§ 137. Returning from this parenthetical discussion, concerning the probable or possible state of things that may arise after Evolution has run its course; and confining ourselves to the changes constituting Evolution, with which alone we are immediately concerned; we have now to inquire whether the cessation of these changes, in common with all their transitional characteristics, admits of à priori proof. It will soon become apparent that equilibration, not less than the preceding general principles, is deducible from the persistence of force.
We have seen (§ 85) that phenomena are interpretable only as the results of universally-coexistent forces of attraction and repulsion. These universally-coexistent forces of attraction and repulsion, are, indeed, the complementary aspects of that absolutely persistent force which is the ultimate datum of consciousness. Just in the same way that the equality of action and re-action is a corollary from the persistence of force, since their inequality would imply the disappearance of the differential force into nothing, or its appearance out of nothing; so, we cannot become conscious of an attractive force without becoming simultaneously conscious of an equal and opposite repulsive force. For every experience of a muscular tension, (under which form alone we can immediately know an attractive force,) presupposes an equivalent resistance—a resistance shown in the counter-balancing pressure of the body against neighbouring objects, or in that absorption of force which gives motion to the body, or in both—a resistance which we cannot conceive as other than equal to the tension, without conceiving force to have either appeared or disappeared, and so denying the persistence of force. And from this necessary correlation, results our inability, before pointed out, of interpreting any phenomena save in terms of these correlatives—an inability shown alike in the compulsion we are under to think of the statical forces which tangible matter displays, as due to the attraction and repulsion of its atoms, and in the compulsion we are under to think of dynamical forces exercised through space, by regarding space as filled with atoms similarly endowed. Thus from the existence of a force that is for ever unchangeable in quantity, there follows, as a necessary corollary, the co-extensive existence of these opposite forms of force—forms under which the conditions of our consciousness oblige us to represent that absolute force which transcends our knowledge.
But the forces of attraction and repulsion being universally co-existent, it follows, as before shown, that all motion is motion under resistance. Units of matter, solid, liquid, aëriform, or ethereal, filling the space which any moving body traverses, offer to such body the resistance consequent on their cohesion, or their inertia, or both. In other words, the denser or rarer medium which occupies the places from moment to moment passed through by such moving body, having to be expelled from them, as much motion is abstracted from the moving body as is given to the medium in expelling it from these places. This being the condition under which all motion occurs, two corollaries result. The first is, that the deductions perpetually made by the communication of motion to the resisting medium, cannot but bring the motion of the body to an end in a longer or shorter time. The second is, that the motion of the body cannot cease until these deductions destroy it. In other words, movement must continue till equilibration takes place; and equilibration must eventually take place. Both these are manifest deductions from the persistence of force. To say that the whole or part of a body’s motion can disappear, save by transfer to something which resists its motion, is to say that the whole or part of its motion can disappear without effect; which is to deny the persistence of force. Conversely, to say that the medium traversed can be moved out of the body’s path, without deducting from the body’s motion, is to say that motion of the medium can arise out of nothing; which is to deny the persistence of force. Hence this primordial truth is our immediate warrant for the conclusions, that the changes which Evolution presents, cannot end until equilibrium is reached; and that equilibrium must at last be reached.
Equally necessary, because equally deducible from this same truth that transcends proof, are the foregoing propositions respecting the establishment and maintenance of moving equilibria, under their several aspects. It follows from the persistence of force, that the various motions possessed by any aggregate, either as a whole or among its parts, must be severally dissipated by the resistances they severally encounter; and that thus, such of them as are least in amount, or meet with greatest opposition, or both, will be brought to a close while the others continue. Hence in every diversely moving aggregate, there results a comparatively early dissipation of motions which are smaller and much resisted; followed by long-continuance of the larger and less-resisted motions; and so there arise dependent and independent moving equilibria. Hence also may be inferred the tendency to conservation of such moving equilibria; since, whenever the new motion given to the parts of a moving equilibrium by a disturbing force, is not of such kind and amount that it cannot be dissipated before the pre-existing motions (in which case it brings the moving equilibrium to an end) it must be of such kind and amount that it can be dissipated before the pre-existing motions (in which case the moving equilibrium is re-established).
Thus from the persistence of force follow, not only the various direct and indirect equilibrations going on around, together with that cosmical equilibration which brings Evolution under all its forms to a close; but also those less manifest equilibrations shown in the re-adjustments of moving equilibria that have been disturbed. By this ultimate principle is proveable the tendency of every organism, disordered by some unusual influence, to return to a balanced state. To it also may be traced the capacity, possessed in a slight degree by individuals, and in a greater degree by species, of becoming adapted to new circumstances. And not less does it afford a basis for the inference, that there is a gradual advance towards harmony between man’s mental nature and the conditions of his existence. After finding that from it are deducible the various characteristics of Evolution, we finally draw from it a warrant for the belief, that Evolution can end only in the establishment of the greatest perfection and the most complete happiness.
[18]. Sir David Brewster has recently been citing with approval, a calculation by M. Babinet, to the effect that on the hypothesis of nebular genesis, the matter of the Sun, when it filled the Earth’s orbit, must have taken 3181 years to rotate; and that therefore the hypothesis cannot be true. This calculation of M. Babinet may pair-off with that of M. Comte, who, contrariwise, made the time of this rotation agree very nearly with the Earth’s period of revolution round the Sun; for if M. Comte’s calculation involved a petitio principii, that of M. Babinet is manifestly based on two assumptions, both of which are gratuitous, and one of them totally inconsistent with the doctrine to be tested. He has evidently proceeded on the current supposition respecting the Sun’s internal density, which is not proved, and from which there are reasons for dissenting; and he has evidently taken for granted that all parts of the nebulous spheroid, when it filled the Earth’s orbit, had the same angular velocity; whereas if (as is implied in the nebular hypothesis, rationally understood) this spheroid resulted from the concentration of far more widely-diffused matter, the angular velocity of its equatorial portion would obviously be immensely greater than that of its central portion.
[19]. See paper “On the Inter-action of Natural Forces,” by Prof. Helmholtz, translated by Prof. Tyndall, and published in the Philosophical Magazine, supplement to Vol. XI. fourth series.
[20]. Until I recently consulted his “Outlines of Astronomy” on another question, I was not aware that so far back as 1833, Sir John Herschel had enunciated the doctrine that “the sun’s rays are the ultimate source of almost every motion which takes place on the surface of the earth.” He expressly includes all geologic, meteorologic, and vital actions; as also those which we produce by the combustion of coal. The late George Stephenson appears to have been wrongly credited with this last idea.