176 THE PRINCIPLES OF HEREDITY 



every island of the Pacific, it is not impossible that malaria 

 was thus introduced. The only alternative is that infected 

 mosquitoes, borne on driftwood or driven on tempests, crossed 

 separately. 



295. Pathogenetic organisms are exposed to climatic and 

 other influences external to the body only during their pas- 

 sage from one human being to another. It is this that 

 bestows on the mode of infection its importance. The 

 parasites of contagious diseases, passing directly from one 

 person to another, are thoroughly protected from external 

 influences, and, therefore, are capable of existing in, or of 

 travelling to, every part of the habitable globe. They may 

 afflict nomadic peoples or small isolated communities almost 

 equally with large and populous centres. At the other 

 extreme the microbes of malaria, which are conveyed from 

 person to person by means of a particular species of mosquito, 

 are infective only within the area frequented by the mos- 

 quito. As, with the possible exception of certain islands, the 

 mosquito is already present in nearly every region suitable to 

 its existence, malaria is now almost incapable of extending its 

 boundaries. Anciently, no doubt, owing to the long duration 

 of the disease, it spread with extraordinary rapidity. Air- 

 borne diseases (measles, chicken-pox, small-pox, whooping- 

 cough, influenza and the like) are almost as well able to 

 travel as contagious diseases. Conveyed to long distances by 

 a succession of infected individuals, or by infected clothing, 

 they spread with extraordinary rapidity on reaching new 

 ground. They quickly infect the whole population, but for 

 that very reason, and because immunity is quickly acquired 

 against them, they soon exhaust the food supply, and, there- 

 fore, are apt to die out among nomadic tribes, or in small and 

 isolated communities. 



296. Of all diseases except those carried by insects, the 

 water- and earth-borne maladies are most under the influ- 

 ence of conditions external to the human body. Cholera and 

 dysentery are natives of warm climates, and seldom, and then 

 only under especially favourable conditions, make incursions 

 into colder regions. A high temperature is favourable also to 

 typhoid. All water-borne maladies are, moreover, essentially 

 crowd and filth diseases. A settled population and a continu- 

 ously infected water supply are necessary conditions of their 

 permanence. When spreading beyond the areas in which 

 they are endemic, they usually travel along the great trade 

 routes, and, therefore, more rarely than the air-borne diseases 

 afflict nomadic peoples and small isolated communities. In 



