An introductory text-book of logic - Mellone, Sydney - Rare Books at ...

OR SCIENTIFIC METHOD. 2/9
tion of two events, and that we have failed to find any
then
instance where one has occurred without the other ;
it is probable that they are causally connected, and the
probability increases with the number and variety of the
negative instances. It presupposes the ordinary Method
"
of Single Agreement before proceeding to marshal the
negative instances
"
; and like that Method, it is appro
priate where exact experiment is not possible.
In illustration of the Double Method of Agreement we
will take Darwin s investigation of the theory that "vege
table mould" is produced by earthworms. He devoted a
special treatise ( Vegetable Mould and Earthworms]
to the
proof that these creatures are thus performing a work of
vast magnitude and importance for the maintenance of life
on the surface of the earth.
The phenomenon to be investigated is the production of
vegetable mould on the surface.
(a) Positive Instances. These were rightly made as num
erous and varied as possible i.e., the surfaces examined
consisted of widely different kinds of land, and the objects
which sunk were of different kinds. Many observations
were made, of which we "
quote a few. In the spring of
1835, a field, which had long existed as poor pasture, and
was so swampy that it trembled slightly when stamped on,
was thickly covered with red sand, so that the whole sur
face appeared at first bright red. When holes were dug
in this field after an interval of about two and a half years,
the sand formed a layer at a depth of three-quarters of an
inch beneath the surface. Seven years after the sand had
been laid on, fresh holes were dug, and now the red sand
formed a distinct layer, two inches beneath the surface."
The original surface-soil, which consisted of black sandy
peat, was found immediately beneath the layer of red sand.
Another instance was that of a Kentish chalk formation.
"
Its surface, from having been exposed for an immense
period to the dissolving action of rain-water, is extremely
irregular, being penetrated by many deep well-like cavities.
During the dissolution of the chalk, the insoluble matter,