J. W. H. Harrison 231 



artificial aberrations of L. dispar. If it does not, then we are dealing 

 with a further Lamarckian effect. 



Similarly one cannot deny that under artificial conditions moisture 

 induces parallel phenomena, and by the same mechanism of scale reduc- 

 tion, but unfortunately such variation, unlike-whafc occurs naturally, is 

 equally liable to be accompanied by albinistic forms. Furthermore, 

 the artificial state is pathological and not hereditary, being coexistent 

 with damaged scales ; in natural melanism such as the moisture-smoke 

 theory seeks to explain the scales are as perfect as in the ordinary 

 form. For the same and other reasons we must not delay to examine 

 the other alleged stimuli. 



Tutt's theory of the united effect of smoke and moisture stands on 

 much firmer ground. Briefly, as summarised^ by its author, it is: 

 Humidity produces^ melanism ; the environment of the particular species 

 determines how far and in what direction melanism may or may not be 

 developed ; natural selection may counteract, modify or intensify the 

 tendency to melanism. 



As a sort of corollary to this, to explain melanism in districts like 

 the west of Ireland and Scotland far removed from smoky manufacturing 

 towns, he suggested that there the necessary blackening of the resting 

 places of the insects, which enabled natural selection to work, was 

 obtained by the direct darkening of rock surfaces from exposure to 

 heavy rainfall. 



Later, however, Tutt appeared to shift his position ; in place of the 

 statement that moisture definitely produces melanism we find him 

 stating* instead that " Moisture plays an important if indirect part " — a 

 vastly different position from the first one of direct cause and effect. 



To include all of the cases of melanism occurring in the British 

 Islands we may summarise Tutt's later views as follows : 



(1) By its indirect action on rocks, etc. moisture blackens their 

 surfaces in districts remote from urban areas. 



(2) In urban areas such influence is augmented by the deposition 

 of soot and its fixation by moisture. 



(3) Natural selection acting progressively in favour of melanochroic 

 forms resting in positions so darkened urges the species towards its 

 culmination in total melanism. 



^ Entomologists' Record, Vol. i. p. 96. 



2 The italics are mine. 



3 British Lepidoptera, Vol. i. p. 65, 1899. 



