IV CHEMICAL ACTION OF LIGHT 145 



which in any case they could not have been adapted by selec- 

 tion without special conditions of life which ought to be 

 explicitly pointed out. Yet, notwithstanding this, the pheno- 

 menon has been assumed without further question to be due 

 to adaptation by selection. 



This interpretation I would meet with the following 

 proposition : 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- photography, 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 light falls, we are brought near to the con- 

 ception that sight, especially the perception of colours by the 

 eye 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 stimu- 

 lation or nervous conductivity might well render comprehen- 

 sible 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. Experiments 

 prove this : after the eyes have been removed, the action of 

 the colour of the environment on the colour of the skin ceases. 



Thus, in many cases, chemical action and stimulation of 

 the optic nerves may be closely connected in the process by 

 which the colours of animals are affected by light. 



Whether simple chemical action of the light upon the 

 skin, or such action taking place indirectly through the eyes, 



L 



