THE MAMMALS. 
237 
The robin is our most common bird; it begins to nest 
before the leaves begin to appear; a pair will build the ex- 
terior of their nest in two or three days, but the lining is 
not finished until after the first egg is laid: hence it is about 
a week before the nest is completed and the first egg laid. 
Class VIII. — Mammalia {Rat, Dog, Monhey, Man). 
General Characters of Mammals. — In this the highest 
class of vertebrates hair takes the place of feathers, and the 
young are nourished with milk. 
Mammals walk on all fours, man j/yv 
being the only truly upright y^j I 
mammal, walking firmly on his r\ /I I 
hinder limbs, and using his fore \ ^\y1y //j I 
limbs as arms for grasping and \.^.^:S/^ //// 
performing all those actions // // 
which minister to his intellectual // // 
wants. /// 
As we ascend in the mamma- ///y 
Man series, the limbs, particu- 
larly the fore-limbs, are vari- 
ously modified. The limbs of Mm 
whales are paddle-like, though ^^^"^^H^^^^ 
the bones of the limbs are identi- 
cal with those of other mammals. ^\%J 
The teet of the seal are webbed, \ ' 
forming flippers; it cannot sup- fig. sso.-Arm-bones of the 
port itself on its limbs, but the <^^^°^bless Monkey UteUs). 
fore-feet have considerable freedom of motion. In the dog 
the fore-limbs have but little motion of the radius on the 
ulna, but the cats have more of this rotary motion, enabling 
them to grasp with the fore-foot. This rotary motion of 
the fore-arm, the fore-foot becoming a hauid, is seen in the 
thumbless monkeys (Pig. 280), and in those provided with 
a thumb, in the gorilla, and especially in man. The ex- 
treme of specialization of all four limbs is seen in the horse. 
