THE LOCAL EXTINCTION OF MOLLUSCS.
91
Limnaea and Succinea are then at a decided disadvantage. I have
carefully noted the case of both species of Limnaea above men-
tioned. Limnaea. truncatula is always ready to take an excursion
on wet grass, but L. palustris will not go beyond the wet mud on
which it delights to repose ; and what is more, it does not here
live with other than a river-communication. Both may be
described as marsh species, and the drying up of marsh land
affects them equally. A small colony of L. palustris, var. Corvus,
is here (at Felstead) reduced to probably less than a dozen
members. I saw one of these pairing with an individual of the
normal type—a sure sign of its diminished numbers. The pond
fauna also, from the circumstance of increasing limitation, is
gradually giving in.8 Limnaea stagnalis of the normal type is now
very uncommon ; the variety fragilis is much more plentiful ;
both have forsaken the river. Planorbis carinatus appears to have
nearly forsaken the river also, although it is common in such
ponds as we have here.
These instances will show that our Freshwater Molluscs
undergo the same mutations of distribution as their land kins-
men. Taking mine as a fair locality, we find that local extinc-
tion is in constant progress.
We have now to turn to the other side of the question, and
see what we have in the way of introduction to counterbalance
the loss of species becoming extinct. It is obvious that where
whole families are unable to exist on account of unfavourable
conditions, there is but little hope of new arrivals establishing
themselves, assuming these to require pretty much the same
conditions as the old members. Nevertheless, we are not quite
without examples of the immigration of species not native of the
locality.
The introduction of a species is not a matter of common
observation, but I have had the good fortune to witness what I
believe to be a great local increase of one species—Helix virgata.
It came in from the north, apparently, and the great army is
still to be found a few miles in the rear of the outposts : but it is
most certainly moving southwards, and, I doubt not, will pass
over this part of Essex like a slow-moving wave. Then, again,
among the aquatic species I have, I think, been able to trace a
8 [" Drainage has, no doubt, been carried out very largely during the past three or four
years. On the other hand, the disuse of the Chalky Boulder Clay as manure has caused the
marlpits to become ponds, which are extremely numerous in a C B. Clay district—as may be
seen N. of Chelmsford."—T. V. H. On this Mr. French remarks, "Our marl-pits at Felstead,
as a rule, do not contain water, they having been dug through to the gravels."J