to the ravages of insects, which have in later years attacked them; but
this is evidently by no means the sole, or even the principal cause of
their decay. In these cases, it is not to the exhaustion of the
particular acres on which the fruit trees have grown that we are to
ascribe their degeneracy, but to a general change in the condition of
the soil or the air; for it is equally impossible to rear them
successfully on absolutely new land in the neighborhood of grounds
where, not long since, they bore the finest fruit.

I remember being told, many years ago, by intelligent early settlers of
the State of Ohio, that the apple trees raised there from seed sown soon
after the land was cleared, bore fruit in less than half the time
required to bring to bearing those reared from seed gown when the ground
had been twenty years under cultivation. Analogous changes occur slowly
and almost imperceptibly even in spontaneous vegetation. In the peat
mosses of Denmark, Scotch firs and other trees not now growing in the
same localities, are found in abundance. Every generation of trees
leaves the soil in a different state from that in which it found it;
every tree that springs up in a group of trees of another species than
its own, grows under different influences of light and shade and
atmosphere from its predecessors. Hence the succession of crops, which
occurs in all natural forests, seems to be due rather to changes of
condition than of climate. See chapter iii., post.]


Uncertainty of Modern Meteorology.

We are very imperfectly acquainted with the present mean and extreme
temperature, or the precipitation and the evaporation of any extensive
region, even in countries most densely peopled and best supplied with