Some physiologists incline to the view that non-living substances must enter into the bioplasm before they undergo metabolism. They consider that the molecules of the non-living substance must at the time when they undergo a chemical change be physically and chemically a part of the living substance. Others take the opposite view: that the living substance does not undergo change, but brings about changes in the non-living substance which is in contact with it, enclosed within its meshes. This is a problem which is not likely to be solved, nor is its solution of great importance in relation to the question which we are discussing. Whichever of the two views be justified, we have to distinguish between the bioplasm of the cell—the machine—and its raw materials and manufactured products. The question to which we want an answer is the following: Must the bioplasm undergo change? There seems to be no reason in the nature of things why it should. It is not, as we have already pointed out, subject to wear and tear. A perfect machine would in the absence of friction, which rubs down its steel and brass, continue to turn out its products so long as it was supplied with raw materials and the energy needed to manufacture them. We could imagine the bioplasm as indestructible, receiving energy from a portion of the foods, and expending this energy in the production of chemical change in the remainder. We could imagine that when once the tissues had attained their full growth they would require no more protein for their own nutrition; they would be occupied in producing heat and motion from the non-nitrogenous foods. But observation shows clearly that this is not the case. The force which energizes the bioplasm, enabling it to evoke metabolism in non-living substance, is obtained at the cost of its own destruction. The bioplasm wastes unless constantly supplied with proteid food.