384 Fatio on the Forms and Colours of Plumage. 



is, as we shall see, different from those of the, ordinary 

 barbules." 



" Moisture developes each part in proportion to its supply of 

 cortical substance. In the optical feathers the barbules, only 

 augment in diameter, and as each segment swells into a little 

 cylinder, more or less regular, we sometimes see the lateral hook- 

 lets swallowed up (noyes) in the distended mass, and disappear 

 more or less completely. In the ordinary feather, the barb 

 possesses the most cortical substance and becomes most 

 developed. Possessing all the qualities of the optical barbule, 

 it becomes swollen and coloured, sometimes immersed in its own 

 matter, but usually expelling its useless barbules." 



" The ordinary barb becomes much dilated like the optical 

 barbule ; but instead of increasing like it, the proportions of 

 its segmentation and separating diaphragms, it loses in its 

 development the character which is associated with the persist- 

 ence of the barbules in mixed feathers. 



"The ordinary barb is simply a fibrous mass. It is at the 

 moment of extreme dilatation of the cortical substance that the 

 pigmentary matter, often extravasated, proceeds to unite and 

 sometimes solder together adjacent barbs, to form certain masses 

 which have a horny appearance, and which we have spoken of in 

 the wax-wing. I have seen several of these accidental solderings 

 in the red frontal feathers of the goldfinch. This enormous 

 dilatation of the barb is only exhibited in those ordinary feathers 

 which are destined to acquire a certain splendour or lustre, and 

 there are many others which exhibit a mean development, and 

 these we call mixed." 



(c Mixed feathers comprise all the feathers which undergo little 

 change of form ; they constitute a great part of the plumage 

 of ordinary birds : they never exhibit metallic reflexions, are 

 often dull, and though sometimes of a certain lustre, rarely 

 show that brilliance which belongs to the ordinary feathers." 



"The barbules of mixed feathers are persistent and in 

 direct communication with the barb that bears them. These 

 barbules dilate and colour themselves at the expense of the 

 barbs, without exhibiting the regular form or the pronounced 

 segmentation of the latter/'' 



" In certain mixed feathers the colouring matter accumulates 

 at the ends of the barbules, which often break at these points, 

 and assume the truncated appearance of optical cylinders. I call 

 these feathers mixed, because they resemble the optical feathers 

 in their mode of development, and the ordinary feather by 

 their mode of colouration." 



" Those feathers which owe their colour entirely to the 

 light, whether optical or enamelled, do not change their in- 

 ternal pigmentation, and only acquire their colours by changes 



