genus (20. Dr. Gunther, Catalogue of Fishes, vol. iii. pp. 221 and
240.), a crest is developed on the head of the male only during the
breeding-season, and the body at the same time becomes more
brightly-coloured. There can be little doubt that this crest serves as
a temporary sexual ornament, for the female does not exhibit a trace of
it. In other species of the same genus both sexes possess a crest, and
in at least one species neither sex is thus provided. In many of the
Chromidae, for instance in Geophagus and especially in Cichla, the
males, as I hear from Professor Agassiz (21. See also A Journey in
Brazil, by Prof. and Mrs. Agassiz, 1868, p. 220.), have a conspicuous
protuberance on the forehead, which is wholly wanting in the females
and in the young males. Professor Agassiz adds, I have often observed
these fishes at the time of spawning when the protuberance is largest,
and at other seasons when it is totally wanting, and the two sexes shew
no difference whatever in the outline of the profile of the head. I
never could ascertain that it subserves any special function, and the
Indians on the Amazon know nothing about its use. These protuberances
resemble, in their periodical appearance, the fleshy carbuncles on the
heads of certain birds; but whether they serve as ornaments must remain
at present doubtful.
I hear from Professor Agassiz and Dr. Gunther, that the males of those
fishes, which differ permanently in colour from the females, often
become more brilliant during the breeding-season. This is likewise the
case with a multitude of fishes, the sexes of which are identical in
colour at all other seasons of the year. The tench, roach, and perch
may be given as instances. The male salmon at this season is marked on
the cheeks with orange-coloured stripes, which give it the appearance
of a Labrus, and the body partakes of a golden orange tinge. The
females are dark in colour, and are commonly called black-fish. (22.
Yarrell, History of British Fishes, vol. ii. 1836, pp. 10, 12, 35.)
An analogous and even greater change takes place with the Salmo eriox
or bull trout; the males of the char (S. umbla) are likewise at this
season rather brighter in colour than the females. (23. W. Thompson, in
Annals and Magazine of Natural History, vol. vi. 1841, p. 440.) The
colours of the pike (Esox reticulatus) of the United States, especially
of the male, become, during the breeding-season, exceedingly intense,
brilliant, and iridescent. (24. The American Agriculturalist, 1868,
p. 100.) Another striking instance out of many is afforded by the male
stickleback (Gasterosteus leiurus), which is described by Mr. Warington
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