When I drive a peg into the ground with my axe or mallet, is the life in my arm any more strictly the source (the secondary source) of the energy expended than is the nut in this case? Of course, the sun is the primal source of the energy in both cases, and in all cases, but does not life exert the force, use it, bring it to bear, which it receives from the universal fount of energy? Life cannot supply energy _de novo_, cannot create it out of nothing, but it can and must draw upon the store of energy in which the earth floats as in a sea. When this energy or force is manifest through a living body, we call it vital force; when it is manifest through a mechanical contrivance, we call it mechanical force; when it is developed by the action and reaction of chemical compounds, we call it chemical force; the same force in each case, but behaving so differently in the one case from what it does in the other that we come to think of it as a new and distinct entity. Now if Sir Oliver or any one else could tell us what force is, this difference between the vitalists and the mechanists might be reconciled. Darwin measured the force of the downward growth of the radicle, such as I have alluded to, as one quarter of a pound, and its lateral pressure as much greater. We know that the roots of trees insert themselves into seams in the rocks, and force the parts asunder. This force is measurable and is often very great. Its seat seems to be in the soft, milky substance called the cambium layer under the bark. These minute cells when their force is combined may become regular rock-splitters. One of the most remarkable exhibitions of plant force I ever saw was in a Western city where I observed a species of wild sunflower forcing its way up through the asphalt pavement; the folded and compressed leaves of the plant, like a man's fist, had pushed against the hard but flexible concrete till it had bulged up and then split, and let the irrepressible plant through. The force exerted must have been many pounds. I think it doubtful if the strongest man could have pushed his fist through such a resisting medium. If it was not life which exerted this force, what was it? Life activities are a kind of explosion, and the slow continued explosions of this growing plant rent the pavement as surely as powder would have done. It is doubtful if any cultivated plant could have overcome such odds. It required the force of the untamed hairy plant of the plains to accomplish this feat.