MELANISM AND MELANOCHROISM. 
5 
melanic variation occurring in the swamps of Tropical Africa, 
in the forests of the Amazon, on the banks of the Mississippi, 
and in many other damp climates, even within tropical regions, 
and I am not aware that this is the case. It has been pointed 
out that no variation in the direction of melanism has been 
found to occur in dry open level tracts, however far north these 
may be ; and this fact, although it tends to show that cold is 
not by itself a potent cause of such variation, is perfectly con- 
sistent with the theory that diminished sunlight exercises a 
certain influence upon the direction in which colour may be 
expected to, and does, vary. The only conclusion that I would 
venture to maintain, in regard to this very wide subject, is that 
variation of colour in the direction of melanism, melanochroism, 
or such suffusion of markings as can appreciably affect the 
rapidity of heat-absorption by the wing-surface of any insect, 
is very generally proportionate to the degree of interference 
with the direct action of sunlight caused by climatic or other 
conditions to which the insect is exposed, and that such inter- 
ference may have a more or less immediate, or only a very 
gradual effect, upon the variation of species. We have yet to 
ascertain how far such effect is assisted by a process of natural 
selection, and whether such natural selection is exercised for 
the sake of protective resemblance, or for the advantage derived 
from an increased power of heat-absorption, or whether it is 
produced by the direct chemical action of the visible or invisible 
rays of the solar spectrum. 
“ I sincerely wish that Mr. Poulton or Mr. Merrifield, or 
some of those gentlemen who have time and opportunity for 
studying this question, would carry out an experiment, origin- 
ally put into my head by Sir John Lubbock, viz., the rearing 
of some of our variable Geometrid^, such as Gnophos ohscu- 
raria, Cidaria russata, or some of the species of Melanippe or 
Hybernia, guarding them in the larval and pupal stages by 
hyposulphide of carbon or bichromate of potash from the 
action of the ultra-violet rays of the sun, which are supposed 
by many to have such a strong chemical effect upon the colour- 
ing matter in the scales, or on the chlorophyll in the pigment 
of the larvae. If it were found that under such conditions any 
appreciable change could be made in the percentage of dark as 
compared with paler varieties, it would go far to prove that 
melanic variation cannot be attributed to the direct influence 
either of cold or moisture, but rather to the indirect influence 
of the two combined ; and the facts, so far as they are at 
