620 H. S. Jennings 



of fission at times went so far as to produce mere monstrosities 

 (parts of ^, Fig. 20), Such monstrosities were rare, while indi- 

 viduals neatly united in pairs or in chains were very common. 



The first occurrence of such unions (Fig. 4, ^°) was on May 10. 

 Cultures were kept in watch glasses from that time till July i 

 (probably about fifty generations); at that date the unions were 

 still abundant. In fifty generations the original individual which 

 underwent the modification causing the union would have pro- 

 duced progeny to a number running far up into the billions. 



Effects of Artificial Selection 



On June 22 I began experiments to determine the effect of selec- 

 tion on this peculiarity. Would it be possible by selection to pro- 

 duce on the one hand a series showing little or no tendency to 

 remain united, on the other hand a series in which most or all 

 the individuals remain in united pairs } 



Two selected cultures were started in watch glasses. The first 

 contained twenty individuals united two by two in ten pairs. 

 The second contained twenty free individuals (descended from 

 the same ancestors as the united pairs). 



Forty-eight hours later (June 24), both sets had multiplied to 

 about 100 specimens. In the first set (ancestors united) there were 

 ten united pairs. In the second set (free ancestors) there were 

 two united pairs. 



From the first set I removed all the free individuals, leaving only 

 the ten united pairs. From the second set the two united pairs 

 were removed, leaving all free. 



The further history was as follows: 



Culture from free ancestors. On June 25 this had multiplied 

 to 200-400; among these were three or four united pairs. I re- 

 moved the latter and retained only 100 of the free individuals. 



On June 26 these had multiplied two to four times but contained 

 no united specimens. This culture was kept for a week or so 

 longer, but developed no more united pairs. Thus, selection had 

 quite removed from this set the tendency to remain united. 



Culture from united ancestors. After the second isolation often 

 united pairs (June 24), the number multiplied to about 50 in 24 



