MELANIC VARIATION IN LEPIDOPTERA. 85 



enabled to absorb more heat from the sun's ra3's, as black is the 

 greatest absorber of heat. 



This hj'pothesis is thus set forth : — 



" Seeing that radiation and absorption alilve involve motion, which may 

 be taken to be the basis of the theory of photoplastic mechanical action, we 

 must not forget a point strongly urged by Prof. Tyndall, namely, the 

 importance to the organic world of the ultra-violet invisible rays of the 

 spectrum on account of their chemical energy.-^- It has been shown by 

 the same author ' that the invisible rays of the sun show a preference for 

 black, which diminishes the reflection.' It is, of course, no new discovery 

 that among colours black is the greatest absorber of heat. In Craven's 

 * Recreations in Shooting,'! the following passage occurs : — ' Colour is well 

 known to influence the rate by which bodies acquire, reflect, or part with 

 heat, and as white is the colour which most readily and perfectly reflects it, 

 and which most difficultly [sic] parts with it, so a body clothed with that 

 colour shall retain heat longest, and therefore be better fitted to exist in the 

 coldest latitudes.' Applying this to the winter plumage of Ptarmigan, he 

 continues, quoting from Daniel (the original passage I have been unable to 

 find) ;— ' If two animals, one of a black colour and the other white, be 

 placed in a higher temperature than that of their own body, the heat will 

 enter the one that is black with the greatest rapidity, and elevate its tem- 

 perature considerably above that of the other ; but when these animals are 

 placed in a situation the temperature of which is considerably lower than 

 their own, the black animal will give out its heat by radiation to every 

 surrounding object colder than itself, and speedily have its temperature 

 reduced, while the white animal will part with its heat at a much slower 

 rate.' Birds and animals living through the winter naturally require to 

 retain in their bodies a sufficient amount of heat to enable them to maintain 

 their existence with unreduced vitality against the severities of the climate. 

 Insects, on the contrary, require rajndli/ to take advantage of transient 

 gleams of sunshine during the short summer season, and may be content 

 to sink into a dormant condition so soon as they have secured the repro- 

 duction of their species ; only to be revived in some instances by a return 

 of exceptionally favourable conditions." 



It is then dealt with in a concrete form by the following 

 illustrations : — 



" We all know how rapidly the pairing of our Lepidoptera is effected. 

 Edwards gives instances of freshly developed males gathering round a 

 femalf pupa to await the emergence of the perfect insect, and the method 



* ' Fragments of Science,' Tyndall, vol. i. p. 32. 

 t ' Recreations in Shooting,' Craven, 1847, p. 101. 



