ART. 15 ANATOMY OF THE EARED AND EARLESS SEALS HOWELL 139 



antagonistic work which the brachial muscles of this animal must 

 needs perform during swimming movements by the hinder portion 

 of the body. These questions, then, may be propounded as follows. 

 Given the habits and form of the two animals dissected, then : 



1. Why should the crotch of the posterior limb be located practi- 

 cally as far distad in Zdophus as in Plwcaf 



2. Why should the superficial division of the biceps femoris be 

 developed in ZalopMis to bind down the shank more firmly than it 

 does in PJioca? 



3. Why should the excessively shortened femur be relatively of 

 the same length in both ? 



4. Wliy should the pes of Zaloyhus be in most respects more 

 specialized than is Phocaf 



5. Why should there be such large cartilagenous extensions of the 

 pedal digits in Zalophiis but none in Phoca^ which apparently would 

 have more use for them. 



If one accept the thesis of a certain amount of retrogressive evolu- 

 tion having taken place then these five points are easily explainable. 

 For the sake of argument then, let us presume that at one time before 

 the Phocidae used the posterior limbs so exclusively for aquatic loco- 

 motion they made great use of the forefeet also. The adjoining 

 muscles would then become highly specialized, a condition which 

 might survive as a relic after disuse had caused great reduction in 

 the size of the forelimb. Similarly, then, it may also be argued that 

 the hind limb of Zalophus maj^ conceivably have been a primary 

 means of aquatic progression in the ver}'^ distant past, and it then 

 might readily have acquired the puzzling details listed above, which 

 subsequent relative disuse has failed to obliterate. In support of such 

 argument is the seeming fact that the terminal digital cartilages of 

 the pes of a juvenile Zalophus appear to be relatively better developed 

 than in an adult. 



This hypothesis of retrogressive evolution has been presented 

 merely because it largely explains matters, and it is the only one 

 which appears to do so. I may eventually come to accept it but at 

 the present time I am, nevertheless, far from convinced that this is 

 the proper explanation. 



It is seen that the present contribution sheds little or no light upon 

 the question of whether the Otariidae or the Phocidae is the " older " 

 family. The evidence is conflicting and the proper weight to accord 

 details of variation is, and probably always will be, a moot question. 

 It is felt, however, that this evidence points to the probability that 

 the Otariidae, although not necessarily the better (or as well) 

 equipped for an aquatic existence, have perhaps departed widely from 

 a typical terrestrial condition in more numerous and profound re- 

 spects than have the Phocidae. 



