cock can, however, have no such use, but must
be rather injurious than beneficial in the bird's ordinary life. The
fact that they have been developed to so great an extent in a few
species is an indication of such perfect adaptation to the conditions of
existence, such complete success in the battle for life, that there is,
in the adult male at all events, a surplus of strength, vitality, and
growth-power which is able to expend itself in this way without injury.
That such is the case is shown by the great abundance of most of the
species which possess these wonderful superfluities of plumage. Birds of
paradise are among the commonest birds in New Guinea, and their loud
voices can be often heard when the birds themselves are invisible in the
depths of the forest; while Indian sportsmen have described the peafowl
as being so abundant, that from twelve to fifteen hundred have been seen
within an hour at one spot; and they range over the whole country from
the Himalayas to Ceylon. Why, in allied species, the development of
accessory plumes has taken different forms, we are unable to say, except
that it may be due to that individual variability which has served as
the starting-point for so much of what seems to us strange in form, or
fantastic in colour, both in the animal and vegetable world.

_Development of Accessory Plumes and their Display._

If we have found a _vera causa_ for the origin of ornamental appendages
of birds and other animals in a surplus of vital energy, leading to
abnormal growths in those parts of the integument where muscular and
nervous action are greatest, the continuous development of these
appendages will result from the ordinary action of natural selection in
preserving the most healthy and vigorous individuals, and the still
further selective agency of sexual struggle in giving to the very
strongest and most energetic the parentage of the next generation. And,
as all the evidence goes to show that, so far as female birds exercise
any choice, it is of "the most vigorous, defiant, and mettlesome male,"
this form of sexual selection will act in the same direction, and help
to carry on the process of plume development to its culmination. That
culmination will be reached when the excessive length or abundance of
the plumes begins to be injurious to the bearer of them; and it may be
this check to the further lengthening of the peacock's train that has
led to the broadening of the feathers at the end