THE TAIL 



As already mentioned it seems to me highly probable that whales have 

 descended from an essentially active ancestry, and that some method of 

 swimming has always been employed which involved movement of the 

 tail in the vertical plane, originally somewhat after the style now to be 

 seen in the river otters. Successive steps in caudal development then 

 would be a lateral flange upon either side, next the broadening of the 

 terminal part of this flange without a corresponding increase in the 

 width of the proximal portion. Following an increased tendency to- 

 ward segregation in the extreme distal portion of the tail of the lateral 

 expansions, there would finally result the graceful, bilobed flukes of the 

 rorquals, which are probably more specialized than those of any other 

 cetacean. 



That an immense length of time has elapsed since the first step in the 

 development of the cetacean flukes is indicated by embryological evi- 

 dence. Ryder showed that in a Delphinapterus fetus of about an inch 

 and a half in length this expansion is apparent, and of spear-shaped 

 form. Indeed, the literature is replete with such evidence. Contrary to 

 what one might expect this caudal expansion does not occur upon the 

 peduncle proper but apparently is confined to the area of the tail tip 

 which will later support the fully-grown flukes (figure 32). This fact 

 may contribute some evidence to the theory that the cetacean tail was not 

 comparable to that of the present manati during any stage of its evolu- 

 tion, but that the lateral expansion had always been confined to the po- 

 sition which it now occupies. 



Naturalists are not entirely in accord regarding the precise manner in 

 which the flukes of whales are used, for opportunities for observation 

 are infrequent and porpoises may move the flukes too rapidly for the 

 human eye to follow. A number of observers have marvelled that when 

 looking down upon a porpoise swimming at speed, as just in front of 

 the bow of a boat, no movement was appreciable, and yet the animal 

 not only maintained its position but easily darted ahead when it so 

 wished. From this it must be inferred that the fluke movement of at 

 least some porpoises is through a short arc, is rapid, and correspondingly 

 very powerful. It has been claimed from time to time that the move- 

 ments of the cetacean flukes are not strictly in the vertical plane but are 

 slightly oblique, first to one side and then the other (scull-like) or even 

 somewhat twisting (partly screw-like) . It is possible that both are used 

 at times, in addition to strictly vertical thrusts, this depending upon the 

 speed, the sort of whale concerned, and the conformation of the flukes. 

 I think it more likely that a cetacean with relatively narrow flukes would 



[201] 



