148 MENDELISM CHAP. 



elusions at which he arrived. If we weigh a 

 number of seeds collected from a patch of plants 

 such as Johannsen's beans we should find that they 

 varied considerably in size. The majority would 

 probably not diverge very greatly from the general 

 average, and as we approached the high or low 

 extreme we should find a constantly decreasing 

 number of individuals with these weights. Let us 

 suppose that the weight of our seed varied between 

 4 and 20 grains, that the greatest number of 

 seeds were of the mean weight, viz. 1 2 grains, 

 and that as we passed to either extreme at 4 

 and 20 the number became regularly less. The 

 weight relation of such a collection of seeds can be 

 expressed by the accompanying curve (Fig. 30). 

 Now if we select for 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 con- 

 tinually 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 be- 

 yond which further selection has no effect. This 

 phenomenon has been long known, and it was 

 customary to regard these variations as of a con- 

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

 homogeneous mass, and the effect of selection was 

 supposed to afford evidence that small continuous 

 variations could be increased 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 



