324 



BOTANY 



PART I 



grow two plants of the same origin under completely identical 

 conditions they would necessarily be indistinguishable. In practice 

 this is never possible, and therefore the homozygotic individuals of a 

 pure line show many quantitative differences even under the most 

 uniform cultivation possible. For example, the seeds of a pure line 

 of Bean can be sorted into a number of groups according to their 

 weights, and the number in each group or category ascertained. The 

 result of such an investigation is the curve in Fig. 271, which shows 

 that the weight- categories that occur most frequently are those 



closest to the average weight, and 

 that the farther a category is 

 from the average the fewer are 

 the individuals belonging to it. 

 Practically all statistical investi- 

 gations of variation conform to 

 this result. The VARIATION 

 CURVES thus obtained agree more 

 or less closely with the so-called 

 curve of chance. This is readily 

 understood, for there are always 

 several external factors acting 

 which may result in either an 

 increase or diminution of the 

 size, number, or weight under 

 consideration. Only chance de- 

 cides which effect takes place. 

 Thus only rarely will all the 

 factors make for diminution or 



. , J 11 the **0 for increase ; more 



of a pure line (JOHANNSEN'S Line K). (After frequently the factors Will be 



BAUR.) combined so as to determine an 



intermediate result. 



If a seed of a pure line is sown it is indifferent whether one 

 starts from a small, medium, or large specimen. The variation curve 

 of the next generation will not differ from that of the generation to 

 which the seed belonged. Similarly the changes resulting from 

 cultivation in alpine regions (Fig. 270) are not inherited. Such 

 modifications persist only as long as, or but little longer than, the 

 action of the causes giving rise to them. 



Practical experience seems at first sight to contradict this 

 result. In the process of SELECTION a plant with special properties 

 is chosen from a large number and the same characters appear 

 to recur frequently in its descendants. This depends on the fact 

 that in this case a single pure line has been isolated from what 

 was really a mixture of a number of different races or lines. The 

 characteristic properties of the selected line are continued in the 



FIG. 271,-Variation curve of the weights of Beans 



