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 vere 
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] 
