much greater than one often realizes. It has
already been noted that a large part of the population dies before it
reaches the age of marriage. Of 1,000 babies born in the United States,
only 750 will reach the average age of marriage; in some countries half
of the thousand will have fallen by that time. These dead certainly will
leave no descendants; but even of the survivors, part will fail to
marry. The returns of the thirteenth U. S. census showed that of the
males 45-64 years of age, 10% were single, while 11% of the females,
35-44 years old, were single. Few marriages will take place after those
ages. Add the number who died unmarried previous to those ages, but
after the age of 20, and it is safe to say that at least one-third of
the persons born in the United States die (early or late) without having
married.

The consideration of those who died before the age of marriage properly
comes under the head of lethal selection, but if attention is confined
to those who, though reaching the age of marriage, fail to marry, sexual
selection still has importance. For instance, it is generally known (and
some statistical proof will be given in Chapter XI) that beauty is
directly associated with the chance of marriage. The pretty girls in
general marry earlier as well in larger percentage; many of the ugly
ones will never find mates. Herbert Spencer argued ingeniously that
beauty is associated with general mental and moral superiority, and the
more exact studies of recent years have tended to confirm his
generalization. A recent, but not conclusive, investigation[65] showed
beauty to be correlated with intelligence to the extent of .34. If this
is confirmed, it offers a good illustration of the action of sexual
selection in furthering the progressive evolution of the race. Miss
Gilmore, studying a group of normal school graduates, found a direct
correlation between intelligence (as judged by class marks) and early
marriage after graduation. Anyone who would take the trouble could
easily investigate numerous cases of this sort, which would show the
effect of sexual selection in perpetuating desirable qualities.

But sexual selection no longer has the importance that it once had, for
nowadays the mere fact of marriage is not a measure of fecundity, to the
extent that it once was. In the old days of unlimited fecundity, the
early marriage of a beautiful, or intelligent, woman meant a probable
perpetuation of her endowments;