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Darwin Correspondence Project

From Alexander Wallace   28 February 1868

Beverley Road House, | Colchester.

Feby 28. 68.

Chas Darwin Esq

Dear Sir

I am gratified to have given you useful information.1 My observations are always freely at your service. There are some points in your letter this morning received that seem to deserve notice from me2—with regard to the preponderance of males & females captured either at sugar light or otherwise. that can be no guide to the real preponderance of sexes as born—or rather (as I believe) as ova— I will cite 2 instances of the fallacy of estimation by captured insects—(1) the common eggar—B. Quercus3—or the Kentish Glories—Endromis versicolor—may be captured ad infinitum by exhibiting a newly born female, but only males will be so attracted—this is an extreme instance but it only shews that the modes of hunting for & finding insects are very limited— 2. Nyssia Hispidaria is or was taken at Richmond Park4 on certain oak trees (the imago) by me rather freely one year—but all males—we very rarely got the female—which is apterous.— One day however, noted in my memory, I and another great friend went down for a stroll on a Good Friday. I was then younger than I am now—and I trust less wise. It was a rare day—a very high wind sunshine hail rain & sleet by turns— the apterous females were out, but either dare not run up the trees as is their wont or were blown down. The result was we captured 60 odd ♀s between us, to about 20 ♂s, a most unusual circumstance— & one that never again recurred to me— Both these are simply unusual instances of the capturing of one sex under favorable conditions & lead to the inference that the habits of the sexes being different, we can best capture the one or the other as our knowledge of their habits & opportunities of getting at them—are combined in ones favour—

It is quite different with bred insects, but on consideration I cannot allow that domestication & protection afforded by man can alter the sex in the larval state—though it may prove more than usually favorable to the development of the insect viz. the female larva is generally larger as the imago is greater than the ♂ imago ∴ the ♀ larva requires more food & more time for development. ∴ She runs a greater risk from enemies accidents of temperature ichneumon. &c.   these causes may operate & probably do to destroy some ♀s & are avoided by the protection afforded by man.5 But when you hint that the future sex of the imago may be influenced by accidental circumstances such [as] food &c in the larval age—I cannot agree. For I apprehend there is one law to all created things as to this point, omne vivum ex ovo aut mas. aut femina” is my motto,6 and I can as soon conceive of an insect, as of a human embryo changing its sex during development by starvation or plenty. Besides were it so my half starved larvae of Cynthia7 fed on celery would then have been all one sex—whereas both males & females in apparently the usual proportions were born from the stunted half starved broods. No, I apprehend the law not yet understood which fixes the sex is the same for all created things and I suppose you are aware of the various theories propounded by various medical & scientific observers. I think the one which seems to agree best with what is observed is that conception follows in the human race menstruation, & if but two or 3 days have intervened after the cessation of menstruation the product is ♀, if more than 3 days, the sex is ♂—& by the bye that reminds me as bearing on the point of preponderance of sexes in the human species ♂s slightly preponderate   How is with horses? the stud book contains the pedigrees of all race horses—& will give statistics as to sex—which preponderates.8   have you investigated this point. & here let me recount an anecdote mentioned to me today by a medical friend a naturalist, with whom I chatted on the point—

Certain factory girls in the North—of loose morals avoid the consequences of illicit intercourse by permitting coition only after 6 clear day’s have elapsed after the cessation of menstruation— Now do these facts in any way—point to a similar law in other created beings. It is worth investigation & experiment—but in insects it could only be worked out by rearing the whole brood from the eggs & marking the result—varying the date of coition with the different females for the period of emergence— Now in the experiment quoted by me—in my last letter, p 207, Ailanthiculture, 101 cocoons were obtained.9 I find a reference to my Diary the following May 22, 1st B Cynthia out a fine ♂. 23rd. ♀ B Cynthia out the 2nd in evening. 24th pair in cop. that evening   she commenced to oviposit—no others being out   She had laid on the 26 42—27th. 49 more. 28th 11. total 102— from this brood, brought up on a tree in a pot in the Greenhouse & a room upstairs, I obtained the 101. cocoons alluded to where there was a very slight preponderance of male insects—

The sexes of B Cynthia are very easily distinguished by the educated eye & as I in the season daily separated f 30. to 60 pairs or upwards retaining only the ♀ & permitting the males to escape, It is not likely that I should confuse the sexes in my observations. all silkworms & indeed I think all insects are easily distinguished by close and interested observers. I am not prepared to admit with you that the ♀ of some Lepidoptera notice the gay colors of their partner. It may be so in some foreign instances, but I have met with no observations on our English Lepidoptera bearing on the point   In fact as to Cynthia I believe I may state the opposite—as I have seen beautiful ♀s fresh & fine ally themselves with males, sometimes ragged torn & faded   at other times with males of very pale dingy colors, and the converse— I think therefore as I stated before that it is all accident.10 Whichever arrives first at the ♀ expectant has the entrée, but I am very willing to bear the point in mind for future observation— The female being mostly stationary the males active & volant, in fact unless the weather is very warm the greater number of pairs are found settled on the cocoons strung up, whence I concluded that the female just emerged, remained stationary until after coition was concluded. I still think that the males preponderate largely & that the experience of breeders to the contrary must be more closely investigated— It may make all the difference whether they have bred from the egg in batches or whether the larvæ have been collected— I will endeavour to procure some information on this point from Dr Knapp11 who has bred largely one individual species from eggs— My larvæ are bred out in the open air as nearly as possible in a state of nature & therefore uninfluenced by external conditions. As I have witnessed the act of calling in many instances I might be permitted to describe it   about 9 pm. at dusk in the June and July evenings the newly born ♀ without altering her position either on the leaf or cocoon, with no action of her wings, extrudes her ovipositor to the extent of 18 inch


