518 



VARIATIONS 



of flowers per plant, number of seeds per pod, weight and dimen- 

 sions of seeds, size and shape of leaves, height of plants, etc., 

 distribute themselves around a mode in the same general way as 

 shown by the illustrations. 



FIG. 468. A demonstration of Quetelet's law of continuous variation in 

 the size of the seeds of a Common Bean. The seeds are grouped with refer- 

 ence to length. The longest column contains those of average length and 

 the columns to the right or left of it are shorter accordingly as the length of 

 Beans in each column are greater or less than the average length. Redrawn 

 from De Vries and Johannsen. 



When fluctuating variations are studied throughout a number 

 of successive generations, it is found that they fluctuate around a 

 mode that is practically constant for successive generations. 

 Thus the variation in the number of leaves per plant, in the 

 length of ears, and so on, in any variety of Corn tends to fluctuate 

 around a mode that is common throughout generations. This 

 feature is not encouraging to one who wishes to improve a race 

 of plants by selection, for it means that plants grown from the 

 seed of parents selected on account of size, high yield, or some 

 other desirable fluctuating variation produce offspring with very 

 little or no better average than that of the generation from which 

 the superior plants as parents were selected. In other words, no 

 matter how marked and desirable a fluctuating variation may 

 be it is commonly lost in succeeding generations. Fluctuating 

 variations seldom breed true. Often, however, the mode of a 

 variation in the offspring of certain parents does show much 

 improvement over the mode of the group from which the parents 

 were selected. In a group of most any of our crop plants or wild 

 plants the individuals differ in their heritage. Some individuals 

 have inherited and therefore transmit to their offspring more for 

 size, high yield, etc., than other individuals. The mode of a 



