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Darwin Correspondence Project

From Alexander Wallace   25 February 1868

Beverley Road House,| Colchester.

Feby 25— 68.

C. Darwin Esq

Dear Sir

Though not a convert to your views, I shall be glad to aid you, by an information I can afford. Possibly however, if you more fully explained the exact point you wished to work out, I could keep it in view in my experiments on Silkworms & aid you by more accurate information—1 So many numerous extraneous accidents interfere with the life of silkworms as to render it very difficult to arrive at accurate conclusions— For instance your 1st question has reference to the proportion of sexes. Now do you wish for information on this point as regards a whole brood brought up from the entire no of eggs laid by two healthy parents only? Or do you apply to your question to the mixed eggs of a no of silkworms exposed to all the accidents of outdoor life— In the former case I apprehend the information might at first sight be more trustworthy but then a probably disturbing cause would come in to operation viz the effect of domestication, a better supply of food, and mans interference.— In the second case, the accidents to silkworms from enemies weather &c are so numerous that information for large numbers could alone in my opinion be trustworthy.

As far as my observation is worth anything I believe the males slightly exceed the females in number—. I know they come out earlier and are abundant before the ♀s are numerous— subsequently the females are more numerous than the males.

I have now 1000 pupæ of B. Cynthea2 sent to me by a friend not of my own rearing: these are out of their cocoon & could perhaps easily be separated, but error might creep in. In my experiment, the cocoons are strung up inside a canary House where the moths emerge and pair as in a state of nature the trees growing inside— It would be easy for me another year perhaps to obtain some more accurate information for you if you wish it—on this point.3

I have seen it stated by French writers on the silkworm that the sexes are evenly distributed: vide also Count Dandolo p. 242: ‘S’il y a plus des males il faut les jeter, s’il y a plus des femelles, on peut leur donner les males, qui aient été deja accouplés: he goes on to say that the offspring of the second union of the same male with a second female—is by many not considered so vigorous, as the offspring of the first union,4 and this I believe, vide also an experiment of mine—Ailanthi culture. p. 203–p 207 is detailed the history of a brood of 101. cocoons, out of which 39 ♀s 46 ♂s emerged, and afterwards ♀ 9. & ♂ 6.— It is a common thing for the same male to unite himself to a second perhaps even to a 3rd female—.5

2 I have seen 2 or more males flying about the same ♀   I never saw them fight. the same thing may be noticed in the common white butterfly—

It is no uncommon thing for me to find in the morning ♀ & ♂ united on a leaf or cocoon, & a 2nd male close by, asleep, with his tail in the direction of the ♀ body, as if he had been just anticipated by the male in possession

3 I do not think the ♀ evince any choice. I have had frequently 300 or more moths at large in my ailanthi, & only there some of the most vigorous ♀ will be found mated with much stunted ♂ and vice versa—though in a less degree—for I apprehend the amount power of attraction in the ♀ depends on her vitality and if she is weakly—the vigorous male will be attracted to a more vigorous neighbour than she—6 But a male with his wings rubbed will probably have also his tarsi broken, & then he cannot effect coition, not being able to alight & steady himself: and this almost always occurs to the male when about 3 days old. Coition therefore to be effectual must take place early—at least in the case of the male— Once knocked about he would be unable to perform his duties. But I do not think that the ♀ is alive & sensible to all this. She is very passive & quiet—extruding her ovipositor—and the first that arrives, suceeds.

Lastly: as to B Cynthia: the ♀ emerges before 8 oclock pm. She generally copulates that night, & remains in cop till the following evening, when she at once begins to deposit her eggs. I think you would be interested in my essay on ailanthiculture, where I have closely detailed the life history of Cynthia.

