I have been led lately from experiments (not published) on dimorphism to reflect much on sterility from hybridism, and partially to change the opinion given in "Origin." I have now letters out enquiring on the following point, implied in the experiment, which seems to me well worth trying, but too laborious ever to be attempted. I would ask every pigeon and fowl fancier whether they have ever observed, in the same breed, a cock A paired to a hen B which did not produce young. Then I would get cock A and match it to a hen of its nearest blood; and hen B to its nearest blood. I would then match the offspring of A (viz., a, b, c, d, e) to the offspring of B (viz., f, g, h, i, j), and all those children which were fertile together should be destroyed until I found one--say a, which was not quite fertile with--say, i. Then a and i should be preserved and paired with their parents A and B, so as to try and get two families which would not unite together; but the members WITHIN each family being fertile together. This would probably be quite hopeless; but he who could effect this would, I believe, solve the problem of sterility from hybridism. If you should ever hear of individual fowls or pigeons which are sterile together, I should be very grateful to hear of the case. It is a parallel case to those recorded of a man not impotent long living with a woman who remained childless; the husband died, and the woman married again and had plenty of children. Apparently (by no means certainly) this first man and woman were dissimilar in their sexual organisation. I conceive it possible that their offspring (if both had married again and both had children) would be sexually dissimilar, like their parents, or sterile together. Pray forgive my dreadful writing; I have been very unwell all day, and have no strength to re-write this scrawl. I am working slowly on, and I suppose in three or four months shall be ready. I am sure I do not know whether any human being could understand or read this shameful scrawl. LETTER 154. TO T.H. HUXLEY. Down, December, 28th [1862].