MELANISM AND MELANOCHROISM. 233 



in some instances, if we confine ourselves to the British Isles, 

 or even to the western shores of Europe under the influence 

 of the Gulf Stream, but, in the hi<:^h latitudes both of Europe 

 Asia, and America, especially at low altitudes, melanism is 

 practically unknown. Mr. Dobree {Entoni. xx. pp. 25-28) 

 thoroughly exposed this fallacy, and showed how incorrect it 

 was, and Mr. Jenner Weir {Entoin. xx. p. 85) supported Mr. 

 Dobrde's facts ; and, since the whole of Lord Walsingham's 

 theory was based on this incorrect assumption, the misleading 

 character of the theory may be easily estimated. The actual 

 fact " that dark colours would be advantageous to insects, 

 owing to the rapidity with which they would absorb heat," is 

 a simple statement of fact that the most elementary student of 

 physics would understand ; but that this is the cause of 

 melanism is another matter, especially when it is assumed, 

 that, as the temperature gets lower, the moths get darker, 

 which is contrary to fact, unless the lower temperature is 

 accompanied by increased humidity. Lord Walsingham, too, 

 allows nothing for the fact that moths generally fly by night, 

 and that radiation would have to be taken into account, as it 

 is another elementary fact in physics that " good absorbers are 

 generally good radiators ; " but still the previous fact, that the 

 moths of northern latitudes are riot generally given to melanic 

 tendencies, renders most of the arguments in the paper equally 

 abortive, 



Mr, Dale, in the Young Naturalist Supplement, Jan. iSgo, 

 PP- ^1^ 38? after pointing out that melanism is essentially 

 characteristic of the most humid districts, asserts that : — " The 

 dry chalk soils of the South absorb a greater amount of heat . 

 than the wet peaty soils of the North, and to that, in conjunc- 

 tion with the fact that there is more rain and consequently less 

 sunshine in the North, is melanism entirely due." Grandly 

 assertive, but entirely unconvincing, this was rather severely 

 handled by Lord Walsingham in the February number of the 

 same Magazine, but Mr. Dale again comes to the front, and 

 begins by denying one of the most elementary laws of physics, 

 viz. : — that dark-coloured substances do absorb more rapidly 

 than light-coloured, and then adds : — " The pith of my argu- 

 ment is this — dark-coloured soils absorb a great deal of 

 moisture and cannot absorb so much heat as light-coloured 

 soils, which are dry." Here Mr. Dale assumes, evidently, that 

 all dark soils are necessarily " heavy " {i.e., retentive) soils, and 

 vice versd, an obvious error. The amount of heat absorbed by 



