are nearly all causes with which we are acquainted. Scarcely any
change can rightly be ascribed to one agency alone, to the neglect of
the permanent or temporary conditions under which only this agency
produces the change. But as it does not materially affect our argument,
we prefer, for simplicity's sake, to use throughout the popular mode of
expression. Perhaps it will be further objected, that to assign loss of
heat as the cause of any changes, is to attribute these changes not to a
force, but to the absence of a force. And this is true. Strictly
speaking, the changes should be attributed to those forces which come
into action when the antagonist force is withdrawn. But though there is
inaccuracy in saying that the freezing of water is due to the loss of
its heat, no practical error arises from it; nor will a parallel laxity
of expression vitiate our statements respecting the multiplication of
effects. Indeed, the objection serves but to draw attention to the fact,
that not only does the exertion of a force produce more than one change,
but the withdrawal of a force produces more than one change.

Returning to the thread of our exposition, we have next to trace,
throughout organic progress, this same all-pervading principle. And
here, where the evolution of the homogeneous into the heterogeneous was
first observed, the production of many effects by one cause is least
easy to demonstrate. The development of a seed into a plant, or an ovum
into an animal, is so gradual, while the forces which determine it are
so involved, and at the same time so unobtrusive, that it is difficult
to detect the multiplication of effects which is elsewhere so obvious.
But, guided by indirect evidence, we may safely conclude that here too
the law holds. Note, first, how numerous are the changes which any
marked action works upon an adult organism--a human being, for instance.
An alarming sound or sight, besides the impressions on the organs of
sense and the nerves, may produce a start, a scream, a distortion of
the face, a trembling consequent on general muscular relaxation, a burst
of perspiration, a rush of blood to the brain, followed possibly by
arrest of the heart's action and by syncope; and if the subject be
feeble, an indisposition with its long train of complicated symptoms may
set in. Similarly in cases of disease. A minute portion of the small-pox
virus introduced into the system, will, in a severe case, cause, during
the first