338 SUMMARY OF THE [CHAP. XIIL 



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 arrived at by 

 many naturalists under the designation of single centres of crea 

 tion, by various general considerations, more especially from the 

 importance of barriers of all kinds, and from the analogical distri- 

 bution of sub-genera, genera, and families. 



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 remem- 

 ber that some forms of life have changed very slowly, enormous 

 periods of time having been thus granted for their migration, 

 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 climatal changes on distribution, 

 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 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 occa- 

 sional 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 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 dif- 

 ferent 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 allowed 

 certain forms and not others to enter, either in greater or lesser 



