The Descent of Man

les. It is an interesting fact, as Dr. Gill remarks
(15. 'The Eared Seals,' American Naturalist, vol.
iv. Jan. 1871.), that in the monogamous species,
"or those living in small communities, there is
little difference in size between the males and
females; in the social species, or rather those of
which the males have harems, the males are
vastly larger than the females."
Amongst birds, many species, the sexes of
which differ greatly from each other, are certainly
monogamous. In Great Britain we see
well-marked sexual differences, for instance, in
the wild-duck which pairs with a single female,
the common blackbird, and the bullfinch which
is said to pair for life. I am informed by Mr.
Wallace that the like is true of the Chatterers or
Cotingidae of South America, and of many other
birds. In several groups I have not been
able to discover whether the species are polygamous
or monogamous. Lesson says that
birds of paradise, so remarkable for their sexual