288 University of California Publications in Zoology. [VOL. 6 



of any series of birds ranging through half a dozen families will 

 give quite contradictory results. The crown will be found white 

 on some, black on others; the breast usually light colored, but 

 sometimes with a spot or chain of spots, or the throat spotted and 

 the breast plain ; and the rump white in some, heavily pigmented 

 in others. Only patient ingenuity could long persist in seeking 

 evidence for the correlation of pigmented areas with regions of 

 fuller circulation; and the seeker for such evidence must shut 

 his eyes to the patent fact that to produce a monochrome plumage 

 such as that of the Leconte thrasher the dermal circulation 

 must be uniform over the entire surface of the body ; the existence 

 of the theory would seem to demand the denial of such a con- 

 dition. 



Cunningham's suggestion (1900, p. 109) of a local stimulus 

 incident to the elevation of plumes, etc., in courtship, is useful 

 in accounting for the special development of certain epidermal 

 features, but it sheds no light on the cause of intensification of 

 color in the breeding male, in the plumage as a whole. The 

 vigor theory attempts to do this, but leaves untouched a large 

 number of instances in which there exists no sexual dimorphism 

 in coloration, and still others in whhh the female takes over not 

 only the color character, but also the courting antics of the 

 male, leaving him the responsibility for incubation, as is the 

 case in the knot (Tringa canutus). 



The other class of intrinsic factors in color evolution grouped 

 under the general term heredity, is one which this paper will 

 merely recognize as existing, a potent but little understood influ- 

 ence. If it can be shown that white markings do not occur 

 indiscriminately on birds of all habits and environments, but are 

 associated with similar habits and ranges among birds not 

 otherwise closely allied, it will be clear that they cannot be 

 regarded solely as evidences of the manifestation of hereditary 

 tendencies, but may fairly be attributed, at least in part, to 

 selective influences. 



Whatever the original cause of pigmentation or absence of 

 pigment and its replacement by structural colors, and whatever 

 the mode of evolution of such colors and combinations, the as- 

 sumption of a principle of natural selection everywhere efficient 



