Good point– I could see where Shelley’s audience might find the idea more plausible, particularly those who were less educated about biology and other sciences. I’m not sure I have a strong enough understanding of what science was like in 1818 to say for sure, though.
I would say that we are absolutely still trying to harness the “power” of the natural world, probably more today than we were then. After all, what else are solar panels and wind turbines? Even fossil fuels are a way of harnessing the natural world’s power!