EVOLUTION OF THE COLORS OF BIRDS. 163 



With regard to this secondary pigmentation, meta- 

 bolism may be introduced as an explanation. When a 

 species is katabolic the surplus of energy might be ex- 

 pended in the formation of pigment. This would be- 

 come distributed in the feathers in accordance with the 

 laws determining the location of growth force. It would 

 become fixed there by the conservative laws of heredity, 

 and then a state of anabolism might follow. At the 

 beginning of the next katabolic period, in accordance 

 with the rhythm of metabolism of Geddes and Thomson, 

 more pigment would be produced and again distributed, 

 this being the secondary pigmentation of Hacker. 



Let us see now how the facts agree with these theories. 

 The first group of feathers on Plate II is a series from 

 the under parts of a Calif ornian woodpecker (Melanerpes 

 formicivorus bairdi) commencing on the sides and 

 extending up on the breast. It will be noticed at once 

 in how many respects it tallies with the ideal instance 

 previously considered. The feathers of the belly are 

 white, but along the sides and the upper portion of the 

 belly there is a black streak down the shaft. In some 

 instances the shaft alone is thus marked, but generally 

 the base of the barbs is similarly affected. On the 

 feathers located on the lower part of the breast the line 

 has become broader, and black pigment has appeared on 

 the borders of the feathers. Both areas of black next in- 

 crease, although the inner one spreads most rapidly, until 

 the white area has been confined to two small islands 

 which ultimately disappear in the clear black feathers 

 which form the patch upon the upper chest. This 

 sequence of stages is in accordance with the law of suc- 

 cessional relation (see ante, pp. 77-78), and undoubtedly 

 indicates the phylogeny of the black feathers. Certain 

 it is that the feathers of the abdomen are less developed 

 with regard to pigmentation than those on the flanks; 



