stant and discoverable,
so that the different units of force are separately measurable, the
force thus irregular in its action can never be placed in any scientific
category. Evolution, then, cannot proceed from any innate organic
impulse, unless the force that tends to exact reproduction, and the
force that induces a change be equally and separately cognizable. Change
must proceed according to some law which accounts for the change, and
distinguishes between the normal exertion of power and that exertion
which causes a deviation. Science, to be science, must explain apparent
exceptions as fully as the regular operation of forces, and that which
causes the irregularity must be as distinctly cognizably by itself as
the force which acts regularly. Anything less than this is not science.
The discovery of Neptune was the result of the application of this
principle; it was a successful attempt to discriminate the force which
caused variation from the force which operated regularly.

Each species represents the operation of certain vital forces, and one
cannot physically pass into another except by the increase of this
force, or at least by a change in the manner of its manifestation; and
this increase in amount or this change in direction must separately be
accounted for. Nor does it matter, for the purposes of this discussion,
as to the genesis of this added increment, further than to show that its
origin must be exterior to the organism by which its presence is
manifested; for vital energy acting through an organism is a unit, and
cannot, even in thought, be separated into distinguishable portions.
Change in the direction of vital energy indicates that the original
impulse has been modified in its action by encountering another force,
for nothing but force can change the direction of force. It does not
fall within the range of this paper to determine the nature of this
exterior force which is thus distinguishable from that acting through
the vital organization, and therefore capable of separate objective
representation. Metaphysically we may say that force is resolvable into
will, but will being purely personal is incapable of material
representation, and thus cannot enter into the determinations of
physical science, which does not seek to discover the origin of force,
but deals solely with its presence.

As the logician must assume his premises, and, as a logician, cannot
question their truth, so the physicist must assum