CHAP. I.] 
INTRODUCTORY. 
5 
In the western hemisphere we find equally striking examples. 
The Eastern United States possess very peculiar and interesting 
plants and animals, the vegetation becoming more luxuriant as 
we go south but not altering in essential character, so that when 
we reach the southern extremity of Florida we still find our- 
selves in the midst of oaks, sumachs, magnolias, vines, and 
other characteristic forms of the temperate flora; while the 
birds, insects, and land-shells are almost identical with those 
found further north. But if we now cross over the narrow 
strait, about fifty miles wide, which separates Florida from the 
Bahama Islands, we find ourselves in a totally different country, 
surrounded by a vegetation which is essentially tropical and 
generally identical with that of Cuba. The change is most 
striking, because there is no difference of climate, of soil, 
or apparently of position, to account for it; and when we 
find that the birds, the insects, and especially the land- 
shells are almost all West Indian, while the North American 
types of plants and animals have almost all completely 
disappeared, we shall be convinced that such differences and 
resemblances cannot be due to existing conditions, but must 
depend upon laws and causes to which mere proximity of 
position offers no clue. 
Hardly less uncertain and irregular are the effects of climate. 
Hot countries usually differ widely from cold ones in all their 
organic forms ; but the difference is by no means constant, nor 
does it bear any proportion to difference of temperature. 
Between frigid Canada and sub-tropical Florida there are less 
marked differences in the animal productions than between 
Florida and Cuba or Yucatan, so much more alike in climate and 
so much nearer together. So the differences between the birds 
and quadrupeds of temperate Tasmania and tropical North 
Australia are slight and unimportant as compared with the 
enormous differences we find when we pass from the latter 
country to equally tropical Java. If we compare corresponding 
portions of different continents, we find no indication that the 
almost perfect similarity of climate and general conditions has 
any tendency to produce similarity in the animal world. The 
equatorial parts of Brazil and of the West Coast of Africa are 
