III. The Reserve or Study Systematic Series contains the exceedingly numerous specimens (in many groups the great bulk of the collection) showing the minute distinctions required for working out the problems of variation according to age, sex, season and locality, for fixing the limits of geographical distribution, or determining the range in geological time: distinctions which, in most cases, can only be appreciated when the specimens are kept under such conditions as to admit of ready and close examination and comparison. It is to this part of the collection that naturalists resort to compare and name the animals and plants collected in exploring expeditions, to work out natural-history problems, and generally to advance the knowledge of science. In fact, these reserve collections, occupying comparatively little space, kept up at relatively small cost, and visited by comparatively few persons, constitute, from a scientific point of view, the most important part of the Museum; for by their means new knowledge is obtained, which, given forth to the world in the form of memoirs and books, is ultimately diffused over a far wider area than that influenced even by the exhibited portions of the Museum. Indeed, without the means of study afforded by the reserve series, the order displayed in the arrangement of the exhibition galleries, and the instruction which may be gleaned from the same, would not be possible. It is important to bear in mind that if all the specimens required for enlarging the boundaries of knowledge were displayed in the public galleries, so that each could be distinctly seen, a museum many times larger than the present one would not suffice to contain them; the specimens themselves would be inaccessible to those capable of deriving instruction from their examination, while, owing to the effects of exposure to light upon preserved natural objects, many would lose their chief characteristics. This portion of the collection must, in fact, be treated as are the books in a library and used for consultation and reference by accredited students.[4] In some parts of the Museum the reserve collections are contained in drawers beneath the cases in which the corresponding exhibited portion is placed. This applies principally to the fossil specimens, the shells, and the minerals. The reserve birds and insects have special rooms devoted to them, and the extensive series of reptiles, fishes, and other animals preserved in spirit is kept, for the purpose of safety, in a separate building behind the Museum. In the Botanical Department the reserve collections are kept in the well-known form of an Herbarium, or _Hortus siccus_. [Sidenote: Supplementary Collections.]