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Darwin Correspondence Project

To B. D. Walsh   27 March [1865]1

Down. | Bromley. | Kent. S.E.

Mar 27.

My dear Sir

I have been much interested by your letter.2 I received your former paper on Phytophagic unity, most of which was new to me.3 I have since received your paper on willow-galls: this has been very opportune as I wanted to learn a little about galls.4 There was much in this paper which has interested me extremely, on gradations &c and on your “unity of correlation”.5 This latter subject is nearly new to me, though I collected many years ago some such cases with birds;6 but what struck me most was when a bird-genus inhabits two continents the two sections sometimes display a somewhat different type of colouring. I shd like to hear whether this does not occur with widely ranging insect-genera?

You may like to hear that Wichura has lately published a book which has quite convinced me that in Europe there is a multitude of spontaneous hybrid willows. Wd it not be very interesting to know how the gall-makers behaved with respect to these hybrids?7 Do you think it likely that the ancestor of Cecidomyia acquired its poison like Gnats. (which suck men) for no especial purpose; at least not for gall-making.?8 Such notions make me wish that some one wd try the experiments suggested in my former letter.9 Is it not probable that Guest-flies were aboriginally gall-makers, & bear the same relation to them which Apathus probably does to Bombus?10 With respect to Dimorphism you may like to hear that Dr Hooker tells me that a dioicous parasitic plant allied to Rafflesia has its 2 sexes parasitic on 2 distinct species of the same genus of plants;11 so look out for some such case in the 2 forms of Cynips.12 I have posted to you copies of my papers on Dimorphism.13 Leersia does behave in a state of nature in the provoking manner described by me.14 With respect to Wagner’s curious discovery my opinion is worth nothing:15 no doubt it is a great anomaly, but it does not appear to me nearly so incredible as to you: remember how allied forms in the Hydrozoa differ in their so-called alternate generations: I follow those naturalists who look at all such cases as forms of Gemmation;16 & a multitude of organisms have this power or traces of this power, at all ages from the germ to maturity. With respect to Agassiz’s views there were many, & there are still not a few, who believe that the same species is created on many spots.17 I wrote to Bates & he will send you his mimetic paper, & I dare say others: he is a first rate man.18

Your case of the wingless insects near the Rocky mountains is extremely curious:19 I am sure I have heard of some such case in the old world, I think on the Caucasus. Wd not my argument about wingless insular insects perhaps apply to truly Alpine insects; for wd it not be destruction to them to be blown from their proper home?20 I shd like to write on many points at greater length to you, but I have no strength to spare—

With every good wish believe me yours very sincerely | Charles Darwin

If you publish on wingless insects, kindly inform me or send, if you can, a copy.—


