AQUATIC MAMMALS 



the higher the attainments of any quahfied physicist with whom the 

 questions are discussed the more emphatic is he in his indication that 

 he will have nothing to do with it. It therefore seems entirely im- 

 possible to arrive at an exact conception of the mechanics of swimming 

 by mammals, but certain generalities we know to be facts, and the evi- 

 dence for certain others appears to be sufficiently strong to justify us 

 in tentatively offering hypotheses. 



Stream-line, as I understand it, is that indefinite term by which we 

 designate the precise shape of a given mass in order that it will slip 

 through the water (or air) with less friction or resistance than any 

 other shape of equal mass. But it is perhaps requisite that this body 

 be towed. If it be self-propelled then the propeller will introduce a 

 complication. It therefore is self-evident that no vertebrate can be 

 perfectly stream-line in form. Not only must it have appendageous 

 means of propulsion, but it usually has a steering or equilibrating ap- 

 paratus. Because of feeding or other requirements its head may not 

 be of a shape best adapted for speedy passage through the water. Either 

 to compensate for such external irregularities or because obligated by 

 muscular or visceral requirements, the cross section of the body may 

 depart in one direction or the other from the circle that is ideal in a 

 stream-line form. This being the case, two fish may be equally efficient 

 in body form and yet have a considerably different appearance. Both 

 may be 90 per cent efficient, and yet those details which from one 

 detract 10 per cent may be entirely different from those amounting 

 to the 10 per cent in the other. Or another viewpoint may be accorded 

 this fact by the statement that what proves stream-line for one mode of 

 propulsion does not prove so for some other method of swimming. Not 

 only that but if one animal swims with its entire caudal and lumbar 

 regions involved, the degree of its departure from an ideal stream-line 

 form will not be the same as if it swims by vibrations of the tip of 

 the tail only. 



Thus, in effect, it is found that when we speak of a body as stream- 

 line we mean merely that it tapers gradually, without sharp angles or 

 excrescences that would offer resistance as it passes through the water. 

 But even though our knowledge of what is, or might be, the ideal of 

 stream-line form in any particular case is so vague, the evidence fur- 

 nished by the tendency of every essentially active form of aquatic life 

 to assume certain definite body-shapes is so overwhelming that we know 

 beyond question that this is one of the most fundamental of aquatic 

 stimuli. 



[10] 



