The Descent of Man

so that their crests are actually of greater
length, though narrower, than in the adult male.
(4. Macgillivray, 'Hist. Brit. Birds,' vol. v. pp.
207-214.)
When the young and the females closely resemble
each other and both differ from the males,
the most obvious conclusion is that the males
alone have been modified. Even in the anomalous
cases of the Heliothrix and Mergus, it is
probable that originally both adult sexes were
furnished—the one species with a much elongated
tail, and the other with a much elongated
crest—these characters having since been partially
lost by the adult males from some unexplained
cause, and transmitted in their diminished
state to their male offspring alone, when
arrived at the corresponding age of maturity.
The belief that in the present class the male alone
has been modified, as far as the differences
between the male and the female together with
her young are concerned, is strongly supported