38 
SEXUAL SELECTION. 
Part It 
CHAPTER XIII. 
Secondary Sexual Characters of Birds. 
Sexual differences — Law of Battle — Special weapons — Vocal 
organs — Instrumental music — Love -antics and dances-^ 
Decorations, permanent and seasonal — Double and single 
annual moults — -Display of ornaments by the males. 
Secondary sexual characters are more diversified and 
conspicuous in birds, though not perhaps entailing more 
important changes of structure, than iu any other class 
of animals. I shall, therefore, treat the subject at con- 
siderable length. Male birds sometimes, though rarely, 
possess special weapons for fighting with each other. 
They charm the females by vocal or instrumental music 
of the most varied kinds. They are ornamented by 
all sorts of combs, wattles, protuberances, horns, air- 
distended sacs, topknots, naked shafts, plumes and 
lengthened feathers gracefully springing from all parts 
of the body. The beak and naked skin about the head, 
and the feathers are often gorgeously coloured. The 
males sometimes pay their court by dancing, or by fan- 
tastic antics performed either on the ground or in the air. 
Iu one instance, at least, the male emits a musky odour 
which w T e may suppose serves to charm or excite the 
female ; for that excellent observer, Mr. Ramsay , 1 says 
of the Australian musk-duck (Biziura lobata ) that “ the 
“ smell which the male emits during the summer 
“ months is confined to that sex, and iu some indi- 
“ viduals is retained throughout the year ; I have never 
1 ‘Ibis,’ vol. iff, (new series) 1867, p. 414. 
