THE PELVIC LIMB 



ing of the hind feet alone would otherwise impart to the virtually tail- 

 less body, and the animal accordingly trots through the water. The 

 aquatic rabbits may possibly constitute an exception to this rule. If 

 already the hind feet are not held sufficiently close together during pro- 

 pulsion so that no stabilizing action of the fore feet is necessary to over- 

 come any lateral deviation that would otherwise tend to occur, this 

 might possibly be an eventual adaptation. 



The tendency in these mammals, except the rabbit with its negligible 

 aquatic specialization, is for all four feet to develop equally, limited 

 naturally by inherent functional difference, as, for instance, in the moder- 

 ate webbing of the capybara. But the dependence upon the land is 

 still too great for there to have been much change. 



This four-square method of propulsion is far from efficient and 

 really high speed can never be attained without some adaptation pro- 

 viding for an oblique action against the water by all four feet, such, 

 conceivably, as that now employed by the walrus. For the same reasons 

 as in the walrus it is now utterly impossible to predict whether the an- 

 terior pair will gain the functional, and hence evolutional, velocity, as 

 in the sea-lion, or the posterior pair, as in the seal, if indeed they do 

 not have constitutional limitations that would prevent them from fol- 

 lowing either course to a satisfactory conclusion. 



(c) Under this heading (as of page 274) there should be included 

 Enhydra (which will be left for the last) , the Phocidae, and the Odo- 

 benidae partially. The latter need be given scant attention for in both 

 conformation and function the hind feet of the walrus is fairly inter- 

 mediate between the seals and the sea-lions. In shape these members 

 partake of the characters of both. They may be placed flat upon the 

 ground but the definition of the posterior part of the astragalus indicates 

 the probability that they are developing the limitations in this respect 

 of the Phocidae. The hind feet are said to assist swimming by lateral 

 oscillations like the seals, or they may be used purely for equilibration 

 like these members in the sea-lions. Which pair of limbs will ultimately 

 gain the ascendency is a matter for pure conjecture. 



In the seals (Phocidae) the external features of the part of the hind 

 limb above the foot are very similar to those of the sea-lion, and for 

 the reason that the anatomical differences are more significant when 

 treated comparatively, this part of the phocid limb will be discussed in 

 connection with the otariid. Just why the seal adopted, or could adopt, 

 its present method of swimming it is extremely difficult to say. Its foot 

 is really much less remarkable than that of the sea-lion. Externally the 



[281] 



