You can hardly convince a man of an error in a lifetime, but must content yourself with the reflection that the progress of science is slow.
 Henry David Thoreau, A Week on the Concord and Merrimack Rivers (1849). copy citation

Context

“Society has relaxed a little from its strictness, one would say, but I presume that there is not less religion than formerly. If the ligature is found to be loosened in one part, it is only drawn the tighter in another. You can hardly convince a man of an error in a lifetime, but must content yourself with the reflection that the progress of science is slow. If he is not convinced, his grandchildren may be. The geologists tell us that it took one hundred years to prove that fossils are organic, and one hundred and fifty more, to prove that they are not to be referred to the Noachian deluge.” source