Darwin, always searching for missing evidence, might well have accepted Harrison's Lamarckian interpretation, but in 1926 biologists were skeptical. Although the rate of mutation of a hereditary characteristic can be increased in the laboratory by many methods, Harrison's figures inferred a mutation rate of 8 percent. One of the most frequent mutations in nature is that which causes the disease hemophilia in man; its rate is in the region of .0005 percent, that is, the mutation occurs about once in 50,000 births. It is, in fact, unlikely that an increased mutation rate has played any part in industrial melanism.
At the University of Oxford during the past seven years we have been attempting to analyze the phenomenon of industrial melanism. We have used many different approaches. We are in the process of making a survey of the present frequency of light and dark forms of each species of moth in Britain that exhibits industrial melanism. We are critically examining each of the two forms to see if between them there are any differences in behavior. We have fed large numbers of larvae of both forms on foliage impregnated with substances in polluted air. We have observed under various conditions the mating preferences and relative mortality of the two forms. Finally we have accumulated much information about the melanism of moths in parts of the world that are far removed from industrial centers, and we have sought to link industrial melanism with the melanics of the past.
Our main guinea pig, both in the field and in the laboratory, has been the peppered moth Biston betularia and its melanic form carbonaria. This species occurs throughout Europe, and is probablv identical with the North American Amphidasis cognataria. It has a one-year life cycle; the moth appears from May to August. The moth Hies at night and passes the day resting on the trunks or on the underside of the boughs of J'Oughbarked deciduous trees such as the oak. Its larvae feed on the foliage of such trees from Tune to late October; its pupae pass the winter in the soil.
The dark form of the peppered moth was first recorded in 1848 at Manchester in England. Both the light and dark forms appear in each of the photographs at right and on the next page. The background of each photograph is noteworthy. In the photograph on the next page the background is a lichen-encrusted oak trunk of the sort that today is found only in unpolluted rural districts. Against this background the light form is almost invisible and the dark form is conspicuous. In the photograph at right the background is a bare and blackened oak trunk ill the heavily polluted area of Birmingham. Here it is the dark form which is almost invisible, and the light form which is conspicuous. Of 621 wild moths caught in these Birmingham woods in 1953, 90 percent were the dark form and only 10 percent the light. Today this same ratio applies in nearly all British industrial areas and far outside them.
We decided to test the rate of survival of the two forms in the contrasting types of woodland. We did this bv releasing known numbers of moths of both forms. Each moth was marked on its underside with a spot of quick-drying cellulose paint; a different color was used for each day. Thus when we subsequently trapped large numbers of moths we could identify those we had released and established the length of time they had been exposed to predators in nature.
In an unpolluted forest we released 984 moths: 488 dark and 496 light. We recaptured 34 dark and 62 ligh t, indicating that in these woods the ligh t form had a clear advantage over the dark. We then repeated the experiment in the polluted Birmingham woods, releasing 630 moths: 493 dark and 137 light. The result of the first experiment was completely reversed; we recaptured proportionately twice as manv of the dark form as of the light.