would release their contents as soon as plants had grown enough to furnish them with food. Others would allow their contents to hatch only after certain other varieties had multiplied—to be their food-supply.

When the Ecology-Ship left, it had done a very painstaking job. It had treated the planet to a sort of Russell's Mixture of life-forms. The real Russell's Mixture is that blend of the simple elements in the proportions found in suns. This was a blend of life-forms in which some should survive by consuming the now-habituated flora, others by preying on the former. The planet was stocked, in effect, with everything that it could be hoped would live there.

But only certain things could have that hope. Nothing which needed parental care had any chance of survival. The creatures seeded at this time had to be those which could care for themselves from the instant they burst their eggs. So there were no birds or mammals. Trees and plants of many kinds, fish and crustaceans and tadpoles, and all kinds of insects could be planted. But nothing else.

The Ludred swam away through emptiness.

There should have been another planting centuries later. There should have been a ship from the Zoölogical Branch of the Ecological Service. It should have landed birds and beasts and reptiles. It should have added pelagic mammals to the seas. There should have been herbivorous animals to live on the grasses and plants which would have thriven, and carnivorous animals to live on them in turn. There should have been careful stocking of the planet with animal life, and repeated visits at intervals of a century or so to make sure that a true ecological balance had been established. And then when the balance was fixed men would come and destroy it for their own benefit.

But there was an accident.

Ships had improved again. Even small private space-craft now journeyed tens of light-years on holiday journeys. Personal cruisers traveled hundreds. Liners ran matter-of-factly on ship-lines tens of thousands of light-years long. An exploring-ship was on its way to a second island universe. (It did not come back.) The inhabited planets were all members of a tenuous organization which limited itself to affairs of space, without attempting to interfere in surface matters. That tenuous organization moved the Ecological Preparation Service files to Algol IV as a matter of convenience. In the moving, a card-file was upset. The cards it contained were picked up and replaced, but one was missed. It 

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