68 WATER REPTILES OF THE PAST AND PRESENT 



the struggle. It can never acquire new material, never get new 

 fingers and toes, new organs or parts of organs; all its possibilities 

 lie in the improved and new uses it can make of the material which 

 it received from its ancestors. 



The beginning of aquatic adaptation of the limbs lies in the 

 membranous webs between the toes of frogs, salamanders, ducks, 

 seal, otters, etc., where the feet are used largely or entirely for pro- 

 pulsion through the water, in the absence of a propelhng tail. And 

 this membrane, in the majority of cases, is the extent of aquatic 

 adaptation in air-breathing animals. In those animals, however, 

 such as most of the reptiles described in the following pages, where 

 the tail has developed as the propelling organ, the hmbs lose to a 

 greater or less extent their propelling function and become merely 

 organs of equilibration and control. Of the two pairs of fins of fishes 

 it is evident that the anterior ones have the more important equili- 

 brational function; the hind ones have a much less important use as 

 guiding organs; as a matter of fact, in not a few fishes the hind or 

 pelvic fins have actually migrated forward to supplement the func- 

 tion of the pectoral fins. It is for these reasons that those animals 

 best adapted of all for Hfe in the water — the whales and sirenians — • 

 have lost the hind legs completely. In other tail-propelled air- 

 breathers the hind legs have become progressively smaller and 

 less powerful than the front ones. In all short-tailed water animals, 

 however, where the legs, and especially the hind legs, have the 

 important function of propulsion to subserve, they still retain 

 the large size and firm connections with the body, examples 

 of which will be seen in the seals, sea-otters, marine turtles, and 

 plesiosaurs. 



Because the legs are no longer needed for the support or propul- 

 sion of the body in long-tailed air-breathers, their connection with 

 the body becomes less and less firm, long before their entire dis- 

 appearance. In animals using the legs for crawling or walking 

 the bones of an arm and thigh are elongated, and the joints are 

 always well formed, permitting varied, extensive, and firm move- 

 ments. Just the reverse is the tendency in all those animals that 

 propel themselves by the aid of the tail in the water, since here what 

 is needed is broad, short limbs, not long and slender ones. 



