28 Where did Life Begin f 



♦ 



America. Birds must have shortened their 

 northward migrations and lengthened them 

 southward, and for the most part may have 

 escaped catastrophe — the beasts of prey, cat- 

 tle, and other large mammals of Drift latitudes 

 must also, to a great extent, have moved 

 toward the tropics as the rigors of the ap- 

 proaching ice period began to be felt." If the 

 comparatively swift coming cold of the glacial 

 period could have moved to the southward, 

 over vast areas, all the plants and animals of 

 the northern hemisphere, how much more ade- 

 quate and effectual for the same purpose must 

 have been the gradual lowering of tempera- 

 ture during the earlier and immensely longer 

 periods of the earth's life history. 



IV. 



Let us now see how admirably the earth is 

 adapted by its surface formation and topo- 

 graphy, for a southern migration from a zone 

 surrounding the north pole. In the first 

 place, nearly the whole of the earth's surface 



