GEOGRAPHICAL DISTRIBUTION 201 



But the islands, though in sight of each other, are 

 separated by deep arms of the sea, in most cases wider 

 than the British Channel, and there is no reason to sup- 

 pose that they have at any former period been continu- 

 ously united. The currents of the sea are rapid and 

 sweep between the islands, and gales of wind are extraor- 

 dinarily rare; so that the islands are far more effectually 

 separated from each other than they appear on a map. 

 Kevertheless some of the species, both of those found 

 in other parts of the world and of those confined to the 

 archipelago, are common to the several islands; and we 

 may infer from their present manner of distribution that 

 they have spread from one island to the others. But 

 we often take, I think, an erroneous view of the proba- 

 bility of closely-allied species invading each other's 

 territory, when put into free intercommunication. Un- 

 doubtedly, if one species has any advantage 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, 

 both will probably hold their separate places for almost 

 any length of time. Being familiar with the fact that 

 many species, naturalized through man's agency, have 

 spread with astonishing rapidity over wide areas, we are 

 apt to infer that most species would thus spread; but we 

 should rememoer that the species which become natural- 

 ized in new countries are not generally closely allied to 

 the aboriginal inhabitants, but are very distinct forms, 

 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, differ on the 

 different islands; thus there are three closely-allied species 



