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would not be present every year. The epidemic develops at the time when 

 the leaves are young and only during periods of warm days with frequent 

 even if almost unappreciahle showers which make possible a longer retention 

 of moisture among the plants. Cold, wet summers generally prevent the 

 development of rust epidemics. Similar conditions may be observed in 

 bacterial epidemics. 



Therefore, epidemics arc forms of disease which mature only because of 

 far reaching factors. Only certain weather combinations of longer duration 

 may be considered as the initial cause. Naturally the intensity of the epi- 

 demic will vary locally because local factors will produce special favorable 

 conditions. In this way is explained the occurrence of centres in which the 

 malady appears first and disappears last, in case not all the individuals are 

 killed in a short time. In this way is explained further the retrogression of 

 epidemics into endemics ; that is, into narrowly confined centres of disease. 

 Among the epidemics produced by animal parasites, those caused by grain 

 flies are the most abundant with us. They usually take place during periods 

 of continued warm, dry weather after the winter conditions have been favor- 

 able for the individual grain flies which in some regions are always present. 

 So far as statistics now go, preferred centres and points of departure may 

 often be determined for this plague-like distribution. Thus, for example, 

 the province Posen is proved to be especially favorable soil for grain flies. 

 From Posen as a centre an epidemic usually radiates towards Brandenburg, 

 Pomerania and West Prussia. The whole Eastern part of Germany suffers 

 more from injuries due to flies than does the Western p^^rt. North Western 

 Europe is usually visited more frequently and intensely than South \\^estern 

 and South Eastern Europe. 



According to the point of view here developed any treatment of the 

 epidemics by fighting the symptoms as they appear must ofifer the least pros- 

 pect of success, because these are only the result of initial stages which 

 existed long before. If the parasites arc present in enormous quantities the 

 desire to kill the micro-organisms is seen to be a vain one since no insecticide 

 of fungicide can even approximately reach the main mass and still less cause 

 its death. Thus as the pestilences are induced by general factors acting uni- 

 versally, they must be combatted by broad means wdiich undo the life con- 

 ditions of the parasite and change the constitution of the host, that is, the 

 functional direction. If, for example, long wet periods permit the bacterial 

 rot of potato, which we call "ivet rot," to appear in epidemic proportions, 

 any other means than increased ventilation of the soil can scarcely be used 

 successfully. So far as specific anaerobic bacteria are concerned, the factor 

 favorable to growth (lack of oxygen with excess of carbon dioxid) is re- 

 moved by an increase of oxygen and also by the decrease for them, as well 

 as for other bacteria, of the condition fundamental to their abundant in- 

 crease, an abundance of water. Nature generally works in this way. If, 

 after the rainy periods, dry, windy weather continues for some time so that 

 the soil dries and the air circulates freely, the progress of the disease comes 



