1893] 



THE BRITISH NATURALIST. 



155 



the melanism. Thus, suppose someone should promulgate such a 

 theory as this, as in point of fact, Dr. Buchanan White, Mr. 

 McLachlan, and others have done, The original post glacial insect 

 population of the palaearctic region was for some reason or other — of 

 which we provisionally profess ignorance — melanic in character. The 

 forms we find now numerously in Ireland and in Great Britain, more 

 especially at high elevations and northern latitudes, represent the 

 survivals of that first sub-glacial population, and are melanic now 

 because those their progenitors were melanic then. Succeeding waves 

 of immigration, now of general lowland distribution ; represent, not 

 the original types, out of which by atmospheric or other influences, 

 were manufactured our present melanic forms, but races perhaps 

 ages later in differentiation, at any rate, races always restricted to 

 less rigorous temperatures or perhaps drier areas. 



Now such a theory as this would be perfectly comprehensible, 

 and not conspicuously inadequate ; it would indeed merely push 

 back the necessity for accounting for melanism, to a time when 

 atmospheric conditions were very different from what they are in 

 Europe to-day, but it would render Mr. Tutt's invocation of the 

 humidity of our western districts entirely needless. 



But now, on the other hand, consider Melanism, No. 2, the recent 

 kind. Here no such theory can be advanced. We seem to have seen 

 the species change almost under our eyes. Personally, I think we are 

 at present a long way off a right explanation of this matter. I look 

 on the evidence of association between a smoky and polluted atmos- 

 phere and the occurrence of these forms, to be insufficient and 

 uncritical (not necessarily therefore incorrect). 



Probably the two districts which have been now for the longest time 

 and most completely under the influence of the disturbing effect of manu- 

 factures, mines, &c, are South Staffordshire and East Worcestershire, 

 and the coal-fields of South Wales. Have we any evidence of distinct 

 melanism from these districts ? There can be no doubt but that the 

 pall of smoke, which has now for nearly a century, obscured the sky 

 above that district commonly called the " black country," ought to 

 have had a much greater effect, than the mild and comparatively 

 recent emanations of Derby or Huddersfield. 



Again, Delamere forest has been cited as a special district for the 

 melanic change in several Geometers, but the idea of any special smoke- 

 produced obscuration of the sky above, or defilement of the herbage 

 beneath, to any one who knows those woodlands, is absolutely absurd. 

 The forest of Dean, with its long established iron-works and collieries 

 quite within the circle of its woods, should have been much more 

 effective in the production of melanism, but I do not remember to 

 have heard cases quoted from that district. Possibly Mr. Tutt may 



