AQUATIC MAMMALS 



they are biserial, and that the walrus is but an aberrant and modified 

 otariid, but as in the case of the whales much of the discussion has 

 been of a profitless nature. The meaning of the term biserial or 

 diphyletic is loose and varies with the investigator. It is presumed that 

 all placentals were derived from a common ancestor, and therefore all 

 phyla eventually go back to this point. It is commonly believed that 

 the pinnipeds were derived from the adaptive creodonts, which in turn 

 were probably derived from a single ancestor. It may be pointed out, 

 however, that because the pinnipeds have so far been unable to sever 

 their connection with the land at the time of the birth of the young 



Figure 5. Pinniped postures. Seal {Phocidae) (above), sea-lion {Otariidae) , 

 and walrus {Odobenidae) . 



their latter day (geologically speaking) evolutional velocity may have 

 been at a slow rate, and they may well be nearly as old, phylogene- 

 tically, as the Cetacea. 



Whether or not the otariids and phocids became individually dif- 

 ferentiated before or after their ancestors first took to the water is, 

 with the present paucity of fossil material, a matter that cannot be 

 settled and over which one may waste much valuable time and ink. 

 It has been frequently claimed — first by Mivart, I believe — that cer- 

 tain anatomical resemblances which it is not necessary here to repeat 

 at length indicate that the otariids have descended from the bears and 

 the phocids from the otters. In the first place the otariid stock is al- 



[34] 



