Note 20 — Chapter VI · Geographical Distribution.

I say “large areas” for the sake of argument; but the same correlation between distribution and affinity extends likewise to small areas where only small differences of affinity are concerned. Thus, for instance, speaking of smaller areas, Moritz Wagner says:—"The broader and more rapid the river, the higher and more regular the mountain-chain, the calmer and more extensive the sea, the more considerable, as a general rule, will be the taxonomic separation between the populations"; and he shows that, in correlation with such differences in the degrees of separation, are the degrees of diversification—i. e., the numbers of species, and even of varieties, which these topographical barriers determine.