PLANT DISTRIBUTION.
153
nature such a concurrence of conditions does not often take place.1
But if it should happen that a new species becomes established
in a crowded neighbourhood, then other changes will take place
as a consequence.
The crowded neighbourhood, or full flora, is the resultant of
all the forces of distribution that have been at work in the past,
and anything that disturbs equilibrium in one place will cause
equivalent changes in some other place The stronger plants
becoming invasive, the weaker ones will gradually succumb. It
is not necessary to go into the causes of either stimulation or
weakening. We are very well assured from observation that
such changes do take place. The effect of these constantly
recurring changes is to set up something like a stream of
vegetation, in which some plants are progressing, some are in
culmination, and others are in retreat or decay.
This aspect of the problem enables us to sketch out three
groups into one of which every plant may be placed. Ordinary
observation will enable us to place many of them in their
respective groups, but with the majority of plants there is
difficulty in assigning them to their proper positions. It will
now be necessary to explain at some length the principles under
which we have been enabled to tabulate a fair number of plants,
and to discuss the probabilities under which they have been
allowed to enter.
The district of Felstead has been supplied with plants in the
past by channels of communication some of which have been
altered or do not exist in the present day. That the majority
of plants have come along, or do come along, those lines of com-
munication will appear more or less in the sequel. These lines
of communication in old times were the Rivers (Chelmer and
Ter), the Woods, the Roads, the Commons, and probably a
certain amount of artificial agency. The lines of communication
of to-day are restricted as regards the rivers, which have altered
their character ; the Woods, which are now only conservatories ;
and the Commons, which have disappeared. Artificial agency
has probably greatly increased.
I have placed these lines of communication, so far as I can
tell, in the order of their importance, the rivets being of the first.
In old times the River was not confined to its channel as at
1 I propose having more to say on this subject in a future paper.