The objects of nature undergo changes in time, emerging and vanishing, some quickly and others slowly: is there a universal law by which these changes also are governed ? This was the aim of the research. Mr. Spencer early found that the conception of progress which implies movement in one direction only is erroneous. There is no unbroken march of events; breaks and regressions alternate with advancement, and descending as well as ascending changes have to be accounted for. He therefore rejected the term progress as having erroneous implications, and adopted the term evolution, as more fully indicating the scope of the inquiry and better expressing the strictly scientific nature of his theory. The naturalist Von Baer had already attempted to define and generalize the changes of organic growth, and had formulated them as from the homogeneous germ state to the heterogeneous adult state by a process of differentiation. Mr. Spencer soon found that this formula gave but a very partial account of what takes place in organic development. The change was shown to be not only from uniformity to unlikeness, or a differencing of parts, but from the indefinite to the definite, from the incoherent to the coherent, producing the integration of parts, or increasing unity with increasing complexity.

The conditions and course of changes in which organic evolution consists being ascertained, the question arose as to their extent, and Mr. Spencer became convinced that the law of organic movement is not an isolated fact in nature, but that the process of change gone through by each evolving organism is a process gone through by all things." Science had shown that the universe, past and present, is subject to orderly changes; he discovered that fundamentally this order is one. The nebular hypothesis proposed by Kant, confirmed by Iler-schel and Laplace, and accepted by astronomers, explained the origin and motions of suns and planets by slow condensation from a nebulous mist diffused through space. The geological history of our earth shows that it has undergone a vast series of progressive changes, and, as Prof. Dana says, " was first a featureless globe of fire, then had its oceans and dry land, in course of time received mountains and rivers, and finally all those diversities of surface which now characterize it." The course of organic life, as we have seen, was a progressive unfolding into greater diversity and specialty. Mind is developed with the body, and therefore mental phenomena obey a law of unfolding.

As human society is made up of units that are capable of these changes, it presents in the past a gradual development of intelligence, arts, and institutions, as now embodied in our diverse and complex civilization. By a careful analysis of the phenomena in these widely separated cases, Mr. Spencer showed that they all conform to a great general law, of which individual life is but a special case. Equally in the inorganic, the organic, and the super-organic spheres, the progressive changes are from the homogeneous to the heterogeneous by differentiation. But with increasing divergences there is also increasing definite-ness, coherence, complexity, and integration. 'Evolution is thus a universal law, while the development of the individual and the career of the race, so far from being exceptional phenomena, are but parts of the great system of change to which the whole cosmos conforms. Evolution being thus disclosed as a universal dynamical law, the question next arises, how is it to be interpreted ? Is it an ultimate law like gravitation, or is it a derivative principle deducible as a necessity from the established laws of matter, motion, and force? Mr. Spencer proves that evolution is a resultant of dynamical agencies, and that, given matter as a vehicle of change, motion as the result of change, and force as the cause of change, such are their established laws of interaction that evolution follows as an inevitable consequence.

We can here only touch upon the leading elements of the elucidation, and must refer the reader to Mr. Spencer's "System of Philosophy" for the full elaboration of the subject. Modern science has established the great principles of the indestructibility of matter and the conservation of force. (See Correlation of Forces.) Mr. Spencer maintains that these resolve themselves into the single law of the persistence of force, and that this is the fundamental postulate of evolution. Whatever interpretation is given to the principle, it certainly bevomes a fundamental condition of the changes taking place in nature. If matter and force throughout the universe are neither created nor destroyed, all changes must be changes of transformation. The stock of material and energy being limited, each new effect must be at the expense of something preexisting; and hence in the ongoings of nature one thing is necessarily derived from another, while the problem of advance becomes one of transmutation. Mr. Spencer traces out the several causes of transformation or factors of evolution, and shows that they are all corollaries from the supreme law of the persistence of force. Briefly indicated, these are as follows: 1. The principle of the rhythm of motion.

Under the law of the persistence of forces and the diversity of their forms, there arise constant conflicts of effect, so that motions are not uniform but varying. Action is met by counteraction, and the result is that movements take a rhythmical form. Boughs, for example, sway in the wind, water is thrown into waves, sound arises in vibrations, earthquakes are propagated in shocks, planets swing through eccentric orbits, breathing is recurrent, the heart beats, scarcity alternates with abundance, and prices rise and fall. From the minutest organism throughout the whole frame of things to the most distant systems, from momentary pulses to geological cycles, the agitations of things take the form of thrills and surges, which produce incessant and universal redistributions of matter and force. How are these redistributions directed? 2. They are controlled first by the law of the instability of the homogeneous. The relatively homogeneous is the commencing stage of all evolution, and Mr. Spencer has shown that this is an unstable condition, and under rhythmic disturbance tends constantly to rearrangement and greater complexity.