SHXUAL COLORATION. 263 



male gland on the opposite side. The same kind of herma- 

 phroditisrn has been noticed in other birds. In cases where a 

 female bird has assumed the plumage of the cock, it has been 

 found that the ovary was diseased or atrophied.* 



The second instance which I shall bring forward is that of a 

 rnoth the common Oak Eggar. Dr. Bertkau f has figured 

 and described a hermaphrodite specimen of this insect in 

 which the wings of one side of the body showed the colora- 

 tion, form and size of those of a male, while the opposite 

 couple of wings had the coloration of the female, and were, 

 as in the female moth, larger than the wings coloured after 

 the male pattern. On a dissection, the insect proved to be not 

 hermaphrodite, like the chaffinch, but a female with degenerate 

 organs, some of the parts typically present being absent. It 

 cannot, therefore, be called a hermaphrodite ; it should be 

 remarked that the ovary was more degenerate upon the side 

 of the body on which the wings were those of a male, than 

 upon the other. 



The Theory of Sexual Selection. 



The classical theory to account for these sexual differences 

 is of course Darwin's theory of " sexual selection." 



According to this theory the females, when courted by the 

 males, exercise a decided choice, and are not simply the prey 

 of the most persevering or the most powerful male. Hence, 

 there is a tendency to an increase in the beauty of the colora- 

 tion of the males, and to an increased development in such 

 special appendages of that sex as crests, wattles, etc. The 

 gorgeous tail of the peacock, and the curious crests and other 



* See Mr. Bland Button's " An Introduction to Pathology," for further 

 instances. Also J. H. Gurney (jun.), " Ibis," 1888, p. 22(3. 

 f ArcJi.f. Natwrgeschichts, lv., p. 75. 



