Chap. XI. GEOGRAPHICAL DISTRIBUTION. 305 



(>sscnlially Aniorlcan, though they may be all peculiar species. 

 We may look back to past ages, as shown in tli(j last chapter, 

 and we lind American types then prevailing on the American 

 Continent and in the American seas. AVe see in these fact.s 

 some deep organic bond, throughout space and time, over the 

 same areas of land and water, and indej)endent of physical con- 

 ditions. The naturalist must be dull who is not led to inquire 

 •what this bond is. 



The bond is simply inheritance, that cause which alone, as 

 far as we positive!}' know, produces organisms quite Hke each 

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

 dissimilarity of the inhabitants of different regions may be at- 

 trilnited to modification througli natural selection, and in a 

 subordinate degree to the definite inlluence of different phys- 

 ical conditions. The degree of dissimilarity will depend on 

 the migration of the more dominant forms of life from one re- 

 gion into another having been prevented more or less effectually, 

 at periods more or less remote — oii the nature and number of 

 the former inmiigrants — and on the action of the inhabitants 

 on each other in leading to the preservation of different modifi- 

 cations; the relation of organism to organisin in the struggle 

 for life 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 througli natural selection. Widely- 

 ranging species, abounding in individuals, which have already 

 triumphed over many competitors in their own widelj'-extendcd 

 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 fre(iuently undergo 

 further modification and improvement ; and thus they will be- 

 come still further victorious, and will produce grou[is of modi- 

 fied descendants. On this principle of inheritance with modili- 

 eation, we can uiidci-stand 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 d(?velopment. As the variability of each species is 

 an independent property, and will Ix; taken advantage of by 

 natural selection, only so far as it jirolits each individual in its 

 complex struggle for life, so the amount of modification in 

 difierent species Avill be no uniform (juantity. If a number of 

 species, after having long conipeteil witli each otlier in their 



