Why did Mr. Summers replace the original wood chips with slips of paper?

Mr. Summers replaced the chips with paper because of the large growth in population. Slips of paper fit into box, whereas, the wood chips might not have.

Because so much of the ritual had been forgotten or discarded, Mr. Summers had been successful in having slips of paper substituted for the chips of wood that had been used for generations. Chips of wood, Mr. Summers had argued. had been all very well when the village was tiny, but now that the population was more than three hundred and likely to keep on growing, it was necessary to use something that would fit more easily into he black box.