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
As further instances of mutations among animals which have been observed in nature, we may mention the valezina form of the female of the Silver-washed Fritillary Butterfly (Argynnis paphia) and the helice form of the female Clouded-yellow Butterfly (Colias edusa).
The common jelly-fish is an organism which frequently throws off sports, and some zoologists are of opinion that the medusoid Pseudoclytia pentata arose by a discontinuous variation from Epenthesis folleata or a closely allied form. Thomson discusses this particular case at some length on pages 87-89 of his Heredity, and gives it as his opinion that the evidence in favour of this latter having arisen as a mutation is “exceedingly strong.”
Mutating Species
It is our belief that many species of birds which occur in nature have been derived from other species which still exist, but as no one has ever seen the mutation take place, we cannot furnish any proof thereof. We merely rely on the fact that the species in question differ so slightly from one another that there seems every likelihood that they have suddenly arisen and managed to establish themselves alongside of the parent species.
The Curassows, Crax grayi, C. hecki, each of which is only known by a very few specimens, appear to be mutations of the female of the globose Curassow, Crax globicera. The fact that when a female hecki bred in the London Zoological Gardens with a male globicera, the solitary young one which lived to grow up was a pure globicera, renders the assumption almost certain.
The Chamba Monaul (Lophophorus chambanus) seems to be a mutation of the male of the common Monaul or Impeyan Pheasant (Lophophorus impeyanus), the common species of the Himalayas.