ecome more diverse in structure, every
differentiated part will become the parent of further differences; at
the same time, dissimilar units in the aggregate tend to separate, and
those which are similar, to cluster together ("segregation"); and this
subdivision and dissipation of forces, so long as there are any forces
unbalanced by opposite forces, must end at last in rest; the penultimate
stage of this process "in which the extremest multiformity and most
complex moving equilibrium are established," being the highest
conceivable state. The various derivative laws of phenomenal changes are
thus deducible from the persistence of force. It remains to apply them
to inorganic, organic, and superorganic existences. The detailed
treatment of inorganic evolution is omitted, as we have said, from
Spencer's plan, and he proceeds to interpret "the phenomena of life,
mind, and society in terms of Matter, Motion, and Force."

IV.

The first volume of the "Principles of Biology" consists of three parts,
the first of which sets forth the data of biology, including those
general truths of physics and chemistry with which rational biology must
start. The second part is allotted to the inductions of biology, or, in
other words, to a statement of the leading generalizations which
naturalists, physiologists, and comparative anatomists have established.
The third and final part of the first volume of the "Principles of
Biology" deals with the speculation commonly known as "the development
hypothesis," and considers its _a priori_ and _a posteriori_ evidences.

The inductive evidences for the evolutionary hypothesis, as
contra-distinguished from the special-creation hypothesis, are dealt
with in four chapters. The "Arguments from Classification" are these:
Organisms fall into groups within groups; and this is the arrangement
which we see results from evolution where it is known to take place. Of
these groups within groups, the great or primary ones are the most
unlike, the sub-groups are less unlike, the sub-sub-group still less
unlike, and so on; and this, too, is a characteristic of groups
demonstrably produced by evolution. Moreover, indefiniteness of
equivalence among the groups is common to those which we know have been
evolved, and to those supposed in the volume before us to have been
evolved. There is the further significant fact that divergent groups are
allied through their lowest rather than their highest members. Of the