112 SITVIBIONTICISM AND THE ORIGIN OF SPECIES 



specific explanation that would fully answer the question. 

 Nevertheless, we can appreciate the complexity of the 

 situation when we review the behavior of known micro- 

 organisms and the factors regulating their activities. The 

 reactions of pathogenic microorganisms, perhaps, have 

 been investigated more fully than those of any other group ; 

 their behavior is more significant, also, because of the rela- 

 tionship they bear to other forms of Ufe. A brief discussion 

 will suffice to indicate some of the "pecuHarities" of micro- 

 bial behavior, as well as the importance of variations in the 

 host with respect to infection. 



A large number of pathogenic microorganisms have a 

 limited specificity as regards the plant or animal species 

 in which they are capable of producing disease. The avian 

 tubercle bacillus, for example, is capable of producing tuber- 

 culosis in birds, cattle and swine, but apparently not in 

 man. The bovine strain of the tubercle bacillus produces 

 disease in cattle, man and certain laboratory animals, but 

 not in the horse. Some microorganisms have a marked 

 tendency to localize in certain tissues, for example the 

 globoid bodies of anterior poliomyehtis. The diphtheria 

 bacillus is hmited in its infections to the mucous mem- 

 branes, especially those of the respiratory tract, and to 

 wounds exposed to the air. 



The host also plays an important role in the determina- 

 tion of infection. The tubercle bacillus does not ordinarily 

 produce definite disease in a normal, healthy individual. 

 When vitality is lowered, the organism may become im- 

 planted and develop. The response of the host to an in- 

 fection, or to the presence of a pathogenic organism varies 

 within wide Unfits. In a particular type of infection, one 

 individual may respond with a violent reaction while 

 another host may manifest very Httle or no response. An 

 example of marked tolerance is found in those cases spoken 

 of as ''carriers." Typhoid carriers are rather common 

 and may not be conscious of the infection they harbor. 



