IS MOISTURE THE CAUSE OF MELANISM ? 
BY JOHN E. ROBSON, F.E.S., HARTLEPOOL, 
Editor of “The British Naturalist ” (Read 21st February, 1893.) 
Entomologists have been puzzled for the last half century to 
explain the curious fact that the markings on the wings of Lepidop- 
tera were gradually becoming darker in certain districts. In those 
whose colouring contained any portion of black, the black scales 
increased in number until the surface was more or less suffused 
with them. In those where black did not form a portion of the 
original colouring, then the scales of the darkest hue increased in 
the same way, and the wings became suffused with this darker shade. 
For this phenomenon the term Melanism was adopted, and after a 
little time it was restricted to those species which had become blacker, 
and the term Melanochroism was applied when the species had 
become darker in hue without a suffusion of black scales. The variety 
Doubleday avia of Betularia will serve as an illustration of one, and the 
variety Unicolor of Xerampelina as an example of the other. Believing 
that both these forms of variation are produced by the same exciting 
cause, I propose to avoid needless repetition by using the word 
Melanism as a generic term including the whole. 
The late Mr. Edleston was the first person to call attention to this, 
remarkable phenomenon. Rather more than fifty years ago, he wrote 
to “ Newman’s Entomologist ” in reference to the increasing. numbers 
of what he called the “negro” aberration of Betularia. He had 
exposed some virgin females in his garden, and was surprised to find 
most of the visitors were of this “ negro aberration.” He said this 
form was almost unknown sixteen years before, and, contrasting it 
with its increased numbers when he wrote, said, “ If this goes on for 
a few years, the original type of Betularia will be extinct in this 
locality.” 
I do not find any further allusion to what was going on until 1876 , 
when the late Edwin Birchall wrote a very remarkable paper “On 
Melanism,” in the “ Entomologist’s Monthly Magazine ” (Vol. XIII, 
p. 131 ). In this paper Mr. Birchall condensed into less than three 
pages, almost everything that has been said on the subject from that 
time to the present. He spoke in general terms of the localities 
where Melanism prevailed, viz., the Highlands of Scotland, Ireland, 
the Isle of Man, Durham, South Lancashire, and the West Riding of 
Yorkshire; and pointed out that where it prevailed there was also but 
a meagre Lepidopterous fauna. He suggested “ as not improbably 
among the causes at work in limiting the numbers of Lepidoptera in 
