2 NATURAL HISTORY OF BIRDS. 



turtles and the duck-mole similar beaks, but we know now that teeth were as common 

 in certain groups of extinct birds as they are in reptiles or mammals nowadays. Nor 

 is the laying of eggs and their hatching an exclusive characteristic of the feathered 

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

 vegetable matter heaped upon them, while the latest indications are that the old report 

 of the Monotremes laying eggs, hitherto regarded as a fable, is substantially true. 

 The so-called * pneumacity ' of the bird-skeleton, or the peculiarity of the bones being 

 hollow and filled with air through 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 and some other forms were also filled with air, air-canals being present 

 in nearly all the bones of the skeletons of the larger species, while several recent 

 birds, for instance the kiwis and the penguins, are entirely destitute of pneumacity in 

 any part of the skeleton. 



We will mention one more character which cannot be upheld as peculiar to the 

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

 bones of the skull grow together at an early age, fusing so completely that the borders 

 of the individual bones are completely obliterated, while in most other vertebrates 

 these bones remain separated by sutures during the whole lifetime of the animal. 

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

 which the sutures were permanent, while, on the other hand, all tends to show that 

 the ancient Pterosaurians had the different pieces of the skull fused together as com- 

 pletely and as early as any bird now living. 



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

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

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

 popular verdict would probably be in favor of regarding the hairs and the feathers as 

 more resembling one another than either of them do the scales, particularly when we 

 remember the many hair-like appendages in birds. Scientific investigation, however, 

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

 scales and feathers has been frequently used as a further argument for the close relation- 

 ship between reptiles and birds, the scale-like feathers along the edge of the penguin's 

 wing being regarded as a structure intermediate in character between the two kinds of 

 integument and a proof of their common origin, while much stress was laid upon the 

 differences between hair and feather. True, the latter differ radically, particularly in 

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

 feather originates on the top of a large papilla ; but the homology of the latter with the 

 scales of the reptiles is not therefore a sure thing, and Mr. 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 remarked that 

 the above-mentioned scale-like feathers of the penguin are in every respect true feathers, 

 and not half feather, half scale. 



Young birds, when breaking the egg enclosing them, vary greatly in their develop- 

 ment, some being quite naked, as, for example, most Passeres, Picarias, herons, and 

 cormorants, but soon assuming a more or less full covering of soft down, which again 

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

 is perfected within the egg-shell, while the final feather plumage is put on afterwards ; 

 the former are called Gymnopoedes (gymnos, naked ; paides, children) ; the latter 

 group ; Dasypffides (dasys, downy). All the Gymnopsedes are fed in the nest by the 



