V28 
SEXUAL SELECTION : BIKDS. 
Part II. 
With respect to the slight individual differences which 
are common, in a greater or less degree, to all the 
members of the same species, we have every reason 
to believe that they are by far the most important 
for the work of selection. Secondary sexual characters 
are eminently liable to vary, both with animals in a 
state of nature and under domestication.^^ There is 
also reason to believe, as we have seen in our eighth 
chapter, that variations are more apt to occur in the 
male than in the female sex. All these contingencies 
are highly favourable for sexual selection. Whether 
characters thus acquired are transmitted to one sex 
or to both sexes, depends exclusively in most cases, 
as I hope to shew in the following chapter, on the 
form of inheritance which prevails in the groups in 
question. 
It is sometimes difficult to form any opinion whether 
certain slight differences between the sexes of birds 
are simply the result of variability with sexually- 
limited inheritance, without the aid of sexual selection, 
or whether they have been augmented through this 
latter process. I do not here refer to the innumerable 
instances in which the male displays splendid colours 
or other ornaments, of which the female partakes only 
to a slight degree ; for these cases are almost certainly 
due to characters primarily acquired by the male, 
having been transferred, in a greater or less degree, to 
the female. But what are we to conclude with respect 
to certain birds in which, for instance, the eyes differ 
slightly in colour in the two sexes ? In some cases 
the eyes differ conspicuously; thus with the storks 
On these points see also ‘Variation of Animals and Plants under 
Domestication,’ vol. i. p. 253 ; vol. ii. p. 73, 75, 
See, for instance, on the irides of a Podica and Gallicrex in ‘ Ibis,’ 
vol. ii. I860, p. 206 ; and vol. v. 1863, p. 426. 
