340 TJte Inheritance of Wing Colour in Lepidoptera 



palings, etc. in the daylight ; and at night all moths are melanic. It is 

 true that a species like Gnophos obscurata has a white form confined to 

 the chalk district of Lewes, and a dark form common in the New Forest, 

 where the insect is said to rest on cut peat ; but the contrast between 

 the colour of chalk cliffs and that of peat hags is much greater than 

 between urban and rural districts. Geological formations, moreover, 

 have been able to exert their influence for a much greater period than the 

 works of man. It would be dangerous to say that the black colour of the 

 melanic forms has no protective value, but it is hardly credible that it 

 can be the chief, or only factor in their rapid appearance and increase. 



It might, at first sight appear that if a new form is dominant, as is 

 the case in most melanic varieties, nothing else would be required to 

 ensure that it gradually replaced the type form. It has however been 

 shown on mathematical grounds \ that if a dominant character is intro- 

 duced into a normal population, in which mating takes place at random, 

 the proportion of this form in the second generation will be double what 

 it was in the preceding one, but this proportion will afterwards have no 

 tendency whatever to increase. Some other factor which favours the 

 variety at the expense of the type form must therefore be postulated. 



It appears much more reasonable to suppose that the black colour 

 has little or nothing to do with the matter, but that the real cause 

 lies in a constitutional hardiness, which is correlated with melanism. 

 The black colour is such a striking feature that far too great an effect is 

 likely to be attributed to it. Doncaster^, who first made this suggestion, 

 has observed that the banded variety oi Angerona prunaria, var. sordiata, 

 is less hardy than the type, and Bowater^' states that the melanic variety 

 of Odontoptera bidentata is also the hardier. In the experiments here 

 recorded, as well as in those dealing with T. consonaria, var. nigra, and 

 other melanic forms, the general impression is that the melanics seem 

 to be earlier, stronger and larger than the type forms. Moreover, there 

 is often a slight excess of melanics in most crosses. 



Some explanation is certainly necessary to account for the fact that 

 melanic varieties occur with such frequency in urban and industrial 

 districts. If however it be supposed that many species have " sported," 

 giving melanic varieties, which are hardier and more robust than the 

 type insects, it follows that these melanics will increase rapidly at the 

 expense of the feebler form, wherever the struggle for existence is 



1 Hardy, G. H., Science, N. Y., Vol. xxviii. N. S. No. 706, p. 49, July 10, 1908. 



2 Doncaster, L., Ent. Rec. Vol. xviii. p. 219, 1906. 



» Bowater, W., Journal of Genetics, Vol. iii. p. 299, 1914. 



