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III. Observations on the Laws which appear to influence the Assumption and Changes of 
Plumage in Birds. By Witi1aM Yarrett, Esq., F.L.S. & Z.S8. 
Communicated February 26, and April 23, 1833. 
THE changes of plumage observable in birds at certain seasons, have of late years oc- 
cupied much of the attention of ornithologists ; and among others of our own country- 
men, we are greatly indebted to the late Colonel Montagu for a long series of obser- 
vations, by which the difficulties of tracing specific identity in many instances were suc- 
cessfully cleared up, and various periodical appearances distinguished and described. 
These interesting changes have their origin in various causes, and are produced by 
different means: some are the effect of age, others of sex, season, or disease ;: some- 
times they are produced by moulting, or discharging the old feathers and obtaining new 
ones, but more frequently by one or other of two different processes. 
Before, however, proceeding to explain the manner in which changes are otherwise 
produced in the plumage of birds, it may be considered necessary to say a few words on 
the feathers themselves. 
The bulb or pulp, which is the foundation of each feather, has its origin in a gland or 
follicle of the skin ; and as the pulp lengthens, this gland or follicle is itself absorbed. 
The pulp still lengthening becomes invested on its outer surface with several concentric 
layers of condensed cellular membrane, from which the shaft, the filaments of both 
lateral webs, the colouring matter and the horny quill are severally produced ; but ana- 
tomists appear to differ a little in opinion as to the exact manner in which the growth 
of the various parts takes place. The pulp, which nearly fills the barrel of the quill 
while the feather is forming, is connected with the body of the bird by an aperture at 
that end of the quill which is fixed in the skin, through which aperture or umbilicus a 
portion of the pulp is extended. The whole of the pulp, within as well as without the 
quill, is the only part of the feather which appears to be vascular, and the large feathers 
of the wing may be injected, while growing, from the humeral artery ; but the feathers 
once perfected, the injection can no longer be sent even into the pulp. The membranes 
of which it was composed, the former nidus of vessels now obliterated, dry up, contract, 
and ultimately separating transversely into funnel-shaped portions, (which remain in 
the barrel of each quill,) are well known by the familiar term of pith. 
A perfect feather presents many points of interest, if we consider its various parts, 
form, colour, strength, lightness, durability, and the peculiar manner in which the 
fibres of the web lock in with each other to afford continuity of surface. The accessory 
plume also requires to be noticed. This is usually a small downy tuft, which not only 
