ORGANIC EVOLUTION 



271 



as representing the normal condition for the species. It 

 will be observed that the variations from the normal are 

 here a little more numerous on the side of fewer numbers of 

 spines, but that the curve is nearly symmetrical. It is an 

 approximation to the symmetrical mathematical curve 

 representing the distribution of error. Chance variations 

 fluctuate thus about the normal. 



A count of the ray flowers of 315 heads of the bur mari- 

 gold, gives, when the results are plotted, a curve that is very 

 much askew: 



No. of ray flowers (classes) 345 6 7 891011 



No. times occurring (frequencies) 2 3819522219 o r 



FIG. 166. The 



curve of numerical variation in leaflets of the smooth sumac, 2730 leaves counted. 



