CHAPTER EX. 
THE INFLUENCE OF CLIMATE. 
The Limits of the Distribution of Species defined by Climate—Sub- 
tropical Character of the Middle and Western States during the 
Summer Months—The Limits within which Sorghum may be 
profitably grown defined—The Summer Isotherm of 70° Fahr.— 
The Summer Line of 72° Fahr. probably the Northern Limit of 
the District within which Sugar may be most successfully pro- 
duced. 
WuiILE the nature of the soil determines in a great de- 
gree the situation in which each individual plant is found 
to flourish in a state of nature, climate fixes the wider 
limits of the distribution of each species. The geographical 
range within which the cultivation of the sorghum is capa- 
ble of being successfully extended in this country, may now 
be regarded as pretty accurately settled, at least as respects 
those varieties which have proved the most valuable. This 
range is very extensive, but within its limits the soil and 
situation define much more narrowly the area within which 
alone sugar will become a staple article of production. 
The results of the past eight years, properly interpreted, 
indicate this clearly. The subtropical character of those 
regions of the Old World from whence the several varieties 
of the sorghum were originally derived, might perhaps lead 
to the inference that, in this country, we are attempting to 
carry the limits of the sugar growing region farther north 
than analogy of climate would support. But such conclu- 
sions have no foundation in fact, for it should be remem- 
bered that an extreme, almost tropical temperature is char- 
acteristic of the middle latitudes of the United States, 
(68 ) 
