Breaks in the distribution of forest mammals (of which the best data are for the primates), frogs and cyprinodont fishes and their subspeciations have been attributed by authors to the barriers presumably presented by a savanna gap formed by past extension of the Baoule V in the Côte d'Ivoire, by the present Dahomey Gap and by the course of the lower Niger. The potentials of these three features are considered and their history is discussed in the light of up-to-date geological and palaeo-climatological information. As part of the back-ground, evidence for the rate at which evolution can occur is summarized. The distributional data for each of the three classes and also for birds are then set out. Each class of animal is found to show different problems at taxonomic levels from the subspecies upwards.