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CHAPTER VIII. 



ON THE SENSES OF KST8. 



The Sense of Vision. 



If is, I think, generally assumed not only that the world 

 really exists as we see it, but that it appears to other 

 animals pretty much as it does to us. A little con- 

 sideration, however, is sufficient to show that this is 

 very far from being certain, or even probable. 



In the case of insects, moreover, the mode of vision 

 is still an enigma. They have, at least many of them 

 have, a large compound eye on each side ; and ocelli, 

 generally three in number, situated on the summit of 

 the head. The compound eyes consist of a number of 

 facets, each situated at the summit of a tube, to the 

 base of which runs a fibre of the optic nerve. 



The structure of the ocellus and that of the com- 

 pound eye are essentially different, and it does not seem 

 possible that either the ocellus should be derived from 

 the compound eye, or the compound eye from the ocel- 

 lus. On the contrary, both seem to point back to 

 a less developed ancestral type. Starting from such an 

 origin, an increase of the separate elements and an im- 

 [.rovement of the lens would lead to the ocellus, while 



