280 University of California Publications in Zoology fVor. 13 



Part II 



SYSTEMATIC 



Introduction 



After making a careful study of the modifications of plumage 

 of a single individual bird of a representative species, namely, Circus 

 hudsonius (Chandler, 1914), and after making a general survey of 

 the entire class of birds to find out in how far the phenomena there 

 found are applicable to birds in general, a systematic study was 

 made of each order of birds in succession to find out what, if any, 

 modifications of feather structure were characteristic of, or peculiar 

 to, the order or other group in question, and to determine the ex- 

 tent of variation to be found in the group, and to work out, if 

 possible, the probable phylogenetic relationships on the basis of 

 feather structure. 



1. Intraspecific and Phylogenetic Modifications 



At the outset it was necessary to determine how much individual, 

 seasonal, or sexual modification in structure might exist within a 

 species. Examination of a series of birds, in any group in which 

 this has been attempted, shows conclusively that the corresponding 

 feathers of any individuals of a species normally show no appreci- 

 able variation from each other, providing the age, sex, season, and 

 other conditions of the specimens be comparable. In other words, 

 comparable specimens of a species possess a definite, typical feather 

 structure which is normally invariable, as much so a^ are the muscles, 

 bones, or any other system of the body. Abnormalities and wear 

 may produce considerable changes, but they need not be considered 

 at length here. Fault bars in feathers, for instance, resulting from 

 inadequate nutrition or some other unfavorable condition, produce 

 areas of imperfectly formed barbs; albinism, as shown on page 279, 

 makes a feather which normally possesses a modification in struc- 

 ture for the production of color revert to the normal species type ; 

 wear and soiling often give the minute structure a very different 

 appearance ; and it is possible that other foreign influences may 

 considerably alter the form and structure of feathers, but these all 



