To sum up. Although in his later years, as shown by his correspondence, Darwin came to recognize more fully the swamping effects of free intercrossing, and the consequent importance of "separation" for the prevention of these effects, and although in this connexion he likewise came more clearly to distinguish between the "two cases" of monotypic and polytypic evolution, it is evident that he never worked out any of these matters--"thinking it prudent," as he wrote with reference to them in 1878, "now I am growing old, to work at easier subjects[36]." Therefore he never clearly saw, on the one hand, that free intercrossing, far from constituting a "difficulty" to _monotypic_ evolution by natural selection, is the very means whereby natural selection is in this case enabled to operate; or, on the other hand, that, in the case of _polytypic_ evolution, the "difficulty" in question is so absolute as to render such evolution, by natural selection alone, absolutely impossible. Hence, although in one sentence of the _Origin of Species_ he mentions three forms of isolation (besides the geographical form) as serving in some cases to assist natural selection in causing "divergence of character" (i. e. polytypic evolution[37]), on account of not perceiving how great and how sharp is the distinction between the two kinds or "cases" of evolution, he never realized that, where "two or more new species" are in course of differentiation, _some_ form of isolation other than natural selection must _necessarily_ be present, whether or not natural selection be likewise so. The nearest approach which he ever made to perceiving this necessity was in one of his letters to Wagner above quoted, where, after again appealing to the erroneous analogy between monotypic evolution and "unconscious selection," he says:--"But I admit that by this process (i. e. unconscious selection) two or more new species could hardly be formed within the same limited area: some degree of separation, if not indispensable, would be highly advantageous; and here your facts and views will be of great value." But even in this passage the context shows that by "separation" he is thinking exclusively of _geographical_ separation, which he rightly enough concludes (as against Wagner) need certainly not be "indispensable." Had he gone a step further, he must have seen that separation, _in some form or another, is_ "indispensable" to polytypic evolution. Instead of taking this further step, however, two years later he wrote to Semper as follows:-- [36] _Life and Letters_, vol. iii. p. 161. [37] Page 81. The three forms of isolation mentioned are, "from haunting different stations, from breeding at slightly different seasons, or from the individuals of each variety preferring to pair together."