IGO RUTH L. PHILLIPS 



the time of even laboratory temperature as well as when it was 

 more variable, it would seem that they cannot be entirely due to 

 this factor. In the case of a satisfactory food like A'C, the de- 

 gree of fluctuation tends to be greater than with a food like C, 

 which is incapable of sustaining a high metabolic level in Para- 

 mecium. Aside from this fact, no definite correlation seems to 

 exist between the occurrence of these fluctuations and the type 

 of food used. They appear to be due to intrinsic factors affecting 

 the metabolism irrespective of food or to slight variations in the 

 environment not under control. 



It is of interest to note that the chance mixture M, which 

 throughout the first period appeared to be the most satisfactory 

 kind of food used, showed the least degree of fluctuation of any 

 of the cultures under observation during this period. That this 

 mixture was comparable at this time to any laboratory infusion 

 in which Paramecia thrive, there can be no reasonable doubt. 

 That it did not continue to furnish conditions favorable to the 

 continued life of Paramecium has been demonstrated. The 

 question arises as to whether the high rate of metabolism of the 

 animals in M during this period was due to a preponderance of 

 favorable types of bacteria which prevented the increase of un- 

 favorable types. Since it is well known that bacteria vary in 

 their resistance to drying, is it not possible that some of the more 

 resistant forms are unsuitable food for Paramecium? 



The foregoing observations may be summarized as follows: 

 The data dealing with the behavior of Paramecium when fed in 

 a standard 0.1 per cent timothy hay infusion upon a diet consist- 

 ing of either a pure culture of C, or its mixtures A'C and A'B'C, 

 and of the chance mixture M, demonstrate that it is possible for 

 this animal to live upon a single article of diet, although the 

 metabolism under such conditions is not so high as when a mixed 

 diet is used. The chance mixture M was very much more satis- 

 factory during the first period than either of the artificial mixtiu'es, 

 but such a chance mixture is not easily kept constant, and under 

 the conditions of these experiments failed to maintain a continued 

 normal rate of metabolism in the animals fed upon it. Of the 

 artificial mixtures, A'C proved to be the most satisfactory and 



