OF NATURAL HISTORY. 503 



■whole fyftem of animation. Men, quadrupeds, birds, fifhes, reptiles, 

 infeds, all afford ftriking examples of the migrating principle. From 

 the fame fafts it is equally apparent, that the general motives for 

 migrating are fimilar in every clafs of animals. Food, multiplication 

 of fpecies, and a comfortable temperature of air, are evidently the 

 chief caufes which induce animals to remove from one place to an- 

 other, or, what amounts to the fame thing, from one climate to an- 

 other. Partial emigrations, or emigrations to fmall diftances, are 

 prompted by the fame inftindtive motives which induce animals of a 

 different ftrufture to undertake long and fatiguing excurfjons. But, 

 previous to adual migration, what are the peculiar feelings of diffe- 

 rent animals, and what fhould ftimulate them to proceed uniformly 

 in the diredlion that ultimately leads them to the fituations moft ac- 

 commodated to their wants and their conftitutlons, are myfteries, 

 with regard to which, like every other part of the oeconomy of Na- 

 ture, it is the duty of philofophers, inftead of attempting to pufli 

 their inquiries beyond the bounds of human ability, to obferve a re- 

 fpeiStable filence. 



CHAP. 



