242 CAUSES OF VARIATION 



alcohol to decolorize, and then with iodin, which forms its char- 

 acteristic blue with starch. The spectrum between B and D 1 

 includes the upper part of the red, the orange, and the lower 

 parts of the yellow, the thermic rather than the actinic 

 portion of the spectrum. 



Among both plants and animals light has an important influ- 

 ence upon color. The chlorophyll of plants is formed only in 

 its presence, and it is intimately concerned in the production of 

 pigments in the skin. Not only that, but the arrangement and 

 position of pigmentary matter, whether lying next the surface 

 and well diffused thus giving color to the animal or lying 

 collected in masses deeper in the skin and having little effect 

 upon the color, are due largely to the direct effect of light 

 falling upon the skin of the animal. In this way certain animals, 

 as the chameleon, are capable of exhibiting a considerable range 

 of colors, giving rise to the fiction that they are able to imitate 

 any color near which they may be situated. 2 



It has been customary to cite the fact that cave animals are 

 frequently less highly colored than their congeners of the land, 

 as evidence that color is fundamentally dependent upon light. 

 This cannot be true except in a very general sense. All material 

 substances have some relation to light and therefore have some 

 color. What the color of a body may be is therefore dependent 

 primarily upon its composition, and in this sense its color may 

 be said to be accidental, a remark that is as true of chlorophyll 

 as it is of gold or silver, or of red, white, or yellow brick. 



But when the particular compound happens to be one like 

 chlorophyll, or a pigment that can be formed only in the presence 

 of light) then and then only can color be said to depend upon 

 the presence of light. Deep-sea fishes are often highly colored ; 

 rocks hidden in the earth have their characteristic tint ; the 

 blood of vertebrates is red, not from the presence of light but 

 from the presence of compounds of iron. In all these cases 

 the color arises from a substance in no sense dependent upon 

 light for its formation and existence, and the case is distinct 



1 The vibrations at this point are approximately 525 x io 12 per second (525,- 

 000,000,000,000). 



2 C. B. Davenport, Experimental Morphology, Part I, pp. 192-194. 



