286 University of California Publications in Zoology. IT OL - 6 



synonym were there not good reasons for believing that the 

 white markings exposed in flight serve rather as a clue to the 

 direction taken by disappearing forms of birds, than as an aid 

 to recognition of the species. 



The investigation of the theory of directive markings in birds 

 is hampered by the impracticability of carrying on field experi- 

 ments such as those of Professor Reighard (1908) in pursuance 

 of a similar inquiry in regarding warning coloration in fishes. 

 Obviously it is out of the question to capture and liberate large 

 numbers of birds with a view of detecting natural reactions under 

 such artificial conditions. Field observations can, however, be 

 offered, but these, in the nature of the case, do not -afford evidence 

 which is experimentally precise. Much depends upon the indi- 

 vidual in the field the personal equation. This source of error 

 cannot be set aside by recourse to photographs, for the reason 

 that the crux of the problem of directive markings, in so far as 

 it concerns actual perception, is the effect of the moving white of 

 the wings or other parts of the flying birds. Photographic plates 

 show but the fixed effect of motionless patterns, and tell us 

 nothing as to their eificacy in attracting visual attention to those 

 patterns in motion. 



I have, therefore, narrowed the field to a discussion of the 

 revealing properties of flight-exposed white markings, and a 

 study of the occurrence of such markings throughout the order 

 Passeriformes, deducing from such occurrence a very suggestive 

 correspondence of coloration with habit and feeding range. I 

 have no direct evidence to offer as to the origin and mode of 

 evolution of the white markings in birds. It may not be amiss, 

 however, before proceeding further, to call attention to the 

 limitations of explanations based on intrinsic factors alone. 



INTRINSIC FACTORS IN THE EVOLUTION OF COLOR. 



Riddle ( 1908) has proved that the melanin deposited in 

 barbules varies with nutrition. He has also shown that funda- 

 mental bars in feathers may have their origin in rhythms of blood 

 pressure. He thus establishes a presumption in favor of a 

 continuously acting cause, apart from heredity, governing the 

 degree of pigment deposited in birds' feathers. He is perhaps 



