AQUATIC MAMMALS 



stated, all four limbs are habitually used in diagonal alternation while 

 swimming, but it is not meant to imply that an observer can never de- 

 tect variations of action, just as a human swimmer may play about and 

 keep afloat by several expedients. 



Neither fore nor hind feet of the hippopotamus are appreciably modi- 

 fied in an aquatic direction, and it is readily seen that it would be ex- 

 tremely difficult for a foot of its type, upon which such a ponderous 

 body is imposed, to change into a flattened propulsive organ, at least 

 as long as terrestrial locomotion is a necessity. The fore feet of the 

 capybara are furnished with interdigital membranes which extend prac- 

 tically to the tips of the toes, although these are indented, rather than 

 straight, along their free margins. This is equally the case in the hind 

 feet. 



The fore feet of the otters are chiefly of interest in the present con- 

 nection because I deem it likely that the ancestors of the Cetacea were 

 beasts of somewhat similar conformation, swimming by the same meth- 

 ods. It is usually stated that the manus of the common otter is partly 

 webbed, but this character is not more pronounced than in many terres- 

 trial mustelids. There is no webbing at all in the African clawless otter 

 (Aonyx, fig. 42), while Allen (1924) shows that it is particularly ex- 

 tensive and broad in the African Lutra maculicollis (fig. 42). It is 

 to be expected that during swimming the fore feet of the latter animal 

 are employed in a manner somewhat different from the case of the 

 former. Other genera of otters are presumably variously intermediate 

 between these two extremes. 



The common otter is very nimble and so given to sportiveness while in 

 the water that it is very difficult to determine any really important uses 

 to which the manus may be put while in this element. The anterior 

 limbs are often used for grasping, but hardly as a definite aid to propul- 

 sion, nor apparently for equilibration. Certainly they will eventually 

 become more modified, but in the mean time they may be expected 

 to exhibit some tendency toward reduction of size, as is indeed the case 

 to a slight extent in the sea otter. 



Not a great deal of significance can be said about the manus of the 

 Sirenia. They do not use the fore limbs as a direct aid to propulsion 

 and probably never have since the days when they swam dog-fashion, 

 if the present evidence is trustworthy: nor apparently are the limbs 

 used as equilibrators for elevation and depression of the body after just 

 the same fashion that the flippers of the whale are now employed. 

 Rather should their action be compared to that of the seal, but to a more 

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