268 ANIMAL LIFE 
finally teaches it to fly and to hunt for food for itself. 
Young chickens are not so helpless as the nestling robins, 
but are able to run about, and under the guiding 
care of the hen mother to pick up food for 
themselves. 
Among the mam- 
mals the young are 
always given some 
degree of care. I:x- 
cepting in the case 
of the duck-bills, the 
lowest of the mam- 
mals, the young are 
born alive—that is, 
are not hatched from 
eggs laid outside the 
body—and are nour- 
ished after birth for 
a shorter or longer 
time with milk 
drawn from the 
body of the mother. 
Before birth the 
Fig. si iaceaacte ee sutorius) young undergoes a 
; longer or shorter 
period of development and growth in the body of the 
mother, being nourished by the blood of the mother. The 
nests or homes of mammals present varying degrees of 
elaborateness, from a simple cave-like hole in the rocks 
or ground to the elaborately constructed villages of the 
beavers with their dams and conical several-storied houses 
(Fig. 163). The wood-rat piles together sticks and twigs 
in what seems, from the outside, a most haphazard fashion, . . 
but which results in the construction of a convenient and 
ingenious nest. The moles and pocket-gophers (Fig. 165) 
build underground nests composed of chambers and gal- 
