The Arrangement of Genes in Drosophila Virilis

cism misses the mark. It need only be stated that Castle was wrong in supposing that the cross-overs between yellow and white should be subtracted from the total before the white bifid cross-over value is calculated. If, as Castle states, he cannot conceive of a mechanism "which would tie two genes together in such a way that they will subsequently separate from each other oftener than they will remain together, yet this is what the idea of cross-overs in excess of 50 per cent amounts to," it would seem to follow that he has not really understood the mechanism that we have described, and which he has attacked; for, whether such a mechanism really exists or not, it is nevertheless a conceivable mechanical device that could do just this thing. We have left to Dr. Metz the opportunity to answer Castle's criticism relating to D. virilis. To sum up: we believe that we have met all the pertinent criticisms that Castle has brought forward of our methods and conclusions, and that he has failed to meet our criticism of his three dimensional model.