SOME SPECIFIC DIFFERENCES AMONG PROTOZOA 



WITH RESPECT TO THEIR RESISTANCE 



TO HEAT 



ROBERT H. HUTCHISON 



The Zoological Laboratory, University of Pennsylvania 



TWO FIGURES 



It is well known that different species of bacteria show marked 

 differences in their ability to withstand high temperatures, such 

 differences being of considerable practical importance in bac- 

 teriological work. A temperature of 30°C. is fatal for some 

 forms, while others, including most pathogenic forms, are not 

 killed until 50°C. is reached, and finally there are the so-called 

 thermophilic bacteria, forms whose death temperature is 70° or 

 over. Similar differences are known to exist among the Proto- 

 zoa, many pathogenic forms living normally at temperatures 

 which are fatal to their free living relatives. In all such cases 

 the question of adaptation must play an important part. The 

 powers of adaptation of unicellular organisms to new environ- 

 ments are high, as is shown by the experiments of Dallinger ('87) 

 on acclimatization of Protozoa to high temperatures. The ques- 

 tion arises whether the thermal resistance of any given organism 

 is merely a response to the temperature of its environment, or 

 whether it is something more fundamental, depending upon the 

 nature of the animal itself. If the thermal resistance is inti- 

 mately connected with the original protoplasmic constitution of 

 the animal, one would expect to find considerable and constant 

 differences even among forms living under approximately the 

 same conditions. Is such the case? If so, how much variation 

 is shown within the species? Are the descendants of a single 

 individual less variable than a population selected at random? 



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