ON THE MIGRATION OF BIRDS. 25 
it would seem that, in these instances, the sexes do 
not travel in society. 
Having endeavoured, in the foregoing remarks, to 
prove the migration of the Periodical Summer and 
Winter Birds, I shall here briefly observe that our 
irregular visitors also must migrate; as it is equally 
impossible that they should le torpid during a period 
of several years, or that they should escape the notice 
of observers for so great a length of time. Thus the 
migration of every description of Periodical Birds, 
whose appearance has been difficult to account for, is 
established according to the sound principles of the 
inductive logic. 
