EVOLUTION OF THE COLORS OF BIRDS. 181 



feathers. Still there is probably a general tendency to 

 repeat the same pattern on both sides of the body, just 

 as there seems to be a tendency to repeat a general style 

 of marking on the lower that has been produced on the 

 upper part of the body. The asymmetry of hybrid 

 flickers is an abnormal case, and would hence not be an 

 exception to the rule of bilateral symmetry, which is 

 intended only for normal forms. 



Although a streak may occur either just over or 

 through the eye, I know of no instance where a streak 

 runs near or through the eye at right angles to the su- 

 perciliary stripe. I knowof no species which has either 

 the throat or the top of the head barred, nor can I think 

 of any instance where any definite pattern of color 

 occurs on the back proper. Neither does any instance 

 come to me where a single streak down the middle of 

 the back (as in Dryohates piibesccns) is continued without 

 interruption over the top of the head to the bill, nor 

 where a similar streak on the under parts, extending 

 down the throat and median breast, (as the carmine of 

 Pyrrhuloxia sinuata) extends also down on the abdomen 

 and under tail coverts. I can think of no species 

 in which a single transverse band or bar crosses the back 

 below the nape (where it occurs in Dolichonyx oryzivorus, 

 for example), nor of any instance where such a band or 

 bar crosses the under parts of the body below the breast. 

 No instance is recalled where large spots of any kind 

 occur either on the throat or head. 



It may seem like an idle task to enumerate these forms 

 of marking which do not occur, but by next noting what 

 styles are most common, the two will be brought out in 

 strong relief. The head is by far the most complexly 

 marked part of the body. Of the head markings the 

 superciliary stripe is the most universal, being char- 

 acteristic not merely of birds but also of many reptiles 



