142 THE ORIGIN OF SPECIES 



being, as I 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 individu- 

 als which have already triumphed over many competitors 

 in their own widel3'-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 exposed 

 to new conditions, and will frequently undergo further 

 modification and improvement; and thus they will be- 

 come still further victorious, and will produce groups 

 of modified descendants. On this principle of inheritance 

 with modification we can understand how it is that sec- 

 tions of genera, whole genera, and even fanulies, are con- 

 fined to the same areas, as is so commonly and notori- 

 ously the case. 



There is no evidence, as was remarked in the last 

 chapter, of the existence of any law of necessary devel- 

 opment. As the variability of each species is an inde- 

 pendent 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 modi- 

 fication 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 afterward 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 re- 

 lations with each other and in a lesser degree with the 

 "surrounding physical conditions. As we have seen in 



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