6 THE entomologist's RECORD. 



present known, point, I think, somewhat forcibly to this con- 

 clusion " {Trans. Ent. Soc. Loud., i8go, pp. liv., Iv.). 



These remarks suggest many ideas, and open up new stand- 

 points from which the subject may be discussed, and the 

 following suggestions at once presented themselves to me as I 

 heard the address. 



Although I certainly look on moisture rather as an indirect 

 than a direct cause of melanism, I am not inclined to discount 

 the probability of " 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." To a great extent our knowledge of 

 the fauna of these districts is more or less confined to the 

 larger and more conspicuous of the lepidoptera, especially 

 diurnal lepidoptera, whose habits would render them less 

 susceptible to these influences, their environment leading them 

 to vary mainly in other directions. When the nocturnal lepi- 

 dopterous fauna of these districts is as intimately known as our 

 own, I feel satisfied that numerous cases of melanism will come 

 to light proving that the combined influence (direct and in- 

 direct) of moisture and " natural selection " are of general 

 application. 



Lord Walsingham's idea as to the action of the actinic rays 

 of the sun in producing colour, and the absorption of certain 

 of these rays by clouds, etc., as a probable cause of melanism, 

 is quite new to me, and has proved of great interest, although 

 at present I cannot see my way clearly to accept the sugges- 

 tions to which the idea leads. I suppose that since the solar 

 spectrum consists of heat, luminous and actinic rays, we may 

 perhaps assume that a lessened amount of luminous rays, after 

 having filtered through clouds, etc., is accompanied by a 

 fewer number of actinic rays, and that the clouds may have a 

 corresponding absorptive and diffusive ^ power over these rays as 

 they have over the luminous. Considering, too, the well-known 

 phenomenon of the decomposition of carbon dioxide by the 

 chlorophyll of plants in the presence of water, by the direct 

 action of the actinic rays, there seems a superficial reason for 

 considering that these rays may have, as is observed by Lord 

 Walsingham, some action on the chlorophyll in the pigment of 



^ The absorptive and dispersive power of aqueous vapour on the ultra-red (or heat) 

 rays of the spectrum is well known : see Ganot's Physics, par. 959. It l)ecomes 

 certain, therefore, that the influence of vapour is to increase the quantity of obscure 

 (ultra-red or heat) rays, and to lessen the luminous and ultra-violet (or actinic) rays, 

 in proportion to the increase in the ultra-red. 



