That many black species have arisen as sudden mutations from lighter-coloured animals seems tolerably certain from the facts that in Malacca the black leopard forms a local race; that some of the Gibbon apes are as often black as light coloured; that the American black bear is sometimes brown, while the other bears, when not brown, are almost invariably black. Color Mutations Not uncommon, although rarer than black or melanistic forms, are reddish or chestnut varieties. These occur both among tame and wild animals. Among domesticated creatures, sandy cats, “red” pigeons, buff fowls, chestnut horses, red guinea pigs afford examples of this mutation. Among wild animals many of the species of squirrel, not naturally red, produce red mutations; and some of the grey owls—as, for example, the Indian race of the Scops (_Scops giu_)—throw off a red or chestnut form. As everyone knows, some species are normally red. Green or olive species not unfrequently throw off yellow mutations. As examples of these we may cite yellow canaries, yellow budgerigars (_Melopsittacus undulatus_), goldfish, golden tench, and the golden form of the common carp among captive animals; and among animals in a state of nature, yellow forms have been recorded of the rose-ringed Paroquet (_Palæornis torquatus_), the green woodpecker, the pike, and the eel. These lutinistic forms usually have normally coloured eyes. Sometimes, but only very rarely, these yellow forms throw off white sports—as, for example, the “silver” form of the goldfish. Finn has seen a white variety of the common carp. White canaries are excessively rare, while white budgerigars are unknown. It is worthy of note that entirely yellow species of birds and fish are unknown. We would suggest that the explanation of this is that yellowness is correlated with some physical characteristic unfavourable to an organism exposed to the struggle for existence; hence individuals which are yellow are not permitted to survive. In some species of moths individuals occur in which the parts normally red are yellow. According to Bateson, a chalk pit at Madingly, near Cambridge, has long been known to collectors as a habitat of a yellow-marked form of the six-spot Burnet Moth (_Zygæna filipendulæ_). These lutinistic forms are not confined to one genus of Butterflies. Moreover, in the Pin-tailed Nonpareil Finch (_Eythrura prasina_) of the Eastern Archipelago the red tail and other red parts of the plumage are not infrequently replaced by yellow in wild individuals of either sex and of any age. In the blue-fronted Amazon parrot (_Chrysotis æstiva_)—a most variable bird—the normally red edge of the pinion is sometimes yellow. Bateson, in his _Materials for the Study of Variation_, gives other examples of this kind of variation. Mutations among Invertebrates