402 GEOGRAPHICAL DISTRIBUTION. Chap. XII. 



the several islands, and we may infer from certain facts 

 that these have probably spread from some one island 

 to the others. But we often take, I think, an erro- 

 neous view of the probability of closely allied species 

 invading each other's territory, when put into free 

 intercommunication. Undoubtedly if one species has 

 any advantage whatever over another, it will in a 

 very brief time wholly or in part supplant it; but if 

 both are equally well fitted for their own places in 

 nature, both probably will hold their own places and 

 keep separate for almost any length of time. Being 

 familiar with the fact that many species, naturalised 

 through man's agency, have spread with astonishing 

 rapidity over new countries, we are apt to infer that 

 most species would thus spread ; but we should remem- 

 ber that the forms which become naturalised in new 

 countries are not generally closely allied to the aboriginal 

 inhabitants, but are very distinct species, belonging in a 

 large proportion of cases, as shown by Alph. de Candolle, 

 to distinct genera. In the Galapagos Archipelago, many 

 even of the birds, though so well adapted for flying 

 from island to island, are distinct on each ; thus there 

 are three closely-allied species of mocking-thrush, each 

 confined to its own island. Now let us suppose the 

 mocking-thrush of Chatham Island to be blown to 

 Charles Island, which has its own mocking-thrush : why 

 should it succeed in establishing itself there ? We may 

 6afely infer that Charles Island is well stocked with its 

 own species, for annually more eggs are laid there 

 than can possibly be reared ; and we may infer that the 

 mocking-thrush peculiar to Charles Island is at least as 

 well fitted for its home as is the species peculiar to 

 Chatham Island. Sir C. Lyell and Mr. Wollaston have 

 communicated to me a remarkable fact bearing on this 

 subject ; namely, that Madeira and the adjoining islet of 



