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, 
itis 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 
