26 LUTHER BURBANK 



subject to large variations in a single generation, 

 such variations afford peculiarly good material 

 for the operation of natural selection. Moreover, 

 evolution by mutation would presumably be 

 much more rapid than evolution that depended 

 for its leverage upon minute variations. 



WHAT CAUSES MUTATION? 



Incidentally the idea of relatively rapid evolu- 

 tion, thus given plausibility, answered the objec- 

 tion of certain geologists who had questioned 

 whether the earth had been habitable long enough 

 to permit the evolution of the existing forms of 

 life through the cumulative effect of slight vari- 

 ations. 



The mutation theory is thus in many ways 

 acceptable. But to give the theory finality it is 

 obviously necessary to proceed one step farther 

 and ask this question: What causes mutation? 

 And it is equally obvious that the question must 

 be hard to answer. 



Professor de Vries, to be sure, made the as- 

 sumption that the changes in his evening prim- 

 rose were probably due to altered conditions of 

 nutrition incident to the growth of the plant in a 

 new soil. He further developed a thesis that 

 probably all species are subject to mutation 

 "periods," which recur at more or less regular 



