174 DISEASE IN PLANTS. 



increased in the case postulated above, because 

 two ideals are to be worked up to, and they may 

 conceivably be incompatible. Not necessarily so, 

 however, for breeders have solved such problems 

 before in obtaining early and heavy cropping races 

 of potatoes, wheat, etc., sweet and large grapes, 

 strawberries, etc., hardy and brilliant flowers, and 

 so forth. 



There is, however, another aspect of this 

 question of variability in organisms in this con- 

 nection to be considered. Ever since cultivation 

 began man has probably been cultivating not 

 only the crops he desires, but also the pests 

 which infest them, and if variation of his chosen 

 plants occurs and no one will deny that 

 surely variation of the fungi and insects which 

 live on them also takes place. That this is 

 so can be demonstrated, though, since it is not 

 part of my theme to go into the question 

 of peculiarities of species and races of parasites, 

 the subject must here be passed over with a few 

 remarks only. 



Recent researches have shown not only that 

 fungi vary immensely in form and morphological 

 characters according to the amount and kind of 

 food-materials put at their disposal, thus bringing 

 the whole question of polymorphism into the 

 domain of experimental physiology, but that their 

 capacities for infection, spore formation, etc., are 

 also capable of variation and are dependent 

 on the quality and quantity of food supplies, 

 water, as well as on the temperature, illumination, 



