ABSTRACT
Evolutionary discussion is often greatly concerned with the idea of inter-specific barriers, in the sense of physiological and morphological impediments to effective hybridization. Several conditions would have to be observed in experiments on the hybridization of monkeys. The hybridization of two species or genera might also be effectively prevented by differences in their gestation periods. If the occasional notes about the appearance of the hybrids are to be relied upon, it would seem that the generic and specific characters of monkeys behave in hybridization like varietal differences. The few facts there are relating to the fertility of hybrid monkeys refer only to the F.1 generation of the cross between Macaca mulatta and Macaca irus. In spite of the almost complete absence of data relating to the fertility or sterility of the offspring of inter-generic crosses, the facts throw a new light on the problem of the divergence of the Old World Primates from a common ancestor.
