i6o 



THE BRITISH NATURALIST. 



f August 



Reviewing the whole controversy, it appears to me that the most 

 valuable contribution yet made to it has been that of Lord Walsingham. 

 His suggestion as to the derivative advantage gained for the organism, 

 by the assumption of a dark coloration under conditions of decreased 

 or obscured sunlight seems scientifically sound, theoretically adequate, 

 and in part, at least, confirmed by actual experiment and direct ex- 

 periment in cases of this sort, is of far greater value and importance 

 than mere theorizing or unsupported assertion . One might have 

 anticipated that Mr. Tutt's humidity theory would have been fortified 

 by some experimental evidence however slight. Xylopkasia polyodon is, 

 I believe, a common insect and comes into the category of affected 

 species. Could not Mr. Tutt have selected a normally light coloured 

 pair of this moth, divided the ova, subjecting either part to such opposite 

 conditions of dryness and humidity as he might consider should be 

 effective, and then announce the result to an expectant world. One 

 such carefully conducted experiment resulting in normally light im- 

 agines from the dry larvae, and moths with any distinct trace of melanism 

 from the larvae fed up in a humid atmosphere, would be worth more 

 from an evidential point of view than the whole of Mr. Tutt's book 

 from title page to colophon. This, how T ever, still remains to be done. 



On the other hand, we have Lord Walsingham's actual experi- 

 ments and the well established laws of physics. To me it appears 

 perfectly credible that during the long ages of semiarctic conditions 

 through which Northern Europe passed, after the maximum refrigera- 

 tion of the glacial period had been already assuaged, this principle — 

 that of the superiority of a dark surface in the rapid absorbtion of the 

 feeble heat rays of that time — might have been actually operative, 

 and resulted through natural selection, in a great 'preponderance of 

 melanic forms, and further, I see no objection to believing that our 

 present melanic Alpine, Northern, or Irish forms, may be merely the 

 survivors of such a general archaic melanism. 



I must confess, however, that recent melanism, the melanism of the 

 smoke clouds, is far more difficult to explain satisfactorily. The 

 suddenness of the change seems so surprising, nothing we know of the 

 ordinary processes of natural selection acts with a rapidity comparable 

 to this. And yet by some form of natural selection must the effect 

 have been produced, unless we assume a direct physiological action, 

 and of this we have no evidence. It is true my late friend, Mr. N. 

 Cooke, of Liverpool, did ascribe to solid carbon assimilated (or more 

 properly absorbed) by the larva, the resultant blackness of pigment in 

 the imago, but, with Mr. Tutt, I should at once dismiss such an idea 

 as contrary to everything we know of the physical process involved in 

 in the case. 



My own belief is that we require far more evidence on this class of 



