560 ZOULOGY. 
But it is in the limbs, and especially the feet, of mammal; 
that the skeleton varies most, and always in accordance witl 
the different habits of the creature. The limbs of mammal; 
differ from those of the lower vertebrates in the fact statec 
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 tc 
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 limhs. 
but the fore-feet have consider. 
able motion of the radius on thc 
ulna. In the dog the fore-limbs 
have but little motion of the 
a — oa of the Thumbless radius on the ulna, but the cats 
(Felide) 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 bul 
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 
