AQUATIC MAMMALS 



parable to conditions in the muskrat, desman etc. The hallux is elon- 

 gated and considerably more robust than the other digits, while the fifth, 

 although equally elongated, chiefly by lengthening of the first phalanx, 

 is only very slightly more robust than the three middle toes. 



No mammal swimming by means of the hind limbs alone could at- 

 tain by any thrust-and-recovery method the speed of which the seal is 

 capable, but would be obliged to develop some method whereby the 

 pedes act obliquely upon the water. Mechanically there appear to be 

 only three ways in which such a mammal as the seal could attain this 

 end. One is by employing the hind limbs in the same fashion that the 

 sea-lion uses its anterior flippers, adducting sharply in the transverse 

 plane. This would probably be an impossible specialization. Not 

 only does it seem utterly out of the question for the adductor muscles 

 of the hind limb to evolve into anything comparable in efficiency with 

 the enormous pectorals of the sea-lion, but any such development would 

 almost certainly place the greatest body diameter in the pelvic region, 

 and this would introduce very profound complications in the mechanics 

 of its swimming. The second possibility might be for the feet to be held 

 in the horizontal p'ane, soles up, with their inner borders touching, 

 and oscillated in the vertical plane, as suggested for the sea otter. The 

 third course left to it would be to take the direction which it has actually 

 followed, and to swim by oscillations from side to side of not only the 

 adpressed hind feet but the entire posterior end of the body. 



Just how the animal was enabled to do this is a somewhat puzzling 

 question. It may, however, have depended on some determinant which 

 in itself was very simple. Thus any habits which did not tend to de- 

 velop the back muscles in swimming and did tend to throw the greater 

 work upon the fore limbs would be apt to predispose the sea-lion ancestor 

 toward its present course of evolution. On the other hand some mode 

 of swimming when all four feet were being used for the purpose, tend- 

 ing to develop the back muscles, as they evidently are now being de- 

 veloped in the tailed muskrat, possibly coupled with some slight ana- 

 tomical detail which one might judge of very minor importance, should 

 have been sufficient to start the seal on the road to its present state. 



I have searched the literature and written to everyone likely of whom 

 I could learn, including Alaskans' who have spent years about the Aleu- 

 tian Islands, in an endeavor to ascertain precisely how the sea otter 

 swims. No two accounts have been the same, for probably none of 

 these informants has himself been sufficiently close to a sea otter moving 

 through clear water at speed to be sure of the motions, or having ob- 



[284] 



