COLOUR IN BIRDS AND QUADRUPEDS. 211 



quill. This shaft is now filled with air. After this process, the ob- 

 ject of nature in providing covering and the means of flight to the bird, 

 seems to have been accomplished, and the feather ceases to grow. 



In order to renew the nourishment of the quill, it would be neces- 

 sary to renew the vascular connection. It is doubtful whether any 

 revivescence of colour can take place without this. As long as this 

 lasts, the assimilative powers within the feather may continue to act. 

 When this ceases, these are extinguished with it. As the vascular fluid, 

 then, which fills the sheath and the quill gradually disappears, as also 

 the bulb which nourishes it, after the feather has arrived at full ma- 

 turity and becomes stationary in its growth, it would appear that after 

 this period no further change of colour can take place. This point, 

 however, has not been so satisfactorily investigated by physiologists, as 

 to enable us to express a positive opinion, and, until this is done, we 

 are obliged to resort to an examination of individual species, in order 

 to ascertain how far there is a uniformity in these changes of colour 

 in all birds. 



The fact that birds in winter are, by a wise Creator, furnished with 

 a thicker covering than they possess in summer, does not appear to 

 have received that attention from naturalists who have made inquiries 

 on the subject, that its important bearing on this point seems to require. 

 Several of the birds subject to these semi-annual changes in colour, 

 spend their winter in climates comparatively cold. The Sylvia coro- 

 nata is seen by thousands in Carolina, where the thermometer, during 

 winter, stands for several days in succession, below the freezing point. 

 The Fringilla tristis also braves our coldest winters ; and the Saxicola 

 sialis and Sylvia petechia linger among our copse woods and orange 

 groves. These birds are clothed with a covering suited to the rigorous 

 season. In summer their dress is lighter, as well as more gaudy. If, 

 then, there is no dropping off" of feathers, it would be reasonable to in- 

 quire what becomes of their winter clothing? This would have to be 

 borne through all the heats of summer, till the moulting season; and 

 their thick blanket of down would damp their ardour, and silence many 

 a joyous song. 



I am well aware, however, that we may build theories, and indulge 

 in speculations, which, however plausible they may appear, and however 

 Yi. — 3 c 



