loured varnish, which can be removed by a
touch with a paintbrush. Here we have paint, the result of the urinary
compound laid on the inner surface of the covering, just as the
chromatic ingredients of our glass-painters are laid on our
stained-glass windows.

At other places the skin is coloured in its very substance; the
colouring-matter forms an integral part of it and can no longer be
swept away with a camel-hair brush. Here we have a dyed fabric,
represented in our windows by the panes of coloured glass which the
crucible decorates uniformly with this or that tint, by means of the
incorporated metallic oxides.

Whereas, in these two cases, there is a profound difference in the
distribution of the chromatic materials, is this true of their
chemical nature as well? The suggestion is hardly admissible. The
worker in stained glass dyes or paints with the same oxides. Life,
that incomparable artist, must even more readily obtain an infinite
variety of results by uniformity of method.

It shows us, on the back of the Spurge Caterpillar,[7] black spots
jumbled up with other spots, white, yellow or red. Paints and dyes lie
side by side. Is there on this side of the dividing line a paint-stuff
and on the other side a dye-stuff, absolutely different in character
from the first? While chemistry is not yet in a position to
demonstrate, with its reagents, the common origin of the two
substances, at least the most convincing analogies point to it.

[Footnote 7: The caterpillar of the Spurge Hawk-moth.--_Translator's
Note_.]

In this delicate problem of the insect's colouring, one single point
thus far comes within the domain of observed facts: the progressive
advance of chromatic evolution. The carbuncle of the Dung-Beetle of
the Pampas suggested the question. Let us then inquire of his near
neighbours, who will perhaps enable us to advance a step farther.

Newly stripped of his cast-off nymphal skin, the Sacred Beetle
possesses a strange costume, bearing no resemblance to the ebony black
which will be the portion of the mature insect. The head, legs and
thorax are a bright rusty red; the wing-cases and abdomen are white.
As a colour, the red is almost that of the Spurge Caterpillar, but it
is the result of a dye on which nitric acid has no effect as a
detector of urates. The same chromatic principle must certainly exist
in a more elaborate form and under a different molecular arrangement
in the skin of the abdomen and t