The only obvious respect in which all kinds of Evolution are alike, is, that they are modes of _change_. Every phenomenon to which we apply the term, presents us with a succession of states; and when such succession ceases, we no longer predicate Evolution. Equally in those past forms of it which are more or less hypothetical, and in those forms of it which we see going on around, this is the common characteristic. Note next, that the kind of change which constitutes Evolution, is broadly distinguished from change of an equally general kind, in this, that it is change of internal relations instead of change of external relations. All things in motion through space are the subjects of change; but while in this which we call mechanical motion, the relative position as measured from surrounding objects is continually altered, there is not implied any alteration in the positions of the parts of the moving body in respect to each other. Conversely, a body exhibiting what we call Evolution, while it either may or may not display new relations of position to the things around it, _must_ display new relations of position among the parts of which it is made up. Thus we narrow the field of inquiry by recognizing the change in which Evolution consists, as _a change in the arrangement of parts_: of course using the word parts in its most extended sense, as signifying both ultimate units and masses of such units. Further, we have to remember that this change in the arrangement of parts which constitutes Evolution, is a certain order of such change. As we saw in the last chapter, there is a change in the arrangement of parts which is not Evolution but Dissolution—a destructive change as opposed to a constructive change—a change by which the definite is gradually rendered indefinite, the coherent slowly becomes incoherent, and the heterogeneous eventually lapses into comparative homogeneity. Thus then we reduce that which we have to investigate to its most abstract shape. Our task is to find the cause or causes of a certain order of change that takes place in the arrangement of parts. * * * * * § 60. Evidently the problem, as thus expressed, brings us face to face with the ultimate elements of phenomena in general. It is impossible to account for a certain change in the arrangement of the parts of any mass, without involving—first, the _matter_ which makes up the parts thus re-arranged; next, the _motion_ exhibited during the re-arrangement; and then, the _force_ producing this motion. The problem is a dynamical one; and there can be no truly scientific solution of it, save one given in terms of Matter, Motion, and Force—terms in which all other dynamical problems are expressed and solved.