The Descent of Man, and Selection in Relation to Sex

by Charles Darwin

Twins, tendency to produce, hereditary.

Twite, proportion of the sexes in.

Ugliness, said to consist in an approach to the lower animals.

Umbrella-bird.

Umbrina, sounds produced by.

United States, rate of increase in; influence of natural selection on the progress of; change undergone by Europeans in the.

Upupa epops, sounds produced by the male.

Uraniidae, coloration of the.

Uria troile, variety of (=U. lacrymans).

Urodela.