acquired the habit of taking refuge from danger in holes or crevices in
the rocks.
With respect to the above groups in which the females are conspicuously
coloured and build concealed nests, it is not necessary to suppose that
each separate species had its nidifying instinct specially modified;
but only that the early progenitors of each group were gradually led to
build domed or concealed nests, and afterwards transmitted this
instinct, together with their bright colours, to their modified
descendants. As far as it can be trusted, the conclusion is
interesting, that sexual selection together with equal or nearly equal
inheritance by both sexes, have indirectly determined the manner of
nidification of whole groups of birds.
According to Mr. Wallace, even in the groups in which the females, from
being protected in domed nests during incubation, have not had their
bright colours eliminated through natural selection, the males often
differ in a slight, and 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 one splendidly-coloured
species, the sexes differ so much that they were at first thought to be
specifically distinct. (22. Namely, Cyanalcyon, Gould s Handbook to
the Birds of Australia, vol. i. p. 133; see, also, pp. 130, 136.) Mr.
R.B. Sharpe, who has especially studied this group, has shewn me some
American species (Ceryle) in which the breast of the male is belted
with black. Again, in Carcineutes, the difference between the sexes is
conspicuous: in the male the upper surface is dull-blue banded with
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