The science of paleontology has passed through four periods, the first of them the long _Mystic period_ extending up to the beginning of the seventeenth century, when the idea that fossils were once living things was only rarely perceived. The second period was the _Diluvial period_ of the eighteenth century, when nearly everyone regarded the fossils as remains of the Noachian deluge. With the beginnings of the nineteenth century there arose in western Europe the knowledge that fossils are the “medals of creation” and that they have a chronogenetic significance; also that life had been periodically destroyed through world-wide convulsions in nature. From about 1800 to 1860 was the time of the creationists and catastrophists, which may be known as the _Catastrophic period_. The fourth period began in 1860 with Darwin’s Origin of Species. Since that time the theory of evolution has pervaded all work in paleontology, and accordingly this time may be known as the _Evolutionary period_. _Mystic Period._—The Mystic period in paleontology begins with the Greeks, five centuries before the present era, and continues down to the beginning of the seventeenth century of our time. Some correctly saw that the fossils were once living marine animals, and that the sea had been where they now occur. Others interpreted fossil mammal bones as those of human giants, the Titans, but the Aristotelian view that they were of spontaneous generation through the hidden forces of the earth dominated all thought for about twenty centuries.