356 LUTHER BURBANK 



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 

 Hemisphere 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 clustered 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. 



