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THE AMERICAN NATURALIST 



[Vol. XLII 



in mass breeding of as great importance as any that may 

 be hoped to be obtained by looking for a single mutating 

 type evolved through the method of DeVries. 



I am unable to affirm whether disease resistance, im- 

 munity to disease, is structural or physiological. I be- 

 lieve the latter, for I have been able to develop it in 

 varying degrees of perfection in every strain of potatoes, 

 wheat or flax with which I have worked, and for all of 

 the diseases noted, while the method used has always re- 

 mained the same. There is also good structural evi- 

 dence that it is physiological, as shown in the structural 

 changes caused by the entrance of disease-producing 

 organisms within the tissues of resistant hosts. I 

 have worked with pure strains, centgener progeny, 

 from individuals, with the progeny of cross bred plants, 

 with mixed strains and variations in bulk, and with 

 bulk selections of centgener origin, side by side, and 

 resistance has come at approximately the same rate 

 and grade for a variety through any one of these methods. 

 When the conditions of disease production and conditions 

 of infection for each plant have been held so as to be 

 constant factors, the bulk method of selection, as, for ex- 

 ample, selection to seed weight, color and form from a 

 disease plot of flax, has given final hereditary resistance 

 as rapidly as individual selections from the progeny of 

 individuals of the same strains. The only cases in which 

 resistance has developed irregularly in such strains of 

 flax or wheat have been found to be due to impurity of 

 type or to irregularities as to the constancy and amount 

 of disease infection, or to irregularities in the conditions 

 promoting the development of disease. 



Kesistanee to the diseases named for the crops named 

 can be developed in any variety or strain by either 

 method of selection noted. In any case, it may be in- 

 creased by every proper selection year by year, just in 

 proportion to the perfection with which the disease con- 

 ditions of elimination are maintained. 



The method works in potato selection, where the process 



