The Descent of Man

would be superfluous to give additional instances.
These differences between the young and
the old apparently depend, as in the last two
classes, on the young having retained a former
or ancient state of plumage, whilst the old of
both sexes have acquired a new one. When the
adults are bright coloured, we may conclude
from the remarks just made in relation to the
scarlet ibis and to many herons, and from the
analogy of the species in the first class, that
such colours have been acquired through
sexual selection by the nearly mature males;
but that, differently from what occurs in the
first two classes, the transmission, though limited
to the same age, has not been limited to the
same sex. Consequently, the sexes when mature
resemble each other and differ from the
young.
CLASS IV.