ANIMALS UNDERGROUND 233 



natural habit, or, at any rate, one acquired previously 

 to the use of earths by adult foxes. 



The sand-martins are, however, the most complete 

 examples of creatures which have taken to underground 

 life entirely to protect their young, and abandon it with 

 joy the instant these have flown. How far the king- 

 fisher and the sheldrake contribute to the making of 

 the burrows in which they lay their eggs is doubtful, 

 but it is a very notable change of habit in birds of such 

 strong flight and open-air, active habits. It may be 

 paralleled by the case of the stormy petrels and fork- 

 tailed petrels, true ocean birds, which, nevertheless, 

 abandon the sea and air to dig deep holes in the soil of 

 the Hebridean islets, and rear their young in these 

 dark and tortuous passages. Rabbits, rats, and some 

 other rodents make nursery burrows of a very rudi- 

 mentary kind, having only one opening, which the 

 mothers close up when leaving the nest. This probably 

 gives the clue to the process by which the true 4 under- 

 ground animals ' have been evolved. First they scratched 

 holes in which to shelter their young. Then they made 

 use of the same device to protect themselves, and 

 acquired much greater skill in working, and some 

 modifications of coats and claws to do this with comfort 

 and effect. In time the habit became so easy that its 

 exercise afforded them pleasure ; and thus we have 

 the spectacle of the prairie-dog who digs holes for 



