INTRODUCTION 5 



adjustment which takes place between an organism 

 and its environment. For instance, the cholera 

 bacillus grown on albumin produces toxines or 

 alkaloids, and is pathogenic; on potatoes, it gives 

 rise to a brown pigment and is chromogeuic ; while 

 on sugar it produces butyric acid, and is con- 

 sequently zymogenic or fermentive (Hueppe). The 

 action of gases, heat, light, electricity, and various 

 antiseptics have the power of altering the common 

 properties of a microbe ; but in every case the usual 

 products, etc., are formed when the microbe is once 

 more transferred to its natural mode of life. ' Every 

 organism has more potentialities or modes of action 

 than those which are actually in operation at any 

 given time, and when the environment is changed 

 one or other of these potentialities may come into 

 action, replacing, more or less completely, a former 

 one.' The extent of the powers of adaptation of 

 an organism depend on its potentialities and their 

 capacity of extension, and these vary, in each case, 

 enormously, a view in perfect consonance with the 

 results which experiments have already yielded. 



As microbes differ in their actions, they likewise 

 differ in their dimensions ; and, as a general rule, 

 they vary from about 0*0005 mm. to 0'05 mm. in 

 length or diameter, as the case may be. Dr. F. 

 Cohn calculated that one bacterium (Bacterium 

 termo) weighs 0*000,000,001,571 milligramme, or 

 that six hundred and thirty-six millards of bacteria 

 would weigh one gramme, or six hundred and 

 thirty-six thousand milliards a kilogramme; and 

 the late Professor J. Clerk Maxwell stated that the 



