I went as far as I could, perhaps too far, in agreement with Wagner [i. e. in the last edition of the _Origin of Species_]; since that time I have seen no reason to change my mind; but then I must add that my attention has been absorbed on other subjects[38]. [38] _Life and Letters_, vol. iii. p. 159. And he seems to have ended by still failing to perceive that the explanation which he gives of "divergence of character" in the _Origin of Species_, can only hold on the unexpressed assumption that free intercrossing is in some way prevented at the commencement, and throughout the development, of each diverging type. Lastly, we have to consider Darwin's opinion touching the important principle of "Independent Variability." This, it will be remembered, is the principle which ensures that when a portion (not too large) of a species is prevented from interbreeding with the rest of the species, sooner or later a divergence of type will result, owing to the fact that the average qualities of the separated portion at the time of its separation cannot have been exactly the same as the average qualities of the specific type as a whole. Thus the state of Amixia, being a state of what Mr. Gulick calls Independent Generation, will of itself--i.e. even if unassisted by natural selection--induce divergence of type, in a ratio that has been mathematically calculated by Delbœuf. Darwin wrote thus to Professor Weismann in 1872:-- I have now read your essay with very great interest. Your view of the origin of local races through "Amixia" is altogether new to me, and seems to throw an important light on an obscure question[39]. [39] _Life and Letters_, vol. iii. p. 155. And in the last edition of the _Variation of Animals and Plants_ he adds the following paragraph:-- This view may throw some light on the fact that the domestic animals which formerly inhabited the several districts in Great Britain, and the half-wild cattle lately kept in several British parks, differed slightly from one another; for these animals were prevented from wandering over the whole country and intercrossing, but would have crossed freely within each district or park[40]. [40] _Variation_, &c., vol. ii. p. 262.