1844-] MUTABILITY OF SPECIES. 389 



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 seeing what a vast number 

 of plants will live, for instance in England, if kept free from 

 weeds, and ?mtive 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 proportional 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 element. Hence, with respect to terrestrial pro- 

 ductions, a tract of country, which had oftenest within the 

 late geological periods subsided and been converted into 

 islands, and reunited, I should expect to contain most forms. 

 But such speculations are amusing only to one self, and 

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

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

 which I have unclearly written, I would not have troubled 

 you with the reading of it. Believe me, — at last not hypo- 

 thetically, Yours very sincerely, 



C. Darwin. 



C. Darwin to J. D. Hooker. 



Down, 1844. 



... I forget my last letter, but it must have been a very 

 silly one, as it seems I gave my notion of the number of 

 species being in great degree governed by the degree to 

 which the area had been often isolated and divided ; I must 

 have been cracked to have written it, for I have no evidence, 

 without a person be willing to admit all my views, and then 

 it does follow ; but in my most sanguine moments, all I 

 expect, is that I shall be able to show even to sound Natu- 

 ralists, that there are two sides to the question of the immu- 



