s 
WOODPECKERS. 
A very little extension of the tarsal joints brings 
the centre of action more to the tree, and a very 
little bending of the same joints removes it farther 
away. Nor is this ready adaptation of the centre to 
the greatest effect of the stroke the only result of that 
action of the tarsal joints by which it is produced ; 
for there is the same nice adjustment of the degree of 
hold taken on the bark to the varying stability of the 
position. When the centre of action is removed to 
the greatest distance, the centre of gravity is thrown 
furthest out, in proportion to the line of the axis, and 
therefore the weight tends more to pull the bird from 
the bark ; but the very same action of the tarsal 
joints which produces this causes the claws to take a 
firmer hold of the bark, and also the stiff feathers of 
the tail to bear more against it as a su])port. So also, 
when the centre of action is brousrht nearer to the 
tree, and the hold by the foot not so much required, 
the same extension of the tarsal joint Avhich brings 
the axis more parallel to the tree eases the clutch of 
the foot in exactly the same proportion. 
Thus, by one of the most beautiful instances of the 
harmony of parts with each other, the Woodpecker is 
enabled to work with equal stability and effect, while 
the axis of the body is at all possible angles to the 
line of the tree, within the range that is necessary for 
its habit ; and yet the different parts of this very 
curious and apparently complicated organization are 
so flexible to other purposes, that the Woodpecker can 
at once become a wing-bird or a ground-bird, when 
such a habit is necessary.* 
* Natural Histoiy of Birds. 
