Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/200

180 VERTEBRATA Argii- ments struc- ture. epidermal tract by the infolding of which the nerve-tube of Tunicata and the other Vertebrata is formed. Hence it seems impossible to exclude Balanoglossus from a place in the phylum Vertebrata. The possession of pharyngeal gill-slits alone might not justify the association ; but, when this is combined with the presence of the notochord, though rudimentary and with a special condition and position of the main nerve-centre, it becomes impossible to ignore Balanoglossus in our conception of the Vertebrate phylum. The recent discoveries of Harmer (7) with regard to Cephalo- discus will hereafter render it necessary to associate that form, and in all probability Rhabdopleura also, with Balano- glossus in the Vertebrate phylum. The further conse quences of such an association in regard especially to the affinities of Polyzoa and of Gephyrsza open up a large field of speculation and of consequent embryological and ana tomical research. The Cuvierian Vertebrata, Amphioxus, Tunicata, and Balanoglossus being thus indisputably connected by a re- markable combination of structural points, which admit of no explanation consistent with the principles of evolutional morphology except that of the genetic relationship of the forms thus enumerated, we are at once confronted by those questions as to the ancestral history of Vertebrata which have been already mentioned above as stimulated by Kowalewsky s discoveries. Undoubtedly Amphioxus is lower and simpler in structure than any Fish, Tunicata as low as or lower than Amphioxus, and Balanoglossus, in some respects, more archaic than eithef Amphioxus or the Ascidian tadpole. The first tendency arising from the discovery of the affinities of these simpler forms with the Cuvierian Vertebrata Avas to see in them the representatives of the ancestors of all Vertebrata. Amphioxus has been pointed to by authorities in morphology as the living presentation of our common Vertebrate ancestor ; a similar position corresponding to an earlier stage of development has been admitted by no less an authority than Darwin for the Ascidian. It appears, nevertheless, that all such simple solutions of the problem of Vertebrate ancestry are without warrant. They arise from a very common tend ency of the mind, against which the naturalist has to guard himself, a tendency which finds expression in the very widespread notion that the existing anthropoid apes, and more especially the gorilla, must be looked upon as the ancestors of mankind, if once the doctrine of the descent of man from ape-like forefathers is admitted. A little re flexion suffices to show that any given living form, such as the gorilla, cannot possibly be the ancestral form from which man was derived, since ex hypothesi that ancestral form underwent modification and development, and in so doing ceased to exist. The same considerations apply to the question of the ancestry of Vertebrata. Probably no existing low form of Vertebrate closely represents the an cestral form by the modification of which higher forms have been developed. We have no justification for assum ing that such low forms do more than present to us a col lateral branch of the family, and that collateral branch must, in all probability, have experienced its own special series of modifications of structure. Not only this, but we have no sufficient ground for assuming that, even in respect of the simplicity of their structure, any given animal forms at present existing exhibit a mere survival of a corresponding degree of simplicity in their remote ancestors. Such an assumption was almost universally made, until a more correct view was pressed on the atten tion of naturalists by Dr Anton Dohrn, the founder of the zoological station of Naples (8}. So far from its being the case that simplicity of organization necessarily implies the continuous hereditary transmission of a low stage of structural development from remote ancestors, there are numerous instances in which it is certain that the existing simplicity of structure is due to a process of degeneration, and that an existing form of simple structure is thus de scended from ancestors of far higher complexity of organi zation than itself. Such are various parasitic worms and Cmstacea. The evidence in favour of the occurrence of progressive simplification of structure or degeneration, in place of progressive elaboration, depends (1) upon the com parison of the adult structure of the degenerate organism with that of its nearest allies, by which it is often rendered clear that the ensemble of the organization of the simpler organism cannot be explained on the hypothesis that it represents an ancestral or archaic condition common to it and its more elaborate congeners, and (2) on the direct evidence of individual development or life-history. The latter evidence is conclusive, when we find, as in the case of Cirrhipede Crustaceans and of Ascidian Tunicates, that the embryo on its way to the adult condition passes through stages of development presenting a higher degree of or ganization than that ultimately reached, so that, as in the Cirrhipede larva and the Ascidian tadpole, the young form resembles allied organisms of a higher stage of develop ment, and subsequently degenerates from the point of progressive elaboration to which it had attained, and be comes greatly simplified in the final stage of its growth. Conclusive as such evidence is, there is no law of develop ment which necessitates its preservation. If it be an advantage to the organism, the full force of heredity has play, and what are called the " recapitulative phases " of ancestral development are passed through by the indi vidual in the course of development from the egg. But with remorseless thoroughness all such hereditary tenden cies may be removed when such removal is an advantage to the organism, and the development from the egg may proceed directly to the adult degenerate form. Such is the case with many Tunicata, the young of which never exhibit notochord and tadpole form ; indeed, were it not for the preservation of a few exceptional cases, like that of the Ascidian section of the group, we should have no direct evidence of the degeneration of Tunicata from tad pole-like ancestry. The general result of the considerations which have Hypo- been urged with regard to degeneration is this, that it is tliesis o prima fade as legitimate an hypothesis, that any existing de & en< animal has developed by progressive simplification from more elaborate ancestors, as it is that such an animal has developed by a continuous and unbroken progress in elaboration from simpler ancestors ; and we are specially called upon to apply the hypothesis of degeneration where the animal under consideration is likely from its mode of life to have undergone that process. Such modes of life, tending to degeneration, are parasitism, sessile or adherent habit, burrowing in the sea-bottom, and diffuse feeding. The animal which pursues living prey, and contends with other organisms for the dominion of the regions of earth and water that are flooded with light and richly supplied with oxygen gas, is the animal which represents the outcome of a longer or shorter period of progressive elaboration. It is worth while noting in parenthesis that in all cases the " whirligig of time " has probably brought its revenges, and that the ancestry of a form evolved through a long period of progressive elaboration was at an antecedent period subject to simplification and degeneration, whilst in the past records of the present exemplars of the latter process there must certainly have been long stretches of continuous elaboration. Applying these considerations to the construction of the Genea- genealogical tree of Vertebrata, we find that the task is by logical no means simplified. We cannot with the earliest evolu-

tionists adopt a scale or ladder-like series, placing the