Chap. XII.] GEOGRAPHICAL DISTRIBUTION. 31 S 



we positively know, produces organisms quite like each other, or, 

 as we see in the case of varieties, nearly alike. The dissimilarity of 

 the inhabitants of different regions may be attributed to modification 

 through variation and natural selection, and probably in a sub- 

 ordinate degree to the definite influence of different physical con- 

 ditions. The degrees of dissimilarity will depend on the migration 

 of the more dominant forms of life from one region into another 

 having been more or less effectually prevented, at periods more or 

 less remote ; — on the nature and number of the former immigrants ; 

 — and on the action of the inhabitants on each other in leading to 

 the preservation of different modifications ; the relation of organism 

 to organism in the struggle for life being, as I have already often 

 remarked, the most important of all relations. Thus the high im- 

 portance of barriers comes into play ky 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, will have the best chance of seizing on new places, when they 

 spread into new countries. In their new homes they will be ex- 

 posed to new conditions, and will frequently undergo further modi- 

 fication 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 con- 

 fined 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 advan- 

 tage 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 having 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 distant quarters of the 



