232 Genetical Studies in Moths 



And this, except for those who like Doncaster^ have assumed a non- 

 committal attitude, has come to be the opinion of every lepidopterist 

 who has ventured to make a reasoned enquiry into the matter. For my 

 part I can only say that I disagree completely, because I believe the 

 theory to be erected on erroneous assumptions due to the acceptance of 

 generalisation in place of particularisation as to our climatic conditions, 

 added to a complete misconception of the actual condition of tree trunks 

 and rock surfaces, both in the smoke zones and in other affected regions. 

 In particular Tutt's crucial instance of smoke-blackened greenhouses is 

 an extremely unfortunate choice. The blackening of white paints 

 cannot be compared with the supposed darkening of tree trunks, for the 

 former is strictly chemical and proceeds directly from the action between 

 the sulphuretted hydrogen of coal smoke and the basic lead carbonate 

 in the paint producing black lead sulphide according to the following 

 equation : — 



2PbC03, Pb (0H)2 + SH^S = 3PbS + 200-, + 4H2O. 



No one will venture to assert that light coloured tree barks contain lead 

 salts ! 



Laying aside this particular statement as merely due to oversight 

 or lack of exact chemical knowledge I adduce as the basis of my dis- 

 agreement the appended facts which I shall discuss seriatim. 



(1) Independent of such local melanism as that discovered by my 

 friend Goodwin in Kent affecting Boarmia consonaria and B. consortaria, 

 certain affected districts like that surrounding Middlesbrough (rainfall 

 24*9 inches), that near Newcastle (rainfall 28 inches) and the Moray 

 Firth (rainfall 26 inches) are situated in the driest portions of our 

 country. Alone this fact need not have excited wonder, but when one 

 mentions that in each of the cases the type of melanism -exhibited is 

 peculiar to the neighbourhood the matter becomes of extreme im- 

 portance. 



For instance, near Newcastle and Middlesbrough Boarmia repandata 

 and Oporabia dilutata are both melanic, yet no one could mistake say 

 Middlesbrough repandata for examples from Birtley or Gateshead near 

 Newcastle, so marked is the facies of the insect proper to each district. 

 As one passes from the towns in both cases the melanism disappears 

 and the insect becomes typical. In other words the melanism is strictly 

 localised, and has arisen independently in its own special region irrespec- 



^ Doncaster, L., " A Collective Inquiry as to Progressive Melanism in Lepidoptera," 

 Entomologists' Record, Vol. xvni. Nos. 7 — 10 (1906). 



