VARIATION IN UNICELLULAR ORGANISMS 103 



change that is passed on to later generations. This factor of 

 time, or of something connected with the time, of action, 

 appears of great interest. 



In unicellular organisms such results have been produced, 

 not by one or two external agents only, but by many different 

 types of environmental conditions; by conditions that operate 

 on the organisms in their normal life. Many different types 

 of inherited alterations have thus been produced. Some of 

 them are degenerative ; they leave the organism the worse for 

 their action. Others are distinctly to the advantage of the 

 organism. They are what can be called adaptive changes in 

 the hereditary constitution. Changes produced by a particular 

 environmental agent may have a definite specific relation to 

 the agent that produces them, fitting the organism to survive 

 and thrive under the action of that agent. We shall examine 

 here the main different types. 



The first type of inherited environmental effect brought to 

 light consisted of inherited injuries to the constitution of the 

 organism, weakening it, inducing a degenerate condition. It 

 has often been suggested that in man, life for many genera- 

 tions under evil conditions may produce inherited degenera- 

 tion. In unicellular organisms this is indeed true. It was dis- 

 covered through the classical work on life histories, depres- 

 sion, degeneration and rejuvenescence in Protozoa, begun 

 fifty to sixty years ago by Balbiani and Maupas, and continued 

 by many investigators from that time to this. 



Everyone who has had experience in this kind of work 

 knows that if one cultivates an infusorian for generation after 

 generation in isolation, on glass slides, or under other condi- 

 tions differing much from nature, as a rule the organisms 

 begin after a long time to "run down." They divide less fre- 

 quendy, they do not feed well; they lose resistance to bad 

 conditions; they have digestive and assimilative troubles; 



