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Show 1892.] VARIATIONS IN SECONDARY SEXUAL CHARACTERS. 589 much alter the shape of the curve \ The size of the forceps in females scarcely varies at all, probably less than 1 mm. 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 communion 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 attempted, 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, in respect of a secondary sexual character. II. XYLOTRUPES GIDEON. W e 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 tbe 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 tbe 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 i 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 that 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 |
