PROTOZOAN differences: resistance to heat , 139 



extreme cases out of consideration, it appears that of the com- 

 mon Protozoa Uving under the same conditions of temperature 

 and in the same or approximately the same medium, each species 

 has a resistance peculiarly its own. The thermal resistance is 

 quite constant for the species under given conditions. Of the 

 forms studied, Spirostomum had the highest resistance (42° to 

 43°) while Synura had the least ( < 28°). The constancy of the 

 thermal resistance makes possible a definite expression in the form 

 of curves. This in contrast with their resistance to carbon dioxide, 

 where it was found "a certain amount of individual and cultural 

 variation may occur which prevents the expression of the resist- 

 ance of the species in absolute terms" (Jacobs '12). 



As to the effects of individual and cultural variation upon the 

 death-temperature curve some experimental evidence can be 

 given here. The curve for Paramecium caudatum shown in fig- 

 ure 1 indicates a comparatively large amount of variation. Since 

 different races of Paramecia have been shown to differ in certain 

 physiological respects, such as their tendency to conjugate, and 

 so forth (Jennings '10), it is conceivable that they may be dif- 

 ferent in their ability to withstand heat. If so, the spread of 

 the curve in figure 1 may be explained on the ground that sev- 

 eral strains or races were present in the wild cultures examined. 

 To test this point some pure line cultures were started. A large 

 quantity of hay infusion made up at one time, was divided equally 

 among six jars, thus making the medium the same in all. Each 

 jar was seeded with a single individual transferred from an ordi- 

 nary culture by means of a capillary pipette. Three of these 

 culture were followed with some care for a period of two months 

 at which time some unfavorable condition caused the animals to 

 disappear. The results are summed up in tables 7 to 9. Curves 

 based upon these results are given in figure 2. 



It appears that, by mere chance, a strain (Race 5) of com- 

 paratively low resistance had been found. It will be observed 

 that the mode of the curve for Race 5 is three degrees below 

 that for Race 1 or Race 6. It must be stated here that all these 

 cultures developed rapidly and were in good healthy condition 

 for about a month. Then some unfavorable conditions, prob- 



