MELANISM AND MELANOGHROISM. 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 cHmates, 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.e, such as GnopJios obsai- 

 raria, Cidaria russata, or some of the species of Melanippe or 

 Hybeniia, 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 



