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 THE LOGIC OF CLASSIFICATION. 237 nomenclature apart, the grouping itself is astronomically useful ; and, as it accomplishes the threefold object of aiding the understanding, of displaying coexistences and of helping the memory, it must be pronounced satisfactory and scien- tifically unimpeachable. Again, take an easy instance from Botany. The full classifying scheme of the Natural History sciences will be considered later on ; but, meanwhile, let us illustrate the one point of a luminous principle from the well-known Knot- grasses. Besides other modes of arrangement, a group of these might be formed so as to disclose a serial development in one particular part the flower. At one end would stand Common Knotgrass, with abundant sessile flowers, clustered in the axils of nearly all the leaves on the stem ; at the opposite extreme would stand Persicaria, with its short dense terminal racemes. Between the two would come Climbing Knotgrass and Copse Knotgrass, each possessing the axile floral cluster of Common Knotgrass and the ter- minal raceme of Persicaria : each, too, with the flower coloured like that of Common Knotgrass (viz., green, with a white margin), but with the lustrous seed-vessel of Persicaria. Here we have an obvious evolution of parts which the mere placing of the groups in this relation serves exactly to bring out. So, too, with the three popular species of Primroses Common Primrose, Oxlip and Cowslip, which, when arranged in this order, show a marked gradation in two separate points, the leaf and the flower. The leaves, although all agreeing in being wrinkled and toothed, are easily distinguished by their difference in shape. The flowers are even more sharply differentiated. Those of the Common Primrose are solitary, borne upon longish slender pedicels, which rise apparently direct from the root-stock, having the corolla of a pale yellow colour, with broad flat limb and contracted throat with thickened folds. Those of the Oxlip rise from the root-stock in clusters upon a short stem or peduncle, with corolla of a pale yellow colour, but limb concave, throat open and destitute of folds. Those of the Cowslip are also clustered, but upon longer peduncles ; have corolla small, funnel-shaped and of a buff-yellow colour ; limb cup-shaped ; throat open, with folds obscure. The grouping is obviously instructive, and possesses systematic and scientific value. Once more, let us take the books in a library. These, clearly, might be arranged in several useful ways. They might be grouped according to the subjects of which they