CAUDAL APPENDAGES. 257 



peckers. Here nature throws aside ornament, and 

 makes the tail of these birds not only useful, but 

 absolutely essential to their means of supporting 

 existence ; the loss of it, to a woodpecker, would, 

 in fact, lead to the loss of life. The bird could no 

 longer climb trees in search of food, because it 

 would want that support in a perpendicular position 

 which the tail supplied ; so that, like an American 

 monkey so circumstanced, it would die within sight 

 of ample nourishment. There is still another form 

 under which a great developement of tail is observed, 

 and the use of which is exclusively confined to the 

 flight. No instances of this form are found among 

 quadrupeds, but there is scarcely any family of 

 birds that is without it. In the modifications just 

 described, the shape of the tail is always round or 

 wedge-shaped, but in that we are now speaking of, 

 it is invariably forked. A familiar example of this 

 is seen in the swallow family, where it is most pre- 

 valent, but nowhere is it carried to such an extent 

 as in some of the goatsuckers of tropical America, 

 and the fork-tailed kite of the United States. It 

 does not appear, however, that this structure is so 

 prevalent in all the individuals of a natural genus as 

 those already noticed ; for many of the swallows 

 and most of the goatsuckers have even tails : hence 

 this character, among birds, can rarely be employed 

 otherwise than to designate sections, or perhaps 

 sub-genera : in such cases, however, it becomes 

 essential, or the chief mark by which such forms 

 are to be pointed out. 



(178.) Among the winged insects (PHhta Arist.), 

 great length of the posterior wings, or caudal ap- 



