486 
GEOGRAPHY. 
BOOK V. 
are as certain and immutable as any of those with the nature 
of which we are acquainted. It is probable that temperature 
is the principal cause, from the well-known fact that the 
vegetable productions of hot climates can be successfully cul- 
tivated in cold ones by the aid of heat ; and that the plants of 
cold climates may be cultivated in hotter climates by an arti- 
ficial reduction of temperature. But that other causes also 
operate, is apparent from the impossibility of cultivating the 
plants of any high latitudes in those considerably to the south. 
Thus, when living plants were brought to England from Mel- 
ville Island, no means whatever could be discovered of keeping 
them alive, although the temperature at which they were main- 
tained did not materially vary from that to which they must 
have been often exposed, in the summer season, in their own 
climate. Assuming, however, for the present, that temper- 
ature is the most efficient cause of variety in the distribution 
of plants, the first point to consider is, how far temperature 
and latitude are uniformly the same in either hemisphere. 
This has been discussed, with his habitual skill, by Humboldt, 
of whose observations I must avail myself in nearly all that I 
can say upon the subject. According to this observer, the 
geographical parallels of latitude do not indicate correspond- 
ing temperature, either in the old and new world, or in the 
northern and southern hemispheres. In the new world the 
temperature decreases more rapidly as we recede from the 
equator than in the old world ; and in the southern hemi- 
sphere, beyond the parallel of 34°, the summers are colder than 
in corresponding latitudes of the northern hemisphere ; but 
the winters milder. On this account Humboldt concludes 
that " the lines of equal mean annual heat, which may be 
called isothermal, are not parallel with the equator, but inter- 
sect the geographical parallels at a variable angle." 
The following table shows the difference in the mean annual 
heat of the same latitudes in the old and new worlds : — 
