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1ooo 2o per cent agrarian agricultural algae aluminum amount animal annually approximately average become billion acres billion tons birth rates breeding calories capita carbon carbon dioxide century coal coitus interruptus consumed contraceptive copper cultivation culture death rates decrease difficult diseases duction earth economic effect estimated Europe eventually existence factors farms fertility food production fossil fuels fraction future greater growth human important India industrial society intake iron land limited living machine magnesium major metals mortality natural obtained operation patterns period persons petroleum pig iron plant podsols population density population increase possible potential pounds present probably protein quantities rapidly raw materials regions reproduction rate result soil solar sources South America Soviet Union species stability steel sulphur supply techniques thorium tion tons of coal underdeveloped areas United uranium uranium and thorium utilized valley vegetation waterpower Western Western World