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Darwin Correspondence Project

From Thomas Vernon Wollaston   2 March [1855]

25 Thurloe Square, | Brompton.

March 2.

My dear Sir,

You will I am sure pardon my negligence in having delayed my reply thus long to your letter received on Tuesday last (the 27th.), for I have been so extremely busy in preparing for my departure, & in finishing the arrangment of my collection for the British Museum, that I have been scarcely a free agent. I will now, however, answer your queries in the order in which you have asked them;1 though, had such been possible, a little more time to have thought over a few of them might have been desirable.

1st., then, as regards the quæstio vexata of Zargus Desertæ,2 I have been looking at my specimen again this morning; &, although I would not positively affirm what appears to go so decidedly against experience, I am still inclined to believe it to be a true hybrid, rather than a monstrosity. Altho’ unquestionably more akin to Calathus than Zargus, it does nevertheless seem to me to unite so completely the characters of both that I cannot but imagine (when the circumstances of its capture are taken ⁠⟨⁠into⁠⟩⁠ account) that both genera (!!) have had a hand in producing it. You will however be able any day to see it at the Museum (now that my Collection has been purchased,3 & is to be kept seperate), & so judge for yourself. You will however remark that, not only is one elytroso shorter than the other & resembling that of Zargus, but the very pronotum has the shape peculiar to each species on the respective sides; the mandibles are straightened & elongated (one of the main features of Zargus); & the claws (judging from the 2 only which remain) are but slightly pectinated,—thus uniting, as it were, the structure which obtains in the 2 insects, the serration being extremely powerful in one, & evanescent in the other. With all this however, I admit that it is more nearly related to Calathus Complanatus than to the Zargus Desertæ,—as the sub-pectination of the claws (apparently in all the legs) & the almost entire upper-lip, together with its general facies, would appear to indicate: but still I cannot altogether regard it as monstrous, seeing the amount of evidence which would appear to tell a different tale. Whatever it be however, the specimen was clearly but ill-developed in parts (as I suppose we should a fortiori expect, under the contingencies which I have assumed),—the feet being dark, & brittle in substance, at their extremities (so that the claws broke-off, almost without provocation), & even the ends of the palpi shewing the same tendency.—

2. The Dezerta Grande 4 is about 4 miles long, & varies in breadth from a mere ridge (which it is scarcely possible to walk upon, tho’ it might be ridden across, as on a horse’s back) to about three quarters of a mile. It rises to about 1600 feet, in its highest part, & is perfectly barren ⁠⟨⁠&⁠⟩⁠ uninhabited,—tho’ it is said (& there are indications of the fact) that a small colony once attempted to settle on it from Madeira; but were compelled to abandon it after a time, on account of the want of water. Even now however a small portion of the central valley is cultivated,—a certain no. of men residing every year (in the ruins of one of the old houses, which still exists, & which has been patched-up to receive them) for the purpose of sowing corn, from Funchal, & afterwards making an expedition to cut it.— I just mention this that you may not be surprised (when I come to that question) that I consider some few of even the Dezertan insects as having been introduced.—

3. The 6th. Salvagian5 insect (which you appear to have overlooked) is the opatrum dilatatum (p. 501). I hope to freight a vessel to the Salvages in propriâ personâ this spring, with Mr. Lowe,6 —when I shall be better able to state whether or not these 6 species may be regarded as a fair exponent of the Coleopterous fauna of those rocks. Meanwhile, I am inclined to believe that they are; for, although Mr. Leacock was only one day on the islands, & is moreover no Entomologist, he is a good general observer & collector, & I know that he worked hard. He was however principally in quest of Land-shells (at which, altho’ not a Conchologist in theory, he works admirably); & it may be interesting to you to know that he only succeeded in procuring one species,—since described by Lowe as Helix ustulata.— The “Great Piton” (as it is called, though in reality the smaller ⁠⟨⁠of⁠⟩⁠ the 2 islands) appears to be the most productive of the rocks,—& is described as ⁠⟨⁠a⁠⟩⁠ conical eminence, rising out of a sandy base, & covered with plants. All his species were collected on that island.

