354 ACTION OF THE WOODPECKER. 



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 pro- 

 duced ; 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 farthest 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 strut. So also, when the centre of action is brought 

 nearer to the tree, and the hold by the foot not so 

 much required, the same extension of the tarsal joint 

 which 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 wood- 

 pecker 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 

 organisation 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. 



When on the tree, the woodpecker may be regarded 

 as consisting of three parts, all differently employed ; 

 the first of these is the sternum and its apparatus, 

 which acts as the basal or pectoral part, and has no 

 motion except in bringing the centre of action to the 

 tree, or removing it to a greater distance ; the second 

 or prehensile part which consists of the posterior 



