€HAr. 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 

 w^e 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 

 iind 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 



