SHORTER ARTICLES AND DISCUSSION 



SELECTION, SUGAR-BEETS AND THRIPS 

 A discovery of great importance to students of genetics has 

 recently been made by one of the plant-breeders 1 of the U. S. 

 Department of Agriculture, viz., that beets are regularly cross- 

 pollinated and that an important agent in the process is a minute 

 inconspicuous insect, so small that it readily passes "through the 

 meshes of fine silk chiffon. ' ' 



To understand fully the theoretical importance of this dis- 

 covery one need only recall the large attention given to the 

 sugar-beet in recent adverse criticisms of the selection-theory. 

 De Vries in his "Mutationstheorie," p. 72, cites the case of the 

 sugar-beet as showing the most systematic, refined and elaborate 

 selection known for any cultivated plant, and yet as being with- 

 out any permanent effect in raising the sugar content of the beet. 

 For, although the average sugar content of the beet has by syste- 

 matic selection been practically doubled in the last 60 years, 

 De Vries holds the improved racial condition to be unstable and 

 thinks that the improved race would within a few generations 

 revert to its old level of sugar-content if the selection were dis- 

 continued. His reason for thinking so is the familiar fact that 

 the offspring of the best selected beets are on the average not 

 quite so good as their selected mother-beets, but show a tendency 

 to regress downward toward the old level of sugar content. It 

 should be pointed out, however, that in reality regression is not 

 toward the original average of 7 or 8 per cent, sugar-content, but 

 toward an average twice as high as this. For De Vries 's varia- 

 tion polygon (I. c, Fig. 22) for the sugar content of 40,000 beets 

 shows a nearly symmetrical probability curve about a mode at 

 15.5 per cent. It is to be supposed therefore that regression 

 would occur toward this condition from both the upper and the 

 lower halves of the frequency polygon, rather than toward the 

 old average condition of 7-8 per cent., which, -according to the 

 data of DeVries, is now rarely if ever seen in the improved race. 

 To have doubled the average sugar-content of the beet is cer- 

 tainly something of an achievement for selection; the form of 

 iShaw, Harry B., "Thrips as Pollinators of Beet Flowers," Bun. No. 

 104, U. S. Dept. Agr., July 10, 1914. 



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