202 



GENERAL BIOLOGY 



number. In other words the line that connects the 

 tops of the columns describes the same sort of a 

 curve 1 that we secure when we plot out the heights 

 of a large number of men. 



Nearly all the obvious variation of organisms is 

 of the kind just described. Mathematical analysis 

 gives no clue to the nature of the individual variate, 

 and for this reason such variation is frequently 

 called fortuitous, i.e. random or unpredictable. Nev- 

 ertheless the mathematical values obtained from the 

 analysis of a mass of such data are very accurate 

 and certain . 



-NORMAL CURVE 



K: 



01 



NUMBER OF VEINS MEAN 



a b 



FIG. 73. The Normal Curve. Veins in beech-leaves. (From Daven- 

 port, after Pearson.) 



Types of Variation Curves. The theoretical or 

 normal binomial curve is perfectly symmetrical 

 (tig. 73). The classes may be marked off along the 



1 The curve is that known in mathematics as the binomial curve 

 (the expansion of the expression [p -f- q] n ). 



