By another law, flowing from the same primary principle, the parts of a whole diverge from one another in proportion to their diversity, and group themselves together in proportion to their resemblances. Motions that are alike in direction or intensity, acting on these parts, drive them in the same direction, and with the same velocity, whence results an integration of these parts, while those driven by motions unlike in direction or intensity go in different directions with different velocities, separate from one another, are disintegrated. This is the law of segregation, the application of which brings into prominence the heterogeneous character of the products of change, by giving to their heterogeneity a clearer and more definite nature.
Finally, we note another consequence of the persistence of force. Every change in an aggregation of sensible parts is conditioned by opposing forces, the one representing action, the other reaction; the one the tendency to change, the other resistance; their antagonism can end only when equilibrium has been established, by the dissipation of the excess of the one force over the other. A body subject to any disturbance whatever, owing to a modification of its circumstances, tends toward equilibrium with its new circumstances; and, as the different forces acting on it have not the same intensity, those which are weaker soon find their equilibrium, while those which are stronger continue to give motion to the body, and then the latter presents the spectacle of an aggregate whose parts are in an invariable ratio to each other, while the total aggregate is ever changing its relations to external objects. This is equilibrium mobile, unstable equilibrium, and it serves as a transition to a more perfect equilibrium, or else to a renewal of the internal movements which have already found equilibrium.
The action of these laws of change of objects and their parts leads to two contrary results, according to the mode of distribution of the forces in action. We have evolution, i. e., change with integration of matter, dissipation of internal motion, increase of the number and diversity of the parts, whenever the external forces are not such as to break the bond which unites them; we have dissolution, continuous or discontinuous, i. e., a change with disaggregation of matter; absorption of motion (which, becoming internal, drives the constituent units with greater velocity) and diminution both of the numbers and of the diversity of the parts, whenever the external forces are sufficiently intense to destroy the cohesion of the aggregate and to restore to its parts their original independence.
The work of Mr. Spencer in his "Biology" consists in referring to these general laws the generalizations obtained in the various parts of the domain of biology, and in discerning those which possess the character of necessity. This course has the twofold advantage of giving to these generalizations greater authority, and of introducing into a coordinated system of philosophy the science whose general-
