SWIMMING 



both a long tail and a long neck, and presumably this development could 

 only take place in connection with a body in which each motion during 

 swimming was counterpoised by a corresponding motion on the oppo- 

 site side. 



The Crocodilia, mesosaurs, mosasaurs and longer-bodied ichthyosaurs 

 really represent different steps toward the same goal in the development 

 of their limbs, and in all these there is very little difference in size be- 

 tween the fore and hind feet. The longer sorts were partly anguilliform 

 in movement and the shorter, fusiform. The Crocodilia use the tail 

 exclusively for speedy swimming, and the hind limbs to a considerable 

 extent during sluggish progression. So probably did the mesosaurs, 

 and the mosasaurs largely so. In the latter the limbs had become true 

 paddles, the fore limbs differing but slightly in size from the hinder 

 ones, but the powerful, laterally flattened tail is clearly indicated as the 

 more efficient propulsive organ and the flippers had probably begun 

 to be used merely as equilibrators and accessory organs, as they finally 

 became in the later, shorter-bodied ichthyosaurs. In the latter, with 

 their very powerful tails, the hind limbs were considerably smaller than 

 the fore limbs, and hence it seems evident that they had long since fallen 

 into relative disuse, but not at a sufficiently remote period to have disap- 

 peared altogether. 



From a consideration of aquatic mammals it seems that there are 

 certain mechanical conditions which the more specialized sorts are 

 constrained to endeavor to fulfill during the course of their development. 

 There seems to be a decided tendency for aquatic mammals to develop 

 as the primary means for locomotion a single organ or pair of organs, 

 and if the original conformation of the animal allow and nothing divert 

 it from its goal, the situation of this propelling apparatus will most likely 

 be at the hinder end and medially situated. In other words, the stimulus 

 is for the acquisition of the fish-tail type of propulsion, although if we 

 leave out of consideration the question of comparison of the muscles 

 involved, it makes no difference whether this equipment be in the form 

 of a true tail expanded vertically or horizontally, or of the hind feet 

 held with soles adpressed (Phocidae) . Pending the development of a 

 tail fitted for propulsion the chief means by which swimming is ac- 

 complished will probably, in the majority of cases, be the hind feet. 

 Unless the animal can use the hind feet in swimming after the manner 

 of the Phocidae (and Enhydrinae ?) , however, these members will 

 probably never develop into the final, primary propulsive organ, one 

 reason possibly being that the mammalian foot is likely incapable of 



[21] 



