THE GRAY RAT. 291 



tenant. One at least was caught weekly, where- 

 upon another would immediately take the place of 

 the deceased, and the nuisance continued. 



Thus, when once rats have begun to come to a 

 place, they will continue to do so for so long as 

 food exists there for them a state of affairs which 

 would seem to prove definitely that they follow in 

 each other's steps, distributing themselves in such 

 numbers as the quantity of available food alone 

 determines. 



Hence, if an outhouse furnishes food enough for 

 three rats, the pioneer is quickly followed by two 

 others, which settle with him. Other rats come, 

 and still others ; but though a sifting and changing 

 may take place, the forerunners being ousted by 

 stronger rivals, three rats remain in that outhouse. 

 Those that are turned aside wander on in search of 

 fresh quarters ; one settles here, another there, but 

 each settler is tracked to his lair, which, if capable 

 of sustaining more than one, he is compelled to 

 share. Should the food-supply be adequate for 

 the maintenance of a hundred rats, then more new- 

 comers, and still more, arrive till the limitation of 

 the food-supply is reached, and once again the over- 

 flow goes drifting by to penetrate new territory. 



Were it not that rats are so extremely gregarious 

 in their habits, it would, by this system, take 

 them longer to occupy fruitful territory than 

 to distribute themselves over a region where 

 food was scanty, as the absorbing reservoirs would 

 take longer to fill, and so the flowing tide of 

 rats would move less speedily on its way. This 

 proposition is supported by the known facts. It 

 may take years for rats to penetrate into a region 

 where food is scarce, whereas they occupy a fruit- 

 ful territory in a veritable invading army, sweeping 



