1 90 ANIMAL BIOLOGY 



shown when the cow was classified (p. 9). Their care for 

 the young, their intelligence, and their ability to survive 

 when in competition with other animals, causes the mam- 

 mals to be considered the highest class in the animal 

 kingdom. 



According to these tests, what class of vertebrates should 

 rank next to mammals? Compare the heart, lungs, blood, 

 and parental devotion of these two highest classes of ani- 

 mals. 



M~U 



j? 



FIG. 339. SKELETON OF LION (cat family). 



The first mammals, which were somewhat like small 

 opossums, appeared millions of years ago, when the world 

 was inhabited by giant reptiles. These reptiles occupied 

 the water, the land, and the air, and their great strength 

 and ferocity would have prevented the mammals from 

 multiplying (for at first they were small and weak), but 

 the mammals carried their young in a pouch until able to 

 care for themselves, while the reptiles laid eggs and left 

 them uncared for. The first mammals used reptilian eggs 

 for food, though they could not contend with the great 

 reptiles. Because birds and mammals are better parents 

 than reptiles, they have conquered the earth, and the rep- 



