, so diversity in
+function must be accompanied by particularities in structure; and, in
+consequence also, the number of dissimilar parts must be augmented and
+the complication of the machine increased" (p. 463). Since function
+comes before form there is not always a special organ for every
+function. "It is a grave error to believe that a particular function can
+be performed only by one and the same organ. Nature can arrive at the
+desired result by various ways, and when we look down through the animal
+kingdom from the highest to the lowest forms we see that the function
+does not disappear even when the special instrument provided for the
+purpose in the higher types ceases to exist" (p 470).
+
+Nature, holding fast to the law of economy, does not even always create
+a new organ for a new function; she may simply adapt an undifferentiated
+part to special functions, or she may even convert to other uses an
+organ already specialised (p. 464). So, for example, the function of
+respiration is in the lowest animals diffused indifferently over the
+whole surface of the body, and only as organisation advances is it
+localised in special organs, such as gills. Now suppose that Nature
+wishes to adapt a fish, which breathes by gills, to life in the air; she
+does not create an organ specially for this purpose, but utilises the
+moist gill-chamber (_e.g._, in _Anabas scandens_), modifying it in
+certain ways so that the fish can take advantage of the oxygen it
+contains. But this gill-chamber lung is at best a makeshift, and when
+she comes to the more definitely terrestrial Amphibia Nature gives up
+the attempt to use the gill-chamber as a lung, and creates a new organ,
+the true vertebrate lung, specially adapted for breathing air (p. 475).
+
+But whatever means Nature adopts, her aim is always the same--to
+specialise, to differentiate, to produce diversity from uniformity.
+
+Differentiation not only raises the level of organisation; it usually
+also takes the direction of adaptation to particular habits of life, and
+this is perhaps the most fruitful cause of diversity. Everywhere we find
+animals specialised in adaptation to their environment--to life in air
+or water, or on land--and many of their most striking differences are
+due to this cause. But adaptation may also act in reducing diversity,
+for there necessarily occur many instances of parallel adaptation or
+convergence. So we get the extraordinary parallelism between the
+families of marsupials and the orders of placentals,[307] the remarkable
+similarity between the respiratory organs of land-crabs and
+air-breathing fish--to mention only two out of an immense range of
+analogous facts.
+
+The last cause of diversity that Milne-Edwards adduces is what he calls
+a "borrowing" of peculiarities of structure from another systematic
+group. Thus, "among reptiles, the tortoises seem to have borrowed from
+birds some of their characteristic features of organisation; and among
+the sauroid fishes the piscine type seems to have been influenced by the
+type from which reptiles are derived" (p. 479). So many riddles that, a
+little later on, stimulated the ingenuity of the evolutionists!
+
+Such, then, were the factors which Milne-Edwards considered adequate to
+explain the rich variety of animal forms. We cannot do better than quote
+his own summary of his doctrine:--"To sum up, then, the great
+differences introduced by Nature into the constitution of animals seem
+to depend essentially upon the existence of a certain number of general
+plans or distinct types, upon the perfecting in various degrees either
+of the whole or of parts of each of these structural plans, upon the
+adaptation of each type to varied conditions of existence, and upon the
+secondary imitation of foreign types by certain derivatives of each
+particular type" (p. 480).
+
+We have laid stress on the fact that Milne-Edwards put function before
+form, for this is the mark of the true Cuvierian. With it goes the
+belief that Nature forms new parts to meet new requirements, that she is
+not limited, as Geoffroy thought, to a definite number of "materials of
+organisation," but can produce others at need. Cuvier held, for example,
+that many of the muscles and even the bones of fish were peculiar to
+them, and without homologues in the other Vertebrates, having been
+created by Nature for special ends.[308] So, too, Johannes Mueller, who in
+many ways and not least in his sane vitalism was a follower of the
+Cuvierian tradition, recognised that many of the complicated cartilages
+in the skull of Cyclostomes were specially formed for the important
+function of sucking, and had no equivalent in other fish.[309]
+
+So, too, the embryologists after Cuvier often came across instances of
+the special formation of parts to meet temporary needs. Thus Reichert
+interpreted the "palatine" and "pterygoid," which are formed in the
+mouth of the newt larva by a fusion of conical teeth, as special
+adaptations to enable the little larva to lead a carnivorous life.[310]
+
+Not many years after the publication of Milne-Edwards' _Introduction a
+la zoologie generale_ (1851) there appeared a book by H. G. Bronn in
+which was offered a very similar analysis of organic diversity. The
+curious thing was that Bronn approached the problem from quite a
+different standpoint, from the standpoint, indeed, of
+_Naturphilosophie_. Of this the title of the book is itself sufficient
+proof--_Morphologische Studien ueber die Gestaltungs-gesetze der
+Naturkoerper ueberhaupt und der organischen insbesondere_ (Leipzig and
+Heidelberg, 1858).[311] The linking up of organic with inorganic form is
+characteristic; there is much talk, too, in the book of _Urstoffe_ and
+_Urkraefte_, but underlying the _Naturphilosophie_ we can trace the same
+Cuvierian treatment of form, and see crystallise out laws of progressive
+development that bear no small analogy with the laws established by
+Milne-Edwards.
+
+According to Bronn, the ideal fundamental form of the plant is an ovoid
+or strobiloid[312] body, for a plant reaches out in two directions in
+search of food--towards the sun and towards the earth. Animals differ
+from plants in being endowed with sensation and mobility (_cf._
+Aristotle and Cuvier), and it is this characteristic that gives them
+their distinctive form. The main types of animal form--the Amorphozoa,
+Actinozoa, and Hemisphenozoa--are essentially adaptations to particular
+modes of locomotion. Animals either are fixed, or they move in all
+directions without reference to any definite axis, or they move in one
+main direction.
+
+The Amorphozoa or shapeless animals include many of the Protozoa and
+sponges; they have no typical form, and most of them are sessile. The
+Actinozoa include such animals as the Coelentera, which are fixed, and
+the Echinoderms, which have a central point and move indifferently along
+any radial axis; their form differs from the strobiloid mainly in having
+radiate rather than spiral symmetry. The Hemisphenozoa, or bilaterally
+symmetrical animals, include all those that habitually move forward;
+they have a front end and a hind end, a dorsal surface and a ventral,
+and the mouth, sense-organs and "brain" are concentrated in the front
+end to form a head--all in direct adaptation to this forward movement;
+they make up the vast majority of animals.
+
+The fundamental forms of living things are, however, merely so many
+themes on which a multitude of further variations are woven, through the
+action of the laws which rule the detail of organic diversities. These
+further laws may be set down under four main heads. Under the first
+comes the law of the existence of certain fundamentally distinct
+structural types, which are distinguished from one another by their
+ground-form, by the number of organ-systems, and by the number of
+homotypic organs they possess, but principally by the relative position
+of the organs to one another (principle of connections). The form and
+connections of the nervous system are of particular importance in
+distinguishing the types (_cf._ Cuvier). The second factor in the
+diversity of organic form is the action of certain laws of progressive
+development[313] (_Entwickelungsgesetze_), which bear the same relation to
+the development of the animal kingdom as the laws of individual
+development bear to the development of the embryo, for organs appear in
+the different animal series in much the same order and manner as they
+develop in the individual. These laws are (1) progressive
+differentiation of functions and organs; (2) numerical reduction of
+serially repeated parts; (3) concentration of functions and their organs
+in particular parts of the body; (4) centralisation of organ-systems and
+parts of such, so that they come to depend upon one central organ; (5)
+internalisation of the "noblest" organs, unless these are necessarily
+external, and (6) increase in size of the whole or of parts. Of these
+the law of differentiation is by far the most important, and most of the
+others are in a sense merely special cases of this fundamental law. To
+this law of differentiation is due the increase in complexity or
+perfection of organisation which is shown by all the animal series.
+Bronn himself recognised the great similarity of this law of progressive
+differentiation to Milne-Edwards' principle of the division of labour;
+he seems, however, to have arrived at it independently.
+
+Bronn's third factor in the production of variety of form is adaptation
+to environment, or better, functional response to environment. Bronn
+gives an excellent account of adaptational modifications and calls
+attention, just as Milne-Edwards did, to the numerous analogies of
+structure which adaptation brings about. He works out the interesting
+view that there is some connection between classificatory groups and
+adaptational forms, especially such as are connected with the function
+of locomotion:--"Based upon a common characteristic method of locomotion
+are whole or nearly whole sub-phyla (Hexapoda), classes (mammals and
+reptiles, birds, fishes, gastropods, pteropods, brachiopods, Bryozoa,
+Rotifera, jelly-fish, polypes, sponges), sub-classes (mobile and
+immobile lamellibranchs, echinoderms, walking and swimming Crustacea,
+parasitic and free-living worms, and so on), often, however, only orders
+and quite small groups (snakes, eels, bats, sepias, medusae, etc.)" (p.
+141).
+
+It was characteristic of the 'forties and 'fifties that transcendental
+anatomy, along with Nature-philosophy, went rather out of fashion, its
+false simplicities and premature generalisations being overwhelmed by
+the flood of new discoveries. A few stalwarts indeed upheld
+transcendental views. We have already discussed the morphological system
+built up by Richard Owen in the late 'forties, a system transcendental
+in its main lines. We have seen the vertebral theory of the skull still
+maintained in the 'fifties by such men as Reichert and Koelliker, and we
+find J. V. Carus in 1853[314] taking it as almost conclusively proved.[315]
+
+We may mention, too, as showing clear marks of the influence of
+transcendental ideas, L. Agassiz's work on the principles of
+classification.[316] And Serres, who was Geoffroy's chief disciple,
+recanted not a whit of his doctrine of recapitulation, but re-affirmed
+and expanded it from time to time, and particularly in a lengthy memoir
+published in 1860.[317] But in general we may say that pure morphology in
+the Geoffroyan or Okenian sense was becoming gradually discredited. A
+curious indication of this is seen in the fact that not only the idea
+but the very word "Archetype" came to be regarded with suspicion. Thus
+even J. V. Carus, who had much affinity with the transcendentalists,
+wrote of the vertebrate archetype (which he took over almost bodily from
+Owen)--"It may here be observed that this schema may be used as a
+methodological help, but it is not to be placed in the foreground"
+(_loc. cit._, p. 395). Huxley, who was definitely a follower of von
+Baer, was much more outspoken with regard to ideal types. In an
+important memoir on the general anatomy of the Gastropoda and
+Cephalopoda,[318] he set himself the task of reducing all their complex
+forms to one type. In summing up, he writes:--"From all that has been
+stated, I think that it is now possible to form a notion of the
+archetype of the Cephalous Mollusca, and I beg it to be understood that
+in using this term, I make no reference to any real or imaginary 'ideas'
+upon which animal forms are modelled. All that I mean is the conception
+of a form embodying the most general propositions that can be affirmed
+respecting the Cephalous Mollusca, standing in the same relation to them
+as the diagram to a geometrical theorem, and like it, at once imaginary
+and true" (i., p. 176). Again, in his Croonian lecture on the theory of
+the vertebrate skull, he remarks that a general diagram of the skull
+could easily be given. "There is no harm," he continues, "in calling
+such a convenient diagram the 'Archetype' of the skull, but I prefer to
+avoid a word whose connotation is so fundamentally opposed to the spirit
+of modern science" (_Sci. Memoirs_, vol. i., p. 571).
+
+It is instructive to find that between Serres and Milne-Edwards there
+existed the same antagonism as between von Baer and the German
+transcendentalists. Milne-Edwards was a constant critic of the law of
+parallelism which Serres continued to uphold with little modification
+for over thirty years, just as von Baer was a critic of that form of the
+doctrine which was current in the early part of the century. As early as
+1833, Milne-Edwards, through his studies of crustacean development,[319]
+had come to the conclusion, independently of von Baer, that development
+always proceeded from the general to the special; that class characters
+appeared before family characters, generic characters before specific.
+In an interesting paper published in 1844,[320] he discussed the relation
+of this law of development to the problems of classification, and
+arrived at results almost identical with those set forth by von Baer in
+his Fifth Scholion.
+
+Like von Baer he rejected completely the theory of parallelism and the
+doctrine of the scale of beings; like von Baer he held that the type of
+organisation--of which there are several--is manifested in the very
+earliest stages and becomes increasingly specialised throughout the
+course of further development; like von Baer, too, he sketched a
+classification based upon embryological characters.
+
+These views were further developed in his volume of 1851, and also in
+his _Rapport_ of 1867.
+
+They brought him into conflict with his confrere in the Academy of
+Sciences, Etienne Serres, who in a number of papers published in the
+'thirties and 'forties,[321] and particularly in his comprehensive memoir
+of 1860, still maintained the theory of parallelism and the doctrine of
+the absolute unity of type. His memoir of 1860 shows how completely
+Serres was under the domination of transcendental ideas. Much of it
+indeed goes back to Oken. "The animal kingdom," he writes, "may be
+considered in its entirety as a single ideal and complex being" (p.
+141). His views have become a little more complicated since his first
+exposition of them in 1827, and he has been forced to modify in some
+respects the rigour of his doctrine. But he still holds fast to the main
+thesis of transcendentalism--the absolute unity of plan of all animals,
+vertebrate and invertebrate alike,[322] the gradual perfecting of
+organisation from monad to man, the repetition in the embryogeny of the
+higher animals of the "zoogeny" of the lower.
+
+He recognised, however, that the idea of a simple scale of beings is
+only an abstraction, and that the true repetition is of organs rather
+than of organisms. He was willing even to admit, at least in the later
+pages of his memoir, that there might be not one animal series but
+several parallel series, as had been suggested by Isidore Geoffroy St
+Hilaire (p. 749). In general, his views are now less dogmatic than they
+were in his earlier writings, but they are not for all that changed in
+any essential. For, in summing up his main results, he writes, "The
+whole animal kingdom can in some measure be regarded ideally as a single
+animal, which, in the course of formation and metamorphosis in its
+diverse manifestations, here and there arrests its own development, and
+thus determines at each point of interruption, by the very state it has
+reached, the distinctive characters of the phyla, the classes, families,
+genera, and species" (p. 833).[323]
+
+To settle the dispute pending between two of its most illustrious
+members, the Academy proposed in 1853, as the subject of one of its
+prizes, "the positive determination of the resemblances and differences
+in the comparative development of Vertebrates and Invertebrates." A
+memoir was presented the next year by Lereboullet[324] which met with the
+approval of the Academy in so far as its statements of fact were
+concerned, but seemed to them to require amplification in its
+theoretical part. But even in this memoir Lereboullet was able to show
+that the balance of evidence was greatly in favour of Milne-Edwards'
+views, and his general conclusions in 1854 were that "in the presence of
+such fundamental differences, one is obliged to give up the idea of one
+single plan in the formation of animals; while, on the contrary, the
+existence of diverse plans or types is clearly demonstrated by all the
+facts" (p. 79). To fulfil the Academy's requirements, Lereboullet
+continued his work, and in 1861-63 he published a series of elaborate
+monographs[325] on the embryology of the trout, the lizard and the
+pond-snail _Lymnaea_, and rounded off his work with a full discussion[326]
+of the theoretical questions involved. In this considered and
+authoritative judgment he completely disposed of Serres' theories of the
+unity of plan and the unity of genetic formation. Except in the very
+earliest stages of oogenesis there is no real similarity between the
+development of a Zoophyte, a Mollusc, an Articulate and a Vertebrate,
+but each is stamped from the beginning with the characteristics of its
+type. The lower animals are not, and cannot possibly be the permanent
+embryos of the higher animals. "The results which I have obtained," he
+writes, "are diametrically opposed to the theory of the zoological
+series constituted by stages of increasing perfection, a theory which
+tries to demonstrate in the embryonic phases of the higher animals a
+repetition of the forms which characterise the lower animals, and which
+has led to the assertion that the latter are permanent embryos of the
+former. The embryo of a Vertebrate shows the vertebrate type from the
+very beginning, and retains this type throughout the whole course of its
+development; it never is, and never can be, either a Mollusc or an
+Articulate" (xx., p. 54).
+
+"We are led to establish ... as the general result of our researches,
+the existence of several types, and, consequently, of different plans,
+in the development of animals. These different types are manifested from
+the very beginning of embryonic life; the characters distinguishing them
+are therefore primordial, and we can say with M. Milne-Edwards that
+_everything goes to prove that the distinction established by Nature
+between animals belonging to different phyla is a primordial
+distinction_" (p. 58).
+
+In other directions also von Baer's work was confirmed and extended by
+later observers--those parts of it particularly that had reference to
+the germ-layer theory, and to the concept of histological
+differentiation. His germ-layer theory was accepted in its main lines by
+Rathke, Bischoff and Lereboullet, and applied by them to the multitude
+of new facts they discovered. Rathke, in particular, was a firm upholder
+of the doctrine, and made considerable use of it in his writings.[327]
+Even before the publication of von Baer's book he had interpreted in
+terms of the germ-layer theory sketched by his friend Pander the
+splitting of the blastoderm which occurs in the early development of
+_Astacus_, whereby there are formed a serous and a mucous layer, one
+inside the other--like the coats of an onion, to use his own expressive
+phrase.[328]
+
+An ingenious application of the Pander-Baer theory was made by Huxley,
+who compared the outer and inner cell-layers which form the groundwork
+of the Coelentera with the serous and mucous layers of the vertebrate
+germ.[329] He laid stress, it is true, rather on the physiological than on
+the morphological resemblance. "A complete identity of structure," he
+writes, "connects the 'foundation membranes' of the Medusae with the
+corresponding organs in the rest of the series; and it is curious to
+remark, that throughout, the outer and inner membranes appear to bear
+the same physiological relation to one another as do the serous and
+mucous layers of the germ; the outer becoming developed into the
+muscular system, and giving rise to the organs of offence and defence;
+the inner, on the other hand, appearing to be more closely subservient
+to the purposes of nutrition and generation" (p. 24). Von Baer had
+already hinted at this homology in the second volume of his
+_Entwickelungsgeschichte_ (1837), where he says with reference to the
+separation of the blastoderm of the chick into two layers. "Yet
+originally there are not two distinct or even separable layers, it is
+rather the two surfaces of the germ which show this differentiation,
+just as polyps show the same contrast of an external surface and an
+internal digestive surface. In between the two layers there is in our
+germ as in the polyp an indifferent mass" (p. 67). The terms ectoderm
+and entoderm were introduced by Allman[330] in 1853 for the two
+cell-layers in the Hydrozoa.
+
+Remak is the second great name in the history of the germ-layer theory.
+He had the great advantage over von Baer of being able to make use of
+the cell-theory in interpreting the formation of the germ-layers.
+Microscopical technique also had been greatly improved since 1828.[331]
+
+Remak's greatest service was that he put the germ-layer theory in direct
+relation with the cell-theory by demonstrating the cellular continuity
+from egg-cell to tissue, and by showing that each germ-layer possessed
+distinctive histological characteristics. Hardly less important was his
+clear marking-off of the "middle layer" as a separate and distinct layer
+of the germ. He it was who introduced the modern conception of the
+mesoderm, and cleared up the confusion in which Pander and von Baer had
+left the organs formed between the serous and the mucous layer. Remak's
+middle layer was a different thing from Pander's ill-defined
+"vessel-layer"; it included and unified from a new point of view the
+"vessel" and "muscle" layers of von Baer.
+
+There are in the unincubated blastoderm of the chick, according to
+Remak,[332] two cell-layers, of which the undermost subsequently splits
+into two. Three layers are thus formed--the upper, middle and lower. The
+upper layer differentiates into a medullary plate and an epidermic plate
+(Remak's _Hornblatt_), and gives origin to the medullary tube with all
+its evaginations, and to the skin with all its derivatives and pockets.
+It forms such diverse structures as the brain, the spinal cord, the eye,
+the ear, the mouth, hairs, feathers, nails, sweat-glands, lacrymal
+glands, and so forth. All these parts are connected directly or
+indirectly with sensation, and the upper germ-layer may accordingly be
+called the _sensory_ layer. The lower layer gives rise to the epithelium
+and the proper tissue of the alimentary canal and its derivatives, as
+the liver, lungs, pancreas, kidneys, thyroid, thymus, etc. These parts
+are all concerned in the processes of assimilation and dissimilation,
+and the lower layer may accordingly be called the _trophic_ layer. Now
+between the upper or sensory layer and the lower or trophic layer there
+exists, in spite of their very different functions, a close histological
+likeness, for both are essentially epithelial layers. The resemblance is
+particularly strong if we compare the lower layer with the _Hornblatt_
+of the upper layer--both consist of epithelial tissue, and of its
+derivative, glandular tissue, and form neither vessels nor nerves. The
+middle layer, on the contrary, forms nerves and muscles, vessels and
+connective tissue, and little or no epithelium. It does not form all the
+blood-vessels without exception (and so cannot be called the
+vessel-layer), for the blood-vessels of the central nervous system are
+in all probability formed from the upper layer. So, too, it does not
+form all the nerves and muscles--the optic and auditory nerves and the
+nerves and muscles of the iris probably arise in the upper layer. But,
+in spite of these exceptions, its general histological character is so
+well defined that it may be contrasted with the other two as
+preeminently the layer that forms muscular, nervous, vascular and
+connective tissue. In view of its functional significance, it may be
+called the _motory_ layer, or better, since it forms also the sexual
+glands, the _motor-germinative_ layer. The middle layer, early in its
+history, shows a division into dorsal plates (_Urwirbelplatten_) and
+ventral plates (_Seitenplatten_). The former exhibit almost as soon as
+they are formed the characteristic proto-vertebral segmentation, the
+latter split to form the pleuro-peritoneal or body-cavity. Remak
+describes the latter process as follows:--"In the region of the trunk,
+where a greater independence of the fate of the alimentary canal and its
+annexes becomes necessary for the voluntary executive organs, the
+ventral plates undergo a process of splitting, leading to the formation
+of the sensitive part of the integument (the _Hautplatten_), the
+muscular part of the alimentary tube (the _Darmfaserplatten_), and the
+mother-tissue of the generative organs (the _Mittelplatten_). From the
+_Hautplatten_ there develops, without the dorsal plates seeming to take
+any part in the process, the rudiment of the extremities" (p. 79).
+
+[Illustration: FIG. 12.--Transverse Section of Chick Embryo. (After
+Remak.)]
+
+His _Darmfaserplatten_ form the nervous and muscular tissue of the
+alimentary canal and its dependencies, and also the heart; the
+_Hautplatten_ form the general body-wall (exclusive of the skin) and the
+appendages. In the embryo they line the amniotic cavity. The skeleton
+and peripheral nerves originate wholly within the middle layer.
+
+Remak's conception of the relations of the three germ-layers to one
+another and to the body-cavity is well illustrated in Fig. 12.
+
+In his germ-layer theory Remak's standpoint is histological rather than
+morphological. The distinction which he draws between the sensory and
+trophic layers on the one hand, and the motor-germinative layer on the
+other, is entirely a histological one. The greater part of his book,
+indeed, is devoted to a study of the histogenesis of the different
+organs of the body; he is bent chiefly upon unravelling the part which
+each germ-layer takes in the formation of each tissue and organ.
+
+His generalisation that two of the germ-layers give rise exclusively or
+almost exclusively to one kind of tissue excited great interest at the
+time, and gave the direction to histogenetic research for quite a number
+of years, though in the end it turned out to be insufficiently founded.
+
+Though Remak's germ-layer theory had thus principally a histological
+orientation, it laid down the main lines of the modern morphological
+treatment of the germ-layers.
+
+    [293] _Embryologie des Salmones_, 1842.
+
+    [294] _Die Cellularpathologie in ihrer Begruendung auf
+    physiologische und pathologische Gewebelehre_, Berlin,
+    2nd ed. 1859; Eng. trans., by Chance, 1860.
+
+    [295] _Arch. path. Anat. Phys_., vii., pp. 1-39 (1854).
+
+    [296] _Bericht ueber die Fortschritte der mikroskopischen
+    Anatomie im jahre 1854._ Mueller's _Archiv_, 1855. See
+    also 1856.
+
+    [297] _Hndb. d. Physiol._, i., 1835.
+
+    [298] See Leuckart's reply to Ludwig's criticism, in
+    _Zeit. f. wiss. Zool._, ii., p. 271, 1850.
+
+    [299] Leipzig, 1853.
+
+    [300] _Souvenirs d'un Naturaliste_, 2 vols., Paris, 1854.
+    Eng. Trans. as _Rambles of a Naturalist on the Coasts of
+    France, Spain, and Italy_, 2 vols., 1857.
+
+    [301] Milne-Edwards later published a classical textbook
+    on comparative anatomy and physiology--_Lecons sur la
+    Physiologie et l'Anatomie comparees_, 14 vols., Paris,
+    1857-80.
+
+    [302] Paris, 1834-40. Three volumes of the _Suites a
+    Buffon_.
+
+    [303] Paris, 1865. Two volumes of the _Suites a Buffon_.
+
+    [304] _U. d. Metamorphose der Ophiuren u. Seeigel._,
+    Berlin, 1848. _U. d. Metamorphose der Holothurien u.
+    Asterien._, Berlin, 1851.
+
+    [305] As I have been unable to obtain a copy of the
+    _Introduction_, the passages which follow are taken from
+    the _Rapport_ of 1867, where Milne-Edwards gives a
+    complete exposition of his doctrine, sometimes in the
+    words of the original.
+
+    [306] This principle was first developed by Milne-Edwards
+    in 1827, in the _Dictionnaire classique d'Hist.
+    naturelle_. It was probably suggested to him by his
+    studies on the Crustacea, among which the principle is
+    so beautifully exemplified in the concentration and
+    specialisation of the appendages and the ganglionic
+    chain.
+
+    [307] Studied by Isidore Geoffroy St Hilaire in his paper
+    _Classification parallelique des Mammiferes, C. R. Acad.
+    Sci._, xx., 1845. Remarked upon by Cuvier, _Regne
+    animal_., i., p. 171, 1817, also by de Blainville.
+
+    [308] Cuvier et Valenciennes, _Hist. nat. des Poissons_,
+    i., p. 550, 1828.
+
+    [309] _Myxinoiden_, Th. I. _Abh. k. Akad. Wiss. Berlin_
+    for 1834, pp. 100, 110, 179, etc.
+
+    [310] _Vergl. Entw. Kopf. nackt. Amphibien_, p. 101, 1838.
+
+    [311] I have not seen the companion volume on
+    palaeontological progression, _Unters. ue. d.
+    Entwickelungsgesetze der organischen Welt waehrend der
+    Bildungszeit unserer Erdoberflaeche_, Stuttgart, 1858.
+
+    [312] "Strobiloid" because of its spiral development. The
+    theory of the spiral growth of plants played an
+    important part in botanical morphology about this time.
+
+    [313] _Cf._ Meckel's Principle of progressive Evolution,
+    _supra_, p. 93.
+
+    [314] _System der thierischen Morphologie_, pp. 33, 457.
+    Also C. Bruch, _Die Wirbeltheorie des Schaedels, am
+    Skelette des Lachses geprueft_, Frankfort-on-Main, 1862.
+
+    [315] In France the vertebral theory was advocated by
+    Lavocat in his _Nouvelle Osteologie comparee de la tete
+    des animaux domestiques_, Toulouse, 1864. It seems also
+    that Lacaze-Duthiers held fast to it even in
+    1872--_Arch. zool. exp. gen._, i., p. 51, 1872.
+
+    [316] _An Essay on Classification_, Boston, 1857, London,
+    1859. He considered the classificatory categories to be
+    the categories of the Creator's thought, and hence
+    natural, and in no sense mere conventions.
+
+    [317] "Principes d'Embryogenie, de Zoogenie et de
+    Teratogenie," _Mem. Acad. Sci._, xxv., pp. 1-943, pls.
+    xxv., 1860.
+
+    [318] "On the Morphology of the Cephalous Mollusca,"
+    _Phil. Trans._, 1853, _Sci. Memoirs_, i., pp. 152-92.
+
+    [319] "Observations sur les changements de forme que les
+    divers Crustaces eprouvent," _Ann. Sci. nat._ (1) xxx.,
+    p. 360, 1833.
+
+    [320] "Considerations sur quelques principes relatifs a la
+    classification naturelle des animaux," _Ann. Sci. nat._
+    (3) i., p. 65, 1844.
+
+    [321] _Supra_, pp. 79-83. Also _Precis d'anatomie
+    transcendante, principes d'organogenie_, Paris, 1842.
+
+    [322] The inversion of the organs shown by Vertebrates as
+    compared with Invertebrates is due to the reversed
+    position of the embryo relatively to the yolk! (pp.
+    821-6).
+
+    [323] It is worth while recording that Serres enunciated a
+    "law of symmetry" according to which the embryo is
+    formed by the union of its two symmetrical halves--a law
+    which recalls the "concrescence theory" of His and some
+    modern embryologists.
+
+    [324] "Embryologie comparee du Brochet, de la Perche, et
+    de l'Ecrevisse," _Ann. Sci. nat._ (4), i., p. 237, 1854;
+    ii., p. 39, 1854. _Mem. Savans etrangers_, xvii.
+
+    [325] _Ann. Sci. nat._ (4) xvi., p. 113, 1861; xvii., p.
+    88, 1862; xviii., p. 5, 1862; xix., p. 5, 1863.
+
+    [326] xx., p. 5, 1863.
+
+    [327] Particularly in his _Blennius_ (1833) and _Natter_
+    (1839).
+
+    [328] In the "preliminary notice" of his Crayfish
+    paper--_Isis_, pp 1093-1100, 1825.
+
+    [329] "On the Anatomy and the Affinities of the Family of
+    the Medusae," _Phil. Trans._, 1849; _Sci. Memoirs_, i.,
+    pp. 9-32.
+
+    [330] _Phil. Trans._, cxliii., p. 368, 1853.
+
+    [331] The principle of achromatism was discovered (by
+    Fraunhofer) and achromatic microscopes introduced in the
+    early part of the 19th century. The use of chemical
+    reagents, such as acetic acid, and various hardening
+    fluids, came into fashion not long after. J. Mueller
+    seems to have been one of the first to realise their
+    importance. Remak himself invented one or two fixing and
+    hardening mixtures (pp. 87, 127, 1855), which enabled
+    him to cut excellent hand sections. Section-cutting
+    machines were not invented till later (V. Hensen, 1866,
+    His, 1870).
+
+    [332] _Untersuchungen ueber die Entwickelung der
+    Wirbelthiere_, folio, pp. xxxvii + 195, 12 plates,
+    Berlin, 1850-1855.
+
+
+
+
+CHAPTER XIII
+
+THE RELATION OF LAMARCK AND DARWIN TO MORPHOLOGY.
+
+
+It is a remarkable fact that morphology took but a very little part in
+the formation of evolution-theory. When one remembers what powerful
+arguments for evolution can be drawn from such facts as the unity of
+plan and composition and the law of parallelism, one is astonished to
+find that it was not the morphologists at all who founded the theory of
+evolution.
+
+It is true that the noticeable resemblances of animals to one another,
+the possibility of arranging them in a system, the vague perception of
+an all-pervading plan of structure, did suggest to many minds the
+thought that systematic affinities might be due to blood-relationship.
+Thus Leibniz considered that the cat tribe might possibly be descended
+from a common ancestor,[333] and another great philosopher, Immanuel Kant,
+was led by his perception of the unity of type to suggest as possible
+the derivation of the whole organic realm from one parent form, or even
+ultimately from inorganic matter. In the course of his masterly
+discussion of mechanism and teleology,[334] he writes, "The agreement of
+so many genera of animals in a certain common schema, which appears to
+be fundamental not only in the structure of their bones, but also in the
+disposition of their remaining parts--so that with an admirable
+simplicity of original outline, a great variety of species has been
+produced by the shortening of one member and the lengthening of another,
+the involution of this part and the evolution of that--allows a ray of
+hope, however faint, to penetrate into our minds, that here something
+may be accomplished by the aid of the principle of the mechanism of
+Nature (without which there can be no natural science in general). This
+analogy of forms, which with all their differences seem to have been
+produced according to a common original type, strengthens our suspicions
+of an actual relationship between them in their production from a common
+parent, through the gradual approximation of one animal-genus to
+another--from those in which the principle of purposes seems to be best
+authenticated, _i.e._, from man down to the polype, and again from this
+down to mosses and lichens, and finally to the lowest stage of Nature
+noticeable by us, viz., to crude matter."[335]
+
+So, too, Buffon's evolutionism was suggested by his study of the
+structural affinities of animals, and Erasmus Darwin in his _Zoonomia_
+(1794) brought forward as one of the strongest proofs of evolution, "the
+essential unity of plan in all warm-blooded animals."[336]
+
+But, as a matter of historical fact, no morphologist, not even Geoffroy,
+deduced from the facts of his science any comprehensive theory of
+evolution. The pre-Darwinian morphologists were comparatively little
+influenced by the evolution-theories current in their day, and it was in
+the anatomist Cuvier and the embryologist von Baer that the early
+evolutionists found their most uncompromising opponents.
+
+Speaking generally, and excepting for the moment the theory of Lamarck,
+we may say that the evolution-theories of the 18th and 19th centuries
+arose in connection with the transcendental notion of the _Echelle des
+etres_, or scale of perfection. This notion, which plays so great a part
+in the philosophy of Leibniz, was very generally accepted about the
+middle of the 18th century, and received complete and even exaggerated
+expression from Bonnet and Robinet. Buffon also was influenced by it.
+Towards the beginning of the 19th century the idea was taken up eagerly
+by the transcendental school and by them given, in their theories of the
+"one animal," a more morphological turn. Their recapitulation theory was
+part and parcel of the same general idea.
+
+One understands how easily the notion of evolution could arise in minds
+filled with the thought of the ideal progression of the whole organic
+kingdom towards its crown and microcosm, man. Their theory of
+recapitulation led them to conceive evolution as the developmental
+history of the one great organism.[337] Many of them wavered between the
+conception of evolution as an ideal process, as a _Vorstellungsart_, and
+the conception of it as an historical process. Bonnet, Oken, and the
+majority of the transcendentalists seem to have chosen the former
+alternative; Robinet, Treviranus, Tiedemann, Meckel, and a few others
+held evolution to be a real process.
+
+We have already in previous chapters[338] briefly noticed the relation of
+one or two of the transcendental evolution-theories to morphology, and
+there is little more to be said about them here. They had as good as no
+influence upon morphological theory, nor indeed upon biology in
+general.[339] It is different with the theory of Lamarck, which, although
+it had little influence upon biological thought during and for long
+after the lifetime of its author, is still at the present day a living
+and developing doctrine.
+
+Lamarck's affinity with the transcendentalists was in many ways a close
+one, but he differed essentially in being before all a systematist. Nor
+is the direct influence of the German transcendentalists traceable in
+his work--his spiritual ancestors are the men of his own race, the
+materialists Condillac and Cabanis, and Buffon, whose friend he was. The
+idea of a gradation of all animals from the lowest to the highest was
+always present in Lamarck's mind, and links him up, perhaps through
+Buffon, with the school of Bonnet. The idea of the _Echelle des etres_
+had for him much less a morphological orientation than it had even for
+the transcendentalists, for he was lacking almost completely in the
+sense for morphology. Lamarck's scientific, as distinguished from his
+speculative work, was exclusively systematic, and it was systematics of
+a very high order. He introduced many reforms into the general
+classification of animals. He was the first clearly to separate
+Crustacea (1799), and a little later (1800) Arachnids, from insects. He
+reduced to a certain orderliness the neglected tribes of the
+Invertebrates, and wrote what was for long the standard work on their
+systematics--the _Histoire naturelle des Animaux sans Vertebres_
+(1816-22). His speculative work on biology is contained in three
+publications, the small book entitled _Considerations sur l'organisation
+des corps vivants_ (1802), the larger work of 1809, the _Philosophie
+zoologique_, and the introductory matter to his _Animaux sans Vertebres_
+(vol. i., 1816).
+
+It is no easy matter to give in short compass an account of Lamarck's
+biological philosophy. He is an obscure writer, and often
+self-contradictory.
+
+In the first part of the _Philosophie zoologique_ Lamarck is largely
+pre-occupied with the problem of whether species are really distinct, or
+do not rather grade insensibly into one another. As a systematist of
+vast experience Lamarck knew how difficult it is in practice to
+distinguish species from varieties. "The more," he writes, "we collect
+the productions of Nature, the richer our collections become, the more
+do we see almost all the gaps filled up and the lines of separation
+effaced. We find ourselves reduced to an arbitrary determination, which
+sometimes leads us to seize upon the slightest differences of varieties,
+and form from them the distinctive character of what we call a species,
+and at other times leads us to consider as a variety of a certain
+species individuals a little bit different, which others regard as
+forming a separate species."[340]
+
+For Lamarck, as for Darwin later, the chief problem was not the
+evolution and differentiation of types of structure, but the mode of
+origin of species.
+
+Lamarck is at great pains to show how arbitrary are our determinations
+of species, and how artificial the classificatory groups which we
+distinguish in Nature. Strictly speaking, there are in Nature only
+individuals, "... this is certain, that among her products Nature has in
+reality formed neither classes, nor orders, nor families, nor genera,
+nor constant species, but only individuals which succeed one another and
+resemble those that produced them. Now, these individuals belong to
+infinitely diversified races, which shade into one another under all the
+forms and in all the degrees of organisation, and each of which
+maintains itself without change, so long as no cause of change acts upon
+it" (p. 41).
+
+But there is a natural order in the animal kingdom, a progression from
+the simpler to the more complex organisations, a natural _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.
+2. Polyps.
+3. Radiates.
+4. Worms.
+5. Insects.
+6. Arachnids.
+7. Crustacea.
+8. Annelids.
+9. Cirripedes.
+10. Molluscs.
+11. Fishes.
+12. Reptiles.
+13. Birds.
+14. Mammals.
+
+But the order of Nature is essentially continuous, and the limits of
+even the best defined of these classes are in reality artificial--"if
+the order of Nature were perfectly known in a kingdom, the classes which
+we should be forced to establish in it would always constitute entirely
+artificial sections" (p. 45).
+
+In the same way the lesser classificatory groups represent smaller
+sections of the one unique order of Nature. Note that Lamarck's
+_Echelle_ is in no way a morphological one, and was not intended to be
+such. It is a scale of increasing physiological differentiation, and the
+stages of it are marked by the acquirement of this or that new organ
+(_cf._ Oken). "Observation of their state convinces one that in order to
+produce them successively Nature has proceeded gradually from the
+simpler to the more complex. Now Nature, having had in mind the
+realisation of a plan of organisation which would permit of the greatest
+perfecting (that of the Vertebrates), a plan very different from those
+which she has been obliged to form as a preliminary to reaching it, one
+understands that, among the multitude of animals, one must necessarily
+come across not a single system of organisation which has become
+progressively perfected, but diverse very distinct systems, each of
+which has come into existence at the moment when each primary organ
+first put in its appearance" (p. 171).
+
+For Lamarck this order of Nature was not merely ideal--Nature had
+actually formed the classes successively, proceeding from the simpler to
+the more complex; she had brought about this evolution by transforming
+the primitive species of animals, raising them to higher degrees of
+organisation, and modifying them in relation to the environment in which
+they found themselves.
+
+Lamarck's theory of evolution is worked out in great detail in his
+_Philosophie zoologique_, but the exposition is diffuse and
+disconnected; it is better in giving an account of it to follow the more
+concise, mature and general exposition which he gives in the
+Introduction to his _Histoire naturelle des Animaux sans Vertebres_.[341]
+Near the beginning of the Introduction Lamarck gives us in a few short
+"Fundamental Principles" the main lines of his general philosophy. He is
+a confirmed materialist. Every fact and phenomenon is essentially
+physical and owes its existence or production entirely to material
+bodies or to relations between them. All change and all movement is in
+the last resort due to mechanical causes. Every fact or phenomenon
+observed in a living body is at once a physical fact or phenomenon and a
+product of organisation (p. 19). Life, thought and sensation are not
+properties of matter, but result from particular material combinations.
+
+His thorough-going materialism is most clearly shown in its relation to
+living things in the first three of the "Zoological Principles and
+Axioms," which are developed further on in the book.
+
+These are as follows:--"1. No kind or particle of matter can have in
+itself the power of moving, living, feeling, thinking, nor of having
+ideas; and if, outside of man, we observe bodies endowed with all or one
+of these faculties, we ought to consider these faculties as physical
+phenomena which Nature has been able to produce, not by employing some
+particular kind of matter which itself possesses one or other of these
+faculties, but by the order and state of things which she has
+constituted in each organisation and in each particular system of
+organs.
+
+"2. Every animal faculty, of whatever nature it may be, is an organic
+phenomenon, and results from a system of organs or an organ-apparatus
+which gives rise to it and upon which it is necessarily dependent.
+
+"3. The more highly a faculty is developed the more complex is the
+system of organs which produces it, and the higher the general
+organisation; the more difficult also does it become to grasp its
+mechanism. But the faculty is none the less a phenomenon of
+organisation, and for that reason purely physical" (p. 104).
+
+According to these "axioms" function is a direct and mechanical effect
+of structure.
+
+The curious thing is that in spite of his avowed materialism, Lamarck's
+conception of life and evolution is profoundly psychological, and from
+the conflict of his materialism and his vitalism (of which he was
+himself hardly conscious), arise most of the obscurities and the
+irreductible self-contradiction of his theory.
+
+Lamarck divided animals (psychologically!) into three great
+groups--apathetic or insensitive animals, animals endowed with
+sensation, and intelligent animals. The first group, which comprise all
+the lower Invertebrates, are distinguished from other animals by the
+fact that their actions are directly and mechanically due to the
+excitations of the environment; they have no principle of reaction to
+external influences, but passively prolong into action the excitations
+they receive from without. They are _irritable_ merely. The second group
+are distinguished from the first by their possessing, in addition to
+irritability, a power which Lamarck calls the _sentiment interieur_. He
+has some difficulty in defining exactly what he means by it:--"I have no
+term to express this internal power possessed not only by intelligent
+animals but also by those that are endowed merely with the faculty of
+sensation; it is a power which, when set in action by the feeling of a
+need, causes the individual to act at once, _i.e._, in the very moment
+of the sensation it experiences; and if the individual is of those that
+are endowed with intelligence it nevertheless acts in such a case
+entirely without premeditation and before any mental operation has
+brought its _will_ into play" (p. 24).
+
+It is the power we call instinct in animals (p. 25), and it implies
+neither consciousness nor will. It acts by transforming external into
+internal excitations.
+
+To this second group of animals, possessing the _sentiment interieur_,
+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 interieur_ the faculties of
+intelligence and will.
+
+Now, broadly put, Lamarck's theory of evolution is that new organs are
+formed in direct reaction to needs (_besoins_) experienced by the
+_sentiment interieur_. The _sentiment interieur_ is therefore the cause
+not only of instinctive action but also of all morphogenetic processes.
+Will and intelligence (which are confined to a relatively small number
+of animals) have little or nothing to do directly with evolution.
+
+To understand the working-out of Lamarck's evolution-theory we must
+revert to his conception of the _Echelle des etres_. What he wrote in
+the _Philosophie zoologique_ is here repeated in the work of 1816 with
+little modification.
+
+There is a real progression from the simpler to the more complex
+organisations; Nature has gradually complicated her creatures by giving
+them new organs and therefore new faculties.
+
+It is interesting to note that Lamarck expressly refers to Bonnet (p.
+110), but refuses to accept his view of an _Echelle_ extending down into
+the inorganic. Like Bonnet, however, and like the German
+transcendentalists, Lamarck makes man the goal of evolution (p. 116). He
+makes it quite clear that his _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 (_Anim. s. Vert._, p.
+134). To the latter cause are due the lacunae, the blind alleys, and the
+complications which the otherwise simple scale of perfection shows.
+
+To explain both these aspects of evolution Lamarck propounded in his
+volume of 1816 four laws, which read as follows:--
+
+"_First Law_.--Life, by its own forces, tends continually to increase
+the volume of every body possessing it, and to extend the dimensions of
+its parts, up to a limit which it brings about itself.
+
+"_Second Law_.--The production of a new organ in an animal body results
+from the arisal and continuance of a new need, and from the new movement
+which this need brings into being and sustains.
+
+"_Third Law_.--The degree of development of organs and their force of
+action are always proportionate to the use made of these organs.
+
+"_Fourth Law_.--All that has been acquired, imprinted or changed in the
+organisation of the individual during the course of its life is
+preserved by generation and transmitted to the new individuals that
+descend from the individual so modified" (pp. 151-2).
+
+It is mainly but not entirely by reason of the first of these laws that
+organisation tends to progress, and mainly by reason of the second and
+third that difference of environment brings about diversity of
+organisation. In virtue of the fourth law the acquirements of the
+individual become the property of the race.
+
+Lamarck's exposition of his first law, that life tends by its own powers
+to enlarge and extend its bodily instrument, is vague and difficult to
+understand. He has already explained some pages back how the first
+organisms arose by spontaneous generation in the form of minute
+gelatinous utricles (_cf._ Oken). He conceives that it is in the
+movements of the fluids proper to the organism that the power resides to
+enlarge and extend the body. Nutrition alone is not sufficient to bring
+about extension; a special force is required, acting from within
+outwards (p. 153). In the most primitive organisms the movements of the
+vital fluids are weak and slow, but in the course of evolution they
+gradually accelerate, and, becoming more rapid, trace out canals in the
+delicate tissue which contains them, and finally form organs.
