36G ON THE ORIGIN OF SPECIES 



from the importance of barriers and from the analogical 

 distribution of sub-genera, genera, and families. 



With respect to the distinct species of the same 

 genus, which on my theory must have spread from one 

 parent-source ; if we make the same allowances as before 

 for our ignorance, and remember that some forms of 

 life change most slowly, enormous periods of time being 

 thus granted for their migration, I do not think that 

 the difficulties are insuperable ; though they often are 

 in this case, and in that of the individuals of the same 

 species, extremely great. 



As exemplifying the effects of climatal changes on 

 distribution, I have attempted to show how important 

 has been the influence of the modern Glacial period, 

 which I am fully convinced simultaneously affected the 

 whole world, or at least great meridional belts. 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 the individuals of the 

 same species, and likewise of allied species, have pro- 

 ceeded from some one source ; then I think all the 

 grand leading facts of geographical distribution are 

 explicable on the theory of migration (generally of the 

 more dominant forms of life), together with subsequent 

 modification and the multiplication of new forms. We 

 can thus understand the high importance of barriers, 

 whether of land or water, which separate our several 

 zoological and botanical provinces. We can thus 

 understand the localisation of sub-genera, genera, and 

 families ; 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 in so mysterious a manner linked together by affinity, 

 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 



