WOODPECKERS. 
3 
use of the beak and of the Avings, if these should he 
necessary.^ But if the head were turned downwards, 
or the bony' en much out of the upright position, 
the principle of stability would he changed into a 
cause of falling. Hence, though these birds can run 
upwards, they come down backwards only, and that 
rather slowly and awkwardly ; and they can get round 
only in an ascending spiral, hence they beat and hunt 
the trees from the roots upwards.* 
The bill being required for the performance of a 
peculiar and laborious operation, that of punching or 
picking into the hark or alburnum of trees, is accord- 
ingly adapted hi the most beautiful manner for such 
a purpose. It is, in the typical species, perfectly 
wedge-shaped ; both mandibles are of such an equal 
size and thick- 
ness, that 
when closed, 
the commis- 
sure (or line 
made by the 
joining of the 
two together) 
is perfectly in 
the middle ; the bill gradually becomes smaller from 
the base, and its circumference would be cylindrical 
were it not for certain ridges, which form little pro- 
jecting angles, so that its section appears nearly^ 
hexagonal ; the end has not a sharp point, but a per- 
pendicular edge, like that of a wedge or hatchet.f 
This particular structure is not observed in the Green 
* Feathered Tribes of the British Islands. t Swainson. 
