Our considerations so far have been of a nature to convince us that not isolation, but a constancy in the continual action of like relatively small increments of force in the same directions, is the condition of steady evolution. The less constant and the larger the increments, the nearer the changes involved resemble catastrophe, though the catastrophes themselves may be regarded in another light as forming part of an evolution of a higher order. The changes the sun is undergoing may be regarded as evolution in so far as the influence of the cooling medium is a constant one. The earth as a whole and in its parts may be regarded as passing through a process of evolution towards full stability in so far as the sun's heat is a constant quantity, the periodic changes of seasons and of day and night the same. The relation would seem, therefore, to be one of time--the time-relation involved in the duration of outer conditions as constant with reference to the period required for the attainment of stability. Thus the sun's influence upon the earth might appear approximately constant to the human individual, but might represent a rapid change in relation to some stupendous and long-continued evolution in some other part of the universe. Considerations which we have already noticed forbid our regarding any conditions of "full" as distinguished from absolute stability as anything other than peculiar states single in the system and thus unenduring maxima succeeded by decrease, although the process may be, with reference to any other particular process, so slow, the retrogression from the culminating point so gradual, as to be, with respect to this other process, inappreciable. And while we are busied with matters which involve the whole multiplicity of relations in the universe, just a word with reference to cause and effect. Which one of these myriad material parts interacting at any moment shall we single out as the cause of the succeeding state? The solidarity of the universe as far as the complete interdependence of all its parts is concerned is clear to us. It is true we cannot reckon with all factors of the universe at once; and the concept of cause and effect is therefore a useful one. But the cause of anything must be, from a positive point of view, just what the methods prescribed for its discovery in any particular case shows it to be: namely, a factor, merely, in the manifold conditions determining a following state, the removal of which means the prevention of the succession of exactly that state. Which, for instance, shall we regard as the cause of an evil act--the character of a man or the temptation offered by circumstances? The change or removal of either means the change or removal of the act. Neither is complete without the other, and both are involved in the whole complexity of the universe, through heredity on the one hand and the action of nature external to life on the other.