LUTHER BURBANK 



abundantly prove, the conditions of climate now 

 restricted to the tropics prevailed even in the 

 temperate zones, and the vegetable life was char- 

 acterized by the abundant production of colossal 

 forms. 



In successive ages the climate changed, and 

 it became necessary for the plants that were 

 unable to maintain existence under the changed 

 conditions to adapt themselves in size and in 

 structure to a less bountiful supply of food-stuffs 

 drawn from both soil and air; for the soil of the 

 temperate zone is relatively arid, and the air 

 probably became progressively less rich in car- 

 bon, owing to the permanent storage of vast 

 quantities of this substance in what ultimately 

 became the coal beds. 



So it came about that all the descendants of 

 the colossal plants of the Carboniferous Era 

 formed races that were dwarfs by comparison. 

 Here and there a straggling species, like the 

 California redwoods, preserved a reminiscence of 

 its imposing heritage. But in general the trees 

 that make up our forests in the temperate zone 

 are but insignificant representatives of a lost race 

 of giants. These, then, are the remote ancestors 

 that may be invoked in explanation of tlie rapid 

 growth and relatively gigantic stature of our 

 hybrid walnuts. 



[164] 



