1892.] VARIATIONS IN SECONDARY SEXUAL CHARACTERS. 589 



much alter the shape of the curve \ The size of the forceps in the 

 females scarcely varies at all, probably less than 1 ram. in the whole 

 sample. 



It is perhaps unnecessary to say that this result is of considerable 

 importance to an appreciation of the way in which Variation may 

 occur. There is here a group of individuals living in close com- 

 munion with each other, high and low, under the same stones. No 

 external circumstance can be seen to divide them, and yet they are 

 found to consist of two well marked groups, containing about equal 

 numbers. To those who are acquainted with the chapter on Organic 

 Stability in Galton's ' Natural Inheritance,' this will be recognized 

 as an instance of Variation about two positions of stability, the 

 intermediate position being one of less stability. In the common 

 language of naturalists, the facts of this case suggest that there is, 

 for some wholly unknown reason, a dimorphism among the males of 

 these Earwigs, maintained though all live together. In cases of 

 dimorphism some have thought fit to speculate on the possible 

 utility of the phenomenon. We know no basis of fact from which 

 these discussions may be properly attemf)ted, and we leave these 

 matters to those who are satisfied with such methods of biological 

 inquiry and have leisure and ingenuity to pursue them. 



For the present we are content to recognize that in this case of 

 the Earwig there is evidence of a definite and partially discontinuous 

 Variation, iu respect of a secondary sexual character. 



II. XyLOTRUPES GIDEON. 



We are indebted to Baron A. von Hiigel for a large supply of 

 this species. They were collected by him at a height of 4000-5000 

 feet in the Willis Mountains, Kediri, Java, in February to April, 1878. 

 In fig. 3 (p. 590) the males of this species are represented. As there 

 seen, in this sex two horns are present, the one produced from the 

 head, the other from the thorax. The two horns lie in the same 

 vertical plane, and each ends in a small fork. Taken together, these 

 two horns thus constitute a pair of forceps, which can be opened by 

 depressing the head. The points of the forceps do not exactly 

 meet, but the point of the cephalic horn in high males is overlapped 

 by that of the thoracic horn. As the figures show, there is very 

 great variation in the degree to which these horns are developed in 

 the male, the three drawings representing " high," "medium," and 

 "low" males respectively. In the female neither horn is developed, 

 but there is considerable variation in total length. As may also be 

 seen in the figures, those males which have very large horns are 



^ In most insects having high and low males, the high males are large in 

 every way, while the low males are small. That this is so, generally speaking, 

 in these Earwigs was clear, but it is not possible to get reliable measurements of 

 total length, owing to the fact that the abdominal segments " telescope " into 

 each other. Hence no examination of the correlation between total length and 

 the length of the forceps could be attempted. There is nevertheless no doubt 

 tliat the ratio of the length of the forceps to the total length is higher in high 

 males than in the low. 



Proc. Zool. Soc— 1892, No. XL. 40 



