26 DISTRIBUTION OP BIRDS. 



Birds are thus distributed from the verge of perpetual 

 snow on the mountain-top, to almost the farthest range at 

 sea ; from the depth of the forest to the bleak and treeless 

 hill j from the marsh to the arid waste ; and, in short, in all 

 the extremes of surface or of climate which the country 

 presents. Also, when places change with the seasons, from 

 one character of surface to another, the birds change their 

 habitations ; and if the season does not leave within the 

 country any place adapted for them, they take wing, and 

 migrate to another ; and the extent to which they perform 

 those changes is so varied, that it cannot be reduced to any 

 general law ; therefore the natural history of birds becomes 

 necessarily a matter of detail, except in so far as it derives 

 some little assistance from the arrangement of them into 

 orders and genera. 



