PHYLOGENY, LOCUS OF COLOR VISION 521 



Just as diurnality has surely arisen by slow degrees within the primate 

 group, so also has human color vision developed entirely within the pri- 

 mate order. We might expect to see color vision in the true lemurs — as 

 also in the diurnal squirrels — but it is not there. Only entirely above the 

 lemuroids has the final refinement of color vision been added to the pre- 

 requisite diurnaUty, and it is quite possible that this addition has been 

 made independently in the platyrrhines and the catarrhines (v. s.) . 



It seems necessary to believe that human color vision owes nothing 

 whatever to the product of the teleost and the reptile. But 'human' color 

 vision is already present far below man in the anthropoid stock. It is not 

 necessary to suppose, with Bierens de Haan and Prima, that human color 

 vision has evolved wholly within the genus Homo. True, it was once be- 

 lieved that the ancients of Greece and Egypt had an incomplete color 

 vision as compared with modem man. The situation is now realized, how- 

 ever, to have been due to a simple paucity of words for colors in the lan- 

 guages of archaic and primitive peoples — the Homeric vocabulary, for 

 instance, contained no word for 'blue'. The Japanese use the word ao for 

 both green and blue — but they see a difference between them. 



Locus of Color Vision — We know that whenever color vision did 

 arise, however often it may have done so, it involved a differentiation of 

 several cooperative sensation-processes in the central nervous system, as 

 well as a set of differentially photosensitive chemical substances in the 

 visual cell (see Chapter 4). These latter, however, may be universally 

 present in cones, several such substances being needed in order to fill out 

 neatly the responsivity of the visual cell, to embrace as fully as possible 

 the spectrum which the watery dioptric media of the eye will let through. 

 It seems highly significant that the electrophysiological images of hue- 

 stimuli show the same hue-specific character in achromatic animals (e.g., 

 cats, rabbits) that they show in color-seeing forms. The evolution of a 

 color-vision system very likely entails only the affiliation of specific cen- 

 tral processes of registration and integration with particular photo- 

 chemicals already present in the cones. 



Where, in the central nervous system, are these hue-sensory processes 

 placed? We can say a little, though not much, on that point. In the 

 lower vertebrates, the optic nerves (in their continuation as the optic 

 tracts, quite unmodified since the decussation is total) sweep directly up 

 to the optic tectum, the roof of the mid-brain. A few fibers do terminate 

 in other minor centers; but the connections of the tectum with the centers 



