28 GROWTH OF THE 'ORIGIN.' [1844. 



C. Darwin to J. D. Hooker. 



Down [1844]. 



. . . The conclusion, which I have come at is, that those 

 areas, in which species are most numerous, have oftenest 

 been divided and isolated from other areas, united and again 

 divided ; a process implying antiquity and some changes in 

 the external conditions. This will justly sound very hypo- 

 thetical. I cannot give my reasons in detail ; but the most 

 general conclusion, which the geographical distribution of all 

 organic beings, appears to me to indicate, is that isolation is 

 the chief concomitant or cause of the appearance of new 

 forms (I well know there are some staring exceptions). 

 Secondly, from seeing how often the plants and animals 

 swarm in a country, when introduced into it, and from see- 

 ing what a vast number of plants will live, for instance in 

 England, if kept free from weeds, and native plants, I have 

 been led to consider that the spreading and number of the 

 organic beings of any country depend less on its external 

 features, than on the number of forms, which have been there 

 originally created or produced. I much doubt whether you 

 will find it possible to explain the number of forms by pro- 

 portional differences of exposure ; and I cannot doubt if 

 half the species in any country were destroyed or had not 

 been created, yet that country would appear to us fully 

 peopled. With respect to original creation or production of 

 new forms, I have said that isolation appears the chief ele- 

 ment. Hence, with respect to terrestrial productions, a tract 

 of country, which had oftenest within the late geological pe- 

 riods subsided and been converted into islands, and reunited, 

 I should expect to contain most forms. 



But such speculations are amusing only to one's self, and in 

 this case useless, as they do not show any direct line of obser- 

 vation : if I had seen how hypothetical [is] the little, which I 



