80 DOMESTICATED ANIMALS AND PLANTS 



the yellow should be dissolved away, then the creature would 

 be blue, either wholly or partially, unless indeed it should 

 also secrete red pigment, which would then discharge its own 

 function in fixing tints which, with the blue, would extend to 

 violet or even black. 



Another very different case of the same order is the white or 

 light color quite common to the under scales of snakes and 

 lizards, an effect due principally to the storage there of lime, as 

 we store the same substance in our bones, coming in both cases 

 from the food. With them it is a thoroughly waste product, as 

 it is with us late in life after the bones are finished, when it 

 often makes trouble by collecting in the bladder or kidneys in 

 the form of small stones. 



5. The scintillating effect like the metallic luster of certain 

 plumage is due not to pigment but to strictly mechanical causes. 

 In the humming bird, for instance, the surface of the feathers 

 is covered with minute striae, which, by their unequal reflection 

 and slight refraction of the light rays, give that beautiful play of 

 colors with which we are all familiar, and which is not greatly 

 different in its character from the play of colors in pearl, which 

 is also due to the fact that the pearl consists of exceedingly 

 thin laminae laid one upon another. 



6. There is still one more cause of coloration worth mention- 

 ing here. In a desert where everything is of a dull gray there 

 is practically no white light, because certain rays are absorbed 

 by the universal monotony of nature. If there is no white 

 light, then nothing will appear in its natural colors, but every- 

 thing will appear to be of a dull gray, because there are no 

 other colors at hand to be reflected to the eye, just as in an 

 artificial red light everything appears red, no matter what its 

 color might be in perfect light, because there are no other rays 

 to be reflected. 



The student needs to be exceedingly careful, therefore, in 

 generalizing about color markings and utility. The color, es- 

 pecially of animals, is often highly protective, and then natural 



