to LUTHER BURBANK 



fubjtet to Urge rarUtionf in a single gviention, 

 tucfa ▼amtkms afford peculiarly good material 

 for the operation of natural aeloction. Moreorer* 

 (nrolution by mutatkm would pretumably be 

 much more rapid than evolution that depended 

 for its leverage upon rninutr variations. 



WuAT Lai >i.s MuTATioii/ 



Incidentally the idea of relatively rapid evolu- 

 tioii. thus f^iven plausibility, answered the objec* 

 tion of oertain geologiits who had quetlioiied 

 whether tlie earth had been habitable long enough 

 to permit the evolution of the existing forms of 

 life through the cumulative effect of alight vari* 

 ations. 



The niutation theory is thus in many ways 

 acceptable. But to give the theory thiality it is 

 obviously necessar>' t eed one step farther 



and ask this question. Unit causes mutation? 

 And it is equally ubvions that the question must 

 be hard to answer. 



Professor de Varies, to be sure, made t)i :is 

 sumption that the changes in his evening prim- 

 rose were •»— '^-Hly due to altered conditions of 

 nutrition it i to the growth of the plant in a 



new soil. He further developed a theaia that 

 probably all species are subject to mutation 

 "periods,** which recur at more or less regular 



