PLANT DISTRIBUTION.
155
The roads are now supplemented by the railway-track. The
time is yet too young to give an account of species arriving by
this channel. Its influence, however, must inevitably be felt, as
it supplies continuity of area—a prime factor in plant distribution.
The Commons have undoubtedly brought a few species in the
past that could not very well have come by any other channel,
But because these commons were more or less discontinuous,
and did not occupy a great space in our district, I do not think
their influence at any time to have been great. The few species
probably referable to their agency, and that can only now be
described as lingerers, may be enumerated.
Lastly we come to the Agency of Man. What it was in the
past we do not know. A few of our aromatic plants may be
remnants of that earlier agency, but it would be quite unsafe to
classify them as such. Of its modern agency much more is to
be said. There are plants in the cornfields and gardens that are
never found away from those places. These, therefore, we
presume, are incapable of coping with the native flora, and unless
they were re-introduced from time to time they would become
extinct. This compels us to classify them as comparatively new
arrivals although in point of fact the process may have been
going ou for centuries. It must be of somewhat ancient date,
for we find them described in the botanical lists as inhabitants
of the corn field or garden. That the direct influence of man
otherwise plays but small part in the economy of distribution
(other than destruction) may be shewn by reference to the
number of exotics that are being continually brought into
competition with the native flora and the very small proportion
of these that succeed in obtaining a footing. It is really so small
that we shall be able to adduce but few instances from the
district under consideration.
So far the channels of transmission assist us in placing a
great number of plants in two of their respective Tables, but
there remains another Table, which includes plants in their
culminating stage, to be provided for. Some plants here, as
before hinted, place themselves readily in that list, but a great
many can only be admitted by careful observation and the
weighing of evidence.
A plant of a highly invasive character is never very far from
its culminating stage, even though it be a new arrival. That