314 THE WOKLD OE LIFE 



rapidly that, together with the support given by the hinder 

 portion of the wings and tail, the onward motion is kept up, 

 and the strongest flying birds exhibit hardly any undulation in 

 the course they are pursuing. But very little is said about the 

 minute structure of the feathers themselves, which are what 

 renders perfect flight in almost every change of conditions a 

 possibility and an actually achieved result. 



But there is a further difference between this instrument 

 of flight and all others in nature. It is not, except during 

 actual growth, a part of the living organism, but a mechanical 

 instrument which the organism has built up, and which then 

 ceases to form an integral portion of it — is, in fact, dead mat- 

 ter. Hence, in no part of the fully grown feather is there 

 any blood circulation or muscular attachment, except as re- 

 gards the base, which is firmly held by the muscles and ten- 

 dons of the rudimentary hand (fore limb) of the bird. This 

 beautiful and delicate structure is therefore subject to wear 

 and tear and to accidental injury, but probably more than any- 

 thing else by the continuous attrition during flight of dust- 

 laden air, which, by wearing away the more delicate parts 

 of the barbules, renders them less able to fulfil the various 

 purposes of flight, of body-clothing, and of concealment, as well 

 as the preservation of all those colours and markings which are 

 especially characteristic of each species, and generally of each 

 sex separately, and which, having all been developed under the 

 law of utility, are often as important as structural characters. 

 Provision is therefore made for the annual renewal of every 

 feather by the process called moulting. The important wing- 

 feathers, on which the very existence of most birds depends, 

 are discarded successively in pairs at such intervals as to allow 

 the new growth to be Avell advanced before the next pair are 

 thrown off, so that the bird never loses its power of flight, 

 though this may be somewhat impaired during the process. 

 The rest of the plumage is replaced somewhat more rapidly. 



This regrowth every year of so complex and important a 

 part of a bird's structure, always reproducing in every feather 



