65 



avion. One entomologist at least is on the track of the 

 mystery, but so far I believe it has eluded him. Last year, 

 however, a step was made in advance, the pupa having been 

 discovered in its natural position. 



There seems to be an increasing tendency towards 

 melanism among Lepidoptera in certain parts of the 

 country. In Lancashire, Cheshire, and the western parts of 

 Yorkshire, among species which have been affected may be 

 mentioned Aplecta nebulosa, Cymatophora dnplaris, Odontopera 

 bidcntata, Hibernia marginaria, Macaria liturata, and one of 

 the latest to be noted viz. Acronycta Icporina ; in the London 

 district Amphidasys betularia, Hemevophila abruptaria, and 

 Eupithecia rectangulata. In some of these cases the dark 

 form is replacing the ordinary light form, and in the others 

 also it is possible it is destined to do so. 



In some cases, as in A. nebulosa in Delamere Forest, we 

 find every intermediate form between the usual grey and the 

 extremely dark. Some of these forms are shown on Plate 

 V, for which I am indebted to the kindness of Mr. R. South. 

 Fig. i is from Ireland, Fig. 2 from the New Forest, Figs. 

 3 — 10 from Delamere Forest. 



In other cases, as in C. dnplaris at Knowsley in Lancashire, 

 I believe that only dark forms are found. 



Various theories have been put forward to account for 

 melanism. Of course, it may be arrived at by different 

 methods in different species; e.g. the black form of A. 

 betularia is reached practically in one step, intermediates 

 being rare, while in A. nebulosa in Delamere Forest every 

 gradation occurs, as I have just mentioned. From black 

 parents of this latter species J. Arkle records breeding twenty- 

 four specimens, viz. five typical, five intermediate between 

 the type and robsoni, ten robsoni and four of the extreme 

 form with white fringes. He also states that from wild 

 larvae it has been found that var. robsoni may occur in the 

 proportion of 10 per cent, and the extreme form in the pro- 

 portion of 3 per cent. From a large number of wild larvae 

 collected in 1905 Mr Harrison and myself obtained 11 per 

 cent, of the dark forms. 



It is conceivable that there may be different forces at work 

 in different cases to bring about the same result, but 

 generally one would look for the same influences or com- 

 bination of influences working together on the insects in 

 the same stages of their life-history. This would cause 

 variations of the perfect insects in a certain direction, 

 and if this direction were favourable to the race, by 



5 



