The first thing which we wish to take up here is the use of protein in the repair of protoplasmic wastage and in growth. We saw in Chapter III that protein is manufactured originally by the living cells of green plants. We have also seen that protein is the only material that can repair protoplasmic wastage or that can make new protoplasm. We have omitted to say thus far that the most important place from which we can obtain protein is from living protoplasm. It is true that most seeds store up within themselves nonliving protein to be used by the young sprout as it forms, and seeds make up a large part of our diet; but except in grain and other seed foods we obtain our supplies of protein by eating protoplasm. This protein is to be used by us for repairing our protoplasmic wastage or, in parts of us that are growing, for making new protoplasm. We have already seen that the protein which we eat must go through a process of digestion before we can use it for these purposes, and our present task is to explain just why this is necessary and to show how the protein is actually used in our bodies. A thing about protein which fits it specially to be the chief material of living protoplasm is that it is very much the same sort of substance wherever we find it and yet can differ enough to account for the differences that exist among animals and plants. In spite of the fact that protoplasm analyzes about the same, no matter where it comes from, we are bound to believe that the difference between a dog and an oak tree is at bottom a chemical difference; they are unlike because the protoplasm of one is not the same substance chemically as the protoplasm of the other, and the difference is a difference in the proteins. We can come even nearer home than that and say that the differences between the races of mankind are probably chemical differences between their proteins. Human protein is undoubtedly different from the protein of beef or pork or mutton. What we have then is a substance which can be at the same time similar and different; also since we can make human protein out of beef protein or any other kind which we happen to eat it must be fairly easy to change one into the other.