226 THE SENSES OF ANIMALS 



the algae, and returns to the one spot that exactly fits its 

 shell when the waves retreat. Pigeons can be trained to home 

 from great distances by being sent further and further away 

 at each trial. The limpet explores its surroundings and gets 

 to know its way about its home; and so, too, do homing 

 pigeons, at least to some extent. Similarly wasps and bees 

 find their way home by using landmarks with which they 

 have become familiar. If the landmarks are moved their 

 homing is badly upset and they have to learn the new 

 pattern. Even birds can be completely put out by moving 

 their nest quite a small distance ; they will bring food to the 

 place where the nest was, but leave their young only a few 

 yards away to starve. They react to the old landmarks and 

 cannot quickly learn the new pattern or, indeed, appreciate 

 the altered conditions. 



But some animals show an ability for homing which does 

 not depend upon learning the landmarks and knowing the 

 locality. The Shearwaters that were taken from their nests 

 in Pembrokeshire and flown in closed boxes to America 

 could have had no landmarks to recognize in the thousands 

 of miles across the Atlantic, yet some of them were safely at 

 home a week later. Dogs and cats that have moved house 

 with their owners to distances of a hundred miles and more 

 have been known to disappear and turn up at their old home 

 days or weeks later. Frogs and toads make long journeys 

 each spring to their breeding ponds, overcoming all obstacles 

 and passing by other ponds that seem to us to be as desirable 

 as those to which they go, (See Plate 20.) 



Nobody knows how they do it, although much experiment 

 has been done on the homing and migration of birds. It 

 has been suggested that birds may respond to the earth's 

 magnetic field, or to the Coriolis force produced in a moving 

 body by the rotation of the earth, but experiment has not 

 been able to support such ideas. Much work has been done 

 upon the theory that birds navigate by unconsciously observ- 

 ing the altitude of the sun, and working out their bearing by 

 this and the use of a postulated time-sense or "internal 

 clock". The theoretical internal clock is necessary for obtain- 

 ing longitude by comparing local time with that of the point of 

 departure. The suggestion that there may be some way of 



