UNDER ONE SET OF EXTERNAL CONDITIONS. 



499 



from one original species, it seems to me necessary to suppose 

 two conditions, both of which relate to the state of the species — 

 namely, Separation and Variation. I regard Separation as a con- 

 dition of the species and not of surrounding nature, because it 

 is a state of division in the stock which does not necessarily imply 

 any external barriers, or even the occupation of separate dis- 

 tricts. This may be illustrated by the separation between the 

 castes of India or between different genera occupying the same 

 locality. 



To state the conditions more fully : — 



1st. We must suppose that they possess or have possessed an 

 inherent tendency to variation, so strong that all that is necessary 

 to secure a divergence of types in the descendants of one stock 

 is to prevent, through a series of generations, their intermingling 

 with each other to any great degree. This supposition is not at 

 variance, but rather in accordance, with facts that are observed in 

 analogous cases in the history of man and of domestic animals of 

 one original stock, that are kept entirely apart. But this con- 

 dition alone would not be enough to account for the species of 

 Achatinellince being confined to areas so much smaller than usual ; 

 for if this tendency has produced such results in the distribution 

 of one family, why does it not in all ? 



Migration and Variation opposing Factors in the Limitation 

 of Areas. 



2nd. To account, therefore, for the small areas, we must further 

 suppose that, as compared with other families, there is a dispro- 

 portion between the tendency to variation and the tendency and 

 opportunities to migrate. Either the tendency to variation in 

 this family is very much greater than usual, or their tendency to 

 migrate is weaker and their opportunities fewer than usual. 

 According to a priori reasoning, the areas occupied must vary 

 directly as the tendency, power, and opportunities for migrating, 

 but inversely as the tendency to variation. 



If the amount of migration is greatly expanded in proportion 

 to the tendency to variation, the areas must be expanded ; if, on 

 the other hand, the tendency to variation is expanded as compared 

 with the amount and extent of migration, the areas occupied by 

 the different species must be correspondingly contracted. 



If the power of migrating and the opportunities for being trans- 

 ported are very limited in any family of creatures, we may expect 



