It gradually dawned on the minds of men that the crust of the earth is like a gigantic mausoleum, containing within it the remains of numerous and varied forms of life that formerly existed upon the surface of the earth. The evidence is clear that untold generations of living forms, now preserved as fossils, inhabited the earth, disported themselves, and passed away long before the advent of man. The knowledge of this fossil life, on account of its great diversity, is an essential part of biology, and all the more so from the circumstance that many forms of life, remains of which are exhibited in the rocks, have long since become extinct. No history of biology would be complete without an account of the rise and progress of that department of biology which deals with fossil life. It has been determined by collecting and systematically studying the remains of this ancient life that they bear testimony to a long, unbroken history in which the forms of both animals and plants have been greatly altered. The more ancient remains are simple in structure, and form with the later ones, a series that exhibits a gradually increasing complexity of structure. The study of the fossil series has brought about a very great extension of our knowledge regarding the age of the world and of the conditions under which life was evolved. Strange Views Regarding Fossils.--But this state of our knowledge was a long time coming, and in the development of the subject we can recognize several distinct epochs, "well-marked by prominent features, but like all stages of intellectual growth, without definite boundaries." Fossils were known to the ancients, and by some of the foremost philosophers of Greece were understood to be the remains of animals and plants. After the revival of learning, however, lively controversies arose as to their nature and their meaning. Some of the fantastic ideas that were entertained regarding the nature of fossil remains may be indicated. The fossils were declared by many to be freaks of nature; others maintained that they were the results of spontaneous generation, and were produced by the plastic forces of nature within the rocks in which they were found embedded. Another opinion expressed was that they were generated by fermentations. As the history of intellectual development shows, the mind has ever seemed benumbed in the face of phenomena that are completely misconceived; mystical explanations have accordingly been devised to account for them. Some of the pious persons of that period declared that fossils had been made and distributed by the Creator in pursuance of a plan beyond our comprehension. Another droll opinion expressed was that the Creator in His wisdom had introduced fossil forms into the rocks in order that they should be a source of confusion to the race of geologists that was later to arise.