246 LUTHER BURBANK 



it to take on an altogether abnormal activity. 

 But, as just stated, this result applies only to 

 individual plants, and no one thinks of develop- 

 ing a race of Winter Rhubarbs in this way. 



Mixed Heredities 



The habit of perpetual bearing, as manifested 

 by my perfected varieties of Winter Rhubarb, 

 was explained as a development based on the 

 comparatively recent residence of the ancestors 

 of the plant in a tropical climate. The fluctuat- 

 ing temperatures of the globe in successive ages — 

 a time of tropical warmth being succeeded by 

 an ice age — resulted in subjecting the plant 

 at different periods to wide extremes of 

 temperature. 



A vast number of species were in this way 

 wiped out of existence. 



But those that survived developed powers of 

 resistance which were in many cases subse- 

 quently submerged or lost when the plants mi- 

 grated to the tropics, or when tropical conditions 

 prevailed; but which remained as latent in- 

 fluences in the heredity susceptible of being 

 brought out again under proper conditions of 

 hybridization. 



Thus, in order fully to understand the anom- 

 alous habit of the new Winter Rhubarbs, it is 



