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 the wing-cases which will presently replace white by red. In two or three days the colourless becomes the coloured, a process whose rapidity implies a fresh molecular structure rather than a change of composition. The building-stone remains the same, but is arranged in a different order; and the structure alters in appearance. The Scarabaeus is now all red. The first brown stains show themselves on the denticulations of the forehead and fore-legs, the sign of an earlier maturity in the implements of labour, which are to acquire an exceptional hardness. The smoky tinge spreads more or less all over the insect, replaces the red, turns darker and finally becomes the regulation black. In less than a week the colourless insect turns a rusty red, next a sooty brown and then an ebony black. The process is completed; the insect possesses its normal colouring.