chap, v.] DISPERSAL OF ANIMALS AND PLANTS. 
73 
diffusion of many groups, which we maintain to he 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. — Reptiles 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 . 1 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. 
