2 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



general presence of convolutions on the front part of the 

 brain, the occurrence of a muscular partition or diaphragm 

 between the chest and the abdomen, and so on, as we shall 

 afterwards notice in detail. Most mammals are suited for 

 life on land, but diverse types, such as seals, whales, and sea 

 cows, have taken to the water, while the bats are as markedly 

 suited for aerial life. 



Among the mammalian characteristics of great import- 

 ance are those which relate to the bearing of young, and 

 even a brief consideration of these shows that some 

 mammals are distinguished from others by differences 

 deeper than those which separate whales from carnivores, 

 or rodents from bats. These deep differences may be 



FIG. i. Duckmole (Ornithorhynchus). 



stated briefly as follows : (a) Before birth most young 

 mammals are very closely united (by a complex structure 

 called the placenta) to the mothers who bear them. (/>) But 

 this close connection between mother and unborn young is 

 of rare occurrence, or only hinted at, in the pouched 

 animals or Marsupials, which bring forth their young in a 

 peculiarly helpless condition, as it were prematurely, and in 

 most cases place them in an external pouch, within which 

 they are sheltered and nourished, (c) In the Australian 

 duckmole and its two relatives, the placental connection is 

 quite absent, for these animals lay eggs as birds and most 

 reptiles do. These differences and others relating to 

 structure warrant the division of Mammals into three sub- 

 classes : 



