Equally necessary is the conclusion that the several results of the force expended on any aggregate, must be complementary to each other. It is not less obviously a corollary from the persistence of force, that of the whole incident force the effective is the part which remains after deducting the non-effective; than it is, that of the effective force, whatever does not work permanent results, works temporary results, and that such amount of the permanently effective force as is not absorbed in producing insensible re-arrangements, will produce sensible re-arrangements. CHAPTER XIII. THE INSTABILITY OF THE HOMOGENEOUS.[16] § 109. Thus far our steps towards the interpretation of Evolution have been preparatory. We have dealt with the factors of the process, rather than the process itself. After the ultimate truth that, Matter, Motion, and Force, as cognizable by human intelligence, can neither come into existence nor cease to exist, we have considered certain other ultimate truths concerning the modes in which Force and Motion are manifested during the changes they produce in Matter. Now we have to study the changes themselves. We have here to analyze that re-arrangement in the parts of Matter, which occurs under the influence of Force, that is unchangeable in quantity though changeable in form, through the medium of Motion taking place rhythmically along lines of least resistance. The proposition which comes first in logical order, is, that some re-arrangement must result; and this proposition may be best dealt with under the more specific shape, that the condition of homogeneity is a condition of unstable equilibrium. First, as to the meaning of the terms; respecting which some readers may need explanation. The phrase _unstable equilibrium_ is one used in mechanics to express a balance of forces of such kind, that the interference of any further force, however minute, will destroy the arrangement previously subsisting; and bring about a totally different arrangement. Thus, a stick poised on its lower end is in unstable equilibrium: however exactly it may be placed in a perpendicular position, as soon as it is left to itself it begins, at first imperceptibly, to lean on one side, and with increasing rapidity falls into another attitude. Conversely, a stick suspended from its upper end is in stable equilibrium: however much disturbed, it will return to the same position. The proposition is, then, that the state of homogeneity, like the state of the stick poised on its lower end, is one that cannot be maintained. Let us take a few illustrations.