62 



On the Occasional Abundance of Certain Species of 



Lepidoptera in the British Islands. 



By Mr. R. Adkin. Read April 2\th, 1890. 



Anyone who has paid attention to the lepidoptera of the 

 British Islands, will have noticed that many species are far 

 more abundant in some years than in others ; that those 

 usually accounted rare are occasionally to be found in con- 

 siderable numbers, while others that are recognised as generally 

 common, appear from time to time in utmost profusion. 

 These cases of abnormal abundance are generally confined 

 to a limited number of species at one time, some one or 

 possibly two being simultaneously affected, and it often 

 happens that at such times lepidoptera generally are less 

 common than usual ; the area also to which they extend is 

 sometimes very circumscribed, but at others, includes the 

 whole of the British Isles, and even the Continent of Europe ; 

 indeed, there appears to be good ground for believing that 

 large districts of the palaearctic region are at times simul- 

 taneously affected. 



The question that will naturally present itself to our minds 

 is, what are the causes of these times' of unusual abundance ? 

 The subject has already received a large amount of attention, 

 and numerous theories have been advanced to account for 

 the phenomonen ; these may be grouped into two great 

 classes, which we may, for convenience, term the " migration," 

 and the " local causes " theories ; I propose, after first glancing 

 at the probable origin of our present lepidopterous fauna, to 

 briefly investigate the evidence which we have at our disposal 

 bearing upon the one and the other. 



The British Isles as we now know them, are made up of two 

 main and sundry smaller islands, having altogether a superficial 

 area of some 121,000 square miles ; they lie for the most part 

 between the 50th and 60th degree N. latitude, adjacent to the 

 flow of the Gulf Stream, 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 of lepidoptera 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 



