PREFACE. Xlll 



blackened by the soot which sticks to them throug-h the rain 

 and damp, that the pale forms which rest on trunks of trees, 

 walls, and the dark herbage of the moors, are by the now 

 g-enerally accepted theory of the "survival of the fittest," 

 g'radually eliminated, and only the dark forms left. But 

 that this theory is far from proved is evidenced by exceptions 

 so marked, as sufficient no doubt to cause its rejection by 

 many. As an instance, the form of Acronycta menyanihidis 

 Avhich occurs right in the area of melanism in our smoky 

 South West district, and where it is common, is almost 

 invariably of the palest form we know, and a black specimen 

 is never seen ; whereas on the heaths near York and Selby, 

 where there is practically no smoke, the black form is 

 abundant, and Mr. Samuel Walker informs me that at York 

 no specimen is ever seen so pale as the West Yorkshire 

 moth. The same applies to Abraxas ulmata at Sledmere, 

 the ordinary pale form only occurring in South West York- 

 shire. Melanism is also developed in several other North 

 and East Yorkshire species, but as it is in species which do 

 not occur in the southern districts, the same comparisons 

 cannot be made. 



Moreover, there are at least two species common in South 

 West Yorkshire which are distinctly paler than in most of 

 the southern counties. The yellow colour in the male Fidoiiia 

 piniaria^ so prevalent all over the south of England, is 

 replaced in all our northern counties by pure white ; and the 

 Dover district is the only known locality where Cidana 

 suffiimata occurs as pale as it is in South Yorkshire. York- 

 shire Strenia clathrata too have a pure white ground colour 

 as against a yellow ground colour in the southern counties; 

 and the York Lithosia helveola are paler than elsewhere. 



It is satisfactory to note that the scientific importance of 

 melanism is so great, that the Royal Society has appointed 

 a Committee to, if possible, thoroughly investigate its causes. 



