We have no means of judging how great a weight of earth a single
full-sized worm ejects during a year. Hensen estimates that 53,767 worms
exist in an acre of land; but this is founded on the number found in
gardens, and he believes that only about half as many live in
corn-fields. How many live in old pasture land is unknown; but if we
assume that half the above number, or 26,886 worms live on such land,
then taking from the previous summary 15 tons as the weight of the
castings annually thrown up on an acre of land, each worm must annually
eject 20 ounces. A full-sized casting at the mouth of a single burrow
often exceeds, as we have seen, an ounce in weight; and it is probable
that worms eject more than 20 full-sized castings during a year. If they
eject annually more than 20 ounces, we may infer that the worms which
live in an acre of pasture land must be less than 26,886 in number.
Worms live chiefly in the superficial mould, which is usually from 4 or 5
to 10 and even 12 inches in thickness; and it is this mould which passes
over and over again through their bodies and is brought to the surface.
But worms occasionally burrow into the subsoil to a much greater depth,
and on such occasions they bring up earth from this greater depth; and
this process has gone on for countless ages. Therefore the superficial
layer of mould would ultimately attain, though at a slower and slower
rate, a thickness equal to the depth to which worms ever burrow, were
there not other opposing agencies at work which carry away to a lower
level some of the finest earth which is continually being brought to the
surface by worms. How great a thickness vegetable mould ever attains, I
have not had good opportunities for observing; but in the next chapter,
when we consider the burial of ancient buildings, some facts will be
given on this head. In the two last chapters we shall see that the soil
is actually increased, though only to a small degree, through the agency
of worms; but their chief work is to sift the finer from the coarser
particles, to mingle the whole with vegetable debris, and to saturate it
with their intestinal secretions.
Finally, no one who considers the facts given in this chapter on the
burying of small objects and on the sinking of great stones left on the
surface on the vast number of worms which live within a moderate extent
of ground on the weight of the castings ejected from the mouth of the
same burrow on the weight of all the castings ejected within a known time
on a measured space will hereafter, as I believe, doubt that worms play
an important part in nature.
77