Were trees in ancient times greater than to-day?
If they were, then Homer, Plato, and Demosthenes cannot be equalled in modern times; if they were not, they can.
Fontenelle states the problem in this succinct way at the beginning of the Digression.

The permanence of the forces of Nature had been asserted by Saint Sorlin and Perrault; they had offered no proof, and had used the principle rather incidentally and by way of illustration.

But the whole inquiry hinged on it.

If it can be shown that man has not degenerated, the cause of the Moderns is practically won.

The issue of the controversy must be decided not by rhetoric but by physics.