SOME OF THE PRINCIPLES OF PLANT BREEDING We are inclined to think that plant -breeding is based on old and well-established laws. The fact is, however, that the fundamental principles of plant breeding were not made known until the latter part of the eighteenth century. The sexuality of plants was established experimentally by Camera rius in 1691, and the first hybrid of which we have any record was made by Thomas Fairchild, an English gardener, in 1719, being a cross of the carnation with the sweet William. Hybrids were carefully studied by Koelreuter, but not from a practical breeding standpoint. Plant-breeding had its real beginning with the work of Thomas Andrew Knight, an eminent English plant physiologist, working in the early days of the nineteenth cen tury. About the same time Van Mons, a Belgian horticulturist, also carried out experiments in a similar direction. A large part of our knowledge of plant-breeding has come down to us from these two investigators. Knight worked mainly in hybridization, and in 1806 said : " New varieties of every species of fruit will generally be better obtained by introducing the farina of one variety of pollen into the blossoms of another than by propagating from a single kind." Knight also enunciated what we may call the law of food sup ply, which is now generally recognized. This pred icates that one of the principal factors which causes or induces variation in plants is an increase of food supply or a modification thereof. Van Mons worked mainly in selection, and it is inter esting to note that his experiments were carried out primarily with pears. He preached the doctrine of continuous selection, and produced very many valuable varieties. Van Mons and Knight, there fore, were the exponents of the important factors of selection and hybridization in plant - improve ment. It is probable that a large part of the suc cess of Van Mons' work was due to the fact that pears are normally sterile to their own pollen, requiring cross-fertilization, and, therefore, many of his new varieties were probably hybrids. He
was not aware of this fact, however, and it made no great difference in the establishment of the prin ciple which has since proved to be so important.
In this country very valuable work was done in the improvement of plants and in discovering the principles of plant-breeding, by Carman, Pringle, Hovey, Ricketts, Rogers, and others, and in more recent years by Burbank, Hopkins, Hays, Bailey, and very many others.
The rediscovery of Mendel's now famous law by DeVries and Correns, in 1900, and the publication of DeVries' Mutation Theory in the same year, marked the beginning of a new era in plant-breed ing. No matter what the final conclusions may be regarding Mendel's principles and the mutation theory, the general attention and investigation directed to plant-breeding as the result of these two theories will serve greatly to modify and extend our understanding of the general laws of breeding.
Classification of varieties.
To understand clearly the character of organisms with which we are dealing, we need careful defini tions of the different groups of cultivated plants which are ordinarily known as varieties. We speak of varieties of wheat, corn, apples and pears, yet we know that these varieties differ from each other as natural groups. In order to distinguish clearly these differences, the writer has proposed the fol lowing classification of varieties into races, strains and dons: Races are groups of cultivated plants which have well-marked differentiating characters, and propa gate true to seed except for simple individual vari ations. The different groups of beans, peas, wheat, oats, corn, cotton, and the like, referred to com monly as varieties, are thus in a more restricted sense races. Boone County White. Learning, Reid's Yellow Dent, and the like, would be recognized as races of field corn, and Turkey Red, Fulcaster, Fultz, and the like, as races of wheat.