gradually

adverb2

1. •
   In a gradual manner; making slow progress; slowly.

   • The most likely conclusion, the researchers say, is that the Levallois method was not taken to Eurasia by African migrants, but evolved there gradually and independently. — 2014
2. •
   by degrees

   • Horns, upon the due accession of Moisture, doth gradually become Soft. — 1682