¹ Part of this essay, written to commemorate the centennial of the 1857-1859 Rothamsted experiments on biological nitrogen fixation, was given as the Presidential Address on May 2, 1957, at the 57th Annual Meeting of the Society of American Bacteriologists held in Detroit, Michigan, under the title „In Defense of Imperfection.” For bibliographic as well as other obvious reasons, the present title appears to be more appropriate for the written form.