70 THE THEORY OF THE GENE 



types were recognized at that time. These were the new 

 mutants. 



Now it has turned out that one of these types is due to 

 doubling the number of the chromosomes. It is called 

 gigas (Fig. 42). One is a triploid, semigigas. Several of 

 the types are due to the presence of an extra chromosome. 

 These are called lata and semilata forms. One at least, 

 brevistylis, is a point-mutant, like the recessive mutants 

 of Drosophila. It is, then, to 0. hrevistylis, and to the 

 residue of recessive mutants, that de Vries must appeal.' 

 It appears, now, highly probable that this residue (the 

 recessive mutants) conform to the Drosophila mutant 

 types, but their manner of reappearance in nearly every 

 generation gives a picture entirely different from that of 

 mutation in Drosophila and other animals and plants. A 

 possible interpretation may be found in the presence of 

 lethal genes closely linked with these recessive mutant 

 genes. Only when the recessive gene is released from its 

 near-by lethal through crossing-over is there an opportu- 

 nity for it to come to expression. It has been possible 

 in Drosophila to make up balanced lethal stocks carrying 

 recessive genes that simulate closely Oenothera. Only 

 when crossing-over occurs does the recessive reappear. 

 The frequency of its appearance is dependent on the 

 closeness of the lethal to the recessive gene. 



It has been found that other species of wild Oenotheras 

 behave in the same way as Lamarck's evening primrose, 

 whose peculiarities in inheritance are, therefore, not due 

 to a hybrid origin (as has been sometimes surmised), 

 but due, in the main, to the presence of recessive genes 

 linked to lethal factors. The appearance of the mutant 

 types does not represent the mutation process that pro- 



1 De Vries and Stomps both thought that some of the peculiarities of 

 0. gigas are due to other factors than chromosome number. 



