228 THE SENSES OF ANIMALS 



when they have neither sight nor sound to guide them. 

 Ahhough it is not proved that men or birds have a time- 

 sense which is conveniently labelled an "internal clock", 

 some animals do appear to have an internal rhythm of some 

 sort that can regulate their behaviour. Some American 

 experimenters kept a Ground Squirrel for two years under 

 conditions with no seasonal fluctuations. The temperature 

 was constant at a little above freezing, an artificial day and 

 night of twelve hours each was maintained, and ample food, 

 water and bedding were provided. In spite of the removal of all 

 external seasonal stimuli the squirrel hibernated only at the 

 time that its relations in the wild were hibernating and was 

 active, like them, only from June to September. As nothing 

 external should conceivably have given a clue to the time 

 of year the squirrel must have had some sort of internal 

 seasonal clock to maintain the rhythm. 



Even if the birds are proved to have a similar time 

 mechanism the mystery of their navigation is not solved. 

 Latitude and longitude can be determined with the aid of 

 a sextant and a chronometer (and a nautical almanac) but 

 knowing your position in these terms is no help to naviga- 

 tion unless you have a chart — if not a physical one, at least 

 a mental one. It is essential to know the geographical posi- 

 tion of the place of departure ; further, you must know what 

 place it is you wish to reach and its position, and you must 

 also know that you wish to reach it. Is it conceivable that 

 birds are provided with all these aids to navigation, even 

 subconsciously and figuratively? And do migratory fishes 

 observe the altitude of the sun and possess similar internal 

 clocks and mental charts? 



It seems more probable to me that navigation, and some 

 kinds of homing performed by animals, depend upon some- 

 thing we have not yet discovered. It was not until man 

 invented echo-location — sonar and radar — that he found 

 that animals had been using them for finding their way 

 about for millions of years, utilizing the senses with which 

 he was already familiar. Is it possible that there is some way, 

 that we have not yet discovered, of using the known senses 

 for navigation? Perhaps when some elaborate instrument 

 is invented for the purpose we shall suddenly realize that the 



