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Darwin Correspondence Project

From Louis Agassiz   22 July 1868


July 22nd, 1868.

Dear Sir,

Dr. Gray has forwarded to me your letter of the 2d. of this month. I feel bound to contradict an ugly expression you use in alluding to my estimate of yourself and I do not see a better way of showing you its inapplicability than by answering your questions in full & [inditing] my answer directly to yourself.1 It is true that I am and have been from the beginning an uncompromising opponent of your views concerning the transmutability of species, it is equally true that I hold these views as mischievous, because they lead to a looseness of argumentation which it has been the aim of the great naturalists of our age to eliminate; but there is nothing in this opposition which should blind me to the great value of your original researches; and as to allowing my feelings to get the mastery of my judgement I hope I shall rarely be guilty of such a mistake and your friends here and the warmest supporters of your doctrines owe me the justice to say that I have never expressed an unkind word concerning yourself.2

But let me answer your questions. It is true not only of the Amazonian fishes, but of a large number of the representatives of this class, from every part of the world in the warm as well as the colder Zones, that, as in the class of Birds, the males are more brightly colored than the females and this difference is generaly heightened at the spawning season; so that it may be said of a vast number of fishes that, like Birds, they have a wedding dress.3 The Amazonian fishes which hatch their eggs within the mouth are of this kind; not only are the males generaly brighter than the females, but the difference is greater at the spawning season, than at any other time.4 I have colored drawings of a number of them, taken at different seasons which show these differences beautifully. But it is not only between males & females that such differences obtain, they exist also between young & old and are so conspicuous that they have occasionaly led to specific distinctions, as was the case with many birds half a century ago, even among our european species, as for instance Larus nævius &c.5 To make my statement full, I ought to add that while the sexual differences, as far as color is concerned, run in the direction mentioned, there are species of sombre colors, the bright males of which are much darker than the darker hued females of some brillant species. Again these same differences are uniformely noticed in all the representatives of the family of Chromids whether they lay their eggs in the water among aquatic plants, or deposit them in holes leaving them to come out without further care, or build shallow nests in the river mud for them, over which they sit, as our Pomotis do. It ought not to be overlooked that these sitters are among the brightest in their respective families; Hygrogonus for instance is bright green, with large black ocelli, encircled with the most brillant red.6 The subject of coloration in fishes is full of interest but barely accessible in its generality, because we possess so few figures colored from life. I have had the mortification to find that the colors of the figures of Spix, the ichthyological collection of which I described, are mostly false to an extent which is incredible.7

Your second question relates to the conspicuous protuberance on the forehead of Geophagus. Let me first say that the genus Geophagus is not the only one of the family of Chromides which has such a projection; it occurs in many other members of that family and is most conspicuous in the genus Cichla proper. I have often observed these fishes at the time of spawning when the protuberance is largest and also at other seasons when it is totaly wanting and the two sexes show no difference whatever in the outline of the profile of the head; but I never could ascertain that they subserve any special function. The Indians know nothing about its use.8 They say however that during the spawning season they are often seen rubbing their head against submerged stumps of trees. The fact that these protuberances are transient brings them into the category of those swellings which appear about the head in some birds during the breeding season; they resemble still more the swellings of the hand of some Batrachians with which they hold their female during copulation.9 But I repeat it, I could not satisfy myself that the protuberance of the forehead of Chromids served any distinct purpose.

With much regard | yours truly | Ls. Agassiz

Ch. Darwin Esq.

CD annotations

0.3 Dear … questions. 2.1] crossed pencil
2.1 It is … Batrachians 3.13] enclosed in square brackets blue crayon


Agassiz refers to Asa Gray; CD’s letter to Gray has not been found. For CD’s view of Agassiz’s opinion of him, see the letter to J. D. Hooker, 28 July [1868].
On Agassiz’s opposition to CD’s theory at this time, see S. J. Gould 1979.
CD cited Agassiz for this information in Descent 2: 13.
CD quoted the latter part of this sentence in Descent 2: 20.
The juvenile Larus marinus had been classified as a separate species, L. naevius, by Linnaeus (see Linnaeus 1766–8, 1: 225, and Wiedemann 1817).
CD quoted from this sentence in Descent 2: 21. Cichlidae and damselfishes (Pomacentridae) were once classified as members of a single family, ‘Chromidae’ (Pauly 2004). Pomotis is a genus of sunfishes. Hygrogonus is a synomym of Astronotus. From the description, Agassiz was probably referring to a species now known as Astronotus crassipinnis, which is found in South America. In a list of queries to Albert Günther in DAR 82: B14, CD wrote: ‘Agassiz writes to me that certain chromides, as Pomotis, Hygrogonus &c sit on their nests; do you happen to know whether it is the male, or the female, or indifferently both sexes, which thus sit?’
Agassiz had arranged, described, and added anatomical observations to the illustrations of fish collected by Johannes Baptist von Spix on his expedition to Brazil with Karl Friedrich Philipp von Martius between 1817 and 1820 (Spix 1829).
CD quoted from this passage in Descent 2: 13.
CD repeated Agassiz’s likening of the protuberance on the forehead of male Geophagus to swelling on the heads of certain birds, but concluded that it was doubtful whether they were ornaments, in Descent 2: 13.


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

Gould, Stephen Jay. 1979. Agassiz’ later, private thoughts on evolution: his marginalia in Haeckel’s Natürliche Schöpfungsgeschichte (1868). In Two hundred years of geology in America. Proceedings of the New Hampshire bicentennial conference on the history of geology, edited by Cecil J. Schneer. New Hampshire: University of New England.

Linnaeus, Carolus (Carl von Linné). 1766–8. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. 12th edition. 3 vols. Stockholm: Laurentius Salvius.

Pauly, Daniel. 2004. Darwin’s fishes. An encyclopedia of ichthyology, ecology, and evolution. Cambridge: Cambridge University Press.

Spix, Johann Baptist von. 1829. Selecta genera et species piscium quos in itinere per Brasiliam annis 1817–1820: jussus et auspiciis Maximiliani Josephi I. Bavariae regis augustissimi peracto collegit et pingendos curavit Dr. J. B. de Spix. Edited by K. F. Ph. von Martius. Arranged, described, and with anatomical observations by J. L. A. Agassiz. Munich: C. Wolf.

Wiedemann, Christian Rudolph Wilhelm. 1817. Ein Wort vorläufiger Erinnerung vom Herausgeber. Zoologisches Magazin 1: 1–7.


LA clarifies his opposition to CD’s views, which does not blind him to the great value of CD’s original researches.

Answers CD’s questions regarding sexual coloration of Amazonian fishes and the protuberances on the head of male Geophagus and Cichla during the spawning season [see Descent, pp. 520, 529].

Letter details

Letter no.
Jean Louis Rodolphe (Louis) Agassiz
Charles Robert Darwin
Sent from
Nahant, Mass.
Source of text
DAR 82: B78–9
Physical description
ALS 4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 6286,” accessed on 8 February 2023,

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