382 CLASSICS OF MODERN SCIENCE 

 in virulence till it becomes deadly though a drop of the infective liquid 

 be diluted in a quadrillion times. They have founded thereon the 

 most beautiful practical applications, not suspecting that the bacteria in 

 question have never been certainly demonstrated. 



The original works of Coze and Feltz, as also that of Davaine, are 

 not at my disposal for reference; and I cannot therefore enter into a 

 complete criticism of them. So far, however, as I can gather from 

 the references accessible to me, especially from the detailed notices in 

 Virchow and Hirch's "Jahnesbericht," no complete proof that the viru- 

 elence of septicsemic blood increases from generation to generation 

 seems to have been furnished. Apparently blood more and more 

 diluted was injected, and astonishment was felt when this always 

 acted, the effect being then ascribed to its increasing virulence. But 

 controlling experiments to ascertain whether the septicaemic blood 

 were not already as virulent in the second and third generations as in 

 the twenty-fifth, do not seem to have been made. My experiments so 

 far support and are in accordance with those of Coze, Feltz, and 

 Davaine in that for the first infection of an animal relatively large 

 quantities of putrid fluid are necessary ; but in the second generation, 

 or at the latest in the third, the full virulence was attained, and after- 

 wards remained constant. 



Of my artificial infective diseases the septicaemia of the mouse has 

 the greatest correspondence with the artificial septicaemia described by 

 Davaine. If we were to experiment with this disease in the same man- 

 ner as Davaine experimented, we would, if no controlling experiments 

 were employed, find the same increase in virulence of the disease. It 

 would only be necessary to use blood in slowly decreasing quantities in 

 order to obtain in this way any progressive increase of the virulence 

 that might be desired. I, however, took from the second or third ani- 

 mal the smallest possible quantity of material for inoculation, and thus 

 arrived more quickly at the greatest degree of virulence. Till, there- 

 fore, I am assured that, in the septicaemia observed by Davaine, such 

 controlling experiments were made, I can only look on an increase in 

 virulence as holding good for the earlier generations. In order to ex- 

 plain this we do not, however, require to have recourse to the magical 

 wand of natural selection ; a feasible explanation can be very naturally 

 furnished. Let us take again the septicaemia of mice, as being the 

 most suitable example. 



