It is the merit of Mr. Thayer to have drawn attention to these considerations, and to have carried out some interesting demonstrations of the more frequent significance of colour-markings as means of concealment and illusion than had been recognised before his work. At the same time, it is not possible to consider the yellow and black livery of wasps, of certain evil-tasting grubs, and of poisonous salamanders as anything but a “danger-flag,” a warning to other animals that the yellow and black animal had better not be bitten and tasted. So the previous experience of animals who have bitten yellow and black creatures is appealed to, and ensures the safety of the yellow and black gentry from tentative bites which would kill them. Other recognition marks by which ill-tasting, nauseous butterflies are distinguished, and in consequence of which they escape attack, and, not only that, but are “mimicked” (as the yellow and black poisonous wasp is mimicked by some innocuous flies which thus escape attack) by other pleasant-tasting butterflies which fly with them, are considered by Mr. Thayer to be wrongly interpreted as recognition or “warning” marks. He shows, with more or less success, that the markings of the butterflies known as Heliconiæ are effective as concealment, and is therefore inclined to deny their value as “warning” marks, serving to indicate a noxious quarry best left untasted. It is, of course, quite possible that what are “concealment markings” when viewed by an aggressive bird or lizard at a distance, may be recognised as “warning marks” when seen by the same observers at close quarters, and it is also possible that the latter may have become the more important or only important result of the colour marks of a given butterfly which were once useful as “concealment.” The possible change of significance of colour spots and markings in wild animals may be illustrated by the effect on human beings of the burglar’s crêpe mask. At the present moment probably the most prominent result of the appearance in a house full of people in the dead of night of a man with a crêpe mask over his face would be terror to those who saw him. The mask would be interpreted as a “mark” or “sign” of evil, not to say violent intentions on the part of the masked man. It would be a “warning colour,” and most unathletic individuals would severely avoid it; in fact, retire from it in alarm. But actually, the burglar’s mask—as possibly some noxious insects’ distinctive markings—was not invented for the purpose of causing alarm. Far from it! The burglar, or nocturnal malefactor, dons his crêpe mask in order to cover the white glitter of his face, and so to escape observation. In origin it is a protective coloration leading to invisibility, and only secondarily has it become a “warning colour” or “mark” at close quarters. There will be much more ascertained, and much instructive discussion as to the colours and