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.