XXI] PATHOGENIC ORGANISMS 557 



the large mass of observations that have been made on the 

 relation of the degrees of natural immunity of different 

 tissues and different animals in their normal and abnormal 

 states against saprophytic and specific microbes in different 

 conditions (of virulence) and introduced in different 

 amounts. 



Now, what is the cause of this spontaneous immunity ? At 

 the outset it is necessary to keep this question separate from 

 that of acquired immunity, that is immunity which is pro- 

 duced by one or more previous mild or transitory attacks ; it 

 has been known as long as infectious diseases have been 

 recognised as such that in some at any rate one attack, mild 

 or severe, protects against a second severe attack, and the 

 inoculations against small-pox (brought from the East), 

 Jenner's vaccination against small-pox, " vaccination " against 

 anthrax, fowl cholera, swine erysipelas (Pasteur), are based 

 on that experience. The important fundamental observa- 

 tions that have been made in this field, subsequently to 

 Pasteur's work, by Salmon, Roux and Yersin, Klemperer, 

 Behring and Kitasato, Behring, R. Pfeiffer, and many 

 others show that a specific immunity or resistance of 

 different degrees can be produced against a specific microbe 

 or its toxin according to the strength (virulence) and amount 

 of the living microbe or its toxin previously introduced. 

 Moreover this same principle of "active immunisation" by 

 previous toxin injections holds good also for other than 

 microbic toxins : Ehrlich produced this active immunity 

 against an otherwise fatal dose of Ricin and Abrin respec- 

 tively by previous repeated administration of subfatal and 

 gradually increasing doses of these toxins, Calmette and 

 Eraser the same against snake venom. The point that at 

 present concerns us is the meaning and cause of spon- 

 taneous or natural resistance or immunity of one or another 

 tissue or the animal body against one or another kind of 



