MIGRATIONS. 99 



selves with greater ease, and with the interpo- 

 sition of fewer obstacles, than the quadrupeds, 

 the theatre of whose motions is the earth, inter- 

 sected by rivers and mountain ridges, which 

 renders their periodical transit less easy to 

 accomplish. The number of birds that migrate, 

 if we take Dr. Richardson's scale, for those of 

 North America, as a rule, compared with those 

 that reside the whole year in a country, is about 

 five-sixths, a very large proportion ; but as the 

 summer residents are replaced by winter ones, 

 the difference is less striking, and the desertion 

 less apparent and annoying. The celebrated 

 Dr. Jenner, in a very ingenious posthumous 

 paper, in the Philosophical Transactions for 

 1824, has produced many arguments to prove 

 that the periodical migrations of birds are the 

 result, not of the approach of the cold or hot 

 seasons, but of the absence or presence of a 

 stimulus connected with the original law, " In- 

 crease and multiply." That when they feel it 

 they seek their summer, and when it ceases its 

 action, their winter quarters. In one case, the 

 animal winging its way to a climate and country 

 best suited to the great purpose impressed upon 

 it by its Creator, of producing and rearing a 

 progeny ; and in the other returning to a home, 

 most congenial to its nature, and best supplying 

 its wants. 



The cause of emigration, in both cases, had 