4. My note (p. xiii), on the common-ness of certain insects,7 is intended to mean this: viz. that the 1st. 5 (S. abbreviatus, L. morio, C. complanatus, H. vividus, & H. cinerascens, in the last of which I include the Portosantan H. illotus) are apparently the most generally distributed of the whole, occurring (as I believe) on every rock. I then add (as a seperate remark) that they are also (apart from being universally distributed) amongst the most abundant individually of all the species hitherto observed; & I take the opportunity of stating that 7 other species (although not universally distributed) are likewise amongst the “most abundant individually” (in their respective localities).—viz. T. lineatus, D. obscuroguttatus, O. Maderensis, O ventrosus, H. Pluto & conpertus & A. tristis.— This was merely a selection, put in as a subsidiary remark; & I might have added to them (& perhaps should have done) the Oxytelus complanatus, as you well observe.— Ptinus albopictus might possibly have been included also (being, altho’ generally rare, in profusion on the Ilheu Chão); but no Tarphius (important as they are, as an assemblage) was sufficiently common to be admitted into the list.—

5. With regard to my observation (p. 584) that “transition groups” are usually “variable”,8 I am of course only speaking (from personal knowledge) Entomologically; though of course if such is true in one department of science, we should à priori expect to find it so in another. Whether such is the case or not, I do not presume to decide (not being in a position to do so); &, that not being my province in the present work, I did not consider myself called on to moot the question generally: but in my own department of Zoology I am more & more convinced that the statement is true,—for it seems to me that almost every family partakes at either end (to a certain extent) of the peculiarities of that which immediately precedes & follows it; so that when these families (as is so often the case) become (as science advances) themselves to be subdivided, it follows that the detachments at either end are of necessity transitional,—&, as families, may be regarded as essentially variable, i.e. to say, as combining in a measure the distinguishing features of their opposite allies.— Almost any families of the Coleoptera may be cited in illustration of this,—2 or 3 only being of a sufficiently isolated nature (as the Carabidæ, Trichopterygia & Staphylinidæ,—the 1st. & last of which are consequently made, as I cannot but believe rightly, in the best systems, to commence & wind-up the entire Order) not to suit our purpose. But let us take, for example, the Tomicidæ (the Bostrichi of the Continent): they are sometimes placed in the Rhycophora & sometimes out of it,—& this on account of the structure of their feet (the most important character on which the Rhyncophora, as a department, is based). Thus, we know that the essential features of the Rhyncophora are, that their head sd. be produced & their tarsi pseudotetramerous (i.e. 5-jointed; with the 4th. joint minute & concealed within the lobes of the 3rd. ). Now, in the Tomicidæ, the head is hardly ever really rostrate; &, altho’ the feet have their 4th. articulation usually diminished, as in the normal Rhyncophoræ, the 3rd. is not (or very rarely) bilobed: thus varying in the very minutiæ which serve, when properly carried-out, to constitute the “life-blood” as it were of the great Section into which, from the Cissidæ, they lead us.— This is but one instance out of many, but it may just serve to shew what I mean: & it was on such considerations that my special pleadings on the position of the Anisotomidæ (which, I may add, I am not prepared to defend,—though as far as the reasoning is concerned, I believe it to be a perfectly feasible one) were mainly based.—

6. I believe it to be true in Madeira that, as a mere matter of observation & fact, it is the “commonest” insects (i.e. to say, those which are most widely distributed) which vary most.9 I am aware that we are not in a position to assert this from theory, because we have no means of judging whether the scarcer things are subject to a like range of form: but it so happened, in reality, that I did find much greater instability amongst those insects wh. were observed under all circumstances (I refer, more particularly, to the 1st. 5 at p. xiii) than in those (although in profusion in their respective localities) of a less universal nature: &, hence, I felt myself justified in assuming that the circumstances under which the several species were placed had much to do in modifying their external contour,—the negative proposition, of whether the scarcer kinds would exhibit or not the same degree of inconstancy, were they subject to similar agencies, not necessarily (as being an hypothetical case) coming within my province. (Albeit, from analogy, & such few facts as we possess, I believe that they wd. be governed by the same law).

