within the latitude of health. Disease follows if the cell or the cell complex 

 is so changed that ultimately the whole structure suffers. 



Now, however, the fact, always confirmable by examples, that certain 

 cultivated varieties show a tendency to disease not shown by others under 

 similar conditions of growth, furnishes us proof that in the different individ- 

 uals the organic substance may oppose a dififering amount of resistance to 

 the same attacks. This would mean that more attacks are necessary for one 

 individual than for another in order to carry it out of the latitude of health. 

 If, in an epidemic, only large numbers of individuals always suddenly become 

 sick, besides the especially susceptible ones there must also be others among 

 them for which a greater number of attacks and therefore a longer period 

 of action is necessary, in order that they may become sick. Therefore a 

 longer period of the influences producing the disease must have led up to 

 the outbreak of the epidemic and these irfluences are to be seen in the atmosr 

 pheric factors. 



Therefore, according to our tlieor}-, each epidemic is, so to speak, the 

 explosion of a charge which had been slowly accumulating for some time. 

 Its cause therefore is not to be sought, at least exclusively, in the existing 

 factors of growth present at the moment but in the accumulation of attacks 

 which for some time previously have been effective in the same way. In 

 parasitic epidemics the extensive occurrence of the micro-organism in no 

 way represents the first stage of the phenomenon but is a final effect of long 

 preparation. This preparation consists on the one hand in the gradual pro- 

 duction of life conditions favorable for the enormous increase of the micro- 

 organisms, on the other hand, in the gradual weakening of some functions 

 of the host which we believe are always connected with this and a correlative 

 increase of other functions. 



If, for example, we study the best known fungous epidemic, potato 

 blight, observation shows that a period of warm, dull, sultry days usually 

 precedes the outbreak. The fungus Phytophthora infcstans is always 

 present. Its astonishingly rapid increase, however, takes place out of doors 

 only if abundant atmospheric precipitation and a warm motionless air con- 

 tinuously favor the production and the scattering of the swarm spores. Dur- 

 ing weather of this kind the potato plant develops a greater amount of sugar, 

 a more rapid stem growth and a great number of young leaves; that is, it 

 produces an especially susceptible environment for the development of the 

 fungus which scorns organs that have become old. In this way we find that 

 whole fields may become diseased in a few days. 



On the other hand we do not find the Pytophthora epidemic if the same 

 amount of precipitation occurs in the same space of time but in cold weather. 

 The epidemic cannot develop if, with increased warmth and a clouded sky. 

 l^ersistent strong winds keep blowing. A similar relation is shown in rust 

 epidemics of grains. Like the majority of fungi the grain rusts love con- 

 tinuous moisture. Yet by no means do we always have rust epidemics in wet 

 years, although there might be scarcely one grain field in which the rusts 



