Chap. XI. SINGLE CENTRES OF CREATION. 351 



and even families are confined to the same areas, as is 

 so commonly and notoriously the case. 



I believe, as was remarked in the last chapter, in no 

 law of necessary development. As the variability of 

 each species is an independent property, and will be 

 taken advantage of by natural selection, only so far as 

 it profits the individual in its complex struggle for 

 life, so the degree of modification in different species 

 will be no uniform quantity. If, for instance, a number 

 of species, which stand in direct competition with each 

 other, migrate in a body into a new and afterwards 

 isolated country, they will be little liable to modifica- 

 tion ; for neither migration nor isolation in themselves 

 can do 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 con- 

 ditions. As we have seen in the last chapter that some 

 forms have retained nearly the same character from an 

 enormously remote geological period, so certain species 

 have migrated over vast spaces, and have not become 

 greatly modified. 



On these views, it is obvious, that the several species 

 of the same genus, though inhabiting the most distant 

 quarters of the world, must originally have proceeded 

 from the same source, as they have descended from the 

 same progenitor. In the case of those species, which 

 have undergone during whole geological periods but 

 little modification, there is not much difficulty in believ- 

 ing that they may have migrated from the same region ; 

 for during the vast geographical and climatal changes 

 which will have supervened since ancient times, almost 

 any amount of migration is possible. But in many other 

 cases, in which we have reason to believe that the species 

 of a genus have been produced within comparatively 

 recent times, there is great difficulty on this head. It 



