19 S TJie Descent of Man. Part 11 



transmitted, every possible gradation may be found, even within 

 the same genus, from the Ciosest similarity to the widest dis- 

 similarity between the sexes. With many closely-allied species, 

 following nearly the same habits of life, the males have come to 

 differ from each other chiefly through the action of sexual 

 selection; whilst the females have come to differ chiefly from 

 partaking more or less of the characters thus acquired by the 

 males. The effects, moreover, of the definite action of the 

 conditions of life, will not have been masked in the females, as 

 in the males, by the accumulation through sexual selection of 

 strongly-pronounced colours and other ornaments. The indi- 

 viduals of both sexes, however affected, will have been kept at 

 each successive period nearly uniform by the free intercrossing 

 of many individuals. 



With species, in which the sexes differ in colour, it is possible 

 or probable that some of the successive variations often tended 

 to be transmitted equally to both sexes; but that when this 

 occurred the females were prevented from acquiring the bright 

 colours of the males, by the destruction which they suffered 

 during incubation. There is no evidence that it is possible by 

 natural selection to convert one form of transmission into 

 another. But 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. When, through the law of 

 the equal transmission of characters to both sexes, the females 

 were rendered as conspicuously coloured as the males, their 

 instincts appear often to have been modified so that they were 

 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 transposed, for the 

 females are larger, stronger, more vociferous and brighter 

 coloured than the males. They have, also, become so quarrel- 

 some that they often fight together for the possession of the 

 males, like the males of other pugnacious species for the 

 possession of the females. If, as seems probable, such females 

 habitually drive away their rivals, 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 

 rendered, by sexual selection and sexually-limited transmission, 

 more beautiful than tne males — the latter being left unmodified 

 or only slightly modified. 



Whenever the law cf inheritance at corresponding ages prevails 



