AQUATIC MAMMALS 



norm has been insensibly deliberate, and if we could but have the com- 

 plete picture before us there would be no difficulty in its correct in- 

 terpretation. The change which the abandonment of the atmospheric 

 for the completely aquatic environment necessitated was prodigeous, 

 but nevertheless it is probable that with few exceptions (as the sperma- 

 ceti organ of the cachalot) there have been no fundamentally new de- 

 tails developed, but only the gradual change and specialization of old 

 ones. 



In discussing aquatic modification, and most other things, the majority 

 of writers are prone to consider that there is one, or at most a few, 

 stimuli operating to accomplish precisely the same end. In other words 

 it seems to be a common belief that when mammals take to the water 

 there will be a tendency for them all to develop along precisely the 

 same lines to exactly the same end. This is true to only a definitely 

 limited extent. A dozen, say, types of terrestrial mammals taking 

 to the water may have twelve individually peculiar organizations, differ- 

 ing in their capabilities for variation, amenable to diverse stimuli of 

 dissimilar strength, with twelve combinations of interdependence of 

 parts. The convergence which they will ultimately show will be in 

 two ideal types of body form and toward two (presumably) final 

 methods of swimming. But these ends may be attained in considerably 

 different manners, involving entirely different specialization of details, 

 and one must be cautious in stating that the aquatic life will result in 

 such an item as a shortened neck without a very careful analysis of the 

 factors involved. 



Almost all mammals can swim after a fashion, and even some of 

 those which one would naturally expect to be the least capable of 

 doing so, such as the sloths {Bradypus and Choloepus) can cross 

 broad rivers. To merit the term aquatic, however, a vertebrate must 

 be sufficiently at home in the water so that when near this element it 

 will instinctively seek it for concealment or escape. It must not only 

 be able to swim with adequate speed but must be able to dive with 

 facility and remain submerged. In almost all cases there is necessary 

 the further corollary that it habitually seeks its food in the water. The 

 ability to propel itself through the water does not mean that it has 

 necessarily developed organs that are appreciably more highly special- 

 ized for such use than are found' in its nearest, wholly terrestrial rela- 

 tive, but it does mean that such a vertebrate must have evolved some 

 sort of valvular mechanism for closing both nostrils and ears while 

 submerged. In other words an aquatic vertebrate must have the ability 

 to close all orifices leading within the body. 



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