he Single Method of Agreement,
relies on observation, sequence may not be perceptible in the instances
observed, and then, direct causation cannot be proved by it, but only
the probability of causal connection; and, again, the real cause, though
present, may be so obscure as to evade observation. It has, however, one
peculiar advantage, namely, that if the second list of instances (in
which the phenomenon and its supposed antecedent are both absent) can be
made exhaustive, it precludes any hypothesis of a plurality of causes;
since all possible antecedents will have been included in this list
without producing the phenomenon. Thus, in the above symbolic example,
taking the first set of instances, the supposition is left open that B,
C, D, E, F, G may, at one time or another, have been a condition of _p_;
but, in the second list, these antecedents all occur, here or there,
without producing _p_, and therefore (unless counteracted somehow)
cannot be a condition of _p_. A, then, stands out as the one thing that
is present whenever _p_ is present, and absent whenever _p_ is absent.

Stated in this abstract way, the Double Method may seem very elaborate
and difficult; yet, in fact, its use may be very simple. Tyndall, to
prove that dispersed light in the air is due to motes, showed by a
number of cases (1) that any gas containing motes is luminous; (2) that
air in which the motes had been destroyed by heat, and any gas so
prepared as to exclude motes, are not luminous. All the instances are of
gases, and the result is: motes--luminosity; no motes--no luminosity.
Darwin, to show that cross-fertilisation is favourable to flowers,
placed a net about 100 flower-heads, and left 100 others of the same
varieties exposed to the bees: the former bore no seed, the latter
nearly 3,000. We must assume that, in Darwin's judgment, the net did not
screen the flowers from light and heat sufficiently to affect the
result.

There are instructive applications of this Double Method in Wallace's
_Darwinism_. In chap. viii., on _Colour in Animals_, he observes, that
the usefulness of their coloration to animals is shown by the fact that,
"as a rule, colour and marking are constant in each species of wild
animal, while, in almost every domesticated animal, there arises great
variability. We see this in our horses and cattle, our dogs and cats,
our pigeons and poultry. Now the essential difference between the
conditions of life of domesticated and