40 FORCEPS OF EARWIGS. [IXTROD. 



the moderately low and the moderately high, the forms of mean 

 measurement being comparatively scarce. It is true that the 

 numbers are few, but so little heed is paid to phenomena of this 

 kind that material is difficult to obtain and the present oppor- 

 tunity was indeed wholly exceptional 1 . But taking the evidence 

 for what it is worth, the comparative scarcity of " medium " males 

 in that particular sample is clear, and so far the form is dimorphic, 

 and has two male normals. 



Now such a condition may have arisen in several ways. First, 

 in the past history of the species there may have been a time 

 when the males were horned and were monomorphic, the " medium" 

 form being the most frequent, and the present dimorphic condition 

 may have been derived from this, either continuously or discon- 

 timiously as described above for the case of Stature. Secondly, 

 the dimorphism may date from the first acquisition of the horns, 

 and this character may perhaps have always been distributed in 

 the dimorphic way. In this case the term Discontinuous would 

 be applicable to the Variation by which the groups of "high" and 

 " low " males have been severally produced. I am not acquainted 

 with evidence as to the course of inheritance in these cases, and I 

 do not know therefore whether both " high" and " low" males may 

 be produced by one mother. If this should be shewn to be the 

 case, it would suggest that the separation of the males into two 

 groups was a case of characters which do not readily blend, 

 and are thus exempt from what Galton has called the Law of 

 Regression 2 . 



In the case of a somewhat similar structure found in the Common 

 Earwig (Forjicula auric ularia) the dimorphism is 

 still more definite. In the autumn of 1892 on a 

 visit to the Fame Islands, a basaltic group off the 

 coast of Northumberland, it was found that these 

 islands teem with vast quantities of earwigs. The 

 abundance of earwigs was extraordinary. They 

 ^^^^ lay in almost continuous sheets under every stone 



w and tussock, both among the sea-birds' nests 

 1 B and by the light-keepers' cottages. Among them 



FIG. 3. I, High were males of the two kinds shewn in Fig. 3 ; the 

 male, II, Low male one or high male having forceps of unusual length, 

 of Common Earwig the other or low ma j b e i n g the common form. 

 (Forficula auricu- T . , . , , . , c 



laria) from the It appears that the high male is known from many 



Fame Islands. places in England and elsewhere and that it was 

 made into a distinct species, F. furcipata, by 



1 In the Lucanidffi, of which the Stagbeetlc (L. <-ernt\ is an example, a similar 

 phenomenon occurs, the "high" and "low" males being distinguished by the 

 degree of development of the mandibles. No sufficient number of male Stagbeetles 

 has yet been received to warrant any statement as to the frequency of the various 

 types of males. 



2 Natural Inheritance, pp. 88 110. 



