1877.] 15 



Melanochroism, if-c, in Lepidoptera. — Mr. Fetherstonhaugh (E. M. M., xiii, 215) 

 and Mr. Tugwell {id., 256) have suggested that variation is produced by the impression 

 tlie surrounding colour has upon the female insects, especially during " the period of 

 generation." It would not be difficult to find many arguments against this theory 

 {e. g. the case of nocturnal species, on which colour can probably have little in- 

 fluence), but as it ought to be capable of experimental proof, I leave to its authors 

 the onus of proving it by experiments on insects in confinement, and hope that they 

 will let us know the result. 



The cases of the prevalence of melanochroic forms in the manufacturing districts 

 of Yorkshire, &c. (if, except in the case of one or two species, such prevalence is a 

 fact) may be induced by chemical causes (the impregnation of the air and soil, and 

 consequently of the food-plants, by the various gases discharged from the chimneys 

 of the factories, &c.), and continued and intensified by " natural selection," for, in a 

 region where all things are blackened, a dark individual will have more chances of 

 escaping detection by enemies, and hence of propagating the species and producing 

 a melanochroic race. The same would be the case with the var. perfamaria of 

 Boarmia rJiomhoidaria, which might escape a sharp-eyed sparrow when a lighter- 

 colored typical specimen would be seen and gobbled up. 



Mr. Tugwell brings forward Gnophos obscuraria as the best insect to illustrate 

 his view, and points out that on light soils it is light and on dark it is black. To my 

 thinking, this insect is the worst he could have selected to support his theory, for in 

 it we have a most brilliant example of natural selection. Gnophos olscuraria always 

 rests on or near the ground, often on rocks or stones. On a light soil dai'k specimens 

 would be conspicuous and thus liable to observation by enemies. So in generation 

 after generation the darkest colored individuals would be weeded out before they had 

 time to propagate, and the breed would get lighter and lighter till that exact shade 

 of colour best suited for securing non-detection is attained. On a dark soil, the 

 reverse would take place, till, as Mr. Tugwell remarks, in the New Forest we get 

 specimens "harmonizing with the soil." 



Mr. Fetherstonhaugh says that I appear to reject Mr. Birchall's notion that 

 cold damp climates, with absence of sunshine, may be the cause of the melanochroism, 

 and that I suggest meteorological causes ; but do not cold, damp, and absence of 

 sunsliine belong to meteorology ? I consider that tliey certainly do, and in suggesting 

 that the meteorological differences of one year from another, cause a varying amount 

 of variation in a single locality, I meant the different degrees of humidity or dryness, 

 heat or cold, sunshine or its absence, that occur in one locality in various years, the 

 amount of difference being sometimes very great. If these are the causes why the 

 individuals of one year vary from the individuals of another year in one and the 

 same locality, it is probable that in two localities, more or less widely separated, and 

 hence differing in their meteorology, individuals of the same species will vary in 

 opposite directions, and the variation in course of time become permanent, and henco 

 (let us suppose) in one locality a melanochroic race is established, in the other a 

 leucochroic. That the degree of heat or cold has an influence of the most appre- 

 ciable character has been conclusively shown by Dr. Weissmann (" Studien zur 

 dcscendenz Theorie," vol. i), and by Mr. W. 11. Edwards on certain American 

 butterflies (alluded to by Prof. Westwood in his Address to the Entomological So- 

 ciety, 1877). Further researches on this subject, by Mr. Edwards, will be found in 



