and in so far as it continues, pigmentation may be altered, but once the circulation has ceased beyond the entrance to the base of the shaft, and once that the feather, although still attached to the epidermis, is cut off from the circulation in the deeper living layer of the skin, then the feather is no more likely or able to change the pigment which is responsible for its pattern or its colour than would be the same feather had it been plucked out and kept entirely separate from the bird.
Once the feather is full grown, and the circulation in it stopped, there is no reason to believe that any thing can alter it save sunlight and water, and oil supplied as an external unguent from the oil gland. That appearances are most deceptive in this respect must be allowed. Feathers may be collected from the flanks of hen Grouse which show every possible graduation Pigmentationbetween the almost vermiculate flank feather indicating the perfect winter plumage, and the broad-barred breeding-season flank feather of the summer hen. But it is very much more probable that the growing period of these ambiguous or intermediate feathers is one of great susceptibility to outside conditions, as we know to be the case in respect of the metabolic processes which are taking place within the hen bird at the time. Pigment is indisputably a product of tissue metabolism. It is often probably a mere waste product, but it appears at times to serve a special function notwithstanding. It is also certain that pigment is a production whose appearance, or failure to appear, is open to considerable vicissitudes in consequence of small recognised changes in physiological condition, and of some less easily recognised changes in the general metabolism of the body.
In the hen Grouse during the breeding season we know that pigment production is very actively at work, for we know that a very large amount is being produced for excretion in the pigment glands of the lower part of the oviduct. This pigment, moreover, is precisely of the shade and colour which is characteristic, not of the breeding plumage, but of the winter dress of the hen and the cock Red Grouse. It is normally deposited in abundance on every egg, but on the other hand it may abnormally fail to be deposited or even produced at all, not only in the eggs in the oviduct, but in the circulating blood of the bird's whole system. Thus the feathers, instead of becoming buff or brown, reddish or even black as they proceed in growth, may be any intermediate paler shade of buff, or even white, a character which is due generally to the complete absence of all pigment granules. The place of
