244 BACTERIOLOGY. 



being naturally protected, while it is reported 

 that the small-pox in Central America de- 

 stroyed one-half of the native inhabitants. 



That the different organs of the body pos- 

 sess different degrees of predisposition to 

 disease may be inferred from the fact that 

 diseases which are now known to be infectious 

 were once regarded as organic, as, for instance, 

 inflammation of the lungs and catarrh of the 

 intestine. The striking fact that different 

 organs are attacked by tuberculosis, and that 

 the disease selects different organs at dif- 

 ferent periods of life beautifully illustrates 

 this point. The experience of physicians, and 

 the vast material placed at our disposal in 

 statistics of disease and death have, in short, 

 shown the existence of a predisposition to dis- 

 ease and have made it clear that susceptibility 

 to different diseases varies greatly according 

 to the period of life and according to sex. 



Plant parasites furnish some fine examples 

 of predisposition. Laboulbenia muscce is found 

 only in the house-fly. Cordyceps, on the other 

 hand, occurs in the larvae of different butter- 

 flies and other insects. Phytophtora infestans 

 occurs only upon potatoes, while Phytophtora 

 omnivora attacks a number of other plants but 

 not potatoes. Species of moulds of the genera 

 Pythium and Sclerotinia attack only plants 



