February 
finches, etc., which reverse this order, and 
only come at the approach of cold weather or 
in mid-winter, and disappear in the spring. 
He may be equally unfamiliar with the fact that 
still other species, like some of the thrushes, 
finches, warblers, and greenlets, can be seen 
only for a few weeks at a time at two different 
periods in the year, and is perhaps unaware 
that a very few species are to be found in the 
woods throughout the whole year. 
These various movements are not due to the 
special peculiarities of the several classes, but to 
the law controlling them all equally, and the 
apparent complexity of the law resolves into 
the utmost simplicity when it is understood. 
Bird-migrations are all in the direction of 
north and south, and the underlying cause 
of this is that they are determined chiefly by 
the two considerations of temperature and food- 
supply. With uniform climate and abundant 
subsistence, birds would doubtless remain in 
their several localities, or approximately so, the 
entire year. In that case, such of the war- 
blers as find their summer home in northern 
New England and Canada would also remain 
there throughout the winter, and we should not 
be likely to see them except at the personal in- 
49 
