Studies on the Life History of Protozoa. 459 



had a lower division rate and died out before May 5; that is, 

 after running five months {cf. Diagrams I and Iiy It follows, 

 therefore, that something luas operative in the ex-conjugant that 

 was absent in the stimulated form, and this something could 

 be nothing else than the reorganization which follows conjuga- 

 tion. The accompanying curves show that the periods of 

 depression and death which menaced the regular series in 

 December, 1901, and again in June, 1902, were not paralleled 

 in the cultures of the descendants of the ex-conjugant, and 

 the conclusion is obvious that conjugation provided some stimu- 

 lus which enabled this line of Paramcecium to live through 

 periods in which the allied races were saved only by vigorous 

 treatment and stimulation. There is no doubt at all that, had 

 I tried to revive the race of the ex-conjugant by beef extract 

 at: the end of August, 1902, I could have done so, for there 

 was nothing serious in the nature of the depression at this time, 

 when I allowed them to die without making an effort to save 

 the race. It is now a matter of deep regret to me that I did 

 not try to save them, and see if they would live beyond the time 

 when the allied lines died out in December, 1902. Had they done 

 so, it would have been still more convincing proof that conjuga- 

 tion does actually rejuvenate and overcome the conditions of 

 so-called "old age." I believe that the evidence which I have 

 outlined above is quite sufficient, however, to establish this point, 

 the one questionable factor being the beef extract, and even this, 

 as I have shown, could have only a limited bearing and does not 

 at all outweigh the positive evidence in favor of the conclusion. 



Columbia University, 

 April, 1904. 



