170 ORGANIC EVOLUTION CONSIDERED 



of Cyclops, the various kinds and numbers of simple 

 eyes of insects, spiders, scorpions, and myriapods, 

 the various kinds of compound eyes, the fact that 

 both simple and compound eyes are usually found in 

 insects, the eyes of cuttle-fishes and the eyes of 

 vertebrates. 



From the above it is evident that if eyes have been 

 evolved they have been evolved many times independ- 

 ently of each other. This follows both from their 

 differences in structure, in number, and from the 

 various positions that they occupy in different animals. 



It is evident that the eyes of the Pecten in the edge 

 of the mantle, and of the snail on tentacles, could 

 not have had a common origin ; that the compound 

 eyes with hexagonal facets could not have been 

 evolved from those with four sides; that the ocelli of 

 insects could not have originated from compound 

 eyes; that the eye of the Cuttle-fish could not have 

 been evolved from any known eye, and that the eyes 

 of vertebrates have not originated from any known 

 eyes of invertebrates. 



How are we to account for the independent evolu- 

 tion of so many ~ different eyes and eye-spots? The 

 frequency of its occurrence would seem to indicate 

 that it is a matter of no great difficulty. Romanes 

 attempts to explain the evolution of the eye in the 

 following language: "Take, for example, what is 

 perhaps the most wonderful instance of refined 

 mechanism in nature — the eye of a vertebrated ani- 

 mal. Comparative anatony and embryology combine 

 to testify that this organ had its origin in modifica- 

 tions of the endings of the ordinary nerves of the 

 skin. Now it is evident that from the very first any 

 modification of a cutaneous nerve, whereby it was 

 rendered able, in however small a degree, to be dif- 

 ferently affected by light and by darkness, would be of 



