earth in Europe. These facts appear to Rudolphi inexplicable
on any other hypothesis than on that of the universal
diffusion of plants wherever physical conditions exist adapted
to their nature, independently of original dispersion from any
given centres.
But before we conclude with Rudolphi, that the dispersion
of plants is more extensive than it is possible to explain,
without admitting something closely bordering on the theory
of equivocal production, we must take into the account a
variety of facts, which plainly indicate a very extensive diffusion
of seeds through the atmosphere, as well as through
soils, and the possibility of their being preserved in the earth,
under a variety of circumstances, for an indefinite period of
time, so th a t particular kinds are almost everywhere at
hand, andTeady to make their appearance as soon as conditions
ale present which favour their developement.
I t is well known, that when a piece of ground is laid
bare, it is speedily covered with vegetation, and that this
consists not uniformly of plants which grow in the immediate
neighbourhood, but often of species which are only
found at very distant places. I t happens, indeed, not un-
frequeatly, that in soils newly turned up, ^plants appear
which have not been seen in the same districts within the
memory of man. These would appear to be, partly kinds
which are transported by the atmosphere, and take root on a
new surface which affords them a very congenial-soil, but
principally plants of which the seeds have lain concealedfbr
many years, at g re a t. depths under-ground, until they are
brought up under circumstances favourable to their vegetation.
I t is commonly observed, that clover is ready to spring up
wherever wood-ashes have been strewn, of where weeds
have been burnt. I t is said, that the same thing happens
when soap-lees have been spread upon the ground. Here the
soiHmbued with vegetable alkali, affords a congenial receptacle
to seeds which are very generally diffused through the
air.*
* I am informed by Professor Graham, that after any hill-pasture in Scotland
has been laid dry and limed and the surface broken, -white clover always makes
its appearance.
Many remarkable facts are referable to this or to some
analogous explanation.
The growth of, certain plants is known to prepare soils for
other species, which vegetate after such plants more readily
than elsewhere, and even spring up spontaneously, or without
seeds purposely.deposited. When vast forests are destroyed
by fire in countries previously uncultivated, either by
men with the i view of .bringing the ground under tillage, or
in those accidental conflagrations which break out occasionally,
and spread devastation through whole regions, it is
known, th a t certain kinds of trees *are usually- replaced by
others.' The same trees do not reappear in the same spots,
but they have successors which seem regularlyto take their
place. Thus the pine forests of North America when
burnt, afford room to forests of oak-trees. The particular
kinds of seeds aremeyer long wanting to which the new soil
is •congenial- I f yve had only to find ani explanation óf the
spontaneous appeafance of mushr ooms*or« of mould, we might,
perhaps, be «disposed to admit the theory of equivocal production,
where it is difficult to account for the presence of
seeds; but the production of oak-trees without acorns will
iM&rc^ly h ^ t o p ix i|)i^ ^ th o ii|ff the latter supposition
may, 'perhaps* be not more beyond the sphere of possibility
than the former^. I f the presence of acorns is admitted as
sufficiently indicated by the appearance of oak-forests on a
new surface, a similar admission will be made as to the pre-
'existence óf-seeds in all analogous circumstances;
^ In some instaneesHt appears clear, thattèèeds cannot have
•been newly introduced; and we are obliged to conclude,
•that they had long lain buried beneath the surface of the
soil. This wUs the case in the following instances which I
relate on the authority of Professor Graham of the university
of Edinburgh.
Previous to the year 171 §>> no broom grew in the king’s
park at Stirling, but in that year a camp was formed there,
and the surface of the ground consequently was broken in
many places. Wherever it was broken broom sprang up.
The plantrwas subsequently destroyed; but in 1745 a similar
growth appeared after the ground had been again broken