AQUATIC MAMMALS 



It is probable that the original adoption of a habitat that was largely 

 aquatic usually, if not almost invariably, meant for a terrestrial mammal 

 a diminution in the severity of the competition to which it was ac- 

 customed, else it would not have taken to the water. And it is well 

 known that a life wherein competition is reduced is not conducive to 

 the rapid modification of bodily form. On the other hand, any such 

 drastic change in the functions of the body as is experienced by a terres- 

 trial mammal when it takes definitely to the water is highly conducive 

 to evolutionary changes. To what effect these diametrically opposed 

 tendencies have acted in the past upon the aquatic mammals with which 

 we are now acquainted cannot be conjectured. But we do know, by 

 such paleontological evidence as is at hand, that the course of develop- 

 ment and change in our aquatic mammals has been an extremely slow 

 and lengthy process, probably lasting, in those groups most highly 

 adapted, throughout tens if not scores of millions of years. 



Let us take the hypothetical case of a carnivore that eventually be- 

 comes exclusively marine. For our purpose this must be of an active 

 type that prefers live food, and not such an omnivorous and slightly 

 sluggish animal as the racoon (Procyon), which is slightly aquatic in 

 its habits. Such an active carnivore likes fish and finds that it is easier 

 to catch them in the shallows than to compete with other sorts of preda- 

 tors catching mice in the woods. He will spend an increasing amount 

 of time in the water and throughout successive generations gain increas- 

 ing confidence in swimming considerable distances. During this time 

 he has naturally discovered that few of his terrestrial enemies will fol- 

 low him into the water, and he therefore instinctively seeks this element 

 when startled. But he still journeys overland from one stream to an- 

 other, may hunt on shore occasionally when fish are temporarily shy, and 

 sleeps and raises his family in a hole beneath a bank. While he is 

 content to linger in a small river he can do little else and his bodily 

 modifications will not only be correspondingly circumscribed, but the 

 change throughout long ages will be slow. 



Perhaps in a few million years this carnivore has gained such a facility 

 in swimming that he finds narrow quarters irksome, and seeks the greater 

 freedom of large rivers. He is now able to catch fish in fair chase, 

 and can also escape from his enemies by his speed. The water is his 

 home but although he frequently takes short naps while floating on 

 the surface, he still seeks a hole in the bank, or if large and bulky, a 

 sandbar, for a sound sleep, and his mate must seek the land to raise a 

 family. At this time it makes little difference in the degree of his 



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