In the beginning, it appears that water covered the whole earth. In that day, the living creatures of earth dwelt in the water, and it was the great age of fishes and other aquatic animals. Soon the first land lifted itself timidly above the surface of the ocean, and formed inviting places for land animals and plants. Upon the land came, first, according to the story of the rocks, a class of animals known as amphibians, like frogs, that could live both in water and on land. Associated with these creatures were vast forests of low orders of plants, that cleared the atmosphere of noxious gases, and made it fit for higher forms of life. Then followed an age in which the predominating animals were gigantic reptiles, a step higher than the amphibians, but a step lower than the class of Mammals to which man belongs. During the age of these prehistoric monsters, the earth was yet more fully prepared for higher life. Following the age of reptiles, came the age of mammals, which still persists, though, since the coming of man upon the earth, the geological age has been known as the age of man. This rapid sketch of the geological history of the earth does very poor justice to one of the most complete, wonderful and beautiful stories brought to the knowledge of man. The purpose of this chapter is not, however, to discuss the past ages of the earth. It is, of course, readily understood that such mighty changes as those just described, and the succession of different kinds of organic life, could not have taken place in a few years. Vast periods of time must of necessity have been required for the initiation, rise, domination and final extinction of each class of animals. A year is too small a unit of measurement in geological time; a thousand years or, better, a million years, would more nearly answer the requirements. [Sidenote: The earth is probably millions of years old.] It is possible in various ways to arrive at a conception of the age of the earth since organic life came upon it. For instance, the gorge of the Niagara Falls was begun in comparatively recent days, yet, judging by the rate at which the falls are now receding, it must have been at least 31,000 years since the making of the gorge was first begun, and it may have been nearly 400,000 years.[A] Lord Kelvin, on almost purely physical grounds, has estimated that the earth cannot be more than 100,000,000 years old, but that it may be near that age.[B] It need not be said, probably, that all such calculations are very uncertain, when the actual number of years are considered; but, all human knowledge, based upon the present appearance of the earth and the laws that control known phenomena, agree in indicating that the age of the earth is very great, running in all probability into millions of years. It must have been hundreds of thousands of years since the first life was placed upon earth. [Footnote A: Dana's New Text Book of Geology, p. 375.]