AQUATIC MAMMALS 



character, in which the hind feet are sufficiently larger than the fore 

 feet so that there will be no doubt but that the former will be of greater 

 importance to locomotion while the animal is swimming "dog-fashion". 

 Given the above characters the course of aquatic development will 

 ordinarily be as follows : When the animal first takes to the water, swim- 

 ming will normally be accomplished by the movement of all four feet. 

 The hind feet, however, being larger than the fore feet will be de- 

 pended upon more and will gradually become larger, acquiring web- 

 bing or a fringe of bristles. At this stage the fore feet are only an 

 incidental aid to locomotion. Swimming will normally be by alternate 

 strokes of the hind feet. I accept this as the most efficient method 

 that will almost invariably be employed merely because it is the rule, 

 in birds as in mammals. Swimming by alternate strokes of the hind 

 feet involves wriggling the hinder end of the body, which will cause 

 a sinuous motion of the tail. This latter will aid forward locomotion 

 in degree according to the area of the lateral tail surface. I have no 

 hesitation whatsoever in stating that this horizontal lashing of the tail, 

 so well illustrated in the case of the muskrat (Ondatra) constitutes a 

 strong stimulus for lateral flattening of this member. Why this is so 

 no one knows, but the evidence is overwhelming that just such stimula- 

 tion will initiate development in a useful direction. It may be purely 

 natural selection, it may be chiefly because the friction of the water 

 against the upper and under sides of the tail tends to develop a dorsal 

 and ventral thickening, or a complex of unknown factors may be in- 

 volved. At any rate, as the flattening of the tail progresses it will be of 

 greater and greater proportional importance in swimming, for it is 

 theoretically much more efficient than can be the alternate kicking of 

 the hind feet because the latter necessitates recovery motions. Finally 

 this specialization of the tail will increase possible speed to the point 

 at which movement of the hind feet would be more of a hindrance than 

 a help and use of the feet as a primary, or even secondary, means of 

 speedy aquatic locomotion will be abandoned. The final step in this 

 direction would be the assumption of a fish-like tail comparable to that 

 of the whale, but with flukes vertical instead of horizontal, and pre- 

 sumably the elimination of the hind feet. No mammal is thus devel- 

 oped, for none with horizontal flattening of the tail is independent of 

 the land. All are of rather small size, are inhabitants of streams more 

 properly than of large rivers, and are not yet very highly specialized in 

 an aquatic direction, Potomogale being the most modified of the lot. 

 The above thesis is perfectly consistent with the facts except in two 



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