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Letter 2136

Darwin, C. R. to Gray, Asa

5 Sept [1857]

    Summary Add

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    Encloses an abstract of his ideas on natural selection and the principle of divergence; the "means by which nature makes her species".

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    Discusses varieties and close species in large and small genera, finding some data from AG in conflict with his expectations.

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    Has been observing the action of bees in fertilising kidney beans and Lobelia.


Down Bromley Kent

Sept. 5th

My dear Gray

I forget the exact words which I used in my former letter, but I daresay I said that I thought you would utterly despise me, when I told you what views I had arrived at, which I did because I thought I was bound as an honest man to do so.

I shd have been a strange mortal, seeing how much I owe to your quite extraordinary kindness, if in saying this I had meant to attribute the least bad feeling to you. Permit me to tell you, that before I had ever corresponded with you, Hooker had shown me several of your letters (not of a private nature) & these gave me the warmest feeling of respect to you; & I shd indeed be ungrateful if your letters to me & all I have heard of you, had not strongly enhanced this feeling. But I did not feel in the least sure that when you knew whither I was tending, that you might not think me so wild & foolish in my views (God knows arrived at slowly enough, & I hope conscientiously) that you would think me worth no more notice or assistance. To give one example, the last time I saw my dear old friend Falconer, he attacked me most vigorously, but quite kindly, & told me “you will do more harm than any ten naturalists will do good”— “I can see that you have already corrupted & half-spoiled Hooker”(!!). Now when I see such strong feeling in my oldest friends, you need not wonder that I always expect my views to be received with contempt. But enough & too much of this.—

I thank you most truly for the kind spirit of your last letter. I agree to every word in it; & think I go as far as almost anyone in seeing the grave difficulties against my doctrine. With respect to the extent to which I go, all arguments in favour of my notions fall rapidly away the greater the scope of forms considered. But in animals, embryology leads me to an enormous & frightful range. The facts which kept me longest scientifically orthodox are those of adaptation—the pollen-masses in Asclepias —the misseltoe with its pollen carried by insects & seed by Birds the woodpecker with its feet & tail beak & tongue to climb trees & secure insects. To talk of climate or Lamarckian habit producing such adaptations to other organic beings is futile. This difficulty, I believe I have surmounted. As you seem interested in subject, & as it is an immense advantage to me to write to you & to hear ever so briefly, what you think, I will enclose (copied so as to save you trouble in reading) the briefest abstract of my notions on the means by which nature makes her species. Why I think that species have really changed depends on general facts in the affinities, embryology, rudimentary organs, geological history & geographical distribution of organic beings. In regard to my abstract you must take immensely on trust; each paragraph occupying one or two chapters in my Book. You will, perhaps, think it paltry in me, when I ask you not to mention my doctrine; the reason is, if anyone, like the Author of the Vestiges, were to hear of them, he might easily work them in, & then I shd have to quote from a work perhaps despised by naturalists & this would greatly injure any chance of my views being received by those alone whose opinion I value.—

I have been lately at work on a point which interests me much; namely dividing the species of several Floras into two as nearly as equal cohorts as possible—one with all those forming large genera, & the other with the small genera. Thus in your U. States Flora, I make (with omissions of naturalised & of a few protean genera & Carex from its unusual size) 1005 sp. in genera of 5 & upwards, & 917 in genera with 4 & downwards; & the large genera have 881000 varieties & the small genera only 501000. This rule seems to be general. & Hooker is going to work out some Floras on same plan.— But to my disgust your vars. marked by big-type are only in proportion 481000 to 461000.

Several things have made me confidently believe that “close” species occurred most frequently in the larger genera; & you may remember that you made me the enclosed list. Now to my utter disgust, I find that the case is somewhat the reverse of what I had so confidently expected, the close species hugging the smaller genera. Hence I have enclosed the list. & beg you kindly to run your eye over it, & see whether, not understanding my motive, you cd have attended more to the small than to the large genera: but I can see that this is not probable. And do not think that I want you to “cook” the results for me.— Are the close species very generally geographical representative species: this might make some difference?

Lately I examined buds of Kidney Bean with pollen shed, but I was led to believe that the pollen cd hardly get on stigma by wind or otherwise, except by Bees visiting & moving the wing petals: hence I included a small bunch of flowers in two Bottles, in everyway treated the same: the flowers in one I daily just momentarily moved as if by a Bee; these set 3 fine pods, the other not one. Of course this little experiment must be tried again, & this year in England it is too late, as the flowers seem now seldom to set. If Bees are necessary to this flower's self-fertilisation, Bees must almost cross them, as their dusted right-side of head & right legs constantly touch the stigma.

