It would appear that the majority of the caverns were formed in pluvial periods of early glacial times; the formation had been completed, the subterranean streams had ceased to flow, and the interiors were relatively dry and free from moisture in Fourth Glacial and Postglacial times, when man first entered them. There is no evidence, however, that the cavern depths were generally inhabited, for the obvious reason that there was no exit for the smoke; the old hearths are invariably found close to or outside of the entrance, the only exception being in the entrance to the great cavern of Gargas, where there is a natural chimney for the exit of smoke. There was no cave life, strictly speaking--it was grotto life; the deep caves and caverns were probably penetrated only by artists and possibly also by magicians or priests. It is in the _abris_ or shelters in front of the grottos and in the floors of the caverns that remarkable prehistoric records are found from late Acheulean times to the very close of the Palæolithic, as in the wonderful grotto in front of the cave at Castillo, near Santander. Thus, as Obermaier(60) observes: "In Chellean times primitive man was a care-free hunter wandering as he chose in the mild and pleasant weather, and even the colder climate of the arid 'loess' period of the late Acheulean was not sufficient to overcome his love of the open; he still made his camp on the plains at the edge of the forest, or in the shelter of some overhanging cliff." Only in rare instances, as at Castillo, were the Acheulean hearths brought within the entrance line of the grotto. DIFFERENCES OF OPINION AS TO THE GEOLOGIC AGE OF THE PALÆOLITHIC CULTURE STAGES The right-hand column represents the theory adopted in this volume.