156
PLANT DISTRIBUTION.
condition, therefore, which is one that can easily be made out,
places many plants in the list. But some plants are either not
invasive, or only so to a very small degree, and yet in other
respects they show great signs of vigour and do not appear to be
capable of attaining to greater capacity in their development.
Such plants will live and produce seed under the most untoward
circumstances, and when this is the case we have no hesitation,
whatever may be their numbers, in placing them in the culmina-
ting stage. On the other hand there are plants which are
produced in great quantity, which show certain signs of local
weakness, and although in virtue of the great profusion in which
we find them we place some of them in the table of culminations,
we have reason to think they have passed their stage and are
now on the downward grade. We shall in the tables be able to
give illustrations of all these groups.
A question now naturally presents itself as to the length of
time that any particular species may have been in the locality.
We have to reply that all our measures are relative, and except
for a few recent plants that may from time to time arrive they
will, we fear, remain so. The Alder, the Hazel, and the Oak
were here in Neolithic times, as I have evidence to prove, and
probably a great many more have as high a degree of antiquity,
and while on the other hand a few may he scarcely a century old.
It would seem from what we know of the mutation of plants,
which is ever in progress, that very material changes may occur
in the course of two or three centuries. A few species may have
been characteristic ever since the days that Caesar wrote of
them in his Commentaries but minor changes, which ate great
in the aggregate, have undoubtedly been repeated many times
since those early dates.
We will now introduce the tables with a few words of
explanation. Being an attempt only at part of an abstruse
problem, they are submitted as provisional, and are subject to
corrections and additions. The corrections and omissions are
due to imperfect observation and imperfect knowledge of
all the plants of the district. I may add that the most
valuable corrections would be those obtained from observations
extended over a wider district. These would not only be
authoritative, but they would enhance the value of the tables as
scientific contributions.