To J. D. Hooker   [after 26] November [1862]¹

My dear Hooker

Your last letter has interested me to an extraordinary degree, & your truly parsonic advice “some other wise & discreet person” &c, amused us not a little.—² I will put a “concrete” case to show what I think A. Gray believes about crossing & what I believe.—³ If 1000 pigeons were bred together in cage for ten 1000 years, their number not being allowed to increase by chance killing, then from mutual intercrossing no varieties would arise; but if each pigeon were a self-fertilising hermaphrodite a multitude of varieties would arise. This I believe is common effect of crossing, viz the obliteration of incipient varieties.⁴ I do not deny that when two marked varieties have been produced; their crossing will produce a third or more intermediate varieties. Possibly or probably with domestic varieties, with strong tendency to vary, the act of crossing tends to give rise to new characters; & thus a third or more races, not strictly intermediate, may be produced. But there is heavy evidence against new characters arising from crossing wild forms; only intermediate races are then produced.— Now do you agree thus far? if not, it is no use arguing, we must come to swearing, & I am convinced I can swear harder than you. [THEREFORE] I am right. Q.E.D.—

If the number of 1000 pigeons were prevented increasing, not by chance killing, but by, say, all the shorter-beaked birds being killed, then the whole body would come to have longer beaks. Do you agree?

Thirdly, if 1000 pigeons were kept in hot country, & another 1000 in cold country, & fed on different food & confined in different size aviary & kept constant in number by chance killing, then I sh^d. expect as rather probable that after ten 1000 years, the two bodies would differ slighty in size, colour & perhaps other trifling characters. This I sh^d. call the direct action of physical conditions.⁵ By this action I wish to imply that the innate vital forces are somehow led to act rather differently in the two cases. Just as heat will allow or cause two elements to combine, which otherwise would not have combined.— I sh^d. be especially obliged if you would tell me what you think on this head.—⁶

But the part of your letter which fairly pitched me head over heels with astonishment; is that where you state that every single difference which we see might have occurred without any selection. I do & have always fully agreed; but you have got right round the subject & viewed it from an entirely opposite & new side, & when you took me there, I was astounded. When I say I agree, I must make proviso, that under your view, as now, each form long remains adapted to certain fixed conditions & that the conditions of life are in long run changeable; & 2^d, which is more important that each individual form is a self-fertilising hermaphrodite, so that each hair-breadth variation is not lost by intercrossing. Your manner of putting case would be even more striking than it is, if the mind could grapple with such numbers— it is grappling with eternity— think of each of a thousand seeds bringing forth its plant, & then each a thousand. A globe stretching to furthest fixed star would very soon be covered. I cannot even grapple with idea even with races of dogs, cattle, pigeons or fowls; & here all must admit & see the accurate strictness of your illustration.—

Such men, as you & Lyell thinking that I make too much of a Deus of N. Selection is conclusive against me.—⁷ Yet I hardly know how I could have put in, in all parts of my Book, stronger sentences. The title, as you once pointed out, might have been better.⁸ No one ever objects to agriculturalists using the strongest language about their selection; yet every breeder knows that he does not produce the modification which he selects. My enormous difficulty for years was to understand adaptation, & this made me, I cannot but think rightly, insist so much on N. Selection. God forgive me for writing at such length; but you cannot tell how much your letter has interested me, & how important it is for me with my present Book in hand to try & get clear ideas.⁹ Do think a bit about what is meant by direct action of physical conditions. I do not mean whether they act; my facts will throw some light on this. I am collecting all cases of “bud-variations in contradistinction to ‘seed-variation’” (do you like this term for what some gardeners call “sports”):¹⁰ these eliminate all effect of crossing.— Pray remember how much I value your opinion, as the clearest & most original I ever get.—

Very sincere thanks to you & Oliver for the Books—: Planchon has been very useful:¹¹ they shall be all returned by Rail the first day, probably Tuesday, on which I send to Bromley.—

I see plainly Wellwitschia will be a case of Barnacles.—¹²

Please do not send Oxalis sensitiva.¹³

I have another plant to beg, but I write on separate paper, as more convenient for you to keep.¹⁴ I meant to have said before, as excuse for asking for so much from Kew; that I have now lost two seasons, by accursed nurserymen not having right plants, & sending me the wrong instead of saying that they did not possess.—

Ever yours | My dear Hooker | C. Darwin