io6 GENETIC VARIATIONS 



damaged materials of the depressed individuals multiply 

 themselves in the damaged condition. In merely ten genera- 

 tions the damaged material has produced more than a thou- 

 sand times its original amount; in a hundred generations it 

 has multiplied itself by billions. 



Thus in these unicellular organisms, the condition of the 

 individuals of the present generation as to health and vitality 

 depends on the conditions in v^hich their ancestors have 

 lived; it may depend on the ancestral environment a hundred 

 generations ago. If their ancestors a hundred generations back 

 have been living for many generations under bad conditions, 

 the present individuals are weak and depressed. If their 

 ancestors of a hundred generations ago have been living for 

 many generations under good conditions the present genera- 

 tion is strong and vigorous. We have then in these organisms 

 the "inheritance of acquired characters"; the inheritance of 

 environmental effects, at least insofar as damage to the genetic 

 constitution through bad living conditions is concerned. 



But such inheritance is not limited to injurious effects of 

 the environment. It occurs too when the action of the environ- 

 ment is such as to change the organisms for the better. The 

 environment may produce in unicellular organisms changes 

 that are adaptations to the conditions that produce the 

 changes. When subjected for long periods — for hundreds or 

 thousands of generations — to gradually increasing tempera- 

 tures, infusoria may slowly acquire an increased resistance to 

 high temperatures. The increase may be very great; Dallinger 

 and Drysdale in seven years increased the temperature which 

 certain species of infusoria could resist, from about 25 degrees 

 centigrade up to 70 degrees; or expressed on the Fahrenheit 

 scale, from about 80 degrees to 150 degrees. Before the experi- 

 ments, none of the organisms could endure these high tem- 

 peratures; at the end all live and thrive in such conditions. 



