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Darwin Correspondence Project

From George Fraser   12 April 1871

The Crescent. | 169, Camden Road, N.W.

12 April 1871.

To | Chas. Darwin Esqr.

Sexual Selection—Butterflies.

Dear Sir,

I take the liberty of sending you some notes on sexual differences in British butterflies, which may, as I hope, prove of some little use to you.

In Vol 1. of “The Descent of Man” p. 396 you say, referring to butterflies, that “the lower surface (of the wings) generally affords to entomologists the most useful characters for detecting the affinities of the various species”.1 I think also, sir, that this lower surface should furnish another link in your chain of argument as to Sexual Selection. Thus, for example, in the Cabbage butterflies, the under-surface is alike in both sexes of Pieris Brassicae. The black spots which appear on both surfaces in the female vanish from the upper surface in the male, probably because the female has some dislike to them. There can be no difference of food-plant, of habit, or of protection here; the only explanation seems to be a whim of the female, or a whim of Nature—and we have lately discarded all thought of Nature being freakish. In P. Rapae & P. Napi a similar difference prevails, though less constant & in a degree less marked.2 In Anthocharis Cardamines the under surface of both sexes is alike, notwithstanding the vast difference of their upper surfaces.3 When these butterflies alight & close their wings, the under-surfaces of the hind wings are alone visible; & these are apparently the parts of the insects modified for the sake of protection. The simple yellow in Brassicae and Rapae, the green-veined yellow in Napi, & the green marbling in Cardamines of the under sides of the hind-wings are well fitted to conceal those insects as they settle on the wild-flowers which they prefer.—

Again in Hipparchia Janira the light brown patch so conspicuous on the upper surface of the fore-wing of the female vanishes from that of the male; & in H. Tithonus & H. Hyperanthus a tendency to decrease the quantity of light colour on the upper surface of the male butterfly prevails.4 So it is also with Thecla Betulae—the under-surface still being alike in both sexes of these different species.5 The female butterflies, in this case, would seem to wish their partners to be of a duskier hue than it is granted to themselves to be. The differences mentioned above are so slight that you say at page 387 “with those (butterflies) which are plain coloured, as the meadow-browns (Hipparchiae), the sexes are alike”.6 But I think you will admit, sir, that though these differences are slight, they are yet important, as showing a tendency more or less marked to follow the rule which you have laid down; and every sign of such a tendency strengthens your case.—

In Apatura Iris the under-surface of both sexes is alike, though the male has his upper surface glorified with purple for the delight of his plain brown wife.—7 In the Blues, Polyommatus Alexis, P. Corydon, P. Adonis, P. Ægon, the under-surface of both sexes is also alike, tho’ in the males the blue & in the females the brown of the upper surface forms the background for the spotty design.8 The blue blood is very strong in these butterflies, & will show itself sometimes even in the females: they, if powerless over their own decoration, have at least succeeded in bringing out the innate splendour of their handsome husbands. With the blues as with the Cabbage butterflies, the under-surface of the hind-wings seems specially adapted for protective purposes; every butterfly-hunter knows how difficult it is to distinguish the common blue when it is sitting, shut up, on a scabious flower. It is the same with the small copper which has its under-surface dotted very similarly.9 But burnished copper and dazzling blue are not colours for protection, surely. We may give the under-surface to Mr. Wallace; but I think we must all yield the upper surface to you, sir.—10

