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THE BRITISH NATURALIST. [April 



humidity is a condition which constantly exists without melanic 

 forms. Lord Walsingham has suggested another to which I must 

 now refer. 



In dealing with the " Melanism of High Latitudes " Lord Walsing- 

 ham showed how great an advantage it is for an insect to be dark 

 coloured, under the conditions which obtain in such localities. 

 Where frequent clouds and mists obscure the sun, the insect that by 

 its hue, is enabled to take advantage of every passing gleam, will 

 certainly, in the struggle for existence, compete favourably with those 

 whose hues are less heat-absorbing, will perpetuate its race where 

 they would fad, and transmit, by the laws of heredity, those characters 

 to its offspring which had enabled it to fulfil the purpose of its 

 existence. Though this address was directed to the melanism of 

 High Latitudes only, before concluding it, he referred also to the 

 melanism of our manufacturing districts and of the neighbourhood of 

 our large towns. I have already quoted part of the passage, previous 

 to which he said : "We should expect to find varieties tending to 

 assume absorbent colouring not only in the Arctic and Alpine regions, 

 where the amount of cloud and mist must greatly diminish the 

 incidence of direct sunlight, but also under any conditions which 

 would limit it in an artificial manner. Dr. J. Hann strongly insists 

 upon the chemical energy and potentially of diffused sunlight ; 

 corroborating and supplementing Tyndall's observations to this effect." 

 Here, in my opinion, is the true solution. Whatever obstructs the 

 direct rays of the sun, will increase the tendency to melanism, and 

 though various conditions may be found, the action is the same and 

 the result is the same. Whether these direct rays are obstructed by 

 cloud and mist in the North and in mountainous regions by the 

 sunless skies of Ireland and the Isle of Man, or by the smoky 

 pall that hangs over our great cities, and our manufacturing 

 districts, the effect is the same. The sun's rays are obscured, and 

 heat-absorbing colours are found on the wings of our Lepidoptera. 

 Why Ireland and the Isle of Man are so sunless, I do not know. I 

 have tried to think that the action of the Gulf Stream might produce 

 such results, but Mr. Barrett's account of the pure blue sky and 

 perfectly clear atmosphere at Pembroke, shows that it is not so. 

 Besides, the effects of the Gulf Stream are not confined to the narrow 

 limits given by Mr. Tutt, they extend over all our western and 

 southern shores, and even to the coast of France. I leave the 

 explanation for the meteorologists of Ireland. The facts we know 

 are that the skies are comparatively sunless, and that melanism is a 

 marked characteristic of the Lepidoptera. 



In the manufacturing districts of the North of England the 

 appearance of melanic forms there, exactly coincides with the great 



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