Dear Sir,

D^r. Gray has forwarded to me your letter of the 2^d. 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 | L^s. Agassiz

Ch. Darwin Esq.