236 SEXUAL ' SELE CTIOK : BIRDS. Part II. 
With the species^ in which the sexes differ in colour^ 
it is possible th^t 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. 
But it wuuld 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. When, 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- 
