Introduction

In the eastern half of Africa the forests are mainly restricted to mountains and surrounded by savanna or even semi-desert. They have very appropriately been compared with an archipelago by White (1981). Their isolation and the high degree of endemism of flora and fauna raise questions about the evolutionary and geographic history of the forests as a whole and of the species living there. As with real islands the central questions are:

1. 1. From the geographic point of view, have these islands always been isolated or is there a history of interconnections?

2. 2. From the biological point of view, how are the species distributed and related, and where do their sister species live?

Starting from an allopatric speciation model there is a causative correlation between the two questions in such a way that the geographic history of the islands must have influenced the evolution of the species living in the islands. Biogeography is concerned with this correlation. To put it in a simple way, the biogeographic question is:

1. 3. Have the species of a particular island originated on the spot (and if so, how about the ancestors) or are they colonists from elsewhere (either by jump dispersal or following a range expansion of the habitat), or a mixture of both (and if so, what are the proportions)?

With real islands the ecological difference between the island and its surroundings (the sea) is so extreme that an origin of the great majority of the terrestrial island organisms from the sea is most unlikely if not out of question.