THE WAY OF THE WILD 79 
2. Closely akin to this is a second cause of color, namely, a 
body surface so delicate and transparent that the color of the 
creature is fixed by that of the internal parts, as in certain carth- 
worms, in which the color is due to the blood showing through 
the transparent skin, while in related species a dark surface pig- 
ment obscures the blood and gives its own color to the worm. 
In other cases, as frequently in larvae, which do not have red 
blood, the contents of the digestive tract show through the skin 
and give color to the insect. In this way all leaf-feeding larvee 
that have transparent skins are green in color ; that is, they look 
green, though that which gives the color is only the chlorophyll 
of their food, 
3. Very similar to the above is a class of cases in which the 
pigment, instead of being fixed at the surface, as in hair or skin, 
is contained in irregular-shaped cells extending from the surface 
to considerable depths beneath the skin. When the creature is 
at rest or in its normal conditions, the pigment lies near the 
surface and gives its color to the animal ; but if it be paralyzed 
with sudden fright, the surface layers of the skin contract and 
drive the coloring matter into the deeper layers and out of sight, 
so that the creature undergoes a blanching process quite akin 
to the sudden paling of the face when the blood is driven out 
of the surface veins by sudden fright. 
4. A fourth cause of color, especially in animals, is the storing 
away in the cells of the body of certain waste materials taken in 
with the food and not digested or otherwise eliminated from the 
body. A common example of this is the color of many butter- 
flies whose larvee feed freely upon leaves. If the chlorophyll is 
not digested or otherwise changed, it is packed away, especially 
in the cells of the wings, either uniformly or in more or less 
regular patterns dependent on the structure. In such a case 
the butterfly would necessarily be green. 
If, however, it should secrete some material that would dis- 
solve out the blue part of the green, either wholly or in part, the 
butterfly would be yellow, either all over or in spots. If, however, 
