that they depend, according to the hypothesis of pangenesis, on
gemmules derived from each part of the male being present, though
latent, in the female; their development following on some slight
change in the elective affinities of her constituent tissues.
A few words must be added on changes of plumage in relation to the
season of the year. From reasons formerly assigned there can be little
doubt that the elegant plumes, long pendant feathers, crests, etc., of
egrets, herons, and many other birds, which are developed and retained
only during the summer, serve for ornamental and nuptial purposes,
though common to both sexes. The female is thus rendered more
conspicuous during the period of incubation than during the winter; but
such birds as herons and egrets would be able to defend themselves. As,
however, plumes would probably be inconvenient and certainly of no use
during the winter, it is possible that the habit of moulting twice in
the year may have been gradually acquired through natural selection for
the sake of casting off inconvenient ornaments during the winter. But
this view cannot be extended to the many waders, whose summer and
winter plumages differ very little in colour. With defenceless species,
in which both sexes, or the males alone, become extremely conspicuous
during the breeding-season, or when the males acquire at this season
such long wing or tail-feathers as to impede their flight, as with
Cosmetornis and Vidua, it certainly at first appears highly probable
that the second moult has been gained for the special purpose of
throwing off these ornaments. We must, however, remember that many
birds, such as some of the Birds of Paradise, the Argus pheasant and
peacock, do not cast their plumes during the winter; and it can hardly
be maintained that the constitution of these birds, at least of the
Gallinaceae, renders a double moult impossible, for the ptarmigan
moults thrice in the year. (33. See Gould s Birds of Great Britain. )
Hence it must be considered as doubtful whether the many species which
moult their ornamental plumes or lose their bright colours during the
winter, have acquired this habit on account of the inconvenience or
danger which they would otherwise have suffered.
I conclude, therefore, that the habit of moulting twice in the year was
in most or all cases first acquired for some distinct purpose, perhaps
for gaining a warmer winter covering; and that variations in the
plumage occurring during the summer were accumulated through sexual
selection, and transmitted to the offspring at the same season of the
485