EVOLUTION OP THE COLORS OF BIRDS. 159 



the Mniotiltidte, and blue among the Corvida3. When 

 any of these colors occur in considerable masses in 

 combination with some other color, it is generally with 

 brown, black or olive green, and seldom with any other 

 very pronounced shade. 



• 



PATTERN OF MARKINGS. 



In the preceding discussion of the distribution of color 

 masses it has been found difficult to keep clear of a 

 closely related subject, to which attention must next be 

 directed, viz.: the pattern of markings, both as distin- 

 guished from and as related to their colors. These 

 markings must be regarded in two ways; first, as to the 

 pattern, mode of development, etc., of the markings 

 upon individual feathers, and second, the general effect 

 of feather markings in the pattern of the entire bird. 



In an investigation of color patterns, the laws of growth 

 and structure stated in the introduction, should be kept 

 in mind. The law of growth* force or bathmism is 

 especially important in this connection. Pigment is a 

 definite chemical substance which travels through the 

 various branches of the feather, advancing farthest 

 and most rapidly along the lines of least resistance and 

 accumulating in masses where the resistance is greatest. 

 Now the pigment cells must reach the various parts of 

 the feather by way of the shaft, and we should a priori 

 expect to find that the resistance would be least right 

 down the shaft. It might spread out a very short 

 distance on the barbs but the main tendency would be 

 toward the tip. This would produce a streaked feather 

 as the most primitive form. Having reached the tip it 

 would encounter an obstacle, and then would develop in 

 mass. This route of the pigment might be illustrated 

 by a panic-stricken mob running down a street. They 

 would naturally keep in the road where travel was easiest 



