THE PRESENT EVOLUTION OF mM — PHYSICAL 271 



eiTor obviously obtains as regards such a disease as- 

 tuberculosis, which, as we know, frequently attacks 

 persons hitherto immune, whose vitality has been 

 lowered by other causes. Moreover, evolution against 

 particular zymotic diseases may involve some evolution 

 against zymotic disease in general ; in other words, it 

 is possible that a race which has undergone evolution 

 against many zymotic diseases is thereby endowed with 

 powers which may enable it to resist the microbes of 

 a disease new to it, better than a race that has had 

 little or no experience of zymotic disease. This involves 

 another possible source of error, for if such be the case, 

 adults of a race that has suffered much from zymotic 

 diseases would necessarily prove more resistant to any 

 new zymotic disease than their immature kin. 



A second point of interest presented by zymotic 

 disease is the apparently well- authenticated fact, that 

 continued residence in a country where any such disease 

 is prevalent often appears to confer on the individual 

 acquired powers of resistance against it, even in the 

 absence of illness and recovery ; but these powers are 

 of such a kind that they endure only in the presence of 

 the disease ; so that if an individual who possesses them 

 leave for a time the country where the disease is preva- 

 lent, he is liable on his return to fall a victim to it. 

 Yellow fever is an example, and the natives of Domingo, 

 for instance, who have returned from a sojourn abroad, 

 appear to contract this disease more readily than com- 

 patriots who have never left the infected districts. The 

 only conceivable explanation of this curious phenomenon 

 appears to lie in the hypothesis, that individuals who 

 are already in some degree resistant by njtture — i.e. 

 inborn traits — acquire greater powers of defence as a 

 reaction to continual assaults from the disease; in 

 other words, that those living within the infected areas 

 are continually infected by the pathogenic micro- 



