THE ANATOMY OF A BIRD 3 



glides smoothly and rapidly under water in pursuit of its fishy prey, 

 a continuous coating of feathers is not only a source of additional 

 w;irnith, but offers less resistance to the water ; so, too, with a run- 

 ning bird like the Emu or Ostrich. But in the case of the latter, at 

 any rate, the young nestling has quite distinct tracts and apteria, 

 thus showing that, although nowadays it is incapable of flight, it 

 has descended from an. ancestor that could fly at least, that is 

 the way in which it is customary to interpret such differences in 

 structure between young animals and their parents. The Apteryx 

 also, of New Zealand, is quite analogous. The old bird has a nearly 

 continuous covering of feathers, but the unhatched young show 

 perfectly distinct patches of feathers with bare spaces between. We 

 si mil show on another page that there are other arguments which 

 appear to prove that all these flightless birds have been gradually 

 derived in the course of time from birds that could fly perfectly 

 well. They are an instance, so far, of what is termed degenera- 

 tion. 



The examination of any bird wiU show that it has several kinds 

 of feathers. They are all constructed upon the same plan, but some 

 are larger than others, and the smallest are soft instead of firm to 

 the touch. 



The biggest feathers of all are a set which fringe the wing (see 

 fig. 1) and another set at the end of the tail. These are called 

 respectively the ' Eemiges ' and ' Eectrices,' or the ' rowing ' feathers 

 and the ' steering ' feathers. Their principal use, as may be 

 imagined, is in flight. The remaining feathers are also to some 

 extent used in flight, but their main use appears to be to keep 

 the body warm. An eider-down quilt, as everybody knows, is the 

 warmest kind of coverlet ; the reason being that the feathers are very 

 bad conductors of heat, and do not, therefore, allow the heat of the 

 body to escape. Birds are the hottest of all animals, which is in part 

 duo to their covering of feathers. To understand the structure of a 

 typical feather is perhaps a little difficult ; but possibly the accom- 

 panying figures (figs. 1, 2, 3, 4) will render the explanation easier 

 to follow. The feather consists of a stem which is technically 

 called the rhachis, the word simply signifying stem. From each 

 side of this a row of parallel rodlets arise which are called barbs. 

 These in their turn give rise to another set of processes which 

 are the barbules. This, however, is not all ; the barbules are firmly 

 locked together by other processes, so that the entire feather is 

 quite firm, and can be used as a kind of oar with which to row 



B2 



