THE PELVIC LIMB 



ascendancy for potential development, which, coupled with the fact that 

 the most efficient method for propelling a body through the water is 

 from the rear, introduced the extreme likelihood that they would at 

 some stage of aquatic specialization be of more importance for swimming 

 than the fore limbs, unless the conditions were complicated by other 

 factors. 



In general the function of the hind limbs of a mammal that is ex- 

 clusively aquatic may be of three sorts: that of propulsion, of steering, 

 or of no use at all. Naturally, partial terrestrial dependence will intro- 

 duce complex difficulties that may vary in their influence from 100 per 

 cent to zero, according to the individual case. In a mammal of the shape 

 of a hippopotamus and without a respectable tail, it would be expected 

 that always the development of all four limbs will be at a fairly even 

 rate, neither pair at the expense of the other, as in the case of some of 

 the aquatic reptiles of somewhat similar body conformation. This is so 

 for the reason that the mud-turtle type of swimming is employed, as 

 already discussed. To some extent this is also so in the case of the 

 capybara, and is not so essentially dependent upon the fact that in both 

 animals there is great dependence upon the land. 



Lack of ascendency, or at least delayed ascendency, in the development 

 of the hind limbs may also be attributable to the fact that the tail was 

 already of such a form that it was enabled to take over the function of 

 chief swimming apparatus before the hind feet could become highly 

 modified for this purpose, as seems to have been the case in the river 

 otters and the insectivore otter (Potomogale) . 



If the body be not unduly broad (hippopotamus) or if the tail be not 

 already large and powerful (river otter) , the hind feet may be expected 

 usually to gain ascendency over the fore, and, for a time at least, to be 

 the principal means of swimming. This statement needs qualification, 

 however. The anomolous exception is the sea-lion, and we are utterly 

 incapable of stating whether the development of this order is because 

 of some individual idiosyncracy in its make-up, or whether such a tail- 

 less mammal as the capybara might be expected to follow the same 

 course, if it should ever become pelagic, rather than that of the seal. 

 I judge, however, that the sea-lion is an exception, chiefly for the reason 

 that the most efficient means of propulsion is from the rear. 



In most sorts of mammals that become aquatic the tail is of goodly 

 length but of small diameter at base, and in such there will always be 

 ascendental development of the hind feet. This takes the form of en- 

 largement of these members, and either a growth of stiff, bristly hairs 



[269] 



