EVOLUTION OF THE COLORS OF BIRDS. 169 



along the outer edge. In the ninth primary this outer 

 white patch has vanished, the one on the inner edge 

 persisting in a long line on the edge of all the second- 

 aries. The first secondary is well marked off from the 

 last primary by a spot of white at the terminal end of 

 the white border. A second spot is gradually constricted 

 off from the end of the border, and the first one dwindles 

 into insignificance. 



It is time now to ask the meaning of this successioiial 

 taxology in the color pattern. If the successioiial tax- 

 ology represents different evolutionary stages of pro- 

 gression, as I believe it does in some cases, it is easily 

 understood. For example, in the feathers of Melanerpes, 

 previously described, it is only necessary to assume that 

 the under part of the body is becoming black, but the 

 tendency has been stronger on the upper part of the 

 breast than lower down, to understand this successional 

 relation perfectly, as different degrees of complete- 

 ness in evolution in different parts of the body. This 

 is a beautiful example of Cope's law of acceleration and 

 retardation. The growth force has been accelerated 011 

 the upper breast and retarded on the belly. With re- 

 gard to wing feathers, however, the explanation is not 

 so simple. Here, instead of one part of the body ex- 

 hibiting a general stage of development, and- other parts 

 successively higher stages, we find feather following- 

 feather in uniform steps of transition, when in many 

 cases it is simply impossible to say that one stage is 

 higher than another. Certainly there is not an ideal 

 mode of coloration being approached through successive 

 stages, as is the case with feathers becoming black, but 

 rather a variety of individual patterns combined to 

 produce one effect. 



Thus far little has been said of general effects. The 

 question has been discussed as if it were supposed that 



