ADAPTATION OF ORGANISMS 311 



itself. Indeed, certain species of Bacteria not only do not 

 need free oxygen at all, but are killed when it is present in 

 any considerable amount. All such organisms are termed 

 ANAEROBES. A common example is Bacillus tetani which 

 inhabits garden soil and street dust and produces tetanus, 

 or lockjaw, in Man and certain domesticated animals when it 

 gains entrance to the tissues. 



TEMPERATURE. Although protoplasmic activity is re- 

 stricted to ranges of temperature which do not seriously 

 interfere with the chemico-physical processes involved, it is 

 a commonplace that various species are adapted to different 

 degrees of temperature. The great majority of organisms, 

 however, find their optimum temperature between 20 C. 

 and 40 C., though species inhabiting the polar and tropical 

 regions show adaptations to the temperature extremes of 

 their surroundings. As a matter of fact, it is not possible to 

 state the upper and lower limits beyond which active life is 

 suspended, but some Algae and Protozoa are known to 

 multiply in the water of hot springs, certainly at temperatures 

 higher than 50 C., and others in water until freezing actually 

 occurs. 



But many of the lower forms of life, such aos the Bacteria 

 and Protozoa, have the power of developing, particularly 

 under unfavorable surroundings, protective coverings of 

 various sorts about themselves and of assuming a resting 

 condition in which all the metabolic processes characteristic 

 of active life are reduced to the lowest ebb. (Fig. 160.) 

 In this spore or encysted state they are immune to extremes 

 of temperature and of desiccation to which they readily 

 succumb during vegetative life. Thus some types of Bacteria 

 can successfully withstand a temperature of nearly 200 C. 

 for six months, and about 250 C. for shorter periods, which 

 is a temperature approaching closely that at which no 



