CHAPTER 15 

 THE ANCESTRY OF VERTEBRATES 



One of the many unsolved problems of biology is that of the ancestry 

 of vertebrates. The problem has persisted even after morphologists 

 ceased the vain attempt "to evolve the ancestor out of their inner con- 

 sciousness." While great divergence of opinion still prevails, nevertheless 

 as the result of recent advances in genetics the scope of the field of inquiry 

 has been considerably narrowed. 



Earlier morphologists made the erroneous assumption that phylo- 

 genetic changes have taken place through the transformation of adult 

 organisms. In the light of genetic evidence, this notion can no longer be 

 entertained. The belief that modifications of the body effect representa- 

 tive alterations in the germ cells is no longer tenable. Evolutionary 

 change appears to be initiated within the germ cells and not within the 

 body cells. It must be admitted that the cause of alteration in the genetic 

 constitution of the germ cell remains unknown. But that is another 

 problem. Somehow or other the potencies of germ cells have changed so 

 as produce new phyla with fundamentally different types of organization. 



To this generation of morphologists it no longer seems profitable to 

 try to convert the body of a scorpion into that of a fish. The genetic 

 potencies of highly specialized forms do not become so easily changed. 

 The shift of morphological opinion has led to increased tolerance of the 

 view that the larger subdivisions of the animal kingdom have made their 

 appearance independently and much earlier in the history of organisms 

 than morphologists formerly assumed. This has brought about an 

 increased respect for the value of embryological evidence in its bearing 

 upon the problem of phylogenesis. 



If chorda tes have arisen as suggested in Fig. 374, none of the extant 

 phyla of invertebrates — with the possible exception of Protozoa — can 

 be said to be "ancestors" in a strict sense. If the diagram is correct, the 

 evolution of the chordates has not involved the transformation of the 

 types of invertebrates with which we are familiar, but a series of forms 

 which have disappeared from the earth. The story of chordate phylo- 

 genesis has to be pieced together, as has been done in this text, from frag- 

 ments of embryological and anatomical evidence. All pre-chordate 

 ancestors are therefore highly conjectural. 



To aid the speculative morphologist, however, there is evidence of 

 parallelism in evolution. Some extant non-chordates possess structures 



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