VARIATION IN UNICELLULAR ORGANISMS 115 



action of heat or chemicals ; the longer the action, the greater 

 the resistance produced. The action of the environmental 

 conditions on the materials of the organism is progressive. 

 There is in many cases a similar relation between the length 

 of time the effective agent acts, and the length of time meas- 

 ured in generations, that the induced genetic variation con- 

 tinues to be inherited, after removal from the conditions 

 that produce it. When the environmental condition — the heat 

 or the special chemical or the like — has acted but a short 

 time, usually its effect is not inherited at all. The constitution 

 has not been modified. If, however, the special environment 

 has operated for a longer period, its effect is inherited for a 

 number of generations, but finally disappears. If the action of 

 the special environment has continued for a very long period, 

 through many generations, the inherited effect continues for 

 a great number of generations after removal from the condi- 

 tions that induce it. An example of these relations is seen in 

 the inheritance of an abnormality in the shell of the rhizopod 

 Arcella, as studied by JoUos. The abnormal form of the shell, 

 known as double-lobing, is partly produced by the accumula- 

 tion of waste products in the fluid in which the animals 

 live. If after the abnormality has persisted for many genera- 

 tions, the individuals that are least abnormal are continually 

 selected for propagation, gradually the population becomes 

 normal. When the animals had lived for four months in the 

 condition tending to produce the abnormality, it required 

 two months to bring them back to normal; when they had 

 lived for seven and one-half months in such conditions it 

 required three months to bring them back to normal; when 

 they had lived for twelve months in such conditions, four 

 months were required to bring them back to normal. In these 

 cases there was about one generation or more a day, so that 



