PURE LINES 255 
TasLe XLIII.—Sevection-errect Durina Srx GENERATIONS OF LINE XIX or THE 
Princess Beans. (From Johannsen) 
Wee) Mean weight of progeny seeds of 
Harvest re the select strains Diter: select strains Difference 
vices of beans b-—a ar 
a—minus | b—plus A-—minus B-plus 
1902 219 30 40 10 | 35.83+0.44 | 34.78+0.38 | —1.05+0.58 
1903 200 25 42 17 | 40.21+0.65 | 41.02+0.43 | +0.81+0.78 
1904 590 31 43 12 | 31.89+0.29 | 32.64+0.21 | +1.25+0.36 
1905 1,657 27 39 12 | 38.26+0.16 | 39.15+0.17 | +0.89+0.23 
1906 1,367 30 46 16 | 37.92+0.22 | 39.87+0.16 | +1.95+0.27 
1907) 594 24 47 | 23 | 37.3640.30 | 36.9540.21 | —0.41+0.37 
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 
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