144 THE BOOK OF A NATURALIST 
its appearance. Furthermore, I have now in my 
mind two separate functions, the first of which 
occasionally, perhaps often, passes into and becomes 
one with the other. 
When the common or ring snake pursues a frog, 
the chase would in most cases prove a very vain 
one but for that fatal weakness in the hunted 
animal, which quickly brings its superior activity 
to naught. The snake need not even be seen for 
the effect to be produced, as any one can prove for 
himself by pushing his walking-stick, snake-wise, 
through the grass and causing it to follow up the 
frog’s motions, whereupon, after some futile efforts 
to escape, the creature collapses, and stretching out 
its fore-feet like arms that implore mercy, emits a 
series of piteous, wailing screams. ‘Thus, all that 
is necessary for this end to be reached is that the 
frog should be conscious of something, no matter 
what, pushing after it through the grass. There is 
here, apart from the question in animal psychology, 
a little mystery involved; for how comes it that 
in the course of the countless generations during 
which the snake has preyed on the frog, this 
peculiar weakness has not been eliminated by 
means of the continual destruction of the in- 
dividuals most subject to it, and, on the other 
hand, the preservation of all those possessing it in 
a less degree, or not at all? It is hard for a good 
Darwinian to believe that the frog is excessively 
prolific for the snake’s advantage rather than for 
its own. But this question need not detain us; 
