"Around 8,000 BC, world population was something like five million. By the time of [the birth of Jesus] Christ, it was 200 to 300 million. By 1650, it was 500 million, and by 1800 it was one billion. The population of the world reached two billion by 1930. By the beginning of the '60s it was three billion; in 1975 it was four billion; and after only eleven more years it was five billion. This cannot go on forever; collapse is inevitable. The only question is when."