PURE LINES 



255 



TABLE XLI1I. SELECTION-EFFECT DURING Six GENERATIONS OF LINE XIX OF THE 

 PRINCESS BEANS. (From Johannsen) 



The mean for the progeny of the plus strain is 66.12 + 0.28 and for the 

 progeny of the minus strain, 66.66 + 0.33. The difference is 0.54 + 0.43 

 (the probable error of the difference in all cases being found by taking 

 the square root of the sum of the squares of the two probable errors). 

 In Line I, therefore, there is no positive effect of selection; on the con- 

 trary there would appear to be a slight inverse effect! 



Line XIX was characterized by beans of the least weight. The data for 

 the results of six years of selection in plus and minus directions, particu- 

 larly the difference between the progeny means (B-A), reveal somewhat 

 larger fluctuations in the plus direction than in Line I but it will be noted 

 that the probable errors of the differences are smaller, hence the validity 

 is the more certain. Comparing the means of the means of the progeny 

 seeds as before, for the plus strain we have 37.40 + 0.11 and for the 

 minus strain, 38.83 + 0.15, the difference being +0.57 0.19, which is 

 certainly small although in the plus direction. Now, if we compare the 

 summaries of the data from these experiments, 0.54 and +0.57, we 

 are forced to conclude that selection was without effect in these pure lines. 



Finally Johannsen conducted similar experiments with the Princess 

 beans, using the characters, length and breadth. He came to the same 

 general conclusion, to wit, that he found no trace whatever of selection 

 effect within pure lines and that the variations in pure line individuals are 

 merely fortuitous modifications and are not inherited. 



Conditions Necessary for the Existence of Pure Lines. Johannsen 

 defined a pure line as the progeny of a single self-fertilized individual of 

 homogeneous factorial composition. Unless mutation takes place none of 

 the descendants of such an individual can differ from the parent in their 

 genetic factors. Two important conditions are imposed by this 

 definition, viz., homozygosity and self-fertilization. The latter of these 

 is the more fundamental inasmuch as it is mathematically demonstrable 

 that self-fertilization, if continued generation after generation, leads 



