Note 33 — Chapter VI · A Brief History of Opinions on Isolation as a Factor of Organic Evolution.

The analogy is radically unsound because unconscious selection differs from methodical selection only in the degree of "separation" which it effects. These two forms of selection do not necessarily differ from one another in regard to the number of characters which are being simultaneously diversified; for while it may be the object of methodical selection to breed for modification of a single character alone, it may, on the other hand, be the result of unconscious selection to diversify an originally uniform stock, as Darwin himself observes with regard to horse-breeding. The real distinction between monotypic and polytypic evolution is, not at all with reference to the degree of isolation (i. e. amount of "separation"), but to the number of cases in which any efficient degree of it occurs (i. e. whether in but a single case, or in two or more cases).