The Descent of Man

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 (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.
(17. Dr. Gunther makes this remark; 'Catalogue
of Fishes in the British Museum,' vol. iii. 1861,
p. 141.)
[Fig.31. Plecostomus barbatus. Upper figure,
head of male; lower figure, female.]