Chap. XIV. Variability. 425 



that if the one had possessed any advantage over the other, il 

 would soon have been multiplied to the exclusion of the latter. 

 If, for instance, the male pied ravens, instead of being persecuted 

 by their comrades, had been highly attractive Oike the above 

 pied peacock) to the black female ravens, their numbers would 

 bars rapidly increased. And this would have been a case oi 

 soxual^selection. 



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 behave, 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, as we shall see in the 

 following chapter, on the form of inheritance which prevails. 



It is sometimes difEicuIt to form an opinion whether certain 

 slight differences , between the sexes of birds are simply the 

 result of variabihty with sexually-limited inheritance, without 

 the aid of sexaal selection, or whether they have been augmented 

 through this 'latter process. I do not here refer to the many 

 instances where the male displays splendid colours or other 

 ornaments, of which the female partakes to a slight degree ; for 

 these are almost certainly due to characters primarily acquired by 

 the male having been more or less transferred 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 of the genus .Xenorhynchus, those of the male are blackish- 

 hazel, whilst those of the females are gamboge-yellow; with 

 many hornbills (Buceros), as I hear from Mr. Blyth,** the males 

 have intense crimson eyes, and those of the females are white. 

 In the Buceros biennis, the hind margin of the casque and a 

 Btripe on the crest of the beak are black in the male, but not so 

 in the female. Are we to suppose that these black marks and 

 the crimson colour of the eyes have been preserved or augmented 

 through sexual selection in the males ? This is very doubtful ; 



" On these points see also ' Varia- of a Podiea and Gallicrex in ' Ibis,' 



lior of Anijnals and Plants under vol. ii. 1860, p. 206 ; and vol. v. 



Domestication,' vol. i. p. 253 ; vol. ii. 1863, p. 426. 



jp. 73, 75. *^ See also Jcrdon, ' Birds of 



*^ See, for instance, on the irides India,' vol. i. f>p. 243-245. 



