CENOZOIC ERA: AGE OF MAMMALS 



6oi 



(i) That race was more likely to survive whose members had teeth 

 enabling their possessors to grind up nutritious food, no matter how 

 tough and hard. Particularly was this true if the teeth of suc- 

 cessive generations developed better grinding surfaces, permitting 

 their possessors to take advantage of new food or food that, because 

 of inability to grind it, was unsuited to their ancestors. The efficient 

 grinding teeth of the horse, cow, and elephant, as will be seen, are the 

 result of such an evolution. 



(2) Since the swiftest animals are more likely to escape their 

 enemies, those that possessed limbs constructed for rapid motion 

 were most likely to 

 survive. The leg 

 best suited for this 

 purpose is one in 

 which the foot is 

 lengthened, the joints 

 perfected, and the 

 number of toes re- 

 duced. The one-toed 

 horse may be con- 

 sidered the climax of 

 such evolution. 



(3) Since more sa- 

 gacious animals are 

 better able to find 

 food, escape their 

 enemies, and care for 

 their young, it natu- 

 rally follows that 

 those with large 

 brains (Fig. 542) were 

 more likely to survive. 

 As the various races 



of mammals are discussed, attention will be called to the increase in 

 the size of the brains, and any exceptions will be noted. 



(4) Increased bulk and the strength which usually accompanies 

 size is often a protection against enemies and, in the case of males, 

 results in the destruction of the smaller and weaker members of the 

 same species. As a consequence, it will be seen that the surviving 

 species of a given order often become larger in the course of their 



Fig. 542. — Brains of ancient (on left) compared with 

 modern (on right) mammals: A, creodont; B, dog; 

 C, early amblypod ; D, rhinoceros ; E, highly developed 

 amblypod (Uintatherium) ; F, hippopotamus. Olfactory 

 lobes (dots), cerebral hemispheres (oblique lines), cerebel- 

 lum and medulla (dashes). (After Osborn.) 



