310 Heredity of Melanism in Lepidoptera 



12. The question of the exact method of the heredity of melanism 

 in the familiar Peppered Moth, Amphidasys betularia, is of special 

 interest because the melanic variety, doubled ay aria, in the 65 years 

 since it first appeared has multiplied and spread all over England, and 

 is now far commoner than the type, and is often quoted as a good 

 example of a distinct alteration in a whole species occurring in our own 

 times and apparently not due, directly at any rate, to man's influence. 



In the Type form the body and wings are white, abundantly dusted 

 with black. On the costal margin are five black spots. From the first 

 two arises a duplicate angulated black first line, to the third is attached 

 a black discal spot, the fourth originates the second line, which is angu- 

 lated. On the hind margin is a series of black dots, extending into the 

 white cilia. Hind wings white (especially so in costal area), dusted or 

 spotted with black, and having usually 2 or 3 obscure partial transverse 

 lines. Central spot black. Leg tufts brownish grey, tarsi and antennae 

 in female black spotted with white. In male antennae brown-black. 



In the Melanic form (doubledayaria) the whole insect is almost 

 uniformly smoky black, except lower portion of face which is white, and 

 a round white spot at the extreme base of the costa of each fore-wing. 



Varieties of the type have occurred where the definite markings 

 have been reduced, leaving the whole white surface but scantily dusted 

 with black, and one specimen has occurred almost white. In other cases 

 the typical markings are accentuated and somewhat increased. 



In what are usually termed the intermediate forms, the normal 

 markings are much increased and become partly confluent, so that in 

 many cases the moth is black spotted all over with white dots and 

 minute irregular blotches. The hind-wings are almost always lighter 

 than the fore-wings. 



Between the type and the doubledayaria variety every gi-adation of 

 intermediate has occurred, although the intermediate is the rarest of the 

 three forms. It occurs in nature as well as in bred specimens. The 

 thorax and abdomen are of exactly the same colouration as the wings in 

 all specimens of t5rpe and varieties. 



The black which is velvety in a live specimen diminishes in intensity 

 on death, and has a strong tendency to " rust " on the slightest provoca- 

 tion, and after a time even in spite of precautions. 



The accompanying table consists of those records of genetic value of 

 breeding this moth which I have been able to find. 



In February, 1910, I started an experiment which I hoped would 

 furnish some evidence on this question. The claims of my other 



