EVOLUTION OP THE COLORS OF BIRDS. 137 



Changes of plumage, then, may be classified as fol- 

 lows: 



I. Substitution of feathers. 



1. By renewal (moult). 



2. By addition. 



II. Modification of feathers (aptosochromatism). 



3. By shedding of tip (acraptosis). 



4. By change of pigment (acroptosis). 



( Different pigment. 



a. By addition. < . 



f Intensification of same pigment. 



b. By loss. 



The first class, namely, the substitution of feathers, 

 has not been discussed because its bearing upon our 

 subject is less direct, and because it is more familiarly 

 known. It need only be noted here that all birds have 

 an autumn moult, while some have a more or less com- 

 plete spring moult in addition, either in both sexes or 

 in the male only. 



GENERAL PRINCIPLES OF COLOR IN BIRDS. 



Having devoted so much space to the fundamental 

 problems underlying all transitions in animals, it will 

 not be possible to consider at any length the general 

 principles of coloration in nature. Indeed, the subject 

 has already been touched upon in discussing sexual 

 selection. However, a few words more are necessary. 



Colors may be classed, as Poulton has suggested, as 

 structural and pigmental, depending upon the manner 

 in which they are produced. The structural colors are 

 of three sorts those due to thin plates, one upon an- 

 other; those due to diffraction, where the surface is 

 broken by fine parallel grooves; and those due to re- 

 fraction, where the light passes through a transparent 

 wedge-shaped substance or prism. All of the above 



