30 THE YOUNG NATURALIST. 



new types ; and for these twenty or thirty years between Geffroy and 

 Latreille, although progress was being annually made, it is difficult, 

 if not impossible, to assign to each author his own peculiar share in 

 it. 



(To be continued). 



The Blown-over 



v. 



The Climatic Influence Theories. 



By H. H. CORBETT, Esq., M.R.C.S. 



The phenomenon of the sudden appearance of one or more species 

 of insects, generally more or less rare, in unwonted numbers, has long 

 been a subject of interest to entomologists, and has during the late 

 summer of 1888 been especially brought before our notice in the case 

 of D. galii. Several theories have from time to time been advanced 

 to account for such sudden influxes. The chief of these are known 

 as — First, the blown-over theory : by which it is supposed that in- 

 sects are either voluntarily, by some migratory instinct, or involun- 

 tarily, by stress of weather, carried over from their head-quarters on 

 the continent to our islands. Second, the climatic influence theory : 

 by which it is supposed that through some peculiarity of climate, a 

 species may be so well suited that it increases in numbers far beyond 

 what is usual with it ; or that having laid dormant for years in either 

 the oval or pupal states, it is suddenly awakened, and appears in 

 great numbers as either larvae or imagines. 



Many entomologists appear to adopt one of these two theories, to 

 the exclusion of the other. It seems to be a strong trait of human 

 character to take sides on any question, and to refuse to see any good 

 or truth in the opinion of those whose views are different. But I 

 think that in this, as in many other cases, the truth is to be found in 

 both sides ; and it will be my endeavour to show good reasons for 

 supporting both views. I do not wish to appear dogmatic, or to 

 attempt to set up my own opinion against those of other naturalists, 

 but to throw out a few ideas and suggestions, subject to correction, 

 of what seem to me to be the probabilities of the case. 



Certain species are known to be very irregular in appearance, even 

 among our commoner insects, e.g. C. edusa, P. [cardui, C. promissa, 

 C. . sponsa, P. gamma, and many others. While among the rare 



