My dear Hooker, I fear I shall weary you with letters, but do not answer this, for in truth and without flattery, I so value your letters, that after a heavy batch, as of late, I feel that I have been extravagant and have drawn too much money, and shall therefore have to stint myself on another occasion.
When I sent my MS. I felt strongly that some preliminary questions on the causes of variation ought to have been sent you. Whether I am right or wrong in these points is quite a separate question, but the conclusion which I have come to, quite independently of geographical distribution, is that external conditions (to which naturalists so often appeal) do by themselves VERY LITTLE. How much they do is the point of all others on which I feel myself very weak. I judge from the facts of variation under domestication, and I may yet get more light. But at present, after drawing up a rough copy on this subject, my conclusion is that external conditions do EXTREMELY little, except in causing mere variability. This mere variability (causing the child NOT closely to resemble its parent) I look at as VERY different from the formation of a marked variety or new species.
(No doubt the variability is governed by laws, some of which I am endeavouring very obscurely to trace.) The formation of a strong variety or species I look a as almost wholly due to the selection of what may be incorrectly called CHANCE variations or variability. This power of selection stands in the most direct relation to time, and in the state of nature can be only excessively slow. Again, the slight differences selected, by which a race or species is at last formed, stands, as I think can be shown (even with plants, and obviously with animals), in a far more important relation to its associates than to external conditions.
Therefore, according to my principles, whether right or wrong, I cannot agree with your proposition that time, and altered conditions, and altered associates, are 'convertible terms.' I look at the first and the last as FAR more important: time being important only so far as giving scope to selection. God knows whether you will perceive at what I am driving. Ishall have to discuss and think more about your difficulty of the temperate and sub-arctic forms in the S. hemisphere than I have yet done. But I am inclined to think that I am right (if my general principles are right), that there would be little tendency to the formation of a new species, during the period of migration, whether shorter or longer, though considerable variability may have supervened...
CHARLES DARWIN TO J.D. HOOKER.
December 24th [1856].
...How I do wish I lived near you to discuss matters with. I have just been comparing definitions of species, and stating briefly how systematic naturalists work out their subjects. Aquilegia in the Flora Indica was a capital example for me. It is really laughable to see what different ideas are prominent in various naturalists' minds, when they speak of "species;"in some, resemblance is everything and descent of little weight--in some, resemblance seems to go for nothing, and Creation the reigning idea--in some, descent is the key,--in some, sterility an unfailing test, with others it is not worth a farthing. It all comes, I believe, from trying to define the undefinable. I suppose you have lost the odd black seed from the birds' dung, which germinated,--anyhow, it is not worth taking trouble over. I have now got about a dozen seeds out of small birds' dung. Adios, My dear Hooker, ever yours, C. DARWIN.
CHARLES DARWIN TO ASA GRAY.
Down, January 1st [1857?].
My dear Dr Gray, I have received the second part of your paper ('Statistics of the Flora of the Northern United States.' "Silliman's Journal", 1857.), and though Ihave nothing particular to say, I must send you my thanks and hearty admiration. The whole paper strikes me as quite exhausting the subject, and I quite fancy and flatter myself I now appreciate the character of your Flora. What a difference in regard to Europe your remark in relation to the genera makes! I have been eminently glad to see your conclusion in regard to the species of large genera widely ranging; it is in strict conformity with the results I have worked out in several ways. It is of great importance to my notions. By the way you have paid me a GREATcompliment ("From some investigations of his own, this sagacious naturalist inclines to think that [the species of] large genera range over a larger area than the species of small genera do."--Asa Gray, loc. cit.): to be SIMPLY mentioned even in such a paper I consider a very great honour. One of your conclusions makes me groan, viz., that the line of connection of the strictly alpine plants is through Greenland. I should EXTREMELY like to see your reasons published in detail, for it "riles" me (this is a proper expression, is it not?) dreadfully. Lyell told me, that Agassiz having a theory about when Saurians were first created, on hearing some careful observations opposed to this, said he did not believe it, "for Nature never lied." I am just in this predicament, and repeat to you that, "Nature never lies," ergo, theorisers are always right...
