Dear Darwin,--Some time ago you write that I ought to read a book or paper by Jordan about varieties in relation to sterility. I cannot find any book or paper of his at the L.S. library which treats of this subject; could you give me the name of his essay? I am making arrangements for trying whether there are any degrees of sterility to be found between well-marked and constant varieties of plants. But as I have never done anything in the way of hybridising, perhaps you would be good enough to let me know whether the enclosed plan of experimenting represents the full and proper way of going to work. I know that you do not believe in the object of it, but, even supposing it to be a wild goose chase, there would be no harm in your telling me the best way to run. Then, whether the results prove positive or negative, it will not be open for any one to doubt them on the ground of any fault in the method. Do any objections occur to you _re_ my answer to critics in the 'Nineteenth Century'? Of course I might have said more about the swamping effects of free intercrossing (which appears to me the only point in which I deviate at all from the 'Origin of Species'), but it is much too large a subject to be dealt with in a review. My greatest difficulty here is to conceive the possibility of differentiation (as distinguished from transmutation in linear series) without the assistance of isolation in some form or another. Yours very truly, GEO. J. ROMANES. Dear Darwin,--Criticism of an intelligent kind is what I feel most in need of, and therefore it is no merit on my part to like it when it comes. The point about the combined action of natural and physiological selection is, after all, a very subordinate one, and, as I said in 'Nature' some weeks ago, is the most highly speculative and least trustworthy part of the theory. Moreover, it is the only part that is directly opposed to an expressed conclusion in the 'Origin,' though, even here, the opposition is not real. If natural selection can do anything at all in the way of bringing about sterility with parent forms, it can only do so by acting on the type or whole community (for I quite agree with the reasoning in the 'Origin,' that it cannot do so by acting on individuals); and whether natural selection could in any case act on a type is a question which your father has told me he could never quite make up his mind about, except in the case of social hymenoptera and moral sense of man.