x COLOURS AXD ORXAMEXTS CHARACTERISTIC OF SEX 293 



but must be 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 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 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 forni 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 



