Dear Professor Meldola,--Physiological selection seems to have brought a regular nest of hornets about my head. If I had known there was to have been so much talk about it at the British Association I should have gone up to defend the new-born. If you were there, can you let me know the main objections that were urged? It seems to me there is a good deal of misunderstanding abroad, due, no doubt, to the insufficiency with which my theory has been stated. In 'studying' the paper, therefore, please keep steadily in view that the backbone of the whole consists in regarding mutual sterility as the _cause_ (or at least, the chief _condition_) instead of the _result_ of specific differentiation. This is just the opposite view to that now held by all evolutionists, and, I believe, by Darwin himself. (See 'Origin,' pp. 245-246; 'Variation,' ii. pp. 171-175.) Now, if this view be sound, my theory is obviously not restricted to any one class of causes that may induce mutual sterility. Such cases may be either extrinsic or intrinsic as regards the reproductive system; they may be either direct in their action on that system or indirect (_e.g._ natural selection, or use and disuse, &c., producing morphological changes elsewhere, which in turn react on that system); therefore these causes may act either on a few or on many individuals. Yet Wallace does not seem to see this, but argues in the 'Fortnightly' that they can only act on an individual here and there. I sincerely hope you will give your attention to the subject, because the great danger I now fear is prejudice against the theory on account of people not taking the trouble to understand it. How absurd ----, for example, giving that quotation from 'Origin' in 'Nature,' as evidence of Mr. Darwin's having considered the theory. Read with its context, the passage is arguing (much against the writer's desire) that variations in the way of sterility with parent forms cannot be seized upon (or perpetuated as specific distinctions) by natural selection. But physiological selection says that such variations _do not require to be seized upon by natural selection_. Therefore, so far as the passage in question proves anything, it tends to show that nothing could have been further from the mind of the writer than a theory which would have rendered his whole argument superfluous, and I can scarcely believe that if the theory of physiological selection had ever occurred to him, he would not have mentioned it, if only to state his objections to it, as he has done with regard to so many ideas of a much less feasible character.