36 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
ever they may be. This would appear no easy task when 
one considers that such forces are as complicated as they 
are numerous and that many are exceedingly subtle in 
their influence and, as for instance epidemic diseases, 
quite outside ordinary methods of investigation. A little 
consideration however induced me to think that among 
them all, the weather must be one of the most powerful 
factors, because we find gala by no means a rare insect 
in most parts of the continent, nor does it appear there to 
be so subject to irregularity of appearance. In fact Great 
Britain seems to be as it were the very fringe of its range. 
Now if the cause of the occasional abundance of this 
‘insect here were the temporary cessation of the ravages 
inflicted upon it by say ichneumons, birds, mice, or even 
winter floods, or moving sands, or disease except as induced 
by climatic causes, I can discovered no good reason why 
such influences should not make themselves felt as much 
in Germany as in England. But it is evident that the 
irregular action of the sinister influences whatever they 
may be are a peculiarity of this favoured isle, and the 
weather at once suggests itself. Now the only two forms 
of weather variation which permit themselves to be dealt 
with at all definitely or systematically are rainfall and 
temperature. 
The accompanying (p. 44) table shows how the four years 
of galw abundance were affected by temperature and rain- 
fall. There are accessible records of temperature of every 
month since 1814, but as our authentic entomological 
records do not go beyond 1834 we need not pursue the 
enquiry beyond that point. The statistics of rainfall are 
not so complete, the only set I can obtain for my first 
series of years 1833—1834 are from a British Association 
report on records taken in York, but the monthly means 
for all parts of these islands are proportionally very similar, 
