Here is the paragraph that contains the words:
As Karl Marx and Friedrich Engels saw clearly, the 19th-century business class
created more massive and more colossal productive forces than all preceding generations together. Subjection of Nature’s forces to man, machinery, application of chemistry to industry and agriculture, steam-navigation, railways, electric telegraphs, clearing of whole continents for cultivation, canalisation of rivers, whole populations conjured out of the ground—what earlier century had even a presentiment that such productive forces slumbered in the lap of social labor?
What does "conjured out" mean in this context? What is it that the whole populations conjured out of the ground?