2 GENERAL SURVEY OF THE ANIMAL KINGDOM. 
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. In another direction the 
bats are markedly adapted 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 
stated briefly as follows:—(a) Before birth most young 
mammals are very closely united (by a complex structure 
Fic. 1.—Duckmole ( Ornzthorhynchus). 
called the placenta) to the mothers who bear them. (4) 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 :— 
(a) Eutheria, Monodelphia, or Placentals—those in which there is 
a close (placental) union between the unborn embryo and its 
mother, e.g. Ungulates, Carnivores, Monkeys. 
