Note 3 — Chapter II

These fraternal twins, which show no special resemblance to each other, are like the multiple births of other animals, and there is no disposition for them to be of the same sex. In the sheep, for example, statistics show that the frequency of pairs of twins, male and female, is approximately double that of the frequency of pairs, both male or both female, as it should be if the sex-distribution were fortuitous. For instance Bernadin (La Bergerie de Rambouillet, 1890, p. 100) gives the following figures for twin-lambs in Merinos: both male, 87; both female, 83; sexes mixed, 187. The 9-banded Armadillo (Dasypus novemcinctus), in which the young born in one litter are said to be always of one sex, is the only known exception in Vertebrates, and is presumably a genuine case of normal polyembryony (see especially, Rosner, Bull. Ac. Soc. Cracovie, 1901, p. 443, and Newman and Patterson, Biol. Bull., XVII, 1909, p. 181), and an important paper lately published by H. H. Newman and J. T. Patterson, Jour. Morph., 1911, XXII, p. 855.