21] But in the majority of cases the males _follow_ the
female, hovering over her in a position which would render it almost
impossible for her to see the particular colours or patterns on his
upper surface; to do so the female should mount higher than the male,
and fly towards him--being the seeker instead of the sought, and this is
quite opposed to the actual facts. I cannot, therefore, think that this
suggestion adds anything whatever to the evidence for sexual selection
of colour by female butterflies. This question will, however, be again
touched upon after we have considered the phenomena of sexual colour
among the vertebrata.

_Sexual Coloration of Birds._

The general rule among vertebrates, as regards colour, is, for the two
sexes to be alike. This prevails, with only a few exceptions, in fishes,
reptiles, and mammalia; but in birds diversity of sexual colouring is
exceedingly frequent, and is, not improbably, present in a greater or
less degree in more than half of the known species. It is this class,
therefore, that will afford us the best materials for a discussion of
the problem, and that may perhaps lead us to a satisfactory explanation
of the causes to which sexual colour is due.

The most fundamental characteristic of birds, from our present point of
view, is a greater intensity of colour in the male. This is the case in
hawks and falcons; in many thrushes, warblers, and finches; in pigeons,
partridges, rails, plovers, and many others. When the plumage is highly
protective or of dull uniform tints, as in many of the thrushes and
warblers, the sexes are almost or quite identical in colour; but when
any rich markings or bright tints are acquired, they are almost always
wanting or much fainter in the female, as we see in the black-cap among
warblers, and the chaffinch among finches.

It is in tropical regions, where from a variety of causes colour has
been, developed to its fullest extent, that we find the most remarkable
examples of sexual divergence of colour. The most gorgeously coloured
birds known are the birds of paradise, the chatterers, the tanagers, the
humming-birds, and the pheasant-tribe, including the peacocks. In all
these the females are much less brilliant, and, in the great majority of
cases, exceptionally plain and dull coloured birds. Not only are the
remarkable plumes, crests, and gorgets of the birds of paradise entirely
wanting in the females, but these latter are usually without