x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 293 
but must bo rather injurious than beneficial in the bird’s ordi¬ 
nary life. The fact that they have been developed to so great 
an extent in a few species is an indication of such perfect adapta¬ 
tion 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 Avay 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 them¬ 
selves 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 genera¬ 
tion. 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 
