GENERATION OF ANIMALS, V. i. 



fluid ; therefore, whenever Nature cannot make the 

 fluid in both eyes tally, either by concocting it or 

 by not concocting it in both, but instead of that 

 concocts it in one and not in the other, the result is 

 odd-coloured eyes. 



The fact that some animals are keen-sighted and Two senses 



, , /. r> II 1 >> of "keen. 



others not is due to two sets ot causes, tor keen 

 here has practically two meanings (so it has when 

 appUed to hearing and smelling). Thus, keen sight 

 means (a) ability to see from a distance, (6) distin- 

 guishing as accurately as possible the differences " of 

 the objects which are seen ; and these faculties do 

 not occur together in the same persons. The man 

 who shades his eye with his hand or looks through a 

 tube will not distinguish any more or any less the 

 differences of colours, but he will see further ; at any 

 rate, people in pits and wells sometimes see the stars. 

 So that if any animal has a considerable projection 

 over his eyes, while the fluid in his pupils is not pure 

 nor suitably proportionate to the movement coming 

 from without, and if the skin on the surface of them 

 is not thin, then that animal will not have accuracy of 

 vision in so far as differences of colours are concerned, 

 but he will be able to see from a distance (just as he 

 would from close quarters) better than animals which 

 though they have pure fluid in their eyes and a pure 

 covering round it, yet have no projecting brow at all 

 in front of their eyes. The reason is that (a) the 

 cause of being keen-sighted enough to distinguish the 

 differences <of colour) lies in the eye itself, since just 

 as quite small stains are plain and distinct on a pure, 

 clean shirt, so quite small movements are plain and 



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