Dlstrihution of Marine Animals. 121 



A few Batrachians, Mr. Boulenger tells me, live in 

 brackish or salt water^ such as Rana limnocharis, Bafo halo- 

 pkila, and to a certain extent the European Bufo viridis and 

 Bufo calamita ; but as their eggs only develop in fresh water, 

 their o))portunities for oceanic distribution are limited and 

 need not at present be further dealt witli. 



In addition to the semimarine Iguanids — Amblyrhynchus , 

 ^vhich enters the sea (by diving) to feed on seaweeds, Tropi- 

 durus, and the various turtles, — marine reptiles are only found 

 amongst the snakes, if the estuarine crocodiles and Trionychoids, 

 which occasionally wander some miles seawards, are passed 

 by. As Mr. Boulenger * observes, " no better instance of 

 gradual modification from terrestrial into marine forms could 

 be found than in the snakes living at the present day, amongst 

 which are also to be found the only recent reptilian types 

 that, being viviparous, never leave the water." Tliese arc 

 the llydrophids or sea-snakes, the largest of which is about 

 ]2 fe^i long. They are, as described by the author just 

 mentioned, found in the Indian and western South Pacific 

 Oceans, ranging from the Persian Gulf to North Australia, 

 one species [Hydrus bicolor) stretching throughout the Indian 

 and tropical Pacific Oceans, the extreme points being tlie 

 Cape of Good Hope and Guayaquil. 



As snakes are most abundant in tropical and subtropical 

 regions, it would appear that certain land-snakes in these 

 parts had gradually adapted themselves, probably in con- 

 nexion with food, to marine life — so much so that some are 

 never known to leave the water. Yet their distribution has 

 been limited, perhaps partly by temperature, though they 

 probably have extended considerably from their original 

 centre. It may be also that they are kept in check by the 

 large predatory forms, such as Elasmobranchs and Cetaceans. 



The marine fishes are, perhaps, more actively and charac- 

 teristically pelagic than any other group. As already shown, 

 the obstacles which oppose the distribution of land-animals 

 are absent — food and temperature chiefly requiring con- 

 sideration, though the abundance of the former in every sea 

 almost removes it from such a category. Another factor, 

 it is true, is the pelagic or demersal condition of the eggs, 

 since the latter habit might be supposed to have the effect of 

 making the proximity of the shores, or at least of the bottom, 

 a necessity at certain seasons. Yet one of the best known 

 and most widely distributed amongst pelagic fishes, the 

 herring, has demersal eggs. 



* iS'at. Science, vol. i. p. 4o (1892). 



