310 TROPICAL NATURE, AND OTHER ESSAYS. 



frogs and toads are most completely shut out by an 

 ocean barrier ; then follow snakes, but as these are only 

 found in the Galapagos and are very like South American 

 species, they may possibly have been conveyed in boats 

 or by floating trees. Lizards, however, are so wide- 

 spread over almost all the warmer islands of the great 

 oceans, that they must have some natural way of passing 

 over, but the exact mode in which this is effected has 

 not yet been discovered. Birds, as we have seen, are 

 liable to be carried by winds and storms over great 

 widths of sea, but this only applies to certain groups ; 

 and large numbers which feed on the ground or which 

 inhabit the depths of the forests, are almost as strictly 

 confined to their respective countries by even a narrow 

 arm of the sea as are the majority of the mammalia. 



This sketch of the mode in which the various kinds 

 of islands have been stocked with their animal inhabitants 

 forms the best introduction to the study of those changes 

 in our continents which have led to the existing distri- 

 bution of animals. It demonstrates the importance of 

 the sea as a barrier to the spread of all the higher 

 animals ; and we are thus naturally led on to inquire, 

 how far and to what extent such barriers have in past 

 time existed between lands which are now united, and 

 on the other hand what existing oceanic barriers are of 

 comparatively recent origin. In pursuing this inquiry 

 we shall have to take account of those grand views of 

 the course of nature associated with the names of Lyell 

 and Darwin of the slow but never-ceasing changes in 

 the physical conditions, the outlines and the mutual 

 relations of the land-surfaces of the globe ; and of the 

 equally slow and equally unceasing changes in the 



