origin than are the denizens of populous regions, and especially of cities having commercial relations with all parts of the world. But when an exotic pestilential disease is first introduced among people who have previously enjoyed an immunity from it, on account of their isolation, it is usually very fatal, owing to the great susceptibility of a virgin population. This is due to the fact that there is no individual immunity resulting from a previous attack, and also to a relatively great race susceptibility as compared with a people among whom the disease has prevailed for many years. It is evident that the continued prevalence of an infectious disease in a given area will have a tendency to reduce the susceptibility of the population, in accordance with the laws of natural selection and survival of the fittest.
In illustration of this I may mention the comparative immunity of the African race to malarial fevers, which are so fatal to Europeans who visit the malarious regions of the African coast and interior; and the immunity of the native ("creole") population of those cities where yellow fever prevails as an endemic disease, as at Havana, Vera Cruz, and Rio de Janeiro.
What has been said will suffice to show that the geographic distribution of infectious diseases is to some extent influenced by the relative susceptibility of the population in various regions. The prevalence of the strictly contagious diseases also depends to some extent upon climatic conditions, although to a far less degree than is the case in our second group, which includes diseases in which the germ may multiply external to and independently of infected individuals.
In general, contagious diseases are more likely to spread in northern latitudes, and during the winter season, because the climatic conditions lead to the aggregation of individuals in towns and in closed apartments, while in southern latitudes and during the summer season a larger proportion of the population live in the open air during the daytime and sleep in well-ventilated rooms at night.
The influence of season upon the prevalence of smallpox, a strictly contagious disease, has been referred to by numerous authors, and is insisted upon by Hirsch in his Handbook of Geographical and Historical Pathology. In a table contained in the monumental work of Hirsch the season is given in which ninety-nine epidemics of smallpox reached their height. In sixty-seven it was during the cold season and in thirty-two during the warm season. The same thing is shown by the mortuary statistics of various civilized countries. The immunity resulting from vaccination has largely influenced the geographic distribution of smallpox epidemics, which are now almost unknown in Germany and are comparatively infre-
