416 Dk. E. J. SALISBURY ON THE 
The action of the plough in arable land naturally tends to obscure, although 
it does not completely destroy, the soil gradient, and earthworms in a minor 
degree perform for uncultivated soils the same function. 
It is now over forty years since Darwin called attention to the importance 
of earthworms affecting a natural cultivation of the soil (ef. C. Darwin, 
‘Vegetable Mould and Earthworms, London, 1881). He found from 
observations in four separate locations that the amount of soil brought to the 
surface as wormeasts ranged in weight from 7:56 tons to 18:12 tons per acre 
per year. These results were based on continuous collection of the worm- 
casts from definite areas, and are open to the objection that removal may 
have stimulated a more than normal deposition at the surface. There is at 
all events some reason to believe that the wormcast on the surface serves as 
a protection to the orifice of the worm burrow. ‘The values which Darwin 
obtained, however, represent an even layer of soil of from ‘09 in. to about 
0:10 in. depth per acre per year, which is considerably less than the corre- 
sponding value deduced by Darwin from the depth of burial of objects after 
a period of years (0:19—0:83 in. per ann.). 
During adverse seasons, namely the cold of winter and the heat of summer, 
earthworms have been known to descend to a depth of as much as eight feet, 
though they are not usually met with below three feet. The cultivating 
action thus mainly affects the upper layers of the soil, and in natural soils it 
would seem that worms feed mainly quite close to the surface. In the great 
majority of instances where the writer has dug up worms in woodlands, 
chalk downs, and heaths, they have been found feeding within the top few 
inches. 
In view of what has already been said with regard to the reaction gradient 
in undisturbed soils, it is important to know whence the soil is derived which 
passes through the worm and is deposited at the surface. If the wormcasts 
show a different reaction to the underlying surface soil, the question at once 
arises as to whether the wormeasts represent soil from the more alkaline (or 
less acid) layers of the subsurface or subsoil transported from below, or are 
they derived from the regions of maximum acidity near the surface ? 
The obvious test te employ for this purpose is the organic content which, 
as we have seen, diminishes with increasing depth. Some of the organic 
material is doubtless decomposed in the passage of the soil through the 
digestive tract ; but if it can be shown that the organic content of wormeasts 
is higher than that of the subsurface, then any change of reaction as com- 
pared with that of the surface must clearly be attributed to direct action of 
the Earthworm, and not to mere transport of soil from a stratum having a 
different reaction in the sense observed. 
The appended data (Table IT. & fig. 1), which represent losses on ignition, 
corrected for the carbon dioxide evolved from the carbonates, show clearly 
that in the eight locations investigated the wormeasts were derived from 
surface soil, The average values for the organic content of wormeasts in 