7. And now, touching the question of wings,10 I may just preface my remarks by stating that perhaps I ought not properly to have spoken of insects as “apterous”, but rather as “with the wings obsolete”,—because I doubt if in any cases we should not be able to perceive the 1st. rudiments of wings, on close inspection,—even in those species which have connate elytra, & which are usually looked-upon (since such to all practical purposes they are) as literally apterous. However, assuming that you will not deny the “Alæ amplæ” & “alæ obsoletæ” as correctly expressive of the 2 extremes, I have not the least hesitation in expressing my belief that all the intermediate grades may be (& occasionally are) found in the same species,—or, in other words, that the wings are essentially a variable organ, to be developed more or less according to the circumstances of the case. It certainly is so with many insects in our own country; but I never realised this principle so completely as in Madeira, where nearly everything which I have usually looked upon as winged in more northern latitudes was there practically “apterous”. If therefore you will grant my premises, that an organ (however essential in some instances) may be variable (i.e. to say, may be capable of developement according as required), I would go a step further & subscribe heartily to your admirable solution,11 in the case of insular species, which chimes in better with my idea of adaptation than it has ever occurred to me to think of,—for it is clear that where the sphere for dissemination is prescribed, great powers of locomotion would be worthless: & why may we not therefore assume that, since the period of their isolation, the means of flight (which would more often get them into trouble than not) have been withdrawn. I do not think there can be any danger in this theory, since it does not touch the question of development in its larger sense, or in any way border on transmutation,—each species having only certain limits to vary between (greater or less according to the kind), beyond which (either in a progressive or retrograde movement) it cannot pass.—12

(N.B. If this appears dogmatical, do not laugh at it,—for you have of course studied this subject far more than I have).

I have not gone sufficiently into the question to measure the degree of variation in the wings of the same species on the same island, having been contented with the general fact (of which, in my own judgment, I had satisfied myself) that they were subject to almost indefinite variation. A priori, however, I should imagine that the range of development would not be very great, geographical causes seeming to be one of the main points by which (in the present instance) it has been regulated; and, although we may easily detect small differences almost alongside each other, I suspect that we must view the same insect in countries far distant (or, at any rate, very oppositely constituted, as regards land & water) to see this tendency to variation in the light of a bonâ fide reality.—

8. Next, as regards poor Forbes’s Atlantis,13 I dare not dogmatise much (& indeed you must be getting tired of my meanderings); I may however add a few remarks, of a loose & desultory nature, which may bear indirectly on the subject. How far this great question can be solved by the comparison of the fauna of the present islands, inter se, I will not venture to suggest; but, assuming that some light may be thrown upon it in this manner, my own observations & general train of ideas would rather tend, I think, to uphold Forbes’ theory (to a certain degree) than to undermine it. In the 1st. place then, I start with the admission that these are not the days of migration (in the sense that would have been understood in ante- miocene times); or, in other words, that the period when Forbes’s plants came across from the Germanic plains to England, his Iberian Saxifrages from Gallicia, & my insects wandered northwards through this Atlantic region (whenever these successive æras may have been) was marked by a more rapid provision for the self- dissemination of life than now; & I likewise assume that (in the same manner as Forbes’s “British” plants came 1st. into existence in a country beyond the British isles,—i.e. on the great Germanic plain) this “miocene continent” was specially rich in “centres of diffusion created expressly for itself”, & that the general breaking up of the land of passage must have taken place at no very remote period after its insect fauna came into being; or, at any rate, that the ⁠⟨⁠gre⁠⟩⁠ater portion of it must have been destroyed before the several species had permeated far. With these crude & general ideas (which are of course mere theory, & on none of which would I presume to insist), we will just turn for an instant to a few facts, & see how far they are countenanced by observation. So far as I know of the Insect fauna of the Canaries, the Salvages, Madeiras, the opposite coast of Africa, & the Spanish peninsular, there do certainly seem to be traces of a general migration of the species, in a north-easterly direction, from several successive centres of radiation. Thus, for instance, take the genus Hegeter,—which finds its maximum in the Canary islands (the land of Hegeters). 2 species (out of about 20) have found their way to the opposite coast of Africa; one of these 2 (probably of rapid dissemination) had reached the Madeira Islands, & even the S.W. of Portugal, in those migratory days; whilst the Salvages (the nearest of all to the Canaries) produces another (different to those “2”), which is probably identical with a species described from Teneriffe. Let us now go a little more northward, to the Madeiras, & take another case. Like Hegeter, in the Canaries, Madeira may be called “the land of Tarphii. There are 15 species of this genus in Madeira proper, but in the Canary Islands none have ever yet been observed. One however appears (like the 3 Hegeters from the Canaries) to be “given-off”, in a north-easterly direction,—&, after attaining its maximum in Po. So.,14 turns-up again in Sicily!!—

