VISION VERSUS PHOT ORECEPT ION 5 



to the central nervous system all of the information that human brains 

 receive from human eyes. That does not mean at all, however, that the 

 same use and value is ever made and obtained from that information. 



Many vertebrates with perfectly good eyes, as complex as our own, 

 may not see anything. In explanation of this perhaps surprising state- 

 ment, it may be enough to point out that the portion of the brain in 

 which human visual impulses terminate and are integrated — in which, 

 in other words, vision seems to reside — is not present in the brains of 

 fishes at all. A fish may have a knowing look in his eye as he passes up 

 one kind of fly and avidly seizes another, but we have no right whatever 

 to assume that he sees either fly, or indeed anything else. It is quite 

 possible that he is acting, like the worm, only reflexly and without con- 

 scious accompaniments to patterns of shade and hue which, given a 

 brain capable of the analysis ours can perform, would be mental pictures 

 to him as they are to us. 



When therefore, elsewhere in this book, such questions are raised as : 

 "Do dogs see colors?" and "Can fishes tell a square from a triangle?" 

 the reader must visualize 'see' and 'tell' in tell-tale quotation marks, and 

 bear with the writer if he seems to lapse into anthropocentrism and to 

 attribute conscious visual acts to animals whose dim minds we cannot 

 read. It is easiest to compare the visual potentialities of one ocular 

 mechanism with those of another as though behind each there lay a brain 

 like that of man; but it is hoped that without further frequent reminder, 

 the reader will forever remember this : 



Human vision, so valuable and so kaleidoscopic, is the product of a 

 complex brain teamed with a relatively simple eye; and when we some- 

 times encounter more complex eyes (which are always connected with 

 simpler brains) we must not assume that they afford their owners any- 

 thing so informative of the environment as does the vision we experience. 

 "Nothing is in the mind which is not first in the senses" — but the sense- 

 organs, and particularly the eye, may offer the mind much more than 

 the latter can assimilate. 