+
+Subtle fluids play a great part in Lamarck's biology: they take the
+place of the soul or entelechy which the vitalists would postulate to
+explain organic happenings. Lamarck seems in this to follow certain of
+the old materialists, who conceived the soul to be formed of a matter
+more subtle than the ordinary.[342]
+
+In his second law Lamarck's essentially vitalistic attitude comes out
+very clearly, for it states that a psychological moment enters into all
+new production of form, that the ultimate cause of the development of
+new form is the need felt by the organism. This need is of course not a
+conscious one, it is a need perceived by the _sentiment interieur_.
+
+In the large group of apathetic or insensitive animals, which do not
+possess this faculty, needs cannot be experienced; accordingly new
+organs are here formed directly and mechanically, by the movements of
+the vital fluids set in action by excitations from without--the
+evolution, like the behaviour, of these animals is due to the direct and
+physical action of the environment. "But this is not the case with the
+more highly organised animals which possess _feeling_. They experience
+needs, and each need felt, acting upon their 'inner feeling,'
+immediately directs the fluids and the forces to the part of the body
+where action can satisfy the need. Now, if there exists at this point an
+organ capable of performing the required action, it is quickly
+stimulated to act; and if the organ does not exist and the need is
+pressing and sustained, bit by bit the organ is produced and developed
+in proportion to the continuity and the energy of its use" (p. 155).
+
+In intelligent animals the _sentiment 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 employed by
+Nature to diversify species, a means which comes into play whenever the
+environment changes. The cause of the great diversity shown by animal
+species is indeed ultimately to be sought in the environment. As the
+imperfect and earliest forms developed they spread over the earth and
+invaded the utmost corners of it:--"One can imagine what an enormous
+variety of habitats, stations, climates, available foods, environing
+media, etc., animals and plants have had to endure, as the existing
+species were forced to change their place of abode. And although these
+changes have taken place with extreme slowness ... their reality,
+necessitated by various causes, has none the less induced the species
+affected by them slowly to change their manner of life and their
+habitual actions. Through the effects of the second and third of the
+laws cited above, these induced activity-changes must have brought into
+being new organs, and must have been able to develop them further if
+more frequent use was made of them; they must in the same way have been
+capable of bringing about the degeneration and finally the complete
+disappearance of existing organs which had become useless" (p. 161).
+
+On the other hand, if the environment does not change, species remain
+constant.
+
+It is to be noted that change in environment is rather the occasion than
+the cause of modification; the environment induces the organism to
+change its habitual way of life; it sets up new needs, to satisfy which
+the organism must modify its structure. It is the organism that takes
+the active part in all this, the action of the environment is indirect.
+
+Of Lamarck's fourth law, which asserts the transmission of acquired
+characters, little need here be said in the way of exposition. Upon the
+truth of it depends of course Lamarck's whole theory. He himself never
+dreamed that anyone would ever dispute it.
+
+Lamarck sums up as follows:--"By the four laws which I have just
+enunciated all the facts of organisation seem to me to be easily
+explained; the progression in the complexity of organisation of animals,
+and in their faculties, seems to me easy to conceive; so, too, the means
+which Nature has employed to diversify animals, and bring them to the
+state in which we now see them, become easily determinable" (p. 168).
+
+It is never made quite clear, we may note in passing, how far his second
+and third laws tend to bring about an increase in complexity, in
+addition to diversifying animals.[343]
+
+"The function creates the organ," this would seem to be the kernel of
+Lamarck's doctrine. But how does he reconcile this essentially
+vitalistic conception with his strictly materialistic philosophy?
+
+We have seen that irritability, the _sentiment 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 _Philosophie zoologique_ and
+the introduction to the _Animaux sans Vertebres_ 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 modifications of the nervous
+fluids) upon the parts where a new organ is needed. But if it is itself
+only a result of the movement of nervous fluids? Again, how can a need
+be "felt" by a nervous fluid? This is an entirely psychological notion
+and cannot be applied to a purely material system. Whence arises the
+power of the _sentiment 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 _sentiment 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 same time to believe that new
+organs are formed in direct response to needs felt by the organism.
+Lamarck could never resolve this antinomy, and his speculations were
+thrown into confusion by it. To this cause is due the frequent obscurity
+of his writings.
+
+Should we be right in laying stress upon the psychological side of
+Lamarck's theory, and disregarding the materialistic dress in which,
+perhaps under the influence of the materialism current in his youth, he
+clothed his essentially vitalistic thought? Everything goes to prove
+it--his constant preoccupation with psychological questions, his tacit
+assimilation of organ-formation to instinctive behaviour, his constant
+insistence on the importance of _besoin_ and _habitude_.
+
+Let us not forget the profundity of his main idea, that, exception made
+for the lower forms, the animal is essentially active, that it always
+_reacts_ to the external world, is never passively acted upon. Let us
+not forget that he pointed out the essentially psychological moment
+implied in all processes of individual adaptation. With keen insight he
+realised that conscious intelligence counts for little in evolution, and
+focussed attention upon the unconscious but obscurely psychical
+processes of instinct and morphogenesis.
+
+Not without reason have the later schools of evolutionary thought, who
+developed the psychological and vitalistic side of his doctrine, called
+themselves Neo-Lamarckians.
+
+We shall say then that Lamarck, in spite of his materialism, was the
+founder of the "psychological" theory of evolution.
+
+Lamarck stood curiously aloof and apart from the scientific thought of
+his day.[344] He took no interest in the morphological problems that
+filled the minds of Cuvier and Geoffroy; he had indeed no feeling at all
+for morphology. He did not realise, like Cuvier, the _convenance des
+parties_, the marvellous co-ordination of parts to form a whole; he had
+little conception of what is really implied in the word "organism." He
+was not, like Geoffroy, imbued with a lively sense of the unity of plan
+and composition, and of the significance of vestigial organs as
+witnesses to that unity. He seems not to have known of the
+recapitulation theory, of which he might have made such good use as
+powerful evidence for evolution. Even with the German
+transcendentalists, with whom in the looseness of his generalisations he
+shows some affinity, he seems not to have been specially acquainted.
+
+He was interested more in the problems suggested to him by his daily
+work in the museum. He wanted to know why species graded so annoyingly
+into one another; he wanted to examine critically his haunting suspicion
+that species were really not distinct, and that classification was
+purely conventional. The question, too, of the adaptation of species to
+their environment, the problem of ecological adaptation, in distinction
+to that of functional adaptation which interested Cuvier so greatly,
+came vividly before him as he worked through the vast collections of the
+museum. He was the first systematist to occupy himself in a
+philosophical manner with the problems of general biology. He introduced
+new problems and a new way of looking at old. With Lamarck the problem
+of species and the problem of ecological adaptation enter into general
+biology.
+
+The one point in which he does definitely carry on the thought of his
+predecessors is his conception of the animal kingdom as forming a scale
+of (functional) perfection. He did not go to the same extreme as Bonnet;
+he did not even consider that the animal series was a continuation of
+the vegetable series; in his opinion they formed two diverging scales.
+He recognised, too, that among animals there was no simple and regular
+gradation from the lowest to the highest, but that the orderly
+progression was disturbed and diverted by the necessity of adaptation to
+different environments. It is interesting to note that in developing
+this idea he arrived at a roughly accurate distinction between
+homologous and analogous structures. More importance, he thought, was to
+be attributed in classifying animals to characters which appeared due to
+the "plan of Nature" than to such as were produced by an external
+modifying cause (p. 299). But he did not formulate the distinction in
+any strictly morphological way.
+
+As his ideas developed he laid less stress upon the simplicity and
+continuity of the scale; in his supplementary remarks to the
+Introduction of 1816 he admits that the series is really very much
+branched, and even that there may be two distinct series among animals
+instead of one. His last schema of the course of evolution shows no
+little analogy with the genealogical trees of Darwinian speculation. It
+is headed "The presumed _Order_ of the formation of Animals, showing two
+separate partly-branching series," and it reads as follows:--
+
+   I.--_Series of Non-articulated_    II.--_Series of Articulated_
+                 _Animals_.                      _Animals_.
+"
+I    |--         Infusoria.
+n    |               |
+s A  |             Polyps.
+e n  |               |
+n i  |        ----------------
+s m  |        |              |
+i a  |     Ascidians.     Radiates.                   Worms.
+t l  |        |                                         |
+i s  |        |                                  --------------
+v .  |        |                                  |            |
+e    |        |                                  |         Epizoa.
+"    |--      |                                  |            |
+              |                                  |            |
+"    |--      |                                  |            |
+S A  |     Acephala.                          Annelids.    Insects.
+e n  |        |                                               |
+n i  |        |                                               |
+s m  |     Molluscs.                                  -------------
+i a  |                                                |           |
+t l  |                                                |       Arachnids.
+i s  |                                            Crustacea.
+v .  |                                                |
+e    |                                                |
+"    |--                                          Cirripedes.
+
+I
+n    |--
+t A  |
+e n  |                          Fishes.
+l i  |                          Reptiles.
+l m  |                          Birds.
+i a  |                          Mammals.
+g l  |
+e s  |--
+n .
+t
+
+It is interesting to note that Vertebrates are placed between the two
+series, and are now not linked on directly to any Invertebrate group.
+
+Lamarck's theory had little success. There is evidence, however, that
+both Meckel and Geoffroy owed a good many of their evolutionary ideas to
+Lamarck, and Cuvier paid him at least the compliment of criticising his
+theory,[345] not distinguishing it, however, very clearly from the
+evolutionary theories of the transcendentalists. But, speaking
+generally, Lamarck's theory of evolution exercised very little influence
+upon his contemporaries. This was probably due partly to the obscurity
+and confusion of his thought, partly to his lack of sympathy with the
+biological thought of his day, which was preponderatingly morphological.
+
+It was not that men's minds were not ripe for evolution, for in the
+early decades of the 19th century evolution was in the air. There were
+few of von Baer's contemporaries who had not read Lamarck;[346] Erasmus
+Darwin's _Zoonomia_ ran through three editions, and was translated into
+German, French and Italian;[247] German philosophy was full of the idea of
+evolution.
+
+There was no unreadiness to accept the derivation of present-day species
+from a primordial form--if only some solid evidence for such derivation
+were forthcoming. Cuvier and von Baer, as we have seen, combated the
+current evolution theories on the ground that the evidence was
+insufficient, but von Baer at least had no rooted objection to
+evolution. In an essay of 1834, entitled _The Most General Law of Nature
+in all Development_,[348] von Baer expressed belief in a limited amount of
+evolution. In this paper he did not admit that all animals have
+developed from one parent form, and he refused to believe that man has
+descended from an ape; but, basing his supposition upon the facts of
+variability and upon the evidence of 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 himself to a belief in
+any extended or comprehensive process of evolution.
+
+Imbued as he was with the idea of development von Baer saw in evolution
+a process essentially of the same nature as the development of the
+individual. Evolution, like development, was due to a _Bildungskraft_ or
+formative force. The ultimate law of all becoming was that "the history
+of Nature is nothing but the history of the ever-advancing victory of
+spirit over matter" (p. 71). In a later essay (1835) in the same volume
+he says that all natural science is nothing but a long commentary on the
+single phrase _Es werde!_. (p. 86).
+
+As we shall see, von Baer adopted in later years the same attitude to
+Darwinism as he did to the evolution theories in vogue in his youth.
+
+Although in the twenty or thirty years before the publication of the
+_Origin of Species_ (1859) no evolution theory of any importance was
+published, and although the great majority of biologists believed in the
+constancy of species, there were not wanting some who, like von Baer,
+had an open mind on the subject, or even believed in the occurrence of
+evolutionary processes of small scope. Isidore Geoffroy St Hilaire, the
+son of the great Etienne Geoffroy St Hilaire, seems to have held that
+species might be formed from varieties. The law which L. Agassiz thought
+he could establish,[349] of the parallelism between palaeontological
+succession, systematic rank, and embryological development, tended to
+help the progress of evolutionary ideas. J. V. Carus, who afterwards
+became a supporter of Darwin, seems already, in 1853, to have inferred
+from Agassiz's law the probability of evolution.[350]
+
+But no evolution theory was taken very seriously before 1859, when the
+_Origin of Species_ was published.
+
+Like Lamarck, Charles Darwin was, neither by inclination nor by
+training, a morphologist. In his youth he was a collector, a sportsman
+and a field geologist. His voyage round the world on the _Beagle_
+aroused in him keen interest in the problem of species--their variety,
+their variation according to place and time, their adaptedness to
+environment. The conviction gradually took possession of his mind that
+the puzzling facts of geographical range and geological succession which
+he observed wherever he went were explicable only on the hypothesis that
+species change. He was not satisfied with the theories of evolution that
+had been proposed by his grandfather, by Lamarck, and by E. Geoffroy St
+Hilaire--he did not indeed understand these theories any too well. He
+resolved to work out the problem in his own way, for his own
+satisfaction. He tells us all this very clearly in his autobiography.
+"During the voyage of the _Beagle_ I had been deeply impressed by
+discovering in the Pampean formation great fossil animals covered with
+armour like that on the existing armadillos; secondly, by the manner in
+which closely allied animals replace one another in proceeding
+southwards over the continent; and thirdly, by the South American
+character of most of the productions of the Galapagos archipelago, and
+more especially by the manner in which they differ slightly on each
+island of the group; some of the islands appearing to be very ancient in
+a geological sense.
+
+"It was evident that such facts as these, as well as many others, could
+only be explained on the supposition that species gradually become
+modified; and the subject haunted me. But it was equally evident that
+neither the action of the surrounding conditions, nor the will of the
+organisms (especially in the case of plants) could account for the
+innumerable cases in which organisms of every kind are beautifully
+adapted to their habits of life--for instance, a woodpecker or a
+tree-frog to climb trees, or a seed for dispersal by hooks or plumes. I
+had always been much struck by such adaptations, and until these could
+be explained it seemed to me almost useless to endeavour to prove by
+indirect evidence that species have been modified."[351]
+
+All Darwin's varied subsequent work revolved round these, for him,
+essential problems--How do species change, and how do they become
+adapted to their environment? He never ceased to be essentially a field
+naturalist, and his theory of natural selection would have been an empty
+and abstract thing if his vast knowledge and understanding of the "web
+of life" had not given it colour and form. He never lost touch with the
+living thing in its living, breathing reality--even plants he rightly
+regarded as active things, full of tricks and contrivances for making
+their way in the world. No one ever realised more vividly than he the
+delicacy and complexity of the adaptations to environment which are the
+necessary condition of success in the struggle for existence. Almost his
+greatest service to biology was that he made biologists realise as they
+never did before the vast importance of environment. He took biology
+into the open air, away from the museum and the dissecting-room.
+
+Naturally this attitude was not without its drawbacks. It led him to
+take only a lukewarm interest in the problems of morphology. It is true
+he used the facts of morphology with great effect as powerful arguments
+for evolution, but it was not from such facts that he deduced his theory
+to account for evolution. It is questionable indeed whether the theory
+of natural selection is properly applicable to the problems of form. It
+was invented to account for the evolution of specific differences and of
+ecological adaptations; it was not primarily intended as an explanation
+of the more wonderful and more mysterious facts of the _convenance des
+parties_ and the interaction of structure and function. Perhaps Darwin
+did not realise this inner aspect of adaptation quite so vividly as he
+did the more superficial adaptation of organisms to their environment.
+It was, perhaps, his lack of morphological training and experience that
+led him to disregard the problems of form, or at least to realise very
+insufficiently their difficulty.
+
+It is in any case very significant that only a small part of his _Origin
+of Species_ is devoted to the discussion of morphological
+questions--only one chapter out of the fourteen contained in the first
+edition.
+
+Though the theory of natural selection took little account of the
+problems of form, Darwin's masterly vindication of the theory of
+evolution was of immense service to morphology, and Darwin himself was
+the first to point out what a great light evolution threw upon all
+morphological problems. In a few pages of the _Origin_ he laid the
+foundations of evolutionary morphology.
+
+We have here to consider his interpretation of morphological facts and
+its relation to the current morphology of his time.
+
+The sketch of his theory, written in 1842,[352] shows a very significant
+division into two parts--the first dealing with the positive facts of
+variability and the theory of natural selection, the second with the
+general evidence for evolution. It is in the second part that the
+paragraphs on morphological matters occur. In paragraph 7, on affinities
+and classification, Darwin points out that on the theory of evolution
+homological relationship would be real relationship, and the natural
+system would really be genealogical. In the next paragraph he notes that
+evolution would account for the unity of type in the great classes, for
+the metamorphosis of organs, and for the close resemblance which early
+embryos show to one another. It is of special interest to note that he
+definitely rejects the Meckel-Serres theory of recapitulation. "It is
+not true," he writes, "that one passes through the form of a lower
+group, though no doubt fish more nearly related to foetal state" (p.
+42). The greater divergence which adults show seems to him to be due to
+the fact that selection acts more on the later than on the embryonic
+stages. He realises very clearly how illuminative the theory of
+evolution is when applied to the puzzling facts of embryonic
+development. "The less differences of foetus--this has obvious meaning
+on this view: otherwise how strange that a horse, a man, a bat should at
+one time of life have arteries, running in a manner which is only
+intelligibly useful in a fish! The natural system being on theory
+genealogical, we can at once see why foetus, retaining traces of the
+ancestral form, is of the highest value in classification" (p. 45).
+
+Abortive organs, too, gain significance on the evolutionary hypothesis.
+"The affinity of different groups, the unity of types of structure, the
+representative forms through which foetus passes, the metamorphosis of
+organs, the abortion of others, cease to be metaphorical expressions and
+become intelligible facts" (p. 50).
+
+In general, organisms can be understood only if we take into account the
+cardinal fact that they are historical beings. "We must look at every
+complicated mechanism and instinct as the summary of a long history of
+useful contrivances much like a work of art" (p. 51).[353]
+
+Already in 1842 Darwin had seized upon the main principles of
+evolutionary morphology: the indications then given are elaborated in
+the thirteenth chapter of the _Origin of Species_ (1st ed., 1859). A
+good part of this chapter is given up to a discussion of the principles
+of classification, only a few pages dealing with morphology proper. But,
+as Darwin rightly saw, the two things are inseparable.
+
+We note first that there is no hint of the "scale of beings"--Darwin
+conceives the genealogical tree as many branched. Animals can be classed
+in "groups under groups," and cannot be arranged in one single series.
+
+He discusses first what kind of characters have the greatest
+classificatory value. Certain empirical rules have been recognised, more
+or less consciously, by systematists--that analogical characters are
+less valuable than homological, that characters of great physiological
+importance are not always valuable for classificatory purposes, that
+rudimentary organs are often very useful, and so on. He finds that as a
+general rule "the less any part of the organisation is concerned with
+special habits, the more important it becomes for classification" (p.
+414), and adduces in support Owen's remark that the generative organs
+afford very clear indications of affinities, since they are unlikely to
+be modified by special habits. These rules of classification can be
+explained "on the view that the natural system is founded on descent
+with modification; that the characters which naturalists consider as
+showing true affinity ... are those which have been inherited from a
+common parent, and, in so far, all true classification is genealogical;
+that community of descent is the hidden bond which naturalists have been
+unconsciously seeking, and not some unknown plan of creation, or the
+enunciation of general propositions, and the mere putting together and
+separating objects more or less alike" (p. 420).
+
+In general, then, homological characters are more valuable for
+classificatory purposes because they have a longer pedigree than
+analogical characters, which represent recent acquirements of the race.
+
+Coming to morphology proper, Darwin takes up the question of the unity
+of type, and the homology of parts, for which the unity of type is but a
+general expression.
+
+He treats this on the same lines as E. Geoffroy St Hilaire, and Owen,
+referring indeed specifically to Geoffroy's law of connections. "What
+can be more curious," he asks, "than that the hand of a man, formed for
+grasping, that of a mole for digging, the leg of a horse, the paddle of
+the porpoise, and the wing of the bat, should all be constructed on the
+same pattern, and should include similar bones, in the same relative
+positions? Geoffroy St Hilaire has strongly insisted on the high
+importance of relative position or connection in homologous parts; they
+may differ to almost any extent in form and size, and yet remain
+connected together in the same invariable order" (p. 434).
+
+The unity of plan cannot be explained on teleological grounds, as Owen
+has admitted in his _Nature of Limbs_, nor is it explicable on the
+hypothesis of special creation (p. 435). It can be understood only on
+the theory that animals are descended from one another and retain for
+innumerable generations the essential organisation of their ancestors.
+"The explanation is to a large extent simple on the theory of the
+selection of successive slight modifications--each modification being
+profitable in some way to the modified form, but often affecting by
+correlation other parts of the organisation. In changes of this nature,
+there will be little or no tendency to alter the original pattern or to
+transpose the parts.... If we suppose that the ancient progenitor, the
+archetype as it may be called, of all animals, had its limbs constructed
+on the existing general pattern, for whatever purpose they served, we
+can at once perceive the plain significance of the homologous
+construction of the limbs throughout the whole class" (p. 435).
+
+We may note three important points in this passage--first, the
+identification of the archetype with the common progenitor; second, the
+view that progressive evolution is essentially adaptive, and dominated
+by natural selection; and third, the _petitio principii_ involved in the
+assumption that adaptive modification brings inevitably in its train the
+necessary correlative changes.
+
+In his section on morphology Darwin shows clearly the influence of Owen,
+and through him of the transcendental anatomists. He refers to the
+transcendental idea of "metamorphosis," as exemplified in the vertebral
+theory of the skull and the theory of the plant appendage, and shows
+how, on the hypothesis of descent with modification, "metamorphosis" may
+now be interpreted literally, and no longer figuratively merely (p.
+439).
+
+Very great interest attaches to Darwin's treatment of development, for
+post-Darwinian morphology was based to a very large extent on the
+presumed relation between the development of the individual and the
+evolution of the race. Just as he kept clear of the notion of the scale
+of beings, so he avoided the snare of the Meckel-Serres theory of
+recapitulation, according to which the embryo of the highest animal,
+man, during its development climbs the ladder upon the rungs of which
+the whole animal series is distributed, in its gradual progression from
+simplicity to complexity. The law of development which he adopts is that
+of von Baer, which states that development is essentially
+differentiation, and that as a result embryos belonging to the same
+group resemble one another the more the less advanced they are in
+development. There can be little doubt that he was indebted to von Baer
+for the idea, and in the later editions of the _Origin_ he acknowledges
+this by quoting the well-known passage in which von Baer tells how he
+had two embryos in spirit which he was unable to refer definitely to
+their proper class among Vertebrates.[354]
+
+Not only are embryos more alike than adults, because less
+differentiated, but it is in points not directly connected with the
+conditions of existence, not strictly adaptive, that their resemblance
+is strongest (p. 440)--think, for instance, of the arrangement of aortic
+arches common to all vertebrate embryos. Larval forms are to some extent
+exceptions to this rule, for they are often specially adapted to their
+particular mode of life, and convergence of structure may accordingly
+result. All these facts require an explanation. "How, then, can we
+explain these several facts in embryology--namely, the very general, but
+not universal, difference in structure between the embryo and the
+adult--of parts in the same individual embryo, which ultimately become
+very unlike and serve for different purposes, being at this early period
+of growth alike--of embryos of different species within the same class,
+generally but not universally, resembling each other--of the structure
+of the embryo not being closely related to its conditions of existence,
+except when the embryo becomes at any period of life active and has to
+provide for itself--of the embryo apparently having sometimes a higher
+organisation than the mature animal, into which it is developed" (pp.
+442-3). Obviously all these facts are formally explained by the doctrine
+of descent. But Darwin goes further, he tries to show exactly how it is
+that the embryos resemble one another more than the adults. He thinks
+that the phenomenon results from two principles--first, that
+modifications usually supervene late in the life of the individual; and
+second, that such modifications tend to be inherited by the offspring at
+a corresponding, not early, age (p. 444).
+
+Thus, applying these principles to a hypothetical case of the origin of
+new species of birds from a common stock, he writes:--"... from the many
+slight successive steps of variation having supervened at a rather late
+age and having been inherited at a corresponding age, the young of the
+new species of our supposed genus will manifestly tend to resemble each
+other much more closely than do the adults, just as we have seen in the
+case of pigeons"[355] (pp. 446-7).
+
+Since the embryo shows the generalised type, the structure of the embryo
+is useful for classificatory purposes. "For the embryo is the animal in
+its less modified state; and in so far it reveals the structure of its
+progenitor" (p. 449)--the embryological archetype reveals the ancestral
+form. "Embryology rises greatly in interest, when we thus look at the
+embryo as a picture, more or less complete, of the parent form of each
+great class of animals" (p. 450)--a prophetic remark, in view of the
+enormous subsequent development of phylogenetic speculation.
+
+We may sum up by saying that Darwin interpreted von Baer's law
+phylogenetically.
+
+The rest of the chapter is devoted to a discussion of abortive and
+vestigial organs, whose existence Darwin naturally turns to great
+advantage in his argument for evolution. Throughout the whole chapter
+Darwin's preoccupation with the problems of classification is clearly
+manifest.
+
+On the question as to whether descent was monophyletic or polyphyletic
+Darwin expressed no dogmatic opinion. "I believe that animals have
+descended from at most only four or five progenitors, and plants from an
+equal or lesser number.... I should infer from analogy that probably all
+the organic beings which have ever lived on this earth have descended
+from one primordial form, into which life was first breathed" (p. 484).
+
+Darwin rightly laid much stress upon the morphological evidence for
+evolution,[356] which he considered to be weighty. It probably contributed
+greatly to the success of his theory. Though he himself did little or no
+work in pure morphology, he was alive to the importance of such work,[357]
+and followed with interest the progress of evolutionary morphology,
+incorporating some of its results in later editions of the _Origin_, and
+in his _Descent of Man_ (1871).
+
+In his morphology Darwin was hardly up to date. He does not seem to have
+known at first hand the splendid work of the German morphologists, such
+as Rathke and Reichert; he pays no attention to the cell-theory, nor to
+the germ-layer theory. His sources are, in the main, Geoffroy St
+Hilaire, Owen, von Baer, Agassiz, Milne-Edwards, and Huxley.
+
+Perhaps his greatest omission was that he did not give any adequate
+treatment of the problem of functional adaptation and the correlation of
+parts. It is not too much to say that Darwin not only disregarded these
+problems almost entirely, but by his insistence upon ecological
+adaptation and upon certain superficial aspects of correlation,
+succeeded in giving to the words "adaptation" and "correlation" a new
+signification, whereby they lost to a large extent their true and
+original functional meaning.
+
+It is true that Darwin himself, as well as his successors, believed that
+natural selection was all-powerful to account for the evolution of the
+most complicated organs, but it may be questioned whether he realised
+all the conditions of the problem of which he thus easily disposed. He
+says, rightly, in an important passage, that "It is generally
+acknowledged that all organic beings have been formed on two great
+laws--Unity of Type, and the Conditions of Existence. By unity of type
+is meant that fundamental agreement in structure which we see in organic
+beings of the same class, and which is quite independent of their habits
+of life. On my theory, unity of type is explained by unity of descent.
+The expression of conditions of existence, so often insisted upon by the
+illustrious Cuvier, is fully embraced by the principle of natural
+selection. For natural selection acts by either now adapting _the
+varying parts of each being to its organic and inorganic conditions of
+life_:[358] or by having adapted them during past periods of time: the
+adaptations being aided in many cases by the increased use or disuse of
+parts, being affected by the direct action of the external conditions of
+life, and subjected in all cases to the several laws of growth and
+variation. Hence, in fact, the law of the Conditions of Existence is the
+higher law; as it includes, through the inheritance of former variations
+and adaptations, that of Unity of Type" (_Origin_, 6th ed., Pop.
+Impression, pp. 260-1). It is clear that Darwin took the phrase
+"Conditions of Existence" to mean the environmental conditions, and the
+law of the Conditions of Existence to mean the law of adaptation to
+environment. But that is not what Cuvier meant by the phrase: he
+understood by it the principle of the co-ordination of the parts to form
+the whole, the essential condition for the existence of any organism
+whatsoever (see above, Chap. III., p. 34).
+
+Of this thought there is in Darwin little trace, and that is why he did
+not sufficiently appreciate the weight of the argument brought against
+his theory that it did not account for the correlation of variations.
+
+Darwin's conception of correlation was singularly incomplete. As
+examples of correlation he advanced such trivial cases as the relation
+between albinism, deafness and blue eyes in cats, or between the
+tortoise-shell colour and the female sex. He used the word only in
+connection with what he called "correlated variation," meaning by this
+expression "that the whole organisation is so tied together during its
+growth and development, that when slight variations in any one part
+occur, and are accumulated through natural selection, other parts become
+modified" (6th ed., p. 177). He took it for granted that the "correlated
+variations" would be adapted to the original variation which was acted
+upon by natural selection, and he saw no difficulty in the gradual
+evolution of a complicated organ like the eye if only the steps were
+small enough. "It has been objected," he writes, "that in order to
+modify the eye and still preserve it as a perfect instrument, many
+changes would have to be effected simultaneously, which, it is assumed,
+could not be done through natural selection; but as I have attempted to
+show in my work on the variation of domestic animals, it is not
+necessary to suppose that the modifications were all simultaneous, if
+they were extremely slight and gradual" (6th ed., p. 226).
+
+In post-Darwinian speculation the difficulty of explaining correlated
+variation by natural selection alone became more acutely realised, and
+it was chiefly this difficulty that led Weismann to formulate his
+hypothesis of germinal selection as a necessary supplement to the
+general selection theory.
+
+The change in the conception of correlation which Darwin's influence
+brought about has been very clearly stated by E. von Hartmann,[359] from
+whom the following is taken:--"While the correlation of parts in the
+organism was before Darwin regarded exclusively from the standpoint of
+morphological systematics, Darwin tried to look at it from the
+standpoint of physiological and genealogical development, and in so
+doing he put the standpoint of morphological systematics in the shade.
+But the more we are now beginning to realise that systematic
+relationship does not necessarily imply genetic affinity the more must
+the correlation of parts come back into favour as a systematic
+principle. While Darwin only, as it were, against his will, relied on
+the law of correlation as a last resort when all other help failed, this
+law must be regarded, from the standpoint of the orderly inner
+determination of all organic form-change, as having the rank of the
+highest principle of all, a principle which rules parallel, divergent
+and convergent evolution" (pp. 47-8).
+
+Further on, following 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. With morphological correlation in _different_ types Darwin
+troubles himself not at all, nor with correlation in the normal
+development of a type" (p. 49).
+
+Cuvier's conception of the _convenance des parties_, essential to all
+biology, remained on the whole foreign to Darwin's thought, and to the
+thought of his successors.
+
+It was indeed one of their boasts that they had finally eliminated all
+teleology from Nature. The great and immediate success which Darwinism
+had among the younger generation of biologists and among scientific men
+in general was due in large part to the fact that it fitted in well with
+the prevailing materialism of the day, and gave solid ground for the
+hope that in time a complete mechanistic explanation of life would be
+forthcoming. "Darwinismus" became the battle-cry of the militant spirits
+of that time.
+
+It was precisely this element in Darwinism that was repugnant to most of
+Darwin's opponents, in whose ranks were found the majority of the
+morphologists of the old school. They found it impossible to believe
+that evolution could have come about by fortuitous variation and
+fortuitous selection; they objected to Darwin that he had enunciated no
+real _Entwickelungsgesetz_, or law governing evolution. They were not
+unwilling to believe that evolution was a real process, though many drew
+the line at the derivation of man from apes, but they felt that if
+evolution had really taken place, it must have been under the guidance
+of some principle of development, that there must have been manifested
+in evolution some definite and orderly tendency towards perfection.[360]
+
+No one expressed this objection with greater force than did von Baer, in
+a series of masterly essays[361] which the Darwinians, through sheer
+inability to grasp his point of view, dismissed as the maunderings of
+old age. In these essays von Baer pointed out the necessity for the
+teleological point of view, at least as complementary to the
+mechanistic. His general position is that of the "statical"
+teleology--to use Driesch's term--of Kant and Cuvier. His attitude to
+Darwinism is determined by his teleology. He admits, just as in 1834, a
+limited amount of evolution; he criticises the evolution theory of
+Darwin on the same lines exactly as forty or fifty years previously he
+had criticised the recapitulation and evolution-theories of the
+transcendentalists--principally on the ground that their deductions far
+outrun the positive facts at their disposal. He rejects the theory of
+natural selection entirely, on the ground that evolution, like
+development, must have an end or purpose (_Ziel_)--"A becoming without a
+purpose is in general unthinkable" (p. 231); he points out, too, the
+difficulty of explaining the correlation of parts upon the Darwinian
+hypothesis. His own conception of the evolutionary process is that it is
+essentially _zielstrebig_ or guided by final causes, that it is a true
+_evolutio_ or differentiation, just as individual development is an
+orderly progress from the general to the special. He believed in
+saltatory evolution, in polyphyletic descent, and in the greater
+plasticity of the organism in earlier times.
+
+The idea of saltatory evolution he took from Koelliker, who shortly after
+the publication of the _Origin_ promulgated in a critical note on
+Darwinism a sketch of his theory of "heterogeneous generation."[362]
+
+Koelliker's attitude is typical of that taken up by many of the
+morphologists of the day.[363] He accepts evolution completely, but
+rejects Darwinism because it recognises no _Entwickelungsgesetz_, or
+principle of evolution. For the Darwinian theory of evolution through
+the selection of small fortuitous variations he would substitute the
+theory of evolution through sudden, large variations, brought about by
+the influence of a general law of evolution. This is his theory of
+heterogeneous generation. "The fundamental idea of this hypothesis is
+that under the influence of a general law of evolution creatures produce
+from their germs others which differ from them" (p. 181). It is to be
+noticed that Koelliker laid more stress upon the _Entwickelungsgesetz_
+than upon the saltatory nature of variation, for he says a few pages
+further on--"the notion at the base of my theory is that a great
+evolutionary plan underlies the development of the whole organised
+world, and urges on the simpler forms towards ever higher stages of
+complexity" (p. 184). Saltatory evolution was not the essential point of
+the theory:--"Another difference between the Darwinian hypothesis and
+mine is that I postulate many saltatory changes, but I will not and
+indeed cannot lay the chief stress upon this point, for I have not
+intended to maintain that the general law of evolution which I hold to
+be the cause of the creation of organisms, and which alone manifests
+itself in the activity of generation, cannot also so act that from one
+form others quite gradually arise" (p. 185). He put forward the
+hypothesis of saltatory variation because it seemed to him to lighten
+many of the difficulties of Darwinism--the lack of transition forms, the
+enormous time required for evolution, and so on. It should be noted that
+Koelliker regarded his principle of evolution as mechanical.
+
+It would take too long to show in detail how a belief in innate laws of
+evolution was held by the majority of Darwin's critics. A few further
+examples must suffice.
+
+Richard Owen, who in 1868[364] admitted the possibility of evolution, held
+that "a purposive route of development and change, of correlation and
+interdependence, manifesting intelligent Will, is as determinable in the
+succession of races as in the development and organisation of the
+individual. Generations do not vary accidentally, in any and every
+direction; but in pre-ordained, definite, and correlated courses" (p.
+808).
+
+He conceived change to have taken place by abrupt variation, independent
+of environment and habit, by "departures from parental type, probably
+sudden and seemingly monstrous, but adapting the progeny inheriting such
+modifications to higher purposes" (p. 797). He believed spontaneous
+generation to be a phenomenon constantly taking place, and constantly
+giving the possibility of new lines of evolution.
+
+E. von Hartmann in his _Philosophie des Unbewussten_ (1868) and in his
+valuable essay on _Wahrheit und Irrtum im Darwinismus_ (1874) criticised
+Darwinism in a most suggestive manner from the vitalistic standpoint. He
+drew attention to the importance of active adaptation, the necessity for
+assuming definite and correlated variability, and to the evidence for
+the existence of an immanent, purposive, but unconscious principle of
+evolution, active as well in phylogenetic as in individual development.
+
+In France H. Milne-Edwards[365] stated the problem thus:--"In the present
+state of science, ought we to attribute to modifications dependent on
+the action of known external agents the differences in the organic types
+manifested by the animals distributed over the surface of the globe
+either at the present day, or in past geological ages? Or must the
+origin of types transmissible by heredity be attributed to causes of
+another order, to forces whose effects are not apparent in the present
+state of things, to a creative power independent of the general
+properties of organisable matter such as we know them to-day?" (p. 426)
+
+He concluded that the action of environment, direct or indirect, was
+insufficient to account for the diversity of organic forms, and rejected
+Darwin's theory completely. He thought it likely that the successive
+faunas which 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 intervention of a hidden cause, of a power of some higher
+order, seems to him necessary" (p. 429).
+
+The criticism of Darwinism exercised by the older currents of thought
+remained on the whole without influence. It was under the direct
+inspiration of the Darwinian theory that morphology developed during the
+next quarter of a century.
+
+    [333] Radl, _loc. cit._, i., p. 71.
+
+    [334] _Kritik der Urtheilskraft_, 1790.
+
+    [335] Eng. Trans. by J. H. Bernard, p. 337, London, 1892.
+
+    [336] H. F. Osborn, _From the Greeks to Darwin_, p. 145,
+    New York and London, 1894.
+
+    [337] See Meckel, _supra_, p. 93; _cf._ Tiedemann,
+    _Zoologie_, p. 65, 1808. "Even as each individual
+    organism transforms itself, so the whole animal kingdom
+    is to be thought of as an organism in course of
+    metamorphosis." Also p. 73 of the same book.
+
+    [338] Chapters vii. and ix.
+
+    [339] On early evolution-theories see, in addition to
+    Osborn and Radl, J. Arthur Thomson, _The Science of
+    Life_, 1899, and the opening essay in _Darwin and Modern
+    Science_, Cambridge, 1909.
+
+    [340] _Phil. zool._, ed. Ch. Martins, vol. i., p. 75,
+    1873.
+
+    [341] Quotations in the text are from the 2nd Edit.
+    (Deshayes and Milne-Edwards), i., Paris, 1835.
+
+    [342] For instance, Lucretius:--
+
+    "Is tibi nunc animus quali sit corpore et unde
+    constiterit pergam rationem reddere dictis. Principio
+    esse aio persubtilem atque minutis perquam corporibus
+    factum constare."
+
+    --_De Rerum Natura_, iii., vv. 177-80.
+
+    [343] Contrast Treviranus--"In every living being there
+    exists a capability of an endless variety of
+    form-assumption; each possesses the power to adapt its
+    organisation to the changes of the outer world, and it
+    is this power, put into action by the change of the
+    universe, that has raised the simple zoophytes of the
+    primitive world to continually higher stages of
+    organisation, and has introduced a countless variety of
+    species into animate Nature." Quoted by Haeckel in
+    _History of Creation_, i., p. 93, 1876.
+
+    [344] There is no evidence that he was influenced by
+    Erasmus Darwin, who forestalled his evolution theory, and
+    was indeed more aware of its vitalistic implications. See
+    S. Butler, _Evolution, Old and New_, London, 1879, for an
+    excellent account of Erasmus Darwin.
+
+    [345] As did also Lyell in his _Principles of Geology_,
+    1830.
+
+    [346] K. E. von Baer, _Reden_, i., p. 37, Petrograd, 1864.
+
+    [347] Radl, _loc. cit._, i., p. 296.
+
+    [348] Reprinted in his _Reden_, i., 1864.
+
+    [349] See Huxley's criticism of it in a Royal Institution
+    lecture of 1851, republished in _Sci. Mem._, i., pp.
+    300-4. On its relation to Haeckel's biogenetic law, see
+    below, p. 255.
+
+    [350] _System der thierischen Morphologie_, p. 5, 1853.
+
+    [351] _Life and Letters of Charles Darwin_, ed. F. Darwin,
+    i., p. 82, 3rd ed., 1887.
+
+    [352] _The Foundations of the Origin of Species, a Sketch
+    written in 1842_. Ed. F. Darwin, Cambridge, 1909.
+
+    [353] _Cf._ a parallel passage in the _Origin_, 1st ed.,
+    pp. 485-6.
+
+    [354] In the 1st ed. (p. 439), Darwin makes the curious
+    mistake of attributing this story to Agassiz.
+
+    [355] In which nestlings of the different varieties are
+    much more alike than adults. Darwin attached much
+    importance to this idea, see _Life and Letters_, i., p.
+    88, and ii., p. 338.
+
+    [356] See his _Letters, passim_.
+
+    [357] Writing to Huxley on the subject of the latter's
+    work on the morphology of the Mollusca (1853), he
+    says:--"The discovery of the type or 'idea' (in your
+    sense, for I detest the word as used by Owen, Agassiz &
+    Co.) of each great class, I cannot doubt, is one of the
+    very highest ends of Natural History."--_More Letters_,
+    ed. F. Darwin and A. C. Seward, 1903, i., p. 73.
+
+    [358] Italics mine.
+
+    [359] _Das Problem des Lebens. Biologische Studien_. Bad
+    Sacha, 1906. See also E. Radl, _Biol. Centralblatt_,
+    xxi., 1901.
+
+    [360] See the excellent treatment of the difference
+    between the "realism" of Darwin and the "rationalism" of
+    his critics, in Radl, ii., particularly pp. 109, 135.
+    The most elaborate criticism of Darwinism from the older
+    standpoint was that given by A. Wigand in _Der
+    Darwinismus und die Naturforschung Newtons und Cuviers_,
+    3 vols., Braunschweig, 1872.
+
+    [361] In vol. ii. of his _Reden_, St Petersburg
+    (Petrograd), 1876--_Ueber den Zweck in den Vorgaengen der
+    Natur; Ueber Zielstrebigkeit in den organischen Koerpern
+    insbesondere_; and _Ueber Darwin's Lehre_.
+
+    [362] "Ueber die Darwinische Schoepfungstheorie," _Zeits.
+    f. wiss. Zool._, xiv., pp. 74-86, 1864. Elaborated in
+    _Anat. u. syst. Beschreibung d. Alcyonarien_, 1872.
+
+    [363] _Cf._ for instance Naegeli's theory of a perfecting
+    principle, first developed in his _Entstehung u. Begriff
+    der naturhistorischer Art_, Muenchen, 1865.
+
+    [364] _Anatomy of Vertebrates_, iii., 1868.
+
+    [365] _Rapport sur les Progres recents des Sciences
+    zoologiques en France_. Paris, 1867.
+
+
+
+
+CHAPTER XIV
+
+ERNST HAECKEL AND CARL GEGENBAUR
+
+
+At the time when Darwin's work appeared there already existed, as we
+have seen, a fully formed morphology with set and definite principles.
+The aim of this pre-evolutionary morphology had been to discover and
+work out in detail the unity of plan underlying the diversity of forms,
+to disentangle the constant in animal form and distinguish from it the
+accessory and adaptive. The main principle upon which this work was
+based was the principle of connections, so clearly stated by Geoffroy.
+The principle of connections served as a guide in the search for the
+archetype, and this search was prosecuted in two directions--first, by
+the comparison of adult structure; and second, by the comparative study
+of developing embryos. It was found that the archetype was shown most
+clearly by the early embryo, and this embryological archetype came to be
+preferred before the archetype of comparative anatomy. It became
+apparent also that the parts first formed (germ-layers) were of primary
+importance for the establishing of homologies.
+
+While practically all morphologists were agreed as to the main
+principles of their science, they yet showed, as regards their general
+attitude to the problems of form, a fairly definite division into two
+groups, of which one laid stress upon the intimate relation existing
+between form and function, while the other disregarded function
+completely, and sought to build up a "pure" or abstract morphology. In
+opposition to both groups, in opposition really to morphology
+altogether, a movement had gained strength which tended towards the
+analysis and disintegration of the organism. This movement took its
+origin in the current materialism of the day, and found expression
+particularly in the cell-theory and in materialistic physiology.
+
+The separation between morphology as the science of form and physiology
+as the science of the physics and chemistry of the living body had by
+Darwin's day become well-nigh absolute.
+
+The morphology of the 'fifties lent itself readily to evolutionary
+interpretation. Darwin found it easy to give a formal solution of all
+the main problems which pre-evolutionary morphology had set--he was able
+to interpret the natural system of classification as being in reality
+genealogical, systematic relationship as being really
+blood-relationship; he was able to interpret homology and analogy in
+terms of heredity and adaptation; he was able to explain the unity of
+plan by descent from a common ancestor, and for the concept of
+"archetype" to substitute that of "ancestral form."
+
+The current morphology, Darwin found, could be taken over, lock, stock
+and barrel, to the evolutionary camp.
+
+In what follows we shall see that the coming of evolution made
+surprisingly little difference to morphology, that the same methods were
+consciously or unconsciously followed, the same mental attitudes taken
+up, after as before the publication of the _Origin of Species_.
+
+Darwin himself was not a professional morphologist; the conversion of
+morphology to evolutionary ideas was carried out principally by his
+followers, Ernst Haeckel and Carl Gegenbaur in Germany, Huxley,
+Lankester, and F. M. Balfour in England.
