Page:Popular Science Monthly Volume 9.djvu/700

672 the limit of the earth's orbit, before that separate existence could begin; for before then the earth must have formed part of the fiery mass of the sun. This calculation, like the others, falls short by nearly two hundred millions of years of the period estimated by Sir Charles Lyell for the commencement of life upon the earth.

But it would not be satisfactory to see a theory upset, if with the theory the means of accounting for observed facts were also destroyed. One great reason which weighs with geologists in assigning an almost incalculable age to the earth is, that among the fossils of the latest glacial epoch there are found the remains of tropical plants and animals, deposited in alternate strata with the remains of temperate climates, and this not once, but many times over. A hot climate prevailed at one time, and the earth became peopled with the flora and fauna appropriate to those conditions: after a lapse of many ages, the land subsided, and became the bed of the ocean; a vast period of upheaval then ensued, and dry land once more appeared: the climate gradually changed and ice set in: after ages more there was another slow subsidence, another equally slow upheaval, and another change of climate; and so on without end. Seeing the slow way in which the land sinks or is upheaved nowadays, it naturally appeared that no conceivable lapse of time could be enough to explain that which had obviously taken place.

Mr. Croll, however, has recently afforded an explanation at once beautiful, simple, and complete. About the facts to be accounted for there can be no doubt. The land has been many times under the sea, and the most violent changes of climate have succeeded one another. Mr. Croll's explanation is partly astronomical, and partly rests on geological dynamics. The beat of the sun is great in proportion to his distance from the earth. This distance is greater at one time of the year than another. The orbit of the earth is not quite circular, but its eccentricity Varies slowly from century to century. It is just now very small, and the summer of the northern hemisphere happens when the earth is at its greatest distance from the sun. Both these circumstances tend to produce in Europe a moderate climate. But the longitude of the perihelion, as this state of things is called, is constantly changing, and the line joining the solstices moves round the orbit in about twenty-one thousand years. It follows that every ten thousand years, or thereabouts, the winter of the northern hemisphere will occur when the earth is at its farthest from the sun; and, if at that time the earth's orbit is very eccentric, the two causes combined will produce a very severe climate. Eleven thousand years hence the northern hemisphere will be nearest to the sun in summer, and farthest from him in winter. Now, if, when that state of things occurred, the eccentricity of the earth's orbit happened to be very great—if the earth in winter-time was at a part of her orbit several millions of miles farther from, and in summer-time was very much nearer, the