368 THE ORIGIN OF SPECIES 



often a species may have ranged continuously over a wide area, 

 and then have become extinct in the intermediate tracts — the 

 difficulty is not insuperable in believing that all the individuals 

 of the same species, wherever found, are descended from common 

 parents. And we are led to this conclusion, which has been ar- 

 rived at by many naturalists under the designation of single 

 centres of creation, by various general considerations, more es- 

 pecially from the importance of barriers of all kinds, and from 

 the analogical distribution of subgenera, genera, and famihes. 



With respect to distinct species belonging to the same genus, 

 which on our theory have spread from one parent-source; if we 

 make the same allowances as before for our ignorance, and re- 

 member that some forms of life have changed very slowly, enor- 

 mous periods of time having been thus granted for their migra- 

 tion, the difficulties are far from insuperable; though in this case, 

 as in that of the individuals of the same species, they are often 

 great. 



As exemplifying the effects of climatical changes on distribu- 

 tion, I have attempted to show how important a part the last 

 Glacial period has played, which affected even the equatorial 

 regions, and which, during the alternations of the cold in the 

 north and the south, allowed the productions of opposite hemi- 

 spheres to mingle, and left some of them stranded on the moun- 

 tain-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 geographical distribution are explicable on the theory of migra- 

 tion, together with subsequent modification and the multiplica- 

 tion 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 prov- 

 inces. 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 



