0.0 | Yellow, spot 0.7 | Lethal I 1.1-| Lethal Ib 1.1 | White, eosin, cherry 2.2 | Facet 2.4 | Abnormal 2.6 | Notch 3.3 | Lethal Ia | 6.3 | Bifid | | | | | | 12.5 | Lethal II 14.6 | Lethal sb 16.7 | Club 17.5 | Lemon 17.8 | Shifted 18.0 | Depressed 19.5 | Lethal IIIa | | 23.7 | Lethal sa | 26.5 | Lethal III | | | 33.0 | Vermilion 33.+/- | Dot 34.7 | Reduplicated 36.1 | Miniature 38.0 | Furrowed | | 43.0 | Sable | | | | | | 55.1 | Rudimentary 56.5 | Forked 57.0 | Bar 59.5 | Fused | | | 66.2 | Lethal sc DIAGRAM I. {23} Diagram I has been constructed upon the basis of all the data summarized in table 65 (p. 84) for the first or X chromosome. It shows the relative positions of the gens of the sex-linked characters of _Drosophila_. One unit of distance corresponds to 1 per cent of crossing-over. Since all distances are corrected for double crossing-over and for coincidence, the values represent the _total_ of crossing-over between the loci. The uncorrected value obtained in any experiment with two loci widely separated will be smaller than the value given in the map. It may be asked what will happen when two factors whose loci are more than 50 units apart in the same chromosome are used in the same experiment? One might expect to get more than 50 per cent of cross-overs with such an experiment, but double crossing-over becomes disproportionately greater the longer the distance involved, so that in experiments the observed percentage of crossing-over does not rise above 50 per cent. For example, if eosin is tested against bar, somewhat under 50 per cent of cross-overs are obtained, but if the distance of bar from eosin is found by summation of the component distances the interval for eosin bar is 56 units. In calculating the loci of the first chromosome, a system of weighting was used which allowed each case to influence the positions of the loci in proportion to the amount of the data. In this way advantage was taken of the entire mass of data.