The year is established by the relationship between this letter and the letter from B. D. Walsh, 1 March 1865.
Letter from B. D. Walsh, 1 March 1865.
The reference is to the first part of Walsh 1864–5, in which Walsh discussed the influence of insects’ food-plants on insect variation and speciation, arguing that feeding on distinct food-plants could, over time, produce distinct species of insects, or what he called phytophagic species. CD’s indexed and heavily annotated copy of the first part of Walsh 1864–5, inscribed by Walsh, is in the Darwin Pamphlet Collection–CUL. Walsh coined the term ‘phytophagic unity’ in Walsh 1864b, p. 635, to convey his observation that when the larvae of one species of gall-making insect feed on a given genus of plants, many more species of the same genus will also be found on that genus of plants. CD discussed Walsh’s findings in both papers in Origin 4th ed., pp. 55–6 and 187. See also letter from B. D. Walsh, 1 March 1865 and n. 14.
Walsh 1864b; there is a heavily annotated copy of this paper in the Darwin Pamphlet Collection–CUL. CD cited Walsh 1864b in his discussion of galls in Variation 2: 282–5. CD used galls, swollen tissues that are produced on plants by, as an example of how external agencies may affect structure.
A handwritten index appended to CD’s copy of the first part of Walsh 1864–5, in addition to CD’s annotations on page 428, indicate that CD was particularly interested in Walsh’s remarks on gradation in structure and colour from phytophagic varieties to phytophagic species (Walsh 1864–5, pp. 427–9; see Origin 4th ed., pp. 55–6). See also CD’s handwritten index appended to his copy of Walsh 1864b, and Walsh 1864b, pp. 567, 642–3. CD also refers to Walsh’s law of ‘Unity of Coloration’ (Walsh 1864b, pp. 635–6), according to which identical shades of colour and patterns of design in a group of species indicated a genetic connection. Emma Darwin, who wrote most of this letter at CS’s dictation, evidently wrote ‘correlation’ in error, a mistake that was corrected to ‘coloration’ by Walsh.
CD had been considering variations of colour in birds for some time (see, for example, Notebooks and Ornithological notes); however, the particular cases he is referring to have not been identified. See also Origin, pp. 25, 132, 133, and Variation 1: 195–7. CD discussed gradations in colour, pattern, and structure in bird plumage as an indication of descent from pre-existing species in Descent 2: 135–53; CD’s notes for this discussion, mostly dated between 1866 and 1868, part of a portfolio on sexual selection, are in DAR 84.1 and 84.2.
CD refers to Max Ernst Wichura and to Wichura 1865; see letter to M. E. Wichura, 3 February [1865]. There is an annotated copy of Wichura 1865 in the Darwin Library–CUL (see Marginalia 1: 871–3). CD’s curiosity regarding gall-making insects and hybrid willows was also stimulated by Walsh’s discussion of which gall-making insect species inhabited particular species of willows (see Walsh 1864b, pp. 544–6, 638–41). In Walsh 1866, which was the second part of Walsh 1864b, Walsh investigated CD’s query and made further observations on various gall-making insects inhabiting different species of willow. CD cited Walsh 1864b and 1866 in his discussion of galls (see Variation 2: 282–3). An annotated copy of Walsh 1866 is in the Darwin Pamphlet Collection–CUL.
Cecidomyia is a genus of gall-forming midges. Walsh discussed them in Walsh 1864b (see also letter from B. D. Walsh, 1 March 1865).
CD refers to his suggestion that Walsh experiment with inserting insect and other poisons into plant tissues (see Correspondence vol. 12, letter to B. D. Walsh, 21 October [1864] and n. 6). See Variation 2: 283–4 for CD’s conclusions regarding the poisons and plant tissues.
Walsh discussed the guest-flies, or inquilines, whose larvae inhabited the galls of other insects in Walsh 1864b, pp. 619–34; he noted that guest gall-gnats usually lived in the galls of species to which they were closely allied (ibid., p. 634). Walsh also discussed several different species of guest-flies in Walsh 1866, arguing that the inquilinous saw-flies, for example, were ‘primordially identical’ with the gall-making saw-fly (Walsh 1866, p. 279; see also pp. 237–8). Apathus (now known as Psithyrus) is a genus of humble-bee that closely resembles the humble-bee genus Bombus and lays its eggs in the latter’s nests. Walsh had discussed Apathus and Bombus in Walsh 1864c, pp. 246–7; there is an annotated copy in the Darwin Pamphlet Collection–CUL. CD described the parasitic habits of Apathus in Natural Selection, p. 509.
Joseph Dalton Hooker may have given CD this information during his visit to Down House between 4 and 6 March 1865. See also letter to J. D. Hooker, 4 May [1865].
Walsh had published a paper on dimorphism in Cynips, a genus of gall-wasps, in March 1864 (Walsh 1864a). See also letter from B. D. Walsh, 1 March 1865 and n. 20.
CD refers to ‘Dimorphic condition in Primula’ and ‘Two forms in species of Linum’; Walsh had requested that CD send the former after receiving a presentation copy of ‘Three forms of Lythrum salicaria’ (see letter from B. D. Walsh, 1 March 1865).
CD refers to his discussion of Leersia oryzoides in ‘Three forms of Lythrum salicaria’ (see letter from B. D. Walsh, 1 March 1865 and n. 13).
CD refers to Nikolai Petrovich Wagner’s observations of paedogenesis in the larvae of Miastor metraloas (Wagner 1862 and 1863). See letter from B. D. Walsh, 1 March 1865 and nn. 15 and 17.
CD added this statement to Origin 4th ed., p. 518, noting Wagner’s discovery (see n. 15, above). Gemmation, or budding, is a form of asexual reproduction whereby a new individual is formed by the protrusion and complete or partial separation of a part of the parent (OED); the nineteenth-century meaning of alternation of generations may have included gemmation (see letter from B. D. Walsh, 1 March 1865 and n. 17).
CD alludes to Walsh’s discussion of Louis Agassiz’s views expressed in J. L. R. Agassiz 1857–62 and 1863 (see letter from B. D. Walsh, 1 March 1865 and nn. 25–7).
CD refers to Henry Walter Bates and to Bates 1861.
See letter from B. D. Walsh, 1 March 1865 and nn. 33–5.
CD refers to his discussion of the development of wingless insects on the island of Madeira in Origin, pp. 135–6. See letter from B. D. Walsh, 1 March 1865 and nn. 34 and 37.


Comments on BDW’s papers ["On certain entomological speculations of the New England school of naturalists", Proc. Entomol. Soc. Philadelphia 3 (1864): 207–49; "On insects inhabiting the galls of certain species of willow", ibid. 3 (1864): 543–644]; much is new to CD.

Asks about wide-ranging insect genera,

Rocky Mt. wingless insects,

willow hybrids,


and other subjects.

Letter details

Letter no.
Charles Robert Darwin
Benjamin Dann Walsh
Sent from
Source of text
Field Musuem of Natural History, Chicago (Walsh 3)
Physical description
5pp †

Please cite as

Darwin Correspondence Project, “Letter no. 4797,” accessed on 17 September 2019,

Also published in The Correspondence of Charles Darwin, vol. 13