BIRDS-SUMMARY. 493 



ences will have afforded an excellent ground-work for the action 

 of sexual selection. 



The laws of inheritance, irrespectively of selection, appear to 

 have determined whether the characters acquired by the males 

 for the sake of ornament, for producing various sounds, and for 

 fighting together, have been transmitted to the males alone or to 

 both sexes, either permanently, or periodically during certain 

 seasons of the year. Why various characters should have been 

 transmitted sometimes in one way and sometimes in another, is 

 not in most cases known; but the period of variability seems 

 often to have been the determining cause. When the two sexes 

 have inherited all characters in common they necessarily resemble 

 each other; but as the successive variations may be differently 

 transmitted, every possible gradation may be found, even within 

 the same genus, from the closest 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 selec- 

 tion; whilst the females have come to differ chiefly from partak- 

 ing 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-pro- 

 nounced colors and other ornaments. The individuals of both 

 sexes, however affected, will have been kept at each successive 

 period nearly uniform by the free intercrossing of many individ- 

 uals. 



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

 or probable that some of the successive variations often tended 

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

 curred the females were prevented from acquiring the bright 

 colors of the males, by the destruction which they suffered dur- 

 ing 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 difliculty in rendering a female dull- 

 colored, the male being still kept bright-colored, 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 conspicu- 

 ously colored 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 col- 



