AQUATIC MAMMALS 



The probabilities certainly are that the tail of the muskrat, for in- 

 stance, will continue to expand vertically for some time to come, for 

 we have every reason to believe that the stimulus that started it on its 

 present course of development will continue. It is therefore improbable 

 that the present direction of flattening would change through an angle 

 of 90 degrees, so that eventually its tail would ever resemble that of the 

 beaver. Evidently the stimuli for the two sorts of caudal flattening 

 exhibited by these two mammals were always different. At least this 

 thesis can be accepted as a likely one, and also that the tail of the muskrat 

 will continue throughout the ages, to grow higher. 



Bilateral symmetry among vertebrates is almost invariable, while 

 dorso-ventral symmetry, in its exact meaning, is very rare, but still, 

 after a general fashion, is sometimes encountered. Thus, the dorsal 

 and ventral halves of some sorts of fish are often exactly alike in shape. 

 The difficulties arising from the situation were the tail of a whale longer 

 and wider on one side than the other are too apparent to need discus- 

 sion. But asymmetry in the vertically expanded tail of some vertebrate 

 is a different matter. In a lop-sided whale of the former character all 

 manner of gyrations would be performed because the asymmetry of 

 the tail would throw the body to the side and there is present no pair 

 of vertical rudders, one above and the other below the body, to counter- 

 act the uneven influence. But were the tail expanded vertically instead 

 of horizontally asymmetry need have no disturbing influence that could 

 not easily (theoretically) be overcome by compensating tilting of the 

 fore limb rudders, or proper curvature of the body. Tails dorso-ventrally 

 asymmetrical with which their owners get along very well indeed are 

 seen in certain newts and sharks. 



But asymmetry of the sort exhibited by the newt's tail might be pas- 

 sively developed, as far as concerns swimming. Could there be any 

 active stimulus connected with swimming that could cause asymmetry 

 in a tail that is vertically expanded.'' Undoubtedly yes. There might 

 be sufficient dorso-ventral asymmetry of the body, advantageous for 

 feeding or other habits, so that there would eventually and naturally 

 follow a compensating asymmetry of the tail to counteract it: or there 

 might be asymmetrical influences introduced by an organism swimming 

 habitually near the surface or near the bottom. Obviously, if a shark 

 swam at the surface it would be disadvantageous to have one lobe of 

 the tail projecting into the air, or conversely, a dependent tail-fork drag- 

 ging over the bottom would be a handicap. From another aspect, swim- 

 ming near the surface or bottom respectively introduces uneven degrees 

 of water friction operating upon the upper and lower parts of the body. 

 [190] 



