From A. C. L. G. Günther   [late December 1867 or early January 1868]1

My dear Sir

There is a whole family of fishes (Cyprinodontidæ) exhibiting sexual differences as remarkable as any in Reptiles or Birds.2 It was very stupid in me not to think of it when you were in the Museum.3 I have given the specimens of two of the most remarkable forms to my artist, who will make drawings of them.4

As soon as they are finished, I will send you proofs of the two plates.5

With the compliments of the season | Yours sincerely | A Günther

CD annotations

Top of letter: ‘A mere crest or uncoloured ornament cd not cause more danger to female than male.— | Fresh Water?’6 pencil; ‘Musical Fish S. Brazil’7 ink
Bottom of 1st page: ‘How about numbers of males in other fishes in which males differ’8 pencil, del ink
End of letter: ‘Colours?? | Female lays first, & never attends to eggs or young— Are males greatly in excess?? | Polygamy’ ink; ‘— Stickle-backs9 | Get names of other Fish’10 pencil
1
The date is established by the seasonal greeting and the relationship between this letter and the letter from A. C. L. G. Günther, 13 May 1868 (Correspondence vol. 16).
2
In his letter of 19 December 1867, Günther had provided information on sexual differences in reptiles. CD cited Günther on sexual differences in size and ornamentation in the Cyprinodontidae in Descent 2: 7, 9–10. There are many contemporary notes by CD on sexual differences in animals, including fish, reptiles, and birds, in DAR 82.
3
Günther was employed at the British Museum. CD had visited London from 28 November until 7 December 1867 (CD’s ‘Journal’ (Correspondence vol. 15, Appendix II)).
4
The reference is to George Henry Ford. The two forms Günther refers to are probably Mollienesia petenensis and Xiphophorus hellerii: CD cited Günther 1859–70 and Günther 1864–6 on the differences between the sexes of these two Cyprinodont species in Descent 2: 9–10 and figure 29.
5
Chapter 12 of Descent includes illustrations of four examples of sexual differences in fish. Drawings for the plates were executed by Ford, who also drew reptiles and amphibians for chapter 12 under Günther’s supervision (Descent 2: 4–11). See also Correspondence vol. 16, letter from A. C. L. G. Günther, 13 May 1868.
6
In Descent 2: 18–23, CD concluded that ornamentation in fish occurs more frequently in males, and considered this in relation to their vulnerability to predation. In Descent 2: 9, CD noted that fish of the Cyprinodontidae inhabit fresh water.
7
CD was aware of fish in Brazil that made melodious sounds (see letter from Fritz Müller, 17 July 1867 and n. 18). CD mentioned the topic in Descent 2: 23, 331.
8
In Descent 1: 307–9, CD discussed the numerical proportions of the sexes in fish; he discussed numerical proportions of the sexes in relation to sexual selection in ibid., pp. 263–5. CD’s notes on the numerical proportions of the sexes in animals and humans are in DAR 85 and 86; a note on the proportion of Cyprinodont sexes is in DAR 86: A49.
9
In Descent 2: 19, CD noted that the fertilised ova of most fish are left unprotected by both sexes after oviposition. On numerical proportions of the sexes, see n. 8, above. In Descent 1: 271 and 2: 2, CD referred to the polygamous habit of the common stickleback, Gasterosteus leiurus (now G. aculeatus aculeatus).
10
In Descent 2: 14, CD referred to sticklebacks, together with tench, roach, perch, salmon, and pike, as examples of fish in which striking colour develops during the breeding season in males only.

Please cite as “DCP-LETT-5734,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-5734