58 PERIODICITY IN ORGANIC LIFE. 



further examples from the insect world, as both the above- 

 mentioned species are such excellent illustrations of periodicity, in 

 the sense I have used the term. Insects are, however, one of the 

 best classes of animals for working up this subject, as there are so 

 many species which, during their period of abundance, are so very 

 numerous in individuals that the changes may be the more readily 

 observed than if the specimens were less frequent and more difficult 

 to obtain. 



I have no doubt that many of our entomological friends will be 

 able to call to mind further examples of this periodicity in abundance, 

 for all must have heard of a " Clouded Yellow year," a " Camberwell- 

 Beauty year," a " Convolvuli year," and so on. 



All of these years of abundance have not been due, in the case of 

 many insects, to the parents being blown over from the Continent, 

 although it is just possible that this cause may explain the occasional 

 apparition of great numbers of " Clouded Yellow "' and " Camberwell 

 Beauty " butterflies. 



Occasionally it has happened, and perhaps more frequently than 

 hitherto noticed, that the period of abundance of a species in this 

 country coincides with the abundance of the species throughout the 

 continent of Europe. This was notably so, a few years since, with 

 regard to the "Gamma-moth " (Plusia gamma). Now in this instance, 

 it could neither be the suitability of the season nor the abundance 

 of food, as these causes could not have produced the same effect in 

 the hotter regions of the South of Europe, and the cooler climate of 

 Scandinavia. For this mysterious something, then, I use the term 

 " Periodicity," not at all as an explanation, but simply as a term to 

 note the fact. 



It is easy to say in explanation of the immense hordes of locusts 

 which occur in many parts of the world, that they are migrating for 

 the purpose of finding food. But is this an explanation of periodicity 

 ill these insects? How is it they are in such abundance that they 

 must migrate? They are always in the districts from whence they 

 start on their migrations, and to which they never return. How is it 

 they do not find it necessary to migrate every year? It cannot be 

 in consequence of meteorological causes. Nor is it a question of 

 food, because frequently years elapse between one period of 

 abundance and another, and at other times for several years in 

 succession swarms follow swarms. When I come to mention the 

 Lemming, we shall find the same difficulty in explaining the 



