ructure and
environment) has been the cause, and not the effect, of the similarity
or differences of the sexes as regards colour. When the confirmed habit
of a group of birds, was to build their nests in holes of trees like the
toucans, or in holes in the ground like the kingfishers, the protection
the female thus obtained, during the important and dangerous time of
incubation, placed the two sexes on an equality as regards exposure to
attack, and allowed "sexual selection," or any other cause, to act
unchecked in the development of gay colours and conspicuous markings in
both sexes.

When, on the other hand (as in the Tanagers and Flycatchers), the habit
of the whole group was to build open cup-shaped nests in more or less
exposed situations, the production of colour and marking in the female,
by whatever cause, was continually checked by its rendering her too
conspicuous, while in the male it had free play, and developed in him
the most gorgeous hues. This, however, was not perhaps universally the
case; for where there was more than usual intelligence and capacity for
change of habits, the danger the female was exposed to by a partial
brightness of colour or marking might lead to the construction of a
concealed or covered nest, as in the case of the Tits and Hangnests.
When this occurred, a special protection to the female would be no
longer necessary; so that the acquisition of colour and the modification
of the nest, might in some cases act and react on each other and attain
their full development together.

_Exceptional Cases confirmatory of the above Explanation._

There exist a few very curious and anomalous facts in the natural
history of birds, which fortunately serve as crucial tests of the truth
of this mode of explaining the inequalities of sexual colouration. It
has been long known, that in some species the males either assisted in,
or wholly performed, the act of incubation. It has also been often
noticed, that in certain birds the usual sexual differences were
reversed, the male being the more plainly coloured, the female more gay
and often larger. I am not, however, aware that these two anomalies had
ever been supposed to stand to each other in the relation of cause and
effect, till I adduced them in support of my views of the general theory
of protective adaptation. Yet it is undoubtedly the fact, that in the
best known cases in which the female bird is more conspicuously coloured
than the male, it