“That isn’t what I mean at all,” Wotherspoon interrupted, “I want you to look at it in another way. I suppose you had your usual style of dinner to-day. Just think of the items: soup, fish, meat, bread, and so on. Your soup was made from bones and vegetables; your fish course was originally an animal; so was your joint; your sweet was probably purely vegetable; and your dessert certainly was a plant product. Now don’t you see what I mean?” “No, I confess I don’t.” “Haven’t I just shown you that everything you ate comes from either the animal or vegetable kingdom? You don’t bite bits out of the crockery, like the Mad Hatter. Everything you use to keep your physical machine alive is something which has already had life in it? Isn’t that so? You never think of having a meal of pure chemicals, do you?” “It never occurred to me; and I doubt it I shall begin now. It doesn’t sound very appetising.” “It would be worse than that; but follow my argument further. Take the case of your joint. Presumably that came from an ox or a sheep. Where did the animal, whatever it was, get _its_ food? From the vegetable kingdom, in the form of grass. Isn’t it clear that everything you yourself eat comes, either directly or indirectly, from the plants? And aren’t all animals on the same footing as yourself--they depend ultimately on the vegetables for their sustenance, don’t they? A fox may live on poultry; but the chickens he kills have grown fat by eating grain; and so you come back to the plants again. If you like to look on it in that way, we are all parasites on the plants; we cannot live without them. Our digestive machinery is so specialised that it will assimilate only a certain type of material--protoplasm--and unless it is supplied with that material, we starve. We can convert the protoplasm of other animals or of plants to our own use; but we cannot manufacture protoplasm from its elements. We have to get it ready-made from the vegetables, either directly or indirectly. “Now the foundation-stone of protoplasm is the element nitrogen. The plants draw on the store of nitrogenous compounds in the soil in order to build up their tissues; and then we eat the plants and thus transfer this material to our own organisms. What happens next? Do we return the nitrogen to the soil? Not we. We throw it into the sea in the form of sewage. So you see the net outcome of the process is that we are gradually using up the stores of nitrogen compounds in the soil, with the result that the plants have less and less nitrogen to live on.” “Well, but surely four-fifths of the atmosphere is nitrogen? That seems to me a big enough reserve to be drawn on.”