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.



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.



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.



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.