KINE TO GENESIS. 



353 



all thoroughly aquatic Vertebrata. This implies the 

 immobility of the articulations, which is due to the 

 loss of their condylar surfaces. This may be traced 

 to disuse of such articulations. This disuse would be 

 at first voluntary, the limb being held stiffly while used 

 as an oar in the act of swimming. Loss of power of 

 extension and flexion is well 

 known to result from disuse. 

 It is well known that the flex- 

 ors and extensors of the manus 

 have become atrophied in the 

 Cetacea. Not so, however, 

 with the flexors and extensors 

 of the humerus, which become 

 those of the entire limb. In 

 the whales the first segment of 

 the fore limb is enclosed within 

 the integument of the body, 

 so that its motion being much 

 restricted, the insertional 

 crests are reduced in size. In 

 the eared seals (Otariidae) the 

 hind limbs are somewhat free 

 from the body integument, so 

 that they can be turned for- 

 ward when on land. They are 

 further enclosed in the true 



seals (Phocidae) so that their motion is very slight and 

 they cannot be used for progression on land, and are 

 available only for swimming. 



Fig. no. — Balirna mysticetus 

 fore limb : from Cuvier, Oss. Fos 

 silcs. 



b. Abortion of Phalanges in Utigulata. 



In the heavy Ungulata the longitudinal diameter 

 of the phalanges is greatly reduced in relation to their 



