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THE AMERICAN NATURALIST [Vol. XLII 



raised to the boiling point. This method was never 

 varied during the entire period of the cultures and the 

 salt content of the water, as shown by weekly analyses, 

 did not vary beyond a very slight fraction of one part 

 to one hundred thousand. ' ' 



The only condition present in the life of this culture 

 which could not be encountered by a wild Paramecium 

 was the boiled condition of the culture medium, but this 

 was essential in the experiments in order to prevent the 

 introduction of an encysted or active wild individual into 

 the culture. 



The objection might be made that the environment was 

 changed too frequently and too greatly to afford natural 

 conditions. But, if the hundreds of millions of wild in- 

 dividuals which are derived from a single wild Para- 

 mecium are considered, and if the environment of any 

 four lines of descent of four individuals existing at the 

 end of twelve months in the wild state is taken into ac- 

 count, it will be readily appreciated that the surviving 

 organisms have probably experienced far more changes 

 of medium than have the four lines of the culture under 

 consideration. It may be that only those individuals 

 which have encountered varied conditions have survived 

 without resorting to conjugation. 



It is obvious, indeed, that all the variations in environ- 

 ment which may be experienced by a wild Paramecium 

 have not been supplied; for in a pond considerable 

 changes of temperature occur during the year, and 

 periods of rest by encystment or lack of food are afforded. 

 It is believed, however, that the conditions under which 

 this culture has been carried approach nearer to those 

 encountered by the majority of wild individuals than has 

 been the case in previous investigations. 



A glance at the plotted curve of the life history shows 

 that at no ten-day period during the life of the culture up 

 to the present time has the average rate of division of the 

 culture fallen as low as one division in two days, the 

 nearest approach to this rate being attained at period 



