REPTILES, BIRDS, AND MAMMALS 



463 



of the new abundance of plant food provided by the angiosperms ; others 

 retained the more primitive omnivorous or carnivorous food habits. 



Much of the evolutionary history of the placentals was controlled by 

 the interplay between carnivore and herbivore. Since the slower, more 

 stupid, and less well protected of the herbivores were the easiest prey, 

 selection continually operated to make the survivors speedier and more 

 alert, or to give them improved means of protection. Some stocks evolved 

 into fast runners; others developed great size and strength, with tough 



Fig. 28.30. Pleistocene life in southern California, reconstructed from the fossils of the tar 

 pits at Rancho La Brea in the suburbs of Los Angeles. Great numbers of animals were 

 trapped in the sticky tar when they came to drink at the pools or to feed upon mired 

 creatures. Among the forms evidently common in the region were mastodons, horses, lions, 

 saber-toothed tigers, dire wolves, giant vultures, and many smaller mammals and birds. 

 {Courtesy Carnegie Museum., Pittsburgh.) 



skins ; protective weapons appeared in wide variety, the commonest being 

 horns and striking feet. Meanwhile the carnivorous types kept pace. 

 Some, like the wolves, specialized on the long chase; others, like the cats, 

 on the pounce from concealment; size and strength grew to match the 

 size and strength of the prey. All this is reminiscent of what we have seen 

 among the dinosaurs — a race between offense and defense. But among the 

 mammals one result of this competition was something quite new — the 

 development of social behavior. Family groups and organized herds 

 among the herbivores, and hunting groups among the carnivores, have 

 had no counterparts among the reptiles. 



