226 SEXUAL SELECTION: BIRDS. [Part II. 



the law of the equal transmission of characters to both 

 sexes, the females have been rendered as conspicuously 

 colored as the males, their instincts have often been modi- 

 fied, and they have been led to build domed or concealed 

 nests. 



In one small and curious class of cases the characters 

 and habits of the two sexes have been completely trans- 

 posed, for the females are larger, stronger, more vociferous, 

 and brightly-colored than the males. They have, also, 

 become so quarrelsome that they often fight together like 

 the males of the most pugnacious species. If, as seems 

 probable, they habitually drive away rival females, and 

 by the display of their bright colors or other charms en- 

 deavor to attract the males, we can understand how it is 

 that they have gradually been rendered, by means of sex- 

 ual selection and sexually-limited transmission, more beau- 

 tiful than the males — the latter being left unmodified or 

 only slightly modified. 



Whenever the law of inheritance at corresponding 

 ages prevails, but not that of sexually-limited transmission, 

 then if the parents vary late in life — and we know that 

 this constantly occurs with our poultry, and occasionally 

 with other birds — the young will be left unaffected, while 

 the adults of both sexes will be modified. If both these 

 laws of inheritance prevail and either sex varies late in 

 life, that sex alone will be modified, the other sex and the 

 young being left unaffected. When variations in bright- 

 ness or in other conspicuous characters occur early in life, 

 as no doubt often happens, they will not be acted on 

 through sexual selection until the period of reproduction 

 arrives ; consequently, if dangerous to the young, they 

 will be eliminated through natural selection. Thus we 

 can understand how it is that variations arising late in 

 life have so often been preserved for the ornamentation 

 of the males, the females and the young being left almost 



