VEGETABLE PRODUCTIONS. 
181 
the tropics ; this occurring not because the elevation is sufficient 
to warrant the growth, but that the lower level of the Isthmus 
is cooled much below the average temperature of its latitude by 
the constant northeast winds, by the great humidity of the north- 
ern slope, an,d by the proximity of the lofty table-lands and mount- 
ain summits which cool the land in their vicinity. The differ- 
ent families of plants are, for these reasons, more scattered with 
apparent indiscrimination, and the mountain regions, with few 
exceptions, are clothed with species found in the alluvial basins ; 
nor is it an uncommon thing to see the pines which stud the 
loftiest and coldest summits decorated with the fimbriated 
orcMdea, or trellised with the scitamenem of the plains. 
The mean annual temperature of the Gulf shore of the Isthmus 
is 81°, as is that of the Pacific coast ; and the isotheral line, or 
the curve of mean summer temperature of 8° of Humboldt, 
passes over the Isthmus. When this is extended into Africa it 
passes through the countries lying south of the Mediterranean, 
Algiers, Tunis, Barca", and Egypt, along lat. 30° 1ST. In Asia it 
passes through Central Persia, Thibet, and China, and thence 
into the Chinese sea, north of the island of Formosa. 
The curve of mean winter temperature, or isocMmenal line of 
70°, passes through the northern parts of Oaxaca and Tobasco, 
and the southern part of Yera Cruz, in lat. 18° ; passes south- 
ward into Africa, through Sahara and Southern JSTubia, thence 
along Arabia Felix through Hindostan in lat. 15° and at Tonquin, 
passes into the ocean about lat. 17°, indicating that the summer 
heat is that of latitude 12 degrees more 'northerly in Africa and 
Western Asia, and the winter heat that of its own latitudes even 
on large continents. In other words, it has a cooler summer 
and more moderate winter than similar latitudes, and it is this 
extreme equality of climate which gives to these lands the beauty 
and profusion of vegetation with which they are clothed. 
The curve of mean annual temperature, or the isothermal line, 
of 75° passes through the Isthmus, through Southern Egypt and 
Arabia in latitude 25°, and Persia in latitude 28°, showing the 
mean temperature to be that of 12° more north in Central Asia. 
It is on the outside of the limits of the equatorial zone, and its 
productions are those of a tropical zone, which is an advantage 
