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 the change 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] 



