172 SEXUAL SELECTION: BIRDS. [Part II. 



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, while young, 

 acquire, through variation or some unknown cause, the 

 characters of the male. 30 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 female; their 

 development following on some slight change in the elec- 

 tive 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, etc., of egrets, herons, and 

 jnany other birds, which are developed and retained only 

 during the summer, serve exclusively for ornamental or 

 nuptial purposes, though common to both sexes. The fe- 

 male is thus rendered more conspicuous during the period 

 of incubation than during the winter ; but such birds as 

 herons and egrets would be able to defend themselves. 

 As, however, plumes would probably be inconvenient and 

 certainly of no use during the winter, it is possible that 

 the habit of moulting twice in the year may have been 

 gradually acquired through natural selection for the sake 

 of casting off inconvenient ornaments during the winter. 



Audubon's ' Ornitholog. Biography,' vol. i. p. 229. On the Palaeornis, 

 Bee, also, Jerdon, ' Birds of India,' vol. i. p. 263. On the wild-turkey, 

 A-udubon, ibid. vol. i. p. 15 : I hear from Judge Caton that in Illinois the 

 female very rarely acquires a tuft. 



30 Mr. Blyth has recorded (Translation of Cuvier's ' Regne Animal,' p. 

 158) various instances with Lanius, Ruticilla, Linaria, and Anas. Audu- 

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

 Tyranga cestiva. 



