Races of Paramecium. 537 



one to fifty-eight fissions. The race C2, which is of the same size 

 as k, multiplies throughout the experiment less rapidly than k, 

 with six to nine fissions per week, and with a total of thirty-four 

 to forty-four in the entire thirty-seven days. The two smaller 

 races c and i have a much more rapid rate, dividing eleven to fifteen 

 times per week, with totals in the thirty-seven days of from sixty- 

 three to seventy-three fissions. There is no clear difference in 

 rate of fission between c (larger) and i (smaller). 



In the caudatum group there appear to be at most but two sets 

 so far as rate of fission is concerned. L^ and 20 show essentially the 

 same rate as k in the am*elia group, with nine to twelve fissions per 

 week, and forty- three to fifty-nine in the period of thirty-seven 

 days. The race D appears at first view to have a very different rate 

 of fission, the totals for thirty-seven days being but thirty-five and 

 thirty-six and the rate per week being sometimes as low as four 

 to six. The details of the experiment seem to indicate however 

 that the difference between D and the other races is rather a matter 

 of lack of adaptation of these specimens of D to continued exist- 

 ence in the culture medium used, than to a natural difference in 

 the rate of fission. In the first week, three of the hues of D gave the 

 same rate as did the other races of caudatum (ten to eleven per 

 week), and with two of the lines this continued another week. 

 But now all the lines of D evidently sickened ; monstrosities were 

 produced, the fission became irregular or stopped for long periods, 

 and many of the individuals died. Finally the fines from one of 

 the two original individuals died out completely, while the other 

 was barely saved, the individuals being of swollen irregular form, 

 with many abnormalities and deaths. Further, four other 

 series from D were later started (April 30) and these all gave 

 during the first two weeks rates of fission of ten to twelve or 

 thirteen per week. Thus the normal rate of fission is apparently 

 the same for D as for the other races of caudatum. The experi- 

 ment then shows no racial diversity in rate of fission within the 

 caudatum group, though it shows three diverse sets within the 

 aurelia group. 



