XIV 



ECONOMICAL 



161 



sowing only that seed which weighs over 1 2 grains, we shall 

 find that in the next generation the average weight of the 

 seed is raised and the curve becomes somewhat shifted to 

 the right as in the dotted line of Fig. 30. By continually 

 selecting we can shift our curve a little more to the right, 

 i.e. we can increase the average weight of the seeds until 

 at last we come to a limit beyond which further selection 

 has no effect. This phenomenon has been long known, 

 and it was customary to regard these variations as of a 

 continuous nature, i.e. as all chance fluctuations in a 

 homogeneous mass, and the effect of selection was sup- 



4 6 8 10 12 14 16 18 20 

 Weight of individual seeds 



FIG. 31. 

 Curves to illustrate the conception of pure lines in a population. 



posed to afford evidence that small continuous variations 

 could be irlcreased by this process. But Johannsen's 

 results point to another interpretation. Instead of our 

 material being homogeneous it is probably a mixture of 

 several strains each with its own average weight about 



