Fatio on Feathers — their Decoloration. 173 



" If the tissues which are entirely filled with a coloured 

 solution are able to distend themselves under the influence of 

 moisture, no extravasation will occur until the new spaces are 

 filled by the freshly dissolved pigment. But there is a limit 

 to the cortical development of a feather, nearer or more re- 

 mote. Optical feathers,* which possess most of this cortical 

 substance, exhibit a later and less complete extravasation, while 

 mixt feathers, possessing less of this substance, exhibit it more 

 speedily." 



" A young gull, Larus ridibwidus, has in the summer a first 

 plumage which is almost entirely brown, and in its first spring 

 it is nearly all white, without any true moult having occurred 

 in the greater part of its feathers. An observation of the 

 plumage of this bird at the end of autumn discloses all the 

 transitions between the two states of colour, and the micro- 

 scope explains the cause of the transformation. The brown barbs 

 and barbules are filled with a very diffused brown pigment ; 

 a brown dust covers the exterior of each part of the feather in 

 proportion to the extent of the decoloration, not being seen 

 on those which are quite brown or quite white. The decolor- 

 ation pursues an opposite course to that of the coloration ; it 

 proceeds from the base towards the extremities, and from the 

 centre to the periphery, instead of moving from the borders 

 towards the middle of the feather. It is a continuous change 

 of a colourless grease for a coloured pigment, of which at last 

 only a little remains in the centres, giving a slight tint to the 

 feather. 



" Similar phenomena occur in most of our birds, and to 

 such actions we must ascribe the case mentioned by Brehm of 

 some starlings, in which a black colour was hidden under a 

 white external dust. - " If we place under the microscope, be- 

 tween two pieces of glass, a feather in process of decoloration, 

 and a drop of oil, the process may be seen going on, made more 

 active by slight heat and retarded by cold. 



" In spring, we often see young gulls whose white livery 

 is much less advanced than that of others of the same age : 

 they are individuals in which the decoloration commenced in 

 the autumn Las been arrested by the extreme cold. The 

 deeper a feather lies, and the more it is sheltered, the more 

 promptly extravasation takes place during the life of the bird. 

 The grease proceeding from the bird's body produces a colora- 

 tion from the periphery towards the centre, when it meets with 

 pigment that is already dissolved, or tissues already filled, be- 

 cause only one exchange is possible ; and the same grease pro- 

 duces a coloration from the periphery towards the centre, 

 because it is at the extremities that it finds first, and chiefly, 



* For an explanation of these terms, see the previous paper, Deo. No., p. 377. 