+
+It was in 1866 that Haeckel's chief work appeared, a _General Morphology
+of Organisms_,[366] which was intended by its author to bring all
+morphology under the sway and domination of evolution.
+
+It was a curious production, this first book of Haeckel's, and
+representative not so much of Darwinian as of pre-Darwinian thought. It
+was a medley of dogmatic materialism, idealistic morphology, and
+evolution theory; its sources were, approximately, Buechner, Theodor
+Schwann, Virchow, H. G. Bronn, and, of course, Charles Darwin.
+
+It was scarcely modern even on its first appearance, and many regarded
+it, not without reason, as a belated offshoot of _Naturphilosophie_.
+
+Its materialism is of the most intransigent character. The form and
+activities of living things are held to be merely the mechanical result
+of the physical and chemical composition of their bodies. The simplest
+living things, the Monera, are nothing more than homogeneous masses of
+protein substance. "They live, but without organs of life; all the
+phenomena of their life, nutrition and reproduction, movement and
+irritability, appear here as merely the immediate outcome of formless
+organic matter, itself an albumen compound" (p. 63, 1906).
+
+Teleology, the Achilles' heel of Kant's (otherwise sound!) philosophy,
+is to be regarded as a totally refuted and antiquated doctrine,
+definitely put out of court by Darwinism.
+
+Haeckel works out his materialistic philosophy of living things very
+much after the fashion of Schwann. There is the same talk of cells as
+organic crystals, of crystal trees, of the analogy between assimilation
+by the cell and the growth of crystals in a mother liquid. Heredity and
+adaptation are shown equally as well by crystals as by organisms; for
+heredity, or the internal _Bildungstrieb_ (!), is the mechanical effect
+of the material structure of the crystal or the germ, and adaptation, or
+the external _Bildungstrieb_, is a name for the modifications induced by
+the environment. Adaptation so defined comes to be synonymous with the
+fortuitous variation which plays so great a part in Darwin's theory of
+natural selection.
+
+It goes without saying that Haeckel allowed to the organism no other nor
+higher individuality than belongs to the crystal, and took no account at
+all of that harmonious interaction of the organs which Cuvier called the
+principle of the "conditions of existence." The concept of correlation
+had simply no meaning for Haeckel. The analysis and disintegration of
+the organism was pushed by him to its logical extreme, and in this also
+he was a child of his time.
+
+A no less important influence clearly visible in the _General
+Morphology_ is the idealistic morphology of men like K. G. Carus and H. G.
+Bronn. In previous chapters we have seen how K. G. Carus attempted to
+work out a geometry of the organism, and how Bronn tried in a modest way
+to found a stereometrical morphology, but had the grace not to push his
+stereometry _a l'outrance_, recognising very wisely that the greater
+part of organic form is functionally determined. Haeckel took over this
+idea[367] and pushed it to wild extremes, founding a new science of
+"Promorphology" of which he was the greatest--and only--exponent.[368]
+
+This "science" dealt with axes and planes, poles and angles, in a
+veritable orgy of barbarous technical terms. It was intended to be a
+"crystallography of the organic," and to lay the foundations of a
+mechanistic morphology, or morphography at least.
+
+How it was to be linked up with the physics and chemistry of living
+matter on the one hand and with the ordinary morphology of real animals
+on the other, was never made quite clear.
+
+The science of Promorphology has no historical significance; it is
+interesting only because it illustrates Haeckel's close affinity with
+the idealistic morphologists.
+
+Another abortive science of Haeckel's, the science of Tectology, was
+equally a heritage from idealistic morphology. Tectology is the science
+of the composition of organisms from individuals of different orders.
+There were six orders of individuals:--(1) Plastids (Cytodes and cells);
+(2) Organs (including cell-fusions, tissues, organs, organ-systems); (3)
+Antimeres (homotypic parts, _i.e._, halves or rays); (4) Metameres
+(homodynamic parts, _i.e._, segments); (5) Persons (individuals in the
+ordinary sense); (6) Corms (colonial animals).
+
+The thought is essentially transcendental, and recalls the "theory of
+the repetition of parts," of which so much use was made by the German
+transcendentalists, such as Goethe,[369] Oken, Meckel and K. G. Carus, as
+well as by 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. We are interested only in the way he
+applied evolution to morphology, what modifications he introduced into
+the principles of the science, and in general in what way he interpreted
+the facts and theories of morphology in the light of the new knowledge.
+
+We find that he repeats very much what Darwin said, giving, of course,
+more detail to the exposition, and elaborating, particularly in his
+recapitulation theory or "biogenetic law," certain doctrines not
+explicitly stated by Darwin.
+
+Like Darwin he held that the natural system is in reality genealogical.
+"There exists," he writes, "one single connected natural system of
+organisms, and this single natural system is the expression of real
+relations which actually exist between all organisms, alike those now in
+being on the earth and those that have existed there in some past time.
+The real relations which unite all living and extinct organisms in one
+or other of the principal groups of the natural system, are
+genealogical: their relationship in form is blood-relationship; the
+natural system is accordingly the genealogical tree of organisms, or
+their genealogema.... All organisms are in the last resort descendants
+of autogenous Monera, evolved as a consequence of the divergence of
+characters through natural selection. The different subordinate groups
+of the natural system, the categories of the class, order, family,
+genus, etc., are larger or smaller branches of the genealogical tree,
+and the degree of their divergence indicates the degree of genealogical
+affinity of the related organisms with one another and with the common
+ancestral form" (ii., p. 420).
+
+The degree of systematic relationship is thus the degree of genealogical
+affinity. It follows that the natural system of classification may be
+converted straightway into a genealogical tree, and this is actually
+what Haeckel does in the _General Morphology_. The genealogical trees
+depicted in the second volume (plates i.-viii.) are nothing more than
+graphic representations of the ordinary systematic relationships of
+organisms, with a few hypothetical ancestral groups or forms thrown in
+to give the whole a genealogical turn.
+
+If the genealogical tree is truly represented by the natural system, it
+would seem that for each genus a single ancestral form must be
+postulated, for each group of genera a single more primitive form, and
+so in general for each of the higher classificatory categories, right up
+to the phylum. Species of one genus must be descended from a generic
+ancestral form, genera of one family from a single family _Urform_, and
+so on for the higher categories.
+
+This consequence was explicitly recognised by Haeckel. "Genera and
+families," he writes, "as the next highest systematic grades, are
+extinct species which have resolved themselves into a divergent bunch of
+forms (_Formenbueschel_)" (ii., p. 420).
+
+The archetype of the genus, family, order, class and phylum was thus
+conceived to have had at some past time a real existence.
+
+The natural system of classification is based upon a proper appreciation
+of the distinction between homological and analogical characters.
+Haeckel, following Darwin, naturally interprets the former as due to
+inheritance, the latter as due to adaptation, using these words, we may
+note, in their accepted meaning and not in the abstract empty sense he
+had previously attributed to them.[370] Similarly the "type of
+organisation," in von Baer's sense, was due to heredity, the "grade of
+differentiation" to adaptation.
+
+So far Haeckel merely emphasised what Darwin had already said in the
+_Origin of Species_. But by his statement of the "biogenetic law," and
+particularly by the clever use he made of it, Haeckel went a step beyond
+Darwin, and exercised perhaps a more direct influence upon evolutionary
+morphology than Darwin himself.
+
+Haeckel was not the original discoverer of the law of recapitulation. It
+happened that a few years before the publication of Haeckel's _General
+Morphology_, a German doctor, Fritz Mueller by name, stationed in Brazil,
+had been working on the development of Crustacea under the direct
+inspiration of Darwin's theory, and had published in 1864 a book[371] in
+which he showed that individual development gave a clue to ancestral
+history.
+
+He conceived that progressive evolution might take place in two
+different ways. "Descendants ... reach a new goal, either by deviating
+sooner or later whilst still on the way towards the form of their
+parents, or by passing along this course without deviation, but then
+instead of standing still advancing still farther" (Eng. trans., p.
+111). In the former case the developmental history of descendants agrees
+with that of the ancestors only up to a certain point and then diverges.
+"In the second case the entire development of the progenitors is also
+passed through by the descendants, and, therefore, so far as the
+production of a species depends upon this second mode of progress, the
+historical development of the species will be mirrored in its
+developmental history" (p. 112).
+
+Of course the recapitulation of ancestral history will be neither
+literal nor extended. "The historical record preserved in developmental
+history is gradually _effaced_ as the development strikes into a
+constantly straighter course from the egg to the perfect animal, and it
+is frequently _sophisticated_ by the struggle for existence which the
+free-living larvae have to undergo" (p. 114).
+
+It follows that "the primitive history of a species will be preserved in
+its developmental history the more perfectly the longer the series of
+young stages through which it passes by uniform steps; and the more
+truly, the less the mode of life of the young departs from that of the
+adults, and the less the peculiarities of the individual young states
+can be conceived as transferred back from later ones in previous periods
+of life, or as independently acquired" (p. 121).
+
+Applying these principles to Crustacea, he concluded that the shrimp
+_Peneus_ with its long direct development gave the best and truest
+picture of the ancestral history of the Malacostraca, and that
+accordingly the nauplius and the zoaea 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. Mueller.
+
+In much the same form as it was propounded by Mueller it was adopted by
+Haeckel, and made the corner-stone of his evolutionary embryology.
+Haeckel gave it more precise and more technical formulation, but added
+nothing essentially new to the idea.
+
+It is convenient to use his term for it--the biogenetic law
+(_Biogenetische Grundgesetz_)--to distinguish it from the laws of
+Meckel-Serres and von Baer, with which it is so often confused.
+
+Haeckel's statement of it may best be summarised in his own words,
+"Ontogeny, or the development of the organic individual, being the
+series of form-changes which each individual organism traverses during
+the whole time of its individual existence, is immediately conditioned
+by phylogeny, or the development of the organic stock (phylon) to which
+it belongs.
+
+"Ontogeny is the short and rapid recapitulation of phylogeny,
+conditioned by the physiological functions of heredity (reproduction)
+and adaptation (nutrition). The organic individual (as a morphological
+individual of the first to the sixth order) repeats during the rapid and
+short course of its individual development the most important of the
+form-changes which its ancestors traversed during the long and slow
+course of their palaeontological evolution according to the laws of
+heredity and adaptation.
+
+"The complete and accurate repetition of phyletic by biontic development
+is obliterated and abbreviated by secondary contraction, as ontogeny
+strikes out for itself an ever straighter course; accordingly, the
+repetition is the more complete the longer the series of young stages
+successively passed through.
+
+"The complete and, accurate repetition of phyletic by biontic
+development is falsified and altered by secondary adaptation, in that
+the bion[372] during its individual development adapts itself to new
+conditions: accordingly the repetition is the more accurate the greater
+the resemblance between the conditions of existence under which
+respectively the bion and its ancestors developed" (ii., p. 300).
+
+The last two propositions, it will be observed, are taken over almost
+verbally from F. Mueller.
+
+Now we have seen that the natural system of classification gives a true
+picture of the genealogical relationships of organisms, that the smaller
+and larger classificatory groups correspond to greater or lesser
+branches of the genealogical tree. If ontogeny is a recapitulation of
+phylogeny, we must expect to find the embryo repeating the organisation
+first of the ancestor of the phylum, then of the ancestor of the class,
+the order, the family and the genus to which it belongs. There must be a
+threefold parallelism between the natural system, ontogeny and phylogeny
+(ii., pp. 421-2).
+
+It will be observed that there is here implied an analogy between the
+biogenetic law and the law of von Baer, for both assert that development
+proceeds from the general to the special, that the farther back in
+development you go the more generalised do you find the structure of the
+embryo; both assert, too, that differentiation of structure takes place
+not in one progressive or regressive line, but in several diverging
+directions.
+
+But the analogy between the biogenetic law and the Meckel-Serres law is
+even more obvious, and the resemblance between the two is much more
+fundamental. It is a significant fact that in his theory of the
+threefold parallelism Haeckel merely resuscitated in an evolutionary
+form a doctrine widely discussed in the 'forties and 'fifties,[373] and
+championed particularly by L. Agassiz,[374] a doctrine which must be
+regarded as a development or expansion of the Meckel-Serres law.[375] It
+is the view that a parallelism exists between the natural system,
+embryonic development, and 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 arrangement of the animal kingdom was a mere perversion of
+reality, and that a branching arrangement of groups more truly
+represented the real relations of animals to one another, he could not
+of course entertain the Meckel-Serres theory in its original form. But
+he accepted the main tenet of it when he asserted that each stage of
+ontogeny had its counterpart in an adult ancestral form. Such ancestral
+forms might or might not be in existence as real species at the present
+day; they might or might not be discoverable as fossils. That they had
+real existence either now or at some past epoch Haeckel never doubted.
+In his construction of phylogenetic trees he was so confident in the
+truth of his biogenetic law that he largely disregarded and consistently
+minimised the importance of the evidence from 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 hypothetical, being deduced directly from a study of ontogeny and
+systematic relationships. The hypothetical ancestral forms which the
+theory thus postulated naturally took their place in the natural system,
+for they were merely the concrete projections or archetypes of the
+classificatory groups.
+
+The transcendentalists, of course, conceived evolution, whether real or
+ideal, as a uniserial process, whereas Haeckel conceived it as
+multiserial and divergent. It is here that the superficial agreement of
+the biogenetic law with the law of von Baer comes in.
+
+We might almost sum up the relation of the biogenetic law to the laws of
+von Baer and Meckel-Serres by saying that it was the Meckel-Serres law
+applied to the divergent differentiation upheld by von Baer instead of
+to the uniserial progression believed in by the transcendentalists.
+
+How near in practice Haeckel's law came to the recapitulation theory of
+the transcendentalists may be seen in passages like the following, with
+its partial recognition of the _Echelle_ idea:[276]--"As so high and
+complicated an organism as that of man ... rises upwards from a simple
+cellular state, and as it progresses in its differentiating and
+perfecting, it passes through the same series of transformations which
+its animal progenitors have passed through, during immense spaces of
+time, inconceivable ages ago.... Certain very early and low stages in
+the development of man, and other vertebrate animals in general,
+correspond completely in many points of structure with conditions which
+last for life in the lower fishes. The next phase which follows on this
+presents us with a change of the fish-like being into a kind of
+amphibious animal. At a later period the mammal, with its special
+characteristics, develops out of the amphibian, and we can clearly see,
+in the successive stages of its later development, a series of steps of
+progressive transformation which evidently correspond with the
+differences of different mammalian orders and families."[377]
+
+The biogenetic law went beyond both the Meckel-Serres law and the law of
+von Baer in that it recognised that the ancestral history of the species
+accounts in part for the course which the development of the individual
+takes, that in a certain sense, though not in the crude way supposed by
+Haeckel, phylogeny is the cause of ontogeny. This thought, that the
+organism is before all an historical being, is of course implied in the
+evolution idea, is indeed the essential core of it. Take away this
+element from the biogenetic law--not a difficult matter--and it becomes
+merely a law of idealistic morphology, applicable to evolution
+considered as an ideal process, as the progressive development in the
+Divine thought of archetypal models.
+
+As a book, the _General Morphology_ suffers a good deal from the arid,
+schematic, almost scholastic manner of exposition adopted. Haeckel's
+Prussian mania for organisation, for absolute distinctions, for
+iron-bound formalism, is here given full scope. A treatment less
+adequate to the variety, fluidity and changeableness of living things
+could hardly be imagined.
+
+His doctrine, though it remains essentially unchanged, receives in his
+later works a less formal and more concrete expression, and, in
+particular, his views on the biogenetic law undergo some small
+modification.
+
+Even in the _General Morphology_ Haeckel had recognised that ontogeny is
+neither a complete nor an entirely accurate recapitulation of phylogeny;
+he had admitted, following F. Mueller, that the true course of
+recapitulation was frequently modified by larval and foetal adaptations.
+As time went on, he was forced to hedge more and more on this point, and
+finally in his _Anthropogenie_ (1874) and his second paper on the
+Gastraea theory (1875),[378] he had to work out a distinction between
+palingenetic and cenogenetic characters, of which much use was made by
+subsequent writers.
+
+The distinction may be given in Haeckel's own words:--"Those ontogenetic
+processes," he writes, "which are to be referred immediately, in
+accordance with the biogenetic law, to an earlier completely developed
+_independent ancestral form_, and are transmitted from this by
+_heredity_, obviously possess _primary_ importance for the understanding
+of the casual-physiological relations; on the other hand, those
+developmental processes which appear subsequently through _adaptation_
+to the needs of embryonic or larval life, and accordingly can _not_ be
+regarded as repeating the organisation of an earlier independent
+ancestral form, can clearly have for the understanding of the ancestral
+history only a quite subordinate and _secondary_ importance.
+
+"The first I have named _palingenetic_, the second _cenogenetic_.
+Considered from this critical standpoint, the whole of ontogeny falls
+into two main parts:--First, _palingenesis_, or 'epitomised history'
+(_Auszugsgeschichte_), and second, _cenogenesis_, or 'counterfeit
+history' (_Faelschungsgeschichte_). The first is the true ontogenetic
+epitome or short recapitulation of past evolutionary history; the second
+is the exact contrary, a new foreign ingredient, a falsifying or
+concealing of the epitome of phylogeny."[379]
+
+As examples of palingenetic processes in the development of Amniotes,
+for instance, may be quoted the separation of two primary germ-layers,
+the formation of a simple notochord between medullary tube and
+alimentary canal, the appearance of a simple cartilaginous cranium, of
+the gill-arches and their vessels, of the primitive kidneys, the
+primitive tubular heart, the paired aortae and the cardinal veins, the
+hermaphroditic rudiment of the gonads, and so on. Cenogenetic processes,
+on the other hand, include such phenomena as the formation of yolk and
+the embryonic membranes, the temporary allantoic circulation, the navel,
+the curved and contracted shape of the embryo, and the like.
+
+The most important phenomena to be included under the general heading of
+cenogenesis are, first, the occurrence of food-yolk, and second, those
+anomalies of development which are classed by Haeckel as heterochronies
+and heterotopies.
+
+It is to the influence of the different amounts of yolk present in the
+egg that are due the great differences in the segmentation and
+gastrulation processes, which almost mask their true significance.
+
+Heterochronic processes are such as arise through the dislocation of the
+proper phylogenetic order of succession: heterotopic processes in the
+same way are caused by a wandering of cells from one germ-layer to
+another. The two classes of phenomena are disturbances either of the
+proper spatial or of the proper temporal relation of the parts during
+development.
+
+Heterochrony shows itself, as a rule, either as an acceleration or as a
+retardation of developmental events, as compared with their relative
+time of occurrence during phylogeny. Thus the notochord, the brain, the
+eyes, the heart, appear earlier in the ontogenetic than in the
+phylogenetic series, while, on the other hand, the septum of the
+auricles appears in the development of the higher Vertebrates before the
+ventricular septum, which is undoubtedly a reversal of the phylogenetic
+order.
+
+Cases of heterotopy, or of organs being developed in a position or a
+germ-layer other than that in which they originally arose in phylogeny,
+are not so easy to find. According to Haeckel, the origin of the
+generative products in the mesoderm is a heterotopic phenomenon, for he
+considers that they must have originated phylogenetically in one of the
+two primary layers, ectoderm or endoderm.
+
+It is worthy of note that the help of comparative anatomy is admittedly
+required in deciding what processes are palingenetic and what
+cenogenetic (p. 412).
+
+Haeckel's morphological notions, and particularly his biogenetic law,
+excited a good deal of adverse criticism from men like His, Claus,
+Salensky, Semper and Goette. Nor was his principal work, the _General
+Morphology_, received with much favour. Nevertheless, since he did
+express, though in a crude, dogmatic and extreme manner, the main
+hypotheses upon which evolutionary morphology is founded, his historical
+importance is considerable. He cannot perhaps be regarded as typical of
+the morphologists of his time--he was too trenchantly materialistic, too
+much the populariser of a crude and commonplace philosophy of Nature. In
+point of concrete achievement in the field of pure research he fell
+notably behind many of his contemporaries.
+
+His friend, Carl Gegenbaur, who gained a great and well-deserved
+reputation by his masterly studies on vertebrate morphology,[380] was a
+sounder man, and probably exercised a wider and certainly a more
+wholesome influence upon the younger generation of professional
+morphologists than the more brilliant Haeckel. It is true that in his
+famous _Grundzuege der vergleichenden Anatomie_, the second edition of
+which, published in 1870, soon came to be regarded as the classical
+text-book of evolutionary morphology, Gegenbaur enunciated very much the
+same general principles as Haeckel, and referred to the _Generelle
+Morphologie_ as the chief and fundamental work on animal morphology. But
+in Gegenbaur's pages the Haeckelian doctrines are modified and subdued
+by the strong commonsense and thorough appreciation of the older
+classical or Cuvierian morphology that characterise Gegenbaur's work.
+According to Haeckel,[381] Gegenbaur was greatly influenced by J. Mueller,
+who, as we know, laid as much stress on function as on form.
+
+The "General Part" of Gegenbaur's text-book is in many ways a
+significant document and deserves close attention.
+
+We note first of all that physiology and morphology are considered by
+Gegenbaur to be entirely distinct sciences, with different
+subject-matter and different methods. "The task of physiology is the
+investigation of the functions of the animal body or of its parts, the
+referring back of these functions to elementary processes and their
+explanation by general laws. The investigation of the material
+substratum of these functions, of the form of the body and its parts,
+and the explanation of this form, constitute the task of Morphology"
+(2nd ed., p. 3).
+
+Morphology falls naturally into two divisions--comparative anatomy and
+embryology. The method of comparative anatomy is _comparison_ (p. 6),
+and in employing this method account is to be taken of "the spatial
+relations of the parts to one another, their number, extent, structure,
+and texture." Through comparison one is enabled to arrange organs in
+continuous series, and it comes out very clearly during this proceeding
+"that the physiological value of an organ is by no means constant
+throughout the different form-states of the organ, that an organ,
+through the mere modification of its anatomical relations, can subserve
+very different functions. Exclusive regard for their physiological
+functions would place morphologically related organs in different
+categories. From this it follows that in comparative anatomy we should
+never in the first place consider the function of an organ. The
+physiological value comes only in the second place into consideration,
+when we have to reconstruct the relations to the organism as a whole of
+the modification which an organ has undergone as compared with another
+state of it. In this way comparative anatomy shows us how to arrange
+organs in series; within these series we meet with variations which
+sometimes are insignificant and sometimes greater in extent; they affect
+the extent, number, shape, and texture of the parts of an organ, and can
+even, though only in a slight degree, lead to alterations of position"
+(p. 6).
+
+Geoffroy St Hilaire would have subscribed to every word of this
+vindication of his "principle of connections."
+
+Between comparative anatomy and embryology there exists a close
+connection, for the one throws light on the other. "While in some cases
+the same organ shows only slight modifications in its development from
+its early beginnings to its perfect state, in other cases the organ is
+subjected to manifold modifications before it reaches its definitive
+form; we see parts appear in it which later disappear, we observe
+alterations in it in all its anatomical relations, alterations which may
+even affect its texture. This fact is of great importance, for those
+changes which an organ undergoes during its individual development lead
+through states which the organ in other cases permanently shows, or at
+the least the first appearance of the organ is the equivalent of a
+permanent state in another organism. If then the fully developed organ
+is in any special case so greatly modified that its proper relation to
+some organ-series is obscured, this relation may be cleared up by a
+knowledge of the organ's development. The earlier state indicated in
+this way enables one to find with ease the proper place for the organ
+and so insert it into an already known series. The relations which we
+observe in an organ-seriation are then the equivalent of processes which
+in certain cases take place in a similar manner during the individual
+development of an organ. Embryology enters therefore into the closest
+connection with comparative anatomy.... It teaches us to know organs in
+their earliest states, and connects them up with the permanent states of
+others, whereby they fill up the gaps which we meet with in the various
+series formed by the fully developed organs of the body" (pp. 6-7).
+
+This recognition of the parallelism between comparative anatomy and
+embryology is, of course, the kernel of the Meckel-Serres law. For
+Gegenbaur it had a very definite evolutionary meaning--he subscribed to
+the evolutionary form of it, the biogenetic law. How near his conception
+of the relation between ontogeny and phylogeny came to the old
+Meckel-Serres law may be gauged from the following passage, taken from a
+later work:--"Ontogeny thus represents, to a certain degree,
+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"[382] (p. 6).
+
+It is worth noting that in Gegenbaur's opinion comparative anatomy was
+prior in importance to embryology, that embryology could hardly exist as
+an independent science, since it must seek the interpretation of its
+facts always in the facts of comparative anatomy (_Grundzuege_, pp. 7-8).
+
+While Gegenbaur was at one with all "pure" morphologists, whether
+evolutionary or pre-evolutionary, in minimising as far as possible the
+importance of function in the study of form, he was too cautious and
+sober a thinker not to recognise the immense part which function really
+plays. Thus he classified organs, according to their function, into
+those that established relations with the external world and those that
+had to do with nutrition and reproduction, very much as Bichat had done
+before him.
+
+Like Darwin, Haeckel and most evolutionists, he interpreted the
+homological resemblances of animals as being due to heredity, their
+differences as due to adaptation,[383] but he did not adopt Haeckel's
+crude and shallow definition of these terms. For Gegenbaur heredity was
+a convenient expression for the fact of transmission, and was not
+explained offhand as the mere mechanical result of a certain material
+structure handed down from germ to germ. Adaptation he defined in a way
+which took the fullest account of function, and was as far as possible
+removed from Haeckel's definition of it as the direct mechanical effect
+of the environment upon the organism. "The organism is altered," writes
+Gegenbaur, "according to the conditions which influence it. The
+consequent _Adaptations_ are to be regarded as gradual, but steadily
+progressive, changes in the organisation, which are striven after during
+the individual life of the organism, preserved by transmission in a
+series of generations, and further developed by means of natural
+selection. What has been gained by the ancestor becomes the heritage of
+the descendant. Adaptation and Transmission are thus alternately
+effective, the former representing the modifying, the latter the
+conservative principle.... Adaptation is commenced by a change in the
+function of organs, so that the _physiological relations_ of organs play
+the most important part in it. Since adaptation is merely the material
+expression of this change of function, the modification of the function
+as much as its expression is to be regarded as a gradual process. In
+Adaptation, the closest connection between the function and the
+structure of an organ is thus indicated. Physiological functions govern,
+in a certain sense, structure; and so far what is morphological is
+subordinated to what is physiological" (_Elements_, pp. 8-9). Gegenbaur
+recognised also that morphological differentiation depended largely on
+the physiological division of labour (_Grundzuege_, p. 49).
+
+It is clear that Gegenbaur realised vividly the importance of function,
+and in this respect, as in others, he is far beyond Haeckel. The same
+thing comes out markedly in his treatment of correlation. Haeckel had no
+slightest feeling for the true meaning of correlation. For him, as for
+Darwin, it reduced itself to a law of correlative variation, according
+to which "actual adaptation not only changes those parts of the organism
+which are directly affected by its influence, but other parts also, not
+directly affected by it."[384] Such "correlative adaptation" was due to
+nutrition being a "connected, centralised activity."
+
+Gegenbaur, on the contrary, had a firm grasp of the Cuvierian
+conception, and expressed it in unmistakable terms. "As indeed follows
+from the conception of life as the harmonious expression of a sum of
+phenomena rigorously determining one another, no activity of an organ
+can in reality be thought of as existing for itself. Each kind of
+function (_Verrichtung_) presupposes a series of other functions, and
+accordingly every organ must possess close relations with, and be
+dependent on, all the others" (_Grundzuege_, p. 71). The organism must be
+regarded as an individual whole which is as much conditioned by its
+parts as one part is conditioned by the others. For an understanding of
+correlation a knowledge of functions, and of the functional relations of
+the organism to its environment, is clearly indispensable.
+
+Gegenbaur's morphological system was out-and-out evolutionary. "The most
+important part of the business of comparative anatomy," in Gegenbaur's
+eyes, "is to find indications of genetic connection in the organisation
+of the animal body" (_Elements_, p. 67).
+
+The most important clue to discovering this genetic connection is of
+course that given by homology; it is indeed the main principle of
+evolutionary morphology that what is common in organisation is due to
+common descent, what is divergent is due to adaptation. "Homology ...
+corresponds to the hypothetical genetic relationship. In the more or the
+less clear homology, we have the expression of the more or less intimate
+degree of relationship. Blood-relationship becomes dubious exactly in
+proportion as the proof of homologies is uncertain" (_Elements_, p. 63).
+
+It is worth noting that while Gegenbaur agrees with Haeckel generally
+that morphological relationships are really genealogical, that, for
+instance, each phylum has its ancestral form, he enters a caution
+against too hastily assuming the existence of a genetic relation between
+two forms on the basis of the comparison of one or two organs. "In
+treating comparative anatomy from the genealogical standpoint required
+by the evolution-theory," he writes, "we have to take into consideration
+the fact that the connections can almost never be discovered in the real
+genealogically related objects, for we have almost always to do with the
+divergent members of an evolutionary series. We derive, for instance,
+the circulatory system of insects from that of Crustacea ... but there
+exists neither a form that leads directly from Crustacea to insects nor
+any organisatory state (_Organisationszustand_), which as such shows the
+transition. Even when one point of organisation can be denoted as
+transitional, numerous other points prevent us from regarding the whole
+organism strictly in the same light" (_Grundzuege_, p. 75). The real
+ancestral forms cannot, as a rule, be discovered among living species,
+nor often as extinct. "When we arrange allied forms in series by means
+of comparison, and seek to derive the more complex from the simpler, we
+recognise in the lower and simpler forms only similarities with the
+ancestral form, which remains essentially hypothetical" (p. 75).
+
+The facts of development, Gegenbaur goes on to say, help us out greatly
+in our search for ancestral forms, for the early stages in the ontogeny
+of a highly organised animal give us some idea of the organisation of
+its original ancestor. Characters common to the early ontogeny of all
+the members of a large group are particularly important in this respect
+(_cf._ von Baer's law).
+
+Gegenbaur distinguishes homologous or morphologically equivalent
+structures from such as are analogous or physiologically equivalent,
+just as did Owen and the older anatomists. Like von Baer he recognises
+homologies, as a rule, only within the type.
+
+He contributed, however, to the common stock a useful analysis of the
+concept of homology, and established certain classes and degrees of it.
+He distinguished first between general and special homology, in quite a
+different sense from Owen.
+
+General homology, in Gegenbaur's sense, relates to resemblances of
+organs within the organism, and includes four kinds of resemblance,
+homotypy, homodynamy, homonomy and homonymy. Right and left organs are
+homotypic, metameric organs are homodynamic; homonomy is the relation
+exemplified by fin-rays or fingers, which are arranged with reference to
+a transverse axis of the body; homonymy is a sort of metamerism in
+secondary parts (not the main axis) of the body, and is shown by the
+various divisions of the appendages (_Grundzuege_, p. 80).
+
+Special homology, on the other hand, relates to resemblances between
+organs in different animals. The interesting thing is that Gegenbaur
+defines it genetically. Special homology is the name we give "to the
+relations which obtain between two organs which have had a common
+origin, and which have also a common embryonic history" (_Elements_, p.
+64). This is his definition; but, in practice, Gegenbaur establishes
+homologies by comparison just as the older anatomists did, and infers
+common descent from homology, not homology from common descent.
+
+"Special homology," he continues, "must be again separated into
+sub-divisions, according as the organs dealt with are essentially
+unchanged in their morphological characters, or are altered by the
+addition or removal of parts" (p. 65). In the former case the homology
+is said to be "complete," in the latter "incomplete." Thus the bones of
+the upper arm are completely homologous throughout all vertebrate
+classes from Amphibia upwards, while the heart of a fish is incompletely
+homologous with the heart of a mammal.
+
+Independently of Gegenbaur, Sir E. Ray Lankester proposed in 1870 a
+genetic definition of homology.[385] He proposed, indeed, to do away with
+the term homology altogether, on the ground that it included many
+resemblances which were obviously not due to common descent--as, for
+instance, the resemblance of metameres. So, too, organs which were
+homologous in the ordinary sense, as the heart of birds and mammals,
+might have arisen separately in evolution. He proposed, therefore, that
+"structures which are genetically related, in so far as they have a
+single representative in a common ancestor," should be called
+_homogenous_(p. 36). All other resemblances were to be called
+_homoplastic_. "Homoplasy includes all cases of close resemblance of
+form which are not traceable to homogeny, all details of agreement not
+homogenous, in structures which are broadly homogenous, as well as in
+structures having no genetic affinity" (p. 41). Serial homology, for
+instance, was a case of homoplasy.
+
+The term "analogy" was to be retained for cases of functional
+resemblance, whether homogenetic or not.
+
+The attempt was an interesting one, but most morphologists wisely
+adhered to the old concept of homology, in spite of Lankester's
+declaration that this belonged to an older "Platonic" philosophy, and
+ought to be superseded by a term more consonant with the new philosophy
+of evolution.
+
+    [366] _Generelle Morphologie der Organismen. Allgemeine
+    Grundzuege der organischen Formenwissenschaft, mechanisch
+    begruendet durch die von Ch. Darwin reformierte
+    Descendenztheorie_. Berlin, 1866. Reprinted in part as
+    _Prinzipien der generellen Morphologie der Organismen_.
+    Berlin, 1906.
+
+    [367] He mentions as his predecessors in this field,
+    Bronn, J. Mueller, Burmeister, and G. Jaeger.
+
+    [368] In _Grundriss einer Allgemeinen Naturgeschichte der
+    Radiolarien_, Berlin, 1887, and _Kunstformen der Natur_,
+    Suppl. Heft, Leipzig.
+
+    [369] Haeckel had an intense admiration for Goethe's
+    morphological work. It is a curious coincidence that the
+    work of Goethe, Oken and Haeckel was closely associated
+    with the town of Jena.
+
+    [370] But he himself would not admit this! See _Gen.
+    Morph._, ii., p. 11.
+
+    [371] _Fuer Darwin_, 1864. Eng. trans, by Dallas as _Facts
+    and Arguments for Darwin_, London, 1869.
+
+    [372] The bion is the physiological, as the morphon is the
+    morphological, individual.
+
+    [373] See Vogt, _Embryologie des Salmones_, p. 259, 1842,
+    and _supra_, p. 230.
+
+    [374] _An Essay on Classification_, London, 1859.
+
+    [375] It was hinted at by Tiedemann. "It is clear that,
+    proceeding from the earlier to the more recent strata, a
+    gradation in fossil forms can be established from the
+    simplest organised animals, the polyps, up to the most
+    complex, the mammals, and that accordingly the animal
+    kingdom as a whole has its developmental periods just
+    like the single individual organism. The species and
+    genera which have become extinct during the evolutionary
+    process may be compared with the organs which disappear
+    during the development of the individual animal" (p. 73,
+    1808).
+
+    [376] _The History of Creation_, vol. i., p. 310, 1876.
+    Translation of the _Natuerliche
+    Schoepfungsgeschichte_, 1868.
+
+    [377] _Cf._ a parallel passage from Serres, _supra_, p.
+    82.
+
+    [378] _Jenaische Zeitschrift_, ix., pp. 402-508, 1875.
+
+    [379] _Loc. cit._, ix., p. 409.
+
+    [380] _Untersuchungen zur vergl. Anatomie d.
+    Wirbelthiere_, Leipzig, i., 1864; ii., 1865; and iii.,
+    1872.
+
+    [381] "U. d. Biologie in Jena waehrend des 19
+    Jahrhunderts," _Jenaische Zeitschrift_, xxxix., pp.
+    713-26, 1905.
+
+    [382] _Grundriss der vergl. Anatomie_, 1874, 2nd ed.,
+    1878. Trans. by F. Jeffrey Bell, revised by E. Ray
+    Lankester, as _Elements of Comparative Anatomy_, London,
+    1878.
+
+    [383] "This theory (evolution) shows that what was
+    formerly called 'structural plan' or 'type' is the sum
+    of the dispositions (_Einrichtungen_) of the animal
+    organisation which are perpetuated by heredity, while it
+    explains the modifications of these dispositions as
+    adaptive states. Heredity and adaptation are thus the
+    two important factors through which both the unity and
+    the variety of organisation can be understood"
+    (_Grundzuege_, p. 19).
+
+    [384] _History of Creation_, i., pp. 241-2.
+
+    [385] "On the use of the term Homology in Modern Zoology,
+    and the distinction between Homogenetic and Homoplastic
+    agreements," _Ann. Mag. Nat. Hist._ (4), vi., pp. 35-43,
+    1870.
+
+
+
+
+CHAPTER XV
+
+EARLY THEORIES ON THE ORIGIN OF VERTEBRATES
+
+
+Haeckel and Gegenbaur set the fashion for phylogenetic speculation, and
+up to the middle 'eighties, when the voice of the sceptics began to make
+itself heard, the chief concern of the younger morphologists was the
+construction of genealogical trees. The period from about 1865 to 1885
+might well be called the second speculative or transcendental period of
+morphology, differing only from the first period of transcendentalism by
+the greater bulk of its positive achievement. It must be remembered that
+the later workers (at least towards the end of this period) had immense
+advantages over their predecessors in the matter of equipment and
+technique; they possessed well-fitted laboratories in the university
+towns and by the sea; they had at their command perfected microscopes
+and microtomes; while the whole new technique of microscopical anatomy
+with its endless variety of stains and reagents made it possible for the
+tyro to confirm in a day what von Baer and Mueller had taken weeks of
+painful endeavour to discover.[386] But the democratisation of morphology
+which followed upon the facilitation of its means of research left an
+evil heritage of detailed and unintelligent work to counterbalance the
+very great and real advances which technical improvements alone rendered
+possible.
+
+This period of rapid development, which set in soon after the coming of
+evolution and multiplied the concrete facts of morphology an
+hundredfold, may for our present purpose be conveniently divided into
+two somewhat overlapping periods, of which the second may be said to
+begin with the enunciation by Haeckel of his 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 Coelom theories.
+
+In the first period we might again distinguish two main tendencies,
+according as speculations were based mainly upon anatomical or mainly
+upon embryological considerations, and it so happens that these two
+tendencies are very well illustrated by the various theories as to the
+origin of Vertebrates which began to appear towards the 'seventies. We
+shall accordingly, in this chapter, consider very briefly the history of
+the earlier views on the phylogeny of the vertebrate stock.
+
+In the early days, before the other claimants to the dignity of
+ancestral form to the Vertebrates--_Balanoglossus_, Nemertines and the
+rest--had put in an appearance, there were two main views on the
+subject, one upheld by Haeckel, Kowalevsky and others, to the effect
+that the proximate ancestor of Vertebrates was a form somewhat
+resembling the ascidian tadpole, the other supported principally by
+Dohrn and Semper that Vertebrates and Arthropods traced their descent to
+a common segmented annelid or pro-annelid ancestor. The former view is
+historically prior, and arose directly out of the brilliant
+embryological investigations of A. Kowalevsky, who proved himself to be
+a worthy successor of the great comparative embryologist Rathke. His
+work was indeed a true continuation of Rathke's. It was not directly
+inspired by evolution, though it supplied much useful confirmation of
+the theory--you may read Kowalevsky's earlier memoirs and not realise
+that they were written several years after the publication of the
+_Origin of Species_.
+
+His first paper of evolutionary importance was a note in Russian on the
+development of Amphioxus, published in 1865. This subject was followed
+up in two papers which appeared in 1867[387] and 1877.[388] In his
+papers on Amphioxus Kowalevsky made out the main features in the
+development of this primitive form, and showed that the chief organs
+were formed in essentially the same way as in Vertebrates; he described
+the formation of the archenteron by invagination, the appearance of the
+medullary folds, which coalesced to form the neural canal, the formation
+of the notochord and of the gill-slits. At first he made the mistake of
+supposing that the body-cavity arose from the segmentation-cavity, but
+in his later paper he rightly surmised that it was formed from the
+cavities of the "primitive 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,[389] Kowalevsky's work at once made the development of
+Amphioxus the key to vertebrate embryology, the typical ontogeny with
+which all others could be compared.
+
+Meanwhile, in 1866 and 1871, Kowalevsky had communicated memoirs of even
+greater interest,[390] in which he showed that the simple Ascidians
+developed in an extraordinarily similar way to Amphioxus and hence to
+Vertebrates in general. His proof that Ascidians also develop on the
+vertebrate type aroused great interest at the time, and was naturally
+acclaimed by the evolutionists as a striking piece of evidence in favour
+of their doctrine. The systematic position of the Ascidians was at that
+time quite uncertain; they were grouped, as a rule, with the Mollusca,
+and certainly no one suspected that their well-known tailed 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 ectodermal folds just as it did in
+Amphioxus and Vertebrates, how gill-slits were formed in the walls of
+the pharynx, and how there existed in the ascidian larva a structure
+which in position and mode of development was the strict homologue of
+the vertebrate notochord. In his second paper he entered into much more
+detail, and published some excellent figures, often reproduced since
+(see Fig. 13), but the proof of the affinity between Vertebrates and
+Ascidians was in all essentials complete in his paper of 1866.
+
+[Illustration: FIG. 13.--Development of the Ascidian Larva. (After
+Kowalevsky.)]
+
+Kowalevsky's results were accepted by Haeckel, Gegenbaur, Darwin,[391]
+and many others as conclusive evidence of the origin of Vertebrates
+from a form resembling the ascidian tadpole; they were extended and
+amplified by Kupffer[392] in 1870, later by van Beneden and Julin[393]
+and numerous other workers; they were adversely criticised by
+Metschnikoff[394] and von Baer,[395] as well as by H. de
+Lacaze-Duthiers and A. Giard.[396] Lacaze-Duthiers and von Baer both
+held fast to the old view that Ascidians were directly comparable with
+Lamellibranch molluscs; they denied the homology of the ascidian
+nervous system with that of Vertebrates, von Baer being at great pains
+to show that the ascidian nerve-centre was really ventral in position.
+He pointed out also that the "notochord" was confined to the tail of
+the ascidian larva. Giard's attitude was by no means so
+uncompromising, and the criticisms he passed on the Kowalevsky theory
+are both subtle and instructive. He admits that there exists a real
+homology between, for instance, the notochord of Vertebrates and that
+of Ascidians. "But," he adds, "it is too often forgotten that homology
+does not necessarily mean an immediate common origin or close
+relationship. There exist, doubtless, homologies of great atavistic
+importance--I consider as such, for example, the formation of the
+cavity of Rusconi [the archenteron] in Ascidians and lower
+Vertebrates. But there are also adaptive and purely analogical
+homologies, such as the interdigital palmation of aquatic birds,
+amphibians and mammals. These are not purely analogous organs, for
+they can be superposed one on another, which is not the case with
+simply analogous structures (the bat's wing, for example, cannot be
+superposed on the bird's wing); they are homologous formations,
+resulting from the adaptation of the same fundamental organs to
+identical functions. Such is, in my opinion, the nature of the
+homology existing between the tail of the ascidian tadpole and that of
+Amphioxus or of young amphibians. The ascidian larva, having no cilia
+and being necessarily motile, requires for the insertion of its
+muscles or contractile organs ... a central flexible axis, a true
+chorda dorsalis analogous to that of Vertebrates" (pp. 278-9). This
+point of view is strengthened by the fact that in _Molgula_, studied
+by Lacaze-Duthiers, the embryo is practically stationary, and forms no
+notochord, nor ever develops sense-organs in the cerebral vesicle.
+
+Giard's general conclusion is that "the true homology with Vertebrates
+ceases after the formation of the cavity of Rusconi and the medullary
+groove: the homologies established by Kowalevsky for the notochord and
+the relations of the digestive tube and nervous systems are not
+atavistic, but adaptive, homologies" (p. 282). There is accordingly no
+close genetic relationship between Ascidians and Vertebrates.
+
+Giard's criticisms did not avail to check the vogue of the new theory,
+which soon became an accepted article of faith in most morphological
+circles.[397] The fall of the Ascidians from their larval high estate
+provided the text for many a Darwinian sermon.
+
+Some years after the genetic relationship of Ascidians and Vertebrates
+had been established, a rival theory of the origin of Vertebrates made
+its appearance--a theory which was practically a rehabilitation in a
+somewhat altered form of the old Geoffroyan conception that Vertebrates
+are Arthropods walking on their backs. This was the so-called Annelid
+theory of Dohrn and Semper. Both Dohrn and Semper started out from the
+fact that Annelids and Vertebrates are alike segmented animals, and it
+was an essential part of their theory that this resemblance was due to
+descent from a common segmented ancestor. Both laid great stress on the
+fact that the main organs in Vertebrates are arranged in the same way as
+in an Annelid lying on its back, the nervous system being uppermost, the
+alimentary system coming next, and below this the vascular.
+
+Dohrn's earlier views are contained in the fascinating little book
+published in 1875, which bears the title _Der Ursprung der Wirbelthiere
+und das Princip des Functionswechsel_ (Leipzig). He followed this up by
+a long series of studies on vertebrate anatomy and embryology,[398] in
+which he modified his views in certain details. We shall confine our
+attention to the first sketch of his theory.
