418 



DXPEBIMENrS WITH PLANTS 



the series represented by the upright lines in Fig. 237. 

 The highest pile is the one containing those with 

 twenty -one rays, and the size of the piles decreases 

 in both directions from this: those which contain 

 daisies with five and thirty -seven rays are the smallest 

 and represent the two extremes. When we draw a 

 line over the tops of the piles, we get a curve of 



u zt 2s u, 27 Ja zg jo 31 Ji « ja jo m, jt 



237. Curve of variation ; the result of sorting daisies into 

 piles according to the number of rays they possess. 



very characteristic form. It has been found that, no 

 matter what feature of an organism we study in this 

 way, we get practically the same sort of curve (the 

 only apparent exception being partial curves and 

 double curves). This curve, therefore, becomes of 

 great interest, especially as it is found that it can 

 be expressed by a mathematical formula and that 

 the variation obeys certain mathematical laws. The 

 mathematical, or statistical, study of variation has 

 now become an important branch of biology. 



