The Descent of Man

occasionally in a considerable degree from the
females. This is a significant fact, for such differences
in colour must be accounted for by some
of the variations in the males having been from
the first limited in transmission to the same sex;
as it can hardly be maintained that these differences,
especially when very slight, serve as a
protection to the female. Thus all the species in
the splendid group of the Trogons build in
holes; and Mr. Gould gives figures (21. See his
Monograph of the Trogonidae, 1st edition.) of
both sexes of twenty-five species, in all of
which, with one partial exception, the sexes
differ sometimes slightly, sometimes conspicuously,
in colour,—the males being always
finer than the females, though the latter are
likewise beautiful. All the species of kingfishers
build in holes, and with most of the species the
sexes are equally brilliant, and thus far Mr. Wallace's
rule holds good; but in some of the Australian
species the colours of the females are
rather less vivid than those of the male; and in