Darwin's Theory of Sexual Selection 175 
been a double or mutual process of sexual selection; the 
more vigorous and precocious females selecting the more 
attractive and vigorous males, the latter rejecting all except 
the more attractive females. But from what we know of the 
habits of animals, this view is hardly probable, for the male 
is generally eager to pair with any female. It is more prob- 
able that the ornaments common to both sexes were acquired 
by one sex, generally the male, and then transmitted to the 
offspring of both sexes. If, indeed, during a lengthened 
period the males of any species were greatly to exceed the 
females in number, and then during another lengthened 
period, but under different conditions, the reverse were to 
occur, a double but not simultaneous process of sexual selec- 
tion might easily be carried on, by which the two sexes might 
be rendered widely different.” 
The improbability of such a process is so manifest that 
the suggestion can scarcely be looked upon as anything more 
than pure speculation. We shall have occasion later to re- 
turn to the same subject, and point out its bearing more 
explicitly. 
Nearly the whole animal kingdom is passed in review by 
Darwin from the point of view of the sexual selection theory. 
There is brought together a large number of extremely inter- 
esting facts, and if the theory did no more than merely hold 
them together, it has served, in this respect, a useful end. 
We may select some of the most instructive cases by way of 
illustrating the theory. 
In many of the lower animals in which the sexes are sep- 
arated, and these alone, of course, can be supposed to come 
within the range of the theory, there are no striking differ- 
ences between the sexes, in regard to ornamentation, although 
in other respects differences may exist. 
“ Moreover it is almost certain that these animals have too 
imperfect senses and much too low mental powers, to appre- 
ciate each other’s beauty or other attractions, or to feel rivalry. 
