50 THE ESSEX NATURALIST.
cretins who at forty look very like a boy of eight or ten. But
these are true dwarfs. The snails are not ; they are healthy
and show a normal growth when put under favourable con-
ditions. Among plants, however, a similar great retardation
of growth occurs whenever seedlings are sown too thickly, and,
as with the snails, growth starts again as soon as they are planted
out singly.
On one other point I have observed a parallelism between
the snails and plant seedlings. If mustard and cress or any other
seeds are sown thickly they come up at first all approximately
of the same size. If kept for a long time, in some cases they
remain like this, in others a few seedlings begin to grow and
overtop the others. In time quite marked inequalities appear, so
the tallest may be three or four times the height of the shortest.
With snails both these cases occur. I have kept some nurseries
for two years with all the snails remaining approximately equal,
the largest perhaps double or a little more than double the
length of the smallest. More often a greater inequality appears,
one or two markedly outgrowing the remainder ; sometimes
also, but not so often, one or two remain markedly smaller than
all the rest.
To what is this dwarfing or retarding of growth due ? Semper,
in 1874, published the results of some elaborate experiments on
the growth of L. stagnalis, and a good shorter account is given
in his well-known book Animal Life. He grew snails in jars of
various sizes, as well as a varying number of snails in jars of the
same size. He showed that snails taken from one brood and
bred in jars of different sizes grew at a rate proportional to the
size of the jar in which they lived. Since then at least five
observers, belonging to as many different nations, have carried
out investigations, and all, except the last, agree substantially
with Semper as to the facts, but all favour different theories as
to the explanation. Like all other animals whose growth has
been recorded snails grow at first slowly, then with increasing
rapidity for some weeks. The first half of the growth curve
approximates to a geometrical progression, that is to say, the
length gains in a nearly constant ratio each week. After half the
adult size is reached the rate of growth falls off. In some of my
experiments L. pereger has doubled its length in four weeks. In
one series of L. stagnalis several doubled their length in one week.