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 



EPHALS 

 CROSSOPTERYGII 



HOLOCEPHALI 



ELASMOBRANCHS 



? CYCLOSTOMES 

 PRIM. ELAS 



70STRACODERMS 

 dOBRANCHS 



FIG. 221. Lines of descent of the different groups of vertebrates; 



tendency has been to regard the mammals as descendants frorr/ 

 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. 



