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THE AMERICAN NATURALIST [Vol. LI 



cation to the reader that all variations have the same co- 

 efficients of heredity, that a bud-variation is simply a 

 wide fluctuation imposed by external conditions. If this 

 were true the whole organic world would be chaos. But 

 species and varieties do exist. They may be ' ' judgments ' ' 

 in one sense, but in another they are concrete things. In 

 fact we learn this further on in this volume when it suits 

 Bailey's purpose to have asexually propagated varieties 

 very constant. He says (p. 353) : 



tree fruits, being propagated by buds, is really but a multiplication of one 

 original plant, and all the trees which spring from this original are ex- 

 pected to reproduce its characters. 



3. The Effects of Disease.— The influence of disease is 

 in many ways like that of malnutrition, in that it is wholly 

 an effect on the physiological efficiency of the reproducing 

 cells. This fact is fairly clear when dealing with diseases 

 with outstanding symptoms. In many instances, how- 

 ever, diseases are not easily diagnosed. There may even 

 be no suspicion that disease is present. In such cases it 

 is rather hard to believe that selection is not accomplish- 

 ing a positive and radical improvement. A good ex- 

 ample of this is the selection of potato tubers. No one 

 consciously selects a seed potato infected with blight. In- 

 dependent of the probability of reinfection, there is the 

 likelihood that the diseased tuber will not be able to pro- 

 duce a normal plant because of the effect the fungus has 

 had on its own cells. One doesn't usually believe, how- 

 ever, that rejection of this tuber and selection of the 

 healthy sister is going to lead to the formation of a new 

 race. Yet numerous experiments on potatoes in which it 

 is shown that successive selections have raised the| 

 average yield over that of the unselected tubers, are prob- 

 ably of just this type. The race is kept up by the re- 

 jection of diseased tubers, but there is no evidence what- 

 ever that it is improved. I am not going to argue that 

 desirable asexual variations may not occur during this 

 time, and be retained. I say only that any improvement 