+
+If the Vertebrate is conceived to have evolved from a primitive Annelid
+which took to creeping or swimming ventral surface uppermost, a
+difficulty at once arises with regard to the relative positions of the
+"brain" and the mouth. In Vertebrates the brain, like the rest of the
+nervous system, is dorsal to the mouth and the alimentary canal; in an
+inverted Annelid, however, the brain is ventral to the mouth and is
+connected with the dorsal nerve cord by commissures passing round the
+oesophagus. It would seem, therefore, that the primitive Vertebrate must
+have acquired either a new brain or a new mouth. Dohrn took the latter
+view. He supposed that the original mouth of the primitive ancestor lay
+between the _crura cerebelli_ in the _fossa rhomboidea_, and that in
+Vertebrates this mouth has been replaced functionally by a new ventrally
+placed mouth, formed by the medial coalescence of a pair of
+gill-slits.[399] Probably the two mouths at one period co-existed, and the
+older one was ousted by the growing functional importance of the newer
+mouth.
+
+The gill-slits were considered by Dohrn to be derived from the segmental
+organs of Annelids, which were present originally in every segment of
+the primitive ancestor. The gills were at first external, like the gills
+of many Chaetopods at the present day. For their support cartilaginous
+gill-arches naturally arose in the body-wall, and the superficial
+musculature became attached to these bars. "There existed in all the
+segments of the Annelid-ancestors of Vertebrates gills with
+cartilaginous skeleton and gill-arches in the body wall. Each gill had
+its veins and arteries, each had its branch of the ventral nerve-cord,
+and between each successive pair of gills a segmental organ opened to
+the exterior" (p. 14, 1875). The paired fins and limbs of the Vertebrate
+arose by the functional transformation of two pairs of these gills. The
+anterior gills became the definitive internal gills of the Vertebrate,
+for they gradually shifted into the mouths of the anterior segmental
+organs, which had already acquired an opening into the pharynx and had
+been transformed into true gill-slits. The posterior gills degenerated
+and disappeared, but their arches remained as ribs. Gill-arches and ribs
+were accordingly homologous structures and formed a _parietal_ skeleton.
+The vertebrate anus, like the mouth, was probably secondary and formed
+from a pair of gill-slits, the post-anal gut of vertebrate embryos
+hinting that the original anus was terminal as in Annelids. The unpaired
+fins of fish were originally paired and possibly arose from the
+coalescence of rows of parapodia. Dohrn assumed also that the primitive
+Annelid ancestor must have possessed a notochord to give support in
+swimming.
+
+If Vertebrates arose from primitive Annelid ancestors, how account for
+Amphioxus and the Ascidians, which seem to be the most primitive living
+Vertebrates and yet show no particular annelidan affinities? Dohrn tries
+to answer this awkward question by showing that these forms are not
+primitive but degenerate. He points out first that Cyclostomes are
+degenerate fish, half specialised and half degraded in adaptation to a
+parasitic mode of life. He thinks that if an _Ammocoetes_ were to become
+sexually mature and degenerate still further, forms would result which
+would resemble Amphioxus, and ultimately, if the process of degeneration
+went far enough, larval Ascidians. Amphioxus therefore might well be
+considered an extremely simplified and degenerate Cyclostome, and the
+ascidian larva the last term of this degeneration-series. Both Amphioxus
+and the Ascidians would accordingly be descended from fish, instead of
+fish being evolved from them.
+
+Dohrn conceived that the transformation of the Annelid into the
+Vertebrate took place mainly by reason of an important transforming
+principle, which he calls the principle of function-change. Each organ,
+Dohrn thinks, has besides its principal function a number of subsidiary
+functions which only await an opportunity to become active. "The
+transformation of an organ takes place by reason of the succession of
+the functions which one and the same organ possesses. Each function is a
+resultant of several components, of which one is the principal or
+primary function, while the others are the subsidiary or secondary
+functions. The weakening of the principal function and the strengthening
+of a subsidiary function alters the total function; the subsidiary
+function gradually becomes the chief function, the total function
+becomes quite different, and the consequence of the whole process is the
+transformation of the organ" (p. 60). Examples of function-change are
+not difficult to find. Thus the stomach in most Vertebrates performs
+both a chemical and a mechanical function, but in some forms a part of
+it specialises in the mechanical side of the work and becomes a gizzard,
+while the remaining part confines its energies to the secretion of the
+gastric juice. So, too, it is through function-change that certain of
+the ambulatory appendages of Arthropods have become transformed into
+jaws--their function as graspers of food has gradually prevailed over
+their main function as walking limbs. In the evolution of Vertebrates
+from Annelids the principle came into action in many connections--in the
+formation of a new mouth from gill-slits, in the transformation of gills
+into fins and limbs, of segmental organs into gill-slits, and so on.
+Dohrn tells us that the principle of function-change was suggested to
+him by Mivart's _Genesis of Species_ (1870), and he points out how it
+enables a partial reply to be made to the dangerous objection raised
+against the theory of natural selection that the first beginnings of new
+organs are necessarily useless in the struggle for existence.
+
+We may note in passing that a somewhat similar idea was later applied by
+Kleinenberg to the explanation of some of the ancestral features of
+development. He pointed out in his classical memoir on the embryology of
+the Annelid _Lopadorhynchus_[400] that many embryonic organs seem to be
+formed for the sole purpose of providing the necessary stimulus for the
+development of the definitive organs. Thus the notochord is the
+necessary forerunner of the vertebral column, cartilage the precursor of
+bone. "From this point of view," he writes, "many rudimentary organs
+appear in a different light. Their obstinate reappearance throughout
+long phylogenetic series would be hard to understand were they really no
+more than reminiscences of bygone and forgotten stages. Their
+significance in the processes of individual development may in truth be
+far greater than is generally recognised. When in the course of the
+phylogeny they have played their part as intermediary organs
+(_Vermittelungsorgane_) they assume the same function in the ontogeny.
+Through the stimulus or by the aid of these organs, now become
+rudimentary, the permanent parts of the embryo appear and are guided in
+their development; when these have attained a certain degree of
+independence, the intermediary organ, having played its part, may be
+placed upon the retired list."[401]
+
+Dohrn was well aware of the functional, or as he calls it, the
+physiological, orientation of his principle, and he rightly regarded
+this as one of its chief merits. He held that morphology became too
+abstract and one-sided if it disregarded physiology completely; he saw
+clearly that the evolution of function was quite as important a problem
+as the evolution of form, and that neither could be solved in isolation
+from the other. "The concept of function-change is purely
+physiological;" he writes, "it contains the elements out of which
+perhaps a history of the evolution of function may gradually arise, and
+for this very reason it will be of great utility in morphology, for the
+evolutionary history of structure is only the concrete projection of the
+content and course of the evolution of function, and cannot be
+comprehended apart from it" (p. 70).[402]
+
+It is very instructive in this connection to note that Dohrn was not,
+like so many of his contemporaries, a dogmatic materialist, but upheld
+the commonsense view that vital phenomena must, in the first instance at
+least, be accepted as they are. "It is for the time being irrelevant,"
+he writes, "to squabble over the question as to whether life is a result
+of physico-chemical processes or an original property (_Urqualitaet_) of
+all being.... Let us take it as given" (p. 75).
+
+Semper's speculations on the genetic affinity of Articulates and
+Vertebrates are contained in two papers[403] which appeared about the same
+time as Dohrn's. He openly acknowledges that his work is essentially a
+continuation of Geoffroy's transcendental speculations, and gives in his
+second paper a good historical account of the views of his great
+predecessor. It is a significant fact that evolutionary morphologists
+very generally held that Geoffroy was right in maintaining against
+Cuvier[404] the unity of plan of the whole animal kingdom, for they saw in
+this a strong argument for the monophyletic descent of all animals from
+one common ancestral form.
+
+In his first paper Semper does little more than break ground; he insists
+on the fact that both Annelids and Vertebrates are segmented animals,
+and he points out how close is the analogy between the nephridia or
+"segmental organs" of the former and the excretory (mesonephric) tubules
+of the latter, upon which he published in the same volume an extensive
+memoir. At this time he considered _Balanoglossus_--by reason of its
+gill-slits (its notochord he did not know)--to be the nearest living
+representative of the ancestral form of Vertebrates and Annelida.
+
+His second paper is a more exhaustive piece of work and deals with every
+aspect of the problem, both from an anatomical and from an embryological
+standpoint. It is consciously and admittedly an attempt to apply
+Geoffroy's principle of the unity of plan and composition to the three
+great metameric groups, the Annelida, Arthropoda, and Vertebrata. Semper
+follows Geoffroy's lead very closely in maintaining that it is not the
+position of the organs relative to the ground that must be taken into
+account in establishing their homologies, but solely their spatial
+relations one to another. He holds that dorsum and venter are terms of
+purely physiological import, and he proposes to substitute for them the
+terms neural and cardial (better, 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
+bigemina_), the dorsal muscle-plates forming a tube above the notochord,
+the ventral plates a tube below the notochord, whereas Articulates do
+not possess this axis, and form only one tube, namely, that round the
+"vegetative" organs (_evolutio gemina_). Semper is at pains to prove
+that _evolutio bigemina_ is characteristic also of Annelidan
+development.
+
+[Illustration: FIG. 14.--Transverse Section (Inverted) of the Worm
+_Nais_. (After Semper.)]
+
+He gets his facts from an elaborate study of the process of budding in
+the _Naidae_, making the somewhat risky assumption that regeneration
+takes essentially the same course as embryonic development.
+
+He succeeds in showing--to his own satisfaction at least--that in the
+formation of new segments in _Nais_ and _Chaetogaster_ a strand of cells
+appears between the alimentary canal and the nerve-cord, and that from
+this axial strand the 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
+_Chaetogaster_ 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 _Chaetogaster_, which open from the pharynx to the exterior.
+In Sabellids he describes an elaborate system of gill-canals, with a
+supporting cartilaginous framework which forms a real _Kiemenkorb_ or
+gill-basket, comparable with that of Amphioxus.
+
+Gill-slits, notochord, relation of nervous system, mesonephric tubules,
+are thus common to Annelids and Vertebrates--what further proof could
+one desire of the close relationship of these groups? Yet Semper enters
+into refinements of comparison, seeing, for instance, in the lateral
+portions of the ventral ganglia (Fig. 14, _sp. g._) the homologues of
+the spinal ganglia of Vertebrates, and comparing the lateral line of
+sense organs in Annelids with the lateral line in Anamnia.
+
+He will not admit that Amphioxus and the Ascidians show a closer
+resemblance to Vertebrates than his beloved Annelids. Amphioxus, he
+thinks, is not a Vertebrate, and Ascidians, though sharing with Annelids
+the possession of a notochord, gill-slits, and a "dorsal" nervous
+system, yet are further removed from Vertebrates than the latter by
+reason of their lacking that essential characteristic of Vertebrates,
+metameric segmentation.
+
+Not content with establishing the unity of plan of Annelids, Arthropods,
+and Vertebrates, Semper tries to link on the Annelids, as the most
+primitive group of the three, to the unsegmented worms, and particularly
+to the Turbellaria. His speculations on this matter may be summed up
+somewhat as follows:--The common ancestor of all segmented animals is a
+segmented worm-like form, not quite like any existing type, resembling
+the Turbellaria in having two nerve strands on the dorsal side and no
+oesophageal ring, potentially able to develop either the Vertebrate or
+the Annelid mouth, and so to give origin both to the Articulate and to
+the Vertebrate series. The common ancestor alike of unsegmented worms
+and of all segmented types is probably the trochosphere larva, which in
+the Vertebrates is represented by the simple _Keimblase_ or blastula.
+
+The Annelid theory of Dohrn and Semper was perhaps not so widely
+accepted as the rival Ascidian theory, but it counted not a few
+adherents and gave a certain stimulus to comparative morphology. F. M.
+Balfour, who pointed out about the same time as Semper the analogy
+between the nephridia of Annelids and the mesonephric tubules of
+Vertebrates,[405] while not accepting the actual theories of Dohrn and
+Semper, took up a distinctly favourable attitude to the general idea
+that Annelids and Vertebrates were descended from a common segmented
+ancestor. Discussing this question in his classical work on the
+development of Elasmobranch fishes,[406] Balfour came to the conclusion
+"that we must look for the ancestors of the Chordata, not in allies of
+the present Chaetopoda, but in a stock of segmented forms descended from
+the same unsegmented types as the Chaetopoda, but in which two lateral
+nerve-cords, like those of Nemertines, coalesced dorsally instead of
+ventrally to form a median nervous cord. This group of forms, if my
+suggestion as to their existence is well founded, appears now to have
+perished."[407]
+
+He held that while there was much to be said for the interchange of
+dorsal and ventral surfaces postulated by Dohrn and Semper, the
+difficulties involved in the supposition were too great; he preferred,
+therefore, to assume that the present Vertebrate mouth was primitive,
+and not a secondary formation.
+
+His views as to the phylogeny of the Chordata and the genetic relation
+of the various classes to one another are exhibited in the following
+schema,[408] names of hypothetical groups being printed in capitals, names
+of degenerate groups in italics:--
+
+
+ Mammalia.                   Sauropsida.
+    |                            |
+    |____________________________|
+                    |
+               Proto-Amniota.         Amphibia.
+                    |                     |
+                    |_____________________|
+                             |
+                  Proto-Pentadactyloidei.
+                             |
+      Teleostei.             |
+          |                  |
+       Ganoidei.             |____________Dipnoi
+          |                  |
+          |__________________|
+                             |
+                       Proto-Ganoidei.
+                             |
+                             |____________Holocephali.
+                             |
+                             |____________Elasmobranchii.
+                             |
+                    Proto-Gnathostomata.
+                             |
+         ____________________|
+         |                   |
+   _Cyclostomata_.           |
+                             |
+                             |
+                     Proto-Vertebrata.
+                             |
+                             |
+                             |
+                             |
+         ____________________|______________________
+         |                                         |
+  _Cephalochorda_.    Protochordata.          _Urochorda_.
+
+
+The hypothetical ancestral forms (Protochordata) possessed a notochord,
+a ventral suctorial mouth and numerous gill-slits, and were presumably
+descended from the common ancestor of Annelids and Vertebrates.
+Amphioxus and the Ascidians found their place in this schema as
+degenerate offshoots of the ancestral Protochordates, while the
+Cyclostomes were in the same way the degenerate modern representatives
+of the ancestral Protovertebrates.
+
+Balfour's suggestion, that the nervous system in Annelids and
+Vertebrates might have arisen by the dorsal or ventral coalescence of
+the lateral nerve cords found in their common ancestor, bore fruit in
+the speculations of Hubrecht,[409] on the relation of Nemertines to
+Vertebrates.
+
+The Annelid theory was firmly supported by Eisig, who in his elaborate
+monograph on the _Capitellidae_[410] maintained against Fuerbringer the
+genetic identity of the Annelidan nephridia with the kidney tubules of
+Vertebrates. The independent discovery by E. Meyer[411] and J. T.
+Cunningham,[412] of an internal segmental duct in _Lanice_, into which
+several nephridia opened, seemed to strengthen this view.
+
+Following Ehlers,[413] Eisig found the homologue of the notochord in the
+accessory intestine of the _Capitellidae_ and _Eunicidae_, which he
+supposed might easily be transformed, according to the principle of
+function-change, from a respiratory to a supporting organ. He finally
+disposed of the alternative notion that the notochord was represented in
+Annelids by the "giant-fibres" or neurochordal strands which lie close
+above the nerve-cord, a view held by Kowalevsky,[414] and for a time by
+Semper. These strands were shown by Eisig, and by Spengel, to be the
+neurilemmar sheaths of thick nerve fibres which had in many cases
+degenerated. The view that the content of the neurochordal tubes was
+nervous in nature was first promulgated by Leydig in 1864.
+
+Much difference of opinion reigned as to the true homologies of the
+brain and mouth of Annelids and Vertebrates. Beard[415] and others got
+over the difficulty of the 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 hypophysis represents the old Annelidan mouth.
+
+Van Beneden and Julin[416] assumed that in the ancestors of Vertebrates
+the oesophagus shifted forward between the still unconnected lobes of
+the brain to open on the haemal surface.
+
+The fundamental assumption of the Annelid theory, that dorsal and
+ventral surfaces are morphologically interchangeable, seemed rather bold
+to many zoologists, and Gegenbaur[417] voiced a common opinion when he
+rejected as unscientific the comparison of the ventral nerve cord of
+Articulates with the dorsal nervous system of Vertebrates.
+
+The _Balanoglossus_ theory of Vertebrate descent also belongs, at least
+in its first form, to the earlier group of evolutionary speculations.
+The gill-slits of _Balanoglossus_ were discovered by Kowalevsky as early
+as 1866.[418] _Tornaria_ was discovered by J. Mueller 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.[419] That it
+had some relationship with Vertebrates was recognised by Semper,
+Gegenbaur and others, but the full working-out of its Vertebrate
+affinities is due to Bateson.[420]
+
+Bateson broke completely with the Dohrn-Semper view that the metamerism
+of Articulates and Vertebrates must be put down to inheritance from a
+common ancestor. He held that metamerism was merely a special
+manifestation of the general property of repetition, common to all
+living things (_cf._ Owen's "vegetative force"), and that accordingly
+"however far back a segmented ancestor of a segmented descendant may
+possibly be found, yet ultimately the form has still to be sought for in
+which these repetitions had their origin" (p. 549). The meaning of the
+phenomenon was obscure, but he was convinced that the explanation was
+not to be found in ancestry. "This much alone is clear," he wrote, "that
+the meaning of cases of complex repetition will not be found in the
+search for an ancestral form, which, itself presenting this same
+character, may be twisted into a representation of its supposed
+descendant. Such forms there may be, but in finding them the real
+problem is not even resolved a single stage; for from whence was their
+repetition derived? The answer to this question can only come in a
+fuller understanding of the laws of growth and of variation, which are
+as yet merely terms" (pp. 548-9). It was in following up this line of
+thought that Bateson produced his monumental _Materials for the Study of
+Variation_ (1894).
+
+He found a strong positive argument for his theory that Vertebrates are
+descended from unsegmented forms in the fact that the notochord arises
+as an unsegmented structure. With the notochord he homologised the
+supporting rod in the proboscis of _Balanoglossus_, which like the
+notochord arises from the dorsal wall of the archenteron, and has a
+vacuolated structure. The gill-slits of _Balanoglossus_, with their
+close resemblance in detail to those of Amphioxus, Bateson also used as
+an argument in favour of the phylogenetic relationship of the
+Enteropneusta and Vertebrata, together with the formation from the
+ectoderm of a dorsal nerve tube.
+
+Bateson's views attracted considerable attention, and were thought by
+many to lighten appreciably the obscurity in which the origin of
+Vertebrates was wrapped. Thus Lankester wrote in his article on
+Vertebrates[421] in the _Encyclopedia Britannica_:--"It seems that in
+_Balanoglossus_ we at last find a form which, though no doubt
+specialised for its burrowing sand-life, and possibly to some extent
+degenerate, yet has not to any large extent fallen from an ancestral
+eminence. The ciliated epidermis, the long worm-like form, and the
+complete absence of segmentation of the body-muscles lead us to forms
+like the Nemertines. The great proboscis of _Balanoglossus_ may well be
+compared to the invaginable organ similarly placed in the Nemertines.
+The collar is the first commencement of a structure destined to assume
+great importance in _Cephalochorda_ and _Craniata_, and perhaps
+protective of a single gill-slit in _Balanoglossus_ before the number of
+those apertures had been extended. Borrowing, as we may, the nephridia
+from the Nemertines, and the lateral in addition to the dorsal nerve, we
+find that _Balanoglossus_ gives the most hopeful hypothetical solution
+of the pedigree of Vertebrates."
+
+Much doubt was cast upon the Chordate affinities of the Enteropneusta by
+Spengel in his monograph of the group,[422] but when the development of
+the coelom came to be more thoroughly worked out in _Balanoglossus_ and
+Amphioxus, the striking resemblance in this respect between the two
+forms gave additional support to the Batesonian view.[423]
+
+    [386] The stages in the development of microscopical
+    technique are well summarised by R. Burckhardt,
+    _Geschichte der Zoologie_, p. 121, Leipzig 1907.
+
+    [387] "Entwickelungsgeschichte des Amphioxus lanceolatus,"
+    _Mem. Acad. Sci. St Petersbourg_ (Petrograd) (vii.),
+    xi., No. 4, 1867, 17 pp., 3 pls.
+
+    [388] "Weitere Studien ue. die Entwickelungsgeschichte des
+    Amphioxus lanceolatus," _Arch. fuer mikr. Anat._, xiii.,
+    pp. 181-204, 1877.
+
+    [389] Particularly by Hatschek (1881) and Boveri (1892).
+
+    [390] "Entwickelungsgeschichte der einfachen Ascidien,"
+    _Mem. Acad. Sci. St Petersbourg_ (Petrograd), (vii.),
+    x., No. 15, 1866, 19 pp., 3 pls. "Weitere Studien ue. die
+    Entwicklung der einfachen Ascidien," _Arch. f. mikr.
+    Anat._, vii., pp. 101-130, 1871.
+
+    [391] _Descent of Man_, i., p. 205, 1871.
+
+    [392] _Arch. f. mikr. Anat._, vi., 1870, and viii., 1872.
+
+    [393] _Archives de Biologie_, 1884, 1885, and 1887.
+
+    [394] _Bull. Acad. Sci. St Petersbourg_ (Petrograd) xiii.,
+    1869, and _Zeits. f. wiss. Zool._, xxii., 1872.
+
+    [395] _Mem. Acad. Sci. St Petersbourg_(Petrograd)(7),
+    xix., 1873.
+
+    [396] Giard, _Arch. zool. exper. gen._, i., 1872, and
+    Lacaze-Duthiers, _ibid._, iii., 1874.
+
+    [397] For the later history of the Amphioxus-Ascidian
+    theory the reader may be referred to A. Willey's
+    well-known work, _Amphioxus and the Ancestry of the
+    Vertebrates_, New York and London, 1894, and to Delage
+    et Herouard, _Traite de Zoologie concrete_, Tome viii.,
+    Paris, 1898.
+
+    [398] "Studien zur Urgeschichte des Wirbelthierkoerpers,"
+    _Mittheil. Zool. Stat. Neapel_, 1882-1907.
+
+    [399] Leydig (_Vom Baue des thierischen Koerpers_,
+    Tuebingen, 1864), who, in a measure, forestalled Dohrn
+    and Semper by comparing Vertebrates with reversed
+    Arthropods, specially insects, supposed the old mouth to
+    pass between the _crura cerebri_.
+
+    [400] _Zeits. f. wiss. Zool._, xliv., 1886.
+
+    [401] Quoted by E. B. Wilson, _Wood's Holl Biological
+    Lectures for 1894_, p. 121.
+
+    [402] _Cf._ Metschnikoff, _Quart. Journ. Microsc. Sci._,
+    xxiv., pp. 89-111, 1884.
+
+    [403] "Die Stammesverwandschaft der Wirbelthiere und
+    Wirbellosen," _Arb. zool.-zoot. Instit. Wuerzburg_, ii.,
+    pp. 25-76, 1875; "Die Verwandschaftsbeziehungen der
+    gegliederten Thiere," _Ibid._, iii., pp. 115-404,
+    1876-7.
+
+    [404] Abuse of Cuvier also dates from the early days of
+    evolution, see Radl, ii., pp. 12-17.
+
+    [405] "On the origin and history of the urino-genital
+    organs of Vertebrates," _Journ. Anat. Phys._, x., 1876.
+    The conclusions of Balfour and Semper were adversely
+    criticised by M. Fuerbringer (_Morph. Jahrb._, iv.,
+    1878), and were negatived by later research.
+
+    [406] _A Monograph on the Development of Elasmobranch
+    Fishes_, London, 1878.
+
+    [407] _A Treatise on Comparative Embryology_, vol. ii., p.
+    311, London, 1881.
+
+    [408] _Loc. cit._, vol. ii., p. 327.
+
+    [409] "On the Ancestral Form of the Chordata," _Q.J.M.S._,
+    xxiii., 1883. "The Relation of the Nemertea to the
+    Vertebrata," _ibid._, xxvii., 1887. Hubrecht gives the
+    credit for the first indication of the relationship of
+    Nemertines and Vertebrates to Harting (_Leerboek van de
+    Grondbeginselen der Dierkunde_, 1874).
+
+    [410] "Monographie der Capitelliden des Golfes von
+    Neapel," _Fauna u. Flora des Golfes von Neapel_, Monog.
+    xvi., Berlin, 1887.
+
+    [411] _Mitt. Zool. Stat. Neapel_, vii., 1887.
+
+    [412] _Nature_, xxxvi., p. 162, 1887.
+
+    [413] "Nebendarm und Chorda dorsalis," _Nachr. Ges. Wiss.
+    Goettingen_, p. 390, 1885.
+
+    [414] "Embryologische Studien an Wuermern u. Arthropoden,"
+    _Mem. Acad. Sci. St Petersbourg_ (Petrograd), (7), xvi.,
+    1870. And in _Arch. f. mikr. Anat._, vii., p. 122, 1871.
+
+    [415] "The Old Mouth and the New," _Anat. Anz._, iii.,
+    1888. _Nature_, xxxix., 1889.
+
+    [416] "Recherches sur la Morphologie des Tuniciers,"
+    _Arch. de Biol._, vi., 1887.
+
+    [417] "Die Stellung u. Bedeutung der Morphologie," _Morph.
+    Jahrb._, i., pp. 1-19, 1876.
+
+    [418] "Anatomie des Balanoglossus," _Mem. Acad. Sci. St
+    Petersbourg_ (Petrograd), (7), x., 1866.
+
+    [419] _Zeit. f. wiss. Zool._, xx., 1870. For a recent view
+    of the relation of the Enteropneusta to the Echinoderma,
+    see J. F. Gemmill, _Phil. Trans._ B., ccv., pp. 213-94,
+    1914.
+
+    [420] In a series of papers published in 1884-6, the
+    speculative results being discussed in his memoir on
+    "The Ancestry of the Chordata," _Q.J.M.S._ (n.s.), xxvi.,
+    pp. 535-71, 1886.
+
+    [421] Reprinted in _Zoological Articles_, London, 1891.
+
+    [422] "Die Enteropneusten des Golfes von Neapel," _Fauna
+    und Flora des Golfes von Neapel_, Monog. xviii., Berlin,
+    1893.
+
+    [423] See Macbride, "A Review of Prof. Spengel's Monograph
+    on Balanoglossus," _Q.J.M.S._, xxxvi., 1894, and "The
+    Early Development of Amphioxus," _Q.J.M.S._, xl., 1898.
+
+
+
+
+CHAPTER XVI
+
+THE GERM-LAYERS AND EVOLUTION
+
+
+In his papers of 1866 and 1867 Kowalevsky had remarked upon the
+widespread occurrence of a certain type or fundamental plan of early
+embryonic development, characterised by the formation, through
+invagination, of a two-layered sac, whose cavity became the alimentary
+canal. This developmental archetype was manifested in, for instance,
+_Sagitta_,[424] _Rana_,[425] _Lymnaea_,[426] _Astacus_,[427]
+_Phoronis_,[428] _Asterias_,[429] _Ascidia_,[428] the _Ctenophora_,[428]
+and _Amphioxus_.[428] He noticed also that the invagination-opening
+often became the definitive anus. Further instances of this mode of
+development were later observed by Metschnikoff[430] and by
+Kowalevsky[431] himself, but it was left to Haeckel to generalise these
+observations and build up from them his famous Gastraea theory. This was
+first enunciated in his monograph of the calcareous sponges,[432] and
+worked out in detail in a series of papers published in 1874-76.[433]
+
+Haeckel maintained that the "gastrula" stage occurred in the development
+of all Metazoa, and that it was typically formed, by invagination, from
+a hollow sphere of cells or "blastula." This typical formation might be
+masked by cenogenetic modifications caused chiefly by the presence of
+yolk. The gastrula stage was the palingenetic repetition of the
+ancestral form of all Metazoa, the Gastraea.
+
+From the Gastraea theory there followed at once two consequences, (1)
+that ectoderm and endoderm, invagination-cavity (_Urdarm_) and
+gastrula-mouth (_Urmund_ or _Protostoma_), were, with all their
+derivatives, homologous, because homogenous, throughout the Metazoa, and
+(2) that the descent of the Metazoa had been monophyletic, since all
+were derived from the ancestral Gastraea. Huxley's suggestion (_supra_,
+p. 208) that the outer and inner layers in Coelentera were homologous
+with the ectoderm and endoderm of the germ was thus fully confirmed and
+greatly extended.
+
+The great importance of the 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 implications 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 _Kalkschwaemme_.[434]
+
+_Monophyletic Genealogical Tree of the Animal Kingdom, based upon the
+Gastraea Theory and the Homology of the Germ Layers_.
+
+_______________________________________________________________________
+|      |                                                       |    . |
+|      |                                                       |    m |
+|      |                               _Vertebrata_.           |    o |
+|    . |                                    |                  |    l |
+|    m |               _Arthropoda_.        |                  |    e |
+|    r |                     |              |                  |    o |
+|    e |                     |              |                  |    c |
+|    d |_Echinoderma_.       |              |  _Mollusca_.     |      |
+|    d |      |              |              |       |          |    a |
+|    n |      |              |   Sagitta.   \______ | ______/  |  |   |
+|    e |      |              |      |              \|/         | .  d |
+|      |      |              |      |               |          | a  n |
+|    y |      |              |      | Nematoda.   Himatega.    | i  a |
+|    b |      |              |      |     |            |       | r    |
+|      |      |              |      |     |            |       | a  d |
+|      |      |              |      |     |            |       | t  o |
+|      |      \______________|______|_  __|____________|_____/ | a  o |
+|      |                              \/                       | m  l |
+|      |                                                       | ae  b |
+|      |                            _Coelomati_                | H    |
+|      |                    (worms with body-cavity}.          |  | h |
+|      |                             \      /                  |    t |
+|      |                              \    /                   |    i |
+|      |                               \  /                    |    W |
+|      |________________________________\/_____________________|______|
+| .    |                                |                      |      |
+| )  d |                                |                      |    . |
+| s  e |    _Zoophyta_                  |      Plathelminthes. |    m |
+| l  n |   (Coe;enterata).              |               |      |    o |
+| a  i |                                |               |      |    l |
+| m  l |                Acalephae.       \______________ |_____/|    e |
+| i    |                    |                          \/      |    o |
+| n  , |  Spongiae.          |                  _Acoelomi_      |    c |
+| a  t |     |              |                (Worms without    |      |
+|    u | Archispongia.   Archydra.            body cavity).    |    o |
+| t  g |     |              |                         |        |    n |
+| u    |     |              |                         |        |      |
+| G  e |     \______  ______/                         |        |  | d |
+| (  u |            \/                                |        | a  n |
+|    r |         Protascus.                      Prothelmis.   | i  a |
+|    t |             |                                |        | r    |
+|      |             |                                |        | a  d |
+|    A |       Gastraea radialis            Gastraes bilateralis | ae  o |
+|  |   |          (sedens).                       (repens).    | n  o |
+| a  . |             |                                |        | A  l |
+| o  s |             |                                |        |  | b |
+| z  r |             \_______________  _______________/        |      |
+| a  e |                             \/                        |    o |
+| t  y |                         _Gastraea_                     |    N |
+| e  a |                   (Ontogeny : Gastrula).              |      |
+| M  l |                             |                         |      |
+|  |   |                             |                         |      |
+|    m |                             |                         |      |
+|    r |                             |                         |      |
+|    e |                             |                         |      |
+|    g |                             |                         |      |
+|      |                             |                         |      |
+|    y |                             |                         |      |
+|    r |                             |                         |      |
+|    a |                             |                         |      |
+|    m |                             |                         |      |
+|    i |                             |                         |      |
+|    r |                             |                         |      |
+|    P |                             |                         |      |
+|      |                             |                         |      |
+|    o |                             |                         |      |
+|    w |                             |                         |      |
+|    T |                             |                         |      |
+|______|                    _________|_________________________|______|
+|      |                    |                                         |
+|      |          __________|                                         |
+|      |          |                                                   |
+|    . |          |                                                   |
+|    t |      Planaeada               Acinetae.             Ciliata.   |
+|    u | (Ontogeny : Planula).            |                    |      |
+|    g |          |                       \_________  _________/      |
+| >    |          |                                 \/                |
+| i  o |          |                             Infusoria.            |
+| /  n |          |                                  |                |
+| <    |          |                                  |                |
+| a  , |      Synamoebae            Gregarinae         |                |
+| o  s | (Ontogeny : Morula).          |             |                |
+| z  r |          |                    |             |                |
+| o  e |          |                    \_____  ______/                |
+| t  y |          |                          \/                       |
+| o  a |          |                        Amoebina.                  |
+| r  l |          |                           |                       |
+| P    |          \____________  _____________/                       |
+| >  m |                       \/                                     |
+| i  r |                    _Amoebae_                    ?   ?   ?     |
+| <  e |              (Ontogeny : Ovulum).              |   |   |     |
+|    g |                        |                       |   |   |     |
+|      |                        |                       |   |   |     |
+|    o |                    _Monera_                     Monera.      |
+|    N |              (Ontogeny : Monerula).                          |
+|      |                                                              |
+|______|______________________________________________________________|
+
+
+The scheme is in many respects an interesting and important one. The
+great contrast between the Protozoa, or animals with neither gut nor
+germ-layers, and the Metazoa, which possess both structures, is for the
+first time clearly brought out. The derivation of all the Metazoa from a
+single ancestral form, the 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
+_archenteron_ (Lankester, 1875), and an ectoderm and endoderm which are
+homologous throughout all the Metazoan phyla. Finally, in the separation
+of the sponges, Coelenterata and Acoelomi as animals lacking a body
+cavity or coelom[435] from the four higher phyla, which are essentially
+Coelomati, there is contained the germ of a conception which later
+became of importance.
+
+Somewhat similar views as to the importance of the germ-layer theory for
+the phylogenetic classification of animals were published by Sir E. Ray
+Lankester in 1873.[436] He distinguished three grades of animals--the
+Homoblastica, Diploblastica, and Triploblastica. The first included the
+Protozoa, the second the Coelenterata, the third the other five phyla,
+distinguished by the possession of a third layer, the mesoderm, and a
+"blood-lymph" cavity enclosed therein. He used the germ-layer theory to
+prove the essential unity of type of all the Triploblastica.
+
+The 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.[437] These were the monerula, cytula, morula, blastula, and
+gastrula (see Fig. 15). The monerula was the fertilised ovum after the
+disappearance of the germinal vesicle;[438] it was the equivalent of
+the primordial anucleate Monera which are the ancestors of all
+animals. The ovum after the nucleus had been re-formed became the
+cytula, which was the ontogenetic counterpart of the amoeba. The
+morula, a compact mulberry-like congeries of segmentation-cells,
+corresponded to the synamoeba, or earliest association of
+undifferentiated amoeboid cells to form the first multicellular
+organism. The blastula, or hollow sphere of segmentation cells,
+usually ciliated, was reminiscent of the planaea, an ancestral
+free-swimming form whose nearest living relation is the spherical
+_Magosphaera_. The gastrula, finally, is the two-layered sac formed
+from the blastula, typically by invagination of its wall. It repeats
+the organisation of the gastraea, which is the common ancestor of all
+Metazoa, and finds its nearest living counterpart in the simple
+"sponges" _Haliphysema_ and _Gastrophysema_.[439] The ancestral line
+of all the higher animals begins with the five hypothetical forms of
+the moneron, amoeba, synamoeba, planaea, and gastraea.
+
+[Illustration: FIG. 15.--The Five Primary Stages of Ontogeny. (After
+Haeckel.) 1. Monerula. 2. Cytula. 3. Morula. 4. Blastula. 5. Gastrula.]
+
+We may take the following account[440] of the phylogeny of the human
+species, from the gastraea stage onwards, as typical of Haeckel's
+speculations on the evolution of the higher forms. The progenitors of
+man are, after the Gastraeada:--
+
+
+1. Turbellaria.
+*2. Scolecida. (Worms with a coelom, probably represented
+        at the present day by _Balanoglossus_.)
+*3. Himatega. (Evolved from Scolecida by formation of
+        dorsal nerve-tube and chorda, and resembling tailed
+        larvae of Ascidians.)
+4. Acrania. (With metameric segmentation. Including
+        Amphioxus.)
+5. Monorrhina. (Cyclostomes.)
+6. Selachia.
+7. Dipneusta.
+8. Sozobranchia. (Amphibia with permanent gills.)
+9. Sozura. (Tailed Amphibia.)
+*10. Protamnia.
+*11. Promammalia.
+12. Marsupialia.
+13. Prosimiae.
+14. Menocerca. (Tailed apes.)
+15. Anthropoides.
+16. Pithecanthropi.
+17. Homines.
+
+It will be noticed that except for the hypothetical forms (marked with
+an asterisk), which are themselves generalised classificatory groups,
+the ancestral forms belong to long-recognised classes. The whole course
+of the evolution follows well-worn systematic lines. This is typical of
+Haeckel's phylogenetic speculations.
+
+A more abstractly morphological scheme of the evolution of Vertebrates
+is given in the _Systematic Phylogeny_ of 1895.[441] The ontogenetic and
+ancestral stages are arranged in parallel columns thus:--
+
+Cytula.                            Cytaea (Protozoa).
+Morula.                            Moraea (Coenobium of Protozoa).
+Blastula.                          Blastaea (_Volvocina_, etc.).
+Depula (invaginated blastula).     Depaea.
+Gastrula.                          Gastraea (cf. _Olynthus_, _Hydra_, and
+                                      primitive Coelentera).
+Coelomula (with one pair           Coelomaea (cf. _Sagitta_, _Ascidia_,
+  of coelom-pockets).                 and primitive Helminthes).
+Chordula (with medullary           Chordaea (_cf._ Ascidian larva and
+  tube and chorda).                   larva of Amphioxus).
+Spondula (with segmented           Prospondylus (Primitive Vertebrate).
+  mesoderm).
+
+This scheme differs from the earlier one chiefly in taking into account
+certain advances, notably as regards the cytology of the fertilised ovum
+and the true nature of the coelom, which had been made in the interval
+of some twenty years.
+
+Haeckel's Gastraea theory, though it exercised a great influence upon the
+subsequent trend of phylogenetic speculation, was by no means
+universally accepted _telle quelle_. Opinions differed considerably as
+to the primitive mode of origin of the two-layered sac which was very
+generally admitted to be of constant occurrence in early embryogeny. Ray
+Lankester, in his paper of 1873, and more fully in 1877,[442] propounded a
+"Planula" theory, according to which the ancestral form of the Metazoa
+was a two-layered closed sac formed typically by delamination, less
+often by invagination. He denied that the invagination opening (which he
+named the blastopore) represented the primitive mouth,[443] holding that
+this was typically formed by an "inruptive" process at the anterior end
+of the planula, which led to the formation of a "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,[444] without, however, any very definite conclusion being
+reached. He held that both processes could be proved in certain cases to
+be purely secondary or adaptive, and that accordingly there was nothing
+to show that either of them reproduced the original mode of transition
+from the Protozoa to the ancestral two-layered Metazoa (p. 342). He by
+no means rejected the theory that the 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 Gastraea 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. But it was not Haeckel himself who enunciated the coelom
+theory.
+
+It will be remembered that Remak introduced in 1855 the conception of
+the mesoderm as an independent layer derived from the endoderm. The
+pleuro-peritoneal or body-cavity was formed as a split in the "ventral
+plates" of the mesoderm. Haeckel's "coelom" corresponded to the
+"pleuro-peritoneal cavity" of Remak, but his view of the origin of the
+mesoderm brought him much closer to von Baer's conception of the origin
+of _two_ secondary layers from ectoderm and endoderm respectively than
+to Remak's conception of the mesoderm as a single independent layer.
+
+Much uncertainty reigned at the time as to the exact manner of origin of
+the mesoderm;[445] some held that it developed from the ectoderm, others
+that it originated in the endoderm, while still others, and among them
+Haeckel, considered that part of it came from the ectoderm and part from
+the endoderm (pp. 23-4, 1874).
+
+The solution of the problem came from those observations on the
+development of the lower forms to which we have just alluded.
+
+The early history of these discoveries and of the theory which grew out
+of them has been well summarised by Lankester,[446] and may conveniently
+be given in his own words:--
+
+"As far back as 1864 Alexander Agassiz ("Embryology of the Star-fish,"
+in _Contributions to the Natural History of the United States_, vol. v.,
+1864) showed in his account of the development of Echinoderma that the
+great body-cavity of those animals developed as a pouch-like outgrowth
+of the archenteron of the embryo, whilst a second outgrowth gave rise to
+their ambulacral system; and in 1869 Metschnikoff (_Mem. de l'Acad.
+imperiale des Sciences de St Petersbourg_, series vii., vol. xiv.,
+1869), confirmed the observations of Agassiz, and showed that in
+Tornaria (the larva of Balanoglossus) a similar formation of
+body-cavities by pouch-like outgrowths of the archenteron took place.
+Metschnikoff has further the credit of having, in 1874 (_Zeitsch. wiss.
+Zoologie_, vol. xxiv., p. 15, 1874), revived Leuckart's theory of the
+relationship of the coelenteric apparatus of the Enterocoela to the
+digestive canal and body-cavities of the higher animals. Leuckart had in
+1848 maintained that the alimentary canal and the body-cavity of higher
+animals were united in one system of cavities in the Enterocoela
+(_Verwandschaftsverhaeltnisse der wirbellosen Thiere_, Brunswick, 1848).
+Metschnikoff insisted upon such a correspondence when comparing the
+Echinoderm larva, with its still continuous enteron and coelom, to a
+Ctenophor, with its permanently continuous system of cavities and
+canals. Kowalevsky, in 1871, showed that the body-cavity of Sagitta was
+formed by a division of the archenteron into three parallel cavities,
+and in 1874 demonstrated the same fact for the Brachiopoda. In 1875
+(_Quart. Journ. Micr. Sci._, vol. xv., p. 52) Huxley proposed to
+distinguish three kinds of body-cavity: the schizocoel, formed by the
+splitting of the mesoblast, as in the chick's blastoderm; the
+enterocoel, formed by pouching of the archenteron, as in Echinoderms,
+Sagitta and Brachiopoda; and the epicoel.... Immediately after this I
+put forward the theory of the uniformity of origin of the coelom as an
+enterocoel (_Quart. Journ. Micr. Sci._, April, 1875).... My theory of
+the coelom as an enterocoel was accepted by Balfour and was greatly
+strengthened by his observations on the derivation of both notochord and
+mesoblastic somites from archenteron in the Elasmobranchs, and by the
+publication in 1877 by Kowalevsky of his second paper on the development
+of Amphioxus--in which the actual condition which I had supposed to
+exist in the Vertebrata was shown to occur, namely, the formation of the
+mesoblast as paired pouches in which a narrow lumen exists, but is
+practically obliterated on the nipping-off of the pouch from the
+archenteron, after which process it opens out again as coelom" (pp.
+16-18).
+
+The enterocoelic theory was taken up by O. and R. Hertwig as an
+essential part of their _Coelomtheorie_.[447] In a lengthy series of
+monographs these workers made a comparative study of the mode of
+formation of the middle layer, and arrived at a coherent theory of its
+origin. They distinguished in the middle layer two quite distinct
+elements, the mesoblast proper, formed by the evagination of the walls
+of the archenteron, and the mesenchyme, formed by free cells budded off
+from the germ-layers. The following passage gives a good idea of their
+views and of the phylogenetic implications involved:--"Ectoblast and
+entoblast are the two primary germ-layers which arise from the
+invagination of the blastula; they are always the first to be laid down,
+and they can be directly referred back to a simple ancestral form, the
+Gastraea; they form the limits of the organism towards the exterior and
+towards the archenteron. The parietal and visceral mesoblast, or the two
+middle layers, are always of later origin, and arise through evagination
+or plaiting of the entoblast, the remainder of which can now be
+distinguished as secondary entoblast from the primary. They form the
+walls of a new cavity, the enterocoel, which is to be regarded as a
+nipped-off diverticulum of the archenteron. Just as the two-layered
+animals can be derived from the Gastraea, so can the four-layered animals
+be derived from a Coelom form. Embryonic cells, which become singly
+detached from their epitheliar connections we consider to be something
+quite different from the germ-layers, and accordingly we call them by
+the special name of mesenchyme germs or primary cells of the mesenchyme.
+They may develop both in two-layered and in four-layered animals. Their
+function is to form between the epithelial limiting layers a secreted
+tissue (_Secretgewebe_) or connective tissue with scattered cells, which
+cells can undergo, like the epithelial elements, the most varied
+modifications.... This secreted tissue in its simple or in its
+differentiated state, with all its derivatives, we call the mesenchyme"
+(p. 122).
+
+The important point for us is that, just as all Metazoa were considered
+by Haeckel to be descended from the Gastraea, so all Coelomati were held
+by the Hertwigs to be derived from an original coelomate _Urform_. In
+both cases an embryological archetype becomes a hypothetical ancestral
+form.
