terior,
cooling faster than the interior, will become different in temperature
from it. And the lapse into heterogeneity of temperature, so obvious in
this extreme case, is ever taking place more or less in all cases. The
actions of chemical forces supply other illustrations. Expose a
fragment of metal to air or water, and in course of time it will be
coated with a film of oxide, carbonate, or other compound: its outer
parts will become unlike its inner parts. Thus, every homogeneous
aggregate of matter tends to lose its balance in some way or
other--either mechanically, chemically, thermally or electrically; and
the rapidity with which it lapses into a non-homogeneous state is simply
a question of time and circumstances. Social bodies illustrate the law
with like constancy. Endow the members of a community with equal
properties, positions, powers, and they will forthwith begin to slide
into inequalities. Be it in a representative assembly, a railway board,
or a private partnership, the homogeneity, though it may continue in
name, inevitably disappears in reality.

The instability thus variously illustrated becomes still more manifest
if we consider its rationale. It is consequent on the fact that the
several parts of any homogeneous mass are necessarily exposed to
different forces--forces which differ either in their kinds or amounts;
and being exposed to different forces they are of necessity differently
modified. The relations of outside and inside, and of comparative
nearness to neighbouring sources of influence, imply the reception of
influences which are unlike in quantity or quality or both; and it
follows that unlike changes will be wrought in the parts dissimilarly
acted upon. The unstable equilibrium of any homogeneous aggregate can
thus be shown both inductively and deductively.

And now let us consider the bearing of this general truth on the
evolution of organisms. The germ of a plant or animal is one of these
homogeneous aggregates--relatively homogeneous if not absolutely
so--whose equilibrium is unstable. But it has not simply the ordinary
instability of homogeneous aggregates: it has something more. For it
consists of units which are themselves specially characterized by
instability. The constituent molecules of organic matter are
distinguished by the feebleness of the affinities which hold their
component elements together. They are extremely sensitive to heat,
light, electricity, and the chemical ac