498 THE ZOOLOGIST. 
largely in the work of concealment when the ruins are so placed 
as to permit of the action of these subsidiary causes. 
The assistance which worms lend to the process of denudation 
is of special importance in the case of flat or gently inclined 
surfaces, for here it is not improbably the chief agent at work. 
Castings thrown up during or shortly before rain flow for a short 
distance down an inclined surface, and the finest earth is washed 
completely away. Again, during dry weather the disintegrated 
castings roll as little pellets, and, even on a level field, a strong 
wind will blow them all to leeward. 
In his closing chapter, Mr. Darwin thus summarises the chief 
benefits confirmed by worms on man as a tiller of the soil. 
‘ Worms,” he says, “ prepare the ground in an excellent manner 
for the growth of fibrous-rooted plants and for seedlings of all 
kinds. They periodically expose the mould to the air, and sift it, 
so that no stones larger than the particles which they can swallow 
are left in it. They mingle the whole intimately together, like a 
gardener who prepares fine soil for his choicest plants. In this 
state it is well fitted to retain moisture and to absorb all soluble 
substances, as well as for the process of nitrification. ‘The bones 
of dead animals, the harder parts of insects, the shells of land- 
mollusks, leaves, twigs, &c., are before long all buried beneath 
the accumulated castings of worms, and are thus brought in a 
more or less decayed state within reach of the roots of plants. 
Worms likewise drag an infinite number of dead leaves and other 
parts of plants into their burrows, partly for the sake of plugging 
them up and partly as food. The leaves which are dragged into 
the burrows as food, after being torn into the finest shreds, par- 
tially digested, and saturated with the intestinal and urinary 
secretions, are commingled with much earth. This earth forms 
the dark-coloured, rich humus, which almost everywhere covers 
the surface of the land with a fairly well-defined layer or mantle. 
When we behold a wide, turf-covered expanse, we should 
remember that its smoothness, on which so much of its beauty 
depends, is mainly due to all the inequalities having been slowly 
levelled by worms. It is a marvellous reflection that the whole 
of the superficial mould over any such expanse has passed, and 
will again pass, every few years through the bodies of worms.” 
In the face of such testimony as that adduced by Mr. Darwin’s 
untiring industry in this extremely interesting volume, it may 
