It has been found that when _certain_ characters enter a cross together (_i. e._, from the same parent) their factors tend to pass into the same gamete of the hybrid, with the result that other ratios than the chance ratios described by Mendel are found in the F_2 generation. {6} Such cases of linkage have been described in several forms, but nowhere on so extensive a scale as in the pomace fly, _Drosophila ampelophila_. Here, over a hundred characters that have been investigated as to their linkage relations are found to fall into four groups, the members of each group being linked, in the sense that they tend to be transmitted to the gametes in the same combinations in which they entered from the parents. The members of each group give free assortment with the members of any of the other three groups. A most significant fact in regard to the linkage shown by the _Drosophila_ mutants is that _the number of linked groups corresponds to the number of pairs of the chromosomes._ If the gens for the Mendelian characters are carried by the chromosomes we should expect to find demonstrated in _Drosophila_ that there are as many groups of characters that are inherited together as there are pairs of chromosomes, provided the chromosomes retain their individuality. The evidence that the chromosomes are structural elements of the cell that perpetuate themselves at every division has continually grown stronger. That factors have the same distribution as the chromosomes is clearly seen in the case of sex-linked characters, where it can be shown that any character of this type appears in those individuals which from the known distribution of the X chromosomes must also contain the chromosome in question. For example, in _Drosophila_, as in many other insects, there are two X chromosomes in the cells of the female and one X chromosome in the cells of the male. There is in the male, in addition to the X, also a Y chromosome, which acts as its mate in synapsis and reduction. After reduction each egg carries an X chromosome. In the male there are two classes of sperm, one carrying the X chromosome and the other carrying the Y chromosome. Any egg fertilized by an X sperm produces a female; any egg fertilized by a Y sperm produces a male. The scheme of inheritance is as follows. +------------------------------+ | | | Eggs X--X | | Sperm X--Y | +------------------------------+ | Daughter XX | | Son XY | | | +------------------------------+