LETTER 188. TO F. MULLER. Down, January 11th .
I received your interesting letter of November 5th some little time ago, and despatched immediately a copy of my "Journal of Researches." I fear you will think me troublesome in my offer; but have you the second German edition of the "Origin?" which is a translation, with additions, of the third English edition, and is, I think, considerably improved compared with the first edition. I have some spare copies which are of no use to me, and it would be a pleasure to me to send you one, if it would be of any use to you. You would never require to re-read the book, but you might wish to refer to some passage. I am particularly obliged for your photograph, for one likes to have a picture in one's mind of any one about whom one is interested. I have received and read with interest your paper on the sponge with horny spicula. (188/1. "Ueber Darwinella aurea, einen Schwamm mit sternformigen Hornnadeln."--"Archiv. Mikrosk. Anat." I., page 57, 1866.) Owing to ill-health, and being busy when formerly well, I have for some years neglected periodical scientific literature, and have lately been reading up, and have thus read translations of several of your papers; amongst which I have been particularly glad to read and see the drawings of the metamorphoses of Peneus. (188/2. "On the Metamorphoses of the Prawns," by Dr. Fritz Muller.--"Ann. Mag. Nat. Hist." Volume XIV., page 104 (with plate), 1864. Translated by W.S. Dallas from "Wiegmann's Archiv," 1863 (see also "Facts and Arguments for Darwin," passim, translated by W.S. Dallas: London, 1869).) This seems to me the most interesting discovery in embryology which has been made for years.
I am much obliged to you for telling me a little of your plans for the future; what a strange, but to my taste interesting life you will lead when you retire to your estate on the Itajahy!
You refer in your letter to the facts which Agassiz is collecting, against our views, on the Amazons. Though he has done so much for science, he seems to me so wild and paradoxical in all his views that I cannot regard his opinions as of any value.
LETTER 189. TO A.R. WALLACE. Down, January 22nd, 1866.
I thank you for your paper on pigeons (189/1. "On the Pigeons of the Malay Archipelago" (The "Ibis," October, 1865). Mr. Wallace points out (page 366) that "the most striking superabundance of pigeons, as well as of parrots, is confined to the Australo-Malayan sub-region in which...the forest-haunting and fruit-eating mammals, such as monkeys and squirrels, are totally absent." He points out also that monkeys are "exceedingly destructive to eggs and young birds."), which interested me, as everything that you write does. Who would ever have dreamed that monkeys influenced the distribution of pigeons and parrots! But I have had a still higher satisfaction, for I finished your paper yesterday in the "Linnean Transactions." (189/2. "Linn. Soc. Trans." XXV.: a paper on the geographical distribution and variability of the Malayan Papilionidae.) It is admirably done. I cannot conceive that the most firm believer in species could read it without being staggered. Such papers will make many more converts among naturalists than long-winded books such as I shall write if I have strength. I have been particularly struck with your remarks on dimorphism; but I cannot quite understand one point (page 22), (189/3. The passage referred to in this letter as needing further explanation is the following: "The last six cases of mimicry are especially instructive, because they seem to indicate one of the processes by which dimorphic forms have been produced. When, as in these cases, one sex differs much from the other, and varies greatly itself, it may happen that individual variations will occasionally occur, having a distant resemblance to groups which are the objects of mimicry, and which it is therefore advantageous to resemble. Such a variety will have a better chance of preservation; the individuals possessing it will be multiplied; and their accidental likeness to the favoured group will be rendered permanent by hereditary transmission, and each successive variation which increases the resemblance being preserved, and all variations departing from the favoured type having less chance of preservation, there will in time result those singular cases of two or more isolated and fixed forms bound together by that intimate relationship which constitutes them the sexes of a single species. The reason why the females are more subject to this kind of modification than the males is, probably, that their slower flight, when laden with eggs, and their exposure to attack while in the act of depositing their eggs upon leaves, render it especially advantageous for them to have some additional protection. This they at once obtain by acquiring a resemblance to other species which, from whatever cause, enjoy a comparative immunity from persecution." Mr. Wallace has been good enough to give us the following note on the above passage: "The above quotation deals solely with the question of how certain females of the polymorphic species (Papilio Memnon, P. Pammon, and others) have been so modified as to mimic species of a quite distinct section of the genus; but it does not attempt to explain why or how the other very variable types of female arose, and this was Darwin's difficulty. As the letter I wrote in reply is lost, and as it is rather difficult to explain the matter clearly without reference to the coloured figures, I must go into some little detail, and give now what was probably the explanation I gave at the time. The male of Papilio Memnon is a large black butterfly with the nervures towards the margins of the wings bordered with bluish gray dots. It is a forest insect, and the very dark colour renders it conspicuous; but it is a strong flier, and thus survives. To the female, however, this conspicuous mass of colour would be dangerous, owing to her slower flight, and the necessity for continually resting while depositing her eggs on the leaves of the food-plant of the larva. She has accordingly acquired lighter and more varied tints. The marginal gray-dotted stripes of the male have become of a brownish ash and much wider on the fore wings, while the margin of the hind wings is yellowish, with a more defined spot near the anal angle. This is the form most nearly like the male, but it is comparatively rare, the more common being much lighter in colour, the bluish gray of the hind wings being often entirely replaced by a broad band of yellowish white. The anal angle is orange-yellow, and there is a bright red spot at the base of the fore wings. Between these two extremes there is every possible variation. Now, it is quite certain that this varying mixture of brown, black, white, yellow, and red is far less conspicuous amid the ever- changing hues of the forest with their glints of sunshine everywhere penetrating so as to form strong contrasts and patches of light and shade. Hence ALL the females--one at one time and one at another--get SOME protection, and that is sufficient to enable them to live long enough to lay their eggs, when their work is finished. Still, under bad conditions they only just managed to survive, and as the colouring of some of these varying females very much resembled that of the protected butterflies of the P. coon group (perhaps at a time when the tails of the latter were not fully developed) any rudiments of a prolongation of the wing into a tail added to the protective resemblance, and was therefore preserved. The woodcuts of some of these forms in my "Malay Archipelago" (i., page 200) will enable those who have this book at hand better to understand the foregoing explanation."), and should be grateful for an explanation, for I want fully to understand you. How can one female form be selected and the intermediate forms die out, without also the other extreme form also dying out from not having the advantages of the first selected form? for, as I understand, both female forms occur on the same island. I quite agree with your distinction between dimorphic forms and varieties; but I doubt whether your criterion of dimorphic forms not producing intermediate offspring will suffice, for I know of a good many varieties which must be so called that will not blend or intermix, but produce offspring quite like either parent.
I have been particularly struck with your remarks on geographical distribution in Celebes. It is impossible that anything could be better put, and would give a cold shudder to the immutable naturalists.
And now I am going to ask a question which you will not like. How does your journal get on? It will be a shame if you do not popularise your researches.