_4 Chester Place, Regent's Park, N.W. March 19-24, 1868._ My dear Wallace,--I have sent your query to Cambridge to my son. He ought to answer it, for he got his place of Second Wrangler chiefly by solving very difficult problems. I enclose his remarks on two of your paragraphs: I should like them returned some time, for I have not studied them, and let me have your impression. I have told E. Edwards to send one of my large photographs to you addressed to 76-1/2 Westbourne Grove, not to be forwarded. When at home I will send my carte. The sterility is a most [? puzzling] problem. I can see so far, but I am hardly willing to admit all your assumptions, and even if they were all admitted, the process is so complex and the sterility (as you remark in your note) so universal, even with species inhabiting quite distinct countries (as I remarked in my chapter), together with the frequency of a difference in reciprocal unions, that I cannot persuade myself that it has been gained by Natural Selection, any more than the difficulty of grafting distinct genera and the impossibility of grafting distinct families. You will allow, I suppose, that the capacity of grafting has not been directly acquired through Natural Selection. I think that you will be pleased with the second volume or part of Lyell's Principles, just out. In regard to sexual selection. A girl sees a handsome man, and without observing whether his nose or whiskers are the tenth of an inch longer or shorter than in some other man, admires his appearance and says she will marry him. So, I suppose, with the pea-hen; and the tail has been increased in length merely by, on the whole, presenting a more gorgeous appearance. Jenner Weir, however, has given me some facts showing that birds apparently admire details of plumage.--Yours most sincerely, C. DARWIN. * * * * * _Hurstpierpoint. March 24, [1868?]._ Dear Darwin,--Many thanks for the photo, which I shall get when I go to town. I return your son's notes with my notes on them. Without going into any details, is not this a strong general argument?-- 1. A species varies occasionally in two directions, but owing to their free intercrossing they (the variations) never increase. 2. A change of conditions occurs which threatens the existence of the species, but the _two varieties_ are adapted to the changing conditions, and, if accumulated, will form two new _species adapted to the new conditions_. 3. Free crossing, however, renders this impossible, and so the species is in danger of extinction. 4. If _sterility_ could be induced, then the pure races would increase more rapidly and replace the old species. 5. It is admitted that _partial sterility_ between _varieties_ does occasionally occur. It is admitted the _degree_ of this sterility _varies_. Is it not probable that Natural Selection can accumulate these variations and thus save the species?