CAUDAL APPENDAGES. 957 
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 ( Ptilota Arist. ), 
great length of the posterior wings, or caudal ap- 
s 