Ditylus, again, does not occur in Madeira, but there are 2 or 3 species common in Teneriffe: now one of these (or, at any rate, a species closely allied to it,—& probably only a “species”, so called, thro’ the modifying effects of isolation!) had reached the Salvages (the movement likewise northerly);—& the same may be said of an Acalles wh. I have described from those rocks.— I will not however multiply cases; suffice it to observe that I think I can trac⁠⟨⁠e a⁠⟩⁠ decided northerly movement throughout the whole insect-fauna of this region; or, in other words, that where the more southern portions impart 20 species to those north of them, not one seems to have passed from north to south: & hence (if this be true) I should join issue with you as to the necessity of the Spanish species crossing over to Madeira15 even had a miocene continent ever existed to constitute a land of passage; whilst the fact of my species being found (in a gradually diminishing ratio) as you proceed to the N.E. (or towards the Mediterranean bason) would become in part intelligible.

And here I may just say a word on your 9th. question, as to what is my idea concerning the total absence of certain large (& all but universal) groups from the Madeiran fauna. I do not believe that it is through any physical peculiarity in the country itself,—seeing that there are abundant conditions in a land of flowers for the sustenance of the Cetoniadæ (indeed they wd. seem almost, were it not for the amount of Hymenoptera & Diptera, to be required), in the deep mossy ravines for the Pselaphi, & at the roots of the grass, in the large open tracts of a high elevation, for the larvæ of the Elateridæ. I can only therefore conclude that this was not the region of their creation, & that they had not migrated to these spots of ground (either from S. or East) before the breaking-up of the surrounding land.—16 I could add a ⁠⟨⁠go⁠⟩⁠od deal more (of a far more interesting nature), concerning the direction ⁠⟨⁠of migra⁠⟩⁠tion, gathered from the Land-Mollusca (at wh. I worked hard), but I fear I have not time now: suffice it therefore to state that they tell the same story as the insects,—only (if I understand them aright) far more clearly.—

10. Of the Dezertan species which you mention, 3 (Ceutorhynchus quadridens, Haltica subtilis & Philonthus sordidus) are certainly introduced (as they are into Madeira proper), & these are winged: the other 3 (Ptinus fragilis, & Tychius robustus & albosquamosus) are indigenous & with the wings obsolete.—

This, I think notices all your queries, (I cannot say replies to them): if in my haste I have not expressed myself clearly everywhere, you must forgive me, for I have really not had time to think sufficiently over your observations; & if I have anywhere committed myself, you will I doubt not be charitable, & will not “shew me up”.— I perfectly delight in islands; for, being little worlds in themselves, everything within them is suggestive; whilst each individual fact which one gathers becomes of greater interest, where there is a theory to establish, than twenty times the number on the mainland. I hope to start for Madeira either (by the Steamer, fr. Southampton) on Friday next (in which case I should leave Town on Thursday); or else by the Eclipse (from London) on the 15th—.— Anything more that I can do for you in situ, I shall be very glad, if you will only let me know what it is.

And now I must relieve you from this state of purgatory, which I feel has been protracted beyond its proper limits; &, hoping that you will make allowances for my tremendous haste, Believe me, | my dear Sir, | Your’s very sincerely | T V Wollast⁠⟨⁠on⁠⟩⁠

P.S. You are much too liberal in your judgment concerning my opusculum,—which is, after all, whatever may have been the labour spent on it, but “a drop dissevered fr. the boundless sea”.17 That it should afford you the slightest amount of “old information, freshly put” (for I cannot call it new) is very satisfactory to me.—

