VARIATION 



43 



offer a more hopeful line of approach than statistical 

 generalizations. It is better to become acquainted 

 with the real parent than to evolve a hypothetical 

 "mid-parent" mathematically. In this connection 

 it is well always to bear in mind the warning of 

 Johanssen, himself a past master in biometry, when 

 he writes : '^ Mit Mathematik niclit als Mathematik 

 treiben wir unsere Studien." 



6. Fluctuating Variation 



With respect to any measurable character there 

 are bound to be deviations from an average con- 

 dition. According to the mathematical laws of 

 chance these deviations sometimes are plus and 

 sometimes minus, and consequently they may be 

 termed fluctuating variations. 



Pearson gives as a simple illustration of fluctuating 

 variation the number of ribs present in two sets of 

 beech-leaves, as shown below. These sets were taken 

 from two different trees, and each contains twenty- 

 six leaves. 



It will at once be seen that, while certain leaves 

 might well belong to either tree, as, for example, those 

 with sixteen ribs, the entire group of leaves from 



