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Chapter 18
The Interpretation of Evolution

§146. Is this law ultimate or derivative? Must we rest satisfied with the conclusion that throughout all classes of concrete phenomena such is the course of transformation? Or is it possible for us to ascertain why such is the course of transformation? May we seek for some all-pervading principle which underlies this all-pervading process? Can the inductions set forth in the preceding four chapters be reduced to deductions?

Manifestly this community of result implies community of cause. It may be that of the cause no account can be given, further than that the Unknowable is manifested to us after this mode. Or, it may be that this mode of manifestation is implied by a simpler mode, from which these many complex effects follow. Analogy suggests the latter inference. Just as it was possible to interpret the empirical generalizations called Kepler's laws, as necessary consequences of the law of gravitation; so it may be possible to interpret the foregoing empirical generalizations as necessary consequences of some deeper law.

Unless we succeed in finding a rationale of this universal metamorphosis, we obviously fall short of that completely unified knowledge constituting Philosophy. As they at present stand, the several conclusions we have lately reached appear to be independent. There is no demonstrated connexion between increasing definiteness and increasing heterogeneity, or between both and increasing integration. Still less proof is there that these laws of the re-distribution of matter and motion, are necessarily correlated with those laws of the direction of motion and the rhythm of motion, previously set forth. But until we see these now separate truths to be implications of one truth, our knowledge remains imperfectly coherent.

§147. The task before us, then, is that of exhibiting the phenomena of Evolution in synthetic order. Setting out from an established ultimate principle, it has to be shown that the course of transformation among all kinds of existences, cannot but be that which we have seen it to be. It has to be shown that the re-distribution of matter and motion, must everywhere take place in those ways, and produce those traits, which celestial bodies, organisms, societies, alike display. And it has to be shown that in this universality of process, is traceable the same necessity which we find in each simplest movement around us, down to the accelerated fall of a stone or the recurrent beat of a harp-string.

In other words, the phenomena of Evolution have to be deduced from the Persistence of Force. As before said — "to this an ultimate analysis brings us down, and on this a rational synthesis must build up." This being the ultimate truth which transcends experience by underlying it, furnishes a common basis on which the widest generalizations stand; and hence these widest generalizations are to be unified by referring them to this common basis. Already the truths that there is equivalence among transformed forces, that motion follows the line of least resistance or greatest traction and that it is universally rhythmic, we have found to be severally deducible from the persistence of force; and this affiliation of them on the persistence of force has reduced them to a coherent whole. Here we have similarly to affiliate the universal traits of Evolution, by showing that, given the persistence of force, the re-distribution of Matter and Motion necessarily proceeds in such ways as to produce these traits. By doing this we shall unite them as correlative manifestations of one law, at the same time that we unite this law with the foregoing simpler laws.

§148. Before proceeding it will be well to set down some principles that must be borne in mind. In interpreting Evolution we shall have to consider, under their special forms, the various resolutions of force or energy which accompany the re-distributions of matter and motion. Let us glance at such resolutions under their most general forms.

Any incident force is primarily divisible into its effective and non-effective portions. In mechanical impact the entire momentum of a striking body is never communicated to the body struck: even under those most favourable conditions in which the striking body loses all its sensible motion, there still remains with it some of the original momentum under the shape of that insensible motion produced among its particles by the collision. Again, of the light or heat falling on any mass, a part, more or less considerable, is reflected; and only the remaining part works molecular changes in the mass. Next it is to be noted that the effective force is itself divisible into the temporarily effective and the permanently effective. The units of an aggregate acted on may undergo only those rhythmical changes of relative position which constitute increased vibration; or they may also undergo changes of relative position which are not from instant to instant neutralized by opposite ones. Of these the first, disappearing in the shape of radiating undulations, leave the molecular arrangement as it originally was; while the second conduce to one form of that re-arrangement characterizing compound Evolution. Yet a further distinction has to be made. The permanently effective force works out changes of relative position of two kinds — the insensible and the sensible. The insensible transpositions among the units are those constituting molecular changes, including what we call chemical composition and decomposition; and it is these which largely constitute the qualitative differences that arise in an aggregate. The sensible transpositions are such as result when certain of the units — molar units as well as molecular units — instead of being put into different relations with their immediate neighbours, are carried away from them and deposited elsewhere.

Concerning these divisions and subdivisions of any force affecting an aggregate, the fact which it chiefly concerts us to observe is, that they are complementary to one another. Of the whole incident force, the effective must be that which remains after deducting the non-effective. The two parts of the effective force must vary inversely as each other: where much of it is temporarily effective, little of it can be permanently effective; and vice versa. Lastly, the permanently effective force, being expended in working both the insensible re-arrangements which constitute molecular modification, and the sensible re-arrangements which result in structure, must generate of either kind an amount that is great or small in proportion as it has generated a small or great amount of the other.