belonging to two distinct Orders, have the extraordinary habit of
hatching within their mouths, or branchial cavities, the eggs laid by
the females. (38. Prof. Wyman, in Proc. Boston Soc. of Nat. Hist.
Sept. 15, 1857. Also Prof. Turner, in Journal of Anatomy and
Physiology, Nov. 1, 1866, p. 78. Dr. Gunther has likewise described
other cases.) I am informed by Professor Agassiz that the males of the
Amazonian species which follow this habit, not only are generally
brighter than the females, but the difference is greater at the
spawning-season than at any other time. The species of Geophagus act
in the same manner; and in this genus, a conspicuous protuberance
becomes developed on the forehead of the males during the
breeding-season. With the various species of Chromids, as Professor
Agassiz likewise informs me, sexual differences in colour may be
observed, 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, over which they
sit, as our Pomotis does. It ought also to be observed that these
sitters are among the brightest species in their respective families;
for instance, Hygrogonus is bright green, with large black ocelli,
encircled with the most brilliant red. Whether with all the species of
Chromids it is the male alone which sits on the eggs is not known. It
is, however, manifest that the fact of the eggs being protected or
unprotected by the parents, has had little or no influence on the
differences in colour between the sexes. It is further manifest, in all
the cases in which the males take exclusive charge of the nests and
young, that the destruction of the brighter-coloured males would be far
more influential on the character of the race, than the destruction of
the brighter-coloured females; for the death of the male during the
period of incubation or nursing would entail the death of the young, so
that they could not inherit his peculiarities; yet, in many of these
very cases the males are more conspicuously coloured than the females.
In most of the Lophobranchii (Pipe-fish, Hippocampi, etc.) the males
have either marsupial sacks or hemispherical depressions on the
abdomen, in which the ova laid by the female are hatched. The males
also shew great attachment to their young. (39. Yarrell, History of
British Fishes, vol. ii. 1836, pp. 329, 338.) The sexes do not
commonly differ much in colour; but Dr. Gunther believes that the male
Hippocampi are rather brighter than the females. The genus Solenostoma,
however, offers a curious exceptional case (40. Dr. Gunther, since
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