TEE MAMMALS. 2B7 



The robin is otir 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, Monkey, 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 

 being the only truly upright 

 mammal, walking firmly on his 

 hinder limbs, and using his fore 

 limbs as arms for grasping and 

 performing all those actions 

 which minister to his intellectual 

 wants. 



As we ascend in the mamma- 

 lian series, the limbs, particu- 

 larly the fore-limbs, are vari- 

 ously modified. The limbs of 

 whales are paddle-like, though 

 the bones of the limbs are identi- 

 cal with those of other mammals. 

 The teet of the seal are webbed, 

 forming flippers; it cannot sup- fiq. sso.-Arm-bones of the 



port itself on its limbs, but the ">™W'='^^ donkey (Atele.). 



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 han(d, is seen in the 

 thumblsss monkeys (Fig. 280), and in those provided with 

 a thumb, in the gorilla, and especially in man. The ex- 

 i:-eme of specialization of all four limbs is seen in the horse. 



