I. An Elementary or Introductory Series, by which the study of every group should commence. In this series, limited, so far as the cases in the Central Hall are concerned, to Vertebrated Animals and Botany, the leading features of the structure, and, so far as may be, the development of the various parts of some of the more typical members of each group, are demonstrated in a simple manner, and the terms used in describing and defining them explained by means of illustrative examples. This idea is carried out in the Department of Minerals in a series of cases placed on the north or left-hand side of the gallery containing the rest of the collection. [Sidenote: Exhibited Systematic Series.] II. The Exhibited Systematic Series, in which the more important types of animals, plants, or minerals are shown, by means of specimens, arranged in a systematic manner; or one which exhibits, so far as may be, their natural relations to each other. Classification is an important feature in this series, which properly should be so extensive and so arranged as to enable visitors to the Museum, without recourse to assistance from the officials, to find every well-known and markedly distinct type of animal, plant, or mineral, and satisfy themselves about, at least, its external characters. In practice, with the amount of space available, and the resources at the disposal of the authorities, it has, however, been found impracticable to carry out this ideal in anything like its entirety, and in most instances only a selection of specimens is in consequence exhibited. While the two series above mentioned have for their object the _diffusion_ of scientific knowledge, the next ministers mainly to its _advancement_, so that between them the twofold object of a National Museum of Natural History is carried out. [Sidenote: Reserve or Study Systematic Series.]