Page:Popular Science Monthly Volume 77.djvu/198

192 true to its type because of self-pollination, and during the growing season the plants can be compared and any desirable type selected for future propagation. In a cross-pollinated plant like maize this is not the case. The pollen is carried by the wind through long distances and varieties planted close together are continually intercrossed. The isolation of a particular type is not simple as in the case of wheat, but may be prolonged through many generations. Each prize ear selected for future planting will have had at least a few and possibly many of its seeds fertilized by pollen from less desirable strains. When these seeds are grown they of course again fertilize the seeds of the desirable plants with a frequency proportionate to their number.

In the case just cited recourse may be had to artificial self-pollination. Several hundred seeds are thus produced at one operation and the work of isolating the new variety is made materially easier. But suppose we are dealing with red clover where the flowers are small, almost sterile with their own pollen and produce only one seed. In this crop, the long and tedious method of continuous selection just mentioned must be used, for there is no other way. This method is often called the pedigree-culture method. The main idea of the plan is that the seeds of single plants are grown in isolated plots, and the character of the mother plant judged by the characters of the progeny. This method has given much better results than the so-called German method, which consisted in planting a mixed lot of seeds from several of the best plants. For example, the German sugar-beet raisers have for years analyzed large numbers of sugar-beets and have grown their seed from the mother beets showing the highest percentage of sugar. No particular attention was paid to the general average of the progeny of each beet; those were bred from which appeared to be the best as shown by the polariscope sugar test. In this way the amount of sugar produced per acre was gradually increased, but progress was slow and cessation of selection immediately caused the sugar content to decline.

To see the real reason of this we must go back to the time of Darwin. The data from which Darwin proved the doctrine of descent came in large measure from domestic animals and cultivated plants. He saw that plants varied among themselves and that by selection of the variants new types were gradually produced. From these facts he argued that all evolution had taken place by the selection of minute variations and generally through the selective agency of a contest for life taking place among all living organisms. This he called the agency of natural selection. Later, however, Bateson, Korshinsky and de Vries called attention to the fact that many new types of animals and plants are known to have originated suddenly. There was no gradual evolution of the type; it simply appeared fully formed. This hypothesis, called the "mutation theory," found great favor among plant breeders