84 ANIMAL ECOLOGY 



of underground runways, they do not usually clash in any way 

 in their activities ; for while the gerbilles come out exclusively 

 at night, leaving their burrows after sunset and returning always 

 before dawn, the mongooses and suricats feed only during the 

 day, and retire to earth at night.'^^ It appears that a mongoose 

 never attacks a gerbille under ordinary circumstances because 

 the two creatures do not usually meet : they have different 

 hours for business. It is only when the gerbilles are smitten by 

 an epidemic of some disease like plague, at which times they 

 wander out of their holes in the daytime, that they are attacked 

 and eaten by the mongoose. This last fact has a practical 

 importance, since the South African plague investigator is 

 able by examining the excreta of the mongoose to find out with 

 tolerable certainty whether the gerbilles have been dying of 

 plague, a fact which is rather difficult to establish easily in 

 any other way. If the mongoose excreta contain gerbille fur, 

 then there is strong evidence of epidemic amongst the latter.'^^ 



In this case the alternation of day and night has the effect 

 of separating almost completely two animals which live in 

 the same place, and although the phenomenon and its results 

 happen to have an important practical bearing, it is only one 

 example among thousands which might be given, all of which 

 go to show that the phenomenon is of general occurrence in 

 nearly all animal communities. 



2. The environment even in the same place is always 

 changing rhythmically and more or less violently ; some of 

 these changes being regular, like the alternation of day and 

 night, of high and low tides, or the annual succession of the 

 seasons, while others are more irregular, like the fluctuations 

 in weather from day to day and week to week. These changes 

 all leave a corresponding impress upon the arrangement and 

 composition of animal communities. Just as animals tend to 

 become specialised for life in certain places, so also most 

 of them are active only at certain times. There are various 

 ways of meeting the onset of unfavourable conditions. If 

 the latter last only for a short time, the animal may merely 

 retire to some hiding-place or become inactive wherever it 

 happens to be at the moment. Every one must have noticed 



