140 CALIFORNIA ACADEMY OF SCIENCES. 



necessary to depend upon appearances, and here there 

 are many facts that seem to support the view. Thus 

 Plates XVIII and XIX seem to be in accordance with it. 

 It will be noticed that in the genus Dendroica olive green 

 is, especially in the female and young, the prevailing 

 color of the group. It is not a little significant that in 

 coloring these two plates it was found that in order to 

 produce this color all that was necessary was to mix 

 black with gamboge yellow, in about equal proportions 

 — black and gamboge being the two most characteristic 

 colors of the specialized males. 



It might at first sight appear as if this was a modifi- 

 cation from complexity to simplicity, but this is not 

 really the case. If the system of the bird from which 

 the present genus Dendroica was evolved normally pro- 

 duced two pigments which were lodged in the integument, 

 unless some controlling and distributing force were 

 brought to bear upon them, they would naturally be 

 mixed and combined at first, and only after a long pro- 

 cess of selection would the real colors become apparent. 



This principle may be termed the Law of Assortment 

 of Pigments. If true at all it will be found to have a 

 very wide applicability in the evolution of colors of 

 birds, and, indeed, to underlie all the other principles 

 of color differentiation, although, of course, conditioned 

 by all the more general laws of evolution. The best 

 examples of it are to be found, as might be expected, 

 among the most highly specialized genera, where the 

 colors have changed from some dull hue to distinctly 

 colored patterns. Thus woodpeckers are generally col- 

 ored black and white and scarlet. The combination of 

 the black and scarlet would produce a brown, which is 

 still the body color of some species, as jDrijobnJcs urizoiw. 

 In the genus Tyrannus the original color was probably 

 an olive green, although in many instances where the 



