12 INTRODUCTION. 



In those birds which have a habit of flirting up the tail, the 

 under coverts are longer and stronger than in those which 

 have no such habit. 



The tail feathers require no reference. They vary much in 

 their numbers and length, and also in the form of their ter- 

 minations, and the motions which the bird can communicate 

 to them. In some birds the whole, or at least great part, 

 of those feathers appear to be merely ornamental, which of 

 course means nothing more than that their uses have not 

 been observed. In general, however, they act both as a 

 rudder in flying, and as a means of directing the motion 

 upwards or downwards. Analogy would indeed lead us to 

 suppose that their chief use is in the upward and downward 

 motion, because their greatest surface is generally horizontal. 

 That analogy is further confirmed by the fact, that many of 

 the low-flying birds have the breadth of the closed tail in the 

 vertical plane, though these also can, in general, spread it out 

 like a fan when they fly. 



The other feathers are to be considered rather as the cloth- 

 ing of the bird than as active instruments in its flight, or as 

 auxiliaries in its motions upon the land or the water. But 

 still they are not less worthy of notice, both in the distin- 

 guishing of one bird from another, and in tracing how well 

 all the parts of birds are adapted to their general habits and 

 their haunts. These ordinary feathers are imbricated, that is, 

 they are placed one over the edges of two, as slates or plain 

 tiles are in a covering of a roof. By means of this arrange- 

 ment, all the parts of the bird are equally feathered, and so 

 they are equally proof to the action of the atmosphere. The 

 lines in which the several rows of feathers are placed form 

 very curious curves, and their shafts diverge or converge so 

 naturally, and with so perfect agreement to the surfaces which 

 they cover, that no line of separation can be traced. 



The feathers of birds, the coverings of the featherless parts, 



