VARIABILITY. 419 



ter has caused the bird to be ranked by several ornithologists 

 as a distinct species under the name o£ U. lacrymans, but it is 

 now known to be merely a variety. It often pairs with the com- 

 mon kind, yet intermediate gradations have never been seen; nor 

 is this surprising, for variations which appear suddenly, are often, 

 as I have elsewhere shown,*^ transmitted either unaltered or not 

 at all. We thus see that two distinct forms of the same species 

 may co-exist in the same district, and we cannot doubt that if 

 the one had possessed any advantage over the other, it 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 (like the above pied 

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

 rapidly increased. And this would have been a case of sexual 

 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 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 favorable for sexual 

 selection. Whether characters thus acquired are transmitted to 

 one sex, or to both sexes, depends, as we shall see in the fol- 

 lowing chapter, on the form of inheritance which prevails. 



It is sometimes difficult to form an 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 many instances 

 where the male displays splendid colors or other ornaments, of 

 which the female partakes to a slight degree; for these are al- 

 most 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 color 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 



*2 'Variation of Animals and Plants under Domestication,' vol. ii. 

 p. 92. 



*^ On these points see, also, 'Variation of Animals and Plants under 

 Domestication,' vol. i. p. 253; vol. ii. pp. 73, 75. 



" See, for instance, on the irides of a Podica and Galliorex in 'Ibis,' 

 vol. ii. 1860, p. 206; and vol. v. 1S63, p. 426. 



