Chap. XVI. SUMMARY. 235 



during a lengthened period have produced some definite 

 effect on both sexes, the more important result will have 

 been an increased tendency to fluctuating variability or 

 to augmented individual differences; and such differ- 

 ences will have afforded an excellent groundwork for 

 the action of sexual selection. 



The laws of inheritance, irrespectively of selection, 

 appear to have determined whether the characters ac- 

 quired by the males for the sake of ornament, for pro- 

 ducing various sounds, and for fighting together, have 

 been transmitted to the males alone or to both sexes, 

 either permanently or periodically during certain sea- 

 sons of the year. Why various characters should some- 

 times have been transmitted in one way and sometimes 

 in another is, in most cases, not known ; but the period 

 of variability seems often to have been the determining 

 cause. When the two sexes have inherited all charac- 

 ters in common they necessarily resemble each other; 

 but as the successive variations may be differently trans- 

 mitted, every possible gradation may be found, even 

 within the same genus, from the closest similarity to 

 the widest dissimilarity 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 

 in a greater or lesser degree 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 case of 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. 



