192 SOME MINOR PROBLEMS CONCERNED IN THE LOCAL
In considering this list, I believe that we are quite clear of recent
introductions, as some of these species are found fossil, and others
are scattered in isolated directions, often far apart, and precluding
by their remoteness the idea of modern introduction. This distribu-
tion agrees with the localities given in conchological works, and we
thus find that the anomaly is pretty general the country throughout.
Why should some forms, therefore, thus prefer the river to the pond,
and vice versa ? That it is a matter of preference appears from the
two or three cases already quoted of distribution in the Chelmer, for
we may say with regard to them that they have the passage open to
other districts, where the conditions of sustenance and increase
appear to be the same, although the water is not so still and deep,
nor perhaps so warm. In the case of ponds there is the same
element of still water, etc, but the way is not so open to allow of
migrations. Considering the various agencies that are still at work
in the dispersal of Fresh-water Molluscs, it is not safe to conjecture
how the present state of distribution has been brought about;
yet it is instructive to look back at the known history of the county
as furnished by the river Alluvium, and to see in that past history
how the work of distribution was immensely facilitated for these
forms. With the river consisting of a series of standing pools, it
would seem that all the pond molluscs might there have been repre-
sented, whilst a way remained open for their easy distribution.
We will now turn to the Fish fauna of the same localities.
Observations of over thirty years bear witness to the great changes
in the species of this class in the Chelmer at Felstead. Forty years
ago the river was full of Pike; now there are none. Trout then were
extremely rare; now they are common. Thirty years ago Roach
were plentiful, and were much in excess of Dace; now the reverse is
the case. At that time the river literally swarmed with Minnows;
now they have diminished by at least nine-tenths. Twenty to
twenty-five years ago Perch were very common; now they are rare.
The causes that are said to have operated may be thus stated.
The Pike of forty years ago were artificially introduced. These
throve for some time amazingly, and were thought to have destroyed
the Trout previously existing. After a time the Pike died out, and
the Roach, Minnows, and other small fish increased to an extent
unknown before. The Perch are said to have been artificially intro-
duced, and they flourished well for a time, but in a few years died
out. The paucity of small fish now is said to be due to the increase