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.