ADAPTATIONS FOR PERCHING 63 



from which the powerful leg-muscles send down their 

 tendons, which bend the knee and the toes, and, so 

 doing, thrust the body forward. The thigh is short 

 and the knee buried in the body's side. What appears 

 to be the backwardly pointing knee is in reality the 

 ankle-joint, and separates the shank from those bones 

 which in ourselves lie flat, but which in a bird are 

 drawn up to form a vertical column extending the 

 length of the limb, and giving it additional power of 

 movement. Just as in a horse or antelope great 

 speed is gained by the larger joints of shank and foot 

 bone, so in a bird the corresponding bones are 

 drawn out into an elongated zigzag. In the most 

 perfect runner amongst birds — the ostrich — the reduc- 

 tion in the number of toes and the elastic padding 

 for treading over sand have followed precisely on the 

 lines adopted by the camel. 



The majority of birds possess feet suitable for 

 perching or swimming, and the ease with which they 

 securely alight, sleep resting on one leg, or paddle 

 through water, involves a mechanism as perfect in 

 its adaptation to these ends as is that of the wings for 

 flight. Of the four toes usually present, one corre- 

 sponding to the first of lizards is commonly opposed to 

 the rest, so that the claw of the middle toe in front over- 

 laps that of the hind toe and holds the twig between. 

 Now by a labour-saving mechanism this hold, appar- 

 ently so precarious, is rendered automatically secure. 

 Along the front of the knee there runs a tendon which 

 passes down the leg and ends on the under-side of the 



