It will be observed that there is here implied an a.n.a.logy between the biogenetic law and the law of von Baer, for both a.s.sert 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 a.s.sert, too, that differentiation of structure takes place not in one progressive or regressive line, but in several diverging directions.

But the a.n.a.logy 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. Aga.s.siz,[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 Aga.s.siz stated it, the doctrine applied neither to types, nor as a general rule to cla.s.ses, 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 princ.i.p.al 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 a.s.serted 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 circ.u.mstance 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 cla.s.sificatory 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 pa.s.sages like the following, with its partial recognition of the _ech.e.l.le_ 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 pa.s.ses through the same series of transformations which its animal progenitors have pa.s.sed through, during immense s.p.a.ces 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. Muller, 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' (_Falschungsgeschichte_). The first is the true ontogenetic epitome or short recapitulation of past evolutionary history; the second is the exact contrary, a new foreign ingredient, a falsifying or concealing of the epitome of phylogeny."[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 ca.n.a.l, 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 cla.s.sed 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 cla.s.ses 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 r.e.t.a.r.dation 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 princ.i.p.al 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 _Grundzuge der vergleichenden Anatomie_, the second edition of which, published in 1870, soon came to be regarded as the cla.s.sical 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 cla.s.sical or Cuvierian morphology that characterise Gegenbaur's work.

According to Haeckel,[381] Gegenbaur was greatly influenced by J. Muller, 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 doc.u.ment 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, const.i.tute 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 pa.s.sage, taken from a later work:--"Ontogeny thus represents, to a certain degree, palaeontological development abbreviated or epitomised. The stages which are pa.s.sed 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 (_Grundzuge_, 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 cla.s.sified 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 h.o.m.ological 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 (_Grundzuge_, 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" (_Grundzuge_, 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 h.o.m.ology; 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. "h.o.m.ology ...

corresponds to the hypothetical genetic relationship. In the more or the less clear h.o.m.ology, we have the expression of the more or less intimate degree of relationship. Blood-relationship becomes dubious exactly in proportion as the proof of h.o.m.ologies 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 a.s.suming 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" (_Grundzuge_, 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 h.o.m.ologous or morphologically equivalent structures from such as are a.n.a.logous or physiologically equivalent, just as did Owen and the older anatomists. Like von Baer he recognises h.o.m.ologies, as a rule, only within the type.

He contributed, however, to the common stock a useful a.n.a.lysis of the concept of h.o.m.ology, and established certain cla.s.ses and degrees of it.

He distinguished first between general and special h.o.m.ology, in quite a different sense from Owen.

General h.o.m.ology, in Gegenbaur's sense, relates to resemblances of organs within the organism, and includes four kinds of resemblance, h.o.m.otypy, h.o.m.odynamy, h.o.m.onomy and h.o.m.onymy. Right and left organs are h.o.m.otypic, metameric organs are h.o.m.odynamic; h.o.m.onomy is the relation exemplified by fin-rays or fingers, which are arranged with reference to a transverse axis of the body; h.o.m.onymy 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 (_Grundzuge_, p. 80).

Special h.o.m.ology, on the other hand, relates to resemblances between organs in different animals. The interesting thing is that Gegenbaur defines it genetically. Special h.o.m.ology 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 h.o.m.ologies by comparison just as the older anatomists did, and infers common descent from h.o.m.ology, not h.o.m.ology from common descent.

"Special h.o.m.ology," 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 h.o.m.ology is said to be "complete," in the latter "incomplete." Thus the bones of the upper arm are completely h.o.m.ologous throughout all vertebrate cla.s.ses from Amphibia upwards, while the heart of a fish is incompletely h.o.m.ologous with the heart of a mammal.

Independently of Gegenbaur, Sir E. Ray Lankester proposed in 1870 a genetic definition of h.o.m.ology.[385] He proposed, indeed, to do away with the term h.o.m.ology 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 h.o.m.ologous 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 _h.o.m.ogenous_(p. 36). All other resemblances were to be called _h.o.m.oplastic_. "h.o.m.oplasy includes all cases of close resemblance of form which are not traceable to h.o.m.ogeny, all details of agreement not h.o.m.ogenous, in structures which are broadly h.o.m.ogenous, as well as in structures having no genetic affinity" (p. 41). Serial h.o.m.ology, for instance, was a case of h.o.m.oplasy.

The term "a.n.a.logy" was to be retained for cases of functional resemblance, whether h.o.m.ogenetic or not.