EFFECT OF OVERCROWDING OX GROWTH OF WATER SNAILS. 49
in a two pound jar, liberally supplied with weed, Elodea. Three
days later twelve of them were removed to eight large jars,
four as singles, four as pairs. Three died, but the remainder
all grew up to adult size by the middle of August, and had laid
eggs. Young snails had hatched out in every jar by Aug. 30th,
57 days after the hatching of their parents. On Sept. 6th, 64
days after hatching, I took this photograph. The 44 snails
below were all the survivors left in the nursery. The large snail
in the middle is of exactly the same age as its brothers, but
between five and six times the size, linear measurement, and
therefore more than hundred times larger in volume. Its
young are only a little smaller than their aunts ; and if I had
waited a few days longer before taking the picture, would have
exceeded them in size. And this difference is all due to giving
the snails a larger jar to grow up in ; there was no question of
depriving the others of water weed.
This experiment works quite regularly ; I have repeated it
hundreds of times and under considerably varied conditions, and
it always works the same way.
Now it is interesting to see next what happens if the experi-
ment is continued longer. Many have worked at this problem,
and several experimenters have spoken of it as dwarfism. If by
dwarf we mean an abnormal individual who will not grow up even
in good surroundings the term is a misnomer, for the snails will
grow up whenever removed from the nursery. The longest
experiment I have made lasted over three years. One snail
laid eggs in August, 1922, and 200 young hatched out. From
this stock I planted out snails at various dates during 1923-4 and
1925. The rest remaining in the nursery diminished in number,
partly from this cause, but still more owing to deaths. By
March, 1925, only four survived, but these all grew up and
bred when planted out in two jars, a pair in each jar. Not all
the shells have been saved, but plate 4, fig. 2, shows in separate
columns those which died in 1923, 1924 and 1925 respectively.
All the larger ones had been planted out during the years shown
under each column. The smaller ones are those which died
in the nursery.
Among human beings and all the domestic animals one can
roughly judge the age at sight, because the size corresponds to
the age within slight limits. It is true that there are a few