i v INTRODUCTION. 



creatures of the air, and their organization has been fitted for the 

 purpose ; the larger birds, as the Pelican and others, are specially 

 organized for carrying their weight by air sacs under their breasts, 

 besides the bones in their body being rilled with air, which makes 

 them more buoyant, and facilitate respiration under various pressures 

 of the atmosphere. 



Just as is the hair or fur of a mammal or the scales of a snake the 

 feather is a horny production of the epidermis. According to Professor 

 Huxley, it is devolved within sacs from the surface of a conical papilla 

 of the dermis. The external surface of the dermal papilla, whence a 

 feather is to be developed, is provided upon its dorsal surface with a 

 median groove which becomes shallower towards the apex of the 

 papilla. From this median groove lateral furrows proceed at an open 

 angle, and passing round upon the under surface of the papilla, 

 become shallower until, in the middle line opposite the dorsal 

 median groove, they become obsolete. Minor grooves run at 

 right angles to the lateral furrows. Hence the surface of the papilla 

 has the character of a kind of mould, and if it were repeatedly 

 dipped in such a substance as a solution of gelatine and withdrawn 

 to cool until its whole surface was covered with an even coat of 

 that substance, it is clear that the gelatine would be thickest at 

 the basal or anterior end of the median groove, at the median 

 ends of the lateral furrows, and those ends of the minor grooves 

 which open into them ; whilst it would be very thin at the apices 

 of the median and lateral grooves and between the ends of the 

 minor grooves. If, therefore, the hollow cone of gelatine, removed 

 from its mould, were stretched from within, or if its thinnest part 

 became weak by drying, it would tend to give way along the inferior 

 median line opposite the rod-like casts of the median groove and 

 between the ends of the casts of the lateral furrows as well as 

 between each of the minor grooves, and the hollow cone would 

 expand into a flat feather-like structure with a median shaft as 

 a "vane" formed of barbs and barbules. In point of fact, in the 

 development of a feather, such a cast of the dermal papilla is formed, 

 though not in gelatine, but in the horny epidermic layer developed 

 upon the mould, and as this is thrust outwards it opens out in the 

 manner just described. After a certain period of growth, the papilla 



