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 ; 



