40 6 
The necessity for the continuance of the same external 
conditions, which have modified an organism, is well seen 
from the following consideration :—“ Not rarely a long 
period of dry or moist weather exercises a considerable 
influence on the size of the following generation. Imme 
diately after a continuous dry summer butterflies are 
always smaller than after a moderately moist season. The 
second generation of Argynnis selene, which takes flight 
in the height of summer, is always smaller than the first 
generation.” Seasons, however, vary, and no permanent 
change takes place. But if butterflies pass into a climate 
like Australia, which is permanently dry, “ the diminution 
which occurs remains permanent in all subsequent gener 
ations in their new habitat, and the deteriorated condition 
of the species is established for ever.”* 
Change of food also produces the most marked results. 
“ The Tiger-moth (Chelonia caja), whose caterpillar is the familiar 
Woolly-bear, is almost the classical instance of the effects of food 
upon colour. It appears to be in any case a most variable species. 
The pages of The Entomologist, and other journals devoted to 
entomology, contain numerous records of varieties, some of which 
have been traced to food, while others have not a known history. 
The orange-red of the hind wings may be replaced by yellow, and 
the proportions of brown and white in the fore wings are subject to 
immense fluctuation. Eimer (Organic Evolution, p. lyo) quotes 
from Koch a number of colour changes which accompany, and are 
probably caused by a change of diet. Thus, if fed upon lettuce, the 
white ground of the fore wings predominates over the brown ; the 
precise contrary is produced by feeding the larvae upon the leaves of 
the deadly nightshade ; in moths bred from larvae which have been 
fed upon the leaves of this plant, the white becomes almost ob 
literated, while the bluish marks upon the hind wings fuse together 
and displace the orange-yellow ground colour. Koch concludes the 
account of his observations with the following remarks : ‘ Must not 
similar processes occur equally, and even on a larger scale, in the 
natural life of the countless forms of the class in question ? When a 
great number of individuals perish through an occasional scarcity 
Eimer. Organic Evolution, p.152.