PROGEESSIVE MELANISM IN LEPIDOPTERA. 



165 



Collective Inquiry as to Progressive Melanism in Lepidoptera. 



Summary of Evidence prepared by L. Doncaster. 



It is well-known to collectors that dark varieties of several common 

 moths have recently come into existence, and that, in several species, they 

 have, within living memory, largely increased in frequencj^ sometimes 

 almost, or quite, superseding the lighter types in certain localities. 

 Such an evolutionary change obviously possesses features of great 

 scientific interest, and, had the process been carefully watched from the 

 beginning, information of extraordinary value could have been obtained. 

 Though that opportunity has been, in great measure, lost, the trans- 

 formation is still going on, and, in the hope of recording its progress, 

 the Evolution Committee of the Eoyal Society instituted a collective 

 inquiry, in the year 1900. Circulars were addressed to collectors all 

 over England, and a few to persons abroad. It was the object of these 

 inquiries to awaken interest in the matter, and to obtain, as far as 

 possible, an approximate record of the present state of the afiected 

 population in the various districts, which might serve as a point of 

 departure for future comparisons. Questions were also asked respecting 

 any changes witnessed within the recorder's own memory. 



The northern midlands of England and the metropolitan area are 

 well-known to be the regions most afiected by the change, and, respect- 

 ing these districts, much information has been elicited. With regard 

 to unaffected districts precise statements would, in view of future 

 possibilities, be of value, but it has been difficult to obtain records of 

 this class, doubtless on account of the smaller interest attaching to 

 negative experience. 



To ensure greater accuracy of description, a coloured plate Avas 

 issued to those willing to take part in the enquiry, exhibiting the more 

 important variations from the respective types. The specimens there 

 represented were very kindly lent by Mr. W. H. B. Fletcher for the 

 purpose. The species represented on the plate were : Odontopera 

 hidentata, Hewerophila abrtq)taria, Hyhernia j^rof/enunana [marginaria), 

 Eupithecia rectangulata, Pkigalia pilosaria, Amphidasys hetularia, 

 Boarmia repandata, Boarmia rlwmhoidaria, Acidalia aversata, Venusia 

 cainbrica, Acronycta psi, Xylophasia polyodon (vwnoglypha), J\fiana 

 ■■<trigilh, Polia chi, and Aplecta nebulosa. 



Since the inquiry was instituted, the volume of Barrett's Lepi- 

 doptera, dealing with the (reonietridae — the family most conspicuously 

 affected — has been published. A large mass of facts bearing on the 

 problem is therein contained, and, in the summary which follows, 

 Barrett's collection of evidence has been freely incorporated with that 

 resulting from the committee's enquiry. 



Though the evidence is somewhat meagre in amount, and falls far 

 short of that which might be obtained by further efforts, it seemed 

 well to no longer delay publication of a summary, in the hope that the 

 attention of collectors may be more attracted to the great interest of 

 the problem. 



One of the questions raised by the facts, relates to the degree of 

 suddenness with which the transformation is effected. It might a 

 priori be possible for a general melanism to come about in one of twO' 

 ways. Either the population, as a whole, might gradually and 

 progressively become darker, or dark individuals and dark strains. 

 July 1st, 1906. 



