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Show 586 MESSRS. BATESON AND BRINDLEY ON [Nov. 15, The investigation we proposed to ourselves was as follows. Taking a species in which the horns 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 Englishmen, group themselves around a mean form in such a way that the curve representing 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 tall 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 tall persons were common and short persons were common, but persons of middle height were rare, such a group might be 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 which is the commonest, but rather tbe 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 breeding-place 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 chief 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 the 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 |