OF NATURAL HISTORY. 495 



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, uni- 

 formly purfue a ftraight line, unlefs they are forced to turn afide by 

 fome unfurmountable obftacle. If they meet with a rock, they firft 

 try to pierce it, and, after difcovering the attempt to be impradi- 

 cable, they go round it, and then refume the ftraight line. Even a 

 lake does not interrupt their paflage ; for they either traverfe it in a 

 ftraight line or perifti in the attempt; and, if they meet with a bark 

 or other veflel, they do not alter their diredion, bat 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 marlhy 

 grounds in queft of infeds. The numbers of young frogs which 

 fuddenly make their appearance in the plains induced Rondeletius, 

 and many other naturalifts, to imagine that they were generated in 

 the clouds and fhowered down upon the earth. But if, like the 

 worthy and intelligent Mr Derham, they had examined the fitua- 

 tion of the place with regard to ftagnating waters, and attended to 

 the nature and transformation of the animals, they would foon have 

 difcovered the real caufe of the phenomenon. 



Of all migrating animals, particular kinds of fifties make the 

 longeft journies, and in the greateft numbers. The multiplication 

 of the fpecies, and the procuring of food, are the principal motives 

 of the migration of fifties. The falmon, a fifti which makes regu- 

 lar 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. It is found in fome of the rivers of France 



that 



