by little and little

一点一点地，逐渐地

by air, by all means, by all nerves, by analogy, by and by, by and large, by any means, by chance, by choice, by comparison, by courtesy of, by error, by far, by force, by hand, by heart, by instinct, by invitation, by itself, by leaps and bounds, by means of, by mistake, by name, by nature, by night, by no means, by oneself, by pressed for, by reason of, by reason that, by request, by the moment, by the name of, by the same token, by the standards of, by the way, by turns, by virtue of, by way of, by worthy of,