AQUATIC MAMMALS 



toral fins at a certain angle to the body axis. It is by no means unlikely 

 that there is now the same stimulus, but in reverse order, operating in the 

 case of aquatic mammals, to place the axis of the fore limbs at some par- 

 ticular angle. Such a stimulus is impossible of analysis, and the question 

 seems too speculative to follow further. 



In the case of a mammal such as the sea-lion which swims exclusively 

 by means of the anterior limbs, there are certain principles of efficiency 

 which must govern the motions employed. This does not concern low 

 speed, during which several sorts of makeshifts may be employed, some 

 for relaxation and some in pure fun. During leisurely progress the 

 flippers may be advanced and then brought rearward with a broad sweep, 

 this corresponding to the overhand stroke employed by human swim- 

 mers. It is true that during action of this sort there is an active recov- 

 ery motion which, during the forward stroke, must substantially increase 

 water resistance, but what resistance is added at this time is subtracted 

 during the backward stroke and after much thought I have come to the 

 conclusion that as far as concerns only resistance it makes no practical 

 difference whether the flippers, during slow progression, are held with 

 their forward borders in the same transverse plane, or whether they are 

 alternately advanced and retarded. 



The above "overhand" movement is not employed by sea-lions for 

 speedy progression, so far as my experience shows. The reason for this 

 seems to be that when a certain degree of speed has been passed, the 

 animal is incapable of operating the long flippers on the backward stroke 

 sufficiently fast to supply any propulsive force. The principle is some- 

 what the same as that which correlates running speed with the celerity 

 with which the legs may be moved. Another deterent is the fact that 

 during such a backward stroke the broad surface of the flippers would 

 be presented in an antero-posterior direction. Hence, even though the 

 tip of the flipper could be retarded sufficiently quickly to supply some 

 propulsion, the broad part of the wrist would be so situated as to act as 

 a strong brake. By this method of overhand swimming, therefore, 

 there appears to be a certain unknown speed limit which sea-lions are 

 incapable of exceeding. So something more efficient must be evolved — 

 some way of utilizing an oblique, rather than a direct, thrust against the 

 water. 



As a result the Otariidae now swim at high speed not by flexing and 



extending the arm, but by adducting and abducting it, with hardly any 



antero-posterior action at all so far as I can determine. As the flipper is 



adducted it is turned obliquely or "feathered," so that the thicker an- 



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