Now, although I allow that Darwin never attributed to this principle of Amixia, or Independent Variability, anything like the degree of importance to which, in the opinion of Delbœuf, Gulick, Giard, and myself, it is entitled, the above passage appears to show that, as soon as the "view" was clearly "suggested" to his mind, he was so far from being unfavourably disposed towards it, that he added a paragraph to the last edition of his _Variation_ for the express purpose of countenancing it. Nevertheless, later on the matter appears to have entirely escaped his memory; for in 1878 he wrote to Semper, that he did "not see at all more clearly than I did before, from the numerous cases which he [Wagner] has brought forward, how and why it is that a long isolated form should almost always become slightly modified[41]." I think this shows entire forgetfulness of the principle in question, because, if the latter is good for explaining the _initial_ divergence of type as between separated stocks of "domesticated animals," much more must it be competent to explain the _further_ divergence of type which is "almost always" observable in the case of "a long isolated form" under nature. The very essence of the principle being that, when divergence of type has once begun, this divergence must _ipso facto_ proceed at an ever-accelerating pace, it is manifestly inconsistent to entertain the principle as explaining the first commencement of divergence, and then to ignore it as explaining the further progress of divergence. Hence, I can only conclude that Darwin had forgotten this principle altogether when he wrote his letter to Semper in 1878--owing, no doubt, as he says in the sentence which immediately follows, to his having "not attended much of late years to such questions." [41] _Life and Letters_, vol. iii. p. 161. * * * * * So much, then, for Darwin's opinions. Next in order of time we must consider Moritz Wagner's essays on what he called the "Law of Migration[42]." The merit of these essays was, first, the firm expression of opinion upon the swamping effects of free intercrossing; and, second, the production of a large body of facts showing the importance of geographical isolation in the prevention of these effects, and in the consequent differentiation of specific types. On the other hand, the defect of these essays was, first, not distinguishing between evolution as monotypic and polytypic; and, second, not perceiving that geographical isolation is only one among a number of other forms of isolation. From these two radical oversights--which, however, were shared by all other writers of the time, with the partial exception of Darwin himself, as previously shown--there arose the following and most lamentable errors. [42] _Die Darwin'sche Theorie und das Migrationsgesetz_ (1868): _Ueber den Einfluss der geographischen Isolirung_, &c. (1870).