310 



EEPTILES. 



[Chap. IX. 



on the western shores of tropical America. And if, as 

 has been stated 1 , they have been seen on a late occasion 

 in considerable numbers in the Bay of Panama, the fact 

 can only be regarded as one of the rare instances, in 

 which a change in the primary distribution of a race of 

 animals has occurred, either by an active or a passive 

 immigration. Being exclusively inhabitants of the sea, 

 they are liable to be swept along by the influence of 

 currents ; but to compensate for this they have been 

 endowed with a wonderful power of swimming. The 

 individuals of all the groups of terrestrial serpents are 

 observed to be possessed of this faculty to a greater or 

 a less degree ; and they can swim for a certain distance 

 without having any organs- specially modified for the 

 purpose ; except, perhaps, the lung, which is a long sac 

 capable of taking in a sufficient quantity of air, to keep 

 the body of the snake above water. Nor do we find any 

 peculiar or specially adapted organs even in the fresh- 

 water-snakes, although they can catch frogs or fishes 

 while swimming. But in the hydrophids, which are 

 permanent inhabitants of the ocean, and which in an 

 adult state, approach the beach only occasionally, and 

 for very short times, the tail, which is rounded and 

 tapering in the others, is compressed into a vertical 

 rudder-like organ, similar to, and answering all the 

 purposes of, the caudal fin in a fish. When these snakes 

 are brought on shore or on the deck of a ship, they 

 are helpless, and struggle vainly in awkward attitudes. 

 Their food consists exclusively of such fishes as are 

 found near the surface ; a fact which affords ample 

 proof that they do not descend to great depths, although 



1 Proc. Zool. Soc. 1858. 



