SELECTION IN PLANT BREEDING 195 



To my mind this work should clear up the strife between the critics 

 and the adherents of evolution by mutation. It is evident that there 

 are variations that are inherited and variations that are not inherited. 

 If we call the one a mutation and the other a fluctuation, we have a 

 distinction that will stand analysis. Why should a further distinction 

 be made? De Vries believes mutations to be qualitative, fluctuations 

 quantitative. Nevertheless, quantitative changes that are transmis- 

 sible occur in much greater numbers than do qualitative changes. Op- 

 ponents of mutation believe wide jumps appear too seldom to have been 

 a factor in organic evolution, but they can not deny that they do occur. 

 There are too many authentic cases in variation under domestication. 

 Yet no one who has had experience in breeding plants will deny that 

 small variations (not fluctuations) occur with much greater frequency. 

 While it is impossible to prove it, I believe that the mathematical law 

 of error controls the transmissible variations as well as fluctuations. 

 If one could collect a random sample of variations that are inherited 

 he would probably find that a great many forces act as the causes, and 

 therefore as in ordinary probability, the extreme changes — that is, the 

 great variations — occur with less frequency. One should remember, 

 however, that in our present state of physiological knowledge, he can 

 not know with much certainty which of two changes that apparently 

 differ greatly in magnitude is really the greater in the light of the 

 plant's economy. 



It might be well before leaving this part of the subject to speak of 

 one other point. In a strain that has been self-fertilized for several 

 generations, gradual progress has sometimes been made by selection. 

 This probably comes about because the parent plant is still hybrid in 

 regard to certain characters, and it is to their recombinations that the 

 intensification or reduction of certain apparently single characters but 

 which are really combinations of separately heritable characters, is due. 

 According to the law of chance with repeated self-fertilizations any 

 strain approaches a constant condition in all of its characters when 

 unseleeted, but one can not say when this state is reached unless he 

 knows the exact number of hybrid characters in the beginning and can 

 recognize each. 



If we were to take up the crops of the United States which owe their 

 present excellence and future prospects in large measure to the isolation 

 of superior strains by selection, we should cover a great majority of the 

 agricultural wealth of the country. Of course natural cross-fertiliza- 

 tion and even occasional artificial hybridization have played important 

 parts by causing recombinations of characters, but selection has been the 

 main cause of improvement. Two of the important crops, tobacco and 

 wheat, are very seldom cross-pollinated naturally; nevertheless new 

 types are continually appearing in the fields. To make new varieties 



