J. W. H. Harrison 245 



consonaria and B. consortaria in Kent. It is quite possible that there 

 London smoke deposits its last impurities, but not likely in spite of the 

 fact that the Lancashire smoke affects Lake District lichens seventy 

 miles away. So small is the area affected that I think we are dealing 

 with the effect of some trivial, local cause ^ of an obscure nature leading 

 to the necessary metabolic upset. Here perhaps it is best to remark 

 that the very great similarity of the melanic forms of the closely related 

 species Tephrosia histortata, T. crepuscular-ia, Boarmia consonaria, B. 

 consortaHa, B. repandata and B. gemmaria suggests that we are dealing 

 with an atavistic character, and that the chemical change in the nucleus 

 necessary for the reappearance of the old common form may not be a 

 very profound one — a view that receives some little confirmation from 

 the fact that the cognataria form of Amphidasys hetularia found in all 

 three of the known relict stations of Miocene forms, China, Eastern 

 North America and Turkestan, shows much more pigment than type 

 hekdaria and thus shows some approach to the melanic double day aria. 



Lastly we have to deal with the melanism and melanochroism of such 

 species as Camptogramma hilineata, Dianthoecia conspersa, Triphaena 

 conies'^, Melanippe fluctuata, Emmelesia albulata and others in habitats 

 along the west and north-west coasts and islands of Ireland and Scotland 

 in particular, and to a less marked extent along coasts elsewhere. All 

 the species feed on low-growing plants, and all show their melanism 

 chiefly in coastal habitats ; none are woodland species. Not even our 

 very impressionable 0. dilutata is included ; had the melanism been of 

 the usual type it was bound to join in, yet in these very areas wherever 

 woodlands approach the sea the perverse insect, as if to defy the moisture 

 theory, will persist in giving us the silveriest of silvery forms. 



What have they in common ? All live on spray-washed food and all 

 therefore receive great quantities of the salts found in sea water of the 

 type of sodium and magnesium chlorides, bromides, iodides, sulphates 

 and similar salts so that conditions approaching those in urban areas are 

 set up, possibly with the same effects. This, too, may be the cause of 

 the melanochroism exhibited by other coast Noctuidae, its possible 

 accentuation in the more insular Scotch and Irish stations being brought 

 about by their more exposed and isolated positions giving greater scope 

 for its advent and its subsequent predominance by inbreeding. 



1 It cannot be chance, for two species are affected. 



2 I know that T. comes is melanic at certain inland stations, but these forms only 

 exist where infiltration from the coast is possible ; the metropolis, so to speak, of the 

 melanism of that species lies on the Scotch coast between Burghead and the Orkney 

 Islands. 



16—2 



