who brought it forward because natural selection, operating
solely through a differential death-rate, seemed inadequate to account
for many phases of evolution. By sexual selection he meant that an
individual of one sex, in choosing a mate, is led to select out of
several competitors the one who has some particular attribute in a high
degree. The selection may be conscious, and due to the exercise of
aesthetic taste, or it may be unconscious, due to the greater degree of
excitation produced by the higher degree of some attribute. However the
selection takes place, the individual so selected will have an
opportunity to transmit his character, in the higher degree in which he
possesses it, to his descendants. In this way it was supposed by Darwin
that a large proportion of the ornamental characters of living creatures
were produced: the tail of the peacock, the mane of the lion, and even
the gorgeous coloring of many insects and butterflies. In the early
years of Darwinism, the theory of sexual selection was pushed to what
now seems an unjustifiable extent. Experiment has often failed to
demonstrate any sexual selection, in species where speculation supposed
it to exist. And even if sexual selection, conscious or unconscious,
could be demonstrated in the lower animals, yet the small percentage of
unmated individuals indicates that its importance in evolution could not
be very great.[96]

[Illustration: HOW BEAUTY AIDS A GIRL'S CHANCE OF MARRIAGE

FIG. 32.--Graph showing the marriage rate of graduates of a
normal school, correlated with their facial attractiveness as graded by
estimates. The column of figures at the left-hand side shows the
percentage of girls who married. Of the prettiest girls (those graded 80
or over), 70% married. As the less attractive girls are added to the
chart, the marriage rate declines. Of the girls who graded around 50 on
looks, only about one-half married. In general, the prettier the girl,
the greater the probability that she will not remain single.]

In man, however, there is--nowadays at least--a considerable percentage
of unmated individuals. The Census of 1910 shows that in the United
States one-fourth of all the men between 25 and 44 years of age, and
one-sixth of all the women, were unmarried. Many of the men, and a
smaller number of the women, will still marry; yet at the end there
will remain a large number, particularly in the more highly educated
classes, who die celibate. If the