THE simplest possible form of statistical classification is "division" (as the logicians term it) "by dichotomy," i.e. the sorting of the objects or individuals observed into one or other of two mutually exclusive classes according as they do or do not possess some character or attribute; as one may divide men into sane and insane, the members of a species of plants into hairy and glabrous, or the members of a race of animals into males and females. The mere fact that we do employ such a classification in any case must lnot of couirse be held to imply a natural and clearly defined bouindary between the two classes; e.g. sanity and insanity, hairiness and glabrousness, mav pass into each other by such fine gradations that judgments mnay differ as to the class in which a given individual should be entered. The judgment must however be finally decisive; intermediates not being classed as suich even when observed.