132 ANIMALS AT WORK AND PL A Y 



development of the mental factors in their case. 

 The usual distance at which terrestrial species use 

 their eyes is limited by the ground horizon. But 

 in the case of the soaring birds, such as vultures and 

 eagles, the horizon, the natural limit of sight, is 

 enormously extended. Macgillivray early noted that 

 though birds of prey have orbits of great size the 

 eyeball of the common buzzard being ij in. in 

 diameter they do not, as a rule, soar when seeking 

 their prey. The eagle, when hunting, flies low, just 

 as do the sparrow-hawk and the hen-harrier. Yet 

 the vultures and condors, birds which admittedly do 

 soar when seeking food, have been proved to find 

 carrion by sight. A carcase was covered with canvas, 

 and some offal placed upon it. The vultures saw 

 this, descended and ate it, and then sat on the 

 covered portion within a few inches of a putrid 

 carcase. When a hole was made in the covering 

 they saw and attacked the food below. But the 

 rapid congregation of vultures from a distance to a 

 carcase, is probably due to their watching their neigh- 

 bours, each of which is surveying a limited area ; 

 Charles Darwin pointed out that in a level country 

 the height of sky commonly noticed by a mounted 

 man is not more than fifteen degrees above the 

 horizon ; and a vulture on the wing at the height 

 of between 3000 ft. and 4000 ft. would probably 



