In other words, the phenomena of Evolution have to be deduced from thePersistence of Force. As before said -- "to this an ultimate analysisbrings us down, and on this a rational synthesis must build up." Thisbeing the ultimate truth which transcends experience by underlying it, furnishesa common basis on which the widest generalizations stand; and hence thesewidest generalizations are to be unified by referring them to this commonbasis. Already the truths that there is equivalence among transformed forces,that motion follows the line of least resistance or greatest traction andthat it is universally rhythmic, we have found to be severally deduciblefrom the persistence of force; and this affiliation of them on the persistenceof force has reduced them to a coherent whole. Here we have similarly toaffiliate the universal traits of Evolution, by showing that, given the persistenceof force, the re-distribution of Matter and Motion necessarily proceeds insuch ways as to produce these traits. By doing this we shall unite them ascorrelative manifestations of one law, at the same time that we unite thislaw with the foregoing simpler laws. §148. Before proceeding it will be well to set down some principlesthat must be borne in mind. In interpreting Evolution we shall have to consider,under their special forms, the various resolutions of force or energy whichaccompany the re-distributions of matter and motion. Let us glance at suchresolutions under their most general forms.
Any incident force is primarily divisible into its effective and non-effectiveportions. In mechanical impact the entire momentum of a striking body isnever communicated to the body struck: even under those most favourable conditionsin which the striking body loses all its sensible motion, there still remainswith it some of the original momentum under the shape of that insensiblemotion produced among its particles by the collision. Again, of the lightor 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 itis to be noted that the effective force is itself divisible into the temporarilyeffective and the permanently effective. The units of an aggregate actedon may undergo only those rhythmical changes of relative position which constituteincreased vibration; or they may also undergo changes of relative positionwhich are not from instant to instant neutralized by opposite ones. Of thesethe first, disappearing in the shape of radiating undulations, leave themolecular arrangement as it originally was; while the second conduce to oneform of that re-arrangement characterizing compound Evolution. Yet a furtherdistinction has to be made. The permanently effective force works out changesof relative position of two kinds -- the insensible and the sensible. Theinsensible transpositions among the units are those constituting molecularchanges, including what we call chemical composition and decomposition; andit is these which largely constitute the qualitative differences that arisein an aggregate. The sensible transpositions are such as result when certainof the units -- molar units as well as molecular units -- instead of beingput into different relations with their immediate neighbours, are carriedaway from them and deposited elsewhere.
Concerning these divisions and subdivisions of any force affecting anaggregate, the fact which it chiefly concerts us to observe is, that theyare complementary to one another. Of the whole incident force, the effectivemust be that which remains after deducting the non-effective. The two partsof the effective force must vary inversely as each other: where much of itis temporarily effective, little of it can be permanently effective; andvice versa. Lastly, the permanently effective force, being expended in workingboth the insensible re-arrangements which constitute molecular modification,and the sensible re-arrangements which result in structure, must generateof either kind an amount that is great or small in proportion as it has generateda small or great amount of the other.
