802 Comparative Morphology of Chordates 



The tendency of "all to be everywhere" is illustrated in Fig. o<)7. 



"degeneration" 



Specialization by reduction or "degeneration' of structures seems 

 to have occurred along many lines in the vertebrate series. The 

 results of it greatly complicate the problem of discovering genetic 

 relationships. In many instances, it is difficult to ascertain whether 

 some weakly developed organ is at an early stage of progressive evolu- 

 tion or at a late stage of regression. This is especially true of the rela- 

 tion between cartilage and bone. It has long been a widely accepted 

 idea that primitive elasmobranchs had an all-cartilaginous endo- 

 skeleton and that this primitive condition persists in modern shark-like 

 fishes. It would then seem to be reasonable to regard the "ganoid" 

 fishes, with their more or less incompletely ossified skeletons, as inter- 

 mediate between elasmobranchs and the bony Teleostei. In modern 

 urodele amphibians such as Necturus, there is much persistent em- 

 bryonic cartilage in the adult skeleton. This invites interpretation as a 

 primitive condition. The widely accepted, but perhaps overworked, 

 idea that "ontogeny repeats phytogeny" interprets the cartilaginous 

 skeleton of the vertebrate embryo as a heritage from shark ancestors. 

 There is, however, much paleontologic evidence that the endoskeletons 

 of early fishes and amphibians were ossified to a much greater extent 

 than those of modern "ganoids" and amphibians, indicating, there- 

 fore, a reduction or "degeneration" in the degree of ossification. A 

 question is raised, then, as to the phylogenetic status of modern 

 elasmobranchs, "ganoids," and amphibians (especially the urodeles). 

 To what extent do they retain the ancient primitive characteristics, 

 and how far may they have undergone a regressive evolution which 

 has resulted in anatomic characteristics deceptively primitive in 

 appearance? The solution of these problems awaits more evidence 

 from paleontology. 



The Vertebrate Named (by Himself) Homo Sapiens 



All Orders of mammals except two are definable on the basis of 

 highly specialized anatomic characteristics which are of predominant 

 importance in the adaptation of the animal to its particular habitat 

 and habits. One of the exceptional Orders is the Insectivora. They are 

 commonly described as "generalized" or "primitive" — they are "just 

 mammals." It is true that the mole is specialized for burrowing, but 

 its digging claws and degenerate eyes are features of the mole as a 

 I'ossorial animal and not of the Order as a whole. The hedgehog's sharp 

 spines are specialized hairs, but among Insectivora they are confined to 



