92 GEOGRAPHICAL DISTRIBUTION. 



the fields and gardens, ruin the harvest, and only 

 what is kept in the houses escapes them, into 

 these happily they never enter. Their number 

 is so prodigious, that, when they die, the air is 

 infected, and much sickness is the consequence. 

 All this tribe of mice appear to live on roots, 

 bulbs, grain, nuts, &c. and have generally a very 

 short tail. 



The Campagnol, 1 or short-tailed rat of Pennant, 

 is equally destructive ; in some years their num- 

 bers are so prodigious, that they overflow, as it 

 were, a whole district, and by their ravages pro- 

 duce famine and desolation. This effect is stated 

 to have been produced in certain parts of France 

 where an extent of forty square leagues was de- 

 vastated by them. In their progress these ani- 

 mals are preyed upon by the predaceous quadru- 

 peds and birds, by whose incessant attacks their 

 numbers, in ordinary seasons, are kept within the 

 bounds assigned them by the Creator, as are the 

 Locusts by the Locust-eating Thrush, 2 and the 

 Aphides or Plant-lice which may be denominated 

 the Locusts of Britain, and which are stated 

 sometimes almost to darken the air, by the lady- 

 birds and aphidivorous flies. 



All these migrations are produced by a different 

 cause from those periodical ones which take place, 

 after certain intervals, or at certain seasons, in 

 various other animals of every grade ; and though 



1 Arvicola arvalis. a Turdus gryllivorus. 



