102 GENETIC VARIATIONS 



rate of multiplication, and in other respects. Other Protozoa 

 can be readily altered in similar ways. 



But usually, when the animals are restored to the original 

 conditions, in the offspring there produced the same old 

 form, structure and functioning return. I have myself made 

 extensive experimental studies on this, in Paramecium and in 

 DifHugia, by subjecting them to diverse chemical and other 

 agents, which changed them greatly. But on removal from 

 the chemical, in the next generation the animals returned to 

 their original state; the acquired characters were not inherited. 

 This is the common result of such experiments. The sub- 

 stances of the organisms have not been modified in constitu- 

 tion; it is only the superficial characteristics that have been 

 changed. The effects are like those of changing the shape of 

 a crystal, by filing off its angles or carving it into new shapes. 

 If such changed crystals are dissolved and recrystallized, the 

 original crystal form returns, for the chemical constitution 

 has not been changed. 



Most effects of the environment are of this kind. They do 

 not change the constitution of the fundamental materials of 

 the organism; therefore they produce no genetic variation; no 

 lasting effect on its characteristics. 



But in the unicellular organisms, not all environmental 

 action is of this sort. Sometimes, by environmental action, the 

 underlying substances are changed in constitution, so that a 

 genetic variation is induced, a variation that is carried on to 

 later generations. 



The difference in effect appears to be connected with the 

 length of time that the effective agent continues to act. If the 

 organisms are subjected for generation after generation suc- 

 cessively to the special conditions, so that these act for a very 

 long time, then the constitution may become changed; there 

 is produced an alteration in the inherited characteristics, a 



