132 GENETIC VARIATIONS 



tory body; it disappears at intervals, and when it disappears, 

 the acquired characteristics dependent on it must disappear 

 also. Such acquired characters then are only transitory. They 

 play an important role in the life of the organisms; they last 

 for generations, but they do not permanently alter the charac- 

 teristics of the species. For such modifications JoUos' term of 

 Dauermodifikationen is an appropriate term; they are long- 

 standing modifications, but destined to disappear when the 

 macronucleus disappears. 



But it is certain that not all environmental modifications 

 are limited to the macronucleus. In Paramecium, the organ- 

 ism most studied with respect to these matters, it was unex- 

 pectedly found, after much work had been done, that the 

 macronucleus disappears at regular rather short intervals, 

 being replaced in each case from the micronucleus. This is 

 the process known as endomixis. In Paramecium aurelia this 

 disappearance and replacement occurs every three or four 

 weeks; in Paramecium caudatum the interval is about twice 

 as great. 



Yet most of the acquired modifications do not disappear 

 at these periods. Though the macronucleus is lost, they re- 

 main. It is clear therefore that they are not modifications 

 limited to the genie materials of the macronucleus. This is 

 true of the depression, or injurious modifications, induced by 

 poor living conditions. Paramecium continues to "run 

 down," becoming more and more depressed, in spite of 

 repeated replacements of the macronucleus from the micro- 

 nucleus. This is true also in many cases of the inherited 

 resistance induced by subjection to heat or to chemicals; this 

 often does not disappear when the macronucleus disappears. 

 On the contrary, it continues to be inherited for many gener- 

 ations, during which there are repeated disappearances and 

 replacements of the macronucleus. 



