vi FORM AND FUNCTION 149 



as is sometimes stated, that the barbules themselves 

 interlock. It is the hooklets that fasten one barbule 

 to another, and this they do in such a way that, while 

 keeping a firm grip, they increase the elasticity 

 natural to the material of which the feather is made 

 (Fig. 39A). The edges of the barbules, that have to 

 be laid hold of by the hooklets of those opposite to 

 them, are folded over. Below this folded edge is a 

 channel between the two adjacent barbules that lie 

 parallel to one another. The hooklet keeps hold of 

 the edge, and at the same time is able to move up and 

 down in the channel. Hence the wonderful play of 

 the vane of a wing or tail feather when pressure is 

 applied to it. In the softer, partly plume-like feathers 

 the mechanism is not so perfect ; in some cases the 

 hooklets do not exist. Such feathers are not imper- 

 vious to air, and they are much less strong and much 

 less elastic. All feathers, it will be noticed, are con- 

 cave underneath, a form that adapts them for resisting 

 pressure from below and not from above. 



In nearly every case there is a small after-shaft (S 2, 

 fig. 38) arising just below the small pit at the top of 

 the quill. Generally it is insignificant and escapes 

 notice, unless attention is specially called to it. In the 

 Pigeon it is minute ; in the Cassowary, on the contrary, 

 it is as large as the main shaft. It is curious that in the 

 embryo feathers of this bird there is no after-shaft at 

 all. In no bird except the Emeu does it appear till 

 the feather proper grows, and this has been thought 

 to show that in primitive birds after-shafts were not 

 found. 



The development of the feather now demands our 



