240 THE PRESENT EVOLUTION OF MAN— PHYSICAL 



vary little in type for people of the same races in dif- 

 ferent 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 co 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 con- 

 ditions, 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 chang- 

 ing; 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 differ- 

 ences 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, ifec, are com- 

 paratively mild in type when attacking races that they 

 have long afflicted, but comparatively severe in type 



