56o SCIENCE PROGRESS. 



traversing exhausted ground. We are still at the very 

 besfinnino-. 



In what follows I purpose to take a few examples, most 

 of them easily accessible, in illustration of the work in view, 

 hoping that those who propose to devote themselves to such 

 questions may be thereby assisted in the choice of a subject, 

 and that these illustrations may afford some indication of 

 the methods which are required. While specialists will 

 already be familiar with most of the subjects chosen, to 

 students of evolution there will be nothing novel in the 

 statement of method. 



Variations so definite as to suggest that they may possess 

 something of specific distinctness may roughly be divided 

 into two classes. In the one the variety is found associated 

 with the type, mingling and, we must suppose, breeding 

 freely with it ; in the other the variety is found partly or 

 entirely separate from the type. It is perhaps especially in 

 dealing with cases belonging to the second category that 

 we stand in the greatest need of the co-operation of the 

 general collector ; but in order to explain more clearly the 

 particular treatment which is needed in cases of the second 

 class, two examples of the first and simpler kind may be 

 conveniently spoken of. 



The first is an example of variation familiar to all ento- 

 mologists, that of AnipJiidasys betularia L. and its variety 

 doiibledayaria, Milliere. The typical form is the well- 

 known Peppered Moth, a large insect having the whole 

 upper surface of the wings of a white ground colour, almost 

 uniformly speckled with black. The sexes are similar. Of 

 this insect a unicolorous black form, the variety doiibledayaria, 

 is now commonly distributed over the northern and mid- 

 land areas of England. The special interest of this case 

 lies in the fact that it is one of the few examples in which a 

 variety is known to have come into prominence within 

 recent times. Though the case is notorious in entomo- 

 logical circles its importance has scarcely been recognised 

 by evolutionists. A short summary of the evidence is 

 given by Barrett (i), see also Chappell (3). Briefly the 



