Page:Popular Science Monthly Volume 67.djvu/202

196, and the two can be separated again at will. So it is in the Belgian produced by cross-breeding with an albino. The albino character is there, though unseen, and will appear as a distinct entity when the cross-bred reproduces, for it will be represented in approximately half of the sex-cells formed by the cross-bred animal, the alternative or Belgian character being represented in the other half. It is as if the two pieces of glass, combined originally to illustrate the formation of a cross-bred animal, were separated again to illustrate the formation of the reproductive elements by the cross-bred. For every element formed having the opaque character, there will be another

having the transparent character, but there will be no elements of an intermediate nature.

This simple principle, that in alternative inheritance sex-cells of two sorts are formed by cross-bred individuals, constitutes one of the most important discoveries ever made in the study of heredity. The discovery was made about forty years ago by an Austrian monk, Gregor Mendel, who was engaged in the study of cross-bred garden peas. It, however, attracted little attention at the time and was soon forgotten. Meanwhile, a great body of workers was studying with great minuteness the material basis of heredity, the sexual elements. Their investigations disclosed in the cell a complete basis for just this kind of alternative inheritance and led up to the rediscovery of Mendel's law simultaneously by several different experimental breeders, foremost among whom was the Dutch botanist, de Vries.