MELANIC VARIATION IN LEPIDOPTERA. 121 



If, in Mr. Lewis' words,* ' blackness is a structure formed 

 [mechanically] by heat rays,' why should Melanippe nwntanaia 

 be black and white, instead of a uniform colour ? Why should 

 these rays act differently upon different portions of the wing of 

 any insect ? And should we not expect that northern insects 

 exposed to a less amount of heat than those of the south would 

 assume a less instead of a greater amount of pigment ? 



Why, again, should the colours of dead insects exposed to 

 light have a tendency to fade ? If the scales have lost their 

 supposed plasticity, would they not at least retain their estab- 

 lished form ? 



Must we not search for some law which will better account 

 for the undoubted facts we are dealing with ? 



Mr. Lewis himself indicates what he rightly describes as 

 another line of evolution, and Mr. Forbes in his paper above 

 referred to,t after quoting Von Tschudi % as to the melanic 

 tendency of Alpine Coleoptera, suggests as the most probable 

 conclusion that ' darkness of colouration is in some mysterious 

 way correlated with a constitution better fitted to encounter 

 unfavourable conditions of life more especially meteorological.' 

 Can we endeavour in some degree to solve the mystery which 

 surrounds the question of what is the precise method of 

 evolution, if any, which secures to our dark northern varieties 

 and species a constitution better fitted to enable them to 

 encounter unfavourable meteorological conditions through 

 advantages derived from their peculiar colouration ? 



Seeing that radiation and absorption alike 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 



* Transactions of Entomological Society, 1882, p. 517. 

 t Entomologists' Monthly Magazine, xiv, p. 16. 

 J ' Monde des Alpes,' p. 394. 



