2 NATURAL 11 1 STORY UF lilUDS. 



turtles ami tlic <liK-k-uu)k' siiiiihir beaks, but we know imw iliat teetli were as euininon 

 in eertain grouiis of extinct birils as they are in rej)liles or niainnials nowadays. Nor 

 is the layiiij; of eggs anil their hateliing an exclusive characteristic of the feathered 

 tribes, fur we have birds which leave tlie hatching to be done by the heat of decaying 

 vegetable mutter hea]ied upon them, while the latest indications are that the old re)port 

 of the ilonotremes laying eggs, hitherto regardetl as a fable, is substantially true. 

 The so-called '])neumacity ' of the bird-skeleton, or the jieculiarity of the bones being 

 hollow an<l filled with air thniugh the canals in connection with the respiratory organs, 

 has also been regarded as belonging to the birds only, but the bones of the extinct 

 Pterosaurians ami some other forms were also filled with air, air-canals being jiresent 

 in nearly all the bones of the skeletons of the larg<'r species, while seviral recent 

 birils, for instance the kiwis and the j)enguins, are entirely destitute of ]ineumacity in 

 any part of the skeleton. 



We will mention one more character which cannot be uplieM as ])ecidiar to the 

 birds in view of our present knowledge. It is well known that in liirds the different 

 bones of the skull grow together at an early age, fusing so conii>letely that tiie borders 

 of the individual bones are completely obliterated, while in most other vertebr.-ites 

 these bones remain separateil by sutm'es dining the whole lifetime of the animal. 

 Still there have been found remains of an extinct bird, the remarkable (iustornin, in 

 which the sutures were j)ermanent, while, on the other hand, all tends to show that 

 the ancient Pterosaurians had the different jiieees of the skidl fused together as com- 

 pletely and as early as any bird now living. 



.Since we thus have to fall back ujioii the feathers as the most distinctive feature 

 of a bird, a brief comment upon their structure and origin may not be out of place. 

 CoHijiaring the scales of reptiles, the feathers of birds, and the hairs of mammals, the 

 jiojpiilar verdict would jirobably lie in favor of rcganling the hairs and the featliers as 

 more resembling one another than either of them do the scale's, partictdarly when we 

 remember tlie many hair-like ajijiendages in birds. Scientilie investigation, however, 

 seemed to prove the correctness of quite the ojiposite view, and the alleged identity of 

 scales and feathers has been frequently used as a further argument for the elo.se relation- 

 ship between reptiles and birds, the sc:de-like feathers along the edge of the jiengnin's 

 wing bein"- regarded as a structure intermediate in character between the two kinds of 

 integument and a j)roof of their common origin, while much stress was laid upon the 

 dilTerences between hair and feather. True, the latter differ radically, jiarlicularly in 

 their early stages, for a hair is formed in a solid ingrowth of the ejiidermis, while tlie 

 feather originates on the top of a large ]>apilla ; but the homology of tlie latter with the 

 scales of the rejitiles is not therefore a sure thing, and }ih: J. A. .Jeffries has recently 

 brought forward arguments which indicate a different nature of the two structures, the 

 strongest being that feathers may grow upon scuta. It should also be remarkeil that 

 the above-mentioned scale-like feathers of the jienguin are in every res].ect true feafhei-s, 

 and not half feather, half scale. 



Young birds, when breaking the egg enclosing them, vary greatly in their develo|)- 

 ment, some being quite naked, as, for examjile, most Passeres, Picaria-, herons, and 

 cormorants, but soon .issuming a more or less fidl covering of soft down, which .igain 

 is replaced by (inner feathers; other kinds are not hatched before the downy clothing 

 is jierfeeted within the egg-shell, while the final feather jiliimage is juit on afterwards; 

 the former are called Gyniiiop:edes (f/>/i/iiios, naked; paiifes, children); the latter 

 group ; Dasypa;des (dasys, downy). All the Gymnopa'des are fed in the nest by the 



