More Letters of Charles Darwin — Volume 1. Darwin Charles

one. I was so struck with the distribution of the Galapagos organisms, etc., and with the character of the American fossil mammifers, etc., that I determined to collect blindly every sort of fact which could bear any way on what are species. I have read heaps of agricultural and horticultural books, and have never ceased collecting facts. At last gleams of light have come, and I am almost convinced (quite contrary to the opinion I started with) that species are not (it is like confessing a murder) immutable. Heaven forfend me from Lamarck nonsense of a "tendency to progression," "adaptations from the slow willing of animals," etc.! But the conclusions I am led to are not widely different from his; though the means of change are wholly so. I think I have found out (here's presumption!) the simple way by which species become exquisitely adapted to various ends. You will now groan, and think to yourself, "on what a man have I been wasting my time and writing to." I should, five years ago, have thought so...(13/3. On the questions here dealt with see the interesting letter to Jenyns in the "Life and Letters," II., page 34.)

      LETTER 14. TO J.D. HOOKER. {November} 1844.

      ...What a curious, wonderful case is that of the Lycopodium! (14/1. Sir J.D. Hooker wrote, November 8, 1844: "I am firmly convinced (but not enough to print it) that L. Selago varies in Van Diemen's Land into L. varium. Two more different SPECIES (as they have hitherto been thought), per se cannot be conceived, but nowhere else do they vary into one another, nor does Selago vary at all in England.")...I suppose you would hardly have expected them to be more varying than a phanerogamic plant. I trust you will work the case out, and, even if unsupported, publish it, for you can surely do this with due caution. I have heard of some analogous facts, though on the smallest scale, in certain insects being more variable in one district than in another, and I think the same holds with some land-shells. By a strange chance I had noted to ask you in this letter an analogous question, with respect to genera, in lieu of individual species, — that is, whether you know of any case of a genus with most of its species being variable (say Rubus) in one continent, having another set of species in another continent non-variable, or not in so marked a manner. Mr. Herbert (14/2. No doubt Dean Herbert, the horticulturist. See "Life and Letters," I., page 343.) incidentally mentioned in a letter to me that the heaths at the Cape of Good Hope were very variable, whilst in Europe they are (?) not so; but then the species here are few in comparison, so that the case, even if true, is not a good one. In some genera of insects the variability appears to be common in distant parts of the world. In shells, I hope hereafter to get much light on this question through fossils. If you can help me, I should be very much obliged: indeed, all your letters are most useful to me.

      MONDAY: — Now for your first long letter, and to me quite as interesting as long. Several things are quite new to me in it — viz., for one, your belief that there are more extra-tropical than intra-tropical species. I see that my argument from the Arctic regions is false, and I should not have tried to argue against you, had I not fancied that you thought that equability of climate was the direct cause of the creation of a greater or lesser number of species. I see you call our climate equable; I should have thought it was the contrary. Anyhow, the term is vague, and in England will depend upon whether a person compares it with the United States or Tierra del Fuego. In my Journal (page 342) I see I state that in South Chiloe, at a height of about 1,000 feet, the forests had a Fuegian aspect: I distinctly recollect that at the sea-level in the middle of Chiloe the forest had almost a tropical aspect. I should like much to hear, if you make out, whether the N. or S. boundaries of a plant are the most restricted; I should have expected that the S. would be, in the temperate regions, from the number of antagonist species being greater. N.B. Humboldt, when in London, told me of some river (14/3. The Obi (see "Flora Antarctica," page 211, note). Hooker writes: "Some of the most conspicuous trees attain either of its banks, but do not cross them.") in N.E. Europe, on the opposite banks of which the flora was, on the same soil and under same climate, widely different!

      I forget (14/4. The last paragraph is published in "Life and Letters," II., page 29.) my last letter, but it must have been a very silly one, as it seems I gave my notion of the number of species being in great degree governed by the degree to which the area had been often isolated and divided. I must have been cracked to have written it, for I have no evidence, without a person be willing to admit all my views, and then it does follow.

      (14/5. The remainder of the foregoing letter is published in the "Life and Letters," II., page 29. It is interesting as giving his views on the mutability of species. Thus he wrote: "With respect to books on this subject, I do not know any systematical ones, except Lamarck's, which is veritable rubbish; but there are plenty, as Lyell, Pritchard, etc., on the view of the immutability." By "Pritchard" is no doubt intended James Cowles "Prichard," author of the "Physical History of Mankind." Prof. Poulton has given in his paper, "A remarkable Anticipation of Modern Views on Evolution" (14/6. "Science Progress," Volume I., April 1897, page 278.), an interesting study of Prichard's work. He shows that Prichard was in advance of his day in his views on the non-transmission of acquired characters. Prof. Poulton also tries to show that Prichard was an evolutionist. He allows that Prichard wrote with hesitation, and that in the later editions of his book his views became weaker. But, even with these qualifications, we think that Poulton has unintentionally exaggerated the degree to which Prichard believed in evolution.

      One of Prichard's strongest sentences is quoted by Poulton (loc. cit., page 16); it occurs in the "Physical History of Mankind," Ed. 2, Volume II., page 570: —

      "Is it not probable that the varieties which spring up within the limits of particular species are further adaptations of structure to the circumstances under which the tribe is destined to exist? Varieties branch out from the common form of a species, just as the forms of species deviate from the common type of a genus. Why should the one class of phenomena be without end or utility, a mere effect of contingency or chance, more than the other?"

      If this passage, and others similar to it, stood alone, we might agree with Prof. Poulton; but this is impossible when we find in Volume I. of the same edition, page 90, the following uncompromising statement of immutability: —

      "The meaning attached to the term species, in natural history, is very simple and obvious. It includes only one circumstance — namely, an original distinctness and constant transmission of any character. A race of animals, or plants, marked by any peculiarities of structure which have always been constant and undeviating, constitutes a species."

      On page 91, in speaking of the idea that the species which make up a genus may have descended from a common form, he says: —

      "There must, indeed, be some principle on which the phenomena of resemblance, as well as those of diversity, may be explained; and the reference of several forms to a common type seems calculated to suggest the idea of some original affinity; but, as this is merely a conjecture, it must be kept out of sight when our inquiries respect matters of fact only."

      This view is again given in Volume II., page 569, where he asks whether we should believe that "at the first production of a genus, when it first grew into existence, some slight modification in the productive causes stamped it originally with all these specific diversities? Or is it most probable that the modification was subsequent to its origin, and that the genus at its first creation was one and uniform, and afterwards became diversified by the influence of external agents?" He concludes that "the former of these suppositions is the conclusion to which we are led by all that can be ascertained respecting the limits of species, and the extent of variation under the influence of causes at present existing and operating."

      In spite of the fact that Prichard did not carry his ideas to their logical conclusion, it may perhaps excite surprise that Mr. Darwin should have spoken of him as absolutely on the side of immutability.

      We believe it to be partly accounted for (as Poulton suggests) by the fact that Mr. Darwin possessed only the third edition (1836 and 1837) and the fourth edition (1841-51). (14/7. The edition of 1841-51 consists of reprints of the third edition and three additional volumes of various dates. Volumes I. and II. are described in the title-page as the fourth edition; Volumes III. and IV. as the third edition, and Volume V. has no edition marked in the title.) In neither of these is the evolutionary point of view so strong as in the second edition.

      We have gone through all the passages