192 
SEXUAL selection: birds. 
Part II. 
they will almost always have been exposed will cause 
them to undergo, judging from a widely-spread analogy, 
a certain amount of fluctuating variability. In this 
case sexual selection, which depends on an element 
eminently liable to change — namely the taste or admi- 
ration of the female — will have had new shades of colour 
or other difierences to act on and accumulate ; and as 
sexual selection is always at work, it would (judging 
from what we know of the results on domestic animals 
of man’s unintentional selection), be a surprising fact if 
animals inhabiting separate districts, whicli can never 
cross and thus blend their newly-acquired characters, 
were not, after a sufiRcient lapse of time, differently 
modified. These remarks likewise apply to the nuptial 
or summer plumage, whether confined to the males or 
common to both sexes. 
Although the females of the above closely-allied 
species, together with their young, differ hardly at all 
from each other, so that the males alone can be distin- 
guished, yet in most cases the females of the species 
within the same genus obviously differ from each other. 
The differences, however, are rarely as great as between 
the males. We see this clearly in the whole family of 
the Gallinaceae : the females, for instance, of the com- 
mon and Japan pheasant, and especially of the gold and 
Amherst pheasant — of the silver pheasant and the wild 
fowl — reseinble each other very closely in colour, whilst 
the males differ to an extraordinary degree. So it is 
with the females of most of the Cotingidm, Fringillidae, 
and many other families. There can indeed be no doubt 
that, as a general rule, the females have been modi- 
fied to a less extent than the males. Some few birds, 
however, offer a singular and inexplicable exception ; 
thus the females of Paradisea apoda and P. papuana 
differ from each other more than do their respective 
