114 GENETIC VARIATIONS 



great role of environmental conditions in producing inherited 

 variations is not in doubt. 



Thus in the unicellular organisms in general — bacteria and 

 Protozoa — we find that environmental conditions may pro- 

 duce genetic variations of many sorts. We may classify them 

 under two heads. On the one hand they may produce injury. 

 The materials of the organisms are damaged, but continue to 

 multiply in the damaged condition, so that a long series of 

 depressed or degenerate generations is produced. On the other 

 hand, the environment may produce adaptations; it may 

 result, not in damage, but improvement. The organism under 

 the influence of the environment acquires power to do certain 

 things that it could not before do; to bring about chemical 

 reactions of which it was before incapable, and so to live 

 under conditions under which it formerly could not live. 

 These acquired powers are inherited for many generations 

 after removal from the conditions which induced them. 



In producing such results, there are certain important rela- 

 tions between the length of time that the special environmen- 

 tal condition has acted, and the intensity and duration in 

 generations of the genetic variation that it produces. In many 

 cases the degree or intensity of the inherited effect is propor- 

 tional to the length of time that the effective environmental 

 condition acts. This is notably the case with the inherited 

 damage resulting from living under bad conditions; the dam- 

 age does not appreciably begin till the conditions have existed 

 for many generations; it increases in amount as the bad condi- 

 tions last longer, and finally may result in destruction of the 

 stock. The constitution is progressively more modified the 

 longer the conditions last. The same relations are found in 

 many cases in the production of inherited resistance by the 



