1 66 STUDIES IN LIFE AND SENSE. 



wood, or the chance dispersal of their eggs by other animals or upon 

 plants, exemplify the accidental methods of diffusion. The minute 

 eggs of fishes have been known to adhere to the plumage of aquatic 

 birds ; even water-insects may transport fish-ova. The young of 

 shell-fish, like the cockle and oyster, at first swim freely in the sea, 

 and may migrate to vast distances ; and certain shell-fish (e.g. fresh- 

 water snails) deposit their eggs upon aquatic plants, which may like- 

 wise be conveyed for many miles by currents. That the feet ot 

 aquatic birds may convey minute or embryonic shell-fish to great 

 distances is rendered probable by observations of Mr. Darwin ; and 

 the same high authority remarks on the agency of aquatic birds in 

 conveying seeds which are contained in the mud of ponds adhering 

 to their feet. With regard to the dispersal of insects, the power of 

 flight is seen to confer obvious advantages upon this class of animals. 

 Even quadrupeds appear to possess occasional powers of dispersal, 

 which may account for their presence in situations that at first sight 

 would seem inaccessible to the race. The tiger is known to be a 

 powerful swimmer; and the pig, popularly credited with being 

 inefficient in the water, has been proved to be a swift swimmer 

 likewise. Quadrupeds may also be conveyed long distances on 

 driftwood, and may thus chance to be deposited in localities far 

 removed from their original habitat. 



There is little difficulty in accounting for the mechanical means 

 and conditions whereby the dispersal of animals and plants is secured. 

 Hence, returning to the question of island-population, we find in the 

 Azores a collection of animals and plants, obviously derived from 

 adjoining areas, and which has as yet had but little time to develop, 

 through variation, a general distinctness of its own. 



The Galapagos Islands present, as we have seen, the common 

 features of "oceanic" islands, in the absence of native quadrupeds 

 and amphibians, and in the fact that they are of volcanic origin. 

 They differ from the Azores, however, in that they possess two species 

 of snakes, lizards, and land-tortoises the latter being of large size. 

 A single mouse exists in these islands ; but this quadruped belongs to 

 an American genus, and was probably introduced, since these islands 

 have been largely visited for 300 years back by sailors. The tortoises 

 are regarded as having been derived from the American continent, 

 and the lizards, of which there are five, are likewise typically American 

 in their character. That tortoises and lizards can travel for long 

 distances by water cannot be doubted ; and the fact that snakes 

 occur in the Galapagos, and may have reached these islands by 

 swimming seeing that they are related to South American serpents 

 is explained by the knowledge that snakes may swim for long distances. 

 A boa- constrictor has been known to swim to St. Vincent from the 

 South American coast, a distance of at least 200 miles. The birds 



