200 
SEXUAL SELECTION: BIRDS. 
Pakt II. 
tions in brightness which occurred in the females or in 
the youug would have been of no service to them, and 
would not have been selected ; moreover, if dangerous, 
would have been eliminated. Thus the females and the 
young will either have been left unmodified, or, and 
this has much more commonly occurred, will have been 
partially modified by receiving through transference from 
the males some of the successive variations. Both sexes 
have perhaps been directly acted on by the conditions 
of life to which they have long been exposed ; but the 
females from not being otherwise much modified will 
best exhibit any such effects. These changes and all 
others will have been kept uniform by the free inter- 
crossing of many individuals. In some cases, especially 
with ground birds, the females and the young may pos- 
sibly have been modified, independently of the males, 
for the sake of protection, so as to have acquired the 
same dull-coloured plumage. 
Class II. When the adult female is more conspicuous 
than the adult male^ the young of both sexes in their first 
plumage resemhle the adult male, — This class is exactly 
the reverse of the last, for the females are here more 
brightly coloured or more conspicuous than the males ; 
and the young, as far as they are known, resemble 
the adult males instead of the adult females. But the 
difference between the sexes is never nearly so great 
as occurs with many birds in the first class, and the 
cases are comparatively rare. Mr. Wallace who first 
called attention to the singular relation which exists 
between the less bright colours of the males and their 
performing the duties of incubation, lays great stress on 
this point, as a crucial test that obscure colours have 
13 ‘Westminster Review,’ July,. 1867, and A. Murray, ‘Journal of 
Travel,’ 1868, p. 83. 
