118 THE STORY OF REPTILE LIFE. 



rivers, and the fresh-water forms which occasion- 

 ally wander out to sea, there is this difference, 

 that whilst the latter show only a slight modifi- 

 cation in the shape of a somewhat compressed 

 "body, the marine forms have acquired an eel-like 

 form, the tail being flattened into an oar-shaped 

 organ. 



That these marine forms, which are believed 

 never to leave the water, have been derived from 

 several distinct fresh- water types is an extremely 

 interesting point, especially in view of the fact 

 that, though unrelated, all have acquired the 

 same peculiarly modified body. Here we see again 

 the principle of convergent evolution at work 

 that is to say, the effect of a similar environment 

 acting on similar structures. Just as the burrow- 

 ing lizards and the burrowing snakes have come 

 to resemble one another in the same way. 

 Further, we have to remark a resemblance of 

 another kind between these marine snakes, inas- 

 much as certain harmless kinds have come to 

 bear so close a resemblance to poisonous species 

 in their coloration that they can only be dis- 

 tinguished after careful discrimination. On this 

 point we shall have something to say when dis- 

 cussing the phenomena of mimicry. 



Comment has frequently been made through- 

 out these pages on the fact that, while certain 

 animals have preserved a remarkable versatility 

 of locomotion, or the ability to subsist upon a 

 varied diet, others have, by what we may call 

 a process of concentration, acquired a peculiar 

 facility in one direction only in the matter of 

 movement, or a special adaptability for subsist- 



