192 CALIFORNIA ACADEMY OF SCIENCES. 



course of time these inner marks might become of suffi- 

 cient size for natural selection to make use of, and then 

 they would rapidly increase in size. The tail of the 

 belted kingfisher (Ceryle alcyon) is an interesting in- 

 stance of this sort. When the tail is in a normal posi- 

 tion of rest, the two outer tail feathers almost or quite 

 touch. The bars on these two feathers are so symmet- 

 rically placed that they invariably meet, forming one 

 continuous line. On the outer web, just opposite the 

 tip of each bar, is a spot of white. This complex ar- 

 rangement has obviously been due to selection, for by 

 no law of pigment distribution could the bars on one 

 feather be made to match the bars on the opposite feather 

 with such perfect accuracy. On the innermost feather 

 of the tail the bars are present, but so very faint as to 

 be scarcely discernable, and on the successive feathers 

 passing outward they become more and more sharply 

 defined. 



Certain birds display a curiously converse form of tail 

 marking, having the greatest specialization on the two 

 inner tail feathers. This is markedly the case with 

 some of the woodpeckers, such as Sphyrapicus varius, 

 which has the outer tail feather barred distinctly on its 

 outer web, and less so on the inner web, this marking 

 repeated but much less complete on the next feather 

 within, followed by two feathers entirely unmarked and 

 with the two inner feathers the most strongly barred of 

 all. Sometimes these two inner feathers are barred on 

 both outer and inner web, the black predominating on 

 the former, and the white on the latter (young); while 

 sometimes the outer web is black without marks, and 

 the inner web white with narrow bars of black (adult). 

 Of course, this instance is at variance with the rules, but 

 becomes intelligible when it is seen that the inner webs 

 of the upper tail-coverts, and sometimes the entire 



