EVOLUTION OF THE COLORS OF BIRDS. 195 



Let us now consider these various classes and see how 

 they may help in the elucidation of the color problem. 

 Omitting the more inclusive divisions as sufficiently 

 clear, let us commence immediately with (1) Procryptic 

 colors. This includes all of that large class of forms 

 whose colors harmonize with their environment for the 

 purpose of protecting them from enemies. In many 

 cases it is extremely difficult to say whether an apparent 

 adaptation to a given environment is due solely to nat- 

 ural selection, or solely to the direct action of the envi- 

 ronment, or to the co-operation of these two factors. In 

 many instances, such as the white of Arctic animals and 

 the pale color of desert forms, the latter view will prob- 

 ably be found to be correct; but as the influences of the 

 environment are to be discussed later, nothing more need 

 be said upon the subject at present. 



These protective colors may vary with a changing 

 environment, as is the case with the ptarmigan (Lago- 

 pus), which is dark in summer and white in winter, 

 and possibly with some of the goldfinches such as Spinus 

 psaltria, which is bright yellow and olive green during 

 the summer when the yellow Compositse, upon which 

 it is almost invisible, are in bloom, and buffy and olive 

 brown in winter when it is almost equally inconspicuous 

 among the dry weeds. It seems probable that this is 

 a case of a bird which acquired its j^ellow breast largely 

 by sexual selection, and afterwards this character was 

 made use of by natural selection as a variable protec- 

 tive color. The house-finch (Garpodacus mexicanus fron- 

 talis) varies with the seasons also, and its winter coat is 

 a protective one, but the summer dress does not happen 

 to be of a color which can be turned to account. 



By far the greater number of general protective colors 

 are constant, and harmonize with the most prevalent 

 environment onlv. Nearlv all of the Gallinaceous birds 



