OF THE DIFFERENT FEATHER-TRACTS. 21 



they emit a larger or smaller branch at this spot. Tn Oriolus, and other genera, these tracts 

 dwindle away at the inner margin, whilst in most they are very strong at that part. It is only in 

 the genus Cincltis, in this family, that down-feathers occur between the contour-feathers, and 

 also usually on the spaces. 1 Lastly, in Ocypterus (Plate III, fig. 4) alone, we find the singular 

 powder-down spots, to which I shall refer hereafter. But I have now said enough about these 

 differences. 



The species of the same genus of birds usually agree with one another in the design of the 

 feather-tracts when the genus has natural limits, and its species are sufficiently known. But if it 

 has too wide a compass, and contains numerous dissimilar species, it cannot be surprising if 

 differences are to be found with respect to the feather-tracts, corresponding to the groups of 

 species. Thus I have found striking specific distinctions in the genera Falco, Alcedo, and Bucco, 

 and their allies. 



In the numerous individuals of one species no distinctions, except sexual ones, occur in the 

 feather- tracts. When these occur it is not merely the structure of the feathers (as in Paradised), 

 but also their position, that appears to be changed, if I have been right in my observations. (See 

 below, the genus Paradisea among the Passerine birds.) I have never met with accidental 

 differences. 



CHAPTER V. 

 OF THE DIFFERENT FEATHER-TRACT'S. 



HAVING already named the various feather-tracts observed by me, and explained their general 

 properties and differences, I now pass to the detailed description of each tract. 



I. THE SPINAL TRACT (Pteryla spinalis). 



This extends along the whole vertebral column, upon the middle of the whole of the back 

 (in the sense of Illiger's noiaum), from the nape of the neck to the tail, and is bounded by the 

 lateral spaces of the neck and trunk. To the portion of it from the nape to between the 

 shoulders I give the name of the anterior (pars antica), or free (apricd) part ; to the rest, which 

 is situated upon the back, that of the posterior (postica) or covered (tecta) part. The contour- 

 feathers of the anterior part on the nape and between the shoulders always remain freely visible, 

 whilst those of the posterior part are usually covered by the wings when folded up and drawn to 

 the body. 



In its form the spinal tract presents very great variations. In a very few birds only it 



Thus this genus agrees with the water birds, not only in its mode of life, but also in a 

 remarkable detail of organization. 



