What key idea does the text below suggest? No one would have believed in the last years of the nineteenth century that this world was being watched keenly and closely by intelligences greater than man's and yet as mortal as his own; that as men busied themselves about their various concerns they were scrutinised and studied, perhaps almost as narrowly as a man with a microscope might scrutinise the transient creatures that swarm and multiply in a drop of water. Humans were observed as if they were insects under a microscope. Humans were engaged in important but unnecessary work. The advanced human society had the tools to detect those spying on them. The creatures swarming in water are as visible as the Martian inhabitants were.