CD annotations

1.1 You will … desirable.— 1.7] crossed pencil
2.1 1st… . than Zargus, 2.5] ‘Hybrid’ added brown crayon
2.16 With all this … a different tale. 2.21] ‘Hybrid’added brown crayon
3.1 2… . that island. 4.14] crossed pencil
3.1 2… . barren 3.4] ‘Descript of Dezerta Grande’ added brown crayon
4.1 3… . island. 4.14] ‘Descrip of Salvage’ added brown crayon
5.1 4… . the list.— 5.15] crossed pencil; ‘On Commonness of Insectsadded brown crayon
6.1 “transition … “variable”, 6.2] underl pencil
6.2 I am … generally: 6.7] crossed pencil
6.15 2 or 3 … purpose. 6.18] square brackets added pencil
6.18 But let … Tomicidæ] underl pencil
6.23 pseudotetramerous] cross bars of ‘t’s’ added pencil
6.27 varying] del pencil; ‘differing’ added pencil
7.1 6… . circumstances 7.6] ‘On Variability in relation to wide range’ added brown crayon
7.7 profusion] ‘f’ superimposed for clarity, pencil
8.8 I have not … species,— 8.10] underl brown crayon
8.11 It certainly … country; 8.12] underl brown crayon
8.19 where the sphere … powers of 8.20] underl brown crayon
8.21 & why … regulated; 10.6] crossed brown crayon
8.23 think … danger in] underl pencil
10.7 although we … each other 10.8] underl brown crayon; double scored brown crayon
11.1 8… . the several species 11.20] crossed pencil; ‘Assumptions to account for Forbes Theory’ added brown crayon
11.22 we will … radiation. 11.26] double scored brown crayon
11.29 one of these … observed. 11.36] ‘Direction of Migration’ added brown crayon
11.31 whilst the Salvages … Teneriffe. 11.33] double scored pencil
12.9 & hence … fauna. 13.3] ‘These are chiefly endemic from whence Travel North’ added pencil
13.8 this was … land.—13.10] double scored brown crayon
14.1 10… . Wollast⁠⟨⁠on⁠⟩⁠ 16.4] crossed brown crayon; ‘29’ added brown crayon
17.1 P.S. … me.— 17.4] crossed pencil


CD had read Wollaston’s Insecta Maderensia (Wollaston 1854) in the first months of 1855 (Correspondence vol. 4, Appendix IV, 128: 10). The page references in the letter refer to this work. The book was evidently later given to CD by Wollaston, as an inserted slip in the copy in the Darwin Library–CUL reads: ‘Charles Darwin esqre., from the Author. 25. Thurloe Square, Brompton. March 10th., 1855.’ CD’s extensive notes on Wollaston 1854 are in DAR 197.1 and 197.2. CD frequently cited the work in Natural selection.
Zargus desertae was one of three species found by Wollaston on Madeira that he identified as comprising a new genus. He described it as the most interesting new genus in his collection, since it appeared intermediate in character between the Harpalides and Chlœniidea (Wollaston 1854, pp. 22–3). CD’s reference is to Wollaston’s account of a hybrid between Z. desertae and Calathus complanatus in Wollaston 1854, p. 25.
Wollaston’s collection of four thousand Coleoptera from Madeira and the Salvage Islands, arranged and described by the collector, was purchased by the British Museum in 1855 (British Museum 1904–6, 2: 562).
Wollaston stated that Zargus desertae and the hybrid beetle were found only on the island of Dezerta Grande in the Madeiras (Wollaston 1854, p. 25).
CD had evidently counted the number of species listed as inhabiting only the Salvage Islands, small rocky islets forming part of the Madeira group. The discovery of Opatrum dilatatum by T. S. Leacock of Funchal is described in Wollaston 1854, p. 501.
Richard Thomas Lowe, a friend and collaborator of Wollaston, lived on Madeira until 1852.
Wollaston had listed twelve species that inhabited all the islands of the group and stated that he believed these to be ‘the most abundant individually of all with which we are concerned.’ (Wollaston 1854, p. xiii).
Actually Wollaston 1854, pp. 483–4, with reference to the taxonomic position of the family Anisotomidae. Wollaston considered these to be ‘very interesting and anomalous little beetles’ in which the number of tarsi (joints) in the legs were extremely variable (p. 483). Reasoning from this phenomenon, he claimed ‘so completely is variableness the very essence of a transition group, that, when we find great mutability to exist in any series of characters, we are even à priori led to suspect that the assemblage in which it occurs is in all probability connective between some two others.’ (p. 484). See also the letter from G. R. Waterhouse, [after 2 March 1855].
CD had recently expanded his survey of the number of species in aberrant genera to investigate the number of species and the geographical range of common or successful genera. He believed that organisms which ranged over a large geographical area and were individually numerous would be the forms most likely to vary and adapt to changing conditions (Browne 1980).
Wollaston stated that the two principal features of the Madeiran Coleoptera were the general dullness of the colouring and a tendency to be wingless (Wollaston 1854, p. xii). CD discussed the origin of apterous insects, with special reference to Wollaston’s findings, at length in Natural selection, pp. 291–3. See also letter from J. D. Hooker, [before 17 March 1855].
CD, reasoning from the principle of selection, maintained that in locations like the islands of the Madeira group, which were small in size and extremely windy, wings would endanger species by exposing them to the possibility of being blown out to sea. This explanation for the existence of wingless insects is proposed in the letter to J. D. Hooker, 7 March [1855]. See also Natural selection, p. 291. Statistics on apterous beetles are cited from Wollaston 1854 in Origin, pp. 135–6.
Wollaston, like most contemporary natural historians, interpreted the variations in wing development mainly in terms of adaptive use and disuse within the limits of species type. Wollaston, with his typological concept of species, was strongly opposed to the Origin when it was published and gave it one of the more hostile reviews ([Wollaston] 1860).
The popular name given to Edward Forbes’s belief in a land-bridge linking the Iberian peninsula with the Azores and with Ireland in the Miocene era as an explanation for the similarity in the flora and fauna of the regions (Forbes 1846, pp. 348–9). Wollaston had commented on the apparent relationship between some of the endemic species on Madeira and those of south-west Britain and Ireland and cited Forbes’s paper, stating that ‘it is impossible to deny that, so far as the Madeiras betoken, everything would go to favour this grand and comprehensive idea.’ (Wollaston 1854, p. xiii).
The island of Porto Santo.
CD had evidently suggested that if there had been a land connection between the mainland and Madeira there ought to be organisms of Spanish origin presently living on Madeira. Wollaston argued that any migration in the Miocene period would have been in the opposite direction, that is, from the Mediterranean basin towards north-west Ireland, without necessarily encompassing Spain (Wollaston 1854, pp. xiii–xiv).
Wollaston described his views on the creation of insects on Madeira in Wollaston 1857. He divided the insects into those created on the island, those which migrated there during former geological periods, those which were naturalised, and introduced species (p. ix).
The concluding sentence of the introduction to Wollaston 1854, p. xviii.


