There is, however, a fifth department, which owes its separate existence to a time when the terms Zoology and Botany were limited to the study of the existing forms of animal and plant life, and the extinct or fossil forms were associated with minerals, rather than with their living representatives. This arrangement prevailed in the British Museum until the year 1857. The fossils were then severed from this incongruous connection, and placed in a separate department to which the name of “Geology” was given.[3] The result is that there are two distinct zoological and botanical collections in the building, one containing the remains of the animals and plants which lived through successive ages of the world’s history from the earliest dawn of life down to close upon the present time, and the other including those living at the particular period in which we dwell. Notwithstanding the objections which may be urged against this separation, it prevails largely in museums, and (owing to certain conveniences, as well as to the difficulty and expense of rearranging extensive collections and reorganising the staff in charge of them) will probably be retained for some time to come. It should, however, be mentioned that a few specimens illustrating some of the more important extinct forms have been intercalated among the recent Mammals and Reptiles; while, conversely, skeletons and other specimens of recent animals have been introduced among the fossil Vertebrates in the Geological Department. Again, the more important remains of extinct Cetaceans are now shown in the Whale-Room, and some of the specimens of recent Elephants, as well as all the Sea-Cows, in the Geological Department. [Sidenote: Introductory Collection.] Besides the five Departments, into which the collection is divided for the purposes of custody and administration, each of which is under the charge of an officer styled “Keeper” and a staff of Assistants, there is a sixth, under the supervision of the Director, and arranged in the Central Hall, some of the specimens in which are intended to serve as an introduction to those exhibited in the others. [Sidenote: The specimens all arranged in three series.] Inclusive of the last-named collection, the whole of the specimens contained in the Museum, whether Animal, Vegetable, or Mineral, are arranged in the three following distinct series, as follows:-- [Sidenote: Introductory or Elementary Series.]