^BT. 15 ANATOMY OF THE EARED AND EARLESS SEALS — HOWELL 137 



to be older than the ursine, as mentioned by Kellogg (1922), and 

 the phocid line may well prove to antedate the lutrine. 



At any rate the typical otariid is now very different from the 

 typical phocid of the present day. Comparative anatomy should 

 contribute much evidence informative of the degree of phyletic 

 divergence that they have experienced, and it is found to do so, but 

 the evidence is often of so contradictory a character that it is far 

 less satisfactory than had been hoped. The significance of some of 

 the more important of the differences as they exist, however, may 

 be discussed. 



The methods of swimming now employed by the otariids and 

 phocids are fundamentally different. These differences may date 

 from the time when the first ancestors of the two families managed 

 to swim across a river, or it is very possible that the ancestral 

 otariid and ancestral phocid followed the same method of swimming 

 until they were as well fitted for aquatic life as, say, the otter 

 (Lutra), or conceivably more so. The two stocks then acquired 

 steadily increasing adaptations, but of diverging sorts, throughout 

 the ages, or else one or the other of them experienced retrogressive 

 changes of indeterminate duration. In other words, it is not 

 impossible that there was a considerable stretch of time when the 

 otariid employed also the hind feet and the phocid also the forefeet 

 primarily as accessory propulsive organs, and that this time lasted 

 sufficiently long for strong anatomical evidence of the fact to re- 

 main at the present day. And indications, as enumerated shortly, 

 are not lacking that retrogressive changes of this sort may actually 

 have been experienced by both families. Furthermore, it is ex- 

 tremely probable that all parts of the body which show aquatic modi- 

 fications have not evolved at the same rate or velocity. Thus, the 

 fact that the external ear of the phocid has disappeared while that 

 of the otariid has not may merely indicate that slight differences 

 in habits have operated to retain the external ear in the latter. 



As evidence for or against close relationship of the two pinniped 

 stocks such matters as general body form, external details of the 

 eyes, nose, and ears migration distad of the external axilla and of 

 the crotch of the posterior limb and craniad of the anterior border 

 of the pectoralis origin, form of the innominate, and shortening of 

 certain segments of the limb, are not particularly illuminating, for 

 these are items which conform to usual or eventual requirements of 

 aquatic adaptation and they might be due largely to convergence. 

 On the whole, however, Phoca exhibits in these respects a somewhat 

 greater divergence from fissiped conditions than does Zalophiis. 



The following differences may be considered as attributable chiefly 

 to the dissimilarity in the modes of progression characteristic of the 

 two families: differences in the form of the neck; in the length 



