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SOME MINOR PROBLEMS CONCERNED IN THE
LOCAL DISTRIBUTION OF ANIMALS AND
PLANTS IN THE NEIGHBOURHOOD OF
FELSTEAD, ESSEX.
By J. FRENCH.
THE distribution of animal forms in a great river which flows
through several degrees of latitude occurs under circumstances
of which we can more or less give an account. Climatal changes
there affect the distribution, both directly and indirectly. In small
streams, such as our Essex rivers, this disturbing element is elimi-
nated, yet inequalities of distribution still prevail, and it is clear,
therefore, that some other explanation must be sought.
Our first case of this inequality of distribution will be that of the
Fresh-water Molluscs. In the account of the Chelmer Expedition
(Essex Naturalist, vol. v., p. 253), among the list of aquatic
mollusca observed, is Planorbis corneus. This shell is not now to
be found above Hartford End, either recent or fossil. We may,
perhaps, therefore regard it as a new introduction. In the same list
occurs Neretina fluviatilis. If this shell now lives above Hartford
End it is exceedingly rare. I believe it to be extinct. As a fossil, in
the Alluvium it is common, and we may therefore suppose it to have
retreated down stream. Ancylus lacustris I have never taken in the
Chelmer above Hartford End, and I believe it to be rare, or quite
extinct, there, although it is frequent enough in the ponds about
Felstead. These instances are sufficient to show that there is a
difference in the distribution of the molluscs in the Chelmer as the
estuary of the river is approached.
Some other cases of local distribution appear to have a bearing
on this question. The following table, compiled for the neighbour-
hood of Felstead, will illustrate cases :—