C. Tepandata spends the day on pine trunks, where the light form is almost invisible. The dark form is somewhat more easily seen. By noting at dawn the spot where an insect had come to rest, and then revisiting the tree later in the day, we were able to show that on some davs more than 50 percent of the insects had moved. Subsequently we found that because of disturbances such as ants or hot sunshine they had had to By to another tree trunk, usually about 50 yards away. I saw large numbers of these moths on the wing, and three other observers and I agreed that the dark form was practically invisible at a distance of more than 20 yards, and that the light form could be followed with ease at a distance of up to 100 yards. In fact, we saw birds catch three moths of the light form in Bight It is my belief that when it is on the wing in these woods the dark form has an advantage over the light, and that when it is at rest the reverse is true.
This may be one of many ways in which melanism was useful in the past. It may also explain the balance between the light and dark forms of Cleam Tepandata in the Black Wood of Rannoch. In this case a melanic may have been preserved for one evolutionary reason but then have spread widely for another.
The melanism of moths occurs in many parts of the world that are not industrialized, and in environments that are quite different. It is found in the mountain rain forest of New Zealand's South Island, which is wet and dark. It has been observed in arctic and subarctic regions where in summer moths must By in daylight. It is known in very high mountains, where dark coloration may permit the absorption of heat and make possible increased activity. In each case recurrent mutation has provided the source of the change, and natural selection, as postulated by Darwin, has decided its destiny.
Melanism is not a recent phenomenon but a very old one. It enables us to appreciate the vast reserves of genetic variability which are contained within each species, and which can be summoned when the occasion arises. Had Darwin observed industrial melanism he would have seen evolution occurring not in thousands of years but in thousands of days-well within his lifetime. He would have witnessed the consummation and confirmation of his life's work.