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 earth- 

 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 larvae 

 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 comrnon example of this is the color of many butter- 

 flies whose larvae 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, 



