But now, what is the ultimate meaning of the conclusion that a force producesdifferent changes throughout a uniform mass, because the parts of the massstand in different relations to the force? Fully to understand this, we mustcontemplate each part as simultaneously subject to other forces -- thoseof gravitation, of cohesion, molecular motion, etc. The effect wrought byan additional force, must be a resultant of it and the forces already inaction. If the forces already in action on two parts of any aggregate, aredifferent in their resultant directions, the effects produced on these twoparts by equal additional forces must be different in their directions. Whymust they be different? Because such unlikeness as exists between the twosets of factors, is made by the presence in the one of some specially-directedforce that is not present in the other; and that this force will producean effect, rendering the total result in the one case unlike that in theother, is a necessary corollary from the persistence of force. Still moremanifest does it become that the dissimilarly-placed parts of any aggregatemust be dissimilarly modified by an incident force, when we remember thatthe quantities of the incident force to which they are severally subject,are not equal, as above supposed, but are nearly always unequal. Look againat the above examples. The amounts of any external radiant force which thedifferent parts of an aggregate receive, are widely contrasted: we have thecontrast between the quantity falling on the side next the radiating centre,and the quantity, or rather no quantity, falling on the opposite side; wehave contrasts in the quantities received by differently-placed areas onthe exposed side; and we have endless contrasts between the quantities receivedby the various parts of the interior. Similarly when mechanical force isexpended on any aggregate, either by collision, continued pressure, or tension,the amounts of strain distributed throughout the mass are manifestly unlikefor unlike positions. And it is obvious that ordinary chemical action affectssurface more than centre, and often one part of the surface more than another.
But to say the different parts of an aggregate receive different quantitiesof any force capable of changing them, is to say that if they were beforehomogeneous they must be rendered to a proportionate extent heterogeneous;since, force being persistent, the different quantities of it falling onthe different parts, must work in them different quantities of effect-differentchanges. Yet one more kindred deduction is required to complete the argument.
Even apart from the action of any external force, the equilibrium of a homogeneousaggregate must be destroyed by the unequal actions of its parts on one another.
That mutual influence which produces aggregation (not to mention other mutualinfluences) must work different effects on the different parts; since theyare severally exposed to it in unlike amounts and directions. This will beclearly seen on remembering that the portions of which the whole is madeup, may be severally regarded as minor wholes; that on each of these minorwholes, the action of the entire aggregate then becomes an external incidentforce; that such external incident force must, as above shown, work unlikechanges in the parts of any such minor whole; and that if the minor wholesare severally thus rendered heterogeneous, the entire aggregate is renderedheterogeneous.
The instability of the homogeneous is thus deducible from that primordialtruth which underlies our intelligence. One stable homogeneity only, is hypotheticallypossible. If centres of force, absolutely uniform in their powers, were diffusedwith absolute uniformity through unlimited space, they would remain in equilibrium.
This however, though a verbally intelligible supposition, is one that cannotbe represented in thought; since unlimited space is inconceivable. But allfinite forms of the homogeneous -- all forms of it which we can know or conceive,must inevitably lapse into heterogeneity; and the less heterogeneous mustlapse into the more heterogeneous. In three several ways does the persistenceof force necessitate this. Setting external agencies aside, each unit ofa homogeneous whole must be differently affected from any of the rest bythe aggregate action of the rest upon it. The resultant force exercised bythe aggregate on each unit, being in no two cases alike in both amount anddirection, and usually not in either, any incident force, even if uniformin amount and direction, cannot produce like effects on the units. And asthe various positions of the parts in relation to any incident force, preventsthem from receiving it in uniform amounts and directions, a further differencein the effects wrought on them inevitably arises.
One further remark is needed. The conclusion that the changes with whichEvolution commences, are thus necessitated, has to be supplemented by theconclusion that these changes must continue. The absolutely homogeneous (supposingit to exist) must lose its equilibrium; and the relatively homogeneous mustlapse into the relatively less homogeneous. That which is true of any totalmass, is true of the parts into which it segregates. The uniformity of eachsuch part must as inevitably be lost in multiformity, as was that of theoriginal whole; and for like reasons. And thus the continued changes characterizingEvolution, in so far as they are constituted by the lapse of the homogeneousinto the heterogeneous, and of the less heterogeneous into the more heterogeneous,are necessary consequences of the persistence of force.
[A small change in the definition of Evolution indicated in a note atthe end of Chapter XVII of this part, must be recalled as involving a correlativechange in this chapter. Here, as before, the required change, though alreadyimplied (page 367), has not been sufficiently emphasized, and lack of theemphasis invites misinterpretation. For reasons like those before given,the requisite explanations cannot be made in this place. The reader willfind them in Appendix A.
Replies to certain criticisms on the general doctrine set forth in thischapter will be found in Appendix C.]
