8 Introduction 



present, manifests a permanent or transitory refractory state as regards 

 this specific organism. As soon however as the cause of this immunity 

 ceases to act, the micro-organism gets the upper hand and sets up the 

 specific disease. It is thus in diabetic persons that boils make their ap- 

 pearance as the result of the development of Staphylococcus pyogenes, 

 a micro-organism that is almost always found in abundance on the 

 skin and mucous membranes of the human subject. The diabetes is, 

 in these cases, the cause of the suspension of the immunity which 

 exists in the healthy individual. 



People who carry the Pneumococcus on their mucous membranes 

 may remain for long without being attacked by fibrinous pneumonia 

 or any of the other maladies due to this micro-organism. But often, 

 in consequence of some special circumstance, a cold for example, the 

 refractory state gives way to a more or less marked susceptibility. 



It is unnecessary to multiply the number of such examples; they 

 demonstrate in the clearest fashion that, in addition to the causes 

 of disease which come from the outer world and which are represented 

 by the micro-organisms, there are yet other causes which lie within 

 the organism itself. When these internal factors are powerless to 

 prevent the development of the morbific germs, a disease is set up ; 

 when, on the other hand, they resist the invasion of the micro- 

 organisms properly, the organism is in a refractory condition and 

 exhibits immunity. 



Diseases in general and infective diseases in particular were 

 developed on the earth at a very remote epoch. Far from being 

 peculiar to man, animals and the higher plants, they attack inferior 

 forms and are widely distributed among unicellular organisms, In- 

 [9] fusoria and Algae. Diseases undoubtedly play an important role in 

 the history of life on our planet, and it is very probable that they 

 have contributed in a marked degree to the extinction of certain 

 species. When we observe the ravages produced by parasitic Fungi 

 among the young fish which we are trying to rear, or the destruction 

 of crayfish in certain countries in consequence of the rapid increase 

 of epizootic germs, we are involuntarily led to the conclusion that 

 pathogenic micro-organisms must have brought about the disappear- 

 ance of certain animal and vegetable species. 



Darwin 1 , in the chapter on the extinction of species in his book 

 On the Origin of Species, states upon the authority of several 

 observers that insects so annoy elephants that these large mammals 

 1 " On the Origin of Species," 6th ed., London, 1872, Chapter xr, p. 277. 



