362 CLASS xvi. 



As in mammals, before changing the milk-teeth, the rudi- 

 ments of the permanent teeth are present, so on the moulting of 

 birds there already exists in the base of the dermal cell a vesicle 

 under the old feather, from which subsequently the new feather 

 proceeds. As in the annual decadence of the horns in stags so also 

 in that of the feathers, the occurrence is in close connexion with the 

 sexual function. In like manner as castrated stags no longer shed 

 their horns, so birds moult no more almost, when they have been 

 rendered incapable of propagating. The relation between the 

 sexual function and the feathers is moreover clearly indicated by 

 the well-known difference of plumage in the different sexes, whilst 

 finally, certain female birds, which from age have ceased to lay 

 eggs, sometimes assume a clothing, which resembles that of the males 

 of their species, as, for example, has been observed in pheasants 

 and ducks 1 . 



The moulting of birds usually occurs about the beginning of 

 autumn, when the brooding time has passed; many birds, those, 

 namely, which have a winter dress, change their feathers twice a 

 year, casting their winter dress in spring before the time of pairing. 



The feathers may be distinguished as contour-feathers (pennce} 

 and as down-feathers (plumulce] . The first are furnished with a perfect 

 quill and with firm branches and rays. The down-feathers have an 

 imperfect, short quill, and the rays, destitute of little hairs or hooks, 

 exhibit under the microscope darker knotty swellings, remote from 

 each other, and causing them to appear as though divided into long 

 joints. There are feathers, which, from the entire absence or slight 

 development of the vane, almost resemble hairs, and which are 

 distinguished as a third kind under the name of thread-feathers 

 (filoplumce). The quill-feathers are mostly placed in certain de- 

 finite regions (pterylce NITZSCH), between which other fields (apteria) 

 are situated, which are covered with down-feathers only, or are 



Ueber die Federbildung, REIL'S Archiv f. d. Physiol. xn. s. 37 96, with figures. 

 Compare also F. CUVIER Mem. du Mus. xm. pp. 327 368. (KECLAM De plumarum 

 pennarumque evolutione, Ace. Tab. in. Lipsiae, 1846, and G-. SCHRENK De formatione 

 pennce, Dorpati, 1848, are known to me only from REICHERT'S Jahresbericht in MUEL- 

 LER'S Archiv, 1847, s - 2 9> an( l l %49> s ' 2I 



1 BLUMENBACH Institutiones physiol., p. 508 (ed. 1798); ISID. GEOFFR. SAINT- 

 HILAIRE Mem. du Mus. d'Hist. not. xn. pp. 220 231, and Essais de Zoologie generate, 

 Paris, 1841, pp. 493 5 14, PI. 4 , 5. 



