QUETELET'S LAW 



519 



variation determined by considering all individuals of the group 

 IS less than the mode if only the better individuals were con- 

 sidered and greater than the mode if only the poorer individuals 

 were considered. If the individuals having most in their heritage 



Fig. 469. — A curve constructed to show the variation in weight of tubers 

 harvested from 6 hills of Irish potatoes. The spaces in the vertical lines rep- 

 resent the number of tubers in each column and the numbers at the base of 

 each line give the range of weight in ounces of the tubers in the columns repi'e- 

 sented by the vertical lines. 



for the variation are selected from the group to be the parents of 

 the next generation, the mode of the variation in the next genera- 

 tion is thereby improved. Through the selection from fields of 

 Wheat, Oats, Barley, and other crops, the individuals having most 

 in their heritage for a desirable variation, better strains of plants 

 have been obtained. In fact this is one of the chief ways of 

 improving races of plants, but it is evident that the improvement 

 is due to the selection of hereditary variations and not to the 

 selection of fluctuating variations caused directly by environ- 

 ment. It has been demonstrated that very little or nothing is 

 gained by the selection of variations if all the individuals of the 

 group from which the selection is made are aUke in their heritage. 

 Johannsen, a Danish botanist, has demonstrated that, if one 

 starts with a pure hne, that is, with the offspring of a single 

 individual produced by self-fertilization, and keeps the genera- 

 tions pure by preventing cross-pollination, the mode of a fluctua- 

 ting variation cannot be increased. He clearly demonstrated 

 this fa'ct with Beans. Beans commonly self-pollinate and hence 

 remain pure. He attempted to increase the average size of the 

 seeds of a certain variety of Beans by selecting the plants 



