INTRODUCTION. 



that there exists another yellow-shafted species with red mystacal strips but not red nuchal 

 crescent, viz., Colaptes chrysoides. If this theory be correct, we would have a clew to 

 another class of dichromatic species, viz., those which now are stereotyped into two 

 invariable forms or species, separated geographically, but still identical in structure. 

 We shall only mention an example recently brought forward by Mr. Ridgway, that of 

 the scarlet and the white ibises (Gruara rubra and alba), of which he very character- 

 istically remarks that they are now so different in color that probably nobody would 

 deny their specific distinction, though structurally so alike that a specimen of the 

 white one dyed scarlet would be indistinguishable from G. rubra. The question which 

 finally impresses itself upon the inquirer, in view of the above facts, is this: Are not 

 the two or three ' phases ' of dichromatic or trichromatic species ' incipient species,' 

 the final fate of which will be that of the white and the scarlet ibises? 



We have enlarged considerably upon this subject, because it is one of the most 

 perplexing, and, consequently, most interesting questions in modern ornithology. It 

 shows what we know, and particularly what we do not know ; it shows that ornithology 

 means more than a mere description and naming of birds, that one of its aims is to con- 

 tribute to the solution of the great problem of the age : " The origin of species," 



Besides feathers, we recognize in birds other epidermal appendages, as the horny 

 sheaths of the beak, the teeth in some extinct forms, the scaly covering of the feet, spurs, 

 and nails. Most of these different structures will be more advantageously treated of in 

 other connections, and under the head of such groups in which they may be of special 

 interest, although we wish here to call attention to the fact that parts of the horny 

 beak and the nails of the toes may be shed in a way analogous to that of the molt 

 of the feathers, referring, as we do, to the deciduous nature of the basal parts of the 

 bill in several members of the auk family (puffins and dwarf-auks), to the ' centre- 

 board ' of the white pelican's bill, and to the seasonal claw-molt in the grouse-family, 

 particularly the ptarmigans. The most primitive form of the horny covering of the 

 feet seems to be its division into uniform hexagonal scales, and is called reticulate ; the 

 next stage is when some of these scales fuse together, forming what is termed scuta, 

 or scutella, which particularly cover the anterior part of the tarsus and the upper sur- 

 face of the toes ; still further specialization is indicated by the tarsal scuta fusing into 

 a continuous covering which, in its extreme development, embraces both the front and 

 the back of the tarsus, as in some of the higher group of passerine birds ; such a 

 tarsus is said to be ' booted.' 



It has already been remarked that the skin has no sudoriferous glands nor sebaceous 

 follicles; but we cannot dismiss the dermal system before having mentioned the bilobed 

 oil-gland placed at the base of the tail-feathers on the ' pope's nose,' and seldom miss- 

 ing, as it is in the ostriches apd some few other birds. When 'preening' their 

 feathers, birds press the fatty substance out of this oil-box with their beaks, and by 

 p:issing each feather between the mandibles, anoint the whole plumage in order to 

 keep it in repair and protect it against getting wet, as particularly noticeable in water 

 birds. 



Turning now to the other structural systems of the bird's body, it is not our inten- 

 tion to enlarge upon or even mention such general features as are regularly found in 

 the text-books, only those being deemed worth our attention, in the present connection, 

 which are of particular importance for an intelligent understanding of modern orni- 

 thological classification, or questions which at present are most occupying the lovers 

 of our beautiful science. 



