:>1 LEPIDOPTEROUS FAUNA OF PORTLAND. 



1860, p. 140). If it is, as I believe it to be, much like its nearest 

 allies, it is a very fast flying, strong-winged insect, and might have 

 come from a considerable distance. It would also be very much 

 affected by the attraction of light. Foreign, and indeed exotic 

 moths and butterflies do occasionally appear in most unexpected 

 quarters from unknown causes. 



In the case of the above moths, which also occur abroad, we 

 assume from the great general resemblance (for we rarely, if ever, 

 find exact resemblances in nature) that the Portland and foreign 

 moths have had common ancestors which resembled themselves, 

 and not that, for instance, the Dorset Butalis siccella is descended 

 from the D. variella of the Dorset heaths, and has altered its form 

 gradually through the difference of its surroundings, and that the 

 German B. siccella has had an independent origin. B. siccella 

 must, therefore, at some time or other have occupied for a greater 

 or less period every intermediate space of ground on a line, probably 

 a very crooked one, drawn between the place where it is here found 

 and some other present locality, and must have disappeared again 

 from every part of this line. If B. siccella were the only example 

 of a moth not found elsewhere in this country, there would be less 

 evidence against a theory that it and the Dorset heath B. variella 

 were descended from the same source, and that siccella had become 

 smaller and darker, and otherwise altered its form, either through 

 natural selection or from the fact that its new food plants would 

 only produce small dark specimens. The latter has a parallel case 

 in the common Hypsipetes elutata, which has a small dark form, the 

 larva of which feeds on bilberry in Yorkshire, sallow being the 

 ordinary food plant. If we adopted this theory we should have to 

 say that the German B. siccella had come to the same form as the 

 British one independently, which would be a most extraordinary 

 coincidence, considering that the climate and surroundings must be 

 very different in the two cases. But I think that on the whole it 

 is much more simple to conclude that the resemblance between the 

 (H THI an and English B. siccella is not accidental, but that they 

 belong to the same .stock, though it is difficult to account for the 



