SEXUAL SELECTION : BIEDS. 
1118 
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1 it! 
With respect to the slight individual differences ^ ■ 
are common, in a greater or less degree, to ah 
members of the same species, we have every 
to believe that they are by far the most imp 0 ' ^ 
for the work of selection. Secondary sexual char® , 4 
are eminently liable to vary, both with animals 
state of nature and under domestication . 40 The 1 ® 
also reason to believe, as we have seen in our 
in 1 
chapter, that variations are more apt to occur - j e , 
male than in the female sex. All these conting e °, , f 
are 
highly favourable for sexual selection. W 
7 b# 
characters thus acquired are transmitted to cue . 
or to both sexes, depends exclusively in most c9 ^ 
as I hope to shew in the following chapter, 
on 
form of inheritance which prevails in the group® 
if 
question. ^ 
It is sometimes difficult to form any opinion wb e jj 
l 
certain slight differences between the sexes of ^ 
are simply the result of variability with seX 
.viu 1 
Hf 
limited inheritance, without the aid of sexual se 
or whether they have been augmented through ^ 
latter process. I do not here refer to the innuine 19 
instances in which the male displays splendid 0 u 
Ofl 1 ' 
or other ornaments, of which the female partake® . a 
to a slight degree; for these cases are almost cei't il h. 
due to characters primarily acquired by the 
having been transferred, in a greater or less degff^i 
the female. But what are we to conclude with red 1 ,, f 
to certain birds in which, for instance, the eyes ' 1 jt ; 
slightly in colour in the two sexes ? 41 In some ^ 
the eyes differ conspicuously; thus with the 
s to r * 
¥ 
■ii 1 On these points see also ‘Variation of Animals and Plan ts 
Domestication,’ vol, i. p. 253 ; vol. ii. p. 73, 75. , fF 
41 See, for instance, on the irides of a Podica and GallicreX i n 
vol. ii. I860, p. 206 ; and vol. v. 1863, d. 426. 
