#^ AND ^"HV 



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 



