Luther Burbank: His Methods and Discoveries (1914)
An interesting experiment in causing the progeny of a certain plant to vary in opposite directions through selection, has been made at the Illinois Agricultural Station. Here the quality under consideration was the protein content—that is to say the amount of nitrogeneous matter—in the kernels of a given variety of corn.
The specimen with which the experiment started showed on analysis 10.92 per cent. of protein. Selection was made, among the ears of corn grown from this seed, of the individual specimens having the highest protein content on one hand, and those having the lowest protein content on the other.
By continuing this double selection for ten generations, two races of corn were developed, one of which produced seed having an average protein content of 14.26 per cent., while the other, grown in the same field, showed a decrease to 8.64 per cent.
This experiment illustrates the possibility of selecting out and fixing new races varying widely as to a single important quality of grain among the descendants of a parent plant of relatively fixed strain. In point of fact no plant is so fixed that its individual members do not show variation; none so fixed that it does not supply material with which the experimenter may work in producing new varieties.
Another illustration of the same thing was given by an allied series of experiments at the Illinois Station at which selection was made with reference to the height of the ear on the corn stalk. Seed from the same cob was planted in two fields and grown always under closely similar conditions.
But in one field selection was made for breeding purposes from stalks having the ears higher from the ground than the average; and in the other field from ears that were lower than the average. At the end of five years the two fields were so widely diversified that the average height of the ear from the ground in one of them was less than three feet (33.2 inches), whereas in the other field the average height of the ears was fully six feet (72.4 inches).
One could not well ask a more striking illustration than this of the possibility of developing new races, differing as to some conspicuous character, by simple selection from a given stock.
"An experiment with plants that is often held up as being particularly convincing was launched in 1903 at the Illinois Agricultural Experiment Station and continued until 1927. A population of corn was grown, the parent for the next generation always being the plants with ears closest to the ground. At the start, the average height of ears above the ground ranged from 43 to 56 inches. At the end of twenty-four years, the average had become a mere eight inches, and this trait bred true. As a check, plants with the highest ears were also selected and bred; in this case the average height rose to 120 inches —10 feet — by the end of the experiment."
Taylor, Gordon Rattray. Great Evolution Mystery (London: Sacker and Warburg, 1983), p. 34.