AN AMICRONUCLEATE OXYTRICHA. II 



139 



carried was obviously limited, it was decided to carry three at a 

 time, isolating the animals as soon as fission occurred and keeping 

 all progeny for six or seven generations. Selection was then 

 made of the most typical twins in these generations. After three 

 generations the twin which had divided to give the eight animals 

 showing most promise of breeding true was used as the starting- 

 point of the succeeding culture. Thus a continued process of 

 selection was used, and the result indicates that a strain of twins 



TABLE 1 

 A summary of a series of cultures carried in the experiment to determine the effect of 

 artificial selection in twin lines taken originally from animals of series I {culture 

 AT). The continuity of the various subcultures is shown in columns 1 and 3 of 

 the table. The percentage of animals remaining twins throiighout the series is 

 shown graphically in figure 7. 



was obtained which showed less tendency to separate, and toward 

 the end of the life of series I, one culture was obtained which 

 through the seventh generation produced 100 per cent twins. 



A study of table 1 shows further that during the progress of 

 this selection the tendency to remain united became gradually 

 more and more marked. Unfortunately, when a strain of twins 

 was thus obtained which gave every promise of continuing to 

 breed true the whole series died out. It is believed that the cul- 

 ture medium was largely responsible for the death of the series, 



