EXCURSIONS OF AN EVOLUTIONIST 



sun are perpetually altering. First, owing to 

 the great revolution known as the " precession 

 of the equinoxes," the earth's perihelion 10,500 

 years ago came in midsummer in the northern 

 hemisphere, and it will come so again 10,500 

 years hence. In this state of things the winter 

 half of the year was and will be eight days 

 longer than the summer half. Secondly, the 

 shape of the earth's orbit changes from time 

 to time, under the influence of the variously 

 compounded attractions exerted upon it by its 

 companion planets. These changes occur at 

 irregular intervals, but they admit of accurate 

 calculation, and have been computed for 3,000,- 

 ooo years in the past and 1,000,000 years in 

 the future by Mr. Croll, from formulas fur- 

 nished by Leverrier. It has thus been ascer- 

 tained that at three several times within the 

 past 3,000,000 years the earth's orbit has be- 

 come very much elongated, so that the differ- 

 ence between its greatest and least distances 

 from the sun has been between four and five 

 times as great as at present, that is, it has 

 been from 12,000,000 to 14,000,000 miles. 

 The first of these periods of high eccentricity 

 began 2,650,000 years ago and lasted 200,000 

 years; the second began 880,000 years ago, 

 and lasted 1 80,000 years ; the third began 240,- 

 ooo years ago, and lasted 1 60,000 years. For 

 the last 50,000 years, the departure of the 



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