﻿THE ENTOMOLOGIST. 



Vol. XXIIL] JUNE, 1890. [No. 325. 



ON THE OCCASIONAL ABUNDANCE OF CERTAIN SPECIES 

 OF LEPIDOPTERA IN THE BRITISH ISLANDS.* 



By R. Adkin, F.E.S. 



The sporadic appearance in these Islands of certain species 

 of Lepidoptera is a subject that has received a large amount of 

 attention, and many theories have been advanced to account for 

 the phenomenon : these may be grouped into two great classes, 

 which may for present purposes be termed the " migration " and 

 the " local causes " theories. 



The British Islands, as we now know them, have a superficial 

 area of some 121,000 square miles, and enjoy a temperate 

 climate, but such favourable conditions were not always existent ; 

 at some former period this country was a land of snow and ice, 

 probably not differing much from what Greenland now is, and 

 the majority of our present species could not have lived under 

 such climatic conditions : it is probable that at this time these 

 Islands formed a portion of the continent of Europe, and that 

 upon the glacial conditions giving way to a period of milder 

 temperatures, a fauna suitable to the altered circumstances of 

 climate would follow in due course. If we compare represent- 

 atives of our more generally distributed species with those 

 inhabiting Continental Europe, we find the majority of them are 

 identical, and we arrive at the conclusion that they are of common 

 origin. Further, the conviction is forced upon us that our present 

 insect fauna has been derived from the Continent, and that it has 

 been brought about by the several species gradually extending 

 the area of their distribution. This would appear to indicate an 

 initial migratory condition which it is easily conceivable would 

 be subject to such modification as became necessary from time to 

 time in the economy of a species; and the cutting off of the 



* Abstract of a Paper read before the South London Entomological and Natural 

 History Society, April 24, 1890. 



ENTOM. — JUNE, 1890. O 



