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. ISTor 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. 
