122 SEXUAL SELECTION : I5IRDS [Part IL 



In various parts of the northern seas a remarkable 

 variety of the common Guillemot {Uria troile) is found ; 

 and "in Feroe, one out of every five birds, according to 

 Graba's estimation, consists of this variety. It is charac- 

 terized " by a pure white ring round the eye, with a curved 

 narrow white line, an inch and a half in length, extending 

 back from the ring. This conspicuous character has caused 

 the bird to be ranked by several ornithologists as a dis- 

 tinct species under the name of U. lacrt/mans, but it is 

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

 the common kind, yet intermediate gradations have never 

 been seen ; nor is this surprising, for variations which ap- 

 pear suddenly are often, as I have elsewhere shoAvn," 

 transmitted either unaltered or not at all. We thus see 

 that two distinct forms of the same species may coexist in 

 the same district, and we cannot doubt that if the one had 

 possessed any great 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 and driven away by their comrades, had been 

 highly attractive, like the pied peacock before mentioned, 

 to the common black females, their numbers would have 

 rapidly increased. And this would have been a case of 

 sexual selection. 



With respect to the slight individixal dittl-rences 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 impcn-tant for the work of selec- 

 tion. 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 



^* Graba, ibid. s. 54. Macgillivray, ibid. vol. v. p. 327. 



^^ 'Variation of Animals and I'lant.-^ under Domestication,' vol. ii. 

 p. 02. 



*° On these points sec also ' Variation of Animals and Plants under 

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