I have, also, lately been reobserving daily Lobelia fulgens— this in my garden is never visited by insects & never sets seeds, without pollen be put on stigma. (whereas the small blue Lobelia is visited by Bees & does set seed); I mention this because these are such beautiful contrivances to prevent the stigma ever getting its own pollen; which seems only explicable on the doctrine of the advantage of crosses.

I forget whether I ever said I had received safely Mr Watson's papers. & your Lesson in Botany, for which very many thanks & which I am now reading. But I have never had the last part of your paper on Naturalised Plants. If you have a spare copy (which is not likely) I shd be very glad of it: otherwise I will borrow Hooker's. I ought to feel ashamed of the length of this letter, knowing how busy you are.

My dear Dr Gray | Believe me with much | sincerity Your's truly | C. Darwin

I will try if I can anyhow get seed of the Adlumia cirrhosa & observe it next summer. Perhaps they have it at Kew.


I. It is wonderful what the principle of Selection by Man, that is the picking out of individuals with any desired quality, and breeding from them, and again picking out, can do. Even Breeders have been astonished at their own results. They can act on differences inappreciable to an uneducated eye. Selection has been methodically followed in Europe for only the last half century. But it has occasionally, and even in some degree methodically, been followed in the most ancient times. There must have been, also, a kind of unconscious selection from the most ancient times,—namely in the preservation of the individual animals (without any thought of their offspring) most useful to each race of man in his particular circumstances. The “rogueing” as nurserymen call the destroying of varieties, which depart from their type, is a kind of selection. I am convinced that intentional and occasional selection has been the main agent in making our domestic races. But, however, this may be, its great power of modification has been indisputably shown in late times. Selection acts only by the accumulation of very slight or greater variations, caused by external conditions, or by the mere fact that in generation the child is not absolutely similar to its parent. Man by this power of accumulating variations adapts living beings to his wants,—he may be said to make the wool of one sheep good for carpets and another for cloth &c.—

II. Now suppose there was a being, who did not judge by mere external appearance, but could study the whole internal organization— who never was capricious,—who should go on selecting for one end during millions of generations, who will say what he might not effect! In nature we have some slight variations, occasionally in all parts: and I think it can be shown that a change in the conditions of existence is the main cause of the child not exactly resembling its parents; and in nature geology shows us what changes have taken place, and are taking place. We have almost unlimited time: no one but a practical geologist can fully appreciate this: think of the Glacial period, during the whole of which the same species of shells at least have existed; there must have been during this period millions on millions of generations.

III. I think it can be shown that there is such an unerring power at work, or Natural Selection (the title of my Book), which selects exclusively for the good of each organic being. The elder De Candolle, W. Herbert, and Lyell have written strongly on the struggle for life; but even they have not written strongly enough. Reflect that every being (even the Elephant) breeds at such a rate, that in a few years, or at most a few centuries or thousands of years, the surface of the earth would not hold the progeny of any one species. I have found it hard constantly to bear in mind that the increase of every single species is checked during some part of its life, or during some shortly recurrent generation. Only a few of those annually born can live to propagate their kind. What a trifling difference must often determine which shall survive and which perish—

IV. Now take the case of a country undergoing some change; this will tend to cause some of its inhabitants to vary slightly; not but what I believe most beings vary at all times enough for selection to act on. Some of its inhabitants will be exterminated, and the remainder will be exposed to the mutual action of a different set of inhabitants, which I believe to be more important to the life of each being than mere climate. Considering the infinitely various ways, beings have to obtain food by struggling with other beings, to escape danger at various times of life, to have their eggs or seeds disseminated &c. &c, I cannot doubt that during millions of generations individuals of a species will be born with some slight variation profitable to some part of its economy; such will have a better chance of surviving, propagating, this variation, which again will be slowly increased by the accumulative action of Natural selection; and the variety thus formed will either coexist with, or more commonly will exterminate its parent form. An organic being like the woodpecker or misletoe may thus come to be adapted to a score of contingencies: natural selection, accumulating those slight variations in all parts of its structure which are in any way useful to it, during any part of its life.

V. Multiform difficulties will occur to everyone on this theory. Most can I think be satisfactorily answered.— “Natura non facit saltum” answers some of the most obvious.— The slowness of the change, and only a very few undergoing change at any one time answers others. The extreme imperfections of our geological records answers others.—

VI. One other principle, which may be called the principle of divergence plays, I believe, an important part in the origin of species. The same spot will support more life if occupied by very diverse forms: we see this in the many generic forms in a square yard of turf (I have counted 20 species belonging to 18 genera),—or in the plants and insects, on any little uniform islet, belonging almost to as many genera and families as to species.— We can understand this with the higher, animals whose habits we best understand. We know that it has been experimentally shown that a plot of land will yield a greater weight, if cropped with several species of grasses than with 2 or 3 species. Now every single organic being, by propagating so rapidly, may be said to be striving its utmost to increase in numbers. So it will be with the offspring of any species after it has broken into varieties or sub-species or true species. And it follows, I think, from the foregoing facts that the varying offspring of each species will try (only few will succeed) to seize on as many and as diverse places in the economy of nature, as possible. Each new variety or species, when formed will generally take the places of and so exterminate its less well-fitted parent. This, I believe, to be the origin of the classification or arrangement of all organic beings at all times. These always seem to branch and sub-branch like a tree from a common trunk; the flourishing twigs destroying the less vigorous,—the dead and lost branches rudely representing extinct genera and families.

