254 



GENETICS IN RELATION TO AGRICULTURE 



The above analysis not only demonstrates that Johannsen's material 

 was a mixture of different "weight types" but it also gives striking proof 

 that selection within a single pure line has no effect. Johannsen points 

 out that in certain lines (7, X, XI) there seems to be a slight effect but 

 that in others (77, IX, XII, etc.,) an opposite tendency appears; while 

 still others (77, 777, 7777) are irregular. Generally speaking then no 

 effect of selection is seen for there is no significant difference between the 

 means of the several groups in each pure line. The apparent indications 

 of selection effects are merely fortuitous variations. In each of these 

 lines, therefore, the offspring of plus and minus variants exhibit complete 

 regression to the mean of the particular line. In short, individual varia- 

 tions were not inherited, only the characteristic modifiability of the particu- 

 lar line was inherited. 



Johannsen did not rest here but continued to test his pure lines of 

 beans during successive years. He found a certain amount of seasonal 

 fluctuation in the range of variation and in the variation constants, yet 

 each pure line maintained its own individuality as indicated by the varia- 

 tion in weight of beans produced. And this maintenance of entity was 

 accomplished in spite of repeated selections of smallest and largest beans 

 so that each year every pure line was represented by two lots of plants, a 

 "plus strain" grown from the largest beans and a "minus strain" grown 

 from the smallest beans. Complete failure of such repeated selection to 

 cause significant change in the mean weight of either strain was observed 

 in each pure line. As illustrations the data on Lines I and XIX are 

 presented in Tables XLII and XLIII. 



From these data it is evident that six years of selection of plus and 

 minus strains within Line I produced no permanent departure in either 

 direction. In fact the last column (B-A) actually shows an inverse effect 

 during three of the six years. Moreover, if the average of the means for 

 the six years in both strains be compared this conclusion is verified. 



TABLE XLII. SELECTION-EFFECT DURING Six GENERATIONS IN LINE I OF THE 

 PRINCESS BEANS. (From Johannsen) 



