LUTHER BURBANK 
It is easy to see how the altered conditions of 
temperature made the struggle for existence un- 
duly hard for many species, because there is a 
tangibility about the coming of a glacial period 
that finds an analogy in the coming of winter in 
the regular sequence of seasons. The fact that a 
plant which thrives in the summer in northern 
regions cannot survive through the winter unless 
protected is so familiar as to give us a concrete 
example of the destruction of species through 
changed climatic conditions in the geological eras. 
But the struggle for existence that goes on all 
about us among plants of every species is so much 
less tangible that it is not so easily visualized. 
Not unlikely the climate of the northern hemis- 
phere is changing now year by year as rapidly as 
it ever changed in any era of the past. 
The alteration is so slight within the span of 
any single life as to be unappreciable. But when 
we look back, aided by the studies of the geologist, 
and think of thechange of climate that transformed 
the flora of the Mesozoic time, we see things clus- 
tered in perspective, and in our mental vision the 
picture of the transformation from tropical to 
arctic conditions corresponds rather to the onset of 
winter in our annual experience, than to the true 
picture of a change of climate that required not 
merely centuries but millenniums. 
[204] 
