322 



GENERAL ZOOLOGY 



Tests of the pure-line idea by other workers seem to show that 

 in the protozoan, Difflugia, selection within a pure line may be 

 effective in increasing the number of spines on the shell, the 

 number of teeth on the mouth of the shell, and the dimensions 



Pure Line 



Fig. 183. — Diagram to illustrate five pure lines derived from a population of 

 beans assorted according to weight. Tubes containing beans of the same weight 

 are placed in the same vertical view. {From Walter, Genetics, copyright, The 

 Macmillan Company, after Johannsen. By permission.) 



of the shell as a whole. The differences in these results and 

 those of Johannsen may be due to a difference in the organisms 

 as to the frequency of variation. It would seem that if an 

 organism is in a period of active mutation, selection of maximal 

 individuals will extend the variation range at its maximal end. 

 The outcome of selection, in any given case, all boils down to 



