{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.
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