202 



THE AMERICAN MONTHLY 



[November, 



brane, the other had none. This same 

 result was obtained in inoculating liq- 

 uids from gelatin cultures. This mi- 

 crobe also grows more vigorously in 

 nutrient liquids, forming, after one or 

 two weeks, an abundant deposit. Cul- 

 tures of the bacterium of swine-plague 

 contain but a very slight deposit at the 

 end of the same period. 



Another diftbrence was observed in 

 gelatin tube cultures. The bacterium 

 under consideration failed to grow at 

 first, both in tubes and on plates, 

 while the bacterium of swine-plague, 

 sown on the same plate in lines by its 

 side, was visible to the naked eye in 

 two days. When another prepara- 

 tion of gelatin was used, which had 

 become faintly turbid on boiling, both 

 bacteria grew equally well, and the 

 bacterium of swine-plague much bet- 

 ter than formerly. The favorable 

 change was simply due to a greater 

 alkalinity of the culture medium, the 

 more sensitive of the two bacteria be- 

 ing the one under consideration. This 

 microbe also failed to induce the dis- 

 ease in two guinea-pigs, which are 

 very susceptible to the bacterium of 

 swine-plague. 



In summing up we find that this 

 microbe differs from the bacterium of 

 swine-plague in forming a membrane 

 on liquids, in failing to grow in neu- 

 tral gelatin, and in not being fatal to 

 guinea-pigs. Several pigs inoculated 

 with this new microbe failed to con- 

 tract the disease. We may safely as- 

 sume, however, that it was the cause 

 of swine-plague in Nebraska, since it 

 is quite difficult to produce swine- 

 plague by subcutaneous inoculation. 



These two microbes may, at least 

 for the pi-esent, be regarded as varie- 

 ties of the same bacterium. We may 

 also assume that the apparenth' un- 

 important biological differences, a 

 greater demand for oxygen, and a 

 more alkaline medium, may have a 

 very important, though still unknown, 

 bearing upon the disease in the svis- 

 eeptible animals. 



It is unnecessary to reopen here any 

 discussion as to the propriety of sep- 



arating bacteria into well-defined spe- 

 cies. The view of Nageli, that it is 

 difficult or quite impossible to make 

 any such distinctions, has been com- 

 pletely set aside by the facts which 

 new methods have established. The 

 possibility of making pure cultures of 

 bacteria, and of determining thereby 

 that certain definitely reappearing 

 forms are constantly allied to certain 

 well-marked, easily distinguished bi- 

 ological and pathogenic properties, 

 makes a distinction into species not 

 only properbut necessarv for the time 

 being. The facts presented point to 

 a variation of bacteria, so well estab- 

 lished among higher forms of life, 

 but not yet noted among bacteria. 

 Whether the variation may be ascribed 

 to both forms or only to one ; whether 

 one is better adapted to a parasitic ex- 

 istence than the other ; whether the 

 differences are brought about by causes 

 external to the susceptible animal — in 

 other words, by a saprophytic life of 

 which the bacterium is capable to a 

 certain extent ; these questions can 

 only be presented and not answered 

 in the present state of our knowledge. 

 It is not necessary, in order to pro- 

 duce the same pathological effect, that 

 two bacteria should be of the same 

 species from a phylogenetic stand- 

 point, i. e., that they should have de- 

 scended from the same ancestral form. 

 Morphological differences must be 

 subordinated to physiological ones. 

 If a microbe has acquired the power 

 of living in a limited supply of oxy- 

 gen and of producing certain sub- 

 stances which act as poisons to the 

 living cell, it matters very little as to 

 its form. Hence it is quite conceiv- 

 able that two microbes which are 

 morphologicallv different may have 

 acquired, in the course of long pe- 

 riods of time, the same physiological 

 or pathogenic powers. I say con- 

 ceivable, for no two have yet been 

 found which produce precisely the 

 same disease, if we except the va- 

 rious pus-producing organisms. The 

 two forms before us are, however, 

 morphologically identical. They can- 