I shall be at all times willing to render you what aid I can

Yrs truly | Alex Wallace—

CD annotations

1.1 Though … outdoor life— 1.9] crossed pencil
1.6 Now … only? 1.8] scored blue crayon
1.8 Or do … outdoor life— 1.9] scored blue crayon
2.1 As far … number—. 2.2] double scored blue crayon
2.2 I know … males. 2.3] double scored red; ‘So the latter ♀ could hardly breed.’ added pencil
2.2 the ♀s are numerous— 2.3] double underl blue crayon; ‘see p 487 of Dr W Pamphlet | See last Page of this note’7 added red crayon
3.1 I have now … point. 3.6] crossed pencil
4.2 “S’il … first union, 4.6] scored red crayon
4.4 second union] underl red crayon
4.6 this I believe] underl red crayon
4.6 Ailanthi culture.... 6.— 4.8] scored blue crayon; ‘= 48 ♀ 52 ♂’ added pencil
5.1 2] enclosed in brackets blue crayon
5.1 2 … she— 7.5] heavily crossed pencil
7.1 3] enclosed in brackets blue crayon
7.2 & only … degree— 7.3] scored blue crayon; ‘good quote [see] Walsh [illeg]added blue crayon
7.5 neighbour than she—] closing square bracket blue crayon
7.2 some of … stunted ♂ 7.3] ‘The reverse case, apparently is not so common; he believes that vigorous males would pass over weakly females & pair with those [possessed] most in vitality.’ interl ink
7.5 But … Cynthia 8.4] crossed pencil
7.5 But a male … himself: 7.7] scored red crayon
7.6 & then … early— 7.9] scored red crayon
7.9 at least … duties. 7.10] scored red crayon
Top of letter: ‘Similarity in size of male & female [3 words illeg]pencil del pencil; ‘(Sexes)’ pencil, square brackets in MS; ‘I think sexes do not differ in colour in B. cynthia & are protective ∴ experiment of females choosing not good.—’ ink


CD’s letter to Wallace has not been found. CD had mentioned that he intended to write to Wallace for information on silkworms in his letters to H. T. Stainton, 21 February [1868], and Roland Trimen, 21 February [1868].
Bombyx cynthia is now Samia cynthia (the Ailanthus silk-moth).
CD reported the results of three subsequent breeding experiments by Wallace in Descent 1: 313.
Wallace quotes from the treatise on silkworms by Vincenzo Dandolo. The first Italian edition is Dandolo 1815. There were numerous French editions, and an English edition (Dandolo 1825). The quotation may be found on p. 230 of the third French edition (Dandolo 1830), and reads: ‘If there are more males they must be thrown away; but if there is an excess of females, males must be given to them, that have already coupled.’ The passage on the vigour of the offspring is in Dandolo 1830, pp. 234–5. CD had discussed silk-moths at length in Variation and had referred extensively to French literature on the subject, as well as to Dandolo 1825 (see Variation 1: 300–4).
Wallace’s experiment is described in ‘Ailanthiculture; or, the prospect of a new English industry’ (A. Wallace 1866, pp. 203–7). Ailanthus glandulosa (now Ailanthus altissima, tree of heaven) is the tree on which Samia cynthia feeds.
Wallace’s observations were reported in Descent 1: 401.
CD refers to another paper by Wallace, ‘On some variations observed in Bombyx Cynthia, in 1866’ (A. Wallace 1867); it is cited in Descent 1: 310, 346.


Dandolo, Vincenzo. 1815. Dell’arte di governare i bachi da seta per trarre costantemente da una data quantità di foglia di gelso la maggior copia di ottimi bozzoli e dell’influenza sua sull’aumento annuo di ricchezza sì domestica che nazionale. Milan: Sonzogno.

Dandolo, Vincenzo. 1825. The art of rearing silk-worms. London: John Murray.

Dandolo, Vincenzo. 1830. L’art d’élever les vers à soie pour obtenir constamment d’une quantité donnée de feuilles de mûrier la plus grande quantité possible de cocons de première qualité, et de l’influence de cet art sur l’augmentation annuelle des richesses des particuliers et des nations. Translated by François Philibert Fontaneilles. 3d edition. Lyon: Bohaire.

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

Variation: The variation of animals and plants under domestication. By Charles Darwin. 2 vols. London: John Murray. 1868.

Wallace, Alexander. 1866. Ailanthiculture; or, the prospect of a new English industry. Transactions of the Entomological Society of London 3d ser. 5 (1865–7): 185–245.

Wallace, Alexander. 1867. On some variations observed in Bombyx Cynthia, in 1866. [Read 4 February 1867.] Transactions of the Entomological Society of London 3d ser. 5 (1865–7): 485–92.


Asks CD to make his queries about proportion of sexes more precise so he can keep them in mind in his experiments with silkworms. [see Descent 1: 313.]

Letter details

Letter no.
Alexander Wallace
Charles Robert Darwin
Sent from
Source of text
DAR 85: B69–70
Physical description
4pp ††

Please cite as

Darwin Correspondence Project, “Letter no. 5933,” accessed on 23 January 2021,

Also published in The Correspondence of Charles Darwin, vol. 16