BIOMETRY 63 



The action of several heredity factors which are indepen- 

 dent of each other produces a curve of the same sort; and 

 so do several environmental factors independent of each 

 other; in most cases of variation agencies of both sorts are 

 at work. But in some cases the causes which tend to pro- 

 duce plus variation may be stronger or weaker than those 

 which tend to produce minus variation. The result is an 



Tails 123456789 10 



Fig. 12. A graphic presentation of the data contained in Table 2. After Lock. 



unsymmetrical or " skew " variation curve. Thus among 

 Harvard students the causes which tend to produce variation 

 in weight above the normal are apparently stronger than 

 those causes which tend to produce weight below the normal, 

 as is indicated by Fig, 10. The same was found to be true still 

 more emphatically of adult males in England, according to 

 data tabulated by Yule. 



In some cases, biological variation is exclusively in one 

 direction from the mode, i. e., all the causes of variation 

 which are operative tend in one direction. Thus the common 

 buttercup varies in number of petals from five upward but 

 very rarely in the reverse direction. Five is the commonest 

 or modal number, but the observed variation curve is one- 

 sided. See Fig, 14, H 1887. 



It is evident that to describe the character of variation in 

 any case it will not suffice to name the mode; we must also 



