The Descent of Man

The practice of polygamy leads to the same
results as would follow from an actual inequality
in the number of the sexes; for if each male
secures two or more females, many males cannot
pair; and the latter assuredly will be the
weaker or less attractive individuals. Many
mammals and some few birds are polygamous,
but with animals belonging to the lower classes
I have found no evidence of this habit. The intellectual
powers of such animals are, perhaps,
not sufficient to lead them to collect and guard
a harem of females. That some relation exists
between polygamy and the development of
secondary sexual characters, appears nearly
certain; and this supports the view that a numerical
preponderance of males would be eminently
favourable to the action of sexual selection.
Nevertheless many animals, which are
strictly monogamous, especially birds, display
strongly-marked secondary sexual characters;
whilst some few animals, which are polygamous,
do not have such characters.