GEOGRAPHICAL DISTRIBUTION 207 



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 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 belong 

 ing 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 multipli- 

 cation of new forms. We can thus understand the high 

 importance of barriers, wliether of land or water, in not 

 only separating, but in apparently forming the several 

 zoological and botanical provinces. We can thus under- 

 stand 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 des- 

 erts, 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 en- 

 tered one of the regions, or both; according to the nature 



— Science — -26 





