ers of which it cannot be deprived, and which cannot be added to nor lessened. It is this that secures the permanence of Nature, keeping it unchanged in its power or powers amid all changes of action. This energy, disappearing in one form, appears necessarily in another, and gives us what Spencer calls the "persistence of force." This ever enduring force gives rise to development. Going out from one body, it is manifested in another. The fact is, all causation, all physical action, is evolution. The substances and powers in the agents acting as the cause are found, though in a modified form, in the effects. Proceeding on this very principle, Mayer says: "Forces are causes; accordingly, we may in relation to them make full application of the principle causa equat effectum" and he thus elaborated the grand scientific truth, the most important discovered in our day, that the sum of energy in the universe is always the same.
2. I admit that this power becomes more and more differentiated, that is, takes more and more diverse forms, and thus imparts an ever increasing multiplicity and variety to the universe, and will continue to do so till the diversity breaks it up, and "the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also, and the works that are therein shall be burned up." Mr. John S. Mill has been successful in showing that there is usually more than one antecedent or agent in a cause. "A man takes mercury, goes out-of-doors, and catches cold. We say, perhaps, that the cause of his taking cold was exposure to the air. It is clear, however, that his having taken mercury may have been a necessary condition of his catching cold; and though it might consist with usage to say that the cause of his attack was exposure to the air, to be accurate we ought to say that the cause was exposure to the air while under the effect of mercury." He concludes, "The real cause is the whole of these antecedents." Now, I hold that in physical Nature causes are not only usually, but invariably, of this dual or plural nature. I go a step farther, and have shown, I think, that the effects are also of the same dual or plural character. The effect, in fact, consists of the same agents or substances as the cause, but now in a new state. A picture falls from a wall and breaks a table; we say that the breaking of the table was the effect of the fall of the picture. But the true effect embraces both the picture and the table, the picture having lost its momentum, and the table being broken. It follows from all this that the new combination of agents, acting as the causes, must produce more and more varied effects, as the effects joining with other effects become causes, and ramify into branches and branchlets. The sum of the powers is one and the same, but they appear in an ever-increasing number and diversity of forms. The conservation of force thus gives a unity to Nature, while the mutual action and interaction give it its multiplicity. I remember how deeply I was interested in that paper (I read it when it appeared) of Von Baer, in which he shows