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portion of this chapter is exclusively devoted to the plant, the treatment of which naturally comes first.
Variation. It has been recognized for at least a century that wheat is capable of variations. These may be peculiar to the plant itself, and may occur although the environment remains constant. Variation in this sense became established only with the theory of evolution, and refers to those changes which tend to become permanent through inheritance. Such variations are assumed to be the manifestations of a natural tendency inherent to all organic life.
The theory of common descent for all living beings found its first great advocator in Lamark at the beginning of the nineteenth century. Fifty years later Darwin assembled enough evidence in support of the theory to enable it to gain general acceptation. Darwin assumed that the great variation involved in the theory proceeded in the main by slow and gradual changes. He recognized, however, that species may also originate in nature by leaps and sports. The theory that all variation occurs by sudden mutations has been held by a minority of scientists. Cope and De Vries' are among those who have most recently increased the evidence in this direction. A defence of discontinuous evolution has also been made by various other scientists, such as the paleontologist Dollo, the zoologist Bateson, and the botanist Korshinsky. In general, it may be said that if the followers of Darwin have been open to the criticism of under-emphasizing sudden change, the supporters of the theory of mutations have certainly erred more widely in the opposite extreme.
Variations may also be induced. In this process two different methods may be used, hybridization and change of environment. Only those variations which may occur or be induced independently of environment are considered in this chapter. Others are treated in subsequent chapters. Variations may include differences in habit of growth, chemical composition, periods of development, appearance, form, yield, prolificacy, tion of variation may be, the scientific wheat grower utilizes the process in two different ways, by the simple process of selection, or by the compound process of selection, hybridization and selection.
1 An able criticism of the theory of mutations has been made by Prof W. F. R. Weldon, "Professor De Vries on the origin of species," Biometrika, 1:365, 1902. A study of this theory is interesting in conjunction with the more elaborate theory of homotyposis developed by Prof Karl Pearson in his work at University College, England, but space forbids a discussion of the matter here.
Selection is an unfailing means for the modification of form and tendency in organic life. It augments the power of variation by successively selecting the most marked variations in any direction. While conscious selection is a modern process which has attained commercial importance at a comparatively recent date, there is no doubt of selection having been one of the most powerful influences from the very first in developing wheat, although men were not aware of its operation. Whatever protection or cultivation early man bestowed upon the cereal plants was naturally bestowed upon the grasses and wheats which produced the most food in return, and not upon those comparatively less important as food. The very essence of the importance attached to wheat has always been its food yielding quality. It is a perfectly sound inference that those varieties of wheat which had this quality in the highest degree had an advantage which aided them to survive other varieties. This, however, is only the operation of the prime factor of selection, or, as Darwin calls it, the “law of the preservation of the favorable individual differences and variations, and the destruction of those which are injurious."
Selection and cultivation, in the ordinary sense, were the processes of domestication. After domestication, varieties continue to be propagated in a similar manner. The results have been attained none the less advantageously and certainly on account of the fact that man was unconsciously the selecting agent. To this force of artificial selection was added that of natural selection in early development, which was a result of the coincidence that the quality of wheat as a human food and the reproductive functions of the plant were both united in its seed. The plant producing the greatest number of seeds was most apt to survive, not only because man was most likely to give it his fostering care, but also because of the increased chances of reproduction. In wheat artificially sown, care must be exercised lest this force of natural selection operate disadvantageously, for fewer seeds are no longer a disadvantage in reproduction. If for any reason, such as being brought to a
new climate, wheat shows an unusual tendency to vary, it changes, and if the better yielding plants are crowded out the change results in a lower yield. Virgil' mentions selection of seed before the time of Christ, and noticed its advantages. A Scottish agriculturist, Shireff, made discoveries pertaining to the selection of wheat as early as 1819. The Belgian horticulturist, Van Mons, scientifically practiced selection before 1835. The works of Le Couteur, the English breeder, show that selection in wheat was early practiced, but. never long continued or repeated. One of the early experiments in selection of wheat was that of Hallett in England, begun in 1857. He selected the best heads and kernels. The following table gives his results.
Thus by means of repeated selection alone, the length of the ear was doubled, the number of grains per head was nearly trebled, and the tillering power was increased over fivefold. It is only within recent years that wheat experiments of this nature have been carried on in America. The most extensive and successful of these were begun in 1892 at the Minnesota experiment station under the direction of Prof. W. M. Hays. From 1891 to 1896 experiments were made in Kansas with light, common and heavy seed, and seed from selected heads. The light seed uniformly gave a lower yield, but common seed gave the highest yield during three years. At the Minnesota station from 1895 to 1898, No. 169, a wheat selected on principles similar to those of Hallett, gave an average yield of 28.3 bushels per acre, while during the same years the un
Georgics I., lines 286-288.
2 Neb. Bul. 32. p. 91.
selected parent sort yielded only 22.5 bushels, an increase during four years of 5.8 bushels per acre. In ten years nearly 25 per cent in yield was gained.'
Ninety-six tests of selected wheat seed during the years 1900 to 1902 at the Canada experiment farms gave an average gain of about 3.6 per cent in favor of selection. Principles differing somewhat from those usually followed in selection were
CROSSING AS A CAUSE OF VARIATION: Yield in grain of 100 plants, showing greater variation in yield of hybrid ttan
of parents. Yield of hybrid shown by x line. (After Hays.)
utilized by Lyon. His selections were for quality rather than quantity. He experimented with the smallest and lightest kernels on account of their high nitrogen content. Heavy seed planted at the rate of 1.5 bushels per acre gave a greater yield of wheat the first year than light seed sowed at the same rate. Selecting heavy seed grown from the heavy wheat and light from the light wheat, the difference in yield in 3 or 4 years was small. After the first year of the separation, the light seed gave much the greater amount of proteids per acre. Lyon points out, however, that proteid nitrogen is no index to the amount of gluten, which is the better basis for improvement. It is not yet decided whether selection should be for plants with large heads or for plants with a large number of medium-sized heads. In general, the results of many experiments seem to favor the selection of large seed.'
1 Hays, Plant Breeding, p. 10.