LEPIDOPTEROUS FAUNA OF PORTLAND. 55 



survival of our small, but flourishing, isolated colony. There 

 is one other possible explanation namely, that the moths were 

 in some way imported direct at a comparatively recent period. 

 But it is hard to see how such an insect could have crossed over 

 from the Continent, and that it should, in the first place, have done 

 this, and in the second place arrived at the very patch of ground 

 where alone it could survive, is, in the highest degree, improbable. 

 Had it crossed over in this way one would certainly have expected 

 that it would not have been so extremely local, but if it had 

 survived at all, would have spread to where its food plants occur 

 in the neighbourhood. I prefer, therefore, to adopt the theory 

 that it is a survival from ancient times of the common ancestors of 

 itself and the German B. siccella. 



One of the most interesting moths at Portland is Scoparia 

 mercurella, a very generally distributed and common species, of 

 which the larva feeds in moss ; but at Portland, and nowhere else 

 in the world, so far as I am aware, is found the very striking variety 

 of it, called Portlandica. This variety has two pure white bars 

 crossing the fore- wing, which, in ordinary specimens, are dark grey. 

 The extreme variety is only moderately common at Portland, the 

 bulk of the specimens being intermediate between it and the typical 

 dark form, which is, perhaps, at Portland the least common of all. 

 What appears to be the most simple explanation of the existence of 

 this white variety is that a specimen with this tendency being less 

 visible on the rocks than its darker relatives, is less likely to be eaten 

 by birds, and would, therefore, probably leave more descendants 

 than the dark form, as it would be likely to have a longer period 

 of life during which to lay its eggs ; the tendency to whiteness 

 would in this way increase in every generation. If the light form 

 has, as seems probable, a real advantage over the dark form at 

 Portland we might expect that the latter would in time die out 

 there altogether. Why this has not already occurred I propose to 

 consider later on. 



I should mention here that there is another allied species, 

 Scoparia dulitalis (of which three varieties are figured in our last 



