128 YORKSHIRE NATURALISTS' UNION. 



This would also in another way tend to assist that process 

 of evolution in the direction of melanism which I have attempted 

 to indicate. Those insects which most rapidly and most com- 

 pletely acquire vital energy through the absorption of heat 

 would be the most likely to possess under favourable conditions 

 a superabundance of such energy. 



If their organic chemistry, as in the case of plants, permits 

 this surplus to be devoted to the deposition of molecular sub- 

 stance, would it not be applied by this natural process to the 

 formation of pigment ? 



On this assumption M. Nicolas Wagner's electrical experi- 

 ments are not unintelligible. What is more certain to produce 

 ' free energy by vibration of the cell contents ' than electricity ? 

 And where a large surplus was thus artificially created, we not 

 unnaturally find increased deposition of colouring matter in the 

 hollow cell-like scales. 



If this is possible, it may be taken to be an important 

 factor in the development of varietal colouration, acting and re- 

 acting for the advantage of the species. As in the colder lati- 

 tudes black appears to be the colour most beneficial to their 

 interests, so in tropical regions other colours may play their 

 part in various ways, either in defending them by reflection 

 from too intense heat-rays,* or in protecting them from enemies 

 by assimilation to surrounding objects. Without implying any 

 distinct act of volition on the part of the subject, we may fairly 

 admit the objective power of Nature in a thousand unknown 

 ways to produce results enabling all existing species, in the 

 absence of prohibitive conditions, to maintain a balance pro- 

 portionate to their importance in the general system. 



Shall I be too bold if I would endeavour to account for 

 the acquisition of summer plumage in birds, and darker colour- 

 ing in arctic animals during the summer months, by supposing 



* The insects of hot deserts are very frequently white or pale yellow. 



Trans. V.N.U., 1883 (pub. 18S5). Series D 



