THE GRAY RAT. 293 



smallest mountain-burn, are the first to be occupied, 

 as migrating rats are fond of following water first, 

 because they are habitually thirsty animals ; and, 

 second, because its proximity affords them additional 

 shelter. In winter most, if not all, of the rat popu- 

 lations of these outposts drift back to civilisation ; 

 nor do they return each year in the same numbers. 

 All this would seem to indicate that the rats 

 remember from year to year their various feeding- 

 grounds, and since they have by now penetrated 

 to the utmost corners of the country-side, only 

 new buildings are immune from them during the 

 summer months. Old buildings are most favoured, 

 not only because their construction suits the rats' 

 mode of living, but also because old buildings 

 have their established records among the rat 

 populace. 



MIGRATION. 



Some light has already been thrown upon the 

 why and the wherefore of the rat migrations that 

 take place during certain seasons. Rats are said 

 to move occasionally en masse from one point to 

 another, and many such migrations have been 

 witnessed. The failing of food-supplies, or the 

 superabundance of foes in one locality, quickly 

 decides the rats to move elsewhere, but the rate 

 of their going is governed by necessity. If the 

 food-supply gradually gives out, the rats gradually 

 dwindle away ; but should the famine be sudden, 

 should the assault of their enemies be fierce and 

 effective, should water or fire invade their terri- 

 tory, then they seek strength in their unity of 

 purpose and migrate in a body. One can be quite 

 sure, however, that they know where they are 

 going, and the horde will very soon split up into 



