102 MENTAL EVOLUTION IN ANIMALS. 



such wise that, if it has been cooled, it over-estimates the 

 temperature of any object it may touch, and vice versd. This 

 is taken to be analogous to the appearance of warm colours 

 in the eyes when closed immediately after having been 

 exposed to intense cold colours, and vice versd. So, too, it is 

 with simultaneous contrasts. It is well known that if a 

 small colourless surface is enclosed between two surfaces of 

 cold or warm colours, the small surface will appear inversely 

 coloured warm or cold, as the case may be ; and Professor 

 Preyer has found by experiment, that if a small portion of the 

 skin be enclosed by cold or warm surfaces on either side, the 

 small enclosed area will feel cool if the neighbouring parts 

 are heated, and vice versd. 



After showing that in his view illumination is to the 

 sense of colour what contact is to the sense of temperature, 

 and pointing out several subordinate analogies which I have 

 no space to mention, Professor Preyer goes on to remark an 

 important fact in relation to his theory, viz., that different 

 parts of the skin manifest in their estimations of tempera- 

 ture great differences in their estimates of what he calls the 

 " neutral point," i.e., the point at which it cannot be said that 

 a body is felt to be either hot or cold. The retina, then, being 

 supposed to be merely a nerve-expansion having a much 

 higher " neutral point " in the appreciation of temperature 

 (ethereal vibrations) than has any nerve-expansion of the 

 skin, colour-blindness is explained by supposing that the 

 retina of the individual so affected has a neutral point either 

 above or below the normal. " An over- warm eye must be 

 blind to yellow and blue ; an over cool one must be blind to 

 red and green." Total colour-blindness, which is a physio- 

 logical characteristic among certain nocturnal animals, has its 

 parallel in the pathological condition sometimes met with in 

 man, of a total absence of the sense of temperature without 

 impairment in the sense of touch. 



Lastly, it is observed that the first condition to the 

 validity of any physiological hypothesis is that it should 

 accord with morphological fact. But this is not the case with 

 the theory of Young and Helmholtz, which ascribes the 

 colour-sense to the functions of three retinal elements ; for it 

 has been proved that the number of fibres in the optic nerve 

 immediately before it enters the retina is much smaller than 

 the number of rods and cones in the retina. 



