350 GEOGRAPHICAL DISTRIBUTION. Chap. XI. 



the American continent and in the American seas. 

 We see in these facts some deep organic bond, prevail- 

 ing throughout space and time, over the same areas of 

 land and water, and independent of their physical con- 

 ditions. The naturalist must feel little curiosity, who 

 is not led to inquire what this bond is. 



This bond, on my theory, is simply inheritance, that 

 cause which alone, as far as we positively know, pro- 

 duces organisms quite like, or, as we see in the case 

 of varieties nearly like each other. The dissimilarity of 

 the inhabitants of different regions may be attributed 

 to modification through natural selection, and in a quite 

 subordinate degree to the direct influence of different 

 physical conditions. The degree of dissimilarity will de- 

 pend 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 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 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 exposed to new conditions, and will frequently 

 undergo further modification and improvement ; and 

 thus they will become still further victorious, and will 

 produce groups of modified descendants. On this prin- 

 ciple of inheritance with modification, we can under- 

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



