GENE MUTATIONS 55 



effect, due to a slight mutation of a particular gene. Subjec- 

 tion of those changed descendants to the high temperature 

 for another generation is reported to increase the effect and 

 alter the gene still more. By continued subjection to high 

 temperatures for many generations a series of gradations is 

 thus produced, each one hereditary at ordinary temperatures, 

 but passing from slight changes in earlier generations to very 

 great ones in later generations. Thus, in one series of experi- 

 ments, a certain gene was so altered, in the first generation 

 subjected to heat, as to cause a slight darkening of the body 

 color. Subjection to heat in later generations increased the 

 effect on this gene until finally the body was black. In another 

 series, the red color of the eye was, by subjection of successive 

 generations to heat, caused to become lighter and lighter 

 until it was yellowish, and until finally entirely white eyes 

 were produced. The mutations to which these changes in 

 eye color were due were all in a certain gene, that is perhaps 

 the best known gene in the fruit fly. Thus what could be 

 called directed mutations were induced; series of changes in 

 a certain direction for successive generations; what might be 

 called experimental orthogenesis. 



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

 tions was dependent to some extent on the nature of the con- 

 ditions in 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- 



