$8 HERBERT SPENCER

matter are regarded as falling from positions of unstable
equilibrium to positions of stable equilibrium; on the other
hand, " they give out in their falls certain momenta—
momenta that may be manifested as heat, light, electricity,
nerve-force, or mechanical motion, according as the con-
ditions determine." It follows from the law of the Con-
servation of Energy that " whatever amount of power an
organism expends in any shape, is the correlate and equiva-
lent of a power which was taken into it from without."

Metabolism.—" The materials forming the tissues of
plants as well as the materials contained in them, are
progressively elaborated from the inorganic sub-
stances ; and the resulting compounds, eaten, and
some of them assimilated by animals, pass through
successive changes which are, on the average, of
an opposite character : the two sets being constructive
and destructive. To express changes of both these
natures the term * metabolism9 is used; and such of
the metabolic changes as result in building up from
simple to compound are distinguished as ' anabolic,'
while those which result in the falling down from
compound to simple are distinguished as ' katabolic.'"

" Regarded as a whole, metabolism includes, in the first
place, those anabolic or building-up processes specially
characterising plants, during which the impacts of ethereal
undulations are stored up in compound molecules of unstable
kinds; and it includes, in the second place, those katabolic
or tumbling-down changes specially characterising animals,
during which this accumulated molecular motion (contained
in the food directly or indirectly supplied by plants) is in
large measure changed into those molar motions constituting
animal activities. There are multitudinous metabolic changes
of minor kinds which are ancillary to these—many katabolic
changes in plants and many anabolic changes in animals—but
these are the essential ones."

De/imtion 0/I,z/L—Spencer's first definition of lifete or dictate under