VERTEBRATA 



175 



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 Gephalo- 

 discus will hereafter render it necessary to associate that 

 form, and in all probability Bhabdopleura 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 Gephyrxa open up a large field 

 of speculation and of consequent embryological and ana- 

 tomical research. 

 Ances- The Cuvierian Vertebrata, Amphioxus, Tunicata, and 

 *™1^*^°™ Balanoglossus being thus indisputably connected by a re- 

 brates ' 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 either Amphioxus or the 

 Ascidian tadpole. The first tendency arising from the 

 discovery of the affinities of these simpler forms with the 

 Cuvierian Vertebrata was 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 Da»win 

 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 sufiices to show that any given living form, such as 

 the gorilla, cannot possibly be the ancestral form frotai 

 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 

 Argu- series of modifications of structure. Not only this, but 

 ments we have no wifficient ground for assuming that, even in 

 fromsiin-j.ggpg(j^ of the simplicity of their structure, any given 

 '^lJ°^amm&\ 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 



stric- 

 ture. 



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 

 Crustacea. 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 thesis of 

 prima facie as legitimate an hypothesis, that any existing <iege'>e- 

 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 difiuse 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. 



Appljang 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 



