From another point of view the contents of caves are peculiarly valuable. They have been used as places of shelter, not merely by man, but by wild animals, from the time they first became accessible to the present day. In the same way, therefore, as now they contain, in their superficial layers, the bones of sheep, oxen, and horses, foxes, rabbits, and badgers, so in their deeper strata lie buried the remains of the animals which were living in Europe long before the historic times. In other words, they enable us to make out the groups of animals inhabiting the neighbouring districts, and which in many cases have either forsaken their original abodes or have become extinct. And since those which are extinct, or which have migrated, could not have lived where their remains are found under the present conditions of life, an inquiry into their history leads us into the general question of the ancient European climate and geography. It is obvious, for example, that the spotted hyæna, which formerly inhabited the caves of Sicily, could not have crossed over to that island after it was separated from Africa and Italy; and it would be impossible for the musk-sheep, the most arctic of the herbivora, to live as far south as Auvergne under the present climatal conditions. The presence, therefore, of these animals in these districts is proof in the one case of a geographical, and in the other of a climatal, change. The discussion of all these questions is comprehended under the second, or biological, division of cave-hunting, which may be defined as an inquiry into the remains of man and animals found in the caves, and into the conditions under which they lived in Europe. _The three Classes of Bone-caves._