294 Animal Life and Intelligence. 



example can distinguish between light and darkness by 

 the general surface of the skin. M. Dubois, by a number 

 of experiments on the blind Proteus of the grottoes of 

 Carniola, has shown that the sensitiveness of its skin to 

 light is about half that of its rudimentary eyes ; and, 

 further, that this sensibility varies with the colour of the 

 light employed, being greatest for yellow light.* 



We have not been able to do more than make a rapid 

 survey of the sense of sight as it seems to be developed in 

 the invertebrates and lower animals. The visual organs 

 differ, not only in structure, but in principle. We may, I 

 think, distinguish four types. 



1. Organs for the mere appreciation of light or dark- 

 ness (shadow), exemplified by pigment-spots, with or with- 

 out concentrating apparatus. 



2. Organs for the appreciation of the direction of light 

 or shadow, with or without a lens. The simple retinal eyes 

 of gasteropods, and perhaps in some cases the ocelli of 

 insects, probably belong to this class. 



3. True eyes, or organs in which a retinal image is 

 formed, through the instrumentality of a lens, as in 

 vertebrates and cephalopods. 



4. The facetted eyes of insects, in which a stippled 

 image is formed, on the principle of mosaic vision. 



Unfortunately, all these are called indiscriminately 

 eyes, or organs of vision. An infusorian or a snail is said 

 to see. But the terms "eye," "vision," "sight," imply 

 that final excellence to which only the higher animals, 

 each on its own line, have attained. 



This final excellence probably has its basis and earliest 

 inception in the fact that the functional activity of proto- 

 plasm is heightened in the presence of setherial vibrations. 

 If, then, we imagine, as a starting-point, a primitive 

 transparent organism with a general susceptibility to the 

 influence of light-vibrations, the formation within its 

 tissues of pigment-granules absorbent of light will render 

 the spots where they occur specially sensitive to the 

 . * See Nature, vol. xli. p. 407. 



