114 CLAUDE W. MITCHELL 



medium of F. This series, then in the twenty-third generation, 

 had assumed the humped type during the Oxytricha experiments 

 (generations fifteen to twenty-one inclusive), but had returned 

 to the saccate type two generations previous to the Euglena 

 experiment. The influence of the new diet was immediate; for 

 in the first generation of young the mutation had occurred. That 

 such an abrupt transition should have occurred is indeed remark- 

 able. The size of the adult offspring became double that of the 

 parent, while the number of young produced, as shown by table 9, 

 was also nearly doubled. In like manner the new food was sup- 

 plied to series G, now saccate and in its seventh generation, but 

 which had been humped in its second and third generations. In 

 this series also the transition followed in the next generation 

 after the food change. 



Influence of Euglena upon E 



In strong contrast to series F and G, series E had proved a 

 weak race with slow and sparing reproduction (fourteen genera- 

 tions of E to twenty-three of F) and had developed only saccate 

 individuals. Euglena was therefore given to this race to deter- 

 mine whether it also had the ability to develop the humped type. 

 That the Euglena were injested was proven by examination of 

 stomach contents. The transition did indeed occur, and a glance 

 at table 8 will reveal how marked it was; but it will also be ob- 

 served that there was no corresponding increase in the number 

 of offspring. Nevertheless, the experiment proves that the 

 ability to mutate is inherent even in individuals of weak races, 

 though dependent for its manifestation upon favorable diet or, 

 more probably, favorable food change. 



Influence of Euglena on the old Series D and D 2 



Changed diet having thus shown a powerful influence over the 

 preceding series, it was next applied to the old series, D and D 2, 

 which had shown at this time an unbroken succession of fifty- 

 seven and fifty-nine generations respectively of the pure saccate 

 type. Here no less than in the previous briefer series the transi- 

 tion was complete and immediate (tables 11 and 12). 



