Mutation 115 



than an odd one. This, together with the fact that 

 there are several species groups in which the chromo- 

 some count of some of the members is just twice that of 

 the others, suggests that tetraploidy may ha\'e played 

 a considerable role in evolution. Tetraploidy commonly, 

 but not always, brings gigantism. 



Blakeslee now puts the finishing touches on this 

 tetraploidy conception by more work on Datura. In 

 addition to the abnormal forms with twenty-five chromo- 

 somes, he has discovered one completely triploid (thirty- 

 six chromosomes) and one tetraploid form (forty-eight 

 chromosomes). These latter both seem to be in a 

 "better-balanced" condition than the non-disjunctional 

 (twenty-five chromosome) forms, since they are more 

 ''normal" with respect to their vegetative features and 

 fertility. . 



The beauty of the situation arises from the fact that 

 the tetraploid type contains a previously known Mende- 

 lian factor. In normal diploid forms a hybrid of the 

 composition Aa will give a 3: i ratio of purple flowered 

 and white flowered in the F2. The tetraploid hybrid 

 AAaa gives gametes in the ratio 1 AA:^ AA; i aa. 

 Chance matings among these results in an F^ of 35 

 purple: I white. The F3 and later generations behave 

 according to expectations on this basis. 



As stated before, the term "mutation" is now commonly 

 restricted to locus changes. The author has not discovered the 

 conventional term to include all of the foregoing cases unless it be 

 merely "germinal variations." 



The bearing of these phenomena upon evolution miglil be 

 considered briefly. Until a few years ago the general belief on 

 evolution included the following notions: inheritance of acquired 

 characters has been exploded; Darwinian variations are rather 



