282 University of California Publications in Zoology [Vol. 13 



able, in some cases being scarcely noticeable, wliilc in others it is 

 very apparent. In the Steganopodes, for instance, the difference in 

 feather morphology in some of the different families is very great, 

 while in the families of Passeriformes, which, as a matter of fact, 

 are hardly more than supergenera, it is extremely difficult to dis- 

 tinguish between even widely separated ones. This difference in 

 degree of differentiation also holds true for groups of higher rank. 

 As intimated above, to be comparable the feathers whose parts are 

 to be compared must be approximately similar, since there is fre- 

 quently more variation between different kinds of feathers on a 

 single body than between corresponding feathers of birds of different 

 orders. For example, the barbules of a remex of Larus differ in 

 their minute structure from those of a breast feather of the same 

 genus far more than they differ from those of a remex of a loon, 

 for instance. 



2. Classification Adopted 



The problem of what recognized system of classification to follow 

 in the study of comparative feather morphology presents itself at 

 this point. To the mind of the writer the system which represents 

 most clearly the true relationships of birds according to the present 

 status of our knowledge concerning them, and one that is coming 

 into very general favor with ornithologists in this country as 

 well as in Europe, is that presented by Knowlton and Ridgway 

 in the Birds of the World (1909). This classification, as stated by 

 Knowlton, is essentially the same as that used by Gadow (1891), 

 modified in some details by the later researches of ornithological 

 workers. Although this classification was adopted in the present 

 study as a mere working basis, it was found that as far as feather 

 morphology was concerned it is apparently a more natural grouping 

 than any other ; yet, as will be shown in the following pages, there 

 are some possible changes in it suggested by feather structure, and 

 a hypothetical revision of it, based primarily on the latter, will be 

 suggested at the close of this paper. 



In the systematic discussion of the various groups, the grouping 

 and succession used by Knowlton has been used with onlj^ two ex- 

 ceptions. The Struthioniformes, Rheiformes, Casuariiformes and 

 Apterygiformes have been included under a common heading Ratitae, 

 as has usually been done, while the Crypturiformes have been dis- 

 sociated from these and placed immediately after the Galliformes, 