With regard to the Ghost moth (Hepialus humuli) of which, & some others of the moth kind you say at p. 399— “It is difficult to conjecture what the meaning can be of these differences between the sexes in the shades of darkness or lightness; but we can hardly suppose that they are the result of mere variability, with sexually-limited inheritance, independently of any benefit thus derived”.—11 The female Ghost follows your rule that females are most conservative of the features of kinship. She is, in her colouring, closely allied to the other Hepialidae. And the male, notwithstanding his shining shroud, keeps to the same sober under-colouring as his mate.— I believe H. humuli is more nocturnal in its habits than any of the other species in the genus Hepialus; for I have caught H. hectus & H. lupulinus flying in bright sunshine, but I have never seen the ghost moth until dusk was far advanced.12 Might it not be that here sexual selection came into play by the female choosing the whitest male, as he would be the most distinguished at the hour when all colour fades into dimness? She would not be able to discover elaborate patterns in gold and amber, but a snow-white surface would then be quite visible.13 One would think the large expanse of white somewhat dangerous to the male ghost at a time when his enemies the bats and owls are so vigilant: but perhaps his very distinction protects him—or his eccentric flight, which is puzzling enough to his human enemies.

I shall be very glad to hear if you think there is anything valuable in these suggestions which I have ventured to lay before you. Should you think so, will you permit me to send a copy of this note & of your reply to it for insertion in Nature?—14

I am, dear sir, | yours most respectfully | George Fraser

CD annotations

2.6 The black … to them. 2.8] scored red crayon
3.1 Again … male; 3.2] scored red crayon
3.7 The differences … your case.— 3.12] scored red crayon
4.1 In … design. 4.5] scored red crayon
4.10 every … sir.— 4.15] scored red crayon
5.1 With regard] after opening square bracket red crayon

Footnotes

Descent 1: 396–7.
Pieris brassicae is the cabbage butterfly, or cabbage white. Pieris rapae is the small cabbage white. Pieris napi is the green-veined white butterfly. Fraser uses ‘cabbage butterflies’ as a group name to describe all three species.
Anthocharis cardamines is the orange-tip butterfly. CD had discussed the markings of male and female A. cardamines in Descent 2: 388 and 409.
Hipparchia janira is now Maniola jurtina, the meadow-brown butterfly. Hipparchia tithonus is now Pyronia tithonus, the gatekeeper, and H. hyperanthus is now Aphantopus hyperantus, the ringlet.
Thecla betulae is the brown hairstreak butterfly.
Descent 1: 387. Fraser uses the word ‘hipparchia’ here informally to describe the group of meadow-browns.
Apatura iris is the purple emperor.
Polyommatus alexis is now Polyommatus icarus, the common blue; P. corydon is now Lysandra coridon, the chalkhill blue; P. adonis is now L. bellargus, the adonis blue; P. aegon is now Plebejus argus, the silver-studded blue.
The small copper is Lycaena phlaeas.
Alfred Russel Wallace believed that coloration in butterflies was a protective mechanism developed through natural selection, while CD attributed it at least in part to sexual selection. CD published a discussion of their differing views in Descent 1: 387–415.
Descent 1: 399.
Hepialus hectus is now H. hecta, the gold swift moth; H. lupulinus is the common swift moth.
CD cited Fraser’s comments on ghost moths (published as G. Fraser 1871) in Descent 2d ed., p. 316 n. 21.
Fraser published this letter (without a reply from CD) in Nature, 20 April 1871 (G. Fraser 1871); CD cited it in Descent 2d ed., pp. 312, 316 n. 21.

Bibliography

Descent 2d ed.: The descent of man, and selection in relation to sex. By Charles Darwin. 2d edition. London: John Murray. 1874.

Descent: The descent of man, and selection in relation to sex. By Charles Darwin. 2 vols. London: John Murray. 1871.

Fraser, George. 1871. Sexual selection. Nature, 20 April 1871, p. 489.

Summary

On sexual selection in butterflies. [See GF’s article in Nature 3 (1870–1): 489; also Descent (1875): 312.]

Letter details

Letter no.
DCP-LETT-7677
From
George Fraser
To
Charles Robert Darwin
Sent from
London, Camden Rd, 169
Source of text
DAR 89: 100–2
Physical description
6pp †

Please cite as

Darwin Correspondence Project, “Letter no. 7677,” accessed on 4 July 2020, https://www.darwinproject.ac.uk/letter/DCP-LETT-7677.xml

Also published in The Correspondence of Charles Darwin, vol. 19

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