i2o THE DARWINIAN THEORY 



objects to which they are attached ; for instance, it 

 was shown that the pupa of the small tortoise-shell 

 butterfly, when placed on a dark background, 

 became itself very dark; but when placed on a 

 white background it became light-coloured. This 

 susceptibility to change of colour is greatest at the 

 stage when the larva first fixes itself before changing 

 to the pupa. The effect is produced through the 

 skin generally, and not through the eyes. 



Again, trout change colour according to that of 

 the bottom of the stream they inhabit ; so also do 

 minnows ; hence the interior of a minnow-can is painted 

 white, so that the bait may be light-coloured, and 

 more conspicuous to a pike or perch. Changes of 

 colour depend on the eye in the case of fish, and a 

 blind trout remains dark, the pigment cells relaxing 

 and becoming flattened, thereby exposing their 

 maximum amount of surface area. Cave animals, 

 on the other hand, become pale, because the pigment 

 which is now useless degenerates and disappears. 



The effect of cold in causing change of colour 

 indirectly through the nervous system, has been 

 demonstrated by suddenly exposing animals to cold 

 which had previously been protected from it for 

 some time, the result being distinct blanching. 

 Melanism, or dark coloration, is common on 

 oceanic islands, and humidity of the atmosphere is 

 as a rule associated with the darkening of colours. 

 So it is with increased elevation, and it is possible 

 that the object of this is to increase the absorption of 

 heat. Brilliant colours are not dependent on or 

 proportionate to the amount of light. 



