OCCASIONAL ABUNDANCE OF INSECTS. 21 
arrested by so singular a spectacle, and because I failed to 
find in the various theories promulgated that vera causa 
which I felt to be wanting I was led to investigate the 
matter a little more closely and from another point of view, 
and it is the result or absence of result of this enquiry to 
which I am now directing attention. 
The problem to be attacked stands somewhat thus. 
A certain Lepidopteron Deilephila galwi, which discoy- 
ers the antiquity of its pedigree and the perfection of its 
developmental adjustment in a range which extends 
almost all over the northern temperate zone, here in 
Great Britain is normally one of the rarest of our insects, 
but as far back as authentic entomological records go has 
appeared during the last sixty years on four occasions in 
phenomenal abundance. These years were 1834, 1859, 
1870, and 1888. So scarce is this imsect that in all 
the years amounting to fifty-four which made up the 
intervals been these anni mirabiles, probably not a score 
of specimens in any stage have been captured. Yet in 
those particular summers Dedlephila gal turns up in 
every place in which it has ever appeared. That is to say 
almost wherever the Galiwm grows, there in a “‘ Gali” year 
is D. galic to be found, and such spots are virtually wherever 
round these islands there are sandhills to grow Galiwm 
and entomologists to record the moths or the larve. 
Now there must be some reason for this, science knows 
not the idea which men call chance. Some sufficing 
cause vaust have produced these abundant crops of 
Deilephila galw in those particular years, and it is to dis- 
cover this cause that my efforts are directed. 
I cannot find that previous to 1888 any attempt at an 
explanation had been offered, but that year, being the 
occasion of the most abundant manifestation yet recorded 
saw several theories broached. Most of these were by 
