INTRODUCTORY 15 



so difficult to pull the barbs of a feather apart. In the vane of a 

 Crane's quill, in a piece of web 15 inches long, no less than 650 

 of these barbs were counted, each of which bore about 600 pairs of 

 barbules — that is, about 800,000 for the inner web alone, and more 

 than a million for the whole feather : and all these are necessary to 

 hold the vane together. 



This system of interlocking is most perfect in the wing and tail 

 quills. In some feathers, such as have a loose, hairy texture, as in 

 the body feathers of many birds, these hooklets are but feebly, if at 

 all, developed. The loose feathers of the Ostrich tribe also lack 

 them, but here they were once all perfectly developed ; when the 



D.R. R. 



/ p. R. 



Fig. 6.— Section through two rows parallel to the Distal Radii or 

 Barbules. (After Pycraft.) 



D.R. Distal Radii; P.R. Proximal Radii or Barbules. 



birds ceased to fly, the feathers degenerated, and the interlocking 

 was lost. 



What are known as semi-plumous feathers are degenerate feathers. 



Down feathers differ considerably from *' contour feathers " in 

 structure, having little or no shaft, all the barbs arising from a common 

 base. These barbs are, further, very long, and have only very minute 

 barbules. In the Ducks and some other birds these barbules take 

 the form of triangular nodules ; while in other birds again they are 

 knot-like. 



The iilo-plumes have a long, slender shaft with a minute vane 

 at the tip. They are, apparently, a degenerate form of contour 

 feather, judging from the fact that during the earlier part of their 

 development many more barbs are present than are to be found in 

 the fully grown filo-plume. 