This sketch is most imperfect; but in so short a space I cannot make it better. Your imagination must fill up many wide blanks.— Without some reflexion it will appear all rubbish; perhaps it will appear so after reflexion.— | C. D.

This little abstract touches only on the accumulative power of natural selection, which I look at as by far the most important element in the production of new forms. The laws governing the incipient or primordial variation (unimportant except as to groundwork for selection to act on, in which respect it is all important) I shall discuss under several heads, but I can come, as you may well believe, only to very partial & imperfect conclusions.—

    Footnotes Add

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    f1 2136.f1
    The year is established by the relationship to the letter to Asa Gray, 20 July [1857], and by the reference to CD's experiments on controlling fertilisation in kidney beans (see n. 12, below).
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    f2 2136.f2
    See letter to Asa Gray, 20 July [1857].
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    f3 2136.f3
    Hugh Falconer.
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    f4 2136.f4
    Letter from Asa Gray, [August 1857].
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    f5 2136.f5
    In his chapter on the possibility of all organic beings crossing, CD described the ‘beautiful adaptation’ of the Asclepiadeae and Orchideae that ensured that the pollen masses adhered to the backs of insects that visited the flowers (Natural selection, p. 53).
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    f6 2136.f6
    Vestiges of the natural history of creation ([Chambers] 1844) had run to ten editions by 1853. Each one was revised in the light of criticisms and new information.
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    f7 2136.f7
    A. Gray 1856a.
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    f8 2136.f8
    CD refers to the varieties considered by Gray to be doubtful, and distinguished in his catalogue (A. Gray 1856a) by being printed in bold type.
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    f9 2136.f9
    CD had asked Gray to prepare a list of ‘close species’ in genera of North American plants, which Gray did in August 1855 (see Correspondence vol. 5, letters to Asa Gray, 8 June [1855] and 24 August [1855]). The list is now in DAR 165: 92/3.
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    f10 2136.f10
    Gray's response to this query has not been found. In DAR 16.1: 126 there is a sheet headed ‘Asa Gray. Close Species’ that appears to indicate Gray's comments. It reads, in part: He marked the close. 160 in number (omitting Salix), but several of these were allied to 2 others, these I have omitted, as they were difficult to compare with any standard & which has injured the result for me. Nor have I much confidence in result of any kind, as Dr G. told me in subsequent letter, not to trust much. Moreover they seem to cling to the smaller genera.
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    f11 2136.f11
    See letter to Asa Gray, 20 July [1857].
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    f12 2136.f12
    A note in DAR 49: 48 headed ‘Aug 19th 1857.’ describes this experiment. See also letter to Gardeners' Chronicle, 18 October [1857].
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    f13 2136.f13
    See letter from Asa Gray, 7 July 1857.
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    f14 2136.f14
    A. Gray 1857a.
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    f15 2136.f15
    Gray had already sent CD the third and final part of his ‘Statistics of the flora of the northern United States’ (A. Gray 1856–7), but CD continued to think (perhaps because of what Gray had told him in the letter from Asa Gray, 4 November 1856) that he had not received a further part that discussed introduced plants (see letters to Asa Gray, 9 May [1857] and 18 June [1857]).
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    f16 2136.f16
    See letter from Asa Gray, [August 1857]. CD was able to obtain the seeds from Kew (see letter to J. D. Hooker, 14 [November 1857]).
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    f17 2136.f17
    The enclosure up to and including the initials ‘C. D.’, is in the hand of CD's copyist, Ebenezer Norman. CD corrected the copy and filled in gaps left by the copyist. Only those changes that appear to have been made by him are recorded in the Manuscript alterations and comments section. The draft from which the copy was made is in DAR 6 and is reprinted in Correspondence vol. 7, Appendix III. This draft, along with extracts from CD's 1844 species essay (DAR 7; Foundations, pp. 57–255), formed the basis of the paper read to the Linnean Society on 1 July 1858 as CD's part of the joint contribution with Alfred Russel Wallace entitled ‘On the tendency of species to form varieties; and on the perpetuation of varieties and species by natural means of selection’ (Darwin and Wallace 1858).
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