100 THE AMERICAN BOTANIST 



are gaining rapidly, some plants are gaining slowly, and some 

 plants are in a resting stage as far as the development of dis- 

 ease resistance is concerned. If the degree of infection be in- 

 creased so that some but not all plants are killed, those that 

 will be killed are those which happen at the time to be in a 

 resting stage. As all plants, superior and inferior alike, have 

 to go through resting stages at irregular intervals, it will be 

 seen that causing such deaths is not a process of weeding out 

 inferior ones. 



Disease resistant plants have been produced under vary- 

 ing circumstances at different experiment stations, but the 

 one circumstance which cannot be omitted is that they must 

 be grown under disease conditions. A plant or an animal de- 

 velops disease resisting power in fighting a disease, and dis- 

 sease resistance is a heritable thing. The obvious inference 

 is that the acquired resistance is inherited. The stock argu- 

 ment against that is that disease kills the weaker plants and 

 leaves the stronger, and that this selection brings about the 

 improvement. It has just been shown that selection does not 

 operate as it is assumed to operate, but the main argument 

 against the selection theory is that killing a weak plant does 

 not add to the strength of a strong plant, and the thing ob- 

 tained is increase of disease-resisting power. No one has 

 shown any process of increasing any power in any other wa\ 

 than by exercising the powers previously in existence. 



