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 transmissible occur in much greater numbers than do qualitative changes. Opponents 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-fertilization 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
