vary little in type for people of the same races in different times and places; whereas diphtheria, typhoid, and cholera, the microbes of which are more capable of existing outside the body, vary much more in relation to time and space. This is of course exactly what might be expected on à priori grounds. For since the microbes of disease are unicellular organisms, and as such presumably capable of transmitting acquired traits in whole or part to descendants, it follows that diseases, the microbes of which exist under the more stable conditions, should be more stable in type than diseases, the microbes of which exist under less stable conditions; and therefore that such a disease as syphilis, the microbes of which exist solely in the human body, under conditions of temperature, moisture, &c. that are constantly nearly the same, should be more stable in type than such a disease as malaria, the microbes of which normally exist outside the body under conditions of temperature, moisture, &c. that are constantly changing; so also diseases intermediate in the scale—e.g. measles and scarlatina, cholera and diphtheria—should approach syphilis in stability, or malaria in instability, accordingly as the environment in which the pathogenic organisms exist resembles that of the microbes of syphilis in its equability, or that of malaria in its changefulness.
But, so far as we are able to institute comparisons, all zymotic diseases, even the most stable, show differences of type when attacking different races of men, always being least severe as regards each disease in races that have had much and prolonged experience of it, and most severe in races that have had little or no previous experience of it. Thus measles, scarlatina, small-pox, tuberculosis, syphilis, malaria, &c., are comparatively mild in type when attacking races that they have long afflicted, but comparatively severe in type
