ON MUSBURY CASTLE. 69 



increase of speed in the pursuer is followed 

 by an increase of speed in the pursued, since 

 only the swiftest will now escape ; every 

 sharp-toothed squirrel opens still harder and 

 harder nuts, and thus leaves the very hardest 

 alone to produce future trees. The squirrel 

 survives because it can crack nuts which 

 other squirrels must refuse ; the nut survives 

 because it can baffle the squirrel which can 

 crack so many other nuts. 



