DICHOTOMY, literally a cutting asunder, the technical term for a form of logical division, consisting in the separation of a class into two subclasses, one of which has and the other has not a certain quality or attribute (Gr. 5L a, apart, r I.WELY, to cut). Men may be thus divided into white men and men who are not white ; each of these may be subdivided similarly. On the principle of contradiction this division is both exhaustive and ex clusive; there can be no overlapping, and no members of the original genus or the lower groups are omitted. This method of classification, though formally accurate, has slight value in the exact sciences, partly because at every step one of the two groups is merely negatively characterized and may be unreal ; it is useful, however, in setting forth clearly the gradual descent from the most inclusive genus (summum genus) through species to the lowest class (in Pima species), which is divisible only into individ ual persons or things. (See DrinsioN.) In astronomy the term is used for the aspect of the moon or of a planet when apparently half illuminated, so that its disc has the form of a semicircle. DICK, ROBERT (1811-1866), Scottish geologist and bot anist, was born in Tullibody, Clackmannanshire, the son of an officer of excise. He became a baker and worked at his trade until his death in Thurso on Dec. 24, 1866. He never published any thing, but from 1835 onwards, when he first discovered fossils in the Caithness flags, he sent his specimens to Hugh Miller and others. His herbarium, which consisted of about 200 folios of mosses, ferns and flowering plants, is now stored, with many of his fossils, in the museum at Thurso.
See Samuel Smiles, Robert Dick, Baker of Thurso, Geologist and Botanist 0878).