174 



THE AGE OF MAMMALS 



evolution of an animal runs to the development of tusks and horns, prob- 

 ably favored by sexual selection, the grinding teeth are apparently neg- 

 lected and are apt to show arrested development. The widespread belief 

 that bulky animals tend to disappear first is inconsistent with the fact that 

 the small phenacodonts became extinct long before the large Amblypoda. 



Among the modernized Eocene Herbivora of Europe, several of the 

 small Artiodactyla became extinct very soon after the period which marked 

 the extinction of the bulky lophiodonts. Thus bulk is chiefly fatal where 



Fig. 72. — Evolution of mammals in North America. (In solid black) Archaic mammals 

 which became extinct in the Eocene and (creodonts) Oligocene Epochs, namely : multituber- 

 culates, creodonts, tillodonts, tseniodonts, condylarths, and amblypods. (In hollow lines) Ap- 

 pearance and extinction of archaic and modernized mammals which survive to Pleistocene or 

 recent time, namely : marsupials, insectivores, carnivores, rodents, edentates, primates, peris- 

 eodactyls, proboscideans. 



correlated with inadequate feeding mechanism, with brain power not 

 adequate to enabling the females to defend and care for the young as 

 well as to meet new conditions of life, and with inadequate defensive 

 organs. 



The competition of the archaic Creodonta with the diminutive pro- 

 Carnivora in Eocene times may be only remotely compared to the extinc- 

 tion of the Tasmanian wolf (Thylacinus) and Tasmanian devil {Sar- 

 cophilus) through the introduction of the true dog {Cards dingo) on the 

 Australian main land. 



The steady increase in size of the creodonts as displayed in Patriofelis 

 and in the enormously powerful Harpagolestes is a fact which may be 

 placed parallel with the increasing size of the Amblypoda. 



It is noteworthy that the only archaic Carnivora which persisted into 

 the Lower Oligocene are the hysenodonts, in which the brain actually in- 



