AQUATIC MAMMALS 



likely to remain so and never be of economic value to the animal, in 

 which case the limbs will be the members developed for propulsion. 

 If the tail be long it is extremely likely that it will eventually become the 

 prime means of swimming, unless it be diverted at a rather early stage 

 in its development by the necessity for fulfilling some other function. If 

 it be of intermediate length one of two things may happen, depending 

 upon its exact size originally and the other conf ormational features of the 

 mammal. In this must be included inherent capacity for change in the 

 desired direction. It may become shorter and cease to be of consequence 

 or it may become longer and of greater importance. In illustration of 

 the possibilities in this line it may be mentioned that in the case of a 

 mammal with moderate length of tail we can never predict what the de- 

 velopment will be because the antagonistic stimuli may be too nearly 

 equal. Although the modification is entirely according to law it appears 

 to us as fortuitous whether the tail or the hind limbs gains the initial 

 ascendency and implied evolutional velocity of the chief swimming or- 

 gan. 



In approaching this subject of caudal evolution one must clear his 

 mind of all idea that this member changed to the form in which it may 

 now occur in the most specialized of aquatic mammals for some single 

 fundamental reason. True, its development has followed definite laws 

 and in most cases it is predictable what form the tail will finally assume 

 merely by observing the methods of swimming which any mammal now 

 employs, but the development is, nevertheless, step by step. Thus the 

 current belief that the cetacean tail is flattened vertically so that the ani- 

 mal may more readily ascend to the surface for breathing is not only 

 erroneous in practice, but involves an improper mental approach to the 

 entire subject of aquatic specialization. Not only can an animal with 

 horizontally flattened tail ascend as easily for the reason that elevation of 

 the body is accomplished by the equilibrating rather than the propelling 

 mechanism, but no animal could start on its evolutional career with any 

 such particular end in view. The direction in which the tail will finally 

 be flattened is dependent not upon the tail itself but upon the direction in 

 which it is involuntarily (usually) moved during swimming movements 

 of the unspecialized ancestral form. 



For ease in discussion the tails of aquatic mammals may be divided 

 into two classes, which involve two fundamentally different principles: 

 Those which are narrow in the horizontal plane or which will eventually 

 become so; and those which are flattened in the vertical plane or will 

 eventually become so. In these two classes are included even such tails 



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