22 Outline of Genetics 



ing his crop into sicker and sicker soil in the later genera- 

 tions, he finally obtained a fully resistant strain from the 

 pure non-resistant strain with which he started. Such a 

 strain, he says, will not lose its resistance if planted 

 progressively in more infected soils. He gives the fol- 

 lowing theoretical explanation of his results: 



''Either (i) the so-called unit character of resistance 

 was present in undeveloped form and becomes stronger 

 from year to year under conditions of disease; or (2) there 

 never was any character present which is entitled to be 

 called a unit character, but it began to develop the 

 first year the parent plant came in contact with the 

 disease, and the protoplasmic nature of the ancestors of 

 the plants which we now have has been such that they 

 accumulated more and more the resisting power from 

 year to year, just as they had opportunity to develop 

 resistance against a constantly acting factor of disease, 

 which, when too powerful, acts as an eliminating factor." 



BoLLEY inclines to the second alternative. This 

 general conception seems to explain why home-grown 

 seed is regularly more resistant than seed from the same 

 variety which has had a vacation away from home for 

 several years. It has kept in training like a football 

 player. Bolley says that if these conclusions are cor- 

 rect, there are probably no unit characters which are not 

 fluctuating, and there are no fluctuating characters 

 which may not readily be fijced. 



These results are striking enough, but their signifi- 

 cance depends entirely upon the purity of the strains 

 which were used originally, and also upon the preserva- 

 tion of purity during the experiment. Bolley's phrase 

 "elimination factor," which he uses repeatedly, might 



