U.
Unicellular organisms, reproduction of, [16];
action of environment on, [23], [147] et seq.;
potentially immortal, [23];
natural selection and the, [24], [57], [114];
and the origin of hereditary individual variations, [100].
V.
Variation, see Congenital variations, [Acquired characters], &c.;
Darwin on the causes of, [102];
Weismann on the origin of, [153].