io6 THE COURTSHIP OF ANIMALS 



Bird of Paradise, the general coloration being of a dull 

 brown hue ; but even here, a pair of wing-coverts are 

 produced into long, broad streamers, unique among birds ; 

 while the feathers of the throat and flanks are of a mar- 

 vellous metallic green, the flank-feathers being produced 

 to form a long, pointed tuft. 



To what factors must we attribute the growth ot these 

 wonderful colours, these strange outgrowths, frills, and 

 tufts, and streamers, the like of which is almost un- 

 paralleled ? In a group numbering some fifty or more 

 species there is not one that does not display some strange 

 feature. We cannot attribute it to the environment, for 

 in such case the results should have produced uniformity ; 

 nor can we invoke the aid of sexual selection save in a 

 very indirect manner, and in a sense other than generally 

 understood by this term. It seems, then, not unreasonable 

 to suggest that they are the expression points of the 

 internal metabolism : the manifestations of that tendency 

 to vary which is inherent in every fibre of the organism. 

 But no attempt shall be made to elaborate this theory till 

 more evidence has been taken. The humming-birds, and 

 the game-birds, are perhaps the only other groups which 

 exhibit quite such a prodigality of ornament ; of the 

 latter, instances have already been cited. 



So far the displays which have been described have been 

 such as are confined to the use of more or L.s resplen- 

 dent plumage. There are, however, many species which 

 contrive to secure most startling results, not so much by 

 the parade of coats of many colours as by grotesque changes 

 of shape produced by wind-bags of various kinds. The 

 Pouter-pigeon affords a case in point. This bird possesses 

 the power of inflating the gullet to an enormous size, so 

 as to produce a strangely distorted form, at any rate, 



