GEOGRAPHICAL DISTRIBUTION 399 



of seizing on new places, when they spread into new coun- 

 tries. In their new homes they will be exposed to new con- 

 ditions, and will frequently undergo further modification and 

 improvement; and thus they will become still further vic- 

 torious, and will produce groups of modified descendants. 

 Or. this principle of inheritance with modification we can 

 understand how it is that sections of genera, whole genera, 

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

 commonly and notoriously the case. 



There is no evidence, as was remarked in the last chapter, 

 of the existence of any 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 each individual in its complex struggle for 

 life, so the amount of modification in different species will 

 be no uniform quantity. If a number of species, after hav- 

 ing 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. 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 or at all modified. 



According to these views, it is obvious that the several 

 species of the same genus, though inhabiting the most dis- 

 tant quarters of the world, must originally have proceeded 

 from the same source, as they are descended from the same 

 progenitor. In the case of those species which have under- 

 gone during whole geological periods little modification, 

 there is not much difficulty in believing that they have mi- 

 grated from the same region ; for during the vast geographi- 

 cal and climatal changes which 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 compara- 

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



