Descent of Man [ 1871]

In the very distinct family of the Cyprinodontidae- inhabitants of
the fresh waters of foreign lands- the sexes sometimes differ much
in various characters. In the male of the Mollienesia petenensis,* the
dorsal fin is greatly developed and is marked with a row of large,
round, ocellated, bright-coloured spots; whilst the same fin in the
female is smaller, of a different shape, and marked only with
irregularly curved brown spots. In the male the basal margin of the
anal fin is also a little produced and dark coloured. In the male of
an allied form, the Xiphophorus Hellerii (see fig. 30), the inferior
margin of the caudal fin is developed into a long filament, which,
as I hear from Dr. Gunther, is striped with bright colours. This
filament does not contain any muscles, and apparently cannot be of any
direct use to the fish. As in the case of the Callionymus, the males
whilst young resemble the adult females in colour and structure.
Sexual differences such as these may be strictly compared with those
which are so frequent with gallinaceous birds.*(2)

  * With respect to this and the following species I am indebted to
Dr. Gunther for information: see also his paper on the "Fishes of
Central America," in Transact. Zoological Soc., vol. vi., 1868, p.
485.
  *(2) Dr. Gunther makes this remark, Catalogue of Fishes in the
British Museum, vol. iii., 1861, p. 141.

  In a siluroid fish, inhabiting the fresh waters of South America,
the Plecostomus barbatus* (see fig. 31), the male has its mouth and
interoperculum fringed with a beard of stiff hairs, of which the
female shows hardly a trace. These hairs are of the nature of
scales. In another species of the same genus, soft flexible
tentacles project from the front part of the head of the male, which
are absent in the female. These tentacles are prolongations of the
true skin, and therefore are not homologous with the stiff hairs of
the former species; but it can hardly be doubted that both serve the
same purpose. What this purpose may be, is difficult to conjecture;
ornament does not here seem probable, but we can hardly suppose that
stiff hairs and flexible filaments can be useful in any ordinary way
to the males alone. In that strange monster, the Chimaera monstrosa,
the male has a hook-shaped bone on the top of the head, directed
forwards, with its end rounded and covered with sharp spines; in the
female "this crown is altogether absent," but what its use may be to
the male is utterly unknown.*(2)

  * See Dr. Gunther on this genus, in Proceedings of the Zoological
Society, 1868, p. 232.
  *(2) F. Buckland, in Land and Water, July, 1868, p. 377, with a
figure. Many other cases could be added of structures peculiar to
the male, of which the uses are not known.

  The structures as yet referred to are permanent in the male after he
has arrived at maturity; but with some blennies, and in another allied
genus,* 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,*(2) 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.

   * Dr. Gunther, Catalogue of Fishes, vol. iii., pp. 221 and 240.
  *(2) See also A Journey in Brazil, by Prof. and Mrs. Agassiz,
1868, p. 220.

  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."* 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.*(2) 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.*(3) Another striking instance out of many is afforded by
the male stickleback (Gasterosteus leiurus), which is described by Mr.
Warington,*(4) as being then "beautiful beyond description." The
back and eyes of the female are simply brown and the belly white.
The eyes of the male, on the other hand, are "of the most splendid
green, having a metallic lustre like the green feathers of some
humming-birds. The throat and belly are of a bright crimson, the
back of an ashy-green, and the whole fish appears as though it were
somewhat translucent and glowed with an internal incandescence." After
the breeding-season these colours all change, the throat and belly
become of a paler red, the back more green, and the glowing tints
subside.