DISTRIBUTION OF ANIMALS AND PLANTS IN FELSTEAD, ESSEX. 203
the rank vegetation is cleared away year by year, the hedgerow will
be prolific of species and individuals. Any botanical collector can
verify this statement. In a new clearing certain species will some-
times become very prominent and give one the idea of great vigour
(Jack-by-the-Hedge, Sisymbrium alliaria, is a good example). Yet on
reflection it must be seen that such plants are only tenants at will,
and if they had no assistance from the hand of man (in removing
obstructions) they must inevitably in a few generations become locally
extinct. Thus we see that in our restricted areas the struggle for
life by itself ultimately results in the local extinction of many forms
and in the preservation of a few.
These notices of plants might be expanded to almost any extent.
I have quoted a well-known family, the Crowfoots, and, in opposition,
a family less well-known, the Orchids. We are in a better position
sometimes to deal historically with the less well-known. Notices of
their occurrence are in most lists definitely stated, whereas the
common forms are disposed of with a passing notice. This is un-
fortunate, for the mere collation of present with past lists would often
reveal incipient changes of distribution which could not be other
than instructive. It becomes therefore a question as to whether
a chronicle of abundant forms and the conditions under which they
maintain that abundancy is not a desideratum.
To recapitulate briefly, we have found that the various species of
Freshwater and Land Mollusca are distributed unevenly in localities
separated by only a few miles and where direct communication is
often possible. That in time not long past their distribution was
greatly different from that at present obtaining.
As regards the Fish, that in the river where communication is
still open certain species affect different parts of that river and that
the food supply appears to take no part in this separation. That,
withal, fish show much adaptability to circumstances, as seen in cases
of pond fish. These pond fish may be from their mode of occurrence
descendants of ancestors many generations removed.
That among Reptiles Snakes are diminishing in a certain tract in
Essex from causes which cannot be certainly stated, and that the
extinction of Adders in the same tract may be due to the enclosure
and cultivation of the Commons.
As regards the Plants, that they seem to show for any particular
locality a period of culmination and decay. That sporadic forms
seem to be connected with the preservation of germs which takes place