error 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 prevalent, 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 compatriots 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 nature—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-
