MORPHOLOGY OP COMPOUND EYES OF ARTHROPODS. 145 



" But in order that light may produce chemical eflFects (upon 

 protoplasm), it must be absorbed; it must be spent in doing 

 the chemical work. Accordingly, the first step towards the 

 formation of an organ of vision is the differentiation of a 

 portion of protoplasm into a pigment at once capable of 

 absorbing light and sensitive to light — i.e. undergoing decom- 

 position upon exposure to light. An organism, a portion of 

 whose protoplasm had thus become differentiated into such a 

 pigment, would be able to react towards light. The light 

 falling on the organism would be in part absorbed by the pig- 

 ment, and the rays thus absorbed would produce a chemical 

 action and set free chemical substances which before were not 

 present. We have only to suppose that the chemical substances 

 are of such a nature as to act as a stimulus to the protoplasm 

 of other parts of the organism (and we have manifold evidence 

 of the exquisite sensitiveness of protoplasm in general to 

 chemical stimuli), in order to see how rays of light falling on 

 the organism might excite movements in it or modify move- 

 ments which were being carried on, or might otherwise affect 

 the organism in whole or part. Such considerations as the 

 foregoing may be applied to even the complex organ of vision 

 of the higher animals. If we suppose that the actual termina- 

 tions of the optic nerve are surrounded by substances sensitive 

 to light, then it becomes easy to imagine how light, falling on 

 these sensitive substances, should set free chemical bodies 

 possessed of the property of acting as stimuli to the actual 

 nerve-endings, and thus give rise to visual impulses in the 

 optic fibres." 



Lubbock' advances essentially the same idea as Balfour's in 

 his recent work on the subject, illustrated with some lucid 

 diagrams. " In the simple forms,^' he says, '^the whole surface 

 is more or less sensitive. Suppose, however, some solid and 

 opaque particles of pigment deposited in certain cells of the 

 the skin. Their opacity would arrest and absorb the light, 

 thus increasing its effect, while their solidity would enhance 



1 'On the Senses, Instinct, and Intelligence of Animals,' Inter. Sci, 

 Series, vol. Ixix, 1888. 



