Further Data on Linkage in Gammarus

T H E work of Morgan and his pupils has finally proved for the Dipteran Drosophila (i) that the factors of Mendelian heredity are contained in the chromosomes; (2) that they are disposed within the chromosomes in a constant and linear arrangement; (3) that a special mechanism exists for ensuring that factors within the same chromosome can become separated from each other—the mechanism of crossing-over. Of these three statements, we may regard (1) as universally valid ; (2) as probably also valid universally, although it will take much work before this can be definitely established. As to (3), we know that wherever factors have been found to be linked—i.e. in the same chromosome—they have always been found to exhibit crossingover, at least in one sex. However, in no organism except Drosophila do we know anything accurate as to the detailed mechanism of the process, although it appears probable that in other organisms, e.g. the Sweet Pea (Punnett, 1923), the mechanism is not dissimilar.