236 SEXUAL SELECTION: BIEDS. Part IL 



Witli the species, in wliieli the sexes differ in colour, 

 it is possible that at first there existed a tendency to 

 transmit the successive variations equally to both sexes ; 

 and that the females were prevented from acquiring the 

 bright colours of the males, on account of the danger to 

 which they would have been exposed during incubation. 

 l)ut it would be, as far as I can see, an extremely diffi- 

 cult process to convert, by means of natural selection, one 

 form of transmission into another. On the other hand 

 there would not be the least difficulty in rendering a 

 female dull-coloured, the male being still kept bright- 

 coloured, by the selection of successive variations, which 

 were from the first limited in their transmission to the 

 same sex. Whether the females of many species have 

 actually been thus modified, must at present remain 

 doubtful. AYhen, through the law of the equal trans- 

 mission of characters to both sexes, the females have 

 been rendered as conspicuously coloured as the males, 

 their instincts have often been modified, 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 voci- 

 ferous and brightly-coloured than their 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 colours or 

 other charms endeavour to attract the males, we can 

 understand how^ it is that they have gradually been ren- 

 dered, by means of sexual selection and sexually-limited 

 transmission, more beautiful 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 trans- 



