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Darwin Correspondence Project

From Albert Günther   26 February 1874

British Museum

Febr. 26th. 1874

My dear Sir

The sexual difference in Mallotus has been known for a long time, as you will see on looking over the references in my Catalogue Vol. VI. p. 170, but I do believe I never thought of this fish, when giving you instances of this kind some years ago. The fish is called “Capelin”, & belongs to the family of Salmonidæ, order Physostomi.1

The genus Monacanthus belongs to the family Balistidæ or “Trigger-fishes”, order Plectognathi widely different from the former.2

What you say about the use of these brushes in mature males, astonishes me: and I must look up the article in the American Naturalist.3 It must be very strong evidence, indeed, from a reliable source, to remove any doubts I still entertain about it. There is nothing known of the propagation of Monacanthus. Excrescences homologous to the brush of Monacanthus, but in the form of spines or tubercles are found in Acanthurus, Naseus etc, which belong to the order Acanthopterygii.4 But in these genera they are equally developed in both sexes, but less so in the young than in the adult.

Who is your authority for the remarkable observation that 2 males attend 1 female among Cyprinidæ? I have watched them spawning over & over again, and never observed anything of the kind: they generally spawn in shoals, where all is confusion.5 I have seen single female Barbels attended by 2, 3, 4 & probably more males.6

Yours very truly | A Günther

I am, at present, deep in the Cephalopods, & had under my hand the very specimens you watched in St Jago, & the habits of which you describe in the “Journal”. The species is Octopus rugosus.7

I have come across a specimen in which the entire oral apparatus is absent; & there is no trace of an injury. The central opening leads direct into the stomach. This defective nutrition led to a very anomalous development of the various parts: arms of nearly normal size, suckers very small & retracted, body reduced to a small appendage.

CD annotations

4.1 Who … Cyprinidæ? 4.2] ‘Descent vol. I. p 309’ added pencil
6.1 … appendage. 7.5] crossed pencil


See letter to Albert Günther, 25 February 1874. Günther refers to his Catalogue of acanthopterygian fishes in the collection of the British Museum (A. C. L. G. Günther 1859–70). Mallotus (the genus of capelins) is now in the family Osmeridae in the order Osmeriformes.
Monacanthus is now in the family Monacanthidae (file-fishes), order Tetraodontiformes.
Acanthurus is the lancetfish or surgeonfish, and is now in the order Perciformes. The genus Naseus is now Naso (unicorn fishes), also in the order Perciformes, and in the same family, Acanthuridae, as Acanthurus.
In Descent 1: 309 n. 55, CD had cited Yarrell 1836 for this observation, and said that it was a well-known fact. He did not alter the text in the second edition.
In Britain, the fish known as the barbel is Barbus barbus, a member of the Cyprinidae family. Other barbels are mostly also in the genus Barbus.
CD described the behaviour of octopuses he found at St Jago (Santiago, Cape Verde) in Journal of researches 2d ed., pp. 6–8. Octopus rugosus is now O. vulgaris.


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

Günther, Albert Charles Lewis Gotthilf. 1859–70. Catalogue of acanthopterygian fishes in the collection of the British Museum. 8 vols. London: by order of the Trustees.

Journal of researches 2d ed.: Journal of researches into the natural history and geology of the countries visited during the voyage of HMS Beagle round the world, under the command of Capt. FitzRoy RN. 2d edition, corrected, with additions. By Charles Darwin. London: John Murray. 1845.

Yarrell, William. 1836. A history of British fishes. 2 vols. London: John Van Voorst.


Comments on several points in Descent,

doubts facts about Monacanthus brushes

and the two Cyprinidae males attending the female when spawning.

Letter details

Letter no.
Albrecht Carl Ludwig Gotthilf (Albert) Günther
Charles Robert Darwin
Sent from
British Museum
Source of text
DAR 89: 26–7
Physical description
ALS 4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 9316,” accessed on 1 October 2023,

Also published in The Correspondence of Charles Darwin, vol. 22