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 strize, 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 laminz laid one upon another. 
6. There is still one more cause of coloration worth mention- 
ing here. Ina 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 
