He did not do this.

He did not take into consideration, for instance, that climatic conditions may vary from age to age as well as from country to country.
4.
Having established the natural equality of the Ancients and Moderns, Fontenelle inferred that whatever differences exist are due to external conditions--( 1) time; (2) political institutions and the estate of affairs in general.
The ancients were prior in time to us, therefore they were the authors of the first inventions.

For that, they cannot be regarded as our superiors.

If we had been in their place we should have been the inventors, like them; if they were in ours, they would add to those inventions, like us.

There is no great mystery in that.