In the biological branch of the subject the caves will be treated first which are comprehended within the limits of history; then we shall pass on to the investigation of Prehistoric caves, or those which have been inhabited in the interval that separates history from the remote geological era, which is characterized by the existence of the extinct mammalia in Europe. And, lastly, those will be examined which have furnished the remains of the extinct animals, and which are termed by the geologists Pleistocene, from the fact that a larger percentage of existing species were then living than in the preceding Pleio-, Meio-, and Eocene periods. The equivalent terms “Quaternary,” used by many French geologists, and the “Post-pleiocene division of the Post-tertiary Formation,” used by Sir Charles Lyell, are not adopted in this work, because they imply a break in the continuity of life, which does not exist. “Pleistocene” was invented and subsequently discarded by Sir C. Lyell,[3] and is at present used by many eminent writers, such as Forbes, Phillips, Gervais, and others. The ossiferous caves will therefore be divided into the Historic, Prehistoric, and Pleistocene groups. And it will be more convenient to work backwards in time from the basis offered by history, than to begin with the Pleistocene, or oldest division, and bring the narrative down to the present day. This classification, founded in part on the principle of change in the animal world, and partly on the basis offered by history, coincides, only in part, with that of the archæologists based on the remains of man’s handiwork. The Pleistocene age is the equivalent of the Palæolithic, or that of rude unpolished stone; the Prehistoric represents the ages of polished stone, bronze, and iron in part, or those stages in human progress when the use of these materials became general for the purposes of every-day life; while the Historic covers merely the later portion of that of iron. _History of Cave-Exploration in Europe._