

LEPIDOPTEROUS FAUNA OF PORTLAND. 57 



Portlandica. This is not the case of a species which has an 

 endless variety of forms, of which scarcely two are alike, like 

 Hypsipetes elutata, in which the variation seems to have not two, 

 but many extreme forms, so that if one tries to arrange a long 

 series of, say 200, different forms in a natural order, so that the 

 grades of variation between consecutive individuals are almost 

 imperceptible, one finds that one gets into great confusion j and if 

 one takes any two extreme forms and picks out a series connecting 

 them, one only uses a very small number, perhaps 20 out of the 

 whole 200 the rest have no place in this line of variation, but 

 belong to some other line between two other extreme forms, and 

 often the same specimen will be rightly placed in two or more of 

 these lines, the fact being that there are a great many distinct 

 points of variation which exist quite independently of each other 

 in different parts of the same individual. In S. mercurella at 

 Portland, however, the only important variation is that from the 

 dark form in the direction of var. Portlandica, which greatly 

 simplifies matters, so that a long series of Portland specimens could 

 be arranged in a continuous line between the extremes of the type 

 and the variety. This leads us naturally to the conclusion that all 

 these intermediate forms are produced by the same cause that 

 produces var. Portlandica, and that no other important cause of 

 variation is acting on this species at Portland. As the dark form 

 is the one that occurs everywhere else, we may fairly assume that at 

 some distant period it was also the only form which occurred in 

 Portland, and that from the light colour of the rocks, or from some 

 other definite cause or causes, which gave to the lighter specimens a 

 slight advantage over the dark ones, the var. Portlandica has been 

 developed. But one would naturally expect that as Portland is 

 only a small locality, the interbreeding that would necessarily take 

 place would prevent much variation in the shade of the moths at 

 the same period, and that the whole of them would gradually get 

 lighter together until all were of the white variety. But, instead 

 of this, we find all shades. It seems to me that the most simple 

 way of accounting for this fact is that specimens of the ordinary 



