48 ANIMALS AT WORK AND PLAY 



trunk. This instinct survives even with animals in 

 captivity. When the prairie-dogs at the Zoo occupied 

 a small paddock, instead of the cages which are now 

 their home, they always kept a sentinel on duty, though 

 he seldom uttered his warning whistle, having learnt, 

 probably, that the visitors would not come inside the 

 railings. The prairie-dogs at the Jardin d'Acclimata- 

 tion at Paris observe the same precaution. Wild geese 

 and wild swans take turns at ' sentry-go/ the former 

 when feeding on land, the latter on the water. Of 

 the former birds, St John says : ' They seem to act 

 in so organised and cautious a manner when feeding 

 or roosting as to defy all danger. When a flock of 

 wild geese has fixed on a field of newly-sown grain 

 to feed in, before alighting they make numerous 

 circling flights, and the least suspicious object pre- 

 vents them from pitching. Supposing that all is 

 right and they do alight, the whole flock for the 

 space of a minute or two remains motionless, with 

 erect head and neck, reconnoitring the country 

 round. . . . They now appear to have made up 

 their minds that all is safe, and are contented to 

 leave one sentry, who either stands on some elevated 

 part of the field, or walks slowly with the rest- 

 never, however, venturing to pick up a single grain 

 of corn, his whole energies being employed in watch- 

 ing/ After describing the march of the geese across 



