152 SUMMARY [ch. x 



members of the paratyphoid group of bacilli, are open to the 

 same criticism. In the first place, a temporary variation in one 

 character alone — namely in agglutination properties — would 

 sufficiently explain the results obtained. In the second place, 

 these results may have been due to a secondary invasion — in 

 other words, it is conceivable that there may have been a pre- 

 existing but unrecognised infection in the animals utilised for 

 the experiments. This hypothesis we have shown, from the 

 records of other investigators and by analogy with other pro- 

 cesses of infection, to be not improbable ; while the writer's 

 experiments further demonstrate the ease with which such a 

 secondary invasion may be overlooked. 



In none of these experiments, therefore, can the occurrence 

 of transmutation be regarded as proved, nor, on close ex- 

 amination, does its occurrence appear probable. 



A theory which we propose to discuss in conclusion suggests 

 a via media by means of which organisms might conceivably 

 exchange many of their characters and functions without them- 

 selves undergoing transmutation. This is the Enzyme theory 

 of disease. 



