348 POPULAR SCIENCE MONTHLY 



tainly had to do with the reduction of the hallux. The hallux has, 

 however, been retained by most flying birds because it is used in 

 perching, in prehension and in swimming. As the legs of birds are 

 set well forward on the body, they are more widely separated above 

 than below. This position has thrown the greatest strains upon the 

 outer or fourth digit, which is always longer than the second toe. 

 Now in the foot of running birds like the emu, the digits are used 

 only in locomotion and the hallux has disappeared, as it was useless 

 in progression. But the digitigrade posture of the emu is the same 

 as that of the flying birds ; the ball of the foot touches the ground and 

 the remaining three digits, being all functional, are well developed. 

 The ostrich, however, has increased its swiftness by running (and 

 walking) upon the tips of its toes. This posture would throw the 

 weight of the body and the work of locomotion upon the longer third 

 and fourth digits. As the foot of the ostrich is used only in locomotion, 

 and as the birds traverse the smooth floor of the desert, the shorter 

 second digit would fail to reach the ground and eventually disappear. 

 As a result we find that the ostrich has only two functional digits. 



The digits of birds, therefore, show structural changes which are 

 exactly paralleled by those exhibited by various ungulates, and the 

 digital reductions which have taken place may be attributed to the 

 same factors in each case. 



