SUMMARY. 399 



the grand leading facts of geographical distribution are 

 explicable on the theory of migration, together with subse- 

 quent modification and the multiplication of new forms. 

 We can thus understand the high importance of barriers, 

 whether of land or water, in not only separating but in 

 apparently forming the several zoological and botanical 

 provinces. We can thus understand the concentration of 

 related species within the same areas; and how it is that 

 under different latitudes, for instance, in South America, the 

 inhabitants of the plains and mountains, of the forests, 

 marshes and deserts, are linked together in so mysterious a 

 manner, and are likewise linked to the extinct beings which 

 formerly inhabited the same continent. Bearing in mind 

 that the mutual relation of organism to organism is of the 

 highest importance, we can see why two areas, having nearly 

 the same physical conditions, should often be inhabited by 

 very different forms of life ; for according to the length of 

 time which has elapsed since the colonists entered one of the 

 regions, or both ; according to the nature of the communi- 

 cation which allowed certain forms and not others to enter, 

 either in greater or lesser numbers ; according or not as 

 those which entered happened to come into more or less 

 direct competition with each other and with the aborigines ; 

 and according as the immigrants were capable of varying 

 more or less rapidly, there would ensue in the two or more 

 regions, independently of their physical conditions, infi- 

 nitely diversified conditions of life ; there would be an 

 almost endless amount of organic action and reaction, and 

 we should find some groups of beings greatly, and some only 



: slightly, modified ; some developed in great force, some exist- 

 ing in scanty numbers — and this we do find in the several 

 great geographical provinces of the world. 



On these same principles we can understand, as I have 

 endeavored to show, why oceanic islands should have few 

 inhabitants, but that, of these, a large proportion should be 



'•. endemic or peculiar ; and why, in relation to the means of 

 migration, one group of beings should have all its species 

 peculiar, and another group, even within the same class, 

 should have all its species the same with those in an adjoin- 

 ing quarter of the world. We can see why whole groups of 

 organisms, as batrachians and terrestrial mammals, should 



f be absent from oceanic islands, while the most isolated 

 islands should possess their own peculiar species of aerial 

 mammals or bats. We can see why, in islands, there should 



