32 
GENETICS: PF. E. CASTLE 
corresponding parts of the other twin. It is evident from the experimental 
data, notably that of Muller,^ that new connections are not formed with any 
torn fragment of chromosome which happens to come into the proper position, 
but that connections are always formed at exactly corresponding points with 
homologous systems of genes.^ It is like the replacement of one chemical 
radicle with another within a complex organic molecule and it seems highly 
probable that such is its real nature. 
^ The distances shown in Morgan's chromosome map in excess of 50 (admittedly not 
obtained experimentally but only by summation) are therefore too large. Accordingly, if 
one clings to the assumption that the arrangement of the genes is linear, it must be, not 
that the longer distances are too short, as Morgan has assumed, but that the short dis- 
tances are too long. Therefore, any hypotheses framed to account for an apparent short- 
ening of the long distances are superfluous. The long distances given by direct experi- 
ment are long enohigh; they approach the limit of the possible, viz., 50%. Thufe in table 65 
of Morgan and Bridges, we find the following high cross-over percentages given by direct 
experiment: — yellow-bar, 47.9; white forked, 45.7; and white-lethal sc, 46.0. What is needed 
therefore, if the linear arrangement hypothesis is retained, is a secondary h)^othesis to 
explain why the short distances given by experiment are too long. 
But if we abandon the hypothesis of linear arrangement, all secondary hypotheses are 
unnecessary. The experimentally obtained cross-over percentages may be accepted at 
their face value, which in every case fall within the limits of the possible, 0 and 50. 
2 Morgan, T. H., and Bridges, C. B., Sex-linked inheritance in Drosophila. Carnegie 
Inst. Washington, PubL, No. 237, 1916, (88 pp., 2 pi.). 
3Weinstein, A., Genetics, 3, 1918, (135-172). 
^ MuUer, H. J., Amer. Nat., Lancaster, Pa., 50, 1916. 
5 The case of 'deficiency' studied by Bridges {Genetics, 2, pp. 445-460, Sept. 1917) forms 
an apparent exception to the rule. Here a certain segment of the linkage system was as 
regularly wanting as it is commonly present. The regularity of the process, however, shows 
that the principle of union at particular points still holds. In the deficiency race, a new 
and simplified linkage system had been established and this persisted. 
THE LINKAGE SYSTEM OF EIGHT SEX-LINKED CHARACTERS 
OF DROSOPHILA VIRILIS {DATA OF METZ^) 
By W. E. Castle 
BussEY Institution, Harvard University 
Communicated December 9, 1918 
In an earlier paper it has been shown that the arrangement of the genes in 
the sex-chromosome of Drosophila ampelophila is probably not linear, and a 
method has been developed for constructing a model of the experimentally 
determined linkage relationships. From such a model one may by direct 
measurement ascertain what other undetermined linkage values are likely to 
be. In order to test the utility of this method, it is desirable that it be tried 
out as widely as possible and the results for different cases compared with each 
other. For such use, suitable material is found in a paper by Metz^ dealing 
