In view of all these facts, it is not surprising that concerning the history of many mammalian groups we have but scanty information, or none at all, while in the case of others the story is wonderfully full and detailed. The latter are, very generally, the groups which were not only numerically abundant at all stages of their history, but also had skeletons that were strong enough to resist destruction; while the groups as to which there is little or no information are chiefly of small and fragile animals, or such as were always rare. For example, a great deal has been learned regarding the development of horses and rhinoceroses in North America, but the history of the tapirs is very unsatisfactorily known, because, while horses and rhinoceroses were common, tapirs were solitary and rare. In Europe bats have been found in the Eocene, Oligocene and Miocene, and there is no reason to suppose that they were not equally ancient and equally abundant in America; but none have been found in the western hemisphere in any formation older than the Pleistocene. All things considered, the extraordinary fact is, not that so many forms have irretrievably perished, but that so much has been preserved, escaping all the chances of destruction. As to the degree of preservation in fossil mammals, we have to do almost entirely with bones and teeth. With very rare exceptions, and those all of late geological date, the viscera, muscles, skin, hair, horns, hoofs and claws have been completely destroyed and have vanished without leaving a trace. In northern Siberia the gravel soil is permanently frozen to a depth of several hundred feet and contains the intact carcasses of elephants and rhinoceroses of Pleistocene date and notably different from any species of these animals now in existence. Sometimes such a carcass is disinterred from a bluff by the cutting action of a stream and is in a state of nearly complete preservation, with hide, hair and flesh almost as in an animal freshly killed. From these remains it has been learned that the †Mammoth was an elephant densely covered with hair and wool, just as he was depicted in the carvings and cave-paintings of Pleistocene Man in Europe, where †Mammoth bones have been abundantly found, and also that there were Siberian rhinoceroses similarly protected against the cold. †Mammoth remains with hide and flesh, but much less complete, have likewise been found in Alaska.