Fatio on the Forms and Colours of Plumage. 383 



extreme barbules are already large, well coloured, and even 

 showing metallic reflexions. As these feathers finish their 

 growth, as well as after their complete moult, they show the 

 same difference — an inferiority of development and. colour in 

 the barbule of the ordinary feather as compared with an 

 optical one. If I look at the feathers of the same birds in 

 spring, I am struck by a series of differences which multiply 

 more and more between the ordinary and the optical feathers, 

 as I enter more minutely into their details of development, 

 structure, and colouration. In the first place to the naked eye 

 merely, the linnet's feather is shortened through the loss of its 

 brown extremities, and has become red by a more complete 

 solution of its internal pigment, whilst the starling's feather 

 which has lost its white tip, and the ends of its upper lateral 

 barbs has taken a more thread-like aspect (effilee) whilst its 

 reflexions are much more brilliant and extensive. Under the 

 microscope, in the ordinary feather, the barbule, instead of 

 developing, is seen in many places to have fallen off, whilst the 

 barb or secondary axis is enlarged and coloured, especially at 

 its extreme portions. In the optical feather of the starling 

 the barb has slightly changed, while the barbules, or tertiary 

 axis are much developed both in size and colour." 



" The barbs, or branches of the single stem, are in general 

 formed of superimposed segments, more or less persistent, and 

 the barbules constructed on a similar plan, are implanted in the 

 barbs by their basilar segments. This base of the barbule is 

 larger and stronger according to the period, and the nature of 

 the feathers; but internal communications appear always to 

 exist, which may render possible a mingling of pigments 

 between the two axes of optical feathers, and between the same 

 parts of ordinary mixed feathers. All the barbules are, like 

 the barbs, composed of larger or smaller segments, arranged 

 end to end and enveloped in a common epidermis ; but as their 

 segmentation has much more importance than that of the barbs, 

 let us see the chief differences which exist between them in 

 spite of their unity of structure." 



" 1 . The lines of demarcation between the segments of optical 

 barbules are much more decided than in ordinary barbules • or, 

 to speak explicitly, separating diaphragms of these different parts 

 are stronger in the former than in the latter. 2. The granular 

 pigment deposits ordinarily diffused in the central nucleus of the 

 segments, are always more abundant and more regularly dis- 

 tributed in the first than in the second. 3. We distinguish 

 around these dark nuclei a layer of parallel or longitudinal fibres, ' 

 much thicker in the optical barbule than in the ordinary one. 

 4. The lateral hooklets of the optical barbules are in general 

 more developed in autumn than in spring, and their function 



