“And Man grew a thumb for that he had need of it,
  And developed capacities for prey. 
  For the fastest men caught the most animals,
  And the fastest animals got away from the most men. 
  Whereby all the slow animals were eaten,
  And all the slow men starved to death.”

[Illustration]

HOW THE SOIL IS MADE

(FROM THE FORMATION OF VEGETABLE MOULD.)

BY CHARLES DARWIN.

[Illustration:  W]

Worms have played a more important part in the history of the world than most persons would at first suppose.  In almost all humid countries they are extraordinarily numerous, and for their size possess great muscular power.  In many parts of England a weight of more than ten tons (10,516 kilogrammes) of dry earth annually passes through their bodies and is brought to the surface on each acre of land; so that the whole superficial bed of vegetable mould passes through their bodies in the course of every few years.  From the collapsing of the old burrows the mould is in constant though slow movement, and the particles composing it are thus rubbed together.  By these means fresh surfaces are continually exposed to the action of the carbonic acid in the soil, and of the humus-acids which appear to be still more efficient in the decomposition of rocks.  The generation of the humus-acids is probably hastened during the digestion of the many half-decayed leaves which worms consume.  Thus the particles of earth, forming the superficial mould, are subjected to conditions eminently favorable for their decomposition and disintegration.  Moreover, the particles of the softer rocks suffer some amount of mechanical trituration in the muscular gizzards of worms, in which small stones serve as mill-stones.

[Illustration:  DIAGRAM OF THE ALIMENTARY CANAL OF AN EARTH-WORM.]

The finely levigated castings, when brought to the surface in a moist condition, flow during rainy weather down any moderate slope; and the smaller particles are washed far down even a gently inclined surface.  Castings when dry often crumble into small pellets and these are apt to roll down any sloping surface.  Where the land is quite level and is covered with herbage, and where the climate is humid so that much dust cannot be blown away, it appears at first sight impossible that there should be any appreciable amount of sub-aerial denudation; but worm castings are blown, especially while moist and viscid, in one uniform direction by the prevalent winds which are accompanied by rain.  By these several means the superficial mould is prevented from accumulating to a great thickness; and a thick bed of mould checks in many ways the disintegration of the underlying rocks and fragments of rock.

[Illustration:  A WORM CASTING, FROM NICE. (Natural Size.)]