454 AUSTIN RALPH MIDDLETON 



and fast lines can, by selection, be isolated among the descend- 

 ants of a single parent. I have carried out this work for Sty- 

 lonychia pustulata. The animals were kept isolated and trans- 

 ferred daily to fresh culture medium; for the fast lines the indi- 

 viduals that divide first are uniformly selected; for the set to 

 be developed into lines having a slow rate of fission the indi- 

 viduals that divide last are taken. 



No attempts have ever been made heretofore to test the effect 

 on the fission rate of selection among the progeny of a single 

 individual. It appeared possible, though hardly probable, that 

 such a physiological character might give results differing from 

 those obtained from studies of the mainly structural characters 

 hitherto examined. The work was undertaken at the sugges- 

 tion of Prof. H. S. Jennings, to whom my sincere thanks are 

 due for assistance throughout the work. 



The fundamental questions for examination are then as fol- 

 lows: Can we, with respect to the character examined, get from 

 a single genotype by selection two genotypes that differ charac- 

 teristically from each other under identical conditions; and that 

 retain these differences from generation to generation? Is selec- 

 tion of small variations, such as appear within the pure strain 

 or clone an effective evolutionary procedure? 



TECHNIQUE 



In any investigation of a physiological character which is so 

 delicately respi^nsive to all environmental changes as is the 

 fission rate of infusoria, the statement of Calkins ('02, p. 141) — 

 "A correct method is the sine qua non of successful experiments 

 with Protozoa;" — applies with peculiar force. In order that re- 

 sults may be of any value conditions must be uniform throughout. 



Jennings ('13) has pointed out that to secure this uniformity 

 the bacterial content must not vary. In addition to uniformity 

 the technique used in work on the fission rate must insure the 

 experimenter against the introduction of a 'wild' individual into 

 the cultures and against the contamination of the lines inter se. 

 These results have been secured by the adoption of the follow- 



