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THE BRITISH NATURALIST. 



(March 



address to the Yorkshire Naturalists' Union " Some probable causes 

 of a tendency to Melanic variation in Lepidoptera of High Latitudes." 

 In this paper he proved, as the result of actual experiment that dark 

 coloured Lepidoptera absorbed heat more rapidly than did those of 

 paler hues. He showed how great an advantage was derived by 

 darker insects under special climatic conditions. From their power 

 of more quickly absorbing invigorating warmth they developed more 

 rapidly and were able to take advantage of any transient gleam of 

 sunshine. As this important pamphlet is no doubt familiar to most 

 of you, I need not quote from it at length, but it sought to establish 

 the fact that wherever the direct rays of the sun are interrupted, 

 whether through the mist and cloud of Alpine regions or from any 

 other cause, that there we might reasonably expect to find Melanic 

 forms, and did so. "The smoke of our furnaces m the manufacturing 

 districts and of our chimneys in large towns undoubtedly deprive our 

 urban population of their fair share of undiffused sunlight within the 

 radius affected by them. Accordingly we find several instance of 

 more or less Melanic forms occurring in such situations." The 

 illustrations I need not quote. He then goes on : "We should expect 

 insects occurring in dense forests to be darker than those of the open 

 country, and so far as my Californian experiences goes this is uncloubt- 

 eldy the case." He also tell us that " all fishes from the very deep sea, 

 inaccessible to the sun's rays, are either colourless or entirely black." 



This was the first attempt to show why Melanic forms obtained a 

 preference in the struggle for existence and gave us another step 

 towards a knowledge of the causes of variation. Two years later 

 Mr. D'Obree published a paper on the subject in the "Entomologist " 

 (Vol. XX., p. 25), in which he claimed to have destroyed Lord Wal- 

 singham's argument. But Mr. D'Obree was a student of Noctuae 

 only, and no conclusion based on study of an entire order could be 

 upset by reference to one portion only of such order. Mr. D'Obree's 

 conclusions were that " Melanism is primarily due to the peculiar 

 geographical position of these islands. Tins wave of migration was 

 arrested on reaching our most northern shores, with the natural 

 outcome of stunted forms, and Melanism became the result of the 

 want of accustomed succulent food and of a climate warmer certainly 

 than that within the Arctic circle, but still sufficiently cold and wuth 

 sufficiently short summers to arrest the development capable in the 

 milder climate of the insect's original home. The appearance of 

 Melanism in the high mountainous regions of Continental Europe, 

 where the same causes obtain, as in our extreme north, helps to 

 support this view. Yorkshire Melanism " (in which I suppose he 

 includes Lancashire and Durham also) " may be treated as a merely 

 local aberration not affecting the general position." 



