and the Darwinian Theory. 163 



" If a number of species, after having long competed with 

 each other in their old home, were to migrate in a body into 

 a new and afterwards isolated country, they would be little 

 liable to modification, for neither migration nor isolation in 

 themselves effect anything. These principles come into play 

 only by bringing organisms into new relations with each 

 other, and in a lesser degree with the surrounding physical 

 conditions." [' Origin of Species,' on the fourth and fifth 

 pages of the first chapter on " Geographical Distribution."]* 

 " Each separate island of the Galapagos Archipelago is 

 tenanted, and the fact is a marvellous one, by many distinct 

 species ; but these species are related to each other in a very 

 much closer manner than to the inhabitants of the American 

 continent or of any other quarter of the world. This is what 

 might have been expected, for islands situated so near each 

 other would almost necessarily receive immigrants from the 

 same original source and from each other. But how is it 

 that many of the immigrants have been differently modified, 

 though only in a small degree, in islands situated within 

 sight of each other, having the same geological nature, the 

 same height, climate, &c. ? This long appeared to me a 

 great, difficulty : but it arises in chief part from the deeply 

 seated error of considering the physical conditions of a country 

 as the most important ; whereas it cannot be disputed that 

 the nature of the other species with which each has to com- 

 pete is at least as important, and generally a far more im- 

 portant element of success. Now if we look to the species 

 which inhabit the Galapagos Archipelago, and are likewise 

 found in other parts of the world, we find that they differ 

 considerably in the several islands." [' Origin of Species,' 

 near the middle of the second chapter on " Geographical 

 Distribution."] | 



The implication in both these passages is that if the repre- 

 sentatives of the same species are surrounded by the same 

 organic forms, as well as by the same physical conditions in 

 isolated countries, they will not undergo divergent modifica- 

 tion. This is in complete accord with the third paragraph 

 quoted near the beginning of this paper from the fourth 

 chapter of the ' Origin of Species.' 



Divergent Forms of Sexual Selection. 



assages last quoted there is no men 

 to the principle that difference in e. 



See ed. 6, p. 319. t See ed. 6, p. 355. 



In the passages last quoted there is no mention of any 

 exception to the principle that difference in external con- 



