672 THE POPULAR SCIENCE MONTHLY. 



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 near- 

 ly 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 ani- 

 mals, deposited in alternate strata with the remains of temperate 

 climates, and this not once, but many times over. A hot climate pre- 

 vailed 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 up- 

 heaval 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. CrolPs 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 con- 

 stantly 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 



