GENERAL DISTRIBUTION. 
171 
we do find are mostly sea-fowl. Usually a special zone 
is inhabited by a number of species, which gradually 
become rarer as we approach the confines of that zone, 
until at last they disappear altogether. The lesser seas 
and lakes do not interfere with the regular changes 
occurring among the land-birds inhabiting such zones; 
and the same may he said of tracts of land equally 
insignificant in extent. Where, however, we meet with 
expanses of territory destitute of water, or even broad 
oceanic tracts, a deviation from these regular changes 
takes place. Those who have sailed across the ocean, or 
wandered over the pathless plains of the Sahara or 
through the American steppes, find, at the end of their 
journey, that the whole animal creation before them has 
become changed as if by magic, and the conditions of 
existence entirely altered. 
As a rule, the zone of distribution extends further 
east and west, than from north to south. In the 
former direction, a difference in this respect may be 
observed covering hundreds of miles ; while, in the latter, 
scarcely any change whatever is to be remarked. But, 
in following out this hypothesis, one must carefully 
remember that the resemblance between the birds of 
America and the Old World, in both hemispheres, 
invariably decreases the further we go south in either 
country; and, indeed, a corresponding area of demarcation 
between these continents holds good. Yet, nevertheless, 
we find, in spite of the wide expanse of water which 
separates the two hemispheres, some species repre¬ 
sentative of each other, alike in the New as in the Old 
World; and these resemble one another so closely that 
the most careful observation is necessary to determine 
the difference between them. 
2 a 
