The Descent of Man

In a siluroid fish, inhabiting the fresh waters of
South America, the Plecostomus barbatus (18.
See Dr. Gunther on this genus, in 'Proceedings
of the Zoological Society,' 1868, p. 232.) (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, it 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 hookshaped
bone on the top of the head, directed