+
+The Coelom theory was considerably modified, extended and developed by
+later workers, particularly as regards the relations to the coelom of
+the genital organs and ducts and the nephridia, but no special
+methodological interest attaches to these further developments.[448] We
+shall here focus attention upon one interesting line of speculation
+followed out in this country particularly by Sedgwick--the theory of the
+Actinozoan ancestry of segmented animals. Its relation to the Coelom
+theory lies in the fact that Sedgwick regarded the segmentation of the
+body as moulded upon the segmentation of the mesoblast, which in its
+turn, as Kowalevsky and Hatschek had shown, was a consequence of its
+mode of origin as a series of pouches of the archenteron. In other
+respects Sedgwick's speculations link on more closely to the Gastraea
+theory, for one of his main contentions is that the blastopore or
+_Urmund_ is homologous throughout at least the three metameric phyla. In
+following up Balfour's observations on the development of
+_Peripatus_,[449] Sedgwick was struck with the close resemblance existing
+between the elongated slit-like blastopore of this form (giving rise to
+both mouth and anus), with its border of nervous tissue, and the
+slit-like mouth of the Actinozoan (functioning both as mouth and anus),
+round which, as the Hertwigs had shown, there lies a special
+concentration of nerve cells and nerve fibres. He found another point of
+resemblance in the gastric pouches of the Actinozoa, which he
+homologised directly with the enterocoelic pouches of the Coelomati. He
+was led to enunciate the following theses:--[450] (1) that the mouth and
+anus of Vermes, Mollusca, Arthopoda, and probably Vertebrata, is derived
+from the elongated mouth of an ancestor resembling the Actinozoa; (2)
+that somites are derived from a series of archenteric pouches, like
+those of Actinozoa and 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 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 Coelenterate-like animal with
+a pouched gut, the pouching having arisen as a result of the necessity
+for an increase in the extent of the vegetative surfaces in a rapidly
+enlarging animal (for circulation and respiration)" (p. 47). "I have
+attempted to show," he writes further on, "that the majority of the
+Triploblastica ... are built upon a common plan, and that that plan is
+revealed by a careful examination of the anatomy of Coelenterata; that
+all the most important organ-systems of these Triploblastica are found
+in a rudimentary condition in the Coelenterata; and that all the
+Triploblastica referred to must be traced back to a diploblastic
+ancestor common to them and the Coelenterata" (p. 68). The main
+assumption was that the neural or blastoporal surface must be homologous
+throughout the Metazoa, though it was dorsal in the Chordata, ventral in
+the Annelida and Arthropoda. He derived the central nervous system of
+the Chordata from the circumoral ring of the common ancestor by means of
+the hypothesis that both the pre-blastoporal and the post-blastoporal
+parts of it disappeared.[451]
+
+The characteristic relation of the central nervous system to the
+blastopore in Annelida and Vertebrates had already been pointed out by
+Kowalevsky,[452] who had also sketched a theory of the common descent of
+these two phyla from an ancestral form in which the nervous system
+encircled the blastopore.
+
+In 1882, before the publication of Sedgwick's papers, A. Lang[453] had put
+forward the somewhat similar view that the stomach-diverticula of the
+Turbellaria, which he had found to be segmentally arranged in certain
+Triclads, were the morphological equivalents of the enterocoelic pouches
+of higher animals. This view, however, he soon gave up.[454] Sedgwick's
+views found a supporter in A. A. W. Hubrecht,[455] who utilised them in
+connection both with his speculations on the relation of Nemertines to
+Vertebrates, and with his exhaustive work on the early development of
+the Mammalia. He postulated as the far-back ancestor of Vertebrates, "an
+actinia-like, vermiform being, elongated in the direction of the
+mouth-slit" (p. 410, 1906), and derived the central nervous system from
+the circum-oral ring of this primitive form, the notochord from its
+stomodaeum, and the coelom from the peripheral parts of the gastric
+cavity (p. 169, 1909).
+
+    [424] Gegenbaur, _Zeits. f. wiss. Zool._, v., 1853.
+
+    [425] Remak, _loc. cit._, p. 183, pl. xii.
+
+    [426] Lereboullet, _Ann. Sci. nat._ (4) xviii., pp. 118-9,
+    1862.
+
+    [527] Lereboullet, in Remak, p. 183 f.n.
+
+    [428] Kowalevsky, _Mem. Acad. Sci. St
+    Petersbourg_ (Petrograd), (7), x. and xi., 1866 and 1867.
+
+    [429] A. Agassiz, _Contrib. Nat. Hist. United States_, v.,
+    1864.
+
+    [430] _Mem. Acad. Sci. St Petersbourg_ (Petrograd), (7),
+    xiv., 1869.
+
+    [431] "Embryolog. Studien an Wuermern u. Arthropoden,"
+    _Mem. Acad. Sci. St Petersbourg_ (Petrograd), (7), xvi.,
+    1870.
+
+    [432] _Die Kalkschwaemme_, 3 vols., Berlin, 1872. General
+    chapters translated in _Ann. Mag. Nat. Hist._ (4), xi.,
+    pp. 241-62, 421-30, 1873.
+
+    [433] "Die Gastraea-Theorie, die phylogenetische
+    Classification des Thierreichs und die Homologie der
+    Keimblaetter." _Jenaische Zeitschrift_, viii., pp. 1-55,
+    1874. "Die Gastrula und die Eifurchung der Thiere,"
+    _ibid._, ix., pp. 402-508, 1875. "Die Physemarien,
+    Gastraeaden der Gegenwart," and "Nachtraege zur
+    Gastraea-Theorie," _ibid._, x., pp. 55-98, 1876.
+    Republished in _Biologische Studien_, 2nd part, _Studien
+    zur Gastraea-Theorie_, 270 pp., 14 pls., Jena, 1877.
+
+    [434] See _Ann. Mag. Nat. Hist._ (4), xi., p. 253.
+
+    [435] Term first introduced in _Die Kalkschwaemme_, p. 468,
+    1872.
+
+    [436] "On the Primitive Cell-layers of the Embryo as the
+    Basis of Genealogical Classification of Animals, and on
+    the Origin of Vascular and Lymph Systems," _Ann. Mag.
+    Nat. Hist._ (4), xi., pp. 321-38, 1873.
+
+    [437] First distinguished in _Die Kalkschwaemme_, i., p.
+    465.
+
+    [438] Even in the 'seventies it was still believed by many
+    that the egg-nucleus disappeared on fertilisation. The
+    true nature of the process was not fully made out till
+    1875, when O. Hertwig observed the fusion of egg- and
+    sperm-nuclei in _Toxopneustes (Morph. Jahrb._, i.,
+    1876).
+
+    [439] _Studien z. Gastraea-Theorie_, p. 214, 1877. These
+    forms were known even in 1870 (Carter, _Ann. Mag. Nat.
+    Hist._ (4), vi., pp. 346-7), to be Foraminifera. The
+    figures of supposed collar-cells, etc., do credit to
+    Haeckel's imagination.
+
+    [440] _History of Creation_, Eng. Trans., ii., pp. 278 ff.
+
+    [441] _Systematische Phylogenie_, iii., p. 41, Berlin,
+    1895.
+
+    [442] "Notes on the Embryology and Classification of the
+    Animal Kingdom," _Q.J.M.S._ (n.s.), xvii., pp. 399-454,
+    1877.
+
+    [443] It was "part of the non-historic mechanism of
+    growth" (_loc. cit._, p. 418).
+
+    [444] _Treatise on Comparative Embryology_, ii., chap.
+    xiii., 1881. For a modern discussion of this problem,
+    see Hubrecht, _Q.J.M.S._, xlix., 1906.
+
+    [445] See Balfour, _loc. cit._, Chapter xiii.
+
+    [446] _A Treatise on Zoology_, Pt. ii., 1900. Introduction
+    by Sir E. Ray Lankester.
+
+    [447] _Studien zur Blaettertheorie_, Jena, 1879-80. "Die
+    Coelomtheorie, Versuch einer Erklaerung des mittleren
+    Keimblattes," _Jenaische Zeitschrift_, xv., pp. 1-150,
+    1882.
+
+    [448] For an historical account of this work, see
+    Lankester, _loc. cit._, pp. 21-37.
+
+    [449] _Proc. Roy. Soc._, 1883, and _Q.J.M.S._, xxiii.,
+    1883.
+
+    [450] "Origin of Metameric Segmentation," _Q.J.M.S._,
+    xxiv., pp. 43-82 1884.
+
+    [451] See further the same author's article "Embryology"
+    in the _Ency. Brit._, vol. xi., 11th ed., Cambridge,
+    1910.
+
+    [452] _Arch. f. mikr. Anat._, xiii., pp. 181-204, 1877.
+
+    [453] "Der Bau von Gunda segmentata," _Mitth. Zool. Stat.
+    Neap._, iii., pp. 187-250, 1882.
+
+    [454] "Die Polycladen," _Fauna u. Flora des Golfes von
+    Neapel_, Monog. v., Leipzig, 1884, and "Beitraege zu
+    einer Trophocoeltheorie," _Jen. Zeits._, xxxviii., pp.
+    1-373, 1904 (which see for a modern account of theories
+    of metamerism).
+
+    [455] "Die Abstammung der Anneliden u. Chordaten," _ Jen.
+    Zeits._, xxxix., pp. 151-76, 1905. "The Gastrulation of
+    the Vertebrates," _Q.J.M.S._, xlix., pp. 403-19, 1906.
+    "Early Ontogenetic Phenomena in Mammals," _Q.J.M.S._,
+    liii., pp. 1-181, 1909.
+
+
+
+
+CHAPTER XVII
+
+THE ORGANISM AS AN HISTORICAL BEING
+
+
+"Of late the attempt to arrange genealogical trees involving
+hypothetical groups has come to be the subject of some ridicule, perhaps
+deserved. But since this is what modern morphological criticism in great
+measure aims at doing, it cannot be altogether profitless to follow this
+method to its logical conclusions. That the results of such criticism
+must be highly speculative, and often liable to grave error, is
+evident."
+
+The quotation is from Bateson's paper of 1886, and it is symptomatic of
+the change which was soon to come over morphological thought. New
+interests, new lines of work, began to usurp the place which pure
+morphology had held so long.
+
+This is accordingly a convenient stage at which to take stock of what
+has gone before, to consider the relation of evolutionary morphology to
+the transcendental and the Cuvierian schools of thought which preceded
+it, and to make clear what new element evolution-theory added to
+morphology.
+
+The close analogy between evolutionary and transcendental morphology has
+already been remarked upon and illustrated in the last three chapters.
+We have seen that the coming of evolution made comparatively little
+difference to pure morphology, that no new criteria of homology were
+introduced, and that so far as pure morphology was concerned, evolution
+might still have been conceived as an ideal process precisely as it was
+by the transcendentalists. The principle of connections still remained
+the guiding thread of morphological work; the search for archetypes,
+whether anatomical or embryological, still continued in the same way as
+before, and it was a point of subordinate importance that, under the
+influence of the evolution-theory, these were considered to represent
+real ancestral forms rather than purely abstract figments of the
+intelligence. The law of Meckel-Serres was revived in an altered shape
+as the law of the recapitulation of phylogeny by ontogeny; the natural
+system of classification was passively inherited, and, by a _petitio
+principii_, taken to represent the true course of evolution. It is true
+that the attempt was made to substitute for the concept of homology the
+purely genetic concept of homogeny, but no inkling was given of any
+possible method of recognising homogeny other than the well-worn methods
+generally employed in the search after homologies.
+
+There was a close spiritual affinity between the speculative
+evolutionists and the transcendentalists. Both showed the same
+subconscious craving for simplicist conceptions--the transcendentalists
+clung fast to the notion of the absolute unity of type, of the ideal
+existence of the "one animal," and the evolutionists did precisely the
+same thing when they blindly and instinctively accepted the doctrine of
+the monophyletic descent of all animals from one primeval form. Geoffroy
+persisted in regarding Arthropods as being built on the same plan as
+Vertebrates: Dohrn and Semper did nothing different when they derived
+both groups from an ancestor combining the main characters of both. The
+determination to link together all the main phyla of the animal kingdom
+and to force them all into a single mould was common to evolutionary and
+pre-evolutionary transcendentalists alike.
+
+From the fact that all Metazoa develop from an ovum which is a simple
+cell, the evolutionists inferred that all must have arisen from one
+primordial cell. From the fact that the next step in development is the
+segmentation of the ovum, they argued that the ancestral Metazoa came
+into being through the division of the primal Protozoon with aggregation
+of the division-products. From the fact that a gastrula stage is very
+commonly formed when segmentation has been completed, they assumed that
+all germ-layered animals were descended from an ancestral Gastraea.
+
+They quite ignored the possibility that a different explanation of the
+facts might be given; they seized upon the simplest and most obvious
+solution because it satisfied their overwhelming desire for
+simplification. But is the simplest explanation always the
+truest--especially when dealing with living things? One may be permitted
+to doubt it. It is easy to account for the structural resemblance of the
+members of a classificatory group, by the assumption that they are all
+descended from a common ancestral form; it is easy to postulate any
+number of hypothetical generalised types; but in the absence of positive
+evidence, such simplicist explanations must always remain doubtful. The
+evolutionists, however, had no such scruples.
+
+Phylogenetic method differed in no way from transcendental--except
+perhaps that it had learnt from von Baer and from Darwin to give more
+weight to embryology. The criticisms passed by Cuvier and von Baer upon
+the transcendentalists and their recapitulation theory might with equal
+justice be applied to the phylogenetic speculations which were based on
+the biogenetic law. There was the same tendency to fix upon isolated
+points of resemblance and disregard the rest of the organisation. Thus,
+on the ground of a presumed analogy of certain structures to the
+vertebrate notochord, several invertebrate groups, as the Enteropneusta,
+the Rhabdopleura, the Nemertea, were supposed to be, if not ancestral,
+at least offshoots from the direct line of vertebrate descent. And if
+other points of resemblance could in some of these cases be discovered,
+yet no successful attempt was made to show that the total organisation
+of any of these forms corresponded with that of the Vertebrate type.
+With the possible exception of the Ascidian theory, all the numerous
+theories of vertebrate descent suffered from this irremediable defect,
+and none carried complete conviction.
+
+In spite of the efforts of the evolutionists, as of those of the
+transcendentalists, the phyla or "types" remained distinct, or at best
+connected by the most general of bonds.
+
+The close affinity of transcendentalists and evolutionists is shown very
+clearly in their common contrast in habits of thought with the Cuvierian
+school. It is the cardinal principle of pure morphology that function
+must be excluded from consideration. This is a necessary and unavoidable
+simplification which must be carried out if there is to be a science of
+pure form at all. But this limitation of outlook, if carried over from
+morphology to general biology becomes harmful, since it wilfully ignores
+one whole side of life--and that the most important. The functional
+point of view is clearly indispensable for any general understanding of
+living things, and this is where the Cuvierian school has the advantage
+over the transcendental--its principles are applicable to biology in
+general.
+
+Geoffroy and Cuvier in pre-evolutionary times well typified the contrast
+between the formal and the functional standpoints. For Geoffroy form
+determined function, while for Cuvier function determined form. Geoffroy
+held that Nature formed nothing new, but adapted existing "materials of
+organisation" to meet new needs. Cuvier, on the other hand, was always
+ready to admit Nature's power to form entirely new organs in response to
+new functional requirements.
+
+The evolutionists followed Geoffroy rather than Cuvier. They laid great
+store by homological resemblances, and dismissed analogies of structure
+as of little interest. They were singularly unwilling to admit the
+existence of convergence or of parallel evolution, and they held very
+firmly the distinctively Geoffroyan view that Nature is so limited by
+the unity of composition that she can and does form no new organs.
+
+By no one has this underlying principle of evolutionary morphology been
+more explicitly recognised than by Hubrecht, who in his paper of 1887,
+after summarising the points of resemblance between Nemertines and
+Vertebrates which led him to assume a genetic connection between them,
+writes as follows:--"At the base of all the speculations contained in
+this chapter lies the conviction, so strongly insisted upon by Darwin,
+that new combinations or organs do not appear by the action of natural
+selection unless others have preceded, from which they are gradually
+derived by a slow change and differentiation.
+
+"That a notochord should develop out of the archenteric wall because a
+supporting axis would be beneficial to the animal may be a teleological
+assumption, but it is at the same time an evolutional heresy. It would
+never be fruitful to try to connect the different variations offered,
+_e.g._, by the nervous system throughout the animal kingdom, if similar
+assumptions were admitted, for there would be then quite as much to say
+for a repeated and independent origin of central nervous systems out of
+indifferent epiblast just as required in each special case. These would
+be steps that might bring us back a good way towards the doctrine of
+independent creations. The remembrance of Darwin's, Huxley's, and
+Gegenbaur's classical foundations, and of Balfour's and Weismann's
+brilliant superstructures, ought to warn us away from these dangerous
+regions" (p. 644).
+
+This same prejudice lies at the root of the idea of _Functionswechsel_,
+in spite of the general functional orientation of that idea.
+
+Dohrn's constant assumption is that Nature makes shift with old organs
+wherever possible, instead of forming new ones. He derives gill-slits
+from segmental organs, fins and limbs from gills, ribs from gill-arches,
+and so on, instead of admitting that these organs might quite as well
+have arisen independently. He objects on principle to the origin of
+organs _de novo_. Thus, rebutting the suggestion that certain organs
+which are not found in the lower Vertebrates might have arisen as new
+formations, he writes:--"Against this supposition the whole weight of
+all those objections can be directed that are to be brought in general
+against the method of explanation which consists in appealing without
+imperative necessity to the _Deus ex machina_, 'New formation,' which is
+neither better nor worse than _Generatio equivoca_" (p. 21).
+
+Of a similar nature was the objection to convergence.[456]
+
+Why, we may ask, were morphologists so unwilling to admit the creative
+power of life? Dohrn, for instance, was fully aware of the great
+transforming influence exerted by function upon form--his theory of
+_Functionswechsel_ regards as the most powerful agent of change the
+activity of the animal, its effort to make the best use of its organs,
+to apply them at need in new ways to meet new demands. Why then did he
+not go a step further and admit that the animal could by its own
+subconscious efforts form entirely new organs? Why did most
+morphologists join with him in belittling the organism's power of
+self-transformation?
+
+The reasons seem to have been several. There is first the fundamental
+reason, that the idea of an active creative organism is repugnant to the
+intelligence, and that we try by all means in our power to substitute
+for this some other conception. In so doing we instinctively fasten upon
+the relatively less living side of organisms--their routine habits and
+reflexes, their routine structure--and ignore the essential activity
+which they manifest both in behaviour and in form-change.
+
+We tend also to lay the causes of form-change, of evolution, as far as
+possible outside the living organism. With Darwin we seek the
+transforming factors in the environment rather than within the organism
+itself. We fight shy of the Lamarckian conception that the living thing
+obscurely works out its own salvation by blind and instinctive effort.
+We like to think of organisms as machines, as passive inventions[457]
+gradually perfected from generation to generation by some external
+agency, by environment or by natural selection, or what you will. All
+this makes us chary of believing that Nature is prodigal of new organs.
+
+Other causes of the unwillingness of morphologists to admit the new
+formation of organs are to be sought in the main principle of pure
+morphology itself, that the unity of plan imposes an iron limit upon
+adaptation, and in the powerful influence exercised at the time by
+materialistic habits of thought. Teleology had become a bugbear to the
+vast majority of biologists, and all real understanding of the Cuvierian
+attitude seems, in most cases, to have been lost, although, curiously
+enough, teleological conceptions were often unconsciously introduced in
+the course of discussions on the "utility" of organs in the struggle for
+existence.
+
+Evolutionary morphology, being for the most part a form of pure or
+non-functional morphology, agreed then in all essential respects with
+pre-evolutionary or transcendental morphology.
+
+But it contained the germ of a new conception which threw a new light
+upon the whole science of morphology. This was the conception of the
+organism as an historical being.
+
+We have seen this thought expressed with the utmost clearness by Darwin
+himself (_supra_, p. 233). In his eyes the structure and activities of
+the living thing were a heritage from a remote past, the organism was a
+living record of the achievements of its whole ancestral line. What a
+light this conception threw upon all biology! "When we no longer look at
+an organic being as a savage looks at a ship as something wholly beyond
+his comprehension; when we regard every production of Nature as one
+which has had a long history; when we contemplate every complex
+structure and instinct as the summing-up of many contrivances, each
+useful to the possessor, in the same way as any great mechanical
+invention is the summing-up of the labour, the experience, the reason,
+and even the blunders of numerous workmen; when we thus view each
+organic being, how far more interesting--I speak from experience--does
+the study of natural history become!" (_Origin_, 6th ed., pp. 665-6).
+
+Sedgwick expressed the same thing from the morphological point of view
+when he wrote, with reference to the ancestral significance of the
+blastopore:--"If there is anything in the theory of evolution, every
+change in the embryo must have had a counterpart in the history of the
+race, and it is our business as morphologists to find it out" (p. 49,
+1884).
+
+By the evolution-theory the problems of form were linked indissolubly
+with the problem of heredity. Unity of plan could no longer be explained
+idealistically as the manifestation of Divine archetypal ideas; it had a
+real historical basis, and was due to inheritance from a common
+ancestor. The evolution-theory gave meaning and intelligibility to the
+transcendental conception of the unity of plan; in particular it
+supplied a simple and satisfying explanation of those puzzling vestigial
+organs, whose existence was such a stumbling-block to the teleologists.
+It enabled the biogenetic law to be substituted for the laws of
+Meckel-Serres and von Baer, as being in some measure a combination and
+interpretation of both.
+
+Where the concept of evolution proved itself particularly useful was in
+the interpretation of structures which were not immediately conditioned
+by adaptation to present requirements, such as, for instance, the
+arrangement of gill-slits and aortic arches in the foetus of land
+Vertebrates. Such "heritage characters" could only be explained on the
+hypothesis that they had once had functional or adaptational meaning.
+Why, for instance, should the blastopore so often appear as a long slit,
+closing by concrescence, unless this had been the original method of its
+formation in remote Coelenterate ancestors?
+
+The point hardly requires elaboration, since it has become an integral
+part of all our thinking on biological problems. It may be as well,
+however, for the sake of continuity, to give one or two examples of the
+historical interpretation of animal structures. The first may
+conveniently be the phylogenetic interpretation of the contrast between
+"membrane" and "cartilage" bones.
+
+In his _Grundzuege_ of 1870, Gegenbaur made the suggestion that the
+investing or membrane bones were derived phylogenetically from
+integumentary ossifications, and this was worked out in detail a few
+years later by O. Hertwig.[458]
+
+Many years before, several observers--J. Mueller, Williamson, and
+Steenstrup--had been struck with the resemblance existing between the
+placoid scales and the teeth of Elasmobranch fishes. Hertwig followed up
+this clue, and came to the conclusion not only that placoid scales and
+teeth were strictly homologous, but also that all membrane bones were
+derived phylogenetically from ossifications present in the skin or in
+the mucous membrane of the mouth, just as cartilage bones were derived
+from the cartilaginous skeletons of the primitive Vertebrates. In some
+cases this manner of derivation could even be observed in ontogeny, as
+Reichert had seen in the Newt, where certain bones in the roof of the
+mouth are actually formed by the concrescence of little teeth, (_supra_,
+p. 163). Hertwig considered that the following bones were originally
+formed by coalescence of teeth--parasphenoid, vomer, palatine,
+pterygoid, the tooth-bearing part of the pre-maxillary, the maxillary,
+the dentary and certain bones of the hyo-mandibular skeleton of
+Teleosts. All the investing bones (_Deckknochen_) of the skull were of
+common origin, and could be traced back to integumentary skeletal
+plates, which in the ancestral fish formed a dense carapace.
+
+These conclusions were accepted by Koelliker himself, who wrote in his
+_Entwickelungsgeschichte_ (1879)--"The distinction between the primary
+or primordial, and the investing or secondary bones is from the
+morphological standpoint sharp and definite. The former are
+ossifications of the (cartilaginous) primordial skeleton, the latter are
+formed outside this skeleton, and are probably all ossifications of the
+skin or the mucous membrane" (p. 464).
+
+Gegenbaur[459] consistently upheld the phylogenetic derivation of
+investing bones from dermal ossifications, and even went further and
+derived substitutionary bones as well from the integument, thus
+establishing a direct comparison between the skeletal formations of
+Vertebrates and Invertebrates. Investing bones were actual integumentary
+ossifications which had gradually sunk beneath the skin to become part
+of the internal skeleton; substitutionary bones were produced by cells
+(osteoblasts) which were ultimately derived from the integument.[460]
+
+A further instance of the historical interpretation of animal structure,
+taken from quite a different field, is afforded by the speculations of
+Dollo[461] on the ancestral history of the Marsupials. In a brilliant
+paper of 1880[462] Huxley made the suggestion that the ancestors of
+Marsupials were arboreal forms. "I think it probable," he wrote, "from
+the character of the pes, that the primitive forms, whence the existing
+Marsupialia have been derived, were arboreal animals; and it is not
+difficult, I conceive, to see that, with such habits, it may have been
+highly advantageous to an animal to get rid of its young from the
+interior of its body at as early a period of development as possible,
+and to supply it with nourishment during the later periods through the
+lacteal glands, rather than through an imperfect form of placenta" (p.
+655). Dollo followed up this suggestion, which had in the meantime been
+strengthened by Hill's discovery of a true allantoic placenta in
+_Perameles_, by demonstrating in the foot of present-day Marsupials
+certain features which could only be interpreted as inherited from a
+time when the ancestors of Marsupials were tree-living animals. These
+were the occurrence of an opposable big toe (when this was present at
+all), the great development of the fourth toe, the reduction and partial
+syndactylism of the second and third toes, and in some cases the
+regression of the nails. These characters were shown to be typical of
+arboreal Vertebrates, and their occurrence in forms not arboreal
+indicated that these were descended from tree-living ancestors. Traces
+of an arboreal ancestry could be demonstrated even in the marsupial mole
+_Notoryctes_.
+
+These are only two examples out of hundreds that might be given. Present
+day structure was interpreted in the light of past history; the common
+element in organic form was seen to be due to common descent; the
+existence of vestigial and non-functional organs was no longer a riddle.
+
+There was even a tendency to concentrate attention upon the historical
+side of structure, upon what the animal passively inherited rather than
+upon what it personally achieved. Homologies were considered more
+interesting than analogies, vestigial organs more interesting than
+foetal and larval adaptations. Convergence was anathema. The dead-weight
+of the past was appreciated at its full and more than its full value;
+and the essential vital activity of the living thing, so clearly shown
+in development and regeneration, was ignored or forgotten.
+
+But evolutionary morphology for all practical purposes was a development
+of pure or idealistic morphology, and was powerless to bring to fruit
+the new conception with which evolution-theory had enriched it. The
+reason is not far to seek. Pure morphology is essentially a science of
+comparison which seeks to disentangle the unity hidden beneath the
+diversity of organic form. It is not immediately concerned with the
+causes of organic diversity--that is rather the task of the sciences of
+the individual, heredity and development. To take an example--the
+recapitulation theory may legitimately be used as a law of pure
+morphology, as stating the abstract relation of ontogeny to phylogeny,
+and the probable line of descent of any organism may be deduced from it,
+as a mere matter of the ideal derivation of one form from another; but
+an explanation of the reason for the recapitulation of ancestral history
+during development can clearly not be given by pure morphology unaided.
+From the fact that the common starfish shows in the course of its
+development distinct traces of a stalk[463] it is possible to infer,
+taking other evidence also into consideration, that the ancestors of the
+starfish were at one stage of their existence stalked and sessile
+organisms. But this leaves unanswered the question as to how and why the
+starfish does still repeat after so many millions of years part of the
+organisation of one of its remote ancestors. Why is this feature
+retained, and by what means has it been conserved through countless
+generations? It is clear that the answer can be given only by a science
+of the causes of the production and retention of form, by a causal
+morphology, based upon a study of heredity and development.
+
+From the point of view of the pure morphologist the recapitulation
+theory is an instrument of research enabling him to reconstruct probable
+lines of descent; from the standpoint of the student of development and
+heredity the fact of recapitulation is a difficult problem whose
+solution would perhaps give the key to a true understanding of the real
+nature of heredity.
+
+To make full use of the conception of the organism as an historical
+being it is necessary then to understand the causal nexus between
+ontogeny and phylogeny.
+
+We shall see in the next chapter that the transformation of morphology
+from a comparative to a causal science did take place towards the end of
+the century, and that some progress was made towards an understanding of
+the relation between individual development and ancestral history,
+particularly by Roux and Samuel Butler, working with the fruitful
+Lamarckian conception of the transforming power of function.
+
+    [456] The importance of convergence came to be realised
+    after the vogue of phylogenetic speculation had
+    passed--see Friedmann, _Die Konvergenz der Organismen_,
+    Berlin, 1904, and A. Willey, _Convergence in Evolution_,
+    London, 1911. Also L. Vialleton, _Elements de
+    morphologie des Vertebres_, Paris, 1912.
+
+    [457] From this point of view there is a very profound
+    analogy between artificial and natural selection. Upon
+    the theory of natural selection organisms are lifeless
+    constructs which are mechanically perfected by external
+    agency, just as machines are improved by a process of
+    conscious selection of the most successful among a
+    number of competing models. (_Cf._ passage quoted below,
+    on p. 308.)
+
+    [458] _Arch. f. mikr. Anat._, xi. (suppl.), 1874; _Morph.
+    Jahrb._, ii., 1876, v. 1879, and vii., 1882.
+
+    [459] _Vergleich. Anat. d. Wirbelthiere_, i., pp. 200-1,
+    1898.
+
+    [460] For a full historical account of work on membrane
+    and cartilage bones (as well as on the theory of the
+    skull) see E. Gaupp, "Altere und neuere Arbeiten ueber
+    den Wirbelthierschaedel," _Ergeb. Anat. Entw._, x., 1901,
+    and "Die Entwickelung des Kopfskelettes," in Hertwig's
+    "_Handbuch vergl. exper. Entwickelungslehre d.
+    Wirbelthiere_," iii., 2, pp. 573-874, 1905.
+
+    [461] "Les Ancetres des Marsupiaux etaient-ils
+    arboricoles?" _Trav. Stat. zool. Wimereux_, vii., pp.
+    188-203, pls. xi.-xii., 1899. See also Bensley, _Trans.
+    Linn. Soc._ (2) ix., pp. 83-214, 1903.
+
+    [462] _Proc. Zool. Soc._, pp. 649-62, 1880. _Sci. Mem._,
+    iv., pp. 457-72.
+
+    [463] J. F. Gemmill, _Phil. Trans. B_, ccv., p. 255, 1914.
+
+
+
+
+CHAPTER XVIII
+
+THE BEGINNINGS OF CAUSAL MORPHOLOGY
+
+
+Until well into the 'eighties animal morphology remained a purely
+descriptive science, content to state and summarise the relations
+between the coexistent and successive form-states of the same and of
+different animals. No serious attempt had been made to discover the
+causes which led to the production of form in the individual and in the
+race.
+
+It is true that evolution-theory had offered a simple solution of the
+great problem of the unity in diversity of animal forms, but this
+solution was formal merely, and went little beyond that abstract
+deduction of more complex from simpler forms, which had been the main
+operation of pre-evolutionary morphology. Little was known of the actual
+causes of ontogeny, and nothing at all of the causes of phylogeny; it
+was, for instance, mere rhetoric on Haeckel's part to proclaim that
+phylogeny was the mechanical cause of ontogeny.
+
+Animal physiology, on its side, had developed in complete isolation from
+morphology into a science of the functioning of the adult and finished
+animal, considered as a more or less stable physico-chemical mechanism.
+Since the days of Ludwig, Claude Bernard and E. du Bois Reymond, the
+physiologists' chief care had been to analyse vital activities into
+their component physical and chemical processes, and to trace out the
+interchange of matter and energy between the organism and its
+environment. Physiologists had left untouched, perhaps wisely, the much
+more difficult problem of the causes of the development of form. For all
+practical purposes they took the animal-machine as given, and did not
+trouble about its mode of origin. They held indeed that form-production
+was due to a complex of physico-chemical causes, which they hoped some
+day to unravel;[464] but this future physiology of development remained
+quite embryonic.
+
+Physiology then had not really come into contact with the problems of
+form, and it could give the morphologist no direct help when he turned
+to investigate the causes of form-production. It had, however, a
+determining influence upon the methods of those who first broke ground
+in this No Man's Land between morphology proper and physiology. But it
+is significant that it was a morphologist and not a physiologist that
+did the first spade-work.
+
+The pioneer in this field, both as investigator and as thinker, was W.
+Roux, who sketched in the 'eighties the main outlines of a new science
+of causal morphology, to which he gave the name of
+_Entwicklungsmechanik_. The choice of name was deliberate, and the word
+implied, first, that the new science was essentially an investigation of
+the development of form, not of the mode of action of a formed
+mechanism, and second, that the methods to be adopted were
+mechanistic.[465]
+
+Though Roux was the only begetter of the science of
+_Entwicklungsmechanik_, he was, of course, not the first to investigate
+experimentally the formative processes of animal life. Study of
+regeneration dates back to Trembley (1740-44), Reaumur (1742), Bonnet
+(1745), and Spallanzani (1768-82),[466] and in the years preceding Roux's
+activity good work was done by Philipeaux. A beginning had been made
+with experimental teratology by E. Geoffroy St Hilaire and others, and
+the work of C. Dareste[467] remains classical. Back in the 18th century,
+some of John Hunter's experiments had a bearing upon the problems of
+form; his work on transplantation was followed up in the 19th century by
+Flourens, P. Bert, Ollier and many others. In founding in 1872 the
+_Archives de Zoologie experimentale et generale_ H. de Lacaze-Duthiers
+put forward in his introduction a powerful plea for the use of the
+experimental method in zoology.
+
+In some ways more directly connected with _Entwicklungsmechanik_ was
+His's attempt in 1874[468] to explain on mechanical principles the
+formation of certain of the embryonic organs by the bendings and
+foldings of tubes or plates of cells. "His compared the various layers
+of the chick embryo to elastic plates and tubes; out of these he
+suggested that some of the principal organs might be moulded by mere
+local inequalities of growth--the ventricles of the brain, for instance,
+the alimentary canal, the heart--and he further succeeded in imitating
+the formation of these organs by folding, pinching, and cutting
+india-rubber tubes and plates in various ways."[469]
+
+But Roux was undoubtedly the first to make a systematic survey of the
+problems to be solved and to work out an organised method of attack. His
+earliest work deals with the important problem of functional
+adaptation--its importance to the organism, and its possible mechanistic
+explanation. The first paper[470] was a study of the branching and
+distribution of the arteries in the human body (1878), and a second
+paper on the same subject followed in 1879.[471]
+
+In these papers Roux showed how the development of the blood-vascular
+system was largely determined by direct adaptation to functional
+requirements, and he inferred the existence in the vascular tissues of
+certain vital properties, in virtue of which the functional adaptation
+of the blood-vessels came about. Thus the intima or inner lining must
+possess the faculty of so reacting to the friction set up by the
+blood-current as to oppose the least possible resistance to its flow;
+the muscular coats must react to increased pressure by growing thicker,
+and so on.
+
+These papers were followed in 1881 by his well-known book, _Der Kampf
+der Theile im Organismus_, which contained the working-out of his
+mechanistic explanation of functional adaptation, and most of the
+elements of his general "causal-analytical" theory of form production.
+The significance of the book was popularly considered at the time to lie
+in its supposed application of the selection idea to the explanation of
+the internal adaptedness of animal structure--in the theory of "cellular
+selection," and the book owed its success to its fitting in so well with
+the prevalent Darwinism of the day. But its real importance, as a big
+step towards causal morphology, was naturally not so fully appreciated.
+
+During the next few years Roux continued his studies on functional
+adaptation,[472] and at the same time made a new departure by
+inaugurating, almost contemporaneously with the physiologist Pflueger,
+the study of experimental embryology. Isolated observations had
+previously been made upon the development of single blastomeres or parts
+of blastulae, by Haeckel and Chun for instance,[473] but Roux[474] and
+Pflueger[475] were the first to investigate the subject systematically,
+choosing for their work the egg of the frog.[476] Roux continued for many
+years to follow up this line of work.[477]
+
+In 1890 he drew up a programme and manifesto[478] of
+_Entwicklungsmechanik_ as "an anatomical science of the future," and in
+1895 he founded the famous _Archiv fuer Entwicklungsmechanik_,[479]
+publishing in the same year the two large volumes of his collected
+papers,[480] of which the first volume dealt with functional adaptation,
+the second with experimental embryology.
+
+His subsequent work includes several important general papers;[481]
+besides a number of special memoirs dealing with the factors of
+development, and with his original subject, functional adaptation.[482]
+
+In our sketch of his views we shall have occasion to refer particularly
+to his publications of 1881, 1895 (the _Einleitung_), 1902, 1905, and
+1910.
+
+Although Roux's biological philosophy is out-and-out mechanistic, he yet
+recognises the difficulty, even the impossibility, of straightway
+reducing development to the physico-chemical level. He tries to steer a
+course midway between the simplicist conceptions of the materialists and
+the "metaphysics" of the neo-vitalist school, which the experimental
+study of development and regeneration soon brought into being. In 1895
+he writes:--"The too simple mechanistic conception on the one hand, and
+the metaphysical conception on the other represent the Scylla and
+Charybdis, between which to sail is indeed difficult, and so far by few
+satisfactorily accomplished; it cannot be denied that with the increase
+of knowledge the seduction of the second has lately notably increased"
+(p. 23).
+
+The _via media_ adopted by Roux is the analysis of development, not
+directly into simple physico-chemical processes, but into more complex
+organic processes dependent upon the fundamental properties of living
+matter. The aim of _Entwicklungsmechanik_ is defined by Roux to be the
+reduction of developmental events to the fewest and simplest
+_Wirkungsweisen_, or causal processes.[483] Two classes of causal
+processes may be distinguished, as "complex components" and "simple
+components" of development. The latter are directly explicable by the
+laws of physics and chemistry; the former, while in essence
+physico-chemical, are yet so very complicated that they cannot at
+present be reduced to physico-chemical terms. The ultimate aim of
+_Entwicklungsmechanik_ is to reduce development to its "simple
+components," but its main task at the present day and for many years to
+come is the analysis of development into its "complex components."
+
+These complex components must be accepted as having much of the validity
+of physical and chemical laws. They are mysterious in the sense that
+they cannot yet be explained mechanistically, but they are constant in
+their action, and under the same conditions produce always the same
+effect--hence they may be made the subject of strictly scientific study.
+They represent biological generalisations, in their way of equal
+validity with the generalisations of physics and chemistry.
+
+The principal "complex components" which Roux recognises are somewhat as
+follows:--First come the elementary cell-functions of assimilation and
+dissimilation, growth, reproduction and heredity, movement and
+self-division (as a special co-ordination of cell-movements). Then at a
+somewhat higher level, self-differentiation, and the trophic reaction to
+functional stimuli. Components of even greater complexity may also be
+distinguished, as, for instance, the biogenetic law. The various
+tropisms exhibited in development may be regarded as "directive" complex
+components. There must be added, not as being itself a component, but
+rather as a mode or peculiar property of all functioning, the
+omnipresent faculty of self-regulation.
+
+It will be noticed that Roux's "complex components" are simply the
+general properties or functions of organised matter.
+
+Expressing Roux's thought in another way, we might say that life can
+only be defined functionally, _i.e._, by an enumeration of the "complex
+components" or elementary functions which all living beings manifest,
+even down to the very simplest. "Living beings," writes Roux, "can at
+present be defined with any approach to completeness only functionally,
+that is to say, through characterisation of their activities, for we
+have an adequate acquaintance with their functions in a general way,
+though our knowledge of particulars is by no means complete" (p. 105,
+1905). Defined in the most general and abstract way, living things are
+material objects which persist in spite of their metabolism, and, by
+reason of their power of self-regulation, in spite also of the changes
+of the environment. This is the "functional minimum-definition of life"
+(pp. 106-7, 1905).
+
+We may now go on to consider the relation of function to form throughout
+the course of development. Roux distinguishes in all development two
+periods, in the first of which the organ is formed prior to and
+independent of its function, while in the second the differentiation and
+growth of the organ are dependent on its functioning. Latterly (1906 and
+1910) Roux has distinguished three periods, counting as the second the
+transition period when form is partly self-determined, partly determined
+by functioning. As this conception of Roux's is of the greatest
+importance we shall follow it out in some detail.
+
+The idea was first elaborated in the _Kampf der Theile_ (1881), where he
+wrote:--"There must be distinguished in the life of all the parts two
+periods, an embryonic in the broad sense, during which the parts
+develop, differentiate and grow of themselves, and a period of completer
+development, during which growth, and in many cases also the balance of
+assimilation over dissimilation, can come about only under the influence
+of stimuli" (p. 180). There is thus a period of self-differentiation in
+which the organs are roughly formed in anticipation of functioning, and
+a period of functional development in which the organs are perfected
+through functioning and only through functioning. The two periods cannot
+be sharply separated from one another, nor does the transition from the
+one to the other occur at the same time in the different tissues and
+organs.
+
+The conception is more fully expressed in 1905 as follows:--"This
+separation (of development into two periods) is intended only as a first
+beginning. The first period I called the embryonic period [Greek: kat'
+exochen] or the period of organ-rudiments. It includes the 'directly
+inherited' structures, _i.e._, the structures which are directly
+predetermined in the structure of the germ-plasm, as, for instance, the
+first differentiation of the germ, segmentation, the formation of the
+germ-layers and the organ-rudiments, as well as the next stage of
+'further differentiation,' and of _independent_ growth and maintenance,
+that is, of growth and maintenance which take place without the
+functioning of the organs.
+
+"This is accordingly the period of direct fashioning through the
+activity of the formative mechanism implicit in the germ-plasm, also the
+period of the self-conservation of the formed parts without active
+functioning.
+
+"The second period is the period of 'functional form-development.' It
+includes the further differentiation and the maintenance in their
+typical form of the organs laid down in the first period; and this is
+brought about by the exercise of the specific functions of the organs.
+This period adds the finishing touches to the finer functional
+differentiation of the organs, and so brings to pass the 'finer
+functional harmony' of all organs with the whole. The formative activity
+displayed during this period depends upon the circumstance that the
+functional stimulus, or rather the exercise by the organs of their
+specific functions, is accompanied by a subsidiary formative activity,
+which acts partly by producing new form and partly by maintaining that
+which is already formed.... Between the two periods lies presumably a
+transition period, an intermediary stage of varying duration in the
+different organs, in which both classes of causes are concerned in the
+further building-up of the already formed, those of the first period in
+gradually decreasing measure, those of the second in an increasing
+degree" (pp. 94-6, 1905).
+
+In the first period the organ forms or determines the function, in the
+second period the function forms the organ, or at least completes its
+differentiation. It is characteristic that in the first period
+functionally adapted structure appears in the complete absence of the
+functional stimulus.
+
+The explanation of the difference between the two periods is to be found
+in the different evolutionary history of the characters formed during
+each. First-period characters are _inherited_ characters, and taken
+together constitute the historical basis of the organism's form and
+activity; second-period characters are those of later acquirement which
+have not yet become incorporated in the racial heritage.
+
+Inherited characters appear in development in the absence of the
+stimulus that originally called them forth; acquired characters are
+those that have not yet freed themselves from this dependence upon the
+functional stimulus. First-period characters were originally, like
+second-period characters, entirely dependent for their development upon
+the functional stimuli in response to which they arose, and only
+gradually in the course of generations did they gain that independence
+of the functional stimulus which stamps them as true inherited
+characters. Speaking of the formative stimuli which are active in
+second-period development, Roux writes:--"These stimuli can also produce
+new structure, which if it is constantly formed throughout many
+generations finally becomes hereditary, _i.e._, develops in the
+descendants in the absence of the stimuli, becomes in our sense
+embryonic" (p. 180, 1881). Again, "form-characteristics which were
+originally acquired in post-embryonic life through functional adaptation
+may be developed in the embryo without the functional stimulus, and may
+in later development become more or less completely differentiated, and
+retain this differentiation without functional activity or with a
+minimum of it. But in the continued absence of functional activity they
+become atrophied ... and in the end disappear" (p. 201, 1881).
+
+This conception of the nature of hereditary transmission is an important
+one, and constitutes the first big step towards a real understanding of
+the historical element in organic form and activity. It supplies a
+practical criterion for the distinguishing of "heritage" characters from
+acquired characters, of palingenetic from cenogenetic--a criterion which
+descriptive morphology was unable to find.[484] The introduction of a
+functional moment into the concept of heredity was a methodological
+advance of the first importance, for it linked up in an understandable
+way the problems of embryology, and indirectly of all morphology, with
+the problem of hereditary transmission, and gave form and substance to
+the conception of the organism as an historical being.
+
+It is this element in Roux's theories that puts them so far in advance
+of those of Weismann. Weismann did not really tackle the big problem of
+the relation of form to function, and he left no place in his mechanical
+system of preformation for functional or second-period development; he
+conceived all development to be in Roux's sense embryonic, and due to
+the automatic unpacking of a complex germinal organisation. Roux himself
+was to a certain extent a preformationist, for the development of his
+first-period characters is conditioned by the inherited organisation of
+the germ-plasm, and is purely automatic. It was indeed his experiments
+on the frog's egg (1888) that supplied some of the strongest evidence in
+favour of the mosaic theory of development. The number of _Anlagen_
+which he postulates in the germ is however small, and the germ-plasm in
+his conception of it has a relatively simple structure (p. 103, 1905).
