LUTHER BURBANK 



All these trees grew far to the north, and 

 luxuriated, as has been said, in a temperature that 

 we of to-day would call sub-tropical, but which the 

 inhabitants of that time, had there been a human 

 population, would have described as arctic; 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 diverging 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 

 habit of the individual. Plants put forth mobile 

 seeds, and devise many strange ways of insuring 

 their wide dissemination. They are always seek- 

 ing new territories, and, granted proper conditions, 

 always finding them. 



[184] 



