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acquire fresh chlorophyll from the leaves. The passage of an adven
titious colouring matter on into a second generation is a very
remarkable phenomenon.”—.(Poulton. The Colour of Animals,
ftp- 79~8°-)
In this case, the green leaf causes the concealment which
the caterpillar enjoys at once, just as the colour of the
chemist’s bottle would be altered by the alteration of its
contents. But that is not all. It colours the eggs which
the caterpillar will lay when it becomes a perfect insect.
Nor is that all; for the young caterpillar of the next
generation is hatched green,—it is protected from the
hour of its emergence from the egg ere it has gained the
protection of the leaves on which it feeds. Of course, all
the organisms undergo this change. How can this case
be cited as a most favourable illustration of the action of
Natural Selection?
But sometimes, in a most marvellous manner, the green
caterpillar changes its tints pari passu with the changing-
colour of the autumnal leaf. In order to understand how
this transmutation has been brought about, we must
remember that the leaves of different plants contain, in
various proportions, a principle known as tannin; and
botanists tell us that the autumnal tints of leaves are due
to the changes which tannin undergoes on exposure to
light and air. These tints are the most splendid in the
case of trees and shrubs rich in tannin. In such species
a leaf or a part of a leaf accidentally screened from light
retains its green summer colour, whilst those around have
turned into a rich yellow, red or brown. Now, it has been
discovered that certain insects also contain this principle
of tannin. In the year 1810, M. Penaut, a pharmaceutical
chemist of Bourges, found in the body of corn-weevils
gallic acid, a compound kindred to tannin. M. Villon,
who has been engaged for a long time in searching for