350 GENETICS 



in the gene "white," at the locus 1.5 in the X-chromosome; 

 this is perhaps the best known gene in the fruit-fly. Thus 

 what could be called directed mutations were induced, se- 

 ries of changes in a certain direction for successive genera- 

 tions : what might be called experimental orthogenesis. 



Furthermore, according to Jollos, the nature of the mu- 

 tations was dependent to some extent on the nature of the 

 conditions under which the organisms were subjected to the 

 heat. Those in moist conditions for example gave a different 

 set of mutations from those in dry conditions. 



These rather revolutionary conclusions are however not 

 confirmed by the extremely extensive and long-continued 

 work of Plough and Ives.^ These authors confirm the fact 

 that high temperatures increase the frequency of mutations. 

 But they do not verify the other conclusions of Jollos; and 

 particularly they find no indications of the directed muta- 

 tions, or orthogenetic series reported by Jollos. As matters 

 stand, the more radical conclusions of Jollos cannot be con- 

 sidered established. 



When a mutation has occurred, is the change a perma- 

 nent one? Or may the genetic system later return to its 

 original condition? 



The great majority of mutations appear to be perma- 

 nent changes. They are inherited by the progeny of the 

 mutated individuals, and such inheritance continues for an 

 indefinite number of generations. 



But in recent years a considerable number of cases have 

 been discovered in which the change was not permanent.^ 

 Having mutated in a certain way, producing its usual effect 

 on the characteristics, the gene in a later generation returns 

 to its original condition, so that the original characteristics 

 are restored. In some cases such a reversion to normal is 

 produced by subjection of the mutated genes to radiation. 

 In other cases the mutated genes show a marked tendency to 



