There are at least two such systems among the factors in the first chromosome in _Drosophila_. The first of these includes the factor for white eyes, that for eosin eyes, and that for cherry eyes, and of course that allelomorph of these factors present in the wild fly and which when present gives the red color. In this instance the normal {12} allelomorph dominates all the other three, but in mice the mutant factor for yellow dominates the wild or "normal" allelomorph. The other system of multiple allelomorphs in the first chromosome is a triple system made up of yellow (body-color), spot (on abdomen), and their normal allelomorph--the factor in the normal fly that stands for "gray." In general it may be said that there are two principal ways in which it is possible to show that certain factors (more than two) are the allelomorphs of each other. First, if they are allelomorphs only two can exist in the same individual; and, in the case of sex-linked characters, while two may exist in the same female, only one can exist in the male, for he contains but one X chromosome. Second, all the allelomorphs should give the same percentages of crossing-over with each other factor in the same chromosome. It is a question of considerable theoretical importance whether these cases of multiple allelomorphs are only extreme cases of linkage or whether they form a system quite apart from linkage and in relation to normal allelomorphism. It may be worth while, therefore, to discuss this question more at length, especially because _Drosophila_ is one of the best cases known for such a discussion. The factors in the first chromosome are linked to each other in various degrees. When they are as closely linked as yellow body-color and white eyes crossing-over takes place only once in a hundred times. If two factors were still nearer together it is thinkable that crossing-over might be such a rare occurrence that it would require an enormous number of individuals to demonstrate its occurrence. In such a case the factors might be said to be completely linked, yet each would be supposed to have its normal allelomorph in the homologous chromosome of the wild type. Imagine, then, a situation in which one of these two mutant factors (a) enters from one parent and the other mutant factor (b) from the other parent. The normal allelomorph of a may be called A. It enters the combination with b, while the normal allelomorph B of b enters the combination with a. Since b is completely linked to A and a to B, the result will be the same as though a and b were the allelomorphs of each other, for in the germ-cells of the hybrid aBAb the assortment will be into aB and Ab, which is the same as though a and b acted as segregating allelomorphs.