j. 
JOURNAL OF VARIATION 
No. 5. VoL. II. 
July I5th, 1891. 
MELANISM AND MELANOCHROISM IN BRITISH 
LEPIDOPTERA. 
By J. W. TUTT, F.E.S. 
{Contmiied from page 8o.) 
jAViNG now considered this subject from many points, 
I think we may safely assume that the general cause 
of melanism is to be sought for in meteorological in- 
fluences, and that this cause must act in some way 
on the larva. It is during the larval existence that the insect is 
constitutionally strengthened or weakened, and that it is during 
this period that some active exciting cause acts, is evident, other- 
wise we should not get a sudden development of dark forms in 
certain seasons. Mr. J. Jenner Weir writes: — “You are 
quite correct about the colour of the wings being determined 
by the condition under which the larva feeds. This is quite 
true of the horeomorphic species of Rhopalocera in India, 
Java and other Eastern districts ” {in litt.), vide also E 7 itom. 
Record, etc., vol. i., pp. 30 and 31. Mr. Merrifield — as I have 
pointed out before — also found the larval, the stage most 
affected by external conditions. I think we may safely 
assume that the larval, is the stage in which the melanism- 
producing factor exerts the greatest influence. 
To account for this, no theories have as yet been advanced 
worthy of the name. I have two, however, one written by 
Mr. T. D. A. Cockerell, the other by Dr. Chapman, two of our 
most original and thoughtful lepidopterists. Mr. Cockerell 
assumes what I have proven, that the change is accompanied 
by a change in humidity. He then supposes that moist air 
may absorb some melanism-producing product of decomposi- 
tion, and then he adds : — “ A moderately moist air in a town 
might produce the same effect as very moist air — say, in 
