GEOGRAPHICAL DISTRIBUTION 361 



gales 01 wind are extraordinarily rare ; so that the 

 islands are far more effectually separated from each 

 other than they appear to be on a map. Nevertheless 

 a good many species, both those found in other parts 

 of the world and those conlined to the archipelago, 

 are common to 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 erroneous 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 re- 

 member 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 Gala- 

 pagos 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 safely 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 ; 



