My dear Wallace

I have been considering the terrible problem.— Let me first say that no man could have more earnestly wished for the success of N. selection in regard to sterility, than I did; & when I considered a general statement, (as in your last note) I always felt sure it could be worked out, but always failed in detail.² The cause being as I believe, that natural selection cannot effect what is not good for the individual, including in this term a social community.— It w^d take a volume to discuss all the points; & nothing is so humiliating to me as to agree with a man like you (or Hooker)³ on the premises & disagree about the result—

I agree with my son’s argument & not with rejoinder.⁴ The cause of our difference, I think, is, that I look at the number of offspring as an important element (all circumstances remaining the same) in keeping up the average number of individuals within any area.— I do not believe that the amount of food by any means is the sole determining cause of number. Lessened fertility is equivalent to a new source of destruction. I believe if in one district a species produced from any cause fewer young the deficiency w^d be supplied from surrounding districts— This applies to your Par. (5).⁵ If the species produced fewer young from any cause in every district, it w^d. become extinct unless its fertility were augmented through natural selection. (see H. Spencer)⁶

I demur to probability & almost to possibility of Par. (1), as you start with two forms, within the same area, which are not mutually sterile, & which yet have supplanted the parent-form.—

(Paragraph 6.) I know of no ghost of a fact supporting belief that disinclination to cross accompanies sterility. It cannot hold with plants, or the lower fixed aquatic animals. I saw clearly what an immense aid this would be, but gave it up. Disinclination to cross seems to have been independently acquired probably by nat. selection; & I do not see why it would not have sufficed to have prevented incipient species from blending to have simply increased sexual disinclination to cross.—

(Par. 11) I demur to a certain extent to amount of sterility & structural dissimilarity necessarily going together, except indirectly & by no means strictly.— Look at vars. of Pigeons, Fowls & Cabbages.—

I overlooked advantage of the half-sterility of reciprocal crosses; yet, perhaps from novelty, I do not feel inclined to admit probability of nat. selection having done its work so queerly.—

I will not discuss second case of utter sterility; but your assumptions in Par. 13 seem to me much too complicated. I cannot believe so universal an attribute as utter sterility between remote species, was acquired in so complex a manner. I do not agree with your rejoinder on grafting: I fully admit that it is not so closely restricted as crossing; but this does not seem to me to weaken the case as one of analogy. The incapacity of grafting is likewise an invariable attribute of plants sufficiently remote from each other, & sometimes of plants pretty closely allied.—⁷

The difficulty of increasing the sterility through nat. selection of two already sterile species seems to me best brought home by considering an actual case. The cowslip & Primrose are moderately sterile, yet occasionally produce hybrids: now, these hybrids, two or three or a dozen in a whole Parish, occupy ground, which might have been occupied by either pure species, & no doubt the latter suffer to this small extent. But can you conceive that any individual plants of the Primrose & cowslip, which happened to be mutually rather more sterile (i.e. which when crossed yielded a few less seed) than usual, would profit to such a degree as to increase in number to the ultimate exclusion of the present primrose & cowslip.— I cannot.—

My son, I am sorry to say, cannot see full force of your rejoinder, in regard to second head of continually augmented sterility. You speak in this rejoinder & in Par. (5) of all the individuals becoming in some slight degree sterile in certain districts; if you were to admit that by continued exposure to these same conditions the sterility would inevitably increase there would no need of Nat. Selection. But I suspect that the sterility is not caused so much by any particular conditions, as by long habituation to conditions of any kind. To speak according to Pangenesis the gemmules of Hybrids are not injured, for Hybrids propagate freely by buds; but their reproductive organs are somehow affected so that they cannot accumulate the proper gemmules, in nearly same manner as the reproductive organs of a pure species become affected when exposed to unnatural conditions.⁸

This is a very ill-expressed & ill-written letter— Do not answer it, unless the spirit urges you— Life is too short for so long a discussion— We shall, I greatly fear, never agree.— My dear Wallace | Most sincerely yours | Ch. Darwin