502 AUSTIN RALPH MIDDLETON 



difference between the two sets, showing that the effect of selec- 

 tion was cumulative. 



This result was, at various intervals, subjected to the most 

 rigid tests possible, by balanced selection throughout long periods ; 

 by mass culture without selection, and by reversed selection. 

 In every case the results were corroborated. The hereditary 

 differences induced continued through periods of balanced selec- 

 tion lasting longer than the periods of direct selection by which 

 they were induced ; they did not disappear save under the effects 

 of reversed selection. 



These results were first reached with the progeny of a single 

 individual multiplying asexually. They were then confirmed by 

 beginning anew with a single individual from among this set 

 and obtaining the same results among its progeny. A third set, 

 derived from a wild individual quite unrelated to the first two 

 series, gave the same results. In this third series conjugation 

 occurred within each of the diverse sets produced through selec- 

 tion, and it was found that the hereditary differences persisted 

 through and after conjugation. 



Thus in Stylonychia, from a single clone of given genotype it 

 is possible to obtain through long continued selection during 

 reproduction by fission, two sets (clones?) of diverse genotype, 

 differing characteristically from each other in rate of fission, 

 under identical conditions; and retaining these differences from 

 generation to generation. The selection of small variations, 

 such as appear within the 'pure strain' or clone, is then an effec- 

 tive evolutionary procedure. 



LITERATURE CITED 



Agar, W. E. 1913 The transmission of environmental effects from parent to 



offspring in Svnocephalus vetulus. Phil. Trans., London, Series B, 



V. 203, pp. 319-350. 



1914 Experiments on Inheritance in Parthenogenesis. Phil. Trans., 



London, Series B, v. 205, pp. 421-489. 

 Barber, M. A. 1907 On heredity in certain micro-organisms. Kansas I^ni- 



versity Science Bull., v. 4, no. 1, 48 pp. 

 Calkins, G.N. 1902 Studies on the life-history of Protozoa. 1. The life-cycle 



of Paramecium caudatum. Arch. f. Entwicklungsmechanik der Org., 



V. 15, pp. 1.39-186. 



