The Descent of Man

have occurred at different periods of life, even
sometimes with species of the same group, we
do not know, but with respect to the form of
transmission, one important determining cause
seems to be the age at which the variations first
appear.
From the principle of inheritance at corresponding
ages, and from any variations in colour
which occurred in the males at an early age not
being then selected—on the contrary being often
eliminated as dangerous—whilst similar
variations occurring at or near the period of
reproduction have been preserved, it follows
that the plumage of the young will often have
been left unmodified, or but little modified. We
thus get some insight into the colouring of the
progenitors of our existing species. In a vast
number of species in five out of our six classes
of cases, the adults of one sex or of both are
bright coloured, at least during the breedingseason,
whilst the young are invariably less