24 Where did Life Begin? 



III. 



Let us now pass to a consideration of the 

 present condition of the earth and the Hfe 

 upon it, and see how far things as we find 

 them tally with the conclusions we have now 

 reached. Before doing so, however, one or 

 two preliminary suggestions may be advisable. 

 There is a very interesting, and at least 

 plausible, theory gaining ground, that from the 

 eccentricity of the earth's orbit, and other 

 causes too numerous to state here, the north- 

 ern and southern hemispheres are alternately 

 submero^ed and drained off as the vast ice 

 accumulations first around one pole and then 

 around the other change slightly the centre 

 of gravity of the earth alternately to the north 

 and the south of the plane of the equator, 

 and that each hemisphere makes the round 

 of these changes in a long period or year, 

 consisting theoreticall}^ of about 26,000, but 

 practically, owing to the reverse movement 

 of the precession of the equinoxes, of about 

 22,500 of our common years. 



