EVOLUTION OP THE COLORS OF BIRDS. 179 



We come next to the general patterns of bird colors. 

 These are so infinitely varied and complex that it will 

 be almost impossible to bring them under any universal 

 law or laws, but at least a few generalizations must be 

 made. Eimer has stated^' '' that the appearance of new 

 characters always takes place at definite parts of the 

 body, usually the posterior end, and during develop- 

 ment — with age — passes forwards, while still newer 

 characters follow after from behind. Thus during life, 

 e. g. in lizards, a series of markings pass in succession 

 over the body from behind forwards, just as one wave 

 follows another, and the anterior ones vanish while new 

 ones appear behind." This development from posterior 

 to anterior is illustrated by many examples among birds, 

 but seems to me to have far too many exceptions to be 

 held as as a general law of development. The succes- 

 sional taxology of markings on the under parts of many 

 of the hawks, conforms to this rule. Upon the breast 

 the markings are simple streaks, but from this stage 

 they pass into spots and from spots into bars which 

 are most distinctly marked upon the lower part of the 

 flanks and belly. This is especially well shown in the 

 western red-tailed hawk {Bideo bovealis cahtrus). It is 

 also well illustrated by a large number of the wood- 

 peckers, especially in the genera Dryobates and Picoides 

 which have the flanks barred and the breast streaked or 

 plain. The reverse is true with the Californian wood- 

 pecker, where, as we have already seen, the markings 

 simplify instead of specialize as we apj^roach the abdo- 

 men (see ante p. 163 and Plate II, figs. 1-6). So also in 

 the lark bunting (Calamospiza riielanocorys), the specializ- 

 ation begins at the anterior end. In immature males the 

 throat first becomes black, and in a series of males it is 

 found that the tendency is for it to spread from this 



* Organic Evolution, p. 28. 



