198 SUMMARY OF THE [Chap. XIII. 



even the equatorial regions, and which, during the al- 

 ternations 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 difficulties 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 geo- 

 graphical distribution are explicable on the theory 

 of migration, together with subsequent modiiication 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 form- 

 ing 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 dif- 

 ferent latitudes, for instance in South America, the in- 

 habitants 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. Bear- 

 ing 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 communication which 



