AQUATIC MAMMALS 



pie tilting of the flippers. Being of importance in this regard to a mam- 

 mal capable of high aquatic speed it is to be expected that they would 

 readily become specialized in a variety of ways according to the needs 

 of the particular species or genus concerned. 



There here may arise the question of whether the pectoral limbs of 

 the Cetacea ever constituted a primary swimming organ. Beddard 

 (1900) considered this to be quite likely and others have entertained 

 the notion. No one can deny this conclusively but in view of what evi- 

 dence there is it is not at all likely. There is nothing in either the oste- 

 ology or myology of the Cetacea to support such a theory, purely me- 

 chanical reasons would prevent it from being a likelihood, and it is not 

 probable that any aquatic mammal capable of developing such a per- 

 fected propeller as the cetacean flukes would have so used the flipper to 

 any marked extent. On the other hand, the osteology of the dugong does 

 render it possible that at some comparatively recent time its flippers could 

 have been used for propulsion in quite efficient fashion; but it seems 

 more probable that osteological development has been merely conver- 

 gent and that its brachial musculature has been put to some other use 

 than as even a moderately important aid to swimming. 



It is seen from the above that in a mammal that is highly specialized 

 for an aquatic life the anterior limb should normally have either one of 

 two functions; that of steering and equilibration more often, their im- 

 portance in this function depending upon other bodily details as well as 

 habits, and that of primary organs for propulsion more rarely. 



If the fore limb be not of great importance in steering (or swimming) , 

 it may never become essentially fin-like. If it is constantly used for this 

 purpose, or to apply the propulsive force in swimming, it will finally 

 assume the characters of an efficient paddle or fin. In either case effi- 

 ciency demands that there should be a surface of broad area which may 

 be brought into action against the water, and a relatively thin border, 

 which may cut through the water with the least amount of resistance that 

 is practicable in connection with requisite strength. As an ideal, how- 

 ever, the cross section of this paddle or flipper will not show two lines 

 that are perfectly parallel, but rather will such a cross section be of a 

 fusiform shape, as is the wing of a bird or of an aeroplane. For one 

 thing the anterior border can not be too thin because of needed strength. 

 The posterior not only can be but should be thinner to allow for greater 

 limberness, and to reduce suction or partial vacuum as the flipper passes 

 through the water. 



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