IV ORIGIN OF VISUAL ORGANS 155 



which remam unpigmented become sight -rods. I have 

 described such superficial pigment-spots representing eyes in 

 their simplest form, in Medusae, e.g. in Aurelia aurita.-^ By 

 their means, in my opinion, the simplest kind of sight is 

 effected in this way, that of the rays coming from any object, 

 one passes through each such sight-rod and is separated from 

 neighbouring rays, and so the lights and shades of the object 

 are communicated to the nervous system in a number of 

 separate points, so that a representation of the object is 

 obtained in the nervous system by a flat image composed of 

 points. The lighter and darker points of the object are each 

 represented in the nervous system, as when an object is seen 

 through a number of minute holes in a card. This kind of 

 sight by points can only be effected by these lowly developed 

 eyes in such low forms as the Medusae, not, as most zoologists, 

 following Johannes MuUer, assume, by the highly developed 

 complex eyes of insects, in which images must certainly be 

 formed. The origin of eyes capable of this kind of sight 

 depends then on the deposition of pigment around touch-cells 

 for the purpose of separating the different light-rays received 

 from one another. We have already seen what an importance 

 light has in the production of pigment. Without the stimulus 

 of light, the pigment so essential to the formation of an eye 

 could not be produced ; without the continuation of the 

 stimulus, that is, without constant use, the eye cannot con- 

 tinue to exist at all ; but by constant use it is improved and 

 perfected. The same stimulus which it is the function of 

 the eye to receive, namely, light, created and still maintains 

 the essential and fundamental element of the organ. 



Exactly the same statement holds also for the other 

 sensitive cells, for the cells of smell, taste, and hearing, as 

 for those of touch and for sight-rods. They have all been 

 ultimately evolved from indifferent epidermic cells, and the 



^ Th. Eimer, Bie Meclusen. 



