THE PRESENT EVOLUTION OF MAN — PHYSICAL 235 



not well satisfied in countries where the heat renders 

 necessary the admission of large volumes of fre'sh air 

 into the dwellings of the inhabitants, nor in colder 

 countries, where the art of building has so little 

 advanced that large volumes of air enter the dwellings 

 in spite of the mistaken endeavours of the inhabitants. 

 Again, as regards scarlatina, undiscovered causes in the 

 environment are unfavourable to the existence of the 

 virus in Arabia; the disease is therefore almost un- 

 known in that country, though its inhabitants are not 

 immune when they travel abroad. 



In the third class are included all zymotic diseases of 

 which the microbes are able to subsist for an indefinite 

 time outside the living body on non-living organic 

 matter, the microbes of which, in fact, are essentially 

 saphrophitic, and have their normal habitat outside 

 the living body, but are capable of existence, should 

 occasion serve, within it. Examples of such are the 

 malarial fevers. Unlike the diseases of the first two 

 classes, the prevalence of their germs in any territory 

 is quite independent of the human population, except 

 in so far as it affects the outside conditions through 

 drainage, deforestry, &c., and therefore the inhabitants 

 of the most sparsely peopled lands may be afflicted by 

 them. 



These three classes of diseases are not sharply defined 

 from one another, but shade, the • first into the second, 

 and the second into the third. From diseases, the 

 germs of which are practically incapable of existence 

 outside the living body, to diseases, the germs of which 

 have their normal habitat external to it, every degree 

 of ability to exist outside the body may be observed. 

 The microbes of some diseases, — e. g. syphilis, — practic- 

 ally speaking, cannot exi?t outside the living body ; the 

 microbes of others — e.g. tuberculosis — can exist, but 

 cannot multiply outside it ; the microbes of others — e. g. 



