the formation of vegetable mould-第15章
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sand； namely； the molluscan Onchidium and many Echinoderms。 ｛37｝
If earth were swallowed only when worms deepened their burrows or
made new ones； castings would be thrown up only occasionally； but
in many places fresh castings may be seen every morning； and the
amount of earth ejected from the same burrow on successive days is
large。 Yet worms do not burrow to a great depth； except when the
weather is very dry or intensely cold。 On my lawn the black
vegetable mould or humus is only about 5 inches in thickness； and
overlies light…coloured or reddish clayey soil： now when castings
are thrown up in the greatest profusion； only a small proportion
are light coloured； and it is incredible that the worms should
daily make fresh burrows in every direction in the thin superficial
layer of dark…coloured mould； unless they obtained nutriment of
some kind from it。 I have observed a strictly analogous case in a
field near my house where bright red clay lay close beneath the
surface。 Again on one part of the Downs near Winchester the
vegetable mould overlying the chalk was found to be only from 3 to
4 inches in thickness； and the many castings here ejected were as
black as ink and did not effervesce with acids； so that the worms
must have confined themselves to this thin superficial layer of
mould； of which large quantities were daily swallowed。 In another
place at no great distance the castings were white； and why the
worms should have burrowed into the chalk in some places and not in
others； I am unable to conjecture。
Two great piles of leaves had been left to decay in my grounds； and
months after their removal； the bare surface； several yards in
diameter； was so thickly covered during several months with
castings that they formed an almost continuous layer； and the large
number of worms which lived here must have subsisted during these
months on nutritious matter contained in the black earth。
The lowest layer from another pile of decayed leaves mixed with
some earth was examined under a high power； and the number of
spores of various shapes and sizes which it contained was
astonishingly great； and these crushed in the gizzards of worms may
largely aid in supporting them。 Whenever castings are thrown up in
the greatest number； few or no leaves are drawn into the burrows；
for instance the turf along a hedgerow； about 200 yards in length；
was daily observed in the autumn during several weeks； and every
morning many fresh castings were seen； but not a single leaf was
drawn into these burrows。 These castings from their blackness and
from the nature of the subsoil could not have been brought up from
a greater depth than 6 or 8 inches。 On what could these worms have
subsisted during this whole time； if not on matter contained in the
black earth？ On the other hand； whenever a large number of leaves
are drawn into the burrows； the worms seem to subsist chiefly on
them； for few earth…castings are then ejected on the surface。 This
difference in the behaviour of worms at different times； perhaps
explains a statement by Claparede； namely； that triturated leaves
and earth are always found in distinct parts of their intestines。
Worms sometimes abound in places where they can rarely or never
obtain dead or living leaves； for instance； beneath the pavement in
well…swept courtyards； into which leaves are only occasionally
blown。 My son Horace examined a house； one corner of which had
subsided； and he found here in the cellar； which was extremely
damp； many small worm…castings thrown up between the stones with
which the cellar was paved； and in this case it is improbable that
the worms could ever have obtained leaves。 Mr。 A。 C。 Horner
confirms this account； as he has seen castings in the cellars of
his house； which is an old one at Tonbridge。
But the best evidence； known to me； of worms subsisting for at
least considerable periods of time solely on the organic matter
contained in earth； is afforded by some facts communicated to me by
Dr。 King。 Near Nice large castings abound in extraordinary
numbers； so that 5 or 6 were often found within the space of a
square foot。 They consist of fine； pale…coloured earth； containing
calcareous matter； which after having passed through the bodies of
worms and being dried； coheres with considerable force。 I have
reason to believe that these castings had been formed by species of
Perichaeta； which have been naturalized here from the East。 ｛38｝
They rise like towers； with their summits often a little broader
than their bases； sometimes to a height of above 3 and often to a
height of 2。5 inches。 The tallest of those which were measured was
3。3 inches in height and 1 inch in diameter。 A small cylindrical
passage runs up the centre of each tower； through which the worm
ascends to eject the earth which it has swallowed； and thus to add
to its height。 A structure of this kind would not allow leaves
being easily dragged from the surrounding ground into the burrows；
and Dr。 King； who looked carefully； never saw even a fragment of a
leaf thus drawn in。 Nor could any trace be discovered of the worms
having crawled down the exterior surfaces of the towers in search
of leaves； and had they done so； tracks would almost certainly have
been left on the upper part whilst it remained soft。 It does not；
however； follow that these worms do not draw leaves into their
burrows during some other season of the year； at which time they
would not build up their towers。
From the several foregoing cases； it can hardly be doubted that
worms swallow earth； not only for the sake of making their burrows；
but for obtaining food。 Hensen； however； concludes from his
analyses of mould that worms probably could not live on ordinary
vegetable mould； though he admits that they might be nourished to
some extent by leaf…mould。 ｛39｝ But we have seen that worms
eagerly devour raw meat； fat； and dead worms； and ordinary mould
can hardly fail to contain many ova； larvae； and small living or
dead creatures； spores of cryptogamic plants； and micrococci； such
as those which give rise to saltpetre。 These various organisms；
together with some cellulose from any leaves and roots not utterly
decayed； might well account for such large quantities of mould
being swallowed by worms。 It may be worth while here to recall the
fact that certain species of Utricularia； which grow in damp places
in the tropics； possess bladders beautifully constructed for
catching minute subterranean animals； and these traps would not
have been developed unless many small animals inhabited such soil。
The depth to which worms penetrate； and the construction of their
burrows。Although worms usually live near the surface； yet they
burrow to a considerable depth during long…continued dry weather
and severe cold。 In Scandinavia； according to Eisen； and in
Scotland； according to Mr。 Lindsay Carnagie； the burrows run down
to a depth of from 7 to 8 feet； in North Germany； according to
Hoffmeister； from 6 to 8 feet； but Hensen says； from 3 to 6 feet。
This latter observer has seen worms frozen at a depth of 1。5 feet
beneath the surface。 I have not myself had many opportunities for
observation； but I have often met with worms at depths of 3 to 4
feet。 In a bed of fine sand overlying the chalk； which had never
been disturbed； a worm was cut into two at 55 inches； and another
was found here at Down in December at the bottom of its burrow； at
61 inches beneath the surface。 Lastly； in earth near an old Roman
Villa； which had not been disturbed for many centuries； a worm was
met with at a depth of 66 inches； and this was in the middle of
August。
The burrows run down perpendicularly； or more commonly a little
obliquely。 They are said sometimes to branch； but as far as I have
seen this does not occur； except in recently dug ground and near
the surface。 They are generally； or as I believe invariably； lined
with a thin layer of fine； dark…coloured earth voided by the worms；
so that they must at first be made a little wider than their
ultimate
