245 THE AMEEICAN NATURALIST [VOL. XLV 



sis in which the degree of vigor is assumed to depend 

 upon the number of dominant elements present rather 

 than the number of heterozygous elements. While all 

 of my data thus far are in perfect accord with my own 

 hypothesis, and I know of no instance in which self-fertil- 

 ization of a corn-plant of maximum vigor has not re- 

 sulted in a less vigorous progeny, it is quite possible 

 that I have still insufficient data from which to distin- 

 guish between the results expected under these two hy- 

 potheses. However, for the purpose of the present dis- 

 cussion, it is not necessary to decide which of these two 

 hypotheses (if either) is correct. Both of them are 

 based upon the view that the germ-cells produced by any 

 plant whose vigor has been increased by crossing are not 

 uniform, some possessing positive elements or genes not 

 possessed by others. 



Several different characters which are more or less 

 dependent upon physiological vigor have been taken into 

 account in my work, each of which gives its own support 

 to the conception upon which both of these hypotheses 

 are based. The number of rows of grains on the ears 

 which has been most extensively used as a measure of 

 variability, and as a guide in selection, is found to be 

 somewhat affected by the vigor of the individual, and it 

 is due to this fact, no doubt, that the row-number is a 

 fluctuating character, even in the pure genotype. An- 

 other characteristic which has been used as a measure of 

 vigor has been the yield of corn computed in bushels per 

 acre. 7 A third characteristic, which was not taken into 

 account at the beginning of the experiments but which 

 has given confirmatory data in the later years, is the 

 height of the stalks, a character which was much used by 

 Darwin as a measure of vigor in his study of the effects 

 of cross- and self-fertilization in plants. 



7 It should be understood that this method of stating yields is seriously 

 defective, in that it implies the existence of a much smaller probable error 

 than is actually present, since each of my pedigrees has usually occupied 

 only about one one-hundredth of an acre. However, I believe that this 

 defect is more than offset by the advantage of using a unit of yield with 

 which all readers are familiar. 



