MELANISM AND MELANOCHROISM. 87 



,i^rounds by every westerly wind ; and the wind is westerly ^ 

 for nine months out of the twelve. This may have effected a 

 change in the climate {Entom. x., p. 94). And yet, after 

 coming to this reasonable and satisfactory conclusion, Mr. Cooke 

 would not allow that " natural selection " had any hand in 

 the matter, because he could not see the difference between 

 "sexual selection" and "natural selection." Had Mr. Cooke 

 seen the line of argument taken by D. F. B. White {Entom. x., 

 p. 126), I think he must have been driven to the conclusion 

 that I have (in common, I believe, with many others) arrived 

 at. Dr. White {Entom. x., p. 127) says himself: "But what 

 is the exciting cause of this tendency to variation ? I think it 

 is (in some cases, at least) meteorological, that is to say, cold 

 or heat, dryness or dampness, presence or absence of sunshine, 

 etc." So that even in these earlier discussions, atmospheric 

 effects held a place, but generally a most subordinate one. In 

 Entom. X., pp. 152, 153, Mr. Cooke also writes : " I have also 

 lately become acquainted with the fact that TepJirosia biundii- 

 laria of the dark form, occurs in the "black district" of 

 Staffordshire — in Burnt Wood — not far from which an immense 

 quantity of smoke is produced, by the manufacture of iron and 

 pottery." Criticising Mr. Cooke's article {Entom, x., pp. 92-96) 

 Dr. White says : "This aberration Delamerensis var. of biundu- 

 laria, and the ab. Doiibledayaria, Mill., of AmpJiidasys betularia 

 are, however, exceptional cases, and cannot be considered as 

 throwing much light on the origin of the majority of melano- 

 chroic forms " {Entom. x., p. 128). 



Here I differ from Dr. White. They throw a great deal of 

 light on the origin of the melanism in tree frequenting species. 

 The colours of these species lend themselves more readily than 

 those of most species to this peculiar form of variation. Being 

 of a white ground colour with the principal markings black, it 

 follows, that any darkening of surface will be more favourable 

 to those specimens which have an excess of dark colour, than 

 to those which have an excess of light. " Natural selection " 

 will complete the work, and "hereditary tendency" will ensure 

 its intensification. Again, the same species has undergone the 

 same change at Derby {Ent. Mo. Mag., vol. xxiii., p. 6), and 

 has lately exhibited itself to some extent at Birmingham, and 

 in the neighbourhood of Mansfield, Notts. 



To show that these are not isolated cases of melanism of 

 this particular class, and that the results obtained in Boarmia 

 ^ South-westerly would have been more correct. — J.W.T. 



