My dear Sir,

I have to thank you very much for kindly ordering a copy of your new work to be sent to me, and which I have just received.² There is so much matter in it, that it will take a long while to digest; but I think from a few parts which I have read, that my conclusions—I gave in “Nature”—are really in harmony with yours.³ I have paid no attention to Conspicuous flowers, so that my term “self-fertilisation” had sole reference to small inconspicuous flowers; and as far as they are concerned your book certainly seems to corroborate me. I was hoping to find you had experimented on such flowers but you confess not to have done so. I mean to try next year to see what I can do.

In your list of plants fertile without insect-aid pp. 365–369 are included a few of those which I have noticed as being adapted for self-impregnation; & of which I have detected the pollen-tubes: Thus. Fumaria off^s: In this, the 3 Anther-cells of each group form a neat little 3-sided box into which the “horn”-like stigmas are shut, so that the pollen-tubes pour into it while the grains are still in situ. From this I inferred (& you have proved) that it is self-fertilising.⁴ I have also detected the pollen-tubes in Trifolium procumbens & minus.⁵ Nicotiana Tabacum as growing in the Kew Gardens seemed to be proterandrous, but N. rusticum with smaller, greenish flowers was certainly fertilising itself.⁶ I have also noticed the pollen tubes in several small-fl^d. sp. of Veronica. With regard to sp: of Solanum (Note p. 387) S. nigrum I take to be decidedly self-f^g. for the anther cells begin by pores, but slit all the way down & so the pollen fall easily on the stigma, the flower being pendulous.⁷

The conclusion I drew was that these self-fertilising inconspicuous flowers, had acquired the habit—or rather regained it, in consequence of the reduction of the corolla or by suppression of nectariferous affairs or both: for I found (as Bennett had also, though I was not aware of his observations at the time) that as the weather grows colder Two Things followed; (1) buds have a tendency to remain closed (2) the corollas are dwarfed.⁸ The former is seen well, e.g. in Spergula arvensis⁹ which goes on “budding” all through the winter but never “blossoming” & yet produces an abundance of seed:— I therefore greatly rejoiced when I came across the foot-note to p. 420—where you say:—“I am greatly surprized &c for you include Viola tricolor (p. 358) in your list of “Plants sterile without insect aid” Now, these cases of Delphinium and Viola anticipate what I had determined to experimentize & have already asked friends to help me:—viz. to see if flowers which are usually proterandrous will not become self-fertilising when their corollas are early removed; so far then, these cases quite corroborrate my deductions!¹⁰

On p. 385. you speak of the reduction of the size of the corollas of some papilionacae species:— the Cause, judging by analogy, I take to be change of climate; for the reduction of temperature seems at once to affect & reduce the size of the corolla or perianth. A case was mentioned a few weeks ago in the Gardner’s Chronicle where a Lapageria had a branch which had somehow escaped into the air—the blossoms on which only differed “in being smaller”.¹¹ Similarly I have noticed the corollas of conspicuous flowers get reduced in the autumn; but it would seem that the sexual processes are in no way interfered with. For example, I found last September & October several plants of Tradescantia erecta,¹² at Kew which earlier in the season produced conspicuous flowers, but in which the corollas were completely arrested, shriveled: the styles were consequently bent down on the side of the ovary & the stigma thus kept in contact with the anthers. Not a bud expanded, though they were very numerous, yet all set seed: & I have preserved about 100 for next year’s sowing   the embryos are perfectly well formed!

You say you can hear of no instance of a species with all its flowers rendered permanently cleistogamous:¹³ Perhaps the two following may prove to be so: Cerastium glomeratum and Corrigiola littoralis¹⁴ Because, while Spergula arvensis keeps its buds shut in cold weather, yet expands them on a warm sunny day, yet I have found Cer: glom: perfectly closed on a broiling hot day last June in an open hayfield. It grew abundantly, but not a single bud was expanded yet seed was shed in profusion. Cor: litt: was quite closed last Sept^r at Kew: whether it opens on hot days I can’t say.—

The fact that so many Conspicuous flowers are self-fertilising, but benefitted by crossing; and that to be absolutely sterile with its own pollen is a fact decidedly in the minority, also seems to corroborrate my view that self-fertilisation was the primordial condition of flowering plants; but that in proportion as the corollas or perianths became conspicuous & insect adaptation set in, sexual differentiations were set up & self-fertilisation correspondingly “went to the wall”; but as species spread & got into colder regions than their ancestors: the corollas were checked: & self-fertilisation became again the rule for such inconspicuous flowers:

But I must not bore you with any more of my notions:

Again thanking you, | I beg to remain | Y^rs very truly | Geo: Henslow

Ch. Darwin Esq F.R.S.