British Museum (Natural History). 1904–6. The history of the collections contained in the natural history departments of the British Museum. 2 vols. London: the Trustees.

Browne, Janet. 1980. Darwin’s botanical arithmetic and the ‘principle of divergence’, 1854–1858. Journal of the History of Biology 13: 53–89.

Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 29 vols to date. Cambridge: Cambridge University Press. 1985–.

Forbes, Edward. 1846. On the connexion between the distribution of the existing fauna and flora of the British Isles, and the geological changes which have affected their area, especially during the epoch of the Northern Drift. Memoirs of the Geological Survey of Great Britain, and of the Museum of Economic Geology in London 1: 336–432.

Natural selection: Charles Darwin’s Natural selection: being the second part of his big species book written from 1856 to 1858. Edited by R. C. Stauffer. Cambridge: Cambridge University Press. 1975.

Origin: On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. By Charles Darwin. London: John Murray. 1859.

Wollaston, Thomas Vernon. 1854. Insecta Maderensia; being an account of the insects of the islands of the Madeiran group. London: John van Voorst.

Wollaston, Thomas Vernon. 1857. Catalogue of the coleopterous insects of Madeira in the collection of the British Museum. London: By order of the Trustees.

[Wollaston, Thomas Vernon]. 1860a. Review of Origin of species. Annals and Magazine of Natural History 3d ser. 5: 132–43. Reprinted in Hull 1973, pp. 127–40. [Vols. 6,7,8]


Hybrid insects.

Description of the Salvages.

Variability of "transition groups" of insects; relation of variability to ranges of insects. The variability of wings, even within species. Reduction of flying ability on isolated islands.

Forbes’s "Atlantis" theory and insect fauna of the Atlantic islands, considered with regard to insect migrations.

Letter details

Letter no.
Thomas Vernon Wollaston
Charles Robert Darwin
Sent from
Source of text
DAR 181: 136
Physical description
ALS 12pp †, CD notes 6pp

Please cite as

Darwin Correspondence Project, “Letter no. 1640,” accessed on 4 March 2024,

Also published in The Correspondence of Charles Darwin, vol. 5