Chapter Eleven 



The Telvic Limb 



IN SOME respects the tendencies followed by the hind limbs of aquatic 

 mammals are similar to those shown by the anterior appendages, and in 

 others they are very different. In at least one detail the stimuli for the 

 aquatic specialization of the posterior limbs are less complicated than 

 of the anterior pair for the reason that the latter are often, or at least 

 for a long time, of great service in the securing and consumption of food, 

 while the hind limbs never have this function. Thus there are three 

 primary stimuli acting upon these members of an aquatic mammal ; the 

 influence introduced by the mode of terrestrial locomotion, aquatic loco- 

 motion, and equilibration, but unless specialization be very far advanced 

 the two latter are often very intimately combined. Terrestrial locomo- 

 tion introduces a variety of factors, for when an animal first takes to 

 the water its hind limbs have been fashioned not only for a particular 

 mode of walking, but as a support, and differ according as the weight is 

 heavy or light, the foot plantigrade or digitigrade, et cetera. Hence 

 when a mammal first swims it already has an equipment that may pre- 

 dispose it to certain definite motions of the limbs to accomplish swim- 

 ming, and prevent or handicap it in indulging in other kinds of ac- 

 tions. Naturally the difficulties which a shrew must overcome in evolv- 

 ing a paddle from its feet are very different indeed from those which an 

 elephant would be obliged to surmount. For these reasons it is entirely 

 impracticable to unravel the separate factors and be sure in all cases 

 that we know just what we are talking about. But average trends may 

 be evaluated and probabilities advanced. 



It seems certain that in practically all of the terrestrial ancestors of 

 living aquatic mammals strictly quadrupedal locomotion was employed, 

 and that none of them was a leaper, or a flyer, or any other sort of ani- 

 mal in which one pair of limbs was used chiefly in progression. Hence, 

 during the first inefficient attempts at swimming all four limbs were 

 kicked with as nearly equal force as the muscular equipment of the 

 animal would allow. In most mammals the hind legs are not only 

 appreciably more powerful than the fore limbs, but the hind feet have 

 a larger area. Hence from the very start these members had inherent 



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