198 SUMMARY OF THE [Chap. JUL 



even the equatorial regions, and which, during the 

 alternations of the cold in the north and south, allowed 

 the productions of opposite hemispheres to mingle, and 

 left some of them stranded on the mountain-summits 

 in all parts of the world. As showing how diversified 

 are the means of occasional transport, I have discussed 

 at some little length the means of dispersal of fresh- 

 water productions. 



If the diificulties be not insuperable in admitting 

 that in the long course of time all the individuals of 

 the same species, and likewise of the several species 

 belonging to the same genus, have proceeded from 

 some one source ; then all the grand leading facts of 

 geographical distribution are explicable on the theory 

 of migration, together with subsequent modification and 

 the multiplication of new forms. We can thus under- 

 stand 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 



