ESSAYS ON BACTERIOLOGY. 157 



Let us see, now, leaving out of consideration ques- 

 tions of susceptibility affecting the body, how far bac- 

 teriology has thrown light upon these problems. 



We expose, in an ordinary room, a series of culture 

 plates; after a time they are placed in an incubator, 

 and some are found to be infected with the microbes 

 of erysipelas, suppuration, tubercxilosis, or diphtheria, 

 while others remain sterile or show only ordinary non- 

 pathogenic bacteria. The microbes have, in the terse 

 phrase of Welch, hit some and missed others. Imag- 

 ine now these plates to be living persons, and we at 

 once see one reason why, ajuong a number seemingly 

 similarly exposed, some are attacked while others 

 escape. 



Again examining our plates, we shall see another 

 significant fact: Some have developed but few col- 

 onies, while others show many. In a word, some have 

 been hit by many microbes, others by but few ; hence 

 upon some a more pronounced growth than others. 

 Here, too, therefore, we see, in the number of bacteria 

 which fall upon the soil, a second possible explanation 

 of our problems. More direct experiment has abun- 

 dantly shov^n that the inference just suggested is cor- 

 rect. Let a series of similar lesions be infected with 

 bacteria taken from the same culture, but in varying 

 numbers, and, other things being equal, it will be 

 found that those lesions entertaining the larger num- 

 ber of microbes are more likely to be successfully in- 

 fected, and in a ratio of probability increasing witli 



