138 ANIMAL COLOEATION. 



in that family of butterflies (Vanessidee) to which the Tortoise- 

 shell belongs, is puzzling. The very name " chrysalis " or 

 " aurelia," as some of the older entomologists termed it, is 

 derived from the golden colour which is so distinctive a feature 

 of many pupte. Mr. Poiilton has most ingeniously suggested 

 that the angular shajDe of these pup^e combined with their 

 gilding might be protective when the pupa was attached to 

 surfaces of rocks containing glittering particles of mica. 



It has been suggested that natural selection has played no 

 part in the production of this sensibility to colour, — that it is 

 purely a question of the direct influence of light. Prof. Eimer 

 suggests * that " The substance composing the envelope of the 

 pupa possesses, as a fact, the property of being changed by 

 light, like a photographic plate; and the relation of this pro- 

 perty to the outer world may be useful, but it does not neces- 

 sarily owe its origin to selection. That this substance is so 

 constituted as to exhibit in action a process of colour photo- 

 graphy, the goal of so much human longing and striving, leads 

 to another consideration. Since the discovery of visual red 

 in the retina of the eye, a substance which quickly bleaches 

 under the action of light after death, and which is situated 

 in the very cells of the retina on which the light falls, we 

 are brought near to the conception that sight, especially the 

 perception of colours by the eyes of the higher animals and of 

 man, is likewise a chemical process, a kind of photography. A 

 short step farther in the specialisation of nervous stimulation 

 or nervous conductivity might well render comprehensible 

 the wonderful fact above referred to, that the colours of the 

 environment of an animal may be reflected in the colours of its 

 skin. For it is self-evident that the path of action of the 

 coloured light is principally through the eyes. Expei'iments 

 ' Eimer, hic. cif., p. 145. 



