191 ] Tracy: White Markings in Birds. 287 



right in asserting that further evolutionary studies of bird 

 coloration should take their departure from this point. Neverthe- 

 less, if this be true, the gap between fundamental bars in feathers 

 and the varied patterns in which white appears in every con- 

 ceivable combination, often accompanied by intensification of 

 pigment deposits in adjacent plumage, is one which can only 

 be filled by difficult, expensive, and long continued research. In 

 the meantime the discoveries of Thayer (1909) have greatly 

 simplified the explanation from the selective standpoint, while 

 those of Reighard point to an intrinsic tendency toward variety 

 of pattern, needing only immunity such as that afforded by a 

 coral reef, to develop colors and contrast that are neither adaptive 

 nor due to sexual selection, and for which the physiological ele- 

 ments of nutrition, temperature, etc., do not account. Thus it is 

 probable that the production of bars and stripes in feathers, and 

 the rate of pigment deposit, are but secondary processes in a 

 larger scheme. They may themselves be dependent on selective 

 agencies which, if they exist at all, operate upon the whole 

 organism. 



If we were concerned here with the problem of bird color- 

 ation in general, we might linger upon examples of an apparent 

 physiological basis for dark coloration, such as that of most 

 species of Corvidae; these are largely omnivorous feeders, active 

 and of exuberant vigor. But we should certainly be wrong if we 

 failed to take account of the great degree of immunity which 

 these birds enjoy from the attacks of raptorial birds, because of 

 their size and aggressiveness. It is doubtful whether a seed- 

 eating bird of delicate flesh and harmless disposition could have 

 been permitted to develop such a black plumage as that of the 

 raven, even had the physiological excuse for pigment excretion 

 been as great in its case. Some other way would have been found, 

 we may reasonably say, for the excretion of melanin, or else that 

 type of bird would have become extinct for lack of protective 

 coloration. 



Similar difficulties involve such special varieties of the 

 physiological explanation of color patterns as are connected with 

 color distribution on the breast, crown, rump, etc., of birds, 

 regarded as centers of high or of deficient circulation. A study 