+
+The transmission of acquired characters forms, of course, an integral
+part of Roux's conception of heredity and development, for without this
+transmission second-stage characters could not be transformed into
+first-stage characters. He discusses this difficult question at some
+length in the _Kampf der Theile_, coming to the conclusion that such
+transmission takes place in small degree and gradually, and that many
+generations are required before a new character can become hereditary.
+He thinks that acquired characters are probably transmitted at the
+chemical level. It is conceivable that acquired form-changes are
+dependent on chemical changes, or are correlative with such, and that,
+since the germ-cells stand in close metabolic relations with the soma,
+these chemical changes may soak through to the germ-cells and so modify
+them that a predisposition will appear in the descendants towards
+similar form-changes.[485] From this point of view the problem of
+transmission might be merged in the broader problem of the production of
+form through chemical processes--the central problem of all development.
+
+Inherited characters develop by an automatic process of
+self-differentiation, and the separate parts of the embryo show during
+this first period a surprising functional independence of one another.
+But this state of things changes progressively as the second period is
+reached, until finally all form-production and maintenance and all
+correlation depend upon functioning. It is in the first period of
+automatic development through internal "determining" factors that the
+"developmental" functions in the strict sense, _e.g._ automatic growth,
+division and self-differentiation, are most clearly shown. In the second
+or "functional" period the formative influence of function upon
+structure comes into play, and development becomes largely a matter of
+"functional adaptation" to functional requirements.
+
+All structure, according to Roux, is either functional or
+non-functional. The former includes all structure that is adapted to
+subserve some function. "Such 'functional structures' are, for example,
+the composition of striated muscle fibres out of fibrillae and these out
+of muscle-prisms, or again the length and thickness of the muscles, the
+static structure of the bones, the composition of the stomach and the
+blood-vessels out of longitudinal and circular fibres, the external
+shape of the vertebral centra and of the cuneiform bones of the foot"
+(p. 73, 1910). Indeed, as Cuvier had already pointed out, practically
+every organ in the body shows a functional structure which is accurately
+and minutely adjusted to the function it is intended to perform. Thus,
+to take some further examples, the arteries are admirably adapted as
+regards size of lumen, elasticity of wall, direction of branching, to
+conduct the blood to all parts of the body with the least possible waste
+of the propelling power through frictional resistance. So, too, the
+spongy substance of the long bones is arranged in lamellae which take the
+direction of the principal stresses and strains which fall upon the
+bones in action.
+
+Functional structure may be formed either in the first or in the second
+period of development, may be either inherited or acquired, but it
+reaches its full differentiation only in the second period, _i.e._,
+under the influence of functioning. Practically speaking, functional
+structure is directly dependent for its full development and for its
+continued conservation upon the exercise of the particular function
+which it serves. In the second period, but not in the first, increased
+use leads to hypertrophy of the functional structure, disuse to atrophy.
+
+From functional structure is to be distinguished nonfunctional
+structure, which has no relation to the bodily functions--is neither
+adapted to perform any of these, nor has arisen as a by-product of
+functional activity. "To this category belong, for example, among
+typical structures, the triangular form of the cross-section of the
+tibia, the dolicocephalic or brachycephalic shape of the skull, most of
+the external characters distinguishing genera and species, many of the
+external features of the embryo which change in the course of
+development, besides most of the abnormal forms shown by monstrosities,
+tumours, etc." (p. 74, 1910). Non-functional structure is not affected
+by functional adaptation, and may accordingly be left out of
+consideration here.
+
+Now the influence of functioning upon the form and structure of an organ
+is twofold. There is first the immediate change brought about by the
+very act of functioning--for example, the shortening and thickening of
+skeletal muscles when they act. This is a purely temporary change, for
+the organ at once returns to its normal quiescent state as soon as it
+ceases to function. Such temporary functional change, brought about in
+the moment of functioning, is usually dependent for its initiation upon
+some neuro-muscular mechanism, though it may be elicited also by a
+chemical stimulus. It is thus always a phenomenon of "behaviour." "From
+such temporary changes are sharply to be distinguished all permanent
+alterations which first appear in perceptible fashion through
+oft-repeated or long-continued, enhanced functional activity. These
+produce a new and lasting internal equilibrium of the organ, consisting
+in an insertion of new molecules or a rearrangement of old. For this
+reason they outlast the periods of functional form-change, or, if as in
+the case of the muscles they themselves alter during functional
+activity, they regain their state when the organ ceases to function" (p.
+72, 1910). "Oft-repeated exercise or heightened exercise of the specific
+functions, or repeated action of the functional stimuli which determine
+them, produces, as we have said before, true form-changes as a
+by-product. These are of two kinds. In so far as these form-changes
+facilitate the repetition of the specific functions, I have called them
+_functional adaptations_.... Such as do not improve the functioning of
+the organ are indeed by-products of functioning, but without adaptive
+character; they do not belong to the class of functional adaptations at
+all" (p. 75, 1910).
+
+We may now enquire in what way functional adaptations can arise as
+by-products of functioning.
+
+It is clear that natural selection in the sense of individual or
+"personal" selection cannot adequately explain the origin of functional
+structure and the functional harmony of structure, for thousands of
+cells would have to vary together in a purposive way before any real
+advantage could be gained in the struggle for existence, and it is in
+the highest degree unlikely that this should come about by chance
+variation.[486] The development of purposive internal structure is only to
+be explained by the properties of the tissues concerned.
+
+In illustration and proof of the statement that functional adaptation is
+due to the properties of the tissues we may adduce the development and
+regulation of the blood-vascular system, which has been thoroughly
+studied from this point of view by Roux and Oppel (1910).
+
+It appears that only the very first rudiments of the vascular system are
+laid down in the short first period of automatic non-functional
+development. All the subsequent growth and differentiation of the
+blood-vessels falls into the second period, and is due wholly or in
+great part to direct functional adaptation to the requirements of the
+tissues. Thus from the rudiments formed in the first period there sprout
+out the definitive vessels in direct adaptation to the food-consumption
+of the tissues they are to supply. The size, direction and intimate
+structure of these vessels are accurately adjusted to the part they play
+in the economy of the whole, and this adjustment is brought about in
+virtue of the peculiar properties or reaction-capabilities of the
+different tissues of which the blood-vessels are composed.
+
+The properties which Roux finds himself compelled to postulate in the
+vascular tissues, after a thorough-going analysis of the different kinds
+of functional adaptation shown by the blood-vessels, are summarised by
+him as follows:--
+
+"(1) The faculty--depending on a direct sensibility possessed by the
+endothelium and perhaps also by the other layers of the intima--of
+yielding to the impact of the blood, so far as the external relations of
+the vessel permit. In this way the wall adapts itself to the
+haemodynamically conditioned 'natural' shape of the blood-stream, and
+reaches this shape as nearly as possible." Through this faculty of the
+lining tissue of the blood-vessels, the size of the lumen and the
+direction of branching are so regulated as to oppose the least possible
+resistance to the flow of the blood.
+
+"(2) The faculty possessed by the endothelium of the capillaries of each
+organ of adapting itself qualitatively to the particular metabolism of
+the organ." This adaptedness of the capillaries is, however, more
+usually an inherited state, _i.e._, brought about in the first period of
+development.
+
+"(3) The faculty possessed by the capillary walls of being stimulated to
+sprout out and branch by increased functioning, _i.e._, by increased
+diffusion, and their power to exhibit a chemically conditioned
+cytotropism, which causes the sprouts to find one another and unite. A
+similar process can be directly observed in isolated segmentation-cells,
+which tend to unite in consequence of a power of mutual attraction.
+
+"(4) The faculty of developing normal arterial walls in response to
+strong intermittent pressure, and normal venous walls in response to
+continuous lesser pressure." It has been shown, for instance, by Fischer
+and Schmieden that in dogs a section of vein transplanted into an artery
+takes on an arterial structure, at least as regards the circular
+musculature, which doubles in thickness.
+
+"(5) The power to regulate the normal[487] length of the arteries and
+veins, in adaptation to the growth of the surrounding tissues, in such a
+way that the stretching action of the blood-stream brings the vessel to
+its proper functional length.
+
+"(6) The power to form, in response to slight increases in longitudinal
+tension, new structural parts which take their place alongside the
+existing longitudinal fibres.
+
+"(7) The power to regulate the width of the circular musculature
+according to the degree of food-consumption by the tissues, in response
+to nerve impulses initiated in these tissues.
+
+"(8) The power possessed by the circular musculature of responding to
+such continuous functional widening, by the formation of new structural
+parts in the circular musculature, and so of widening the vessel
+permanently or by this new formation of muscular fibres thickening the
+circular musculature.
+
+"(9) The faculty of being stimulated by increased blood-pressure to
+produce the same structural changes as mentioned in par. 8, though here
+the response is otherwise conditioned" (pp. 126-7, 1910).
+
+It is by virtue of the tissue-properties detailed above that the complex
+functional adaptations of the blood-vessels come about.
+
+The development of the vascular system is no mere automatic and
+mechanical production of form, apart from and independent of
+functioning; it implies a living and co-ordinated activity of the
+tissues and organs concerned, a power of active response to foreseen and
+unforeseen contingencies. Form is then not something fixed and
+congealed--it is the ever-changing manifestation of functional activity.
+"Since most of the structure and form of the blood-vessels arises in
+direct adaptation to function, the vessels of adult men and animals are
+no fixed structures, which, once formed, retain their form and
+structural build unchanged throughout life; on the contrary, they
+require even for their continued existence the stimulus of functional
+activity.... The fully formed blood-vessels are no static structures,
+such as they appear to be according to the teaching of normal histology,
+and such as they have long been taken to be. Observation and description
+of normal development never shows us anything but the visible side of
+organic happenings, the _products_ of activity, and leaves us ignorant
+of the real processes of form-development and form-conservation, and of
+their causes" (p. 125, 1910).
+
+The real thing in organisation is not form but activity. It is in this
+return to the Cuvierian or functional attitude to the problems of form
+that we hold Roux's greatest service to biology to consist. The
+attitude, however, seems to smack of vitalism, and Roux, as we have
+seen, is no vitalist. He holds that the marvellous and apparently
+purposive tissue-qualities which underlie all processes of functional
+adaptation have arisen "naturally," in the course of evolution, by the
+action of natural selection upon the various properties, useful and
+useless, which appeared fortuitously in the primary living organisms. He
+is, moreover, deeply imbued with the materialistic philosophy of his
+youth, and it is indeed one of the chief characteristics of his system
+that he states the fundamental properties or qualities of life in terms
+of metabolism. A vital quality is for Roux a special process or mode of
+assimilation. The faculty of "morphological assimilation" whereby form
+is imposed upon formless chemical processes is the ultimate term of
+Roux's analysis--"the most general, most essential, and most
+characteristic formative activity of life" (p. 631, 1902).
+
+We have now to consider very briefly the early results achieved by
+Roux's fellow-workers in the field of causal morphology. As D. Barfurth
+points out,[488] the years 1880-90 saw a general awakening of interest in
+experimental morphology, and it is hard to say whether Roux's work was
+cause or consequence. "There fall into this period," writes Barfurth,
+"the experimental investigations by Born and Pflueger on the sexual
+difference in frogs (1881), by Pflueger 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. Pflueger and W. Roux: Pflueger
+publishing his researches on 'the influence of gravity on
+cell-division,' Roux his experimental investigations on 'the time of the
+determination of the chief planes in the frog-embryo.'... In the same
+year appeared A. Rauber's experimental studies 'on the influence of
+temperature, atmospheric pressure, and various substances on the
+development of animal ova,' which have brought many similar works in
+their train. The following year (1884) saw a lively controversy on
+Pflueger's gravity-experiments with animal eggs, in which took part
+Pflueger, Born, Roux, O. Hertwig and others, and in this year appeared
+work by Roux dealing with the experimental study of development, and in
+particular giving the results of the first definitely localised
+pricking-experiments on the frog's egg (in the _Schles. Gesell. f.
+vaterl. Kultur_, 15th Feb. 1884), also the important researches of M.
+Nussbaum and Gruber (followed up later by Verworn, Hofer and Balbiani)
+on Protozoa, and other experimental work" (pp. xi.-xii.).
+
+In 1888 appeared a famous paper by W. Roux,[489] in which he described how
+he had succeeded in killing by means of a hot needle one of the two
+first blastomeres of the frog's egg, and how a half-embryo had developed
+from the uninjured cell. Some years before[490] he had enunciated, at
+about the same time as Weismann, the view that development was brought
+about by a qualitative division of the germ-plasm contained in the
+nucleus, and that the complicated process of karyokinetic or mitotic
+division of the nucleus was essentially adapted to this end. He
+conceived that development proceeded by a mosaic-like distribution of
+potencies to the segmentation-cells, that, for instance, the first
+segmentation furrow separated off the material and potencies for the
+right half of the embryo from those for the left half. He had tried to
+show experimentally that the first furrow in the frog's egg coincided
+with the sagittal plane of the embryo,[491] and his later success in
+obtaining a half-embryo from one of the first two blastomeres seemed to
+establish the "mosaic theory" conclusively.
+
+Roux's needle-experiment aroused much interest, especially as Weismann's
+theory of heredity was then being keenly discussed. Chabry had published
+in 1887 some interesting results on the Ascidian egg,[492] which strongly
+supported the Roux-Weismann theory. Considerable astonishment was
+therefore caused by Driesch's announcement in 1891[493] that he had
+obtained complete larvae from single blastomeres of the sea-urchin's egg
+isolated at the two-celled stage. He followed this up in the next
+year[493] by showing that whole embryos could be produced from one or more
+blastomeres isolated at the four-cell stage. Similar or even more
+striking results were obtained by E. B. Wilson on _Amphioxus_,[494] and
+Zoja on medusae.[495] Driesch succeeded also in disturbing the normal
+course and order of segmentation by compressing the eggs of the
+sea-urchin between glass plates, and yet obtained normal embryos.
+Similar pressure-experiments were carried out on the frog by O.
+Hertwig,[496] and on _Nereis_ by E. B. Wilson,[497] with analogous results.
+
+In 1895 O. Schultze[498] showed that if the frog's egg is held between two
+plates and inverted at the two-celled stage there are formed two embryos
+instead of one. In the same year T. H. Morgan[499] repeated Roux's
+fundamental experiment of destroying one of the two blastomeres, but
+inverted the egg immediately after the operation--a whole embryo of half
+size resulted. A year or two later Herlitzka[500] found that if the first
+two blastomeres of the newt's egg were separated by constriction, two
+normal embryos of rather more than half normal size were formed.
+
+The main result of the first few years' work on the development of
+isolated blastomeres was to show that the mosaic theory was not strictly
+true, and that the hypothesis of a qualitative division of the nucleus
+was on the whole negatived by the facts.
+
+Evidence soon accumulated that the cytoplasm of the egg stood for much
+in the differentiation of the embryo. A number of years previously Chun
+had made the discovery that single blastomeres of the Ctenophore egg,
+isolated at the two-celled stage, gave half-embryos. This was in the
+main confirmed by Driesch and Morgan in 1896,[501] and they made the
+further interesting discovery that the same defective 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,[502] who removed the anucleate
+"yolk-lobe" from the egg of the mollusc _Ilyanassa_ at the two-celled
+stage, and obtained larvae which lacked a mesoblast. This result was
+brilliantly confirmed and extended some years later by E. B. Wilson,[503]
+working on the egg of _Dentalium_. He found that if the similar
+anucleate "polar lobe" of this form is removed at the two-celled stage,
+deficient larvae are formed, in which the post-trochal region and the
+apical organ are absent. He further showed that in the unsegmented but
+mature egg prelocalised cytoplasmic regions can be distinguished, which
+later become separated from one another through the segmentation of the
+egg. The segmentation-cells into which these cytoplasmic substances are
+thus segregated show a marked specificity of development, giving rise,
+even when isolated, to definite organs of the embryo. Wilson concluded
+that the cytoplasm of the egg contains a number of specific
+organ-forming stuffs, which have a definite topographical arrangement in
+the egg. Development is thus due in part to a qualitative division not
+of the nucleus but of the cytoplasm. Corroborative evidence of the
+existence of cytoplasmic organ-forming stuffs has been supplied for
+several other species, _e.g._, _Patella_ (Wilson), _Cynthia_ (Conklin),
+_Cerebratulus_ (Zeleny), and _Echinus_ (Boveri).
+
+It is interesting to recall that so long ago as 1874 W. His[504] put
+forward the theory that there exist in the blastoderm and even in the
+egg prelocalised areas, which contain the formative material for each
+organ of the embryo, and from which the embryo is developed by a simple
+process of unequal growth.
+
+The experimental study of form was prosecuted in many other directions
+besides that of experimental embryology. The study of regeneration and
+of regulatory processes attracted many workers, among whom may be
+mentioned T. H. Morgan, C. M. Child, and H. Driesch. In an interesting
+series of papers C. Herbst applied the principles of the physiology of
+stimulus to the interpretation of development.[505] The formative power of
+function was studied in Germany by Roux and his pupils, Fuld, O. Levy,
+Schepelmann and others, particularly by E. Babak. In France, F. Houssay
+inaugurated[506] an important series of memoirs by himself and his pupils
+on "dynamical morphology," the most important memoir being his own
+valuable discussion of the functional significance of form in fishes.[507]
+The principles of his dynamical morphology were first laid down in his
+book _La Forme et la Vie_ (1900).
+
+The famous experiments of Loeb, Delage and others on artificial
+parthenogenesis may also be mentioned, though their connection with
+morphology is somewhat remote.
+
+The period was characterised also by the lively discussion of first
+principles, in which Driesch took a leading part. Materialistic methods
+of interpretation were upheld by perhaps the majority of biologists, but
+vitalism found powerful support.
+
+    [464] See Carus's remark, referred to on p. 194, above.
+
+    [465] Roux, _Die Entwicklungsmechanik_, p. 26, Leipzig,
+    1905.
+
+    [466] T. H. Morgan, _Regeneration_, p. 1, New York and
+    London, 1901.
+
+    [467] _Recherches sur la production artificielle des
+    Monstruosites_, Paris, 1877, and many later papers.
+
+    [468] _Unsere Koerperform und das physiologische Problem
+    ihrer Entstehung_, Leipzig, 1874.
+
+    [469] J. W. Jenkinson, _Experimental Embryology_, p. 3,
+    Oxford, 1909.
+
+    [470] "Ueber die Verzweigungen der Blutgefaesse des
+    Menschen," _Jen. Zeit_., xii., 1878.
+
+    [471] "Ueber die Bedeutung der Ablenkung des
+    Arterienstammes bei der Astabgabe," _Jen. Zeit_., xiii.,
+    1879.
+
+    [472] "Beitraege zur Morphologie der funktionellen
+    Anpassung. I. Struktur eines hochdifferenzierten
+    bindgewebigen Organes (der Schwanzflosse des Delphin),"
+    _Arch. Anat. Physiol._ (_Anat. Abt._) for 1883. II.
+    "Ueber die Selbstregulation der 'morphologischen' Laenge
+    der Skeletmuskeln des Menschen," _Jen. Zeit._, xvi.,
+    1883. III. "Beschreibung ... einer
+    Kniegelenkeknochenankylose," _Arch. Anat. Physiol._
+    (_Anat. Abt._) for 1885.
+
+    [473] In 1869 and 1877 respectively (Roux, p. 53, 1905).
+
+    [474] _Ueber die Zeit. der Bestimmung der Hauptrichtungen
+    des Froschembryo_, Leipzig, 1883.
+
+    [475] "Ueber den Einfluss der Schwerkraft auf die Teilung
+    der Zellen," Pflueger's _Archiv_, xxxi., 1883. Also
+    subsequent papers in same journal.
+
+    [476] For an account of the classical experiments on the
+    frog's egg, see T. H. Morgan, _The Development of the
+    Frog's Egg_, New York, 1897.
+
+    [477] In a series of "Beitraege zur Entwicklungsmechanik
+    des Embryo," published in various journals from 1884 to
+    1891, all dealing with the frog's egg. Also in many
+    papers in the _Archiv f. Entw. mech._, from 1895
+    onwards.
+
+    [478] _Die Entwicklungsmechanik der Organismen, eine
+    anatomische Wissenschaft der Zukunft_, Wien, 1890.
+
+    [479] The first volume contains the important _Einleitung_
+    or general Introduction.
+
+    [480] _Gesammelte Abhandlungen ueber Entwicklungsmechanik
+    der Organismen_, 2 vols., Leipzig, 1895.
+
+    [481] "Fuer unser Programm und seine Verwirklichung,"
+    _A.E.M._, v., pp. 1-80 and 219-342, 1897. "Ueber die
+    Selbstregulation der Lebewesen," _A.E.M._, xiii., pp.
+    610-5, 1902. "Die Entwicklungsmechanik, ein neuer Zweig
+    der biologischen Wissenschaft," Heft I. of the _Vortraege
+    u. Aufsaetze ueber Entwicklungsmechanik der Organismen_,
+    Leipzig, 1905. Oppel and Roux, "Ueber die gestaltliche
+    Anpassung der Blutgefaesse," Heft x., of the _Vortraege u.
+    Aufsaetze_, Leipzig, 1910.
+
+    [482] "Ueber d. funkt. Anpassung des Muskelmagens der
+    Gans," _A.E.M._, xxi., pp. 461-99, 1906.
+
+    [483] The exact quantitative formulation of a
+    _Wirkungsweise_ constitutes a law. The word itself is
+    perhaps most conveniently rendered as "causal process."
+
+    [484] M. Fuerbringer, perhaps under the influence of Roux,
+    emphasised the importance, from a morphological point of
+    view, of studying post-embryonic (functional)
+    development, _Unters. z. Morph. u. Syst. der Voegel_,
+    ii., Amsterdam, p. 925, 1888.
+
+    [485] See, for the development of this idea, Oppel, in
+    Roux-Oppel, 1910.
+
+    [486] _Cf._ the controversy between Herbert Spencer and
+    Weismann on the subject of "coadaptation" in the
+    _Contemporary Review_ for 1893 and 1894. See also
+    Weismann's paper in _Darwin and Modern Science_,
+    Cambridge, 1909.
+
+    [487] That is, the length they take up when separated from
+    the body.
+
+    [488] "Wilhelm Roux zum 60. Geburtstage," _Arch. f.
+    Entw.-Mech._, xxx. _Festschrift fuer Prof. Roux_, Pt. i,
+    1910.
+
+    [489] Virchow's _Archiv_, cxiv., 1888. First announced in
+    Sept. 1887.
+
+    [490] _Ueber die Bedeutung der Kernteilungsfiguren_,
+    Leipzig, 1883.
+
+    [491] _Bresl. aertz. Zeitschr._, 1885.
+
+    [492] _Journ. de l'Anat. et de la Physiologie_, xxiii.,
+    1887.
+
+    [493] _Zeits. f. wiss. Zool._, liii., 1891 and 1892.
+
+    [494] _Journ. Morph._, viii., 1893.
+
+    [495] _Arch. f. Ent.-Mech._, i., 1895; ii., 1896.
+
+    [496] _Arch. f. mikr. Anat._, xliii., 1893.
+
+    [497] _Arch. f. Ent.-Mech._, iii., 1896.
+
+    [498] _Arch. f. Ent.-Mech._, i., 1895.
+
+    [499] _Anat. Anz._, x., 1895.
+
+    [500] _Arch. f. Ent.-Mech._, iv. 1897.
+
+    [501] _Arch. f. Ent.-Mech._, ii., 1896.
+
+    [502] _Arch. f. Ent.-Mech._, iii., 1896.
+
+    [503] _Journ. exper. Zool._, i., 1904.
+
+    [504] _Unsere Koerperform_, p. 19, Leipzig, 1874.
+
+    [505] _Biolog. Centrlbl._, xiv., 1894, xv., 1895.
+    _Formative Reize in der thierischen Ontogenese_,
+    Leipzig, 1901.
+
+    [506] "La Morphologie dynamique," No. i. of the
+    _Collection de Morphologie dynamique_, Paris, 1911.
+
+    [507] "Forme, Puissance et Stabilite des Poissons," No.
+    iv. of the _Collection_, Paris, 1912.
+
+
+
+
+CHAPTER XIX
+
+SAMUEL BUTLER AND THE MEMORY THEORIES OF HEREDITY
+
+
+We have laid stress upon the distinction established by Roux between the
+two stages of development--the automatic and the functional--because of
+the light which it seems to throw upon the phylogenetic relation of form
+to function. We have pointed out, too, the paramount 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 19th century. He was not a professional biologist, and much of his
+biological work is, for that reason, imperfect. But he brought to bear
+upon the central problems of biology an unbiassed and powerful
+intelligence, and his attitude to these problems, just because it is
+that of a cultivated layman, is singularly illuminating.
+
+He was not well acquainted with biological literature; he seems to have
+hit upon the main ideas of his theory of life and habit in complete
+independence of Lamarck, and only later to have become aware that
+Lamarck had in a measure forestalled him. He puts this very beautifully
+in the following passage from his chief biological work _Life and Habit_
+(1877[508]):--"I admit that when I began to write upon my subject I did
+not seriously believe in it. I saw, as it were, a pebble upon the
+ground, with a sheen that pleased me; taking it up, I turned it over and
+over for my amusement, and found it always grow brighter and brighter
+the more I examined it. At length I became fascinated, and gave loose
+rein to self-illusion. The aspect of the world changed; the trifle which
+I had picked up idly had proved to be a talisman of inestimable value,
+and had opened a door through which I caught glimpses of a strange and
+interesting transformation. Then came one who told me that the stone was
+not mine, but that it had been dropped by Lamarck, to whom it belonged
+rightfully, but who had lost it; whereon I said I cared not who was the
+owner, if only I might use it and enjoy it. Now, therefore, having
+polished it with what art and care one who is no jeweller could bestow
+upon it, I return it, as best I may, to its possessor" (p. 306). In one
+of his later works, however, Butler made up for his first neglect of his
+predecessors by giving what is undeniably the best account in English
+literature of the work of Buffon, Lamarck, and Erasmus Darwin--in his
+_Evolution, Old and New_ (1879). Many of his facts he took from Charles
+Darwin, whose theory of natural selection he bitterly opposed, in the
+two books just mentioned and in _Unconscious Memory_ (1880) and _Luck or
+Cunning_ (1887).
+
+Butler's main thesis is that living things are active, intelligent
+agents, personally continuous with all their ancestors, possessing an
+intense but unconscious memory of all that their ancestors did and
+suffered, and moving through habit from the spontaneity of striving to
+the automatism of remembrance.
+
+The primary cause of all variation in structure is the active response
+of the organism to needs experienced by it, and the indispensable link
+between the outer world and the creature itself is that same "sense of
+need" upon which Lamarck insisted. "According to Lamarck, genera and
+species have been evolved, in the main, by exactly the same process as
+that by which human inventions and civilisations are now progressing;
+and this involves that intelligence, ingenuity, heroism, and all the
+elements of romance, should have had the main share in the development
+of every herb and living creature around us" (_Life and Habit_, p. 253).
+Variations are indubitably the raw material of evolution--"The question
+is as to the origin and character of these variations. We say they
+mainly originate in a creature through a sense of its needs, and vary
+through the varying surroundings which will cause those needs to vary,
+and through the opening-up of new desires in many creatures, as the
+consequence of the gratification of old ones; they depend greatly on
+differences of individual capacity and temperament; they are
+communicated, and in the course of time transmitted, as what we call
+hereditary habits or structures, though these are only, in truth,
+intense and epitomised memories of how certain creatures liked to deal
+with protoplasm" (p. 267).
+
+Butler's theory then is essentially a bold and enlightened Lamarckism,
+completed and rounded off by the conception that heredity too is a
+psychological process, of the same nature as memory.
+
+In seeking to establish a close analogy between memory and heredity
+Butler starts out from the fact of common experience, that actions which
+on their first performance require the conscious exercise of will and
+intelligence, and are then carried out with difficulty and hesitation,
+gradually through long-continued practice come to be performed easily
+and automatically, without the conscious exercise of intelligence or
+will.
+
+He tries to show that this is a general law--that knowledge and will
+become intense and perfect only when through long-continued exercise
+they become automatic and unconscious--and he applies this conception to
+the elucidation of development.
+
+Developmental processes, especially the early ones (of Roux's first
+stage) are automatic and unconscious, and yet imply the possession by
+the embryo of a wonderfully perfect knowledge of the processes to be
+gone through, and an assured power of will and judgment. Is it
+conceivable, says Butler, that the embryo can do all these things
+without knowing how to do them, and without having done them before?
+"Shall we say ... that a baby of a day old sucks (which involves the
+whole principle of the pump, and hence a profound practical knowledge of
+the laws of pneumatics and hydrostatics), digests, oxygenises its blood
+(millions of years before Sir Humphrey Davy discovered oxygen), sees and
+hears--all most difficult and complicated operations, involving a
+knowledge of the facts concerning optics and acoustics, compared with
+which the discoveries of Newton sink into utter insignificance? Shall we
+say that a baby can do all these things at once, doing them so well and
+so regularly, without being even able to direct its attention to them,
+and without mistake, and at the same time not know how to do them, and
+never have done them before?" (p. 54). Assuredly not.
+
+The only possible explanation is that the embryo's ancestors have done
+these things so often, throughout so many millions of generations, that
+the embryo's knowledge of how to do them has become unconscious and
+automatic by reason of this age-long practice. This implies that there
+is in a very real sense actual personal continuity between the embryo
+and all its ancestors, so that their experiences are his, their memory
+also his. "We must suppose the continuity of life and sameness between
+living beings, whether plants or animals, to be far closer than we have
+hitherto believed; so that the experience of one person is not enjoyed
+by his successor, so much as that the successor is _bona fide_ but a
+part of the life of his progenitor, imbued with all his memories,
+profiting by all his experiences--which are, in fact, his own--and only
+unconscious of the extent of his own memories and experiences owing to
+their vastness and already infinite repetitions" (p. 50). It is very
+suggestive in this connection, he continues--"I. That we are _most
+conscious of, and have most control over_, such habits as speech, the
+upright position, the arts and sciences, which are acquisitions peculiar
+to the human race, always acquired after birth, and not common to
+ourselves and any ancestor who had not become entirely human.
+
+"II. That we are _less conscious of, and have less control over_, eating
+and drinking, swallowing, breathing, seeing and hearing, which were
+acquisitions of our prehuman ancestry, and for which we had provided
+ourselves with all the necessary apparatus before we saw light, but
+which are, geologically speaking, recent, or comparatively recent.
+
+"III. That we are _most unconscious of, and have least control over_,
+our digestion and circulation, which belonged even to our invertebrate
+ancestry, and which are habits, geologically speaking, of extreme
+antiquity.... Does it not seem as though the older and more confirmed
+the habit, the more unquestioning the act of volition, till, in the case
+of the oldest habits, the practice of succeeding existences has so
+formulated the procedure, that, on being once committed to such and such
+a line beyond a certain point, the subsequent course is so clear as to
+be open to no further doubt, to admit of no alternative, till the very
+power of questioning is gone, and even the consciousness of volition"
+(pp. 51-2).
+
+The hypothesis then, that heredity and development are due to
+unconscious memory, finds much to support it--"the self-development of
+each new life in succeeding generations--the various stages through
+which it passes (as it would appear, at first sight, without rhyme or
+reason), the manner in which it prepares structures of the most
+surpassing intricacy and delicacy, for which it has no use at the time
+when it prepares them, and the many elaborate instincts which it
+exhibits immediately on, and indeed before, birth--all point in the
+direction of habit and memory, as the only causes which could produce
+them" (p. 125). The hypothesis explains, for instance, the fact of
+recapitulation:--"Why should the embryo of any animal go through so many
+stages--embryological allusions to forefathers of a widely different
+type? And why, again, should the germs of the same kind of creature
+always go through the same stages? If the germ of any animal now living
+is, in its simplest state, but part of the personal identity of one of
+the original germs of all life whatsoever, and hence, if any now living
+organism must be considered without quibble as being itself millions of
+years old, and as imbued with an intense though unconscious memory of
+all that it has done sufficiently often to have made a permanent
+impression; if this be so, we can answer the above questions perfectly
+well. The creature goes through so many intermediate stages between its
+earliest state as life at all, and its latest development, for the
+simplest of all reasons, namely, because this is the road by which it
+has always hitherto travelled to its present differentiation; this is
+the road it knows, and into every turn and up or down of which it has
+been guided by the force of circumstances and the balance of
+considerations" (pp. 125-6).
+
+The hypothesis explains also the way in which the orderly succession of
+stages in embryogeny is brought about, for we can readily understand
+that the embryo will not remember any stage until it has passed through
+the stage immediately preceding it. "Each step of normal development
+will lead the impregnated ovum up to, and remind it of, its next
+ordinary course of action, in the same way as we, when we recite a
+well-known passage, are led up to each successive sentence by the
+sentence which has immediately preceded it.... Though the ovum
+immediately after impregnation is instinct with all the memories of both
+parents, not one of these memories can normally become active till both
+the ovum itself and its surroundings are sufficiently like what they
+respectively were, when the occurrence now to be remembered last took
+place. The memory will then immediately return, and the creature will do
+as it did on the last occasion that it was in like case as now. This
+ensures that similarity of order shall be preserved in all the stages of
+development in successive generations" (pp. 297-8).
+
+Abnormal conditions of development will cause the embryo to pause and
+hesitate, as if at a loss what to do, having no ancestral experience to
+guide it. Abnormalities of development represent the embryo's attempt to
+make the best of an unexpected situation. Or, as Butler puts it, "When
+... events are happening to it which, if it has the kind of memory we
+are attributing to it, would baffle that memory, or which have rarely or
+never been included in the category of its recollections, _it acts
+precisely as a creature acts_ _when its recollection is disturbed, or
+when it is required to do something which it has never done before_" (p.
+132). "It is certainly noteworthy that the embryo is never at a loss,
+unless something happens to it which has not usually happened to its
+forefathers, and which in the nature of things it cannot remember" (p.
+132).
+
+Butler's teleological conception of organic evolution was of course
+completely antagonistic to the naturalistic conceptions current in his
+time. In one of his later books he repeats Paley's arguments in favour
+of design, and to the question, "Where, then, is your designer of beasts
+and birds, of fishes, and of plants?" he replies: "Our answer is simple
+enough; it is that we can and do point to a living tangible person with
+flesh, blood, eyes, nose, ears, organs, senses, dimensions, who did of
+his own cunning, after infinite proof of every kind of hazard and
+experiment, scheme out and fashion each organ of the human body. This is
+the person whom we claim as the designer and artificer of that body, and
+he is the one of all others the best fitted for the task by his
+antecedents, and his practical knowledge of the requirements of the
+case--for he is man himself. Not man, the individual of any given
+generation, but man in the entirety of his existence from the dawn of
+life onwards to the present moment" (_Evolution, Old and New_, p. 30,
+1879).
+
+Butler's theory of life and habit remained only a sketch, and he was
+perhaps not fully aware of its philosophical implications. Since
+Butler's time, a new complexion has been put upon biological philosophy
+by the profound speculations of Bergson.
+
+But it is not impossible that the future development of biological
+thought will follow some such lines as those which he tentatively laid
+down.
+
+Butler was not the first to suggest that there is a close connection
+between heredity and memory--it is a thought likely to occur to any
+unprejudiced thinker. The first enunciation of it which attracted
+general attention was that contained in Hering's famous lecture "On
+Memory as a general Function of organised Matter."[509] Butler was not
+aware of Hering's work when he published his _Life and Habit_, but in
+_Unconscious Memory_ (1880) he gave full credit to Hering as the first
+discoverer, and supplied an admirable translation of Hering's lecture.
+As far as the assimilation of heredity to memory is concerned Hering and
+Butler have much in common, but Hering did not share Butler's Lamarckian
+and vitalistic views, preferring to hold fast, for the practical
+purposes of physiology at all events, to the general accepted theory of
+the parallelism between psychical and physical processes. He was
+inclined to regard memory in the ordinary sense as a function of the
+brain, and memory in general as a function of all organised matter.
+Speaking of the psychical life, he says, "Thus the cause which produces
+the unity of all single phenomena of consciousness must be looked for in
+unconscious life. As we know nothing of this except what we learn from
+our investigations of matter, and since in a purely empirical
+consideration, matter and the unconscious must be regarded as identical,
+the physiologist may justly define memory in a wider sense to be a
+faculty of the brain, the results of which to a great extent belong to
+both consciousness and unconsciousness."[510] Hering's views were
+supported by Haeckel.[511]
+
+In 1893 an American, H. F. Orr,[512] tried to work out a theory of
+development and heredity based upon the fundamental idea "that the
+property which is the basis of bodily development in organisms is the
+same property which we recognise as the basis of psychic activity and
+psychic development." He tried also to explain the recapitulation of
+phylogeny by ontogeny as due to habit.
+
+The neo-Lamarckian school of American palaeontologists were also in
+sympathy with the memory idea, and this was expressed most clearly
+perhaps by Cope.[513]
+
+In 1904 appeared the work on this subject which has attracted the most
+attention--R. Semon's _Die Mneme_.[514] This was an elaborate treatment of
+the question from the materialistic point of view, the main assumption
+of Semon's theory being that the action of a stimulus upon the organism
+leaves a more or less permanent material trace or "engramm," of such a
+nature as to modify the subsequent action of the organism.
+
+Applied to the explanation of heredity and development, Semon's theory
+comes to very much the same as Weismann's, with engramms substituted for
+determinants, but it has the great advantage of allowing for the
+transmission of acquired characters. The application of the concept of
+stimulus is valuable and suggestive, but it seems to us that the memory
+theory of heredity can be properly utilised only by adopting a frankly
+Lamarckian and vitalistic standpoint, and this standpoint Semon
+expressly combats. As Ward[515] points out in his illuminating lecture on
+heredity and memory--"Records or memoranda alone are not memory, for
+they presuppose it. _They_ may consist of physical traces, but memory,
+even when called 'unconscious,' suggests mind; for, as we have seen, the
+automatic character implied by this term 'unconscious' presupposes
+foregone experience.... The mnemic theory then, if it is to be worth
+anything, seems to me clearly to require not merely physical records or
+'engrams,' but living experience or tradition. The mnemic theory will
+work for those who can accept a monadistic or pampsychist interpretation
+of the beings that make up the world, who believe with Spinoza and
+Leibniz that 'all individual things are animated albeit in divers
+degree'" (pp. 55-6).
+
+Perhaps the best and most ingenious treatment of memory and heredity
+from a physical standpoint is that offered by E. Rignano in his book,
+_Sur la transmissibilite des caracteres acquis_.[516] Rignano seeks to
+construct a physico-chemical "model" which will explain both heredity
+and memory.
+
+His system, which is based more firmly upon the facts of experimental
+embryology than Semon's, postulates the existence of "specific nervous
+accumulators." The essential hypothesis set up is that every functional
+stimulus is transformed into specific vital energy, and deposits in the
+nucleus of the cell a specific substance which is capable of
+discharging, in an inverse direction, the nervous current which has
+formed it, as soon as the dynamical equilibrium of the organism is
+restored to the state in which it was when the original stimulus acted
+upon it. These specific nuclear substances, different for each cell, are
+accumulated also in the nuclei of the germinal substance, constituting
+what Rignano calls the central zone of development. That is to say, each
+functional adaptation changes slightly the dynamical equilibrium of the
+organism, and this change in the system of distribution of the nervous
+currents leads to the deposit in the central zone of development of a
+new specific substance. In the development of the next individual this
+new specific element enters into activity, and reproduces the nervous
+current which has formed it, as soon as the organism reaches the same
+conditions of dynamical equilibrium as those obtaining when the stimulus
+acted on the parent.
+
+Development can thus be regarded as consisting of a number of stages, at
+each of which new specific elements enter automatically into play and
+lead the embryo from that stage to the stage succeeding. The germinal
+substance on this theory of Rignano's is to be regarded as being
+composed of a large number of specific elements, originally formed as a
+result of each new functional adaptation, but now forming part of the
+hereditary equipment.
+
+The theory represents an advance upon the more static conceptions of
+Semon. It owes much to Roux's influence.
+
+In this country, the mnemic theories have been championed particularly
+by M. Hartog[517] and Sir Francis Darwin.[518]
+
+    [508] The quotations are taken from the 1910 reprint,
+    London, Fifield.
+
+    [509] _Ueber das Gedaechtnis als eine allgemeine Funktion
+    der organisierten Materie_, Wien, 1870.
+
+    [510] Eng. trans, in E. Hering, _Memory_, p. 9, Chicago
+    and London, 1913.
+
+    [511] _Die Perigenesis der Plastidule_, Jena, 1875.
+
+    [512] _A Theory of Development and Heredity_, New York,
+    1893.
+
+    [513] _The Primary Factors of Organic Evolution_, Chicago,
+    1896.
+
+    [514] _Die Mneme als erhaltendes Prinzip im Wechsel des
+    organischen Geschehens_, Leipzig, 1904; 2nd ed., 1908.
+
+    [515] _Heredity and Memory_, Cambridge, 1913.
+
+    [516] Paris, 1906. Also in Italian and German. Eng. trans.
+    by B. C. ,H. Harvey, Chicago, 1911.
+
+    [517] See _Problems of Life and Reproduction_, London,
+    1913.
+
+    [518] _Presidential Address to the British Association_,
+    1908.
+
+
+
+
+CHAPTER XX
+
+THE CLASSICAL TRADITION IN MODERN MORPHOLOGY
+
+
+To write a history of contemporary movements from a purely objective
+standpoint is well recognised to be an impossible task. It is difficult
+for those in the stream to see where the current is carrying them: the
+tendencies of the present will only become clear some twenty years in
+the future.
+
+I propose, therefore, in this concluding chapter to deal only with
+certain characteristics of modern work on the problems of form which
+seem to me to be derived directly from the older classical tradition of
+Cuvier and von Baer.
+
+The present time is essentially one of transition. Complete uncertainty
+reigns as to the main principles of biology. Many of us think that the
+materialistic and simplicist method has proved a complete failure, and
+that the time has come to strike out on entirely different lines. Just
+in what direction the new biology will grow out is hard to see at
+present, so many divergent beginnings have been made--the materialistic
+vitalism of Driesch, the profound intuitionalism of Bergson, the
+psychological biology of Delpino, 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. Mueller. It may well be that the intransigent materialism of
+the 19th century is merely an episode, an aberration rather, in the
+history of biology--an aberration brought about by the over-rapid
+development of a materialistic and luxurious civilisation, in which
+man's material means have outrun his mental and moral growth.
+
+Two movements seem significant in the morphology of the last decade or
+so of the 19th century--first, the experimental study of form, and
+second, the criticism of the concepts or prejudices of evolutionary
+morphology.
+
+The period was characterised also by the great interest taken in
+cytology, following upon the pioneer work of Hertwig, van Beneden and
+others on the behaviour of the nuclei in fertilisation and
+maturation.[519] This line of work gained added importance in connection
+with contemporary research and speculation on the nature of hereditary
+transmission, and it has in quite recent years received an additional
+stimulus from the re-discovery of Mendelian inheritance. Its importance,
+however, seems to lie rather in its possible relation to the problems of
+heredity than in any meaning it may have for the problems of form. More
+significant is the revolt against the cell-theory started by Sedgwick[520]
+and Whitman,[521] on the ground that the organism is something more than
+an aggregation of discrete, self-centred cells.
+
+The experimental work on the causes of the production and restoration of
+form infused new life into morphology. It opened men's eyes to the fact
+that the developing organism is very much a living, active, responsive
+thing, quite capable of relinquishing at need the beaten track of normal
+development which its ancestors have followed for countless generations,
+in order to meet emergencies with an immediate and purposive reaction.
+It was cases of this kind, cases of active regulation in development and
+regeneration, that led men like G. Wolff and H. Driesch to cast off the
+bonds of dogmatic Darwinism and declare boldly for vitalism and
+teleology.
+
+There was the famous case of the regeneration of the lens in Amphibia
+from the edge of the iris--an entirely novel mode of origin, not
+occurring in ontogeny. The fact seems to have been discovered first by
+Colucci in 1891, and independently by G. Wolff in 1895.[522] The
+experiment was later repeated and confirmed by Fischel and other
+workers. Wolff drew from this and other facts the conclusion that the
+organism possesses a faculty of "primary purposiveness" which cannot
+have arisen through natural selection.[523] And, as is well known, Driesch
+derived one of his most powerful arguments in favour of vitalism from
+the extraordinary regenerative processes shown by _Tubularia_ and
+_Clavellina_ in the course of which the organism actually demolishes and
+rebuilds a part or the whole of its structure. But under the influence
+of physiologists like Loeb many workers held fast to materialistic
+methods and conceptions.
