18 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
oped and apparently stably conditioned a group as the 
mammals, the farmers of Dumfries and Galloway have 
lately had reason to their cost to know. But it is to 
certain extraordinary instances of its occurrence among 
the Insecta that I wish on this occasion to refer. Now 
examples of what I allude to, of the sudden profusion of 
some particular species are so common that I need 
scarcely recapitulate them. Cases of the kind are met 
with to whatever order of insects attention may be direc- 
ted. But as the attention of most entomologists is, and 
for a long time past has been directed more especially to 
the Lepidoptera, and as amongst the Lepidoptera some 
of the most remarkable cases are to be found, I will for the 
present restrict consideration to that order. 
The common white cabbage butterfly Pieris rape, may 
be confidently expected to make its appearance each year 
with the first warm sunshine of April. At first it is the 
harbinger of spring, later however this insect outstays its 
welcome and degenerates into a garden pest. However 
cold or wet the season, this species never fails and 1s nearly 
always and in all places equally abundant. But the 
garden white has a not very far distant relative Colias 
edusa. Here in the north of England this is one of the 
rarest of British Diurni, at any rate had been so till the 
year 1877 and then, suddenly, apparently without cause, 
in that late and wet summer came this butterfly’s epip- 
hany. On that occasion C. edusa might have been seen 
in every clover field and by every grassy road side 
throughout this district. 
This was 15 years ago and since that summer with the 
exception of some single captures reported in September 
1888, not one has been so much as seen. And yet con- 
trasting such an erratic insect as this with our unfailing 
garden whites, no casual investigation can discover in 
