WHA T ANIMALS SEE 133 



be two miles distant, and invisible. Those which 

 descend rapidly and appear to have come from 

 beyond the range of human sight, were perhaps 

 hovering vertically over the hunter when he killed 

 his game. There remains one undoubted instance 

 in which bird-vision is far keener than that of man. 

 The great grey shrike, Lanius excuHtor, is habitually 

 used by the men who catch falcons at Valhenswaard 

 to give notice of the approach of a hawk. The 

 bird sees it far sooner than the men, and at once 

 gives notice of its approach. This is a single instance 

 in which the specialised acuteness of sight may be 

 due to the fact that the bird in question much 

 resembles in colour the pigeons, which are the falcon's 

 favourite food. But long-sight does not seem a 

 common property of bird-vision. The gannets, 

 which catch fish at sea, descend from a considerable 

 height, but they kill their prey on the surface of 

 the water or near it. Nocturnal birds and animals, 

 though able to see with little light, have no enhanced 

 powers when the light is more powerful ; and those 

 animals which, like deer, feed by night or day in- 

 differently, have only developed a keenness of vision 

 from constant fear and vigilance. Horses and cattle, 

 which have the same power of sight by night, have 

 never increased their visual range. Dogs habitually 

 rely on another sense, that of scent, in preference to 



