§ 78. Throughout Evolution under all its phases, this truth of course invariably holds. Every successive change or group of changes forming part of it, is of necessity limited by the conditions thus implied. The forces which any step in Evolution exhibits, must be affiliable on the like or unlike forces previously existing; while from the forces so generated must thereafter be derived others more or less transformed. And besides recognizing the forces at any time existing, as necessarily linked with those preceding and succeeding them, we must also recognize the amounts of these forces successively manifested as determinate,—as necessarily producing such and such quantities of results, and as necessarily limited to those quantities. Involved as are the phenomena of Evolution, it is not to be expected that a _definite_ quantitative relation can in each case, or indeed in any case, be shown between the forces expended in successive phases. We have not adequate data for this; and probably shall never have them. The antecedents of the simpler forms of Evolution, belong to a remote past respecting which we can have nothing but inferential knowledge; while the antecedents of the only kind of Evolution which is traceable from beginning to end (namely, that of individual organisms) are too complex to be dealt with by exact methods. Hence we cannot hope to establish _equivalence_ among the successive manifestations of force which each order of Evolution affords. The most we can hope is to establish a qualitative correlation that is indefinitely quantitative—quantitative in so far as involving something like a due proportion between causes and effects. If this can be done, however, some progress will be made towards the solution of our problem. Though it may be beyond our power to show a measurable relation between the force or group of forces which any phase of Evolution displays, and the force or group of forces immediately succeeding it; yet if we can show that there always are antecedent forces, and that the effects they produce always become the antecedents of further ones—if while unable to calculate how much of each change will be produced, we can prove that a change of that kind was necessitated—if we can discern even the vaguest correspondence between the amount of such change and the amount of the pre-existing force; we shall advance a step towards interpreting the transformation of the simple into the complex. With the view of attempting this, let us now reconsider the different types of Evolution awhile since delineated: taking them in the same order as before. * * * * *