284 COLOUR VISION 



of which causes the sensations of yellow and blue and white only, 

 remains in that stage of development in which only the first step of 

 differentiation has been effected and the frequent cases of bichromatic 

 vision, in which yellow and blue and white seem to be the only sensa- 

 tions that can be aroused by stimulation of the retina, are cases of 

 reversion to or arrested development in this more recent ancestral 

 condition. 



" If we try to picture the further evolution of the colour-sense, 

 the process that would seem to be likely to give the best results, and 

 therefore the one most likely to be effected by the factors that have 

 controlled the origin and development of species, is a repetition of the 

 process of differentiation such as gave rise to the blue and yellow 

 systems, but occurring within either the blue or the yellow system. 

 For reasons, which we can hardly hope to determine, this differentiation 

 has proceeded in the yellow system. The light of the two ends of 

 the warm half of the spectrum must be supposed to have begun to set 

 free, within the retinal elements of the yellow apparatus, two different 

 substances in addition to the yellow substance, and with these two 

 new substances, the red and the green, we must assume the concurrent 

 differentiation of the red and green retino-cerebral systems. Then just 

 as it was obviously advantageous that yellow and blue sensation- 

 elements, when excited together, should fuse to give white, so obviously 

 it would have been advantageous that red and green sensations when 

 excited together should fuse to give yellow, else the primitive white and 

 the original yellow sensation would again be lost. As these two new 

 colour-systems became developed in the retina from the fovea centralis 

 outwards, the primitive yellow apparatus would lose its importance 

 and would probably undergo atrophy in this central region of most 

 highly developed colour sensibility, just as the primitive white apparatus 

 has become lost in the very centre, the fovea centralis itself. That the 

 primitive white apparatus remains functional throughout the rest of 

 these parts of the retina in which the colour-systems are developed is 

 probably due to its having assumed the special function of vision under 

 dim illumination, while no analogous functioning has justified the 

 continued existence of the primitive yellow system in the area of red and 

 green sensibility." 



