To J. D. Hooker   19 July [1856]¹

My dear Hooker

I thank you warmly for the very kind manner with which you have taken my request.² It will in truth be a most important service to me; for it is absolutely necessary that I sh^d. discuss single & double creations, as a very crucial point on the general origin of species, & I must confess, with the aid of all sorts of visionary hypotheses, a very hostile one.—

I am delighted that you will take up possibility of crossing;³ no Botanist has done so which I have long regretted, & I was glad to see that it was one of A. Decandolles desiderata.⁴ By the way he is curiously contradictory on subject.— I am far from expecting that no cases of apparent impossibility will be found; but certainly I expect that ultimately they will disappear; for instance Campanulaceæ seemed a strong case, but now it is pretty clear that they must be liable to crossing. Sweet Peas—Bee-orchis, & perhaps Hollyocks are, at present, my greatest difficulties; & I find I cannot experimentise by castrating Sweet Peas, without doing fatal injury. Formerly I felt most interest on this point as one chief means of eliminating varieties; but I feel interest now in other ways.—⁵

One general fact makes me believe in my doctrine, is that no terrestrial animal in which semen is liquid is hermaphrodite except with mutual copulation: in terrestrial plants in which the semen is dry there are many hermaphrodites.— Indeed I do wish I lived at Kew or at least so that I could see you oftener.

If you were to take up crossing, I w^d. look over my notes, which perhaps w^d. guide you to some of the most difficult cases.⁶

To return again to subject of crossing; I have been inclined to speculate so far, as to think (my!?) notion (I say my notion, but I think others have put forward nearly or quite similar ideas) perhaps explains the frequent separation of the sexes in Trees, which I think I have heard remarked, (& in looking over the mono- & diœcious Linnean classes in Persoon seems true⁷ ) are very apt to have sexes separated; for a tree having a vast number of flowers on same individual or at least same stock, each flower if only hermaphrodite on common plan would generally get its own pollen or only pollen from another flower on same stock,—whereas if sexes were separate there would be better chance of occasional pollen from another distinct stock. I have thought of testing this in your New Zealand Flora, but I have no standard of comparison & I found myself bothered by bushes— I sh^d. propound that some unknown causes had favoured development of trees & bushes in New Zealand, & consequent on this, there had been a development of separation of sexes to prevent too much intermarriage— I do not of course suppose the prevention of too much intermarriage the only good of separation of sexes.— But such wild notions are not worth troubling you with the reading of.—

With respect to Ægilops I subscribe to your very just & new to me remarks; I did not know Æ. triticoides was rare in wild state: what I remarked on was in relation to the French objectors.⁸

With respect to the Pringlea I am ready to admit any theory whatever; & generally I w^d. observe that I would admit a continental extension in any few cases, when the facts required it more than in the generality of cases, but it seems to me that you will have to admit continental extensions to every island whatever, & that I cannot swallow. Indeed even one continental extension is an awful gulp to me. I never made a continent for my Coral Reefs.— But how I am running on: it is the greatest temptation to me to write ad infinitum to you.—

I have been much tempted by your invitation for Friday & sh^d. like it extremely, but I have been having a bad 3 weeks & do not think I could stand the fatigue; but I sh^d. have enjoyed such a party extremely.—

I am very sorry that so much of your time sh^d. be taken up with thy young Doctors.⁹

My dear Hooker | Yours affectly | C. Darwin

My poor wife keeps as wretched as ever, but now with general oppression & not nausea.—