EVOLUTION OF THE COLORS OF BIRDS. 165 



The back of Sphyrapicus varius is barred, and this 

 barred stage in the progression from the primitive 

 streak to the uniform black pigmentation may well be 

 due to correlation of pattern. The bars may be of utility 

 on the back, whereas they cannot be on the breast, be- 

 cause they are so concealed as to be almost invisible. In 

 Williamson's sapsucker {Sphyrapicus thyroideus), the 

 barred markings surrounding the black patch on the 

 breast of the female assert themselves very strongly, but 

 this may be simply an advanced form of correlation. 

 They are also much more conspicuous on the back in 

 this instance, and we should therefore expect to find 

 them correspondingly developed on the breast if it was 

 a case of correlation. In the Californian woodpecker, in 

 which the breast feathers do not pass through the barred 

 stage in becoming black, it is to be noted that the back 

 is never barred at any stage of its development. Figures 

 14-17 of Plate II show the transition from the barred 

 to the uniform black type taken from the border of the 

 black throat-patch. It may be remarked that these in- 

 stances illustrate Cope's law of parallelism previously 

 stated (see ante, p. 78), this last instance being especi- 

 ally instructive in this respect, because in the male 

 these parallel stages are no longer repeated as in the 

 female. 



The most systematic study of successional taxology in 

 the pigmentation of feathers can be made upon the wing 

 and tail,wdiere the exact relation of feathers is fixed. Here 

 the interesting fact is disclosed that all changes in the pat- 

 tern occur according to a defininite orderly sequence. 

 Plate III represents the feathers from the wing of a spar- 

 row hawk {Falco sparverius) in their natural order. Fig- 

 ures 1-5 are the feathers of the spurious wing. In fig. 1 the 

 barring is evident, but has not yet strongly asserted itself, 

 being irregular on the upper part of the feather. The sec- 



