PLANT EVOLUTION 245 



agricultural crops had originated by gradual variations, but it 

 is now very generally believed that they are true mutants. 



The mutation theory of DeVries does not contradict Dar- 

 win's theory of natural selection. Darwin believed that all kinds 

 of variations could be inherited and that those that were useful 

 to the plant would be intensified through succeeding genera- 

 tions, thus giving rise to new and distinct types. But, according 

 to the DeVries theory, the mutants only possess the characters 

 by which new types are established. However, it remains for 

 natural selection to determine which of the mutants shall persist 

 and which will be lost in the struggle for existence. 



The cause of variations is as yet unexplained. Of course, 

 plants will vary in size, vigor and many other characters as a 

 result of amount of water, food,, light, temperature and many 

 other environmental factors, but these variations do not appear 

 to be inherited. Mutations appear to be due to changes within 

 the individual. 