+
+The great variety of regulative response of which the organism showed
+itself capable made it very difficult for the morphologist to uphold the
+generalisations which he had drawn from the facts of normal undisturbed
+development. The germ-layer theory was found inadequate to the new
+facts, and many reverted to the older criterion of homology based on
+destiny rather than origin. The trend of opinion was to reject the
+ontogenetic criterion of homology, and to refuse any morphological or
+phylogenetic value to the germ-layers.[524]
+
+The biogenetic law came more and more into disfavour, as the developing
+organism more and more showed itself to be capable of throwing off the
+dead-weight of the past, and working out its own salvation upon original
+and individual lines.[525] A. Giard in particular called attention to a
+remarkable group of facts which went to show that embryos or larvae of
+the same or closely allied species might develop in most dissimilar ways
+according to the conditions in which they found themselves.[526] His
+classical case of "poecilogeny" was that of the shrimp _Palaemonetes
+varians_, the fresh-water form of which develops in an entirely
+different way from the salt-water form.
+
+Experimental workers indeed were inclined to rule the law out of
+account, to disregard completely the historical element in development,
+and this was perhaps the chief weakness of the neo-vitalist systems
+which took their origin in this experimental work.
+
+From the side also of descriptive morphology the biogenetic law
+underwent a critical revision. It was studied as a fact of embryology
+and without phylogenetic bias by men like Oppel, Keibel, Mehnert, O.
+Hertwig and Vialleton,[527] and they arrived at a critical estimate of it
+very similar to that of von Baer.
+
+Theoretical objections to the biogenetic law had been raised from time
+to time by many embryologists, but the positive testing of it by the
+comparison of embryos in respect of the degree of development of their
+different organs starts with Oppel's work of 1891.[528] He studied a large
+number of embryos of different species at different stages of their
+development, and determined the relative time of appearance of the
+principal organs and their relative size. His results are summarised in
+tabular form and have reference to all the more important organs. He was
+led to ascribe a certain validity to the biogenetic law, but he drew
+particular attention to the very considerable anomalies in the time of
+appearance which are shown by many organs, anomalies which had been
+classed by Haeckel under the name of heterochronies.
+
+Oppel's main conclusions were as follows:--"There are found in the
+developmental stages of different Vertebrates 'similar ontogenetic
+series,' that is to say, Vertebrates show at definite stages
+similarities with one another in the degree of development of the
+different organs. Early stages resemble one another, so also do later
+stages; equivalent stages of closely allied species resemble one
+another, and older stages of lower animals resemble younger stages of
+higher animals; young stages are more alike than old stages.... The
+differences which these similar series show (for which reason they
+cannot be regarded as identical) may be designated as temporal
+disturbances in the degree of development of the separate organs or
+organ-systems. Some organs show very considerable temporal dislocations,
+others a moderate amount, others again an inconsiderable amount. Among
+the developmental stages of various higher animals can be found some
+which correspond to the ancestral forms and also to the lower types
+which resemble these ancestral forms. On the basis of the tabulated data
+here given there can be distinguished with certainty in the ontogeny of
+Amniotes a pro-fish stage, a fish-stage, a land-animal stage, a
+pro-amniote stage, and following on these a fully developed reptile,
+bird or mammal stage."[529]
+
+Oppel's methods were employed by Keibel[530] in his investigations on the
+development of the pig, which formed the model for the well-known series
+of _Normentafeln_ of the ontogeny of Vertebrates which were issued in
+later years under Keibel's editorship. Keibel was more critical of the
+biogenetic law than Oppel, and he held that the ancestral stages
+distinguished by Oppel could not be satisfactorily established. He
+suggested an interesting explanation of heterochrony in development,
+according to which the premature or retarded appearance of organs in
+ontogeny stands in close relation with the time of their entering upon
+functional activity. Thus in many mammals the mesodermal part of the
+allantois often appears long before the endodermal part, though this is
+phylogenetically older. This Keibel ascribes to the fact that the
+endodermal part is almost functionless. "One can directly affirm," he
+writes, "that the time of appearance of an organ depends in an eminent
+degree upon the time when it has to enter upon functional activity. This
+moment is naturally dependent upon the external conditions. Among the
+highest Vertebrates, the mammals, the traces of phylogeny shown in
+ontogeny are to a great extent obliterated through the adaptation of
+ontogeny to the external conditions, and through the modifications which
+the germs of more highly organised animals necessarily exhibit from the
+very beginning as compared with germs which do not reach such a high
+level of development" (p. 754, 1897).
+
+Study of individual variation in the time of appearance of the organs in
+embryos of the same species was prosecuted with interesting results by
+Bonnet,[531] Mehnert,[532] and Fischel.[533] Fischel found that variability
+was greatest among the younger embryos, and became progressively less in
+later stages. Like von Baer (_supra_, p. 114) he inferred that
+regulatory processes were at work during development which brought
+divergent organs back to the normal and enabled them to play their part
+as correlated members of a functional whole.
+
+Important theoretical views were developed by Mehnert[534] in a series of
+publications appearing from 1891 to 1898. Like Keibel, Mehnert
+emphasised the importance of function in determining the late or early
+appearance of organs, but he conceived the influence of function to be
+exerted not only in ontogeny, but also throughout the whole course of
+phylogeny, by reason of the transmission to descendants of the effects
+of functioning in the individual life.
+
+In his paper of 1897 Mehnert details the results of an extensive
+examination of the development of the extremities throughout the Amniote
+series. He finds that in all cases a pentadactylate rudiment is formed,
+even in those forms in which only a few of the elements of the hand or
+foot come to full development. But whereas in forms with a normally
+developed hand, _e.g._ the tortoise and man, all the digits develop and
+differentiate at about the same rate, in forms which have in the adult
+reduced digits, _e.g._ the ostrich and the pig, these vestigial digits
+undergo a very slow and incomplete differentiation, while the others
+develop rapidly and completely. He draws a general distinction between
+organs that are phylogenetically progressive and such as are
+phylogenetically regressive, and seeks to prove that progressive organs
+show an ontogenetic acceleration and regressive organs a retardation.[535]
+The acceleration or retardation affects not only the mass-growth of the
+organs, but also their histological differentiation.
+
+Now between progression and functioning and between regression and
+functional atrophy there is obviously a close connection. Loss of
+function is well known to be one of the chief causes of the degeneration
+of organs in the individual life, and on the other hand, as Roux has
+pointed out, all post-embryonic development is ruled and guided by
+functioning. It is thus in the long run functioning that brings about
+phylogenetic progression, absence of functional activity that causes
+phylogenetic regression. This comes about through the transmission of
+acquired functional characters, a transmission which Mehnert conceives
+to be extraordinarily accurate and complete.
+
+In general Mehnert adopts the functional standpoint of Cuvier, von Baer,
+and Roux. His considered judgment as to the phylogenetic value of the
+biogenetic law closely resembles that formed by von Baer, for he admits
+recapitulation only as regards the single organs, not as regards the
+organism as a whole. He has, however, much more sympathy with the law
+than either Keibel or Oppel, though he agrees that it cannot be used for
+the construction of ancestral trees. But he ascribes to it as a fact of
+development considerable importance. The following passage gives a good
+summary of his view as to the scope and validity of the law. "The
+biogenetic law has not been shaken by the attacks of its opponents. The
+assertion is still true that individual organogenesis is exclusively
+dependent on phylogeny. But we must not expect to find that all the
+stages in the development of the separate organs, which coexisted in any
+member of the phylogenetic series, appear _at the same time_ in the
+individual ontogeny of the descendants, because each organ possesses its
+own specific rate of development. In this way it comes about naturally
+that organs which become differentiated rapidly, as, for example, the
+medullary tube, as a rule dominate earlier periods of ontogeny than do
+the organs of locomotion. For the same reason the cerebral hemispheres
+of man are almost as large in youth as in maturity. The picture which an
+embryo gives is not a repetition in detail of one and the same
+phylogenetic stage; it consists rather of an assemblage of organs, some
+of which are at a phyletically early stage of development, while others
+are at a phyletically older stage."[536]
+
+A different line of attack was that adopted by O. Hertwig in a series of
+papers, which contain also what is perhaps the best critical estimate of
+the present position and value of descriptive morphology.[537]
+
+It had not escaped the notice of many previous observers that quite
+early embryos not infrequently show specific characters even before the
+characters proper to their class, order and genus are developed--in
+direct contradiction of the law of von Baer. Thus L. Agassiz[538] had
+remarked in 1859 that specific characteristics were often developed
+precociously. "The Snapping Turtle, for instance, exhibits its small
+crosslike sternum, its long tail, its ferocious habits, even before it
+leaves the egg, before it breathes through lungs, before its derm is
+ossified to form a bony shield, etc.; nay, it snaps with its gaping jaws
+at anything brought near, when it is still surrounded by its amnion and
+allantois, and its yolk still exceeds in bulk its whole body" (p. 269).
+
+Wilhelm His,[539] in the course of an acute and damaging criticism of the
+biogenetic law as enunciated by Haeckel, showed clearly that by careful
+examination the very earliest embryos of a whole series of Vertebrates
+could be distinguished with certainty from one another. "An identity in
+external form of different animal embryos, despite the common
+affirmation to the contrary, does not exist. Even at early stages in
+their development embryos possess the characters of their class and
+order, nay, we can hardly doubt, of their species and sex, and even
+their individual characteristics" (201).
+
+This specificity of embryos was affirmed with even greater confidence by
+Sedgwick in a paper critical of von Baer's law.[540] He wrote:--"If v.
+Baer's law has any meaning at all, surely it must imply that animals so
+closely allied as the fowl and duck would be indistinguishable in the
+early stages of development; and that in two species so closely similar
+that I was long in doubt whether they were distinct species, viz.,
+_Peripatus capensis_ and _Balfouri_, it would be useless to look for
+embryonic differences; yet I can distinguish a fowl and a duck embryo on
+the second day by the inspection of a single transverse section through
+the trunk, and it was the embryonic differences between the Peripatuses
+which led me to establish without hesitation the two separate
+species.... I need only say ... that a species is distinct and
+distinguishable from its allies from the very earliest stages all
+through the development, although these embryonic differences do not
+necessarily implicate the same organs as do the adult differences" (p.
+39).
+
+Hertwig interprets this fact of the specific distinctness of closely
+allied embryos in the light of the preformistic conception of heredity.
+According to this view the whole adult organisation is represented in
+the structure of the germ-plasm contained in the fertilised ovum, from
+which it follows that the ova of two different species, and also their
+embryos at every stage of development, must be as distinct from one
+another as are the adults themselves, even though the differences may
+not be so obvious. If this be the case there can be no real
+recapitulation in ontogeny of the phylogeny of the race, for the
+egg-cell represents not the first term in phylogeny, but the last. The
+egg-cell _is_ the organism in an undeveloped state; it has a vastly more
+complicated structure than was possessed by the primordial cell from
+which its race has sprung, and it can in no way be considered the
+equivalent of this ancestral cell.
+
+Hertwig puts this vividly when he says that "the hen's egg is no more
+the equivalent of the first link in the phylogenetic chain than is the
+hen itself" (p. 160, 1906, b).
+
+If ontogeny is not a recapitulation of phylogeny, how is it that the
+early embryonic stages are so alike, even in animals of widely different
+organisation? Hertwig's answer to this is very interesting. He takes the
+view that many of the processes characterising early embryonic
+development are the means necessarily adopted for attaining certain
+ends. Such are the processes of segmentation, the formation of a
+blastula, of cell-layers, of medullary folds where the nervous system is
+a closed tube, the formation of the notochord as a necessary condition
+of the development of the vertebral column, and so on. "Looked at from
+this standpoint it cannot surprise us that in all animal phyla the
+earliest embryonic processes take place in similar fashion, so that we
+observe the occurrence both in Vertebrates and Invertebrates of a
+segmentation-process, a morula-stage, a blastula and a gastrula. If now
+these developmental processes do not depend on chance, but, on the
+contrary, are rooted in the nature of the animal cell itself, we have no
+reason for inferring from the recurrence of a similar
+segmentation-process, morula, blastula, and gastrula in all classes of
+the animal kingdom the common descent of all animals from one
+blastula-like or gastrula-like ancestral form. We recognise rather in
+the successive early stages of animal development only the manifestation
+of special laws, by which the shaping of animal forms (as distinct from
+plant forms) is brought about" (p. 178, 1906, b).
+
+"The principal reason why certain stages recur in ontogeny with such
+constancy and always in essentially the same manner is that they provide
+under all circumstances the necessary pre-conditions through which alone
+the later and higher stages of ontogeny can be realised. The unicellular
+organism can by its very nature transform itself into a multicellular
+organism only by the method of cell-division. Hence, in all Metazoa,
+ontogeny must start with a segmentation-process, and a similar statement
+could be made with regard to all the later stages" (p. 57, 1906, a).
+
+Similarities in early development are therefore no evidence of common
+descent, and in the same way the resemblances of adult animals, subsumed
+under the concepts of homology and the unity of plan, are not
+necessarily due to community of descent, but may also be brought about
+by the similarity or identity of the laws which govern the evolution of
+these animals. In the absence, therefore, of positive evidence as to the
+actual lines of descent (to be obtained only from 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 explanation in
+common descent or in the common laws that rule organic development" (p.
+151, 1906, b). As A. Braun long ago pointed out, "It is not descent
+which decides in matters of morphology, but, on the contrary, morphology
+which has to decide as to the possibility of descent."[541]
+
+Hertwig, in a word, reverts to the pre-evolutionary conception of
+homology. "We see in homology," he writes, "only the expression of
+regularities (_Gesetzmaessigkeiten_) in the organisation of the animals
+showing it, and we regard the question, how far this homology can be
+explained by common descent and how far by other principles, as for the
+present an open one, requiring for its solution investigations specially
+directed towards its elucidation" (p. 179, 1906, b).
+
+Holding, as he does, that no definite conclusions can be drawn from the
+facts of comparative anatomy and embryology as to the probable lines of
+descent of the animal kingdom, Hertwig accords very little value to
+phylogenetic speculation. It is, he admits, quite probable that the
+archetype of a class represents in a general sort of way the ancestral
+form, but this does not, in his opinion, justify us in assuming that
+such generalised types ever existed and gave origin to the present-day
+forms. "It is not legitimate to picture to ourselves the ancestral forms
+of the more highly organised animals in the guise of the lower animals
+of the present day--and that is just what we do when we speak of
+Proselachia, Proamphibia and Proreptilia" (p. 155, 1906, b).
+
+He rejects on the same general grounds the evolutionary dogma of
+monophyletic or almost monophyletic descent, and admits with Koelliker,
+von Baer, Wigand, Naegeli and others that evolution may quite well have
+started many times and from many different primordial cells.
+
+There is indeed a great similarity between the views developed by O.
+Hertwig and those held by the older critics of Darwinism--von Baer,
+Koelliker, Wigand, E. von Hartmann and others. It is true the
+philosophical standpoint is on the whole different, for while many of
+that older generation were vitalists Hertwig belongs to the mechanistic
+school.
+
+But both Hertwig and the older school agree in pointing out the _petitio
+principii_ involved in the assumption that the archetype represents the
+ancestral form; both reject the simplicist conception of a monophyletic
+evolution (which may be likened to the "one animal" idea of the
+transcendentalists); both admit the possibility that evolution has taken
+place along many separate and parallel lines, and explain the
+correspondences shown by these separate lines by the similarity of the
+intrinsic laws of evolution; finally, both emphasise the fact that we
+know nothing of the actual course of evolution save the few indications
+that are furnished by palaeontology, and both insist upon the unique
+importance of the palaeontological evidence.[542]
+
+It was a curious but very typical characteristic of evolutionary
+morphology that its devotees paid very little attention to the positive
+evidence accumulated by the palaeontologists,[543] but shut themselves up
+in their tower of ivory and went on with their work of constructing
+ideal genealogies. It was perhaps fortunate for their peace of mind that
+they knew little of the advances made by 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 _Archaeopteryx_, for example--but these do not
+always represent the actual phylogenetic links. There is an almost
+complete absence of the archetypal ancestral forms which are postulated
+by evolutionary morphology. Amphibia do not demonstrably evolve from an
+archetypal Proamphibian, nor do mammals derive from a single generalised
+Promammalian type. Few of the hypothetical ancestral types imagined by
+Haeckel have ever been found as fossils. The great classificatory groups
+are almost as distinct in early fossiliferous strata as they are at the
+present day. As Deperet says in his admirable book,[544] in the course of
+a presentation of the matured views of the great Karl von Zittel, "We
+cannot forget that there exist a vast number of organisms which are not
+connected by any intermediate links, and that the relations between the
+great divisions of the animal and vegetable kingdoms are much less close
+than the theory [of evolution] demands. Even the Archaeopteryx, the
+discovery of which made so much stir and appeared to establish a genetic
+relation between classes so distinct as Birds and Reptiles, fills up the
+gap only imperfectly, and does not indicate the point of bifurcation of
+these two classes. Intermediate links are lacking between Amphibia and
+Reptiles. Mammals, too, occupy an isolated position, and no zoologist
+can deny that they are clearly demarcated from other Vertebrates;
+indeed, no fossil mammal is certainly known which comes nearer to the
+lower Vertebrates than does Ornithorhynchus at the present day" (p.
+115).
+
+To take a parallel from the Invertebrata, B. B. Woodward,[545] after
+discussing the phylogeny of the Mollusca as worked out by the
+morphologists and comparing it with the probable actual course of the
+evolution of the group, as evidenced by fossil shells, sums up as
+follows:--"The lacunae 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
+palaeontological history of any hint of passage forms between the classes
+themselves, or between the Mollusca and their nearest allies. Nor is
+this hiatus confined to the Molluscan phylum; it is the same for all
+branches of the animal kingdom. There is circumstantial evidence that
+transitional forms must have existed, but of actual proof none whatever.
+All the classes of Mollusca appear fully fledged, as it were. No form
+has as yet been discovered of which it could be said that it in any way
+approached the hypothecated prorhipidoglossate mollusc, still less one
+linking all the classes" (p. 79).
+
+Pointing in the same direction as the absence of transitional forms is
+the undeniable fact that all the great groups of animals appear with all
+their typical characters at a very early geological epoch. Thus, in the
+Silurian age a very rich fauna has already developed, and
+representatives are found of all the main Invertebrate groups--sponges,
+corals, hydroid colonies, five types of Echinoderms, Bryozoa,
+Brachiopods, Worms, many types of Mollusca and Arthropoda. Of
+Vertebrates, at least two types of fish are present--Ganoids and
+Elasmobranchs. In the very earliest fossiliferous rocks of all, the
+Precambrian formation, there are remains of Molluscs, Trilobites and
+Gigantostraca, similar to those which flourished in Cambrian and
+Silurian times.
+
+The contributions of 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.[546] This is the tendency universally shown by embryos to
+reproduce the characters of their ancestors at earlier and earlier
+stages in their development.
+
+The most valuable contribution made by 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 represented groups. Classical examples were the researches of
+Hilgendorf[547] on the evolution of _Planorbis multiformis_ in the
+lacustrine deposits of Steinheim, those of Waagen[548] on the phylogeny of
+_Ammonites subradiatus_, and the work of Neumayr and Paul[549] on
+_Paludina_ (_Vivipara_).
+
+These investigations demonstrated that it was possible to follow out
+step by step in superjacent strata the actual evolution of fossil
+species and to establish the actual "phyletic series."
+
+To take an example from among the Vertebrates, Deperet has shown (_loc.
+cit._, pp. 184-9), that the European Proboscidea, belonging to the three
+different types of the Elephants, Mastodons and Dinotheria, have evolved
+since the Oligocene epoch along five distinct but continuous lines. The
+Dinotherian stock is represented at the beginning of the Miocene by the
+relatively small form _D. cuvieri_; this changes progressively
+throughout Miocene times into _D. laevius_, _D. giganteum_, and _D.
+gigantissimum_. Among the Mastodons two quite distinct phyletic series
+can be distinguished, the first commencing with _Palaeomastodon
+beadnelli_ of the Oligocene, and evolving between the Miocene and
+Pliocene into _Mastodon arvernensis_, after traversing the forms _M.
+angustidens_ and _M. longirostris_, the second starting with the _M.
+turicensis_ of the Lower Miocene and evolving through _M. borsoni_ into
+the _M. americanus_ of the Quaternary. The phyletic series of the true
+elephants in Europe are relatively short, and go back only to the
+Quaternary, _Elephas antiquus_ giving origin to the Indian elephant, _E.
+priscus_ to the African.
+
+The careful study of phyletic series brought to light the significant
+fact that these lines of filiation tend to run for long stretches of
+time parallel to, and distinct from one another, without connecting
+forms. This is clearly exemplified in the case of the Proboscidea, and
+many other examples could be quoted. Almost all rich genera are
+polyphyletic in the sense that their component species evolve along
+separate and parallel lines of descent.[550] "Such great genera as the
+genus _Hoplites_ among the Ammonites, the genus _Cerithium_ among the
+Gastropoda, the genus _Pecten_ or the genus _Trigonia_ among the
+Lamellibranchs, each comprise perhaps more than twenty independent
+phyletic series" (Deperet, p. 200).
+
+Variation along the phyletic lines is gradual[551] and determinate, and
+appears to obey definite laws. The earliest members of a phyletic series
+are usually small in size and undifferentiated in structure, while the
+later members show a progressive increase in size and complexity. Rapid
+extinction often supervenes soon after the line has reached the maximum
+of its differentiation.
+
+The general picture which 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
+palaeontologists in particular--Cope, Hyatt, Ryder, Dall, Packard,
+Osborn--have worked out a complete neo-Lamarckian theory based upon the
+fossil record.
+
+The functional point of view was well to the fore in the works of those
+great palaeontologists, L. Ruetimeyer (1825-1895) and V. O. Kowalevsky
+(1842-83), who seem to have carried on the splendid tradition of Cuvier.
+Speaking of Kowalevsky's classical memoir, _Versuch einer natuerlichen
+Classification der fossilen Hufthiere_, Osborn[552] writes:--"This work is
+a model union of the detailed study of form and function with theory and
+the working hypothesis. It regards the fossil not as a petrified
+skeleton, but as having belonged to a moving and feeding animal; every
+joint and facet has a meaning, each cusp a certain significance. Rising
+to the philosophy of the matter, it brings the mechanical perfection and
+adaptiveness of different types into relation with environment, with
+changes of herbage, with the introduction of grass. In this survey of
+competition it speculates upon the causes of the rise, spread, and
+extinction of each animal group. In other words, the fossil quadrupeds
+are treated _biologically_--so far as is possible in the obscurity of
+the past" (p. 8). The same high praise might with justice be accorded to
+the work of Cope on the functional evolution of the various types of
+limb-skeleton in Vertebrates, and on the evolution of the teeth as well
+as to the work of other American palaeontologists, including Osborn
+himself.
+
+Osborn's law of "adaptive radiation," which links on to Darwin's law of
+divergence,[553] constitutes a brilliant vindication of the functional
+point of view. "According to this law each isolated region, if large and
+sufficiently varied in its topography, soil, climate, and vegetation,
+will give rise to a diversified mammalian fauna. From primitive central
+types branches will spring off in all directions, with teeth and
+prehensile organs modified to take advantage of every possible
+opportunity of securing food, and in adaptation of the body, limbs and
+feet to habitats of every kind, as shown in the diagram [on p. 363]. The
+larger the region and the more diverse the conditions, the greater the
+variety of mammals which will result.
+
+"The most primitive mammals were probably small insectivorous or
+omnivorous forms, therefore with simple, short-crowned teeth, of
+slow-moving, ambulatory, terrestrial, or arboreal habit, and with short
+feet provided with claws. In seeking food and avoiding enemies in
+different habitats the limbs and feet radiate in four diverse
+directions; they either become _fossorial_ or adapted to digging habits,
+_natatorial_ or adapted to _amphibious_ and finally to _aquatic_
+habits, _cursorial_ or adapted to swift-moving, terrestrial progression,
+_arboreal_ or adapted to tree life. Tree life leads, as its final stage,
+into
+
+                        LIMBS AND FEET.
+                                                        Volant.
+                                                          /
+            Fossorial.                               Arboreal.
+                \                                       /
+        Short-limbed, plantigrade,      }       Ambulatory
+          pentadactyl, unguiculate      }           or
+          Stem.                         }       Terrestrial.
+                /                                       \
+          Natatorial.                                 Cursorial
+         Amphibious.                                 Digitigrade.
+             /                                             \
+         Aquatic                                       Unguligrade.
+
+
+                                  TEETH.
+                                Omnivorous.
+                                                              { Grass.
+                        { Fish.     |                         { Herb.
+Carnivorous             { Flesh.    |   Herbivorous           { Shrub.
+            \           { Carrion.  |        /                { Fruit.
+             \                      |       /                 { Root.
+              \                     |      /
+               \                    |     /   Myrmecophagous.
+                \                   |    /        / Dentition reduced.
+                 \                  |   /        /
+                  \                 |  /        /
+                   \                | /        /
+                    \               |/        /
+                        Stem: Insectivorous.
+
+
+the parachute types of the flying squirrels and phalangers, or into the
+true flying types of the bats.... Similarly in the case of the teeth,
+insectivorous and omnivorous types appear to be more central and ancient
+than either the exclusively carnivorous or herbivorous types. Thus the
+extremes of carnivorous adaptation, as in the case of the cats, of
+omnivorous adaptation, as in the case of the bears, of herbivorous
+adaptation, as in the case of the horses, or myrmecophagous adaptation,
+as in the case of the anteaters, are all secondary" (_loc. cit._, pp.
+23-4).
+
+We have now reached the end of our historical survey of the problems of
+form. What the future course of morphology will be no one can say. But
+one may hazard the opinion that the present century will see a return to
+a simpler and more humble attitude towards the great and unsolved
+problems of animal form. Dogmatic materialism and dogmatic theories of
+evolution have in the past tended to blind us to the complexity and
+mysteriousness of vital phenomena. We need to look at living things with
+new eyes and a truer sympathy. We shall then see them as active, living,
+passionate beings like ourselves, and we shall seek in our morphology to
+interpret as far as may be their form in terms of their activity.
+
+This is what Aristotle tried to do, and a succession of master-minds
+after him. We shall do well to get all the help from them we can.
+
+    [519] See E. B. Wilson's masterly book, _The Cell in
+    Development and Inheritance_, New York and London, 1900.
+
+    [520] _Q.J.M.S._, xxvi. 1886.
+
+    [521] _Wood's Holl Biological Lectures_ for 1893.
+
+    [522] _Arch. f. Ent.-Mech._, i., pp. 380-90, 1895.
+
+    [523] _Beitraege zur Kritik der Darwinschen Lehre_,
+    Leipzig, 1898.
+
+    [524] See E. B. Wilson, "The Embryological Criterion of
+    Homology," _Wood's Holl Biological Lectures_, Boston,
+    pp. 101-24, 1895; Braem, _Biol. Centrblt._, xv., 1895;
+    T. H. Morgan, _Arch. f. Ent.-Mech._, xviii.; J. W.
+    Jenkinson, _Mem. Manchester Lit. Phil. Soc._, 1906, and
+    _Vertebrate Embryology_, Oxford, 1913; A. Sedgwick,
+    article "Embryology" in _Ency. Brit._, p. 318, vol. xi.,
+    11th Ed. (1910).
+
+    [525] For a detailed treatment of this important point see
+    the remarkable volume of E. Schulz (Petrograd),
+    _Prinzipien der rationellen vergleichenden Embryologie_,
+    Leipzig, 1910.
+
+    [526] "La Poecilogonie," _Bull. Sci. France et Belgique_,
+    xxxix., pp. 153-87, 1905.
+
+    [527] _Un probleme de l'evolution. La loi biogenetique
+    fondamentale_, Paris and Montpellier, 1908.
+
+    [528] _Vergleichung des Entwickelungsgrades der Organe zu
+    verschiedenen Entwickelungszeiten bei Wirbeltieren_,
+    Jena, 1891.
+
+    [529] Quoted by Keibel, _Ergebn. Anat. Entwick._, vii., p.
+    741.
+
+    [530] "Studien zur Entwickelungsgeschichte des Schweines,"
+    Schwalbe's _Morphol. Arbeiten_, iii., 1893, and v.,
+    1895.
+
+      _Normentafeln zur Entwickelungsgeschichte des
+      Schweines_, Jena, 1897.
+
+      "Das biogenetische Grundgesetz und die Cenogenese,"
+      _Ergebn. Anat. Entw._, vii., pp. 722-92, 1897.
+
+      "U. d. Entwickelungsgrad der Organe," _Handb. vergl.
+      exper. Entwick. der Wirbelthiere_, iii., 3, pp. 131-48,
+      1906.
+
+    [531] "Beitraege zur Embryologie der Wiederkaeuer," _Arch.
+    Anat. Entw._, 1889.
+
+    [532] "Die individ. Variation d. Wirbeltierembryo,"
+    _Morph. Arbeit._, v., 1895.
+
+    [533] "U. Variabilitaet u. Wachstum d. embryonalen
+    Koerpers," _Morph. Jahrb._, xxiv., 1896.
+
+    [534] "Gastrulation u. Keimblaetterbildung der _Emys
+    lutaria taurica_," _Morph. Arbeit._, i., 1891.
+    "Kainogenese," _Morph. Arbeit._, vii., pp. 1-156, 1897,
+    and also separately. _Biomechanik, erschlossen aus dem
+    Prinzipe der Organogenese_, Jena, 1898.
+
+    [535] This law was foreshadowed by Reichert in 1837, when he
+    wrote:--"We notice in our investigation of embryos of different
+    animal forms that it is those organs, those systems, which in the
+    fully developed individual are peculiarly perfect, that in their
+    earliest rudiments and also throughout the whole course of their
+    development appear with the most striking distinctness" (Mueller's
+    _Archiv_, p. 135, 1837). See also his _Entwick. Kopf. nackt.
+    Amphib._, p. 198, 1838. So, too, Rathke notes how the elongated
+    shape of the snake appears even in very early embryonic stages
+    (_Entwick. Natter._, p. 111, 1839).
+
+    [536] Quoted by Keibel (p. 790, 1897) from the
+    _Biomechanik_.
+
+    [537] _Die Zelle und die Gewebe_, Jena, 1898, and the
+    subsequent editions of this text-book, published under
+    the title of _Allgemeine Biologie. Die Entwickelung der
+    Biologie im neunzehnten Jahrhundert_, Jena, 1900, 2nd
+    ed., 1908. "Ueber die Stellung der vergl.
+    Entwickelungslehre zur vergl. Anatomie, zur Systematik
+    und Descendenztheorie," _Handb. vergl. exper.
+    Entwickelungslehre der Wirbeltiere_, iii., 3, pp.
+    149-80, Jena, 1906. (1906, b). Also in Pt. I. of Vol. I.
+    (1906, a).
+
+    [538] _An Essay on Classification_, London, 1859.
+
+    [539] _Unsere Koerperform_, Leipzig, 1874.
+
+    [540] _Q.J.M.S._, xxxvi., pp. 35-52, 1894.
+
+    [541] Quoted by Hertwig. See also K. Goebel, "Die
+    Grundprobleme der heutigen Pflanzenmorphologie," _Biol.
+    Centrbl._, xxv., pp. 65-83, 1905.
+
+    [542] This is also emphasised by Fleischmann in his critical study of
+    evolutionary morphology entitled _Die Descendenztheorie_, Leipzig,
+    1901.
+
+    [543] The same remark applies to the bulk of speculation as to the
+    factors of evolution, with the exception of the contributions made
+    to evolution theory by the palaeontologists by profession, such as
+    Cope.
+
+    [544] _Les Transformations du Monde animal_, Paris, 1907.
+
+    [545] "Malacology _versus_ Palaeoconchology," _Proc.
+    Malacological Soc._, viii., pp. 66-83, 1908.
+
+    [546] Particularly by E. Perrier, "La Tachygenese," _Ann.
+    Sci. nat._ (_Zool._) (8), xvi., 1903.
+
+    [547] _Monatsber. k. Akad. Wiss._, Berlin, pp. 474-504,
+    1866.
+
+    [548] _Geognost. u. Palaeont. Beitraege_, ii., Heft 2, pp.
+    181-256, 1869.
+
+
+    [549] _Abhand. k.k. Geol. Reichsanstalt_, vii., Wien,
+    1875.
+
+    [550] The case for polyphyletism is very strongly put by
+    G. Steinmann in his book, _Die geologischen Grundlagen
+    der Abstammungslehre_, Leipzig, 1908.
+
+    [551] The steps in this chronological variation were
+    termed by Waagen "mutations."
+
+    [552] _The Age of Mammals in Europe, Asia, and North
+    America_, New York, 1910.
+
+    [553] _Origin of Species_, 6th ed., Chap. IV.
+
+
+
+
+INDEX
+
+
+ACTINOZOAN THEORY of Vertebrate Descent, 299-300
+
+Adaptation as Conservative Principle--
+  Cuvier, 39, 76
+
+Adaptation, Ecological--
+  Von Baer, 123
+  H. Milne-Edwards, 199
+  Lamarck, 221, 222, 223, 224, 227
+  Treviranus, 225 f.n.
+  C. Darwin, 231-2, 235, 239
+  Haeckel, 248, 263
+  Gegenbaur, 263
+  V. O. Kowalevsky, 362
+  Osborn, 362-4
+
+Adaptation, Ecological, and Classification--
+  Bronn, 203
+
+Adaptation of Parts. _See_ "Correlation, Functional," and "Conditions of
+  Existence"
+
+Adaptive Radiation (Osborn), 362-4
+
+Agassiz, A., 288 f.n., 295
+  On Coelom, 296
+
+Agassiz, L.--
+  Criticism of Vertebral Theory of Skull, 157
+  Membrane and Cartilage Bones, 164
+  Transcendentalism, 203
+  Classification, 203 f.n.
+  Three-fold Parallelism, 230, 255
+  Influence on Darwin, 238
+  Specific Distinctness of Embryos, 353
+
+Albertus Magnus, 17
+
+Alcmaeon, 1
+
+Aldrovandus, 18
+
+Allman, 209
+
+Analogy. _See also_ Homology.
+  Aristotle, 8-10
+  Owen, 108
+  Haeckel, 251
+  Gegenbaur, 266
+  Lankester, 267
+
+Anaxagoras, 14
+
+Anaximander, 14
+
+Anaximenes, 1
+
+Animal and Vegetative Lives--
+  Aristotle, 16, 32
+  Buffon, 26-7
+  Bergson, 26 f.n.
+  Cuvier, 26, 32
+  Bichat, 27-9
+  Oken, 94
+  K. G. Carus, 94
+  Von Baer, 116, 123, 131
+  Remak (Sensory and trophic layers), 210
+  Gegenbaur, 263
+
+Annelid Theory of Vertebrate Descent, 274-85, 301
+
+Archetype, Anatomical, 246, 302-3
+  E. Geoffroy, 54, 67
+  Owen, 104-7, 110
+  J. V. Carus, Huxley, 204
+  C. Darwin, 238 f.n.
+
+Archetype, Anatomical, as Ancestral--
+  C. Darwin, 235, 247
+  Haeckel, 251
+  Gegenbaur, 265
+  Sedgwick, 300
+  Criticism of this idea--
+    O. Hertwig, 355-7
+
+Archetype, Embryological, 168, 246, 302-3
+  Von Baer, 126, 132
+  Reichert, 139, 147, 149
+  Rathke, 151, 153
+  Huxley, 159-61
+
+Archetype, Embryological, as Ancestral--
+  C. Darwin, 233, 236-7
+  Haeckel, 254, 289-91
+  Gegenbaur, 266
+  O. and R. Hertwig, 298
+  Sedgwick, 300
+  A. Kowalevsky, 300
+
+Arendt, 162
+
+Aristotle, 2-16, 17, 345, 364
+  _Historia Animalium_, 2
+  _De Partibus Animalium_, 2, 9
+  Knowledge of Animals, 3, 4
+  Comparative Embryology, 4
+  Classification of Animals, 4-6
+  Unity of Plan, 6-7, 10
+  Homology and Analogy, 7-10
+  Teleology and Correlation, 10-12
+  Law of Compensation, 11
+  Division of Labour, 12
+  Degrees of Composition--homogeneous and heterogeneous parts, 12-14, 169
+  Law of Development (Von Baer), 14
+  Scale of Beings, 14-16
+  Functional attitude, 15-16, 197
+  Animal and Vegetative Lives, 16, 32
+
+Ascidian Theory of Vertebrate Descent, 269-73, 304
+
+Atomists, 16
+
+Atomists, "Biological," 192-4
+
+Audouin, V.--
+  Unity of plan in Arthropods, 85-6
+  Law of Compensation, 86
+  Marine Zoology, 195
+
+Autenrieth, 90, 96
+
+Avicenna, 17
+
+
+BABAK, E., 333
+
+Baer, K. E. von, 113-32, 133, 251, 304, 345, 356
+  Founder of Embryology, 113
+  _Entwickelungsgeschichte der Thiere_, 114
+  Regulation of Development, 114, 350
+  Development as Differentiation, 115, 128
+  Germ-Layer Theory, 115-6, 118-119, 208-9, 296
+  Morphological Differentiation, 116-7
+  Histological Differentiation, 117-8
+  Tissues and Germ-Layers, 118
+  Double symmetrical Development, 118, 279
+  Criticism of Meckel-Serres Law, 120-3, 304
+  Theory of Types, 123-4, 289, 291
+  Law of Development, 124-6
+  Embryological Criterion, 126-8, 132, 138
+  Embryological Archetype, 126, 132
+  Types of Development, 127-8
+  Von Baer and Cuvier, 128-30
+  Functional attitude, 129
+  Relation to Transcendentalists, 129, 131
+  Criticism of Scale of Beings, 130
+  Vertebral Theory of Skull, 131, 142
+  Serial Homology, 131-2
+  Gill-slits, Gill-arches and Aortic arches, 135-6, 146
+  Membrane and Cartilage Bones, 162-3
+  Degrees of Composition, 172
+  Ova of Mammals, 175-6
+  Segmentation of Ovum, 186
+  Criticism of Evolution Theory, 229, 242
+  Influence on Darwin, 236, 238
+  Criticism of Darwinism, 242
+  Teleology and Correlation, 242
+  On Ascidians, 271
+
+Baer's Law. _See_ "Development, Von Baer's Law"
+
+Bagge, 187
+
+_Balanoglossus_ Theory of Vertebrate Descent, 285-7
+
+Balbiani, 330
+
+Balfour, F. M., 247, 299
+  Annelid Theory, 282-4
+  Gastrulation and Gastraea Theory, 295
+  Mesoderm, 296 f.n.
+  Coelom, 297
+
+Barfurth, D., 330
+
+Barry, M., 186, 188
+
+Bateson, W.--
+  Metamerism, Vegetative Repetition, 286
+  _Balanoglossus_ Theory, 286-7
+  On Phylogenetic Speculation, 302
+
+Beard, J., 285
+
+Belon, 18
+
+Beneden van, and Julin, 271, 285, 346
+
+Bensley, A. B., 311 f.n.
+
+Bergmann, 187
+
+Bergson, H., 26 f.n., 341, 345
+
+Bernard, Claude, 195, 314
+
+Bert, P., 315
+
+Bichat, X., 27-30, 118, 132, 169, 178, 263
+  Animal and Vegetative Lives, 27-9
+  "General Anatomy," 29-30
+  _Vie propre_ of Tissues, 30
+
+Biogenetic Law. _See_"Development, Haeckel's Law"
+
+Bischoff, 138
+  Segmentation, 186, 188
+
+Blainville, de, 96, 128, 141, 199 f.n.
+
+Bojanus, 96, 97
+
+Bonnet, C.--
+  Scale of Beings, 22-3, 220, 227
+  Evolution, 215
+  Regeneration, 315
+
+Bonnet, R., 350
+
+Bonnier, G., on Albertus Magnus, 17
+
+Born, G., 330
+
+Boveri, T., 270 f.n., 333
+
+Braem, 347 f.n.
+
+Braun, A., 355
+
+Breschet, 138, 173
+
+Bronn, H. G., 200-3, 248
+  _Naturphilosophie_, 201
+  Functional attitude, 201-3
+  Geometry of Organism, 201, 249
+  Theory of Types, 202
+  Principle of Connections, 202
+  Intrinsic Laws of Evolution, 202
+  Division of Labour, 202
+  Ecological  Adaptation and Classification, 203
+
+Brown, R., 171
+
+Bruch, C., 203 f.n.
+
+Buechner, 194, 248
+
+Buffon, 24-7, 336
+  Scale of Beings, 24, 215
+  Unity of Plan, 24
+  Evolution, 24-5, 214
+  Classification, 25-6
+  Animal and Vegetative Lives, 26-7
+  Homology and Analogy, 27
+
+Burckhardt, R., 3 f.n., 268 f.n.
+
+Burdin, 96
+
+Burmeister, 249 f.n.
+
+Butler, S., 226 f.n., 313, 335-42
+  Relation to Lamarck, 335-7
+  Psychological Vitalism, 336-41
+  Heredity and Memory, 337-41
+  The Two Stages of Development, 337-9
+  Consciousness and Habit, 337-9
+  Recapitulation Theory, 339-40
+  Teleology, 341
+
+
+CABANIS, 215
+
+Camper, P., 45, 46
+
+Carter, 293 f.n.
+
+Carus, J. V.--
+  Criticism of Embryological Criterion, 167
+  Morphology and  Physiology, 194
+  Vertebral Theory of Skull, 203
+  On Archetype, 204
+  Evolution, 230
+
+Carus, K.G.--
+  Law of Parallelism, 94, 249
+  Vertebral Theory, 96
+  Geometry of Skeleton, 98-100
+  Splanchnoskeleton, 98, 140
+
+Causal Morphology, 312-3, 315-34
+
+Cell-Theory--
+  Schwann, 169, 173-86, 188
+  C. F. Wolff, 170
+  Schleiden, 170-2
+  Criticism of Schwann-Schleiden Theory, 185-8
+    Virchow, Leydig, 188
+
+Cell-Theory and Germ-Layer Theory--
+  Remak, 209-12
+
+Cell-Theory as Disintegrative--
+  Schwann, 180-5, 248
+  Vogt, 190-1
+  Virchow, 191
+  Haeckel, 248
+  Criticism of this idea--
+    Reichert, 192-3, 194
+    J. V. Carus, 194
+    Sedgwick, Whitman, 346
+
+Cell-Theory, Influence on Morphology, 190
+
+Cenogenesis, 258-9, 323
+
+Chabry, 331
+
+Child, C. M., 333
+
+Chun, C, 317, 332
+
+Classification of Animals--
+  Aristotle, 4-6
+  Rondeletius, Aldrovandus, Gesner, 18
+  Linnaeus, 22
+  Buffon, 25-6
+  Cuvier, 39-41
+  E. Geoffroy, 60
+  L. Agassiz, 203 f.n.
+  Lamarck, 216-7, 227, 228
+
+Classification and Ecological Adaptation (Bronn), 203
+
+Classification as Genealogical--
+  Buffon, 24-5
+  Lamarck, 218, 228
+  C. Darwin, 233, 234, 247
+  Haeckel, 250-1, 254
+  Criticism of this idea, 303, 304,
+    O. Hertwig, 356
+
+Classification, Phylogenetic--
+  Haeckel's, 289-94
+
+Claus, 259
+
+Co-adaptation, 326 f.n.
+
+Coelom--
+  Remak, 211
+  A. Kowalevsky, 270, 295, 297
+  Haeckel, 291, 295, 296
+  Lankester, 291, 297
+
+Coelom, Theory of, 295-301
+
+Cohen, 189
+
+Coiter, 18
+
+Colucci, 346
+
+Compensation, Law of--
+  Aristotle, 11
+  Goethe, 49
+  E. Geoffroy, 72-3
+  Audouin, 86.
+  German Transcendentalists, 100
+
+Condillac, 215
+
+Conditions of Existence, Principle of--
+  Cuvier, 34, 75-6, 239
+  Gegenbaur, 263-4
+  Roux, 324, 326
+  Spencer, Weismann, 326 f.n.
+  Disregard for--
+    Lamarck, 226
+    C. Darwin, 232, 238-41
+    Haeckel, 248, 264
+
+Conklin, 333
+
+Connections, Principle of--
+  Goethe, 47
+  E. Geoffroy, 53-4, 62-3, 71, 74, 261
+  Audouin, 85
+
+Connections, Principle of--_contd._
+  German Transcendentalists, 100
+  J. F. Meckel, 101
+  Owen, 107-8
+  Bronn, 202
+  C. Darwin, 234-5
+  Gegenbaur, 261
+  Semper, 279
+  In Embryology, 168
+  Main Principle of Morphology, 246, 302
+
+Convergence--
+  Milne-Edwards, 199
+  I. Geoffroy St Hilaire, 199 f.n., 206
+  C. Darwin, 236
+  Friedmann, Willey, Vialleton, 306 f.n.
