THE ORIGIN OF THE VERTEBRATES. 



217 



quently regarded as the ancestors of the amphibia, a view 

 which receives its chief support from the existence of lungs 

 in both forms. In their skeleton and in other features the 

 two groups seem widely remote. Again, in recent years the 



MAMMALS 



BIRDS 

 REPTILES 



URODELES 



GOCEPHALS 

 CROSSOPTERYGfl 



HOLOCEPHALl 



ELASMOBRANCHS 



? CYCLOSTOMES ? OSTRACODERMS 

 PRIM. ELASMOBRANCHS 



FlG. 221. Lines of descent of the different groups of vertebrates. 



tendency has been to regard the mammals as descendants from 

 theromorphous reptiles, a view which receives its chief evidence 

 from paleontology. More lately still the tendency is to revert 

 to the older view of an amphibian ancestry. 



