628 The Animal Kingdom 



a few marsupials are left in either continent. In the United States 

 there is but one species, the Virginia opossum. 



The first true placental mammals appeared during the Cretaceous 

 and of course were completely overshadowed by the doomed dinosaurs. 

 The first placental mammals must have been small, shy forms which 

 fed mainly on soft vegetation and insects. The present-day members 

 of the order Insectivora which includes the shrews are closest to 

 these ancestral forms. From this primitive insectivore, all the many 

 successful mammals gradually emerged. The present-day mammals 

 have diversified and now occupy nearly all possible habitats : some such 

 as the bats have taken to the air ; the whales and porpoises have be- 

 come marine animals ; the primates took to the trees and secondarily 

 descended; swarms of ungulates, rodents, and many carnivores popu- 

 late the ground, and some insectivorous forms and rodents burrow 

 under the soil. 



The great success of the mammals is attributable to many different 

 features. Among these is the homoiothermic condition, intrauterine de- 

 velopment, care of the young, the efficient respiratory, circulatory, and 

 excretory systems, and finally the extreme development of the ner- 

 vous system. The brain of mammals sets them apart from all other 

 living groups. This development reaches its highest in man, and per- 

 haps even greater potentialities exist than can be imagined. 



Characteristics of the Class. — The most obvious characteristic 

 which distinguishes mammals from all other animals is their posses- 

 sion of hair. In some forms, the amount of hair is extremely re- 

 duced; in the whale, for example, hair is present only on the embryo. 

 The hairy covering is molted periodically and replaced. It is an effici- 

 ent insulator, thus is important in the maintenance of the homoio- 

 thermic condition. In addition to the hair, the skin is rich in glands, 

 including sweat, scent, sebaceous, and the mammary. 



The jaws are provided with differentiated teeth in sockets. The 

 type and number of teeth present vary with the food habits of the ani- 

 mal. Ordinarily the tongue is quite movable. The ears are provided 

 with a fleshy pinna, and the eyelids are movable. The skull has two 

 occipital condyles, and there are ordinarily only seven cervical vertebrae. 



Various structures of the body are modified in relation to the habits 

 of the animals ; for example, the two pairs of legs may be modified for 

 swimming, walking, running, digging, grasping, or even fiying. 