+
+Convergence, Rejected by Evolutionary Morphologists, 305, 312
+  Hubrecht, 305-6
+
+Cope, E. D., 342, 357 f.n., 361, 362
+
+Correlation, Functional--
+  Aristotle, 10-12
+  Cuvier, 35-8, 239, 241
+  E. Geoffroy, 77
+  Von Hartmann, 240-1
+  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 Existence, 34, 75-6, 239
+  Correlation, 35-8, 239, 241
+  Metabolism, 38
+  Adaptation as Conservative Principle, 39, 76
+  Classification, 39-41
+  Principle of Subordination of Characters, 40
+  Criticism of Scale of Beings, 39-40, 130
+  Type Theory, 41, 124, 289, 291
+  Criticism of Evolution-Theory, 41-4, 129, 304
+  Variation, Limits of, 42
+  Palaeontological Succession, 43
+  Polemic with Geoffroy, 64-5, 74-8
+  Criticism of Vertebral Theory of Skull, 97-8
+  Influence on J. F. Meckel, 101
+  Criticism of Meckel-Serres Law, 129-30, 304
+  As Embryologist, 130
+  Criticism of Lamarck, 228
+
+Cytology, 346
+
+Cytoplasm of Egg, Organ-forming Stuffs, 332-3
+
+
+DALL, 361
+
+D'Alton, 113
+
+Dareste, C., 315
+
+Darwin, Charles, 78, 230-41, 271, 304, 307, 336, 362
+  Systematist and Field Naturalist, 230, 231
+  Palaeontological Succession, 231
+  Ecological Adaptation, 231-2, 235, 239
+  Species Problem, 231
+  Functional Adaptation, Disregard for, 232, 238-41
+  Classification as genealogical, 233, 234, 247
+  Unity of Plan due to Community of Descent, 233, 234-5, 239, 247
+  Embryological Archetype as ancestral, 233, 236-7
+  Rejects Meckel-Serres Law, 233, 236
+  Interpretation  of  Vestigial Organs, 233, 237
+  Organism as Historical Being, 233, 308
+  Rejects Scale of Beings, 234
+  Homology, 234-5, 247
+  Principle of Connections, 234-5
+  Anatomical Archetype as ancestral, 235, 247
+  Von Baer's Law interpreted phylogenetically, 236-7
+  Modifications inherited at corresponding age, 237
+  Monophyletism and Polyphyletism, 238
+  Causes of Success, 238, 241
+
+Darwin, Erasmus, 214, 226 f.n., 229, 336
+
+Darwin, Sir Francis, 344
+
+Daubenton, 26
+
+Degrees of Composition--
+  Aristotle, 12-14, 169
+  Glisson, 19
+  Malpighi, 20
+  Bichat, 29-30
+  Cuvier, 32-3,
+  Dujardin, 169, 188
+  Von Baer, 172
+  Effect of Invention of Microscope, 20
+  Relation to Cell-Theory, 169
+
+Delage, 333
+
+Delage and Herouard, 273 f.n.
+
+Delpino, 345
+
+Demaillet, 44
+
+Democritus, 16
+
+Deperet, C, 357
+  On Cuvier, 43
+  Absence of intermediary forms in Palaeontology, 358
+  Phyletic series and Polyphyletism, 360-1
+
+Development, Von Baer's Law--
+  Aristotle, 14
+  Von Baer, 124-6
+  Prevost and Dumas, 125 f.n.
+  Reichert, 149-50, 351 f.n.
+  Milne-Edwards, 205-8
+  Lereboullet, 206-8
+  Criticised by--
+    Agassiz, 352-3
+    His, 353
+    Sedgwick, 353
+    O. Hertwig, 354
+  Phylogenetic Interpretation of--
+    Darwin, 236-7
+    Gegenbaur, 266
+  Relation to Haeckel's Law, 254, 256, 257
+
+Development, Biogenetic Law (Haeckel)--
+  Haeckel, 251, 253-9, 291-4
+  F. Mueller, 252-3, 254, 257
+  Gegenbaur, 262
+  Roux, 319
+  Butler, 339-40
+  Orr, 342
+  Criticism of--
+    Vialleton, 348
+    Oppel, 348-9
+    Keibel, 349-50
+    Mehnert, 350-2
+    O. Hertwig, 352, 354-5
+    His, 353
+  Relation to Laws of Meckel-Serres and Von Baer, 254, 256, 257, 303, 309
+  Relation to Heredity and Development, 312-3
+  Influence of Causal Morphology, 347-8
+  Palaeontological Evidence for, 359
+
+Development, Meckel-Serres Law--
+  Harvey, 18
+  Hunter, 22
+  E. Geoffroy, 69-70, 72
+  Serres, 80-3, 94, 203-4, 205-6
+  Kielmeyer, Autenrieth, Oken, 90
+
+Development, Meckel-Serres Law-_contd._
+  Tiedemann, 91
+  J. F. Meckel, 91-3
+  K. G. Carus, 94
+  Criticism of--
+    Von Baer, 120-3, 304
+    Cuvier, 129-30, 304
+    Milne-Edwards, 205
+    Lereboullet, 206-8
+    C. Darwin, 233, 236
+  Analogy with Biogenetic Law, 254-7, 262, 303, 304, 309
+
+Development, Meckel-Serres Law, Theory of Three-fold Parallelism--
+  L. Agassiz, 230, 255
+  Tiedemann, Vogt, 255 f.n.
+  Haeckel, 254-5
+
+Development, The two periods of--
+  Roux, 320-4, 325, 327, 335
+  Butler, 337-9
+
+Diogenes of Apollonia, 1
+
+Disintegration. _See_ "Cell-Theory," and "Materialistic Attitude"
+
+Division of Labour, Principle of--
+  Aristotle, 12
+  Milne-Edwards, 197-8
+  Bronn, 202
+  Gegenbaur, 264
+
+Dohrn, A., 269, 274-8
+  Annelid Theory of Vertebrate Descent, 274-7, 303
+  Principle of Function-Change, 276-8, 307
+  Functional Attitude, 277-8, 307
+  Formal Attitude, 306
+
+Doellinger, 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
+  Membrane and Cartilage Bones, 163
+
+Dujardin, 169, 188
+
+Dumas. _See_ Prevost and Dumas
+
+Dumeril, 96
+
+Dumortier, 173
+
+Dutrochet, 99 f.n., 130, 134
+
+Duverney, 19
+
+
+EAR-OSSICLES, Homology of--
+  E. Geoffroy, 56
+  Spix, 100
+  Rathke, 141, 150
+  Reichert, 144-7
+
+_Echelle des etres. See_ "Scale of Beings."
+
+Ehlers, 284
+
+Eisig, H., 284, 285
+
+Embryology, Comparative, Early Workers--
+  Aristotle, 4, 113
+  Fabricius, Harvey, 18, 113
+  Malpighi, 20, 113
+  Oken and Kieser, 90, 113
+  Haller, C. F. Wolff, J. F. Meckel, Tiedemann, 113
+
+Embryology, Experimental, 317, 318, 330-3
+
+Embryological Archetype. _See_ "Archetype, Embryological"
+
+Embryological Criterion of Homology, 133-168, 347
+  Goethe, 49
+  E. Geoffroy, 72, 110
+  Cuvier, 75, 110, 130
+  Owen, 110-1
+  Von Baer, 126-8, 132, 138
+  Rathke, 138, 140-1
+  J. Mueller, 138
+  Reichert, 138-9, 144-7, 163
+  Vogt, 156-7
+  Huxley, 158-9, 166
+  Koelliker, 165-6
+  Criticised by--
+    Owen, J. V. Carus, 167
+
+Empedocles, 1, 15
+
+Engramm (Semon), 343
+
+_Entwicklungsgesetz._ _See_ "Evolution, Intrinsic Laws of"
+
+_Entwicklungsmechanik_, 315
+
+Erasistratus, 17
+
+Evolution Theory--
+  Lucretius, 16
+  Buffon, 24-5, 214
+  Cuvier's criticism, 41-4, 129, 304
+  E. Geoffroy, 66-9, 73, 228
+  J. F. Meckel, 92-3, 215, 228
+  Leibniz, 213
+  Kant, 213-4
+  Erasmus Darwin, 214, 229
+  C. Bonnet, Oken, Robinet, Treviranus, 215
+  Tiedemann, 215, 255 f.n.
+  Lamarck, 215-29
+  Von Baer, 229, 242
+  I. Geoffroy St Hilaire, J. V. Carus, 230
+  Charles Darwin, 230-41
+  Von Hartmann, 240-1, 244, 356
+  Koelliker, 243
+  Owen, 244
+  Milne-Edwards, 244-5
+  Haeckel, 250-9
+  Gegenbaur, 265
+  The Organism as an Historical Being, 308-13
+    C. Darwin, 233, 308
+    Haeckel, 252, 257
+    Sedgwick, 308
+    Roux, 313, 322-4
+    Butler, 313, 336-41
+
+Evolution-Theory, Influence on Morphology, 302-13
+
+Evolution, Intrinsic Laws of, 241
+  J. F. Meckel, 93
+  Bronn, 202
+  Von Baer, 229, 242, 356
+  Koelliker, Naegeei, 243, 356
+  Owen, 244
+  Von Hartmann, 244, 356
+  Milne-Edwards, 244-5
+  O. Hertwig, 354-5, 356-7
+  Wigand, 356
+  Deperet, 361
+
+
+FABRICIUS, 18, 113
+
+Fallopius, 18
+
+Fischel, 346, 350
+
+Fischer, 328
+
+Fleischmann, 357 f.n.
+
+Flourens, 46, 315
+
+Fontana, 172
+
+Forbes, E., 196
+
+Formal Attitude, 246, 305
+  Goethe, 49
+  E. Geoffroy, 62-3, 71, 75-8, 305
+  Haeckel, 249, 257, 260
+  Gegenbaur, 261, 263
+  Semper, 279
+  Adopted by Evolutionary Morphologists, 302-8, 311-2, 314
+  Hubrecht, 305-6
+  Dohrn, 306
+
+France, R., 345
+
+Friedmann, 306 f.n.
+
+Fuld, 333
+
+Functional Adaptation, 316-7, 318, 320-9, 333, 344, 351
+
+Functional Attitude--
+  Aristotle, 15-6, 197
+  Bichat, 27-9
+  Cuvier, 31-6, 65, 75-8, 200, 305
+  Goethe, 49-50
+  J. F. Meckel, 101
+  Owen, 109, 110, 111
+  Von Baer, 129
+  Milne-Edwards, 195, 197-200
+  J. Mueller, Reichert, 200
+  Bronn, 201-3
+  Lamarck, 222-6, 307, 335
+  Gegenbaur, 260, 263-4
+  Dohrn, 277-8, 307
+  Roux, 320-9, 335
+  Houssay, 333
+  Butler, 336-41
+  G. Wolff, 346
+  Driesch, 346-7
+  Giard, 347
+  E. Schulz, 347 f.n.
+  Keibel, 349-50
+  Mehnert, 350-1
+  American Palaeontologists, 361, 362
+  Ruetimeyer, 361
+  V. O. Kowalevsky, 361-2
+  Osborn, 362-4
+
+Function-Change, Principle of--
+  Dohrn, 276-8, 306, 307
+  Eisig, 284
+
+Fuerbringer, M., 282 f.n., 284, 323 f.n.
+
+
+GALEN, 17
+
+Gastraea Theory, 269, 288-95, 298, 299-3O1, 303
+
+Gastrula, Discovery of, 288
+
+Gaupp, E., 310 f.n.
+
+Gegenbaur, C, 247, 260-7, 271, 285, 286, 288 f.n.
+  Division of Egg-nucleus, 188
+  Functional Attitude, 260, 263-4
+  Formal Attitude, 261, 263
+  Principle of Connections, 261
+  Embryology and Comparative Anatomy, 261-2, 263
+  Biogenetic and Meckel-Serres Laws, 262
+  Homology, 261, 263, 265, 266-7
+  Adaptation and Correlation, 263-4
+  Archetype as ancestral, 263 f.n, 265
+  On Phylogenetic Speculation, 265-6
+  Embryological Archetype, 266
+  Membrane and Cartilage Bones, 309, 310
+
+Gemmill, J. F., 285 f.n., 312 f.n.
+
+Geoffroy, Etienne, St Hilaire, 40, 52-78, 141
+  Unity of Plan, 52-65, 70 ff., as conservative, 75, 78
+  Principle of Connections, 53-4, 62-3, 71, 74, 261
+  Unity of Composition, 54, 70-1, 75-6, 200, 305
+  Archetype, 54, 67
+  Metastasis, 55-6, 59, 74
+  Opercular Bones, 56
+  Unity of Composition of Sternum, 57-60
+  Classification, 60
+  Vertebrates and Articulates, 60-4, 274, 278-9, 303
+  Formal Attitude, 62-3, 65, 71, 75-8, 305
+  Cephalopods and Vertebrates, 64-5
+  Scale of Beings, 64
+  Polemic with Cuvier, 64-5, 74-8
+  Evolution, 66-9, 73, 228
+  Biogenetic Law, 69
+  Teratology, 69, 315
+  Meckel-Serres Law, 70, 72
+  Criteria of Homology, 71, 72, 110
+  Law of Compensation, 72-3
+  Criticism of his Principles, 74
+  Relation to German Transcendentalists, 89, 100-1
+  Vertebral Theory of Skull, 96, 97
+  Influence on Darwin, 234-5, 238
+
+Geoffroy, Isidore, St Hilaire, 65 f.n., 199 f.n., 230
+
+Geometry of the Organism, 33
+  K. G. Carus, 98-100, 249
+  Bronn, 201, 249
+  Haeckel, J. Mueller, Burmeister, G. Jaeger, 249
+
+Germinal Vesicle (Egg-nucleus), 175-7, 188, 291 f.n.
+
+Germ-Layer Theory--
+  Von Baer, 115-6, 118-9, 208-9, 296
+  Pander, 119-20, 209
+  C. F. Wolff, 119-20
+  Rathke, 136, 208
+  Lereboullet, Bischoff, 208
+  Huxley, 208, 289
+  Remak, 209-12, 296
+
+Germ-Layers and Gastraea Theory--
+  Haeckel, 289-95
+  Lankester, Balfour, 295
+
+Germ-Layer Theory, Influence of Causal Morphology on, 347
+
+Gesner, 18
+
+Giard, A.--
+  On Ascidian Theory, 271-3
+  Adaptive Homology, 273
+  Poecilogeny, 347-8
+
+Glisson, F., 19
+
+Gluge, 173
+
+Goebel, K., 356 f.n.
+
+Goethe, 45-51, 65, 89, 250
+  Unity of Plan, 45-7, 51
+  Homology, 47
+  Principle of Connections, 47
+  Formal and Functional Attitudes, 48-50
+  Teleology, 48
+  Metamorphosis of Plants, 48
+  Repetition of parts, 48-9
+  Vertebral Theory of Skull, 49, 96, 97
+  Law of Compensation, 49
+  Embryological Criterion, 49
+  Organisms as Nature's Works of Art, 50
+
+Goette, 259
+
+Graaf, von, 175
+
+Grew, N., 169
+
+Gruber, 330
+
+HAECKEL, Ernst, 247-60, 271, 314, 342, 353, 357
+  His sources, 248-50
+  Materialism, 248, 250
+  On Teleology, Heredity and Adaptation, 248, 263
+  Correlation, Disregard for, 248, 264
+  Geometry of the Organism (Promorphology), 249
+  Repetition of Parts (Tectology), 249-50
+  Classification as Genealogical, 250-1, 254
+  Archetype as ancestral, 251
+  Homology and Analogy, 251
+  Biogenetic  Law, 251, 253-9, 291-4
+  Three-fold parallelism, 254-5
+  Scale of Beings, 255, 256-7
+  Organism as an Historical Being, 257
+  Prussianism, 257
+  Palingenesis, 258
+  Cenogenesis, 258-9
+  Heterotopy, Heterochrony, 259
+  Gastraea Theory, 269, 288-95
+  Phylogenetic Classification, 289-94
+  Criticism of Theory of Types, Monophyletism, 289, 291
+  Gastraea Theory and Biogenetic Law, 291-4
+  Primary stages of Ontogeny and Phylogeny, 291-3
+  Coelom, 291, 295, 296
+  Experimental Embryology, 317
+
+Haller, 113
+
+Harting, 284 f.n.
+
+Hartmann, E. von--
+  On Darwin's conception of correlation, 240-1
+  Evolution, 244, 356
+
+Hartog, M., 344
+
+Harvey, 18, 113
+
+Hatschek, 270 f.n., 299
+
+Helmholtz, H. von, 195
+
+Henle, 172
+
+Hensen, V., 209 f.n.
+
+Herbst, C., 333
+
+Herder, 46
+
+Heredity and Memory, 336-44
+
+Hering, E., 341-2
+
+"Heritage" Characters, 309, 322
+
+Herlitzka, 332
+
+Herophilus, 17
+
+Hertwig, O., 163, 330, 331, 346
+  On C. F. Wolff, 119
+  Fertilisation, 291 f.n.
+  Membrane and Cartilage Bones, 309-10
+  Biogenetic Law, 352, 354-5
+  Von Baer's Law, 354
+  Intrinsic Laws of Evolution, 354-5, 356-7
+  Homology not necessarily Homogeny, 355-7
+  Unity of Plan not necessarily due to Community of Descent, 355-7
+  On Phylogenetic Speculation, 356
+
+Hertwig, O. and R.--
+  Coelom Theory, 297-8
+  Nervous System of Coelentera, 299
+
+Heterochrony, 259, 348, 349-52
+
+Heterogeneous Generation (Koelliker), 243
+
+Heterotopy, 259
+
+Hilgendorf, 359
+
+Hill, 311
+
+Hippocratic Treatises, 2
+
+His, W., 206 f.n., 209 f.n.
+  Causal Morphology, 316
+  Cytoplasm of Egg, Organ-forming Stuffs, 333
+  Specific Distinctness of Embryos, 353
+
+Histological Differentiation (von Baer), 117-8
+
+Histology. _See also_ "Cell-Theory"
+  Malpighi, 20
+  Stensen, 21
+  Bichat, 29-30, 169, 178
+  Von Baer, 117-8
+  Schwann, 178
+  Remak, 209-12
+
+Hofer, B., 330
+
+Hofmeister, 185
+
+Homogeny, 267, 303, 355
+
+Homology, 168, 303, 355-7. _See also_ "Connections, Principle of," and
+   "Embryological Criterion"
+  Aristotle, 7-10
+  Belon, 18
+  Buffon, 27
+  Goethe, 47
+  E. Geoffroy, 53, 71
+  Serres, 80
+  Owen, 107-9
+  Lamarck, 227
+  C. Darwin, 234-5, 247
+  Haeckel, 251
+  Gegenbaur, 261, 263, 265, 266-7
+  Giard, 273
+  Semper, 279
+  O. Hertwig, 355-7
+  Braun, 355
+
+Homology, Genetic Definition of--
+  Gegenbaur, 266
+  Lankester, 267
+  O. Hertwig's criticism, 355-7
+
+Homoplasy, 267
+
+Hooke, R., 20, 169
+
+Houssay, F., 19 f.n., 333
+
+Hubrecht, A. A. W., 284, 295 f.n., 301, 305-6
+
+Hunter, J., 22, 315
+
+Huschke, 134-5, 136, 141, 146
+
+Huxley, T. H., 157, 238, 247
+  On Rathke, 154 f.n.
+  Embryological Criterion, 158-9, 166
+  Embryological Archetype, 159-61
+  Criticism of Vertebral Theory of Skull, 161-2
+  Membrane and Cartilage Bones, 166-7
+  On Archetype, 204
+  Germ-Layer Theory, 208, 289
+  Criticism of Three-fold Parallelism, 230 f.n.
+  Coelom, 297
+  Ancestry of Marsupials, 311
+
+Hyatt, A., 359, 361
+
+
+INSTINCT and Morphogenesis, Analogy of, vi., 307, 312
+  Lamarck, 220, 226
+
+JACOBSON , 164
+
+Jaeger, G., 249 f.n.
+
+_Jardin des Plantes_, Paris, 19
+
+Jenkinson, J. W., 347 f.n.
+  On His, 316
+
+Jones, Wharton, 138, 176
+
+Julin, C., 271, 285
+
+Jussieu, de, 40
+
+
+KANT, I.--
+  Teleology, 35, 213, 242
+  Unity of Plan, 46, 213-4
+  Evolution, 213-4
+
+Keibel, F., 348, 349-50
+
+Kerkring, 131
+
+Kielmeyer, 89, 90, 96
+
+Kieser, 90
+
+Kleinenberg, N., 277
+
+Kohlbrugge, J., 44 f.n., 65 f.n.
+
+Koelliker, A.--
+  On C. F. Wolff, 119
+  Vertebral Theory of Skull, 157
+  Membrane and Cartilage Bones, 164-6, 310
+  Embryological Criterion, 165-6
+  Cell-division, 187
+  Intrinsic Laws of Evolution, 243, 356
+  Saltatory Variation, 243
+
+Kowalevsky, A., 269-71, 284, 285, 299, 300
+  Development of Amphioxus, 270
+    Ascidians, 270-1
+  Coelom, 270, 295, 297
+  Gastrula, 288
+
+Kowalevsky, V. O., 361-2
+
+Krause, 176
+
+Kupffer, 271
+
+
+LACAZE-DUTHIERS, H. de, 203 f.n., 315-6
+  On Ascidians, 271, 273
+
+Lamarck, 44, 66, 78, 215-29
+  Relation to Buffon, 215
+  Scale of Beings, 215-8, 220-1, 227-8
+  As Evolutionary, 218, 220
+  Classification, 216-7, 227, 228
+  Species Problem, 216, 227
+  Materialism, 218-9, 222-3, 225-6
+  Psychological Vitalism, 219, 220-6, 307, 335
+  _Sentiment interieur_, 219-20, 222-3, 225
+  Ecological Adaptation, 221, 222, 223, 224, 227
+  Laws of Evolution, 221-5
+  Transmission of Acquired Characters, 221-2, 224
+  Subtle Fluids, 222
+  Use and Disuse, 223-4
+  Independence of Current Thought, 226-7
+  Homology and Analogy, 227
+  Reception of his Theory, 228-9
+  Lamarck and Butler, 335-7
+
+Lang, A., 301
+
+Lankester, Sir E. Ray, 247
+  Homology, Homogeny, Homoplasy, and Analogy, 267
+  _Balanoglossus_ Theory of Vertebrate Descent, 287
+  Germ-Layer Theory and Phylogenetic Classification, 291
+  Planula Theory, 295
+  On Coelom Theory, 296-7, 299 f.n.
+
+Latreille, 86, 100
+
+Laurencet, 64
+
+Lavocat, 203 f.n.
+
+Leeuenhoek, 20, 21, 169
+
+Leibniz, 23, 213, 343
+
+Lereboullet--
+  Von Baer's Law, 206-8
+  Germ-layer Theory, 208
+  Gastrula, 288 f.n.
+
+Leucippus, 16
+
+Leuckart, 193 f.n., 194, 297
+
+Levy, O., 333
+
+Leydig, 187, 188, 275 f.n., 285
+
+Linnaeus, 22
+
+Loeb, J., 333, 347
+
+_Loi de Balancement_. _See_ "Compensation, Law of"
+
+Loven, 186, 196
+
+Lucretius, 16
+  On the Soul, 222 f.n.
+
+Ludwig, 193, 194, 314
+
+Lyell, Sir C., 228 f.n.
+
+Lyonnet, 22
+
+
+MACBRIDE, E. W., 287 f.n.
+
+M'Kendrick, J.--
+  On Fontana, 172
+
+Mackenzie, W., 345
+
+Malpighi, M., 20-1, 113, 169
+
+Marine Zoology, Rise of, 195-6
+
+Materialistic Attitude, 246-7, 345, 364
+  Schwann, 180-5
+  Vogt, 190-1
+  Virchow, 191
+  Ludwig, 193
+  Materialistic Physiology, 193-4, 314-5, 347
+  Lamarck, 218-9, 222-3, 225-6
+  The Darwinians, 241, 308
+  Haeckel, 248, 250
+  Roux, 315, 317, 318-9, 329
+  Semon, 343
+  Rignano, 344
+  Loeb, 347
+  Criticism of this attitude--
+    Reichert, 192-3
+
+Meckel, D. A., 95
+
+Meckel, J. F., 113
+  Meckel-Serres Law, 91-3
+  Evolution, 92-3, 215, 228
+  Teratology, 93-4
+  Repetition of Parts, 95
+  Vertebral Theory of Skull, 96
+  Eclecticism, 101
+
+Meckel's Cartilage, 141, 145
+
+Meckel-Serres Law. _See_ "Development, Meckel-Serres Law"
+
+Mehnert, E., 348, 350-2
+
+Membrane and Cartilage Bones, 162-7, 309-10
+
+Memory and Heredity, 336-44
+
+Mendelism, 346
+
+Mesenchyme, 298
+
+Mesoderm, 209-11, 296, 297, 298
+
+Metabolism--
+  Cuvier, 38
+  Schwann, 182-5
+  Roux, 324, 329
+
+Metamerism, 94, 95, 100, 109, 131-2, 266-7, 274-5, 279, 282, 286, 299, 301
+
+Metamorphosis of Plants, 48, 235
+
+Metastasis, Principle of--
+  E. Geoffroy, 55-6, 59, 74
+  Owen, 106
+
+Metschnikoff, E., 278 f.n., 285, 288
+  Criticism of Ascidian Theory, 271
+  Coelom, 295, 296, 297
+
+Meyen, 170, 185
+
+Meyer, E., 284
+
+Meyranx, 64
+
+Microscope, Invention of, 19
+
+Milne-Edwards, H., 12, 86, 238
+  Marine Zoology, 195
+  Functional Attitude, 195, 197-200
+  Unity of Plan, 197
+  Division of Labour, 197-8
+  Ecological Adaptation, Convergence, 199
+  Von Baer's Law, Polemic with Serres, 204-8
+  Evolution, 244-5
+
+Mirbel, 170, 171
+
+Mivart, St G., 277
+
+Mohl, von, 170, 185
+
+Moldenhawer, 170
+
+Moleschott, 194
+
+Moquin-Tandon, A., 87
+
+Morgan, T. H., 317 f.n., 332, 333, 347 f.n.
+
+Mosaic Theory of Development, 330-3
+
+Mueller, F., Biogenetic Law, 252-3, 254, 257
+
+Mueller, H., 166
+
+Mueller, J., 136, 209 f.n., 260, 285, 309, 345
+  Embryological Criterion, 138
+  Vertebral Theory of Skull, 142-4, 154, 157
+  On Reichert, 150
+  Cell Theory, 172-3
+  Division of Egg-nucleus, 188
+  Vitalism, 192
+  Marine Zoology, 196
+  Functional Attitude, 200
+
+Mutations (Waagen), 361 f.n.
+
+
+NAEGELI, 185, 243 f.n., 356
+
+_Naturphilosophie._ _See_ "Philosophy of Nature"
+
+Nesbitt, R., 162
+
+Neumayr, 357, 360
+
+Nussbaum, M., 330
+
+
+OKEN, L., 89, 113, 131, 134, 149
+  Meckel-Serres Law, 90-1
+  Teratology, 91
+  Repetition of Parts, 94-5
+  Serial Homology, 95-6, 100
+  Vertebral Theory, 96, 97, 98
+  On Geoffroy, 100
+  Influence on Serres, 205
+  Evolution, 215
+
+Ollier, 315
+
+Oppel, A., 318 f.n., 324 f.n., 327, 348-9
+
+Orr, H. F., 342
+
+Osborn, H. F., 214 f.n., 361
+  On V. O. Kowalevsky, 362
+  Functional Attitude, 362-4
+  Law of Adaptive Radiation, 362-4
+
+Owen, R., 97, 102-12, 204
+  Eclecticism, 102
+  Vertebral Theory of Skeleton, 103-7
+  Archetype of Vertebrate Skeleton, 104-7, 110
+  Vertebral Theory of Skull, 104-6
+  Metastasis, 106
+  Principle of Connections, 107-8
+  Anatomy and Embryology, 108
+  Homology and Analogy, 108
+  Classes of Homology, 108-9, 266
+  Functional Attitude, 109, 110, 111
+  Embryological Criterion, 110, 167
+  Homological and Teleological Compoundedness, 110-1
+  Vegetative Repetition of Parts, 111, 286
+  Unity of Plan as Conservative Principle, 112
+  Influence on Darwin, 234, 235, 238
+  Evolution, 244
+
+
+PACKARD, 361
+
+Palaeontological Record, 357-61
+  Absence of connecting forms, 357-9
+  Biogenetic 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_ "Development, Meckel-Serres Law"
+  Three-fold. _See_ "Development, Meckel-Serres Law"
+
+Paris Museum of Natural History, 19, 89, 101
+
+Paul, 360
+
+Pauly, A., 345
+
+Perrault, C., 19
+
+Perrier, E., 88, 359 f.n.
+
+Pflueger, E., 317, 330
+
+Philipeaux, 315
+
+"Philosophy of Nature," 89, 94, 98, 203, 248
+
+Phyletic Series, 359-61
+
+Physiology, Separation from Morphology, 194, 247, 260, 314
+
+Physiology of Development, 315
+
+Planula Theory (Lankester), 295
+
+Plato, 15
+
+Pockels, 138
+
+Poecilogeny (Giard), 347-8
+
+Poli, 175
+
+Polyphyletism--
+  Darwin, 238
+  Von Baer, 242, 356
+  Koelliker, 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, 186
+
+Promorphology (Haeckel), 249
+
+Protoplasm, 169, 188-9
+
+Purkinje, 172, 173, 175, 176, 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
+  Embryological Archetype, 151, 153
+  Membrane and Cartilage Bones, 163, 166
+
+Rauber, A., 330
+
+Reaumur, 22, 315
+
+Recapitulation Theory. _See_ "Development, Biogenetic Law"
+
+Regeneration, 315, 318, 333, 346
+
+Regulatory Processes in Development, 114, 319, 333, 346-7, 350
+
+Reichert, C. B., Embryological Criterion, 138-9, 144-7, 163
+  Archetype, 139, 147, 149
+  Homologies of Gill-arches and Ear-ossicles, 144-7
+  Vertebral Theory of Skull, 147-9, 157
+  Von Baer's Law, 149-50, 351 f.n.
+  Membrane and Cartilage Bones, 163, 165, 166, 310
+  Criticism of "Biological Atomists," 192-3, 194
+  Functional Attitude, 193, 200
+
+Remak, R., 118, 288 f.n.
+  On Vertebrae, 157
+  Cell Theory, 173, 187-8, 209
+  Microscopical Technique, 209 f.n.
+  Germ-Layer Theory, 209-12, 296
+  Cells, Tissues and Germ-Layers, 209-12
+  Mesoderm, 209-11
+  Coelom, 211, 296
+
+Repetition of Parts within the Organism, Theory of. _See also_
+   "Vertebral Theory of Skull"
+  Goethe, 48-9
+  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, Roesel von, 22
+
+Roux, W., 313, 315-29, 344, 351
+  _Entwicklungsmechanik_, 315, 317-8
+  Materialistic Attitude, 315, 317, 318-9, 329
+  Functional Adaptation, 316-7, 318, 320-9, 333
+  Experimental Embryology, 317, 318, 330-1
+  Simple and Complex Components, 318-20
+  Functional Definition of Life, 320
+  Functional Attitude, 320-9, 335
+  The Two Periods of Development, 320-4, 325, 327, 335
+  Mosaic Theory of Development, 323, 330-1
+  Metabolism, 324, 329
+  Structure, Functional and Non-functional, 324-6
+  Functional Unity of Organism, 326
+  Functional Adaptation of Blood-vessels, 326-9
+  Form as manifestation of Activity, 329
+
+Ruini, C., 18
+
+Rusconi, 133-4, 186
+
+Ruetimeyer, L., 361
+
+Ryder, 361
+
+
+SACHS, J. von, 170
+
+St Ange, M., 146
+
+Salensky, 259
+
+Saltatory Variation--
+  E. Geoffroy, 78
+  Von Baer, 242
+  Koelliker, 243
+  Owen, 244
+
+Sarcode, 169
+
+Sars, M., 186, 196
+
+Savigny, J. C., 83-5, 100, 137, 271
+
+Scale of Beings, 89, 206, 214-5
+  Aristotle, 14-6
+  Anaximander, Anaxagoras, 14
+  Empedocles, Plato, 15
+  Albertus Magnus, 17
+  C. Bonnet, 22-3
+  Robinet, 23
+  Buffon, 24
+  E. Geoffroy, 64
+  Lamarck, 215-8, 220-1, 227-8
+    As Evolutionary, 218, 220
+  Haeckel, 256-7
+  Criticism of this idea--
+    Cuvier, 39-40, 130
+    Von Baer, 130
+    Milne-Edwards, 205
+    Lereboullet, 207
+    Darwin, 234
+    Haeckel, 255
+    Relation to Evolution-Theory, 214-5
+
+Schepelmann, 333
+
+Schleiden, 170-2
+
+Schmieden, 328
+
+Schults, C. H., 173
+
+Schultze, Max, 189
+
+Schultze, O., 331
+
+Schulz, E., 347 f.n.
+
+Schwann, Theodor, 169, 173-86, 248
+  Physiological Standpoint, 173, 179, 180, 182
+  Development of Cells, 174-5, 179-80
+  Cellular Nature of Ovum, 175-7
+  Development of Tissues from Cells, 177-8
+  Histology, 178
+  Materialism and Teleology, 180-3, 185
+  Cell-metabolism, 182-5
+  Cells as organic Crystals, 184-5
+
+Sedgwick, A., 347 f.n.
+  Actinozoan Theory of Vertebrate Descent, 299-300
+  Metamerism, 299
+  Embryological Archetype, 300
+  Organism as Historical Being, 308
+  Cell-Theory, 346
+  Von Baer's Law, 353
+
+Segmentation of Ovum, 186-8
+
+Seiler, 138
+
+Selection, Natural and Artificial, 307 f.n.
+
+Self-Differentiation (Roux), 319, 320-1, 322, 323, 324, 327
+
+Self-Regulation (Roux), 319
+
+Semon, R., 342-3
+
+Semper, C., 259, 269, 278-82, 284, 286
+  Annelid Theory, 274, 278-82
+  Metamerism, 274, 279, 282
+  Follower of Geoffroy, 278
+  Unity of Plan and Composition, 279, 303
+  Principle of Connections, 279
+  Formal Attitude, 279
+
+_Sentiment interieur_ (Lamarck), 219-20, 222-3, 225
+
+Serial Homology. _See_ "Metamerism"
+
+Serres, E., 79-83, 91, 100, 205-6, 257 f.n.
+  Criteria of Homology, 80
+  Law of parallelism, 80-3, 94, 203-4, 205-6
+  Law of Multiple Formation, 80-1
+  Unity of Plan, 83, 205, 206
+  Teratology, 83
+  Meckel's Cartilage, 145 f.n.
+  Transcendentalism, 205-6
+  Concrescence Theory, 206 f.n.
+
+Severino, 18
+
+Sharpey, 162, 176
+
+Siebold, von, 186
+
+Skull, Development of, 139-62.
+  _See also_ "Vertebral Theory"
+
+Spallanzani, 315
+
+Species-Problem--
+  Cuvier, 42
+  Lamarck, 216, 227
+  Darwin, 231
+
+Spencer, H., 326 f.n.
+
+Spengel, 285, 287
+
+Spinoza, 343
+
+Spix, 96, 97, 100, 141
+
+Stannius, 165
+
+Steenstrup, 309
+
+Steinmann, G., 357, 360 f.n.
+
+Stensen (Steno), 21
+
+Swammerdam, 20, 21-2
+
+
+TACHYGENESIS, 359
+
+Technique, Microscopical, 209 f.n., 268
+
+Tectology (Haeckel), 249
+
+Teleology--
+  Aristotle, 10
+  Cuvier, 33-5
+  Kant, 35, 213, 242
+  Von Baer, 242
+  Owen, Von Hartmann, 244
+  Butler, 341
+  G. Wolff, Driesch, 346
+  Criticism of--
+    Goethe, 48
+    Schwann, 180-2
+    The Darwinians, 241
+    Haeckel, 248
+    Evolutionary Morphologists, 308
+
+Teratology, 69, 83, 91, 93, 315
+
+Thienemann, 23 f.n.
+
+Thompson, D'Arcy W., 2 f.n.
+
+Thomson, A., 176
+
+Thomson, J. Arthur, 215 f.n.
+
+Tiedemann, 91, 113, 215, 255 f.n.
+
+Tissues and Germ-Layers, 118, 209-12
+
+Transcendental Anatomy, Relation to Evolutionary Morphology, 302-8, 312
+
+Transcendentalism, French and German Schools, 89, 100
+
+Trembley, 22, 315
+
+Treviranus, 141, 170, 215, 225 f.n.
+
+Turpin, 173
+
+Types, Theory of (Cuvier and Von Baer)--
+  Cuvier, 41, 124, 289, 291
+  Von Baer, 123-4, 289, 291
+  Bronn, 202
+  Lereboullet, 207
+
+Types, Theory of (Cuvier and Von Baer)--_contd._
+  Criticised by--
+    E. Geoffroy, 60
+    Haeckel, 289, 291
+    Lankester, 291
+
+Type-Theory and Evolution, 304
+
+
+UNGER, 185
+
+Unity of Composition, Principle of, Geoffroy, 54, 70-2, 75-6, 200, 305
+
+Unity of Plan, 88, 241, 278-9, 303, 312. _See also_ "Archetype"
+  Aristotle, 6-7, 10
+  Belon, Severino, 18
+  Perrault, 19
+  Robinet, 23
+  Buffon, 24
+  Cuvier, 41
+  Goethe, 45-7, 51
+  Vicq D'Azyr, 45
+  Camper, 45, 46
+  Herder, 46
+  Kant, 46, 213-4
+  E. Geoffroy, 52-65, 70 ff.
+  Serres, 83, 205, 206
+  Savigny, 83
+  Audouin, 85-6
+  Latreille, 86
+  Duges, 86-7
+  J. F. Meckel, 101
+  Milne-Edwards, 197
+  Semper, 279
+  Haeckel, 289, 291
+  Lankester, 291
+
+Unity of Plan as due to Community of Descent--
+  Darwin, 233, 234-5, 239, 247
+  Haeckel, 250-1
+  Gegenbaur, 263 f.n., 265
+  Criticism of this idea--
+    O. Hertwig, 355-7
+
+Unity of Plan as Conservative Principle--
+  E. Geoffroy, 75, 78
+  Owen, 112
+  Gegenbaur, 263-4
+  Evolutionary Morphologists, 307
+
+
+VALENTIN, 138, 173, 176
+
+Variation, Limits of, Cuvier, 42
+
+Vegetative Repetition of Parts--
+  Owen, 111, 286
+  Bateson, 286
+
+Velpeau, 138
+
+Vertebral Theory of Skull, 49, 96-9, 104-6, 131, 141-4, 147-9, 154-7,
+  161-2, 165, 203, 235, 310 f.n.
+
+Vertebrate Descent, 269-87, 299-301, 304
+
+Verworn, M., 330
+
+Vesalius, 18
+
+Vestigial Organs, 233, 237, 309, 312
+
+Vialleton, L., 306 f.n., 348
+
+Vicq d'Azyr, 45, 95
+
+Virchow, R., 188, 191
+
+Vitalism, Psychological--
+  Lamarck, 219, 220-6, 307, 335
+  Butler, 336-41
+  Orr, Cope, 342
+  Ward, 343
+  Delpino, France, Pauly, A. Wagner, Mackenzie, 345
+
+Vogt, C.--
+  Criticism of Vertebral Theory, 156-7
+  Capillaries, 179
+  Segmentation, 186
+  Materialistic Attitude, 190-1
+  Threefold Parallelism, 255 f.n.
+
+
+WAAGEN, 359, 361 f.n.
+
+Wagner, A., 345
+
+Wagner, R., 176
+
+Ward, J., 343
+
+Weber, 138
+
+Weismann, A., 240, 323, 326 f.n., 330-1, 343
+
+Werneck, 173
+
+Whitman, C. O., 346
+
+Wigand, A., 242 f.n., 356
+
+Willey, A., 273 f.n., 306 f.n.
+
+Williamson, 309
+
+Willis, 19
+
+Wilson, E. B., 331, 332-3, 346 f.n., 347 f.n.
+
+Wolff, C. F., 113
+  Germ-layer Theory, 119-20
+  Cells, 170
+
+Wolff, G., 346-7
+
+Woodward, B. B., 358
+
+Wotton, E., 17
+
+
+ZELENY, 333
+
+Zittel, K. von, 357, 358
+
+Zoja, 331
+
+       *       *       *       *       *
+
+PRINTED BY
+
+OLIVER AND BOYD,
+
+EDINBURGH, SCOTLAND
+
+       *       *       *       *       *
+
+HEREDITY. By J. Arthur Thompson, M.A., LL.D., Regius Professor
+  of Natural History in the University of Aberdeen. With numerous
+  Illustrations. 9s. _net_.
+
+CONTENTS
+
+Heredity and Inheritance: Defined and Illustrated--The Physical Basis
+  of Inheritance--Heredity and Variation--Common Modes of
+  Inheritance--Reversion and allied Phenomena--Telegony and other
+  Dispute Questions--The Transmission of Acquired Characters--Heredity
+  and Disease--Statistical Study of Inheritance--Experimental Study of
+  Inheritance--History of Theories of Heredity and Inheritance--Heredity
+  and Development--Heredity and Sex--Social Aspects of Biological
+  Results--Bibliography--Subject-Index to Bibliography--Index.
+
+"We all know books of science which we ought to read with pleasure,
+but to which we turn with shrinking. Full, perhaps, of new facts and
+ideas, they are so expressed as to bore consumedly. 'Heredity' belongs
+to another category. He who runs may read, even if he be a beginner,
+and he who reads will probably not cease to run until he has traversed
+the last page."--_Nature_.
+
+"This is certainly the best modern book on heredity to recommend to
+the student and the intelligently curious."--_Science Progress_.
+
+"May be regarded as the standard work of reference on this subject. As
+a judicial summary of an exceedingly difficult and controversial
+subject it is masterly, while in the matter of clearness of exposition
+it has no rival."--_Knowledge_.
+
+
+RECENT ADVANCES IN THE STUDY OF VARIATION, HEREDITY AND EVOLUTION.
+  By Robert H. Lock, M.A., sometime Fellow of Gonville and Caius
+  College, Cambridge, late Assistant-Director of the Royal Botanic
+  Gardens, Ceylon. With Portraits and other Illustrations. Revised by L.
+  Doncaster, D.Sc., F.R.S. With a Biographical Note by Bella Sidnay
+  Woolf (Mrs R. H. Lock). Crown 8vo.
+
+
+MICROSCOPY. The Construction, Theory, and Use of the Microscope. By
+  Edmund J. Spitta, F.R.A.S., F.R.M.S., etc. With numerous Diagrams and
+  Illustrations. Second Edition. 12s. 6d. _net_.
+
+"Let us hasten to urge every student of the microscope who wishes to
+gain a thorough understanding of its principles and possibilities and
+its defects, and every user of the instrument who desires a work of
+reference to which he may turn for an explanation of some unexplained
+optical phenomenon, or for particulars of up-to-date apparatus, to
+procure a copy of Mr Spitta's book without delay."--_Nature_.
+
+
+CONVERGENCE IN EVOLUTION. By Arthur Willey, D.Sc. (Lond.); Hon. M.A.
+  (Cantab.); F.R.S. With Diagrams. Demy 8vo. 7s. 6d. _net_.
+
+This work brings together some scattered facts of parallel development
+of outward form and internal structure in the Animal kingdom,
+introducing new cases and fresh interpretations. It is, taken as a
+whole, an original contribution to the theory of organic evolution,
+with special reference to the forms of Animal life.
+
+THE HEREDITY OF ACQUIRED CHARACTERS IN PLANTS. An aspect of the true
+  Darwinism based on Personal Observations and Experiments. By the Rev.
+  Prof. George Henslow. With Illustrations. Demy 8vo. 6s. _net_.
+
+
+A HANDBOOK OF PHYSIOLOGY. By W.D. Halliburton, M.D., F.R.S., Professor
+  of Physiology, King's College, London. Eleventh Edition, being the
+  Twenty-fourth of Kirkes'. With nearly Seven Hundred Illustrations,
+  including some Coloured Plates. Large Crown 8vo. 15s. _net_.
+
+"One of the best manuals for the student which we possess ... the book
+is an eminently trustworthy one, and will prove a valuable foundation
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+THE BACTERIOLOGY OF MILK. By Harold Swithinbank, of the
+  Bacteriological Research Laboratory, Durham, and Sir George Newman,
+  M.D., F.R.S.E., D.P.H., Chief Medical Officer, Board of Education.
+  With Special Chapters also by Dr Newman on Spread of Disease by Milk
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+  (Edin.), D.P.H., Chief Medical Officer, Board of Education. With
+  Illustrations. Medium 8vo. 21s. _net_.
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+THE REALM OF NATURE. An Outline of Physiography. By H.R. Mill, D.SC,
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+NATURE AND ORIGIN OF FIORDS. By J.W. Gregory, D.Sc., F.R.S., Author of
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+  _net_.
+
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+MECHANISM, LIFE AND PERSONALITY. An Examination of the Mechanistic
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