False Atavism 207 



no changing influence of the climate are re- 

 quired for an adequate explanation of the 

 facts. 



In the observation quoted, what astonishes us 

 most, is the great rapidity of the change, and the 

 short time necessary for the offspring of the 

 accidental crosses to completely supplant the in- 

 troduced type. In the lecture on the selection 

 of elementary species, closely analogous cases 

 were described. One of them was the wild oat 

 or Avena fatua which rapidly supplants the 

 cultivated oats in bad years in parts of the 

 fields. Other instances were the experiments of 

 Risler with the " Galland " wheat and the ob- 

 servation of Eimpau on " Rivett's bearded >: 

 wheat. 



Before leaving the question of vicinism and 

 its bearing on the general belief of the insta- 

 bility of varieties, which when tested with due 

 care, prove to be quite stable, it may be 

 as well to consider the phenomena from 

 another point of view. Our present knowledge 

 of the effects of crosses between varieties ena- 

 bles us to formulate some general rules, which 

 may be used to calculate, and in some way to 

 predict, the nature of the impurities which nec- 

 essarily attend the cultivation of allied species 

 in close vicinity. And this mode of cultivation 

 being in almost universal use in the larger nur- 



