OF LEPIDOPTERA IN GREAT BRITAIN. 65 
find the same law holds good. Belgium for instance has, 
according to Sramyton (Trans. Ent. Society of London), 94 
species, while in Silesia there are 124 gpecies. The number 
also diminishes from 8. to N., but not in the same proportion, 
for there are said to be 77 species in Lapland, while in Scotland 
there are only 34. 
What is, then, the cause of this decrease in the number of 
species as we go Westward? We may, I think, dismiss the 
Geological view, for granting that during the Glacial period the 
Northern and N. Western portion of the Island remained sub- 
merged longer than the 8. Hastern, there is no reason why 
Butterflies with such powerful wings as A. /ris and ZL. Szbylla 
should not have migrated North when the feeble C. Phigas and 
L. Alsus did so, unless other causes prevented them. Nor can 
it be want of the suitable food plants for the larve, for as a 
general rule these are sufficiently abundant in the districts where 
the insects do not occur. In the case of the two first Butterflies 
mentioned above the food plants are poplar and willow, and 
honeysuckle, all surely common enough in our district. 
The only remaining -cause which appears to me probable is 
climate, and it remains for us to consider in what manner this 
can influence the distribution of Lepidoptera. Most people are 
aware that owing to the position of these Islands on the extreme 
West of the Continent of Europe, they are the first to encounter 
the moisture-laden winds that sweep across the Atlantic. These, 
on reaching the high lands that fringe our Western shores, are 
partially condensed into clouds and rain, and pass on Eastwards, 
parting with more and more moisture as they go, till as they 
reach the centre and East of Europe comparatively little 
moisture is left in the atmosphere. The consequence of this is 
- that not only is the rainfall greater, but, owing to the power 
of aqueous vapour to absorb radiant heat, the sun’s rays have 
much less power in this country (and especially on the Western 
side of the Island) than further to the East. And the same 
power of checking radiation causes the cold in winter to be as 
much greater to the E. as is the heat in summer. As, however, 
we are only concerned with the summer temperature, we need 
not concern ourselves with this last point. 
If the above statement be true, we should expect to find the 
day temperature in summer increasing as we go from W. to E., 
and this is precisely what takes place, and to an extent, con- 
sidering the narrow limits of our Island, that is somewhat 
surprising. In order to illustrate this point I have selected 
four places from the list published by Mr. Gutatsuer in his 
Quarterly Returns. These are-- 
1.—Blackheath, in Kent, representing S.E. England. 
2.—Plymouth + Siw. ys 
3.—Leeds 8 N.E. * 
4.—Liverpoul oe NSW fees 
