180 SEXUAL SELECTION: BIRDS. Part IL 



furnislied with a tuft of bristles on the breast, but in 

 two-year-old birds the tuft is about four inches long in 

 the male and hardly apparent in the female ; when, 

 however, the latter has reached her fourth year, it is 

 from four to five inches in length.^® 



In these cases, the females follow a normal course of 

 development in ultimately becoming like the males ; and 

 such cases must not be confounded with those in which 

 diseased or old females assume masculine characters, 

 or with those in which perfectly fertile females, whilst 

 young, acquire through variation or some unknown cause 

 the characters of the male.^° But all these cases have 

 so much in common that they depend, according to the 

 hypothesis of pangenesis, on gemmules derived from each 

 part of the male being present, though latent, in the fe- 

 male; their development following on some slight change 

 in the elective affinities of her constituent tissues. 



A few words must be added on changes of plumage 

 in relation to the season of the year. From reasons 

 formerly assigned there can be little doubt that the 

 elegant plumes, long pendant feathers, crests, &c., of 

 egrets, herons, and many other birds, Avhich are deve- 

 loped and retained only during the summer, serve 

 exclusively for oi'uamental or nuptial purposes, though 



-9 On Ardetta, Translation of Cuviers ' Eegne Animal,' by Mr. Blytli, 

 footnote, p. 159. On the Peregrine Falcon, Mr. Blyth, in Charles- 

 worth's ' Mag. of Nat. Hist.' vol. i. 1837, p. 301. On Dicrurus, ' Ibis,' 

 1863, p. 44. On the Piatalea, ' Ibis,' vol. vi. 1864, p. 366. On the 

 BomV)ycilla, Audubon's ' Ornitholog. Biography,' vol. i. p. 229. On 

 the Palseornis, see, also, Jerdon, ' Birds of India,' vol. i. p. 263. 

 On the wild tm-key, Audubon, ibid. vol. i. p. 15 : I hear from Judge 

 Caton that in Illinois the female very rarely acquires a tuft. 



^^ Mr. Blyth has recorded (Translation of Ouvier's ' Eegne Animal,' 

 p. 158) various instances with Lanius, Kuticilla, Linaria, and Anas. 

 Audubon has also recorded a similar case (' Ornith. Biog.' vol. v, p. 519) 

 with Tyranga sestiva. 



