118 LUTHER BURBANK 



and myrtles and ivies that are represented by 

 existing descendants of allied forms. 



And there were magnolias and tulip trees of 

 which the existing tulip tree of the United States 

 is an obviously direct and not very greatly modi- 

 fied descendant. 



All these trees grew far to the north, and 

 luxuriated, as has been said, in a temperature 

 that we of to-day would call subtropical, for in 

 that day it is probable that the North Pole was 

 tilted far toward the sun, and that the conditions 

 that we now think of as tropical existed only in 

 the region of the pole itself. 



Then there came the slow progressive period 

 of refrigeration. The tropical climate of the 

 pole was succeeded by an age of ice, and the 

 successive ice sheets slowly pressed southward, 

 driving the plants no less than the animals 

 before them along all parallels of longitude, 

 until the flowers and faunas that intermingled 

 in the arctic region were scattered along diverg- 

 ing paths to people the continents separated 

 by the wide stretches of the Atlantic and the 

 Pacific oceans. 



It may seem strange to speak of plants fleeing 

 before the ice sheet. But it must be understood 

 that the plant is a migratory being, when consid- 

 ered as a race, notwithstanding the stationary 



