MELANIC VARIATION IN LEPIDOPTERA. 131 



In the case of Arctic insects it would be not only important 

 that they should become adapted to avail themselves of any ad- 

 vantages they could derive from unfrequent and transient gleams 

 of sunlight, uninterrupted by cloud or mist, but that any similar 

 advantage in a less degree through the prolonged hours of or- 

 dinary daylight should not be lost to them. 



In photography it is well known that intense light produces 

 a far more rapid chemical effect than that which is more or less 

 subdued, nevertheless a long exposure to the latter may give in 

 the end approximately as good results. 



The case of Gnophos obscuraia, and many other similar 

 instances of insects assuming the colour of the soil or of the 

 objects which surround them, might almost tempt us to believe 

 that a kind of natural photographic process capable of imparting 

 colour by reflection was not impossible, for although this pro- 

 tective resemblance is not usually acquired so rapidly as in the 

 case of Mr. James Angus' white spider, * which became yellow 

 on being transferred from a white to a yellow flower; yet it may 

 be questioned whether the gradual change extends through a 

 sufficient number of generations to justify the assumption that 

 it is produced by a system of natural selection. The suggestion 

 may indicate a line of enquiry to some one better qualified than 

 myself to follow or to discard it. 



If I have rightly accounted for the melanic tendency in 

 our northern Lepidoptera by supposing it to be due to the ad- 

 vantages derived by the darkest insects under special climatic 

 conditions, from their more rapid absorption of invigorating 

 warmth ; and perhaps to the application of surplus vital energy, 

 through increased vibration of cell contents, to the deposition 

 of pigment; we should be led to expect certain evidences in 

 its favour, in addition to those I have already mentioned. The 



* 'American Naturalist,' vol. xvi, p. loio. 



