MELANISM AND MELANOCHROISM. 229 



the seasonal forms were dimorphic in colour ; hut I helieve 

 myself, that the influence (whatever it may he) which produces 

 a dark coloration in these species, acts hut very little (if at all) 

 in that stage, and that the larval stage is the one most affected 

 by the exciting influence. If this darkening takes place by 

 retardation in the pupal stage, then, the insects of northern 

 Europe and America, instead of being, as they are, more than 

 usually pallid, would be excessively dark, for many north- 

 country species appear normally to pass two years in this 

 stage. It is in mountainous and other humid districts, that 

 we find dark-coloured insects developed, and, since the spring 

 specimens of several seasonally dimorphic species {Tcplirosia 

 crepusmlaria, Selenia illustraria, etc.) are darker coloured than 

 the summer specimens, I think it more than likely, that delay 

 and environment have infinitely more influence in the larval 

 stage, than the same influences can possibly have on the 

 comparatively quiescent pupal form. It is, therefore, advisable 

 to note, with regard to these dimorphic seasonal forms, that 

 Mr. Merrifield writes : — " In the double-brooded species, the 

 governing consideration is, which brood am I to belong to — 

 the summer form or the winter form ? and that decision can 

 only be arrived at in the larval (growing) stage, and may be 

 then controlled by external influences, e.g., temperature.^ I 

 do not believe that temperature^ can ever convert the one form 

 (summer or winter) into the other, unless brought to bear on 

 the larval stage" (/// litt.). Again, after all the care that Mr. 

 Merrifield has taken with his "temperature" experiments, he 

 writes : — " Like you, I am by no means prepared to accept the 

 position that cold causes melanism. / t}ii7ik it quite probable 

 that, in some cases, it may produce an opposite effect " {in litt.). 



I will now quote one or two more of Mr. Merrifield's obser- 

 vations, which appear to me of importance, and the value of 

 these remarks is enormously increased, when we consider, 

 that, having his attention directed especially to " cold" as the 

 cause of the darkening, he could not reconcile his results as 

 being borne out by differences of temperature. He writes : — 

 " So far as my experiments on pupae have gone, the results of 

 them appear in close accordance with those of Professor 

 Weismann, but other experiments seem to indicate that the 

 temperature^ to which the larva is exposed in its growing stage 



^ If accompanied Ijy humidity.— J. W.T. 



^ Mr. Merrifield's experimenls were based on temperature. He therefore assumed 

 that all his results were produced by temperature. — j.W.T. 



^ I consider these differences caused by the more " wet" condition. — J.W.T. 



