BUTTERFLIES AND MOTHS
to what is called environment—and it is here that the butterflies and moths provide excellent illustration of evolution.
When I was a boy the common peppered moth was known to produce a rare black variety. The growth of the manufacturing districts of Lancashire and Yorkshire has greatly increased the volume of the smoke there, which, carried by the prevalent southwest winds, has done deadly work over a wide area, killing the gray lichens and leaving the treetrunks dark and sooty. Resting on bark like this the peppered moth would be more conspicuous to the eye of a bird seeking food than the black variety, and accordingly for many years this black form has entirely replaced the other form in these northern tracts. The others, being more easily seen, have been eaten. And the peppered moth is not the only species that shows change; several other bark-haunting moths have also become much darker in the same strip of country and during the same short period. Furthermore, similar changes have been observed in the moths of other smoke-producing areas in this country and on the Continent. Harrison has recently shown that some of these moths have become dark after their caterpillars have been fed for many generations on plants contaminated with salts of manganese, such as are contained in smoke. Inasmuch as the effects were transmitted in Mendelian proportions we must conclude that the salts acted upon the germ cells.
A still better but less well-known example of change in colour is found among the butterflies of tropical America. In each district these insects and some of the day-flying moths form groups that are of similar pattern and colouring but that are composed of species having very different degrees of relationship. Among the groups of any locality one species is generally predominant in numbers and is among the most
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