DOI link for Geographical Distribution
Geographical Distribution book
In considering the distribution of organic beings over the face of the globe, the first great fact which strikes us is, that neither the similarity nor the dissimilarity of the inhabitants of various regions can be accounted for by their climatal and other physical conditions. The dissimilarity of the inhabitants of different regions may be attributed to modification through Natural Selection, and in a quite subordinate degree to the direct influence of different physical conditions. Thus the high importance of barriers comes into play by checking migration; as does time for the slow process of modification through Natural Selection. Widely ranging species, abounding in individuals, which have already triumphed over many competitors in their own widely extended homes will have the best chance of seizing on new places, when they spread into new countries. Sir C. Lyell in a striking passage has speculated, in language almost identical with mine, on the effects of great alternations of climate on geographical distribution.