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Darwin Correspondence Project

To B. D. Walsh   31 October 1868

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

Oct 31. 1868

My dear Sir

I am very much obliged for the extracts about the “drumming” which will be of real use to me.1

I do not at all know what to think of your extraordinary cases of the cicadas.2 Prof. Asa Gray & Dr Hooker were staying here & I told them of the facts.3 They thought that the 13-year & the 17-year forms ought not to be ranked as distinct species unless other differences besides the period of development could be discovered.4 They thought the mere rarity of variability in such a point was not sufficient, & I think I concur with them.

The fact of both the forms presenting the same case of dimorphism is very curious.5 I have long wished that some one would dissect the forms of the male stag-beetle with smaller mandibles to see if they were well developed, i.e. whether there was an abundance of spermatozoa, & the same observations ought, I think, to be made on the rarer form of your cicada. Could you not get some observer such as Dr Hartman6 to note whether the females flocked in equal numbers to the “drumming” of the rarer form as to the common form?

You have a very curious & perplexing subject of investigation & I wish you success in your work—

My dear Sir | yours very sincerely | Charles Darwin


For one of the extracts sent by Walsh, see the memorandum from B. D. Walsh, [before 31 October 1868].
See letter from B. D. Walsh, 29 August 1868. CD also commented on the cicadas in his letter to Walsh of 21 September 1868.
A group of guests, including Asa Gray and Joseph Dalton Hooker, stayed at Down House from October 24 (see letter to John Tyndall, 20 October 1868 and n. 1).
Walsh had described another difference between the two cicada forms in the shape of the genital hooklet (see letter from B. D. Walsh, 29 August 1868).
No letter from Walsh containing such a reference to dimorphism has been found.
CD refers to William Dell Hartman (see memorandum from B. D. Walsh, [before 31 October 1868]).


Thanks BDW for extracts about "drumming" [of male Cicada to attract females].

Asa Gray and Hooker doubt that 13–year and 17–year Cicada forms should be considered distinct species. CD is inclined to agree with them.

Suggests observations be made of ratio of females to males in the rarer form.

Letter details

Letter no.
Charles Robert Darwin
Benjamin Dann Walsh
Sent from
Source of text
Field Museum of Natural History, Chicago (Walsh 16)
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 6437,” accessed on 20 January 2022,

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