CHAP. V DISPERSAL OF ANIMALS AND PLANTS 



75 



form a broad isthmus connecting Asia and America as far 

 south as the parallel of 60° N. Continuity of land there- 

 fore may be said to exist already for all parts of the world 

 (except Australia and a number of large islands, which 

 will be considered separately), and we have thus no 

 difficulty in the way of that former wide 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 mi- 

 grate 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 Ee^Mes. — 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 the 

 reptile 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 



^ Lyell's Principles of Geology, ii,, p. 369. 



