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. 
