These cross-over values were plotted and gave a bimodal curve with modes 7 units apart.
"Sex-linked Inheritance in Drosophila"
Thomas Hunt Morgan Calvin B. Bridges
Errors of sampling may similarly account for some, though not all, of the curves of the bimodal type shown in Figure 29C. Some variation in the curves might be ascribed to the variations in kinds of species comprising the individual flights at different times at different places, provided that it could be demonstrated that different species of birds show dissimilar temporal patterns.
"A Quantitative Study of the Nocturnal Migration of Birds. Vol.3 No.2"
George H. Lowery.