EXTERNAL PARTS OF BIRDS. — FEATHERS. 89 



condition, or as due to particular diet, or to direct artificial tincture ; this has been called 

 heterochrosis (Gr. fTepos, heteros, other, and xP«s«). The principal abnormal conditions are : 

 1. Albinism, in which the bird is white, wholly or in part; a " white Blackbird " is no mis- 

 nomer, and white Crows or Ravens are well known. This is the commonest affection of the 

 kinds now under consideration ; any bird may, and many birds do, become entirely pure white, 

 from failure of pigment ; such are called albinos. 2. Melanism, or abnormal blackness, from 

 excess of dark pigment. It is much less common than albinism, but by no means rare. 

 3. Xanthism (Gr. ^avdos, xatithos, yellow), or yellowness, as when a red, orange, or green 

 bird turns out more or less yellow. 4. Erytlirism (Gr. epvdpos, eruthros, red), or redness. 

 Both the last two cases are somewhat special ones, considered as abnormities. Feeding upon 

 cayenne pepper may produce erythrism ; in Brazil, where counterfeit species of Chrysotis, a 

 genus of Parrots, are fashionable, they "are produced by the rubbing in of the cutaneous 

 secretion of a toad, Biifo tructorius, into the budding feathers of the head, which then turn out 

 yellow instead of green" (Newton, Diet., p. 99). It should be noted that all these hetero- 

 chroses are abnormal; normal changes of plumage with age or season, and normal differences 

 of plumage, are treated beyond. Neither dechromatism nor aptosochromatism is here iu 

 question. 



Feather Oil Gland. — Birds do not perspire, and cutaneous glands, corresponding to the 

 sweat-glands and sebaceous follicles so common in Mammalia, are hardly known among them. 

 But their " oil-can " is a kind of sebaceous follicle, which may be noticed here in connection 

 with other teguinentary appendages. This is a two-lobed or rather heart-shaped gland, sad- 

 dled upon the " pope's nose," at the root of the tail, and hence sometimes called the uropyrjial 

 (Lat. uropygiiim, rump), or ruinp gland ; is also known as the elceodoclion (Gr. iKaiohoxoi, 

 elaiodochos, containing oil). It is composed of numerous slender tubes or follicles which se- 

 crete a greasy tluid, the ducts of which, uniting successively in larger tubes, finally open by 

 one or more pores, commonly upon a little nipple-like elevation. Birds press out a drop of oil 

 with the beak and dress the feathers with it, in the well-known operation called "preening." 

 The gland is present in most birds ; it is large and always present in aquatic birds, which have 

 need of waterproof plumage ; smaller in land-birds, as a rule, and wanting in some. The 

 presence or absence of this singular structure, and whether or not it is surmounted by a partic- 

 ular circlet of feathers, distinguishes certain groups of birds, and has become much used in 

 classification, as it was supposed to be related in some occult manner to the coeca of the 

 intestine. 



Pterylography. — Feathered Tracts and Unf eathered Spaces. — Excepting certain 

 birds having obviously naked spaces, as about the head or feet, all would be taken to be 

 fully feathered. So they are all covered with feathers, but it does not follow that feathers are 

 everywhere implanted upon the skin. On the contrary, a uniform and continuous pterylosis 

 is the rarest of all kinds of feathering ; though such occurs, almost or quite perfectly, among 

 certain birds, as Ostriches and their allies Penguins, and Toucans. If we compare a bird's skin 

 to a well-kept park, part woodland, part lawn, then where feathers grow is the woodland, 

 wliere they do not grow is the lawn. The former places are called tracts or pterylce (Gr. 

 nrepov, pteron, aphune, and v\r], huh, woods; literally, " feather-forests") ; the latter, spaces, 

 or «/??ena (Gr. a privative, and TrrepoV) ; they reciprocally distinguish certain definite areas. 

 Not only are pterylee and apteria thus definite, but their size, form, and arrangement mark 

 whole families and even orders of birds, so that pterylosis becomes available, and is indeed 

 found to be important, for purposes of classification. Pterylography, or the description of this 

 matter, was first (1833) made a special study by the celebrated Nitzsch, who laid down the 

 general plan of pterylosis which obtains in the great majority of birds, as follows : 1. The 



