560 



ZOOLOGY. 



But it is in the limbs, and especially the feet, of mammals 

 that the skeleton varies most, and always in accordance with 

 the different habits of the creature. The limbs of mammals 

 differ from those of the lower vertebrates in the fact stated 

 by Gegenbaur, that the planes in which the angles of the 

 limbs of either side are set are parallel to the vertical me- 

 dian plane of the body, thus giving greater independence to 

 the limbs, which now become supports for the body, since 

 they raise it from the ground. Beside this, the angles be- 

 tween the equivalent portions in each limb do not agree 



with each other, as in the rep- 

 tiles, but point in an opposite 

 direction in the case of the fore 

 and hind limbs respectively 

 (Gegenbaur). As we ascend in 

 the mammalian series, the limbs, 

 particularly the fore-limbs, are 

 variously modified. The limbs 

 of whales are paddle-like, though 

 the bones of the limbs are homo- 

 logous with those of other mam- 

 mals. The feet of the seal are 

 webbed, forming flippers ; it can- 

 not support itself on its limbs, 

 but the fore-feet have consider- 

 able motion of the radius on the 

 ulna. In the dog the fore-limbs 

 have but little motion of the 

 radius on the ulna, but the cats 

 (Felidai) have more of this rotary 

 motion, enabling them to grasp with the fore-foot. This 

 rotary motion of the fore-arm, involving the modification 

 of the fore-foot into a hand, is seen in the thumbless mon- 

 keys (Fig. 485), and in those provided with a thumb, in the 

 gorilla, and especially in man. The extreme of specializa- 

 tion of all four limbs is seen in the horse, which has but 

 one digit, and walks on its single toe-nail. In the bat, the 

 ulna and radius are fused together as one bone, and the 

 last three fingers are greatly lengthened. The liberation of 



Fig. 485. Arm of the Thumbless 

 Monkey (Aides). 



