154 RELATIONS OF OCELLUS AND EYE. 



On the Eelation of the Eye to the Ocellus. 



In considering the relation of the eye to the ocellus, 

 it is obyious that we cannot regard either as derived 

 from the other. They are, as Grenacher says, " sisters," 

 and derived from a common origin. 



The ocellus consists of a single lens in front of a 

 larger or smaller number of visual rods. The com- 

 pound eye consists of a number of facets, each in front 

 of a single rod ; which is produced by from four to 

 sixteen cells : in some cases each cell at first produces 

 a separate rod, and these then subsequently coalesce 

 more or less completely. Starting, then, from a simple 

 form of eye consisting of a lens and a nerve-fibre, 

 which would be capable of perceiving light, but would 

 give no picture of the external world, we should 

 arrive at the compound eye by bringing together a 

 number of such eye-spots, and increasing the number 

 of lenses, while the separate cells beneath each com- 

 bined to form a single cone and rod ; while, on the 

 other hand, by increasing the size of the lens, and 

 multiplying the nervous elements behind it, we should 

 obtain the ocellus of an insect, or the typical eyes of 

 a vertebrate animal. 



There is, indeed, no need to suppose that these two 

 eyes are derived from a common origin. We know 

 that, while very similar eyes occur in distant groups of 

 animals, on the other hand nearly allied species often 

 difi'er greatly in the structure of their eyes; that, 

 indeed, eyes of very different types often occur even in 

 the same animal, so that we have strong reasons for 

 assuming that they had an indeoendent and separate 

 origin. 



