Fontenelle saw the weakness of this reasoning.

He saw that it was necessary to prove that the trees, no less than human brains, have not degenerated.

But his a priori proof is simply a statement of the Cartesian principle of the stability of natural processes, which he put in a thoroughly unscientific form.

The stability of the laws of nature is a necessary hypothesis, without which science would be impossible.
But here it was put to an illegitimate use.

For it means that, given precisely the same conditions, the same physical phenomena will occur.
Fontenelle therefore was bound to show that conditions had not altered in such a way as to cause changes in the quality of nature's organic productions.