232 : 
SEXUAL selection: birds. 
Paet II.,- 
covered with snow. On the other hand we have reason 
to believe that whiteness has been gained by many birds 
as a sexual ornament. We may therefore conclude that 
an early progenitor of the Ardea aslia and the Buphus 
acquired a white plumage for nuptial purposes, and 
transmitted this colour to their young ; so that the 
young and' the old became white like certain existing 
egrets; the whiteness having afterwards been retained 
by the young whilst exchanged by the adults for more 
strongly pronounced tints. But if we could look still 
further backwards in time to the still earlier progenitors 
of these two species, we should probably see the adults 
darkrcoloured. I infer that this would be the case, from 
the analogy of many other birds, which are dark whilst 
young, and when adult are white ; and more especially 
from the case of the Ardea. gularis, the colours of which 
are the reverse of those of A, asha, for the young are 
dark-coloured and the adults white, the young having 
retained a former state of plumage. It appears there- 
fore that the progenitors in their adult condition of the 
Ardea aslia, the Buphus, and of some allies, have under- 
gone, during a long line of descent, the following changes 
of colour: firstly a dark shade, secondly pure white, 
and thirdly, owing to another change of fashion (if I 
may so express myself), their present slaty, reddish, or 
golden-buff tints. These successive changes are in- 
telligible only on the principle of novelty having been 
admired by birds for the sake of novelty. 
Summary of the Four Chapters on Birds. — Most male 
birds are highly pugnacious during the breeding-season, 
and some possess weapons especially adapted for fight- 
ing with their rivals. But the most pugnacious and the 
best-armed males rarely or never depend for success 
solely on their power to drive away or kill their rivals,. 
