2 GENERAL SUR VE Y OF THE ANIMAL KINGDOM. 



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 enables us to see 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 

 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, {b) But 

 this close connection between mother and unborn young is 

 only hinted at in the kangaroos and other 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 duck- 

 mole and its two relatives, the placental connection is quite 

 absent, for these animals lay eggs as birds and most 

 reptiles do. Besides these differences in the bearing of 

 young, there are others relating to structure which are of 

 great importance, and which seem to warrant the division of 

 Mammals into three sub-classes : — 



1. Prototheiia, Ornithodelphia, or JMonotiemes — the egg laying 



duckmole [Ornithorhynchus], Echidna and Proechidna. 



2. Metatheiia, Didelphia, or Marsupials— the prematurely bearing, 



usually pouch possessing kangaroos and their relatives. 



3. Eutheria, Monodelphia, or Tlacentals — those in which there is a 



close (placental) union between the unborn embryo and its 

 mother, e.g.. Ungulates, Carnivores, Monkeys. 



Birds. 



There can be no hesitation as to the class which we 

 should rank next to Mammals. For Birds are in most 

 respects as highly developed as Mammals, though in a 

 divergent direction. They are characterised by their feathers 

 and wings, and many other adaptations for flight, by their 

 high temperature, by the frequent sponginess and hollow- 



