2i6 THE BIOLOGY OF BIRDS 



which the male moves when sexually excited depends on the 

 peculiarities of the species previously evolved, and its habits 

 in ordinary life. Thus a flying bird will fly about when 

 courting, like a humming-bird ; a ground bird will strut 

 like a peacock. The male, when he has no voice like that 

 of a singing bird, moves about and agitates his feathers, 

 partly because his excitement finds vent that way, partly 

 to show the female what he desires. Thus for each species 

 particular gestures and erection of particular feathers become 

 habitual, and if the male bird, from his polygamous habits 

 and entire freedom from the cares of incubation or nursing, 

 devotes himself with increasing energy to the practice of 

 these special gestures and movements, the result will in 

 course of time be visible in the corresponding excessive 

 growth and development of the feathers and organs stimu- 

 lated " (1900, p. III). 



This is an ingenious theory, deserving careful considera- 

 tion, and we may call attention to some difficulties, (i) It 

 must be noted that the male's peculiarities are often sharply 

 defined, such as the elongation of the ninth primary feathers 

 in an African Night-jar (Macrodipteryx) to a length of twenty- 

 six inches, or the great elongation and terminal expansion 

 of two tail feathers in a Bird of Paradise. Is it easy to 

 suppose an erection so precise that ic specially stimulates the 

 growth of two feathers in a group ? (2) Is it so certain as 

 Cunningham indicates, that the erection of a feather must 

 stimulate its growth, or the growth of its successor ? (3) In 

 many birds the reproductive period is sharply punctuated, 

 and the courting behaviour begins suddenly when the 

 feathers are fully formed. (4) It is not merely a question of 

 exaggerated growth of the feathers used in display ; in many 

 cases there are marked diff"erences in physical structure and 

 in pigmentation. (5) The correlation between the hormones 

 from the testes and the assumption of the decorative plumage 

 is certain ; this does not seem to us to harmonise well with 

 the view that the excessive growth is due to energetic 

 practice in special gestures. (6) The theory involves an 

 acceptance of the hypothesis that individually acquired 



