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 
unselected, 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. ‘I'wo of the important crops, tobacco and 
wheat, are very seldom cross-pollinated naturally; nevertheless new 
tvpes are continually appearing in the fields. To make new varieties 
