To Alphonse de Candolle 17 June 
Down. | Bromley. | Kent. S.E.
My dear Sir
I am extremely much obliged for your kind & very interesting letter.1 I am pleased that you are interested by the Primula case. Your questions & remarks show that you have gone to the root of the matter. I am now trying various analogous experiments on several plants & on the seedlings raised from the so-called heteromorphic & homomorphic unions; & the results (as far as I have yet seen; for the capsules are gathered, but not yet examined) are interesting; Whenever I publish I will do myself the pleasure of sending you a copy.2 I am particularly obliged for your information on Alkanna. I have examined the small imperfect flowers of Viola & Oxalis: the case is very different both functionally & structurally from that of Primula.—3
You kindly enquire about my larger work; it does make progress, but very slowly owing to my own weak health & ill-health in my family.4 I have, also, been seduced to publish a small work on the Fertilisation of Orchids, which has taken up nearly ten months. As Mr Bentham & Asa Gray think well of this Book, I have sent by this post a copy for you.5 One main object has been to show how wonderfully perfect the structure of plants is; another regards close breeding in & in, to which I see you have attended.— I am not at all surprised that you are not willing to admit natural selection: the subject hardly admits of direct proof or evidence.6 It will be believed in only by those who think that it connects & partly explains several large classes of facts: in the same way opticians admit the undulatory theory of light, though no one can prove the existence of ether or its undulations.—
I hope you will publish on Quercus, & I shall be most grateful for a copy; the genus has long appeared to me preeminently interesting under the point of view to which you refer.7 I am, also, rejoiced to hear that you have the intention of again returning to Geographical Distribn. I believe few, or no one, can have read your truly great work with more care than I have;8 & no one can feel more respect & admiration for it & its author.—
Pray believe me, my dear Sir | Yours sincerely & respectfully | Ch. Darwin
Is pleased that AdeC is interested in the Primula case ["Dimorphic condition of Primula", Collected papers 2: 45–63]. Is pursuing analogous experiments on other plants and on seedlings raised from the unions.
CD’s "large work" progresses slowly owing to ill health and his work on Orchids.
CD is not surprised that AdeC is unwilling to admit natural selection – "the subject hardly admits of direct proof or evidence. It will be believed in only by those who think that it connects & partly explains several large classes of facts".
Hopes AdeC will publish on Quercus
and rejoices that he intends to return to the study of geographical distribution. No one can claim to have read AdeC’s truly great work on that subject [Géographie botanique (1855)] with more care than CD.