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SEXUAL SELECTION: BIKDS. 
Part II. 
exhibit a marked preference or antipathy for certain 
individual males. 
If it be admitted that the females prefer, or are 
unconsciously excited by the more beautiful males, then 
the males would slowly but surely be rendered more 
and more attractive through sexual selection. That it 
is this sex which has been chiefly modified we may infer 
from the fact that in almost every genus in which the 
sexes difler, the males differ much more from each other 
than do the females ; this is well shewn in certain closely- 
allied representative species in which the females can 
hardly be distinguished, whilst the males are quite dis- 
tinct. Birds in a state of nature offer individual difier- 
ences which would amply sufSce for the work of sexual 
selection ; but we have seen that they occasionally pre- 
sent more strongly-marked variations which recur so 
frequently that they would immediately be fixed, if 
they served to allure the female. The laws of variation 
will have determined the nature of the initial changes, 
and largely influenced the final result. The grada- 
tions, which may be observed between the males of 
allied species, indicate the nature of the steps which 
have been passed through, and explain in the most 
interesting manner certain characters, such as the 
indented ocelli of the tail-feathers of the peacock, and 
the wonderfully-shaded ocelli of the wing-feathers of 
the Argus pheasant. It is evident that the brilliant 
colours, top-knots, fine plumes, &c., of many male 
birds cannot have been acquired as a protection ; 
indeed they sometimes lead to danger. That they 
are not due to the direct and definite action of the 
conditions of life, we may feel assured, because the 
females have been exposed to the same conditions, and 
yet often difier from the males to an extreme degree. 
Although it is probable that changed conditions acting 
