JOURNAL OF VARIATION. 
No. 3. VoL. II. June 1st, 1891. 
MELANISM AND MELANOCHROISM IN BRITISH 
LEPIDOPTERA. 
By J. W. TUTT, F.E.S. 
{Continued from page 35.) 
HE earliest explanations of melanic forms were generally 
physiological or had a physiological basis, and the 
supposition was, that the exciting cause had a 
phytophagic origin, and that the larva was affected 
in some unknown way by the chemical elements in its 
food, or by certain external substances taken with its food. 
Thus peaty soils, soils impregnated with iron, leaves covered 
with soot, etc., have all been supposed in some mysterious 
manner to have affected lepidoptera in the direction of 
melanism. Now it is well known that a plant takes the same 
inorganic and organic substances for food wherever it may 
be grown, and that the proportions of these substances vary 
only in the very narrowest degree. Any deficiency of a food- 
substance in the soil is at once visible in the vegetation, and 
stunted growth is the first result of deficiency. Total deficiency 
means “ barrenness ” for any^ plant requiring the absent sub- 
stance. Now, whether a plant be grown on a sandy ” or a 
“ calcareous ” soil, the chemical analysis will prove that the 
plant has its tissues made up of the same substances and in 
almost exactly the same proportions on either soil. I would 
ask therefore : — How can the food influence colour ? If a soil 
has large supplies, in a soluble condition, of all the necessary 
foods that a particular plant requires, we get a luxuriant growth. 
On the other hand, if a soil has but a small quantity of the 
available foods, then that particular plant will be stunted and 
sparse in its growth, but the plant on either soil will still be 
composed of the same chemical substances. Phytophagic 
