THE STRUCTURE OF A FEATHER 263 



In many feathers, especially body feathers, there Is 

 a second shaft, the aftershaft, which springs close to 

 the scar from the top of the barrel. The aftershaft is 

 usually smaller, often much smaller than the main 

 shaft, but in the Emu and Cassowary it is of nearly the 

 same length. In these large, flightless Birds the 

 feathers serve only for defence and warmth, and here 

 the double shaft is of distinct advantage, allowing 

 twice as many shafts to be crowded into the same 

 surface of skin. I cannot explain why the Ostriches 

 and the little Kiwi of New Zealand have no aftershaft, 

 or none that signifies, but I have long been familiar 

 with negative exceptions to every kind of natural con- 

 trivance. At first the enquirer is much shaken in his 

 interpretation of a natural structure when he finds that 

 it is wanting altogether in a species which seems to 

 need it as much as any other. But the constant 

 occurrence of such cases where there can be no doubt 

 of the use of the structure leads at length to a settled 

 conviction that Nature has many ways of accomplish- 

 ing her ends, and can dispense with any organ or any 

 adaptation, often for reasons which are altogether 

 inscrutable to us. 



The next thing is to examine the minute structure 

 of the vane. It resembles at first sight a woven fabric. 

 Cut out a square piece, hold it up against the light, 

 and gently pull it across the grain. We see that it is 

 made up of fibres (barbs). The barbs are held 

 together by a multitude of finer fibres (barbules). 

 The barbules will resist a direct pull pretty well, but if 

 the barbs are slid along sideways, they can be 

 detached without violence. The)" are not truly 



