280 University of California Publications in Zoology Vor. 18 
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 as 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 strue- 
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 
