132 [November, 



Yorkshire, Lancashire, and Durham — those very districts where me- 

 Lanism especially occurs — the air is polluted by mephitic exhalations 

 from furnaces and chemical works, the sun is obscured by clouds of 

 coal smoke, and the vegetation defiled and destroyed by deposits of 

 soot ; in some of the worst districts, such as St. Helen's and Bradford, 

 Lepidoptera scarcely exist at all. In the Highlands of Scotland, 

 although the air be purity, and the sunshine brightness, the long, cold, 

 wet winter and the late spring can hardly fail to check the multipli- 

 cation of those forms of life which have their metropolis in warmer 

 southern lands. 



Now, Mr. Darwin has shewn us that great constitutional differences 

 both in animals and plants are correlated with differences of colour, 

 and he gives many curious and interesting examples proving that 

 black and dark coloured animals escape many diseases, are less liable 

 to the attacks of parasites, and will stand changes of temperature 

 which prove fatal to the lighter coloured A^arieties ( " Animals and 

 plants under domestication, vol. ii, chap. 21 and 25). 



I will further quote a passage from Mr. A. R. Wallace's recent 

 address to the Biological section of the British Association at Glasgow : 

 " Few, if any, wild animals are wholly white, the head, the face, or at 

 " least the muzzle or the nose are generally black, and there is reason 

 "to believe that dark pigment is essential to good hearing, as it 

 " certainly is to perfect vision. 



" If the prevalence of white coloration is generally accompanied 

 " with some deficiency in the acuteness of the most important senses, 

 " this colour becomes doubly dangerous ; for it not only renders 

 " its possessor more conspicuous to its enemies, but at the same time 

 " makes it less ready in detecting the presence of danger. 



" Hence also is a reason why Albinoism, although freely occurring 

 " in captivity, never maintains itself in a wild state, ichile Melanism does. 



" In the xanthochroic races of man, we find a high development 

 " of intellect, accompanied by a slight deficiency in the acuteness of 

 " the senses, as compared with the darker forms." 



As it thus appears certain that greater strength of constitution, 

 and more powerful and acute perceptive faculties arc, from some yet 

 unknown cause, associated with dark colours in the Vertehnita, may we 

 not presume that insects are subject to the same law, and that dark 

 varieties of Lep)idoptera are able to spread and increase under adverse 

 conditions, whilst the lighter coloured types fail to do so, and are 

 consequently eliminated in the struggle for life, and that the occur- 

 rence of melanic forms may be thus reasonably explained as a simple 

 case »f the " survival of the fittest ? 'i 



