362 University of California Publications in Zoology [Vol. 11 



a bird's body are taken into consideration, and it is realized that 

 all the feathers are renewed at least once a year, we may get 

 some idea of the astonnding productivity of the epidermis of a 

 bird. 



Briefly reviewing the ultimate feathers produced by the 

 different kinds of structure described, we tind that while in the 

 majority of cases it is easy to understand their function, and 

 we marvel at their wonderful adaptation for serving it, there are 

 a few cases where no adequate explanation for their presence 

 has been found. ]\Iost noticeable of the feathers in this category 

 are the filoplumes, especially when we consider their persistanee, 

 not only throughout most of the plumage of this species, but 

 throughout the entire order of birds. The circlet of especially 

 modified feathers surrounding the apex of the oil gland is equally 

 mysterious so far as its function is concerned. 



The perfect adaptation of the under covering of down feathers 

 to serve as a heat-insulation has already been mentioned. The 

 especially modified "powder-down" feathers are still an enigma 

 so far as their use in avian economy is concerned. Various 

 theories have been offered, but none of them seem reasonably 

 convincing. 



In the remiges we find the highest degree of efficiency 

 attained, and not the slightest detail of the mechanism of these 

 most important feathers has been omitted in their specialization. 

 Not only has the minute structure attained its highest develop- 

 ment and the maximum of perfection, in order to produce a 

 strong, resistant feather surface, but every detail of their 

 macroscopic structure and interrelations has been unerringly 

 attended to, so that they are thoroughly adapted to meet the 

 physical forces brought to play upon them during flight, and to 

 perform their work with the least amount of friction and wear. 

 The third and fourth primaries are elongated to produce a 

 pointed wing, suitable for the kind of flight indulged in by the 

 species ; the shaft, and with it the feather as a whole, is bent 

 inward and downward to produce a shallow concavity on the 

 under surface of the wing, which, of course, serves the same 

 purpose as the "spoon" of an oar; near the tip of the feather 

 the vanes are somewhat rotated on their axes so that during 



