COLOR AND PATTERN IX ANIMALS 403 



them and looking over some of the evidence adduced for their 

 support, as well as some of the criticism leveled at them, \ve 

 may advisedly look to the actual physical causation of color 

 in animals. Whatever the use or significance of color, our 

 understanding of this use must be based on a knowledge of the 

 method or modes of its actual production. 



Color in organisms is produced as color in inorganic nature 

 is. Certain substances have the capacity of selective absorp- 

 tion of light rays, so that when white light falls on them, 

 certain colors (light waves of certain length) are absorbed, while 

 certain others (light waves of certain other lengths) are re- 

 flected. An object is red because the substance of which it is 

 (superficially) composed, reflects the red rays and absorbs the 

 others. Certain other objects or substances may produce color 

 (be colored) because of their physical rather than their chemical 

 constitution; their surface may be composed of superposed 

 lamellae, or it may be so striated or scaled that the various 

 component rays of white light are reflected, refracted, and 

 diffracted in such varying manner (at different angles and 

 from different depths) that complex interference effects are 

 produced, resulting in the practical extinguishing of certain 

 colors (waves of certain length) or the reflection of some at 

 angles so as not to fall on the eye of the observer, and so on. 

 Such colors will change with changes in the angle of observa- 

 tion, and are the so-called metallic or iridescent colors. These 

 two categories of color have been aptly called chemical and 

 physical: chemical color depending on the chemical make-up 

 of the body, physical on its structural or physical make-up. 

 As a matter of fact we shall find that most animal colors 

 are due to a combination of these two kinds. 



(Substances that produce color by virtue of their capacity 

 to absorb certain colors, and reflect only certain others, we 

 may call, in our discussion of color production, "pigments"; 

 and "pigmental" may be used as practically synonymous with 

 " chemical" in referring to colors thus produced, while "struc- 

 tural' may be used as synonymous with "physical' in 

 referring to colors dependent on superficial structural character 

 of the insect body. For colors produced by the cooperation 

 of both pigment and structure, "combination' or "chemico- 

 physical" may be used as a defining name.) 



Now in all animals, color depends on the presence and ar- 



