MELANIC VARIATION IN LEPIDOPTERA. II5 



Mr. W. A. Forbes remarks, * ' In looking through Dr. 

 Staudinger's catalogue I was much struck by the fact that in 

 nearly every case where a local, form (whether a variety or 

 aberration) from the Alps is noticed, it is characterised as being 

 obscurior or midto-obsciirior, or with some of the markings 

 obsoletaJ He goes on to notice the number of normally black 

 or dark species in the Alps, e.g., Erebiae, Psodos, and some 

 Pyralides. 



Various theories have been advanced at different times 

 and by different authors to account for variation in animals, 

 birds, and insects, and it has been attempted to apply some of 

 these to the phenomena which I propose to consider. 



I. Since Darwin drew attention to this cause, the theory 

 of protective resemblance has been most commonly made use 

 of to account for the varied colouration of insect forms. It has 

 been proved almost to demonstration in many instances, that by 

 more or less gradually developed assimilation to surrounding 

 objects, those varieties best adapted to escape the observation 

 of natural enemies have become perpetuated. 



II. Many instances of varied colouring have been re- 

 ferred to an archaic ori2;in ; that is, to the preservation of such 

 varieties by hereditary transmission from an ancestral source. 



III. The influence of quality and quantity of food has 

 been brought forward to account for modifications of normal 

 colouring. 



IV. Retarded or accelerated development depending 

 upon climatic conditions acting upon insects in their larval or 

 pupal stages has also been quoted as a cause for variation. 



V. Insular varieties have been attributed to the effects of 

 long isolation and segregation tending to establish special races. 



* 'Entomologists' Monthly Magazine,' xiv, 16. 



