397 



tract of country was actually a range of coast sands, at a compara- 

 tively recent point of the post-glacial period, while the great valley 

 of the fens was still submerged ;" and that the species in 

 question have occupied the ground there from that time to the 

 present. Nor is this the only remarkable circumstance. " They 

 have remained," it is stated, " unchanged in form, and even in 

 colour, all through the changing conditions of life occurring during 

 the upheaval of the fen valley, and the consequent alteration of the 

 coast line, and particularly those caused by the change from the 

 saline influences of the neighbouring sea, to those of a warm inland 

 district." * 



This fact is of importance in all discussions about the variation 

 and origin of species, and the more so from the circumstance of 

 most of the lepidopterous insects above mentioned " belonging to 

 large genera of closely allied and abundant species (Agrotis, 

 Mamestra, Gelechia), genera such as have been pointed out as most 

 likely to produce new species by natural selection, dominant groups 

 in fact." 



And this leads to the subject of variation in insects, and the 

 causes which affect it, while it is suggestive of the amount of infor- 

 mation that might be got on these points by local entomologists if 

 they would take up the inquiry. 



It was formerly the practice of entomologists to pay very little 

 attention to varieties, and to collect species only, or what were 

 considered such. But it is now found that however fixed in their 

 characters some species may be, as in the case above mentioned, or 

 even whole genera, the greater part are more or less unstable ; 

 stability being probably due to the forms in which it prevails 

 having been for a long time back cut off and isolated from their 

 congeners, combined with some peculiar circumstances in the 

 localities to which they have been since confined. Where we find 

 stable and unstable species living together in the same area of 

 dispersal, and exposed to the same physical influences, we must 

 look to their economy and habits, or to peculiarities of structure, if 

 existing, as affording the only probable hope of explanation of the 

 phenomenon. 



* Trans, of Norfolk and Norwich Naturalists' Society, 1870-71, p. 61. 



