THE GEOLOGICAL EECOBD 303 



many of the mammalian groups in Tertiary times have run to 

 great size ; most of the larger forms, such as the primitive 

 ungulate, Tinoceras (Fig. 150), of the American Eocene, and the 

 giant ground sloth, Megatherium, of the American Pleistocene, 

 are already extinct, but it must not be forgotten that the existing 

 whales are amongst the largest animals that have ever lived, and 

 in bulk at any rate will bear comparison with thje largest of 

 the great extinct reptiles. 



The last term in the evolutionary series of the Mammalia is 

 man, whose advent, so far as we at present know, dates back only 



Nat. size. 



FIG. 149. Mandible of PlasfoUtlnrium lucMandi ; x 3. (From Smith 

 Woodward's "Vertebrate Palaeontology," after Goodrich.) 



to about the commencement of the Pleistocene or end of the 

 Pliocene epoch. 1 



There is one more point that is well worth emphasizing about the 

 evolution of the Vertebrata, as indicated not only by the geological 

 record but also by the facts of comparative 'anatomy. Each 

 successive great group appears to have arisen, not from the 

 most highly specialized members of some preceding great 

 group, but from comparatively undifferentiated forms. Thus the 

 Amphibia arose, not from bony fishes, but from primitive dipncids 

 or ganoids ; the reptiles arose, not from frogs or toads, but from 

 primitive stegocephalian amphibia ; the birds arose, not from 

 pterodactyls, but from comparatively unspecialized reptiles ; the 

 mammals also arose from the more primitive reptilian forms, and 

 man himself, whose advent undoubtedly marks the commencement 

 of a fresh line of evolution, belongs to the order Primates, which 

 in respect of bodily organization, as seen, for example, in the 



, Vide Chapter XXVII. 



