144 ROBERT H. HUTCHISON 



3. Different strains or biotypes of Paramecium caudatiim may 

 have quite different powers of thermal resistance. Each biotype 

 shows considerably less variation than a population selected at 

 random. 



4. The thermal resistance of the forms studied was not per- 

 ceptibly affected by the age of the culture and its accompanying 

 changes. 



5. The ordinary changes in the acidity of the medium seemed 

 to have no influence on the resistance of the animals to heat. 



6. The thermal resistance of a species may be somewhat modi- 

 fied by raising the salt content of the medium. 



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