Page:Popular Science Monthly Volume 64.djvu/115

Rh and dark elements. But at some time in their later history, and presumably at the time when the egg sends off its polar bodies and at the time when the four spermatozoa are formed, a separation of the dark from the white elements occurs, so that two cells of the one kind and two of the other are formed. Thus the germ-cells are, as it were, purified, and consist of those that contain only white and of those that contain only dark elements. This is supposed to be the condition of the germ-cells in the ovary and in the testis of the primary hybrids.

Suppose now that these hybrids breed together, the white and the black spermatozoa will meet the white and the black eggs, and since it is a question of chance alone how they will come together, all possible combinations will be made. When a white germ-cell meets with a white one, a white individual results, and since it contains only white elements all its descendants will be white (if it is bred, of course, to white individuals). If a gray germ-cell meets a gray germ-cell, a gray individual will result, and all its purely bred descendants will be gray. If, however, a white and a gray germ-cell unite, the individual that develops will contain both elements in all its body-cells and, since the gray always dominates in such combinations, the individual will be gray, but will have the white as a recessive character that may crop out in subsequent generations. On the theory of chance combinations there will be twice as many of these gray-white individuals as of the white or of the pure gray. The series stands 2:1:1. Since in outward appearance all the gray-white mice are like the pure gray, we get three grays to every one white.

Let us now return to Castle's theory and see how he tries to make an application of Mendel's principle to sex. Just as there are two kinds of mice in our illustration, white and black, there are two kinds cf sexual individuals, males and females. It is now assumed that the germ-cells, when they reach their final divisions, separate their male from their female elements, giving pure male and pure female eggs, and pure male and pure female spermatozoa. If, as in the mice, all chance combinations of the germ-cells are possible, there will result three kinds of individuals in the proportion of 2:1:1. The first of these, that are twice as common as either of the other two, would be sex-ybrids. If we assume, as in the mice, that one character always dominates in such a combination, the male let us say, there would be twice as many males of this hybrid kind as there are individuals of either of the other two pure kinds, and since there are as many pure males as there are pure females, there would be in all three times as many males born as females. Since we know that there is no such disproportion of one sex to the other, it appears absurd to attempt to