314 ^S. U.ITASE. 



8\ibstaiK"OS wliich before were not present. We have only to 

 suppose tliiit tlii^ chemical substances are of such a nature as to 

 act as a stiimihis to the protoplasm of otiier parts of the organism 

 (and we have inanifold evidence of the exquisite sensitiveness of 

 protoplasm in <jeneral to chemical stimuli), in order to see how 

 rays of light falling on the organism might excite movements in 

 it, or modify movements which wore being carried on, or might 

 otherwise att'ect the organism in whole or in part. Such consid- 

 erations as the foregoing may be applied to even the complex 

 organ of vision of the higher animals. If we suppose tliat the 

 actual terminations 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 skin. Tiieir opacity 

 would arrest and absorb the light, thus increasing its efl'eet, while 

 their solidity would enhance the efl'eet of external stimulus. A 

 further step might be a depression in the skin at this point, which 

 would serve somewhat to protect these difierentiated and more 

 sensitive cells, while the deeper this depression the greater would 

 be the protection." 



That such steps of gradual development of visual organs have 

 actually taken place in some forms is quite probable. In 

 Arthropods, it seems to me worthy of remark that the omma- 

 tidium of the lateral eye of Limulus makes the nearest approach 

 to this primitive condition. It is nothing more and nothing less 

 than a depression in the skin, with the thickened chitinous cuticle 

 fitting in the open cavity and acting as a lens to condense the light. 

 The cells which form the sensory part of the structure are modi- 

 fied ectodermic cells, and, like the rest of tiie ectodermic cells 

 lying on the surface of the body, secrete the chitinous cuticle on 



' On the Senses, Inalincl, and Intelligence of Animals, Inter. Soie. Series, 

 Vol. L.Xl.X, 1888. 



