The Descent of Man

In some analogous cases, namely with birds
having a different summer and winter plumage,
but with the two sexes nearly alike, certain
closely-allied species can easily be distinguished
in their summer or nuptial plumage, yet
are indistinguishable in their winter as well as
in their immature plumage. This is the case
with some of the closely-allied Indian wagtails
or Motacillae. Mr. Swinhoe (6. See also Mr.
Swinhoe, in 'Ibis,' July 1863, p. 131; and a previous
paper, with an extract from a note by Mr.
Blyth, in 'Ibis,' January, 1861, p. 25.) informs me
that three species of Ardeola, a genus of herons,
which represent one another on separate continents,
are "most strikingly different" when ornamented
with their summer plumes, but are
hardly, if at all, distinguishable during the winter.
The young also of these three species in
their immature plumage closely resemble the
adults in their winter dress. This case is all the
more interesting, because with two other species
of Ardeola both sexes retain, during the