14 MR. W. YARRELL ON THE LAWS WHICH APPEAR TO INFLUENCE 
assumes a very different character in the feathers of different species, but is even very 
dissimilar in the feathers of different parts of the body of the same bird. The accessory 
plume is situated at the distal end of the quill, at the aperture through which the shaft 
and its lateral fibres have passed out, and at the central point from which the two lines 
of the web begin to diverge. In the strong feathers peculiar to the wings and tail, it 
remains a small tuft of down, as at first mentioned ; but in the feathers of the body in 
the Hawks, Grouse, Ducks, Gulls, and some others, it is to be found of all sizes, aug- 
mented in some species to the full extent of the feather from which it emanates. The 
four species of Struthious birds afford remarkable instances of the variety that occurs 
in this accessory plume, even in subjects so closely allied; and the rich Menagerie of 
the Society enables me to speak of them from personal examination upon living speci- 
mens. In the Ostrich the feathers have no accessory plume. In the Rhea there is a 
tuft of down. In the Emu the accessory plume is augmented to the full size of the 
principal shaft and web, and the feather of this bird is constantly and correctly repre- 
sented as having two plumes on one quill. In the Cassowary, besides the double shafts 
and webs from a single quill, as in the Emu, there is still an accessory plume, thus 
forming three distinct parts; and a feather so constructed is figured by M. Guérin in 
his ‘ Iconographie du Régne Animal’. (Oiseaux, pl. 48.) 
In young birds the first feathers are preceded in their passage through the skin by 
filaments of down; but after the first plumage, at the regular period of moulting, each 
old feather is the pioneer (gubernaculum) of that which is to follow. If the shaft of a 
principal feather becomes broken off, the bird ejects the stump with difficulty; a 
certain portion of shaft appears to be necessary to enable the bird to get rid of the 
feather. Though perfectly able to throw off the old feather if entire, it seems unable to 
cast off the smaller but mutilated portion, and no new feather comes through the skin, 
the orifice being occupied. Inflammation of the vessels in the part of the bird, and in- 
creased adhesion of the stump, are the consequences; and whether these portions of 
the feathers are allowed to remain, or too many of them drawn out at the same time, 
disease and some risk to life are the result ;—in the first instance from continued irri- 
tation, in the second from too great and sudden a demand upon the vital powers of the 
animal. The natural moult proceeds by degrees, and the large quill-feathers of the 
wings and tail are generally shed by pairs. 
The state of the plumage in birds, like that of the productions of the cuticle in other 
animals, man not excepted, is in general a good criterion by which to judge of the 
health of the body. 
The principal peculiarities of the feather thus briefly premised, it may be added that 
the time required to obtain that state of plumage which is considered characteristic of 
the adult bird, varies according to the species from one year to five; and that several 
birds build nests, and rear young, before they attain their adult plumage. 
Baron Cuvier has stated, that when the adult female bird differs from the male in the 
