Chapter Nine 



<Tbe Tail 



1 HE QUESTION of the physiological development of the hinder end of 

 an aquatic mammal for the function of primary propulsion is an in- 

 volved one. Of certain points one can be sure, while regarding others 

 there is some uncertainty and only probabilities may be advanced, this for 

 the reason that there are indications that in particular cases the develop- 

 ment has not been in a straight line but has been somewhat by trial and 

 error. As a fundamental concept, however, I have no hesitation in mak- 

 ing the unequivocal statement that the evolutional tendency when a mam- 

 mal takes to a free-swimming type of water habitat is always for it even- 

 tually to develop the rear end into the primary, oscillating organ for its 

 propulsion through the water. If it does not do so it is a sign that it 

 originally made a wrong start or that it has encountered antagonistic 

 stimuli of such strength that it was diverted from the most efficient evo- 

 lutionary development. In theory it makes no difference whether this 

 ideal propulsive force is furnished by the flattened tail as in the whale or 

 by the adpressed hind feet of the seal. In practice, however, it is prob- 

 able that the muscles of the seal that are involved in swimming can never 

 become as homogeneously specialized for a single function as are those 

 of the whale's tail. I do not know and consider that speculation on this 

 point would be well nigh useless. 



The important point is that although swimming by the whale and the 

 seal entails widely different muscular action, the principle is in theory 

 the same, and this principle is the only one that a mammal can adopt 

 which is thoroughly economical in practice, for it is the only one by which 

 there is no lost motion or energy. In the case of other sorts of swimming 

 motion by the hind feet, or by the fore feet, either recovery motions are 

 necessary or a part of the flippers must overcome water resistance al- 

 ternatives which detract by just so much from the propulsive strokes. 



For a proper consideration of the situation some brief recapitula- 

 tion is necessary. When a mammal first takes to the water it has a caudal 

 equipment that may be divided into three categories. It is either without 

 a functional tail, like the bears for instance, it has a long tail, or this 

 member is of intermediate length. If the tail originally was short it is 



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