104 



Nairn districts, in which latter he considers them rather darker than 

 elsewhere, and he has found them throughout the whole of the 

 central and northern counties of Scotland, " always excepting the 

 higher mountains," in varying proportions to the type, of which the 

 following table gives his estimate : 



Inverness (South), Ross, and Sutherland not sufficiently worked to 

 give reliable percentages, but we know that Mr. Salvage took a con- 

 siderable number of curtisii in the last-named county. 



At Rannoch, an exposed district in Perthshire, some 800 to 1000 

 feet above sea-level, the species is scarce, and the only record of 

 curtisii that I have been able to obtain is a single example taken 

 by Mr. Reid ; but he agrees with Messrs. Salvage and McArthur 

 that here, as in the case of the other more exposed inland localities, 

 the prevailing dark form is of an almost unicolorous deep brick-red, 

 a considerable proportion of the examples found being of this form. 



With regard to local meteorological influence, it is conceded that 

 both in Forres, in the parts of Sutherland, and the Hebrides, where 

 the species occurs, and in Orkney, the climate is mild. Comparison, 

 however, of the last-named place, w^here the species is represented 

 only by an extremely dark form of the curtisii variety, with the Scilly 

 Islands, where the curtisii v^x\Q.iy does not occur, maybe interesting, 

 as the two areas have so much in common meteorologically. Both 

 are insular, both have equable and humid climates ; but the chief 

 point of difference between them is that the mean annual tem- 

 perature of Orkney, 45° F., is exactly the same as the mean winter 

 temperature of Scilly. If, then, humidity were the one and only 

 " motive power " in producing the dark forms, and temperature had 

 nothing to do with the question, why should we not find curtisii in 

 Scilly as well as in Orkney ? On the other hand, if the lower tem- 

 perature alone were responsible for the dark forms of T. comes in 

 Orkney, why should not the few scattered examples of the species 

 found on the bleak Rannoch moors be also of the dark form ? 



Whatever the "motive power" in producing these dark forms may 

 be, it must be sought for beyond the simple questions of humidity 

 and temperature. Is not the first great principle in nature the 

 possibility of existence ? If, then, the prevailing conditions in a given 



* Probably this estimate would prove too low if the district were more 

 worked. 



