ON MELANISM. 69 



reasonable to dismiss Yorkshire melanism as " merely local 

 aberration," nor do I think we need do so, even for the sake of 

 our theory. I do not know the degree of humidity of the 

 Yorkshire atmosphere, but should not be surprised to learn that 

 it is greater than in the southern parts of the kingdom. The 

 black variety of the common field slag (Limax agrestis var. nigra 

 Butl.) is very frequent about Wakefield and Beverley, and 

 probably elsewhere in Yorkshire, but, except for a single example 

 at Stroud, in Gloucestershire, it has never been known to occur 

 in the South of England, nor has it been discovered anywhere 

 on the Continent. Among Lepidoptera many dark Yorkshire 

 forms are known, notably Ewpithecia albipunctata var. ajigelicata, 

 Prest., from Selby. Sea-coast specimens are also frequently 

 dark. Cistela sulphurea, L., is very dark near the sea, and 

 particularly so at Deal.* 



The negro vai'iety of the human race probably had its origin 

 in the very humid, though anything but cold region of Central 

 Africa, and among European races those of damp and maritime 

 countries are darker than the flaxen-haired Teutonic tribes, 

 which came from the dry regions of Central Europe and Asia. 



Whether this view will ultimately hold good or not it is 

 impossible to say, but I think it has at least as much in its favour 

 as the others that have been advanced. It would appear that 

 humidity of atmosphere is the main factor in producing melanism, 

 though contact with water, as in the case of aquatic and 

 amphibious species, has no such effect. Much, however, needs 

 to be known, not only as to the geographical distribution of 

 varieties, but also the physical conditions under which they exist ; 

 and I would appeal to those who so frequently record melanic 

 and other varieties, not to omit, as they have so frequently done 

 in the past, all notice of the conditions under which the variation 

 occurred. I think that as our knowledge increases, some form of 

 classification by varietal nomenclature will become more and 

 more necessary for the adequate arrangement of our facts, and 

 for convenience sake I should be glad if simie varietal name 

 might be adopted which would serve universally for all melanic 

 forms. 



5, Priory Eoad, Bedford Park, Chiswick, February 5, 1887. 



* Wollaston, ' The Variation of Species,' p. 60. 



