castings may be seen in extraordinary numbers on commons and chalk-
downs, so as almost to cover the whole surface, where the soil is
poor and the grass short and thin. But they are almost or quite as
numerous in some of the London parks, where the grass grows well
and the soil appears rich. Even on the same field worms are much
more frequent in some places than in others, without any visible
difference in the nature of the soil. They abound in paved court-
yards close to houses; and an instance will be given in which they
had burrowed through the floor of a very damp cellar. I have seen
worms in black peat in a boggy field; but they are extremely rare,
or quite absent in the drier, brown, fibrous peat, which is so much
valued by gardeners. On dry, sandy or gravelly tracks, where heath
with some gorse, ferns, coarse grass, moss and lichens alone grow,
hardly any worms can be found. But in many parts of England,
wherever a path crosses a heath, its surface becomes covered with a
fine short sward. Whether this change of vegetation is due to the
taller plants being killed by the occasional trampling of man and
animals, or to the soil being occasionally manured by the droppings
from animals, I do not know. {9} On such grassy paths worm-
castings may often be seen. On a heath in Surrey, which was
carefully examined, there were only a few castings on these paths,
where they were much inclined; but on the more level parts, where a
bed of fine earth had been washed down from the steeper parts and
had accumulated to a thickness of a few inches, worm-castings
abounded. These spots seemed to be overstocked with worms, so that
they had been compelled to spread to a distance of a few feet from
the grassy paths, and here their castings had been thrown up among
the heath; but beyond this limit, not a single casting could be
found. A layer, though a thin one, of fine earth, which probably
long retains some moisture, is in all cases, as I believe,