Studies on the Life History of Protozoa. 449 



optimum rate and such conditions can by no stretch of the imagin- 

 ation be called identical with those of the ponds. Yet the "nor- 

 mal conditions" may, after all, be but a matter of definition. If 

 we leave a hay infusion to stand exposed to the air, Paramcecium 

 will ultimately appear in it, and will ultimately die out from it. 

 The appearance and disappearance cannot be called artificial, it is 

 as much normal for Paramcecium to appear in such an infusion as 

 it is normal for the bacteria upon which the animals feed to be 

 there. City life for man may be called an artificial life as opposed 

 to the "normal" original or pastoral life, but it is no less normal 

 now than the primitive life was, even if it is found that the average 

 length of life is shorter for urban than for country-dwelling people. 

 The course of human life, or the history of the race, physiologi- 

 cally speaking, is no less normal for being rapid. In the same 

 way we may argue for the race of Paramcecium and its life in the 

 culture chambers of the laboratory. The life pursues a normal 

 course, although possibly faster than in nature, and the ultimate 

 results obtained in cultures may be confidently expected to obtain 

 sooner or later in the natural habitats. Seven hundred and forty- 

 two generations represent a long time for organisms to live and 

 develop in a medium that is not normal, and the mere fact that 

 they do so live is sufliicient evidence to prove the point. It seems 

 to me, therefore, perfectly legitimate to take the phenomena of 

 vitality in Paramcecium in culture as practically identical in 

 outcome with the phenomena in natural surroundings, and as 

 indicative of what goes on in living protoplasm under "normal 

 conditions." 



Looked at from this point of view, the experiments teach (i) 

 that a given form together with the race derived from it will 

 exhibit periodic depressions in vital activity; (2) That such 

 depressions can be overcome by artificial means ( and probably 

 but not surely, in nature by opportune changes in the immediate 

 environment). Further than this, however, the experiments 

 teach, (3) that these depressions are not all of the same type, nor 

 due to the same causes. They give reason for the belief that 

 periods of depression may ensue wherein different functions give 

 out, and that when this occurs, as for example when the cortical 



