AQUATIC MAMMALS 



the marine turtles the tendency is to use the hind limbs as equilibrators 

 and the anterior pair as propellers, these being operated in unison with 

 a sort of flying motion. 



The same principle as forces the mud turtle to trot through the water 

 operates to oblige all slightly specialized aquatic mammals with insig- 

 nificant tails to employ all four feet in swimming, especially if the body 

 be given to corpulency. So apparently swims the hippopotamus, whose 

 legs are far apart, and the capybara, which has no tail ; also the polar 

 bear. But I am inclined to think that the culminating development in 

 such mammals would not be the acquisition of four paddles of almost 

 equal efficacy, as marine reptiles so often show, but rather the ascendency 

 of one pair of limbs over the other, such as is now encountered in the 

 Otariidae or Phocidae. 



In this connection it may be of interest to mention that aquatic rep- 

 tiles seem to have had modifications which it is probable that no mammal 

 could develop. Thus reptiles can increase the number of cervical verte- 

 brae to a phenomenal degree a procedure which a mammal cannot 

 effect. The tendency in marine reptiles always was to develop four 

 paddles, while in mammals but one pair usually becomes very highly 

 modified. Similarly there must be many other reptilian body details 

 which respond to definite stimuli in an entirely different manner than 

 do the same details in mammals. I therefore regard it as very unsafe 

 to draw conclusions anent the Mammalia from data provided by the 

 Reptilia. 



The extinct plesiosaurs followed the turtle path for a considerable 

 distance. Their bodies were heavy and broad, as well as somewhat 

 flattened, and it is not improbable that this shape was accentuated in 

 their ancestors. They were equipped with four long paddles, and as 

 they employed a four-cornered method of propulsion they steered with 

 the feet, kicking according as they wished to turn to one side or the 

 other, and neither the tail nor the neck needed to function as a rudder. 

 These two extremities could therefore develop in response to other 

 stimuli without much regard to any duty in swimming. The result was 

 that some sorts were short tailed and short necked, while others had 

 a long tail and a neck that was relatively much longer than any other 

 vertebrate has ever had, so far as we know. They were rather sluggish 

 beasts, evidently, and the long neck was doubtless developed so that 

 the head might dart here and there in pursuit of active prey in spite of 

 the clumsy body. No other instance is known either in' reptiles (ap- 

 parently) or mammals in which a highly specialized aquatic form had 



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