“4. In the generality of cases coloring-matters, such as indigo, Indian yellow, madder-lake, and the like, do not separately exist in the substance of vegetables, but the pigments are disengaged through fermentation or oxygenation. Again, alizarin itself is reddish yellow, but alkaline solutions strike it a rich violet just as we find them to act towards the substance which Mr. Sorby calls aphidilutein. “5. Mr. Sorby’s four stages of the changes effected by the oxidation of aphideine produce four different substances.” =Chemical and physical nature of the pigment.=—Researches in this difficult field of inquiry have been made by Landois (1864), Sorby (1871), Meldola (1871), by Krukenberg (1884), and more recently by Coste, Urech, Hopkins, and Mayer, and the subject is of fundamental importance in dealing with mimicry and protective coloration, the primary causes of which appear to be due to the action of physical and chemical agents. Over twenty years ago Meldola observed that the yellow pigment of the sulphur-yellow butterfly (_Gonopteryx rhamni_) was soluble in water, and showed that its aqueous solution had an acid reaction. Besides the yellow uranidin found by Krukenberg in different beetles and lepidopterous pupæ, still other coloring-matters, which are very constant in different species are readily recognized by the spectroscope. “Thus there appear in the brownish yellow lymph of _Attacus pernyi_, _Callosamia promethea_ and _Telea polyphemus_, after saponification of the precipitated soap readily effected by ether, or incompletely or not removed by benzine, a chlorophane-like lipochrome; and in the yellowish green lymph of _Saturnia pyri_ and of _Platysamia cecropia_ besides this pigment still another whose spectrum shows a broad band on D, but which disappears with the addition of acetic acid or ammonia, as also after a long heating of the lymph up to 66° C.” Coste, and more especially Urech, have shown that many of the pigments may be dissolved out of the scales by means of chemical reagents, giving colored solutions, and leaving the scales white or colorless. They have also shown that some of these pigments may be changed in color by the action of reagents, and then restored to their original color by other reagents. They have proved that reds, yellows, browns, and blacks are always due to pigments, and in a few cases greens, blues, violets, purples, and whites, and not, as is usually the case, to structural conditions, such as striæ on the scales (Mayer). They confined themselves solely to the chemical side of the problem, not considering the structure of the scales themselves. Urech has also discovered a beautiful smaragd-green coloring-matter in the wings (not in the scales) of the pupa of _Pieris brassicæ_. It is not chlorophyll, and Urech suggests that it may be either the germinal substance of the pigments of the scales or its bearer. It is not the pigment of the blood.