MELANISM AND MELANOCHROISM. 85 



give us a greater proportion of darker specimens of many 

 species that naturally rest on tree-trunks, than do compara- 

 tively open woods of more mixed growth. If we add to these 

 natural conditions the presence of chemical fumes, smoke, and 

 other atmospheric impurities w'hich are specially characteristic 

 of large manufacturing districts, we can see how intensified the 

 tendency to melanism would become in such districts. Only 

 those insects that become positively black, would have in many 

 places the slightest chance of escape, and " natural selection," 

 by weeding out the paler specimens, would leave only the 

 darker, and " hereditary tendency " would play its part in the 

 intensification of the colour of the species. I would now cite 

 examples which I think will tend to prove this view. 



Thanks to the generosity of Mr. Sheldon and Mr. John Hill 

 of Derby, most of us I suppose, who are interested in this 

 subject, possess specimens of TepJirosia biiindularia from this 

 locality. Those I had some years ago from Mr. Sheldon are 

 dark, but comparatively pale compared with some sent me 

 a few years later by Mr. Hill. Some time ago Mr. Sheldon, 

 who is one of our most observant lepidopterists, told me 

 what he considered the history of the melanism of Tephrosia 

 biiindularia in the neighbourhood of Derby,^ and for the 

 purpose of this paper, I wrote to Mr. Sheldon, asking him 

 to give me the particulars. I now quote his statement, 

 which is as follows : — " Some 12 or 13 years ago, I found 

 that biundularia occurred freely in a wood near Derby. The 

 growth was composed of spruce, and Scotch firs, larch, oak, 

 etc., and would be then some twenty years of age. The 

 biundularia were of all shades, from the light southern type to 

 a form about as dark as typical T. punctulata. A large 

 percentage, 50 per cent., would be the light form. This was 

 from 1876-1879. In 1880 I left the district, and did not do 

 any more collecting in it till 1885, when I found a difference in 

 the biicndiilaria ; not only w^ere the extreme dark forms darker 

 than any I had previously noticed, but the percentage of pale 

 ones was much less, not more than about 15 to 20 per cent, of 

 the whole. I have not done any collecting there since, but I am 

 told by a resident collector that it is now difficult to find the light 

 type, and so the darkening process appears, to be going on." 

 With respect to the cause of this, Mr. Sheldon writes : — " I 

 consider it to result from two reasons : — (i). The gradual growth 

 of the trees makes the wood darker than formerly. (2). The dark 



^Vide Ent, Mo. Mag., vol. xxiii., p. 6.' 



