ORGAXIC EVOLUTION 



27J 



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 sym^metrical 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 i 



Fig. 166, The curve of numerical variation in leaflets of the smooth sumac, 2730 leaves counted. 



