18 



STRUCTURE AXD CLASSIFICATION OF BIRDS 



group are found in all the divisions of birds founded upon 

 the distribution of the downs. This view throws a side 

 light upon the Struthiones. The feathers of those birds have 

 been called intermediate between contour feathers and 

 downs. It may be that they are primitive, and that the 

 struthious birds have arisen from some ancient type in 

 which the modern bird's feather had hardly been evolved- 

 Among nearly related families the details of pterylosis do at 

 least sometimes afford indications of resemblance. Thus, 



for instance, there are certain small 

 likenesses between the barbets, 

 toucans, and woodpeckers (see 

 below), which help in establishing 

 the near kinship between the three 

 families. 



The size or the presence or 

 absence of the after shaft appears to 

 be of little use for systematic pur- 

 poses. Among the ducks, for ex- 

 ample, some have it and some have 

 it not. It is as large as the main 

 feather in the emus and totally 

 absent in Rhea. Facts like these, 

 which might be multiplied, throw 

 doubts upon the value of this 

 structure in classification. So too 

 with the oil gland ' and its feather- 

 ing or absence of a tuft. Cancroma, 

 which in other points of its structure conforms to the heron 

 type, is alone in that group in having a nude oil gland. The 

 gland is absent in some parrots, present in others. GAEROD 

 at one time thought that he could correlate among the 

 Pico-Passeres a nude oil gland with small caeca, and a tufted 

 oil gland with the absence of ca3ca ; to the vast majority 

 of picarian birds there is no doubt that the correlation does 



1 A. PILLIET, ' Sur la Glande Sebacee des Oiseaux,' &c., Bull. Soc. Zool. Fr. 

 xiv. 1889, p. 115 E. KOSSMANX, ' Ueber Talgdriisen der Viigel,' ZeitscJn: f. 

 iriss. Zool. 1871, p. 5lis. 



FIG. 4. FEATHEK SHOWING 



AFTEESHAFT (AFTER SCLATEB). 



