My dear D^r. Gray

Your last letter, like all its predecessors has been very valuable to me; & every word in it has interested me.² When I said that your remarks on your alpine plants “riled” me; I did not mean to doubt them, except in the Agassian sense that they went against some theoretic notions of mine. These notions are too long to give & indeed not worth giving, as far as America is concerned, & I can see from your letter that we sh^d. take very much the same view. I am very glad to hear that you think of discussing the relative ranges of the identical & allied U. States & European species, when you have time. Now this leads me to make a very audacious remark in opposition to what I imagine Hooker has been writing & to your own scientific conscience. I presume he has been urging you to finish your great Flora, before you do anything else. Now I would say it is your duty to generalise as far as you safely can from your as yet completed work. Undoubtedly careful discrimination of species is the foundation of all good work; but I must look at such papers as yours in Silliman as the fruit.³ As careful observation is far harder work than generalisation & still harder than speculation; do you not think it very possible that it may be overvalued? It ought never to be forgotten that the observer can generalise his own observations incomparably better than anyone else.

How many astronomers have laboured their whole lives on observations & have not drawn a single conclusion; I think it is Herschel who has remarked how much better it would be, if they had paused in their devoted work & seen what they could have deduced from their work.⁴ So do pray look at this side of question, & let us have another paper or two like the last admirable ones. There, am I not an audacious dog!

You ask about my doctrine which led me to expect that Trees would tend to have separate sexes.⁵ I am inclined to believe that no organic being exists which perpetually self-fertilises itself. This will appear very wild, but I can venture to say that if you were to read all my observations on this subject, you would agree it is not so wild as it will at first appear to you, from flowers said to be always fertilised in bud &c &c &c.— It is a long subject to which I have attended to for 18 years! Now it occurred to me that in a large tree with hermaphrodite flowers, we will say it w^d be ten to one that it would be fertilised by the pollen of its own flower, & a thousand or ten-thousand to one that if crossed, it would be crossed only with pollen from another flower of same tree, which would be opposed to my doctrine. Therefore on the great principle of “Nature not lying”⁶ I fully expected that trees would be apt to be dioicous or monoicous (which as pollen has to be carried from flower to flower everytime, would favour a cross from another individual of the same species) & so it seems to be in Britain & N. Zealand. Nor can the fact be explained by certain families having this structure & chancing to be trees, for the rule seems to hold both in genera & families, as well as in species.

I give you full permission to laugh your fill at this wild speculation; & I do not pretend but what it may be chance which, in this case, has led me apparently right. But I repeat that I feel sure that my doctrine has more probability, than at first it appears to have. If you had not asked, I sh^d not have written at such length, though I cannot give any of my reasons.

The Leguminosæ are my greatest opposers; yet if I were to trust to observations on insects made during many years, I sh^d. fully expect crosses to take place in them; but I cannot find that our garden varieties ever cross each other. I do not ask you to take any trouble about it, but if you should by chance come across any intelligent nurserymen, I wish you w^d. enquire whether they take any pains in raising the vars. of papilionaceous plants apart to prevent crossing. (I have seen statement of naturally formed crossed Phaseoli near N. York) The worst is that nurserymen are apt to attribute all variations to crossing.—

Finally I incline to believe that every living being requires an occasional cross with distinct individual; & as trees from mere multitude of flowers offer obstacle to this, I suspect this obstacle is counteracted by tendency to have sexes separated. But I have forgotten to say that my maximum difficulty is trees having papilionaceous flowers: some of these, I know, have their Keel-petals expanded when ready for fertilisation; but Bentham does not believe that this is general:⁷ nevertheless on principe of nature not lying I suspect that this will turn out so, or that they are eminently sought by Bees dusted with pollen. Again I do not ask you to take trouble, but if strolling under your Robinias when in full flower, just look at stamens & pistils whether protruded & whether Bees visit them.— I must just mention a fact mentioned to me the other day by Sir W. Macarthur, a clever Australian gardener, viz how odd it was that his Erythrinas in N. S. Wales would not set a seed, without he imitated the movements of the petals which Bees cause.—⁸ Well, as long as you live, you will never after this fearfully long note ask me why I believe this or that.

I am particularly obliged for your information about Protean genera; as this is one of the greatest of my many great puzzles; viz to know or conjecture whether the great variability of such genera is due to their conditions of existence, or whether it is apt to be innate in them at all times & places (I am aware that this cannot be strictly predicated of any genus, for all have some fixed species).— Now I have thought that you would not object to my sending the latter half of your note with list of such American genera to M^r. H. C. Watson, of whose great clearness of mind & acuteness I have from long correspondence the highest opinion.— I have sent his note (1)⁹ & Essay (2)¹⁰ for the chance of your liking to see them; but if more busy even than usual, do not read them; & I daresay in course of half year, you will be able to return them together with your own list; but pray do not lose them. There is only one point, to which for myself, I wish to call your attention, viz whether you rightly understood that my question did not refer to genera having very close species, but to genera having very variable species. Watson thought that you might not have understood me.— If you do read Watsons papers & have anything to remark on subject, I need not say, how very much I sh^d. like to hear it.¹¹ I am sure I do not know whether I have acted wisely in sending Watsons letters, but I repeat again pray do not read them, without you feel inclined: as, I suppose, in course of few months you would be sending some parcel to England, it will not cost you trouble to return them.

With hearty thanks for all your kindness, & begging forgiveness for length of this letter, which is chiefly your own fault (as far as trees are concerned), believe me, Your’s most sincerely | Ch. Darwin