586 MESSRS. BATESON AND BRINDLEY ON [NoV. 15, 



The investigation we proposed to ourselves was as follows. 

 Taking a species in which the liorns of the male are sometimes 

 small and sometimes of great size, we wished to see if individuals 

 having any one degree of development of horns are more frequent 

 than those having other degrees of development. The high males 

 are an extreme form, and the low males are again an extreme form : 

 would it then be found, on inquiry, that the mean form between 

 these two is the commonest ? 



To those who are acquainted with the statistics set forth by 

 Galton in 'Natural Inheritance' and elsewhere, it will be well 

 known that measurements of certain quantities, as, for example, 

 those of the stature of EngUshmen, group themselves around a 

 mean form in such a way that the curve ref)resenting the frequency 

 of occurrence of the several measurements has the form known as 

 a curve of Frequency of Error. In other words, there is a mean 

 stature for that group of persons, and variations from this mean are 

 rarer in proportion as they depart from it. Persons of mean stature 

 are common, while the tali and the short are rarer. This group of 

 individuals may then be described as monomorphic in respect of 

 stature. If, on the other hand, it were found that tali persons were 

 common and short persons were common, but persons of middle 

 height were rare, such a group might he called dimorphic in respect 

 of stature, and the curve representing the frequency of their various 

 statures would not form one Curve of Error with one peak, but would 

 have two peaks. In two of the three examples about to be described, 

 the statistics showed that such dimorphism does actually exist, and 

 that it is not the mean form, wliich is the commonest, but rather the 

 moderately high and the moderately low. After these remarks we 

 mav now describe our observations. 



I. FoRFicuLA AURicuLARiA. {The Common Earwig.) 



In a visit to the Fame Islands off the coast of Northumberland it 

 was noticed by one of us that these small rocks were inhabited by 

 vast quantities of Earwigs. The Fames are a group of basaltic 

 islands about 3-5 miles out to sea, few of them having human 

 habitations. They are a well known breetling-j)lace for sea-birds of 

 many kinds. Above high-water mark most of the rocks are covered 

 with a deep, black vegetable mould in which Silene maritima grows 

 luxuriantly, constituting the cliief vegetation, and it is in this that 

 the burrows of the Puffins are for the most part made. 



The abundance of Earwigs was most extraordinary. Under every 

 stone or tussock there was an almost continuous sheet of Earwigs. 

 This was the case not only among the sea-birds' nests, but also 

 round the light-keepers' houses where no birds build. It did not 

 seem, therefore, that the excessive quantity of Earwigs was necessarily 

 connected with the presence of tlie nests. 



It was at once seen that amongst these Earwigs were many males 

 with extremely long forceps, like that shown in fig. 1, II. The 

 usual form is seen in fig. 1, I, both figures being natural size. We 



