154 PLANT DISTRIBUTION
present, but, being obstructed in its course, kept the bed of the
valley in a state of almost perpetual flood. It was under these
conditions, combined with a water-logged state of the country
generally, that the Pond-flora of remote situations probably arose.
There would be no chance whatever of furnishing these ponds
with species that are obviously in a lingering condition under the
present aspect of the country. In further connection with the
old river system are the plants bordering on the upper reaches
of its tributaries. Many of these have severed their connection
with the main stream, and they, therefore, fall in the same
category of lingering species.
The rivers of to-day have added, and are adding, their quota
of plants. Some of these species that occur on the river-banks,
or near neighbourhood, have not penetrated far up the tribu-
taries, nor have they spread to the interior of the country.
These, therefore, fall to be classified as new, or comparatively
new, arrivals.
We will now take the case of the Woods. Just in proportion
that they were more continuous than they are now, and less con-
tinuous than the river, such has been their influence in the
transmission of species. It is impossible to measure what that
influence was in the past. That it was great and gave us a fair
proportion of our present species is a circumstance that helps us
somewhat in tabulating certain forms. If these forms are rarely
or never found away from the wood, then we may know that
they are remnants of an ancient flora, and we place them as
such.
The next important channel of communication for the
district seems to have been the Roads. These are of very ancient
date, so much so that they have become possessed as it were
with a flora of their own, which is indicated as "wayside" in
the botanical lists. Along this line have doubtless been intro-
duced many species that have now spread themselves into the
interior, although we should expect to find them in greater force
by the roadside. When these occur so generally and widely
distributed we are inclined to class them as older arrivals, parti-
cularly if they show no signs of being of an invasive character. If,
on the other hand, we rarely or never find them away from the
road, then we are surely justified in classing them as new or
comparatively new arrivals.