thus— this can be easily seen. if touched by a pencil it retracts but is again shortly extended— the male attracted by this (I presume unknown to us) sense—flutters about around above—alights on the abdomen of the ♀ and if possible embraces her as it were with his pretty feet abd. to abd. the curved tail applies itself to the ♀ organ and the act is commenced. afterwards fatigued he relaxes his hold and hangs down sometimes suspended at other times in the easiest position. coition continues for 24 hours—

One word more, I have heard a curious statement   possibly you can deny or confirm it. 1 Wheat, 2 Barley 3. Oats, if one of the three be planted (I forget which) and allowed to produce a crop & be cut down, a crop of No 2 will be produced next year—if this again be similarly treated: No 3 will be produced the 3rd year— I cannot believe it yet as I said I have seen it in print & not denied. Surely these plants are annuals & cannot live two years?—12

Believe me yrs in haste | Alex Wallace

CD annotations

1.1 I am … favour— 1.22] crossed pencil
1.18 Both … favour— 1.22] scored blue crayon
2.9 I cannot … motto, 2.11] scored blue crayon; ‘sheep. [Kroll]—Theory’ added blue crayon
2.9 For I apprehend … insects— 3.14] crossed pencil
2.22 How is it witih horses?] underl blue crayon
4.1 The sexes … eye] scored blue crayon
4.1 as I … observers. 4.5] crossed pencil
4.5 I am not … partner 4.6] scored blue crayon; opening square bracket, pencil
4.8 as to Cynthia … accident. 4.11] scored blue crayon
4.12 but I am … future] underl pencil
4.16 I still think … investigated— 4.17] scored blue crayon
4.17 It may … collected— 4.19] double scored blue crayon
4.19 I will … her wings, 4.25] crossed pencil
5.1 One word … years?— 5.6] crossed pencil
5.1 One word … confirm it. 5.2] ‘X’ in margin, blue crayon
Top of letter: ‘Letter 2.’ blue crayon

CD note:

[‘Let’ del] Abstract of Letter 2. | Dr Wallace

p. 1. argues against estimation of sexes from capturing & breeding. | from number of males caught when female exposed. *(see in my Var. under Dom. case of female moths not attractive to males— under Sterility from confinement.) [square brackets in MS]13 &c &c not important

p. 2 Habits of sexes being different will account for different frequency

p. 3 female larva being larger may require more *& better [interl pencil] food & moisture for development & may perish more freely under nature. [‘p. 3 … nature.’ double scored red crayon]

His half-starved larvæ of B. cynthia fed unnaturally yielded apparently usual proportion of sexes.

p. 5. The sexes of B. cynthia easily distinguished by educated eye

p. 7. still thinks males of B. cynthia preponderate largely.

p. 7. All the difference whether eggs or young caterpillars collected— *(I suppose because finer & larger caterpillars wd be collected [double scored ink] [‘p. 2 … collected’ crossed pencil]


CD’s letter has not been found.
Wallace refers to the oak eggar (Bombyx quercus, now Lasiocampa quercus).
Nyssia hispidaria is now Apocheima hispidaria (the small brindled beauty); it is common in England and feeds on oak. Richmond Park is a large royal park in Richmond, Surrey, about twelve miles from central London.
CD cited Wallace on female caterpillars in Descent 1: 311.
Omne vivum ex ovo aut mas aut femina: all life is from the egg either male or female. CD had also inquired about the influence of external conditions on sex in his letter to H. T. Stainton, 21 February [1868].
Wallace refers to Bombyx cynthia (now Samia cynthia).
CD received information on the proportion of sexes in racehorces from the Racing Calendar (see letter from W. B. Tegetmeier, [before 15 February 1868] and n. 5).
Dr Knapp has not been further identified.
CD had previously received reports of the transmutation of various cereal plants (see, for example, Correspondence vol. 14, letter from G. S. Gibson, 7 July 1866, and letter from Thomas Rivers, 14 October 1866). For CD’s view, see Correspondence vol. 10, letter to J. B. Innes, 22 December [1862].
In Variation 2: 157–8, CD reported a number of cases in which insects had failed to breed in confinement.


Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 27 vols to date. Cambridge: Cambridge University Press. 1985–.

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.


Proportion of sexes in insects, captured and bred. [see Descent 1: 313.]

Letter details

Letter no.
Alexander Wallace
Charles Robert Darwin
Sent from
Source of text
DAR 85: B41–5
Physical description
8pp †, CD note

Please cite as

Darwin Correspondence Project, “Letter no. 5953,” accessed on 29 November 2021,

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