'266 FEET OF THE CREEPER 



opposition to the two remaining ones, that is, to the 

 interior front one and the back; and the joint by 

 which the two are articulated on the tarsus admits of 

 so much and such varied motion, that the one part of 

 the foot can act in any direction against the other. 

 Owing to this, the action of the foot is universal as to 

 direction, parallel to the axis of the body, or almost 

 at any angle to it. Thus the birds can run upon the 

 bark of trees in any direction, holding on upon the 

 irregularities of the bark with each foot, and at the 

 same time embracing the convexity with both. This 

 can be easily understood, when it is considered that 

 both the recurved toes of each foot press inward 

 towards the centre of the body ; and this action of 

 the two feet in opposition to each other converts 

 them into a sort of one clutching foot, which acts 

 without any direct exertion of muscles, and therefore 

 holds on for a long time without being in the least 

 fatigued. 



Anisodactylicfeet are thus formed for concentrating 

 their action upon the centre of the body, without any 

 reference to the direction in which the weight presses, 

 just in the same way as the crab feet of swifts con- 

 centrate their action so as to support the centre of 

 gravity in the most effective manner. 



The best native example which we have of a bird 

 with this clever species of foot is the creeper, or the 

 common tree creeper {Certhia familiaris). It is a very 

 small bird, the smallest that we have in these islands, 

 with the exception of the crested wrens, which, like 

 it, are tree birds, and very clever on their feet, al- 

 though they are perchers and not climbers. It is a 

 resident bird in Britain, and by no means a rare one, 

 and its motions upon the trees are well worthy of 

 observation. The direction in which it runs (for its 



