Chap. XVI.] SUMMARY. 225 



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

 seems often to have been the determining cause. Wlien 

 the two sexes have inherited all characters in common 

 they necessarily resemble each other ; but, as the suc- 

 cessive variations may be diiferently transmitted, every 

 possible gradation may be found, even within the same 

 genus, from the closest similarity to the widest dissimi- 

 larity between the sexes. With many closely-allied spe- 

 cies, following nearly the same habits of life, the males 

 have come to differ from each other chiefly through the 

 action of sexual selection ; while the females have come to 

 differ chiefly from partaking in a greater or lesser degree 

 of the characters thus acquired by the males. The efiects, 

 moi'eover, of the definite action of the conditions of life, 

 will not have been masked in the females, as in the case 

 of the males, by the accumulation through sexual selection 

 of strongly-pronounced colors and other ornaments. The 

 individuals of both sexes, however aftected, will have been 

 kept at each successive period nearly uniform by the free 

 intercrossing of many individuals. 



With the sjiecies, in which the sexes differ in color, 

 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 acquu-ing the 

 bright coloi-s of the males, on account of the danger to 

 which they would have been exposed during incubation. 

 But it would be, as far as I can see, an extremely difiicult 

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

 of transmission into another. On the other hand, 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 modi- 

 fied, must at present remain doubtful. When, through 



