PARTHENOGENESIS AND PURE LINES 207 



tubers, cuttings, etc. East has studied tlie effect of selec- 

 tion of tubers of certain races of the common potato. A 

 race was first grown from a single tuber. By boring holes 

 into the tubers enough material could be obtained for a 

 chemical test of the amount of nitrogen in them. The 

 rest of each tuber could, if desired, be cut into pieces of 

 standard size and planted. Ten tubers, high in nitrogen, 

 and ten, low in nitrogen, were selected. The tubers of the 

 next generation showed that there was no relation found 

 between the amount of nitrogen in the original tuber and 

 in those that came from it. A repetition of the experi- 



Fis. 95. — A wingless aphid to the left and a winged to the right, both belonging to 

 the same species. (After Webster and Phillips.) 



ment in another generation gave only meagre results 

 owing to drought. As far as the facts went, this genera- 

 tion, too, showed no effect of selection. 



Most of the protozoa propagate by dividing into equal 

 or nearly equal parts — i.e., by a process of cell-division. 

 Jennings has studied the effect of selection in a culture 

 of Paramecium, all members of which had descended from 

 a single individual. No change was induced. Later, how- 

 ever, working on another protozoon, Difflugia corona, 

 Jennings found that selection brought about changes in 

 the direction of selection. In this case, the method of 

 division may possibly include irregular distribution of the 

 chromatin material, and the recent work of Hegner indi- 

 cates that such an interpretation is not improbable. Pos- 



