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, 
