256 MICRO-ORGANISMS AND DISEASE. [CH. xx. 



of resisting the immigration of a new generation of the same 

 organism. There is absolutely nothing that I know of in 

 favour of such a theory ; it is impossible to imagine that the 

 cells of the connective tissues, of the blood and of other 

 organs, owing to a past attack of scarlatina, become possessed 

 of new functions or of some new power, as, for instance, a 

 greater power of oxidising or the like. Connective tissue- 

 cells, blood-corpuscles, liver-cells, and other tissues are, so 

 far as we know, possessed of precisely the same characters 

 and functions after an attack of scarlatina as before. 



On the whole then, we may, it seems, take it as probable, 

 that owing to the presence in the normal blood and tissues 

 in a living animal of a chemical substance inimical to the 

 growth of a particular micro-organism, this animal is un- 

 susceptible to the disease dependent on the growth and 

 multiplication of this micro-organism ; and further, that in 

 those infectious maladies in which one attack gives im- 

 munity against a second attack of the same kind, one attack 

 produces a chemical substance in the blood and tissues 

 which acts inimically to a new immigration of the same 

 organism ; hence the animal becomes unsusceptible to a 

 new attack, or is " protected." This is not the case with all 

 infectious maladies, for, as is well known, in a good many 

 instances a single attack does not protect against a second ; 

 and, as is also well known, a first attack may protect but 

 only for a limited period, or for a period greatly differing in 

 different individuals. All this would be explained by our 

 theory in the same way as it is explained by the other 

 theories; viz., when one attack does not protect, no inhibi- 

 tory chemical substance has been produced ; while in those 

 diseases in which one attack does protect only for a limited 

 period, the necessary inhibitory substance has only lasted 

 for a limited period, and so on. 



