168 
BIRD-LIFE. 
met with in such countless numbers as are some of those 
which inhabit the water. Unite the two dominions of 
earth and water, and then we discover a concourse of 
species as well as of individuals. Thus all lakes and 
swamps, especially those situated in warm climates, are 
exceedingly rich in birds belonging to many different 
species. 
In support of this view of the case, we may regard the 
land, taken in conjunction, in some measure, with the 
equator, as the generator of many species ; and water in 
the Polar Regions may, on the other hand, be regarded 
as the maintainer and animator of multitudes of one 
species—that is to say, within certain bounds: for on 
heat becoming fervid it fails to generate, and burns 
instead; also the higher degrees of cold numb, instead 
of preserving life. On land these conditions are to be 
met with in a comparatively circumscribed limit, when 
we draw a parallel between the forest and the field, the 
mountains and the plains. On the waters this is much 
more difficult, or perhaps impracticable: to fix limits to 
the raging flood cannot well be accomplished. For this 
reason, it is impossible to assign a circle of distribution 
with reference to an aquatic species with the same 
certainty and facility as is done in regard to land 
species. Although the bird, gifted as it is with the 
means of flight, is of all animals the one best adapted for 
travelling, there exist circumstances which confine it to 
certain localities, with which, in our minds, it is always 
more or less associated. 
The different conditions under which certain birds live 
are due to the characteristics of the locality in which 
they are found; and as we must look upon every animal 
as the result of the climate and soil of its native home, 
