A natural language is an organic living thing; and meanings change as words take on new, and shed old, connotations. Recent (post-Fregean) philosophy of language has paid little attention to the growth of meaning; radical philosophers like Feyerabend and Rorty have suggested that meaning-change undermines the pretensions of science to be a rational enterprise. Thinkers in the classical pragmatist tradition, however -Peirce in philosophy of science and, more implicitly, Holmes in legal theory- both recognized the significance of growth of meaning, and understood how it can contribute to the progress of science and to the adaptation of a legal system to changing circumstances. This paper develops these insights, and illustrates them by reference to (1) the growth of meaning of "DNA" from the identification of "nuclein" to the discovery of mtDNA almost a century later, and (2) the growth of meaning of "the establishment of religion" in the First Amendment to the U.S. Constitution from its ratification in 1791 to the present day. Arguing that the growth of meaning can indeed contribute to rationality, it also shows why narrowly formal models are inadequate both to science and to law.