LECTURE IV. 



IN this discussion I have referred to various bacteria as 

 distinguished into species by essential differences of form 

 and function. In these latter days it has become fash- 

 ionable to speak of these minute organisms as transient 

 modifications, due to incidents of their environment, of 

 one and the same organism. Nageli, indeed, would in- 

 clude not only bacteria, but also some of the higher 

 fungi in this hypothesis. As this seems as yet a specu- 

 lation, based not so much upon direct demonstration, as 

 upon deductions, it will not require discussion here. 



An essential element of this theory, however, the so- 

 called accommodative cultivation of bacteria, seems to 

 be supported by certain experimental evidence. This 

 assumes that the physiological characteristics may be 

 modified by contact with unusual influences by a change 

 of environment, in other words as to render the de- 

 scendants of a given bacterium which is capable of 

 successful contest with the living animal tissues impo- 

 tent to maintain such combat ; and conversely to confer 

 upon a previously harmless bacterial species the power 

 to invade and destroy a living animal. This hypothesis 

 is so fascinating, the solution of many difficult problems 

 is rendered thereby so simple, the reconciliation of con- 

 flicting observations and opinions becomes so easy, that 

 every man becomes at once his own bacteriologist. 

 Diphtheria, on this hypothesis, is not due to a specific 

 bacterium, but to some of those usually guileless organ- 

 isms which ordinarily inhabit the healthy throat, incapa- 

 ble of harm ; but which, excited into unusual and per- 

 verse activity by unknown influences of atmosphere, etc., 

 invade the body with disastrous results. The application 

 of this assumed principle is evidently limited only by the 

 fancy and ingenuity of the individual ; we have been al- 

 ready amply entertained by theories ascribing typhoid 



