general law previously pointed out, it was
an example of deductive physiology. The particular was concluded from
the universal.

We here propose in the first place to show, that there is another
general truth closely connected with the above; and in common with it
underlying explanations of all progress, and therefore the progress of
organisms--a truth which may indeed be considered as taking precedence
of it in respect of time, if not in respect of generality. This truth
is, that _the condition of homogeneity is a condition of unstable
equilibrium_.

The phrase _unstable equilibrium_ is one used in mechanics to express
a balance of forces of such kind, that the interference of any further
force, however minute, will destroy the arrangement previously existing,
and bring about a different arrangement. Thus, a stick poised on its
lower end is in unstable equilibrium: however exactly it may be placed
in a perpendicular position, as soon as it is left to itself it begins,
at first imperceptibly and then visibly, to lean on one side, and with
increasing rapidity falls into another position. Conversely, a stick
suspended from its upper end is in stable equilibrium: however much
disturbed, it will return to the same position. Our meaning is, then,
that the state of homogeneity, like the state of the stick poised on its
lower end, is one that cannot be maintained; and that hence results the
first step in its gravitation towards the heterogeneous. Let us take a
few illustrations.

Of mechanical ones the most familiar is that of the scales. If
accurately made and not clogged by dirt or rust, a pair of scales cannot
be perfectly balanced: eventually one scale will descend and the other
ascend--they will assume a heterogeneous relation. Again, if we sprinkle
over the surface of a liquid a number of equal-sized particles, having
an attraction for one another, they will, no matter how uniformly
distributed, by and by concentrate irregularly into groups. Were it
possible to bring a mass of water into a state of perfect homogeneity--a
state of complete quiescence, and exactly equal density throughout--yet
the radiation of heat from neighbouring bodies, by affecting differently
its different parts, would soon produce inequalities of density and
consequent currents; and would so render it to that extent
heterogeneous. Take a piece of red-hot matter, and however evenly heated
it may at first be, it will quickly cease to be so: the ex