78 INTRODUCTION. 



delicate filaments. The former may be denominated the fila- 

 ments, the latter the filamentules, corresponding to the barbs 

 and barbules of the feather. These filamentules have the 

 same relation to the filament, their shaft, that the barbules of 

 the feathers have to their barb, and are, in general, equally 

 distichous ; but they enter into no connection, remaining per- 

 fectly loose, and, owing to the manner in which the shafts are 

 twisted, have the appearance of coming off all round them. 

 Unlike the barbules of the feather, they generally come oflf at 

 right angles to the filament. 



As the down is well known to be a bad conductor of caloric, 

 it is presumed that it serves in the aquatic birds, and particu- 

 larly in those of cold climates, to retain the heat generated in 

 their bodies. In birds which are not furnished with down, 

 but which yet inhabit cold countries, the deficiency might be 

 supposed to be supplied by the downy feathers which w^e ob- 

 serve in those birds, as in the Snowy Owl, White-tailed Sea 

 Eagle, and Golden Eagle. In the Gallinaceous birds, the ac- 

 cessory feather might, in like manner, be imagined to be sub- 

 servient to this purpose. But when we reflect that the Eagles, 

 Owls, and Gallinaceous birds of cold climates are at least not 

 much better furnished with down or downy feathers than 

 species of the same genera inhabiting warm climates, we na- 

 turally look for some other reason for which birds are fur- 

 nished with down. In most of the land birds there is no uni- 

 versal layer of down immediately covering the skin. 



In the Falconine family, and especially the larger spe- 

 cies, besides down of the above nature, there exist plumules of 

 the following description. From the upper part of a short 

 tube there issue two filamentary shafts, which are flattened 

 and exceedingly delicate. From these there branches out on 

 either side a series of extremely delicate filaments, having each 

 two lateral series of filamentules. The whole has the appear- 

 ance of a single tuft of extreme fineness, and silky texture. 

 The filaments have a considerable degree of elasticity. The 

 tube is open above, between the two shafts, there being a direct 

 continuation of it on either side into the shafts, and at this 

 opening the pith comes out and terminates. These plumules 



