An introductory text-book of logic - Mellone, Sydney - Rare Books at ...

280 THE THEORY OF INDUCTION
including a vast number of unrolled flints of all sizes, has
been left on the surface and forms a bed of stiff red clay,
from six to fourteen feet in thickness. Over the red clay,
wherever the land has long remained as pasture, there is a
layer a few inches in thickness of dark-coloured vegetable
mould." In another case chalk spread over the surface of
a field was buried seven inches in thirty years ; in another
a field whose surface had been originally thickly covered
with flints of various sizes, was in thirty years covered with
compact turf growing out of vegetable mould, beneath which
lay the flints. In the latter case, also, the worm-castings
increased in numbers as the pasture improved. In yet
another case, objects such as chalk, cinders, pebbles, &c.,
of different degrees of heaviness, were tried on the same
land ; and it was found that they sank to the same depth
in the same time, being covered by vegetable mould. The
only material circumstance common to all the different
cases of the formation of vegetable mould on the surface,
is the presence of earthworms which are estimated, on the
basis of careful observation and calculation, to number from
thirty to upwards of fifty thousand in an acre, and to yield
castings weighing in the mass from seven and a half to
over eighteen tons in an acre. There is therefore no doubt
of the adequacy of the cause which the Method of Single
Agreement suggests.
(b) Negative Instances. The suggestion was found to be
confirmed as follows. Boulders, of sufficient size to keep
the earth beneath them dry, do not sink, although the sur
face of the ground is raised all round their edges. But in
permanently dry earth very few earthworms exist. In one
case a stone in length about five feet and in breadth three,
had only sunk two inches in thirty-five years ; but
"
on
digging a large hole to a depth of eighteen inches where the
stone had lain, only two worms and a few burrows were
seen, although the soil was damp and seemed favourable
for worms. There were some large colonies of ants beneath
the stone, and possibly since their establishment the worms
had decreased in number." Among other negative instances
recorded, is the case of a dense forest of beech -trees, in
Knole Park. "The
ground," says Darwin, "was thickly