CHAP, v.] 



DISPERSAL OF ANIMALS AND PLANTS. 



73 



diffusion of many groups, which we maintain to be the only- 

 explanation of most anomalies of distribution other than such 

 as may be connected with unsuitability of climate. 



The Dispersal of Birds. — Wherever mammals can migrate 

 other vertebrates can generally follow with even greater facility. 

 Birds, having the power of flight, can pass over wide arms of 

 the sea, or even over extensive oceans, when these are, as in the 

 Pacific, studded with islands to serve as resting places. Even 

 the smaller land-birds are often carried by violent gales of wind 

 from Europe to the Azores, a distance of nearly a thousand 

 miles, so that it becomes comparatively easy to explain the 

 exceptional distribution of certain species of birds. Yet on the 

 whole it is remarkable how closely the majority of birds follow 

 the same laws of distribution as mammals, showing that they 

 generally require either continuous land or an island-strewn sea 

 as a means of dispersal to new homes. 



The Dispersal of Reptiles. — Eeptiles appear at first sight to be 

 as much dependent on land for their dispersal as mammalia, but 

 they possess two peculiarities which favour their occasional 

 transmission across the sea — the one being their greater tenacity 

 of life, the other their oviparous mode of reproduction. A 

 large boa-constrictor was once floated to the island of St. 

 Vincent, twisted round the trunk of a cedar tree, > and was so 

 little injured by its voyage that it captured some sheep before 

 it was killed. The island is nearly two hundred miles from 

 Trinidad and the coast of South America, whence it almost 

 certainly came.-^ Snakes are, however, comparatively scarce 

 on islands far from continents, but lizards are often abundant, 

 and though these might also travel on floating trees, it 

 seems more probable that there is some as yet unknown mode 

 by which their eggs are safely, though perhaps very rarely, 

 conveyed from island to island. Examples of their peculiar 

 distribution will be given when we treat of the fauna of some 

 islands in which they abound. 



The Dispersal of Amphibia and Fresh-water Fishes. — 

 The two lower groups of vertebrates, Amphibia and fresh- 

 water fishes, possess special facilities for dispersal, in the fact of 

 1 Lyell's Principles of Geology^ II., p. 369. 



