A CASE OF PERSISTENT MELANISM

Abstract

The occurrence of melanism is a phenomenon of wide distribution in nature, being recorded among animals belonging to a great many classes and orders. Our records show that while the occurrences are frequent and are found in species belonging to many of the larger zoological groups, yet the actual numbers of melanic individuals found among the individuals of any one species, in any one region is usually extremely small in comparison with the total number of normally colored individuals of the same species found in the same region. Because of this rarity of these black-colored individuals the appearance of melanic forms has been very generally regarded as being due to sporadic though at times oft-repeated, sporting. Such melanic forms do not usually persist racially. It was Darwin who years ago noted that sports of almost all kinds were ruthlessly eliminated in the struggle for existence; and sporting in the form of melanism apparently has offered no exception to this general rule. Perhaps one of the best examples of the racial persistence of melanism is that of the melanic form of the moth, Amphidasys betularia, which existed as a rarity in England some years ago, but which now has replaced the typical form about some of the manufacturing districts. This persistence of melanism has been explained on account of the environment in these districts being changed by the smoke from factories which darkens the vegetation in general, by the killing of lichens and by the depositing of black soot, and in this manner gives an advantage to the melanic forms by making them less conspicuous than the normally colored individuals.