Hedgerow Life 
their ranks. If allowed to increase too 
far, they can do an immense amount of 
damage to crops. This has happened in 
Australia, into which country they have 
been introduced ; and as there were not 
sufficient natural enemies to keep them 
in check, they have become a most 
serious nuisance. 
Rabbits are clever enough to provide 
themselves with a back door to their 
burrows, so that they can escape by that 
way when danger knocks at the front 
entrance. And in this respect animals 
show themselves to be more intelligent 
than birds (I should say, perhaps, 
mammals, not animals, because you 
should plainly understand that a bird is 
an animal quite as much as a dog or any 
other creature); for you will not find a 
back door to any bird’s nesting-hole, 
