THE ANIMAL CLIMAX-THE CHORDATES 



341 



The important mammalian groups 



The most primitive modern mammals, 

 that is, the ones that resemble ancestral 

 forms most closely, are the insect feeders 

 ( Insectivora ) . While there are some fairly 

 large, highly specialized forms such as the 

 moles in this group, the most typical are 

 the tiny shrews. These mouse-like creatures 

 are probably similar to the stock that gave 

 rise to higher mammals including man, 

 himself. Their mouths are armed with nee- 

 dle-like teeth which aid in securing their 

 specialized diet of insects. They are ex- 

 tremely active animals, in fact, so active 

 that it requires a volume of food equal to 



Fig. 13-59. The young of the opossum (top picture) are 

 born in a very immature stage and must continue 

 their development in the marsupium. Here they are 

 shown clinging to the teats in the pouch. The young 

 of higher mammals are well developed at birth. This 

 calf (bottom picture) is able to stand and even run 

 alongside its mother, though only a few minutes old. 



Fig. 13-60. One of the smaller carnivores, the raccoon 

 (Procyon lotor). Its diet consists of many things, 

 among them crayfish, which is the goal of the pres- 

 ent search. 



tlieir body weight each day to satisfy theii' 

 needs. Most of them live in burrows and 

 are such secretive animals that they are sel- 

 dom seen by the casual observer; only the 

 biologist armed with cleverly devised traps 

 is able to capture and study them. It is this 

 secretive habit, reminiscent of their ances- 

 tral cousins, that made it possible for them 

 to survive up to the present. They are as 

 isolated in their burrows as if they were 

 separated by impassable water or mountain 

 barriers. Some, however, live in trees much 

 the same as the ancient forms that gave 

 rise to the tree-loving primates (Fig. 

 13-67). 



The next group significant in the evolu- 

 tion of mammals are the Hesh-eaters, the 

 carnivores (Carnivora) (Figs. 13-60, 13-61, 

 13-62). These are very successful today, as 

 shown by the numbers and kinds of such 

 animals as cats, dogs, weasels, bears, civets, 

 and the marine forms such as the walruses 

 and seals. All our present dogs and wolves 

 are supposed to have arisen from one form, 

 Cynodictis, which resembled a weasel as 

 much as a dos;. The carnivores have teeth 

 well adapted to the rending and tearing 

 of flesh. The large canines readily tear 

 through tough skin and the shearing molars 

 cut the flesh into pieces sufficiently small 

 to be swallowed. Furthermore, since meat 

 is easily digested, the alimentary canals are 



