The Making of Species 
mitted 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 suc- 
cessive variations may be differently transmitted, 
every possible gradation may be found, even 
within the same genus, from the closest simi- 
larity to the widest dissimilarity between the 
sexes.” 
This statement, although it does not throw any 
light upon the problem, is somewhat damaging 
to the theory of sexual selection. If it be 
admitted that dissimilarity between the sexes 
is due to the fact that the males have varied in 
one way and the females in another way, there 
seems no necessity for invoking the aid of 
feminine preference. 
Even greater is the difficulty presented by 
those species in which the males alone are pro- 
vided with horns or antlers. ‘ When,” writes 
Darwin (Descent of Man, p. 767), “the males 
are provided with weapons which in the females 
are absent, there can hardly be a doubt that 
these serve for fighting with other males; and 
that they were acquired through sexual selection, 
and were transmitted to the male sex alone. 
It is not probable, at least in most cases, that 
the females have been prevented from acquiring 
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