378 EXPERIMENTAL STUDY OF INHERITANCE 



isolate the best pure lines, it cannot get beyond the extremes 

 which the included pure lines illustrate. 



One of Johannsen's main results was to show that a pure line 

 is very constant from generation to generation. There are 

 many individual differences, but these do not tend to be repro- 

 duced in the offspring. They appear to be modifications due to 

 peculiarities in the " nurture " of individuals. There do not 

 appear to be the numerous germinal variations which are so 

 often postulated. If a big hereditary change occurs, it comes 



JOS 



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FIG. 4oa. Pure Lines in Par^mcecium. (From Jennings.) 



