If, however, we turn from the considera tion of this most universal aspect of nature to a study of the various types of natural proc esses which are observable when we consider large masses, or series of natural phenomena, we come hereupon to the principles characteris tic of what may be called the synthetic unity of nature, that is, of that unity which appears when we consider the entire lives of organisms, or the histories of races of living creatures, or the history of planets, of solar systems or, in another realm, of societies. These processes which the persistence of force render possible are of two general types. They are, namely, either processes of evolution or processes of dissolution. Evolution occurs when great ag gregations, such as have just been mentioned, appear and grow and become organized. The formation of a stellar system, the solidification of a planet, the gradual building up and diversi fication of the earth's crust, the growth of a race of animals or of plants, the life history of an individual or of a nation — these are proc esses of evolution. Dissolution is the reverse of any of these processes. When an aggregate is distintegrated, dissolved or otherwise de stroyed, when an organization passes away or disappears, we are, therefore, dealing with dis solution.
Evolution is of course the more interesting of these two processes. It possesses, according to Spencer, two characteristic forms, primary evolution and secondary evolution — the latter being based upon the former, but occurring in less frequent instances, although these instan ces are very highly interesting and important. The characteristic of the primary evolutions is that they involve "an integration of matter and a dissipation of motion." Such processes occur when masses of matter cool, contract and solid ify—as, for instance, when planets are formed, The processes of secondary evolution are ren dered possible by the fact that some of the bodies subject to primary evolution do not merely contract and solidify, but also pass through a stage or a series of stages of their existence, in which they predominantly possess a °plastic" character. "Plastic bodies" are such as are sufficiently free from a close coherence of parts to permit of easy change in conse quence of change in their environment, while at the same time these plastic bodies are suffi ciently coherent to retain the traces of dis-: turbances which have happened to them. Thus, a mountain range, subject to erosion, not only yields in numerous ways to the action of water and of weather, but retains, in a great variety of ways, the traces of such erosions as have oc curred in the past, instead of being swept away at once by a single flood, as a smaller mass might be. Again, a living organism is a plastic body, so full of internal motion, or of stored up energy, as to be greatly and easily disturbed by its environment, while at the same time it is so far possessed of rigidity in its structure and in its way of responding to external influences, that traces of what has happened to it remain, in great variety, and for a long time, or even throughout the life of the organism. The power of an organism to adjust itself to new conditions, in combination with the power to form and retain habits, constitutes the basis for all the higher development of life. All such powers depend physically upon the char acteristics of the "plastic bodies," and the proc ess of organic evolution is altogether to be de scribed as due to the accumulation of such sec ondary evolution as these properties of plastic organisms make possible. Since the brain it
self, such as any higher animal possesses, is a plastic body, the result of processes of aggrega tion, and is itself on the way toward a greater solidification (such as occurs in old age), while the brain, during youth and early maturity, is at once extremely sensitive to disturbances and very apt to retain the results of former dis turbance — it follows that the entire evolution of the mind, which so far as the mind is know able, accompanies the development of the brain, is itself to he described in terms of the proper ties of the plastic bodies.
In consequence of the fact that evolution, so far as it is a process of the knowable world, is thus a result of "the integration of matter and the dissipation of motion," and, in case of the secondary evolutions, is a result of the for mation of the plastic bodies, it follows that all evolutionary processes must possess certain common features. The primary evolution, otherwise called °simple evolution," consists altogether of the formation of "coherent aggre gates," such as result whenever bodies solidify and part with their internal energies. The secondary evolution, also called "compound," includes changes that are due to "differences in the circumstances in the different parts of aggregate." The most notable result of these internal differences is a "transformation of the homogeneous into the heterogeneous." Wherever evolution takes place in a plastic body such increasing differentiation is inevitable, the reason being that the traces of previous changes are retained, and are made the basis of increas ing variety, as the traces of new changes are superposed. In the more complex instances of secondary evolution the transition from the ho mogeneous to the heterogeneous is long con tinued and extremely complicated. It is ex hibited not only in the structure of an aggre gate, but in the functions which belong to this aggregate, in case the aggregate is, like a living organism, the seat of a great variety of ener gies. The increasing complication and differ entiation of the functions and habits of a living organism is thus the result of the same kind of process as makes its appearance even in inor ganic nature wherever the results of former changes accumulate in a plastic body. Wher ever integration and increasing heterogeneity are long continued and important, they are also accompanied by an increasing definiteness of structure and function, which Spencer describes as due to the process called by him °segrega tion." Segregation is exemplified wherever the outlines of things grow shalper, wherever the like things of the natural world tend to be grouped near together or the unlike things to be kept apart. For certain natural forces, whose presence in the world Spencer exten sively illustrates, are working along with what ever tends to differentiate phenomena and are so working as to tend to sort out, or to segre gate, like and unlike things. Such forces are manifest in the case of the formation of geo logical strata through sedimentary deposits, and in the case of the grouping together of similar organisms in societies, and in large numbers of other instances. Segregation com bines with differentiation, and with integration, to characterize the whole evolutionary process.