Zoo Hypothesis / Prime Directive

I found an early reference to something much like the zoo hypothesis (or Star Trek's Prime Directive) in Olaf Stapledon's 1937 novel Star Maker:

Their little island universe, their outlying cluster of stars, had come wholly under their control. It contained many natural planetary systems. Several of these included worlds which, when the early Arachnoid explorers visited them telepathically, were found to be inhabited by native races of pre-utopian rank. These were left to work out their own destiny, save that in certain crises of their history the Symbionts secretly brought to bear on them from afar a telepathic influence that might help them to meet their difficulties with increased vigour. Thus when one of these worlds reached the crisis in which Homo Sapiens now stands, it passed with seemingly natural ease straight on to the phase of world-unity and the building of utopia. Great care was taken by the Symbiotic race to keep its existence hidden from the primitives, lest they should lose their independence of mind. Thus, even while the Symbiotics were voyaging among these worlds in rocket vessels and using the mineral resources of neighbouring uninhabited planets, the intelligent worlds of pre-utopian rank were left unvisited. Not till these worlds had themselves entered the full utopian phase and were exploring their neighbour planets were they allowed to discover the truth. By then they were ready to receive it with exultation, rather than disheartenment and fear.
Thenceforth, by physical and telepathic intercourse the young utopia would be speedily brought up to the spiritual rank of the Symbiotics themselves, and would co-operate on an equal footing in a symbiosis of worlds.
Some of these pre-utopian worlds, not malignant but incapable of further advance, were left in peace, and preserved, as we preserve wild animals in national parks, for scientific interest. Aeon after aeon, these beings, tethered by their own futility, struggle in vain to cope with the crisis which modern Europe knows so well. In cycle after cycle civilization would emerge from barbarism, mechanization would bring the peoples into uneasy contact, national wars and class wars would breed the longing for a better world-order, but breed it in vain. Disaster after disaster would undermine the fabric of civilization. Gradually barbarism would return. Aeon after aeon, the process would repeat itself under the calm telepathic observation of the Symbiotics, whose existence was never suspected by the primitive creatures under their gaze. So might we ourselves look down into some rock-pool where lowly creatures repeat with naive zest dramas learned by their ancestors aeons ago.
The Symbiotics could well afford to leave these museum pieces intact, for they had at their disposal scores of planetary systems. (pp. 154-5).