EXTERNAL INFLUENCES AS CAUSES OF VARIATION 



243 



from one in which the color is due to a substance formed only 

 by the aid of light. It is only in this latter case that light may 

 truly be said to be a direct external cause of variation. 



Examples of this are found in the coloring of fruit, either 

 under normal conditions or in " fruit photography," a process 

 by which pictures may be made to appear on highly colored 

 fruit by shading with a screen derived like a negative from the 

 picture to be transferred. 1 



The sun is supposed to exert a direct effect upon the skin, 

 ranging from the tan of the white man to the dark color of the 

 tropical races. This seems an ill adaptation, and so it is as 

 regards the heat, for black objects are warmer than white ones ; 

 but the adaptation is not to the heat rays but to the chemical, 

 for black pigment is almost totally non-actinic. Hence we may 

 say that dark-skinned people have lost something in heat resist- 

 ance, but they have gained what is of more consequence, a 

 screen against the actinic rays. 2 



Light exhibits its most characteristic effect upon the eye of 

 higher animals. It here gives rise to two remarkable actions, 

 muscular contraction of the iris, by which the amount of light 

 admitted is regulated, and a nerve stimulus, which forms a defi- 

 nite image on the retina, as upon a mirror, and which is perfectly 

 comprehended by the mind. Whether the colors and images 

 seen by all eyes are absolutely identical is obviously a matter 

 that can never be determined. It is of course safe to assume 

 that the images, in so far as form is concerned, are identical, 

 because the outlines are due to the mechanical laws of refrac- 

 tion, but the colors as comprehended may be due in part, if not 

 entirely, to physiological peculiarities. That is, the color which 

 to one is red may look to him as yellow does to another, a 

 supposition entirely plausible when we remember that with 

 some individuals sound always suggests color as well, so that 

 the name Jones immediately suggests black, or red, or some 

 other color, differing with different individuals. What relation 

 or coordination between the auditory and the optic nerves can be 

 responsible for this sort of mixed impression we do not know. 



1 Literary Digest, September 16, 1905, p. 381. 



2 Ibid. October 7, 1905, p. 485. 



