146 THE BOOK OF A NATURALIST 



spirit returns, he moves about this way and that, 

 to see all round him, and each time he turns his 

 bright eyes away the spider rapidly moves a little 

 nearer; but when the fly looks again, appears 

 motionless as before. In this way, little by little, 

 the space is lessened, and yet the fly, still turning 

 at intervals to regard the suspicious-looking object, 

 does not make his escape, simply because he does 

 not know that the space has been lessened. Seeing 

 the spider always motionless the illusion is pro- 

 duced that it has not moved: the dividing distance 

 has been accurately measured once for all, and no 

 second act of judgement is required; the fly, know- 

 ing his own quickness and volatile powers, feels 

 himself perfectly safe; and this goes on until by 

 chance he detects the motion and instantly flies 

 away, or else fails to detect it and is caught. Cats 

 often succeed in capturing birds by a similar 

 stratagem. 



The snake, unlike the spider and cat, cannot 

 make the final spring and rush, but must glide up 

 to within striking distance: this he is able to do 

 by means of the faculty he possesses of progressing 

 so gradually and evenly as to appear almost 

 motionless; the tongue which he exserts and 

 rapidly vibrates at intervals when approaching his 

 victim helps in producing the deception. 



Long observation has convinced me that a 

 snake on the ground, moving or resting, is not a 

 sight that violently excites birds, as they are 

 excited by the appearance of a fox, cat, weasel, 



