GEOGRAPHICAL DISTRIBUTION 315 



physical conditions. The degree of dissimilarity will 

 depend on the migration of the more dominant forms 

 of life from one region into another having been effected 

 with more or less ease, at periods more or less remote ; 

 — on the nature and number of the former immigrants ; 

 — and on their action and reaction, in their mutual 

 struggles for life ; — the relation of organism to organism 

 being, as 1 have already often remarked, the most im- 

 portant of all relations. Thus the high importance of 

 barriers comes into play by checking migration ; as 

 does time for the slow process of modification through 

 natural selection. Widely-ranging species, abounding 

 in individuals, which have already triumphed over many 

 competitors in their own widely-extended homes wil: 

 have the best chance of seizing on new places, when 

 they spread into new countries. In their new homer 

 they will be exposed to new conditions, and will fre 

 quently undergo further modification and improvement ; 

 and thus they will become still further victorious, and 

 will produce groups of modified descendants. On 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. 



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 



