272 THE ENTOMOLOGIST. 



origin ; they are not mentioned by Mr. Webb in 1891, and their 

 names date from a period not before Clark's paper in 1901. 



These melanic forms are abs. alhonigrana, nigrocostana, 

 rufinigrana, merlana, jansoniana, atrana, ulotana, fiammeana, 

 nigrocristana, and nigrosubvittana. 



If the total number of specimens of all the melanic forms in 

 my list is taken, and a calculation made of their relation to 

 the total of specimens captured of all forms, the proportions 

 work out in 1915 about 31 per cent., in 1916 about 40 per 

 cent., and in 1917 about 37 per cent. ; and taking the average 

 of the three years together, over 35 per cent,, or more than one 

 third. 



These statistics give an idea of the rate at which P. cristana 

 has progressed towards becoming, in the New Forest, a melanic 

 species ; but of course to fix the approximate date when this 

 development is likely to culminate, if it ever does, it would be 

 necessary to collect statistics for a much greater number of 

 years than I have been able to do ; but there can be no doubt 

 as to the present tendency. 



It is interesting to speculate on the manner by which the 

 change now going on has been brought about, and how the new 

 forms have originated. Two theories seem to me possible. 

 According to the first of these, a specimen of a melanic form — 

 say, ab. nigrana — may be the direct issue of a specimen of any 

 one of the non-melanic forms. According to the second theory, 

 which seems to have evidence in support of it, each melanic 

 form is descended from a certain non-melanic form, or group 

 of forms. Take first ab. nigrana. Mr. Webb says (* Ent.,' xliii, 

 p. 267) it "is only an intensified form of ab. jjrofanana.'" I am 

 afraid I cannot agree with this view. In the first place, ab. 

 profanana is — now, at any rate — a very rare form in the New 

 Forest. Then ab. i^rofanana is a plain form, without any darker 

 markings — except the button — or cloudings, according to Fab- 

 ricius. Ab. nigrana, on the other hand, is by no means a plain 

 insect, in spite of its blackness. Clark's description reads : 

 "Anterior wings blackish, mottled over with darker colour." I 

 think that its parents were probably ab. cristalana, and perhaps 

 ab. semiustana, and other strongly clouded and variegated forms ; 

 and in support of this theory I may say that Mr. Charles 

 Gulliver, who has collected P. cristana in the New Forest for 

 over forty years, informs me that previous to the advent of ab. 

 mgrana, ab. cristalana was by far the most abundant form. At 

 the present date, so far as the evidence of my list shows, it has 

 receded in number to the fourth place. 



Of course, once ab. nigrana had evolved, we know from our 

 experience during the past half century of other species of 

 Lepidoptera displaying a melanic tendency, how very dominant 

 it would be likely to become. This tendency, with the remark- 



