Abstract
The tendency for insects in certain regions to be darker in color than their relatives elsewhere was first pointed out by H. W. Bates among the butterflies of the Amazonas. Similar tendencies to regional have since been noted in other regions, but the biological basis of the condition is still a matter of surmise except in a few cases which have been investigated in detail. The most thoroughly studied example of regional is that of the moths of the midlands of England. Here about 70 species are now known to have melanic mutant forms which are in the process of replacing the normal light forms of the species. Kettlewell (1958) and others have shown that this industrial melanism is a dramatic illustration of natural selection acting upon genetic mutations. In most cases, hoxvever, the darkened condition has been tacitly attributed to the action of environment. There is direct experimental evidence that low temperatures favor melanin formation in in-
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