INTRODUCTION TO THE STUDY OF PERSISTENCE FACTORS 211 



that gives due credit to each of the variants in the two generations. 

 One method commonly used is the graphic method. Thus each seed 

 of the parent generation is measured and plotted on a graph in which 

 the horizontal line (the abscissa) represents a series of size classes vary- 

 ing from the smallest ones at one end and the largest at the other, and 

 the perpendicular line (the ordinate) represents the frequencies of indi- 

 viduals in the varying size classes. A line connecting high points in 

 each of the size classes will form a variation curve which will be charac- 

 teristic of the group. Such a curve has a high point near the middle 

 (called the mode), and the curve slopes gradually toward each end. 

 This curve not only represents the distribution of the different sizes in 

 the group but shows the most commonly occurring size class, the modal 

 class. A similar curve is made for the offspring generation, and the 

 parent curve is compared with the offspring curve. The two may be 

 compared with respect to mode, mean, average, and standard devia- 

 tion, and a great deal may be learned, that can be learned in no other 

 way, about the variation and heredity of such graduated or fluctuating 

 characters as weight. 



For further information about statistical methods in genetics, the 

 reader is referred to a short chapter in the Appendix (chap, xliii). It 

 will hardly be necessary here to do more than indicate that pure-line 

 work, such as that of Johannsen, Jennings, Tower, and Wright, dis- 

 cussed in the next chapter, could not have been done without the use 

 of biometrical methods. 



