18 CONCLUSIONS ON ANCIENT CLIMATES. 
nations, in spite of their jealousies and their perpetual wars, 
sometimes contrive to exchange the products of provinces very 
widely separated from each other. The mounds of Ohio con- 
tain pearls, thought to be marine, which must have come from 
the Gulf of Mexico, or perhaps even from California, and the 
knives and pipes found in the same graves are often formed of 
far-fetched material, that was naturally paid for by some home 
product exported to the locality whence the material was 
derived. The art of preserving fish, flesh, and fowl by drying 
and smoking is widely diffused, and of great antiquity. The 
Indians of Long Island Sound are said to have carried on a 
trade in dried shell fish with tribes residing very far inland. 
From the earliest ages, the inhabitants of the Faroe and Orkney 
Islands, and of the opposite mainland coasts, have smoked wild 
fowl and other flesh. Hence it is possible that the animal and 
the vegetable food, the remains of which are found in the 
ancient deposits [am speaking of, may sometimes have been 
brought from climates remote from that where it was consumed. 
The most important, as well as the most trustworthy con- 
clusions with respect to the climate of ancient Europe and 
Asia, are those drawn from the accounts given by the classical 
writers of the growth of cultivated plants; but these are by no 
means free from uncertainty, because we can seldom be sure 
of an identity of species, almost never of an identity of race or 
variety, between vegetables known to the agriculturists of 
Greece and Rome and those of modern times which are thought 
most nearly to resemble them. Besides this, there is always 
room for doubt whether the habits of plants long grown in 
different countries may not have been so changed by domestica- 
tion or by natural selection, that the conditions of temperature 
and humidity which they required twenty centuries ago were 
different from those at present demanded for their advan- 
tageous cultivation.* 
* Probably no cultivated vegetable affords so good an opportunity of study- 
ing the laws of acclimation of plants as maize or Indiancorn. Maize is grown 
from the tropics to at least lat. 47° in Northeastern America, and farther 
