OF NATURAL HISTORY. 497 



heaven. The celebrated Linnaeus, who paid great attention 

 to the oeconomy of thefe migrating rats, remarked, that they 

 appeared in Sweden periodically every eighte-en or twenty 

 years. When about to migrate, they leave their wonted 

 abodes, and aflemble together in numbers inconceivable. In 

 the courfe of their journey, they make tracks in the earth 

 of two inches in depth ; and thefe tracks fometimes occupy 

 a breadth of feveral fathoms. "What is fingular, the rats, in 

 their march, uniformly purfue a flraight line, unlefs they are 

 forced to turn alide by fome unfurmountable obftacle. If they 

 meet with a rock, they firft try to pierce it, and, after difcov- 

 ering the attempt to be impracticable, they go round it, and 

 then refume the ftraight line. Even a lake does not inter- 

 rupt their pafTage ; for they either traverfe it in a ftraight 

 Hne or perifh in the attempt ; and, if they meet with a bark 

 or other veiTel, they do not alter their direction, but climb 

 up the one fide of it and defcend by the other. 



Frogs, immediately after their transformation from the 

 tadpole ftate, leave the water, and migrate to the meadow or 

 marfhy grounds in queft of infects. The numbers of young 

 frogs which fuddenly make their appearance in the plains in- 

 duced Rondeletius, and many other naturalifts, to imagine 

 that they were generated in the clouds and fliowered down 

 upon the earth. But if, Uke the worthy and intelligent Mr, 

 Derham, they had examined the fituation of the place with 

 regard to ftagnating waters, and attended to the nature and 

 transformation of the animals, they would foon have difcov- 

 ered the real caufe of the phenomenon. 



Of all migrating animals, particular kinds of fifhes make 

 the longed journies, and in the greateft numbers. The muL 

 tiplication of the fpecies, and the procuring of food, are the 

 principal motives of the migration of fiflies. The falmon, a 

 filh which makes regular migrations, frequents the northern 

 regions alone. It is unknown in the Mediterranean fea, and 

 in the rivers which fall into it both from Europe and Africa- 



