AQUATIC MAMMALS 



duplicating the mechanical conditions embodied in the foot of a grebe 

 or duck, which is not the most efficient method of swimming anyway. 

 Swimming by oscillations of the hinder end of the body (by feet or 

 tail) is the most economical method partly for the reason that all motions 

 are propulsive ones, without obligation of any braking action by recovery 

 motions, and because each stroke of the rhythmic cycle is of approxi- 

 mately equal power. 



If the tail be too small to be adapted for a propulsive organ it is 

 then likely that the fore feet will finally be modified for taking over 

 this function exclusively (as in the Otariidae) . Theoretically it is not 

 quite so efficient a method of swimming, for it can never be brought 

 to as high a state of perfection as can that involving the transformation 

 of the tail into flukes, but when a stage will have been reached compar- 

 able to that at which the sea-lions now are, the fore limbs will constitute 

 very effective swimming organs indeed, any braking action by recovery 

 motions being almost entirely overcome, as discussed in a later chapter. 

 Whether a seal can actually swim faster than a sea-lion is unknown, for 

 there are no trustworthy figures on the subject. Both are capable of 

 high speed ; not to the extent of 60 miles an hour, of course, as I have 

 heard claimed by some illiterate fisherman, nor is it likely that they 

 could outstrip some of the speedier porpoises. On theoretical grounds 

 the seal should prove the faster. If it is not it may be because it does 

 not need to be. 



Before closing the present consideration of aquatic progression it will 

 be well to emphasize the fact that there will usually be a regular sequence 

 of swimming methods employed during the evolution of any aquatic 

 vertebrate. This is conveniently illustrated by the alligator. This rep- 

 tile may "walk" slowly through the water with all four feet, or may 

 progress by means of the pedes alone, while during more rapid progres- 

 sion the appendages are pressed to the body while the tail is lashed from 

 side to side. This is entirely illustrative of a sequence in methods of 

 progression according as one part after another of a vertebrate becomes 

 differentiated for special use. It is also illustrative of another and 

 fundamentally important consideration with which every aquatic verte- 

 brate is obliged to contend at some point in its evolution, and which 

 the reader will readily appreciate after having read this chapter. No 

 method of swimming involving forthright pushing backward of the 

 water by any pair of appendages is ever truly efficient, for the speed 

 thereby attained is limited by the inertia of the limbs and their inability 

 to repeat the rhythmic motions sufficiently fast. For really high speed 



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