OTHER THEORIES 283 



by the excitement of a second concurrently differentiated retino- 

 cerebral apparatus would add the sensation of yellow to that of white. 

 If we then consider the state of a species in which the visual apparatus 

 has achieved this degree of development we shall see that it would 

 obviously be an advantageous arrangement that, when the retina was 

 stimulated by white light, i.e., by light containing rays of all wave- 

 lengths, the two new colour-systems, the yellow and the blue, both 

 being excited in addition to the white system, should have the sensa- 

 tion-elements determined by them fused in consciousness to white. 

 This compound white sensation-element would then add itself to and 

 so reinforce the sensation of white due to the excitement of the older 

 white apparatus. For suppose that the yellow and blue sensations 

 neutralised each other when the yellow and blue systems were excited 

 together, this would leave a sensation of white but would involve a 

 waste of the energy that, under the other arrangement, would go to 

 reinforce the white sensation. Or suppose the third possibility, 

 namely, that yellow and blue when excited together fused to give a 

 new kind of sensation. Then stimulation by mixed light would result 

 in a sensation compounded of white and this new yellow-blue sensation ; 

 the ancient and primitive sensation of pure white or grey would have 

 been lost, it could never again be experienced, and in place of the three 

 perfectly distinct kinds of sensation yellow and blue and white yielded 

 by the first arrangement, there would be possible only two, yellow and 

 blue, and a mixture, bluish-yellow. To illustrate this by an example — 

 it is obvious that a species or a variety endowed with the sensations 

 of red and blue and white, would in this respect have an advantage 

 over one endowed with red and blue and purple only. 



" Further, it is obvious that the original white apparatus would 

 not be likely to undergo much further development if the yellow and blue 

 systems developed in importance and in the intensity of the impression 

 produced by them in consciousness, for they would yield when excited 

 together a white sensation of correspondingly developed intensity. 



" It seems not unnatural to suppose that the developing differentia- 

 tion of the colour-sensibility of the retina should have proceeded out- 

 wards from the centre, the region of acutest vision, and the one that 

 is most used. In the peripheral zone of the human retina we have then 

 the perpetuation of the primitive monochromatic stage of development 

 of the eye, while the very rarely occurring monochromatic eyes are 

 cases of reversion to, or arrested development in, this remote ancestral 

 condition. In the same way the zone of tlie human retina, stimulation 



