24 
_ We now come to atmospheric influences, and here we find that 
in degrees of temperature acting upon the chrysalis, we possess 
the most powerful agent yet discovered to produce variation ; but 
before discussing this, let us dispose of the other atmospheric. 
conditions which tend to modify ihe perfect insect. 
Easterly wind will sometimes delay the appearance of an insect. 
until long after its proper time for emergence, perhaps compelling, 
it to lie over for another year, and thus indirectly, by prolonging 
the chrysalis state, be the cause of variation, for it is well known 
that certain chrysalids, especially those who have fed internally, as 
caterpillars, will wriggle themselves towards daylight, with the 
intention of coming out, and then, having had a taste of a north- 
easter, alter their minds and beat a hasty retreat. 
Rain or humidity would seem to operate on the insect about to. 
emerge. By moisture we try to account for the darkness of some 
species taken in the West of England, Ireland, and the Isle of 
Man, because it cannot be accounted for by cold, as in the case of 
Scotch butterflies and moths. We know that if moisture gets. 
between the scales, displacing the air, the colour is altered, and a 
curious enquiry into the cause of the green of the underside of a 
green hair streak turning red when wetted, has lately taken place. 
Electricity is supposed to be a cause of variation amongst the 
blues, more particularly those which emerge during a thunderstorm,,. 
these being of a beautiful lilac instead of the ordinary blue colour,. 
but it is just possible that the enriching of the colour is due to the 
oppressive heat which generally precedes a summer storm, and we- 
shall see further on that one of ihe effects of heat is to convert blue 
into violet owing to the formation of red pigment. Mr. Sydney 
Webb, however, says that tortoiseshell butterflies which emerge 
before a storm are paler, while those appearing after it are darker. 
(Examples exhibited by Mr. Sydney Webb.) 
The French have suggested an apparatus for subjecting chrysalids- 
to the effects of an artificial storm by isolating them in a little. 
sulphur cell and placing them between the electrodes of a battery 
to represent an electrical condition of atmosphere, and also effecting 
occasional discharges to represent lightening, but the results have. 
been negative, and the insects which emerged did not seem any the. 
worse for this treatment. 
Heat and Cold.—A good many years ago some notes in the- 
French Annals, especially one by M. Bellier, de la Chavignerie, 
written in 1858, attracted my attention. M. Bellier’s note ran. 
thus: ‘‘ One day a friend of mine brought me several tortoiseshell 
butterflies, which, owing to the wings being excessively dark, and 
adorned with large, intensely black spots, immediately excited my 
admiration. He assured me that these remarkable specimens had 
