THE PURE LINE AND SELECTION 141 



treme individuals selected from any pure line do not re- 

 produce extreme sizes like themselves, but instead, a 

 progeny varying according to the laws of chance 

 around the average standard of the particular line 

 from which it came. Thus quite independently of 

 Johannsen, Jennings arrived at the same general con- 

 clusion, namely, that selection within a pure line is with- 

 out effect. 



But with Difflugia, another protozoan that secretes 

 for itself a jug-like shell, Jennings, after a characteris- 

 tically careful and prolonged study, has a different 

 story to tell. Difflugia proved to be a more favorable 

 form to study than Paramecium because it has numer- 

 ous distinctive shell characters which are all inheritable 

 to a high degree but are unchanged by growth and en- 

 vironment during the life of the individual, although 

 presenting variations from parent to offspring. 



Jennings selected for (1) the number of spines on 

 the shell ; (2) the length of the spines ; (3) the diameter 

 of the shell; (4) the depth of the shell; (5) the number 

 of teeth surrounding the mouth; (6) the diameter of 

 the mouth. In two families, "one (#303) including 

 495 descendants of a single individual, and the other 

 (#314) including 1049 descendants of the original 

 parent, selection was effectire." 



C. PARTHENOGENETIC PROGENY 



Parthenogenetic animals furnish even better material 

 than unisexual clonal animals for testing the effective- 

 ness of selection in an unmixed line but here again the 



