by little and little
  慢慢地,逐漸
  Lit·tle, n.
  1. That which is little; a small quantity, amount, space, or the like.
     Much was in little writ.   --Dryden.
     There are many expressions, which carrying with them no clear ideas, are like to remove but little of my ignorance.   --Locke.
  2. A small degree or scale; miniature. “ His picture in little.”
     A little, to or in a small degree; to a limited extent; somewhat; for a short time. =\“ Stay a little.”\=
     The painter flattered her a little.   --Shak.
  -- By little and little, or Little by little, by slow degrees; piecemeal; gradually.